JP3518275B2 - Photomask and pattern forming method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the formation of fine patterns in a lithographic process in the manufacture of semiconductor integrated circuits.

[0002]

2. Description of the Related Art In recent years, semiconductor design rules have been miniaturized, and semiconductor chips of 0.25 μm level have already begun to appear on the market. With such a trend of miniaturization, the exposure wavelength in lithography has been shortened, and the line has changed from g-line (436 nm) to i-line (365 nm) to KrF excimer laser (248 nm). ArF excimer laser (193n
Although the stepper using m) as the exposure light is under development, its development is delayed due to the problem that the lens material absorbs ultrashort wavelength light such as ArF excimer laser light. Therefore, various super-resolution techniques have been studied for the lithography technique using the KrF excimer laser.

Generally, the limit resolution of optical lithography by the reduced projection exposure method is proportional to the exposure wavelength and inversely proportional to the numerical aperture of the projection lens. Conventionally, formation of a pattern of about 0.3 μm has been achieved using a KrF excimer laser (wavelength 248 nm) and a projection lens having a numerical aperture of 0.4 to 0.5.

Among the super-resolution techniques for improving the resolution limit in the reduction projection exposure method, one of the techniques showing excellent resolution is a method using a Levenson phase shift mask.
An example of pattern formation using a conventional Levenson phase shift mask will be described below.

FIGS. 7A to 7C are process sectional views of a pattern forming method using a conventional phase shift mask. In these figures, 21 is a positive resist, 2 is
Reference numeral 2 is a substrate, 23A and 23B are exposure lights, and 24 is a phase shift mask. 25 is a light shielding area, 26A and 26
B is a transparent area, and transparent area 26B is a transparent area 26A.
On the other hand, the phase of the exposure light 23A is set to be different by 180 degrees. The exposure light 23A illuminates the mask 24, and the exposure light 23B passing through the transparent region of the mask is exposed to the resist 21.
Image on.

In FIG. 7A, first, a positive resist 21 is applied on a substrate 22. The positive resist 21 was a chemically amplified resist for KrF excimer laser and was applied to a film thickness of 0.5 μm. Next, the positive resist 21 was exposed through the phase shift mask 24.

The exposure conditions of the exposure apparatus (stepper) are an exposure wavelength λ = 248 nm, a numerical aperture NA = 0.48, and a coherent factor σ = 0.30. As shown in FIG. 7B, the Levenson phase shift mask has a transparent region 26.
The quartz B is dug in, and the phase of the light transmitted therethrough is inverted by 180 degrees with respect to the light transmitted through the transmission region 26A.

First, the exposure light 23A illuminates the mask 24,
Light is diffracted according to the pattern density. In the case of the Levenson phase shift mask, since the phases of the transmissive regions on both sides are 180 degrees different from each other through the light shielding region, the 0th order light and the even order light are canceled in the periodic pattern. Further, the odd-order lights of ± 1, 3, 5, etc. are usually diffracted at a half angle of the mask. Generally, the angle of light that can pass through the projection lens is finite,
The resolution of the pattern can be said to be the pattern period that can pass through the lens. Since the Levenson phase shift mask diffracts light at an angle half that of a normal mask, ideally it is possible to obtain a resolution twice as high as that of a normal mask.

In order to realize a high resolution with the Levenson phase shift mask, it is necessary to align the spatial light phases (enhance the coherency). The NA of the projection lens and the NA of the illumination system as a unit showing the degree of coherency.
The ratio σ (coherent factor) of is used. This σ
The smaller the value of, the higher the light coherency. Generally, when the mask is illuminated by a stepper, the coherent factor of the optical system is set to about σ = 0.5 to 0.8, but when a Levenson phase shift mask is used, σ =
It should be about 0.2 to 0.4.

After the pattern exposure shown in FIG. 7B, P
EB (Post Exposure Baking) was performed, and development was performed for 60 seconds with a normal alkaline aqueous solution to form a resist pattern (FIG. 7C). According to this pattern forming method, it was possible to resolve a line and space pattern of 0.16 μm, which is much finer than the exposure wavelength of 248 nm (0.248 μm).

[0011]

Generally, the Levenson phase shift mask is effective in a fine periodic pattern, and therefore its application to a DRAM device including many periodic patterns has been studied. However, the Levenson phase shift mask has a problem that the dimension of the resist transferred onto the wafer changes if the distance between adjacent patterns is different even with the same line width. For this reason, it is very important to control the line width of the gate pattern of the logic device, but since the logic device contains many random patterns, its application to the logic device has not been studied so much.

