Measurement of Wafer Surface Using Shadow Moiré Technique With Talbot Effect

Abstract

In this paper, a modified shadow moiré technique is applied to measure surface topology of wafers. When a wafer is sliced, either by an inner-diameter (ID) saw or wiresaw, the surface needs to be measured to ensure the consistency of quality. Two important characteristics of the wafer surface measurements are the warpage and total thickness variation (TTV). Currently, the most commonly used method of wafer measurement employs a pair of capacitive measuring probes which sample points on the surface of a rotating wafer to obtain the contours of surface. Many sampling points on the surface are needed for more accurate measurements; however, this will require more time for the inspection of wafers during production. An innovative alternative for full-field, whole-wafer measurement is developed using a laser light source and the modified shadow moiré technique. This methodology enables one to examine the whole wafer surface quickly and simultaneously. In this study, a 40 lines/mm (1000 lines/inch) reference grating is employed as the standard to create a shadow moiré pattern. In addition, the Talbot effect is utilized to adjust the gap, or the so-called Talbot distance, between the grating and the wafer surface such that a fringe pattern of good quality can be obtained. By using the phase shifting technique, the resolution (or sensitivity) can be enhanced by two order of magnitude. The results show that not only the full view of the whole wafer surface can be obtained, but enhanced surface resolution and accuracy can also be realized. In addition, warpage due to excessive residual stresses can be observed distinctly with fringe patterns because of the global and interconnected moiré fringes. This process is faster, especially when dealing with wafers with diameter larger than 200 mm (8”). Experimental results of both 200 mm single crystalline and 100 × 90 mm polycrystalline wafers are presented. The system can also be fully automated to become an on-line inspection tool.