In-situ height correction for laser scanning of semiconductor wafers

Abstract

An <i>in situ</i> technique for servo control of surface height during laser scanning of semiconductor wafers is described. The scheme corrects any macroscopic height changes due to tilt and bow in the wafer and rejects local and pattern-dependent changes of height and reflectivity. The waist of the scanning beam is imaged on a slit aperture placed in front of a position-sensitive photodiode, leading to an ac signal at the scanning frequency. This ac signal then undergoes synchronous detection using a reference signal at the scanning frequency. This detection scheme leads to a reduced sensitivity to low-frequency electronic and thermal drifts. Normalization circuitry provides means for excluding the effects of reflectivity which can vary over four orders of magnitude on patterned wafers. The height signal, so obtained, is used to drive a PZT stage to a nominal height position in closed loop. On patterned wafers, an rms height accuracy of better than 0.1 μm has been achieved in 20-Hz bandwidth.