JP2600311B2 - Method for detecting misalignment between mask and wafer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for detecting a positional shift between a mask and a wafer, in particular,
The present invention relates to a method for detecting a positional shift between a mask and a wafer applicable to a line exposure apparatus.

[Technical environment]

In recent years, semiconductors tend to be highly integrated as typified by DRAMs, and the minimum line width of VLSI patterns is also approaching the micro to sub-micro region.

Under such circumstances, in a conventional optical semiconductor exposure apparatus using ultraviolet g-line and i-line, the resolution limit is said to be about 0.5 μm depending on the wavelength of light.
There is a strong demand for a next-generation exposure apparatus that can handle the following patterns. As a next-generation exposure apparatus, an X-ray exposure apparatus is currently considered promising, and research and development are proceeding.

[Conventional technology]

Conventional techniques include, for example, B.Fa
y) et al. from the Journal of Vacuum Science Technolo
gy) Vol. 16, No. 6, pp. 1954-1958, November / December, 1979
cal Alignment System for Submicron X-rar Lithogra
There is a mask-wafer alignment method using a linear Fresnel zone plate (LFZP) reported in "phy."

Conventional alignment uses a cylindrical lens that uses diffraction of light called LFZP as a mask mark,
This is performed by using a linear diffraction grating as a wafer mark, irradiating the LFZP with a laser beam, and detecting diffracted light from the linear diffraction grating.

Next, a conventional alignment method will be described in detail with reference to the drawings.

FIG. 4 is an explanatory view showing an alignment method using SFZP.

A diffraction grating 17 is imprinted on the wafer 16 and the wafer 16
The mask 18 faces the upper surface with a predetermined gap therebetween. An LFZP 19 having a focal length equal to the gap between the mask and the wafer is drawn on the mask 18.

FIG. 5 is a plan view showing the structure of the mask mark LFZP.

LFZP has a structure in which stripes of various widths and intervals are arranged, and when the distance from the center of the mark is r _n Is represented by Here, f is the focal length, and λ is the wavelength of the laser used for alignment. Although the center stripe of the LFZP shown in the figure is transparent, the opposite configuration is also possible.

FIG. 6 is a plan view showing a diffraction grating of a wafer mark.

The diffraction grating has a structure in which rectangles having the same size are arranged at equal intervals, and the diffraction angle is determined by the pitch P of the diffraction grating.

In FIG. 4, the parallel laser beam 20 incident from above the mask 18 is condensed by the LFZP 19, focuses on the surface of the wafer 16, and forms a slit-like image. When the imaged slit and the diffraction grating 17 on the wafer surface overlap on a straight line,
The laser beam is diffracted, passes through the LFZP 19 again, becomes parallel light, and is detected as an alignment signal.

FIG. 7 is a perspective view showing a conventional automatic overlaying apparatus using an alignment method by LFZP disclosed in Japanese Patent Application Laid-Open No. 55-43598.

7 includes a table 21, a piezoelectric transducer 22 for moving the table in the x direction, a wafer 16 mounted on the table and having a diffraction grating 17 previously engraved thereon, And a mask 18 on which an LFZP 19 is preliminarily drawn, which is arranged oppositely with a distance of several tens of μm.

The apparatus further comprises a laser light source 23, wherein said laser light source 23
The parallel laser beam emitted from the laser beam is focused by a first lens 24 and excited by a generator 25 by a motor 26.
The light is reflected by the galvanometer mirror 27 driven by the light source, passes through the second lens 28, and irradiates the LFZP 19 on the mask 18.

With respect to the second lens 28, the irradiation position on the galvanometer mirror 27 and the irradiation position on the mask 18 are paired. Further, the first lens 24 and the second lens 28 constitute an afocal optical system.

Therefore, the angle of incidence of the laser beam on the mask 18 can be changed by the galvanomirror 27. As shown in FIG.
Scan above. Diffracted light from the diffraction grating 17 on the wafer 16 is spatially separated from incident light and detected by the detector 29.

A signal from the detector 29 and a signal from the generator 25 are input to a phase detector 30, and a phase shift signal h is obtained by detecting a phase difference between the two signals. This displacement signal h
Is input to the power source 31 of the piezoelectric converter 22, moves the stage according to the positional shift, and superposes the mask 18 and the wafer 10.

By configuring the optical system and the control system as described above, a closed loop automatic superposition apparatus can be realized.

[Problems to be solved by the invention]

In the above-described conventional method for detecting the positional shift between the mask and the wafer, a galvanomirror is used in the scanning optical system to obtain a positional shift signal, and the galvanomirror driving signal is used as a reference signal for phase detection. When the deflection angle of the mirror drifts, the displacement signal also drifts.

