JP4924931B2 - Stencil mask inspection method and apparatus - Google Patents

Description

  The present invention relates to a stencil mask inspection method and apparatus used in semiconductor manufacturing.

  In recent years, semiconductor devices have been miniaturized, and for example, in an exposure process, an exposure apparatus using a charged particle beam capable of forming a finer pattern is used. The stencil mask is used in such an apparatus, and miniaturization is progressing also in the dimensions of various patterns or holes formed. In such a stencil mask, if there is foreign matter on the surface or in the vicinity of the pattern, various effects can be considered in actual use. For example, the foreign matter is dropped on the wafer during semiconductor manufacturing, or the defective pattern due to foreign matter on the pattern Transfer, generation of charge-up when a charged particle beam is irradiated on the mask surface, or mask destruction due to local charging, and the like. For this reason, in the stencil mask manufacturing process, foreign matter inspection and management are performed as appropriate, and the quality of the mask is assured for semiconductor device manufacturers.

  Conventional mask inspection is a cell comparison method that extracts differences by comparing patterns when there is a part where the same pattern is formed, and compares the pattern shape obtained from design data with the actual pattern image A database comparison method was used. However, in the stencil mask, basically the same cell does not exist, and in the database comparison, the pattern produced from the actual manufacturing process and the pattern generated from the design data are greatly different, and it is difficult to perform a comparative inspection.

On the stencil mask, various fine patterns and holes of several μm or 1 μm or less are formed based on design data so that the charged particle beam can pass. Passes and is reduced or projected at the same magnification on the wafer. In the process of manufacturing a stencil mask having such a fine pattern, surface inspection is performed as appropriate, and appropriate cleaning and correction are performed depending on the type of foreign matter or defect on the detected surface. Through the above processes, the reliability of the mask is improved, which contributes to semiconductor manufacturing with higher precision and higher density.
As a conventional stencil mask inspection method, a method mainly using a microscope is used. For example, visual inspection using a dark field function of a microscope is performed.
As such a stencil mask inspection method, there is an inspection method capable of performing appearance inspection such as short dimension inspection and observation of minute results without generating dirt and dust (see Patent Document 1).
JP 2004-241740 A

Therefore, with the conventional stencil mask inspection method, it is possible to inspect the foreign matter on the mask with high sensitivity, but it is necessary to increase the magnification in order to detect minute foreign matters and defects. There are problems such as narrowing the field of view and taking a very long time to do so.
In addition, as a problem of visual inspection, there is a fear of new foreign matter adhering from wear or the like due to manual inspection, and variation in detection performance due to a decrease in performance due to work.
Regardless of visual inspection, microscope image processing using laser or white light, and automatic inspection methods to detect feature points by devising the optical system have been devised, but stencils with various patterns formed As for the minute foreign matter on the mask surface, the pattern portion is also erroneously recognized as a foreign matter or a defect, so that it is difficult to extract only the foreign matter.
Further, in a method called a cell comparison method, the same cell is not formed in the stencil mask, so there is no cell serving as a reference for comparison, and inspection is impossible.
Also, in an apparatus that uses a method called a design data comparison method in automatic inspection, it is very difficult to convert design data into a two-dimensional image. For example, even if two-dimensional data can be converted into an image by using an image processing technique, it does not completely match a captured image of a mask pattern that has undergone an actual process, and is therefore used as a reference image for an inspection apparatus. It was impossible.
As described above, in the conventional stencil mask inspection method, it takes a very long time for a human to visually detect a minute foreign matter on the mask, and in the automatic inspection using a laser or white light, erroneous detection or comparison is made. There are problems such as lack of standards, making it difficult to use in manufacturing.

  In order to solve the above-described problems, an object of the present invention is to provide a stencil mask inspection method and apparatus capable of realizing stencil mask foreign matter inspection with high accuracy.

In order to achieve the above object, a stencil mask inspection method of the present invention is a stencil mask inspection method for inspecting foreign matter on the surface of a stencil mask used in semiconductor manufacturing,
An irradiation step of irradiating the stencil mask surface obliquely with a sheet beam by a laser; and
A specific area defined based on a light cutting line image obtained by reflection from the stencil mask surface is monitored, and a light cutting line image obtained by reflection from the stencil mask surface in the specific area is detected. The method includes a monitoring step and a detection step of detecting a foreign matter on the surface of the stencil mask based on a light section line image detected in the specific area.

