JPH09288346A - Photomask - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the depth of focus of isolated patterns and to decrease the optical proximity effect at the ends of the isolated patterns or periodic patterns by arranging patterns for supplement near main patterns which have specific transmittance and are inverted in phase. SOLUTION: The main patterns 2 transferred to the surface of a semiconductor substrate by projection exposure in this photomask formed with the patterns by light shielding curtains on a transparent substrate 1 are formed of material films having the transmittance of <40%. The patterns 3, 3a for supplement formed of the light shielding films having light shieldability to irradiation light for exposure are disposed in the peripheral parts of the main patterns 2. The size of the patterns 3, 3a for supplement is preferably smaller than the size of the main patterns 2. The phase difference between the irradiation light for exposure passing the regions not formed with the patterns 2, 3, 3a of the transparent substrate 1 and the irradiation light for exposure passing the main patterns 2 is so adjusted as to attain 180 deg..

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure photomask used in a projection exposure apparatus for manufacturing a semiconductor device, and more particularly to an exposure photomask in which an auxiliary pattern is arranged near a main pattern. is there.

[0002]

2. Description of the Related Art The degree of integration of a semiconductor integrated circuit represented by a DRAM has been steadily increasing, and accordingly, the line width of a pattern has become extremely small. Along with this, in the lithography process for forming a circuit pattern on a semiconductor substrate, transfer of a finer pattern is required. In the current photolithography process, a circuit pattern on a photomask is printed by ultraviolet light on a photoresist coated on a semiconductor substrate by a reduction projection exposure device (stepper or the like) to form a pattern. Ideally, the surface of the substrate should be aligned with the image plane of the projection lens for pattern formation, but there is a gap between the two due to the steps that occur due to element formation or because the substrate is not flat. .
In order to form a pattern even if it is slightly deviated from the ideal image plane, a certain depth of focus (= range in the optical axis direction in which pattern formation is possible) is necessary, and it is important to secure the required depth of focus together with high resolution. Becomes

[0003] In general, the resolution R and the depth of focus DOF are given by the following equations called Rayleigh's equation.

R = k ₁ λ / NA (1) DOF = k ₂ λ / (NA) ² (2) where λ is the exposure wavelength, NA is the lens numerical aperture, and k ₁ and k ₂
Indicates the process coefficient.

From the equation (1), it is understood that the resolution limit R is improved by shortening the wavelength λ and increasing NA.
However, shorter wavelength and higher NA will reduce the depth of focus DOF than the equation (2). At present, the depth of focus is sharply reduced with the improvement in resolution, and it is becoming difficult to secure a necessary depth of focus. On the other hand, (2)
As can be seen from the equation, the decrease in the depth of focus is more gradual in the shorter wavelength than in the higher NA. Therefore, the wavelength is being shortened, and the g-line (λ = 43) of a mercury lamp is used as an exposure light source.
6 nm) to i-line (λ = 365 nm), and further KrF excimer laser (λ = 248 nm) is being used.

It is possible to cope with the miniaturization of the circuit pattern described above by adding a device to the optical system itself without further shortening the wavelength and increasing the NA, and the method is generically referred to as a super-resolution technique. being called. Representative super-resolution techniques can be broadly classified into the following three methods depending on where the device is devised. That is, one that changes the shape of the light source of the projection optical system (deformed illumination method), one that changes the pupil plane of the projection lens (pupil filter method), and one that changes the photomask (phase shift method, halftone method, auxiliary pattern). Law).
The effects of these super-resolution techniques differ depending on the type of pattern on the photomask.

The phase shift method and the modified illumination method utilize two-beam interference, and can improve the resolution and depth of focus of a periodically arranged pattern.

The pupil filter method is a method of changing the spatial frequency distribution of the pupil plane of the projection lens to multiplex the image planes, and is effective for both a periodically arranged pattern and an isolated pattern. However, since this pupil filter method changes the pupil plane of the projection lens, it is not easy to introduce it into the actual process.

The halftone method is a method for enhancing the contrast of the light intensity on the wafer over a wide focal range by emphasizing the high-order diffracted light components in the mask. The feature of this method is that the depth of focus can be improved for both the periodically arranged pattern and the isolated pattern.

