JP3615988B2 - Focus correction method for scanning microscope and scanning microscope - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a surface image of a sample such as a semiconductor wafer having a plurality of regions (dies) having the same shape corresponding to a semiconductor chip on the surface, and an optical part constituted by a microscope and the sample relative to each other. The present invention relates to a scanning microscope generated by performing scanning to be moved and a focus correction method thereof.
[0002]
[Prior art]
In semiconductor manufacturing, as shown in FIG. 1, the same circuit pattern is formed on a plurality of regions (dies) 101 corresponding to semiconductor chips on a semiconductor wafer 100, and an electrical test is performed by a tester. Each die is separated and attached to a lead frame or the like. Usually, several hundred dies 101 are formed on the semiconductor wafer 100. In a semiconductor manufacturing process, a die is made by forming various layers of various circuit patterns on the surface of a semiconductor wafer. Semiconductor manufacturing consists of several hundred processes, and it takes a long time from the start of manufacturing to completion. Therefore, even if a process failure occurs in the initial process, the defect is discovered when an electrical test is performed by a tester. As a result, not only is it wasted, but also the problem is that the planned production quantity cannot be obtained. Therefore, quality control is performed by observing the surface of the semiconductor wafer at the time when the main processes of semiconductor manufacturing are completed, examining defects in the formed pattern, and feeding back the information. Optical microscopes are widely used. An apparatus that generates a surface image of a semiconductor wafer and finds a defective portion is called an inspection machine, and this term is also used here.
[0003]
In the inspection machine, an optical image is converted into an image signal by a TV camera or a line sensor, and the presence or absence of a defect is determined by processing the image signal. In recent years, semiconductor devices have been increasingly integrated, and circuit patterns have been miniaturized accordingly, and inspection machines are required to generate high-resolution images. Therefore, a high resolution image signal is obtained by scanning with a highly integrated line sensor. Scanning is usually performed by moving a sample placed on a stage at a constant speed. When an image of one die 101 is generated by one scan and an image with sufficient resolution cannot be obtained, two scans 102 and 103 are performed as shown in FIG. In general, a defective portion is found by comparing image data of corresponding portions of adjacent dies. In FIG. 1, after performing a scan 102 to inspect the lower half of each die, a scan 103 is performed to inspect the upper half of each die.
[0004]
FIG. 2 is a diagram illustrating a basic configuration of the inspection machine. As shown in FIG. 2, a semiconductor wafer 100 as a sample is held by a Z stage 12 that moves in the vertical direction. The Z stage 12 is held by an XY stage 11 that moves in two horizontal axes. The movement amount of the XY stage 11 and the Z stage 12 is precisely controlled by a stage movement control device 16. The inspection device 13 performs processing such as an optical microscope that forms an enlarged surface image of the semiconductor wafer 100, an image sensor that converts the image into an electrical signal, the presence / absence of a defect by processing the image signal, its position, and defect classification. It has an image processing device to perform. Detailed description of the configuration of this part is omitted.
[0005]
In order to form a good image with an optical microscope, the surface of the semiconductor wafer 100 needs to be focused. The amount of deviation from the focus is detected by the focus sensor 14 combined with the optical microscope, and the focus signal control circuit 15 processes the output of the focus sensor 14 to generate a focus signal (Z signal) indicating the amount of deviation from the focus. Output to the stage drive controller 16. The stage drive control device 16 controls the movement of the Z stage 12 based on the Z signal. The stage drive control device 16 controls the XY stage 11 to move at a predetermined speed during scanning. With the above configuration, an autofocus device (autofocus mechanism: AF mechanism) is configured that is controlled so that the focus of the optical microscope automatically matches the surface of the semiconductor wafer. Even when scanning is performed by the AF mechanism, the surface of the semiconductor wafer is always in focus and a good image signal is generated.
