Preliminary analysis of laser-pulse-induced pressure variation for immersion lithography

Abstract

We describe an assessment of the pressure rise that may be induced by the short-duration but high-power pulses associated with the immersion lithography exposure process. A conservative model provides an upper bound on the pressure rise related to the expansion of the fluid near the wafer due to rapid heating. This rapid heating process is simulated as a constant heat flux from the substrate. The resulting temperature rise causes a change in pressure that propagates into the fluid at the speed of sound. The net change in the mass of the fluid contained within the pressure wavefront must be zero. This continuity requirement allows an estimate of the pressure rise and its penetration depth into the gap. For the nominal conditions associated with 193-nm immersion lithography, the model predicts that the pressure near the wafer surface may rise by as much as 7.3 kPa during the laser pulse. At the end of the laser pulse, this pressure rise will extend nominally 75 μm into the gap. Following the laser pulse, this pressure rise will rapidly decay as the pressure wavefront continues to propagate across the gap and eventually out of the under-lens region.