JP2001085305A - Exposure apparatus aberration measurement method and aberration measurement system - Google Patents

Abstract

(57) [Problem] To measure various aberrations of an exposure apparatus by a simple procedure without using a test reticle. A reticle pattern being used by an exposure apparatus is exposed on a wafer at each position while moving the wafer position in a vertical direction. For each position, a pattern portion to be line-targeted is taken from the exposed pattern on the wafer. Image processing is performed on the captured line symmetric pattern portion, and the area of the line symmetric pattern exposed on the wafer is measured. The best focus position and various aberrations are obtained based on the obtained area.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a method and a system for measuring various characteristic values that affect exposure, such as a best focus position and various aberrations, in an exposure apparatus used for manufacturing a semiconductor element or a liquid crystal display panel.

[0002]

2. Description of the Related Art An exposure apparatus used for manufacturing a semiconductor device or the like is an apparatus that forms a pattern on a submicron level by reducing and projecting a pattern formed on a reticle onto a wafer. With the improvement of LSI manufacturing technology in recent years, pattern miniaturization has progressed, and the dimensional accuracy required for patterns has become stricter year by year. Therefore, very high precision is also required for a lens for reducing and projecting a pattern.

More specifically, it is necessary to control the amount of aberration of the lens so as not to adversely affect the pattern to be formed, and to minimize them. However, depending on the type of aberration or the pattern to be formed, the influence of aberration may be remarkable. Therefore, in terms of quality control of the manufactured product and maintenance of the apparatus, it is necessary to accurately measure the aberration amount of the exposure apparatus periodically or as needed.

Conventionally, a test reticle has been used to measure each aberration of the exposure apparatus. For this reason, the production line was once stopped to measure the aberration, and the aberration was measured after the test reticle was mounted on the exposure apparatus.

[0005] On the test reticle, a resolution chart for measuring aberration is formed in advance. As shown in FIG. 13, for example, the resolution chart is configured by a line & space pattern whose scale changes stepwise. Such a line & space pattern is reduced and exposed on a wafer by an exposure apparatus, and the line width of the pattern actually formed on the wafer is determined by, for example, SEM (Scanning Electron-
The amount of aberration affecting the exposure pattern was calculated by measuring with a beam microscope (scanning electron microscope).

[0006]

However, in the conventional method, as described above, the production has to be interrupted and the reticle must be replaced every time the aberration is measured, which causes a reduction in the production efficiency.

Further, according to the pattern to be actually formed,
Test reticles of different types of resolution charts must be prepared, and preparations for performing aberration measurement are complicated.

Furthermore, the procedure of measurement and calculation differs depending on the type of aberration to be measured, so that the process of measuring aberration is complicated, and time and labor are required.

SUMMARY OF THE INVENTION An object of the present invention is to provide an aberration measuring method capable of easily measuring the characteristic value of an exposure apparatus during actual device manufacture without using a test reticle.

It is another object of the present invention to provide a method and a system which can easily measure various aberrations of an exposure apparatus during actual device manufacturing without using a test reticle.

Still another object of the present invention is to provide an aberration measuring system capable of easily detecting various aberrations of an exposure apparatus during actual device manufacture and feeding back the detection results.

[0012]

In order to achieve the above object, in the aberration measuring method of the present invention, the wafer position is moved up and down in a stepwise manner, and a reticle pattern is exposed on the wafer at each position. Next, at each position, a line symmetric pattern is selected and taken in from the exposed patterns on the wafer. From the captured image, the area of the line symmetric pattern exposed on the wafer is determined. The aberration is measured based on the obtained area.

According to this method, various aberrations can be easily measured using a line-symmetrical pattern from the device pattern during the actual device manufacture without replacing the test reticle. Can be.

The area of the line symmetric pattern may be obtained after image processing of the captured image. The image processing is, for example, black and white processing by simple binarization. In this case, the area is obtained from the number of dots of the image that has been subjected to black and white processing.