In addition, the outer periphery of the LFZP of the mask mark is a line and space of about 0.5 μm, and there is a disadvantage that extremely high processing accuracy is required to give the LFZP a sufficient light-collecting characteristic.

[Means for solving the problem]

According to the method for detecting the positional deviation between a mask and a wafer according to the present invention, a plurality of first linear diffraction gratings are disposed in such a manner that the mask and the wafer are opposed to each other with a predetermined gap therebetween and are arranged in parallel on the mask at regular intervals. And a plurality of second linearly arranged on the wafer in parallel with the first linear diffraction grating and at equal intervals.
Providing a linear diffraction grating, irradiating the first and second linear diffraction gratings with a slit beam while scanning the first and second linear diffraction gratings,
And a first AC signal obtained by the diffracted light from the first linear diffraction grating and a diffracted light from the second linear diffraction grating. The method includes detecting a phase difference of the second AC signal.

〔Example〕

Next, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a perspective view for explaining one embodiment of the present invention.

The method for detecting the displacement between the mask and the wafer shown in FIG.
The mask 1 and the wafer 2 are set apart from each other by a predetermined gap S, and a plurality of linear diffraction gratings 3 are arranged on the mask 1 at regular intervals such that the longitudinal direction is orthogonal to the displacement detection direction A.
A plurality of linear diffraction gratings 4 are arranged on the wafer 2 in parallel with the linear diffraction grating 3 at equal intervals, and a laser beam 5 is focused on a slit in an area where the linear diffraction gratings 3 and 4 are drawn. Irradiate from oblique direction, scan the slit spot of the laser beam 5 in the displacement detection direction A at a constant speed, detect the reflected diffraction light from the linear diffraction gratings 3 and 4, and diffract from the linear diffraction grating 3. Detecting the phase difference between the AC component of the optical signal and the AC component of the diffracted optical signal from the linear diffraction grating 4 is configured.

The slit spot of the laser beam 5 is a linear diffraction grating
When the laser beam 5 overlaps the points 3 and 4, the laser beam 5 is reflected and diffracted. Therefore, when a slit-like spot scans an area where linear diffraction gratings arranged at equal intervals are drawn at a constant speed, if a diffracted light from the linear diffraction grating is detected, a signal having an AC component of a constant frequency is detected. Is obtained.

When the diffracted light signals from the linear diffraction grating 3 on the mask 1 and the linear diffraction grating 4 on the wafer 2 are separately detected, the phase difference between the AC components of the respective diffracted light signals becomes It shows a typical displacement.

By the above, the width w1, w2 and spacing d ₁ of the linear diffraction gratings 3, 4, d ₂ is desirably equal design to the width w3 of the slit-like spot.

2A and 2B are optical path diagrams showing an optical system for realizing the embodiment shown in FIG.

FIG. 2 (a) is an optical path diagram viewed from the direction B shown in FIG. A beam expander 7 for expanding the diameter of a laser beam emitted from a laser light source 6 and a laser beam 5 expanded by the beam expander 7
A galvanomirror 8 for deflecting the laser beam 5, an image forming lens for forming an image of the deflected laser beam 5, and a cylindrical lens 10 for forming an image forming spot of the image forming lens 9 into a slit shape.
And an objective lens 11 for projecting an imaging spot formed by the imaging lens 9 onto the mask 1 and the wafer 2.

The slit spot projected by the objective lens 11 scans the mask 1 and the wafer 2 according to the deflection angle of the galvanometer mirror 8. Therefore, in order to make the scanning speed on the mask 1 and the wafer 2 equal, it is desirable that the drive signal of the galvanomirror 8 is a triangular wave. A scanning optical system using a polygon mirror and an f · θ lens is also effective.

FIG. 2 (b) is an optical path diagram viewed from the direction C shown in FIG. The optical axis of the objective lens 11 is the mask 1 and the wafer 2
It is inclined at an angle to the plane. Therefore, the linear diffraction grating 3 on the mask 1 and the linear diffraction grating 4 on the wafer 2 can be arranged so as to be located in the image plane of the objective lens 11.

FIG. 3 is an enlarged view of the vicinity of the slit spot on the mask 1 and the wafer 2 of the optical system shown in FIG. 2B.

The mask 1 and the wafer 2 are placed in parallel with a gap S of several tens of μm, but since the optical axis is inclined by an angle θ from the vertical direction, the linear diffraction gratings 3 and 4 are focused on the objective lens 11 to form an image. It can be arranged on the plane D. At this time, the linear diffraction gratings 3 and 4 are provided shifted in the horizontal direction by an interval t = S / tan θ.

Since the laser beam 5 is applied obliquely, the mask 1
In addition, the zero-order reflected light from the wafer 2 does not enter the objective lens 11, and only the diffracted light entering the pupil of the objective lens 11 is detected.

