JP4797751B2 - Stencil mask inspection method and apparatus - Google Patents

Description

  The present invention relates to a stencil mask inspection method and apparatus used for charged particle beam exposure and the like. In particular, it relates to the inspection of foreign matter on the membrane surface.

  The stencil mask has been studied and used as a mask for an exposure apparatus using charged particle beams such as electron beam exposure and ion exposure. Also, in the ion implantation process of semiconductor manufacturing, attempts have been made to implant ions using a stencil mask instead of photolithography, and the stencil mask has been studied for practical use in the semiconductor manufacturing field. ing.

  For example, on a stencil mask for an electron beam exposure apparatus, a large number of various fine patterns or holes of several μm or 1 μm or less are formed on the basis of design data so that an electron beam can pass and electron beam drawing can be performed. In semiconductor manufacturing, an electron beam passes therethrough and is subjected to reduced projection exposure on a wafer. In the manufacturing process of the stencil mask having such a minute pattern, surface inspection is appropriately performed, and cleaning or correction is performed by the detected foreign matter or defect on the surface. Through the above, the reliability of the mask is improved, contributing to the manufacture of semiconductors with higher precision and higher density.

  In recent years, semiconductor devices have been miniaturized, and an electron beam exposure apparatus capable of forming a finer pattern is used in the exposure process. Also in the stencil mask used in the apparatus, various patterns or hole dimensions formed therein are being miniaturized.

In such a stencil mask, there are concerns about various adverse effects when foreign matter is present on the surface or in the pattern. For example, in semiconductor manufacturing processes, foreign matter drops on the wafer, charge-up phenomenon when the mask is irradiated with an electron beam, mask destruction due to local thermal distortion, foreign matter is transferred, etc. It was the cause.
For this reason, various foreign matter and defect inspections are performed in the manufacturing process of the stencil mask, and mask inspection is also required in semiconductor device manufacturers.

Conventionally, as a method for inspecting a stencil mask, a method mainly using a microscope has been used. For example, visual inspection using a dark field function of a microscope has been performed.
According to this method, it is possible to inspect the mask over a wide range, but it is necessary to increase the magnification when detecting minute foreign matters and defects, in which case the field of view becomes narrow for visual inspection. Therefore, there is a drawback that it takes a very long time.

  In addition, as a problem of visual inspection, there is a concern that new foreign matter may adhere from clothes or the like, and detection performance may vary due to a decrease in work ability due to manual inspection.

  Microscopic image processing technology using laser or white light, or automatic inspection method that devised the optical system is also used, regardless of visual inspection, but minute foreign matters and defects on the mask surface on which various patterns are formed In the detection, there is a problem because the pattern portion is also erroneously recognized as a foreign object or a defect.

  Also, in a method called a chip comparison method, the same pattern is hardly formed on the stencil mask, and inspection is impossible because there is no standard for comparison.

  In addition, it is difficult to convert design data into a two-dimensional image with an apparatus that uses a method called a design data comparison method in automatic inspection, even if an image processing technique is introduced and converted into an image. Since there is a difference from an image obtained by a camera using a pattern that has undergone an actual process, it is difficult to use the pattern as a reference image for an inspection apparatus.

  Therefore, the present inventor has proposed an inspection method using a resist pattern at the time of mask manufacture as a reference image (see, for example, Patent Document 1).

JP 2004-193318 A

As described above, in the conventional stencil mask inspection method, it takes a very long time to visually detect minute foreign matters on the mask by humans, and in the automatic inspection by laser or white light, false detection or comparison reference It was difficult to use for production.
Further, since the method of Patent Document 1 is based on the resist pattern or the etching pattern upstream of the process, if the cause of the defect is in these, it is not suitable as the standard, and the pattern becomes thicker as it goes through the process. However, there is a problem in that thinning occurs, the reference image does not necessarily match the image to be inspected, and a pseudo defect occurs.

  Therefore, the present invention proposes an automatic high-precision inspection method for detecting defects on a stencil mask, particularly for detecting a surface foreign matter, without relying on a visual inspection, and can obtain a reference image for performing a comparative inspection. Provide inspection equipment.

The first aspect of the present invention is a method for inspecting the surface of a stencil mask used in such a charged particle beam exposure, which was a method of inspecting a stencil mask, characterized by have the following steps There is .
1) A step of acquiring the first image Mn on the surface of the membrane portion of the mask to be inspected as a bright field or dark field image and recording the image in the memory unit M.
2) reversing the front and back of the mask to be inspected,
3) A step of acquiring the second image Nn on the back surface of the membrane part of the mask to be inspected as a bright field or dark field image and recording the image in the memory unit N.
4) A step of taking out one of the images Mn or Nn from the memory unit M or N and performing horizontal inversion,
5) performing a difference process between the Mn image and the Nn image, and recording the resulting image Ln in the memory unit L;
6) A step of taking out the Ln image from the memory unit L, performing a predetermined threshold process on the Ln image, and confirming and recording the coordinates of the foreign matter,
7) A step of repeating the above steps 1) and 3) to 6) n times for the number of images,
8) A step of displaying the result of the above step.

