The desire to reduce cost in volume manufacturing has driven up the throughput in the lithographic exposure machines.
As a result the power transmitted in the projection optics increases. Although small, the absorption levels in the lens
materials are not zero, which leads to localized heating of the lens and hence lens aberrations. To squeeze out the
maximum process windows, the pupil shapes have transformed from simple annular shapes to shapes with very
concentrated poles. As a result, the exposure energy transported through the lens is no longer equally distributed over the
lenses of the projection options. Instead only a fraction of the lens gets to transport the total power. This concentration of
power further aggravates the lens heating induced aberrations and enhances the importance of advanced lens heating
control schemes which are available on ASML scanners.
To analyze the effects of lens heating on the final imaging, a model was developed by the lens manufacturer Carl Zeiss
SMT GmbH, and incorporated into a litho simulation environment by ASML BRION. This tool can be used to analyze
the impact of dose/throughput, illumination shapes and reticle layout on aberrations. It provides a means to assess
potential lens heating issues even before production masks are manufactured. Moreover, this computational tool opens
the possibility to calculate parameters for lens heating correction, rather than measuring them, saving valuable machine
time. In this paper, the performance of the novel computational lens heating control is demonstrated on wafer and
compared with the traditional way of measuring the relevant parameters. In addition, a modeling study is performed to
assess possible lens heating effects for freeform or non-traditional source shapes, thereby demonstrating the advanced
correction potential of ASML latest aberration manipulator, called FlexWaveTM.