Overlay mark image quality measurement method and device using signal shape index, and method for optimizing measurement options of overlay measurement device by using same

Abstract

The present invention relates to an overlay mark image quality measurement method for measuring the quality of an overlay mark image including periodic pattern elements with a designed spatial frequency, and provides an overlay mark image quality measurement method using a signal shape index, comprising the steps of: a) acquiring an overlay mark image; b) selecting a measurement region including the periodic pattern elements from the overlay mark image; c) obtaining a periodic graph representing the measurement region; d) changing the periodic graph into a frequency domain graph; e) finding an amplitude value of a first frequency component having a first spatial frequency in the frequency domain graph; f) finding an amplitude value of a second frequency component having a second spatial frequency, which differs from the first spatial frequency in the frequency domain graph; g) calculating a signal shape index on the basis of the amplitude value of the first frequency component and the amplitude value of the second frequency component; and h) evaluating the quality of the overlay mark image on the basis of the signal shape index.

Description

A method and device for measuring overlay mark image quality using a signal shape index, and a method for optimizing measurement options of an overlay measurement device using the same.

[0001] The present invention relates to a method and apparatus for measuring the quality of an overlay mark image, and more particularly, to a method and apparatus for measuring the quality of an overlay mark image having periodic pattern elements using a new quality metric called a 'signal shape index'.

[0002] Multiple pattern layers are sequentially formed on a semiconductor substrate. Furthermore, through processes such as double patterning, a single layer of circuitry can be divided into two patterns. These pattern layers, or multiple patterns within a single layer, must be precisely formed at predetermined locations to manufacture the desired semiconductor device.

[0003] Therefore, to ensure that the pattern layers are aligned accurately, overlay marks that are formed simultaneously with the pattern layers are used.

[0004] The method for measuring overlay errors using overlay marks is as follows. First, a structure that is part of the overlay mark is formed simultaneously with the pattern layer formed in a previous process, such as an etching process. Then, in a subsequent process, such as a photolithography process, the remaining structure of the overlay mark is formed on photoresist.

[0005] And, through the overlay measurement device, an image of the overlay structure of the pattern layer formed in the previous process (an image is obtained by penetrating the photoresist layer) and the overlay structure of the photoresist layer are obtained, and an offset value between the centers of these images is measured to measure the overlay error.

[0006] More specifically, Japanese Patent Application Laid-Open No. 2020-112807 discloses a method for capturing an image of an overlay mark formed on a substrate, selecting a plurality of working zones from the captured image, forming a signal having information about each of the selected working zones, and comparing them to determine relative misalignment between different layers or different patterns.

[0007] Fig. 1 is a plan view of an example of an overlay mark. The overlay mark (1) illustrated in Fig. 1 has four working zone sets (4, 5, 6, 7). Each working zone set (4, 5, 6, 7) has two working zones that are arranged diagonally from each other. Each working zone set (4, 5, 6, 7) is used to measure an overlay error in the X-axis or Y-axis direction of a pattern layer formed together with the corresponding working zone set. In order to prevent interference from occurring, structures (2) formed together with the first pattern layer and structures (3) formed together with the second pattern layer are arranged so as not to overlap each other.

[0008] Each working zone includes bars (periodic pattern elements) arranged at regular intervals from the center of the overlay mark (1) to the periphery of the overlay mark (1). Therefore, using an overlay measurement device, periodic signals, as illustrated in Fig. 2, can be obtained from two working zones belonging to a set of working zones (4, 5, 6, 7). The graph of Fig. 2 can be obtained, for example, from a selected portion (8) of Fig. 1.

[0009] In the graph of Fig. 2, peaks appear in areas where bars are arranged. Since the overlay mark (1) in Fig. 1 has bars arranged periodically, the acquired signal also has periodicity. Furthermore, the overlay error is measured through correlation analysis of two periodic signals acquired from two selected regions (8, 8').