The second aspect of the present invention, when there is a deviation in the image Mn and Nn after horizontal inversion, the image Mn, stencil mask according to claim 1, characterized in that have a means to match the Nn This is an inspection method.

According to a third aspect of the present invention, there is provided illumination means for illuminating the surface of the stencil mask, imaging means for capturing a membrane part image obtained by the illumination means, and a membrane part image obtained from the imaging means. A stencil having computing means for image processing, a memory section for recording the computation results, and stage means capable of inspecting the entire surface of the stencil mask and moving to a preset coordinate position A mask inspection device,
A stencil mask inspection apparatus characterized in that the calculation means performs processing and control according to claims 1 and 2 .

  According to the present invention, it is possible to inspect when foreign matter adheres to the membrane of the stencil mask, and the detected foreign matter can be managed and corrected, which contributes to improving the reliability of the mask. In addition, since the mask can be automatically inspected, there is no risk of manual adhesion of foreign matters or mask destruction, which was a problem at the time of visual inspection, which contributes to an improvement in yield.

The processing for the foreign matter inspection method for the stencil mask is shown in the flowchart of FIG. Here, the first and second images are generic names of acquired images at the movement coordinate points P and Q.
The content to be described below is one embodiment, and the order of each process can be changed without departing from the gist of the present invention.

  First, at the start of inspection, the stencil mask is mounted on the apparatus stage with the membrane surface side, that is, the side opposite to the support substrate facing up. (Fig. 1 (a))

First, in order to acquire the first image, the stage moves to a predetermined inspection coordinate _Pn . Further, each time an image is acquired, an _Mn image is recorded in the memory M unit 16. (Fig. 1 (b))

Here, FIG. 2 shows an illumination method for acquiring a microscopic image on the membrane surface side when bright field illumination is used and a conceptual diagram of the acquired image. 1 is a microscope objective lens, 2 is bright field illumination, 3 is a foreign object, and 4 is a stencil mask. The acquired image M _n is a bright field image as shown in FIG.

  Further, when dark field illumination is used, it is as shown in FIG. 3, and the pattern edge portion is clearer than bright field illumination as shown in FIG.

  In both bright field illumination and dark field illumination, if there is a foreign matter adhering to the membrane surface side, the presence of the foreign matter can be confirmed together with the pattern portion.

  Image acquisition on the membrane surface side of the stencil mask is completed, and then the stencil mask is turned over and mounted on the inspection apparatus with the membrane back side facing up. (Fig. 1 (c))

In order to acquire the second image, the stage moves to a predetermined inspection coordinate Q _n , acquires an image each time, and records N _n images in the memory N unit 18. (Fig. 1 (d))

  Here, similarly to the membrane surface side, FIGS. 4 and 5 show a conceptual diagram of an illumination method and an acquired image for acquiring a microscopic image on the back side of the membrane when using bright field illumination and dark field illumination, respectively. 10 is a bright field image and 12 is a dark field image.

  When dark field illumination is used, the pattern edge part is clearer than the bright field illumination shown in FIG. 4, but for the back side of the membrane, the stencil mask has a beam-like wall that has been processed on the back side. Therefore, as shown in the figure, light may be blocked and it may be difficult to obtain the effect of dark field illumination. Therefore, attention should be paid by testing beforehand whether the illumination is uniform by changing the angle of illumination.

  In the inspection, these FIG. 4 or FIG. 5 is used as a reference image in the image processing.

  FIG. 6 shows a conceptual diagram of a system that performs the image processing.

When the first and second image acquisition on both sides of the membrane of the stencil mask mounted on the inspection apparatus is completed, the computer first calculates all the second images N _n recorded in the memory N unit 18 of FIG. As shown in FIG. 7, the process of inverting in the horizontal direction is performed, and the processed image is stored again in the memory N unit 18 one by one. (Fig. 1 (e))

Next, the computer calls the first image M _n recorded in the memory M unit 16 in FIG. 6 and the second image N _n recorded in the memory N unit 18 in the arithmetic unit, and performs image-like alignment. Although the M _n image and the N _n image are misaligned when the stencil mask is mounted on the inspection apparatus and there is a mechanical error in the stage operation, even if the images are acquired at the same location, they are shown in FIG. As shown, there is a slight amount of misalignment in the plane.

Therefore, since they never match with each other as they are, for example, the _Mn image is slightly shifted in the X and Y directions with reference to the _Nn image, rotated by θ °, or pattern matching so that the pattern images of the images match each other. The image operation is performed so as to satisfy the matching criterion obtained in advance.

Thereafter, the difference processing of the M _n image with respect to the N _n image is performed, and the image is set as the L _n image. This conceptual diagram is shown in FIG. Since the patterns are completely coincident with each other as described above, it is possible to eliminate all the pattern portions by performing this process. Therefore, what remains is image information other than pattern information, that is, image information of foreign matter and surface defects as indicated by 92 in FIG. 9, and this image is stored in the memory L unit 20. (Fig. 1 (f))

Next, the computer provides a detection threshold for the L _n image in the image memory L unit 20 in advance, and, for example, an intensity distribution in the image contrast acquisition line 101 of the L _n image 100 as shown in FIG. If a predetermined level B is obtained, the pattern remaining after the pattern difference process is also targeted. However, if level A is set, only foreign objects and defects can be recognized. , recording the coordinates and size of the L _n image at this time. This process is performed for all acquired images. (Fig. 1 (g))

  Finally, the coordinates recognized above are calculated from the image acquisition position at that time, and the coordinates are output or displayed on the display unit 15 of FIG. 6 as a foreign substance / defect map of the stencil mask.

