JP2016136563A - Overlay measurement apparatus and overlay measurement method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To highly accurately acquire a deviation amount between layers from a vernier pattern image through relatively simple processing.SOLUTION: An overlay measurement apparatus acquires a deviation amount between a first layer and a second layer that are formed on a substrate, from a vernier pattern image. The first layer includes a first pattern that is a main scale of a vernier pattern, and the second layer includes a second pattern that is a sub scale. The overlay measurement apparatus acquires a plurality of element pair images indicating an element pair that is a combination of each of first pattern elements of the first pattern and a second pattern element of the second pattern corresponding to the first pattern element (step S22). An evaluation value indicating symmetry of the element pair is acquired from the plurality of element pair images (step S23). On the basis of a plurality of evaluation values, an evaluation value function is acquired that indicates a relation between a position of the element pair in an array direction and the evaluation value (step S24). The deviation amount is acquired from a position where the evaluation value is maximum in the evaluation value function (step S25).SELECTED DRAWING: Figure 6

Description

  The present invention relates to an overlay measurement technique for acquiring a shift amount between two layers on a substrate from an image showing a vernier pattern.

  In manufacturing a semiconductor device, a plurality of layers including a circuit pattern are stacked on a substrate. At this time, a relative distance from an ideal position between layers (hereinafter referred to as “shift amount”) has been conventionally measured. The amount of deviation is precisely the amount of deviation of the relative position between the pattern included in one layer and the pattern included in another layer. The measurement of the deviation amount is called overlay measurement, and the measurement result is used for quality control, manufacturing process improvement, and the like. As a method of overlay measurement, a method is known in which a main scale pattern is included in one layer and a vernier pattern is included in another layer, and a deviation amount is acquired from an image including these patterns. For example, Patent Document 1 discloses a vernier pattern that can improve the accuracy of reading a deviation amount.

  Normally, “vernier” refers to a vernier, but in the following description, a pattern including both the main and vernier is referred to as a “vernier pattern”, and an image including the vernier pattern is referred to as a “vernier pattern image”. Call.

Japanese Patent Laid-Open No. 04-000587

  By the way, in the measurement using the vernier pattern, the position where the position of the main scale pattern element and the position of the vernier pattern element coincide with each other is obtained by visual observation. In order to automate this measurement, for example, first, the edge of each pattern element of the main scale is obtained, and the position of the main scale center line that is the center line of the pattern element is obtained from the edge. For the vernier, the position of the vernier center line, which is the center line of the pattern element, is obtained from the edge of each pattern element. Then, the combination of the closest main scale center line and vernier center line is specified.

  However, the amount of calculation increases in the above processing. Further, in practice, some pattern elements may be lost or deformed due to defects during pattern formation or the influence of the imaging system. As a result, there is a possibility that measurement results greatly different from the case of visual observation may be obtained due to automation of measurement.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to acquire a shift amount between layers with high accuracy from a vernier pattern image by a relatively simple process.

  In the first aspect of the invention, the first pattern includes a plurality of first pattern elements arranged at equal intervals in a predetermined arrangement direction, and the second pattern is arranged at equal intervals in the arrangement direction. Imaging the first pattern included in the first layer formed on the substrate, and the second pattern included in the second layer overlapping the first layer, the array including a plurality of second pattern elements arranged. An overlay measurement apparatus for acquiring a shift amount between the first layer and the second layer in the arrangement direction from the vernier pattern image obtained in the above-described arrangement, wherein the first pattern element and the first first An element pair image acquisition unit that acquires an element pair image indicating an element pair that is a combination with a second pattern element corresponding to one pattern element from the vernier pattern image, and an element pair image that is perpendicular to the arrangement direction. Around the axis An evaluation value acquisition unit that acquires an evaluation value indicating the symmetry of an element pair, and an evaluation value function that indicates the relationship between the position in the arrangement direction and the evaluation value based on a plurality of evaluation values obtained for a plurality of element pairs The function acquisition unit that acquires the position, and the evaluation value function acquires a position where the evaluation value is maximum in the arrangement direction, and acquires a shift amount between the first layer and the second layer from the position. A quantity acquisition unit.

  The invention according to claim 2 is the overlay measurement apparatus according to claim 1, wherein, in the evaluation value acquisition unit, the axis is the first pattern element and the second pattern element of the element pair. It is determined on the basis of the larger width in the arrangement direction.

  A third aspect of the present invention is the overlay measurement apparatus according to the first or second aspect, wherein, in the evaluation value acquisition unit, the axis is one of the first pattern element and the second pattern element of the element pair. It is determined based on the smaller density difference from the background.

  The invention according to claim 4 is the overlay measurement apparatus according to claim 2 or 3, wherein, among the first pattern element and the second pattern element, a pattern element serving as a reference for determining the axis is Symmetric about the axis.

  The invention according to claim 5 is the overlay measurement apparatus according to any one of claims 1 to 4, wherein the evaluation value acquisition unit inverts the element pair image around the axis, and The degree of coincidence between the image and the inverted image is acquired as the evaluation value.

  The invention according to claim 6 is the overlay measurement apparatus according to any one of claims 1 to 5, wherein the function acquisition unit sets the plurality of evaluation values corresponding to the positions of the plurality of element pairs. The evaluation value function is obtained by fitting a model function including at least one unknown coefficient to determine a value of the at least one unknown coefficient.

