Novel Design Methods for High-quality Lithographic Objectives Novel Design Methods for High-quality Lithographic Objectives

Celor doua Doamne din viata mea: mamaie si mama 7 Preface This thesis discusses some of the challenges of optical design for Extreme Ultraviolet (EUV) mirror systems and illustrates novel design techniques and their application to, preponderantly, EUV systems and, in addition, some Deep Ultraviolet (DUV) lithographic objectives. The main added value of this thesis is the fact that it shows how scientific results can be used for the design of complex lithographic systems. Photolithography has been the driving force of the semiconductor industry in the last decades. Projection exposure tools, used nowadays, have been subjected to continuous changes in order to satisfy the demands for higher-resolution microchips, improved reliability and lower overall cost. In fact, in terms of performance, the technology enabled the shrinking of critical dimensions to 45 nm and it will go even further to 32 nm and below as of 2009. The first chapter of this thesis covers the basics of lithography technologies. Special attention is paid to the technique, which will be used to print feature sizes below 32 nm. Extreme Ultraviolet Lithography or EUVL has been proposed as the future-generation lithography to replace more conventional optical technology that uses refractive surfaces. In recent years the research has been focused on improving the resolution and the depth of focus of the photolithographic process. The continuous demand for high numerical aperture projection optics can not easily be met by existing design techniques and improvements and innovation are needed in this field. The general problem of optical system design for EUVL and the issues such as merit function construction or constraints are discussed in Chapter 2 of this thesis. Chapter 3 presents a special challenge in the design of EUV projection optics: the sensitivity. When such systems, with high-order aspheric surfaces, are optimized, 8 they often enter into highly unstable regions of the parameter space. A strategy to stabilize the optimization process is discussed. Chapter 4 illustrates a global search method, based on saddle point detection, with applications of this method to EUV mirror systems. The algorithm has been used to obtain additional knowledge about the merit function space. The applicability of the method for practical purposes is also shown and results for six-mirror EUV projection optics are discussed. Optical designers often insert and split lens to force a break through in the design process. Chapter 5 presents an alternative that is based on the use of …