Abstract:
A defect inspection apparatus includes an illumination optical system which sets a transmission illumination region and a reflection illumination region on an inspection target surface of a mask, first and second imaging units having first and second visual fields which are set on the inspection target surface, an imaging optical system that provides images, which are present on the first and second visual fields, on the first and second imaging units, a defect detection unit which detects a defect of the mask on the basis of the images provided on the first and second imaging units, and a control unit which controls a positional relationship between setting positions of the transmission illumination region and the reflection illumination region and setting positions of the first and second visual fields.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-284395, filed Sep. 29, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a defect inspection apparatus which inspects a defect of a mask.  
         [0004]     2. Description of the Related Art  
         [0005]     With an increase in integration density of a semiconductor device such as an LSI, a mask pattern that is formed on a mask, such as a reticle, has become finer. Accordingly, a defect inspection apparatus for a mask pattern is required to have higher performance. There has been proposed a defect inspection apparatus which detects a defect by acquiring an image of a mask pattern by means of an imaging device such as a CCD or a line sensor, and comparing the acquired image with a reference image (see, e.g. JP-A 10-177246 (KOKAI)).  
         [0006]     Normally, when an image of a mask pattern is acquired, both transmission illumination and reflection illumination are used. The transmission illumination and reflection illumination have merits and demerits. By using both the transmission illumination and reflection illumination, their characteristics are made complementary.  
         [0007]     However, if the positions of a transmission illumination region and a reflection illumination region are fixed, a desired image, in some cases, cannot be obtained. For example, an optical system has aberration and distortion. Although an image with high precision is obtained at a center of a visual field of an objective lens, but an image with high precision is hardly obtained at positions away from the center of the visual field of the objective lens. Consequently, if the transmission illumination region is positioned at the center of the visual field of the objective lens, the precision of a transmission illumination image is enhanced but the precision of a reflection illumination image is degraded. Conversely, if the reflection illumination region is positioned at the center of the visual field of the objective lens, the precision of a reflection illumination image is enhanced but the precision of a transmission illumination image is degraded.  
         [0008]     As described above, if the positions of the transmission illumination region and reflection illumination region are fixed, such a problem arises that a desired image can hardly be acquired with high precision and a defect inspection cannot be performed with high precision.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     According to an aspect of the present invention, there is provided a defect inspection apparatus comprising: an illumination optical system which sets a transmission illumination region and a reflection illumination region on an inspection target surface of a mask; first and second imaging units having first and second visual fields which are set on the inspection target surface; an imaging optical system that provides images, which are present on the first and second visual fields, on the first and second imaging units; a defect detection unit which detects a defect of the mask on the basis of the images provided on the first and second imaging units; and a control unit which controls a positional relationship between setting positions of the transmission illumination region and the reflection illumination region and setting positions of the first and second visual fields. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0010]      FIG. 1  schematically shows an example of the structure of a defect inspection apparatus according to an embodiment of the present invention;  
         [0011]      FIG. 2  shows an example of the positional relationship between the setting positions of a transmission illumination region and a reflection illumination region, on one hand, and the setting positions of visual fields of imaging sensors, on the other hand;  
         [0012]      FIG. 3  shows another example of the positional relationship between the setting positions of the transmission illumination region and reflection illumination region, on one hand, and the setting positions of the visual fields of the imaging sensors, on the other hand;  
         [0013]      FIG. 4  shows still another example of the positional relationship between the setting positions of the transmission illumination region and reflection illumination region, on one hand, and the setting positions of the visual fields of the imaging sensors, on the other hand;  
         [0014]      FIG. 5  shows still another example of the positional relationship between the setting positions of the transmission illumination region and reflection illumination region, on one hand, and the setting positions of the visual fields of the imaging sensors, on the other hand;  
         [0015]      FIG. 6  shows still another example of the positional relationship between the setting positions of the transmission illumination region and reflection illumination region, on one hand, and the setting positions of the visual fields of the imaging sensors, on the other hand; and  
         [0016]      FIG. 7  shows still another example of the positional relationship between the setting positions of the transmission illumination region and reflection illumination region, on one hand, and the setting positions of the visual fields of the imaging sensors, on the other hand. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     An embodiment of the present invention will now be described with reference to the accompanying drawings.  
         [0018]      FIG. 1  schematically shows an example of the structure of a defect inspection apparatus according to an embodiment of the present invention.  
