Optimizing 193-nm Optics
06/01/1997
Optimizing 193-nm optics
As the semiconductor industry looks forward to 193-nm lithography, most discussions of optics have concentrated on the projection lens. While the lens is certainly crucial, other parts of the beamline can also affect system performance and reliability. Mirrors and beam splitters can absorb UV radiation, reducing the energy actually reaching the wafer plane. Moreover, UV radiation below 220 nm generates ozone from oxygen. Ozone, in turn, attenuates UV energy and may attack some optical components. Prolonged ozone exposure is a health hazard, even at concentrations as low as 1 ppm.
A recent study at Excimer Laser Systems, Wayland, MA, assessed ozone control measures and optical coatings for 193-nm beamline components. The test system (Fig. 1) used interconnected beam tubes and modular cubes to create a completely sealed enclosure for the beam. Experiments with inert purge gases showed that pressurizing the beamline with nitrogen at 10 psi eliminated ozone attenuation of the beam. Pulse energy at 100 Hz was about 4 mJ in the unpurged beamline, but jumped to more than 10.5 mJ with nitrogen purging.
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Figure 1. Schematic of experimental 193-nm laser beamline enclosure.
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Figure 2. Typical 193-nm components, coatings, and specifications.
Optical studies on the same system compared two types of UV mirrors, both supplied by Acton Research, Acton, MA. The first type of mirror used a broadband UV coating, while the second type used a multilayer dielectric (MLD) coating. MLD coatings alternate high and low refractive index dielectrics to optimize reflectance for a particular wavelength. In this case, the 193-nm optimized coatings used 49 layers of Al2O3 (high index) and SiO2 (low index). The difference in reflectance between the two mirrors was small, but accumulated differences throughout the optical system produced significant changes in efficiency. Figure 2 summarizes the components and coatings used in the experimental beamline. - K.D.