JP6032828B2 - Synchronizer and inspection device - Google Patents

Description

  The present invention relates to a synchronizer that synchronously outputs two types of images captured through the same objective lens, and an inspection apparatus including such a synchronizer.

  As an inspection apparatus for inspecting defects in a photomask, the incident light illumination system is used to illuminate the photomask, and the transmitted light and reflected light emitted from the photomask are received to capture the transmitted image and the reflected image. In addition, an inspection apparatus that detects a defect based on a reflected image is known (for example, see Patent Document 1). In this inspection apparatus, since a diffraction action occurs at the pattern edge, the detection sensitivity at the pattern edge is lowered, and it is difficult to detect a defect near the pattern edge.

As a mask inspection apparatus that solves the above-described problems, a mask inspection apparatus that captures a composite image composed of a transmission image and a reflection image of a photomask and a transmission image and detects defects based on the composite image and the transmission image is known. (For example, see Patent Document 2). In this known mask inspection apparatus, the field of view of the objective lens is divided into two in the direction orthogonal to the main scanning direction of the stage, and the reflected illumination light and the reflected illumination light are simultaneously projected onto one field area to reflect the reflected image and the transmitted image. And a transmitted image is captured by projecting transmitted illumination light onto the other adjacent visual field region. Then, a photomask to be inspected is placed on the stage, the stage is moved in a zigzag shape, and a composite image and a transmission image are captured over the entire surface of the photomask.
U.S. Patent No. 7664310 JP 2008-190938 A

  In the mask inspection apparatus that captures the composite image and transmission image of the photomask described above, both transmitted illumination light and reflected illumination light are incident on the pattern area of the photomask, and an appropriate amount of illumination light is incident on the edge of the pattern. Since it is incident, there is an advantage that defects existing in the pattern edge portion can be detected with higher detection sensitivity. However, since the composite image and the transmission image are individually captured with a time shift, in order to synchronize the threshold inspection based on the composite image and the threshold inspection based on the transmission image, output from the two image sensors. Therefore, a synchronization device that synchronizes the generated image signal is required.

  As a method of synchronizing the image signals output from the two image sensors, a method of temporarily capturing the image signals output from the image sensors into the image memory and outputting the image signals stored in the memory to the defect detection apparatus in synchronization Is assumed. However, in the method of once fetching into the image memory and synchronously outputting, there is a drawback that the threshold inspection cannot be performed in real time using the image signals sequentially output from the image sensor, and the defect inspection throughput is reduced. In addition, a large-capacity memory device is required, and there is a disadvantage that the signal processing device becomes large and the manufacturing cost increases.

An object of the present invention is to realize a synchronization device that can synchronize two kinds of image signals picked up through the same objective lens in real time and is suitable for performing defect inspection in real time.
An object of the present invention is to realize a synchronization device capable of synchronously outputting two types of image signals in real time without using a large-capacity memory device.
Furthermore, another object of the present invention is to provide an inspection apparatus that can synchronize a composite image signal and a transmission image signal that are imaged with a time lag and perform defect inspection in real time.

The synchronizer according to the present invention, which is described as a reference , uses an image of a sample placed on a stage that moves in a zigzag manner in a first scanning direction and a second scanning direction opposite to the first scanning direction. A synchronizer that outputs first and second image signals output from first and second image sensors that respectively image through a lens in synchronization;
Delay means for delaying the input image signal by a predetermined time period;
The first imaging device and the delaying unit are connected during the period in which the stage moves in the first scanning direction, and the stage is connected to the second imaging unit. The stage is disposed between the delaying unit and the first and second imaging devices. A first switch for connecting the second image sensor and the delay means during the period of movement in the scanning direction;
First and second input units connected to the first and second imaging elements, a third input unit connected to the delay means, a first output unit for outputting the first image signal, and A second output unit that outputs a second image signal, and connects the third input unit and the first output unit during a period in which the stage moves in the first scanning direction; And the second output unit, and the first input unit and the first output unit are connected while the stage moves in the second scanning direction, and the third input unit and the second output unit are connected. And a second switch for connecting
The first and second image signals are output in synchronization from the first and second output units.