The above-mentioned aberration measuring method further includes a step of obtaining a best focus position based on the obtained pattern area. Thereby, the best position of the wafer mounted on the exposure apparatus can be known. Further, the amount of astigmatism, spherical aberration, and field curvature of the exposure apparatus can be obtained from the obtained best focus position.

The above method includes a step of determining coma aberration based on the determined area of the line-symmetric pattern.

As described above, once an image of a line symmetric pattern is captured and its area is measured, various characteristic values of the exposure apparatus can be obtained based on the pattern area.

Another feature of the present invention is to provide an aberration measuring system for an exposure apparatus. This system includes an exposure device, a pattern capturing device that selects and captures a line-symmetric pattern from patterns exposed on a wafer, an area measuring unit that measures the area of the line-symmetric pattern from the captured image, And an arithmetic unit for calculating various aberrations from the area of the obtained line symmetric pattern. With this configuration, it is possible to easily obtain various aberrations of the exposure apparatus even during device manufacture by using the line-symmetric pattern of the reticle actually used in the exposure apparatus.

This system further includes a feedback mechanism for automatically adjusting the lens system of the exposure apparatus based on the obtained aberration. This makes it possible to reflect the measured various aberrations on the exposure apparatus and always keep the aberration of the exposure apparatus in the best state.

Other features and effects of the present invention will become more apparent by the embodiments described below.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings.

FIG. 1 is a flowchart showing the flow of the aberration measuring method of the present invention. Before describing the detailed processing flow of aberration measurement based on this flowchart, a general exposure apparatus to which the present invention is applied will be briefly described.

In the exposure apparatus 10 (shown by a broken line) shown in FIG.
Is irradiated onto the reticle 3 via the The pattern formed on the reticle is projected onto the wafer 6 by the projection lens system 4.
The image is reduced and printed on. The wafer 6 is held on a wafer stage 8 and driven in x, y, and z directions by a stage drive unit 19. An alignment device 7 for positioning the wafer 6 is located above the wafer stage 8. Although not shown, the alignment device has, for example, a light emitting unit and a light receiving sensor.

Returning to the flowchart of FIG. 1, first, in step S101, the pattern of the reticle 2 being used by the exposure apparatus is exposed on the wafer 6 while gradually changing the vertical position of the wafer 6 in the vertical direction. For example, the position of the wafer in the Z direction is changed by driving the wafer stage 8 in the vertical direction at intervals of 0.1 μm. At the same time, the pattern transfer position in the xy plane is moved in the horizontal direction by the step method in the same manner as the normal pattern transfer. As a result, the current pattern transfer is performed at a position where the z-direction position slightly changes, adjacent to the area exposed in the previous shot. This is repeated at the z position within a certain range.

Next, in step S103, at each z position, from among the patterns exposed on the wafer 6,
Select a line-symmetric part and capture the pattern of that part. The line-symmetric portion of the pattern can be easily specified because the pattern formed on the reticle being used in the exposure apparatus is known in advance. As the line symmetric pattern, for example, as shown in FIG. 12, any line, such as an isolated line, a block pattern, a hook, a line & space pattern, and a wedge pattern, may be used as long as it is line symmetric in the horizontal direction or the vertical direction.

The pattern can be captured by using, for example, an alignment system 7 provided in the exposure apparatus. In the embodiment, light emitted from the light emitting unit of the alignment system and reflected by the pattern on the wafer 6 is received by the light receiving sensor, and the light intensity distribution can be captured as image information. In addition to the alignment system, for example, a pattern image can be captured by an SEM (scanning electron beam microscope). SEM
Is used, the wafer 6 on which the pattern has been exposed is once removed from the stage 8, moved to a vacuum chamber (not shown), and an image is captured as a scanning image of an electron beam. Therefore, the use of the SEM is suitable for performing aberration data analysis or the like after the end of device manufacturing. In the present embodiment, however, the SEM is usually provided in an exposure apparatus in order to perform aberration measurement in real time during device manufacturing. An image is captured using an alignment optical system. As another example of real-time image capturing, for example, an image capturing device such as a CCD arranged on a wafer stage can be used, and an image can be captured by directly detecting the intensity distribution of exposure irradiation light. .

