JP2712529B2 - Projection exposure apparatus and exposure method using the same - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a projection exposure apparatus, and more particularly to a projection optical system.

[Conventional technology]

Conventionally, reduction projection type exposure equipment has been used to manufacture semiconductor elements consisting of ultra-fine patterns such as LSIs and super LSIs, and a great deal of effort has been made to accurately and stably transfer finer patterns. Have been. For example, the use of shorter wavelength light as the exposure wavelength, or the use of a projection optical system
Efforts are being made to increase NA (numerical aperture). For stable transfer of such a fine pattern, it is necessary not only that the resolution of the projection optical system be excellent, but also that the contrast of the projected image be high. Efforts have been made to find the optimum exposure conditions by studying. Regarding the illumination conditions, by adjusting the so-called σ value corresponding to the ratio of the NA of the illumination optical system to the NA (numerical aperture) of the projection optical system, the two optics are adjusted so as to obtain an appropriate balance between the resolving power and the contrast for a predetermined pattern. Adjusting the NA of the system, for example,
It is known from Japanese Patent Publication No. 151 and the like.

[Problems to be solved by the invention]

However, in the conventional apparatus as described above, although the illumination optical system can be optimized to some extent, as the NA of the projection optical system increases, the depth of focus becomes shallower. However, there is a problem that the focus alignment conditions when exposing and transferring the image are extremely strict, and even if there is a slight change in the focus, the pattern cannot be accurately transferred. Further, the transfer pattern is not only composed of a single periodic structure, but a fine pattern having a different periodic structure coexists. However, depending on a general projection optical system, an optimum focus position (the best focus position) depends on the line width of the fine pattern. Focus position) tends to fluctuate,
Strict adjustment and management in manufacturing were indispensable, such as different best focus positions depending on the fineness of the transferred pattern.

For this reason, in the manufacturing process of the VLSI, it is always necessary to perform extremely strict focus detection and reliable focus position correction, which leads to a complicated and large-sized device, a long processing time, and a reduced throughput. Was causing it.

In order to overcome such problems, a proposal has been made to improve the resolution of a resist image appearing as a result of multiple reflection inside a resist having a thickness by making the spherical aberration of the projection optical system excessive, by the same applicant. (Japanese Patent Application No. 63-320616).

However, it has been clarified that the invention of the above-mentioned application has a disadvantage that a very fine pattern causes "film loss" of the resist. This "film loss"
As shown in FIG. 4, the phenomenon that the resist 60 applied on the wafer 6 is unexposed at a predetermined thickness T of an ideal rectangular shape after exposure and development, as indicated by a broken line in the figure, as shown in FIG. Where a region should remain (in the case of a positive resist), the film thickness is reduced by ΔT, which is a measure of incompleteness in exposure of a fine pattern.

According to a specific example based on the experimental results, the numerical aperture of the projection optical system is set to NA and the exposure wavelength is set to λ, and the line and space (L & S) of the target pattern is expressed by k in the equation L & S = k · λ / NA. At this time, a good resist image is formed for a pattern with k = about 0.8 to 0.6, but “a film loss” occurs for a finer pattern of about k = 0.5 to 0.4, and It has become clear that the overall resolution is reduced.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described problems and to perform extremely stable transfer even when transferring a finer pattern to a resist without significantly reducing the resolving power of the projection optical system. It is an object of the present invention to provide a projection exposure apparatus.

[Means for solving the problem]

The present inventors have conducted intensive tests and researches on various exposure conditions and various resists in order to overcome the above problems, and have obtained the following findings. In other words, regarding the resolution of the fine pattern finally formed on the wafer by photolithography, since there is multiple reflection inside the resist for a photoreceptor with a thickness of resist, the connection of the projection optical system There is a limit to the improvement of the resolution when the image performance is astigmatic, and the resolution is improved when the longitudinal spherical aberration is excessive to some extent, and the aberration-free projection optical system or the same application as this application The above Japanese Patent Application No. 63-32
In a projection optical system in which spherical aberration is generated only in the tertiary aberration region as disclosed in Japanese Patent No. 0616, the optimum focus position tends to fluctuate depending on the fineness of the pattern. The present inventors have found that the stability of the total depth of focus is rather disadvantageous, and have arrived at the present invention.

