JPH0756318A - Pattern forming method and mask used for the same - Google Patents

Abstract

PURPOSE:To provide an exposing method which obviates the generation of pattern projection movement occurring in defocusing by changing the progressing direction of illuminating light of a specific region among patterns and illuminat ing this region diagonally with the optical axis of a projecting optical system. CONSTITUTION:The short wavelength light emitted from a light source 1 consisting of a mercury lamp, etc., illuminates a mask 3 which is a first substrate via a condenser lens 2. This mask 3 is placed on a reticule stage 6. This reticule stage 6 is driven by a reticule stage driving means 19 and is so positioned that the central position of the mask 3 aligns to the optical axis of a projecting lens 7. The patterns 5 drawn on the mask 3 are projected via the projecting lens 7 onto a wafer 8 which is a second substrate. A prism-shaped transparent optical element 4 which bends the direction of the illuminating light is arranged in order to diagonally illuminate only the specific region on the mask 3. The quantity at which the projected images moves according to defocusing is thus corrected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to optical lithography essential for manufacturing semiconductor integrated circuits and the like, and more particularly to a method for transferring a fine pattern and a mask used therefor.

[0002]

2. Description of the Related Art A method called a lithography technique for transferring a pattern drawn on a mask onto a sample substrate (wafer) is widely used for forming a pattern of a semiconductor integrated circuit or the like. In order to perform this pattern transfer, a reduction projection type projection exposure apparatus that reduces and transfers the pattern on the mask is generally used.

With the recent miniaturization of patterns in semiconductor integrated circuits and the like, projection exposure apparatuses are required to have high resolution and capable of transferring fine patterns. Generally, the larger the numerical aperture (NA) of the projection lens or the shorter the wavelength of exposure, the higher the resolution.
However, an extreme increase in NA causes a decrease in the depth of focus, which is a practical disadvantage. In addition, there is no light source capable of coping with the further miniaturization of the required pattern, and the short wavelength of exposure is approaching the limit in view of restrictions of lens materials and resist materials.

Therefore, an attempt has been made to transfer a fine pattern exceeding the conventional resolution limit using the current projection exposure apparatus. Japanese Patent Publication No. 62-50811 discloses that the resolution and the depth of focus are greatly improved by introducing a device for imparting a phase difference of exposure to the mask itself. This technique is extremely effective particularly for a periodic pattern by providing a transparent thin film that changes the phase of transmitted light by approximately 180 degrees at every other light transmitting portion on the mask. On the other hand, when applied to an actual circuit pattern, as a method of avoiding the restriction of the pattern layout in which every other transparent thin film must be provided, three types of phases of the mask transmitted light are 0 degrees, 120 degrees, and 240 degrees. The introduction method is the proceedings of the 53rd JSAP Academic Lecture N
o.2, p. 477 (1992).

[0005]

When the mask disclosed in Japanese Patent Publication No. 62-50811 is used, a fine pattern which substantially exceeds the conventional resolution can be transferred without any change in the projection exposure apparatus. can do. Particularly, when a periodic fine pattern is drawn over the entire surface of the mask, the effect is sufficiently exhibited. However, it does not show a method for appropriately arranging a pattern giving a phase difference for a general pattern having a small periodicity.

On the other hand, in the technique described in Proceedings of the 53rd Annual Meeting of the Japan Society of Applied Physics No. 2, p. 477 (1992), the phase shifter can be easily arranged even if the pattern arrangement becomes complicated. . However, when the light transmitting portions that give the three types of phases as described above are arranged side by side, when the wafer surface deviates from the image plane of the projection lens, that is, in the defocused state, the pattern to be transferred is transferred. However, there is a problem that the image forming position of is moved in the plane. This phenomenon is described in Proceedings of the 40th Joint Lecture on Applied Physics No.2.
p.606 (1993), it means that the pattern position accuracy deteriorates when defocus occurs. Therefore, even if the resolution and the depth of focus are improved by using a mask that introduces a phase difference, the depth of focus is limited in terms of the pattern transfer position accuracy.

An object of the present invention is to provide an exposure method in which pattern projection image movement due to defocus does not occur even if an exposure method using a mask in which light transmitting portions that give at least three types of phases are arranged side by side is adopted. And to provide a mask.

[0008]

The above-mentioned problem is, for example, to correct the amount of movement of a projected image with defocus in a region on a mask where light transmitting portions for giving three kinds of phases are arranged side by side. The pattern is previously illuminated obliquely to the optical axis of the exposure optical system. In order to obliquely illuminate only a specific area on the mask, a prism-shaped transparent optical element that bends the irradiation direction of the illumination light may be arranged above the fixed area. The transparent optical element may be one in which the upper surface of the mask is directly processed to form irregularities, or one in which a transparent member different from the mask is arranged close to or in close contact with the mask may be used. .