In step S105, the captured image is processed. In the present embodiment, black and white processing by simple binarization is performed. By this image processing, the captured image of the line symmetric pattern is displayed as, for example, black dots.

Next, in step S107, the area of the line symmetric pattern at each z position is measured. When the monochrome processing is performed as in the present embodiment, the area of the pattern can be obtained by counting the number of dots in the area corresponding to the pattern. Since the z position is slightly changed, the exposed and printed pattern area changes depending on the degree of defocus at each z position. Various aberrations and the best focus position can be detected based on the area of the line symmetric pattern at each position.

More specifically, in step S109, it is determined whether or not to obtain coma. In the case of YES, the process proceeds to step S119, and the coma is detected based on the pattern area obtained in step S107. If no,
Proceeding to step S111, the best focus position is determined. In step S113, it is determined whether to obtain astigmatism based on the obtained best focus position. YE
In the case of S, astigmatism is obtained in step S115, and NO
In the case of, the spherical aberration is obtained in step S117.

Hereinafter, detection of the best focus position (S1)
11), detection of astigmatism (S115), detection of spherical aberration (S117), and detection of coma (S119) will be described in detail.

<Detection of Best Focus Position (S11)
1) As described above, since the reticle pattern is exposed on the wafer one shot at a time by gradually changing the position of the wafer 6 in the z direction, the irradiation light guided by the projection lens system 4 depending on the position in the z direction. Is changed. In step S107, since the area of the same pattern at each position is obtained from, for example, the number of dots, the pattern area is plotted as a function of the focus position. An example of the plot is shown in FIG.

In the graph of FIG. 2, circles indicate actual pattern measurement data. The solid line is the approximate curve. The z position where the pattern area becomes maximum in the graph is the best focus position where the pattern transfer is most clearly performed.

As described above, without using a test reticle, an image of a line symmetrical portion is taken in from a pattern of a device being manufactured, and appropriate image processing is performed to easily detect a best focus position. Can be.

<Detection of Astigmatism (S113)> The astigmatism is detected based on the detection of the best focus position in step S111. The astigmatism is caused by a shift between the focal point of the light beam included in the meridional surface (meridional focal point) and the focal point of the light beam included in a plane (sagittal surface) orthogonal to the meridional surface (sagittal focal point). Therefore, the line and space (L & S) pattern extending in a certain direction is easily affected by astigmatism.

Therefore, from the pattern area measured in S107, for example, five L & S patterns extending in the vertical direction and five L & S patterns extending in the orthogonal direction (ie, in the horizontal direction).
Using the S pattern, astigmatism is determined based on those area changes.

FIG. 3 shows the astigmatism detection step (S
It is a flowchart which shows 115) in detail. First, in step S201, the best focus position of the vertical L & S pattern is obtained. This is determined by plotting the pattern area as a function of the z position and finding the z position corresponding to the maximum value, as in S111.

Simultaneously with or immediately after this, the best focus position of the lateral L & S pattern is obtained in S203. Similarly, the best focus position of the lateral L & S pattern is determined by obtaining the z position at which the pattern area is maximized. The difference between the best focus position of the vertical pattern and the best focus position of the horizontal pattern thus obtained indicates the amount of astigmatism of the exposure apparatus.

Therefore, in step S205, step S2
From the values detected in 01 and 203, (best focus position of vertical pattern) − (best focus position of horizontal pattern) is calculated to obtain astigmatism.

FIG. 4 is a graph showing the processing shown in the flowchart of FIG. In FIG. 4, the circles indicate z
The area of the vertical pattern as a function of position is shown, the solid line is its approximate curve. The square marks indicate the area of the horizontal pattern, and the broken line is the approximate curve. From the maximum values of these approximate curves, the respective best focus positions are obtained. When there is astigmatism, the best focus positions in the sagittal direction and the meridional direction do not always match.
In FIG. 4, the deviation of the best focus position indicated by the bidirectional arrow A is the amount of astigmatism of the lens system of the exposure apparatus.

