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Chapter 4c:
Specification, Fabrication, Testing, and Mounting of EUVL Optical Substrates
The specification, fabrication, testing, and mounting requirements of optical system substrates are intimately related. This statement may seem self-evident, but the delivery of coated, characterized, and mounted optics that meet functional specifications is often the long straw when it comes to delivering a leading-edge lithographic tool. Even this collection of four tasks is an abbreviated list, because many other aspects of designing and manufacturing a lithographic optical system are also interrelated, including the optical design (e.g., avoiding designs where an optical surface is close to the edge of the substrate), multilayer (ML) coating (e.g., maximizing the amount of smoothing from the deposition process to potentially relax the polishing requirements and minimize coating-added figure errors), and alignment (e.g., offering sufficient degrees of freedom so some aberrations that are constant over the field, such as astigmatism or defocus, can be mitigated by the repositioning of elements). Some aspects of this chapter may be familiar to people with knowledge of delivering an optical system to leading-edge specifications. It is hoped that this chapter has not omitted too many of the details, nor emphasized concerns that have been rendered as standard operating procedures. The intended audience for this chapter includes new personnel entering the optics area for the first time, team members that interact with the optical fabrication team, and the broader lithographic community that depends on the successful performance of the optical system. Here we are concerned with the substrates in particular, while other aspects of the optical elements, such as the ML coatings, are covered in other chapters. The key to constructing any optical system is in formulating a system error budget, which includes all of the contributions to the final set of aberrations and scattered light in the installed system. As a preliminary (and simplistic) example, the system performance specification will lead to a required level of pupil aberrations. These aberrations could be partitioned into figure errors (phase errors) for each of the mirrors in the projection system. The error budget for figure on a mirror would need to be shared among fabrication, coating, metrology uncertainty, and mounting. Other system considerations, such as thermal management of the mirrors, should also be considered. The toughest and most important job of the system engineer is to lead the partitioning of errors among the different contributors, so each has goals that have a reasonable chance of success, i.e., an acceptable level of risk. This ensures that the production yield of the completed substrates is acceptably high and predictable.
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