Image acquisition method and image acquisition device

Abstract

PURPOSE: To provide an image acquisition method with which it is possible to adjust the luminous energy of inspection light entering an imaging sensor so that an image of desired grayscale value can be obtained in relation to the image storage time of a desired element.CONSTITUTION: The image acquisition method in one aspect of the present invention comprises: a step in which the coefficient of a relational expression between a parameter value corresponding to the incident light quantity to an imaging sensor and the output value of the imaging sensor in cases when light is incident on the imaging sensor having a photosensor element for a reference image storage time which the image storage time of the photosensor element is based on, is stored in a storage device; a step in which a desired image storage time is inputted, and a parameter value in order for the desired output value of the imaging sensor to be obtained is calculated using the relational expression after correction using the output value of the imaging sensor that is dependent on the desired image storage time when incident light quantity is zero; a step in which the incident light quantity to the imaging sensor is adjusted to the calculated parameter value; and a step in which a sample is imaged using the imaging sensor with the adjusted incident light quantity and image data having been imaged is outputted.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to an image acquisition method and an image acquisition device, and more particularly to an apparatus and method for capturing an image of a pattern on an exposure mask used in semiconductor manufacturing. [Background technology]

[0002] In recent years, with the increasing integration and capacity of large-scale integrated circuits (LSI), the circuit line width required for semiconductor elements has become increasingly narrow. These semiconductor elements are manufactured by forming circuits by exposing and transferring the pattern onto a wafer using a reduced projection exposure device called a stepper, using an original pattern (also called a mask or reticle, hereinafter collectively referred to as a mask) on which the circuit pattern is formed.

[0003] Improving yield is essential for the manufacture of LSIs, which are very costly to manufacture. One of the major factors that reduces yield is pattern defects in the masks used when exposing and transferring ultra-fine patterns onto semiconductor wafers using photolithography technology. In recent years, as the dimensions of LSI patterns formed on semiconductor wafers have become increasingly miniaturized, the dimensions that must be detected as pattern defects have also become extremely small. This has led to a need for higher-precision pattern inspection equipment to inspect defects in the transfer masks used in LSI manufacturing.

[0004] Inspection techniques include, for example, "die-to-die inspection," which compares optical image data captured of the same pattern at different locations on the same mask, and "die-to-database inspection," which converts pattern-design CAD data into a device input format for input by a drawing device when drawing the pattern on a mask, inputs the converted drawing data (design data) into an inspection device, generates a reference image based on this, and compares it with an optical image that serves as measurement data captured from the pattern.

[0005] To perform such pattern inspection, it is necessary to capture an image of the pattern. The greater the amount of light incident on the image sensor, the greater the gradation value that is output. The smaller the amount of light incident on the image sensor, the smaller the gradation value that is output. Therefore, before the inspection is performed, the amount of inspection light incident on the image sensor is adjusted so that an image with the optimal gradation value for the inspection can be obtained.

[0006] Here, the image sensor is equipped with a photosensor element, and the image accumulation time of this element is set to a predetermined time before capturing an image of the pattern. However, depending on the sample to be inspected and the inspection conditions, it may be necessary to change the image accumulation time of the element. If the image accumulation time of the element is doubled while the light intensity is constant, the incident light information will double, and the output gradation value should double. Therefore, when the image accumulation time of the element is doubled, the light intensity should be halved to obtain the same gradation value. Therefore, the relationship between the incident light intensity and the output value of the image sensor is measured for the image accumulation time of a reference element. Then, when the image accumulation time of the element is changed, a method is being considered to determine the required light intensity from this relationship.

[0007] However, simply changing the light intensity by the ratio of the image accumulation time causes errors in the output values of the image sensor, which results in a deterioration in the accuracy of the obtained image.Furthermore, errors in the output values of the image sensor cause false defects in pattern defect inspection.

[0008] Although not a pattern inspection device for masks or the like, an image recognition device that recognizes objects such as fingers or barcodes has been disclosed that shortens the accumulation time of the sensor when the ambient temperature is high (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2003-032435 JP 2020-42035 JP 2020-42035 JP 2020-42035 JP 2020-42035 Summary of the Invention [Problem to be solved by the invention]

[0010] Therefore, one aspect of the present invention provides an image acquisition method and apparatus capable of adjusting the amount of inspection light incident on an image sensor so that an image with a desired gradation value can be obtained for the image accumulation time of a desired element. [Means for solving the problem]

[0011] An image acquisition method according to one aspect of the present invention includes: a step of storing in a storage device coefficients of a relational expression between parameter values corresponding to the amount of light incident on an image sensor having a photosensor element and output values of the image sensor when light is incident on the image sensor for a reference image accumulation time that is a reference for the image accumulation time of the photosensor element; Input the desired image accumulation time and output the image sensor value depending on the desired image accumulation time when the incident light amount is zero. and the ratio of the output value of the image sensor that depends on the reference image accumulation time when the amount of incident light is zero. calculating parameter values for obtaining a desired output value of the image sensor using the corrected relational expression obtained by correcting the relational expression using adjusting the amount of light incident on the image sensor to the calculated parameter value; capturing an image of the sample using the imaging sensor with the adjusted amount of incident light and outputting the captured image data; The present invention is characterized by the following features.

[0012] Furthermore, it is preferable to use the slope and intercept approximated by a linear function as the coefficients of the relational expression.

[0013] In addition, it is preferable that the relational expression be corrected using the ratio between the reference image accumulation time and the desired image accumulation time, and the ratio between the output value of the image sensor that depends on the reference image accumulation time when the amount of incident light is zero and the output value of the image sensor that depends on the desired image accumulation time when the amount of incident light is zero.

