JPH11258498A - Projective lens and scanning exposure device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provided a projective lens by which the deterioration of the anisotropic imaging characteristic caused by the absorption of the illumination light can be inhibited by performing the initial adjustment so that a first direction (X direction) perpendicular to an optical axis and a second direction (Y direction) perpendicular to the optical axis and the first direction have the different imaging characteristics. SOLUTION: A wafer stage WST comprises an XY stage 18 movable on a base in the Y-axis direction as a scanning direction and an X-axis direction perpendicular to the Y-axis direction, and a Z stage 17 mounted on the XY stage 18. In the initial adjustment of a projective optical system PL forming a scanning exposure device 100, the anistropic distortion is adjusted to be sorting manner. By performing the initial adjustment of the projective optical system PL to estimate and sorting the anistropic imaging characteristic by the absorption of the illumination light of the projective optical system PL, the deterioration of the exposure accuracy caused by the change of the anisotropic imaging characteristic can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a projection lens and a scanning type exposure apparatus, and more particularly, to a scanning method for transferring a mask pattern onto a substrate via a projection optical system while moving the mask and the substrate synchronously. The present invention relates to a mold exposure apparatus and a projection lens suitable as a projection optical system of the scanning exposure apparatus.

[0002]

2. Description of the Related Art Conventionally, various types of exposure apparatuses have been used for manufacturing a semiconductor element or a liquid crystal display element by a photolithography process, and at present, a photomask or a reticle (hereinafter, collectively referred to as a “reticle”). Projection exposure apparatus for transferring the pattern of (1) through a projection optical system onto a substrate such as a wafer or a glass plate having a surface coated with a photosensitive agent such as a photoresist, for example, a so-called step-and-
Repeat type reduction projection exposure apparatus (so-called stepper)
Is used.

In recent years, with the increase in the degree of integration of semiconductor elements, a reticle is illuminated with rectangular or arc-shaped illumination light, and a reticle pattern is formed by synchronously scanning a reticle and a substrate in a one-dimensional direction with respect to a projection optical system. Scanning exposure apparatuses such as a so-called slit-scan method or a so-called step-and-scan method for sequentially transferring images onto a substrate via a projection optical system have been developed. According to such a scanning type exposure apparatus, the reticle pattern can be transferred using only a part (central portion) of the effective exposure field of the projection optical system having the least aberration. It becomes possible to expose a fine pattern with higher precision. Further, according to the scanning type exposure apparatus, the exposure field can be expanded in the scanning direction without being restricted by the projection optical system, so that a large area exposure can be performed. In addition, there is an advantage that the relative scanning of the wafer has an averaging effect, and an improvement in distortion and depth of focus can be expected.

Incidentally, the imaging characteristics such as the projection magnification of a projection optical system used in a projection exposure apparatus include a slight change in temperature of the apparatus, a slight change in atmospheric pressure in a clean room where the projection exposure apparatus is placed, and a change in humidity. The magnification fluctuates near a predetermined magnification due to the change, the irradiation history of the irradiation energy of the projection optical system with the exposure light, and the like. For this reason, recent projection exposure apparatuses are provided with an imaging characteristic correction mechanism for finely adjusting the imaging characteristics of the projection optical system in order to maintain desired imaging characteristics. As the imaging characteristic correcting mechanism, for example, a mechanism for changing the distance between the reticle and the projection optical system, a specific lens element constituting the projection optical system is driven in the optical axis direction, or is driven in the tilt direction. A mechanism or a mechanism for adjusting the pressure in a predetermined closed chamber provided in the projection optical system is known.

[0005]

However, in a scanning exposure apparatus, the projection optical system absorbs illumination light because the illumination area for illuminating the reticle has a rectangular or arc shape elongated in the non-scanning direction. Accordingly, non-rotational symmetry, that is, a change in imaging characteristics (hereinafter, referred to as “anisotropic imaging characteristics” in the present specification) in which changes in two orthogonal directions orthogonal to the optical axis are different.

This will be described in more detail. Here, a case where the illumination area is a rectangle (rectangle) elongated in the non-scanning direction will be described as an example. FIG. 11 shows a view of the projection optical system PL ′ used in the scanning exposure apparatus as viewed from the optical axis direction. An area IA indicated by a dotted line in this figure is an illumination area. In general, a projection optical system absorbs illumination light for exposure during exposure and generates a temperature distribution. However, in a scanning type exposure apparatus, since the illumination area IRA is rectangular, the temperature distribution is as shown in FIG. The distribution depends on the illumination area. As is apparent from FIG. 12, the temperature distribution gradually increases from the periphery to the center in the non-scanning direction and is uniformly illuminated. The temperature changes rapidly from the low temperature part to the high temperature part in the center, and the illumination area is considerably limited. Therefore, the curvature of the projection optical system PL ′ changes in the non-scanning direction as schematically shown in FIG. 13 which is a cross-sectional view taken along the line AA in FIG. As schematically shown in FIG. 14, which is a cross-sectional view taken along the line BB, a change in curvature is large in the illumination region. 13 and 14 show that the projection optical system PL ′ is
Although it is schematically shown as a single lens, it is safe to assume that each lens element constituting the projection optical system causes the above-described thermal deformation, or the projection optical system PL 'itself is replaced by a single large-sized lens. It may be considered that the lens causes a thermal deformation as described above.

Due to the bias of the temperature distribution in the scanning direction and the non-scanning direction in the projection optical system PL 'and the accompanying rotationally asymmetric thermal deformation in the scanning direction and the non-scanning direction of the projection optical system PL', The anisotropic imaging characteristic change as described above occurs. The illumination region I is absorbed by the illumination light of the projection optical system PL ′.
A rectangular distortion occurs in the projected image of the RA on the wafer surface. As shown in FIG. 15, a line and space (L / L) arranged at the center of the illumination area IRA on the reticle R (that is, near the center of the optical axis of the projection optical system PL ′) and having a predetermined period in the scanning direction. S) Consider a case where a composite pattern composed of a pattern (hereinafter referred to as “H pattern”) and an L / S pattern (hereinafter referred to as “V pattern”) having a predetermined period in the non-scanning direction is projected on a wafer. In this case, as described above, since anisotropic temperature distribution is generated by absorption of the illumination light, as shown in FIG. 16, diffracted light in the non-scanning direction generated from the V pattern is relatively emitted by the projection optical system PL ′. An image is formed on a predetermined imaging surface (V-pattern best focus surface) through a surface having a gradual change in curvature. In contrast, FIG.
As shown in FIG. 7, the diffracted light in the scanning direction generated from the H pattern forms an image on a predetermined imaging plane (H pattern best focus plane) through a surface of the projection lens where the curvature change is relatively large. As a result, as shown in Figure 18, the the V pattern best focus plane and H pattern best focus plane, the deviation of only F _CE occurs. That is, the shift of the imaging plane in the two orthogonal directions on the optical axis center of the projection optical system PL ′ (in this specification, the shift of the image forming position in the first direction and the second direction on the optical axis center). The shift is called “center ass”).

The anisotropic imaging characteristics such as the center distortion and the like of the above-mentioned rectangular distortion have not been a problem so far, but a recent device rule of 0.25 μm
In the era, the influence of such anisotropic imaging characteristics on exposure accuracy has come to the fore. However, it is difficult to correct such anisotropic imaging characteristics with the above-described current imaging characteristic correction mechanism.

The present invention has been made under such circumstances, and a first object of the present invention is to provide a projection lens capable of suppressing deterioration of anisotropic imaging characteristics due to absorption of illumination light. It is in.

A second object of the present invention is to provide a scanning type exposure apparatus capable of suppressing a change in anisotropic imaging characteristics due to absorption of illumination light of a projection optical system and improving exposure accuracy. It is in.

Further, a third object of the present invention is to provide a scanning exposure apparatus which can reliably prevent the occurrence of exposure failure due to the deterioration of anisotropic imaging characteristics caused by the absorption of illumination light of a projection optical system. Is to provide.

[0012]

According to a first aspect of the present invention, there is provided a projection lens (PL) for projecting an image of a first object (R) onto a second object (W), the projection lens being orthogonal to an optical axis thereof. The first direction (X direction) and the second direction (Y direction) orthogonal to the optical axis and the first direction are initially adjusted so that imaging characteristics are different.

Usually, the projection lens forms an image of the first object on the second object.
The light is illuminated with illumination light to project it on an object, and the imaging characteristics change due to absorption of the illumination light. This change in the imaging characteristics depends on the temperature distribution of the projection lens due to absorption of the illumination light, and may cause non-rotationally symmetric changes in the imaging characteristics. Therefore, as described in the present invention, by initially adjusting the imaging characteristics so that the imaging characteristics are different between the first direction orthogonal to the optical axis and the second direction orthogonal to the optical axis and the first direction, for example, In some cases, non-rotationally symmetric changes in imaging characteristics can be suppressed.

[0014] In this sense, according to the second aspect of the present invention, when illuminated with illumination light of a predetermined shape, the connection between the first direction and the second direction is considered in consideration of a variation caused by absorption of the illumination light. It is desirable to initially adjust the image characteristics. In such a case, a non-rotationally symmetric change in imaging characteristics due to absorption of illumination light can be reliably suppressed.

In this case, the initial adjustment of the imaging characteristics in the first direction and the second direction is performed in the first direction and the second direction caused by the absorption of the illumination light. It is desirable that the correction be performed by shifting each of the desired imaging characteristics so as to cancel only half of the change amount of the image characteristics. In such a case, the maximum deterioration of the imaging characteristics due to the absorption of the illumination light can be suppressed to substantially half.

