| The increasing cost of photo masks in optical lithography is identified as a key barrier for migrating to sub-100nm features in integrated circuit manufacturing. An economical solution, called optical maskless lithography, has drawn broad interests in the IC industry. In optical maskless lithography, an array of reconfigurable micromirrors replaces the traditional photo mask. However, identifying the optimal mirror configuration has been an outstanding issue in the past few years due to the limited understanding of this problem.; The theory and simulations presented in this work answer this question in two aspects. First, the through-focus effects of micromirrors are analyzed by theoretical modeling and full simulations. Three criteria are developed to examine mirror configurations. The 4-piston mirror is verified as the optimal micromirror configuration for having the lowest through-focus asymmetry and the highest flexibility. Second, an operation strategy utilizing two exposures with compensated mirror configurations is proposed. The key advantage of the strategy is that the through-focus asymmetry can be minimized by two compensated fields, which is verified both by an analytic model and by simulations. Therefore, any types of piston phase-shift mirrors are all good candidates for optical maskless lithography.; The understanding of the through-focus effects is then applied to improving the sensitivity of mirror calibrations. A method using high sensitive interferometric schemes with sensors at defocused planes will be presented. The typical sensitivity improvement is about 3X to 4X when the optimized schemes are used. Then, this thesis will present the design, fabrication, and characterization of an elastomer spatial light modulator (SLM), a type of piston mirror arrays, that can be scaled to meet the requirements of extreme ultraviolet (EUV-13nm wavelength) optical maskless lithography.; Lastly, the advantages and potential of optical maskless lithography based on micromirror array will be summarized together with the conclusion: optical maskless lithography is not only an economic solution to the high mask costs, but also a good platform for flexible and high-resolution patterning. |
