JPH10294267A - Equipment and method for detecting alignment mark - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve accuracy of detection results, by changing an NA (numerical aperture) variable diaphragm provided on an optical path that forms the image of a subject on an image pick up means, performing alignment mark detection plural times, and obtaining the average of the plural detection results. SOLUTION: Light projected from an illuminating light source 6 passes through a lens 3, reaches a semiconductor wafer 1 and is projected on an alignment mark 2. Then, its reflecting light is projected on the image pick up plane of a solid-state image pick up device 5 of an image processing part 7 through the lens 3. The light is converted into an electric signal by the image pick up device 5, and calculation for alignment is performed by a processing unit 8. The calculation results are processed by the computer 10 of a control part 9, and the exposure position of the semiconductor wafer 1 is controlled. The alignment mark detecting equipment is provided with a diaphragm 12, the NA of the lens 3 is changed by the computer 10, detection of the position of the alignment mark 2 is repeated plural times, and the average value is judged as the detected position.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alignment mark for detecting a position of an alignment mark by picking up an image of a subject having an alignment mark by an image pickup means and processing a video signal output from the image pickup means by a video signal processing means. The present invention relates to a detection device and an alignment mark detection method.

[0002]

2. Description of the Related Art In a manufacturing process of a semiconductor device, there are many exposure steps, and for example, alignment of a photomask (or reticle) with respect to a semiconductor wafer is repeated many times. The alignment is performed using the alignment marks formed on the surface of the semiconductor wafer and the alignment marks formed on the photomask as indices.

Therefore, it is necessary to recognize the alignment mark and detect the position of the alignment mark for the alignment.

There are generally two types of alignment mark position detection technologies actually used at present, one of which is to irradiate a laser beam to a certain alignment mark and detect the intensity distribution of the diffracted light. The position of the alignment mark is detected from the waveform indicating the distribution.
This detection technique is suitable when the surface roughness of the mark or the base is small, the symmetry of the alignment mark is good, and the step of the mark is low.

[0005] Another position detection technique is an imaging means (for example, C
The position of the alignment mark is detected by imaging using a CD-type solid-state imaging device or the like, and is suitable for detecting an object having a rough surface, poor mark asymmetry, and a large mark step. I have.

FIGS. 5A to 5C are views for explaining a conventional example of an alignment mark detecting device using an image pickup means, in which FIG. 5A is a schematic configuration diagram, and FIG. 5B is formed on a semiconductor wafer. Plan view showing an example of the alignment mark,
(C) is a waveform diagram showing an ideal video signal by an alignment mark.

In the drawings, 1 is a semiconductor wafer, 2 is an alignment mark formed on the surface of the wafer 2, 3 is a lens that forms an image of a subject on an imaging surface of a solid-state imaging device (CCD) described later, and 4 is an alignment mark. The position of the semiconductor wafer 2 is corrected so that there is no shift on the image pickup surface between the mark and the alignment mark.

Reference numeral 5 denotes a CCD type solid-state imaging device as an image pickup means, 6 denotes a light source for illumination, and 7 denotes a video signal processing means.
D-type solid-state imaging device 5, light source 6 for certification, and imaging device 5
Amplification, waveform shaping, A / D
And a processing unit 8 for performing processing such as conversion. Reference numeral 9 denotes a processing unit which performs various operations on the video data output from the unit 8 and comprises a computer 10. Reference numeral 11 denotes a display for displaying a captured image.

[0009]

By the way, when the alignment mark is made of an aluminum layer, there are many cases where the mark step is large in the past and the surface is largely rough and the mark becomes asymmetric. A detection method for imaging is employed. However, recently, cases where the mark step is reduced are increasing. This is because the cases where the film thickness of the aluminum wiring film or the like becomes thinner with the miniaturization are increasing.

Therefore, it is considerably difficult to detect edges with high accuracy. Further, it is not easy to detect edges with high accuracy even when the step of the alignment mark is considerably large, and there are many problems such as a decrease in the detection accuracy of the alignment mark when noise occurs.

FIGS. 6A to 6C show cases in which a problem occurs. The upper part shows the alignment mark and the lower part shows the waveform of the video signal.

FIG. 1A shows a case where the contrast is small. In this case, the amplitude of the video signal becomes small, and it becomes difficult to accurately detect the position. Insufficient contrast occurs when the color tone between the alignment mark and its base is close. That is, if the contrast is small, the resolution (resolution = K
₁ · λ / NA where K ₁ : constant, NA: numerical aperture number, λ: wavelength of light), the signal has a small amplitude, and it is difficult to distinguish it as if it were seen by the naked eye. .

FIG. 1B shows a case where the edge of the alignment mark is not clear. In this case, a video signal corresponding to the end of the mark is blurred.