In the projection lens according to the second or third aspect of the present invention, various imaging characteristics to be subjected to the initial adjustment are conceivable. For example, as in the invention according to the fourth aspect, the image forming characteristic to be subjected to the initial adjustment is provided. The image characteristic may be a rectangular distortion having the first direction as a long side. In this case, it is preferable that the initial adjustment of the rectangular distortion is performed by largely shifting the magnification in the second direction from a desired magnification as compared with the first direction. The rectangular distortion having the long side in the first direction occurs when the normal illumination area has a shape that is long in the first direction, and the projection lens is almost uniformly illuminated in the first direction, and the projection lens in the second direction. A limited area is illuminated. Therefore, the change in magnification of the projection lens due to the absorption of illumination light is large in the second direction and small in the first direction. Therefore, the magnification in the second direction is largely deviated from the desired magnification as compared with the first direction. Therefore, rectangular distortion can be suppressed.

Further, in the projection lens according to the second or third aspect, as in the invention according to the sixth aspect, the image forming characteristics to be subjected to the initial adjustment include the first direction around the optical axis and the second direction. The deviation of the imaging position in the direction may be used. In this case, as in the invention according to claim 7, the initial adjustment of the shift of the imaging position between the optical axis center in the first direction and the second direction is performed by adjusting the image plane of the periodic pattern in the first direction. It is desirable that this is performed by shifting a predetermined one of the periodic pattern in the second direction and the imaging plane closer to the projection optical system than the other. For example, when the projection lens is illuminated with rectangular illumination light that is long in the first direction, the position of the image plane of the periodic pattern in the second direction is changed in the first direction due to the temperature distribution due to the absorption of the illumination light. Is farther from the projection optical system than the position of the imaging plane, so that this is canceled in advance, that is, the imaging plane of the periodic pattern in the second direction is shifted from the imaging plane of the periodic pattern in the first direction. The worst value of the change in the shift of the imaging position between the optical axis center in the first direction and the second direction can be effectively suppressed by shifting to the direction closer to.

According to the present invention, the mask is illuminated with illumination light of a predetermined shape while the mask (R) and the substrate (W) are synchronously moved, and the pattern is projected onto the projection optical system (PL).
A scanning exposure apparatus for transferring the image on the substrate via a projection lens, wherein the second direction is the synchronous movement direction, and the projection optical system is equipped with the projection lens according to any one of claims 1 to 7. It is characterized by having done.

According to this, since the projection lens according to each of the first to seventh aspects of the present invention is provided as a projection optical system, the first direction (non-scanning direction) and the first direction due to absorption of illumination light by this projection lens. It is possible to suppress the deterioration of the change in the anisotropic imaging characteristic in two directions (synchronous movement direction, that is, the scanning direction), and as a result, it is possible to improve the exposure accuracy.

According to a ninth aspect of the present invention, the mask (R) and the substrate (W) are relatively scanned in synchronization with each other in a predetermined scanning direction, and the mask is illuminated with illumination light of a predetermined shape, thereby forming the pattern. Is transferred onto the substrate via a projection optical system (PL), wherein the projection optical system has different imaging characteristics between the scanning direction and a non-scanning direction orthogonal to the scanning direction. In consideration of the deviation of the imaging position between the non-scanning direction and the scanning direction of the center of the optical axis of the projection optical system, by positioning the substrate, the imaging surface of the projection optical system and the substrate Focus correction devices (12, 17, 19, 2) for adjusting the positional relationship of the projection optical system in the optical axis direction
1, 42).

According to this, during the scanning exposure, the substrate is positioned by the focus correction device in consideration of the deviation of the image forming position between the non-scanning direction and the scanning direction of the optical axis center of the projection optical system. The positional relationship between the imaging plane of the projection optical system and the substrate in the optical axis direction of the projection optical system is adjusted. The adjustment of the positional relationship between the image plane of the projection optical system and the substrate in the optical axis direction, that is, the correction of the focus, is performed, for example, by forming the image plane of the periodic pattern in the non-scanning direction on the mask and the periodic pattern in the scanning direction. The focus target position is set at an intermediate position with respect to the image plane. For this reason, the defocus amount caused by the shift of the imaging position between the scanning direction around the optical axis and the non-scanning direction due to the absorption of the illumination light of the projection optical system during the scanning exposure can be reduced, for example, by half. Exposure accuracy can be improved.

In this case, a change in the anisotropic imaging characteristic due to absorption of the illumination light of the projection optical system (PL) is monitored, and the amount of the change is determined by a predetermined threshold. An imaging characteristic monitoring device (21) for interrupting the exposure operation when the value reaches the value may be further provided. In such a case, the anisotropic imaging characteristics change due to absorption of illumination light of the projection optical system, that is, non-rotationally symmetric imaging characteristics having different values in the scanning direction and the non-scanning direction, by the imaging characteristic monitoring device. Is monitored, and when the amount of change reaches a predetermined threshold, the exposure operation is interrupted. For this reason, it is possible to reliably prevent the occurrence of exposure failure due to the change in the anisotropic imaging characteristic exceeding the allowable value.

In this case, since a projection optical system having a different imaging characteristic between the scanning direction and the non-scanning direction orthogonal to the scanning direction is used, the projection optical system is used in the normal scanning direction and the non-scanning direction. Compared to the case of using a projection optical system adjusted to have the same imaging characteristics, the time until the above-described threshold value, that is, the allowable value is reached can be delayed.

According to the eleventh aspect of the present invention, the mask (R)
The mask is illuminated with illumination light of a predetermined shape while relatively scanning the substrate and the substrate (W) in synchronization with a predetermined scanning direction, and the pattern is transferred onto the substrate via a projection optical system (PL). A scanning exposure apparatus for monitoring a change in anisotropic imaging characteristics due to absorption of the illumination light by the projection optical system, and interrupting the exposure operation when the amount of the change reaches a predetermined threshold. An imaging characteristic monitoring device is provided.

According to this, by the imaging characteristic monitoring device,
The change in the anisotropic imaging characteristic due to the absorption of the illumination light of the projection optical system, that is, the change in the non-rotationally symmetric imaging characteristic having different values in the scanning direction and the non-scanning direction is monitored, and the amount of this change is a predetermined amount. The exposure operation is interrupted when the threshold value is reached. For this reason, it is possible to reliably prevent the occurrence of exposure failure due to the change in the anisotropic imaging characteristic exceeding the allowable value.

In the scanning exposure apparatus according to the tenth or eleventh aspect, various anisotropic imaging characteristics of the object to be monitored can be considered. The anisotropic imaging characteristic may be at least one of a rectangular distortion and a shift of an imaging position of the optical axis center in the non-scanning direction and the scanning direction.

In this case, as in the invention according to the thirteenth aspect, the imaging characteristic monitoring device (21) may determine the change in the rectangular distortion as a difference between a change in magnification in the non-scanning direction and a change in magnification in the scanning direction. The deviation of the imaging position in the non-scanning direction about the optical axis center and the scanning direction in the non-scanning direction and the imaging direction of the periodic pattern in the non-scanning direction formed on the mask (R) are monitored based on The monitoring may be performed based on the difference between the periodic pattern in the scanning direction and the image plane.

In the scanning exposure apparatus according to each of the tenth to thirteenth aspects of the present invention, it is possible to restart the exposure after a predetermined time has elapsed since the interruption of the exposure. If the time is too short, the anisotropic imaging characteristics reach the above threshold immediately after resuming. Conversely, if the above certain time is too long, the exposure interruption time becomes unnecessarily long and the throughput becomes longer. Is unnecessarily worse. Therefore, as in the invention according to claim 14, the imaging characteristic monitoring device (21) continues to monitor the change in the anisotropic imaging characteristic of the projection optical system even after the interruption of the exposure operation, It is desirable that the exposure operation be restarted when the imaging characteristic has attenuated to a predetermined reference. In such a case, there is no inconvenience as described above, and the minimum interruption time can be set.
In addition, it is possible to reliably prevent the occurrence of exposure failure due to the deterioration of the anisotropic imaging characteristics.

According to the ninth aspect of the present invention, there is provided a scanning exposure apparatus according to the fifteenth aspect, which corrects a change in rotationally symmetric imaging characteristics other than the focus of the projection optical system (PL). A characteristic correction device (14, 15) may be further provided. In such a case, the imaging characteristic correction device can correct rotationally symmetric imaging characteristics (magnification, distortion, curvature of field, coma, spherical aberration, etc.) other than focus, so that the line width controllability, Exposure accuracy such as overlay accuracy can be further improved.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIG.
This will be described with reference to FIG.

FIG. 1 shows a schematic configuration of a scanning type exposure apparatus 100 according to an embodiment in which a projection lens according to the present invention is provided as a projection optical system. The scanning exposure apparatus 100 is a projection exposure apparatus of a so-called step-and-scan method.

The scanning exposure apparatus 100 includes an illumination system including a light source 1 and illumination optical systems (2 to 10), a reticle stage R for holding a reticle R as a mask (first object).
ST, a projection optical system PL, an imaging characteristic correction mechanism 14 provided in the projection optical system PL for correcting imaging characteristics such as magnification, a lens controller 15 for controlling the imaging characteristic correction mechanism 14,
A wafer stage WST that holds a wafer W as a substrate (a second object) and moves two-dimensionally in an XY plane and a control system for these are provided.

The illumination system includes a light source 1, a first fly-eye lens 2, a vibration mirror 3, a second fly-eye lens 4, a half mirror 5, an integrator sensor 6, a reticle blind 7, a bending mirror 8, a condenser lens system 9, It is configured to include the reflectance sensor 10 and the like.

