JPH05107739A - Photomask, exposing method, and projection exposure device - Google Patents

Abstract

PURPOSE:To obtain excellent image formation performance over the entire exposure area. CONSTITUTION:Transparent films 1a and 1b which differ in refractive index corresponding to the step structure of a wafer 4 and/or the distortion of an image formation plane by a projection lens 3 are adhered to the pattern formed surface of the photomask 1, and the image formation plane is moved in the direction of the optical axis. At this time, the both sides of the projection lens 3 are preferably telecentric. The transparent films 1a and 1b may be separated from the photomask 1 and an image plane correction member which has a refractive index distribution corresponding to the step structure of the wafer 4 and/or the distortion of the image formation plane may be arranged between the photomask 1 and projection lens 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photomask, a projection exposure method, and a projection exposure apparatus used in a lithography process for manufacturing a semiconductor device or a liquid crystal display device, for example.

[0002]

2. Description of the Related Art In recent years, in a lithography process, as a device for transferring a pattern formed on an original substrate (mask or reticle) onto a photosensitive substrate (semiconductor wafer etc.) with high resolution, a step-and-repeat type reduction projection type exposure apparatus. (Stepper) is often used. In this type of stepper, the projection lens is chromatically corrected with respect to the wavelength of the exposure light, and the reticle and the wafer are arranged to be conjugate with each other under the exposure wavelength. still,
The bottom surface (pattern surface) of the reticle is almost flat (flat surface)
The projection image plane (best image plane) of the projection lens is also substantially flat.

[0003]

However, the conventional device as described above has the following problems. Normal,
The surface of one exposure region on the sensitive substrate (hereinafter referred to as a wafer) is considered to be almost flat, and in this case, there is no problem even if the image plane of the projection optical system is flat. However,
Each exposure region on the wafer may partially have a step (uneven structure). In such a case, in the conventional technique, the image plane of the projection optical system is substantially flat, so that the focus is on the upper surface of the step. That is, if the image forming surface and the upper surface are aligned with each other, the image forming performance on the lower surface of the step deteriorates. In particular, if the depth of focus of the projection optical system is smaller than the step, the lower surface of the step causes poor resolution. On the contrary, when focusing on the lower surface of the step, the imaging performance on the upper surface of the step deteriorates. Further, if the focal point is focused between the upper surface and the lower surface, there is a drawback that sufficient imaging performance cannot be obtained on both the upper surface and the lower surface.

When the unevenness on the wafer is relatively fine, it can be dealt with to some extent by applying a thick resist or using a multilayer resist, but it is very difficult to deal with unevenness over a large area. It was difficult to avoid deterioration of the imaging performance.

Further, even if the wafer is flat, the image plane is distorted over a large area due to aberrations of the projection optical system (curvature of field, etc.) and warpage of the photomask itself. In this case, the image plane and the surface of the exposure area are partially displaced from each other, and the image forming performance is deteriorated. Heretofore, there has been no method of coping with such image plane distortion.

In addition to the above, the minimum transferable size in projection exposure is the numerical aperture (N) of the projection optical system.
Since it is inversely proportional to A) and proportional to the exposure wavelength, the numerical aperture (NA) of the projection optical system tends to increase and the exposure wavelength tends to become shorter in response to the demand for pattern miniaturization. Since the depth of focus of the projection optical system is inversely proportional to the square of the numerical aperture and is proportional to the exposure wavelength, the depth of focus of the projection optical system has been rapidly decreasing in recent years, and it is even more prominent due to unevenness of the wafer and distortion of the image plane. Poor resolution is likely to occur.

The present invention has been made in consideration of the above points, and even if there is a step (unevenness) in the exposure area on the photosensitive substrate or the pattern image by the projection optical system is distorted. An object of the present invention is to obtain a photomask, a projection exposure method, and a projection exposure apparatus capable of performing high-resolution pattern exposure over the entire exposure area.