That is, the ideal waveform is such that when the video signal changes from the background to the edge of the mark, the signal level rapidly changes (in this case, the level drops), and after the edge, the signal level slightly changes in the opposite direction. (In this case the level rises)
A waveform that changes in the opposite direction to the other edge of the mark (in this case, the level drops) when approaching the other edge of the mark, and suddenly returns to the original (corresponding to the background) level at the edge; In other words, the mark goes down once at the edge of the mark, but goes up after it, goes down again when approaching the other edge, and rises sharply when it reaches that edge. And the height of the peak is sufficient.

However, in practice, the height of the mountain is often not sufficient. When such an edge is not clear, the step of the alignment mark is large, so that a sufficient focus margin (DOF = K ₂
Λ / (NA) ² [μm] where K ₂ : constant, λ: wavelength of light beam, NA: numerical aperture cannot be obtained.

FIG. 2C shows a case where noise is generated. In this case as well, it is difficult to detect an edge due to noise on a video signal. Such noise causes severe roughening of a base or the like when aluminum wiring is formed mainly by a high-temperature aluminum forming process, and the roughening causes a severe change in signal level, and the change becomes noise and enters a video signal. It will be lost.

Therefore, assuming that the NA of the optical system has a certain value, the value is optimal in one case and an inappropriate value in another case even if highly accurate detection is possible. Therefore, there is a problem that the detection accuracy is lowered.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem. In an alignment mark detecting apparatus or an alignment mark detecting method for detecting an alignment mark based on a video signal picked up by an image pick-up means, there is provided a method for detecting an alignment mark. An object of the present invention is to prevent or reduce a decrease in alignment mark detection accuracy due to unclear edges, noise generation, and the like.

[0019]

SUMMARY OF THE INVENTION An alignment mark detecting apparatus according to the present invention performs alignment mark detection a plurality of times by changing an NA-variable aperture provided on an optical path for imaging a subject on an image pickup means. It is characterized in that an average of the results can be obtained.

Therefore, according to the alignment mark detection apparatus of the present invention, even if the accuracy of the measurement result under one NA is low, mark detection is performed a plurality of times by changing the NA, and the average value of the plurality of detection results is obtained. , A highly accurate detection result can be obtained. The alignment mark detection method of the present invention performs alignment mark detection a plurality of times by changing an NA-variable aperture provided in an optical path for imaging a subject on an imaging unit, obtains an average of a plurality of detection results, and uses the mark as a mark. It is characterized in that it is recognized as a detection position.

Therefore, according to the alignment mark detection method of the present invention, even if the accuracy of the measurement result under one NA is low, mark detection is performed a plurality of times by changing the NA, and the average value of the plurality of detection results is obtained. , A highly accurate detection result can be obtained.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to an optical path for imaging a subject, for example, a surface on which an alignment mark of a semiconductor wafer is visible on an imaging surface of an imaging means (for example, a CCD solid-state imaging device or the like). Is provided, a mark is detected a plurality of times by changing the NA, and an average value of a plurality of detection results (each result is represented as X, Y coordinates, for example) can be obtained. The average value is recognized as the position of the alignment mark.

[0023]

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

FIG. 1A is a schematic configuration diagram showing a first embodiment of the alignment mark detecting device of the present invention, FIG. 1B is a plan view showing an example of an alignment mark formed on the surface of a semiconductor wafer, and FIG. () Is a waveform diagram showing an ideal video signal obtained by imaging the alignment mark.

In FIG. 1, reference numeral 1 denotes a semiconductor wafer, and 2 denotes an alignment mark formed on the surface of the wafer 2 and has, for example, a planar shape shown in FIG. Reference numeral 3 denotes a lens that forms an image of a subject on an imaging surface of a solid-state imaging device (5) described later, which forms an optical system of the TV camera, and an aperture (12) described later.
Built-in. Reference numeral 4 denotes an index mark serving as an index for recognizing the position of the alignment mark 2, and the position of the semiconductor wafer 1 is corrected so that there is no deviation between the alignment mark 2 and the alignment mark 2 on the imaging surface.

Reference numeral 5 denotes an image pickup means, for example, a CCD type solid-state image pickup device. Reference numeral 6 denotes an illumination light source which irradiates the surface of the semiconductor wafer 2 through the lens 3. Reference numeral 7 denotes an image processing unit which includes a CCD solid-state imaging device 5, a processing unit 8 for performing processing such as amplification, waveform shaping, and A / D conversion on a video signal output from the CCD solid-state imaging device 5. Become. Reference numeral 10 denotes a control unit including a computer 9 for performing various operations on the video data output from the unit 8 (image processing unit 7), and reference numeral 11 denotes a display for displaying a captured image.