The auxiliary pattern method is a method of forming an auxiliary pattern having a size equal to or smaller than the size of the main pattern around the main pattern to improve the resolution and the depth of focus. For details, see, for example, the 1992 Spring Applied Physics Society Proceedings. 30p-L-
16. FIG. 7 shows a top view of a photomask in the case of this conventional auxiliary pattern method. Here, it is assumed that this photomask is used in a stepper that performs projection exposure with a reduction of 1/5. As shown in FIG.
A light-shielding film 52 made of chromium is attached to the surface of a transparent glass substrate 51 serving as a mask substrate. Then, the light shielding film 52 for the circuit pattern which is the main pattern is provided. Further, the light shielding portions 53 and 53a for the auxiliary pattern are formed adjacent to the light shielding portion 52 for the circuit pattern. As described above, in the auxiliary pattern method, an auxiliary pattern having a size smaller than the circuit pattern size is provided adjacent to the circuit pattern formed on the photomask.

[0011]

As described above, in order to improve the resolution and the depth of focus, the modified illumination method, which is relatively easy to introduce into the semiconductor manufacturing process, has been put to practical use in the super-resolution technology. It's starting. This modified illumination method is effective for a circuit pattern having periodicity (hereinafter referred to as “periodic pattern”), but is less effective for an isolated circuit pattern (hereinafter referred to as “isolated pattern”). Also, in the periodic pattern, the shape is deteriorated in the portion where the periodicity disappears, such as the end of the pattern. This is because the light is diffracted uniformly at the end of the isolated pattern or the periodic pattern, so that the effect of two-beam interference cannot be obtained.

In the above-described auxiliary pattern method, the auxiliary pattern is formed adjacent to the main circuit pattern. Therefore, the modified illumination method described above is effective to some extent. However, in the conventional auxiliary pattern method, the line width of the auxiliary pattern needs to be set to be considerably smaller than the main pattern size. Therefore, the effect of improving the resolution and the depth of focus by the conventional auxiliary pattern method is not sufficient.

When the main pattern is designed near the resolution limit, the auxiliary pattern is below the resolution limit, and it becomes difficult to make a mask due to the miniaturization of the pattern. For example, 0.25 μ using KrF excimer lithography
When forming an m-line isolated pattern, the upper limit of the width of the auxiliary pattern is about 0.10 μm. 0.10μ
The formation of the m-mask pattern imposes a heavy burden on the current mask making technology.

The above is a problem concerning the resolution and the depth of focus, but a dimensional difference due to the difference in the density of patterns is also a problem. The dimensional difference between the periodic pattern and the isolated pattern is called the proximity effect, and keeping the proximity effect small is also an important issue. That is, the light intensity distribution upon exposure differs between the periodic pattern and the isolated pattern. Therefore, when the exposure amount is set so that the periodic pattern is as designed when the exposure is performed with a photomask including both, the isolated pattern is designed to have a design dimension. There is a problem of getting out of the way. With respect to this problem, it is difficult to suppress the proximity effect due to the thin line width of the auxiliary pattern in the above-mentioned auxiliary pattern method, and it remains a problem to be solved.

An object of the present invention is to improve the depth of focus of isolated patterns and reduce the optical proximity effect at the ends of isolated patterns and periodic patterns.

[0016]

According to a first aspect of the present invention, in a photomask in which a pattern is formed on a transparent substrate with a light-shielding film, a main pattern transferred to the surface of a semiconductor substrate by projection exposure is transparent. An auxiliary pattern formed of a material film having a ratio of less than 40% and formed of a light-shielding film having a light-shielding property with respect to irradiation light for exposure is provided in the peripheral portion of the main pattern. A photomask that can be obtained is obtained.

According to the second aspect of the invention, the photomask according to the first aspect is obtained in which the size of the auxiliary pattern is smaller than the size of the main pattern.

According to the third aspect of the present invention, the phase difference between the irradiation light for exposure that passes through the region where no pattern is formed on the transparent substrate and the irradiation light for exposure that passes through the main pattern is 180. The photomask according to claim 1 is obtained.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION

[First Embodiment] FIGS. 1 and 2 are views for explaining a first embodiment of the present invention. Here, FIG. 1A is a top view of the photomask of this embodiment, and FIG. 1B is a sectional view thereof. Here, such a photomask is assumed to be used in a stepper for ⅕ reduction projection exposure. Therefore, the circuit pattern having a size five times as large as the transfer pattern to the photoresist on the wafer becomes the size of the pattern on the photomask. The following description will be given when the photoresist is a positive type.