  Image signals output from two image sensors that are spaced apart from each other in the main scanning direction of the stage are output with a time shift corresponding to the distance corresponding to the distance between the image sensors. Therefore, in order to perform signal processing in real time, it is necessary to delay the image signal output from one image sensor by a predetermined time period. Further, when the stage moves in a zigzag shape, it is necessary to switch the image signal directly output from the image sensor. Therefore, in the present invention, the first switch means is disposed between the two image sensors and the delay means, and the second switch means is provided at the subsequent stage of the two image sensors and the delay means. The first switch means switches the image signal input to the delay means according to the scanning direction of the stage. The second switch means switches the image signal output directly from the image sensor without using the delay means according to the scanning direction of the stage. By controlling the switching of these two switch means in accordance with the moving direction of the stage, the two image signals are synchronized even if the stage moves in a zigzag manner in the opposite scanning directions, and the two synchronized image signals are It becomes possible to output in real time.

An inspection apparatus according to the present invention is an inspection apparatus that inspects a photomask having a pattern formation surface on which a pattern portion and a light transmission portion are formed and a back surface opposite to the pattern formation surface,
A stage that supports a photomask to be inspected and moves in a zigzag manner in a first scanning direction and a second scanning direction opposite to the first scanning direction;
Projecting a transmitted illumination beam toward the back surface of the photomask to be inspected, projecting a reflected illumination beam toward the pattern forming surface of the photomask, and a transmitted illumination optical system that illuminates the first area of the photomask; An illumination optical system having a reflective illumination optical system that illuminates a second area that overlaps the first area in the optical axis direction and is smaller than the first area;
A first image sensor that receives a combined light of the reflected light and the transmitted light emitted from the second area of the photomask and captures a combined image obtained by optically combining the transmitted image and the reflected image of the photomask. And a detection system having a second imaging element that receives transmitted light emitted from the remaining third area excluding the second area of the first area and captures a transmission image of the photomask;
A synchronization device that synchronizes the first and second image signals output from the first and second image sensors, respectively, and a defect that processes the image signal output from the synchronization device and outputs data indicating a defect A signal processing device having a detection device,
The synchronization device includes delay means for delaying an input image signal by a predetermined time period;
The first imaging device and the delaying unit are connected during the period in which the stage moves in the first scanning direction, and the stage is connected to the second imaging unit. The stage is disposed between the delaying unit and the first and second imaging devices. A first switch for connecting the second image sensor and the delay means during the period of movement in the scanning direction;
First and second input units connected to the first and second imaging elements, a third input unit connected to the delay means, a first output unit for outputting the first image signal, and A second output unit that outputs a second image signal, and connects the third input unit and the first output unit during a period in which the stage moves in the first scanning direction; And the second output unit, and the first input unit and the first output unit are connected while the stage moves in the second scanning direction, and the third input unit and the second output unit are connected. And a second switch for connecting
The defect detection apparatus includes a signal processing unit that performs signal processing on the first and second image signals, an adding unit that adds the first and second image signals subjected to signal processing to generate an added composite signal, and Comparing means for comparing the added composite signal with a threshold value,
The transmission illumination optical system and the reflection illumination optical system are set so that b2 <b1 when the brightness value of the pattern portion image of the composite image is b1 and the brightness value of the light transmission portion image is b2. And
The signal processing means erases a signal portion having a luminance value b0 or less satisfying the expression b2 <b0 <b1 or a luminance value having a luminance value of b1 or less from the first image signal output from the first image sensor. Limiter means for performing a limiter process to uniformly convert the signal to be shown,
The addition means adds the first image signal subjected to limiter processing and the second image signal subjected to signal processing .

  In the inspection apparatus according to the present invention, the first image signal indicating the composite image composed of the transmission image and the reflection image of the photomask and the second image signal indicating the transmission image are synchronized in real time and are synchronized with each other. Since two image signals are supplied to the defect detection device, it becomes possible to detect the defect in real time. As a result, various signal processing can be performed in synchronization with respect to the image signal indicating the composite image and the image signal indicating the transmission image. In particular, since an added composite signal in which the composite image and the transmission image are added can be formed in real time, useful defect detection can be performed.

