DYNAMIC PERFORMANCE OF A SCANNING X-Y STAGE FOR AUTOMATED ELECTRON-BEAM INSPECTION

The design and performance of an X-Y stage for fast electron-beam inspection of wafers and x-ray masks is described. The inspection technique involves the comparison of images that are acquired by the raster scan acquisition of long swath images recorded while the stage moves at constant velocity. Pairs of images acquired serially must remain registered to about 0.05 mum net accuracy, requiring interferometer controlled motion with very low vibration. The system design requires high-vacuum compatible, nonmagnetic construction, with provision for electron and light optical elements above the stage, and additional electron optics and substrate loading elements below it. Accordingly, an open frame stage with internal linear motors and bearings was selected. High stiffness and particular attention to smooth motion results in very low vibration with a relatively large moving mass. The stage is driven by brushless linear motors inside a 20 Hz bandwidth servo loop closed around high-resolution lambda/256=2.5 nm interferometers. Smooth motion contributes to accurate short term position measurement, allowing residual errors to be corrected by beam deflection. Long range accuracy relies largely upon occasional re-registration using features on the die patterns. At the end of swaths, programmed turnaround trajectories employing position and acceleration feedback allow serpentine paths with a minimum acceleration and overhead time. Using several complementary measurement techniques, overall registration accuracy was shown to be sufficient for detection of 0.05 mum defects.