That is, the present invention relates to a projection exposure apparatus having an illumination optical system for illuminating a reticle having a predetermined pattern, and a projection optical system having a predetermined numerical aperture for projecting a pattern on the reticle onto a wafer surface. As an optimal correction state of spherical aberration relating to the image formation of the pattern on the reticle on the wafer surface by the projection optical system, the spherical aberration has a positive tendency of overcorrection in the tertiary region and a negative correction of insufficient correction in the tertiary region. I found a state with a tendency.

Then, with respect to the longitudinal spherical aberration of such a projection optical system relating to an imaging onto the wafer surface of the reticle on the pattern, and [Delta] S ₁₀ the spherical aberration amount corresponding to 10% of the opening, 7
Assuming that the amount of spherical aberration corresponding to the aperture is ΔS ₇ , the range satisfying the condition of 0 <ΔS ₇ <2.5λ / NA ² (1) −0.5ΔS ₇ <ΔS ₁₀ <1.5ΔS ₇ (2) I found something to do or desirable. Here, λ is the wavelength of the exposure light on which the pattern on the reticle is projected onto the wafer, and NA is the numerical aperture of the projection optical system.

It is preferable to provide a spherical aberration varying means for varying the shape of the spherical aberration curve of the projection optical system according to the illumination conditions, the fineness of the reticle pattern, the thickness of the resist, and the like.

[Action]

Since the optical image formed by the projection optical system is observed as a resist image resulting from multiple reflections inside the resist having the thickness of the photoconductor, it is not always necessary that the optical image formed by the aberration-free optical system forms the best resist image. Not necessarily. By setting the spherical aberration of the projection optical system in an overcorrected state as in the present invention, the substantial depth of focus is increased without significantly lowering the resolution.

FIGS. 2 and 3 show the relationship between the correction state of the spherical aberration and the evaluation by the resist image. FIG. 2 shows the shape of spherical aberration, and FIG.
(B) shows a state where the third-order spherical aberration mainly occurs on the plus side, and (C) shows a state where the third-order spherical aberration occurs on the plus side but fifth-order aberration occurs on the minus side. Each state is shown.

FIG. 3 shows the focus variation ΔDF on the horizontal axis and the line width L of the transfer pattern on the wafer surface on the vertical axis. Shows the relationship. This curve is generally called a CD-focus curve. (A), (B), and (C) of FIG. 3 correspond to the aberration correction states of (A), (B), and (C) of FIG. 2, respectively. In FIG. As a pattern, 1.
The characteristics of each line and space of 6λ / NA, 0.8λ / NA, and 0.5λ / NA are shown.

In each CD-focus curve shown in FIG. 3, the way of setting the horizontal axis is different from the display of a general aberration diagram as shown in FIG. 2, and the positive direction (+) of the horizontal axis is the back of the projection objective lens. This is the direction in which the focus becomes shorter (the direction in which the stage 5 on which the wafer is mounted rises in the configuration of the embodiment shown in FIG. 1). In the negative direction (-) on the horizontal axis, the back focus of the projection objective becomes longer. Direction (the direction in which the stage 5 on which the wafer is placed is lowered).

In the aberration-free state shown in FIG. 2 (A), in a pattern finer than 0.8λNA, as shown in FIG. 3 (A), the CD-focus curve has a tendency to rise to the right and becomes asymmetric. Would. This is a factor that remarkably lowers the depth of focus when the focus range in which the line width variation falls within a certain value is considered as the depth of focus.

As shown in FIG. 2 (B), when the spherical aberration is made positive, the symmetry of the CD-focus curve for 0.8λ / NA line and space is improved, and the same depth of focus as described above is obtained. When thinking, it is understood that the value increases.