  The invention according to claim 7 is the overlay measurement apparatus according to any one of claims 1 to 5, wherein the function acquisition unit has a vertex positioned at each of a plurality of positions set in advance in the arrangement direction. And fitting a model function including at least one unknown coefficient to the plurality of evaluation values corresponding to the positions of the plurality of element pairs to determine a value of the at least one unknown coefficient, A provisional function is acquired, and the evaluation value function is acquired based on the plurality of provisional functions.

  The invention according to claim 8 is the overlay measurement apparatus according to claim 7, wherein the function acquisition unit corresponds to a function in which the plurality of provisional functions are most fitted to the plurality of evaluation values. The position is acquired as a temporary matching position, and the position having the highest evaluation value and the position in the vicinity thereof are selected as a plurality of selection positions from the temporary matching position and the vicinity thereof, and the plurality of selection positions are supported. The evaluation value function is obtained by fitting a model function including at least one unknown coefficient to a plurality of selected evaluation values to determine a value of the at least one unknown coefficient.

  The invention according to claim 9 is the overlay measurement apparatus according to claim 8, wherein the function acquisition unit obtains a deviation of the plurality of selected evaluation values from the evaluation value function, and based on the deviation A value of the at least one unknown coefficient is selected by selecting a part of the plurality of selection evaluation values as a plurality of update selection evaluation values and fitting a model function including at least one unknown coefficient to the plurality of update selection evaluation values. The updated evaluation value function is obtained by determining.

  In the invention according to claim 10, the first pattern includes a plurality of first pattern elements arranged at equal intervals in a predetermined arrangement direction, and the second pattern is arranged at equal intervals in the arrangement direction. Imaging the first pattern included in the first layer formed on the substrate, and the second pattern included in the second layer overlapping the first layer, the array including a plurality of second pattern elements arranged. An overlay measurement method for obtaining a deviation amount between the first layer and the second layer in the arrangement direction from the vernier pattern image obtained in the above, wherein a) each includes one first pattern element and Obtaining from the vernier pattern image a plurality of element pair images indicating an element pair that is a combination with the second pattern element corresponding to the one first pattern element; and b) from the plurality of element pair images. , The array direction A step of obtaining a plurality of evaluation values indicating symmetry of an element pair centered on an axis perpendicular to the axis; and c) showing a relationship between the position in the arrangement direction and the evaluation values based on the plurality of evaluation values. A step of obtaining an evaluation value function; and d) obtaining a position where the evaluation value is maximum in the arrangement direction from the evaluation value function, and an amount of deviation between the first layer and the second layer from the position. And obtaining a process.

  According to the present invention, it is possible to acquire a shift amount between layers with high accuracy from a vernier pattern image by a relatively simple process.

It is a figure which shows schematic structure of an overlay measurement apparatus. It is a figure which shows the structure of a computer. It is a block diagram which shows the function structure of a computer. It is a figure which shows the flow of a preparatory work. It is a figure which shows the example of a vernier pattern image. It is a figure which shows the flow of operation | movement of an overlay measurement apparatus. It is a figure which shows the example of an element pair image. It is a figure which shows the example of a reverse image. It is a figure which shows a mode that the element pair image and the reverse image were piled up. It is a figure which shows the example of an element pair image. It is a figure which shows the example of an element pair image. It is a figure which shows the example of an evaluation value function. It is a figure which shows the other example of a vernier pattern image. It is a figure which shows the example of the relationship between the position of an element pair in case the resolution of a vernier pattern image is low, and an evaluation value. It is a figure which shows the flow of operation | movement of a function acquisition part. It is a figure which shows a provisional function. It is a figure which shows the relationship between the position of the vertex of a provisional function, and a residual sum of squares. It is a figure which shows the evaluation value function before and behind an update.

  FIG. 1 is a diagram showing a schematic configuration of an overlay measuring apparatus 1 according to an embodiment of the present invention. The overlay measuring apparatus 1 acquires a shift amount between a plurality of layers formed on a semiconductor substrate 9 (hereinafter referred to as “substrate”) that is an object. That is, one layer including a circuit pattern is formed on the substrate 9 using a photolithography technique, and another layer including a circuit pattern is formed on the layer using the photolithography technique. The overlay measuring apparatus 1 optically acquires an image of the substrate 9 and acquires a pattern shift amount between layers from the image.

  The overlay measuring apparatus 1 includes a stage 21, a stage driving unit 22, and an imaging unit 3. The stage 21 holds the substrate 9. The stage driving unit 22 moves the stage 21 relative to the imaging unit 3. The stage drive unit 22 includes a ball screw, a guide rail, a motor, and the like. The imaging unit 3 is arranged above the stage 21 and images the substrate 9 to acquire image data. The imaging unit 3 includes an illumination unit 31, an optical system 32, and an imaging device 33. The illumination unit 31 emits illumination light. The optical system 32 guides illumination light to the substrate 9 and allows light from the substrate 9 to enter. The imaging device 33 converts the image of the substrate 9 formed by the optical system 32 into an electrical signal.

  The overlay measuring apparatus 1 is further provided with a computer 5 that performs various arithmetic processes. The computer 5 controls the stage drive unit 22 and the imaging unit 3 so that the target region on the substrate 9 is imaged.

  FIG. 2 is a diagram illustrating the configuration of the computer 5. The computer 5 has a general computer system configuration including a CPU 51 that performs various arithmetic processes, a ROM 52 that stores basic programs, and a RAM 53 that stores various information. The computer 5 includes a fixed disk 54 that stores information, a display 55 that displays various types of information such as images, and a keyboard 56a and a mouse 56b (hereinafter, referred to as an “input unit 56”) that receive input from an operator. ) And a reading unit 57 that reads information from a computer-readable recording medium 8 such as an optical disk, a magnetic disk, or a magneto-optical disk, and a communication unit 58 that transmits and receives signals between other components of the overlay measuring apparatus 1 And further including.