[0014] An image acquisition device according to one aspect of the present invention includes: an imaging sensor having a photosensor element; a storage device that stores coefficients of a relational expression between parameter values corresponding to the amount of light incident on the image sensor and output values of the image sensor when light is incident for a reference image accumulation time that is a reference for the image accumulation time of the photosensor element; Input the desired image accumulation time and output the image sensor value depending on the desired image accumulation time when the incident light amount is zero. and the ratio of the output value of the image sensor that depends on the reference image accumulation time when the amount of incident light is zero. a parameter value calculation unit that calculates parameter values for obtaining a desired output value of the image sensor using the corrected relational expression obtained by correcting the relational expression using a light amount adjustment mechanism that adjusts the amount of light incident on the image sensor to the calculated parameter value; Equipped with The imaging sensor is used with the adjusted amount of incident light to capture an image of the sample, and the captured image data is output. [Effects of the Invention]

[0015] According to one aspect of the present invention, the amount of inspection light incident on the image sensor can be adjusted so that an image with a desired gradation value can be obtained for a desired image accumulation time of an element, thereby suppressing image degradation caused by changes in the image accumulation time. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a configuration diagram showing a configuration of a pattern inspection device according to a first embodiment. [Figure 2] FIG. 2 is a conceptual diagram for explaining an inspection area in the first embodiment. [Figure 3] FIG. 3 is a diagram for explaining a method of adjusting the amount of light in the first embodiment. [Figure 4] FIG. 10 is a diagram showing an example of the relationship between the output of a PD and the output of a TDI sensor with respect to the image accumulation time of a reference element in a comparative example to the first embodiment. [Figure 5]10 is a diagram showing an example of the relationship between the output of a PD and the output of a TDI sensor for each image accumulation time of an element according to the first embodiment. FIG. [Figure 6] FIG. 3 is a flowchart showing an example of main steps of the inspection method according to the first embodiment. [Figure 7] 5 is a diagram showing an example of the relationship between the PD output value and the photosensor array output value in the first embodiment. FIG. [Figure 8] 3 is a block diagram showing an example of the internal configuration of a light intensity calculation circuit according to the first embodiment. FIG. [Figure 9] 10 is a diagram showing an example of the relationship between the PD output and the TDI output for each image accumulation time of the photosensor element in the first embodiment and the comparative example. FIG. [Figure 10] FIG. 3 is a diagram illustrating a filter process according to the first embodiment. [Figure 11] 3 is a diagram illustrating an example of an internal configuration of a comparison circuit according to the first embodiment. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0017] Embodiment 1 Fig. 1 is a configuration diagram showing the configuration of a pattern inspection apparatus according to embodiment 1. In Fig. 1, an inspection apparatus 100 that inspects a substrate to be inspected, for example, a pattern formed on a mask, for defects includes an optical image acquisition mechanism 150 and a control circuit 160.

[0018] The optical image acquisition mechanism 150 includes a light source 103, a light intensity adjustment mechanism 314, a transmitted illumination optical system 170, a reflected illumination optical system 171, a movably arranged XYθ table 102, a magnifying optical system 104, a beam splitter 174, an imaging optical system 176, a mirror 177, an imaging optical system 178, an imaging optical system 179, photosensor arrays 105 and 305, sensor circuits 106 and 306, photoelectric sensors (PD) 310 and 312, a stripe pattern memory 123, a laser length measurement system 122, and an autoloader 130. When only a transmission inspection using transmitted light is performed (when a reflection inspection is not performed), the photosensor array 105, the sensor circuit 106, the reflected illumination optical system 171, the imaging optical system 178, the photoelectric sensor 310, and the beam splitter 174 may be omitted. When only reflection inspection using reflected light is performed (when transmission inspection is not performed), the photosensor array 305, sensor circuit 306, transmission illumination optical system 170, imaging optical system 179, and photoelectric sensor 312 may be omitted.

[0019] A substrate 101 transferred from an autoloader 130 is placed on the XYθ table 102. The substrate 101 includes, for example, a photomask for exposure that transfers a pattern onto a semiconductor substrate such as a wafer. This photomask has a plurality of graphic patterns formed thereon to be inspected. The substrate 101 is placed on the XYθ table 102 with the pattern-formed surface facing downward, for example.

[0020] It is preferable to use, for example, a TDI (time delay integration) sensor as the photosensor array 105, 305 (an example of an imaging sensor). A TDI sensor has a plurality of photosensor elements arranged two-dimensionally. Alternatively, the photosensor array 105, 305 may be, for example, a line sensor having a plurality of photosensor elements arranged one-dimensionally. When capturing an image, a predetermined image accumulation time (sometimes also referred to as a scan time; the same applies hereinafter) is set for each photosensor element. In the TDI sensor, the outputs of the plurality of photosensor elements arranged in the scan direction are integrated and output. The plurality of photosensor elements arranged in the scan direction capture the same pixel at different times in accordance with the movement of the XYθ table 102.

[0021] The PDs 310 and 312 are arranged so as to be movable in a direction perpendicular to the optical axis by a driving mechanism (not shown). In other words, the PDs 310 and 312 move between inside and outside the respective optical paths.

[0022] In the control system circuit 160, a control computer 110 that controls the entire inspection apparatus 100 is connected via a bus 120 to a position circuit 107, a comparison circuit 108, a reference image creation circuit 112, an autoloader control circuit 113, a table control circuit 114, a PD measurement circuit 136, a light intensity calculation circuit 132, a light intensity control circuit 134, a magnetic disk drive 109, a memory 111, a magnetic tape drive 115, a flexible disk drive (FD) 116, a CRT 117, a pattern monitor 118, and a printer 119. The sensor circuits 106 and 306 are connected to a stripe pattern memory 123, which is connected to the comparison circuit 108. The XYθ table 102 is driven by an X-axis motor, a Y-axis motor, and a θ-axis motor. The XYθ table 102 is an example of a stage. The reference image creation circuit 112 is connected to the comparison circuit 108.

[0023] Note that a series of "circuits" such as the position circuit 107, the comparison circuit 108, the reference image creation circuit 112, the autoloader control circuit 113, the table control circuit 114, the PD measurement circuit 136, the light intensity calculation circuit 132, and the light intensity control circuit 134 include a processing circuit. Such processing circuits include an electrical circuit, a computer, a processor, a circuit board, a quantum circuit, or a semiconductor device. Alternatively, different processing circuits (separate processing circuits) may be used. For example, a series of "circuits" such as the position circuit 107, the comparison circuit 108, the reference image creation circuit 112, the autoloader control circuit 113, the table control circuit 114, the PD measurement circuit 136, the light intensity calculation circuit 132, and the light intensity control circuit 134 may be configured and executed by the control computer 110. Input data or calculated results required for the position circuit 107, comparison circuit 108, reference image creation circuit 112, autoloader control circuit 113, table control circuit 114, PD measurement circuit 136, light intensity calculation circuit 132, and light intensity control circuit 134 are stored in a memory (not shown) within each circuit or in memory 111. Input data or calculated results required for the control computer 110 are stored in a memory (not shown) within the control computer 110 or in memory 111. A program for executing a computer or a processor may be recorded on a recording medium such as a magnetic disk device 109, a magnetic tape device 115, a FD 116, or a ROM (read only memory).