[0009]

When the phase difference is given to the transmitted light of the mask, the apparent resolution of the projection lens for projecting the pattern is improved and a finer pattern can be transferred. This principle is
62-50811. That is, as shown in FIG. 7, when the light-transmitting portions 31-1 to 31-5 and 32-1 to 32-5 having a fine width are arranged side by side in the light-shielding portion 30 on the mask, the light transmission is reduced. When the phase of the transmitted light is changed by 180 degrees in the parts 32-1 to 32-5, the contrast of the pattern projection image is improved. However, the pattern shape of the light transmitting portion is complicated. For example, in the case of the pattern shown in FIG.
The rule of giving a 0 degree phase difference cannot be applied. That is, in the example shown in FIG.
And 36-1, and patterns 37-1, 38, and 35-2 are adjacent to each other, but the phase of the transmitted light is the same.

In such a case, three types of phases are introduced as shown in FIG. 9 by the technique described in Proceedings of the 53rd Japan Society of Applied Physics Academic Lecture No. 2, p. 477 (1992). Then the contradiction is resolved. For the patterns 40-1, 40-2, 40-3 of the light transmitting portion, the pattern 4
2, 44-1, 44-2, 46-1, 46-2 is 120
The phase difference, the patterns 41, 43-1, 43-
2, 45-1 and 45-2 give a phase difference of 240 degrees.
Here, considering the cross section along the line AA shown in FIG.
The phases given by the patterns in this cross section are arranged in order from 0 to 120, and 240 degrees periodically from left to right.

The pattern projection image light intensity distribution at this time is
As shown in FIG. 10, the position of the image obtained in the defocused state moves with respect to the image position at the in-focus position. This phenomenon is described in Proceedings of the 40th Joint Lecture on Applied Physics No.
2, p.606 (1993). In particular, under perfect coherent illumination conditions, if the pitch of the light transmitting portions arranged periodically is L, the wavelength of the illumination light is λ, and the defocus amount is Z, the movement amount δ of the position of the projected image is

[0012]

## EQU1 ## δ = (λZ) / (6L) (Equation 1) An ordinary exposure apparatus uses partial coherent illumination, but δ is determined by the value of a parameter (called a coherence factor) σ representing the degree of the partial coherent illumination.
Changes as shown in FIG. On the other hand, the line B shown in FIG.
Considering the cross section along −B, the phases given by the patterns in this cross section are 240 degrees, 1 from left to right.
They are 20 degrees and 0 degrees, which is the opposite of the pattern along the line AA. In this case, the amount of movement δ of the projected image position gives an opposite amount. Even if it is within the nominal depth of focus of the exposure apparatus, since the pattern position accuracy is deteriorated if the amount of movement of these increases, the actual depth of focus is limited even if the phase difference is introduced into the mask pattern.

The above-mentioned movement of the projected image position will be explained as follows. As shown in FIG. 5, the transparent thin films 21-1 and 21-2 have 120
Phase difference of 24 degrees, and the transparent thin films 22-1 and 22-2
Consider a periodic pattern that gives a phase difference of 0 degrees. At this time, the light 20 illuminating the mask pattern is diffracted by the pattern portion to become diffracted lights 23 and 24 that represent the fundamental spatial frequency ν ₀ of the pattern.

Since the diffraction angles θ ₁ and θ _{2 at} this time have different values, the diffraction image 2 formed on the pupil 27 of the projection lens 2
As shown in FIG. 6, 8 and 29 are asymmetric with respect to the center of the pupil 27 of the projection lens. This asymmetry causes the movement of the projected image position depending on the defocus. However, this pattern having the fundamental spatial frequency ν ₀ is a finer pattern than the resolution limit of the projection lens 7. If the phase difference is not introduced, the diffraction image is ν = − outside the pupil 27 of the projection lens.
[nu _0, pattern transfer because it is formed in [nu ₀ a position becomes impossible.

In order to avoid such asymmetry, the pattern part may be previously illuminated obliquely with respect to the optical axis of the projection lens so that the diffraction angles θ ₁ and θ ₂ are the same.

The prism-shaped transparent optical element arranged on the mask as shown in FIG. 2 plays a role of obliquely illuminating the mask pattern below it. The angle of incidence for oblique illumination can be determined by the inclination angle φ ₀ of the surface of the prism-shaped transparent optical element. As described above, the optimum tilt angle changes depending on the coherence factor σ, but σ =
In the case of ₀ , φ ₀ is defined as follows. Now, assuming that the incident angle of the illumination light that obliquely illuminates the pattern portion is φ ₁ and the refractive index of the prism-shaped transparent optical element is n, sin φ ₀ = nsi
The relationship n (φ ₀ −φ ₁ ) holds. Since the angles φ ₀ and φ ₁ are both small and the value of n is about 1.5, the relationship between φ ₀ and φ ₁ can be approximately expressed as follows.