  In the present invention, the image signals output from the two image sensors can be synchronized in real time only by providing one delay memory and two switch means. As a result, defect detection can be performed in real time, and various signal processing can be performed using two types of image signals.

It is a figure which shows the structure of the optical system of the test | inspection apparatus by this invention. It is a figure which shows the illumination area formed of transmitted illumination light and reflected illumination light. It is a figure which shows the outline | summary of a signal processing apparatus. FIG. 4 is a diagram schematically showing the relationship between the first and second image sensors 18 and 19 and the stage movement. It is a figure which shows the enable signal in a test | inspection. It is a figure which shows an example of the synchronizer by this invention. It is a figure which shows the signal form in a 1st test | inspection algorithm. It is a figure which shows the signal form processed in the defect detection. It is a figure which shows the signal form processed in the defect detection. It is a figure which shows the algorithm of a defect detection.

  FIG. 1 is a diagram showing an example of an optical system of an inspection apparatus according to the present invention. A laser light source 1 is used as an illumination light source. The laser beam emitted from the laser light source is reflected by the total reflection mirror 2 via a speckle pattern reduction device (not shown) and is incident on the first beam splitter 3. The laser beam reflected by the first beam splitter 3 forms a transmitted illumination beam, and the laser beam transmitted through the first beam splitter 3 forms a reflected illumination beam. The transmitted illumination beam passes through the attenuator 4 and the quarter-wave plate 5 and enters the ND filter 6. This ND filter is mounted so as to be replaceable, and functions as a means for adjusting the intensity of the beam portion incident on the field of view on one half of the objective lens. That is, the ND filter 6 functions to adjust the luminance value of the transmission image in the composite image, and can adjust the illumination light amount of the transmission illumination according to the inspection purpose. For example, by preparing a plurality of ND filters having different transmittances and exchanging the ND filters, it is possible to adjust the luminance value of the image of the light transmission part in the composite image. Therefore, by replacing the ND filter, the difference between the luminance value of the image of the pattern portion and the luminance value of the image of the light transmitting portion in the composite image is adjusted. The transmitted illumination beam emitted from the ND filter 6 is reflected by the total reflection mirror 7 and enters the condenser lens 8.

  The transmitted illumination beam is incident on the back surface of the photomask 10 to be inspected arranged on the stage 9, and forms a first illumination area on the back surface of the photomask. As a photomask to be inspected, various photomasks such as a binary photomask, a halftone photomask, and a tritone photomask can be used. The illumination area will be described later. The stage 9 is composed of an XY stage, and moves in a zigzag manner in the main scanning direction and the sub-scanning direction orthogonal thereto. The positions of the stage in the main apparatus direction and the sub-scanning direction are detected by a position sensor 11 such as a linear scale (linear encoder) and supplied to a signal processing apparatus described later. The transmitted light that has passed through the photomask 10 is collected by the objective lens 12, passes through the quarter-wave plate 13 and the second beam splitter 14, and enters the total reflection mirror 15. Further, the light is reflected by the total reflection mirror 15 and enters the field dividing mirror 17 through the imaging lens 16. A part of the transmitted light emitted from the objective lens is reflected by the field dividing mirror and enters the first image sensor 18, and the remaining transmitted light passes through the field dividing mirror and enters the second image sensor 19. These image sensors can be composed of TDI sensors. The arrangement direction of the light receiving elements of the TDI sensor is set to a direction orthogonal to the main scanning direction (main movement direction) of the stage, and the charge transfer direction is set to the main scanning direction of the stage. The charge transfer speed and the stage moving speed are synchronized with each other. It is also possible to use a line sensor instead of the TDI sensor.