         [0019]     Light (e.g. a laser beam), which is emitted from a light source  1 , is extended by a beam expander  2  and is converted to an area light source by an optical integrator  3 . A fly-eye lens or a diffusion plate, for instance, is usable as the optical integrator  3 . Light from the optical integrator  3  is Koehler-illuminated by a collimator  4  onto the position of a diaphragm  6  for transmission illumination and the position of a diaphragm  11  for reflection illumination. A beam splitter  5  is disposed on a rear side of the collimator  4 . The light incident on the beam splitter  5  is separated into transmission illumination light and reflection illumination light.  
         [0020]     The transmission illumination light, which is separated by the beam splitter  5 , is Koehler-illuminated on the position of the diaphragm  6  for transmission illumination, as described above. The diaphragm  6  is disposed at a conjugate position with respect to the position of a pattern formation surface (an inspection target surface) of a mask  8  to be inspected (e.g. a photomask such as a reticle). The position of the diaphragm  6  is movable by a diaphragm driving mechanism  23  which is composed of, e.g. a pulse motor. By controlling the position of the diaphragm  6  by the diaphragm driving mechanism  23 , a transmission illumination region is set on the pattern formation surface of the mask  8  that is disposed on a stage  22 .  
         [0021]     Light, which has passed through the diaphragm  6 , is Koehler-illuminated on the pattern formation surface of the mask  8  by a condenser lens  7 . The mask  8  is configured such that a pattern for an LSI is formed on a glass substrate. The illumination light reaches the pattern formation surface through the glass substrate. Thus, the condenser lens  7 , which is to be used, is chosen by taking the thickness of the glass substrate into account. Since the pattern formation surface of the mask  8  is disposed at a conjugate position with respect to the position of the diaphragm  6 , a transmission illumination region, which is adjusted by the diaphragm  6 , is set on the pattern formation surface.  
         [0022]     The reflection illumination light, which is separated by the beam splitter  5 , is Koehler-illuminated on the position of the diaphragm  11  for reflection illumination. The diaphragm  11  is disposed at a conjugate position with respect to the position of the pattern formation surface of the mask  8 . The position of the diaphragm  11  is movable by a diaphragm driving mechanism  24  which is composed of, e.g. a pulse motor. By controlling the position of the diaphragm  11  by the diaphragm driving mechanism  24 , a reflection illumination region is set on the pattern formation surface of the mask  8  that is disposed on the stage  22 .  
         [0023]     Light, which has passed through the diaphragm  11 , is Koehler-illuminated on the pattern formation surface of the mask  8  by a collimator  12  and an objective lens  9 . A beam splitter  10  is disposed between the collimator  12  and the objective lens  9 . Reflective light from the beam splitter  10  reaches the pattern formation surface of the mask  8  via the objective lens  9 . Since the pattern formation surface of the mask  8  is disposed at a conjugate position with respect to the position of the diaphragm  11 , a reflection illumination region, which is adjusted by the diaphragm  11 , is set on the pattern formation surface.  
         [0024]     The above-described beam expander  2 , optical integrator  3 , collimator  4 , beam splitter  5 , diaphragm  6  and condenser lens  7  constitute an illumination optical system for setting a transmission illumination region. The beam expander  2 , optical integrator  3 , collimator  4 , beam splitter  5 , diaphragm  11 , collimator  12 , beam splitter  10  and objective lens  9  constitute an illumination optical system for setting a reflection illumination region. The above-described diaphragm driving mechanisms  23  and  24  constitute an illumination region position control unit which controls the positions of the transmission illumination region and reflection illumination region on the pattern formation surface (inspection target surface) of the mask  8 .  
         [0025]     Light from the transmission illumination region of the mask  8 , that is, image light corresponding to the pattern included in the transmission illumination region, reaches a beam splitter  13  via the objective lens  9 . Similarly, light from the reflection illumination region of the mask  8 , that is, image light corresponding to the pattern included in the reflection illumination region, reaches the beam splitter  13  via the objective lens  9 .  
         [0026]     The image light, which has reached the beam splitter  13 , is separated into transmission light and reflection light by the beam splitter  13 . Of the transmission light emerging from the beam splitter  13 , a light component coming from the transmission illumination region of the mask  8  is imaged on an imaging sensor (imaging unit)  15  via an imaging lens  14 , but a light component coming from the reflection illumination region of the mask  8  is not imaged on the imaging sensor  15 . In addition, of the reflection light reflected by the beam splitter  13 , a light component coming from the reflection illumination region of the mask  8  is imaged on an imaging sensor (imaging unit)  17  via an imaging lens  16 , but a light component coming from the transmission illumination region of the mask  8  is not imaged on the imaging sensor  17 .  
