Design of a gain scheduled PID controller for the precision stage in lithography

Abstract

This paper presents a novel PID gain scheduling controller using a root mean square (RMS) signal for the precise positioning stage in lithography process. Low perturbation in stage regulation is a very important issue and it significantly affects the quality of the lithography process. Perturbation in regulation control is also referred as stage hunting, and usually the stage hunting has a tendency to be decreased if the control bandwidth of the system is reduced. However, in this case the control system will be very weak against small disturbances such as electrical noise or even structural vibration of the building in which the stage is installed. In this paper, a gain scheduling control scheme using a real-time RMS converter chip is proposed to minimize stage hunting and simultaneously maximize immunity to disturbances. For the test of the proposed control scheme, an air levitated precision stage was designed and used for experiments. Simulations were conducted by MATLAB to model and analyze the stage and controller. The simulation and experimental results are presented to verify the effectiveness of the proposed control scheme for the precision positioning stage, as compared with a conventional PID controller.