Immersion lithography: its history, current status and future prospects

Since the 1980's, immersion exposure has been proposed several times. At the end of 1990's, however, these concepts were almost forgotten because other technologies, such as electron beam projection, EUVL, and 157 nm were believed to be more promising than immersion exposures. The current work in immersion lithography started in 2001 with the report of Switkes and Rothschild. Although their first proposal was at 157 nm wavelength, their report in the following year on 193 nm immersion with purified water turned out to be the turning point for the introduction of water-based 193 nm immersion lithography. In February, 2003, positive feasibility study results of 193 nm immersion were presented at the SPIE microlithography conference. Since then, the development of 193 nm immersion exposure tools accelerated. Currently (year 2008), multiple hyper NA (NA>1.0) scanners are generating mass production 45 nm half pitch devices in semiconductor manufacturing factories. As a future extension, high index immersion was studied over the past few years, but material development lagged more than expected, which resulted in the cancellation of high index immersion plans at scanner makers. Instead, double patterning, double dipole exposure, and customized illuminations techniques are expected as techniques to extend immersion for the 32 nm node and beyond.