         [0027]     The imaging sensor  15  is movable by an imaging sensor driving mechanism  25  which is composed of, e.g. a pulse motor. By controlling the position of the imaging sensor  15  by the imaging sensor driving mechanism  25 , the light from the transmission illumination region of the mask  8  can be imaged on the imaging sensor  15 , as described above. In other words, by controlling the position of the imaging sensor  15  by the imaging sensor driving mechanism  25 , the position of a visual field of the imaging sensor  15  on the pattern formation surface of the mask  8  is adjusted. As a result, on the pattern formation surface of the mask  8 , the position of the visual field of the imaging sensor  15  can be made to correspond to the position of the transmission illumination region.  
         [0028]     The imaging sensor  17  is movable by an imaging sensor driving mechanism  26  which is composed of, e.g. a pulse motor. By controlling the position of the imaging sensor  17  by the imaging sensor driving mechanism  26 , the light from the reflection illumination region of the mask  8  can be imaged on the imaging sensor  17 , as described above. In other words, by controlling the position of the imaging sensor  17  by the imaging sensor driving mechanism  26 , the position of a visual field of the imaging sensor  17  on the pattern formation surface of the mask  8  is adjusted. As a result, on the pattern formation surface of the mask  8 , the position of the visual field of the imaging sensor  17  can be made to correspond to the position of the reflection illumination region.  
         [0029]     The above-described objective lens  9 , beam splitter  13  and imaging lens  14  constitute an imaging optical system for providing an image of the transmission illumination region on the imaging sensor  15 . The objective lens  9 , beam splitter  13  and imaging lens  16  constitute an imaging optical system for providing an image of the reflection illumination region on the imaging sensor  17 . The above-described imaging sensor driving mechanisms  25  and  26  constitute a visual field position control unit which controls the positions of the visual fields of the imaging sensors  15  and  17  on the pattern formation surface (inspection target surface) of the mask  8 .  
         [0030]     A computer  21  is connected to the above-described diaphragm driving mechanisms  23  and  24 , the imaging sensor driving mechanisms  25  and  26  and the stage  22  which scans the mask  8 . The operations of these components are controlled by the computer  21 . The diaphragm driving mechanisms  23  and  24  and the imaging sensor driving mechanisms  25  and  26  are automatically controlled by the computer  21 . Thereby, on the pattern formation surface of the mask  8 , the position of the visual field of the imaging sensor  15  can automatically be made to correspond to the position of the transmission illumination region, and the position of the visual field of the imaging sensor  17  can automatically be made to correspond to the position of the reflection illumination region.  
         [0031]     Image data of an image (transmission image) of the transmission illumination region, which is formed on the imaging sensor  15 , is sent to a comparison unit  18  and is compared with reference image data of a reference transmission image which is prestored in a memory unit  19 . Similarly, image data of an image (reflection image) of the reflection illumination region, which is formed on the imaging sensor  17 , is sent to the comparison unit  18  and is compared with reference image data of a reference reflection image which is prestored in a memory unit  20 . Based on comparison results that are thus obtained, a defect on the pattern formation surface of the mask  8  is detected. The detection of a defect may be performed by the comparison unit  18  or by the computer  21 .  
         [0032]      FIG. 2  to  FIG. 6  show the positional relationships between the setting positions of the transmission illumination region and reflection illumination region, on one hand, which are controlled by the diaphragm driving mechanisms  23  and  24 , and the setting positions of the visual fields of the imaging sensors  15  and  17 , on the other hand, which are controlled by the imaging sensor driving mechanisms  25  and  26 .  
         [0033]     In  FIG. 2  to  FIG. 6 , reference numeral  100  denotes an objective visual field (i.e. a visual field on the pattern formation surface of the mask  8 , which is defined by the objective lens  9 ); numeral  101  a transmission visual field (i.e. a visual field of the imaging sensor  15  on the pattern formation surface); numeral  102  a reflection visual field (i.e. a visual field of the imaging sensor  17  on the pattern formation surface);  111  a transmission illumination region on the pattern formation surface; and  112  a reflection illumination region on the pattern formation surface. As shown in  FIG. 2  to  FIG. 6 , the transmission visual field  101  and the reflection visual field  102  are spaced apart. The transmission illumination region  111  and the reflection illumination region  112  are spaced apart. The transmission visual field  101  is positioned within the transmission illumination region  111 , and the reflection visual field  102  is positioned within the reflection illumination region  112 .  