  The laser beam that has passed through the first beam splitter 3 forms a reflected illumination beam. The reflected illumination beam is reflected by the total reflection mirror 20 and enters the field stop 21. The field stop shields one half of the beam portion of the reflected illumination beam and passes only the remaining half beam portion. The reflected illumination beam emitted from the field stop is reflected by the second beam splitter 14, enters the element formation surface of the photomask 10 via the quarter-wave plate 13 and the objective lens 12, and passes through the second illumination area. Form. As shown in FIG. 2, the area of the second illumination area is set to half the area of the first illumination area formed by the transmitted illumination beam, and is formed so as to overlap the first illumination area. The reflected beam reflected by the surface of the photomask (element formation surface) is collected by the objective lens 12 and enters the second beam splitter 14 through the quarter-wave plate 13. Further, the reflected beam passes through the second beam splitter 14 and enters the total reflection mirror 15. Further, the light passes through the imaging lens 16 and the field dividing mirror 17 and enters the second image sensor 19. Image signals output from the first and second imaging elements are supplied to the signal processing device 22. The signal processing device 22 detects defects existing in the photomask using these image signals.

  In the present invention, the field of view of the objective lens is divided into two, the transmitted illumination light is projected on one field of view to capture a transmitted image, and the reflected illumination light and the transmitted illumination light are projected on the other field of view and combined. Take an image. FIG. 2 is a diagram illustrating an illumination area formed by transmitted illumination light and reflected illumination light projected onto a photomask, a field of view of an objective lens, and an image sensor. FIG. 2A shows the field of view and illumination area of the objective lens viewed in the optical axis direction of the objective lens, and FIG. 2B shows the plane including the optical axis of the objective lens and the moving direction of the stage. . In the inspection apparatus according to the present invention, the visual field 30 of the objective lens is divided into two by the dividing line L, the first visual field region 31 captures a composite image of the photomask, and the second visual field region 32 captures a transmission image.

  In order to capture a transmission image and a composite image of a photomask, the inspection apparatus of the present invention transmits a reflection illumination optical system for projecting illumination light for reflection inspection from the front side to the photomask, and transmits from the back side of the photomask. A transmission illumination optical system that projects illumination light for inspection. As shown in FIG. 2B, the transmitted illumination beam emitted from the transmitted illumination optical system forms a first illumination area 33 on the photomask, and the reflected illumination light emitted from the reflected illumination optical system is on the photomask. The second illumination area 34 is formed so as to overlap the first illumination area. The area of the second illumination area is set to be half the area of the first illumination area. The first illumination area is divided into two areas by a field dividing line L, one area constitutes a second illumination area where the reflected illumination light is incident, and the other half area is incident only with the transmitted illumination light. A third illumination area 35 is formed.

  In the second illumination area 34 of the photomask to be inspected, the transmitted illumination beam is incident from the back side and the reflected illumination beam is incident from the front side. Accordingly, the second illumination area 34 is illuminated simultaneously by the reflected illumination light and the transmitted illumination light. Therefore, in the second illumination area, reflected light reflected by the surface of the photomask and transmitted light transmitted through the photomask are emitted. In addition, a transmitted illumination beam is incident on the third illumination area 35 from the back side, and is illuminated only by the transmitted illumination light. Therefore, only the transmitted beam that has passed through the photomask is emitted from the third illumination area. Accordingly, the second illumination area 34 corresponds to the first visual field region 31 that captures the composite image, and the third illumination area 35 corresponds to the second visual field region 32 that captures the transmission image.

  The field division mirror 17 functions to separate the combined light emitted from the second illumination area and the transmitted light emitted from the third illumination area. That is, the transmitted light emitted from the third illumination area of the photomask is reflected by the field division mirror 17 and enters the first image sensor 18. Further, the combined light of the reflected light and the transmitted light emitted from the second illumination area passes through the field division mirror as it is and enters the second image sensor 19. Accordingly, only the transmitted light that has passed through the photomask is incident on the first image sensor 18, and a transmitted image of the photomask is captured. In addition, the reflected light reflected by the photomask and the transmitted light transmitted through the photomask are incident on the second image sensor 19, and the transmitted light emitted from the photomask and the reflected light are optically added. The combined light obtained by optically adding the transmitted light and the reflected light is detected. That is, the light detected by the second image sensor 19 is a light obtained by optically adding the reflected light and the transmitted light, and the second image sensor optically transmits the transmitted image and reflected image of the photomask. It performs the action of adding. Therefore, the second image sensor 19 captures a composite image composed of a transmission image and a reflection image of the photomask.

