Defect inspection apparatus

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.

CROSS-REFERENCE TO RELATED APPLICATIONS

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

1. Field of the Invention

The present invention relates to a defect inspection apparatus which inspects a defect of a mask.

2. Description of the Related Art

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)).

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.

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.

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

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.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1schematically shows an example of the structure of a defect inspection apparatus according to an embodiment of the present invention.

Light (e.g. a laser beam), which is emitted from a light source1, is extended by a beam expander2and is converted to an area light source by an optical integrator3. A fly-eye lens or a diffusion plate, for instance, is usable as the optical integrator3. Light from the optical integrator3is Koehler-illuminated by a collimator4onto the position of a diaphragm6for transmission illumination and the position of a diaphragm11for reflection illumination. A beam splitter5is disposed on a rear side of the collimator4. The light incident on the beam splitter5is separated into transmission illumination light and reflection illumination light.

The transmission illumination light, which is separated by the beam splitter5, is Koehler-illuminated on the position of the diaphragm6for transmission illumination, as described above. The diaphragm6is disposed at a conjugate position with respect to the position of a pattern formation surface (an inspection target surface) of a mask8to be inspected (e.g. a photomask such as a reticle). The position of the diaphragm6is movable by a diaphragm driving mechanism23which is composed of, e.g. a pulse motor. By controlling the position of the diaphragm6by the diaphragm driving mechanism23, a transmission illumination region is set on the pattern formation surface of the mask8that is disposed on a stage22.

Light, which has passed through the diaphragm6, is Koehler-illuminated on the pattern formation surface of the mask8by a condenser lens7. The mask8is 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 lens7, which is to be used, is chosen by taking the thickness of the glass substrate into account. Since the pattern formation surface of the mask8is disposed at a conjugate position with respect to the position of the diaphragm6, a transmission illumination region, which is adjusted by the diaphragm6, is set on the pattern formation surface.

The reflection illumination light, which is separated by the beam splitter5, is Koehler-illuminated on the position of the diaphragm11for reflection illumination. The diaphragm11is disposed at a conjugate position with respect to the position of the pattern formation surface of the mask8. The position of the diaphragm11is movable by a diaphragm driving mechanism24which is composed of, e.g. a pulse motor. By controlling the position of the diaphragm11by the diaphragm driving mechanism24, a reflection illumination region is set on the pattern formation surface of the mask8that is disposed on the stage22.

Light, which has passed through the diaphragm11, is Koehler-illuminated on the pattern formation surface of the mask8by a collimator12and an objective lens9. A beam splitter10is disposed between the collimator12and the objective lens9. Reflective light from the beam splitter10reaches the pattern formation surface of the mask8via the objective lens9. Since the pattern formation surface of the mask8is disposed at a conjugate position with respect to the position of the diaphragm11, a reflection illumination region, which is adjusted by the diaphragm11, is set on the pattern formation surface.

The above-described beam expander2, optical integrator3, collimator4, beam splitter5, diaphragm6and condenser lens7constitute an illumination optical system for setting a transmission illumination region. The beam expander2, optical integrator3, collimator4, beam splitter5, diaphragm11, collimator12, beam splitter10and objective lens9constitute an illumination optical system for setting a reflection illumination region. The above-described diaphragm driving mechanisms23and24constitute 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 mask8.

Light from the transmission illumination region of the mask8, that is, image light corresponding to the pattern included in the transmission illumination region, reaches a beam splitter13via the objective lens9. Similarly, light from the reflection illumination region of the mask8, that is, image light corresponding to the pattern included in the reflection illumination region, reaches the beam splitter13via the objective lens9.

The image light, which has reached the beam splitter13, is separated into transmission light and reflection light by the beam splitter13. Of the transmission light emerging from the beam splitter13, a light component coming from the transmission illumination region of the mask8is imaged on an imaging sensor (imaging unit)15via an imaging lens14, but a light component coming from the reflection illumination region of the mask8is not imaged on the imaging sensor15. In addition, of the reflection light reflected by the beam splitter13, a light component coming from the reflection illumination region of the mask8is imaged on an imaging sensor (imaging unit)17via an imaging lens16, but a light component coming from the transmission illumination region of the mask8is not imaged on the imaging sensor17.

The imaging sensor15is movable by an imaging sensor driving mechanism25which is composed of, e.g. a pulse motor. By controlling the position of the imaging sensor15by the imaging sensor driving mechanism25, the light from the transmission illumination region of the mask8can be imaged on the imaging sensor15, as described above. In other words, by controlling the position of the imaging sensor15by the imaging sensor driving mechanism25, the position of a visual field of the imaging sensor15on the pattern formation surface of the mask8is adjusted. As a result, on the pattern formation surface of the mask8, the position of the visual field of the imaging sensor15can be made to correspond to the position of the transmission illumination region.

The imaging sensor17is movable by an imaging sensor driving mechanism26which is composed of, e.g. a pulse motor. By controlling the position of the imaging sensor17by the imaging sensor driving mechanism26, the light from the reflection illumination region of the mask8can be imaged on the imaging sensor17, as described above. In other words, by controlling the position of the imaging sensor17by the imaging sensor driving mechanism26, the position of a visual field of the imaging sensor17on the pattern formation surface of the mask8is adjusted. As a result, on the pattern formation surface of the mask8, the position of the visual field of the imaging sensor17can be made to correspond to the position of the reflection illumination region.