         [0034]      FIG. 2  shows a position control state with priority on transmission. The center axis of the transmission visual field  101  agrees with the center axis of the objective visual field  100 . In normal defect inspections, in usual cases, the transmission image is mainly used and the reflection image is used as an auxiliary. Thus, if the transmission image is to be used as a main and the precision in detection of the transmission image is to be enhanced, the position setting state as shown in  FIG. 2  is effective.  
         [0035]      FIG. 3  shows a position control state with priority on reflection. The center axis of the reflection visual field  102  agrees with the center axis of the objective visual field  100 . The reflection image is suited to detection of foreign matter such as dust. Thus, if foreign matter such as dust is to be mainly detected and the precision in detection of the reflection image is to be enhanced, the position setting state as shown in  FIG. 3  is effective.  
         [0036]      FIG. 4  shows a position control state with equal priority on transmission and reflection. The center axis of the objective visual field  100  lies between the center axis of the transmission visual field  101  and the center axis of the reflection visual field  102 . The distance between the center axis of the transmission visual field  101  and the center axis of the objective visual field  100  is equal to the distance between the center axis of the reflection visual field  102  and the center axis of the objective visual field  100 .  
         [0037]      FIG. 5  shows a position control state which is an intermediate state between the position control state shown in  FIG. 2  and the position control state shown in  FIG. 4 . The center axis of the objective visual field  100  lies between the center axis of the transmission visual field  101  and the center axis of the reflection visual field  102 . In addition, the distance between the center axis of the transmission visual field  101  and the center axis of the objective visual field  100  is less than the distance between the center axis of the reflection visual field  102  and the center axis of the objective visual field  100 .  
         [0038]      FIG. 6  shows a position control state which is an intermediate state between the position control state shown in  FIG. 3  and the position control state shown in  FIG. 4 . The center axis of the objective visual field  100  lies between the center axis of the transmission visual field  101  and the center axis of the reflection visual field  102 . In addition, the distance between the center axis of the transmission visual field  101  and the center axis of the objective visual field  100  is greater than the distance between the center axis of the reflection visual field  102  and the center axis of the objective visual field  100 .  
         [0039]     In the examples of  FIG. 2  to  FIG. 6 , the transmission illumination region  111  and the reflection illumination region  112  are spaced apart from each other. Alternatively, the transmission illumination region  111  and the reflection illumination region  112  may have a common region.  FIG. 7  shows an example of this case. In  FIG. 7 , the transmission illumination region  111  covers the entire objective visual field  100 . Accordingly, the visual field (transmission visual field)  121  of the imaging sensor  15  lies within the transmission illumination region  111 , and the visual field (transmission/reflection visual field)  122  of the imaging sensor  17  lies within a common region of the transmission illumination region  111  and reflection illumination region  112 . For example, the position control state as shown in  FIG. 7  can be set by greatly displacing the position of the diaphragm  6  for transmission illumination, which is shown in  FIG. 1 , from a region of passage of illumination light.  
         [0040]     As has been described above, in the present embodiment, there are provided the illumination region position control unit comprising the diaphragm driving mechanisms  23  and  24 , and the visual field position control unit comprising the imaging sensor driving mechanisms  25  and  26 . The positional relationship between the setting positions of the transmission illumination region and reflection illumination region, on one hand, and the setting positions of the visual fields of the imaging sensors  15  and  17 , on the other hand, can be controlled by the illumination region position control unit and the visual field position control unit. Accordingly, various position control states, for example, as shown in  FIG. 2  to  FIG. 7 , can be set, and desired images (images for defect detection) according to purposes can be acquired with high precision. Therefore, with use of the defect inspection apparatus according to the present embodiment, high-precision defect inspections can be performed.  
         [0041]     In the above-described embodiment, the diaphragm driving mechanisms  23  and  24  are used as the illumination region position control unit, and the imaging sensor driving mechanisms  25  and  26  are used as the visual field position control unit. Alternatively, the illumination region position and the visual field position may be controlled, for example, by shifting or tilting mirrors, lens, etc. in the optical system.  
         [0042]     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.