JP2018077053A - Pattern inspection method and pattern inspection device - Google Patents

Abstract

PURPOSE: To provide an inspection method that can inspect a pattern dimension of a defective part from an optical image obtained to make closer to a pattern image when transferred by an exposure device.CONSTITUTION: A pattern inspection method of one aspect of the present invention comprises the steps of: acquiring an optical image of a mask substrate on which a pattern is formed; generating a reference image corresponding to the optical image; measuring a second threshold level, used for the reference image, which makes a pattern dimension of a pattern normal part in the optical image, specified using a first threshold level of an image gradation value profile, and a pattern dimension of a corresponding part in the reference image be same dimension; measuring the pattern dimension of the corresponding part in the reference image, which corresponds to a pattern abnormal part in the optical image, using the second threshold level; measuring a pattern dimension of the pattern abnormal part in the optical image using the first threshold level; comparing the pattern dimension measured from the reference image with the pattern dimension measured from the optical image as for the pattern abnormal part, and outputting a comparison result.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a pattern inspection method and a pattern inspection apparatus. For example, the present invention relates to a pattern inspection technique for inspecting a pattern defect of an object as a sample used for semiconductor manufacture, and to an inspection apparatus and method for inspecting a photomask used when manufacturing a semiconductor element or a liquid crystal display (LCD).

  In recent years, the circuit line width required for a semiconductor element has been increasingly narrowed as a large scale integrated circuit (LSI) is highly integrated and has a large capacity. These semiconductor elements use an original pattern pattern (also referred to as a mask or a reticle, hereinafter referred to as a mask) on which a circuit pattern is formed, and the pattern is exposed and transferred onto a wafer by a reduction projection exposure apparatus called a stepper. It is manufactured by forming a circuit. Therefore, a pattern drawing apparatus using an electron beam capable of drawing a fine circuit pattern is used for manufacturing a mask for transferring such a fine circuit pattern onto a wafer. A pattern circuit may be directly drawn on a wafer using such a pattern drawing apparatus. Alternatively, development of a laser beam drawing apparatus for drawing using a laser beam in addition to an electron beam has been attempted.

  In addition, improvement in yield is indispensable for manufacturing an LSI that requires a large amount of manufacturing cost. However, as represented by a 1 gigabit class DRAM (Random Access Memory), the pattern constituting the LSI is on the order of submicron to nanometer. One of the major factors that reduce the yield is a pattern defect of a mask used when an ultrafine pattern is exposed and transferred onto a semiconductor wafer by a photolithography technique. In recent years, with the miniaturization of LSI pattern dimensions formed on semiconductor wafers, the dimensions that must be detected as pattern defects have become extremely small. Therefore, it is necessary to improve the accuracy of a pattern inspection apparatus that inspects defects in a transfer mask used in LSI manufacturing.

  As an inspection method, an optical image obtained by imaging a pattern formed on a sample such as a lithography mask using a magnifying optical system at a predetermined magnification is compared with an optical image obtained by imaging design data or the same pattern on the sample. A method of performing an inspection by doing this is known. For example, as a pattern inspection method, “die to die inspection” in which optical image data obtained by imaging the same pattern at different locations on the same mask is compared, or a pattern is formed using CAD data with a pattern design as a mask. The drawing data (design pattern data) converted into the device input format for input by the drawing device when drawing is input to the inspection device, and a design image (reference image) is generated based on this data and the pattern is captured. There is a “die to database (die-database) inspection” that compares an optical image as measured data. In the inspection method in such an inspection apparatus, the sample is placed on the stage, and the stage is moved so that the light beam scans on the sample and the inspection is performed. The sample is irradiated with a light beam by a light source and an illumination optical system. The light transmitted or reflected by the sample is imaged on the sensor via the optical system. The image picked up by the sensor is sent to the comparison circuit as measurement data. The comparison circuit compares the measured data and the reference data according to an appropriate algorithm after the images are aligned, and determines that there is a pattern defect if they do not match.

  In a semiconductor product with a short product cycle, it is an important item to shorten the manufacturing time. When a defective mask pattern is exposed and transferred onto a wafer, a semiconductor device made from the wafer becomes a defective product. Therefore, it is important to perform a mask pattern defect inspection. Then, the defect found by the defect inspection is corrected by the defect correcting device. However, correcting all the found defects increases the time required for manufacturing and leads to a decrease in product value. With the development of the inspection apparatus, the inspection apparatus determines that there is a pattern defect even if a very small deviation occurs. However, when a mask pattern is transferred onto a wafer with an actual exposure apparatus, the integrated circuit is not caused by a defect such as a circuit disconnection or / and a short circuit on the wafer, and the pattern line width is within an allowable value. Can be used. Therefore, it is desirable to use such a mask for exposure transfer while leaving the state of the defect, instead of correcting the defect each time. For that purpose, it is only necessary to be able to measure the pattern dimension of the pattern image when transferred by the exposure apparatus. However, while the exposure apparatus reduces the mask pattern and forms an image on the wafer, the inspection apparatus enlarges the mask pattern and forms an image on the sensor. Therefore, the configuration of the optical system on the secondary side with respect to the mask substrate is originally different. With this configuration, the inspection apparatus has higher resolution with respect to the mask than the exposure apparatus and can detect minute defects. On the other hand, the shape of the pattern image when transferred by the exposure apparatus can be detected by the inspection apparatus. It is difficult to reproduce.

  Here, a dedicated machine for inspecting an exposure image exposed and transferred by an exposure apparatus using an aerial image is disclosed (see, for example, Patent Document 1).

JP 2001-235853 A

  Here, although it seems that it has not been publicly known at the time of filing of the present application, when an optical image is acquired by an inspection apparatus so as to be close to a pattern image when transferred by the exposure apparatus described above, It has been studied to take an image with a low resolution according to the conditions. However, since the reference image created for comparison with the optical image is based on the design data, the reference image was captured with the optical condition of the exposure apparatus including the illumination shape including the complicated aperture condition in the generation accuracy of the reference image. It is difficult to adjust to an extent that can be compared with an optical image. Therefore, there is a problem that an error occurs in the pattern size in the image when the optical image captured under the optical condition of the exposure apparatus is compared with the reference image. Therefore, an error occurs in the pattern dimension even in a normal part where no defect exists. In such a state, there is a problem that it is difficult to obtain the pattern size of the defective portion with high accuracy.

  Therefore, the present invention provides an inspection method and apparatus capable of inspecting the pattern size of a defective portion from an optical image captured so as to be close to a pattern image when transferred by an exposure apparatus.

The pattern inspection method of one embodiment of the present invention includes:
Obtaining an optical image of the mask substrate on which the pattern is formed;
Creating a reference image corresponding to the optical image;
The second pattern used for the reference image in which the pattern dimension of the normal pattern portion in the optical image specified by using the first threshold level of the image gradation value profile and the pattern dimension of the corresponding portion in the reference image are the same dimension. Measuring a threshold level;
Measuring a pattern dimension of a corresponding portion in the reference image corresponding to a pattern abnormal portion in the optical image using the second threshold level;
Using the first threshold level to measure the pattern dimension of the pattern anomaly in the optical image;
A step of comparing and outputting the pattern dimension measured from the reference image and the pattern dimension measured from the optical image for the pattern abnormal part,
It is provided with.

  It is preferable that the method further includes a step of detecting a plurality of edge pairs in the same direction of the pattern in the optical image.

In addition, as a region of the pattern abnormality portion in the optical image, a defective pixel whose gradation difference between the pixel in the optical image and the corresponding pixel in the reference image exceeds the third threshold and a plurality of pixels around the defective pixel A configured pixel group is used,
When measuring the pattern dimension of the pattern abnormal part in the optical image, the dimension between a plurality of edge pairs in the pixel group is measured,
When measuring the pattern dimension of the corresponding part in the reference image corresponding to the pattern abnormal part in the optical image, the dimension between the plurality of edge pairs corresponding to the plurality of edge pairs in the pixel group is measured,
When comparing the pattern dimension measured from the optical image and the pattern dimension measured from the reference image, the edge pair that maximizes the difference between the optical image and the reference image among the plurality of edge pairs in the pixel group. It is preferable to determine and output the pattern dimension between them as the pattern dimension of the pattern abnormal part.