As shown in FIGS. 1 (a) and 1 (b),
A linear main pattern (isolated pattern) 2 that is a circuit pattern is formed on the surface of a glass substrate 1 that serves as a mask substrate. Here, the line width of the main pattern 2 is 1.25 μm.
It is. Then, the auxiliary patterns 3 and 3a are formed in parallel on both sides of the main pattern 2, respectively. Here, the line widths of the auxiliary patterns 3 and 3a are as follows.
An appropriate value is determined on the condition that a is not resolved. The size of the space between the auxiliary patterns 3 and 3a is 1.2.
It is set to 5 μm.

As shown in FIG. 1B, such auxiliary patterns 3 and 3a are formed by depositing chromium light shielding films 3'and 3'a on the same constituent material as the main pattern 2. It The auxiliary patterns 3 and 3a are formed so as to completely block the irradiation light of the projection exposure of photolithography. On the other hand, the main pattern 2 transmits a predetermined amount of the irradiation light. Further, the phase of light passing through the main pattern 2 is inverted by 180 degrees as compared with that of light passing through the glass substrate 1.

FIGS. 2A to 2D show the conventional auxiliary pattern method, that is, when the main pattern is an isolated line pattern formed of a light-shielding film and the line width of the auxiliary pattern is changed. The light intensity distribution on the wafer obtained by the simulation is shown. 2 (a), 2 (b), 2 (c) and 2 (d), the line width of the above-mentioned auxiliary pattern is 0 (that is, no auxiliary pattern is provided), 0.05. , 0.10, 0.15 μm. The light intensity on the vertical axis is normalized by the intensity in the space area. In this simulation,
In the reduction projection exposure of 1/5, the optical condition is NA = 0.
5, σ = 0.7, λ = 248 nm. Here, σ is a value indicating the coherency of the irradiation light, and is represented by a value obtained by dividing the NA of the optical lens on the irradiation light source side by the NA of the projection lens.

Here, it is 0.25 in the reduction projection exposure of ⅕.
The light intensity at which the line width of the μm line is as designed is It. When this It is the reference light intensity, if the light intensity is smaller than It, the resist pattern remains on the wafer, and if it is larger than It, the resist pattern does not remain. As the value of the line width of the auxiliary pattern increases, the light intensity in the area where the auxiliary pattern is expected to be formed on the wafer monotonically decreases. Since the auxiliary pattern should not remain as a resist pattern, the line width of the auxiliary pattern needs to be suppressed within a range in which the light intensity is larger than It. According to the simulation, the line width of the auxiliary pattern is 0.13
It can be seen that it must be below μm. FIG. 2 (d)
As shown in, the line width of the auxiliary pattern is 0.15 μm.
Then, the resist pattern of the auxiliary pattern portion remains.

FIGS. 2 (e) to 2 (h) are the present embodiment, that is, the main pattern is an isolated line-shaped pattern made of a light-shielding film whose transmittance is appropriately set so as to keep the light intensity on the wafer low. In this case, the light intensity distribution in the simulation when the line width of the auxiliary pattern described above is changed is shown. Here, the transmittance of the main pattern is set to 15%. Here, FIG. 2 (e), FIG. 2 (f), and FIG. 2 (g)
2 (h), the line width of the above-mentioned auxiliary pattern is 0 (that is, no auxiliary pattern is provided), 0.05, 0.
This corresponds to the case of 10, 0.15 μm. The optical conditions of the simulation are the same as those of the conventional auxiliary pattern method described above.

According to this embodiment, a remarkable DOF enlarging effect as described below can be obtained. That is, the light intensity It at which the line width of the line is as designed takes a value smaller than that when the conventional auxiliary pattern method is used. By the same consideration as that of FIGS. 2A to 2D, it is understood that the line width of the auxiliary pattern can be expanded to 0.16 μm.
As a result, the expected DOF is 1.0 μm to 1.5 μm.
It is predicted by simulation that m will be obtained.

When the present invention is used in combination with the modified illumination method, D
The OF is expected to further improve to 2.0 μm or more.

[Second Embodiment] FIG. 3 is a top view of a photomask showing a second embodiment of the present invention. As shown in FIG. 3, when the line-shaped patterns are arranged at a certain pitch, that is, in the case of a periodic pattern of lines, the auxiliary patterns are similarly formed. As shown in FIG. 3, auxiliary patterns 13 and 13 having the same size on both sides of a main pattern (periodic pattern) 12 having a line-to-space width ratio of 1: 1.
a is installed. The line width of these auxiliary patterns and the transmittance of the irradiation light of the main pattern are obtained by the same method as described in the first embodiment.