<Detection of Spherical Aberration (S117)> FIG.
2 is a flowchart showing detailed steps of a spherical aberration detection step S117 in FIG. Spherical aberration is one of the aperture aberrations of the optical system, and is caused by that when light beams having different apertures enter the optical system, the corresponding image points do not form a single point. Therefore, the difference between the best focus positions of line patterns having different line widths indicates spherical aberration. A method for obtaining this will be described with reference to FIG.

In step S301 in FIG. 5, the best focus position FL1 of the line pattern having the line width _L1 is captured. Next, in step S303, a line width L different from L1
2 best focus position _{F L2} of the line pattern of the (L1 ≠ L2) captures. The detection of the best focus position of the line pattern of each line width is as described above. In step S305, the difference between the focus positions, that is, F
Determining the spherical aberration by calculating the _L1 -F _L2.

<Detection of Coma (S119)> If the determination in step S109 of FIG. 1 is YES, step S11
Go to 9 to obtain coma. Of the patterns used in the device body, the stuffing patterns shown in the second row from the top in FIG. 12 are sensitive to coma. An example of such a tightly packed pattern is, for example, a twin hole pattern used as a deep trench in DRAM manufacturing. Therefore, the coma aberration of the exposure apparatus is determined based on a change in the area of a twin hole pattern portion extracted from a pattern exposed using a reticle used in the apparatus.

FIG. 6 shows a change in the exposure pattern when the twin hole pattern on the reticle is exposed on the wafer while changing the z position of the wafer. If the wafer is at the best focus position and there is no coma in the device,
The sizes of the left and right patterns are equal. Assuming that the defocus in the z direction at this time is 0, as the position shifts from the defocus in the z direction, the influence of coma aberration increases,
The sizes of the left and right patterns exposed on the wafer are different and become asymmetric.

Assuming that the area of the left hole on the wafer is La and the area of the right hole is Ra, the amount corresponding to coma is
That is, a normalized frame is obtained by the following equation.

Normalized top = (La−Ra) / (La + Ra) (1) FIG. 7 is a graph in which normalized tops are plotted from the measured values of the actual left and right hole pattern areas. Equation (1) is a value obtained by calculating the ratio of the difference between the left and right pattern areas and the sum from the measurement data, and has no unit. In order to quantify the profile of such a measured value as the amount of coma aberration of the exposure apparatus, a graph that simulates in advance how much aberration is present and how the left and right pattern areas change when there is an aberration is used. I do. The simulation graph of the coma aberration is generated by performing simulation with a known lithography simulator.

FIG. 8 is a graph showing a simulation result of general coma aberration. Λ is used as a unit representing the amount (magnitude) of coma aberration, and the profile of the change of the normalized coma becomes steeper as the aberration amount of the exposure apparatus increases.

For example, comparing the profile of the measured normalized coma shown in FIG. 7 with the simulation result of FIG. 8, it can be seen that the coma amount of this exposure apparatus is substantially equal to 0.4λ. The gentler the profile of the measured normalized coma, the smaller the coma aberration amount of the exposure apparatus. The amount of coma aberration quantified and expressed in the unit λ is used when adjusting the lens system of the exposure apparatus.

<Detection of Field Curvature> As an application example of the method shown in FIG. 1, a method of detecting the field curvature of the exposure apparatus based on the measurement of the best focus position will be described. Also in the case of detecting the field curvature, step S in the flowchart of FIG.
The process up to 111 is the same. To determine the curvature of field, a plurality of best focus positions are required in the effective area of the exposure device. Therefore, as shown in FIG. 10, the effective exposure area on the wafer is divided into a plurality of best focus measurement blocks F (i, j) (1 ≦ i ≦ n, 1 ≦ j ≦ m).
Find the best focus position in each area. in this case,
An arbitrary line symmetric pattern is selected, and the best focus position in each area of the pattern is measured. The field curvature is represented by the width between the maximum best focus position and the minimum best focus position.

FIG. 9 is a flowchart showing a method of detecting a field curvature in this embodiment.