Here, the components of the illumination system will be described together with their functions. Illumination light IL as exposure light generated by the light source 1 passes through a shutter (not shown),
The first fly-eye lens 2 converts the illuminance distribution (intensity distribution) into a substantially uniform light flux. As the illumination light IL, for example, KrF excimer laser light (wavelength: 248 nm) or Ar
F excimer laser light (wavelength 193 nm) or F ₂
Excimer laser light (wavelength 157 nm) or the like is used.

The light beam emitted from the first fly-eye lens 2 passes through a vibrating mirror 3 for smoothing interference fringes and weak speckles generated on the irradiated surface (reticle surface or wafer surface). The light is bent, and the luminous intensity distribution is further made uniform by the second fly-eye lens 4, and reaches the half mirror 5. And this light flux (pulse illumination light)
Most (about 97%) of the IL passes through the half mirror 5 and illuminates the reticle blind 7 with uniform illuminance.

Here, the reticle blind 7 is composed of two movable blinds and a fixed blind disposed near the movable blind and having a fixed opening shape. The width of the slit-shaped illumination area IAR when the reticle R is illuminated by the reticle blind 7 can be set to a desired size.

The light beam that has passed through the reticle blind 7 is
The reticle R reaches the bending mirror 8 and is bent vertically downward to illuminate an illumination area IAR portion of the reticle R on which a circuit pattern and the like are drawn via the condenser lens system 9.

On the other hand, the remaining (about 3%) of the pulsed illumination light IL is reflected by the half mirror 5 and received by the integrator sensor 6. The integrator sensor 6 can detect the amount of illumination light on the reticle R. The light amount signal from the integrator sensor 6 is supplied to the main controller 21.

The reflectivity monitor 10 is used for measuring a wafer reflectivity, which is a basis for calculating fluctuations of imaging characteristics (various aberrations) due to absorption of illumination light of the projection optical system PL, which will be described later, ie, irradiation fluctuations. Then, the amount of light returning from the projection optical system PL via the condenser lens system 9, the bending mirror 8, and the reticle blind 7 is detected. The light amount signal from the reflectance monitor 10 is also supplied to the main controller 21. The wafer reflectance measurement will be described later in detail.

A reticle R is fixed on the reticle stage RST by, for example, vacuum suction. In addition,
The material used for the reticle R needs to be properly used depending on the light source used. That is, when a KrF excimer laser light source or an ArF excimer laser light source is used as a light source, synthetic quartz can be used. However, when an F ₂ excimer laser light source is used, it is necessary to form a fluorite.

The reticle stage RST is driven on a reticle base (not shown) by a reticle driving unit 41 composed of a linear motor or the like, and is adjusted to an optical axis IX of an illumination optical system (which coincides with an optical axis AX of a projection optical system PL described later). ) Can be moved in a predetermined scanning direction (here, the Y-axis direction) within a predetermined stroke within a plane perpendicular to the vertical axis.

The position of the reticle stage RST is, for example, several nm by a reticle laser interferometer system (not shown).
The position information of the reticle stage RST from the interferometer system is sent to the main controller 21. The main controller 21 drives the reticle based on the position information of the reticle stage RST. The reticle stage RST is controlled via the section 41. The reticle laser interferometer system has, for example, two measurement axes in the scanning direction and one measurement axis in the non-scanning direction.

The projection optical system PL is disposed below the reticle stage RST in FIG. 1, and its optical axis AX
The direction (corresponding to the optical axis IX of the illumination optical system) is the Z-axis direction. As the projection optical system PL, here, a refraction optical system including a plurality of lens elements arranged at predetermined intervals along the optical axis AX direction so as to have a telecentric optical arrangement on both sides is used. This projection optical system PL
Is a reduction optical system having a predetermined projection magnification, for example, 1/4 (or 1/5). For this reason, when the illumination area IAR of the reticle R as the first object is illuminated by the illumination light IL from the illumination optical system, the illumination light IL passing through the reticle R causes the reticle R to pass through the projection optical system PL.
A part of the circuit pattern is reduced and projected on a wafer W as a second object having a surface coated with a photoresist.

As described above, the imaging characteristic correcting mechanism 14 is provided inside the projection optical system PL. In the present embodiment, as shown in FIG. 2, a specific plurality (here, five) of lens groups 22 of the plurality of lens elements constituting the projection optical system PL are used as the imaging characteristic correcting mechanism 14 in the present embodiment. , 23, 24, 25, 26 can be independently driven in the optical axis AX direction (Z direction) and the tilt direction with respect to the XY plane by using piezoelectric elements 27, 28, 29, 30, 31 such as piezo elements. This mechanism is used. The lens groups 22, 23, 24, 25, 26 are respectively connected to three piezoelectric elements 27, 2 via respective holders (hardware).
8, 29, 30, and 31 support the lens barrel PP at three points. Therefore, each of the three piezoelectric elements 27, 2
By driving each of 8, 29, 30, and 31 independently, each lens group 22, 23, 24, 25, and 26 can be driven in the optical axis AX direction (Z direction) and the tilt direction with respect to the XY plane. Has become. Further, the piezoelectric elements 27 to 3
The amount of driving of each support point by No. 1 is monitored by a position sensor (not shown). As such a position sensor, for example, a capacitance type non-contact displacement sensor, an eddy current type non-contact displacement sensor, a linear encoder,
Various displacement sensors such as a laser interferometer can be used. By driving or tilting each of the lens groups 22, 23, 24, 25, and 26 in the optical axis direction, various imaging characteristics are changed, and any of the optical axis direction driving and tilt driving of each lens group is changed. By the combination, it is possible to adjust to almost a desired imaging characteristic.

In the present embodiment, as will be described later, five aberrations, specifically, curvature of field, magnification, distortion, coma, and spherical aberration are corrected by the imaging characteristic correcting mechanism 14. The reticle R is formed by the imaging characteristic correcting mechanism 14 and the lens controller 15.
An image forming characteristic correcting device for correcting the image forming characteristic of the pattern image is constructed. The specific contents of the imaging characteristic correction by the imaging characteristic correction device will be described later in detail. here,
In the present embodiment, the image forming characteristic correcting mechanism 14 for correcting the image forming characteristic by driving the lens group is employed. However, instead of or together with the image forming characteristic correcting mechanism, the sealing between specific lens elements is performed. A chamber may be formed and a mechanism for changing the internal pressure in the sealed chamber may be provided.

[0046] In the case of using a KrF excimer laser light or ArF excimer laser light as the illumination light IL is as each of the lens elements constituting the projection optical system PL can be employed including synthetic quartz and the like, F ₂ excimer laser When light is used, fluorite is used as the material of the lenses used in the projection optical system PL.

Returning to FIG. 1, wafer stage WST
Is a scanning direction on a base (not shown) in the Y-axis direction (FIG. 1).
1 and an X-axis direction orthogonal to this (FIG. 1).
XY stage 18 that is movable in the direction perpendicular to the paper surface of FIG.
And the Z stage 1 provided on the XY stage 18
7 is provided.

The XY stage 18 is actually driven in the XY two-dimensional directions on a base (not shown) by a two-dimensional plane motor or the like.
Is driven within a predetermined range (for example, a range of 100 μm) in the Z direction by a drive mechanism (not shown).
In these figures, these two-dimensional planar motors, drive mechanisms, and the like are illustrated as a wafer drive unit 42 as a representative.

The wafer W is suction-held on the Z stage 17 via a wafer holder (not shown). On the Z stage 17, there is provided an irradiation amount sensor 2 for detecting an irradiation amount that passes through the reticle R and the projection optical system PL and reaches the wafer surface.
0 is provided. The detection value of the irradiation amount sensor 20 is supplied to the main controller 21.

X of the Z stage 17 (ie, wafer W)
The position in the Y plane is always measured by a wafer laser interferometer system (not shown) with a resolution of, for example, several nm to 1 nm or less.
Is sent to the main controller 21 and the main controller 2
In 1, the position of the wafer W is controlled in the XY plane via the wafer driving device 42 based on the position information of the Z stage 17.

The length measurement axes of the wafer laser interferometer system are provided, for example, one in the scanning direction and two in the non-scanning direction.

Further, the scanning exposure apparatus 100 of the present embodiment
Then, two autofocus detection systems integrally attached to the projection optical system PL via a holding member (not shown),
That is, a reticle autofocus detection system (hereinafter, referred to as a reticle autofocus detection system)
A “reticle AF system” 12 and a wafer autofocus detection system (hereinafter, “wafer AF system”) 19 are provided.

The wafer AF system 19 includes an irradiation optical system 19a for irradiating the wafer W with a detection beam obliquely, and a light receiving optical system 19b for receiving the reflected light of the detection beam from the surface of the wafer W. The obliquely incident light type focus position detection system for detecting the position in the Z direction is used. As the wafer AF system 19, for example, Japanese Patent Publication No.
The focus position detection system disclosed in Japanese Patent Application Publication No. 1 (1994) and the like is used.

The reticle AF system 12 is provided with a reticle R
An obliquely incident light type focus position detection system including an irradiation optical system 12a for irradiating the pattern surface with a detection beam obliquely and a light receiving optical system 12b for receiving the detection beam reflected from the reticle surface is used. ing. Reticle AF system 12
Is for detecting the position in the Z direction of the optical axis IX of the pattern surface of the reticle R and a region in the vicinity thereof. The reticle AF system 12 is also the above-mentioned Japanese Patent Publication No. 8-21531.
A configuration similar to that disclosed in Japanese Unexamined Patent Publication (Kokai) No. H10-210 can be used.

