Active vibration control of a photolithography platen for increased resolution and throughput

An experimental investigation into active control of bending vibrations in thick plate-like structural elements is described. This work is motivated by vibration problems encountered in manufacturing processes that require greater control authority than is available from conventional surface mounted PZT patches or PVDF films. The focus of this experiment is a surrogate photolithography platen in which PZT stacks are mounted in cutouts on the platen top surface. These actuators provide significant vibration control authority by generating moments in the platen through their compressive loads. A Positive Position Feedback control law is used to significantly augment the damping in the first two bending modes. The implications of the experimental results for photolithography machines are discussed. Introduction In this paper, we develop and demonstrate a design for an active vibration control system suitable for damping thick plate-like structures. In particular, this approach is being used to improve the performance of a recently developed photolithography system featuring a magnetically levitated platen. The manufacture of integrated circuits is still heavily dependent on conventional photolithography processes in which a photosensitive silicon wafer is precisely positioned and a circuit image is projected onto its surface. The achievable circuit density is therefore determined by the precision of the wafer positioning system, and performance is limited by controller induced vibrations of the platen. In the mag-lev system, positioning accuracies on the order of 4 nm are readily achieved using low-bandwidth control. However, as the bandwidth is increased in an effort to increase wafer throughput, the positioning accuracy degrades due to control spillover into the platen flexible body modes. Therefore, the goal of this work is to develop a supplemental flexible body control system to augment the damping of the platen's modes and increase the mag-lev positioning accuracy. To avoid a complete control system redesign, the flexible body controller is designed to operate within the existing mag-lev rigid body positioning loop. In order for this dual control approach to be successful, the flexible body control forces should be orthogonal to the rigid body modes. Externally applied control forces, such as those derived from voice coil actuators, alter the dynamics of the rigid body loop and are therefore not a suitable choice for this application. Conversely, imbedded piezoelectric actuators are attractive since they provide no excitation of rigid body dynamics. Much recent research has focused on the use of imbedded or surface mounted piezoelectric actuators for control of plate-like structures. In particular, active buckling and shape control" have been considered. The application of piezoelectric devices to active vibration control of plates has focused on relatively low frequency modes afforded by thin plates. Controlling the bending vibrations of the relatively thick photolithography platen requires greater control authority than is available from conventional surface mounted djj actuators such as Lead Zirconate Titanate (PZT) patches or Poly vinylidene Fluoride (PVDF) film. Instead the increased force capabilities of d33 PZT stack actuators are exploited to provide the needed control authority. Placed in material cutouts on the platen surface, each of the actuators couples into the platen dynamics though a two-point pre-loaded mounting