JP3369462B2 - Evaluation method of optical characteristics of exposure process - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a semiconductor device,
The present invention relates to a photolithography process, which is one of the manufacturing processes for liquid crystal display elements, thin film magnetic heads, and the like. More specifically, it relates to the optical characteristics of a projection optical image formed by using a projection type exposure apparatus in the exposure process. It is about how to evaluate.

[0002]

2. Description of the Related Art In recent years, miniaturization and high integration of semiconductor device patterns such as ICs and LSIs have progressed, and the manufacturing cost thereof has been increasing due to complication of the device structure and increase in the number of steps. From now on, how to systematically develop new semiconductor devices with high efficiency and low cost,
The key to technology is mass production. One of the keys to such systematization is the semiconductor process / device simulation technology, and one of them is the lithography simulation technology. In the photolithography process, which is often used not only in the above semiconductor devices but also in the manufacture of liquid crystal display elements and thin film magnetic heads, there are physical and chemical parameters related to the processes such as photoresist coating, baking, exposure, and development steps. Very many. For this reason, the lithography simulator has become an indispensable tool for its systematic efficiency improvement.

Here, a conceptual diagram of the photolithography process is shown in FIG. First, as shown in FIG. 1A, a lithography exposure apparatus 6 such as a projection type exposure apparatus (stepper) is used.
Thus, the photoresist 8 applied on the substrate 7 is exposed through the photomask 5 on which a predetermined pattern is formed. At this time, the projected optical image of the pattern of the photomask 5 exposed on the photoresist 8 has, for example, the light intensity distribution as illustrated. Then, FIG.
Then, when the exposed photoresist 8 is developed with a developing solution, the profile of the photoresist 8 becomes according to the light intensity distribution of the projected optical image as shown in FIG.

Next, FIG. 7 shows a typical calculation flow by a lithography simulator which is a tool for simulating the photolithography process. In the simulation by the lithography simulator, first, in FIG. 7, conditions necessary for the photolithography process are input (S1; lithography condition input), and a projection optical image at the position of the photoresist 8 is calculated for a given mask pattern (S2). Light intensity distribution calculation).
Subsequently, for example, when the photoresist 8 is a positive type, the photochemical reaction is calculated assuming that the projection light destroys the inhibitor of the photoresist 8 by exposure (S3; exposure calculation).
To do. Further, the surface shape of the photoresist 8 at the time of development is obtained from the inhibitor concentration distribution obtained by the exposure calculation, and the shape (profile) of the photoresist 8 after development is calculated (S4; development calculation).

On the other hand, as a method for evaluating the light intensity distribution of a projected optical image, there is a method using a physical quantity called contrast. This contrast will be described below by taking a specific mask pattern as an example.

FIG. 8A shows a width of 0.25 μm and a width of 0.5 μm.
Photomask 1 in which a repeating pattern (periodic pattern) of minute pitch lines and spaces of m pitch is formed
1 shows a one-dimensional cross section (relative light intensity distribution in the position coordinate axis direction) of the relative light intensity distribution of the projection optical image calculated by the lithography simulation. For convenience, the pattern and the relative light intensity distribution are shown at the same time. FIG. 2B is a cross-sectional view of the photomask 11 of FIG. The pattern is the opening 11 where the quartz glass such as SiO ₂ which is the base material of the photomask 11 is exposed.
a, and a light shielding portion 11b made of a metal such as Cr and provided on the quartz glass for the purpose of shielding the projection light.

Further, FIG. 9A shows a projection calculated by lithography simulation for the photomask 12 having only one 0.25 μm square rectangular opening pattern (for example, a pattern for forming a contact hole). 2 shows a one-dimensional cross section of a relative light intensity distribution of an optical image. FIG. 2B is a sectional view taken along the line DD of the photomask 12 of FIG. The pattern is the opening 12a
And a light shielding portion 12b.

