JP2785757B2 - Photo mask - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a photomask used in a reduction projection exposure apparatus, and more particularly to a photomask for manufacturing a semiconductor device for forming a fine pattern.

[0002]

2. Description of the Related Art Conventionally, optical lithography has been mainly used in a lithography process which is one of the manufacturing processes of a semiconductor device. At present, in optical lithography, the mass production of semiconductor devices having a reduced line width of about 0.35 μm has become possible by increasing the NA of a projection lens of a reduced projection exposure apparatus. However, although the resolution is improved by increasing the NA of the lens, the depth of focus is reduced. Therefore, considering the formation of a finer pattern of 0.35 μm or less,
It has been difficult to stably mass-produce semiconductor devices only by increasing the NA of the projection lens as in the past.

Therefore, various super-resolution techniques have been studied. In general, the super-resolution method refers to the illumination optical system,
This is a technique for improving the light intensity distribution on the image plane by controlling the transmittance and phase of light in a photomask and a projection lens system. Among various methods, improvement in resolution characteristics by optimizing an illumination system, that is, a so-called deformed illumination method is highly feasible and has attracted particular attention in recent years.

A reduction projection exposure apparatus for this purpose
For example, it is disclosed in JP-A-61-91662. Before describing a modified illumination method and an exposure apparatus for realizing the modified illumination method, a general exposure apparatus will be briefly described.

First, FIG. 9 shows main components of a reduction projection exposure apparatus. An ultra-high pressure mercury lamp 101 is used as a light source, and the emitted light is collected by an elliptical mirror 102 and reflected by a cold mirror 103. After being made parallel by the lens 104, the light passes through the interference filter 105. The ultra-high pressure mercury lamp 101 is a g-line (wavelength: 436 n
m) or i-line (wavelength 365 nm) is designed to emit light efficiently, but generates light with a continuous spectrum from infrared (long wavelength) to ultraviolet (short wavelength) lines. Therefore, the elliptical mirror 102 and the cold mirror 10
At the same time as changing the optical path in step 3, light having a long wavelength (heat ray) is separated, and the interference filter 105 further focuses only on light having a predetermined wavelength. Thereafter, a filler lens 106 is provided to obtain uniformity of illumination. Fly eye lens 10
Reference numeral 6 denotes an optical element in which dozens of generally elongated rectangular single lenses of the same type are bundled in parallel, and each single lens focuses to form a light source group. From the photomask that is the object to be illuminated, the point light source group formed by this fly-eye lens appears to be an illumination light source, not the original ultra-high pressure mercury lamp 101, and only the shape of this point light source group has resolution characteristics. Affect. Therefore, a group of point light sources formed by a fly-eye lens is called an effective light source.

[0006] Immediately after the fly-eye lens 106 (near the focal position), an aperture 107 is provided to determine the shape of the effective light source. The light of this effective light source is
8, the light on the photomask 100 is uniformly illuminated through the optical path of the mirror 109, the condenser lens 110, and the like.
The reason why the filament lens 106 is used is that the uniformity of illumination intensity cannot be obtained because the distance between the light source and each position on the photomask 100 is different with only one point light source. By using a plurality of effective light sources, it is possible to obtain good illumination uniformity over the entire surface of the photomask. Then, the light selectively transmitted through the photomask 100 is imaged by the projection lens system 111 on the semiconductor substrate 112 coated with the photosensitive resin.

Next, the effect of the stop 107 immediately after the fly-eye lens 106 will be described. In the general illumination method shown in FIG. 9, the stop 107 immediately after the fly-eye lens 106 is used.
Is a circular opening. The reason for making the shape circular is to prevent the pattern characteristic from being dependent on the resolution characteristic. For example, if the shape of the aperture is a square, the resolution characteristics of the vertical and horizontal patterns and the oblique (45 °, 135 ° direction) pattern will be different. The size of the aperture 107 determines the NA of the illumination system and affects the resolution characteristics.

Usually, the size of this effective light source is indicated by the coherent factor σ value, which is the ratio of the NA of the projection lens system 111 to the NA of the illumination system, and is 0.3 to 0.5.
A σ value of about 7 is selected according to the photomask pattern. For example, in a fine line and space pattern, the larger the σ value, the larger the depth of focus and the better the resolution characteristics, and the smaller the σ value for an isolated contact hole pattern.

Furthermore, not only optimization of the σ value but also optimization of the effective light source shape for each photomask pattern is performed. This is a modified illumination method generally called, and is classified into several types according to its light source shape (aperture opening shape).