[0006]
As a method for detecting the focus state of an optical microscope, an optical method that can detect the focus state without contacting the surface of the semiconductor wafer is generally used. As the method, a knife edge method, an astigmatism method, Various methods such as a confocal method have been proposed, and any method can be applied in the present invention, and the focus detection method is not limited. Further, the focus position can be adjusted and scanned by fixing the sample and moving the optical microscope of the inspection apparatus 13.
[0007]
[Problems to be solved by the invention]
Various patterns are formed in a number of layers on the surface of the semiconductor wafer. As shown in FIG. In the optical method, an auxiliary beam for focus detection is irradiated as a spot on a semiconductor wafer, and the focus state is detected from the auxiliary beam reflected by the surface. If the surface is uneven as shown in FIG. 3, the Z signal fluctuates due to the influence, making it difficult to detect an absolute focus state.
[0008]
Therefore, as shown in FIG. 4A, height information is obtained at a plurality of specific reliable measurement positions 104 on the semiconductor wafer 100 in advance, and as shown in FIG. Thus, the height distribution of the entire surface of the semiconductor wafer 100 is calculated, and the focal position is adjusted according to the height distribution. The specific measurement position 104 is, for example, a portion between dies that is known to be a flat surface or a specific portion in the die (such as an electrode pad).
[0009]
However, in order to calculate a highly accurate height distribution, it is necessary to measure the height at a large number of measurement points over the entire surface of the semiconductor wafer 100. For this purpose, it is necessary to scan the entire surface of the semiconductor wafer 100, which causes a problem that throughput is reduced.
Also, the specific measurement position 104 is a part that is known to be a flat surface, but when there is no such part in the die, the measurement can be limited to the part between the dies, If it cannot be arbitrarily selected and the interpolation method is used, there is a problem that the interpolation accuracy is lowered.
[0010]
The present invention aims to solve such a problem, and is capable of focusing with high throughput and high accuracy without limiting the measurement points without scanning and measuring the entire sample surface. It aims at realization of the focus correction method of a microscope, and a scanning microscope.
[0011]
[Means for Solving the Problems]
The focus correction method for a scanning microscope according to the present invention focuses on the fact that a plurality of regions (dies) formed on a sample have the same shape, and scans one of the plurality of regions as a reference region. The focus signal at this time is stored as template data, and control is performed so that the same focus signal as the template data is obtained at the corresponding scanning position when scanning is performed to generate a normal image.
[0012]
That is, the focus correction method for a scanning microscope according to the present invention scans the surface of a sample on which a plurality of regions having the same shape are formed, generates a surface image of the sample, and outputs a focus signal indicating a focus state. A focus correction method for a microscope, wherein one of a plurality of regions is scanned with the focus state of the scanning microscope set to a desired state, and a focus signal that changes with the scan is stored as template data. When scanning the surface, template data corresponding to scanning is read for each region, a difference between the focus signal and the template data is obtained, and the focus state of the scanning microscope is corrected so as to eliminate the difference. .
[0013]
The scanning microscope of the present invention is a scanning microscope that generates a surface image of a sample by scanning the surface of the sample on which a plurality of regions having the same shape are formed. A scanning mechanism that outputs a scanning position signal, a focus adjustment mechanism that changes a focus state of the scanning microscope, a focus state detector that outputs a focus signal indicating the focus state of the scanning microscope, and a plurality of regions When one is scanned with the focus state of the scanning microscope in a desired state, a template storage unit that stores a focus signal that changes with the scanning as template data, and when scanning the surface of the sample, The template data is read corresponding to the scanning position, the read template data is compared with the focus signal, and the focus adjustment is made so that there is no difference between the two signals. Characterized in that it comprises a comparator for generating a focus control signal for changing the mechanism.
[0014]
A plurality of regions (dies) formed on the sample (semiconductor wafer) have the same shape. Therefore, if the sample height is the same, the focus signal when scanning each region should be the same. If the sample height is different, the focus signal when scanning each region is the sample height. Only the difference should be different. Therefore, if the focus signal when scanning each region is controlled to be the same as the focus signal when scanning the reference region, scanning is performed in the same focus state as when scanning the reference region.
