JPH0729813A - Method for optimizing projection alignment - Google Patents

Abstract

PURPOSE:To optimize an object to be corrected efficiently by providing an evaluation function through the combination of resolution and distortion in the simulation of an image in order to effect correction of distortion in the image using high resolution exposing method (phase shift method, oblique incidence illumination method and filtered oblique incident illumination method). CONSTITUTION:21 and 22 of a mask 20 represent, respectively, a light untransmissive part and a light transmissive part. An evaluation function and is constituted as follows using the light intensity values a1, a2,...a7 at seven black points (1-7) in a mask pattern. eval = {(a1-a7)*(a1-a7)+(a2-a7)*(a2-a7)+(a3-a7)*(a3-a7)+(a5-a7)*(a5-a7)}/(a4* a4)+(a4+a6)*(a4+a6)/{(a4-a6)*(a4-a6)}. This eval is employed in the optimization of the optical amplitude transmissivity distribution of a filter thus correcting the distortion of pattern.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a large scale integrated circuit (LS).
In the projection exposure method of forming a fine pattern on a wafer through a mask in which a required pattern is drawn at the time of manufacturing I),
The present invention relates to a method for optimizing an illumination optical system and a projection optical system in order to bring the shape of a projected image close to a predetermined design shape.

[0002]

2. Description of the Related Art Conventionally, a projection exposure apparatus for forming a fine pattern such as an LSI on a wafer is required to have a high resolution, and for this reason, a projection lens currently used in the projection exposure apparatus. Has reached a resolution close to the theoretical limit determined by the wavelength of light. Nevertheless, in order to cope with the recent trend toward miniaturization of LSI patterns, efforts have been made to achieve even higher resolution.

As one of the means, a phase difference close to 180 ° is given to the light transmitting portions between the adjacent patterns on the mask so that the light intensity in the light shielding portion is canceled so that it approaches zero. The so-called phase shift method has been proposed to improve the resolution.

As another means, there is used a so-called oblique incident illumination exposure method in which a mask is obliquely illuminated to generate a phase difference for the purpose of producing an effect equivalent to that of a phase shift method using a conventional mask. Therefore, attempts have been made to improve the resolution. In this case, various light source shapes have been proposed to achieve the oblique illumination, for example, ring zones or four-point shapes.

Further, in addition to the grazing incidence illumination, a filter for adjusting the light transmittance is provided in the pupil of the projection optical system according to the shape corresponding to the light source arrangement, and the light transmittance distribution is given to further improve the resolution. Attempts have been proposed.

9 (a), 9 (b) and 9 (c) are model diagrams of an optical barrel in which an illumination optical system and a projection optical system are combined, FIG. 9 (a) being a phase shift method, and FIG. Oblique incidence illumination exposure method, (c) is an oblique incidence illumination exposure method with a filter. In the figure, 1 is a light source, 2 is a mask, 3 is a pupil filter, 4 is an image plane, and ideal lenses 5 are respectively arranged between them, and a typical path of a light ray 6 from the light source 1 is shown. Is.

FIG. 10 shows a plan view of the light source 1, the mask 2 and the pupil filter 3 corresponding to FIG. The pupil filter 3 used in the phase shift method (a) and the oblique incidence illumination exposure method (b) usually has a transmittance of 1. On the other hand, in the case of the grazing incidence illumination exposure method with a filter of (c), as shown in the figure, the transmittance of the annular portion 3a (the shaded portion in the figure) through which the 0th-order light passes is higher than that of the transmitting portion 3b (the white portion in the figure). I'm making it small. This can improve the contrast. At this time, it is known that when the transmittance of the transparent portion 3b is 1, the amplitude transmittance of the annular portion 3a is preferably 0.5. In FIG. 10, 7 is a light-shielding portion and 8 is a shifter.

An image projected and formed by such a high resolution exposure method (the above-mentioned phase shift method, oblique incidence illumination method and oblique incidence illumination method with a filter) has a higher resolution than an image formed by a conventional exposure method. However, the distortion is larger. The generation of such an image-forming distortion greatly affects the accuracy of the fine pattern finally formed as the pattern itself becomes finer, and therefore a means for correcting the distortion is required. As this correcting means, as described in Japanese Patent Application Laid-Open No. 4-179952 "Method for forming fine pattern",
The intensity distribution of the image is obtained by simulation, and the degree of occurrence of imaging distortion is determined from this. This result is fed back to the edge position data of the design pattern to be corrected, and the simulation is repeated.