In FIG. 7C, the pattern 21X is 0.
A line and space pattern of 16 μm, and 21Y is a resist pattern obtained by transferring a mask pattern designed to have a 0.16 μm line / 0.48 μm space. At this time, the actual size of the resist pattern transferred onto each wafer is 0.
16 μm, but the pattern 21Y is 0.20 μm
m, and the dimensional difference between them was 0.04 μm. In the case of forming a normal transistor gate, the dimensional variation is about ± 10% of the line width, so that the line width of 0.16 μm must be suppressed within about 0.03 μm. Therefore, the conventional pattern formation method using a phase shift mask cannot be used for pattern formation of a transistor gate that requires high dimensional accuracy.

In order to solve the above-mentioned problems, the present invention provides a method for forming a fine pattern using a phase shift mask with less variation in line width, and a phase shift mask used therefor.

[0014]

In pattern formation using a Levenson phase shift mask, the phenomenon in which the dimension of a resist pattern transferred onto a wafer changes even if the distance between adjacent patterns is different even if the line width is the same is known as optical proximity. It was found to be due to the effect. In particular, it has been discovered that the distance between adjacent patterns becomes significant in a region where the normalized value is 1.0λ / NA or less.

The photomask of the present invention is a photomask for forming a resist pattern corresponding to the light-shielding region by exposing the resist using a photomask in which the transmission regions on both sides of the light-shielding region differ in phase from each other by 180 degrees. The line width of the light shielding region is corrected according to the interval between adjacent patterns.

Further, the photomask of the present invention is a photomask for forming a line pattern of the same width by exposing a positive resist using a photomask in which the phases of the transmissive regions on both sides of the light shielding region are different from each other by 180 degrees. A light-shielding region is newly provided between the two light-shielding regions having a predetermined distance or more from an adjacent pattern.

As described above, in the pattern forming method of the present invention, by using the above-mentioned photomask, the light intensity and the light profile that have passed through the photomask are changed, or the specific pattern intervals are eliminated, so that different intervals are obtained. Also in the line pattern, the distribution of the widths of the respective light intensity distributions at the threshold is narrowed, and the resist pattern having substantially the same line width can be formed.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A pattern forming method according to an embodiment of the present invention will be described below with reference to the drawings.

First, it will be explained that in the pattern formation using the Levenson phase shift mask, the line width variation increases due to the optical proximity effect.

In the periodic pattern formation using the Levenson phase shift mask, only the ± 1st order light is imaged near the resolution limit. This light intensity distribution is represented by a sine wave. In particular, at the pattern frequency from the incidence of all the primary light to the incidence of the tertiary light, the light intensity remains constant (the amplitude of the sine wave does not change), and the pattern pitch (cycle of the sine wave) Since it spreads, the pattern line width changes greatly. This corresponds to a region where the slope of the normalized pattern interval of 1.0 or less in FIG. 5B is steep. This is an optical proximity effect peculiar to the Levenson phase mask, and the size changes greatly within a very short range as compared with a normal mask. We discovered this phenomenon and found that the normalized pattern spacing of 1.
By correcting the portion of 0 or less, it was possible to reduce the variation in the pattern line width.

(Embodiment 1) FIG. 1 is a process sectional view of a pattern forming method according to Embodiment 1 of the present invention. FIG. 2 is a mask configuration diagram in which a part of the phase shift mask of the present embodiment shown in FIG. 1 is viewed from the wafer side. Further, FIG. 3 shows the distribution of the light intensity imaged on the resist.

In FIGS. 1 and 2, 1 is a positive resist, 2 is a substrate, and 3A and 3B are exposure lights. Exposure light 3
A is light that illuminates the mask, and exposure light 3B is light that passes through the mask and forms an image on the resist. Reference numeral 4 is a photomask, 5, 5A and 5B are light-shielding areas, 6A and 6B are transmissive areas, and the transmissive area 6B is 180 degrees out of phase with respect to the transmissive area 6A. The pattern forming method of the present invention will be described below with reference to the drawings.