[0024] The light intensity adjustment mechanism 314 is configured, for example, by a combination of a light-shielding shutter and multiple filter elements with different transmittances. When a light-shielding shutter is provided, the inspection light from the light source 103 is blocked. In addition, the combination of the multiple filter elements provided is adjusted to adjust the amount of light passing through the light intensity adjustment mechanism 314. This adjusts the amount of light reaching the photosensor arrays 105 and 305. The light intensity adjustment mechanism 314 is controlled by the light intensity control circuit 134.

[0025] In the inspection device 100, a high-magnification transmission inspection optical system is configured by the light source 103, the light amount adjustment mechanism 134, the transmission illumination optical system 170, the XYθ table 102, the magnifying optical system 104, the imaging optical system 176, the mirror 177, the imaging optical system 179, and the photosensor array 305. In addition, a high-magnification reflection inspection optical system is configured by the light source 103, the light amount adjustment mechanism 134, the reflection illumination optical system 171, the beam splitter 174, the magnifying optical system 104, the XYθ table 102, the imaging optical system 176, the imaging optical system 178, and the photosensor array 105.

[0026] The XYθ table 102 is driven by a table control circuit 114 under the control of the control computer 110. It can be moved by a drive system such as a three-axis (XY-θ) motor that drives in the X, Y, and θ directions. These X, Y, and θ motors can be, for example, step motors. The XYθ table 102 can be moved horizontally and rotationally by the motors on the X, Y, and θ axes. The movement position of the substrate 101 placed on the XYθ table 102 is measured by a laser measurement system 122 and supplied to the position circuit 107. The transfer of the substrate 101 from the autoloader 130 to the XYθ table 102 and the transfer process of the substrate 101 from the XYθ table 102 to the autoloader 130 are controlled by an autoloader control circuit 113.

[0027] Drawing data (design data) that is the basis for forming a pattern on the inspected substrate 101 is input from outside the inspection device 100 and stored in the magnetic disk device 109. The drawing data defines a plurality of graphic patterns, and each graphic pattern is usually composed of a combination of a plurality of element graphics. However, a graphic pattern composed of a single graphic may also be present. On the inspected substrate 101, corresponding patterns are formed based on each graphic pattern defined in the drawing data.

[0028] 1 shows components necessary for explaining the first embodiment. It goes without saying that the inspection device 100 may include other components that are normally required.

[0029] 2 is a conceptual diagram illustrating the inspection area in the first embodiment. As shown in FIG. 2, the inspection area 10 (the entire inspection area) of the substrate 101 is virtually divided, for example, in the Y direction, into a plurality of rectangular inspection stripes 20 each having a scan width W of the TDI sensor 105. The inspection device 100 then acquires an image (stripe area image) for each inspection stripe 20. For each inspection stripe 20, a laser beam (inspection light) is used to capture an image of the graphic pattern arranged within the inspection stripe 20 in the longitudinal direction (X direction) of the stripe area. Note that, to prevent missing images, the inspection stripes 20 are preferably set so that adjacent inspection stripes 20 overlap with each other by a predetermined margin width.

[0030] Optical images are acquired while the photosensor arrays 105, 305 move continuously relative to each other in the X direction as the XYθ table 102 moves. The photosensor arrays 105, 305 continuously capture optical images with a scan width W as shown in FIG. 2. In other words, if the photosensor arrays 105, 305 are TDI sensors, they capture optical images of the substrate 101 surface on which multiple graphic patterns are formed while moving relatively in the integration direction of the photosensor arrays 105, 305. If the photosensor arrays 105, 305 are line sensors, they capture optical images of the substrate 101 surface on which multiple graphic patterns are formed while moving relatively in a direction perpendicular to the arrangement direction of the multiple photosensor elements. In the first embodiment, after capturing an optical image of one inspection stripe 20, the next inspection stripe 20 is moved in the Y direction, and then optical images with a scan width W are continuously captured while moving in the reverse direction. That is, imaging is repeated in the forward (FWD) and back-forward (BWD) directions, which are opposite directions on the outward and return paths.

[0031] Furthermore, in actual inspection, the stripe region image of each inspection stripe 20 is divided into a plurality of rectangular frame region 30 images, as shown in FIG. 2. Then, inspection is performed for each frame region 30 image. For example, it is divided into a size of 512 × 512 pixels. Therefore, a reference image to be compared with the frame image 31 of the frame region 30 is also created for each frame region 30.

[0032] Here, the imaging direction is not limited to repeated forward (FWD)-backward (BWD). Imaging may be performed from one direction. For example, FWD-FWD may be repeated. Alternatively, BWD-BWD may be repeated.

[0033] 3 is a diagram for explaining a method of adjusting the light intensity in the first embodiment. In FIG. 3, the transmitted illumination optical system 170 has a mirror 42 and a lens 44. The reflected illumination optical system 171 has a beam splitter 40, a mirror 46, and a lens 48. The inspection light output from the light source 103 passes through the light intensity adjustment mechanism 314 and enters the beam splitter 40. Then, the beam splitter 40 splits the inspection light into inspection light for transmitted inspection and inspection light for reflected inspection.

[0034] The inspection light for transmission inspection is reflected by mirror 42 and illuminates substrate 101 on XYθ table 102 from the front side by lens 44. The transmitted light that passes through the substrate passes through magnifying optical system 104 and beam splitter 174 and enters imaging optical system 176. Imaging optical system 176 forms an image of the transmitted light at the position of mirror 177. The transmitted light reflected by mirror 177 is imaged on photosensor array 305 by imaging optical system 179.