The relative light intensity distributions of FIGS. 8 (a) and 9 (a) are affected by the diffraction phenomenon of the projection light because the patterns are minute as described above. As can be seen from these relative light intensity distributions, the relative light intensity I takes the maximum value in the projection optical image projected through the apertures 11a and 12a which are the light transmitting regions, and the light shielding portions 11b and 12b which are the mask regions. The relative light intensity I takes the minimum value in the projection optical image corresponding to the portion where the projection light is blocked by.
In these relative light intensity distributions, assuming that the maximum light intensity is the maximum relative light intensity I _max and the minimum light intensity is the minimum relative light intensity I _min , the contrast C is C = (I _max −I _min ) / (I _max + I _min ) Defined in (1).

Next, the defocus value (depth of focus), which is the amount of defocus of the projection light in the optical axis direction, is changed, and the value of the contrast C is obtained for each defocus value.
That is, since the relative light intensity distribution of the projected optical image changes depending on the defocus value, the relative light intensity distribution corresponding to each defocus value is obtained, and the contrast C is obtained for each case.

FIG. 2 is a relative light intensity distribution when the defocus value is changed in 5 ways with respect to the above-mentioned minute dimension line & space. As can be seen from the figure, the maximum relative light intensity I increases as the defocus value increases.
_{Since max} becomes smaller and minimum relative light intensity I _min becomes larger, the contrast C becomes smaller from the equation (1).

FIG. 3 is a relative light intensity distribution of each of the minute size rectangular opening patterns when the defocus value is changed in five ways.

FIG. 10 shows the focus characteristics obtained by plotting the relationship between the contrast C and the defocus value. The mark ■ indicates the case of minute size line & space, and the mark ▲ indicates the case of minute size rectangular opening pattern. The range represented by R in the figure is called the focus width in the contrast C. From this focus characteristic, so-called matching is performed to obtain a contrast C at which the focus margin of the photoresist 8 that can be resolved by an actual photolithography process and the focus width match. The contrast C at this time is called a contrast threshold C _th, and when a certain pattern is designed, the contrast C is the contrast threshold C _th.
If the defocus value as described above is adopted, it is estimated that the pattern can be resolved. For example, assuming that the focus margin with which the photoresist 8 can be resolved in an actual process is 0.71 μm, the contrast threshold C _{th at} that time is 0 in the case of a minute dimension line & space from FIG. It becomes 0.75.

[0013]

However, unlike the repetitive pattern of minute dimension lines and spaces, in the case of a minute dimension rectangular opening pattern, its focus characteristic is as shown in FIG.
As shown in 0, the contrast remains large even if the defocus value increases. This is because in the small-sized rectangular aperture pattern, the minimum relative light intensity I _min is close to zero, and therefore the value of the contrast C does not change much even if the defocus value is changed. Therefore, assuming that the contrast threshold C _th is the same as in the case of the minute dimension line & space, the photoresist 8 is determined to be resolvable for all defocus values.

However, the relative light intensity I in the opening 12a of the minute size rectangular opening pattern is smaller than the relative light intensity I in the opening 11a of the minute size line & space, and the absolute light intensity is also small. However, if the defocus value increases as much as possible, there is a high possibility that the small-sized rectangular opening pattern will not be resolved. It is also possible to increase the contrast threshold C _th in order to be able to evaluate the optical characteristics of the photomask 12 which is composed of only isolated minute size rectangular opening patterns. However, in that case, on the contrary, the focus width of the repetitive pattern of minute dimension lines and spaces becomes smaller than necessary. Therefore, such a contrast threshold C _th cannot be adopted in the optimum design of the mask pattern.

That is, since the focus characteristics are different between the minute dimension line & space and the minute dimension rectangular opening pattern as shown in FIG. 10, the relative light intensity distributions for these patterns are uniform using the same contrast threshold C _th. There is an inconvenience that it cannot be evaluated. Although the example in which the minute size line & space and the minute size rectangular opening pattern are compared has been described above, the same can be said for the case where the dense pattern portion and the rough portion are compared.

The present invention has been made in view of the above conventional problems, and an object of the present invention is to evaluate the light intensity distribution of the projected optical image on the same basis regardless of the pattern shape of the photomask. An object of the present invention is to provide a method of evaluating optical characteristics of an exposure process that can be performed.