FIGS. 10 (a) to 10 (d) are a plan view of an aperture and a configuration diagram of an optical element after a fly-eye lens for explaining the principle of the conventional modified illumination method.
(B) is a conventional one, and FIGS. 10 (c) and (d) are shown in Japanese Patent Application Laid-Open No. 61-91662.

As shown in FIGS. 10C and 10D, the central portion of the stop 107B is shielded from light and the fly-eye lens 106 is blocked.
The illumination method using only the peripheral portion is called an annular illumination method. A diaphragm 1 having a normal circular aperture as shown in FIG.
At 07A, there is light that enters the photomask 5 almost perpendicularly as shown in FIG. In order to resolve the pattern of the photomask 5, it is necessary to collect the 0th-order light and +1 or -1st-order light among the diffracted lights. It will not enter the system 111. Therefore, in a fine pattern, the vertically incident light becomes a noise component that does not contribute to the resolution, and lowers the contrast of the light intensity distribution on the image plane. However, when a stop 107B having a ring-shaped opening as shown in FIG. 3C is used, light is incident only obliquely on the photomask 5, and +1 or −.
Any of the first order diffracted light will enter the projection lens system 111, and most of the illumination light will help resolve the pattern. As described above, in the annular illumination method, the resolution and the depth of focus can be improved by removing the vertical incident component of the illumination light that does not contribute to the resolution.

In addition to the above-mentioned modified illumination method, a phase shift mask, which is a super-resolution method by improving the photomask side, has been actively studied. Numerous methods have been proposed for phase shift masks, all of which improve the light intensity distribution profile on the image plane by controlling the phase of light passing through the photomask, and improve resolution and depth of focus. It is a technique to improve. In particular, recently, a phase shift mask of a halftone system has been attracting attention because of its ease of mask formation. Halftone method (attenuation method)
Is a photomask in which a light-shielding film of a normal photomask is a translucent film, leaks about 5 to 10% of light, and makes the phase of the leaked light 180 degrees different from the phase of light in a transparent portion. It is.

However, the illumination conditions are also important when using a phase shift mask. That is, for the halftone mask of the contact hole pattern, the effect of the phase shift method cannot be obtained unless the stop immediately after the fly-eye lens is reduced and illumination conditions with high coherence are set. For a halftone mask for a periodic pattern such as a line and space pattern, a sufficient effect cannot be obtained unless the central part of the stop is shielded from annular illumination. In the annular illumination method, an image is conventionally formed by three light fluxes of the 0th-order light and +/- 1st-order light. On the other hand, since one of the +/- 1st-order lights is discarded, two-beam interference occurs. There is a disadvantage that the contrast is reduced because the intensity of one of the secondary light and the +/- primary light is significantly different. Therefore, the contrast can be improved by combining with a halftone mask and reducing the 0th-order light and balancing it with the 1st-order light.

[0014]

As described above, in the conventional reduced projection exposure, a high σ
In order to change the lighting conditions of / low σ, the diaphragm immediately after the fly-eye lens was removed and replaced. However, in an exposure apparatus used in a general mass production process, there is no mechanism for automatically changing the aperture, so that there is a problem that it is difficult to change the illumination conditions.

For example, the aperture can be replaced by opening the cover of the optical system of the exposure apparatus and performing an operation by an operator, but in this case, there is a possibility that dust is dropped on the optical system of the exposure apparatus. Therefore, it is necessary to perform a foreign substance inspection in the optical system every time the diaphragm is replaced. However, the inspection of foreign matter in the optical system requires several hours, and thus has a problem that mass productivity is extremely reduced. For this reason, the aperture change by the operator has not been performed in the actual mass production site.

Incidentally, in recent exposure apparatuses, a model having an automatic aperture changing function has been developed in consideration of the above-mentioned reasons. For example, a large disk has a plurality of diaphragms on concentric circles, and this disk is rotated to change the diaphragm, or a long plate has a plurality of diaphragms and a mechanism that slides this long plate to replace the diaphragm. is there. However, since the exposure apparatus having these latest functions is expensive, it is difficult to replace the exposure apparatus which is already in use due to cost considerations, and measures for the current exposure apparatus have been desired.

An object of the present invention is to provide a photomask in which illumination conditions can be easily changed even in an existing exposure apparatus.