[0035] On the other hand, the inspection light for reflection inspection is reflected by the mirror 46 and enters the beam splitter 174 via the lens 48. It is then reflected by the beam splitter 174 and enters the magnifying optical system 104. The inspection light for reflection inspection illuminates the back side of the substrate 101 on the XYθ table 102 via the magnifying optical system 104. At this time, the inspection light for reflection inspection illuminates a position slightly shifted from the inspection light for transmission inspection. The reflected light from the substrate 101 passes through the magnifying optical system 104 and the beam splitter 174 and enters the imaging optical system 176. The imaging optical system 176 forms an intermediate image of the reflected light near the position of the mirror 177 and makes it enter the imaging optical system 178. The reflected light is then imaged on the photosensor array 105 by the imaging optical system 178.

[0036] Here, capturing an image using photosensor arrays 105 and 305 and checking the resulting image each time the light intensity is adjusted would take a long time. For this reason, PD 310 is placed in the optical path just before light enters photosensor array 105. Similarly, PD 312 is placed in the optical path just before light enters photosensor array 305. PDs 310 and 312 measure the light intensity and adjust it using light intensity adjustment mechanism 314 to achieve the desired light intensity. The relationship between the output of PD 310 and the gradation values captured by photosensor array 105 with a reference element image accumulation time and output from sensor circuit 106 is measured in advance. Similarly, the relationship between the output of PD 312 and the gradation values captured by photosensor array 305 with a reference element image accumulation time and output from sensor circuit 306 is measured in advance.

[0037] FIG. 4 shows an example of the relationship between the PD output and the TDI sensor output for the image accumulation time of a reference element in a comparative example of the first embodiment. In FIG. 4, the vertical axis represents the PD output, and the horizontal axis represents the TDI sensor output. A negative PD output is also shown, but this is to show the intercept of the vertical axis of the linear proportional function by extending a virtual straight line. The TDI sensor output here represents the gradation value captured by the photosensor array 105 (305) and output from the sensor circuit 106 (306). In FIG. 4, the image accumulation time of the reference element is shown as a scan time of 1X. The comparative example in FIG. 4 shows an example of the results of measuring the relationship between the PD output and the TDI sensor output for which the image accumulation time of the reference element is set through experiments, etc., for the image accumulation time of the reference element. For the image accumulation time of the reference element, the light intensity adjustment mechanism 314 adjusts the light intensity so that the PD output achieves the desired gradation value according to the linear proportional function represented by the scan time of 1X. If the dynamic range of the TDI sensor output is defined as, for example, 256 gradations, for example, in a transmission inspection, the light intensity is adjusted so that the gradation value of the white part (glass part) becomes, for example, 200. Alternatively, for example, in a reflection inspection, the light intensity is adjusted so that the gradation value of the black part (light-shielding film part) becomes, for example, 200.

[0038] In this way, the photosensor arrays 105 and 305 capture patterns after setting the image accumulation time of the elements to a predetermined time. However, depending on the sample being inspected and the inspection conditions, it may be necessary to change the image accumulation time of the elements. For the photosensor arrays 105 and 305, if the image accumulation time of the elements is doubled relative to the image accumulation time of the reference element under a constant light intensity, the incident light information will double, and the output gradation value should double. Therefore, when the image accumulation time of the elements is doubled relative to the image accumulation time of the reference element, the light intensity should be halved to obtain the same gradation value. Therefore, in the comparative example of Figure 4, a linear proportional function represented by a scan time of 2X, where the image accumulation time of the elements is doubled, is defined so that the PD output is half that of the linear proportional function represented by a scan time of 1X. Therefore, the coefficient representing the slope of the linear proportional function represented by a scan time of 1X is halved, and the coefficient representing the intercept of the linear proportional function is also halved.

[0039] Similarly, in the comparative example of Figure 4, the linear proportional function shown for a scan time of 4X, which is four times the image accumulation time of the element, is defined so that the PD output is 1 / 4 of the linear proportional function shown for a scan time of 1X. Therefore, the coefficient indicating the slope of the linear proportional function shown for a scan time of 1X is set to 1 / 4, and the coefficient indicating the intercept of the linear proportional function is also set to 1 / 4.

[0040] For example, when the image accumulation time of the element is changed from t1 to t2, the PD output E at the TDI output I is defined by the following equation (1) using the slope a and intercept b of the linear proportional function when the image accumulation time of the element is t1. (1) E=a(t1 / t2)·I+b(t1 / t2)

[0041] In the comparative example, when the image accumulation time of the element is changed from scan time 1X to scan time 2X, which is doubled, the light intensity adjustment mechanism 314 adjusts the light intensity so that the PD output will provide the desired gradation value, using a linear proportional function represented by the calculated scan time 2X. This should have completed the light intensity adjustment.

[0042] However, simply changing the light intensity by the ratio of the image accumulation times of the elements causes an error in the output value after gradation value conversion of the photosensor array 105 (305) including the TDI sensor. As a result, there is a problem that the accuracy of the obtained image is degraded. Furthermore, if an error occurs in the output value after gradation value conversion of the photosensor array 105 (305) including the TDI sensor, false defects will occur during pattern defect inspection.

[0043] The photosensor array 105 (305) including the TDI sensor has the characteristic of outputting a grayscale value even when no laser light is irradiated due to the influence of dark current. In the comparative example of FIG. 4, the TDI output value when the PD output is zero exhibits a finite value. The longer the image accumulation time of the element, the greater the dark current. Therefore, as shown in the comparative example of FIG. 4, errors occur when the TDI output at dark current is defined as the same value for each image accumulation time of the element.

[0044] FIG. 5 is a diagram showing an example of the relationship between the PD output and the TDI sensor output for each image accumulation time of the element in embodiment 1. In FIG. 5, the vertical axis represents the PD output, and the horizontal axis represents the TDI sensor output. A state in which the PD output is negative is also shown, but this is because a virtual straight line is extended to show the vertical intercept of a linear proportional function. The TDI sensor output here represents the grayscale value captured by the photosensor array 105 (305) and output from the sensor circuit 106 (306). In FIG. 5, the image accumulation time of the reference element is shown as a scan time of 1X. FIG. 5 shows an example of the results of measuring the relationship between the PD output and the TDI sensor output through experiments or the like for the image accumulation time of the reference element. The relationship at a scan time of 1X is the same as that in the comparative example of FIG. 4.