[0017]

According to a first aspect of the present invention, there is provided an optical characteristic evaluation method for an exposure process, wherein a photomask provided with a predetermined pattern using a projection type exposure apparatus is provided in order to solve the above-mentioned problems. By irradiating the
The light intensity distribution of the projection optical image of the pattern projected on the exposed object arranged facing the photomask, the difference between the maximum light intensity and the minimum light intensity of the projection optical image is the maximum light intensity In the optical characteristic evaluation method of the exposure process for evaluating by using the contrast obtained by dividing by the sum of the minimum light intensity, the boundary portion between the mask area and the light transmission area of the pattern is on the exposed object or the Evaluating the light intensity distribution by using the magnitude of the position coordinate differential coefficient of the light intensity of the projected optical image projected on the exposed object as a correction coefficient to multiply the above contrast, and using the corrected result as the corrected contrast. Is characterized by.

In the above invention, the difference between the maximum light intensity and the minimum light intensity in the light intensity distribution of the projected optical image of the pattern on the object to be exposed is first expressed as the sum of the maximum light intensity and the minimum light intensity. Divide to obtain the contrast. Next, the size of the position coordinate differential coefficient of the light intensity of the projected optical image formed by projecting the boundary portion between the mask area and the light transmitting area of the pattern on or in the exposed object, that is, the magnitude of the light intensity gradient. Is used as the correction coefficient. Here, the projection optical image includes not only an image on the object to be exposed (spatial image) but also an image corresponding to the light intensity and the latent image at an arbitrary depth from the surface of the object to be exposed. Then, the corrected contrast is obtained by multiplying the contrast by the correction coefficient.

By correcting the contrast in this way, even when an isolated small size rectangular aperture pattern is projected onto an object to be exposed, the defocus value of the projected light is small because the minimum light intensity of the projected optical image has been small so far. It is possible to give a large change to the contrast that does not change much even when is changed. That is, in this method, the difference in pattern shape is taken into consideration, and the light intensity distribution of the projected optical image of various patterns can be uniformly evaluated.

As a result, it is possible to provide an optical characteristic evaluation method for an exposure process that can evaluate the light intensity distribution of the projected optical image on the same basis regardless of the pattern shape of the photomask.

In order to solve the above-mentioned problems, an optical characteristic evaluation method for an exposure process according to a second aspect of the present invention is the optical characteristic evaluation method for an exposure process according to the first aspect, wherein the object to be exposed is a photo. In the case of a resist, the correction contrast of the correction contrast is adjusted so that the focus width corresponds to the resolvable focus margin of the photoresist in the actual photolithography process from the defocus value dependence of the correction contrast on the projection light. It is characterized in that a corrected contrast threshold value which is a threshold value is obtained.

According to the above invention, the dependency of the correction contrast on the defocus value is used to correspond to the resolvable focus allowance of the photoresist in the actual photolithography process. A corrected contrast threshold with is determined. Therefore, the exposure conditions can be easily optimized.

In order to solve the above-mentioned problems, an optical characteristic evaluation method for an exposure process according to a third aspect of the present invention is the optical characteristic evaluation method for an exposure process according to the first or second aspect, wherein the pattern is It is characterized by being a periodic pattern repeated at a predetermined pitch, or an isolated pattern having openings or masks in a polygonal shape.

According to the above-mentioned invention, the above-mentioned pattern is a basic pattern of a photomask, and if the optical characteristics of the exposure process are evaluated with respect to the above-mentioned pattern, the result is combined with the above-mentioned pattern. It is convenient because it can be applied to many other configured patterns.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of an optical characteristic evaluation method for an exposure process according to the present invention will be described below with reference to FIGS. 1 to 5.

The optical characteristic evaluation method of the exposure process of the present embodiment is performed by using the contrast C derived from the light intensity distribution of the projection optical image formed on or in the photoresist 8 (see FIG. 6), By obtaining the corrected contrast C _p by multiplying by the correction coefficient g, it is possible to uniformly evaluate the optical characteristics regardless of the pattern shape of the photomask.