  In the computer 5, the program 80 is read from the recording medium 8 via the reading device 57 in advance and stored in the fixed disk 54. The CPU 51 executes arithmetic processing according to the program 80 while using the RAM 53 and the fixed disk 54.

  FIG. 3 is a block diagram showing a functional configuration realized by the arithmetic processing of the computer 5. In FIG. 3, the functional configuration realized by the CPU 51, ROM 52, RAM 53, fixed disk 54, and the like of the computer 5 is surrounded by a rectangle denoted by reference numeral 5. The computer 5 includes a pattern image acquisition unit 511, an element pair image acquisition unit 512, an evaluation value acquisition unit 513, a function acquisition unit 514, a deviation amount acquisition unit 515, and a control unit 510 that controls these operations. . Details of these functions will be described later. Each function may be constructed by a dedicated electric circuit, or a partially dedicated electric circuit may be used.

  FIG. 4 is a diagram illustrating a flow of preparation work in the overlay measurement apparatus 1. In the preparatory work, first, the reference substrate 9 is placed on the stage 21, and the reference mark formed on the substrate 9 is imaged by the imaging unit 3 by the operator's operation. When the operator designates the position of the reference mark in the acquired image via the input unit 56, the relative position of the substrate 9 with respect to the stage 21 is acquired in the computer 5.

  Next, the vernier pattern included in one die on the substrate 9 is imaged by the imaging unit 3 by the operator's operation. Thereby, an image including a vernier pattern is acquired. The operator designates the position and range of the vernier pattern via the input unit 56 while watching the display on the display 55. Thereby, the position of the vernier pattern and the reference vernier pattern image are registered in the computer 5 (step S11).

  FIG. 5 is a diagram illustrating an example of a vernier pattern image. FIG. 5 shows a vernier pattern image 601 for acquiring a predetermined shift amount in the x direction and a vernier pattern image 602 for acquiring a shift amount in the y direction perpendicular to the x direction. The vernier pattern image 601 includes a portion showing the first pattern 61 and the second pattern 62 on the substrate 9. In FIG. 5, portions corresponding to the first pattern 61 and the second pattern 62 in the vernier pattern image 601 are denoted by the same reference numerals as those of the first pattern 61 and the second pattern 62 on the substrate 9.

  In the vernier pattern image 601, the first pattern 61 includes a plurality of first pattern elements 611 arranged at equal intervals in the x direction. The second pattern 62 includes a plurality of second pattern elements 621 arranged at equal intervals in the x direction. In FIG. 5, the number of first pattern elements 611 is nine, and the number of second pattern elements 621 is nine. The pitch of the second pattern elements 621 is slightly smaller than the pitch of the first pattern elements 611.

  The first pattern 61 is included in one layer formed on the substrate 9. Hereinafter, the layer including the first pattern 61 is referred to as a “first layer”. Precisely, the first pattern 61 is formed together with the circuit pattern by photolithography using the first layer. The second pattern 62 is included in another layer formed on the substrate 9. Hereinafter, the layer including the second pattern 62 is referred to as a “second layer”. Precisely, the second pattern 62 is formed together with the circuit pattern by photolithography using the second layer. Although the first layer and the second layer overlap, it is not necessary to overlap directly, and other layers may intervene and may overlap indirectly. The pattern may be formed by a technique other than photolithography.

  The first pattern 61 is a so-called main scale, and the second pattern 62 is a so-called sub-scale. Of course, the roles of the main scale and the vernier of the first pattern 61 and the second pattern 62 may be interchanged. Further, the interval between the first pattern elements 611 may be smaller than the interval between the second pattern elements 621.

  When the first layer and the second layer are not shifted in the x direction, that is, when the circuit pattern of the first layer and the circuit pattern of the second layer are not shifted with respect to the x direction, in the vernier pattern image 601, The center position of the first pattern element 611 in the x direction matches the center position of the second pattern element 621 in the x direction. When the first layer and the second layer are shifted in the x direction, the center position of any one of the first pattern elements 611 other than the center and the center position of any other second pattern element 621 are closest. Depending on the amount of deviation, the position of the combination of the first pattern element 611 and the second pattern element 621 whose center position is closest is changed.

  The vernier pattern image 602 also includes a first pattern 61 and a second pattern 62. The first pattern 61 includes a plurality of first pattern elements 611 arranged at equal intervals in the y direction. The second pattern 62 includes a plurality of second pattern elements 621 arranged at equal intervals in the y direction. The interval between the first pattern elements 611 is slightly different from the interval between the second pattern elements 621. The vernier pattern image 602 is the same as the vernier pattern image 601 except that the pattern elements are arranged in the y direction.

  After registration of the vernier pattern images 601 and 602, the operator inputs the number of pairs of the first pattern elements 611 and the second pattern elements 621 included in each vernier pattern image 601 and 602 and the arrangement direction to the computer 5 (step). S12). For the vernier pattern image 601, “9” is input as the element logarithm and “x” is input as the arrangement direction. For the vernier pattern image 602, “9” is input as the element logarithm and “y” is input as the arrangement direction.

  In the above description, the vernier pattern images 601 and 602 and their positions are individually registered in step S11. However, an image including two vernier pattern images 601 may be registered as a pattern group image. In this case, the position and range of each vernier pattern image in the pattern group image are input by the operator. Further, the pitch of the first pattern element 611 or the second pattern element 621 may be input instead of the number of element pairs.