[0045] The graph shows the results of experimentally measuring the relationship between the PD output and the TDI sensor output for an image accumulation time of an element that is twice the image accumulation time of a reference element, with a scan time of 2X. The graph also shows the results of experimentally measuring the relationship between the PD output and the TDI sensor output for an image accumulation time of an element that is four times the image accumulation time of a reference element, with a scan time of 4X. As a result, the TDI output under dark current was gradation value 10 for a scan time of 1X, but became gradation value 11 for a scan time of 2X. Furthermore, it became gradation value 14 for a scan time of 4X. Meanwhile, the slope was 1 / 2 for a scan time of 1X, and 1 / 4 for a scan time of 1X. Therefore, in the first embodiment, a correction term using the TDI output value under dark current is added to the linear proportional function of the image accumulation time of each element shown in the comparative example.

[0046] For example, when the image accumulation time of the element is changed from t1 to t2, if the TDI output value for the dark current changes from α1 to α2, the PD output E at the TDI output I can be defined by the following equation (2) using the slope a and intercept b of the linear proportional function when the image accumulation time of the element is t1. (2) E=a(t1 / t2)·I+b(t1 / t2)(α2 / α1)

[0047] Therefore, if the relationship between the PD output and the TDI sensor output is measured in advance through an experiment or the like for the image accumulation time of a reference element, a linear proportional function for the image accumulation time of a desired element can be obtained by measuring the TDI output value for the dark current for the image accumulation time of the desired element.

[0048] Fig. 6 is a flowchart showing an example of the main steps of the inspection method according to Embodiment 1. In Fig. 6, the inspection method according to Embodiment 1 carries out a series of steps including a reference correlation equation acquisition and storage step (S102), an image accumulation time setting step (S104), a light-shielding shutter installation step (S108), a light-shielding state image acquisition step (S110), a light-shielding state gradation value acquisition step (S112), a PD output calculation step (S114), a light amount adjustment step (S116), a scanning step (S202), a reference image creation step (S204), and a comparison step (S206).

[0049] In the reference correlation equation acquisition and storage step (S102), the relationship between a parameter value corresponding to the amount of light incident on the photosensor array 105 (305) and the output value of the photosensor array 105 (305) when light is incident for a reference image accumulation time t1, which serves as a reference for the image accumulation time of the photosensor element, is acquired. The output value of the PD 310 is used as the parameter value corresponding to the amount of light incident on the photosensor array 105 (305). Similarly, the output value of the PD 312 is used as the parameter value corresponding to the amount of light incident on the photosensor array 305. Specifically, first, the PD 310 is moved into the optical path. Then, the light amount is set to be variable, and the output value of the PD 310 for each amount of light and the output value of the photosensor array 105 for the reference image accumulation time t1 are measured using reflected light from the substrate 101. The light amount is set to be variable by the light amount adjustment mechanism 314 controlled by the light amount control circuit 134. When the light intensity is variably set, it is preferable to include a light-shielded state (PD310 output value = zero) in which light from the light source 103 is blocked by a light-shielding shutter as one of the multiple light intensities to be set. The output value of PD310 is measured by PD measurement circuit 136. The substrate 101 used for measurement may be the substrate to be inspected, or may be another evaluation substrate. For example, measurement may be performed using a black portion (light-shielding film portion) of the substrate. In this case, it is sufficient to obtain an output value of photosensor array 105 that depends on the amount of light incident on photosensor array 105. The grayscale value converted by sensor circuit 106 is used as the output value of photosensor array 105. The output value of PD310 is measured by PD measurement circuit 136. Then, using the obtained measurement value, a relational expression between the output value of PD310 and the output value of photosensor array 105 is approximated by fitting.

[0050] FIG. 7 is a diagram showing an example of the relationship between the PD output value and the photosensor array output value in the first embodiment. The example in FIG. 7 shows a case where approximation is performed using a linear function. The vertical axis represents the PD output, and the horizontal axis represents the TDI gradation value. The TDI gradation value represents the gradation value obtained by converting the output of the photosensor array 105 by the sensor circuit 106. As shown in FIG. 7, by approximating using a linear function, the relational expression y=ax+b can be obtained.

[0051] The coefficients a and b of this relational expression are then stored in a storage device within the light intensity calculation circuit 132.

[0052] Similarly, the PD 312 is moved into the optical path. Then, the light intensity is set to a variable value, and the output value of the PD 312 for each light intensity and the output value of the photosensor array 305 at the reference image accumulation time t1 are measured using light transmitted through the substrate 101. The light intensity is set to a variable value by the light intensity adjustment mechanism 314 controlled by the light intensity control circuit 134. When setting the light intensity to a variable value, it is preferable to include a light-shielded state (PD 312 output value = zero) in which light from the light source 103 is blocked by a light-shielding shutter as one of the set light intensities. For example, it is recommended to measure using a white portion (glass portion) of the substrate. The gradation value converted by the sensor circuit 306 is used as the output value of the photosensor array 305. The output value of the PD 312 is measured by the PD measurement circuit 136. The obtained measurement value is used to approximate the relationship between the output value of the PD 312 and the output value of the photosensor array 305 by fitting. The coefficients a' and b' of this relationship are then stored in a storage device within the light intensity calculation circuit 132. Generally, when photosensor arrays 105 and 305 with the same sensitivity are used, the coefficients will be the same. The fitting process is performed by a processing unit (not shown) in the light intensity calculation circuit 132. Alternatively, it may be performed offline from the inspection device 100.

[0053] As a result, the slope a and intercept b, which are coefficients of the linear function, can be obtained for the reference image accumulation time t1 of the photosensor element.