First, in FIG. 1A, the positive resist 1
Was coated on the substrate 2. The resist was a chemically amplified positive resist for KrF, and the film thickness was set to 0.5 μm. Next, in FIG. 1B, the exposure light 3A illuminates the phase shift mask 4, and the positive resist 1 is exposed with the exposure light 3B that has passed through the mask. The exposure conditions of the stepper are: exposure wavelength λ = 248 nm, numerical aperture NA = 0.48,
A coherent factor σ = 0.40 and a 5: 1 reduction type projection exposure apparatus was used. Phase shift mask 4
Is a digging type in which the phase is changed by 180 degrees by digging in a quartz substrate.

Details of the phase shift mask shown in FIG. 1B will be described with reference to FIG. Shading area 5 in FIG.
A and 5B are locations where a fine resist pattern is formed on the wafer, and xa and xb are respective widths. The light shielding region 5A is a region where a 0.16 μm line and space pattern is formed on the wafer, and the light shielding region 5B
Is a pattern region having a 0.16 μm line / 0.48 μm space interval and exists on the same mask. Exposure is 5
The mask pattern is transferred to the wafer with a reduction of ⅕ because it is performed with the stepper of ⅕ reduction. Therefore, the mask size in FIG. 1B is five times as large as the actually transferred resist pattern size. The width xa of the light shielding area 5A used in the present embodiment is 0.80 μm, and the width xb of the light shielding area 5B is 0.50 μm.

FIG. 3A shows the width Xa of the light shielding area 5A shown in FIG.
Is 0.80 μm, and the width Xb of the light shielding region 5B is 0.50.
.mu.m, and shows the spatial light intensity on the wafer when passing through a mask corrected so that the width of the light-shielding region 5B having a wider mutual distance is smaller than that of the conventional mask shown in FIG. 7B. In FIG. 3B, the mask light-shielding area is set to 0.8 according to the pattern size without performing the mask correction.
It shows the spatial light intensity when 0 μm is set.

In FIG. 3, the broken line indicates the light intensity at which a 0.16 μm line-and-space is formed 1: 1 by adjusting the exposure amount. Since the resist is a positive resist, a resist pattern is formed below the broken line. Therefore, the line width of the formed resist pattern corresponds to the width of the light intensity distribution formed by the broken line. In FIG. 3A, a resist pattern of 0.171 μm is formed in the pattern of 0.16 μm line / 0.48 μm space, but 0.19 μm is formed in the uncorrected mask of FIG. 3B.
It will be 0 μm. By adjusting the width of the light-shielding region of the phase shift mask in this manner, the light intensity on the wafer can be changed and the size can be adjusted.

Thereafter, in FIG. 1C, the resist exposed with such a light intensity is developed by PEB (post-exposure bake treatment) with an alkaline aqueous solution for 60 seconds to form a resist pattern 1.
form x. According to the present embodiment, a resist pattern having a line width of 0.16 μm can be accurately formed with a dimensional accuracy of ± 10% or less.

As described above, according to the embodiment of the present invention, by changing the width of the light shielding region of the phase shift mask, the same line width with different pattern intervals can be accurately formed according to the design dimension.

(Second Embodiment) A pattern forming method according to a second embodiment of the present invention will be described below with reference to the drawings. FIG. 4 shows a case where a phase shift mask in which the phases of the transmissive regions on both sides of the elongated light-shielding region are different by 180 degrees is used as a positive resist.
The relationship between the m line width and the pattern interval is obtained by using a light intensity simulation. The simulation conditions are exposure wavelength λ = 248 nm, numerical aperture NA = 0.60, coherent factor σ = 0.3, and threshold light intensity is set to a value that makes the line and space pattern 1: 1 0.16 μm. is doing.

In FIG. 4, a curve a (black circle) indicates that no mask correction is performed, and the widths of the light-shielding areas on the masks of all patterns are converted into values on the wafer (light-shielding area width × reduction ratio). 0.16 μm similar to the resist pattern
Is. On the other hand, the curve b (white triangle) is set to 0.10 μm in terms of the width of the light-shielding area on the wafer in a pattern having a design value of 0.67λ / NA or more in which the adjacent pattern intervals are standardized. It was done. It can be seen that the difference between the maximum value and the minimum value of the line width is reduced to almost half by correcting the width of the light shielding area of the mask.

As described above, in the present embodiment, the light-shielding area of the mask of the pattern having the designed pattern interval of 0.67λ / NA or more is 0.10 μm in terms of the converted value on the wafer, and the pattern with the interval less than that is set. By providing two light-shielding regions having a width of 0.16 μm on the mask, the variation in the line width of the resist pattern can be reduced to half that of the conventional technique.