Reference numeral 12 denotes an aperture provided in an optical system for forming an image of a subject on the imaging surface of the solid-state imaging device 5 and is controlled by the computer 9. One of the features of the present alignment mark detecting device is that the aperture 12 is provided and the NA of the optical system is adjusted by the computer 9.

Next, the operation of the alignment mark detecting device shown in FIG. 1 will be described. The alignment mark detection device basically operates next. First, the illumination light source 6
The light emitted from the lens reaches the semiconductor wafer 1 via the lens 3 and enters the alignment mark 2. Then, the reflected light is incident on an imaging surface of a solid-state imaging device 5 of, for example, a CCD type as imaging means of the image processing unit 7 via the lens 3.
The reflected light becomes light that forms an image of the alignment mark 2 on the imaging surface by the lens 3. This light is converted by the imaging device 5 into an electric signal [ideally as shown in FIG. ] And the processing unit 8 performs an operation for alignment.

Then, the result of the calculation is processed by the computer 10 of the control section 9, and the exposure position of the semiconductor wafer 1 is controlled. Specifically, such calculation or the like determines the shift between the center of the alignment mark 2 and the center of the index mark 4 and adjusts the position of the semiconductor wafer 1 so that the shift becomes zero. Calculate the quantity.

Further, the present alignment mark detecting device is
An aperture 12 is provided so that the center of the mark can be accurately detected by an image signal obtained by the imaging device 5, that is, a signal forming a contrast between the light reflected by the mark and the light reflected by the base. , N of lens 3
A (numerical aperture) can be changed by the computer 10. Then, the position detection of the alignment mark 2 is repeated a plurality of times (for example, four times, of course, more or less times) by changing the NA, and an average value of the position detection results is obtained. ,
The average value can be determined as the detection position of the alignment mark 2. This is another characteristic of the present alignment mark detecting device.

FIGS. 2A to 2C show the case where the color tone of the base and the alignment mark 2 are similar and the contrast of the signal is small. FIG. 2A shows the mark shape, and FIG. (C) shows a change in the image signal when the NA is changed. If the contrast of the signal is small, the resolution will be reduced, and the boundary between the mark 2 and its base will be difficult to identify, and as a result, the position detection accuracy tends to be reduced. In particular, the tendency is strong when the aperture is strong and the NA is small. Therefore, if the NA is in a small state and the position is recognized as a detected position based on the position detection result in that state, the position detection becomes very low in accuracy.

However, instead of recognizing the detected position based on the position detection result by one NA, the position detection is repeated a plurality of times by changing the NA, and the mark center position of the alignment mark 2 is obtained for each NA. The average value of the coordinate data of the position is obtained, and the average value is used as the coordinate of the mark center position, and the deviation between this and the coordinate of the center position of the index mark 4 is calculated. It is possible to eliminate the possibility that a large alignment error is generated by recognizing the position of the alignment mark 2 based on one detection result having a large amount of error, and improve alignment accuracy.

FIGS. 3A to 3C show a case where a step between the alignment mark 2 and its base is large.
(A) shows a mark shape, (B) shows an image signal,
(C) shows a change in the image signal when the NA is changed.

When the step is large as described above, since a sufficient DOF (depth of focus) cannot be obtained, the edge of the mark is blurred, and the position detection accuracy tends to decrease. In particular, the tendency is strong when NA is large. Therefore, if the NA is in a large state and the position is recognized as the detected position based on the position detection result in that state, the position detection becomes very low in accuracy.

However, instead of recognizing the position as a detected position based on the result of position detection by one NA, the NA is changed and position detection is repeated a plurality of times.
Each time the center position of the alignment mark 2 is obtained, the average value of the coordinate data at that position is obtained, and the average value is used as the coordinate of the center position of the mark.
When the deviation from the coordinates of the center position is calculated, the possibility that a large alignment error is generated by recognizing the position of the alignment mark 2 with one detection result having a large error in a state of insufficient contrast, which has occurred conventionally, is eliminated. And alignment accuracy can be improved.

This can be explained as follows. When the video signal corresponding to the edge of the mark is blurred, the focus margin (DOF) is increased by reducing the NA to improve the appearance of the edge of the alignment mark. , A highly accurate data component can be added to the data to improve the detection accuracy.

Originally, ideally, the level of the video signal suddenly drops when the edge of the mark changes from the background, and the signal level slightly increases after the edge, and again when approaching the other edge of the mark. It is preferable that a waveform change occurs such that the level decreases and suddenly returns to the original level at the edge. In other words, there is a high peak at the corresponding part in the mark, which goes down one end at the edge of the mark but rises after it, goes down again when approaching the other edge, and rises rapidly when it reaches that edge. It is desirable that the height of the peak is sufficiently high. Therefore, if the NA is reduced, the focus margin becomes larger, the height of the ridge generated in a portion corresponding to the mark becomes higher, and a waveform with a clear edge is obtained, so that the accuracy becomes higher. Therefore, the position detection accuracy of the alignment mark is improved. Therefore, it is possible to eliminate the possibility that the detection accuracy, which occurs when the position is recognized only by the result of the position detection by one NA, becomes extremely low.