The AF system is not limited to the obliquely incident light system. For example, an interferometer for measuring the Z position of a wafer surface or a reticle surface, or an autofocus for directly measuring the distance between a projection optical system and a wafer or a reticle. A sensor may be employed.

Further, the scanning exposure apparatus 100 of the present embodiment
Is provided with an environment sensor 36 for detecting a change in environment near the projection optical system PL. Various sensors such as an atmospheric pressure sensor (pressure sensor), a temperature sensor, and a humidity sensor can be provided as the environment sensor 36. However, the exposure apparatus main body including the projection optical system is usually placed in a chamber where the temperature and humidity are strictly controlled. Since it is considered small, the environment sensor 3
It is assumed that a pressure sensor is provided as 6. However, for example, when short-wavelength light such as an ArF excimer laser is used as the light source 1, if oxygen is present in the optical path, a photochemical reaction occurs to generate ozone, which has an adverse effect on the human body or a material for the surface coat of the lens. Depending on the case, it may cause clouding. In order to avoid this, the inside of the projection optical system PL may be filled or flow with nitrogen. Helium may be filled or flown in order to suppress a change in imaging characteristics due to a change in atmospheric pressure. Since helium has a smaller refractive index than air, there is an advantage that the change in imaging characteristics when the atmospheric pressure (the pressure in the chamber) changes is small. As described above, if the inside and outside of the projection optical system PL have different systems of air conditioning, the inside and outside of the projection optical system PL do not always have the same air pressure.

Taking this point into consideration, in the present embodiment, as shown in FIG. 2, an internal pressure sensor 36a for detecting the pressure of the gas inside the projection optical system PL (hereinafter referred to as "internal pressure" as appropriate) is provided. And an external pressure sensor 36b for measuring the atmospheric pressure in the chamber outside the projection optical system PL (hereinafter, appropriately referred to as “external pressure”). In FIG. 1, it is assumed that the internal pressure sensor 36a and the external pressure sensor 36b are representatively shown as the environment sensor 36. A method of calculating a change in the imaging characteristic due to a change in the atmospheric pressure (the above-described outside pressure and inside pressure) will be described later.

When the temperature and humidity dependence of the projection optical system PL itself is large, the environment
A sensor capable of measuring humidity or the like may be provided, and a change in imaging characteristics due to a change in temperature or humidity may be obtained by a calculation similar to a change in imaging characteristics due to a change in atmospheric pressure described later based on the measured value.

In addition, the scanning exposure apparatus 10 of the present embodiment
At 0, an off-axis type alignment system (not shown) for detecting an alignment mark (not shown) attached to each shot area on the wafer W is also provided. The main controller 21 detects the position of the alignment mark on the wafer W using an alignment system prior to the scanning exposure described below, and based on the detection result, the reticle driving unit 41 and the wafer driving unit 42 use the reticle R. (Alignment) between the wafer and the wafer W.

Next, the scanning exposure apparatus 100 of the present embodiment
Will be briefly described. The direction (X) perpendicular to the scanning direction (Y-axis direction) of the reticle R
The reticle R in the illumination area IAR rectangular (slit shape) having a longitudinal direction in the axial direction) is illuminated, the reticle R is scanned at a speed V _R in the -Y direction during exposure (scanning).
The illumination area IAR (the center substantially coincides with the optical axis AX) is projected onto the wafer W via the projection optical system PL, and the illumination area IA
A slit-shaped projection area conjugate to R, ie, an exposure area I
A is formed. Since the wafer W is to the reticle R in inverted imaging relationship, the wafer W is the direction of the velocity V _R is scanned at a speed V _W in synchronization with the reticle R in the opposite direction (+ Y direction), the shot on the wafer W The entire surface of the region can be exposed. At the time of this scanning exposure, the reticle R and the wafer W,
That is, reticle stage RST and wafer stage WS
T is synchronized with the reticle drive unit 41, the wafer drive unit 42, and the main control unit 21 at a speed ratio V _W / V _R (= 1/4 or 1/5) accurately corresponding to the reduction magnification of the projection optical system PL. The pattern in the pattern area of the reticle R is accurately reduced and transferred onto the shot area on the wafer W. By scanning, the entire pattern region on the reticle R is illuminated, and the entire pattern region of the reticle R is sequentially transferred onto the wafer W.

Also, during the above scanning exposure, the pattern surface of the reticle R and the surface of the wafer W are adjusted based on the detection signals of the wafer AF system 19 and the reticle AF system 12.
The main controller 21 drives and controls the Z stage 17 in the Z-axis direction via the wafer driving device 42 so as to be conjugate with respect to, and focus correction is performed. The focus correction will be further described later.

In the scanning exposure apparatus 100 of the present embodiment,
By repeating the transfer of the reticle pattern by the scanning exposure to the shot area on the wafer W as described above and the stepping operation to the scan start position of the next shot area, the exposure of the step-and-scan method is performed, The reticle pattern is transferred to all shot areas on the wafer W.

Next, focus correction in the scanning exposure apparatus 100 will be described. First, the simplest case in which the focus change of the projection optical system PL itself is not considered will be described.

First, in order to obtain the conjugate relationship between the reticle R and the wafer W, the position of the reference Z stage 17 is obtained. Specifically, it is as follows.

That is, a measurement reticle on which a predetermined measurement mark is drawn is mounted on a predetermined position of the reticle stage RST, and the Z mark 17 is step-moved in the Z direction and the X or Y direction to move the measurement mark. The image is transferred onto the wafer W to which the photosensitive agent has been applied. Next, the position Z _best of the Z stage 17 having the _best mark shape (for example, the largest mark shape or the most prominent edge of the mark) obtained by observing the wafer W with an optical microscope is described as follows:
It is found based on the data of the correspondence between each target value Z _i and the position P _i stored in the memory. And the position Z
_{The best} is set as a reference position, and the outputs of the reticle AF system 12 and the wafer AF system 19 when the Z stage 17 is at the reference position are stored in the memory as the respective AF reference positions. Subsequent focus fluctuation correction is managed by the displacement from the reference position.

It is assumed that the best focus plane differs depending on the direction of the pattern due to the anisotropic focus shift described later. Therefore, the focus described here refers to an average best focus surface in consideration of anisotropic focus.

Then, at the time of actual scanning exposure, the main controller 21 controls the outputs of the reticle AF system 12 and the wafer system AF 19 so that they do not fluctuate from the respective AF reference positions described above (that is, the reticle R and the wafer W). The Z stage 17 is drive-controlled in the optical axis direction so that the optical distance of the Z stage 17 is maintained at a constant value. In this manner, focus correction is performed.

More specifically, the detected displacements of the reticle R and the wafer W with respect to the AF reference position are represented by R and R, respectively.
Assuming that z, Wz, and the projection magnification are ML, the focus displacement Δ
F is represented by ΔF = Rz × ML ² −Wz (1) Move the Z stage 17 so that ΔF becomes 0
By moving in the axial direction, the conjugate relationship between the reticle R and the wafer W is maintained. As a specific example, if the reticle R is shifted by 10 μm in the Z-axis direction during scanning, when the projection magnification is 0.25 times (１ ／ times), 10 μm
× (１ ／) ² = 0.625 μm. At this time, the wafer W is intentionally moved by 0.625 in the Z-axis direction.
μm, that is, Wz =
By setting it to be 0.625 μm, ΔF can be set to 0, and the optical distance between the reticle R and the wafer W can be maintained. Of course, the projection magnification of the projection optical system PL is not limited to 1/4, but can be dealt with by multiplying the square of the projection magnification by the displacement amount on the reticle side.

Next, correction when the focus of the projection optical system PL itself fluctuates due to factors such as a change in atmospheric pressure and absorption of illumination light will be described. At this time, by using the following equation (2) obtained by expanding the above equation (1), it is possible to cope with a change in focus of the projection optical system PL itself. ΔF = FL + Rz × ML ² −Wz (2)

In the above equation (2), FL is a focus change of the projection optical system PL itself calculated as described later.

Next, a method of calculating the amount of change in the imaging characteristics (various aberrations) of the projection optical system PL itself including the above-mentioned focus change will be described in detail.

<Change in imaging characteristics due to change in atmospheric pressure>
A method for calculating the amount of change in the imaging characteristics due to a change in the atmospheric pressure will be described. The focus change F _PRESS due to the atmospheric pressure change is calculated based on the following equation (3).

F _PRESS = K _FPIN × ΔP _IN + K _FPOUT × ΔP _OUT (3) where, K _FPIN : Focus change rate due to internal pressure change K _FPOUT : Focus change rate due to external pressure change ΔP _IN : Internal pressure change ΔP _OUT : change in external pressure.

The focus change rate due to the change in the internal pressure and the external pressure may be obtained by optical calculation, or may be measured by experimentally changing the pressure.

By the same method as the above-mentioned focus, other changes in the imaging characteristics can be calculated. That is, the change in the imaging characteristics of the projection optical system PL itself due to the change in the atmospheric pressure can be calculated based on the following equation (4).

[0076]

(Equation 1)

Where F _PRESS : focus change due to atmospheric pressure change CU _PRESS : field curvature change due to atmospheric pressure change M _PRESS : magnification change due to atmospheric pressure change D _PRESS : distortion change due to atmospheric pressure change CO _PRESS : atmospheric pressure change Coma aberration change SA _PRESS : Spherical aberration change due to atmospheric pressure change. The rate of change due to each atmospheric pressure change in the above equation (4) may be obtained by optical calculation, or may be measured experimentally by changing the atmospheric pressure.