A projected optical image of a repetitive pattern such as minute dimension lines & spaces (a periodic pattern repeated at a predetermined pitch) and an isolated minute dimension rectangular opening pattern (an isolated pattern having a polygonal opening). The one-dimensional cross-section (relative light intensity distribution in the position coordinate axis direction) as the light intensity distribution obtained by simulating is as shown in FIG. 4A and FIG. 5A, respectively. In both figures, for convenience, the pattern of the photomasks 1 and 2 and the relative light intensity distribution are shown at the same time. Figure 4 (b)
5B is a cross-sectional view taken along the line AA of the photomask 1 of FIG. 4A and a cross-sectional view taken along the line BB of the photomask 2 of FIG. 5A. The pattern is composed of openings 1a and 2a as light transmitting areas and light shielding portions 1b and 2b as mask areas. The dimensions of the photomask 1 are those of the photomask 11 shown in FIG.
Then, the photomask 2 is the same as the photomask 12 in FIG.

In the pattern of the photomasks 1 and 2,
The relative light intensity distribution is influenced by the diffraction phenomenon of projection light because the pattern is minute. FIGS. 2 and 3 show such relative light intensity distributions obtained for five different defocus values in the range of 0 μm to 1 μm.
FIG. 2 shows the case of a minute size line & space, and FIG. 3 shows the case of a minute size rectangular opening pattern. From these relative light intensity distributions, the maximum relative light intensity I _max and the minimum relative light intensity I _min
And the contrast C is calculated from the equation (1) for each defocus value.

Next, in the relative light intensity distribution for each defocus value, the openings 1a.2 of the photomasks 1 and 2 are shown.
Boundary between a and light-shielding parts 1b and 2b (pattern edge)
Of the relative optical intensity I of the projected optical image formed by the projection of, for example, the relative light intensity I of the point P on the relative light intensity distribution in FIG. 4A and the point Q of the relative light intensity distribution in FIG. 5A. Focus on the relative light intensity I. Then, the magnitude of the position coordinate differential coefficient ∂I / ∂x (= tan α) of the relative light intensity I at the point P, and the position coordinate differential coefficient ∂I / ∂x (= tan β) of the relative light intensity I at the point Q.
, That is, the magnitude of the light intensity gradient (absolute value). Then, the contrast C is multiplied by using the magnitude of this light intensity gradient as the correction coefficient g to obtain the corrected contrast C _p . Therefore, the corrected contrast C _p is represented by C _p = g × C = g × (I _max −I _min ) / (I _max + I _min ) (2).

FIG. 1 shows a result of plotting the corrected contrast C _p obtained from the equation (2) for each defocus value for the minute dimension line & space and the minute dimension rectangular opening pattern. From the figure, it is understood that the correction contrast C _p greatly decreases as the absolute value of the defocus value increases not only in the minute dimension line & space but also in the minute dimension rectangular opening pattern.

According to another experiment, the photoresist 8 on the substrate 7 was resolved by the lithography exposure apparatus 6 (see FIG. 6) according to the pattern dimension when the value of the corrected contrast C _p was 2.2 μm ⁻¹ or more. I know it will be done. Further, in this case, it is also known that the focus margin of the minute dimension line & space is 0.71 μm, and the focus margin of the minute dimension rectangular opening pattern is 0.62 μm. On the other hand, from FIG. 1, the corrected contrast C
When the value of _p is determined a focus width when the 2.2 .mu.m ^-1, in the case of critical dimension line and space was 0.71 .mu.m, in the case of critical dimension rectangular opening pattern 0.62μm
Is. Therefore, a value of above 2.2 .mu.m ^-1 is has minimal correction contrast C _p, i.e., the meaning of the correction contrast threshold C _pth photoresist 8 is resolved pattern scale.

As described above, the focus characteristics of the minute size line & space and the minute size rectangular opening pattern are in good agreement with the actual measurement values, and the correction contrast C _p is used to measure the minute size line & space and the minute size rectangular opening pattern. Optical characteristics can be uniformly evaluated. In the actual photolithography process, the corrected contrast threshold C _pth having the focus width combined with the resolvable focus margin of the photoresist 8 is the same in any pattern. Therefore, if the corrected contrast threshold value C _pth is obtained, the maximum value of the focus width allowed for any pattern when the exposure is actually performed by the lithographic exposure apparatus 6 is known, and the exposure condition is easily optimized. be able to.