  In order to achieve the above-described object, the stencil mask inspection apparatus of the present invention irradiates the surface of the stencil mask with a laser beam at an angle and reflects the reflected light on the stencil mask surface. A light-cutting optical system that obtains a light-cutting line image of the light-cutting line based on a reflection image obtained by the light-cutting optical system, and an image-forming optical system that enlarges the surface of the stencil mask. A scanning mechanism that scans with a sheet beam; and an image processing unit that detects a foreign matter on the surface of the stencil mask by capturing an enlarged optical section line image formed by the imaging optical system and performing image processing with a predetermined algorithm; A control unit for controlling each part including the operation mechanism and the image processing unit, and a position on the surface of the stencil mask where the foreign substance is detected in the image processing unit Characterized by comprising a memory unit for recording data in the order.

  According to such a stencil mask inspection method and apparatus of the present invention, it is possible to inspect foreign matter adhering to the membrane surface of the stencil mask with high accuracy regardless of visual inspection.

(Embodiment 1)
FIG. 1 is a conceptual diagram of a stencil mask that is an inspection target in the present embodiment.
As shown in FIG. 1, the stencil mask 1 basically uses a wafer made of Si or the like as a base material, and has a fine pattern 2 through which a charged particle beam passes. In the present embodiment, a foreign material indicated by reference numeral 3 adheres to the surface of the stencil mask 1 on which the fine pattern as described above is formed, and this is applied using the stencil mask inspection method and apparatus of the present embodiment. It is to detect.

FIG. 2 is a conceptual diagram of foreign object detection by the stencil mask inspection method and apparatus of the present embodiment. The laser beam emitted from the laser oscillator 4 forms a sheet beam 7 through the slit 5 and the incident side condensing lens 6. The apparatus is mechanically adjusted in advance so that the stencil mask 1 to be inspected is set at the focal position.

  In the above description, the sheet beam 7 is reflected by the mask surface, and the reflected lights 8 and 9 enter the inspection imaging camera 11 installed on the opposite side via the reflection-side condenser lens 10. At this time, the focus of the imaging camera 11 is set to a position where the sheet beam 7 is focused on the stencil mask 1.

  FIG. 3 is a conceptual diagram of light cutting in the first embodiment. When the foreign matter 3 is present on the surface of the stencil mask 1, the sheet beam 7 by the laser incident from an oblique direction becomes reflected light 8 from the foreign matter surface and reflected light 9 from the mask surface and is reflected on the opposite side to the oblique incident side. .

FIG. 4 is a conceptual diagram of the contents shown in FIG. 3 viewed from the side.
The reflected light 8 from the surface of the foreign material and the reflected light 9 from the surface of the stencil mask 1 are reflected to the opposite side, but at this time, there is an interval corresponding to the height of the foreign material indicated by reference numeral 13.

FIG. 5 is a block diagram showing a configuration of an inspection apparatus to which the stencil mask inspection method according to the present embodiment is applied. At the start of inspection, the stencil mask 1 is mounted on the XY stage 14 so that the stage system is parallel to the XY coordinates of the pattern on the mask, and the origin of the stage system coincides with the origin of the mask pattern XY coordinates. Suppose that the alignment operation of the apparatus is completed.
The XY stage 14 starts operating simultaneously with the start of inspection and moves to the initial point of the inspection range. Laser irradiation is started from here, and scanning of the sheet beam by the laser is started on the surface of the mounted stencil mask.

  FIG. 6 is an explanatory diagram showing the scanning path of the sheet beam on the surface of the stencil mask. The feed pitch of the XY stage 14 is set so that the image to be inspected always overlaps between the scan line and the adjacent scan line as indicated by reference numeral 20.

  FIG. 7 is a conceptual diagram of a light section line image when no foreign matter is present on the mask surface. Since the XY stage on which the mask is mounted is adjusted with respect to the optical system for irradiating the sheet beam so that the focal length is kept equal without mechanical distortion, the light cutting line in the obtained image is denoted by reference numeral 21. It always appears in the same place as shown.

  FIG. 8 is a conceptual diagram of a light section line image when a foreign substance exists on the mask surface. As the light cutting line, a reflection image 21 on the mask surface and a reflection image 22 from the top of the foreign matter are obtained. In order to detect the presence or absence of foreign matter from the reflected image, an area for image processing is determined in advance before inspection as follows.

Before the inspection, the same mask as the inspection object is set on the stage, a sheet beam by laser is obliquely incident on the mask surface, and the surface reflected light is acquired by the camera.
FIG. 9 shows a light cutting line image when the sheet beam is obliquely incident on the mask surface and the surface reflection light is acquired by the camera, and the light cutting line image when no foreign matter is present on the mask surface is illustrated. FIG. Then, as shown in FIG. 9, the position of the height L of the light cutting line is detected from the obtained image. A range 24 from the position shifted upward by the half-value width of the light cutting line based on the height L to the upper side of the image is set as an image processing range in foreign object detection.

  FIG. 10 is an explanatory diagram showing an optical cutting line image when a foreign substance exists on the mask surface. By setting the range of image processing in foreign object detection as described above, only the reflected light 22 from the foreign object can be the target of image processing.