In this embodiment, as described above, the pressure sensors 36a and 36b are provided inside and outside the projection optical system PL.
Corresponding to this, the change of the imaging characteristic is calculated by different coefficients inside and outside, but the projection optical system PL
In the case where there is no problem in accuracy due to the assumption that the air pressure is interlocked between inside and outside, only one of the calculations is sufficient.

<Change in Imaging Characteristics Due to Illumination Light Absorption> Next, changes in imaging characteristics due to illumination light absorption will be described. First,
A description will be given of a method of measuring the irradiation dose Q, which is a premise.

With the reticle R used for exposure mounted on the reticle stage RST, the reticle blind 7 and the illumination conditions (numerical aperture NA and coherence factor σ)
Value) is set to the state at the time of exposure. The setting of the illumination condition is performed, for example, by the main controller 21 using the projection optical system PL.
The aperture stop (not shown) provided at the position of the pupil plane is adjusted, and the numerical aperture N. A. Is set, and the aperture stop on the illumination system aperture stop plate (not shown) provided near the exit end of the second fly-eye lens 4 is selectively set on the optical path.

Next, main controller 21 drives wafer stage WST such that irradiation amount sensor 20 is located immediately below projection optical system PL. Next, the main controller 21 controls the light source 1
And reticle stage RST and wafer stage W
The output of the dose sensor 20 and the output I of the integrator sensor 6 are synchronously moved under the same conditions as the actual exposure.
By simultaneously taking ₀ at a predetermined sampling interval, the value of the irradiation amount Q ₀ corresponding to the synchronous movement position (scanning position) and the output I ₀ of the integrator sensor 6 corresponding to this are stored in the memory. That is, the irradiation amount Q ₀ and the integrator sensor output I ₀ are stored in the memory as a function corresponding to the scanning position of the reticle R.

The main controller 21 executes such a preparation operation prior to the exposure. Then, based on the irradiation amount Q ₀ and the output I ₀ of the integrator sensor 6 stored in accordance with the scanning position of the reticle R at the time of actual exposure, and the output I ₁ of the integrator sensor 6 at the time of exposure, the irradiation at that time is performed. The quantity Q ₁ is calculated based on the following equation (5) and used for calculating the illumination light absorption.

Q ₁ = Q ₀ × I ₁ / I ₀ (5) According to the equation (5), since the output ratio of the integrator sensor 6 is used for calculation, when the power of the light source 1 fluctuates. Can be calculated without error. Further, since the function is based on the scanning position of the reticle R, the irradiation amount can be accurately calculated even when the reticle pattern is offset in the plane, for example.

In the above description, the output of the irradiation sensor 20 is taken under the illumination condition at the time of actual exposure as a preparatory work. However, for example, the signal may be saturated due to the characteristics of the irradiation sensor 20. ND, not shown
The above-described preparation work may be performed under illumination conditions in which the illumination light amount is intentionally reduced by, for example, selectively putting a filter on the illumination optical path. In this case, the actual may be performed calculation of the dose Q ₁ at the time of exposure in consideration of the extinction ratio of the ND filter.

Next, the wafer reflectance R _W , which is also assumed,
The method of measuring is described. First, the wafer stage W
Two reflectors (not shown) each having a known reflectance _RH and a known reflectance _RL are installed on the ST. Next, similarly to the above-described irradiation light amount measurement, main controller 21 sets exposure conditions (reticle R, reticle blind 7, illumination conditions) in the same manner as in actual exposure, and drives wafer stage WST to install the same. The reflection plate having the specified reflectance R _H is projected onto the projection optical system P.
Move just below L. Next, main controller 21 oscillates light source 1 to cause reticle stage RST and wafer stage WS
While synchronously moving T under the same conditions as the actual exposure, the output V _H0 of the reflectance sensor 10 and the output I _H0 of the integrator sensor 6 are simultaneously taken in at a predetermined sampling interval, so as to correspond to the synchronous movement position (scanning position). The output V _H0 of the reflectance sensor 10 and the corresponding output I _H0 of the integrator sensor 6 are stored in the memory. This allows
The output V _H0 of the reflectance sensor 10 and the output I _{H0 of the} integrator sensor are stored in a memory as a function corresponding to the scanning position of the reticle R. Next, main controller 21 drives wafer stage WST to move the installed reflection plate of reflectance _RL directly below projection optical system PL, and outputs the output of reflectance sensor 10 in the same manner as described above. V _L0 and the output I _L0 of the integrator sensor 6 are stored in the memory as a function corresponding to the scanning position of the reticle R.

The main controller 21 executes such a preparation operation prior to the exposure. Then, at the time of actual exposure, the output of the reflectance sensor and the output of the integrator sensor stored according to the scanning position of the reticle R, the output V ₁ of the reflectance sensor 10 and the output I _{1 of the} integrator sensor 6 at the time of exposure. based on the wafer reflectivity R _W, it is calculated based on the following equation (6), used to calculate the illumination light absorbing

[0087]

(Equation 2)

According to the equation (6), since the output ratio of the integrator sensor 6 is used for the calculation, the wafer reflectance can be accurately calculated even when the power of the light source 1 fluctuates.

Next, a method of calculating the amount of change in focus due to absorption of illumination light will be described. Calculating a focus variation F _HEAT with illumination light absorption in projection optical system PL by using the model function represented by the above manner was irradiated amount Q ₁ obtained by the following equation from the wafer reflectivity R _W (7) .

[0090]

(Equation 3)

Here, F _HEAT : focus change due to illumination light absorption Δt: calculation interval of focus change due to illumination light absorption T _FK : focus change time constant due to illumination light absorption F _HEATK : focus change before time Δt due to illumination light absorption the time constant T _FK component C _FHK: constant T _FK component when the focus change rate with respect to the illumination light absorption alpha _F: a wafer reflectivity dependency.

The model function of the above equation (7) has a form of the sum of three first-order delay systems when the irradiation amount is regarded as input and the focus change is regarded as output. Note that the model function may be changed depending on the amount of illumination light absorbed by the projection optical system PL and the required accuracy. For example, if the illumination light absorption amount is relatively small, the sum of two first-order delay systems may be used, or one primary-order delay system may be used. Further, if it takes time due to heat conduction until the projection optical system PL absorbs the illumination light and appears as a change in the imaging characteristic, a model function of a waste time system may be employed. In addition, the wafer reflectivity dependency is usually 1, but depending on the type of the projection optical system PL, for example, when glass having a large absorptance is used as a material on the side closer to the wafer W, the dependency greatly depends on the reflectivity. is there. This time will be a value greater than 1 is set to alpha _F. Value smaller than 1 is set in the alpha _F when its absorption rate closer to the opposite to the wafer W is adopted small glass. The focus change time constant due to illumination light absorption, the focus change rate with respect to illumination light absorption, and the wafer reflectance dependency are all determined by experiments. Alternatively, it may be calculated by a highly accurate thermal analysis simulation.

By the same method as the above-mentioned focus, the change due to the absorption of the illumination light can be calculated for the other imaging characteristics, that is, the curvature of field, magnification, distortion, coma, and spherical aberration. CU _HEAT : field curvature change due to illumination light absorption M _HEAT : magnification change due to illumination light absorption D _HEAT : distortion change due to illumination light absorption CO _HEAT : coma aberration change due to illumination light absorption SA _HEAT : spherical aberration due to illumination light absorption The change may be calculated based on a model function similar to the above equation (7).

In the above-mentioned focusing, a model function of the sum of three first-order lag systems is required. However, for example, one primary-order lag system may be sufficient for calculating the field curvature. The model function of the illumination light absorption may be changed for each imaging characteristic according to the required accuracy. When the first-order lag system uses two or one model function, there is an effect of reducing the calculation time.

Next, six kinds of rotationally symmetric image forming characteristics of the projection optical system PL, specifically, focus, field curvature, magnification,
A method for correcting distortion, coma, and spherical aberration will be described.

First, in the initial adjustment stage, the focus, field curvature, magnification, distortion, coma aberration, spherical aberration and the like are driven while driving the five lens groups 22 to 26 constituting the imaging characteristic correcting mechanism 14 one by one. The measurement was performed on the six types of image forming characteristics of the aberrations, and the above-mentioned 6 types in the lens groups 22 to 26 were measured.
Previously obtained the type of imaging characteristics variation coefficient (the elements of the right side of the matrix C ₁₁ -C ₆₅ in the following equation (8)).

[0097]

(Equation 4)

Here, G _{1 to} G ₅ are five lens groups 22.
２６26 represents the movement amount. Note that these imaging characteristic change coefficients C _{11 to} C ₆₅ may be obtained by calculation using a highly accurate optical simulation.

The left side of the above equation (8) is the change in focus, curvature of field, magnification, distortion, coma, and spherical aberration described above and is expressed by the following equation (9).

[0100]

(Equation 5)

Then, of the above-mentioned image forming characteristic change coefficients, the following equation (10) is used by using five kinds of image forming characteristic change coefficients excluding focus and the movement amounts (drive amounts) of the five lens groups. A five-element linear simultaneous equation is established.