[0010] In the graph of Fig. 2, the X-axis may be the X-coordinate values ​​of pixels in a selected area (8), and the Y-axis may be the average of the gray values ​​of the pixels.

[0011] To accurately measure overlay errors using the above-described method, an imaging system (overlay measurement device) capable of acquiring (capturing) high-quality overlay mark images and an optimal recipe are required. An optimal recipe may include the optimal focus position, optimal wavelength of illumination, and other factors. Furthermore, to evaluate the imaging system and recipe, the quality of the overlay mark images acquired through them must be assessed.

[0012] For example, if the change in image quality metric according to focus position and wavelength of illumination is displayed as a landscape map in which the X-axis represents focus position, the Y-axis represents wavelength, and the Z-axis represents quality metric, the optimal focus position and wavelength of illumination can be easily selected.

[0013] Various image quality metrics are used to evaluate image quality, including MTF (Modulation Transfer Function), MSE (Mean Squared Error), and RMS (Root Mean Square).

[0014] MTF is a metric used to evaluate how faithfully an imaging system transfers contrast from a subject to the final image. MTF is typically plotted as a function of spatial frequency, representing the contrast transfer ratio (CTR). Spatial frequency refers to the number of repetitions of a single pattern (e.g., one black line and one white line of equal width) within a unit distance. Contrast transfer ratio is the ratio of the contrast of the final image to the contrast of the subject.

[0015] Figure 3 shows examples of various spatial frequencies. The pattern shown in (a) of Figure 3 has the highest spatial frequency, and the pattern shown in (c) has the lowest spatial frequency.

[0016] Figure 4 is a graph showing the change in MTF according to spatial frequency. In the graph of Figure 4, the X-axis represents spatial frequency. Low frequencies indicate large patterns, and high frequencies indicate fine patterns. The Y-axis represents the contrast transmission ratio, ranging from 0 to 1, where 1 indicates perfect contrast reproduction.

[0017] As shown in Figure 4, MTF values ​​decrease as spatial frequency increases. In other words, MTF values ​​depend not only on the quality of the overlay mark image but also on the pitch of the target pattern. Therefore, it is difficult to use MTF to index changes in focus position or light source wavelength. Furthermore, a high MTF does not guarantee high-precision overlay error measurements. MTF is a theoretical evaluation method and does not accurately reflect performance in real-world environments.

[0018] MSE is another quality metric used to assess image quality. MSE is calculated by squaring the pixel value differences between the original and reconstructed images, then averaging this value across all pixels. MSE is used to measure the difference between the reconstructed image and the original. Generally, a lower MSE value indicates better image quality.

[0019] Root mean square root (RMS) is a similar concept to MSE, taking the square root of the MSE value. Generally, a lower RMS value indicates better image quality.

[0020] Although MSE and RMS are useful tools for evaluating the performance of optical systems, they have limitations, just like MTF.

[0021] [Prior Art Literature]

[0022] (Patent Document 1) Japanese Patent Publication No. 2020-112807

[0023] (Patent Document 2) Korean Patent Publication No. 10-2019-0011821

[0024] (Patent Document 3) Korean Patent Publication No. 10-2021-0034038

[0025] The present invention aims to provide a method and apparatus for measuring the quality of an overlay mark image having periodic pattern elements using a new quality metric called a signal shape index.

[0026] In addition, the present invention aims to provide a method for optimizing measurement options of an overlay measurement device by using a method for measuring the quality of an overlay mark image.

[0027] In order to achieve the above-described object, the present invention provides an overlay mark image quality measuring method for measuring the quality of an overlay mark image having periodic pattern elements having a designed spatial frequency, the method comprising the steps of: a) acquiring an overlay mark image; b) selecting a measurement area including the periodic pattern elements in the overlay mark image; c) obtaining a periodic graph representing the measurement area; d) converting the periodic graph into a frequency domain graph; e) finding an amplitude value of a first frequency component having a first spatial frequency in the frequency domain graph; f) finding an amplitude value of a second frequency component having a second spatial frequency different from the first spatial frequency in the frequency domain graph; g) calculating a signal shape index based on the amplitude value of the first frequency component and the amplitude value of the second frequency component; and h) performing a quality evaluation on the overlay mark image based on the signal shape index.

