JP5094048B2 - Appearance inspection device - Google Patents

Description

  The present invention relates to an appearance inspection apparatus that inspects an object to be inspected such as a semiconductor wafer, a photomask, and a printed circuit board, and more particularly to an appearance inspection apparatus that inspects an object to be inspected by reciprocating scanning.

  A description will be given by taking a memory chip on a semiconductor wafer as an example of the object to be inspected. On the wafer, a plurality of chips as individual products are arranged. These chips are continuously arranged at regular intervals in the matrix direction as patterns of the same shape. Taking advantage of the characteristic that the inspection patterns to be compared are arranged in the same shape, at regular intervals, and at regular intervals, the defect inspection detects a defect in the pattern by comparing the detection image in the inspection area with a reference image stored in advance.

  In a current pattern defect inspection apparatus, a detection image and a reference image are generally captured using a one-dimensional image sensor such as TDI in order to increase the inspection speed. The inspection object (or image sensor) is scanned in a direction orthogonal to the pixel arrangement direction of the one-dimensional image sensor to obtain a two-dimensional image. Since the partial image of the chip that can be detected by one scanning of this one-dimensional image sensor is small with respect to the chip height, in order to inspect the entire surface of the chip, a method of scanning by shifting in the height direction of the image sensor a plurality of times Is used.

  For example, Patent Document 1 discloses a defect inspection apparatus that scans by reciprocating a stage on which an object to be inspected is placed.

JP 2002-090311 A

  In order to avoid the influence of the warpage of the inspection object and the stage flatness when holding the inspection object, the optical system of the defect inspection apparatus is usually provided with an automatic focus control mechanism. In such an automatic focus control method, the autofocus area is arranged at a position shifted to some extent from the center of the visual field in the objective lens on the object to be inspected. For this reason, since the stage moves while the auto focus calculates the focal position and changes the focal position at the center of the field of view within the objective lens by the automatic focus control method, there is a possibility that the focal position shift occurs. In the case of an apparatus in which the stage moves in one direction, it is sufficient to provide an autofocus area in front of the center of the visual field in the objective lens. However, when the stage is reciprocated, there is no problem in one direction, but there is a problem in the reverse direction. May occur. An object of the present invention is to provide an appearance inspection apparatus that reduces an error of an automatic focus control means and has a small image difference due to a scanning direction.

The configuration of the present invention for achieving the above object is as follows.
Image detecting means for detecting an image of the inspection object;
Detection area moving means for moving the detection area of the inspection object detected by the image detection means;
A focal position detection means having a focal position detection area for detecting a focal position in an area different from the detection area of the inspection object;
A visual inspection apparatus comprising:
An appearance inspection apparatus provided with a focus position detection position control means for controlling the focus position detection area of the focus position detection means so as to be in front of the movement direction of the detection area of the inspection object by the detection area moving means.

  The image detecting means is means for detecting reflected light of the light irradiated to the inspection object as an image. In addition to a one-dimensional image sensor (line sensor, TDI, etc.), a two-dimensional image sensor (CCD, MOS) or the like can be used for detection. Things can be used. Usually, the image detection sensor cannot capture the entire image of the inspection object at a time, and therefore it is necessary to move the inspection object or the image detection means. The detection area moving means is means for moving the detection area by a reciprocating motion, a rotational motion, a combination thereof, or the like. The focal position detection means is a means for adjusting the focal position based on the information of the reflected light by irradiating the object to be inspected with a light beam. Based on the light information, the position of the optical system of the image detection means or the stage on which the object to be inspected is adjusted so that the image image detected by the image detection means becomes the best.

  The focus position detection position control means is a means for adjusting the focus position detection area so as to be in front of the detection area of the inspection object by the detection area moving means. When the stage reciprocates, control is performed so that the focus position detection area is always forward when the stage moves forward or backward. When the stage rotates, the scanning direction is usually the same direction, but the present invention can be applied to a stage having a reverse rotation mechanism.

  The focus position detection position control means may move the position of the irradiation mechanism of the light beam that irradiates the object to be inspected in order to determine the focus position, or the position of the irradiation mechanism is fixed and reflected in the middle of the optical path. A mirror may be provided so that the position or reflection angle of the reflecting mirror can be changed.

  According to the present invention, when inspecting a plurality of patterns formed on an object to be inspected such as a semiconductor wafer, it occurs due to a difference in scanning direction and an image difference due to a focus position detection error due to a delay in autofocus response. Problems such as false reports and missing defects can be reduced.