[0015]
According to the present invention, as shown in FIG. 4, instead of scanning the entire surface and measuring the height, it is only necessary to scan one area and store the template data, and the throughput is improved. In the method shown in FIG. 4, the height is measured only on a portion between dies that is known to be a flat surface, an electrode pad in the die, and the like. There is no such limitation, and high-precision focusing is possible even if the height of the sample changes within the region.
[0016]
The reference area may be arbitrarily selected. For example, the first area when scanning for image generation is selected. In this case, if there is a change in the height of the sample in the reference area, it will be stored as template data including that change, but each area is usually smaller than the entire surface of the sample, The amount of sample height change is small and does not matter. In addition, when performing inspection by comparing image data between regions, it is preferable to compare images generated in the same focus state, and even if there is a change in the height of the sample in the reference region, a big problem is Does not occur.
[0017]
If the reference region is a region where there is no height change or is very small, for example, the sample is scanned to find a region with a small height change, and that region is set as the reference region. Usually, since a region having the same shape is formed on a very large number of samples (semiconductor wafers), if a reference region of a sample is stored as template data, other samples can be used. Therefore, even if scanning is performed on one sheet to search for an area having a small height change and the template data is stored, the decrease in throughput is negligibly small as a whole.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 5 is a diagram showing an overall configuration of an inspection machine using the scanning microscope according to the embodiment of the present invention. As shown in the figure, the inspection machine of this embodiment has a configuration similar to the conventional configuration of FIG. 2 except that a template storage circuit 17 and a template comparison circuit 18 are provided. The XY stage 11, the Z stage 12, the stage drive control device 16, the inspection device 13, the focus sensor 14, and the focus signal control circuit 15 are the same as the conventional ones. It should be noted that, as in the conventional case, these elements have various modifications, and it goes without saying that these modifications can also be used in this embodiment. Only differences from the conventional example will be described below.
[0019]
The focus signal control circuit 15 processes a focus signal (Z signal) indicating the focus state of the inspection apparatus 13 detected by the focus sensor 14 and outputs a digitized focus signal (Z signal). The template storage circuit 17 stores a digital focus signal (digital / Z signal) when the designated reference area is scanned in correspondence with a signal indicating a position in the area output by the stage drive control device 16. The stage drive controller 16 moves the XY stage 11 at a constant speed during scanning, and outputs a signal indicating the amount of movement from the start point of each region.
[0020]
In this embodiment, as shown in FIG. 6, a large number of dies 101 having the same shape are formed on a semiconductor wafer 100, and an image of each die is generated by dividing into four scans. Compare images. For efficient scanning and inspection, the first region of each die is continuously scanned to produce an image and the images of adjacent dies are compared. In this case, the scanning direction is reversed in adjacent die rows. When the inspection of the first area is completed, the second area is scanned, and this is continued until the fourth area is scanned. In this embodiment, after scanning from the lower left die 111 in the horizontal direction, the next row of dies is scanned in the reverse direction.
[0021]
In this embodiment, the lower left die 111 is set as a reference area, and four areas are scanned as shown in the figure before starting image generation, and stored in correspondence with the position signal output from the stage drive control device 16.
FIG. 7 is a flowchart showing template data storage processing. In step 201, a reference area is selected. As shown in FIG. 6, in the present embodiment, the lower left die 111 is selected as the reference region. In step 202, the operator adjusts the position of the Z stage 12 so that the focus position is optimal with respect to the reference area. In step 203, the reference area (die 111) is scanned while maintaining the position of the Z stage 12 adjusted in step 202, and the digital focus signal output from the focus signal control circuit 15 is output to the stage drive control device 16. Corresponding to the position signal to be stored, it is stored in the template storage circuit 17 as template data.
[0022]
FIG. 8 is a flowchart showing an operation related to focus control when an image is generated and inspected. In step 211, scanning is started. When scanning is started, in step 212, template data corresponding to the position signal output from the stage drive control device 16 is read from the template storage circuit 17. At this time, for example, when scanning is performed in the reverse direction, reading from the template storage circuit 17 is also performed in the reverse direction.