The above-described objective lens9, beam splitter13and imaging lens14constitute an imaging optical system for providing an image of the transmission illumination region on the imaging sensor15. The objective lens9, beam splitter13and imaging lens16constitute an imaging optical system for providing an image of the reflection illumination region on the imaging sensor17. The above-described imaging sensor driving mechanisms25and26constitute a visual field position control unit which controls the positions of the visual fields of the imaging sensors15and17on the pattern formation surface (inspection target surface) of the mask8.

A computer21is connected to the above-described diaphragm driving mechanisms23and24, the imaging sensor driving mechanisms25and26and the stage22which scans the mask8. The operations of these components are controlled by the computer21. The diaphragm driving mechanisms23and24and the imaging sensor driving mechanisms25and26are automatically controlled by the computer21. Thereby, on the pattern formation surface of the mask8, the position of the visual field of the imaging sensor15can automatically be made to correspond to the position of the transmission illumination region, and the position of the visual field of the imaging sensor17can automatically be made to correspond to the position of the reflection illumination region.

Image data of an image (transmission image) of the transmission illumination region, which is formed on the imaging sensor15, is sent to a comparison unit18and is compared with reference image data of a reference transmission image which is prestored in a memory unit19. Similarly, image data of an image (reflection image) of the reflection illumination region, which is formed on the imaging sensor17, is sent to the comparison unit18and is compared with reference image data of a reference reflection image which is prestored in a memory unit20. Based on comparison results that are thus obtained, a defect on the pattern formation surface of the mask8is detected. The detection of a defect may be performed by the comparison unit18or by the computer21.

FIG. 2toFIG. 6show 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 mechanisms23and24, and the setting positions of the visual fields of the imaging sensors15and17, on the other hand, which are controlled by the imaging sensor driving mechanisms25and26.

InFIG. 2toFIG. 6, reference numeral100denotes an objective visual field (i.e. a visual field on the pattern formation surface of the mask8, which is defined by the objective lens9); numeral101a transmission visual field (i.e. a visual field of the imaging sensor15on the pattern formation surface); numeral102a reflection visual field (i.e. a visual field of the imaging sensor17on the pattern formation surface);111a transmission illumination region on the pattern formation surface; and112a reflection illumination region on the pattern formation surface. As shown inFIG. 2toFIG. 6, the transmission visual field101and the reflection visual field102are spaced apart. The transmission illumination region111and the reflection illumination region112are spaced apart. The transmission visual field101is positioned within the transmission illumination region111, and the reflection visual field102is positioned within the reflection illumination region112.

FIG. 2shows a position control state with priority on transmission. The center axis of the transmission visual field101agrees with the center axis of the objective visual field100. 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 inFIG. 2is effective.

FIG. 3shows a position control state with priority on reflection. The center axis of the reflection visual field102agrees with the center axis of the objective visual field100. 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 inFIG. 3is effective.

FIG. 4shows a position control state with equal priority on transmission and reflection. The center axis of the objective visual field100lies between the center axis of the transmission visual field101and the center axis of the reflection visual field102. The distance between the center axis of the transmission visual field101and the center axis of the objective visual field100is equal to the distance between the center axis of the reflection visual field102and the center axis of the objective visual field100.

FIG. 5shows a position control state which is an intermediate state between the position control state shown inFIG. 2and the position control state shown inFIG. 4. The center axis of the objective visual field100lies between the center axis of the transmission visual field101and the center axis of the reflection visual field102. In addition, the distance between the center axis of the transmission visual field101and the center axis of the objective visual field100is less than the distance between the center axis of the reflection visual field102and the center axis of the objective visual field100.

FIG. 6shows a position control state which is an intermediate state between the position control state shown inFIG. 3and the position control state shown inFIG. 4. The center axis of the objective visual field100lies between the center axis of the transmission visual field101and the center axis of the reflection visual field102. In addition, the distance between the center axis of the transmission visual field101and the center axis of the objective visual field100is greater than the distance between the center axis of the reflection visual field102and the center axis of the objective visual field100.

In the examples ofFIG. 2toFIG. 6, the transmission illumination region111and the reflection illumination region112are spaced apart from each other. Alternatively, the transmission illumination region111and the reflection illumination region112may have a common region.FIG. 7shows an example of this case. InFIG. 7, the transmission illumination region111covers the entire objective visual field100. Accordingly, the visual field (transmission visual field)121of the imaging sensor15lies within the transmission illumination region111, and the visual field (transmission/reflection visual field)122of the imaging sensor17lies within a common region of the transmission illumination region111and reflection illumination region112. For example, the position control state as shown inFIG. 7can be set by greatly displacing the position of the diaphragm6for transmission illumination, which is shown inFIG. 1, from a region of passage of illumination light.

As has been described above, in the present embodiment, there are provided the illumination region position control unit comprising the diaphragm driving mechanisms23and24, and the visual field position control unit comprising the imaging sensor driving mechanisms25and26. 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 sensors15and17, 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 inFIG. 2toFIG. 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.

In the above-described embodiment, the diaphragm driving mechanisms23and24are used as the illumination region position control unit, and the imaging sensor driving mechanisms25and26are 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.