[0017]

The Equation 2] φ ₀ = 3φ ₁ (number 2) diffraction pattern 28, 29 shown in FIG. 6 in a symmetrical arrangement,
It may be moved by the amount corresponding the diffraction image 28, 29 [nu _0/6. The incident angle φ ₁ of the illumination light therefor is L ′, where the pitch of the light transmitting portions on the mask is

[0018]

Φ ₁ = λ / (6L ′) (Equation 3) From the equations 2 and 3, the value of the inclination angle φ ₀ of the surface of the prismatic transparent optical element is

[0019]

## EQU4 ## φ ₀ = λ / (2L ') (Equation 4) may be set.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure of a projection exposure apparatus for realizing the pattern forming method of the present invention will be described with reference to FIG. Short-wavelength light emitted from a light source 1 such as a mercury lamp illuminates a mask 3, which is a first substrate, via a condenser lens 2. Hereinafter, the mask 3 is referred to as a reticle. Reticle 3 is
It is placed on the reticle stage 6. The reticle stage 6 is driven by the reticle stage driving means 19 and is positioned so that the center position of the reticle 3 coincides with the optical axis of the projection lens 7. The pattern 5 drawn on the reticle 3 is projected on the wafer 8 which is the second substrate through the projection lens 7. An optical element 4 that is transparent to illumination light, which will be described later, is provided on the reticle 3.

The wafer 8 is mounted on a Z stage 13 movable in the optical axis direction of the projection lens 7, that is, the Z direction, and further mounted on an XY stage 14 movable in the XY plane. Z stage 13 and XY stage 1
4 is driven by the respective drive means 16 and 17 controlled by the system control system 18, so that the projection lens 7
Can be moved to a desired exposure position. The position is accurately monitored by the laser length measuring device 12 as the position of the mirror 11 fixed to the Z stage 13. The position of the surface of the wafer 8 in the Z direction is measured by illuminating the long-wavelength light emitted from the light source 9 so as to be obliquely incident, and detecting the reflected light with the detector 10. The position at which the wafer 8 should be stopped at the time of pattern transfer is determined according to the above detection result, and the XY stage 14 and the Z stage 13 are moved to sequentially position the wafer.

FIG. 2 is an enlarged cross-sectional view of the reticle 3 showing a portion where the transparent optical element 4 according to the embodiment of the present invention is provided. Similar to the reticle shown in FIG. 5, a pattern is defined on the lower side of the transparent optical element 4 by the presence or absence of the light shielding portion 5, and the transparent thin films 21-1 and 21-21 provided on the light transmitting portion are provided.
-2 has a phase difference of 120 degrees and transparent thin films 22-1 and 22-2.
2-2 gives a phase difference of 240 degrees. The light 20 illuminating the mask pattern becomes the light 20-1 bent by the transparent optical element 4 and then diffracted by the pattern portion.

In this embodiment, the magnification is 1/5 and the exposure wavelength is λ.
Of 0.3 μm and a projection lens NA of 0.5 were used to transfer a 0.3 μm line / space pattern (pitch 0.6 μm) onto the wafer. Since the pattern pitch L'on the reticle is 3 μm,
As shown in Formula 4, the inclination angle φ ₀ of the surface of the transparent optical element 4 was set to 0.0608 radian (about 3.5 degrees).

As a result, as shown in FIG. 3, the diffraction angle θ of the diffracted lights 23 and 24 representing the fundamental spatial frequency ν ₀ of the pattern.
₃ and θ ₄ can be made the same, and the diffraction images 25 and 26 formed on the pupil 27 of the projection lens have symmetrical positions.
Here, the basic spatial frequency ν ₀ is a finer pattern than the resolution limit of the projection lens 7, but since a phase difference is introduced, a diffraction image is formed inside the pupil 27 of the projection lens. Transfer is possible. Diffraction image 2
As a result of 5 and 26 being formed at symmetrical positions, it is possible to prevent the movement of the pattern projection image due to defocus that conventionally occurs when there is no transparent optical element 4, and it is possible to prevent the pattern within a large depth of focus. It was possible to secure the positional accuracy of the projected image. When there are three types of phases as in the present embodiment, the inclination angle φ ₀ of the surface of the transparent optical element 4 is expressed by the equation 4, and therefore takes different values depending on the dimensions of the pattern.

As shown in FIG. 2, the transparent optical element 4 may be provided only in a region where the phases of the light transmitting portions are arranged in order of three or more types, and need not be provided over the entire surface of the reticle 3.
Further, in the cross section taken along the line AA and the line BB in FIG. 9, the order in which the phases of light are arranged is opposite, and therefore the inclination of the surface of the transparent optical element 4 is opposite. Also, the direction of the line pattern is not limited to one direction. Therefore, the arrangement position and the inclination direction of the transparent optical element 4 may be appropriately set according to the pattern.