[0023]
In step 213, the template comparison circuit 18 compares the digital Z signal output from the focus signal control circuit 15 with the signal read from the template storage circuit 17. Further, in step 213, the template comparison circuit 18 calculates the difference between the two signals, and if there is a difference, outputs a signal for moving the Z stage 12 so as to eliminate the difference to the stage drive control circuit 16, and in step 215. The stage drive control circuit 16 outputs a signal for moving the Z stage 12 in response to this signal. If there is no difference, the position of the Z stage 12 in that state is maintained. The above operation is continued until the scanning is completed. While performing the above operations, the inspection apparatus 13 performs a comparative inspection between adjacent die images while generating a surface image of each region.
[0024]
It is possible to perform all of the operations in steps 213 to 215 by digital processing. For example, the focus signal control circuit 15 outputs an analog focus signal, and the analog focus signal is output to the template storage circuit 17. An A / D converter for converting to a digital signal and a D / A converter for converting the template data read from the memory to an analog signal are provided, and the difference signal generated by analog operation using the template comparison circuit 18 as an analog subtraction circuit. Can be output to the stage drive control circuit 16.
[0025]
9 and 10 are diagrams illustrating examples of template data, a focus signal (Z signal), and a Z stage control signal in the embodiment. As shown in FIG. 9A, it is assumed that the Z signals when the four regions 121 to 124 of the reference region (die 111) are scanned are signals as shown in FIG. 9B, respectively. Four signals shown in FIG. 9B are stored as template data.
[0026]
FIG. 10 is a diagram for explaining signals when the template data shown in FIG. 9B is stored and then a surface image of each region is generated and a comparative inspection is performed. Here, the case where the 1st area | region of each area | region is scanned is shown. As shown in FIG. 10A, the first region of each region (die) 101 is sequentially scanned. The three dies 101 shown in the figure do not necessarily belong to the same column, and may belong to different columns.
[0027]
Assume that the focus signal (Z signal) when scanning three dies is a signal as shown in FIG. A template signal such as (C) stored corresponding to each Z signal is read and compared. As a result, a signal such as (D) is generated as the Z stage control signal, and the position of the Z stage 12 is controlled so that the same signal as the template data is obtained.
[0028]
FIG. 10 shows an example of a signal when the height of the sample is constant in each die, but this is not always the case. FIG. 11 is a diagram for explaining an example of the focus signal (Z signal) and the Z stage control signal when the height of the sample changes in each die. Assume that the height of the sample changes as shown in FIG. In this case, the Z signal changes as shown in (B). Since the template signal is a signal as shown in (C), when the difference is calculated, it changes as shown in (D), and this becomes a Z stage control signal.
[0029]
The reference area may be arbitrarily selected. In the embodiment, the lower left die 111, which is the first area when scanning for image generation, is selected. If there is a change in the height of the sample in the die 111, it is stored as template data including the change, but each region is usually smaller than the entire surface of the sample, The amount of height change is small and does not matter. In addition, when performing inspection by comparing image data between regions, it is preferable to compare images generated in the same focus state, and even if there is a change in the height of the sample in the reference region, a big problem is Does not occur.
[0030]
When the height distribution of the reference area is known in advance, when the template area is stored by scanning the reference area, control is performed so that an optimum focus state is obtained according to the height distribution known in advance. It is also possible to scan and store template data. If this is the case, even if there is a height distribution in the reference area, template data in focus can be stored. In order to store the template data by such a method, a storage circuit for storing the height distribution data along the scanning trajectory in the reference area is provided in the configuration of FIG. 5, and the reference area is scanned to store the template data. At this time, the stage drive control device 16 reads this data in accordance with the scanning position and controls the height of the Z stage 12.