  The photomask 10 moves in the directions of arrows a and b orthogonal to the dividing line L of the two illumination areas 34 and 35 by main scanning of the stage. Therefore, for example, the photomask is first scanned with the combined illumination beam to capture a composite image of the first visual field region 31, and is scanned with the transmitted illumination beam after a predetermined time has elapsed to transmit the transmitted image of the second visual field region 32. Is imaged. When moving in the reverse direction, scanning is performed with the transmitted illumination beam, and after a predetermined time has elapsed, scanning is performed with the combined illumination beam. On the other hand, in the present invention, in the defect inspection, the synthesized image and the transmission image are added, and the defect is detected by comparing the added synthesized image with a threshold value. Therefore, it is necessary to synchronize the image signals output from the two image sensors.

  FIG. 3 is a diagram showing an outline of the signal processing device 22. The image signals output from the first and second image sensors 18 and 19 and the stage position information output from the position sensor 11 are supplied to the signal processing device 22. The signal processing device includes a synchronization device 40 and a defect detection device 41. The image signals output from the two image sensors 18 and 19 are synchronized with each other by the synchronization device 40, supplied to the defect detection device 41 in a synchronized state, and defect detection is performed based on the two image signals synchronized with each other. .

  Next, the synchronization method according to the present invention will be described. FIG. 4 is a diagram schematically showing the relationship between the first and second image sensors 18 and 19 and the stage movement. It is assumed that two image pickup devices are arranged in the field 30 of the objective lens via an optical system. The stage moves in the first scanning direction, and a specific portion of the photomask reaches the position P1 by the stage scanning, and reading of charges in the inspection region starts. The charges generated at the position P1 of the first image sensor (first TDI sensor) are sequentially transferred by the charge transfer synchronization signal (HD SYNC) formed from the output signal from the linear scale provided on the stage, and the position P2 Is read out. Subsequently, when the stage moves by the distance d, the light emitted from the part enters the position P3 of the second image sensor (second TDI sensor). The charges generated at the position P3 are sequentially transferred and read out at the position P4.

  In this example, if the distance between two image sensors is d and the pixel size on the photomask is Ps, the delay time formed between the two image sensors is the number of transfer lines of the image sensor (TDI sensor). In this case, N = d / Ps line. Therefore, during the period in which the stage moves in the first scanning direction, for example, the image signal output from the first image sensor is delayed by a time period T corresponding to the N = d / Ps line, and the first scanning direction Is output from the two image sensors by delaying the image signal output from the second image sensor during a period of movement in the opposite second scanning direction by a time period T corresponding to the N = d / Ps line. Can be synchronized with each other.

  FIG. 5 shows an enable signal in the first scanning direction. 5A shows an inspection area enable signal of the first image sensor, FIG. 5B shows an inspection area enable signal of the second image sensor, and FIG. 5C shows a delayed image of the first image sensor. (D) shows a defect inspection enable signal. In this example, an enable signal is supplied to the first image sensor and writing of an image signal is started. Subsequently, writing of the second image sensor is started after the time period T = N = d / Ps has elapsed. The image signal output from the first image sensor is supplied to the output defect inspection apparatus after being delayed by time T = N lines by the delay means. Note that during the period of movement in the second scanning direction opposite to the first scanning direction, the first image sensor and the second image sensor are interchanged in FIGS. 5A and 5B. It becomes a state.

  FIG. 6 is a diagram illustrating an example of the synchronization device 40. In this example, the synchronization device 40 is linked to the first switch means 50, the delay means 51 for delaying the input image signal by a time T corresponding to the number of transfer lines of N = d / Ps, and the first switch means 51. Second switch means 52 is included. The control of these switch means is performed using the position information of the stage by a control device (not shown). The first switch means 50 includes two input units 50a and 50b connected to the first and second imaging elements 18 and 19, respectively, and one output unit 50c. When the stage moves in the first scanning direction, the first input unit 50a and the output unit 50c are connected (shown by a solid line), and when moved in the second scanning direction, the second input unit 50b and the output unit 50c. Are connected (indicated by a broken line).