[0028] In addition, the step g) provides a method for measuring the quality of an overlay mark image using a signal shape index, which is a step of calculating a value obtained by subtracting the amplitude value of the second frequency component from the amplitude value of the first frequency component as a signal shape index.

[0029] In addition, the above periodic pattern elements provide a method for measuring the quality of an overlay mark image using a signal shape index, which are periodic elements of a moire pattern.

[0030] In addition, the above periodic pattern elements are periodic elements of a moire pattern, and the step h) provides a method for measuring the quality of an overlay mark image using a signal shape index, wherein the step is a step of evaluating that the quality of the overlay mark image is better as the signal shape index is positive and has a larger size.

[0031] In addition, the step h) provides a method for measuring the quality of an overlay mark image using a signal shape index, which is a step of evaluating the quality of the overlay mark image based on the signal shape index, the amplitude value of the first frequency component, and the amplitude value of the second frequency component.

[0032] In addition, the first spatial frequency is the designed spatial frequency, and the second spatial frequency is a method for measuring the quality of an overlay mark image using a signal shape index that is twice the designed spatial frequency.

[0033] In addition, the present invention provides a method for measuring the quality of an overlay mark image using a signal shape index, wherein the first spatial frequency is a spatial frequency having a smaller frequency value among the two spatial frequencies having the largest amplitude values ​​in the frequency domain graph, and the second spatial frequency is a spatial frequency having a larger frequency value among the two spatial frequencies having the largest amplitude values ​​in the frequency domain graph.

[0034] In addition, a method for measuring the quality of an overlay mark image using a signal shape index is provided, which is a projection step of obtaining a periodic graph representing a change in intensity value according to a change in the first direction coordinate value of the measurement area by obtaining an average or sum of intensities of pixels having the same first direction coordinate value of the measurement area having periodicity in the first direction.

[0035] In addition, the present invention provides an overlay mark image quality measuring device for measuring the quality of an overlay mark image having periodic pattern elements having a designed spatial frequency, comprising: an image detector for acquiring the overlay mark image; The present invention provides an overlay mark image quality measurement device using a signal shape index, comprising a controller capable of communicating with the image detector, the controller configured to perform the steps of: a) acquiring an overlay mark image; b) selecting a measurement area including the periodic pattern elements from the overlay mark image; c) projecting the measurement area to obtain a periodic graph representing the measurement area; d) converting the periodic graph into a frequency domain graph; e) finding an amplitude value of a first frequency component having a first spatial frequency in the frequency domain graph; f) finding an amplitude value of a second frequency component having a second spatial frequency different from the first spatial frequency in the frequency domain graph; g) calculating a signal shape index based on the amplitude value of the first frequency component and the amplitude value of the second frequency component; and h) performing a quality evaluation on the overlay mark image based on the signal shape index.

[0036] In addition, the step g) provides an overlay mark image quality measurement device using a signal shape index, which is a step of calculating a value obtained by subtracting the amplitude value of the second frequency component from the amplitude value of the first frequency component as a signal shape index.

[0037] In addition, the above periodic pattern elements are periodic elements of a moire pattern, and the step h) provides an overlay mark image quality measurement device using a signal shape index, which is a step of evaluating that the quality of the overlay mark image is superior as the signal shape index is positive and has a larger size.

[0038] In addition, the present invention provides a method for optimizing measurement options of an overlay measurement device by comparing the quality of an overlay mark image according to measurement options, the method comprising: forming an overlay mark having periodic pattern elements having a designed spatial frequency; and measuring the quality of the overlay mark image according to a combination of the measurement options, wherein the step of measuring the quality of the overlay mark image is a step performed according to the above-described overlay mark image quality measurement method.