[0008]

A photomask according to claim 1 is a photomask used for exposing a photosensitive substrate with a predetermined pattern through a projection optical system, and in order to achieve the above object. An area having a different refractive index is provided on the pattern formation surface side according to the step structure in the exposure area on the sensitive substrate and / or the distortion of the image plane formed by the projection optical system. ..

In order to achieve the above object, the projection exposure method according to a second aspect of the present invention achieves the above object when the pattern formed on the original drawing substrate is exposed on the sensitive substrate through the projection optical system.
The pattern formation surface side of the original drawing substrate is provided with a step structure in the exposure area on the sensitive substrate and / or a refractive index distribution according to the distortion of the image forming surface by the projection optical system, and the projection optical system is connected. At least a part of the image plane of the projection optical system is moved in the optical axis direction so that the image plane and the surface of the exposure area are substantially aligned with each other.

Further, in the projection exposure apparatus of the third aspect, a projection optical system for image-forming and projecting an image of the pattern formed on the original substrate onto the sensitive substrate, and the surface of the sensitive substrate is imaged by the projection optical system. In order to achieve the above object, there is provided a step structure in the exposure area on the sensitive substrate and / or an image formed by the projection optical system. It has a refractive index distribution according to the distortion of the surface, and moves at least a part of the image plane of the projection optical system in the optical axis direction so that the image plane of the projection optical system and the surface of the exposure region substantially coincide with each other. The image surface correcting member is provided.

[0011]

The operation of the present invention will be described with reference to FIG. 3 (corresponding to the first embodiment of the present invention described later). In the figure, transparent films 1a and 1b having a constant thickness and different refractive indexes are deposited on predetermined portions of the photomask 1 on the pattern formation surface side. The A _1, A ₂ of the pattern formation surface by injection, a transparent film 1
When the light fluxes IL ₁ and IL ₂ transmitted through a and 1b are imaged on B ₁ and B ₂ by the projection optical system 3 having the magnification m,
_Since there is an optical path difference between ₁ and IL ₂ , the image forming positions in the optical axis direction differ by Δd as shown in FIG.
At this time, the refractive indexes of the transparent films 1a and 1b are set to n ₁ , n ₂ (n
₂ > n ₁ ) and the thickness is D, Formula 1 is established. However, Δn = n ₁ −n ₂ , m is the magnification of the projection lens. Δd = {Δn / (n ₁ · n ₂ )} · D · m ² Equation 1

That is, the refractive indexes and the thicknesses of the transparent films 1a and 1b are selected so that the above-mentioned Δd corresponds to the step difference of the exposure region, and the transparent film 1a is placed at a position corresponding to the concave portion of the exposure region. By depositing a portion corresponding to the convex portion of the exposure area, both the concave portion and the convex portion coincide with the image plane. At this time, if the principal rays of the light flux that has passed through the photomask 1 and is incident on the projection optical system 3 and the light flux that is emitted from the projection optical system 3 are inclined with respect to the optical axis, the image plane is oriented in the optical axis direction. Since the size of the image also changes due to the shift, it is desirable that the projection optical system 3 be both-side telecentric.

When the sensitive substrate is flat and the image plane is distorted, the refractive indexes and thicknesses of the transparent films 1a and 1b are set so that Δd corresponds to the difference in the position of the image plane in the optical axis direction. Select
A transparent film 1 having a small refractive index in the portion where the image plane is convex
If a is coated with a transparent film 1b having a large refractive index in a portion where the image forming surface is concave, the distortion of the image forming surface can be corrected and the entire exposure area can be matched with the image forming surface.

In FIG. 3, for simplicity, a transparent film having the same thickness and a different refractive index is applied to the pattern forming surface to move the image forming surface in the optical axis direction. The thickness of the film does not have to be constant.

