Design of a low cost displacement interferometer

Abstract

Lithography machines that manufacture integrated circuits require non-contact and nanometer-scale position measurement, which is led by displacement laser interferometer in some ASML machines. The aim of the project is to design a displacement interferometer using non-polarizing light to meet the requirements and be cost-effective. The design is challenging considering the requirements of 350 mm measurement range, 300 µrad angular range and less than 1 nm periodical non-linearity (PNL) error.

This thesis analyzes all the design requirements first. To fulfill the requirements, efficient design strategies are clarified, and the phase detection method is determined.

Three interferometer designs are investigated and developed in this thesis. The design concepts are collected from ASML internal references. Mathematical models are built to represent the optics and trace the light in each interferometer. Optimization tools are developed to solve and tune the parameters that fulfill all the dimension-related requirements. Geometrical optics simulations are performed to verify the mathematical models and the optimization results. The source and behavior of PNL error in each interferometer are carefully researched. In a design with lenses, sequential mode simulations are performed to calculate the wavefront error. Further calculations based on the wavefront error indicate this interferometer design is not feasible.

Based on the theoretical analysis two interferometer designs can fulfill all the requirements. These two interferometers are built in the cleanroom with off-the-shelf parts to validate the working principle and PNL errors. The optics, optomechanics and electronics used in the tests are explained. The methods of alignment and measurement are introduced. The working principles of the two interferometers are validated, and the behaviors of PNL error match the theoretical analysis.