[0039] According to the present invention, the quality of an overlay mark image can be measured using a new quality metric called a signal shape index.

[0040] Additionally, these overlay mark image quality measurement methods can be utilized to optimize the measurement options of the overlay measurement device.

[0041] Figure 1 is a plan view of an example of an overlay mark.

[0042] Figure 2 shows a signal obtained from one working zone of the overlay mark illustrated in Figure 1.

[0043] Figure 3 shows examples of various spatial frequencies.

[0044] Figure 4 is a graph showing the change in MTF according to spatial frequency.

[0045] FIG. 5 is a schematic diagram of an overlay mark image quality measurement device using a signal shape index according to one embodiment of the present invention.

[0046] FIG. 6 is a flowchart of a method for measuring overlay mark image quality using a signal shape index according to an embodiment of the present invention.

[0047] Figure 7 is a diagram illustrating a step of converting a periodic graph into a graph in the frequency domain.

[0048] Figure 8 is a graph showing changes in signal shape index according to focus position.

[0049] Figure 9 shows a landscape map showing the change in signal shape index according to the focus position and wavelength of illumination.

[0050] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. However, the embodiments of the present invention may be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the art. Therefore, the shapes of elements in the drawings are exaggerated to emphasize a clearer description, and elements indicated by the same reference numerals in the drawings represent the same elements.

[0051] FIG. 5 is a schematic diagram of an overlay mark image quality measurement device using a signal shape index according to an embodiment of the present invention. As illustrated in FIG. 5, the overlay mark image quality measurement device (1000) includes an illumination optical system (1010) that illuminates an overlay mark on a semiconductor wafer (W), an imaging optical system (1020) that collects reflected light from the overlay mark to form an overlay mark image, and an image detector (1030) and a controller (1040) that acquire an overlay mark image formed by the imaging optical system (1020).

[0052] The overlay mark image quality measurement device (1000) can process the acquired overlay mark image to measure the quality of the overlay mark image. In addition, the overlay mark image quality measurement device (1000) illustrated in FIG. 5 can also be used to measure overlay errors between multiple consecutive pattern layers.

[0053] The illumination optical system (1010) may be configured using various optical elements. For example, the illumination optical system (1010) may include an illumination source (1011), a beam splitter (1013), and an objective lens (1015). Additionally, it may further include other optical elements such as other lenses or apertures.

[0054] The illumination source (1011) serves to generate light that illuminates the overlay mark. The illumination source (1011) may include a light source capable of generating light with a wide wavelength band and variable optical filters capable of adjusting the wavelength band of the transmitted light.

[0055] A beam splitter (1013) is placed between the illumination source (1011) and the objective lens (1015), and serves to transmit illumination from the illumination source (1011) to the objective lens (1015).

[0056] The objective lens (1015) focuses light onto a measurement position on the surface of a semiconductor wafer (W) and collects reflected light reflected from the measurement position. The objective lens (1015) is installed on a lens focus actuator (1017). The lens focus actuator (1017) is used to adjust the distance between the objective lens (1015) and the semiconductor wafer (W).

[0057] The imaging optical system (1020) may be configured using various optical elements. For example, the imaging optical system (1020) may include a tube lens (1021). Additionally, the imaging optical system (1020) may use the objective lens (1015) and beam splitter (1013) of the illumination optical system (1010). Additionally, other optical elements such as other lenses or apertures may be included.

[0058] The reflected light collected from the objective lens (1015) passes through the beam splitter (1013) and is then focused onto the image detector (1030) by the tube lens (1021).

[0059] The image detector (1030) receives reflected light from an overlay mark by illumination and generates an overlay mark image. The image detector (1030) may be a CCD camera or a CMOS camera.

[0060] The controller (1040) is installed to enable wired or wireless communication with the image detector (1030). The controller (1040) includes hardware such as a processor and memory, and software installed in the memory. The controller (1040) can cause the processor to perform the steps of FIG. 6 below through software commands. The overlay mark image quality measurement device (1000) can measure the quality of the overlay mark image through this.