[Third Embodiment] FIG. 4 is a top view of a mask showing a third embodiment of the present invention. As shown in FIG. 4, the ratio of the line to the space is 1: 3, and even when the line and the space are arranged at the same pitch, the auxiliary pattern is similarly formed. As shown in FIG. 4, the main pattern (periodic pattern) in which the width ratio of the line to the space is 1: 3.
When 22, 23, 24 and 25 are arranged, auxiliary patterns 26, 27, 28,
29 and 30 are inserted one by one. Note that the line width (distance between patterns) of these auxiliary patterns and the transmittance of the irradiation light of the main pattern are obtained by the same method as described in the first embodiment. It is also possible to reduce the line width of the auxiliary patterns and increase the number of auxiliary patterns arranged between the main patterns. Further, the line-to-space ratio of the present embodiment is 1: 3, but the same can be applied to the case where the space ratio becomes larger.

[Fourth Embodiment] Next, a fourth embodiment will be described.
A method of producing a photomask by means of will be described with reference to the drawings. FIG. 5 is a cross-sectional view of the flow of producing the photomask according to the present embodiment, which corresponds to the photomask according to the first embodiment. As shown in FIG. 5A, a MoSi-based semitransparent film 42, SiO ₂ 43, and chromium 4 are formed on a glass substrate 41.
4. Prepare the one in which the resist 45 is deposited. Where M
The oSi-based semi-transparent film 42 is adjusted to have a film thickness having an appropriate transmittance. Further, the chrome 44 has a film thickness that completely shields light. SiO ₂ 43 is transparent to exposure light when using a photomask. A resist pattern is formed as shown in FIG. 5B, and MoSi is used as a mask.
When the semi-transparent film 42 of the system is etched and the resist is peeled off, it becomes as shown in FIG. A resist is applied again thereon, and a resist pattern is formed so that the resist 46 remains only in the auxiliary pattern as shown in FIG. 5D. By etching only the chrome 44 using the resist 46 as a mask, the photomask according to the present embodiment is completed. Here, SiO ₂ 43 functions as an etching stopper. In this embodiment, the light-shielding film is chromium and the semitransparent film is MoS.
The i-type light-shielding film is used, but the film is not limited to this, and a film having an appropriate light-shielding property and transmittance can be used instead of chromium.

Next, a method of controlling the phase difference between the light passing through the main pattern and the light passing through the space portion in this embodiment will be described. The phase difference between the light passing through the main pattern and the light passing through the space portion needs to be about 180 degrees. Further, since the phase difference is the same even if it changes by one cycle, it may be in the vicinity of 180 ° ± 360 ° n. Since the phase difference is caused by the thickness of the semitransparent film, it is dealt with by adjusting the thickness and optical density of the MoSi-based semitransparent film.

In the embodiment described above, the exposure wavelength is set to 2
48 nm, pattern shape is line pattern, main pattern width is 0.25 μm, auxiliary pattern width is 0.05, 0.1
0, 0.15 μm, the distance between patterns is 0.25 μm, and the transmittance of the main pattern is 15%. However, the exposure wavelength, the pattern shape, and the pattern dimension are not limited to this, and can be changed as necessary. Even if changed, the effect of the present invention can be obtained as in the embodiment. However, when changing, it is necessary to optimize the auxiliary pattern width, the inter-pattern distance, and the transmittance of the main pattern by simulation or experiment. As is apparent from the embodiments, in order to enhance the effect of the present invention, the light intensity of the main pattern region, in particular, the light intensity It at which the line width of the main pattern is as designed is set to a small value. .

Here, reference will be made to the range of possible transmittance. As an example, a case where the present embodiment is applied to a line-shaped isolated pattern (first embodiment) will be described. FIG. 6 shows the DOF simulation calculation results when the transmittance of the main pattern is changed in steps of 5%. Here, in addition to the normal illumination as the illumination system, the case where annular illumination (annular rate 60%) is applied is also shown. The auxiliary pattern line width is set to the maximum value that can be taken when the transmittance of the main pattern is 0% (complete light shielding), and is 0.13 μ for normal illumination.
m and 0.12 μm for annular illumination. However,
It goes without saying that, due to the nature of the present invention, a larger auxiliary pattern line width can be obtained if the transmittance of the main pattern is appropriately set. From FIG. 6, the main pattern transmittance with which the effect of the present invention is obtained is 20 under normal illumination.
% And less than 40% for annular illumination. If the main pattern transmittance greatly exceeds this value, the DOF will drop sharply. Therefore, setting an appropriate transmittance is very important next to setting the auxiliary pattern width.