  The second switch unit 52 includes a first input unit 52a connected to the delay unit 51, a second input unit 52b connected to the first image sensor, and a third input unit connected to the second image sensor. Three input units 52c, a first output unit 52d that outputs an image signal output from the first image sensor, and a second output unit 52e that outputs an image signal output from the second image sensor It has two outputs. In the second switch means 52, when the stage moves in the first scanning direction, the first input section 52a and the first output section 52d are connected and the third input section 52c and the second output section are connected. 52e is connected (indicated by a solid line). Further, when moving in the second scanning direction, the second input unit 52b and the first output unit 52d are connected, and the first input unit 52a and the third output unit 52e are connected ( (Shown with a dashed line). The first and second switch means are controlled under the control of a controller (not shown) using stage position information output from the position sensor.

  In this example, as shown in FIG. 5, the stage moves in the first scanning direction, is supplied to the first image sensor with the inspection area enable signal, and is delayed by the time period T and enabled to the second image sensor. A signal shall be supplied. The image signal output from the first image sensor 18 is supplied to the delay means 51 via the first switch means 50 and is delayed by N = d / Ps. Thereafter, the defect is supplied to the defect inspection apparatus 41 via the first input unit 52a and the first output unit 52d of the second switch means. The image signal output from the second image sensor 19 is supplied to the third input unit of the second switch without passing through the delay unit, and is supplied to the defect inspection apparatus via the second output unit 52e. The As a result, the image signals output from the first and second imaging elements are supplied to the defect inspection apparatus in synchronization with each other.

  Next, the principle of defect inspection according to the present invention will be described. In the present invention, a transmission image output from the first image sensor synchronized with each other and a composite image output from the second image sensor are added, and a defect is detected from the added composite image. 7 and 8 show image signal processing modes. FIG. 7A shows a composite image signal output from the second image sensor 19. In a composite image composed of reflected light and transmitted light from a photomask, a drop image with a sharp drop in brightness is formed at a position corresponding to the edge of the pattern. If detected, a problem that pseudo defects occur frequently occurs. Further, even if the composite image and the transmission image are directly added, a problem that pseudo defects occur frequently occurs. Therefore, signal processing for deleting the drop image from the composite image is required.

  In this example, signal processing for erasing the transmission image and the drop image from the composite image is performed by the limiter process. In order to perform a limiter process for erasing a drop image, in the present invention, when forming a composite image, illumination optics is formed so as to form a difference between the brightness level of the pattern portion image and the light transmission portion image. Set the system. That is, the illumination optical system is set so that b2 <b1, where b1 is the luminance value of the image of the pattern portion in the composite image and b2 is the luminance value of the image of the light transmitting portion. That is, the signal intensity level of the light emitted from the pattern portion is set to about 200 levels of 256 gradations, and the signal intensity level of the light emitted from the light transmitting portion is set to about 150 levels. Thus, by providing a difference in signal level, the image of the pattern portion and the image of the light transmission portion are distinguished in the image signal. Further, when the luminance value of the limit level in the limiter process is b0, the limiter process level is set so that b2 <b0 <b1. Further, in the limiter process, the luminance values of the pixels having the luminance value equal to or lower than the limit level luminance value b0 are deleted or uniformly converted into a signal indicating the luminance value equal to or lower than the luminance value b1. In this example, the luminance value of the pixel having the luminance value equal to or lower than the luminance value b0 is uniformly converted into a signal having the luminance value b0. By this limiter processing, the transmission image and the drop image are erased from the composite image. The combined image subjected to the limiter process is shown in FIG.

  Next, an offset adjustment process is executed for the combined image signal subjected to the limiter process. In the offset adjustment process, the signal level of the combined image subjected to the limiter process is adjusted so as to be matched with a predetermined reference level. For example, the signal level is adjusted so that the offset amount from the reference signal level of the signal level of the image portion subjected to the limiter process matches the predetermined offset amount.