Also, a step of acquiring a second optical image of the mask substrate using a light beam having a numerical aperture higher than a numerical aperture (NA) when the light beam used for the optical image is incident on the objective lens from the mask substrate. When,
Creating a second reference image corresponding to the second optical image;
Comparing the second optical image and the second reference image pixel by pixel;
As a result of comparing the second optical image and the second reference image, using a defective pixel determined to be a defect, identifying a pattern abnormality location in the optical image;
It is preferable to further include

The pattern inspection apparatus according to one aspect of the present invention includes:
An optical image acquisition mechanism for acquiring an optical image of a mask substrate on which a pattern is formed;
A reference image creation unit for creating a reference image corresponding to the optical image;
The second pattern used for the reference image in which the pattern dimension of the normal pattern portion in the optical image specified by using the first threshold level of the image gradation value profile and the pattern dimension of the corresponding portion in the reference image are the same dimension. A threshold level measurement unit for measuring the threshold level;
A first pattern dimension measuring unit that measures a pattern dimension of the corresponding part in the reference image corresponding to the abnormal pattern part in the optical image by using the second threshold level;
A second pattern dimension measuring unit that measures a pattern dimension of a pattern abnormality portion in the optical image using the first threshold level;
A comparison unit that compares the pattern dimension measured from the optical image and the pattern dimension measured from the reference image with respect to the pattern abnormality portion;
It is provided with.

  According to the present invention, it is possible to inspect the pattern size of a defective portion from an optical image captured so as to be close to a pattern image when transferred by an exposure apparatus.

1 is a configuration diagram illustrating a configuration of a pattern inspection apparatus according to a first embodiment. 6 is a conceptual diagram for comparing the numerical aperture in the inspection apparatus and the numerical aperture in the exposure apparatus in the first embodiment. FIG. FIG. 3 is a flowchart showing a part of main processes of the inspection method according to the first embodiment. FIG. 3 is a flowchart showing the remaining part of the main process of the inspection method according to the first embodiment. 6 is a diagram illustrating an example of a configuration of an optical system in a high NA inspection mode according to Embodiment 1. FIG. FIG. 3 is a conceptual diagram for explaining an inspection region in the first embodiment. 6 is a diagram for describing filter processing according to Embodiment 1. FIG. 2 is a block diagram illustrating an internal configuration of a low NA inspection circuit according to Embodiment 1. FIG. 3 is a diagram illustrating an example of a configuration of an optical system in a low NA inspection mode according to Embodiment 1. FIG. 6 is a diagram illustrating an example of an image gradation value profile of a pattern in an optical image and a pattern in a reference image according to Embodiment 1. FIG. FIG. 3 is a diagram showing a defect area image and an example of an edge pair in the first embodiment. 5 is a diagram for explaining a pattern edge recognition method according to Embodiment 1. FIG. FIG. 5 is a diagram illustrating an example of a mechanism for changing the illumination shape in the first embodiment. It is a figure which shows another example of the mechanism in which the illumination shape in Embodiment 1 is changed. It is a figure which shows another example of the mechanism in which the illumination shape in Embodiment 1 is changed.

Embodiment 1 FIG.
FIG. 1 is a configuration diagram showing the configuration of the pattern inspection apparatus according to the first embodiment. In FIG. 1, an inspection apparatus 100 for inspecting a defect of a pattern formed on a mask substrate 101 includes an optical image acquisition mechanism 150 and a control system circuit 160 (control unit).

  The optical image acquisition mechanism 150 includes a light source 103, a transmission illumination optical system 170, an illumination shape switching mechanism 171, a movable XYθ table 102, an objective lens 104, a reflection illumination optical system 172, an aperture stop 180, and an imaging optical system. 178, a photodiode array 105 (an example of a sensor), a sensor circuit 106, a stripe pattern memory 123, and a laser length measurement system 122. A mask substrate 101 is placed on the XYθ table 102. Examples of the mask substrate 101 include a photomask for exposure that transfers a pattern to a wafer. The photomask is formed with a pattern constituted by a plurality of graphic patterns to be inspected. For example, the mask substrate 101 is arranged on the XYθ table 102 with the pattern formation surface facing downward.

  In the control system circuit 160, a control computer 110 serving as a computer is connected via a bus 120 to a position circuit 107, a comparison circuit 108, a development circuit 111, a reference circuit 112, an autoloader control circuit 113, a table control circuit 114, an autofocus (AF). ) The control circuit 140, the inspection mode switching control circuit 144, the low NA inspection circuit 146, the magnetic disk device 109, the magnetic tape device 115, the flexible disk device (FD) 116, the CRT 117, the pattern monitor 118, and the printer 119. . The sensor circuit 106 is connected to the stripe pattern memory 123, and the stripe pattern memory 123 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 reflective illumination optical system 172 includes the objective lens 104 and a polarization beam splitter 174 (polarization element). The polarization beam splitter 174 is moved in and out of the optical path by the drive mechanism 176.

  In the inspection apparatus 100, the light source 103, the XYθ table 102, the transmission illumination optical system 170, the objective lens 104, the photodiode array 105, and the sensor circuit 106 constitute a high-magnification inspection optical system. For example, an inspection optical system having a magnification of 200 to 300 times is configured. The XYθ table 102 is driven by the 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. As these X motor, Y motor, and θ motor, for example, linear motors can be used. The XYθ table 102 can be moved in the horizontal direction and the rotation direction by a motor of each axis of XYθ. The focus position (optical axis direction: Z-axis direction) of the XYθ table 102 is dynamically adjusted on the pattern formation surface of the mask substrate 101 by the AF control circuit 140 under the control of the control computer 110. For example, the focal position of the XYθ table 102 is adjusted by being moved in the optical axis direction (Z-axis direction) by the piezo element 142. The moving position of the mask substrate 101 placed on the XYθ table 102 is measured by the laser length measurement system 122 and supplied to the position circuit 107.

  Design pattern data (drawing data) that is a basis for pattern formation of the mask substrate 101 is input from the outside of the inspection apparatus 100 and stored in the magnetic disk device 109.

  Here, FIG. 1 shows components necessary for explaining the first embodiment. It goes without saying that the inspection apparatus 100 may normally include other necessary configurations.

  FIG. 2 is a conceptual diagram for comparing the numerical aperture of the inspection apparatus and the numerical aperture of the exposure apparatus in the first embodiment. FIG. 2A shows a part of the optical system of the exposure apparatus. In the exposure apparatus, illumination light (not shown) is illuminated on the mask substrate 300, transmitted light 301 from the mask substrate 300 is incident on the objective lens 302, and light 305 that has passed through the objective lens 302 is coupled to the semiconductor substrate 304 (wafer). Image. In FIG. 2A, one objective lens 302 is shown, but it goes without saying that a combination of a plurality of lenses may be used. Here, in the current exposure apparatus, the pattern formed on the mask substrate 300 is reduced to 1/4, for example, and transferred to the semiconductor substrate 304 by exposure. In this case, the numerical aperture NAi (numerical aperture on the image i side) of the exposure apparatus with respect to the semiconductor substrate 304 is set to NAi = 1.2, for example. In other words, the numerical aperture NAi of the objective lens 302 that can pass through the objective lens 302 is set to NAi = 1.2, for example. In the exposure apparatus, since the transmitted light image from the mask substrate 300 is reduced to ¼, the sensitivity of the objective lens 302 to the mask substrate 300 becomes ¼. In other words, the numerical aperture NAo (numerical aperture on the object o side) of the objective lens 302 that can be incident when transmitted light is incident on the objective lens 302 from the mask substrate 300 is ¼ of NAi, and NAo = 0.3. It becomes. Therefore, in the exposure apparatus, a transmitted light image from the mask substrate 300 of a light beam having a numerical aperture NAo = 0.3 is exposed and transferred to the semiconductor substrate 304.

  On the other hand, in the inspection apparatus 100 according to the first embodiment, as shown in FIG. 2B, illumination light (not shown) is illuminated on the mask substrate 101, and transmitted light 190 from the mask substrate 101 is transmitted to the objective lens 104. 192 and the light 192 that has passed through the objective lens 104 form an image on the photodiode array 105 (sensor). At this time, the numerical aperture NAo (numerical aperture on the object o side) of the objective lens 104 that can be incident when the transmitted light 190 enters the objective lens 104 from the mask substrate 101 is set to NAo = 0.9, for example. Therefore, in the inspection apparatus 100, the photodiode array 105 receives a transmitted light image from the mask substrate 101 of a light beam having a numerical aperture NAo = 0.9. Therefore, in the inspection apparatus 100, the defect inspection is performed using light having a higher resolution than the light that is exposed and transferred to the semiconductor substrate 304 by the exposure apparatus. By using such high-resolution light, it is possible to perform high-precision inspection capable of detecting fine defects on the order of nanometers, for example. Although only the objective lens 104 is shown in FIG. 2B, a plurality of lenses (not shown) are arranged between the objective lens 104 and the photodiode array 105.

