Electrostatically Levitated Object Handoff to Minimize Wear and Particle Generation

Abstract

In semiconductor photolithography, feature sizes are reducing towards low nanometers using extreme ultraviolet (EUV) light, which requires exposure in vacuum. Even a few particles on surfaces in the optical path (such as reticles, lenses, and mirrors) pose critical limits on performance, yield, and machine availability. In the semiconductor manufacturing process, object handoff between stages is one of the significant particle generation mechanisms. Vibrations in the handoff and pickup stages generate impact forces on the object and cause relative sliding between contact surfaces, leading to wear and particle generation. Unlike in air, non-contact handling techniques such as Bernoulli grippers do not work in vacuum. We have developed the concept of object handoff by electrostatic levitation, establishing a small upward bow shape to make first contact at the object center and then flattening outward. Our research solution converts the transfer problem from a three-body contact (handoff stage, object, and pickup stage) to a phased two-body contact by electrostatically levitating the object from the handoff to the pickup stage.

This thesis describes the design and modeling of a proof-of-concept handoff system and presents the experimental results. We levitate a 152 by 152 mm square, 400 μm thick aluminum sheet (object) across a 200 μm air gap. We explore various sensing and actuation patterns, develop control strategies, and evaluate methods to avoid electrostatic discharge during stable levitation and handoff. The prototype suspends and stabilizes the object in six degrees-of-freedom (6-DOF) below the pickup stage electrodes. 3-DOF (Z, 𝜃ₓ, and 𝜃ᵧ) are actively controlled and the other 3-DOF in horizontal directions (X, Y, and 𝜃 subscript z) are passively stabilized. The flexible Bow shape of the object is also controlled. The achieved steady-state object positioning noise is < 200 nm-pp in the Z-direction, and < 0.2 mdeg-pp in 𝜃ₓ, 𝜃ᵧ directions, with a 150 Hz maximum bandwidth. The horizontal positions are repeatable to < 0.5 mm in X and Y, and ±0.2 deg in 𝜃 subscript z. We demonstrate pickup-clamp and unclamp-placedown sequences by levitating the object from the resting pins (handoff stage) to the pickup stage electrodes and back. This methodology for object handoff by levitation could be extended, using electrostatic, electromagnetic, acoustic, and pneumatic force fields, to other situations in which wear and particle generation during object handoff are critical.