  FIG. 11 is an explanatory diagram showing a light section line image when a pattern is formed on a mask. According to the above-described method, it is possible to subject only to foreign matter attached to the surface without being affected by the pattern.

  In a range 24 of the image processing unit 16, a change in signal intensity at each pixel is monitored during inspection. For example, a noise level in 256 gradations is set as a threshold, and a signal is output to the computer 17 when a signal exceeding that level is input. The computer 17 records the coordinates of the stage 14 in the memory 18 when the above signal is input.

As described above, the inspection apparatus inspects the foreign matter within a predetermined range of the stencil mask surface. When the inspection is completed, the computer outputs a stencil mask foreign matter map to the display unit of the inspection apparatus based on the recorded coordinates.
In addition, since the coordinates of the output foreign matter are relative coordinates based on the mask center, they can be used for observation in an automatic stage specification microscope or analyzer.
For example, after the inspection work in the inspection apparatus is completed, the recorded foreign object detection coordinates can be transferred to the microscope apparatus, and the detailed inspection by the operator can be performed to classify the foreign objects.

  As described above, the inspection method and apparatus for the stencil mask of the first embodiment is specific to the optical cutting line image obtained by forming the laser in a sheet beam shape, irradiating the mask surface obliquely, and reflecting it. The foreign matter on the mask surface is detected by monitoring this area. For this reason, a slit or cylindrical lens that forms a laser beam in the form of a sheet beam and a condenser lens such as an objective lens are combined, and an optical system is configured so as to focus on the surface of the mask surface obliquely from above. On the surface, it is possible to irradiate the sheet beam with a width of about several μm. In the optical system, the sheet beam irradiated obliquely from above the stencil mask is condensed on the surface and reflected to the opposite side, but the reflected light is imaged on the image sensor or the like at an arbitrary magnification. The optical cutting line image according to the shape of the irradiation site on the surface can be obtained. For example, in the case where minute irregularities exist at the irradiation site on the surface of the stencil mask, a faithful light section line image corresponding to the irregular shape can be captured by forming an image of the reflected light on the imaging element. Therefore, when a foreign object having a minute height and width exists on the surface of the stencil mask, the sheet beam is obliquely incident on the surface of the stencil mask, so that a light cutting line image corresponding to the shape of the foreign object can be obtained on the reflection side. It is.

  Further, the stencil mask inspection apparatus of the first embodiment includes a light cutting optical system that obtains a reflected image by irradiating a sheet beam by a laser obliquely to the mask, and a light cut image at an arbitrary magnification on the reflection side. An automatic stage having an imaging optical system for forming an image, mounted with a stencil mask to be inspected and scanned in the X and Y directions, an image processing unit for capturing and processing a light section line image, and these automatic stage and image A computer for controlling the processing unit as an inspection device and a memory unit for recording coordinates on the surface of the stencil mask where the foreign matter is detected are provided.

In the stencil mask inspection method and apparatus according to the first embodiment, the image intensity of a specific area is always monitored with respect to the obtained optical section line image, and an image intensity of a certain level or more is obtained. The stencil mask foreign matter detection processing algorithm that can recognize the presence of foreign matter on the mask and determine the presence or absence of foreign matter is used.
Normally, when there is no foreign substance on the mask surface in the specific area, the surface is flat, and an image of one horizontal light section line is obtained. If foreign matter exists on the flat surface, a reflection image from the upper part of the foreign matter and the mask surface is obtained as the optical cutting line. Therefore, in order to recognize the presence of foreign matter, scattered light from the foreign matter is detected in an area above the image from one flat light cutting line when no foreign matter is present.
In order to simplify image processing, the entire reflected light cutting line image is not always monitored, but the upper side of the image from the light cutting line when there is no foreign object is set as a monitoring area. Always monitor intensity.

In the stencil mask inspection method and apparatus according to the first embodiment, the height L of the light section line is automatically extracted from the image obtained by reflection from the stencil mask surface, and is monitored using the extracted position as a reference. Use a light-cutting image processing algorithm that automatically determines the specific area to be.
When the mask surface is scanned with a sheet beam by a laser during inspection, there is usually no foreign matter on the mask surface, so that one light cutting line is obtained as an image. In other words, if the XY stage on which the mask is mounted is adjusted so that the focal length remains the same without mechanical distortion with respect to the optical system that irradiates the laser sheet beam, the optical cutting line in the obtained image has a height. It always appears in a certain place at the position of L. Therefore, as a specific inspection area to be monitored, as described above, the height L of a single line image normally obtained in the light section image is obtained by peak detection such as image processing, and becomes larger than this. What is necessary is just to define the range.