[0009]

The correction of the distortion by the above conventional method means the correction of the mask pattern data, and in the recent VLSI, it is necessary to correct it from a huge amount of pattern data. It is necessary to extract the points and correct them, and the time required for this is close to the feasible limit. If the distortion can be reduced by the optimal design of the illumination optical system and the projection optical system on the assumption that the mask of the original design pattern data is used, the economic effect obtained from this will be very large. In this case, since small distortion and high resolution are incompatible, unless there is an appropriate evaluation function to optimize the feedback to the correction target, the current advanced optimization method (for example, global convergence is It is inevitable that trial and error will occur even if the sequential quadratic programming method is used. The correction target may be the light source shape of the illumination optical system, the radiation intensity distribution, or the transmittance distribution of the pupil filter of the projection optical system. These correction objects are divided at appropriate intervals to optimize the correction value of each divided area, but an evaluation function effective for feedback of correction is not known. This situation is the reason why the optimum design example of the light source shape or the pupil filter focusing on the resolution and the distortion has not been reported.

The present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is to provide a high-resolution exposure method (a phase shift method, an oblique incidence illumination method, and an oblique incidence illumination method with a filter). In order to achieve the correction of the image distortion in (1), it is an object of the invention to provide an evaluation function that combines the resolution and the distortion amount in the image simulation, and to provide a method capable of efficiently optimizing the correction target.

[0011]

In order to achieve the above object, the first aspect of the present invention uses a mask having a required pattern shape,
In a method of optimizing a mask projection image formed on a wafer by an illumination and projection optical system, an optimum combination of parameters of the projection optical system is set by calculation by an evaluation function that combines pattern distortion and resolution. . In a second aspect based on the first aspect, a fine line pattern adjacent to a large pattern, a line segment pattern having an end, and a pattern having a hole pattern are used as the pattern shape, and the thin line pattern is obtained from the image intensity distribution of the mask projection image. The resolution of the pattern, line segment pattern, and hole pattern and the light intensity value at the edge position of the design pattern are extracted, and the evaluation function configured from them extracts the light source shape of the illumination optical system and the transmittance of the pupil filter of the projection optical system. It optimizes the distribution.

[0012]

In the present invention, the distortion and the resolution of the image are represented by one evaluation function in one evaluation mask pattern which is likely to cause distortion, so that the feedback to the correction target can be efficiently executed. And is effective in optimizing projection exposure.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the embodiments shown in the drawings. First, optimization of the light transmittance distribution of the pupil filter of the projection optical system according to claim 2 of the present invention will be described. FIG. 1 is a diagram showing a mask pattern figure. In the mask 20, a portion 21 in which a fine horizontal line is drawn is a portion that does not transmit light, and a white portion 22 is a portion that transmits light and forms a light intensity distribution of image formation. A line 23 that divides the entire rectangle into four vertically and horizontally is provided as a guideline for coordinates.
In the calculation, the mask 20 and the image are scaled 1: 1 and the line width of the vertical line 23 at the center of the mask is 0.3 μm. Two of these lines 23 fit in the width of 1/4 of the whole square. The wavelength of light was i-line (0.365 μm).

The evaluation function (eval) is the light intensity values a1 and a at the seven black dots (1 to 7) shown in the mask pattern.
2, ... A7 is used to form the following equation. eval = {(a1-a7) * (a1-a7) + (a2-a7) * (a2-a7) + (a3-a7) * (a3-a7) + (a5-a7) * (a5-a7) )} / (A4 * a4) + (a4 + a6) * (a4 + a6) / {(a4-a6) * (a4-a6)} The first four terms on the right side of the eval are the edge points 1 of the pattern.
It functions to make the light intensity values at 2, 3, 5, and 7 uniform. ev
The last term on the right side of al calculates the reciprocal of contrast from points 4 and 6 and serves to preserve resolution. e
Optimization can be achieved by minimizing val.

An example of optimizing the amplitude transmittance distribution for light of the filter using this eval is shown below. Figure 2
(A), (b) is a figure which shows the inclination distribution of the illumination light in an illumination optical system, and the division | segmentation of the filter to optimize. The illumination light is generated from the annular zone 25 (hatched portion) of 0.6 and 0.7 in units of 1 when the pupil numerical aperture NA of the projection optical system is 0.6. In other words, each point of the mask receives oblique illumination light that is inclined along the edge of the downward cone from the sky. With respect to this illumination light, the amplitude transmittance of each divided portion of the pupil of the projection optical system was optimized using the above evaluation function. The normal partial coherence theory was used for the calculation of the imaging light intensity distribution, and the sequential quadratic programming with global convergence was used for the repeated calculation of optimization.