Note that the minute dimension line &
Space and small dimension rectangular aperture patterns are the base patterns of many other patterns made up of these combinations. Therefore, if the optical characteristics of the projected optical image of the minute dimension line & space and the minute dimension rectangular opening pattern are investigated, the optical characteristic for various patterns can be known, which is highly convenient. From the viewpoint of pattern density, the optical characteristics of the projected optical image can be obtained even for intermediate pattern densities between dense patterns such as minute dimension lines and spaces and coarse patterns such as minute dimension rectangular opening patterns. Can be evaluated in the same way.

Although the opening pattern is rectangular in the above, it may be any polygonal isolated pattern. further,
The optical characteristic evaluation method of the present invention can be similarly applied to a photomask having a microscopic light-shielding pattern obtained by reversing the light transmission region and the mask region of a photomask having a microscopic aperture pattern. However, in this case, since the directions of the curves of the relative light intensity distribution in FIG. 5 are turned upside down, an absolute value is used as the correction coefficient g.

Further, although the object to be exposed in this embodiment is a photoresist, it is not limited to this, and it goes without saying that any photosensitive substance such as photosensitive polyimide may be used.

As described above, according to the optical characteristic evaluation method of the exposure process of this embodiment, the light intensity distribution of the projected optical image is evaluated by the same reference regardless of the pattern shape of the photomask. You can

[0037]

As described above, in the optical characteristic evaluation method of the exposure process according to the first aspect of the present invention, projection light is applied to a photomask provided with a predetermined pattern by using a projection type exposure apparatus. According to the above, the light intensity distribution of the projection optical image of the pattern projected on the exposure object arranged facing the photomask, the difference between the maximum light intensity and the minimum light intensity of the projection optical image is the maximum light. In the optical characteristic evaluation method of the exposure process in which the contrast obtained by dividing the intensity by the sum of the minimum light intensity is used, the boundary portion between the mask region and the light transmission region of the pattern is on the exposed object. Alternatively, the magnitude of the position coordinate differential coefficient of the light intensity of the projection optical image formed in the exposed object is corrected as a correction coefficient by multiplying the contrast, and the corrected result is used as the corrected contrast. It is configured to evaluate the light intensity distribution.

Therefore, by correcting the contrast in this way, even when an isolated minute size polygonal pattern is projected onto the object to be exposed, the minimum light intensity of the projection optical image is so small that the projection light is not projected. Even if the defocus value is changed, the contrast, which does not change much, can be changed greatly. That is, in this method, since the difference in the pattern shape is taken into consideration, the light intensity distribution of the projected optical image can be uniformly evaluated.

As a result, it is possible to provide an optical characteristic evaluation method for an exposure process that can evaluate the light intensity distribution of the projected optical image on the same basis regardless of the pattern shape of the photomask. Play.

As described above, the optical characteristic evaluation method for the exposure process according to the second aspect of the present invention is the optical characteristic evaluation method for the exposure process according to the first aspect, wherein the object to be exposed is a photoresist. From the dependence of the corrected contrast on the defocus value with respect to the projection light, the corrected contrast threshold value is set so that the focus width corresponds to the resolvable focus margin of the photoresist in the actual photolithography process. This is a configuration for obtaining a certain corrected contrast threshold value.

Therefore, by using the dependency of the corrected contrast on the defocus value and corresponding it with the resolvable focus margin of the photoresist in the actual photolithography process, for example, the corrected contrast having the maximum focus width can be obtained. A threshold is required. Therefore, the exposure condition can be easily optimized by using the corrected contrast threshold value.

As described above, the optical characteristic evaluation method for the exposure process according to the third aspect of the present invention is the optical characteristic evaluation method for the exposure process according to the first or second aspect, wherein the pattern has a predetermined pitch. The pattern is a periodic pattern repeated in step 1, or an isolated pattern in which openings or masks are formed in a polygonal shape.