FIG. 4 shows an example in which transparent members 4-1, 4-2, 4-3, 4-4, 4-5 are arranged on the reticle 3 as needed. The transparent optical element 4 is provided in a predetermined area for a predetermined length, but if the transparent optical element 4 is arranged long along the inclination direction, the thickness of the transparent optical element 4 increases. Transparent optical element 4
The role of is to change the direction of the illumination light 20, so a plurality of transparent optical elements 4 shown in FIG. 2 may be arranged. However, in order to prevent light from being diffracted by arranging a plurality of transparent optical elements 4, the arranging pitch is the illumination light 2
It is desirable that the wavelength is sufficiently longer than the wavelength of 0.

Further, as shown in FIG. 12, the transparent optical element 4
Depending on its role, may be provided in close contact with the surface of the reticle 3 (a), or may be unevenness formed on the surface of the reticle 3 itself (b), or may be provided in the vicinity of the reticle 3 ( It may be c). Furthermore, the transparent optical element 4
The surface of may be a smooth inclined surface, or may be realized by a fine stepped surface structure.

In the present embodiment, the case where the phase of the reticle transmitted light is 120 degrees and 240 degrees in addition to 0 degrees is shown. However, the present invention is not limited to this, and when three or more types of phases are defined, their arrangement and The same effect can be obtained by changing the inclination angle of the surface of the transparent optical element 4 according to the phase difference.

[0029]

According to the present invention, when the phase shift exposure method that gives three or more kinds of phases to the reticle transmitted light is adopted, the reduction of the depth of focus, which is restricted by the positional accuracy of the pattern projection image, is solved. be able to. Therefore, there is an effect that the phase shift exposure method, which is less likely to be restricted by the pattern layout, is adopted to improve the resolution, and at the same time, the positional accuracy and the depth of focus of the projected pattern image can be improved.

[Brief description of drawings]

FIG. 1 is a block diagram of a projection exposure apparatus that embodies the present invention.

FIG. 2 is a sectional view showing a part of a mask according to an embodiment of the present invention.

FIG. 3 is a plan view showing an example of a diffraction image formed by the mask pattern of the present invention.

FIG. 4 is a plan view showing an example of a mask according to an embodiment of the present invention.

FIG. 5 is a sectional view showing a part of a conventional mask which gives three kinds of phases.

6 is a plan view showing an example of a diffraction image formed by the mask pattern shown in FIG.

FIG. 7 is a plan view showing a mask pattern that gives a phase difference of 180 degrees to adjacent light transmitting portions.

FIG. 8 is a plan view showing an example of a pattern in which a phase difference of 180 degrees cannot be given to all adjacent light transmitting portions.

FIG. 9 is a plan view showing an example of a pattern in which three kinds of phases are given to a light transmitting portion.

FIG. 10 is an explanatory diagram showing that the pattern projection image moves in a defocused state.

FIG. 11 is a characteristic diagram showing the defocus dependence of a projected pattern image.

FIG. 12 is a sectional view of a mask according to an embodiment of the present invention.

[Explanation of symbols]

1 ... Light source, 2 ... Condenser lens, 3 ... Reticle, 4
... Transparent optical element, 7 ... Projection lens, 8 ... Wafer, 13 ...
Z stage, 14 ... XY stage.

Claims (5)

[Claims] 1. A pattern forming method for illuminating a first substrate and transferring an original image pattern drawn on the first substrate onto a second substrate via a projection optical system, A predetermined pattern including a region that changes the phase of transmitted light is defined on the substrate, and a specific region of the pattern changes the traveling direction of the illumination light and is oblique to the optical axis of the projection optical system. A pattern forming method characterized by being illuminated. 2. The original image pattern defined on one surface of the first substrate according to claim 1, including a region for arranging a light transmitting portion that imparts at least three types of phases to transmitted light, An optical element for generating the optical path difference is provided close to or in close contact with the other surface of the first substrate corresponding to the above. 3. The optical element for generating an optical path difference, which is provided on a surface of the first substrate opposite to a surface on which the original image pattern is defined, constitutes the first substrate. A pattern forming method defined by the unevenness of the surface of a transparent substrate. 4. A desired pattern defined by a light transmission pattern group including a region for changing the phase of transmitted light is defined on one surface, and a predetermined pattern is defined on a predetermined region of the pattern on the other surface. And a means for changing the traveling direction of light by generating the optical path difference of the mask. 5. The pattern defined in the mask according to claim 4, including a region in which light transmitting portions that give at least three kinds of phases are arranged side by side, and the optical path difference is generated on the other surface corresponding to the region. The means for making the mask is a transparent optical element having a predetermined inclination angle which is defined by the unevenness of the surface of the transparent substrate or which is provided close to or in close contact with the surface.