  First, the background of the present invention will be briefly described with reference to the drawings.

  A description will be given by taking a memory chip on a semiconductor wafer as an example of the object to be inspected. On a single wafer as shown in FIG. 1, a plurality of chips as individual products are arranged. These chips are continuously arranged at regular intervals in the matrix direction as patterns of the same shape. Further, a memory cell pattern is formed as a repeated pattern in a constant cycle inside the chip. Taking advantage of the characteristic that the inspection patterns to be compared are arranged in the same shape, at regular intervals, and at regular intervals, the defect inspection uses the following method to detect pattern defects using the inspection area detection image and the pre-stored reference image. To do.

  As shown in FIG. 2, the presence or absence of a defect can be detected from the difference image AB of the reference image B, which is an image having the same shape as the detected image A detected in the inspection region. FIG. 2A is an example in the case where there is no defect, and no difference exists on the difference image, and it can be determined that there is no defect. On the other hand, since the defect information remains in the difference image AB when the defect C exists in one image but has the same shape as shown in FIG. 2B, it can be determined that there is a defect. In general, the pattern defect detection method detects the presence or absence of defects in this way.

  In the pattern defect inspection, a one-dimensional image sensor is used as an image detection means for capturing the detection image and the reference image as described above. As shown in FIG. 3, this one-dimensional image sensor has small photodetectors arranged one-dimensionally. In the pattern defect inspection apparatus, the inspection object is placed in a direction orthogonal to the pixel arrangement direction of the photodetector. Scan to obtain a two-dimensional image. The partial image of the chip that can be detected by one scan of the one-dimensional image sensor is determined by the height a of the image sensor to be detected and the imaging magnification of the imaging optical system that projects the chip image onto the image sensor. This height is called the effective imaging height of the image sensor. FIG. 4 shows an area where an image can be detected by one scan when the inspection area is scanned by the image sensor. In general image sensors, the height of the area that can be detected by one scan of the image sensor is smaller than the chip height, so in order to inspect the entire surface of the chip, scanning is performed by shifting in the height direction multiple times. Method is used.

  In general, an image sensor is fixed, and a stage on which a wafer of an object to be inspected can be moved is used. When image detection is performed by scanning the wafer in the horizontal direction on the wafer to be inspected, the image is first acquired by moving the object to be inspected in the horizontal direction as shown in FIG. After image detection is completed in one scan, the inspection object is moved downward in the vertical direction by the effective imaging height of the image sensor, and the images are sequentially moved by moving the stage so that the scanning direction is reversed. To detect.

As an automatic focus control method for detecting an image using an image sensor, a Through the lens method and an Out of the axis method are generally used. Through the lens method is less susceptible to disturbances such as temperature changes, but the illumination light quantity is reduced by half because a reticle with a pattern in the optical path and the reflected light are split, reducing illumination efficiency. Happens. Also out of the
In the axis method, a separate light source with a different wavelength range is generally arranged, and illumination is performed on a different axis from the illumination system, so there is no loss of illumination light, but it is easily affected by disturbance factors such as temperature drift and should be stable. An error may occur in the focus control method.

  Further, in the conventional autofocus control method as described above, the autofocus region is arranged at a position shifted to some extent from the center of the visual field in the objective lens on the object to be inspected as shown in FIG. For this reason, there is always a response delay before autofocus calculates the focal position and changes the focal position at the center of the field within the objective lens by the automatic focus control method. FIG. 7 shows the shift (trackability) of the focus detection locus due to the difference in the scanning direction when the focus position is calculated in the conventional autofocus region and the surface of the inspection object is tracked. In the conventional autofocus control method in the autofocus area, the difference in the defocus area due to the difference in the scanning direction becomes large. The difference in the tracking performance in the auto-focus scanning direction and the response delay result in defocusing of the image, resulting in a difference in image quality. For example, as shown in FIG. 7, when the defect D on the surface of the object to be inspected is scanned in the positive direction, the defect D can be detected, but when scanned in the negative direction, the defect D appears in the defocus area. Cannot be detected. Due to the difference in scanning direction and response delay, a defocus area is generated not a little, and a defect is missed.