Therefore, if the optical characteristics of the exposure process are evaluated with respect to the above pattern, the result can be applied to many other patterns formed by the combination of the above patterns, which is highly convenient. Produce an effect.

[Brief description of drawings]

FIG. 1 is a graph showing a defocus value dependency of a correction contrast obtained by using an optical characteristic evaluation method of an exposure process according to an embodiment of the present invention.

FIG. 2 is a graph showing a relative light intensity distribution of a projection optical image with respect to a minute dimension line & space for each defocus value.

FIG. 3 is a graph showing a relative light intensity distribution of a projected optical image with respect to a minute size rectangular aperture pattern for each defocus value.

FIG. 4A is an explanatory view showing a method of deriving a corrected contrast from a one-dimensional cross-sectional view of a relative light intensity distribution of a projected optical image with respect to a minute dimension line & space, together with a plan view of a photomask; ) Is A of the photomask in (a)
It is a sectional view taken along line A-A.

FIG. 5A is an explanatory view showing a method of deriving a corrected contrast from a one-dimensional cross-sectional view of a relative light intensity distribution of a projection optical image with respect to a small-sized rectangular aperture pattern, together with a plan view of a photomask; ) Is B of the photomask in (a)
FIG. 6 is a cross-sectional view taken along line B-arrow.

6A to 6C are explanatory views illustrating the concept of a photolithography process.

FIG. 7 is a flowchart illustrating a calculation flow by a lithography simulator.

8A is an explanatory view showing a one-dimensional cross-sectional view of a relative light intensity distribution of a projection optical image with respect to a minute dimension line & space together with a plan view of a photomask, and FIG. 8B is a photo of FIG. 8A. It is CC sectional view taken on the line in the arrow of FIG.

9A is an explanatory view showing a one-dimensional cross-sectional view of a relative light intensity distribution of a projection optical image with respect to a minute size rectangular opening pattern together with a plan view of a photomask, and FIG. 9B is a photo of FIG. 9A. It is a DD line sectional view taken on the line of a mask.

FIG. 10 is a graph showing the defocus value dependence of the contrast obtained by using the conventional optical property evaluation method of the exposure process.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Photomask 1a Opening part (light transmission area) 1b Light shielding part (mask area) 2 Photomask 2a Opening part (light transmission area) 2b Light shielding part (mask area) 6 Lithographic exposure apparatus (projection type exposure apparatus) 7 Substrate 8 Photo Resist (object to be exposed) C Contrast C _th Contrast threshold C _p Correction contrast C _pth Correction contrast threshold g Correction coefficient I Relative light intensity I _max Maximum relative light intensity (maximum light intensity) I _min Minimum relative light intensity ( (Minimum light intensity)

Claims (3)

(57) [Claims] 1. A projection-type exposure apparatus is used to irradiate a photomask provided with a predetermined pattern with projection light, thereby forming a pattern of the pattern projected on an object to be exposed which is arranged to face the photomask. The light intensity distribution of the projected optical image,
In the optical characteristic evaluation method of the exposure process to evaluate using a contrast obtained by dividing the difference between the maximum light intensity and the minimum light intensity of the projection optical image by the sum of the maximum light intensity and the minimum light intensity, The contrast is defined with the magnitude of the position coordinate differential coefficient of the light intensity of the projection optical image formed by projecting the boundary portion between the mask area and the light transmitting area of the pattern on or in the exposed object as the correction coefficient. And an optical characteristic evaluation method for an exposure process, characterized in that the light intensity distribution is evaluated by using the corrected result as a corrected contrast. 2. The exposed object is a photoresist,
From the dependency of the corrected contrast on the defocus value with respect to the projection light, the correction is the threshold value of the corrected contrast so that the focus width corresponds to the resolvable focus margin of the photoresist in the actual photolithography process. The optical characteristic evaluation method for an exposure process according to claim 1, wherein a contrast threshold value is obtained. 3. The optical process according to claim 1, wherein the pattern is a periodic pattern repeated at a predetermined pitch, or an isolated pattern having openings or masks in a polygonal shape. Characteristic evaluation method.