The limitations of high index resists for 193nm hyper-NA lithography

Abstract

This paper will investigate the potential benefits and limitations of increasing the refractive index of the photoresist for water and high-index immersion based lithography. The primary potential benefits are increased exposure latitude due to restoration of the TM polarization component and improved depth of focus due to a delay in the onset of imageinduced top-loss. After first understanding the physical origins of these effects, a series of simulation studies will probe the level of impact they may have for the 32nm and 22nm technology nodes. It is concluded that, although they may provide some process latitude relief, the benefits are minimal for 1.3 5NA water immersion, especially when weighed against the likely required development effort and cost. The benefits are slightly more compelling for high-index immersion (>1.5 NA), but a high index resist does not appear to be critical, provided the resist is at least as large as the immersion fluid index. A comparable benefit can be achieved with a conventional resist by using polarized illumination (a trend already happening for various reasons) and thinning the resist by ∼9% for 1.35NA water immersion and ∼15% for 1.55NA high-index immersion. Additionally, increasing the refractive index is typically accompanied by a corresponding increase in absorption. This will be addressed, concluding the limitations of absorption are likely chemical and not optical in nature. High absorption is likely tolerable, provided the chemistry can be engineered to account for exponential intensity decay. The level of difficulty in doing so is addressed.