[0054] FIG. 8 is a block diagram showing an example of the internal configuration of the light intensity calculation circuit according to the first embodiment. In FIG. 8, a storage device 51 such as a magnetic disk device, an image accumulation time setting unit 50, a light-shielding state gradation value acquisition unit 52, and a PD output calculation unit 54 are arranged within the light intensity calculation circuit 132. Each of the units, such as the image accumulation time setting unit 50, the light-shielding state gradation value acquisition unit 52, and the PD output calculation unit 54, has a processing circuit. Examples of such a processing circuit include an electric circuit, a computer, a processor, a circuit board, a quantum circuit, or a semiconductor device. Each unit may share a common processing circuit (the same processing circuit). Alternatively, each unit may use a different processing circuit (separate processing circuit). Input data or calculation results required for the image accumulation time setting unit 50, the light-shielding state gradation value acquisition unit 52, and the PD output calculation unit 54 are stored in a memory (not shown) within the light intensity calculation circuit 132 or in memory 111.

[0055] The information on the slope a and intercept b, which are coefficients of the linear function, at the reference image accumulation time t1 of the photosensor element input to the light amount calculation circuit 132 is stored in the storage device 51.

[0056] In the image accumulation time setting step (S104), the image accumulation time setting unit 50 sets a desired image accumulation time t2. This sets the image accumulation time t2 of the photosensor elements of the photosensor arrays 105, 305. The image accumulation time of an element corresponds to the imaging time of each photosensor element per pixel. Therefore, the image accumulation time of an element is adjusted by the movement speed of the XYθ table 102 during scanning. If the movement speed of the XYθ table 102 is fast, the image accumulation time of the element will be short. If the movement speed of the XYθ table 102 is slow, the image accumulation time of the element will be long.

[0057] In the light-shielding shutter installation step (S108), a light-shielding shutter is installed on the light path by the light-intensity adjustment mechanism 314 controlled by the light-intensity control circuit 134. This blocks light from the light source 103 that would otherwise reach the photosensor arrays 105 and 305.

[0058] In the light-shielded state image acquisition step (S110), first, the PDs 310 and 312 are moved from their respective optical paths to outside the optical paths. Then, with the light from the light source 103 blocked by the light-shielding shutter (the output values of the PDs 310 and 312 = zero), the optical image acquisition mechanism 150 acquires an optical image of the pattern on the substrate 101. Specifically, the output value of the photosensor array 105 (305) is measured when light is incident for the image accumulation time t2 of the photosensor element. It is not necessary to capture an image of the entire substrate 101 or of one entire inspection stripe 20. It is sufficient to obtain an image with the same number of pixels as the number of photosensor elements in the integration direction of the photosensor arrays 105 and 305 while moving the XYθ table 102 at a stage speed corresponding to the set image accumulation time t2 of the photosensor element. The obtained image data is stored in the stripe pattern memory 123.

[0059] In the light-shielded state gradation value acquisition step (S112), the light-shielded state gradation value acquisition unit 52 acquires the output value of the photosensor array 105 (305) when the amount of incident light is zero at the image accumulation time t2 of the photosensor element from the stripe pattern memory 123. This allows the TDI output value α2 of the dark current at the image accumulation time t2 of the element to be obtained.

[0060] In the PD output calculation step (S114), the PD output calculation unit 54 (parameter value calculation unit) inputs the desired image accumulation time t2 and corrects the above-mentioned relational expression using the output value α2 of the photosensor array 105, 305 that depends on the image accumulation time t2 when the amount of incident light is zero. Here, as shown in equation (2), the relational expression shown in equation (1) is corrected using the ratio (t1 / t2) between the reference image accumulation time t1 and the desired image accumulation time t2, and the ratio (α2 / α1) between the output value α1 of the photosensor array 105, 305 that depends on the reference image accumulation time t1 when the amount of incident light is zero and the output value α2 of the photosensor array 105, 305 that depends on the desired image accumulation time t2 when the amount of incident light is zero.

[0061] Specifically, it operates as follows. First, the relational expression (Equation (1)) using coefficients a and b for the element's reference image accumulation time t1 is corrected as in Equation (2) using the ratios (t1 / t2) and (α2 / α1). This obtains the slope a(t1 / t2) and intercept b(t1 / t2)(α2 / α1) as coefficients of the linear function equation for the photosensor element's reference image accumulation time t2.

[0062] Then, the PD output calculation unit 54 (parameter value calculation unit) uses the corrected relational expression to calculate the PD output value E for obtaining the desired output value I of the photosensor arrays 105 and 305. For example, if the gradation value of the image is defined as 256 gradations, the PD output value E for obtaining, for example, 200 gradations is calculated.

[0063] In the light intensity adjustment step (S116), first, the PDs 310 and 312 are moved from outside the optical path to on the optical path. Then, the light intensity adjustment mechanism 314, controlled by the light intensity control circuit 134, adjusts the amount of light incident on the photosensor arrays 105 and 305 to the calculated PD output value (parameter value). Because the light intensity adjustment mechanism 314 adjusts the light intensity by combining multiple filter elements with different transmittances, an error within an allowable range from the target PD output value may occur. For example, an error within ±10% may be included.

[0064] FIG. 9 shows an example of the relationship between the PD output and the TDI output for each image accumulation time of the photosensor element in the first embodiment and the comparative example. The vertical axis represents the PD output. The horizontal axis represents the TDI output. The gradation value converted by the sensor circuit 106 (305) is used as the TDI output. The units of the vertical and horizontal axes are both in AU. The reference image accumulation time of the photosensor element is represented by a scan time of 1X. The image accumulation time that is four times the reference image accumulation time of the photosensor element is represented by a scan time of 4X. The image accumulation time that is seven times the reference image accumulation time of the photosensor element is represented by a scan time of 7X. The image accumulation time that is 14 times the reference image accumulation time of the photosensor element is represented by a scan time of 14X. Also shown are the actual measurement (◆) results, conversion (■) results, and correction (×) results for each image accumulation time of the photosensor element. The conversion results are obtained by simply interpolating the slope a and intercept b of the linear equation for the reference image accumulation time of the photosensor element using the ratio (t1 / t2) of the image accumulation times. The correction results are shown as in the first embodiment, where the slope a and intercept b of the linear relational expression of the reference image accumulation time of the photosensor element are corrected by the ratio of the image accumulation times (t1 / t2), and intercept b (t1 / t2) is further corrected by the ratio of the TDI outputs at dark currents (α2 / α1). As shown in FIG. 8, the larger the ratio of the image accumulation times, the greater the deviation of the conversion results from the actual measurement results. In contrast, the correction results corresponding to the first embodiment showed a small deviation from the actual measurement values regardless of the ratio of the image accumulation times.