  FIG. 6 is a diagram illustrating a flow of operations of the overlay measurement apparatus 1. When the substrate 9 to be measured is placed on the stage 21, the alignment mark is automatically imaged by the imaging unit 3, and the position of the substrate 9 with respect to the stage 21 is acquired. Then, the pattern image acquisition unit 511 further captures an image of the range in which the vernier pattern is assumed to exist based on the position and range of the vernier pattern registered in advance. A vernier pattern image indicating a vernier pattern is acquired from the acquired image by pattern matching with a reference vernier pattern image (step S21).

  In the case of the present embodiment, a vernier pattern image related to the x direction and a vernier pattern image related to the y direction are acquired. In the following description, these vernier pattern images are given the same reference numerals as in FIG. Further, as described above, when the pattern group image indicating a plurality of vernier patterns and the positions and ranges of the vernier patterns are registered in step S11, the pattern matching between the captured image and the pattern group image, and the pattern group image are performed. Each vernier pattern image is acquired from the captured image based on the position and range of the vernier pattern.

  Next, the element pair image acquisition unit 512 divides the vernier pattern image 601 based on the number of element pairs set in step S12 and the arrangement direction (step S22). Hereinafter, the expression “arrangement direction” simply refers to the arrangement direction of element pairs and pattern elements. A position in the arrangement direction is referred to as an “arrangement direction position”. The vernier pattern image 601 may be divided based on the pitch of the first pattern elements 611. Each divided image includes one element pair, that is, a combination of one first pattern element 611 and one corresponding second pattern element 621. In step S12, the position and range of each element pair in the vernier pattern image 601 may be specified, and an image including each element pair may be acquired from the vernier pattern image 601 based on these pieces of information. Hereinafter, an image showing one element pair is referred to as an “element pair image”.

  FIG. 7A is a diagram illustrating an element pair image 603. The evaluation value acquisition unit 513 generates an image obtained by inverting the left and right of the element pair image 603 as illustrated in FIG. 7B. That is, the evaluation value acquisition unit 513 generates a reverse image 603a obtained by inverting the element pair image 603 around an axis perpendicular to the arrangement direction of the first pattern elements 611. Further, the evaluation value acquisition unit 513 acquires a part of the element pair image 603 as a partial image 631 for pattern matching. Specifically, the lower part of FIG. 7A of the element pair image 603 is acquired as the partial image 631, and the partial image 631 includes only a part of the first pattern element 611. The first pattern element 611 is bilaterally symmetric, and the lower part of the first pattern element 611 is also symmetric.

  The evaluation value acquisition unit 513 performs pattern matching between the partial image 631 and the reverse image 603a. Thereby, the relative position of the element pair image 603 with respect to the reverse image 603a when the first pattern element 611 of the element pair image 603 matches the first pattern element 611 of the reverse image 603a is acquired. As shown in FIG. 7C, the evaluation value acquisition unit 513 uses, as an evaluation value, the matching score of both images when the first pattern element 611 of the element-pair image 603 and the first pattern element 611 of the inverted image 603a are overlapped. Obtain (step S23).

  The matching score is a value indicating the degree of coincidence between the element pair image 603 before inversion and the inverted image 603a, and is one of evaluation values indicating the symmetry of the element pair about the axis perpendicular to the arrangement direction. is there. Various methods may be used as the calculation direction of the matching score. In the present embodiment, for example, the matching score is obtained by the normalized correlation represented by Equation 1 (also referred to as “normalized cross-correlation”).

  In Equation 1, I represents an element pair image 603, T represents an inverted image 603a, and M and N are the numbers of pixels in the x direction and the y direction of a region used for pattern matching.

  Various evaluation values may be used as long as they show the symmetry of element pairs. For example, the degree of coincidence between the element pair image 603 and the reverse image 603a may be simply used as the evaluation value. In addition, the evaluation value may be acquired by performing pattern matching on the basis of the second pattern element 621.

The element pair image 603 may be a color image. For example, in the element pair image 603, when the numerical values R ₁ , G ₁ , B ₁ , R ₂ , G ₂ , B _2. The evaluation value is obtained by regarding the image as a gray image in which three times the number of pixels are arranged. However, when pattern matching is performed between the partial image 631 and the reverse image 603a, the degree of coincidence is acquired while the partial image 631 moves by three pixels in the x direction. By using a color image, a good evaluation value may be obtained even when the contrast is low.

In addition, linear or non-linear contrast correction may be performed on the element pair image 603 before the inverted image 603a is generated. For example, the element pair image 603 is divided into RGB color component images, and each color component image is subjected to contrast correction and then combined and returned to a color image. The hue changes due to the contrast correction. As the contrast correction, for example, the change range of the relationship between the pixel gradation value and the number of pixels may be linearly changed, or nonlinear conversion expressed by y = x ^{1 / γ} may be performed.

  8A and 8B are diagrams illustrating an example of the element pair image 603. FIG. In the element pair 63 in FIG. 8B, the left-right positional deviation between the first pattern element 611 and the second pattern element 621 is larger than that in the element pair 63 in FIG. 8A. Therefore, the evaluation value of FIG. 8B is smaller than the evaluation value of FIG. 8A.

  FIG. 9 is a diagram showing an example of the relationship between the position in the arrangement direction of the element pair 63 and the evaluation value actually obtained by the evaluation value acquisition unit 513. In the example of FIG. 9, the number of element pairs 63 is seven. The position of “0” corresponds to the central element pair 63. The position “1” corresponds to the position of the adjacent element pair 63 in the center, more precisely, the position of the first pattern element 611. Each diamond-shaped point 641 plotted in FIG. 9 indicates an evaluation value obtained from the element pair image 603 at each position.