  The coordinates of foreign matter / defects output here are relative coordinates based on the mask center, so it can be used as a review in automated stage analyzers and other microscopes to identify detected foreign matter. Become.

FIG. 11 is a conceptual diagram of a stencil mask inspection apparatus for carrying out the present invention.
A stage 201 on which a stencil mask is placed so as to automatically move to a preset coordinate position on a pedestal, and an illumination 200 that illuminates the stage surface are provided, and a bright field is provided at a position facing the stencil mask surface above the stage. Alternatively, a CCD camera 202 serving as an imaging means for capturing an image that captures a dark field illuminated microscope image is provided, and in the vicinity of the pedestal, the arithmetic unit 14 that inputs and processes the image obtained from the camera and the processing result are recorded and stored. A computer 13 having memories 16, 18, and 20, a display unit 15 for outputting them, and a control unit (not shown) for overall control of the entire apparatus.

  Depending on the state of the membrane front and back surfaces of various stencil masks, the illumination 200 preferably has an optimal amount of light for each of the first image and the second image. . It is preferable that a mechanism capable of finely adjusting the irradiation angle is provided.

  The present invention can be used for a stencil mask inspection method and apparatus used for charged particle beam exposure and the like.

It is a flowchart explaining the inspection method of this invention. It is the conceptual diagram of the membrane surface test | inspection by bright field illumination, and the acquired image _Mn . It is the conceptual diagram of the membrane surface test | inspection by dark field illumination, and the acquired image _Mn . Is acquired image N _n and schematic diagram of the membrane bottom surface inspection by bright-field illumination. Dark field is acquired image N _n and schematic diagram of the membrane bottom surface inspection by the illumination. It is a figure explaining the computer used with the test | inspection method of this invention. It is a conceptual diagram for inverting the image N _n in the horizontal direction. It is a conceptual diagram related to the alignment of the image M _n and the image N _n. It is a conceptual diagram related to differential processing of the image M _n and the image N _n. It is a conceptual diagram for threshold setting of the image L _n. It is the schematic explaining the inspection apparatus of the stencil mask of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Microscope objective lens 2 ... Bright field illumination 3 ... Foreign material 4 ... Stencil mask 5 ... Bright field image 6 on the membrane surface side ... Dark field illumination 7 ... Membrane surface side Dark field image 8 ... Bright field illumination 9 ... Stencil mask 10 ... Bright field image 11 behind the membrane ... Dark field illumination 12 ... Dark field image 13 behind the membrane 13 ... Computer 14 ... Calculation unit 15 ... Display unit 16 ... Memory M unit 17 ... M image file 18 ... Memory N unit 19 ... N image file 20 ... Memory L unit 21. ..L image file 70 ... Image N _n
71 ... Image N _n after horizontal inversion processing
80 ... Image N _n
81 ... image M _n
90 ... Image M _n
91 ... Image N _n
92 ... image L _n
100 ... image _{L n} enlarged view 101 ... image contrast acquisition line 200 ... lighting 201 ... stage 202 ... camera

Claims (3)

A method for inspecting the surface of a stencil mask used in such a charged particle beam exposure method of inspecting a stencil mask, characterized by have the following steps.
1) A step of acquiring the first image Mn on the surface of the membrane portion of the mask to be inspected as a bright field or dark field image and recording the image in the memory unit M.
2) reversing the front and back of the mask to be inspected,
3) A step of acquiring the second image Nn on the back surface of the membrane part of the mask to be inspected as a bright field or dark field image and recording the image in the memory unit N.
4) A step of taking out one of the images Mn or Nn from the memory unit M or N and performing horizontal inversion,
5) performing a difference process between the Mn image and the Nn image, and recording the resulting image Ln in the memory unit L;
6) A step of taking out the Ln image from the memory unit L, performing a predetermined threshold process on the Ln image, and confirming and recording the coordinates of the foreign matter,
7) A step of repeating the above steps 1) and 3) to 6) n times for the number of images,
8) A step of displaying the result of the above step. If there is a deviation in the image Mn and Nn after horizontal inversion, the image Mn, inspection method according to claim 1 stencil mask, wherein the to have the means to match the Nn. Illuminating means for illuminating the surface of the stencil mask, imaging means for capturing a membrane part image obtained by the illuminating means, computing means for inputting and processing the membrane part image obtained from the imaging means, and recording the computation results A stencil mask inspection apparatus having a memory unit and stage means capable of inspecting the entire surface of the stencil mask and capable of moving to a preset coordinate position,
An inspection apparatus for a stencil mask, wherein the calculation means performs the processing and control according to claims 1 and 2 .