[0065] The above pre-processing completes the light intensity adjustment when changing the image accumulation time of the element, after which the actual inspection processing operation begins.

[0066] In the scanning step (S202), the optical image acquisition mechanism 150 captures an image of the substrate 101 using the photosensor arrays 105, 305 with the adjusted amount of incident light and outputs the captured optical image data. To do this, the optical image acquisition mechanism 150 first scans the inspection stripes 20 with laser light (inspection light) and captures an image of the stripe region for each inspection stripe 20 using the photosensor array 105 (305). Specifically, the operation is as follows: The XYθ table 102 is moved to a position where the target inspection stripe 20 can be imaged. In transmission inspection, light transmitted through the substrate 101 is focused as an optical image on the photosensor array 305 and then incident on it. Alternatively / and in reflection inspection, light reflected from the substrate 101 is focused as an optical image on the photosensor array 105 and then incident on it.

[0067] The pattern image formed on the photosensor array 105 is photoelectrically converted by each photosensor element of the photosensor array 105, and then A / D (analog-to-digital) converted by the sensor circuit 105. At this time, the output after integration of the multiple photosensor elements lined up in the scanning direction by the sensor circuit 105 is converted into a gradation value and output to the stripe pattern memory 123.

[0068] Alternatively, the pattern image formed on the photosensor array 305 is photoelectrically converted by each photosensor element of the photosensor array 305, and further A / D (analog-to-digital) converted by the sensor circuit 305. At this time, the sensor circuit 305 converts the integrated output of the multiple photosensor elements aligned in the scanning direction into a gradation value and outputs it to the stripe pattern memory 123.

[0069] Then, data of pixel values of the inspection stripe 20 to be measured is stored in the stripe pattern memory 123. The measurement data (pixel data) is, for example, 8-bit unsigned data, and represents the brightness gradation (light amount) of each pixel.

[0070] In the reference image creation step (S204), the reference image creation circuit 112 creates a reference image to serve as a reference using graphic pattern data (design data). The creation of the reference image is performed for each inspection stripe 20 in parallel with the scanning operation of the inspection stripe 20. Specifically, the operation is as follows: The reference image creation circuit 112 inputs graphic pattern data (design data) for each frame region 30 of the target inspection stripe 20, and converts each graphic pattern defined in the graphic pattern data into binary or multi-value image data.

[0071] The figures defined in the figure pattern data are, for example, rectangles or triangles as basic figures, and the figure data stored defines the shape, size, position, etc. of each pattern figure using information such as the coordinates (x, y) at the reference position of the figure, the length of the sides, and a figure code that serves as an identifier to distinguish between figure types such as rectangles and triangles.

[0072] When the design pattern data that becomes such graphic data is input to the reference image creation circuit 112, it is expanded into data for each graphic, and the graphic code and graphic dimensions that indicate the graphic shape of the graphic data are interpreted. Then, it is expanded into binary or multi-valued design pattern image data as a pattern to be arranged in a grid with a predetermined quantized dimension as a unit, and output. In other words, the design data is read, the frame area is virtually divided into grids with a predetermined dimension as a unit, and the occupancy rate of the graphic in the design pattern is calculated for each grid, and n-bit occupancy data (design image data) is output. For example, it is preferable to set one grid as one pixel. Then, 1 / 2 is assigned to one pixel. 8 If a pixel has a resolution of (=1 / 256), a small area of 1 / 256 is allocated to the area of the figure placed within the pixel, and the occupancy rate within the pixel is calculated. This is then created as 8-bit occupancy data. The grid (inspection pixel) can be aligned with the pixels of the measurement data.

[0073] Next, the reference image creation circuit 112 performs filtering using a filter function on the design image data of the design pattern, which is image data of the graphic.

[0074] FIG. 10 is a diagram illustrating the filtering process in the first embodiment. The pixel data of the optical image captured from the substrate 101 is filtered due to the resolution characteristics of the optical system used for capturing the image, in other words, is in a continuously changing analog state. Therefore, as shown in FIG. 10, the image intensity (gray value) differs from that of the expanded image (design image), which has a digital value. On the other hand, as described above, the figure pattern data is defined by a figure code, and therefore the image intensity (gray value) of the expanded design image may be a digital value. Therefore, the reference image creation circuit 112 performs image processing (filtering) on the expanded image to create a reference image that approximates the optical image. This allows the design image data, which is image data on the design side with a digital image intensity (gray value), to be matched to the image generation characteristics of the measurement data (optical image). The created reference image is output to the comparison circuit 108.

[0075] FIG. 11 is a diagram showing an example of the internal configuration of a comparison circuit according to the first embodiment. In FIG. 11, comparison circuit 108 includes storage devices 70, 72, and 76, such as magnetic disk devices, a frame image creation unit 74, an alignment unit 78, and a comparison processing unit 79. A series of "units" such as frame image creation unit 74, alignment unit 78, and comparison processing unit 79 each have a processing circuit. Such processing circuits include an electric circuit, a computer, a processor, a circuit board, a quantum circuit, or a semiconductor device. Each "unit" may share a common processing circuit (the same processing circuit), or may use different processing circuits (separate processing circuits). Input data or calculation results required for frame image creation unit 74, alignment unit 78, and comparison processing unit 79 are stored in a memory (not shown) within comparison circuit 108 or in memory 111 each time.

[0076] The stripe data (stripe area image) input to the comparison circuit 108 is stored in the storage device 70. The reference image data input to the comparison circuit 108 is stored in the storage device 72.

[0077] In the comparison step (S206), the comparison circuit 108 (an example of a comparison unit) compares the optical image formed from the optical image data output from the TDI sensor 105 with the reference image. Specifically, the operation is as follows.

[0078] In the comparison circuit 108, the frame image creation unit 74 first generates a plurality of frame images 31 by dividing the stripe region image (optical image) at a predetermined width. Specifically, as shown in FIG. 2, the stripe region image is divided into a plurality of rectangular frame region 30 frame images. For example, the size is divided into 512 x 512 pixels. Data for each frame region 30 is stored in the storage device 76.