[0102]

(Equation 6)

Then, by using the above equation (10),
For example, if it is desired to change to a predetermined magnification, the above equation (1
0) A predetermined amount is set for the imaging characteristic change coefficient of the medium magnification, and a new simultaneous equation is set in which the other four types of image formation characteristic change coefficients are set to “0”, and this simultaneous equation is solved to solve each lens group. Of each lens group 22 to 26 according to the driving amount.
By driving the, field curvature, distortion,
It is possible to control only the magnification to a predetermined value without affecting the coma aberration and the spherical aberration. Here, the case where the magnification is changed has been described. However, the field curvature, distortion, coma aberration, and spherical aberration are the same as above, and the values can be individually changed without affecting the others. .

In the above equation (10), the focus is removed because, when the lens group is driven to correct other image forming characteristics such as magnification, the focus is changed accompanying the driving. This is because the effects need to be considered.

In order to correct the above five types of imaging characteristics,
Assuming that the focus change that occurs as a side effect due to the movement of the five lens groups is FG, FG is given by the following equation (11).
Can be expressed as

[0106]

(Equation 7)

Eventually, the change in focus of the projection optical system PL itself is FL = F + FG (12), including the change in atmospheric pressure, the change in absorption of illumination light, and the change in lens group movement. This FL is converted to the Z stage 1 of the above-described equation (2).
The conjugate relationship (optical distance) between the reticle R and the wafer W is maintained by substituting the Z stage 17 in the Z-axis direction so that ΔF becomes 0 by substituting into the focus correction formula by (7).

Next, the scanning exposure apparatus 100 of the present embodiment
A method for correcting the change in the anisotropic imaging characteristic during irradiation, which is a feature of the method, will be described.

First, the anisotropy of the projection optical system (hereinafter referred to as “projection optical system PL ′” for which the anisotropic imaging characteristic is not initially adjusted in the same manner as in the above-described conventional example by the illumination light absorption). The amount by which the imaging characteristics change is determined. Since the anisotropic imaging characteristic also behaves in the same manner as the above-described illumination light absorption such as focus, it can be obtained in advance by an experiment or a high-precision optical simulation. At this time, the maximum speed of the wafer stage WST of the scanning exposure apparatus 100 of the present embodiment,
The largest energy that can be assumed based on the maximum illuminance of the light source 1 and the sensitivity of the resist applied to the wafer W is given to the projection optical system PL ′, or the change in the anisotropic imaging characteristic is obtained by assuming it.

According to the above experiment or optical simulation, as shown in FIG. 3, for example, as the anisotropic distortion of the projection optical system PL ′, the projected image of the illumination area IRA on the wafer W, ie, the exposure area IA, It is assumed that a state that changes like IA ′ after the absorption of the illumination light is determined. In FIG. 3, the center of the exposure area IA is
The line AA in the non-scanning direction (X direction) passing through the point O is defined as the axis A, the line BB in the scanning direction passing through the point O is defined as the axis B, and further along the scanning direction at the end in the non-scanning direction. The line C-C is taken as the axis C.

FIG. 4 shows the anisotropic distortion at this time plotted at points on each axis. This figure 4
, The horizontal axis is the image height, that is, the center of the optical axis (here,
The vertical axis represents distortion, and the inclination of axes A to C corresponds to a change in magnification with respect to the image height. As is apparent from FIG. 4, the change in magnification with respect to the image height of the A-axis in the non-scanning direction is
It can be seen that the change in magnification with respect to the image height is large in the B and C axes in the scanning direction.

When this is corrected by the combination drive of the lens groups 22 to 26 described above, as shown in FIG. 5, the projected image of the illumination area IRA on the wafer W, that is, the exposure area IA
Is a distortion that changes like IA ″ after the absorption of the illumination light. Here, as in FIG.
As shown in FIG. 6, the magnification is corrected on the axis A in the non-scanning direction, but a magnification correction error remains on the axes B and C in the scanning direction.

Therefore, in the projection optical system PL of the present embodiment, anisotropic distortion due to illumination light absorption is distributed based on the data on the projection optical system PL ′, that is, initial adjustment of the projection optical system PL. At
The projected image of the front illuminating light absorption on the wafer W of the illumination area IRA, i.e. giving distortion like exposure area IA is exposure area IA ₁ as shown by a solid line in FIG. 7 (A) consciously (Distortion is shifted from the ideal lattice). For example, the projection optical system PL
A predetermined process is performed on any one or a plurality of lens elements that constitute the above, and these are combined and adjusted, and an initial adjustment is performed so that distortion as shown in FIG. 7A is generated. It is.

In the state where the projection optical system PL, which has been subjected to the initial adjustment in this way, absorbs the illumination light and the component to be rotated is corrected, the projected image of the illumination area IRA on the wafer W, ie, exposure area IA changes to distortion such that the exposure area IA ₂ as shown in FIG. 7 (B).

When the anisotropic distortion corresponding to FIGS. 7A and 7B is plotted at points on each axis as in FIG. 4, the results are as shown in FIGS. 8A and 8B. In these figures, the horizontal axis is the image height, that is, the distance from the center of the optical axis (here, the point O is assumed to coincide with this), the vertical axis is the distortion, and the inclination of the axes A to C is This corresponds to a change in magnification with respect to the image height. B in FIG. 6 described above.
When the change in magnification on the axis is b, by setting the amount of shift of the magnification of axis B in the initial adjustment of FIG. 8A to −b / 2, the axis after absorption of the illumination light in FIG. The magnification change on B is b / 2.

As described above, the scanning exposure apparatus 1 of the present embodiment
In the projection optical system PL that constitutes No. 00, since the anisotropic distortion is adjusted to be distributed in the initial adjustment, the worst value of the amount of anisotropic distortion caused by the absorption of illumination light is determined in the comparative example (conventional projection lens). It can be reduced to half.

Next, correction of anisotropic focus will be described. Here, for the sake of simplicity, the description will focus on the anisotropy of focus at the center of the projection optical system, that is, center astigmatism.

As described above in connection with the prior art, FIG.
5, which is disposed at the center of the illumination area IAR on the reticle R (that is, near the center of the optical axis of the projection optical system),
When a composite pattern composed of the H pattern and the V pattern is projected and exposed on the wafer W via the projection optical system PL ′ of the comparative example, the projection optical system PL generated due to the illumination area being elongated in the non-scanning direction. As shown in FIG. 18, the V-pattern best focus surface and the H-pattern best focus surface have F
A shift of only _CE occurs. That is, center ass occurs.

Therefore, in the projection optical system PL of this embodiment, based on the data on the above-mentioned projection optical system PL ′, the center astigmatism generation amount _FCE is allocated in the initial adjustment, so that Like the anisotropic distortion described above, the maximum value (worst value) of the center ass is suppressed to half. That is, as shown in FIG.
The initial adjustment is performed such that the V-pattern best focus surface is closer to the projection optical system PL by F _CE / 2 than the H-pattern best focus surface.

Although only the center astigmatism has been described here, the anisotropic focus occurs not only at the center of the projection optical system PL but also over the entire illumination area.
The best focus plane for each pattern direction may be assigned in the initial adjustment in consideration of the entire illumination area.

As described above, according to the present embodiment, the change in the anisotropic imaging characteristic caused by the absorption of the illumination light by the projection optical system PL can be substantially halved. It is possible to sufficiently suppress the deterioration of the exposure accuracy due to the characteristic change, and it becomes possible to expose a finer pattern with higher accuracy than in the past.

In the first embodiment described above, the initial adjustment of the projection optical system PL is performed so that the projection optical system PL can be sorted by anticipating the anisotropic imaging characteristic due to the absorption of illumination light of the projection optical system PL. Although the case where the deterioration of the exposure accuracy due to the change of the isotropic imaging characteristic is suppressed has been described, the projection optical system is not necessarily illuminated with more energy than expected when actually using the apparatus. For example, the power of the light source is higher than the initial adjustment, or the reflectivity of the wafer is
It is possible that it approaches 0%. The following second embodiment has been made in consideration of such a case.

<< Second Embodiment >> Next, a second embodiment of the present invention will be described with reference to FIG. Here, the device configuration and the like of the second embodiment are the same as those of the above-described first embodiment, and only the functions of the main control device 21 are different. Therefore, the following description will focus on this point. I do. The same reference numerals are used for the same or equivalent components as those in the first embodiment. Here, it is assumed that main controller 21 is mainly configured of a first processor and a second processor.

The control operation of the first processor when transferring a reticle pattern to a predetermined number of wafers W will be described below with reference to FIG. The flowchart of FIG. 10 starts when predetermined preparation operations such as reticle loading on reticle stage RST, reticle alignment, and baseline measurement are completed.
As a precondition, it is assumed that an initial value of a first counter (not shown) indicating a wafer number described later and an initial value of a second counter (not shown) indicating a shot number described later are both “0”. Further, the imaging characteristics of the projection optical system PL before the start of the exposure processing (the rotationally symmetric imaging characteristics and the anisotropic imaging characteristics are measured in advance, and the respective values are stored in a memory (not shown) as initial values. It is assumed that a predetermined reference value is set as the focus offset.

First, in step 102, wafer exchange is performed. This wafer exchange is performed by a wafer transfer system (not shown) and a wafer transfer mechanism (not shown) on wafer stage WST in response to an instruction from the first processor. However, when exposing the first wafer W, only the loading of the wafer W into the wafer holder is performed.

In the next step 104, the first counter is incremented by 1 (m ← m + 1). Therefore, 1
When exposing the second wafer W, m is set to “1”.

