Thermal behavior of wafers and its effects on overlay performance in immersion lithography

In semiconductor manufacturing below 45 nm node, immersion lithography is serious technique to increase depth of focus (DOF) at obtainable apertures and to improve resolution with apertures higher than one. However, immersion environment between wafer and lens could be responsible for a net reduction in overlay performance due to wafer temperature instability during exposure. In this paper, we have investigated correlation of thermal behavior of wafer during exposure and overlay performance. It was founded that the thermal effect of concentric distribution on wafer scaling (correctable errors) is lager (0.1 ppm) than uniform distribution. In grid and field residual analysis, the grid residual errors was demonstrated a good correlation (R2 = 0.92) with thermal variation. However, there was little relationship (R2 = 0.3) between the field residual errors and thermal stability. These results indicated that progressed thermal controllability for both spatial and temporal variation can be converted into improved overlay accuracy in immersion lithography.

Thermal stability of wafer during exposure sequence in comarison with different wafer table configuration.Figure optionsDownload as PowerPoint slideHighlights
► The concentric thermal behavior of wafer has a large wafer scaling value.
► The field residual has no significant improvement by better temperature steadiness.
► The grid x and y residual shows 35% and 21% drop with thermal stability improvement.