  In the function acquisition unit 514, a model function having a coefficient set as an unknown in advance is prepared. In the present embodiment, for example, a Gaussian function shown in Equation 2 is prepared as a model function. Then, the values of the unknown coefficients a, b, c, and d are determined so that the model function is closest to the plurality of points 641 indicating the correspondence between the plurality of positions and the plurality of evaluation values.

  Thereby, the function 642 shown in FIG. 9 is acquired. Hereinafter, approaching the model function to the point 641 is appropriately expressed as “fitting the function to the evaluation value”. The function acquisition unit 514 determines a value of the unknown coefficient by fitting a model function including the unknown coefficient to a plurality of evaluation values corresponding to the positions of the plurality of element pairs 63 in the arrangement direction, and evaluates the position and evaluation in the arrangement direction. A function 642 indicating the relationship with the value is acquired (step S24). Hereinafter, the obtained function 642 is referred to as an “evaluation value function”. In the fitting, for example, the value of the coefficient that minimizes the residual sum of squares is obtained.

  Other functions such as a quadratic function shown in Equation 3 may be used as the model function. The model function is convex upward, preferably symmetric. The number of unknown coefficients of the model function may be one. When the number of unknown coefficients is one, for example, only the arrangement direction position of the vertex of the model function is set as the unknown coefficient. The number of unknown coefficients is at least one.

  The deviation amount acquisition unit 515 acquires the arrangement direction position where the evaluation value is maximum in the evaluation value function 642. In the deviation amount acquisition unit 515, the relationship between the maximum position where the evaluation value becomes maximum and the deviation amount between the first layer and the second layer in the arrangement direction is prepared in advance, and the position where the evaluation value becomes maximum. From this, the amount of deviation is acquired (step S25).

  Since the overlay measuring apparatus 1 evaluates the symmetry of the element pair 63 as an evaluation value, the evaluation value can be easily obtained. In particular, as in the present embodiment, by using the matching score between the element pair image 603 and the inverted image 603a, operations such as edge extraction and centerline acquisition are not required, and an evaluation value can be easily acquired. Can do. Further, by using the model function, the position where the evaluation value is maximized can be easily obtained. Further, by using the model function, even if the shape of any element pair 63 is abnormal, the influence of this element pair 63 can be reduced.

  When the displacement amount in the x direction is acquired from the vernier pattern image 601, the same processing is performed on the vernier pattern image 602 in which the arrangement direction of the element pair 63 is the y direction (step S26). Thereby, the deviation | shift amount of the y direction of a 1st layer and a 2nd layer is acquired. In the process for the vernier pattern image 602, the inverted image 603a is an image obtained by inverting the element pair image 603 around the axis in the x direction.

  In the above embodiment, when the evaluation value is obtained, the element pair image 603 and the reverse image 603a are aligned based on the first pattern element 611. In the case of FIG. 7A, this processing is substantially equivalent to obtaining the inverted image 603a by inverting the element pair image 603 around the central axis 632 facing the y direction of the first pattern element 611. Accordingly, the evaluation value becomes smaller as the second pattern element 621 is displaced from the central axis 632 of the first pattern element 611.

  Here, the element pair image acquisition unit 512 acquires the element pair image 603 by dividing the vernier pattern image. At this time, if the vernier pattern image is divided at the pitch of the first pattern element 611, the second pattern element 621 may protrude from any one of the element pair images 603. On the other hand, if the vernier pattern image 601 is divided at the pitch of the second pattern elements 621, the first pattern element 611 may protrude from any of the element pair images 603. The possibility that the pattern element protrudes from the element pair image 603 decreases as the width of the pattern element that does not serve as the reference for the division pitch decreases.

  That is, as in the present embodiment, when the width in the arrangement direction of the first pattern elements 611 is smaller than the width in the arrangement direction of the second pattern elements 621, a vernier pattern image is obtained based on the pitch of the first pattern elements 611. By dividing and acquiring the element pair image 603, the possibility that the second pattern element 621 protrudes from the element pair image 603 can be reduced. In this case, the reverse image 603a is acquired based on the central axis 632 of the first pattern element 611.

  Further, as described above, the inversion of the element pair image 603 may be simply performed without performing pattern matching, and the axis for evaluating the symmetry of the element pair 63 is the central axis of the first pattern element 611. It is not necessary to exactly match 632. Generally speaking, in the evaluation value acquisition unit 513, the axis that becomes the center when evaluating the symmetry of the element pair 63 has a width in the arrangement direction of the first pattern element 611 and the second pattern element 621. It is preferable that the larger one is set as a reference.

  Furthermore, in the case of the present embodiment, the evaluation value becomes smaller as the second pattern element 621 is displaced from the central axis 632 of the first pattern element 611. Therefore, the evaluation value increases as the difference between the pixel value of the second pattern element 621 and the pixel value of the background of the element pair image 603, that is, the density difference increases. Therefore, in the evaluation value acquisition unit 513, the axis that becomes the center when evaluating the symmetry of the element pair 63 is the one of the first pattern element 611 and the second pattern element 621 that has the smaller density difference from the background. It is preferable that it is determined on the basis of.

  When the density difference between the first pattern element 611 and the second pattern element 621 is small, the evaluation value may be obtained by simply performing pattern matching between the element pair image 603 and the reverse image 603a.