Further, in FIG. 3, the unevenness of the exposure area and the distortion of the image plane are dealt with by providing areas having different refractive indexes in the photomask itself, but the transparent films 1a and 1b in FIG.
It is also possible to separate the photomask and the photomask. That is,
By disposing an image plane correction member having a refractive index distribution according to the unevenness of the exposure region or the distortion of the image plane in the vicinity of the photomask of the projection exposure apparatus, the entire exposure region is connected in the same manner as described with reference to FIG. Can be matched to the image plane.

In the present invention, the films (transparent films 1a and 1b in FIG. 3) provided for moving the image plane may be transparent to the exposure light, for example, glass, quartz, PMM.
A, fluororesin, etc., but not limited to these. The desired refractive index distribution can be obtained by injecting an ion beam into a specific part of the transparent film, selectively depositing substances with different refractive indexes using a photoresist as a mask, or conversely etching unnecessary parts. Can be done by doing.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to FIG. First, FIG. 1A is a plan view of the pattern area PA of the photomask used in this embodiment. The formed pattern is a line and space pattern having a dimension on the mask of 2.5 μm.

Next, FIG. 1B is a conceptual diagram showing a state in which projection exposure is performed using the photomask according to this embodiment. In the figure, on the wafer (sensitive substrate) 4 coated with resist, 0.1 μ is applied to the portion corresponding to the dotted line in FIG. 1 (a).
Assuming that a step of m has occurred, projection exposure is performed using the 1/5 reduction projection lens 3. Here, the transparent film 1 having a thickness of 100 μm is formed on the pattern formation surface of the photomask 1.
a and 1b are formed to correspond to the steps of the wafer 4, and the refractive indexes n ₁ and n ₂ of the transparent films 1a and 1b are n ₁ <n ₂ near 1.5, the above formula 1 to n ₁
= 1.5, n ₂ = 1.55, Δn = 0.5. That is, in FIG. 1B, the transparent film 1a having a refractive index of 1.5 is applied to the portion of the photomask 1 corresponding to the concave portion of the wafer 4, and the portion corresponding to the convex portion of the wafer 4 has a refractive index of 1. The transparent film 1b of 55 may be applied.

When projection exposure is performed using such a photomask, the light flux IL _{1 that} has passed through the transparent film 1a forms an image in the concave portion of the wafer 4, and the light flux IL _{2 that} has passed through the transparent film 1b is convex on the wafer 4. An image is formed on the part, and the entire exposure area matches the image forming surface, and good image forming performance is obtained.

On the other hand, FIG. 1 (c) shows a state in which projection exposure is performed using the conventional photomask 101 under the same conditions as in the above-described first embodiment. As shown in the figure, when the image forming surface is aligned with the concave portion of the wafer 4, the convex portion of the wafer 4 does not coincide with the image forming surface, and good image forming performance cannot be obtained in the entire exposure region.

Next, FIG. 2 shows a state in which projection exposure is performed by the same exposure apparatus as that of the first embodiment using the photomask according to the second embodiment of the present invention. In this embodiment, not only the refractive index but also the film thickness of the transparent films 10a and 10b provided on the photomask 10 are changed according to the unevenness of the wafer 4. The film thickness is adjusted by the transparent films 10a and 10b
May be performed at the time of forming the film, or after the film is formed, the film thickness may be reduced only at a specific portion by etching. By adjusting the refractive index and the film thickness, the entire exposure region can be matched with the image plane as in the first embodiment.

Further, when the step in the exposure area of the wafer is relatively large, the step is provided on the photomask itself or a transparent film (having the same refractive index) is provided on a specific portion of the photomask. A method of moving the image plane may be adopted. These methods are proposed by the applicant of the present application in Japanese Patent Application No. 3-200580 (filed on August 9, 1991), and will be briefly described with reference to FIGS. 4 and 5.

First, in FIG. 4B, the luminous flux IL emitted from two points A ₁ and A ₂ whose coordinates in the optical axis direction differ by D.
Assuming that ₁ and IL ₂ are imaged at two points B ₁ and B ₂ whose coordinates in the optical axis direction are different by Δd by the projection lens 3 having a magnification of m, Expression 2 is established. Δd = D · m ² Formula 2 Therefore, if a step of D = Δd / m ² is provided on the photomask 20 ′ itself so that Δd becomes a value corresponding to the step of the wafer, the exposure area with unevenness is connected. The image planes can be matched.

