Design of the ultra precision stage for lithography using VCM

摘要

This paper presents a new design of precision stage for the reticle in lithography process and a low hunting control method for the stage. The stage has three axes for X,Y,θ_Z, those actuated by three voice coil motors individually. The proposed precision stage system has three gap sensors and voice coil motors, and supported by four air bearings, so it do not have any mechanical contact and nonlinear effect such as hysterisis which usually degrade performance in nano level movement. The reticle stage has cross coupled dynamics between X,Y, θ_Z axes, so the forward and inverse kinematics were solved to get an accurate reference position. When the stage is in regulating control mode, there always exist small fluctuations (stage hunting) in the stage movement. Because the low stage hunting characteristic is very important in recent lithography and nano-level applications, the proposed stage has a special regulating controller composed of digital filter, adjustor and switching algorithm. Another importance factor that generates hunting noise is the system noise inside the lithography machine such as EMI from another motor and solenoids. For reducing such system noises, the proposed controller has a two-port transmission system that transfers torque command signal from the DSP board to the amplifier. The low hunting control algorithm and two-port transmission system reduced hunting noise as 35nm(rms) when a conventional PID generates 77nm(rms) in the same mechanical system. The experimental results showed that the reticle system has 100nm linear accuracy and 1 μrad rotation accuracy at the control frequency of 8 kHz.