[0061] FIG. 6 is a flowchart of a method for measuring overlay mark image quality using a signal shape index according to an embodiment of the present invention.

[0062] As illustrated in FIG. 6, a method for measuring the quality of an overlay mark image using a signal shape index according to one embodiment of the present invention begins with a step (S1) of acquiring an overlay mark image.

[0063] The step of obtaining an overlay mark image may be a step of obtaining an overlay mark image using an overlay mark image quality measurement device (1000). For example, it may be a step of obtaining an image as illustrated in FIG. 1.

[0064] When measuring the quality of an overlay mark image for purposes such as recipe optimization, multiple overlay mark images can be acquired at this stage while changing the measurement options of the overlay measurement device, such as the focus position and wavelength of the light.

[0065] The original overlay mark being photographed may be used without limitation as long as it comprises periodic pattern elements. The periodic pattern elements may be not only general periodic pattern elements, but also periodic pattern elements (bars forming a moiré pattern) that form a moiré pattern resulting from the interference effect of overlapping fine periodic patterns.

[0066] Periodic pattern elements have a pre-designed spatial frequency. For example, the periodic pattern elements illustrated in Fig. 1 have a spatial frequency of 7 cycles / unit length, when the X-axis length of the selected region (8, 8`) is considered as a unit length.

[0067] The spatial frequency of a moiré pattern can be designed by controlling the pitches of the fine periodic patterns that form the moiré pattern.

[0068] Next, a measurement area containing periodic pattern elements is selected from the overlay mark image (S2).

[0069] For example, in Fig. 1, each of the working zones can be a measurement area. If the quality of the overlay mark image formed together with the first pattern layer is to be evaluated, at least one of the working zones formed together with the first pattern layer can be selected as a measurement area. One measurement area having a periodicity in the X direction and one measurement area having a periodicity in the Y direction can also be selected.

[0070] And, if you want to evaluate the quality of the overlay mark image formed together with the second pattern layer, you can select at least one of the working zones formed together with the second pattern layer as a measurement area. You can also select one measurement area having a periodicity in the X direction and one measurement area having a periodicity in the Y direction.

[0071] Next, a periodic graph representing the measurement area is obtained (S3).

[0072] This step can be performed, for example, by projection method.

[0073] Projection is a process of obtaining a periodic graph that represents the change in intensity value according to the change in the X-coordinate value of the measurement area by calculating the average or sum of the intensities, for example, gray values, of pixels with the same X-coordinate value in the measurement area that has periodicity in the X direction.

[0074] In the same way, if the measurement area has periodicity in the Y direction, a periodic graph showing the change in intensity value according to the change in Y coordinate value can be obtained.

[0075] The graphs (a-1, b-1, c-1) shown on the left side of Fig. 7 may be graphs obtained from the same measurement areas of overlay mark images obtained by varying shooting conditions for one measurement area of ​​the same overlay mark (e.g., the overlay mark shown in Fig. 1). For example, the graphs (a-1, b-1, c-1) may be obtained from the same measurement areas of overlay mark images obtained by varying the focal lengths to F1, F2, and F3. These graphs (a-1, b-1, c-1) may be graphs obtained through simulation. The X-axis of the graphs (a-1, b-1, c-1) may be the X-coordinate values ​​of the pixels, and the Y-axis may be the average value of the gray values ​​of the pixels.

[0076] As shown in Fig. 7, even when targeting the same overlay mark, different types of periodic graphs (a-1, b-1, c-1) are obtained depending on the shooting conditions.

[0077] Next, the periodic graph is converted into a frequency domain graph (S4).

[0078]

[0079] *In this step, the periodic graph obtained in step S3 is converted into a frequency domain graph using methods such as Fourier Transform and Fast Fourier Transform (FFT).