For example, in FIG. 4A, the projection lens 3
Is one-fifth (m = 0.2), and the wafer 4 has a magnification of 1
If there is a step of μm, the photomask 20 has
A step of m may be provided. At this time, when the pattern of the photomask 20 is connected on both sides of the step portion as shown in FIG. 1A, the step of the wafer 4 is adjusted to the inclination of the step on the wafer 4 (the step of the wafer 4 is generally a right angle). Without inclination)
It is desirable that the steps provided on the photomask 20 also be inclined.

Next, the case of forming transparent films (having a constant refractive index) having different thicknesses on the photomask will be described. In FIG. 5B, the luminous fluxes IL ₁ and I emitted from the two points A ₁ and A ₂ where the film thickness difference of the deposited transparent film 30a ′ is ΔD.
If L ₂ is imaged at two points B ₁ and B _{2 which} are different by Δd in the optical axis direction by the projection lens 3 having the magnification m, the expression 3 is established. However, n is the refractive index of the transparent film 30a ', and 1 is the refractive index of air. Δd = {(n−1) / n} · ΔD · m ² Equation 3 Therefore, if the film thickness difference of the transparent film 30a ′ is adjusted so that Δd becomes a value corresponding to the step of the wafer, there is unevenness. The exposure area and the image plane can be matched.

For example, in FIG. 5A, the projection lens 3
Is 1/5 (m = 0.2), the level difference of the wafer 4 is 0.5 μm, and the refractive index of the transparent film 30a is 1.5, the thickness of the transparent film 30a is 37. It can be seen that 5 μm is sufficient.

Next, FIG. 6 is a schematic diagram showing the schematic construction of a projection exposure apparatus according to the third embodiment of the present invention. In the figure, the illumination system 2 includes a light source such as an ultra-high pressure mercury lamp, an optical integrator, and the like, and irradiates the photomask 40 with the exposure illumination light IL in the wavelength range in which the resist layer is exposed to light with a substantially uniform illuminance. The pattern area P of the photomask 40
The exposure light IL that has passed through A is incident on the both-side telecentric projection lens 3, and the projection lens 3 forms an image of a pattern formed by a light-shielding layer such as chrome on the lower surface of the photomask 40 and a resist layer on the surface. An image is projected on the formed wafer 4. In this embodiment, the photomask 40 itself is used as the image plane correction member in the present invention, which will be described later in detail.

When the magnification of the projection lens 3 is m, the size of the pattern of the photomask 40 is naturally 1 / m times the size of the pattern to be formed on the wafer 4. The projection lens 3 is well corrected for chromatic aberration with respect to the wavelength of the exposure light IL, and the photomask 2 and the wafer 4 are arranged so as to be conjugate with each other under the exposure wavelength.

Further, the wafer 4 is two-dimensionally moved by a motor 6 in a step-and-repeat manner.
When the transfer exposure of the photomask 40 onto one exposure area on the wafer 4 is completed, the wafer is stepped to the position of the next exposure area. Here, the wafer stage 5 holds the wafer 4 so that the surface of the wafer 4 is arranged near the image plane IM of the projection lens 3. The two-dimensional position of the wafer stage 5 is measured by the interferometer 7, for example, 0.
It is always detected with a resolution of about 01 μm.

FIG. 8A shows one area SA of the plurality of exposure areas formed on the wafer, and the four side lines around one exposure area SA usually have street lines with a width of about 50 to 100 μm. ST is formed. Street line S
T is a cutting margin when the chips on the wafer are cut out, and a part of the circuit pattern is not formed to extend there. FIG. 8B is a sectional view taken along the line BB ′ of FIG. 8A and shows that the exposure area SA partially has a step structure. Here, the step (spacing) between the partial areas W ₁₁ and W ₁₂ in the exposure area SA is represented by d.