FIGS. 4A to 4C show the case where the background of the alignment mark 2 is extremely rough and the image signal is noisy. FIG. 4A shows the mark shape, and FIG. 4B shows the image signal. (C) shows a change in the image signal when the NA is changed. In this case, detection of edges tends to be difficult due to noise on the video signal. The cause of the noise is mainly caused by severe roughening of the base when aluminum wiring is formed by a high-temperature aluminum forming process.
The tendency is strong when NA is small. Because NA
This is because if the image size is small and the background is in focus, the imaging information of the background, which is a source of noise, is tapped. Therefore, if the position is recognized only by the result of the position detection by one NA, if the NA is large, the position recognition result with reduced accuracy is generated due to noise. However, the position detection by changing the NA is repeated a plurality of times. When the average value is obtained, the position detection result (the image pickup information on the background is reduced and the noise is reduced) in the state where the NA is large is included in the position detection data, so that the alignment mark detection accuracy is improved. .

As described above, there is a case where a larger NA is originally better depending on the alignment mark and its base, and a case where a smaller NA is better. However, in any case, the NA is changed plural times. If the position detection is repeated and the average value of the position detection results is obtained, there is no possibility that a low-accuracy position detection result will occur, and the position of the alignment mark can be detected with relatively high accuracy.

[0040]

According to the alignment mark detecting device of the present invention, even if the accuracy of the measurement result under one NA is low,
Since the mark detection is performed a plurality of times by changing the NA and the average value of the plurality of detection results can be obtained, a highly accurate detection result can be obtained. According to the alignment mark detection method of the present invention, even if the accuracy of the measurement result under one NA is low, mark detection is performed a plurality of times by changing the NA,
Since the average value of the plurality of detection results is obtained, a highly accurate detection result can be obtained.

[Brief description of the drawings]

FIGS. 1A to 1C are diagrams for explaining a first embodiment of an alignment mark detecting device according to the present invention;
(A) is a schematic configuration diagram, (B) is a plan view of an alignment mark, and (C) is a waveform diagram showing an ideal video signal by the alignment mark.

FIGS. 2A to 2C show the case where the color tone of the base and the alignment mark are similar and the contrast of the signal is small, FIG. 2A shows the mark shape, and FIG. (C) shows a change in the image signal when the NA is changed.

FIGS. 3A to 3C show a case where a step between an alignment mark and its base is large, FIG. 3A shows a mark shape, FIG. 3B shows an image signal, and FIG. NA
5 shows a change in an image signal when the image signal is changed.

4 (A) to 4 (C) show a case where the base of the alignment mark is extremely rough and the image signal has much noise, FIG. 4 (A) shows a mark shape, and FIG. 4 (B) shows an image signal. , (C) show changes in the image signal when the NA is changed.

FIGS. 5A and 5B are diagrams for explaining a conventional example of an alignment mark detection device, in which FIG. 5A is a schematic configuration diagram, and FIG. 5B is an example of an alignment mark formed on a semiconductor wafer; (C) is a waveform diagram showing an ideal video signal by an alignment mark.

FIGS. 6A to 6C are explanatory diagrams showing different cases of the problem to be solved by the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Subject (wafer), 2 ... Alignment mark, 3 ... Optical system, 5 ... Imaging means (CCD type solid-state imaging device), 7 ... Video signal processing means, 12 ... Aperture.

Claims (2)

[Claims] 1. An alignment mark detecting device for capturing an image of a subject having an alignment mark by an image pickup means and processing a video signal output from the image pickup means by a video signal processing means to detect a position of the alignment mark. An optical path for forming an image on the imaging surface of the imaging means, a diaphragm capable of controlling the NA (numerical aperture) by the video signal processing means, performing alignment mark position detection by changing the NA a plurality of times, Alignment mark detection apparatus characterized in that an average value of a plurality of position detection results is obtained. 2. An alignment mark detecting method, wherein an object having an alignment mark is imaged by an imaging means, and a video signal output from the imaging means is processed by a video signal processing means to detect a position of the alignment mark. A stop whose NA (numerical aperture) can be controlled by the video signal processing means is provided on an optical path that forms an image on the imaging surface of the imaging means. Alignment mark detection method characterized by determining an average value of the position detection results of each time and using the average value as the detection position of the alignment mark