[0080] The graphs (a-2, b-2, c-2) shown on the right side of Fig. 7 are graphs that show the periodic graphs (a-1, b-1, c-1) shown on the left side in the frequency domain. The X-axis of these graphs (a-2, b-2, c-2) represents spatial frequency, and the Y-axis represents the amplitude of the corresponding spatial frequency component. As can be seen from the graphs (a-2, b-2, c-2) on the right side of Fig. 7, the amplitude of the spatial frequency components changes depending on the shooting conditions.

[0081] In the upper graph (a-2), only frequency components with a spatial frequency of 7 cycles / unit length are shown, and in the middle graph (b-2), the amplitudes of frequency components with a spatial frequency of 7 cycles / unit length and frequency components with a spatial frequency of 14 cycles / unit length are almost the same. In the lower graph (c-2), both frequency components with a spatial frequency of 7 cycles / unit length and frequency components with a spatial frequency of 14 cycles / unit length are shown, and the amplitude of the frequency component with a spatial frequency of 14 cycles / unit length is larger.

[0082] Next, find the amplitude value of the first frequency component with the first spatial frequency in the frequency domain graph (S5).

[0083] If the designed spatial frequency is known, the designed spatial frequency may be the first spatial frequency. For example, in the frequency domain graphs (a-2, b-2, c-2) illustrated in Fig. 7, the first frequency component is the frequency component whose spatial frequency is 7 cycles / unit length. In this step, the amplitude value (A1) of these first frequency components is found.

[0084] If the designed spatial frequency is not known, the two spatial frequencies with the largest amplitude values ​​can be found in the frequency domain graph, and the smaller spatial frequency among them can be selected as the first spatial frequency.

[0085] For example, in the frequency domain graphs (a-2, b-2, c-2) shown in Fig. 7, the two frequency components with the largest amplitude values, namely the frequency component with a spatial frequency of 7 cycles / unit length and the frequency component with a spatial frequency of 14 cycles / unit length, can be easily found.

[0086] And among the spatial frequencies of these frequency components, a smaller value of 7 cycles / unit length can be selected as the first spatial frequency.

[0087] Next, find the amplitude value of the second frequency component with the second spatial frequency in the frequency domain graph (S6).

[0088] If the designed spatial frequency is known, the second spatial frequency can be twice the designed spatial frequency. For example, in the frequency domain graphs (a-2, b-2, c-2) illustrated in Fig. 7, the second frequency component is a frequency component whose spatial frequency is 14 cycles / unit length. In this step, the amplitude value (A2) of these second frequency components is found.

[0089] If the designed spatial frequency is not known, the two spatial frequencies with the largest amplitude values ​​can be found in the frequency domain graph, and then the larger spatial frequency can be selected as the second spatial frequency.

[0090] For example, in the frequency domain graphs (a-2, b-2, c-2) shown in Fig. 7, 14 cycles / unit length can be selected as the second spatial frequency.

[0091] Next, a signal shape index is calculated based on the amplitude value of the first frequency component and the amplitude value of the second frequency component (S7).

[0092] In this step, for example, as in the mathematical expression 1 below, the value obtained by subtracting the amplitude value (A2) of the second frequency component from the amplitude value (A1) of the first frequency component can be calculated as a signal shape index (SSI).

[0093]

[0094] The signal shape index can be positive, zero, or negative. A positive signal shape index means that the overlay mark image has approximately the designed spatial frequency, and a negative signal shape index means that the overlay mark image has approximately twice the designed spatial frequency. A signal shape index close to zero means that the overlay mark image has both the designed spatial frequency and the twice the designed spatial frequency, and the amplitudes of the two spatial frequency components are similar.

[0095] Fig. 8 is a graph illustrating changes in the signal shape index according to the focus position. The graph illustrated in Fig. 8 can be used, for example, when finding the optimal focus position when the wavelength of the light is determined. The graph in Fig. 8 can be drawn by continuously changing the focus position, obtaining graphs of the same format as the graphs illustrated on the right side of Fig. 7, and then calculating the signal shape index from each graph.