At the next step 106, wafer alignment is performed. Specifically, while moving wafer stage WST in the XY two-dimensional directions via wafer driving device 42, the position of search alignment marks formed at a plurality of locations on wafer W is detected using an alignment system (not shown). After detecting the rotation and center position of the wafer W from the detection result, the wafer stage WST is similarly moved in the XY two-dimensional directions via the wafer driving device 42 while the wafer stage WST is moved over the wafer W using an alignment system (not shown). The positions of a plurality of wafer marks are detected, and the array coordinates of the shot area on the wafer W are calculated by using the detection results and performing a statistical operation using the least squares method disclosed in, for example, Japanese Patent Application Laid-Open No. 61-44429. I do.

In the next step 108, it is determined whether or not the change in the anisotropic imaging characteristic of the projection optical system PL is within a predetermined threshold, that is, within an allowable range. Before the exposure of the first wafer W is started, the measurement of the imaging characteristics of the projection optical system PL has not been started yet, and the initial values of the respective imaging characteristics are stored in the memory. The determination in step 108 is naturally affirmed, and the process proceeds to step 114. In this step 114, the correction of the rotationally symmetric imaging characteristic of the projection optical system PL is executed. However, before the exposure of the first wafer W is started, the initial values of the respective imaging characteristics are stored in the memory, so that no correction is actually performed, and the process proceeds to the next step 116. Then, after setting “1” to a second counter (not shown) indicating the shot number (n ← 1),
Proceeding to step 118, the wafer stage WST is moved to the scan start position for exposure of the n-th shot area based on the position coordinates of the n-th (here, first) shot area obtained in step 106. Moving.

In the next step 120, it is determined whether or not the count value m of the first counter and the count value n of the second counter are both "1". When the exposure for the first shot of the first wafer W is performed, this determination is affirmed, and the routine proceeds to step 122, where a measurement start instruction is given to the second processor. Thereby, the measurement of the change of the imaging characteristics (rotationally symmetric imaging characteristics and anisotropic imaging characteristics) of the projection optical system PL is started by the second processor, and the respective imaging characteristics are measured at intervals of the predetermined time Δt. The calculated values of the imaging characteristics in the memory are sequentially updated at time Δt. The calculation of the imaging characteristic by the second processor is repeatedly performed until the first processor gives an instruction to terminate the measurement described later.

[0130] Here, the second processor, during scanning exposure of step 124 to be described later, receives the output of the integrator sensor 6, obtained by computation of the dose Q ₁ as described above, the dose Q ₁ Is used to calculate the amount of change in the imaging characteristics of the projection optical system PL due to absorption of illumination light. Further, among the imaging characteristics, the method of calculating the amount of change in the rotationally symmetric imaging characteristics is as follows.
This is as described in the first embodiment. The calculation (calculation) of the change amount of the anisotropic imaging characteristic is performed as follows as an example.

Here, as for the method of calculating the amount of change in the anisotropic imaging characteristic, anisotropic distortion (rectangular distortion) and anisotropic focus (center ass) are taken up in the same manner as in the first embodiment. explain. These changes in the anisotropic imaging characteristics are basically calculated by calculating the above-mentioned changes in the rotationally symmetric imaging characteristics (focus, curvature of field, magnification, distortion, coma, spherical aberration) due to absorption of illumination light. It can be determined using exactly the same formula as the formula. However, the method of calculation needs to be devised. That is, for example, to find a rectangular distortion,
The magnification changes in the scanning direction and the non-scanning direction are separately calculated. In addition, the V-pattern best focus plane and the H-pattern best focus plane are separately calculated in order to obtain the center assemblage. Then, the difference between the magnification change in the scanning direction and the non-scanning direction calculated as described above is defined as the amount of change in the rectangular distortion, and the difference between the V-pattern best focus plane and the H-pattern best focus plane is defined as the center astigmatism change amount.

In the next step 124, the above-described scanning exposure is performed to transfer the pattern of the reticle R to the n-th (here, the first) shot area. During this scanning exposure, Z is set via the wafer drive unit 42 based on the detection signals of the wafer AF system 19 and the reticle AF system 12 such that the pattern surface of the reticle R and the wafer surface are conjugate with respect to the projection optical system PL. The stage 17 is driven and controlled in the Z-axis direction to execute focus correction.

After the completion of the scanning exposure in step 124, the process proceeds to step 126, where the second counter is incremented by 1 (n ← n + 1).
28, the count value n of the second counter is
It is determined whether or not it is larger than the above total number of shots N. First
When the scanning exposure of the second shot is completed, this determination is naturally denied, and the process returns to step 118, and the processing determination in steps 118 to 128 is repeated. In the exposure after the second shot on the wafer W, the determination in Step 120 is always denied, and Step 118 →
The loop of 120 → 124 → 126 → 128 is repeated.

When the exposure of all the shots to be exposed on the wafer W (here, N shots) is completed, the determination at step 128 is affirmed, and the routine proceeds to step 130, where the predetermined predetermined number of shots ( Here, it is M)
Is determined by determining whether or not the count value m of the first counter is M or more. When the exposure of the first wafer W is completed, this determination is naturally denied.
02, and repeats the above processing and judgment.

At the time of exposing the second and subsequent wafers W,
Since the measurement of the imaging characteristics of the projection optical system PL by the second processor has been started, the change amount of the anisotropic imaging characteristics stored in the memory, for example, the change amount of the rectangular distortion (scanning) in step 108 It is determined whether or not the difference between the magnification change in the direction and the non-scanning direction) and the amount of change in center ass (the difference between the V-pattern best focus plane and the H-pattern best focus plane) are within predetermined tolerances. . As an allowable value of the change of the anisotropic imaging characteristic,
For example, the maximum allowable change in the imaging characteristic change is obtained in advance from a pattern rule (device rule) to be exposed, and the value is set as an allowable value.

If the determination in step 108 is affirmative in exposing the second and subsequent wafers W, the flow advances to step 114 to correct the imaging characteristics as follows. That is, based on the calculated value of the change amount of each imaging characteristic stored in the memory at that time, the driving amount of each of the lens groups 22 to 26 is calculated using the above-described equation (10) for correcting the rotation-target imaging characteristic. Is calculated, and this value is given to the lens controller 15 to correct the field curvature, magnification, distortion, coma aberration and spherical aberration. ). In addition, focusing is performed by giving the FL of equation (12) described above as a correction value (offset value) to the wafer AF system. And the average value (average image plane) of the H pattern and the best focus plane. As a result, the first processor uses the above-described correction value as an offset during the scanning exposure of the shot based on the above-described equation (2) to set Z.
The Z position of the stage 17 is controlled, and as a result, the focus is corrected.

On the other hand, when the projection optical system PL absorbs the illumination light EL by a predetermined amount or more due to exposure of several wafers W and the determination in step 108 is denied, that is, the anisotropic imaging characteristic If the change is out of the allowable range, the process proceeds to step 110 to repeatedly determine whether the amount of change in the anisotropic imaging characteristic has attenuated by a predetermined amount from the allowable value based on the value in the memory. Waits until the projection optical system PL is naturally cooled to a level at which exposure can be resumed.
Therefore, the exposure is interrupted until the projection optical system PL is naturally cooled to a level at which the exposure can be resumed. Even during the interruption of the exposure, the second processor repeats the above-described imaging characteristics (both rotational symmetry and anisotropy) of the projection optical system PL at intervals of time Δt, and sequentially updates the values in the memory.

Here, the above-mentioned predetermined amount of attenuation may specifically be defined as, for example, an attenuation of 90% or 80%, or an amount of change in the imaging characteristics that deteriorates during exposure of a single wafer. May be calculated in advance and attenuated by the amount of change. The latter change in the imaging characteristics, which deteriorates during single wafer exposure, is calculated from the output of the irradiation amount sensor, the number of shots per wafer, the reflectance of the previous wafer W, the imaging characteristic calculation formula by absorption of illumination light, etc. Can be obtained by

In this way, when the projection optical system PL is naturally cooled to a level at which exposure can be resumed during the interruption of exposure,
The determination at step 110 is affirmed, and the processing and determination after step 114 are restarted.

If the predetermined number (M) of wafers W have been exposed, and the determination in step 130 is affirmative, the flow advances to step 132 to instruct the second processor to terminate the measurement. A series of processes of this routine is ended. The second processor receives the instruction to terminate the measurement and stops the operation of measuring the imaging characteristics.

As described above, according to the second embodiment, the main controller 21 monitors changes in the anisotropic imaging characteristics during the exposure processing, and the amount of the change is determined by the predetermined threshold (allowable value). ), The exposure operation is interrupted, and during the interruption of the exposure, the above-described change in the anisotropic imaging characteristic is continuously monitored, and the exposure is immediately restarted when it is confirmed that the change amount has attenuated to a predetermined value. It is supposed to. For this reason, it is possible to reliably prevent the occurrence of exposure failure due to a change in the anisotropic imaging characteristic, and to perform the exposure immediately after the projection optical system PL is naturally cooled to a level at which the exposure can be resumed. Is resumed, so that there is an effect that deterioration of the throughput can be minimized.

As is clear from the above description, in the first and second embodiments, the reticle AF 12, the wafer AF 19, the Z stage 17, the main control device 21, and the wafer driving device 42 constitute a focus correction device. ,
In addition, the main control device 21 implements an imaging characteristic monitoring device.

In the second embodiment, the case where the main control device 21 is configured to include the first processor and the second processor has been described, but the microcomputer or the workstation constituting the main control device is used. In multi-task processing or time division processing, the first
Of course, the functions of the processor and the second processor may be realized by using a stage controller, an exposure controller, a lens controller, and the like under the control of the processor and the second processor.