After the step of obtaining the best focus in S111, it is determined in S401 whether i = n and j = m. If NO, i or j is incremented, and the process returns to step S111 to measure the best focus position in the next measurement area F (i, j). This is i
= N, j = m. If it is determined that i = n and j = m in step S401 (YE in S401)
S), in step S405, the maximum focus position Max
The difference between (F (i, j)) and the minimum focus position Min (F (i, j)) is calculated to determine the field curvature.

As described above, according to the aberration measuring method of the present invention, the steps up to the step of selecting a line-symmetric pattern and obtaining the area of the pattern are performed in common, and thereafter, the coma aberration, spherical aberration, astigmatism, etc. Can be selected and measured as required, so that aberration measurement is simplified.

Next, an aberration measuring system according to the present invention will be described with reference to FIG.

The aberration measuring system according to the present invention includes an exposure apparatus 1
0, an image processing unit 11, a line-symmetrical pattern area measuring unit 13, an arithmetic unit 15, an optical system control unit 17, a stage control unit 19, and a CPU 21.

The stage controller 19 changes the vertical position (the position in the z-axis direction) of the wafer 6 in a stepwise manner via the stage driver 9 in the exposure apparatus 10. At the same time,
The wafer 6 is shifted in the horizontal direction (x or y direction) for each shot, and the pattern formed on the reticle 3 is reduced-exposed on the wafer 6.

The alignment device 7 of the exposure device 10 functions as a pattern capturing unit at the same time as being an alignment device. In this case, the pattern capturing unit captures the exposure pattern on the wafer, for example, as an image based on the light intensity distribution. The alignment device 7 is connected to the image processing device 11, and performs, for example, a simple binarization black and white process on the image detected and captured by the alignment device 7. The selection of the line symmetric pattern portion from the patterns exposed on the wafer may be performed at the time of capturing an image by the alignment device 7 or may be performed at the stage of image processing by the image processing unit 11.

In the method described so far, the exposure pattern does not need to be the resist pattern after the development step. Even if a so-called latent image immediately after the exposure of the resist is used, the effect of the present invention is not affected at all. It will not change.

Instead of using an alignment device, CC
It is also possible to mount the substrate on which the D elements are arranged on the wafer stage 8. In this case, the CCD element substrate functions as a pattern capturing unit, and directly captures the intensity distribution of the exposure irradiation light transmitted through the reticle pattern. The image processing device 11 is connected to the CCD element substrate and performs image processing on the light intensity distribution information. The pattern data subjected to the image processing is supplied to the line symmetric pattern area measuring unit 13, and the area of the line symmetric pattern is detected for each position in the z direction by, for example, measuring the number of black dots. The line symmetric pattern area measuring unit has, for example, a built-in memory (not shown),
Stores the measured pattern area.

The arithmetic unit 15 obtains various characteristic values of the exposure apparatus using data on the area of the line symmetric pattern. For example, the best focus position, astigmatism, coma, field curvature, spherical aberration, and the like can be obtained.

Each aberration of the exposure apparatus obtained by the arithmetic unit is supplied to the optical system controller 17. Optical system controller 1
Reference numeral 7 also functions as a feedback mechanism, and a corresponding lens (not shown) of the projection lens system 4 almost in real time based on various aberrations of the exposure apparatus supplied from the arithmetic unit.
Adjust the state of. At this time, the amount of various aberrations, the amount of lens adjustment, etc. are associated with the type of the reticle being used, and the CP
It may be stored in a memory (not shown) in U21.

As described above, according to the aberration measurement system of the present invention, the reticle actually used and the device being manufactured can be used in real time without replacing with a test reticle having a resolution chart. Various aberrations and the best focus position of the exposure apparatus are detected, and the lens system of the exposure apparatus can be adjusted to the best state accordingly.

[0061]

As described above, according to the method of the present invention, it is not necessary to replace the reticle with a test reticle.
The aberration state of the exposure apparatus can be measured without interrupting the production line.

Since the area of the line symmetric portion can be obtained from the pattern to be actually exposed, and various aberrations can be obtained based on the area, the procedure of the measurement process is simplified.