  However, as described above, in the inspection apparatus 100, since the photodiode array 105 receives light having higher resolution than the light that is exposed and transferred to the semiconductor substrate 304 by the exposure apparatus, the pattern is exposed and transferred to the semiconductor substrate 304 by the exposure apparatus. In this case, it is difficult for the inspection apparatus 100 to reproduce the same image as the pattern image formed on the semiconductor substrate 304. Therefore, in the first embodiment, the aperture stop 180 is arranged to restrict the light flux of the transmitted light from the mask substrate 101 to adjust the light flux diameter of the transmitted light, and is the same as the light that is exposed and transferred to the semiconductor substrate 304 by the exposure apparatus. The inspection apparatus 100 can generate light having a large numerical aperture NAo (numerical aperture on the object o side) (similar resolution).

  In addition, in the exposure apparatus, the transmitted light from the mask substrate 300 is transferred to the semiconductor substrate 304, but the reflected light is not used. Therefore, no polarizing element is disposed on the optical path from the mask substrate 300 to the semiconductor substrate 304. On the other hand, in the inspection apparatus 100, in addition to the inspection using the transmitted light from the mask substrate 101, the inspection using the reflected light from the mask substrate 101 can be performed. Therefore, in addition to the inspection light for the transmitted light inspection (first inspection light or first illumination light), the inspection light for the reflected light inspection (second inspection light or second illumination) Light) is required. Therefore, in the inspection apparatus 100, the polarization beam splitter 174 (polarization element) is disposed on the optical path for illuminating the mask substrate 300 with the inspection light for the reflected light inspection. Therefore, the photodiode array 105 receives the light that has passed through the polarization beam splitter 174 (polarization element). As a result, the light received by the photodiode array 105 in the inspection apparatus 100 has a polarization component different from the light that forms an image on the semiconductor substrate 304 when the pattern is exposed and transferred to the semiconductor substrate 304 by the exposure apparatus. In this respect, it is difficult for the inspection apparatus 100 to reproduce the same image as the pattern image formed on the semiconductor substrate 304 when the pattern is exposed and transferred to the semiconductor substrate 304 with the exposure apparatus. Therefore, in the first embodiment, the polarization beam splitter 174 (polarization element) is configured to be excluded from the optical path.

  In addition, in the inspection apparatus 100 and the exposure apparatus, dipole, quadrupole illumination, annular illumination, and other arbitrary aperture conditions optimized by computer simulation so as to improve the pattern resolution characteristics of the illumination light, that is, various Exposure in a suitable shape. Furthermore, the exposure light is exposed with the polarization condition of the illumination light set such as direct polarization, azimuth polarization, radial polarization, or no polarization. As a result, the light received by the photodiode array 105 in the inspection apparatus 100 is different in illumination shape and polarization condition from the light imaged on the semiconductor substrate 304 when the exposure apparatus transfers the pattern onto the semiconductor substrate 304 by exposure. Become. In this respect, it is difficult for the inspection apparatus 100 to reproduce the same image as the pattern image formed on the semiconductor substrate 304 when the pattern is exposed and transferred to the semiconductor substrate 304 with the exposure apparatus. Therefore, in the first embodiment, the illumination shape and the polarization condition can be changed.

  The above-described exposure apparatus generates light having the same numerical aperture NAo (numerical aperture on the object o) (same resolution) as the light to be transferred to the semiconductor substrate 304, and the polarizing beam splitter 174 (polarizing element) is placed on the optical path. The inspection apparatus 100 implements the points that are excluded from the above and the point that the illumination shape and the polarization condition are changed to the same shape as those of the exposure apparatus. It is possible to reproduce an image that is substantially the same as or similar to the image that is exposed and transferred. Therefore, in the first embodiment, when a pattern is exposed and transferred to the semiconductor substrate 304 with an exposure apparatus and a high NA inspection mode in which normal defect inspection is performed, an image that is substantially the same as the pattern image formed on the semiconductor substrate 304 or It is configured to be switchable to a low NA inspection mode in which a close image is reproduced and inspected.

FIG. 3 is a flowchart showing a part of main processes of the inspection method according to the first embodiment.
FIG. 4 is a flowchart showing the remainder of the main process of the inspection method according to the first embodiment. 3 and 4, the inspection method according to the first embodiment includes a high NA inspection mode setting step (S102), a high NA scanning (scanning) step (S104), and an autofocus (AF) data creation step (S105). A reference image (1) creation step (S106), a comparison step (S108), a defect region identification step (S110), a low NA inspection mode switching step (S112), and a low NA scanning (scanning) step (S120). ), AF control step (S122), reference image (2) creation step (S124), normal portion measurement image edge pair detection step (S130), normal portion reference image edge pair detection step (S132), and reference Image threshold level measurement step (S134), measurement image edge pair detection step for defective portion (S136), reference image edge pair detection step for defective portion (S136) And 38), and the defect portion of the reference image pattern dimension measuring step (S140), the measurement image pattern dimension measuring step of the defect portion (S142), and comparison step (S144), performing a series of steps referred to.

  In the low NA inspection mode switching step (S112), a polarizing element retracting step (S114), an NA narrowing step (S116), and an illumination shape changing step (S118) are performed as internal steps.

  As the high NA inspection mode setting step (S102), the inspection mode switching control circuit 144 sets each configuration to a high NA inspection mode for performing a normal high NA defect inspection. Specifically, it operates as follows. Upon receiving the control signal from the inspection mode switching control circuit 144, the illumination shape switching mechanism 171 sets the shape of the illumination light (inspection light) to the normal illumination shape for inspection. Further, the inspection mode switching control circuit 144 has a numerical aperture NAo (numerical aperture on the object o side) of the objective lens 104 that can be incident when the transmitted light 190 enters the objective lens 104 from the mask substrate 101, for example, NAo = 0. The aperture value of the aperture stop 180 is controlled to be 9. Alternatively, the aperture stop 180 may be fully opened. Further, the inspection mode switching control circuit 144 controls the driving mechanism 176 to arrange the polarization beam splitter 174 on the optical path. When the polarization beam splitter 174 is located outside the optical path, it is moved onto the optical path. If the polarization beam splitter 174 is on the optical path, its position may be maintained.

  In the high NA scanning (scanning) step (S104), the numerical aperture (NA) that can be incident from the mask substrate 101 to the objective lens 104 that expands the transmitted light or reflected light from the mask substrate 101 on which the pattern is formed is high. The pattern formed on the mask substrate 101 using the photodiode array 105 is scanned by scanning the mask substrate 101 while the photodiode array 105 (sensor) can receive transmitted light or reflected light in the state of NA. A transmitted light image or a reflected light image is captured. In the first embodiment, as described later, a low NA scan is performed. Here, before performing the low NA scan, the optical image acquisition mechanism 150 uses a numerical aperture (NA) when a light beam used for a low NA optical image, which will be described later, enters the objective lens 104 from the mask substrate 101. An optical image (high NA optical image: second optical image) of the mask substrate 101 is acquired using a light beam having a high numerical aperture. Specifically, it operates as follows.

  FIG. 5 is a diagram showing an example of the configuration of the optical system in the high NA inspection mode in the first embodiment. In FIG. 5, laser light (for example, DUV light) having a wavelength in the ultraviolet region or less, which is inspection light, is generated from the light source 103. The generated light is polarized through the polarizing plate 60, and when performing transmitted light inspection, light for transmission inspection illumination (one of p wave and s wave) is allowed to pass through. Then, the light for transmission inspection illumination (one of the p wave and the s wave) is reflected by the polarization beam splitter 61. On the other hand, when the reflected light inspection is performed, the light for reflection inspection illumination (the other of the p wave and the s wave) is allowed to pass. Then, the light for reflection inspection illumination (the other of the p wave and the s wave) passes through the polarizing beam splitter 61. For example, the light passing therethrough can be limited by rotating the polarizing plate 60. Thereby, the transmitted light inspection and the reflected light inspection can be selected.

  When the transmitted light inspection is performed, the transmission light for the inspection inspection (first inspection light) is illuminated on the mask substrate 101 by the transmitted illumination optical system 170. A specific example will be described below based on the example of FIG. In the transmission illumination optical system 170, the light for transmission inspection illumination branched by the polarization beam splitter 61 is reflected by the mirror 63, passes through the lenses 64, 65, 66, and is reflected by the mirror 67. The light reflected by the mirror 67 is imaged by the condenser lens 68 on the pattern forming surface of the mask substrate 101 from the back side opposite to the pattern forming surface of the mask substrate 101. The transmitted light that has passed through the mask substrate 101 enters the imaging optical system 178 via the objective lens 104 and the polarization beam splitter 174. The imaging optical system 178 forms an image on the photodiode array 105 (an example of a sensor) and enters the photodiode array 105 as an optical image. In the imaging optical system 178, the light that has passed through the polarization beam splitter 174 passes through lenses 69 and 70, passes through an aperture stop 180, and is focused on the photodiode array 105 by the lens 72. As the photodiode array 105, for example, a TDI (Time Delay Integration) sensor or the like is preferably used. The photodiode array 105 (sensor) captures an optical image of a pattern formed on the mask substrate 101 while the XYθ table 102 on which the mask substrate 101 is placed is moving.