As described above, in the present embodiment, the width of the light shielding region of the mask pattern is changed with the pattern interval of 0.67λ / NA as a boundary.

(Third Embodiment) A pattern forming method according to a third embodiment of the present invention will be described below with reference to the drawings. FIG. 5 shows the relationship between the line width and the pattern interval when a phase shift mask in which the phases of the transmissive regions on both sides of the elongated light-shielding region are different by 180 degrees is used for the positive resist by light intensity simulation and experiment. FIG. 5B shows the line width and the pattern interval standardized by λ / NA. In FIG. 5B, curve a has NA = 0.60, b has NA = 0.55, and c.
Indicates the case of NA = 0.48. In all cases, the coherent factor is σ = 0.30.

As is apparent from FIG. 5, NA is 0.6.
In the line widths of 0, 0.55, and 0.48, the line width sharply increases until the pattern interval is within the standard value of 0.5 to 1.0, and the resist line width is the most near the standard value of 1.0. Grows. After that, the line width gradually decreases.

For example, the curves a, b, in FIG.
Considering c, the normalized line width is the smallest when the pattern interval is around 0.4λ / NA, and the normalized line width is the maximum when the pattern interval is around 1.0λ / NA.
After that, the line width gradually decreases as the pattern interval increases.

Design rules are set so that there is no pattern interval of the standard value 0.7 or less, the exposure wavelength λ = 248 nm, the numerical aperture NA = 0.48, and the coherent factor σ = 0.40.
With a 5: 1 reduction type projection exposure apparatus.
A resist pattern having a line width of 6 μm was exposed. The dimensional variation obtained by such a phase shift mask is 0.
It could be suppressed to 16 μm ± 10%.

(Fourth Embodiment) A pattern forming method according to a fourth embodiment of the present invention will be described below with reference to the drawings. FIG. 6A is a mask configuration diagram showing a part of the mask showing the present embodiment as seen from the wafer side, FIG. 6B is a mask configuration diagram used for the second exposure, and FIG. ) Indicates a resist pattern transferred by two exposures.

FIG. 8A is a mask configuration diagram of a part of the conventional phase shift mask as seen from the wafer side.
FIG. 8B is a configuration diagram of a mask used for the conventional second exposure, and FIG. 8C shows the transferred resist pattern. In these figures, 15 is a light-shielding area, and 16 and 1
6A and 16B are transmissive regions, and the transmissive region 16B is 180 ° out of phase with the exposure light with respect to the transmissive region 16A. 1A is the formed resist pattern.

The pattern forming method using the phase shift mask of the fourth embodiment will be described in comparison with the conventional example.
In FIGS. 6A and 8A, the fine line patterns on the wafer have adjacent pattern intervals of 6 line widths.
More than twice as far apart. In the present embodiment, as shown in FIG. 6A, in the area where the adjacent pattern intervals are separated by a predetermined distance (six times the line width) or more, as shown in FIG.
A specific limit width x is set in 6B. That is, the regions having a phase difference of 180 degrees are formed as a pair so that the transmission region is not widened.

On the other hand, the conventional mask shown in FIG. 8A does not have such a specific width, and therefore, the transmissive regions 16A and 16B are formed between the adjacent patterns. Therefore, in a pattern having a wide interval, the transmissive region varies depending on the pattern interval, and thus the line width is likely to change.

In the phase shift mask shown in FIG. 6A, since the transmissive regions are formed on both sides of the isolated pattern, the light shielding region is formed in the central portion. For this reason,
Although the light should not be irradiated to the portion that should originally transmit the light, the light may be irradiated to this portion in the subsequent second exposure process for removing the upper and lower portions of the fine pattern. Compared with the conventional mask (FIG. 8B), the mask used for the second exposure used in this embodiment has a position corresponding to the light-shielding region between the isolated patterns of the phase shift mask. It can be seen that the area is a transparent area.

If the width x of a specific transmissive region is determined as in the phase shift mask shown in FIG. 6A, the light intensity passing through the transmissive region does not change even if the pattern interval is different, and the dimensional accuracy is high. A resist pattern can be formed. Further, in the conventional phase shift mask as shown in FIG. 8A, two patterns share one transmissive region, so that it is necessary to always consider the phase when designing the pattern arrangement. However, in the case of FIG. 6A having a pair of left and right transmissive regions in one pattern, there is an advantage that the mask can be freely designed regardless of the phase of the transmissive regions around the light shielding region.