First, a calculation example in the case of not optimizing will be described. The transmittance of the filter that achieves high resolution for the annular illumination is the annular portion of the pupil corresponding to the annular zone, that is, NA = 0.
It has been found that an amplitude transmissivity between 0.6 and 0.7 in units of 6 is 0.5. This is Figure 2
That is, the amplitude transmittance of the dividing unit 7 of the filter is 0.5. The imaging intensity distribution in this case is defocused 0.0
And FIG. 3 show two cases of 0.75 μm and 0.75 μm. Left figures (a-1) and (b-1) are contour maps of the intensity distribution of the image calculated under the present optical conditions for the mask 20 shown in FIG. 1, and the thick line shows the maximum intensity. Shows a contour line of 33% of the values. The figures (a-2) and (b-2) on the right side are
In the contour maps of (a-1) and (b-1), the intensity distribution is shown along the horizontal line marked with x. Defocus 0.0
As you can see from the contour map at, the center line is undulating.

As an example of the correction standard, the result of the conventional data correction is shown in FIG. The arrangement of the figure is the same as that of FIG. 3, (a-1) and (b-1) are contour maps, and (a-2) and (b-2) are intensity distribution maps. The moved pattern edges are 1 to 15 shown in FIG. The amount of movement is as follows: the + sign is thickened and the − sign is thinned. In addition,
Point 0 was used as a reference. Edge 1 = 0.053 µm Edge 2 = 0.003 µm Edge 3 = 0.03 µm Edge 4 = 0.004 µm Edge 5 = -0.003 µm Edge 6 = 0.043 µm Edge 7 = 0.003 µm Edge 8 = 0.053 µm Edge 9 = 0.043 μm Edge 10 = 0.085 μm Edge 11 = 0.1 μm Edge 12 = 0.083 μm Edge 13 = 0.058 μm Edge 14 = 0.058 μm Edge 15 = 0.1 μm

Next, starting from the filter distribution in the case of FIG. 3, the filter distribution was optimized by the above method. However, the value of the dividing unit 7 in FIG. 2B was fixed to 0.5. The amplitude transmittance of each divided portion of the filter shown in FIG. 2 is as follows. Division 1 = 1.0 Division 2 = 1.0 Division 3 = 1.0 Division 4 = 0.9376 Division 5 = 0.7597 Division 6 = 0.09904 Division 7 = 0.5 Division Part 8 = 0.7593 Divided part 9 = 0.6527 Divided part 10 = 0.3944 FIG. 6 shows the image intensity distribution calculated under these optical conditions in the same form as FIG. (A-1) and (b-1) are contour maps,
(A-2) and (b-2) are intensity distribution charts. Figure 3
6 and FIG. 6 are compared, in FIG.
As is clear from the contour diagram at 0, the waviness of the center line is small, and it is understood that the distortion is improved while maintaining the resolution by the optimized filter distribution. As compared with FIG. 4 by the correction by the conventional method, the correction effect in FIG. 6 is inferior due to the line segment having an edge and the hole pattern.

As another non-optimized example, FIG. 7 shows the calculation when the illumination light distribution in FIG. 2 is the same and the transmittance of the filter is 1.0 over the entire pupil aperture. (A-1),
(B-1) is a contour map, and (a-2) and (b-2) are intensity distribution maps. This is called an annular illumination method, and has a resolution that is slightly inferior to the case of using the filter of FIG. 3 for line and space patterns, but has a resolution superior to that of the conventional exposure method (circular illumination light distribution). It is known. It can be seen from FIG. 7 that distortion occurs when the defocus is 0.0 and no distortion occurs when the defocus is 0.75 μm, but the line width is thick.