However, there is a disadvantage that the best focus position of 1.6λ / NA and 0.5λ / NA line and space is different from the best focus position of 0.8λ / NA line and space. This phenomenon
± 1st order diffracted light at 0.5λ / NA line and space,
Since the ± 3rd-order diffracted light in the case of 1,6λ / NA line-and-space passes through a place where the aperture value of the spherical aberration curve is large, the order of 0.
It is understood that it deviates from the best focus position of 8λ / NA line and space. In addition, although it cannot be read from the CD-focus curve, a fine pattern such as the case of 0.5λ / NA line and space has a large aberration of ± 1st-order diffracted light, so that the contrast of the optical image deteriorates. There is also a disadvantage that the film is reduced.

Therefore, the shape of the spherical aberration curve is changed as shown in FIG.
If the configuration has a positive tendency of overcorrection in the tertiary region and a negative tendency of undercorrection in the tertiary region, CD
The focus curve is 0.8 as shown in Fig. 3 (C).
The best focus position of 1.6λ / NA and 0.5λ / NA line and space is matched with that of 0.8λ / NA line and space while maintaining good symmetry of CD-focus curve at λ / NA line and space. It becomes possible.

In addition, although it cannot be read from FIG. 3 (C), 0.5λ / NA
The phenomenon of film thinning in line and space has also been drastically reduced.

As described above, the relationship between the correction state of the spherical aberration and the CD-focus curve varies depending on the illumination conditions, the numerical aperture of the projection optical system, and the types of the wafer and the resist as the object to be exposed, but the above-described tendency. Is almost preserved. From the results of various studies on the relationship between the state of correction of spherical aberration and the CD-focus curve, an appropriate range of the amount of spherical aberration is found as in the above equations (1) and (2).

When the value falls outside the lower limit of the condition of the expression (1), the CD focus curve for the pattern of about 0.8λ / NA line and space becomes remarkably upwardly asymmetric, and it becomes difficult to secure a sufficient depth of focus. If the upper limit of the expression (1) is exceeded, the amount of spherical aberration becomes too large and the resolving power itself is significantly reduced. If the value exceeds the upper limit of the expression (2), it is a standard line and space. Rougher pattern than 0.8λ / NA (1.6λ / NA line and space) and finer pattern (0.5λ / NA
The best focus position in the line and space is shifted to the minus side in the CD-focus curve than that of 0.8λ / NA line and space, and the overall depth of focus including various line widths is reduced. I will invite you. If the value falls outside the lower limit of the expression (2), the direction of the focus shift is reversed, and the overall depth of focus similarly decreases.

〔Example〕

The projection type exposure apparatus according to the present invention as described above will be described based on the configuration of the embodiment shown in FIG.

As shown in FIG. 1, the illumination light for exposure supplied from the illumination optical device 1 uniformly illuminates a reticle 3 having a predetermined projection pattern via a condenser lens 2. The pattern on the reticle 3 is
The image is reduced and projected on the wafer 6 placed on the stage 5.
Here, in the illumination optical device 1, illumination information such as the wavelength λ of the exposure light and the numerical aperture (NA) as an illumination system is input to the arithmetic unit 20 via the illumination information input unit 11 and formed on the reticle 3. Information on the projection pattern relating to the line width of the pattern is input from the projection pattern information input means 12 to the calculation means 20. Further, information on the object to be exposed, such as the material of the wafer, the resist material, and the thickness of the resist, is input to the arithmetic unit 20 by the object information input unit 14. The aperture value (NA) information of the reduction projection objective lens 4 is also transmitted to the aperture information input means 13.
Is input to the calculation means 20 via the.

Based on such various information, the calculating means 20 determines the shape of the optimum spherical aberration curve, and generates the desired spherical aberration by the aberration varying means 40 via the aberration varying driving means 30,
It is possible to set an appropriate depth of focus according to the line width.