In the second embodiment, the same device configuration as that in the first embodiment is used. Therefore, the projection optical system PL also has different imaging characteristics between the scanning direction and the non-scanning direction in the initial adjustment. The case where the projection optical system set as described above is used has been described. However, when such a projection optical system is used, it is possible to lengthen the time until the above-described allowable value of the change of the anisotropic imaging characteristic is reached. This is done in consideration of the fact that the change is possible, that is, the change in the anisotropic imaging characteristic when the same amount of illumination light is absorbed is reduced. However, the present invention is not limited to this. That is, when any projection optical system is used, such as a projection optical system in which the anisotropic imaging characteristic is not adjusted at all, or a projection optical system in which the anisotropic imaging characteristic is adjusted ex post rather than the initial adjustment. Even so, the scanning type exposure apparatus provided with the imaging characteristic monitoring device according to the present invention has the same effect as the second embodiment.

Note that, instead of step 110, a step of waiting for a predetermined time to elapse may be provided as a step for determining whether or not the projection optical system PL has been naturally cooled to a level at which exposure can be resumed. .

Further, the judging steps of steps 108 and 110 may be moved before step 102. In such a case, the wafer exchange time and the decay time of the imaging characteristic change can be overlapped.
It is possible to make the deterioration of the throughput smaller.

Further, in the above-described embodiment, a case has been described in which whether or not the exposure is interrupted is determined each time the exposure of one wafer is completed. However, the present invention is not limited to this. Alternatively, it may be determined whether or not the exposure should be interrupted at an arbitrary time interval, such as every several wafers, one or a plurality of lots, or every several shots.

The scanning exposure apparatus according to the present invention improves the exposure accuracy by suppressing the deterioration of the anisotropic imaging characteristic of the projection optical system as described in the first embodiment. Alternatively, as described in the second embodiment, the exposure optical system is designed to reliably prevent the occurrence of exposure failure due to the deterioration of the anisotropic imaging characteristic caused by the absorption of the illumination light of the projection optical system. Each component of the device is electrical,
They are assembled mechanically or optically.

[0149]

As described above, according to the first to seventh aspects of the present invention, there is provided a projection lens capable of suppressing the deterioration of the anisotropic imaging characteristic due to the absorption of illumination light. can do.

Further, according to each of the eighth, ninth and fifteenth aspects, it is possible to suppress the change in the anisotropic imaging characteristic by absorbing the illumination light of the projection optical system and improve the exposure accuracy. A scanning exposure apparatus can be provided.

Further, according to each of the tenth to fifteenth aspects, it is possible to reliably prevent the occurrence of defective exposure due to the deterioration of the anisotropic imaging characteristic due to the absorption of the illumination light of the projection optical system. It is possible to provide an unprecedented excellent scanning exposure apparatus that can be used.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a schematic configuration of a scanning exposure apparatus according to an embodiment.

FIG. 2 is a partial cross-sectional view illustrating a configuration of an imaging characteristic correction mechanism of FIG.

FIG. 3 is a diagram illustrating an example of an anisotropic distortion of a projection optical system PL ′.

FIG. 4 is a diagram in which the anisotropic distortion of FIG. 3 is plotted at points on each axis.

FIG. 5 is a diagram showing a state after the anisotropic distortion of FIG. 3 has been corrected by an imaging characteristic correction mechanism.

6 is a diagram in which the anisotropic distortion of FIG. 5 is plotted at points on each axis.

FIGS. 7A and 7B are diagrams showing anisotropic distortion before and after absorption of illumination light to explain the initial adjustment of the imaging characteristics of the projection optical system PL ((A), (B)).

FIG. 8 is a diagram in which anisotropic distortions corresponding to FIGS. 7A and 7B are plotted at points on each axis.

FIG. 9 is a diagram for describing initial adjustment of anisotropic focus of the projection optical system PL.

FIG. 10 is a flowchart showing a control algorithm of a first processor in a main controller when transferring a reticle pattern to a predetermined number of wafers in the scanning exposure apparatus according to the second embodiment.

FIG. 11 shows a projection optical system P used in a scanning exposure apparatus.
FIG. 4 is a view of L ′ as viewed from the optical axis direction.

12 is a diagram showing a temperature distribution of the projection optical system PL ′ of FIG.

FIG. 13 is a sectional view taken along line AA of FIG.

FIG. 14 is a sectional view taken along line BB of FIG. 12;

FIG. 15 is a diagram showing a composite pattern composed of an H pattern and a V pattern disposed at the center of an illumination area IAR on a reticle R.

FIG. 16 is a diagram showing a state of projection of a V pattern.

FIG. 17 is a diagram showing a state of projection of an H pattern.

FIG. 18 is a diagram illustrating a state in which center astigmatism has occurred in the projection optical system PL ′.

[Explanation of symbols]

12 (12a, 12b) Reticle AF (part of focus correction device) 14 Imaging characteristic correction mechanism (part of imaging characteristic correction device) 15 Lens controller (part of imaging characteristic correction device) 17 Z stage (focus) 19 (19a, 19b) Wafer AF (part of focus correction device) 21 Main controller (part of focus correction device, imaging characteristic monitoring device) 42 Wafer driving device (one part of focus correction device) Part) 100 scanning exposure apparatus R reticle (first object, mask) W wafer (second object, substrate) PL projection optical system (projection lens) AX optical axis IL illumination light

Claims (15)

[Claims] 1. A projection lens for projecting an image of a first object onto a second object, comprising a first direction orthogonal to the optical axis and a second direction orthogonal to the optical axis and the first direction. A projection lens, which has been initially adjusted to have different image characteristics. 2. The imaging characteristics in the first and second directions are initially adjusted in consideration of fluctuations caused by absorption of illumination light when irradiated with illumination light of a predetermined shape. The projection lens according to claim 1. 3. An initial adjustment of the imaging characteristics in the first direction and the second direction is performed so as to cancel only half of a change amount of the imaging characteristics in the first direction and the second direction caused by absorption of the illumination light. 3. The projection lens according to claim 2, wherein the projection lens is shifted by a desired imaging characteristic. 4. The projection lens according to claim 2, wherein the imaging characteristic to be subjected to the initial adjustment is a rectangular distortion having a long side in the first direction. 5. The projection lens according to claim 4, wherein the initial adjustment of the rectangular distortion is performed by greatly shifting a magnification in the second direction from a desired magnification as compared with the first direction. . 6. The imaging characteristic according to claim 2, wherein the imaging characteristic to be subjected to the initial adjustment is a deviation of an imaging position in the first direction and the second direction with respect to the center of an optical axis. Projection lens. 7. An initial adjustment of a shift of an imaging position between the optical axis center in the first direction and the second direction in the first direction and the second direction is performed by adjusting an imaging plane of the periodic pattern in the first direction and a periodic pattern in the second direction. 7. The method according to claim 6, wherein one of the predetermined position and the image forming plane is shifted to a position closer to the projection optical system than the other.
The projection lens according to claim 1. 8. A scanning exposure apparatus that illuminates the mask with illumination light of a predetermined shape while synchronously moving the mask and the substrate, and transfers the pattern onto the substrate via a projection optical system. A scanning type exposure apparatus comprising the projection lens according to any one of claims 1 to 7 as the projection optical system, wherein the second direction is the synchronous movement direction. 9. The mask is illuminated with illumination light of a predetermined shape while relatively scanning the mask and the substrate in synchronization with a predetermined scanning direction, and the pattern is transferred onto the substrate via a projection optical system. A scanning exposure apparatus, wherein the projection optical system has different imaging characteristics in the scanning direction and a non-scanning direction orthogonal to the scanning direction, and the projection optical system A focus correction device is provided that adjusts the positional relationship between the imaging plane of the projection optical system and the substrate in the optical axis direction of the projection optical system by positioning the substrate in consideration of the shift of the imaging position. A scanning exposure apparatus. 10. An image forming apparatus which monitors a change in anisotropic imaging characteristic due to absorption of the illumination light by the projection optical system, and interrupts an exposure operation when the amount of change reaches a predetermined threshold value. The scanning exposure apparatus according to claim 9, further comprising a characteristic monitoring device. 11. The mask is illuminated with illumination light of a predetermined shape while relatively scanning the mask and the substrate in a predetermined scanning direction, and the pattern is transferred onto the substrate via a projection optical system. A scanning exposure apparatus, wherein a change in anisotropic imaging characteristics due to absorption of the illumination light by the projection optical system is monitored, and the exposure operation is interrupted when the change reaches a predetermined threshold value. A scanning type exposure apparatus, comprising: 12. The anisotropic imaging characteristic of the monitoring target is at least one of a rectangular distortion and a shift of an imaging position of the optical axis center in the non-scanning direction and the scanning direction. The scanning exposure apparatus according to claim 10 or 11, wherein: 13. The imaging characteristic monitoring device monitors a change in the rectangular distortion based on a difference between a change in magnification in the non-scanning direction and a change in magnification in the scanning direction. And monitoring the deviation of the imaging position in the scanning direction based on the difference between the imaging surface of the periodic pattern in the non-scanning direction formed on the mask and the imaging surface of the periodic pattern in the scanning direction. 13. The scanning exposure apparatus according to claim 12, wherein: 14. The image forming characteristic monitoring apparatus continues to monitor a change in the anisotropic image forming characteristic of the projection optical system even after the interruption of the exposure operation, and the image forming characteristic is monitored up to a predetermined reference. The scanning exposure apparatus according to any one of claims 10 to 13, wherein the exposure operation is restarted when the exposure is attenuated. 15. The scanning type exposure apparatus according to claim 9, further comprising an image forming characteristic correcting device for correcting a change in a rotationally symmetric image forming characteristic other than the focus of the projection optical system.