In the above embodiment, the case where one pair of auxiliary patterns are arranged on both sides of the main pattern has been described. However, such an auxiliary pattern may be arranged on the upper, lower, left, and right sides of the main pattern so that the auxiliary pattern surrounds the main pattern, and a plurality of pairs of auxiliary patterns may be arranged around the main pattern. I will mention it. Further, in the embodiment, the case where the exposure photomask is used in the stepper has been described, but it should be noted that the same is formed in the case of the one-to-one projection exposure apparatus.
However, in this case, the pattern size on the photomask is the same as the transfer pattern size.

[0034]

As described above, according to the present invention, a main pattern having a transmittance which is appropriately set so as to suppress the light intensity of the on-wafer region to which the main pattern is transferred and whose phase is inverted is provided in the vicinity of the main pattern. Since the auxiliary pattern is arranged, the line width of the auxiliary pattern in the present invention can be made larger than that of the conventional auxiliary pattern method, so that the optical proximity effect can be reduced more than in the conventional method, and The depth of focus of the isolated pattern can be improved. Furthermore,
When the photomask of the present invention is combined with modified illumination, a greater effect can be obtained.

Further, since the line width of the auxiliary pattern in the present invention can be made larger than that of the conventional auxiliary pattern method, even if the line width of the auxiliary pattern reaches the limit of mask manufacturing due to miniaturization and it is difficult to manufacture. According to the present invention, a photomask having an auxiliary pattern can be provided.

[Brief description of drawings]

1A is a top view of a photomask showing a first embodiment of the present invention, and FIG. 1B is a sectional view thereof.

2 (a) to (d) are the photomasks shown in FIG. 1, and when the main pattern is formed of a light-shielding film, the line width of the auxiliary pattern is changed from 0 μm to 0.15 in increments of 0.05 μm. The light intensity distribution graph of the transmitted light on the wafer when it is made to be, (e) to (h) are the photomasks shown in FIG. 1, and the main pattern is a material whose transmittance is 15% and the phase of the transmitted light is changed by 180 degrees. 9 is a graph of a light intensity distribution of transmitted light on a wafer when the line width of the auxiliary pattern is changed from 0 μm to 0.15 μm in increments of 0.05 μm when it is formed of a film.

FIG. 3 is a photomask top view showing a second embodiment of the present invention.

FIG. 4 is a top view of a photomask showing a third embodiment of the present invention.

FIG. 5 is a fourth method for manufacturing the photomask shown in FIG.
FIG. 6 is a cross-sectional view of a flow of creating a photomask according to the embodiment.

6 is a photomask shown in FIG. 1, where the main pattern is formed of a material film that changes the phase of transmitted light by 180 degrees, the transmittance of the main pattern is changed from 0% to 40% in 5% steps. It is a graph which shows the change of the depth of focus when it changes by.

FIG. 7 is a top view of a photomask when a conventional auxiliary pattern method is applied.

[Explanation of symbols]

 1 glass substrate 2 main pattern (isolated pattern) 3 auxiliary pattern 3a auxiliary pattern 11 glass substrate 12 main pattern (periodic pattern) 13 auxiliary pattern 13a auxiliary pattern 21 glass substrate 22 main pattern (periodic pattern) 23 main pattern (periodic pattern) 24 Main pattern (periodic pattern) 25 Main pattern (periodic pattern) 26 Auxiliary pattern 27 Auxiliary pattern 28 Auxiliary pattern 29 Auxiliary pattern 30 Auxiliary pattern 41 Glass substrate 42 Semi-transparent film 43 Silicon oxide film 44 Light-shielding film 45 Resist 46 Resist 51 Glass substrate 52 Main pattern (isolated pattern) 53 Auxiliary pattern 53a Auxiliary pattern

Claims (3)

[Claims] 1. In a photomask in which a pattern is formed on a transparent substrate with a light-shielding film, a main pattern transferred to the surface of a semiconductor substrate by projection exposure is formed of a material film having a transmittance of less than 40%, and the main pattern is for exposure. 2. A photomask, wherein an auxiliary pattern formed of a light-shielding film having a light-shielding property with respect to the irradiation light is provided in the peripheral portion of the main pattern. 2. The photomask according to claim 1, wherein the size of the auxiliary pattern is smaller than the size of the main pattern. 3. The phase difference between the exposure irradiation light passing through the region where no pattern is formed on the transparent substrate and the exposure irradiation light passing through the main pattern is 180 degrees. The photomask according to claim 1.