An Ultra-precision Measurement System for Next Generation 400 mm Silicon Wafers

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The purpose of this study is to develop an accurate flatness measuring system for large silicon wafers with diameters of up to 400 mm. The system consists of two-dimensional (2D) slope sensor unit, a wafer spindle, and a sensor carriage. The wafer sample is mounted on a vacuum chuck vertically and can be rotated by the spindle. The 2D slope sensor unit consists of two 2D slope sensors. The 2D slope sensor unit is placed on the sensor carriage. It detects the 2D local slopes (X-and Y-directional local slopes) at a point on the wafer surface. The 2D slope sensor is moved by the sensor carriage along X-direction to scan the wafer surface while the wafer is rotating. In this angle-based measuring system the transnational error motions of the sensor carriage (straightness error motions) and the wafer spindle (axial error motion) will not affect in the sensor outputs. Based on the concept of error separation, the 2D local slopes of the wafer surface can be separated from the angular error mo … More tions of the sensor carriage (yaw, roll) and the wafer spindle (angular error motion) using the 2D outputs of the slope sensors. The cross-sectional height profiles of the wafer surface along radial directions can be obtained through integrating the X-directional local slopes of the wafer surface, and the cross-sectional height profiles along concentric circles can be obtained through integrating the Y-directional local slopes. The height profile of the entire wafer surface can thus be evaluated from combining the sectional profiles in these two directions. Two-dimensional slope sensors, which utilizes the principle of autocollimation, were developed to realize the measuring system. The two-dimensional local slopes (angles) are obtained through detecting the corresponding two-dimensional positions of the reflected light spot on the focal plane of the object lens using position-sensing devices. To make the sensor compact in size, it is more effective to improve the sensitivity of angle detection by selecting proper position-sensing devices than using an objective lens with a larger focal distance. It was shown that a quadrant photodiode is the best for highly sensitive two-dimensional slope detection. The compact prototype slope sensors were confirmed to have a resolution of approximately 0.01 arc-seconds. Flatness measurement experiments of large silicon wafers have been conducted to indicate the feasibility of the developed system. Less