Over the past decade, optical lithography's k₁ factor has been continuously eroding despite aggressive improvements in exposure wavelength and numerical aperture. To compensate for the increased difficulty in maintaining adequate process windows at smaller k₁'s, lithographers have gradually introduced mild resolution enhancement techniques (RETs) such as the use of optical proximity correction (OPC) and attenuated phase shifted mask enhanced lithography (attPSM). The penetration of the k₁=0.5 barrier with the 180nm technology node forced serious consideration of strong resolution enhancement techniques such as the use of sub-resolution assist features (SRAFs) with off-axis illumination (OAI), and alternating phase shifted mask enhanced lithography (altPSM). Penetrating into the deep-sub-k₁=0.5 lithography regime for the 130nm and 100nm technology nodes requires broad implementation of these strong RETs in well integrated lithography solutions. After briefly reviewing the history and challenges associated with OPC, this paper will discuss the fundamental concepts and theoretical resolution limits of altPSM and SRAF followed by some experimental results. The paper will then review the challenges facing the implementation of SRAF and altPSM in deep-sub-k₁=0.5 production lithography.
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Lars W. Liebmann (2001) studied this question.