[0079] Next, the alignment unit 78 reads out the corresponding frame image 31 and the corresponding reference image for each frame region 30 from the storage devices 72, 76, and aligns the frame image 31 with the corresponding reference image using a predetermined algorithm. For example, alignment is performed using the least squares method.

[0080] Then, the comparison processing unit 79 (another example of a comparison unit) compares the frame image 31 with the reference image corresponding to that frame image 31. For example, a comparison is made pixel by pixel. Here, the two are compared pixel by pixel according to predetermined judgment conditions to determine the presence or absence of a defect, such as a shape defect. The judgment conditions may, for example, be to compare the two pixel by pixel according to a predetermined algorithm to determine the presence or absence of a defect. For example, a difference value between the pixel values of the two images is calculated pixel by pixel, and if the difference value is greater than a threshold value Th, it is determined to be a defect. The comparison results may then be output to, for example, the magnetic disk device 109, magnetic tape device 115, flexible disk device (FD) 116, CRT 117, pattern monitor 118, or printer 119.

[0081] Although the above example describes the case of die-to-database inspection, die-to-die inspection may also be used. In this case, for frame regions 30 among the plurality of frame regions 30 for which die-to-die inspection is performed, the comparison circuit 108 uses the frame image (optical image) of die 2 acquired for one of the frame regions as a reference (reference image). First, for each frame region 30 for which die-to-die inspection is performed, the alignment unit 78 reads the frame image 31 of die 1 and the frame image of die 2 corresponding to the frame region 30 for which die-to-die inspection is performed from the storage device 76, and aligns the frame image 31 of die 1 with the frame image of die 2 using a predetermined algorithm. For example, the alignment is performed using the least squares method. Then, the comparison processing unit 79 (comparison unit) compares the frame image 31 of die 1 with the frame image of die 2 pixel by pixel for each frame region 30 for which die-to-die inspection is performed.

[0082] As described above, according to the first embodiment, the amount of inspection light incident on the image sensor can be adjusted so that an image with a desired gradation value can be obtained for the image accumulation time of a desired element, thereby suppressing image degradation caused by changes in the image accumulation time.

[0083] Although the embodiments have been described above with reference to specific examples, the present invention is not limited to these specific examples.

[0084] Furthermore, although descriptions of the device configuration, control method, and other parts not directly necessary for the explanation of the present invention have been omitted, the required device configuration and control method can be appropriately selected and used. For example, although the description of the control unit configuration that controls the inspection device 100 has been omitted, it goes without saying that the required control unit configuration can be appropriately selected and used.

[0085] In addition, all image acquisition methods, image acquisition devices, pattern inspection devices and pattern inspection methods that include the elements of the present invention and that can be appropriately modified by those skilled in the art are included within the scope of the present invention. [Explanation of symbols]

[0086] 20 Inspection Stripes 30 Frame Area 31 frame images 40 Beam Splitter 42 Mirror 44 Lens 46 Mirror 48 Lenses 51 Storage device 50 Image accumulation time setting unit 52 Light blocking state gradation value acquisition unit 54 PD output calculation section 70,71,72,76 Storage device 74 Frame image generation unit 78 Alignment section 79 Comparison processing section 100 Inspection equipment 101 Substrate 102 XYθ table 103 Light source 104 Magnifying Optical System 105,305 photosensor array 106,306 Sensor circuit 109 Magnetic disk unit 107 Position circuit 108 Comparison circuit 110 Control computer 111 memory 112 Reference image creation circuit 113 Autoloader control circuit 114 Table control circuit 115 Magnetic Tape Unit 116FD 117 CRT 118 Pattern Monitor 119 Printer 120 Bus 122 Laser length measurement system 123 Stripe Pattern Memory 130 Autoloader 132 Light intensity calculation circuit 134 Light intensity control circuit 136 PD measurement circuit 150 Optical image acquisition mechanism 160 Control Circuits 170 Transmitted illumination optical system 171 Reflected illumination optical system 174 Beam Splitter 176 Imaging Optical System 177 Mirror 178 Imaging Optical System 179 Imaging Optical System 310,312 PD 314 Light intensity adjustment mechanism

Claims

1. storing in a storage device coefficients of a relational expression between a parameter value corresponding to an amount of light incident on an image sensor having a photosensor element and an output value of the image sensor when light is incident on the image sensor for a reference image accumulation time that is a reference for an image accumulation time of the photosensor element; a step of inputting a desired image accumulation time, and calculating a parameter value for obtaining the desired output value of the image sensor using a corrected relational expression obtained by correcting the relational expression using a ratio between an output value of the image sensor that depends on the desired image accumulation time when the amount of incident light is zero and an output value of the image sensor that depends on the reference image accumulation time when the amount of incident light is zero; adjusting the amount of light incident on the image sensor to the calculated parameter value; taking an image of the sample using the image sensor with the adjusted amount of incident light and outputting the imaged image data; An image acquisition method comprising:

2. 2. The image acquisition method according to claim 1, wherein the coefficients of the relational expression are a slope and an intercept approximated by a linear function.

3. 3. The image acquisition method according to claim 1, wherein the relational expression is corrected using a ratio between the reference image accumulation time and the desired image accumulation time, and a ratio between an output value of the image sensor that depends on the reference image accumulation time when the amount of incident light is zero and an output value of the image sensor that depends on the desired image accumulation time when the amount of incident light is zero.

4. an imaging sensor having a photosensor element; a storage device that stores coefficients of a relational expression between a parameter value corresponding to the amount of light incident on the image sensor and an output value of the image sensor when light is incident for a reference image accumulation time that is a reference for the image accumulation time of the photosensor element; a parameter value calculation unit that receives a desired image accumulation time as an input and calculates a parameter value for obtaining the desired output value of the image sensor using a corrected relational expression obtained by correcting the relational expression using a ratio between an output value of the image sensor that depends on the desired image accumulation time when the amount of incident light is zero and an output value of the image sensor that depends on the reference image accumulation time when the amount of incident light is zero; a light amount adjustment mechanism that adjusts the amount of light incident on the image sensor to the calculated parameter value; Equipped with An image acquisition device characterized by using the imaging sensor with an adjusted amount of incident light to image a sample and outputting image data of the image.

Images