  As described above, according to the present embodiment, it is possible to inspect the foreign matter adhering to the membrane surface of the stencil mask with high accuracy regardless of visual inspection. Moreover, it becomes possible to manage and remove the detected foreign matter, and as a result, it contributes to improving the reliability of the stencil mask 1. Further, since the stencil mask 1 can be automatically inspected, there is no risk of manual adhesion of foreign matters or mask destruction, which was a problem during the visual inspection, and this contributes to an improvement in yield.

It is a conceptual diagram of the stencil mask which is a test object in Embodiment 1 of this invention. It is a conceptual diagram of the foreign material detection by the inspection method and apparatus of the stencil mask of Embodiment 1 of this invention. It is a conceptual diagram of the optical cutting | disconnection in Embodiment 1 of this invention. It is the conceptual diagram which looked at the content shown in FIG. 3 in Embodiment 1 of this invention from the side. It is a block diagram which shows the structure of the inspection apparatus to which the inspection method of the stencil mask by Embodiment 1 of this invention was applied. It is explanatory drawing which shows the scanning path | route of the sheet beam of the stencil mask surface by Embodiment 1 of this invention. It is a conceptual diagram of the optical cutting line image when the foreign material does not exist in the mask surface in Embodiment 1 of this invention. It is a conceptual diagram of the optical cutting line image when the foreign material exists in the mask surface in Embodiment 1 of this invention. It is explanatory drawing which shows the optical cutting line image when the foreign material does not exist in the mask surface in Embodiment 1 of this invention. It is explanatory drawing which shows the optical cutting line image when a foreign material exists in the mask surface in Embodiment 1 of this invention. It is explanatory drawing which shows the optical cutting line image in case the pattern is formed on the mask in Embodiment 1 of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Stencil mask, 4 ... Laser oscillator (light cutting optical system), 5 ... Slit (light cutting optical system), 6 ... Incident side condensing lens (light cutting optical system), 10 ... Reflection side collection Optical lens (imaging optical system), 11 ... Imaging camera (imaging optical system), 14 ... XY stage (scanning mechanism, automatic stage), 15 ... Optical system (laser irradiation unit), 16 ... Image Processing unit (extraction means, monitoring area determination means), 17... Computer (control means), 18.

Claims (7)

A stencil mask inspection method for inspecting foreign matter on the surface of a stencil mask used in semiconductor manufacturing,
An irradiation step of irradiating the stencil mask surface obliquely with a sheet beam by a laser; and
A specific area defined based on a light cutting line image obtained by reflection from the stencil mask surface is monitored, and a light cutting line image obtained by reflection from the stencil mask surface in the specific area is detected. A monitoring step;
A detection step of detecting foreign matter on the surface of the stencil mask based on the light cutting line image detected in the specific area;
An inspection method for a stencil mask, comprising:   The monitoring step constantly monitors the image intensity for the optical section line image of the specific area, and the detection step detects the image intensity of the optical section line image of the specific area, and detects the detected image intensity. 2. The method for inspecting a stencil mask according to claim 1, wherein the presence / absence of a foreign substance on the mask surface is determined.   An extraction step for automatically extracting the position of the light section line image from an image obtained by reflection from the surface of the stencil mask, and a specific area to be monitored is automatically determined based on the position of the extracted light section line image. The stencil mask inspection method according to claim 1, further comprising: a monitoring area determination step. A light cutting optical system that irradiates a stencil mask surface obliquely with a laser beam and obtains a reflection image of a light cutting line obtained by reflection on the stencil mask surface; and
An imaging optical system that forms and enlarges a light cutting line image of the light cutting line based on a reflection image obtained by the light cutting optical system;
A scanning mechanism for scanning the stencil mask surface with the sheet beam;
An image processing unit that captures a light cutting line image formed and enlarged by the imaging optical system, and detects a foreign substance on the surface of the stencil mask by performing image processing with a predetermined algorithm;
Control means for controlling each unit including the operation mechanism and the image processing unit;
A memory unit for recording data for specifying a position on the surface of the stencil mask where the foreign matter is detected in the image processing unit;
A stencil mask inspection apparatus characterized by comprising:   The algorithm monitors image intensity with respect to a light cutting line image of a specific area defined based on a light cutting line image obtained by reflection from the surface of the stencil mask, and the light cutting line of the specific area. 5. The stencil mask inspection apparatus according to claim 4, wherein the presence or absence of foreign matter on the mask surface is determined based on the image intensity of the image.   The image processing unit automatically extracts an optical cutting line image position from an image obtained by reflection from the surface of the stencil mask, and a specification to be monitored based on the extracted optical cutting line image position. 5. The stencil mask inspection apparatus according to claim 4, further comprising monitoring area determining means for automatically determining the area.   5. The stencil mask inspection apparatus according to claim 4, wherein the scanning mechanism is an automatic stage that mounts the stencil mask and moves on a plane to scan the surface of the stencil mask with the sheet beam.

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