As described above, by providing the transmission region of the phase shift mask with a specific width, it is possible to form a highly accurate pattern that does not depend on the pattern interval.

In the embodiment of the present invention, the phase shift mask is of the engraved type, but it may be of the transmissive film laminated type.

[0045]

As described above, according to the present invention, the width of the light shielding region of the phase shift mask is changed according to the line width interval.
Alternatively, by adopting a mask configuration in which the pattern interval is a certain value or more, or the interval of the transmissive regions is always kept constant, it is possible to reduce the dimensional variation due to the optical proximity effect that occurs when the phase shift mask is used.

[Brief description of drawings]

FIG. 1 is a process sectional view of a pattern forming method according to a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a mask used in the pattern forming method according to the first embodiment of the present invention.

FIG. 3 is a diagram showing a light intensity distribution of the pattern forming method according to the first embodiment of the present invention.

FIG. 4 is a diagram showing a relationship between a line width and a pattern interval in the pattern forming method according to the second embodiment of the present invention.

FIG. 5 is a diagram showing a relationship between a line width and a pattern interval in the pattern forming method according to the third embodiment of the present invention.

FIG. 6 is a diagram illustrating a pattern forming method according to a fourth embodiment of the present invention.

FIG. 7 is a process sectional view of a conventional pattern forming method.

FIG. 8 is a diagram illustrating a conventional pattern forming method.

[Explanation of symbols]

1 Positive resist 2, 22 Substrates 3A, 3B, 23A, 23B Exposure light 4 Masks 5, 5A, 5B, 15, 25 Masking areas 6, 6A, 6B, 16A, 16B, 26A, 26B Masking areas

Claims (10)

(57) [Claims] 1. A photomask for forming a resist pattern corresponding to a light-shielding region by exposing a resist using a photomask in which the phases of the light-transmitting regions on both sides of the light-shielding region are different from each other by 180 degrees. A photomask in which the line width is corrected according to the interval between adjacent patterns. 2. The photomask according to claim 1, wherein the resist pattern is a pattern having the same width and having an arbitrary interval. 3. The distance between adjacent resist patterns is 0.
The photomask according to claim 2, wherein the width of the light-shielding region of the mask pattern is different at the boundary of 67λ / NA. 4. A photomask for forming a line pattern of the same width by exposing a positive resist by using a photomask in which the phases of the transmissive regions on both sides of the light-shielding region are different from each other by 180 degrees. A new light-shielding structure is provided in which the width of the line pattern can be accurately formed by setting the transmissive region to have a specific width between the two light-shielding regions having a predetermined distance or more, even if the patterns have different intervals. A photomask having a region. 5. The photomask according to claim 4, wherein the interval between adjacent patterns is 6 times or more the pattern line width. 6. A pattern forming method for forming a resist pattern corresponding to a light-shielding region by exposing a resist using a photomask in which the phases of the light-transmitting regions on both sides of the light-shielding region differ from each other by 180 degrees. A pattern forming method using a photomask in which the line width is corrected according to the interval between adjacent patterns. 7. The pattern forming method according to claim 6, wherein the resist pattern is a pattern having the same width and having an arbitrary interval. 8. The distance between adjacent resist patterns is 0.
8. The pattern forming method according to claim 7, wherein a photomask in which the widths of the light shielding regions of the mask pattern are different from each other with 67λ / NA as a boundary is used. 9. A pattern forming method for forming a line pattern of the same width by exposing a positive resist using a photomask in which the phases of the transmissive regions on both sides of the light-shielding region are different from each other by 180 degrees. By setting the transmissive region to a specific width between the two light-shielding regions having a predetermined distance or more, it is possible to accurately form the width of the line pattern even if the pattern interval is different. A pattern forming method characterized by using a photomask provided with a light shielding region. 10. A method of applying a positive resist on a semiconductor substrate, and the phases of the transmissive regions on both sides of a light-shielding region forming a circuit pattern are different from each other by 180 degrees, and the distance between adjacent patterns is two or more. A step of exposing the resist using a photomask in which a light-shielding area is newly provided between the light-shielding areas and a step of newly exposing by using a second exposure mask that shields a portion required for a circuit pattern A method of manufacturing a semiconductor device, including a step of removing an unnecessary resist pattern including a resist pattern corresponding to the shielded region.