  Next, a gain adjustment process is executed for the composite image signal subjected to the offset process. In this gain adjustment, for example, the difference between the luminance value of the image of the pattern portion and the limit value is the difference between the luminance value of the light transmitting portion image and the luminance value of the image of the pattern portion in the transmission image. Adjust the gain so that they are almost the same. FIG. 7C shows the composite image signal whose gain has been adjusted.

  Next, expansion processing is performed on the edge of the image of the light transmission portion of the transmission image. Since the limiter-processed image portion of the composite image is reduced in size by the number of pixels corresponding to the drop image, there is no symmetry between the limiter-processed composite image and the original transmission image. Is added, discontinuity occurs at the edge of the light transmitting portion. Therefore, expansion processing is performed on the transmission image so that the edge of the light transmission portion expands by one pixel to several pixels. As the expansion process, for example, a 3 × 3 pixel matrix can be used, and an expansion process in which the luminance value of the center pixel is replaced with the highest luminance value can be used. A transmission image after the expansion processing is shown in FIG.

  Next, as shown in FIG. 8, addition processing is performed to add the combined image that has been adjusted for offset and gain and the transmitted image that has been subjected to expansion processing, thereby forming an added combined image. By performing the offset processing and gain adjustment processing, the luminance value of the image of the light transmission portion and the luminance value of the image of the pattern portion substantially coincide with each other in the addition composite image. For this reason, it is possible to detect a defect by performing a threshold comparison inspection on the added composite signal. That is, it is possible to detect a defect image by comparing the added composite signal with the first threshold value and determining whether the difference exceeds the second threshold value. In this way, by synchronizing the transmission image signal and the composite image signal, performing a limiter process for erasing the image of the light transmission part and the drop image on the composite image, and adding the composite image and the transmission image subjected to the limiter process As a result, the detection sensitivity of the pattern edge portion increases, and defect inspection that is not affected by the drop image becomes possible.

  FIG. 9 shows the inspection algorithm. Image signals output from the first and second imaging elements 18 and 19 respectively indicating a transmission image and a composite image of the photomask to be inspected are supplied to the synchronization device 40, and the defect detection device 41 is synchronized with each other. To be supplied. An image signal output from the first image sensor indicating the transmission image of the photomask is supplied to the expansion means 60, and the edge of the image of the light transmission portion of the transmission image is expanded by one to several pixels. The expanded image signal is supplied to the adding means 61. The image signal output from the second image sensor 19 that captures the composite image of the photomask is supplied to the limiter 62, and the limiter process is performed. The image signal subjected to the limiter process is supplied to the offset / gain adjusting means 63, and offset adjustment and gain adjustment are performed. The offset and gain adjusted image signal is supplied to the adding means 61.

  The adding means 61 adds the transmitted image signal subjected to the expansion process and the combined image signal subjected to the limiter process and the offset / gain adjustment to form an added combined signal. The added synthesized signal is supplied to the difference means 64, the difference between the added synthesized signal and the first threshold value is detected, and the detected difference value is supplied to the comparing means 65. The comparing means 65 compares the detected difference value with the second threshold value, and if the difference value exceeds the second threshold value, determines that it is a defect and generates a defect detection signal. The defect detection signal is supplied to the memory together with corresponding address information, and the defect and its address are stored in the memory. At this time, the defect image can be stored in another memory. By storing the defect image in the memory, the defect image can be displayed on the monitor for review.

  The present invention is not limited to the above-described embodiments, and various modifications and changes can be made. For example, in the above-described embodiment, a TDI sensor is used as an image sensor, but a line sensor in which a plurality of light receiving elements are arranged in a line shape may be used.

  Further, in the above-described embodiment, the transmission image of the photomask and the composite image are captured. However, the transmission image of the photomask is captured by the first image sensor, and the reflection image is captured by the second image sensor. It is also possible to output the transmitted image and the reflected image in synchronization.