[0018]

According to a first aspect of the present invention, there is provided a photomask having a predetermined pattern formed on a transparent substrate by a first film , and illuminated by transmission illumination to convert the predetermined pattern into an image forming surface. In the photomask formed thereon, a second film having a different light transmittance depending on an incident angle is provided on the entire surface of the front surface or the back surface of the transparent substrate.

[0019]

In this transmittance, light from a specific direction is emphasized and incident on a pattern on the mask by a film having an angle dependency, thereby enabling more appropriate illumination for each mask pattern.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. 1A and 1B are a top view and a sectional view taken along line AA of a photomask according to a first embodiment of the present invention.

As shown in FIGS. 1A and 1B, the photomask 10 includes a quartz transparent substrate 1 and an isolated contact hole pattern formed of a chrome light-shielding film 2 formed on the upper surface of the substrate 1. And a multilayer coating film 3 formed on the lower surface of the substrate. The multilayer coating film 3 causes the transmittance of the mask incident light to have an angle dependence. Here, as shown in FIG. 2, the multilayer coating film is formed of five layers each of a silicon nitride (Si ₃ N ₄ ) film 6 having a thickness of 86.9 nm and a silicon oxide (SiO ₂ ) film 7 having a thickness of 123.8 nm. Have been.

The thickness t of these transparent films (Si ₃ N ₄ , SiO ₂ ) constituting the multilayer coating film 3 is t =
mλ / 4n (λ is the light wavelength, n is the refractive index of the transparent film, and m is an even number 2, 4, 6,...). When this condition is satisfied, the phase of the incident light in each transparent film and the phase of the reflected light are the same with respect to the vertically incident light. That is, as the transmittance of the vertically incident light becomes maximum and the incident angle increases, the phase of the reflected light in the film shifts and the transmittance decreases. The angle dependence is determined by the number of film layers.
The i-line of Si ₃ N ₄ and SiO ₂ used here (λ = 36
5 nm) are 2.099 and 1.47, respectively.
4, the film thicknesses are 86.9 nm and 123.8.
nm.

FIG. 3 shows the angle dependence of the transmittance of the multilayer coating film 3. As shown in the figure, the multilayer coating film 3 has such a characteristic that the transmittance decreases as the incident angle increases with respect to the transmittance of the incident light.
When the incident angle is 5 degrees, the transmittance is 50%. Therefore, for the isolated contact hole pattern,
The oblique incidence component of the illumination light for narrowing the depth of focus can be reduced, and the depth of focus can be increased.

The angle dependence of the transmittance of the multilayer coating film can be set in a wide range by the refractive index, the film thickness and the number of layers of the materials to be alternately laminated. However, if the average transmittance is reduced to some extent, there is a problem of throughput. Therefore, it is necessary to set the transmittance so as not to be 1/3 or less of the original transmittance (92% for quartz). Although using the the Si ₃ N ₄ film and the SiO ₂ film as a multilayer coating film in the first embodiment, is not limited thereto, SnO ₄ layer,
An HfO ₂ film, an Al ₂ O ₃ film, an ITO (indium tin oxide) film, or the like can be used.

FIG. 4 is a top view and a sectional view taken along the line BB of a photomask according to a second embodiment of the present invention.

As shown in FIGS. 4 (a) and 4 (b), the photomask 10A comprises a transparent substrate 1 and a predetermined pattern formed of a light shielding film 2 formed on one surface of the substrate, and the other surface of the substrate. And the multi-layer coating film 3A formed on the substrate. Here, the pattern is a periodic pattern (line and space pattern) having dimensions close to the resolution limit of the exposure apparatus. The multilayer coating film 3A is composed of a Si ₃ N ₄ film and a SiO ₂ film as in the first embodiment, but the angle dependency of the transmittance of the multilayer coating film is opposite to that of the first embodiment. It has become.

Here, the thickness t of the transparent film constituting the multilayer coating film 3A is represented by t = m ₂ λ / 4n (λ is the wavelength of light, n is the refractive index of the transparent film, and m ₂ is an odd number 1, 3, 5 ...
・) At this time, since the phase of the incident light in each transparent film differs by 180 degrees from the vertically incident light, they cancel each other out due to interference, and the intensity decreases. That is, the transmittance of the vertically incident light becomes the lowest, and the phase of the reflected light in the film shifts as the incident angle increases, and the transmittance increases. The angle dependence is determined by the number of film layers. The n values of Si ₃ N ₄ and SiO ₂ used here for the i-line (λ = 365 nm) are each 2.099.
And 1.474, the film thicknesses are 43.5 nm and 61.9 nm. That is, as shown in FIG. 5, the transmittance of vertically incident light is reduced. As described above, the depth of focus can be increased by reducing the vertical incident component that does not contribute to pattern formation as in the annular illumination method.