  On the other hand, when the reflected light inspection is performed, the light for reflection inspection illumination (second inspection light) is illuminated on the mask substrate 101 by the reflection illumination optical system 172. The reflective illumination optical system 172 includes an objective lens 104 and a polarization beam splitter 174 (polarization element), and illuminates the mask substrate 101 with light for reflection inspection illumination using the objective lens 104 and the polarization beam splitter 174, and the mask substrate. The reflected light from 101 is passed. A specific example will be described below based on the example of FIG. In the reflection illumination optical system 172, the light for reflection inspection illumination that has passed through the polarization beam splitter 61 passes through lenses 80, 81, 82, and 83 and is reflected by the polarization beam splitter 174. The light reflected by the polarization beam splitter 174 enters the objective lens 104, and is imaged by the objective lens 104 from the pattern formation surface side of the mask substrate 101 onto the pattern formation surface of the mask substrate 101. The reflected light reflected from the mask substrate 101 passes through the objective lens 104 and the polarization beam splitter 174 and enters the imaging optical system 178. The imaging optical system 178 forms an image on the photodiode array 105 (an example of a sensor) and enters the photodiode array 105 as an optical image. The photodiode array 105 (sensor) captures an optical image of a pattern formed on the mask substrate 101 while the XYθ table 102 on which the mask substrate 101 is placed is moving.

  At this time, a part of the reflected light is branched by the mirror 73 disposed between the lenses 69 and 70, and the branched light is imaged on the AF sensor 76 by the AF optical system 173. The light receiving surface of the AF sensor 76 is disposed so as to have a confocal relationship with the light receiving surface of the photodiode array 105 with respect to the beam splitter 73. An output signal from the AF sensor 76 is transmitted to the AF control circuit 140. The AF control circuit 140 measures the focal position of each position on the mask substrate 101 using the reflected light when the photodiode array 105 receives the reflected light in a state where the numerical aperture is high. Then, the AF control circuit 140 controls the piezo element 142 based on the signal to move the XYθ table 102 in real time in the optical axis direction (Z-axis direction), thereby dynamically forming the pattern of the mask substrate 101. The focal position (optical axis direction: Z-axis direction) of the objective lens 104 is adjusted on the surface. Even when the transmitted light inspection is performed, a part of the reflected illumination is illuminated on the mask substrate 101 for AF by the reflected illumination optical system 172. Then, the mirror 73 reflects the reflected light corresponding to a part of the reflected illumination and forms an image on the AF sensor 76. Thereby, the focus position (optical axis direction: Z-axis direction) of the objective lens 104 can be dynamically adjusted on the pattern forming surface of the mask substrate 101 even in the inspection of only transmission.

  The inspection apparatus 100 can perform one or both of scanning of the transmitted light and scanning of the reflected light. The reflected light reflected from the mask substrate 101 includes pattern height information that is difficult to obtain with the transmitted light from the mask substrate 101. Therefore, the AF control circuit 140 performs AF control using reflected light from the mask substrate 101 instead of transmitted light from the mask substrate 101. When both scanning of transmitted light and scanning of reflected light are performed, either may be performed first. Alternatively, another photodiode array 105 is arranged, and the position of illuminating the mask substrate 101 is shifted between the transmitted light scan and the reflected light scan, and the transmitted light is received by one photodiode array 105, and the reflected light is reflected. The reflected light is reflected by the mirror 73 and then received by the other photodiode array 105, so that both the scanning of the transmitted light and the scanning of the reflected light may be performed simultaneously.

  FIG. 6 is a conceptual diagram for explaining an inspection region in the first embodiment. As shown in FIG. 6, the inspection area 10 (entire inspection area) of the mask substrate 101 is virtually divided into a plurality of strip-shaped inspection stripes 20 having a scan width W, for example, in the y direction. Then, the inspection apparatus 100 acquires an image (stripe region image) for each inspection stripe 20. For each of the inspection stripes 20, a laser beam (DUV light) is used to capture an image of a graphic pattern arranged in the stripe region in the longitudinal direction (x direction) of the stripe region. The mask substrate 101 is moved by the movement of the XYθ table 102. As a result, an optical image is acquired while the photodiode array 105 is continuously moved relatively in the x direction. The photodiode array 105 continuously captures optical images having a scan width W as shown in FIG. In other words, the photodiode array 105 as an example of the sensor captures an optical image (measurement image) of the pattern formed on the mask substrate 101 using the inspection light while moving relative to the XYθ table 102. In the first embodiment, after an optical image in one inspection stripe 20 is captured, the optical image having the scan width W is similarly moved while moving in the y direction to the position of the next inspection stripe 20 and moving in the opposite direction. Take images continuously. That is, imaging is repeated in a forward (FWD) -backforward (BWD) direction in the opposite direction on the forward path and the backward path.

  Here, the imaging direction is not limited to repeating forward (FWD) -backforward (BWD). You may image from one direction. For example, FWD-FWD may be repeated. Alternatively, BWD-BWD may be repeated.

  The pattern image formed on the photodiode array 105 is photoelectrically converted by each light receiving element of the photodiode array 105, and further A / D (analog / digital) converted by the sensor circuit 106. Then, the pixel data of the inspection stripe 20 to be measured is stored in the stripe pattern memory 123. When capturing such pixel data (stripe region image), the dynamic range of the photodiode array 105 is, for example, a dynamic range having a maximum gradation when the amount of illumination light is 60% incident. Further, when acquiring an optical image of the inspection stripe 20, the laser length measurement system 122 measures the position of the XYθ table 102. The measured position information is output to the position circuit 107. The position circuit 107 (calculation unit) calculates the position of the mask substrate 101 using the measured position information.

  Thereafter, the stripe area image is sent to the comparison circuit 108 together with data indicating the position of the photomask 101 on the XYθ table 102 output from the position circuit 107. The measurement data (pixel data) is, for example, 8-bit unsigned data, and represents the brightness gradation (light quantity) of each pixel. The stripe area image output to the comparison circuit 108 is stored in a storage device (not shown).

  In the comparison circuit 108, stripes having a predetermined size (for example, the same width as the scan width W) are arranged in the x direction so as to cut out the frame image of the target frame region 30 from the stripe region image (optical image) of the inspection stripe 20. The area image is divided. For example, the image is divided into 512 × 512 pixel frame images. In other words, the stripe area image for each inspection stripe 20 is divided into a plurality of frame images (optical images) with a width similar to the width of the inspection stripe 20, for example, the scan width W. With this process, a plurality of frame images (optical images) corresponding to the plurality of frame regions 30 are acquired. From the plurality of frame images, one image (measurement image: optical image) data to be compared for inspection is generated.

  As an autofocus (AF) data creation step (S105), the AF control circuit 140 simultaneously performs the focus depending on the height position of the mask substrate 101 surface used for AF control in the above-described scanning with reflected light or transmitted light. The position information is stored in the magnetic disk device 109. Therefore, the magnetic disk device 109 (storage device) reflects the reflected light from the mask substrate 101 illuminated with the light via the reflective illumination optical system 172 in a state where the polarization beam splitter 174 (polarization element) is moved on the optical path. The information of the focal position of each position on the mask substrate 101 measured using is stored.

  Note that when only scanning of transmitted light is performed in the above-described high NA scanning step (S104), it is only necessary to acquire focal position information depending on the height position of the mask substrate 101 surface in advance using reflected light. . When the transmitted light is scanned in the above-described high NA scanning step (S104), AF control may be performed using the focal position information obtained at that time.

  In the reference image (1) creation step (S106), the reference image creation unit creates a reference image (second reference image) corresponding to the high NA optical image (second optical image). First, the development circuit 111 (a part of the reference image creation unit) creates a design image by developing an image based on design pattern data that is a basis for pattern formation of the mask substrate 101. Specifically, design data is read from the magnetic disk device 109 through the control computer 110, and each graphic pattern in the area of the target frame 30 defined in the read design data is converted into binary or multivalued image data ( Image development) to create a design image.

  Here, the figure defined in the design pattern data is, for example, a rectangle or triangle as a basic figure. For example, coordinates (x, y) at the reference position of the figure, side length, rectangle, triangle, etc. Stored is graphic data (vector data) in which the shape, size, position, etc. of each pattern graphic is defined by information such as a graphic code serving as an identifier for distinguishing graphic types.