Further, according to the aberration measuring system of the present invention, the area of the line symmetric pattern being exposed is measured in real time using the reticle actually being used and the device being manufactured, and the best focus is obtained based on the measurement result. The position and various aberrations can be obtained. In addition, it is possible to automatically adjust the projection lens system of the exposure apparatus by feeding back the obtained aberration amount.

[Brief description of the drawings]

FIG. 1 is a flowchart showing a process flow of an aberration measuring method according to an embodiment of the present invention.

FIG. 2 is a graph showing a best focus position determined by the aberration measurement method shown in FIG.

FIG. 3 is a flowchart showing in detail an astigmatism detection step in the flowchart of FIG. 1;

FIG. 4 is a graph showing astigmatism determined according to the astigmatism detection method of FIG. 3;

FIG. 5 is a flowchart showing details of a spherical aberration detection step in the flowchart of FIG. 1;

FIG. 6 is a schematic diagram showing the influence of coma when exposing a line-symmetric pattern.

FIG. 7 is a graph in which the coma aberration determined in the coma aberration detection step in the flowchart of FIG. 1 is plotted as a function of the defocus amount.

8 is a graph quantifying coma aberration determined in a coma aberration detection step in the flowchart of FIG.

FIG. 9 is a flowchart of a method of obtaining a field curvature from a detection result of a best focus position as an application example of the method shown in FIG.

FIG. 10 is a diagram showing a state in which an effective exposure area on a wafer is divided into a plurality of best focus measurement areas when obtaining curvature of field in the processing of FIG. 9;

FIG. 11 is a schematic diagram showing an aberration measurement system of the exposure apparatus according to one embodiment of the present invention.

FIG. 12 is a diagram illustrating an example of a line-symmetric pattern used for aberration detection according to the present invention.

FIG. 13 is a diagram showing an example of a resolution chart used in a conventional aberration detection method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source 2 Condenser lens 3 Reticle 4 Projection lens system 5 Lens drive unit 6 Wafer 7 Alignment device (pattern capture device) 8 Wafer stage 9 Stage drive unit 10 Exposure device 11 Image processing device 13 Line-symmetric pattern area measurement unit 15 Arithmetic unit 17 Optical system control unit (feedback mechanism) 19 Stage control unit 21 CPU

Claims (11)

[Claims] A step of exposing a reticle pattern on a wafer; a step of capturing an image of a line-symmetric pattern among the patterns on the exposed wafer; An aberration measuring method for an exposure apparatus, comprising: a step of calculating an area of a line symmetric pattern that has been exposed to light; and a step of detecting aberration based on the obtained area of the line symmetric pattern. 2. The method of claim 1, further comprising processing the captured image of the line symmetric pattern. 3. The image processing method according to claim 2, wherein the image processing step is black-and-white processing by simple binarization, and the area obtaining step obtains an area from the number of dots of the image subjected to black-and-white processing. the method of. 4. The method according to claim 1, further comprising the step of determining a best focus position based on the determined area of the line symmetric pattern. 5. The method according to claim 4, further comprising determining astigmatism and / or spherical aberration based on the determined best focus position. 6. The method according to claim 4, further comprising determining a curvature of field based on the determined best focus position. 7. The method according to claim 1, further comprising determining a coma aberration based on the determined area of the line-symmetric pattern. 8. The method according to claim 2, wherein said pattern capturing step is performed based on an intensity distribution of an optical image. 9. The method according to claim 9, wherein the pattern capturing step comprises:
The method according to claim 2, wherein the method is performed by M (scanning electron microscope). 10. An exposure apparatus, a pattern capturing section for selecting and capturing a line-symmetric pattern image from a pattern exposed on a wafer, and an image processing section for performing image processing on the captured line-symmetric pattern image. An aberration measurement system for an exposure apparatus, comprising: an area measurement unit that measures an area of a line symmetric pattern from an image-processed line symmetric pattern; and a calculation unit that calculates aberration from the measured area of the line symmetric pattern. 11. The aberration measurement system according to claim 10, further comprising a feedback mechanism for automatically adjusting a lens system of the exposure apparatus based on the obtained aberration.