Since a part of the exposure area SA has a step structure as described above, the exposure area S is used in this embodiment.
Corresponding to the step structure of A, regions having different refractive indexes are formed on the pattern surface of the photomask 40. by this,
A part of the image plane IM of the projection lens 3 (that is, the projected image of the photomask pattern) is shifted (moved) in the direction along the optical axis AX (Z direction), and the image plane IM of the projection lens 3 and the exposure area SA are obtained. The surface of the surface and the surface of the surface substantially coincide with each other (FIG. 9). Therefore, in this embodiment, the photomask 40 having regions having different refractive indexes on the pattern surface side is
It is supposed to be used as an image plane correction member.

FIG. 7A shows an example of a specific structure of a photomask 40 suitable for this embodiment, and FIG. 7B is a sectional view taken along the line AA 'of FIG. 7A. Is. As shown in FIGS. 7A and 7B, on the pattern surface of the photomask 40, regions R ₁₁ having different refractive indices corresponding to the step structure of the exposure region SA,
R ₁₂ is formed. That is, based on the step (distance d) in the exposure area SA shown in FIG. 8B, the area R ₁₁ corresponding to the partial area W ₁₁ and the area R corresponding to the partial area W ₁₂ in the pattern area PA. Transparent films 40a and 40b having different refractive indexes are adhered to ₁₂ respectively. Here, the refractive index difference Δn and the film thickness D of the transparent films 40a and 40b
Is set so as to satisfy the above-mentioned formula 1, but the film thickness of the resist layer formed on the wafer 4, the type of resist,
And Δn, D due to the influence of pattern size and array density.
Since the optimum value of 1 can vary, it is desirable to determine the optimum value for each photomask by conducting experiments or simulations in advance.

Next, a projection exposure apparatus according to the fourth embodiment will be described with reference to FIG. FIG. 10 is a diagram showing an example of the configuration of the apparatus according to the present embodiment. Although the photomask is used as the image plane correction member in the embodiment of FIG. 6,
The present embodiment is different only in that the parallel flat plate glass 51 arranged between the photomask 50 and the projection lens 3 is used as an image plane correction member.

As shown in FIG. 10, in this embodiment, at least one surface (only the upper surface in the drawing) corresponding to the step structure in the exposure area is coated with transparent films 51a and 51b having different refractive indexes. The flat glass plate 51 is arranged between the photomask 50 and the projection lens 3. As a result, also in this embodiment, the image plane of the projection optical system and the surface of the exposure area can be made to substantially coincide with each other over the entire surface. At this time, it is desirable that the parallel plate glass 51 be arranged as close to the photomask 50 as possible. Further, it is desirable that the transparent films 51a and 51b are also formed on the surface closer to the photomask 50, which is effective in clearly separating the step portion of the image plane on the exposure area. In the example shown in the figure, the transparent films 51a and 51b are applied to the parallel flat plate glass so as to have a predetermined refractive index distribution. However, the parallel flat plate glass itself is injected with an ion beam or the like so that the parallel flat plate glass itself has a refractive index. You may make it have a distribution.

By the way, in the above embodiment, the stepped structure on the wafer (exposure region) has two steps (that is, FIG. 8B).
The exposure area SA is divided into two partial areas W ₁₁ and W
_Although it has been described (divided into ₁₂ ), it is naturally possible to easily cope with a step structure having three or more steps having different steps (corresponding to the distance d). In carrying out the present invention, it is desirable that the projection lens 3 be telecentric not only on the wafer side but also on the photomask side. 6 and 1
In the embodiment of No. 0, the stepper has been described as an example, but the present invention is applied to any exposure apparatus other than the stepper as long as it is an exposure apparatus provided with a projection optical system (for example, a mirror projection system may be used). It goes without saying that you can do it. Further, an apparatus may be used which collectively exposes the entire surface of a photosensitive substrate (semiconductor wafer, liquid crystal substrate, etc.) at the same magnification. Further, the image plane correction member in the present invention is not limited to the photomask and the parallel flat plate glass, as long as it can have a refractive index distribution corresponding to the photosensitive substrate step structure or the distortion of the image plane. It doesn't matter.

