1. Field of the Invention
The present invention relates to surface measurement, and more particularly relates to surface measurement using a shadow moire technique with the Talbot effect.
2. Brief Description of the Prior Art
Surface measurement is required in the field of electronic device fabrication including, for example, fabrication of semiconductor devices. At the present time, wafers for fabricating such devices are slice either by an inner diameter saw or a wiresaw, followed by a series of lapping and polishing processes. A measuring device is necessary for measuring the flatness of the surface of the wafers after each manufacturing step. At the present time, such measurements are typically performed using a pair of capacitive probes to sample points on the surface of a rotating wafer.
In the method currently practiced, wafers are handled with a vacuum gripper to hold the wafer at the center and to spin it at high speed while the traversing capacitive probes are used to measure the thickness and surface properties of the wafers. There are a number of disadvantages associated with the current method. In order to obtain accurate measurements, numerous sampling points on the surface are needed, thus increasing the time required for inspection of wafers during production. Further, current techniques may distort the wafer which is being measured during the measurement process. Such distortions are introduced by the stress applied by the vacuum gripper, and also due to dynamic stresses caused by the high angular velocities used when spinning the wafer in order to traverse the capacitive probes.
In view of the foregoing deficiencies with current surface measurement techniques for electronic wafers, it would be desirable to develop an apparatus and method for surface measurement which can rapidly and accurately measure the surface characteristics of an entire wafer at once, without the need for multiple sampling points. Furthermore, it would be desirable if the measurement process did not introduce errors due to vacuum gripping or rotation Yet further, it would be desirable if the process could be automated, using, for example, a computer.
The present invention, which addresses the shortcomings of current systems, provides a method for surface measurement. The method includes the step of providing a specimen having a surface to be measured. A mean surface plane can be defined for the surface. The method further includes the step of supporting a reference diffraction grating at a distance xcex4T from the mean surface plane of the specimen and substantially parallel to the mean surface plane. The distance xcex4T is referred to as the Talbot distance. The reference diffraction grating has a characteristic pitch.
Further steps in the method include causing a beam of light to be directed through the reference grating onto the surface to be measured and then detecting moire fringes produced by the reference grating and the shadow of the reference grating on the surface (which forms an effective specimen grating) when there are irregularities on the specimen surface. In the step of causing the beam of light to be directed through the reference grating, the beam of light, which has a wavelength xcex, casts a reference grating shadow onto the surface which is to be measured. The reference grating shadow forms the effective specimen grating. In the step of detecting the moire fringes, the moire fringes produced by the reference grating and the effective specimen grating can be used to compute the variation in depth of the surface to be measured and are indicative of a condition of the surface to be measured. The orders of the fringes are implicitly detected with the fringes themselves.
The specimen which is to be measured would most typically be a substrate wafer used in microelectronic fabrication, such as for silicon or other semiconductor devices. The beam of light which is provided can be a coherent beam of light, such as that from a laser beam, and the detection of the moire fringes can be accomplished with a camera or video capturing device. The method can be automated by, for example, digitizing the position and order of the moire fringes and entering the position and order into a computer which then calculates the surface depth and compensates for any misalignment between the reference grating and the specimen. The spacing (Talbot distance xcex4Ts) can be adjusted, if desired, automatically, to measure different ranges of surface depths and variations.
An apparatus according to the present invention includes a specimen mount which is adapted to receive a specimen having a surface to be measured. Again, a mean surface plane can be defined for the surface. The apparatus also includes a reference grating which is mounted adjacent to the specimen mount and which is positioned to be substantially parallel to the mean surface plane of the specimen when the specimen is received in the specimen mount. The reference grating is movable with respect to the specimen mount so as to vary the Talbot distance xcex4T between the reference grating and the mean surface plane of the specimen. As before, the reference grating has a characteristic pitch.
The apparatus also includes a light source which is mounted so as to direct a beam of light having a given wavelength through the reference grating onto the surface to be measured when the specimen is received in the specimen mount. The beam of light casts a reference grating shadow onto the surface to be measured. The reference grating shadow forms an effective specimen grating on the surface of the specimen.
The apparatus according to the present invention also includes a detector which is positioned to detect moire fringes produced by the reference grating and the effective specimen grating due to variations in depth of the surface to be measured. For both the method, and apparatus of the present invention, the Talbot distance xcex4T is given by the formula:
xcex4T=n(2p2/xcex),
where: n=1, 2, . . . is a positive integer,
p=pitch of the reference grating, and
xcex=wavelength of the light.
As for the method, the light source (preferably a point light source) can be a coherent light source such as a laser and the detector can be a camera. In one embodiment, the camera is located at a distance L from the reference grating and at a distance D from the point light source, such that a line passing between the camera and the point light source is substantially parallel to the reference grating. The light source provides the coherent beam of light at a projection angle xcex1 with respect to the normal to the grating, while the camera is positioned to detect the moire fringes at a receiving angle xcex2 with respect to the normal to the grating at a location with a depth of w. The apparatus can also include a digitizer/detector for digitizing the position and order of the moire fringes and a computer for performing various calculations and compensations. The computer can also be used to control movement of the reference grating in order to adjust the Talbot distance.
The present invention thus provides an apparatus and method for surface measurement which overcomes the disadvantages of prior systems and methods. In particular, the present method and apparatus permit fast and concurrent measurement of a surface, without any dynamic effects or mounting distortion, over the full-field (i.e., whole surface of the wafer), can be automated, and can be applied to other types of surface measurements as well. Enhancement of the shadow moire technique via the Talbot effect permits measurement of very fine surface features typically encountered with substrate wafers for electronic devices.