By the way, based on the fineness of the pattern on the reticle from the projection pattern information input unit 12 and the information on the illumination condition from the illumination information input unit 11, the arithmetic unit 20 calculates the optimal aperture value of the reduced projection objective lens 4 by calculation. The aperture of the reduction projection objective lens 4 is set to an optimum value by the aperture control means 21,
Alternatively, the σ value can be set to an optimum value by linking the illumination NA control means 22. In this case, the shape of the spherical aberration curve can be set to the optimum value by the aberration varying means 30 based on the optimum aperture value obtained by the calculating means 20 without passing through the aperture information input means 13.

As the spherical aberration varying means, it is possible to adopt a configuration in which the air gap is changed by an optical system composed of a plurality of lenses. Alternatively, it is also possible to prepare a plurality of types of spherical aberration variable optical systems according to the shape of the spherical aberration, and replace them. Alternatively, it is also possible to directly change the air spacing of the lens groups constituting the projection lens 4 by a minute amount using a piezo element or the like.
It is possible to change the shape of the spherical aberration curve.

〔The invention's effect〕

As described above, according to the projection exposure apparatus of the present invention, it is possible to transfer a fine pattern extremely stably even when the line width is various without significantly reducing the resolving power of the projection optical system. Can be. Therefore, the burden on the focus detection and the focus position correction means can be reduced, and the exposure can be performed efficiently without reducing the throughput even in the case of projecting a fine pattern with various line widths, with a simple configuration. It becomes possible.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing one embodiment of a reduction projection exposure apparatus according to the present invention, FIG. 2 is an aberration diagram showing a state of spherical aberration of a projection objective lens, and FIG. 3 is a CD-focus curve of the projection objective lens. FIG. 4 and FIG. 4 are explanatory diagrams showing the state of film thinning. [Description of Signs of Main Parts] 1 ... Illumination optical device, 4 ... Projection objective lens 3 ... Reticle, 6 ... Wafer 20 ... Computing means 30 ... Aberration variable driving means 40 ... Aberration variable means

Continuation of front page (72) Inventor Hiroyuki Tsuchiya 1-6-3 Nishioi, Shinagawa-ku, Tokyo Nikon Oi Works Co., Ltd. (56) References JP-A-2-166719 (JP, A) JP-A-2- 234411 (JP, A) JP-A-63-177124 (JP, A) JP-A-63-12134 (JP, A) JP-A-63-29505 (JP, A)

Claims (8)