When the design pattern information as the graphic data is input to the expansion circuit 111, the data is expanded to data for each graphic, and a graphic code indicating the graphic shape of the graphic data, a graphic dimension, and the like are interpreted. Then, binary or multivalued design image data is developed and output as a pattern arranged in a grid having a grid with a predetermined quantization dimension as a unit. In other words, the design data is read, the occupancy ratio of the figure in the design pattern is calculated for each grid formed by virtually dividing the inspection area as a grid with a predetermined size as a unit, and the n-bit occupancy data is calculated. Output. For example, it is preferable to set one square as one pixel. If a resolution of 1/2 ⁸ (= 1/256) is given to one pixel, 1/256 small areas are allocated by the figure area arranged in the pixel, and the occupation ratio in the pixel is set. Calculate. Then, a design image of 8-bit occupation ratio data is created for each pixel. The design image data is output to the reference circuit 112.

  The reference circuit 112 (other part of the reference image creation unit) creates a reference image (for high NA) by filtering the design image.

  FIG. 7 is a diagram for explaining the filter processing in the first embodiment. The measurement data as an optical image obtained from the sensor circuit 106 is in a state in which the filter is activated by the resolution characteristic of the objective lens 104, the aperture effect of the photodiode array 105, or the like, in other words, in an analog state that continuously changes. By applying the filtering process to the reference design image data which is the image data on the design side whose intensity (shading value) is a digital value, it can be matched with the measurement data. In this way, a reference image to be compared with the frame image (optical image) is created. The created reference image (for high NA) is output to the comparison circuit 108, and the reference image output in the comparison circuit 108 is stored in a storage device (not shown). Thus, the other image (reference image) data to be compared for inspection is generated.

  As a comparison process (S108), the comparison circuit 108 inspects the defect of the pattern by using the optical image of the transmitted light image or the reflected light image of the imaged mask substrate 101. The comparison circuit 101 (comparison unit) compares the frame image (optical image: second optical image) and the reference image (for high NA: second reference image) for each pixel. Specifically, first, a frame image (optical image) to be compared and a reference image to be compared are aligned by a predetermined algorithm. For example, alignment is performed using a least square method. Then, the comparison circuit 108 compares the two for each pixel according to a predetermined determination condition, and determines the presence / absence of a defect such as a shape defect. As a determination condition, for example, both are compared for each pixel according to a predetermined algorithm, and the presence or absence of a defect is determined. For example, it is determined whether the difference between the pixel values of both images is greater than a determination threshold (third threshold). Then, a pixel whose gradation difference between the pixel in the frame image (optical image) and the corresponding pixel in the reference image (for high NA) exceeds the determination threshold (third threshold) is determined as a defective pixel. Then, the comparison result is output. The comparison result may be output from the magnetic disk device 109, the magnetic tape device 115, the flexible disk device (FD) 116, the CRT 117, the pattern monitor 118, or the printer 119.

  Through the above high NA scan inspection (normal inspection), the pixel at the defect position where the shape defect has occurred can be specified. Next, in order to confirm whether a dimensional error caused by such a defect can withstand actual use, the pattern dimension of the defective portion is measured.

  FIG. 8 is a block diagram showing an internal configuration of the low NA inspection circuit according to the first embodiment. 8, in the low NA inspection circuit 146, storage devices 50, 52, 59 such as a magnetic disk device, an area specifying unit 51, an alignment unit 56, a measurement image edge pair detection unit 56, a reference image edge pair detection unit 58, A threshold level measurement unit 40, a measurement image pattern dimension measurement unit 42, a reference image pattern dimension measurement unit 44, and a comparison unit 46 are arranged. What is described as “˜unit” here includes a processing circuit, and an electric circuit, a computer, a processor, a circuit board, a quantum circuit, a semiconductor device, or the like can be used as the processing circuit. In addition, each “˜unit” may use a common processing circuit (the same processing circuit). Alternatively, different processing circuits (separate processing circuits) may be used.

  As the defect area specifying step (S110), the area specifying unit 51 compares the frame image (optical image: second optical image) with the reference image (for high NA: second reference image), and determines that it is a defect. Using the defective pixel thus determined, the region of the pattern abnormality portion in the frame image (optical image: second optical image) is specified. Specifically, information on the inspection result (for example, coordinate information of the defect position) is input from the comparison circuit 108, and an area where a defect is detected (hereinafter, defective area) is specified based on the inspection result information. Examples of the defect area include a frame area 30 including a defect position. However, the present invention is not limited to this, and various region sizes can be selected. Information on the identified defective area is stored in the storage device 59.

  As the low NA inspection mode switching step (S112), the inspection mode switching control circuit 144 switches the setting of each configuration from the above-described high NA inspection mode to a low NA inspection mode for performing a low NA defect inspection. Accordingly, when the pattern is exposed and transferred to the semiconductor substrate 304 by the exposure apparatus, the photodiode array 105 can receive an image substantially similar to or close to the pattern image formed on the semiconductor substrate 304. For this purpose, the following internal processes are performed.

  As the polarizing element retracting step (S114), the inspection mode switching control circuit 144 controls the driving mechanism 176 to move the polarizing beam splitter 174 from the optical path to the outside of the optical path. Thus, the drive mechanism 176 can move the polarizing element from the optical path to the optical path and can move the polarizing element from the optical path to the optical path. In addition, since the reflected illumination light is not used by retracting the polarizing beam splitter 174, the inspection mode switching control circuit 144 allows the polarizing plate 60 to pass only the light for transmission inspection illumination (one of the p wave and the s wave). Rotate.

  As the NA reduction step (S116), the inspection mode switching control circuit 144 switches the numerical aperture NA from the high numerical aperture state to the low numerical aperture state. Specifically, when the transmitted light 190 is incident on the objective lens 104 from the mask substrate 101, the numerical aperture NAo (numerical aperture on the object o side) of the objective lens 104 that can be incident has characteristics similar to those of the exposure apparatus. Control. For example, the aperture value of the aperture stop 180 is controlled so that NAo = 0.3. In Embodiment 1, since the high numerical aperture state is changed to the low numerical aperture state, the aperture of the aperture stop 180 is narrowed down. As described above, the aperture stop 180 can be switched between a high numerical aperture state and a low numerical aperture state where the numerical aperture (NA) at which transmitted light can enter the objective lens 104 from the mask substrate 101. The beam diameter of transmitted light and / or reflected light is adjusted. In the example of FIG. 5, the aperture stop 180 is disposed between the lenses 70 and 72 in the imaging optical system 178, but is not limited thereto. It is only necessary that the light beam diameter can be adjusted by narrowing the light beam incident on the objective lens 104 or the light beam passing through the objective lens 104 and incident on the photodiode array 105. Therefore, the aperture stop 180 may be disposed between the mask substrate 101 and the photodiode array 105. More preferably, the aperture stop 180 is preferably arranged at the pupil position.

  In the illumination shape changing step (S118), the control signal from the inspection mode switching control circuit 144 is received, and the illumination shape switching mechanism 171 uses the illumination light (inspection light) shape similar to the illumination shape used in the exposure apparatus. Change to lighting shape. The illumination shape switching mechanism 171 includes a mechanism that can set a plurality of illumination-shaped aperture stops manufactured in advance by depositing Cr on a glass plate, and a mechanism that mechanically switches them. Further, the polarization condition of the illumination light is set so that the polarization condition of the illumination light becomes the polarization condition of the exposure apparatus.

  In the low NA scanning (scanning) step (S120), the photodiode array 105 can receive light corresponding to the state of the low numerical aperture, and between the mask substrate 101 and the photodiode array 105. In a state where there is no polarizing element on the optical path, the specified defect area of the mask substrate 101 is scanned using predetermined illumination light, whereby the transmitted light of the pattern formed on the mask substrate 101 using the photodiode array 105 is obtained. Take an image. Specifically, it operates as follows.

  FIG. 9 is a diagram illustrating an example of the configuration of the optical system in the low NA inspection mode according to the first embodiment. In FIG. 9, the light generated from the light source 103 is polarized through the polarizing plate 60 and passes only the light for transmission inspection illumination (one of the p wave and the s wave). Then, the light for transmission inspection illumination (one of the p wave and the s wave) is reflected by the polarization beam splitter 61. Light for transmission inspection illumination (first inspection light) is illuminated onto the mask substrate 101 by the transmission illumination optical system 170. At that time, the illumination shape switching mechanism 171 changes the configuration of the transmission illumination optical system 170 to change the illumination shape of light for transmission inspection illumination (first inspection light). Thus, the transmission illumination optical system 170 is configured to be able to change the illumination shape of the light for the transmission inspection illumination (first inspection light). In the example of FIG. 9, after the light for transmission inspection illumination reflected by the polarization beam splitter 61 reaches the lens 65, the illumination shape is changed between the lenses 65 and 66. The light for transmission inspection illumination whose illumination shape is changed is reflected by the mirror 67. The light reflected by the mirror 67 is imaged by the condenser lens 68 on the pattern forming surface of the mask substrate 101 from the back side opposite to the pattern forming surface of the mask substrate 101. Thereby, the mask substrate 101 can be irradiated with light having the same illumination shape as that used in the exposure apparatus. The transmitted light that has passed through the mask substrate 101 enters the imaging optical system 178 via the objective lens 104. The imaging optical system 178 forms an image on the photodiode array 105 (an example of a sensor) and enters the photodiode array 105 as an optical image. At that time, since the numerical aperture is limited by the aperture stop 180, an optical image of low NA light is incident on the photodiode array 105. In the example of FIG. 9, the light that has passed through the objective lens 104 passes through the lenses 69 and 70 in the imaging optical system 178, passes through the aperture stop 180, and is focused on the photodiode array 105 by the lens 72. Thus, the photodiode array 105 receives the transmitted light from the mask substrate 101 that is illuminated with the light for the transmission inspection illumination (first inspection light) while the XYθ table 102 is moving.

