The process window for state of the art chip manufacturing continues to decrease, driven by higher NA exposure tools
and lower k1 values. The benefits of immersion lithography for Depth of Focus (DoF) are well known. Yet even with
this immersion boost, NA=1.35 tools can push DoF into sub-100nm territory. In addition, immersion processes are
subject to new sources of dose and focus variation. In order to realize the full potential of immersion lithography, it is
necessary to characterize, understand and attack all sources of process variation.
Previous work has established our dose/focus metrology capability1, in which we expose Process Monitor Grating
(PMG) targets with high sensitivity to focus, measure the PMGs using scatterometry, and use the Ausschnitt dose/focus
deconvolution approach to determine focus errors to within a few nm and dose errors to within 0.1%. In this paper, we
concentrate on applying this capability to the detailed measurements of immersion photoclusters utilizing ASML
exposure tools. Results will include:
• comparison of Twinscan 1700i and 1900i focus capability
• effectiveness of the Reticle Shape Correction (RSC) for non-flat reticles
• visualization of non-flat wafer chucks, tilted image planes, and other systematic focus error components
• tracking of tool trends over time, using automated monitor wafer flows
The highly systematic nature of the observed focus errors suggest potential for future improvements in focus capability.