[0096] In Fig. 8, when the focus position is F1, as shown in (a-2) of Fig. 7, the value of A1 is very large compared to the value of A2, so the SSI value has a large absolute value and a positive value.

[0097] When the focus position is F2, as shown in Fig. 7 (b-2), the SSI value has a positive value with a small absolute value because the value of A1 is similar to the value of A2.

[0098] When the focus position is F3, as shown in (c-2) of Fig. 7, the value of A1 is very small compared to the value of A2, so the SSI value has a negative value with a large absolute value.

[0099] In the same way, a graph showing the variation of the signal shape index with the wavelength of the light can be obtained. This graph can be used to determine the optimal wavelength of light once the focus position has been determined.

[0100] Figure 9 is a landscape map showing the variation in signal shape index according to the focal position and wavelength of illumination. The landscape map illustrated in Figure 9 can be used to simultaneously find the optimal focal position and wavelength of illumination.

[0101]

[0102] *Next, a quality evaluation is performed on the overlay mark image based on the signal shape index (S8).

[0103] In general, a positive signal shape index is desirable because the overlay mark image reproduces the overlay mark almost exactly. In particular, if the periodic pattern obtained by photographing the overlay mark is a moiré pattern, a positive signal shape index can be evaluated as having good quality of the overlay mark image.

[0104] For example, in the graph of Fig. 8, when the signal shape index has the maximum value, the quality of the overlay mark image can be evaluated as the best.

[0105] Additionally, the quality of overlay mark images belonging to the yellow area with a positive signal shape index in the landscape map of Fig. 9 can be evaluated as excellent.

[0106] However, even if the signal shape index is positive, if its value is too small, there is a disadvantage that the overall signal intensity is weak. In this case, as an exception, if the signal shape index is negative but its absolute value is large, it can be evaluated as a better overlay mark image than when the signal shape index is positive. In addition, even if the signal shape index is close to 0, if the amplitude value of the first frequency component and the amplitude value of the second frequency component are both large, it can be evaluated as a better overlay mark image.

[0107] That is, the quality of the overlay mark image can be evaluated by considering not only the signal shape index but also the amplitude value of the first frequency component and the amplitude value of the second frequency component.

[0108] Utilizing this overlay mark image quality assessment method can optimize the measurement options of an overlay measurement device. Specifically, after acquiring one or more overlay mark images for each combination of measurement options for the overlay measurement device, the signal shape index for each overlay mark image is obtained using the method described above, and by creating and reviewing a graph or map based on changes in measurement options, the optimal combination of measurement options can be found.

[0109] The embodiments described above merely describe preferred embodiments of the present invention, and the scope of the present invention is not limited to the described embodiments, and various changes, modifications, or substitutions may be made by those skilled in the art within the technical spirit and scope of the claims of the present invention, and it should be understood that such embodiments fall within the scope of the present invention.

[0110] [Explanation of symbols]

[0111] 1000: Overlay Mark Image Quality Measurement Device

[0112] 1010: Illumination optics

[0113] 1020: Focusing optical system

[0114] 1030: Image detector

[0115] 1040: Controller

Claims

1. A method for measuring the quality of an overlay mark image having periodic pattern elements having a designed spatial frequency, a) a step of obtaining an overlay mark image, and b) a step of selecting a measurement area including the periodic pattern elements in the overlay mark image; c) a step of obtaining a periodic graph representing the above measurement area, d) a step of converting the above periodic graph into a frequency domain graph, and e) a step of finding the amplitude value of the first frequency component having the first spatial frequency in the above frequency domain graph, f) a step of finding the amplitude value of a second frequency component having a second spatial frequency different from the first spatial frequency in the frequency domain graph; g) A step of calculating a signal shape index by subtracting the amplitude value of the second frequency component from the amplitude value of the first frequency component; h) a step of performing a quality evaluation on the overlay mark image based on the signal shape index, The first spatial frequency is the designed spatial frequency, and the second spatial frequency is twice the designed spatial frequency. Step h) above, If the above signal shape index is positive, it is determined that the periodic pattern element mainly generates a signal of the same frequency as the designed spatial frequency, A method for measuring the quality of an overlay mark image using a signal shape index, wherein if the signal shape index is negative, the periodic pattern element is determined to primarily generate a signal of a frequency twice the designed spatial frequency.

