DESIGN AND SENSORLESS CONTROL OF A PIEZOELECTRIC RETICLE ASSIST DEVICE FOR COUNTERACTING INERTIAL LOADS AND PREVENTING RETICLE SLIP IN LITHOGRAPHY SCANNERS

摘要

This paper presents the design and control techniques of a piezoelectric device for managing the inertial loads on the photoreticle of lithography scanners. Lithography scanners are used to fabricate integrated circuits. Figure 1 shows a simplified diagram of such a system. By moving the reticle and the wafer relative to one another, under a slit of light, the reticle pattern is transferred onto a die. To increase the throughput of the scanners, their scan rate is increased leading to larger inertial forces. The inertial forces can cause nanometer-level pre-sliding slip of the reticle relative to the reticle stage, and thus result in large overlay error. One of the major challenges in increasing the scanners throughput, along with bubble formation in the case of immersion lithography and actuator force density limits, is the reticle slip problem. One set of prior art solutions modify the clamping mechanism to address the reticle slip problem. Another group of solutions use a reticle assist device, which counteracts the inertial load by exerting an opposite force on the reticle.