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Conventionally, highly structured patterns or targets are placed on wafers to facilitate alignment and highly repeatable positioning of the wafer. The authors describe a micro-positioning scheme in which such patterns are replaced either by the device pattern on the front of the wafer, or the ground surface on the rear of the wafer. When illuminated by a visible or near-IR beam of light, both the patterned and the unfinished (i.e., diffusely ground silicon) sides of a wafer scatter prodigious amounts of light. By collecting portions of this light through two apertures and then measuring the phase of their mutual interference, the position of the surface wafer can be repeatedly and unambiguously established with a precision of 1 micro-inch (25 nm) over a range on the order of d equals (lambda) /A, where (lambda) is the wavelength of the light and A is the angle subtended by the apertures at the wafer. Typical values for (lambda) and A are 0.8 micron and 0.1 radian, respectively, in which case d approximately equals 8 microns. This means that if a standard stage can be used to position the wafer to within +/- 4 microns, then the interferometric sensor described here can be used to refine its position to within 25 nm. The ultimate resolution, (Delta) xmin, with which this position can be reestablished is equal to (lambda) /(A'SNR) where SNR is the signal-to- noise ratio of the interference signal. For example, with d equals 8 micrometers and a signal-to-noise ratio of 400, (Delta) xmin approximately equals 25 nm, or just under a micro-inch. The direction of the sensitivity vector direction is defined by the relative orientation of the two apertures, and by the associated optics. By using two measurement locations, angular orientation of the wafer about an axis normal to its surface can also be monitored. Because the measurement is inherently based on interferometric phase, rather than the amplitude, it is highly tolerant to variations in surface reflectivity and/or illumination level. The authors describe the theoretical basis for this measurement technique and present results demonstrating repositioning to better than 50 nm using both the patterned face of a wafer and its diffusely reflecting back side.
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