An example of obtaining the optimum distribution of the filter by the above-described optimization method in order to improve the image forming characteristic will be shown below. However, the dividing portion 7 is fixed to 0.8 on the assumption that the transmittance of the filter in the portion corresponding to the illumination ring zone is as close to 1.0 as possible. The amplitude transmittance of the filter division unit is as follows. Division 1 = 1.0 Division 2 = 1.0 Division 3 = 1.0 Division 4 = 1.0 Division 5 = 1.0 Division 6 = 1.0 Division 7 = 0.8 Division Division 8 = 1.0 Division 9 = 1.0 Division 10 = 0.5234

The intensity distribution of the image in this case is shown in FIG.
(A-1) and (b-1) are contour maps, and (a-2) and (b-2) are intensity distribution maps. Comparing FIG. 7 and FIG. 8, it can be seen that although the distortion at the defocus of 0.0 is not improved in FIG. 8, the thickening of the line width at the defocus of 0.75 μm is slightly improved. Figure 4 by conventional correction
Comparing with, the correction effect is smaller with the line segment with the edge and the hole pattern. The defect that the correction effect is small between the line segment having an edge and the hole pattern needs to be considered as a realistic option as a trade-off with the labor required for correction of the pattern data. Furthermore, by combining with the optimization of the distribution of the illumination light in FIG. 2, there is room for finding an optimal combination that reduces these defects.

The illumination light source and the pupil filter are optimized for the phase shift method by using the mask 20 shown in FIG. 1 which is configured by shifting the phase change of adjacent patterns by π according to the idea of the phase shift method. In the phase shift method, the image is distorted because the distinction from the adjacent pattern having a different phase by π appears too strongly in the image. The illumination source and pupil filter distributions are optimized to mitigate this effect, ie, introducing the inverse effect of grazing incidence illumination.

[0023]

As described above, according to the method of optimizing the projection exposure according to the present invention, the distortion of the image generated in the high resolution exposure method is different from the correction of the enormous pattern data in the conventional method. By constructing the image evaluation function by combining distortion and resolution, it is possible to find a combination of the optimum parameters of the illumination optical system and the projection optical system without trial and error, and as a result, the transmittance of the pupil filter is reduced. And the optimal distribution of the illumination light source can be easily calculated, and the burden required for optimization of projection exposure can be significantly reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing a mask pattern figure used for finding a combination of optimum parameters of an illumination optical system and a projection optical system.

2A and 2B are illumination light distributions of an illumination optical system,
It is a figure which shows division for optimizing the amplitude transmittance of the pupil filter of a projection optical system.

3 (a-1), (b-1), (a-2), (b-
2) is a contour map of the intensity distribution of the image in the standard annular illumination method with a pupil filter and a diagram showing the intensity distribution on the horizontal cut line thereof.

4 (a-1), (b-1), (a-2), (b-
FIG. 2) is a diagram showing contour lines of the image intensity distribution by the correction mask by the conventional pattern data correction method and the intensity distribution on the horizontal cut line.

5 is a diagram showing pattern edges to be moved in the correction of FIG.

6 (a-1), (b-1), (a-2), (b-
FIG. 2) is a contour map of the intensity distribution of an image in the annular illumination method using the optimized pupil filter and a diagram showing the intensity distribution on the horizontal cut line.

7 (a-1), (b-1), (a-2), (b-
2) is a contour map of the image intensity distribution in the standard annular illumination method and shows the intensity distribution on the horizontal cut line.

FIG. 8 (a-1), (b-1), (a-2), (b-
2) is a contour map of the image intensity distribution and its intensity on the horizontal cut line in the annular illumination method by the pupil filter that optimizes the amplitude transmittance of the pupil filter corresponding to the illumination annular zone in the range close to 1.0. It is a figure which shows distribution.

9A, 9B, and 9C are model diagrams of optical barrels of a phase shift method, a grazing incidence illumination method, and a grazing incidence illumination method with a filter.

10 is a plan view of a light source, a mask and a pupil filter corresponding to FIG.

[Explanation of symbols]

 1 light source 2 mask 3 pupil filter 4 image plane 5 ideal lens 20 mask

Claims (2)

[Claims] 1. A method of optimizing a mask projection image formed on a wafer by an illumination and projection optical system using a mask having a required pattern shape, wherein an evaluation function combining pattern distortion and resolution is used.
A method for optimizing projection exposure, which comprises setting an optimum combination of parameters of the projection optical system by calculation. 2. The optimization method of projection exposure according to claim 1, wherein a fine line pattern adjacent to a large pattern, a line segment pattern having an end, and a pattern having a hole pattern are used as a pattern shape, The resolution of the fine line pattern, line segment pattern, and hole pattern and the light intensity value at the edge position of the design pattern are extracted from the image intensity distribution, and the light source shape of the illumination optical system and the projection optical system are extracted by the evaluation function composed of them. A method for optimizing projection exposure, which comprises optimizing the transmittance distribution of the pupil filter of 1.