[0031]
Usually, since a region having the same shape is formed on a very large number of samples (semiconductor wafers), if a reference region of a sample is stored as template data, other samples can be used. In this case, several samples are scanned to search for a region having a small height change, and the template data is stored using the region as a reference region, and is used in common for a plurality of samples. In this case, even if scanning is performed to store the template data and an area having a small height change is searched for, a decrease in throughput is negligibly small as a whole.
[0032]
In addition, there are various variations in scanning for comparison between dies. For example, in the example of FIG. 1, the lower region of all the dies 101 of the semiconductor wafer 100 is compared by scanning 102, and then the upper region of all the dies is compared by scanning 103. However, as shown in FIG. 12, after performing scans 121, 122, 123, and 124 to compare the same region for all dies in each column, the next column may be scanned and compared in the same manner. In such a case, the present invention can be similarly applied.
[0033]
【The invention's effect】
As described above, according to the present invention, the focus of a scanning microscope that can focus with high throughput and high accuracy without scanning and measuring the entire surface of the sample surface and without limiting the measurement points. A correction method and a scanning microscope are realized.
[Brief description of the drawings]
FIG. 1 is a view showing a semiconductor wafer in which a plurality of regions (dies) are formed.
FIG. 2 is a diagram showing a configuration of a conventional example of an inspection machine for inspecting an optical image of a die formed on a semiconductor wafer.
FIG. 3 is a cross-sectional view of a semiconductor wafer having a pattern formed in multiple layers.
FIG. 4 is a diagram illustrating an example in which the height distribution of the entire surface is obtained by measuring the height of a plurality of points on a semiconductor wafer.
FIG. 5 is a diagram showing a configuration of a semiconductor wafer inspection machine according to an embodiment of the present invention.
FIG. 6 is a diagram for explaining scanning for obtaining a reference area and template data on a semiconductor wafer in an embodiment.
FIG. 7 is a flowchart showing processing for storing template data in the embodiment.
FIG. 8 is a flowchart illustrating a focus control process when performing die image generation and comparative inspection in the embodiment.
FIG. 9 is a diagram illustrating an example of template data in the embodiment.
FIG. 10 is a diagram illustrating a signal at the time of focus control processing when there is no height change in the die when performing die image generation and comparative inspection in the embodiment.
FIG. 11 is a diagram illustrating a signal at the time of focus control processing when there is a height change in the die when performing die image generation and comparative inspection in the embodiment.
FIG. 12 is a diagram illustrating another scanning example for die pattern comparison.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... XY stage 12 ... Z stage 13 ... Inspection apparatus 14 ... Focus sensor 15 ... Focus signal control circuit 16 ... Stage drive control device 17 ... Template memory circuit 18 ... Template comparison circuit 100 ... Sample (semiconductor wafer)
101 ... area

Claims (2)

A scanning microscope focus correction method for scanning a surface of a sample on which a plurality of regions having the same shape are formed to generate a surface image of the sample and outputting a focus signal indicating a focus state,
Scanning one of the plurality of regions with a desired focus state of the scanning microscope, and storing the focus signal that changes with the scan as template data,
When scanning the surface of the sample, the template data corresponding to scanning is read for each region, and the difference between the focus signal and the template data is obtained.
A focus correction method for a scanning microscope, wherein the focus state of the scanning microscope is corrected so as to eliminate the difference. A scanning microscope that generates a surface image of the sample by scanning the surface of the sample on which a plurality of regions having the same shape are formed,
A scanning mechanism that performs scanning and outputs a scanning position signal indicating a scanning position;
A focus adjustment mechanism for changing the focus state of the scanning microscope;
A focus state detector that outputs a focus signal indicating the focus state of the scanning microscope;
A template storage unit that stores, as template data, the focus signal that changes with scanning when one of the plurality of regions is scanned with the scanning microscope in a desired focus state;
When scanning the surface of the sample, the template data is read in correspondence with the scanning position of each region, the read template data is compared with the focus signal, and the focus is adjusted so that there is no difference between the two signals. A scanning microscope comprising: a comparator that generates a focus control signal for changing an adjustment mechanism.

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