Source Mask Optimization techniques are gaining increasing attention as RET computational lithography techniques in sub-32nm design nodes. However, practical use of this technique requires careful considerations in the use of the obtained pixilated or composite source and mask solutions, along with accurate modeling of mask, resist, and optics, including scanner scalar and vector aberrations as part of the optimization process. We present here a theory-to-practice case of applying ILT-based SMO on 22nm design patterns.

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With the advent of in-situ aberration measurement, users may now gather a 'snapshot' of a given lens' aberration fingerprint following installation of the exposure tool in their fab. This paper will detail how this information may be used as input data into an optimization routine whose final output is a set of lens adjustments that will yield improved imaging performance. It will be shown that a lens with many reliable degrees of adjustment freedom can offer a wide range of optimization options.For example, for some process levels, a fixed focus latitude requirement is well defined, and it often is desirable to optimize the CD distribution over that available focus range. For other levels, there may be a strict 'image shift' requirement over a specified focus range. In general, such optimization should include some combination of imaging performance targets.Within some limits, the user may specify the imaging performance priorities, and an optimization routine can be executed. The optimization routine may in principle allow manipulation of all available lens adjustment degrees of freedom, ending with a lens adjustment prescription that delivers the optimum result in terms of the user-specified priorities.Examples of such optimization, showing the expected improvement in the imaging performance, will be presented through image simulation results.

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