(57) [Claims] A projection exposure apparatus having an illumination optical system for illuminating a reticle having a predetermined pattern and a projection optical system having a predetermined numerical aperture for projecting a pattern on the reticle onto a wafer surface. In the projection optical system,
Regarding the image formation of the pattern on the reticle on the wafer surface, the tertiary region of the spherical aberration depending on the aperture has a positive tendency of overcorrection, and the quintic region has a negative tendency of undercorrection. A projection exposure apparatus. 2. A projection exposure apparatus comprising: an illumination optical system for illuminating a reticle having a predetermined pattern; and a projection optical system having a predetermined numerical aperture for projecting a pattern on the reticle onto a wafer surface. In the illumination optical system, when the wavelength of exposure light is projected on the wafer by the projection optical system is projected on the wafer by irradiating the reticle onto the reticle, and the numerical aperture of the projection optical system is NA , relative to the longitudinal spherical aberration of the projection optical system relating to an imaging onto the wafer surface of the reticle on the pattern, a spherical aberration amount of spherical aberration corresponding to 10% of the opening and [Delta] S _10, which corresponds to 70% of the opening A projection exposure apparatus characterized by satisfying a condition of 0 <ΔS ₇ <2.5λ / NA ² (1) −0.5ΔS ₇ <ΔS ₁₀ <1.5ΔS ₇ (2) when the amount is ΔS ₇ . 3. A projection exposure apparatus having an illumination optical system for illuminating a reticle having a predetermined pattern, and a projection optical system having a predetermined numerical aperture for projecting a pattern on the reticle onto a wafer surface. In the projection optical system,
With respect to the imaging of the pattern on the reticle on the wafer surface, the third-order region of the aperture-dependent spherical aberration has a positive tendency of overcorrection, and the fifth-order region has a negative tendency of undercorrection, A projection exposure apparatus, wherein the projection optical system includes an aberration varying unit that varies an amount of vertical spherical aberration relating to the imaging of the pattern on the reticle onto the wafer surface within the range of the aberration tendency. . 4. A projection exposure apparatus having an illumination optical system for illuminating a reticle having a predetermined pattern and a projection optical system having a predetermined numerical aperture for projecting a pattern on the reticle onto a wafer surface. In, when the wavelength of the exposure light is projected from the illumination optical system onto the reticle and projected on the wafer by the projection optical system by the projection optical system is λ, and the numerical aperture of the projection optical system is NA,
With respect to the longitudinal spherical aberration of the projection optical system relating to an imaging onto the wafer surface of the reticle on the pattern, a spherical aberration amount of spherical aberration amount corresponding to 10% of the opening and [Delta] S _10, which corresponds to 70% of the opening when to the [Delta] S _7, 0 <the range of the condition of ^{_{ΔS 7 <2.5λ / NA 2 (}} 1) -0.5ΔS 7 <ΔS 10 <1.5ΔS 7 (2), the projection optical system of the reticle on the pattern A projection exposure apparatus, comprising: an aberration varying unit that varies an amount of vertical spherical aberration associated with image formation on the wafer surface. 5. An exposure method wherein a reticle having a predetermined pattern is illuminated by an illumination optical system and a pattern on the reticle is projected onto a wafer surface by a projection optical system having a predetermined numerical aperture. Projection optical system having an overcorrected positive tendency in the third-order region of the spherical aberration depending on the aperture and an insufficiently corrected negative tendency in the fifth-order region with respect to the image formation on the wafer surface described above. Exposure method characterized by using: 6. An exposure method for illuminating a reticle having a predetermined pattern by an illumination optical system and projecting a pattern on the reticle onto a wafer surface by a projection optical system having a predetermined numerical aperture. When the wavelength of exposure light that is projected onto the reticle by the projection optical system and projected onto the wafer by the projection optical system is λ, and the numerical aperture of the projection optical system is NA, the wafer of the pattern on the reticle is Regarding the amount of vertical spherical aberration of the projection optical system with respect to image formation on a surface, when the amount of spherical aberration corresponding to 100% of the aperture is ΔS ₁₀ and the amount of spherical aberration corresponding to 70% of the aperture is ΔS ₇ , An exposure method characterized by satisfying the following condition: 0 <ΔS ₇ <2.5λ / NA ² (1) −0.5ΔS ₇ <ΔS ₁₀ <1.5ΔS ₇ (2) 7. An exposure method in which a reticle having a predetermined pattern is illuminated by an illumination optical system and a pattern on the reticle is projected onto a wafer surface by a projection optical system having a predetermined numerical aperture. Projection optical system having an overcorrected positive tendency in the third-order region of the spherical aberration depending on the aperture and an insufficiently corrected negative tendency in the fifth-order region with respect to the image formation on the wafer surface described above. And a variable amount of vertical spherical aberration relating to imaging of a pattern on the reticle onto the wafer surface within the range of the above-mentioned aberration tendency. 8. An exposure method in which a reticle having a predetermined pattern is illuminated by an illumination optical system and a pattern on the reticle is projected onto a wafer surface by a projection optical system having a predetermined numerical aperture. When the wavelength of exposure light that is projected onto the reticle by the projection optical system and projected onto the wafer by the projection optical system is λ, and the numerical aperture of the projection optical system is NA, the wafer of the pattern on the reticle is Regarding the amount of vertical spherical aberration of the projection optical system with respect to image formation on a surface, when the amount of spherical aberration corresponding to 100% of the aperture is ΔS ₁₀ and the amount of spherical aberration corresponding to 70% of the aperture is ΔS ₇ , Within the range of 0 <ΔS ₇ <2.5λ / NA ² (1) −0.5ΔS ₇ <ΔS ₁₀ <1.5ΔS ₇ (2), the pattern on the reticle of the projection optical system is formed on the wafer surface. Vertical sphere about the statue Exposure method characterized by the aberration amount of the aberration is variable.