  On the other hand, as shown in FIG. 9, since the light for reflection inspection illumination is blocked by the polarizing plate 60, the light for reflection inspection illumination does not enter the reflection illumination optical system 172. Even if light for reflection inspection illumination is incident on the reflection illumination optical system 172, in the reflection illumination optical system 172, since the polarization beam splitter 174 (polarization element) is retracted from the optical path, it is reflected on the mask substrate 101. The light for inspection illumination is not illuminated.

  As described above, the photodiode array 105 receives an image that is substantially the same as or close to the pattern image formed on the semiconductor substrate 304 when the exposure apparatus transfers the pattern onto the semiconductor substrate 304 by exposure. it can.

  The pattern image formed on the photodiode array 105 is photoelectrically converted by each light receiving element of the photodiode array 105, and further A / D (analog / digital) converted by the sensor circuit 106. Then, the pixel data of the defect area to be measured is stored in the stripe pattern memory 123. When capturing such pixel data (defective area image), the dynamic range of the photodiode array 105 is, for example, a dynamic range having a maximum gradation when the amount of illumination light incident is 60%. Further, when acquiring the optical image of the defect area, the laser length measurement system 122 measures the position of the XYθ table 102. The measured position information is output to the position circuit 107. The position circuit 107 (calculation unit) calculates the position of the mask substrate 101 using the measured position information.

  Thereafter, the defective area image is sent to the low NA inspection circuit 146 together with data indicating the position of the photomask 101 on the XYθ table 102 output from the position circuit 107. The measurement data (defective area image: pixel data) is, for example, 8-bit unsigned data, and represents the brightness gradation (light quantity) of each pixel. The defective area image output in the low NA inspection circuit 146 is stored in the storage device 52. As described above, one image (measured image) data to be compared for the low NA inspection is generated.

  Here, in the low NA inspection mode, as described above, since the polarization beam splitter 174 is moved from the optical path to the outside of the optical path, the light via the reflective illumination optical system 172 is not illuminated on the mask substrate 101. Therefore, the reflected light from the mask substrate 101 cannot be obtained. Therefore, the reflected light from the mask substrate 101 cannot be used for AF control. Therefore, the configuration is as follows.

  As the AF control step (S122), the AF control circuit 140 (a part of the focusing mechanism) moves the XYθ table 102 while the polarization beam splitter 174 is moved out of the optical path and moves the photodiode array 105 low. While receiving the transmitted light corresponding to the numerical aperture, the transmitted light is dynamically focused using information on the focal position stored in the magnetic disk device 109. Specifically, the AF control circuit 140 controls the piezo element 142 (part of the focusing mechanism) based on the already measured focal position information stored in the magnetic disk device 109, and the XYθ table 102. Is moved in real time in the optical axis direction (Z-axis direction) to dynamically follow the progress of the XYθ table 102 and dynamically move the focus position of the objective lens 104 on the pattern forming surface of the mask substrate 101 (optical axis direction: Z-axis). Direction).

  In the reference image (2) creation step (S124), the reference circuit 112 (an example of the reference image creation unit) filters the design image to match the defect area image (low NA optical image) and performs a reference image (low NA). For). Here, it is necessary to use a filter operation that reproduces the illumination shape, polarization conditions, and imaging optical conditions of low NA, which is different from the filter operation to be matched with the optical image obtained in the high NA scan (scan in the normal inspection mode). There is an optical filter operation based on partially coherent imaging theory. The created reference image (for low NA) is sent to the low NA inspection circuit 146. The reference image (2) output in the low NA inspection circuit 146 is stored in the storage device 50. Of the reference images (for high NA) of each frame region 30 created in the reference image (1) creation step (S106), the reference image (for high NA) corresponding to the defect region image (low NA optical image). May be used as a simple reference image (for low NA).

FIG. 10 is a diagram illustrating an example of an image gradation value profile of the pattern in the optical image and the pattern in the reference image in the first embodiment. The pattern dimension (CD) of the pattern formed on the mask substrate 101 is created from the tone value profile of the pattern in the optical image obtained by imaging the pattern formation region of the mask substrate 101 and the design data corresponding to the pattern. It is obtained by comparing the gradation value profile of the pattern in the reference image. 10, in the gradation value profile of the pattern in the reference image, the level of the pattern in the optical image at the detection threshold Th ₀ (first threshold level) at which the dimension between EFs in FIG. 10 becomes the desired design width dimension. By measuring the dimension between AB (for example, the width of the space pattern) from the gradation profile, the width dimension CD of the pattern formed on the mask substrate 101 should have been originally obtained.

Here, in the optical image obtained by the high NA scan (scan in the normal inspection mode), if the normal portion is not the defect position, the gradation value profile of the pattern in the optical image and the pattern in the reference image Filter calculation coefficients are set so that the gradation value profiles substantially match. Therefore, the dimension between the EFs of the pattern in the reference image and the dimension between the ABs of the pattern in the optical image match. However, in the optical image obtained by the low NA scan, since the reference image is generated by the filter operation based on the optical theory, the shape of the gradation value profile of the pattern in the optical image and the reference image (for low NA) The shape of the tone value profile of the pattern may not match as shown in FIG. Therefore, the dimension between EFs of the pattern in the reference image (for low NA) and the dimension between AB of the pattern in the optical image do not match. In addition to the reference image (for high NA) that has been smoothed and filtered to match the optical image obtained in the high NA scan (scan in the normal inspection mode), the optical image obtained in the low NA scan is used separately. It is difficult to match the tone value profiles even in a reference image (for low NA) created so as to fit. This is because even if an attempt is made to create a reference image so as to match an optical image obtained by a low NA scan, the accuracy of filter processing is not yet sufficient, and as a result, the tone value profiles may be matched. It has become difficult. Specifically, due to the complexity of illumination conditions and other conditions to approximate the exposure image, a certain difference, that is, an error, remains in the reference image generated by filtering the optical image obtained by the low NA scan. become. For this reason, the pattern dimensions in the optical image obtained by the low NA scan cannot be obtained accurately. Therefore, in the first embodiment, the detection threshold Th ₀ (threshold level) that changes to the desired design width dimension is changed in the gradation value profile of the pattern in the reference image. Specifically, using a normal part pattern that is not a defect position, the original detection threshold Th ₀ (first threshold level), and a tone value profile of a pattern in an optical image obtained by a low NA scan. The detection threshold Th ′ (second threshold level) of the gradation value profile of the pattern in the reference image (for low NA) that matches the obtained dimension between AB is measured. Then, the dimension between E′F ′ is measured from the gradation value profile of the pattern in the reference image at the detection threshold Th ′. On the other hand, the dimension between AB is measured from the gradation value profile of the pattern in the optical image obtained by the low NA scan at the original detection threshold Th ₀ . Since the dimension between E′F ′ should correspond to the desired design width dimension, the width dimension L of the pattern formed on the mask substrate 101 is L = (dimension between E′F ′) / (dimension between AB) × ( (Design width dimension).

  In the normal part measurement image edge pair detection step (S130), first, the alignment unit 56 performs a defect area image (low NA optical image) having a size of the frame area 30 to be compared and a reference image to be compared. Alignment is performed using a predetermined algorithm. For example, alignment is performed using a least square method.

  Next, the measurement image edge pair detection unit 56 detects an edge pair in a pattern in a normal part that is not a defect position stored in the storage device 59 in the defect area image (low NA optical image).