FIGS. 6A and 6B show a technique related to the present invention.
It is the top view of a photomask of operation , and sectional drawing along CC line. As shown in FIGS. 6A and 6B, the photomask 1
Reference numeral 0B denotes a transparent substrate 1 and a contact hole pattern formed of a translucent film 4 formed on the transparent substrate 1. As shown in FIG. 7, the translucent film 4 is formed of a five-layer multilayer coating film of a chromium film 8 having a thickness of 4 nm and a silicon oxide film 7 having a thickness of 60 nm. The light having a phase difference of 180 degrees is generated. Further, the transmittance of incident light has an angle dependency, and as shown in FIG. 8, the transmittance is higher for vertically incident light, and the transmittance is lower as the incident angle is larger.

By using the mask having such a configuration, a conventional exposure apparatus (coherent factor σ =
Also in the range of 0.5 to 0.7), the vertical incidence component of the illumination light can be relatively increased, and the phase shift effect can be sufficiently obtained. When the target pattern is a periodic pattern such as a line-and-space pattern, contrary to the above, a translucent multi-layer coating film having a transmittance that increases as the incident angle increases becomes larger. Make up the membrane. In this case, an effect equivalent to the combination of the modified illumination and the halftone mask can be obtained, and the depth of focus can be further increased.

[0031]

As described above, the present invention can be applied to an existing exposure apparatus in which it is not easy to change illumination conditions by forming a layer having different transmittance depending on the incident angle on a transparent substrate constituting a photomask. This also has the effect that the illumination conditions can be easily optimized. Also, there is an effect that the modified illumination method can be realized in a pseudo manner, and the depth of focus and the resolution can be improved.

[Brief description of the drawings]

FIG. 1 is a top view and a cross-sectional view of a photomask according to a first embodiment of the present invention.

FIG. 2 is a sectional view of a multilayer coating film of the photomask according to the first embodiment.

FIG. 3 is a view showing the angle dependence of the transmittance of a multilayer coating film included in the photomask of the first embodiment.

FIG. 4 is a top view and a cross-sectional view of a photomask according to a second embodiment of the present invention.

FIG. 5 is a diagram showing the angle dependence of the transmittance of a multilayer coating film of a photomask according to a second embodiment.

6A and 6B are a top view and a cross-sectional view of a photomask according to a technique related to the present invention .

FIG. 7 is a cross-sectional view of a translucent film portion of a photomask according to a technique related to the present invention .

FIG. 8 is a view showing the angle dependence of the transmittance of a multilayer coating film included in a photomask of a technique related to the present invention .

FIG. 9 is a configuration diagram of main optical elements of a conventional reduction projection exposure apparatus.

FIG. 10 is a plan view of a stop and a configuration diagram of an optical element for explaining the principle of a conventional modified illumination method.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 transparent substrate 2 light-shielding film 3, 3A multilayer coating film 4 translucent film 5 photomask 6 silicon oxide film 7 silicon nitride film 8 chromium film 10, 10A, 10B, 100 photomask 101 ultra-high pressure mercury lamp 102 elliptical mirror 103 cold mirror Reference Signs List 104 lens 105 interference filter 106 fly-eye lens 107 aperture 108 lens 109 mirror 110 condenser lens 111 projection lens system 112 semiconductor substrate

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-7-253655 (JP, A) JP-A-6-342205 (JP, A) JP-A-6-19109 (JP, A) JP-A-6-209 83027 (JP, A) JP-A-5-232677 (JP, A) JP-A-6-95359 (JP, A) JP-A-4-24638 (JP, A) JP-A-4-45446 (JP, A) JP-A-5-100408 (JP, A) JP-A-7-209849 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G03F 1/08 H01L 21/027

Claims (2)

(57) [Claims] 1. A photomask having a predetermined pattern formed by a first film on a transparent substrate and illuminated by transmitted illumination to form the predetermined pattern on an image forming surface. A photomask comprising a second film having a different light transmittance depending on an incident angle on the entire surface of a front surface or a rear surface of a transparent substrate. 2. A second having angle dependence of the transmittance
2. The photomask according to claim 1, wherein the film is a laminated film made of a plurality of materials having different refractive indexes.