Conventionally, as shown in FIG. 8, the autofocus is, for example, about 10 to 20 μm of the surface of the object to be inspected having a width of 300 mm in order to reduce the influence of the warpage of the object to be inspected and the stage flatness when holding the object to be inspected. The main purpose was to scan the surface of the object to be inspected in a macro manner by scanning the fluctuation of the height in 2 to 3 seconds. In recent years, for example, as shown in FIG. 9, a step in a wafer process such as a memory cell portion becomes large, and a follow-up delay to a microscopic movement greatly affects the defect detection sensitivity and has become a problem. Further, as described above, in the conventional technology, an error caused by a difference in the scanning direction of the image sensor is large. In addition, due to a delay in autofocus response, a pattern with a large step may be defocused. In the above prior art, the autofocus error becomes a defocus factor of the detected image in the band-shaped image imaging region. The defect detection sensitivity is improved by controlling the difference in the scanning direction and the automatic focus control means with high accuracy.

  An example of the problem of the focal position, which is one of the causes of image differences due to differences in scanning direction, will be described. When the surface of the object to be inspected is scanned by the autofocus means and image detection is performed, a defocus area is generated due to a response delay (solid line 1) of the autofocus means as shown in FIG. When the surface of the object to be inspected is scanned in the reverse direction, the way in which the defocus area is generated is also reversed, so that there is a difference in focusing followability between the forward scan and the reverse scan. The difference in the follow-up performance of the focusing is an image difference. As described above, the conventional automatic focus control means detects the focus position at a position slightly shifted from the center of the field of view as shown in FIG. The difference, that is, the difference in the defocus amount increases.

In the conventional defect inspection method, the presence / absence of a defect is sequentially determined for adjacent chips. Scanning is started as shown in FIG. 5, and the detected image of the inspection area Db before and after the folding and the inspection area D- When comparing the reference images detected in a, the scanning direction is reversed, resulting in a difference in follow-up of focusing of the focus detection system, resulting in a difference in image quality, resulting in missed defects, and imaginary defects It causes information (hereinafter referred to as false information). In the image comparison inspection, the difference in the scanning direction becomes a difference in the image quality of the detected image, which causes a problem in the defect inspection. Further, as described above, since the defocus area differs depending on the scanning direction, if there is a characteristic defect in that area, the defect inspection sensitivity varies depending on the scanning direction, which becomes an unstable factor for defect detection.

  As described above, in the prior art, the illumination efficiency is lowered, or the autofocus becomes unstable due to disturbance factors such as temperature drift, and it is difficult to acquire an image with high accuracy.

  In view of the above background, embodiments of the present invention will be described below with reference to the drawings.

  FIG. 10 is a layout diagram of the autofocus area in the appearance inspection that is an embodiment of the present invention. FIG. 12 shows the configuration of an appearance inspection apparatus that is an embodiment of the present invention. FIG. 11 shows the shift of the focus detection locus due to the difference in the scanning direction when the focal position is calculated in the autofocus area arranged in front of the scanning direction from the belt-like image capturing area and the surface of the inspection object is made to follow. Since the focal position information of the surface irregularities of the object to be inspected is obtained in advance in the autofocus area in the scanning direction forward of the band-shaped image capturing area, the image can be acquired at the optimum focal position in the band-shaped image capturing area. It is possible to prevent a difference from occurring in the follow-up of the focal position of the surface irregularities due to the difference.

When the scanning direction is the positive direction, the focal position is calculated in the autofocus area (a) shown in FIG. 10, and image detection is performed at the optimum focal position in the band-shaped image imaging area. When the scanning direction is the negative direction, the focal position is calculated in the autofocus area (b). Autofocus area (a)
Since (b) is arranged in front of the scanning direction of the belt-like image capturing region, it is possible to obtain the focal position information on the surface of the object to be inspected in advance. Using the focal position information obtained in advance, the surface of the object to be inspected can be made to follow as shown in FIG. 11, and an image can be acquired at the optimum focal position in the band-shaped image imaging region.

FIG. 12 shows the configuration of an appearance inspection apparatus according to an embodiment of the present invention. In the appearance inspection apparatus of the present invention, a semiconductor wafer 3 as an object to be inspected is fixed on a Zθ stage 2 installed on an XY stage 1. Above the semiconductor wafer 3, there is a half mirror 5 that directs illumination light from the illumination light source 4 toward the semiconductor wafer 3, and the illumination light reflected by the half mirror 5 illuminates the semiconductor wafer 3 through the objective lens 6. . The reflected light from the semiconductor wafer 3 passes through the objective lens 6 and the half mirror 5 and is received by the defect detection image sensor 10 as detection light. The light branched by the autofocus switching mirror 8 is received by the autofocus image sensor 7. In order to switch the autofocus area accompanying switching of the scanning direction, the autofocus switching mirror 8 that reflects detection light outside the imaging range of the defect detection image sensor 10 is moved as shown in (1) and (2). The image captured by the autofocus image sensor 7 enters the automatic focus determination and control means 9, calculates an optimum focus position that maximizes the contrast value of the image, gives a movement command to the Z stage, and takes a strip image An image can be detected at an optimum focal position in the region. The detection light received by the defect detection image sensor 10 is converted into a digital image signal through the A / D converter 11, and after the image signal detected in the first scanning region is recorded as a reference image by the image recording means 12, The detected image detected by the next chip is compared by the image comparison means 13. Based on the result calculated by the image comparison means 13, the defect position determination means 14 specifies the defect position.