  In the above embodiment, both the first pattern element 611 and the second pattern element 621 are bilaterally symmetric about an axis perpendicular to the arrangement direction. However, in general, regardless of the shape of the pattern element, as the distance between the position of the first pattern element 611, for example, the position of the center of gravity, and the position of the second pattern element 621 increases, the element pair 63 The symmetry becomes smaller and the evaluation value becomes smaller. Accordingly, the first pattern element 611 and the second pattern element 621 are not necessarily symmetrical, and any shape can be adopted. That is, the degree of freedom of the shape of the first pattern element 611 and the second pattern element 621 is increased by using the symmetry evaluation value.

  As described above, when the symmetry evaluation value is used, an appropriate evaluation value can be easily obtained, and as a result, the deviation amount can be acquired with high accuracy. In particular, when symmetry is used as an evaluation value, it is not necessary to perform processing with a large amount of calculation such as extracting an edge from the element pair image 603, and the amount of deviation can be acquired with relatively simple processing.

  Of course, when the symmetry is evaluated with reference to the first pattern element 611, the influence of the shape of the first pattern element 611 on the evaluation value is minimized by the fact that the first pattern element 611 is symmetrical. Can do. In general terms, of the first pattern element 611 and the second pattern element 621, the pattern element serving as a reference for determining the axis that becomes the center when evaluating the symmetry of the element pair 63 is centered on the axis. It is preferably symmetrical, more precisely, line symmetric.

  As shown in FIG. 10, when there are a plurality of vernier patterns that play the same role, a plurality of evaluation values are obtained corresponding to each position. In the case of the example of FIG. 10, since two vernier pattern images 601 and two vernier pattern images 602 are obtained, two evaluation values are obtained from arrangement direction positions that can be regarded as the same in the x direction, and in the y direction. Two evaluation values are obtained from the arrangement direction positions that can be regarded as the same. In this case, the largest value among the plurality of evaluation values obtained from the same position is adopted as the evaluation value at that position. Thereby, the influence of the shape defect of a vernier pattern is reduced.

  By the way, when the resolution of the vernier pattern image is low due to a fine vernier pattern or the like, the evaluation value may meander in the arrangement direction as illustrated in FIG. This meandering is presumed to be caused by interference between the pitch of the element pair 63 and the pitch of the image sensor. As a result, the accuracy of the fitted evaluation value function 642 decreases.

  Further, when generating the reverse image 603 a based on the first pattern element 611, if the second pattern element 621 is very thin, the second pattern element 621 is the second pattern element 621 relative to the first pattern element 611. When shifted by more than half of the width, the second pattern element 621 does not overlap in the element pair image 603 and the reverse image 603a. As a result, the evaluation value is constant regardless of the position of the second pattern element 621.

  Depending on the state of the vernier pattern image such as the color difference due to the film thickness or the chromatic aberration of the image pickup unit 3, the evaluation value monotonously increases or decreases monotonously with respect to the arrangement direction when viewed as a whole, and has an appropriate maximum position. An evaluation value function may not be obtained.

  As described above, when the obtained evaluation value includes something that should not be adopted as the evaluation value, if the deviation amount is obtained using all the evaluation values, the accuracy of the deviation amount is greatly reduced. There is a case.

  FIG. 12 is a diagram showing an operation flow of the function acquisition unit 514 that can obtain the deviation amount with high accuracy, that is, the flow of step S24 in FIG. 6 even in the above case. In the following operation examples, the above-described effects obtained by the operation of the overlay measurement apparatus 1 are the same.

  First, a model function that is convex upward is prepared. For example, a quadratic function or a Gaussian function is prepared as a model function including at least one unknown coefficient. Then, by fitting, the value of the unknown coefficient is determined so that the apex is located at one arrangement direction position where the evaluation value is acquired and the point 641 indicating all evaluation values is approached. The vertical direction of the vertex, that is, the position in the direction of the evaluation value axis is not constrained. Thereby, the provisional function at the position is acquired.

  As shown in FIG. 13, the provisional function 643 is obtained at each arrangement direction position where the evaluation value is acquired (step S241). In principle, the number of provisional functions 643 and the number of evaluation values are equal, but the number of provisional functions 643 may be smaller than the number of evaluation values. In addition, as the provisional function 643, the point 641 indicating the evaluation value and the vertex may be acquired. That is, the position of the vertex in the vertical axis direction may also be constrained. In the example of FIG. 13, the coefficients are limited so that the model function is convex upward, and when the provisional function 643 does not convex upward, all are horizontal straight lines.

  Next, among the provisional functions 643, the provisional function 643 that minimizes the residual sum of squares with the point 641 is specified. FIG. 14 is a diagram illustrating the relationship between the position of the vertex of the provisional function 643 in the arrangement direction and the residual sum of squares. Then, the arrangement direction position of the vertices of the provisional function 643 having the smallest residual square sum is obtained as a provisional coincidence position where the arrangement direction positions of the first pattern element 611 and the second pattern element 621 coincide (step). S242). That is, the function acquisition unit 514 acquires a position corresponding to the one fitted to the plurality of evaluation values among the plurality of provisional functions 643 as a provisional matching position. In the case of FIG. 14, the position “2” is acquired as a temporary matching position.

  The function acquisition unit 514 further selects, as a plurality of selection positions, a position having the highest evaluation value and a position in the vicinity thereof among the provisional matching position and the positions in the vicinity thereof (step S243). For example, in the example of FIG. 14, the evaluation value of the temporary matching position “2” and the two left and right positions “0”, “1”, “3”, and “4” is a total of five positions. The highest position “1” is identified. The number of evaluation value acquisition positions within a range for searching for a position having the highest evaluation value is not limited to five. Then, the position “1” and its two left and right positions, a total of five positions, are selected as selection positions. The number of selection positions is not limited to 5, but may be any number.

