Wafer-surface Flattening Control Using a Wafer Deforming Chuck Driven by Piezoelectric Actuators

A wafer-surface flattening technique has been developed to improve patterning accuracy of VLSI fabrication systems. The technique is especially applicable to X-ray exposure systems. This paper presents the wafer-surface flattening method, including a wafer deforming chuck and its operating system, as well as an evaluation of the system accuracy. The wafer deforming chuck is designed so that a wafer held onto an elastically deformable plate by vacuum suction is flattened by controlling the heights of piezoelectric actuators underneath the deformable plate based on flatness error measured by an electrostatic displacement detector. A unique open loop voltage control method to improve wafer flattening accuracy by eliminating the displacement error due to hysteresis in the piezoelectric actuators is also discussed. The experimental results show this method is capable of flattening a wafer-surface to within ±0.5 μm of a perfect plane, independent of wafer size.