  FIG. 11 is a diagram showing a defect area image and an example of an edge pair in the first embodiment. In the example of FIG. 11, line patterns 12 a, 12 b, 12 c, 12 d extending in the y direction and line patterns 13 extending in the x direction are shown in the defect area image (low NA optical image). Has been. A space pattern 14a is formed between the line patterns 12a and 12b. A space pattern 14b is formed between the line patterns 12b and 12c. A space pattern 15 is formed between the line patterns 12c, 12c, and 13. As shown in FIG. 11, the defect area image (low NA optical image) is composed of an image having the size of the frame area 30 including the pixel 36 determined to be a defective portion (A part). Therefore, in the defect area image (low NA optical image), there is a pattern of a normal part that is not a defective part (A part). The measurement image edge pair detection unit 56 recognizes the position of the edge portion (outer periphery) of the pattern for the normal portion pattern in the defect area image (low NA optical image), and configures the width dimension (CD) of the pattern. An edge pair composed of edges at both ends (outer periphery) is detected. In the example of FIG. 11, a case is shown in which edges a and b at both ends (outer periphery) constituting a CD in a space portion (white portion) between adjacent patterns 12 a and 12 b are detected. One edge pair may be detected in the normal part pattern, but it is preferable to detect a plurality of edge pair groups. In such a case, for example, detection is performed in units of pixels. For example, for the frame region of the x-direction line and space pattern, a pair is detected for each pixel on the outer periphery of the line pattern extending in the y direction. For the frame region of the y-direction line and space pattern, a pair is detected for each pixel on the outer periphery of the line pattern extending in the x direction. For a frame area of a plurality of rectangular patterns, a pair is detected for each pixel on the outer periphery extending in the y direction of each rectangular pattern. For frame regions of a plurality of rectangular patterns, a pair is detected for each pixel on the outer periphery extending in the x direction of each rectangular pattern.

  FIG. 12 is a diagram for explaining a pattern edge recognition method according to the first embodiment. In the example of FIG. 12, a part of the defect area image (low NA optical image) shown in FIG. 11 is shown. For example, at the edge in the line width direction of the line pattern 12c extending in the y direction, the gradation value profile in the x direction changes sharply, whereas the gradation value profile in the y direction maintains a constant value. Become. Similarly, at the edge portion in the extending direction of the line pattern 12d extending in the y direction, the gradation value profile in the y direction changes sharply, whereas the gradation value profile in the x direction maintains a constant value. Become. As described above, there exists an edge portion in a direction in which the inclination is maximum at a position where the inclination of the gradation value profile is maximum. What is necessary is just to recognize such a position as an edge part.

  In addition, in the example of FIG. 11, although it has shown about CD of the space part (white part) between adjacent patterns 12a and 12b, it does not restrict to this. It is preferable that an edge pair is detected even for a black CD having a pattern.

  In the normal portion reference image edge pair detection step (S132), the reference image edge pair detection unit 58 uses a reference image (for low NA) corresponding to the defect region image (low NA optical image) to detect a defective region image (low NA). An edge pair in the reference image corresponding to the edge pair detected from the optical image) is detected.

As the reference image threshold level measurement step (S134), the threshold level measurement unit 40 uses the detection threshold Th ₀ (first threshold level) of the image gradation value profile to identify a defective area image (low NA optical image). The pattern dimension between edge pairs at normal positions in the pattern (for example, dimension between AB) and the pattern dimension between edge pairs at corresponding positions in the reference image (for low NA) are the same dimension (dimension between E′F ′). A detection threshold Th ′ (second threshold level) used for the reference image (for low NA) is measured. When a plurality of sets of edge pairs are detected, a statistical value (for example, an average value, a median value, a maximum value, or a minimum value) of the detection threshold Th ′ (second threshold level) obtained in each set is obtained. It is preferable to calculate. For example, if an average value or a median value is used, measurement errors of the detection threshold Th ′ (second threshold level) can be averaged. Note that the detection threshold Th ′ (second threshold level) is assumed to have a different optimum value depending on the direction. For example, it is preferable to obtain the detection threshold Th ′ for the x-direction dimension and the detection threshold Th ′ for the y-direction dimension.

  As a measurement image edge pair detection step (S136) of a defective portion, the measurement image edge pair detection unit 56 detects an edge pair of a pattern in the region of the pattern abnormality portion in the defect region image (low NA optical image). Even if a defective pixel can be identified by high NA inspection (normal inspection), the maximum value of the pattern dimension of the pattern abnormal part cannot be immediately known. Therefore, in the first embodiment, not only the pattern dimension in which the edge is located on the defective pixel, but also the defective pixel 36 and a plurality of areas around the defective pixel 36 as the region of the pattern abnormality portion in the defective region image (low NA optical image). The pattern dimension of the pixel group 38 composed of the pixels is measured. For example, the pattern dimensions in the 3 × 3 pixel group 38 having the defective pixel 36 as the central pixel are measured. Therefore, the measurement image edge pair detection unit 56 detects a plurality of edge pairs in the same direction of the pattern in the defect area image (low NA optical image). Specifically, a plurality of edge pairs in which at least one edge is included in the pixel group 38 are detected. In the example of FIG. 11, three edge pairs in the same direction in the pixel group 38 in the defect area image (low NA optical image) are shown. If the dimension direction is different, a dimension in a direction different from the dimension of the defect portion to be obtained is also included. Therefore, a plurality of edge pairs in the same direction in the pixel group 38 are detected.

  As the reference image edge pair detection step (S138) of the defective part, the reference image edge pair detection unit 58 performs a reference image corresponding to the edge pair of the pattern detected in the region of the pattern abnormal part in the defect region image (low NA optical image) ( Edge pairs of patterns in (for low NA) are detected. Specifically, the reference image edge pair detection unit 58 detects a plurality of edge pairs corresponding to a plurality of edge pairs in the same direction in which at least one edge is included in the pixel group 38 in the defect area image (low NA optical image). To do. In the example of FIG. 11, three edge pairs in the same direction in the pixel group 38 in the reference image (for low NA) are shown.

  As the reference image pattern dimension measuring step (S140) of the defective part, the reference image pattern dimension measuring part 44 (first pattern dimension measuring part) uses the detection threshold Th ′ (second threshold level) to detect the defect region image. The pattern dimension of the corresponding portion in the reference image (for low NA) corresponding to the pattern abnormal portion in (low NA optical image) is measured. Specifically, the dimension between the edge pairs (dimension between E′F ′) is measured for each of a plurality of detected edge pairs in the same direction in which at least one edge is included in the pixel group 38. The dimension between the edge pairs (dimension between E'F ') corresponds to the design dimension of the pattern. The design dimension can be obtained from design data (drawing data) before image development. As described above, when measuring the pattern dimension of the corresponding part in the reference image (for low NA) corresponding to the pattern abnormal part in the defect area image (low NA optical image), the plurality of edge pairs in the pixel group 38 are measured. The dimensions between corresponding edge pairs are measured.

As the measurement image pattern dimension measurement step (S142) of the defective part, the measurement image pattern dimension measurement part 42 (second pattern dimension measurement part) uses the original detection threshold Th ₀ (first threshold level) to detect defects. The pattern dimension of the pattern abnormal part in the area image (low NA optical image) is measured. Specifically, the dimension between edge pairs (dimension between AB) is measured for each of a plurality of detected edge pairs in the same direction in which at least one edge is included in the pixel group 38. Then, the pattern dimension is measured (approximated) by calculating the pattern dimension L (= (dimension between E′F ′) / (dimension between AB) × design dimension) for each edge pair. As described above, when measuring the pattern dimension of the pattern abnormal portion in the defect area image (low NA optical image), the dimension between the plurality of edge pairs in the pixel group 38 is measured.

  As a comparison step (S144), the comparison unit 46 compares the pattern dimension measured from the reference image (for low NA) and the pattern dimension measured from the defect area image (low NA optical image) for the pattern abnormal part. . Specifically, the comparison unit 46 compares the pattern dimension measured from the defect area image (low NA optical image) with the pattern dimension measured from the reference image (for low NA). Of the pattern dimensions between a plurality of edge pairs, the pattern dimension between edge pairs that maximizes the difference between the defect area image (low NA optical image) and the reference image (for low NA) is determined as the pattern dimension of the pattern abnormal part. Then, a comparison result (determination result) is output. The comparison result may be output from the magnetic disk device 109, the magnetic tape device 115, the flexible disk device (FD) 116, the CRT 117, the pattern monitor 118, or the printer 119. For example, when the pattern dimension measured in the defect area image (low NA optical image) is shifted by more than several percent with respect to the pattern dimension of the reference image, it can be determined as an unusable defect pattern. . For example, when a deviation exceeding 5 to 10% occurs, it can be determined that the defect pattern cannot be used.

  As described above, in the first embodiment, the pattern size of the defective portion is evaluated by comparing with the reference image created based on the design data. For example, since a die-die inspection is difficult for a circuit such as a logic circuit in which the same pattern is often not repeatedly formed in the mask substrate 101, the method of the first embodiment for comparing with a reference image uses such a logic circuit. This is particularly effective when inspecting.