[0036]

As described above, in the present invention, the step structure of the sensitive substrate and / or the step structure of the sensitive substrate is formed on the pattern forming surface side of the photomask.
Or because it has a refractive index distribution according to the distortion of the image plane,
Even if the sensitive substrate has a step structure over a wide range or the image plane is distorted due to the aberration of the projection optical system, it is possible to form a pattern with good image quality over the entire exposure region. can get.

[Brief description of drawings]

FIG. 1 (a) is a plan view of a pattern formed on a photomask used in the first embodiment, and FIG. 1 (b) is a conceptual diagram showing a projection exposure state in the first embodiment. 1 (c) is a conceptual diagram showing a state of projection exposure using a conventional photomask.

FIG. 2 is a conceptual diagram showing a state of projection exposure in a second embodiment of the present invention.

FIG. 3 is a conceptual diagram for explaining the operation of the present invention.

4 (a) and 4 (b) are conceptual views for explaining still another example for moving the image plane in the optical axis direction.

5 (a) and 5 (b) are conceptual diagrams for explaining still another example for moving the image plane in the optical axis direction.

FIG. 6 is a schematic configuration diagram showing a configuration of a projection exposure apparatus according to a third embodiment of the present invention.

7 (a) is a plan view showing the configuration of a photomask (image plane correction member) used in the third embodiment, and FIG. 7 (b) is a sectional view taken along the line AA 'of FIG. 7 (a). Is.

8A is a plan view showing a state of one of a plurality of exposure regions formed on a sensitive substrate, and FIG. 8B is a sectional view taken along line BB ′ of FIG. 8A. is there.

FIG. 9 is a cross-sectional view showing how the image plane is corrected in accordance with the step structure of the sensitive substrate.

FIG. 10 is a main part configuration diagram showing a configuration of a projection exposure apparatus according to a third embodiment of the present invention.

[Explanation of symbols]

1, 10, 20, 30, 40, 50 Photomasks 1a, 1b, 10a, 10b, 30a, 40a, 40
b, 50a, 50b Transparent film 3 Projection optical system 4 Wafer 5 Wafer stage 51 Parallel plate glass IM image plane PA pattern area SA exposure area

Claims (3)

[Claims] 1. A photomask used to expose a predetermined pattern on a sensitive substrate through a projection optical system, wherein a step structure in an exposure area on the sensitive substrate and / or a connection by the projection optical system. A photomask, characterized in that regions having different refractive indices are provided on the pattern formation surface side according to the distortion of the image plane. 2. A method of exposing a pattern formed on an original substrate onto a sensitive substrate via a projection optical system, wherein a step structure on the pattern forming surface side of the original substrate in an exposure area on the sensitive substrate. And / or by providing a refractive index distribution according to the distortion of the image plane formed by the projection optical system so that the image plane of the projection optical system and the surface of the exposure region substantially coincide with each other. An exposure method, wherein at least a part of the surface is moved in the optical axis direction. 3. A projection optical system for image-forming and projecting an image of a pattern formed on an original substrate onto a sensitive substrate, and the sensitive substrate such that the surface of the sensitive substrate is located in the vicinity of the image plane of the projection optical system. In a projection exposure apparatus including a stage for holding a substrate, the projection exposure apparatus has a step structure in an exposure area on the sensitive substrate and / or a refractive index distribution according to distortion of an image plane formed by the projection optical system, A projection comprising an image plane correction member that moves at least a part of the image plane of the projection optical system in the optical axis direction so that the image plane of the optical system and the surface of the exposure region substantially coincide with each other. Exposure equipment.