2. In paragraph 1, The above periodic pattern elements are a method for measuring the quality of an overlay mark image using a signal shape index, which are periodic elements of a moire pattern.

3. In paragraph 1, The above step h) is a method for measuring the quality of an overlay mark image using a signal shape index, which is a step of evaluating the quality of the overlay mark image based on the signal shape index, the amplitude value of the first frequency component, and the amplitude value of the second frequency component.

4. In paragraph 1, The above first spatial frequency is a spatial frequency having a smaller frequency value among the two spatial frequencies having the largest amplitude value in the frequency domain graph, The above second spatial frequency is a method for measuring the quality of an overlay mark image using a signal shape index that is a spatial frequency having a larger frequency value among the two spatial frequencies having the largest amplitude values ​​in the frequency domain graph.

5. In paragraph 1, Step c) above, A method for measuring the quality of an overlay mark image using a signal shape index, which is a projection step for obtaining a periodic graph representing a change in intensity value according to a change in the first direction coordinate value of the measurement area by obtaining an average or sum of the intensities of pixels having the same first direction coordinate value in the measurement area having periodicity in the first direction.

6. An overlay mark image quality measuring device for measuring the quality of an overlay mark image having periodic pattern elements having a designed spatial frequency, An image detector for obtaining the above overlay mark image, A controller capable of communicating with the image detector, The above controller, a) a step of obtaining an overlay mark image, and b) a step of selecting a measurement area including the periodic pattern elements in the overlay mark image; c) a step of obtaining a periodic graph representing the above measurement area, d) a step of converting the above periodic graph into a frequency domain graph, and e) a step of finding the amplitude value of the first frequency component having the first spatial frequency in the above frequency domain graph, f) a step of finding the amplitude value of a second frequency component having a second spatial frequency different from the first spatial frequency in the frequency domain graph; g) A step of calculating a signal shape index by subtracting the amplitude value of the second frequency component from the amplitude value of the first frequency component; h) configured to perform a step of performing a quality evaluation on the overlay mark image based on the signal form index; The first spatial frequency is the designed spatial frequency, and the second spatial frequency is twice the designed spatial frequency. Step h) above, If the above signal shape index is positive, it is determined that the periodic pattern element mainly generates a signal of the same frequency as the designed spatial frequency, An overlay mark image quality measurement device using a signal shape index, wherein if the signal shape index is negative, the periodic pattern element is determined to primarily generate a signal of a frequency twice the designed spatial frequency.

7. In paragraph 6, The above periodic pattern elements are an overlay mark image quality measurement device using a signal shape index which are periodic elements of a moire pattern.

8. In paragraph 6, The above first spatial frequency is a spatial frequency having a smaller frequency value among the two spatial frequencies having the largest amplitude value in the frequency domain graph, The above second spatial frequency is an overlay mark image quality measuring device using a signal shape index that is a spatial frequency with a larger frequency value among the two spatial frequencies with the largest amplitude values ​​in the frequency domain graph.

9. A method for optimizing the measurement options of an overlay measurement device by comparing the quality of overlay mark images according to measurement options. A step of forming an overlay mark having periodic pattern elements having a designed spatial frequency, A step of measuring the quality of an overlay mark image for each combination of the above measurement options is included. A method for optimizing measurement options of an overlay measurement device, wherein the step of measuring the quality of the overlay mark image is performed according to the overlay mark image quality measurement method of any one of claims 1 to 5.

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