1 Laser light source 2, 7, 14, 20 Total reflection mirror
3 First beam splitter 4 Attenuator 45
5 1/4 wavelength plate 6 ND filter
8 Condenser lens 9 Stage 10 Photomask 11 Position sensor 12 Objective lens 13 1/4 wavelength plate 14 Second beam splitter 16 Imaging lens 17 Field division mirror 18 First image sensor 19 Second image sensor 21 Field stop 22 Signal processing apparatus 30 Field of view 31 of objective lens First field area 32 Second field area 33 First illumination area 34 Second illumination area 40 Synchronizer 41 Defect detection apparatus

Claims (4)

An inspection apparatus for inspecting a photomask having a pattern formation surface on which a pattern portion and a light transmission portion are formed and a back surface opposite to the pattern formation surface,
A stage that supports a photomask to be inspected and moves in a zigzag manner in a first scanning direction and a second scanning direction opposite to the first scanning direction;
Projecting a transmitted illumination beam toward the back surface of the photomask to be inspected, projecting a reflected illumination beam toward the pattern forming surface of the photomask, and a transmitted illumination optical system that illuminates the first area of the photomask; An illumination optical system having a reflective illumination optical system that illuminates a second area that overlaps the first area in the optical axis direction and is smaller than the first area;
A first image sensor that receives a combined light of the reflected light and the transmitted light emitted from the second area of the photomask and captures a combined image obtained by optically combining the transmitted image and the reflected image of the photomask. And a detection system having a second imaging element that receives transmitted light emitted from the remaining third area excluding the second area of the first area and captures a transmission image of the photomask;
A synchronization device that synchronizes the first and second image signals output from the first and second image sensors, respectively, and a defect that processes the image signal output from the synchronization device and outputs data indicating a defect A signal processing device having a detection device,
The synchronization device includes delay means for delaying an input image signal by a predetermined time period;
The first imaging device and the delaying unit are connected during the period in which the stage moves in the first scanning direction, and the stage is connected to the second imaging unit. The stage is disposed between the delaying unit and the first and second imaging devices. A first switch for connecting the second image sensor and the delay means during the period of movement in the scanning direction;
First and second input units connected to the first and second imaging elements, a third input unit connected to the delay means, a first output unit for outputting the first image signal, and A second output unit that outputs a second image signal, and connects the third input unit and the first output unit during a period in which the stage moves in the first scanning direction; And the second output unit, and the first input unit and the first output unit are connected while the stage moves in the second scanning direction, and the third input unit and the second output unit are connected. And a second switch for connecting
The defect detection apparatus includes a signal processing unit that performs signal processing on the first and second image signals, an adding unit that adds the first and second image signals subjected to signal processing to generate an added composite signal, and Comparing means for comparing the added composite signal with a threshold value,
The transmission illumination optical system and the reflection illumination optical system are set so that b2 <b1 when the brightness value of the pattern portion image of the composite image is b1 and the brightness value of the light transmission portion image is b2. And
The signal processing means erases a signal portion having a luminance value b0 or less satisfying the expression b2 <b0 <b1 or a luminance value having a luminance value of b1 or less from the first image signal output from the first image sensor. Limiter means for performing a limiter process to uniformly convert the signal to be shown,
An inspection apparatus characterized in that the adding means adds the first image signal subjected to limiter processing and the second image signal subjected to signal processing.   2. The inspection apparatus according to claim 1, wherein the signal processing unit of the defect detection apparatus further performs an expansion process on an edge of an image corresponding to the light transmission portion with respect to the image signal output from the second imaging element. An inspection apparatus comprising expansion means for performing addition, and adding the signal subjected to the limiter process and the signal subjected to the expansion process to output an added composite signal.   3. The inspection apparatus according to claim 2, wherein the signal processing means of the defect detection apparatus includes an offset and gain adjusting means at a subsequent stage of the limiter means, and the first image signal subjected to the limiter processing is an offset adjustment process and a gain. An inspection apparatus which is supplied to the adding means after being adjusted. 4. The inspection apparatus according to claim 1, wherein the first and second imaging elements are configured to capture images at positions separated by a distance d in the main scanning direction of the stage. 5. And
The inspection apparatus delays the input image signal by a time period during which the stage moves by a distance d.

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