MIMO feed-forward design in wafer scanners using a gradient approximation-based algorithm

An experimental demonstration is given of a data-based multi-input multi-output (MIMO) feed-forward control design applied to the motion systems of a wafer scanner. Atop a nominal single-input single-output (SISO) feed-forward controller, a MIMO controller is designed having a finite impulse response (FIR) filter structure. The coefficients in this structure are obtained from a gradient approximation-based algorithm. The aid of the FIR filter structure is mostly through its efficient (and limited) set of perturbed-parameter experiments needed to obtain the FIR coefficients. The effectiveness of the optimized feed-forward design in achieving improved servo tracking performances is demonstrated on a high-speed and nano-accurate MIMO wafer stage of an industrial wafer scanner.