By changing the pits on the linear diffraction gratings 3 and 4, the diffraction angle of the reflected diffraction light from each can be changed.

Therefore, if the diameter of the laser beam in the longitudinal direction of the slit-shaped image spot formed by the objective lens 11 is narrowed in advance, the beam incident on the objective lens 11 and the linear diffraction grating 3 are formed as shown in FIG. And the reflected diffraction light from the linear diffraction grating 4 can be separated into spaces, and it is easy to independently detect the reflected diffraction light from the linear diffraction gratings 3.4. it can.

At this time, spatial filters 12 and 13 are placed at positions conjugate with the mask 1 and the wafer 2 with respect to the objective lens 11,
When only the light that has passed through the spatial filters 12 and 13 is detected by the detectors 14 and 15, a signal having a high S / N ratio can be obtained.

The misregistration detection mark needs to be updated by exposure, so the mark must be placed in the exposure area. At this time, in order to prevent the objective lens 11 from blocking the exposure beam, it is necessary to use an objective lens having a long working distance.

As the objective lens used in the optical system for detecting the positional deviation between the mask and the wafer according to the present invention, a lens having an NA of about 0.20 and a long working distance is suitable. When a He-Ne laser is used as a light source, a slit having a width of about 3 μm is used. Shaped spots can be obtained.

Therefore, the widths w1, w2 and the spacing of the linear diffraction gratings 3, 4
The optimal values of d ₁ and d ₂ are 3 μm, and the period of the AC component of the diffracted light signals obtained from the detectors 14 and 15 is 6 μm.

From the above, if the accuracy of the phase meter is 2-3 °, 0.04 μm
Is obtained, which sufficiently satisfies the overlay accuracy required for submicron device exposure.

In addition, by increasing the number of linear diffraction gratings 3 and 4 as much as possible within the range of the field of view of the objective lens, an averaging effect can be expected, and a high-resolution position shift signal with a high S / N ratio can be obtained. Can be.

〔The invention's effect〕

According to the method for detecting a positional shift between a mask and a wafer according to the present invention, instead of using a drive signal of a galvanomirror as a reference signal for phase detection, a positional shift is performed based on a phase difference between a diffracted light signal from a mask and a diffracted light signal from a wafer. In order to obtain the amount, the drift of the deflection angle of the galvanomirror does not affect the displacement signal, and there is an effect that a stable displacement signal can be obtained.

In addition, since a diffraction grating is used instead of using LFZP for a mask mark, there is an effect that the mark can be easily formed.

[Brief description of the drawings]

FIG. 1 is a perspective view showing one embodiment of the present invention, FIGS. 2 (a) and 2 (b) are optical path diagrams showing an optical system for realizing the embodiment shown in FIG. 1, and FIG. FIG. 2 (b) is an enlarged view of the vicinity of the slit spot, FIG. 4 is an explanatory view showing a conventional alignment method, and FIG.
FIG. 6 is a plan view showing the structure of the LFZP, FIG. 6 is a plan view showing a diffraction grating of a mark for a wafer, and FIG. 7 is a perspective view showing an automatic overlaying apparatus using a conventional alignment method using the LFZP. 1,18 ... mask, 2,16 ... wafer, 3,4 ... linear diffraction grating, 5 ... laser beam, 6,23 ... laser light source, 7 ...
Beam expander, 8,27 …… Galvanomirror, 9 ……
Imaging lens, 10 ... Cylindrical lens, 11 ... Objective lens, 12,13 ... Spatial filter, 14,15,29 ... Detector, 19 ... Linear Fresnel zone plate, 21 ... Stage, 22 ... Piezoelectric transducer, 30 ... Phase detector.

Claims (3)

(57) [Claims] 1. A mask and a wafer are installed facing each other with a predetermined gap therebetween, and a plurality of first linear diffraction gratings arranged in parallel at a predetermined interval on the mask are provided. Providing a plurality of second linear diffraction gratings arranged at equal intervals in parallel with the first linear diffraction grating, and irradiating the first and second linear diffraction gratings with scanning a slit beam,
A reflected diffracted light from the first second linear diffraction grating is detected, and a first AC signal obtained by the diffracted light from the first linear diffraction grating and the second linear diffraction grating are detected. Detecting a phase difference between a second AC signal obtained by the diffracted light of the mask and the wafer. 2. The method according to claim 1, wherein the first linear diffraction grating and the second linear diffraction grating have different pitches. 3. The apparatus according to claim 1, wherein the objective lens having a focal depth smaller than the gap between the mask and the wafer focuses the beam in a slit shape, and the objective lens is arranged obliquely to the normal direction of the plane of the mask and the wafer. Or the method for detecting a positional shift between a mask and a wafer according to 2.