  In the function acquisition unit 514, a model function that is convex on a quadratic function, a Gaussian function, or the like is prepared. As shown in FIG. 15, the five points 641 indicated by the black diamonds corresponding to the selected position are the most. The unknown coefficient of the model function is determined by fitting so as to be close, and the evaluation value function 644 is obtained. That is, an evaluation value function 644 is obtained by fitting a model function including at least one unknown coefficient to a plurality of selection evaluation values corresponding to a plurality of selection positions to determine the value of the unknown coefficient (step S244).

  The above process can be regarded as a process of searching for the center position of the range used for fitting with the provisional coincidence position as a reference and then obtaining an evaluation value function by fitting.

  Here, it is assumed that the arrangement direction position of the vertex of the evaluation value function 644 is the position where the arrangement direction positions of the first pattern element 611 and the second pattern element 621 coincide with each other, and the shift amount between the first layer and the second layer However, in the present embodiment, processing for updating the evaluation value function 644 is performed in order to further improve the accuracy of the deviation amount.

  In updating the evaluation value function 644, a deviation between the evaluation value function 644 and a point 641 indicating each selected evaluation value is obtained. If the deviation exceeds a predetermined threshold, for example, 1.5 times the standard deviation, the corresponding evaluation value is excluded from the selected evaluation value. Thereby, the set of selection positions and selection evaluation values is updated (step S245). Other methods may be used for selecting some of the plurality of selection evaluation values based on the deviation. Hereinafter, the selected selection evaluation values are referred to as “update selection evaluation values”. The function acquisition unit 514 acquires an updated evaluation value function by fitting a model function including at least one unknown coefficient to a plurality of update selection evaluation values and determining a value of the unknown coefficient.

  In the example of FIG. 15, the selected evaluation value at the position “0” is excluded, and the updated evaluation value function 645 is acquired (step S246). The array direction position of the maximum point of the evaluation value function 644 before the update is about 2.1. The array direction position of the maximum points of the evaluation value function 645 after the update is about 1.5. Thereafter, in step S25 of FIG. 6, the deviation amount acquisition unit 515 determines the first layer and the second layer based on the position of the maximum point of the evaluation value function 645, that is, the position where the evaluation value is maximum in the arrangement direction. Get the amount of deviation between.

  For example, when the amount of deviation on the substrate 9 when the position of the maximum point is “1” corresponds to 0.20 μm, the amount of deviation obtained from the evaluation value function 644 is 0.42 μm. The amount of deviation obtained from the evaluation value function 645 is 0.30 μm.

  By acquiring a selection evaluation value from a temporary matching position using the provisional function 643 and further acquiring an evaluation value function 644 using the selection evaluation value, the evaluation value is used in a local region. . As a result, it is possible to reduce an inappropriate influence on the evaluation value due to, for example, the color of the vernier pattern image changing unevenly.

  Furthermore, by updating the selection evaluation value, it is possible to reduce the influence of the evaluation value that is locally high or locally low due to a pattern defect or low resolution. Thereby, an evaluation value function can be acquired using an appropriate evaluation value, the amount of deviation between layers can be obtained, and the accuracy of the amount of deviation can be improved.

  In the above embodiment, the provisional function 643 is acquired for each evaluation value acquisition position, but the number of provisional functions 643 may be larger. That is, by obtaining a large number of provisional functions in which the vertex positions are constrained at a large number of positions set in the arrangement direction, it is possible to obtain the most fitted provisional function as an evaluation value function for obtaining a deviation amount. It is.

  In other words, the function acquisition unit 514 corresponds to the positions of a plurality of element pairs, with the model function including a vertex at each of a plurality of preset positions in the arrangement direction and including at least one unknown coefficient. A plurality of provisional functions are obtained by fitting to a plurality of evaluation values and determining the value of the unknown coefficient. Thereafter, an evaluation value function for acquiring the deviation amount can be acquired by various methods based on the plurality of provisional functions.

  Various changes can be made to the configuration and operation of the overlay measurement apparatus 1 described above.

  The method for calculating the evaluation value may be variously changed. When pattern matching between the element pair image 603 and the reverse image 603a is used, an axis serving as a reference for symmetry exists conceptually as described above, but does not clearly appear in calculation. However, the axis of symmetry may be clearly set for calculation based on the element pair image 603, the first pattern element 611, the second pattern element 621, the element pair 63, and the like.

  In the above embodiment, for convenience of explanation, the combination of the position and the evaluation value is plotted and expressed as a point 641. However, if processing equivalent to the concept of the point 641 or a function is realized, the position and the evaluation value are expressed. Various information storage structures for storing relations and functions with values and arithmetic processing can be employed.

  The operation flow of the overlay measurement apparatus 1 shown in the above embodiment may be changed as appropriate. For example, the calculation process for the vernier pattern image 601 corresponding to the x direction and the calculation process for the vernier pattern image 602 corresponding to the y direction may be performed in parallel. For each element pair 63, the acquisition of the element pair image 603 and the calculation of the evaluation value may be performed sequentially. Some of the preparatory work may be automated. Conversely, a part of the processing up to acquiring the element pair image 603 may be performed by the operator, omitting the preparation work. In other arithmetic processes, auxiliary work by the worker may be included.

  The first pattern element 611 and the second pattern element 621 do not need to be arranged at regular intervals as a whole. For example, a pattern element for measuring only the deviation amount in the (+ x) direction and a pattern element for measuring only the deviation amount in the (−x) direction may be provided as individual vernier patterns. The same applies to the y direction.