  In the embodiment of the present invention, the autofocus area is arranged in the field of view of the objective lens. However, if the autofocus area is arranged in front of the image capturing area in the scanning direction, the arrangement location, the scanning method, and the autofocus control are arranged. This is not a problem.

  In the embodiment of the present invention, the focus position is calculated from the contrast value of the image in the autofocus area within the objective lens field of view as the autofocus mechanism, but the autofocus means is not a problem.

  The embodiment of the present invention uses a defect detection image sensor and an autofocus image sensor as the image detection means, but the means for detecting the image is not a problem.

It is the top view which showed arrangement | positioning of the chip | tip of a semiconductor wafer as an example of a to-be-inspected object. It is a figure explaining the principle which specifies the defect position by a prior art. It is a simplified block diagram of an image sensor. Schematic when scanning an inspection area with an image sensor. It is a top view explaining the scanning locus | trajectory of a to-be-inspected object. The top view explaining the auto-focus area | region in a prior art. It is the top view which showed the tracking locus | trajectory of the to-be-inspected object surface in a prior art. The top view explaining the conventional autofocus role. The top view explaining the auto-focus role in recent years. It is the top view which showed the arrangement | positioning place of the auto-focus area | region within the objective lens visual field explaining the means for solving a subject. It is the top view which showed the tracking locus | trajectory of the to-be-inspected object surface at the time of employ | adopting this invention. It is a block diagram of the external appearance inspection apparatus which is an Example of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... XY stage, 2 ... Z (theta) stage, 3 ... semiconductor wafer, 4 ... illumination light source, 5 ... half mirror, 6 ... objective lens, 7 ... autofocus image sensor, 8 ... autofocus area change mirror, 9 ... focus Position determination and control means 10... Defect detection image sensor 11... A / D converter 12. Image recording means 13. Image comparison means 14.

Claims (3)

Image detecting means for detecting an image of the inspection object;
Detection area moving means for moving a detection area of the inspection object detected by the image detection means;
A focus position detection means for detecting a focus position detection area for determining a focus position from an area different from the detection area of the inspection object;
Focus position detection position control means for controlling the focus position detection area of the focus position detection means so as to be in front of the movement direction of the detection area of the object to be inspected by the detection area moving means. The means is a stage for placing the object to be inspected, and the stage includes a reciprocating mechanism,
The focal position detection position control means has a reflecting mirror,
Further, the focal position detection position control means changes the irradiation position of the light beam for detecting the focal position detection area according to the moving direction of the detection area moving means, and further changes the irradiation position of the light beam to the irradiation position of the light beam. A visual inspection apparatus characterized by being a mechanism for moving the reflecting mirror. In an inspection device for inspecting a substrate,
An illumination optical unit that illuminates the substrate with light;
A scanning unit that scans the substrate relative to the light;
A first detection optical unit for detecting light from the first region for defect inspection;
A processing unit that detects a defect of the substrate using a detection result of the first detection optical system;
A second detection optical unit for detecting light from the second region for focus control;
A focus control unit that changes a focus for the defect inspection according to a detection result of the second detection optical unit, the second detection optical unit according to the direction of the scanning direction. Change the position of area 2
The second region is in a positive direction than the first region in the positive direction of the scanning direction,
The second region is, in the negative direction of the scanning direction, Ri negative direction near than the first region,
Furthermore, the second detection optical system includes:
A mirror that reflects the light in the second region;
A sensor for detecting light reflected by the mirror,
The inspection apparatus, wherein the second detection optical system changes the position of the second region by moving the mirror in accordance with the direction of the scanning direction . The inspection apparatus according to claim 2,
The illumination optical unit illuminates the light via an objective lens,
The first detection optical unit detects light detected by the objective lens,
The inspection apparatus, wherein the first area and the second area are within a detection field of view of the objective lens.

Images