  FIG. 13 is a diagram illustrating an example of a mechanism for changing the illumination shape in the first embodiment. In FIG. 13, the illumination shape switching mechanism 171 moves the polarizing element 90 between the lenses 65 and 66. Thereby, the configuration of the transmission illumination optical system 170 can be changed, and the transmission illumination optical system 170 can change the illumination shape. When the light for transmission inspection illumination passes through the polarizing element 90, the illumination shape of the light for transmission inspection illumination can be changed to polarization illumination in which the polarization direction is manipulated. Note that the illumination shape may be changed to N-pole illumination such as dipole illumination or quadrupole illumination, or annular illumination by replacing the polarizing element 90 with a diffraction element. In the example of FIG. 13, the polarizing element 90 is disposed between the lenses 65 and 66, but is not limited thereto. It is sufficient that the light can be polarized before the light reaches the mask substrate 101. Therefore, the polarizing element 90 may be disposed anywhere on the optical path from the polarizing plate 60 to the condenser lens 68.

  FIG. 14 is a diagram showing another example of a mechanism for changing the illumination shape in the first embodiment. In FIG. 14, the illumination shape switching mechanism 171 may be moved so as to dispose the diaphragm 91 between the light source 103 and the polarizing plate 60. Thereby, the configuration of the transmission illumination optical system 170 can be changed, and the transmission illumination optical system 170 can change the illumination shape. By reducing the stop 91, the illumination shape can be changed to small sigma illumination. In the example of FIG. 14, the diaphragm 91 is disposed between the light source 103 and the polarizing plate 60, but is not limited thereto. What is necessary is just to squeeze before light reaches the mask substrate 101. Therefore, the diaphragm 91 may be disposed anywhere on the optical path from the polarizing plate 60 to the mask substrate 101.

  FIG. 15 is a diagram illustrating another example of a mechanism for changing the illumination shape in the first embodiment. In FIG. 15, the illumination shape switching mechanism 171 may be moved so that a reduction optical system using lenses 92 and 93 is disposed between the light source 103 and the polarizing plate 60. Thereby, the configuration of the transmission illumination optical system 170 can be changed, and the transmission illumination optical system 170 can change the illumination shape. By reducing the light with such a reduction optical system, the illumination shape can be changed to illumination light having different illumination magnifications. In the example of FIG. 11, the reduction optical system using the lenses 92 and 93 is disposed between the light source 103 and the polarizing plate 60, but is not limited thereto. It is sufficient that the light can be reduced before the light reaches the mask substrate 101. Therefore, a reduction optical system using the lenses 92 and 93 may be disposed anywhere on the optical path from the polarizing plate 60 to the condenser lens 68.

  The illumination shape switching mechanism 171 may be configured so that one of the mechanisms shown in FIGS. 13, 140, and 15, any two combinations, or all combinations can be arranged. Or you may comprise by another mechanism. What is necessary is just to arrange | position a required optical element according to the specification of exposure apparatus.

  As described above, according to the first embodiment, it is possible to improve the line width measurement accuracy of the defective portion from the optical image (low NA optical image) captured so as to be close to the pattern image when transferred by the exposure apparatus. it can. Therefore, the pattern dimension of the defective part can be inspected. As a result, it is possible to more accurately evaluate the influence of transferability when the mask pattern is transferred to a semiconductor substrate or the like.

  In the above description, what is described as “˜circuit” includes a processing circuit, and an electric circuit, a computer, a processor, a circuit board, a quantum circuit, a semiconductor device, or the like can be used as the processing circuit. Further, a common processing circuit (the same processing circuit) may be used for each “˜circuit”. Alternatively, different processing circuits (separate processing circuits) may be used. When configured by a processor or the like, the program is recorded on a recording medium such as a magnetic disk device, a magnetic tape device, an FD, or a ROM (read only memory).

  The embodiments have been described above with reference to specific examples. However, the present invention is not limited to these specific examples. For example, when the gradation value of the pixel group 38 including the peripheral pixels of the defective pixel 36 cannot be obtained from one image, such as when the defective pixel 36 is located near the end of the frame region 30, the adjacent frame A low NA scan of the region 30 may also be performed to create an adjacent low NA optical image.

  In addition, although descriptions are omitted for parts and the like that are not directly required for the description of the present invention, such as a device configuration and a control method, a required device configuration and a control method can be appropriately selected and used. For example, although the description of the configuration of the control unit that controls the inspection apparatus 100 is omitted, it goes without saying that the required control unit configuration is appropriately selected and used.

  In addition, all pattern inspection apparatuses and pattern inspection methods that include elements of the present invention and whose design can be appropriately changed by those skilled in the art are included in the scope of the present invention.

DESCRIPTION OF SYMBOLS 10 Inspection area | region 12, 13 Line pattern 14, 15 Space pattern 20 Inspection stripe 30 Frame area 36 Pixel 38 Pixel group 40 Threshold level measurement part 42 Measurement image pattern dimension measurement part 44 Reference image pattern dimension measurement part 46 Comparison part 50, 52, 59 Storage Device 51 Region Identification Unit 54 Positioning Unit 56 Measurement Image Edge Pair Detection Unit 58 Reference Image Edge Pair Detection Unit 60 Polarizing Plate 61 Polarizing Beam Splitter 63 Mirror 64, 65, 66 Lens 67, 73 Mirror 68 Condenser Lens 69, 70, 72 , 80, 81, 82, 83 Lens 90 Polarizing element 91 Aperture 92, 93 Lens 100 Inspection device 101 Mask substrate 102 XYθ table 103 Light source 104 Objective lens 105 Photodiode array 106 Sensor circuit 107 Position circuit 108 Comparison times Path 109 Magnetic disk device 110 Control computer 111 Expansion circuit 112 Reference circuit 113 Autoloader control circuit 114 Table control circuit 115 Magnetic tape device 116 FD
117 CRT
118 Pattern monitor 119 Printer 120 Bus 122 Laser measurement system 123 Stripe pattern memory 140 AF control circuit 142 Piezo element 144 Inspection mode switching control circuit 146 Low NA inspection circuit 150 Optical image acquisition mechanism 160 Control system circuit 170 Transmitting illumination optical system 171 Illumination Shape switching mechanism 172 Reflection illumination optical system 174 Polarization beam splitter 176 Drive mechanism 178 Imaging optical system 180 Aperture stop 190 Transmitted light 192 Light 300 Mask substrate 301 Transmitted light 302 Objective lens 304 Semiconductor substrate 305 Light

Claims (4)

Obtaining an optical image of the mask substrate on which the pattern is formed;
Creating a reference image corresponding to the optical image;
Used for the reference image in which the pattern dimension of the normal pattern portion in the optical image specified by using the first threshold level of the image gradation value profile is the same as the pattern dimension of the corresponding portion in the reference image. Measuring a second threshold level;
Using the second threshold level to measure a pattern dimension of a corresponding portion in the reference image corresponding to a pattern abnormal portion in the optical image;
Using the first threshold level to measure a pattern dimension of a pattern abnormality location in the optical image;
Comparing the pattern dimension measured from the reference image with the pattern dimension measured from the optical image, and outputting the pattern abnormality portion;
A pattern inspection method comprising:   The pattern inspection method according to claim 1, further comprising a step of detecting a plurality of edge pairs in the same direction of the pattern in the optical image. As a region of a pattern abnormality portion in the optical image, a defective pixel in which a gradation difference between a pixel in the optical image and a corresponding pixel in the reference image exceeds a third threshold, and a plurality of pixels around the defective pixel And a pixel group composed of
When measuring the pattern dimension of the pattern abnormal part in the optical image, the dimension between a plurality of edge pairs in the pixel group is measured,
When measuring the pattern dimension of the corresponding part in the reference image corresponding to the pattern abnormal part in the optical image, the dimension between the plurality of edge pairs corresponding to the plurality of edge pairs in the pixel group is measured,
When comparing the pattern dimension measured from the optical image and the pattern dimension measured from the reference image, of the dimensions between the plurality of edge pairs in the pixel group, the difference between the optical image and the reference image The pattern inspection method according to claim 1, wherein a pattern dimension between edge pairs having a maximum is determined as a pattern dimension of the abnormal pattern portion and output. An optical image acquisition mechanism for acquiring an optical image of a mask substrate on which a pattern is formed;
A reference image creation unit for creating a reference image corresponding to the optical image;
Used for the reference image in which the pattern dimension of the normal pattern portion in the optical image specified by using the first threshold level of the image gradation value profile is the same as the pattern dimension of the corresponding portion in the reference image. A threshold level measurement unit for measuring the second threshold level;
A first pattern dimension measuring unit that measures a pattern dimension of a corresponding part in the reference image corresponding to a pattern abnormal part in the optical image by using the second threshold level;
A second pattern dimension measuring unit that measures a pattern dimension of a pattern abnormality portion in the optical image using the first threshold level;
A comparison unit that compares the pattern dimension measured from the reference image and the pattern dimension measured from the optical image for the pattern abnormal part,
A pattern inspection apparatus comprising:

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