  The substrate 9 is not limited to a semiconductor substrate. As long as a fine multilayer pattern such as a glass substrate is formed, the technique of the overlay measuring apparatus 1 can be applied. Further, the imaging unit 3 may be a microscope other than the optical microscope as long as the multilayer film can be observed.

  The configurations in the above-described embodiments and modifications may be combined as appropriate as long as they do not contradict each other.

DESCRIPTION OF SYMBOLS 1 Overlay measuring device 9 Board | substrate 61 1st pattern 62 2nd pattern 63 Element pair 512 Element pair image acquisition part 513 Evaluation value acquisition part 514 Function acquisition part 515 Deviation amount acquisition part 601 Vernier pattern image 603 Element pair image 603a Reverse image 611 1st 1 pattern element 621 2nd pattern element 642, 644, 645 evaluation value function 643 provisional function S21-S26 steps

Claims (10)

A first pattern includes a plurality of first pattern elements arranged at equal intervals in a predetermined arrangement direction, and a plurality of second pattern elements arranged at equal intervals in the arrangement direction. From the vernier pattern image obtained by imaging the first pattern included in the first layer formed on the substrate and the second pattern included in the second layer overlapping the first layer, An overlay measurement apparatus that acquires a shift amount between the first layer and the second layer in the arrangement direction,
An element pair image acquisition unit that acquires, from the vernier pattern image, an element pair image indicating an element pair that is a combination of one first pattern element and a second pattern element corresponding to the one first pattern element;
An evaluation value acquisition unit that acquires an evaluation value indicating the symmetry of an element pair centered on an axis perpendicular to the arrangement direction from an element pair image;
Based on a plurality of evaluation values obtained for a plurality of element pairs, a function acquisition unit that acquires an evaluation value function indicating a relationship between the position in the arrangement direction and the evaluation value;
From the evaluation value function, obtain a position where the evaluation value is maximum in the arrangement direction, and obtain a deviation amount between the first layer and the second layer from the position;
An overlay measuring apparatus comprising: The overlay measurement apparatus according to claim 1,
In the evaluation value acquisition unit, the axis is determined based on a larger one of the first pattern element and the second pattern element of the element pair with respect to the arrangement direction. . The overlay measurement apparatus according to claim 1 or 2,
In the evaluation value acquisition unit, the axis is determined based on a smaller density difference from the background of the first pattern element and the second pattern element of the element pair. . The overlay measurement apparatus according to claim 2 or 3,
Of the first pattern element and the second pattern element, the pattern element serving as a reference for determining the axis is symmetric with respect to the axis. The overlay measurement apparatus according to any one of claims 1 to 4,
The overlay measurement apparatus, wherein the evaluation value acquisition unit inverts the element pair image with the axis as a center, and acquires a degree of coincidence between an image before inversion and an image after inversion as the evaluation value. The overlay measurement apparatus according to claim 1,
The function acquisition unit determines a value of the at least one unknown coefficient by fitting a model function including at least one unknown coefficient to the plurality of evaluation values corresponding to the positions of the plurality of element pairs. An overlay measurement apparatus that acquires the evaluation value function. The overlay measurement apparatus according to claim 1,
The function acquisition unit is configured to select a model function having a vertex at each of a plurality of preset positions in the arrangement direction and including at least one unknown coefficient corresponding to the positions of the plurality of element pairs. Overlay, wherein a plurality of provisional functions are obtained by fitting the evaluation values to determine a value of the at least one unknown coefficient, and the evaluation value functions are obtained based on the plurality of provisional functions Measuring device. The overlay measurement apparatus according to claim 7,
The function acquisition unit acquires, as a temporary matching position, a position corresponding to the most fitted to the plurality of evaluation values among the plurality of temporary functions, and among the temporary matching position and its neighboring positions Selecting a position having the highest evaluation value and a position in the vicinity thereof as a plurality of selection positions, fitting a model function including at least one unknown coefficient to a plurality of selection evaluation values corresponding to the plurality of selection positions, and An overlay measuring apparatus that obtains the evaluation value function by determining a value of at least one unknown coefficient. The overlay measurement apparatus according to claim 8,
The function acquisition unit obtains deviations of the plurality of selection evaluation values from the evaluation value function, selects a part of the plurality of selection evaluation values based on the deviation as a plurality of update selection evaluation values, Overlay measurement characterized in that an updated evaluation value function is obtained by fitting a model function including at least one unknown coefficient to the updated selection evaluation value of and determining a value of the at least one unknown coefficient apparatus. A first pattern includes a plurality of first pattern elements arranged at equal intervals in a predetermined arrangement direction, and a plurality of second pattern elements arranged at equal intervals in the arrangement direction. From the vernier pattern image obtained by imaging the first pattern included in the first layer formed on the substrate and the second pattern included in the second layer overlapping the first layer, An overlay measurement method for obtaining a shift amount between the first layer and the second layer in the arrangement direction,
a) A plurality of element pair images each representing an element pair, which is a combination of one first pattern element and a second pattern element corresponding to the one first pattern element, are acquired from the vernier pattern image Process,
b) obtaining a plurality of evaluation values indicating symmetry of an element pair centered on an axis perpendicular to the arrangement direction from the plurality of element pair images;
c) obtaining an evaluation value function indicating a relationship between the position in the arrangement direction and the evaluation value based on the plurality of evaluation values;
d) obtaining, from the evaluation value function, a position where the evaluation value is maximum in the arrangement direction, and obtaining a shift amount between the first layer and the second layer from the position;
An overlay measurement method comprising:

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