Absolute Heterodyne Interferometer for Strongly Aspherical Mirrors

摘要

The structure of this thesis is as follows: Chapter 1 will contain a brief introduction to the field of extreme ultraviolet lithography, serving as motivating factor for the research undertaken. Key challenges will be mentioned qualitatively, and the approach to the problem will be sketched. Chapter 2 deals with the concept of the interferometer, reviewing several earlier concepts and conclusions, and presenting the specific measurement approach culminating in the design of the prototype interferometer. Chapter 3 is devoted to the theoretical challenge of interpreting the raw measurement data to yield a useful measurement of the mirror substrate's shape. Largely mathematical in nature, it develops- and compares the performance of- several inverse propagation algorithms (IPAs) to fulfill this end. Chapter 4 treats the design and construction of the multiple wavelength light source required by the interferometer. Chapter 5 deals with several types of sensors which could be used in the interferometer. The various methods required to acquire and subsequently process measurements with these sensors will be discussed. Chapter 6 focuses on the design and construction of the interferometer itself, including the frame which connects the mirror substrate under test with the other interferometer components, and includes a discussion of the optical fibers used in the interferometer. Chapter 7 gives the details and results of measurements on a particular mirror substrate using our instrument. These results are discussed in Chapter 8, together with other, more general considerations and suggestions for improvements and future work. Finally, the appendix contains specifications of materials and equipment used.