DUV Coatings Withstand High Fluences
Multilayer dielectric coatings for 157-nm and 193-nm laser optics have demonstrated lifetimes of 7 billion pulses.
By: Kristin Lewotsky
As microlithography gears up for the shift to 193 nm, optical components remain a significant challenge(see Calcium Fluoride Optics Rise to the Deep UV Challenge). The optics in lithographic steppers must withstand fluences ranging from 0.1 mJ/cm2 to 1 mJ/cm2 or more per pulse, and production line systems generate several million pulses per day. Issues range from optical compaction to high absorption to availability of exotic materials such as calcium fluoride.
Coating technology is critical to the performance of deep ultraviolet (DUV) systems. Projection optics for 193-nm lithography consist of hybrid refractive/reflective designs, while beam-delivery optics incorporate primarily reflective components. Thus, systems require dielectric or metal-dielectric thin film stacks that convert components into mirrors, highly-transmissive optics, beamsplitters, attenuators, or bandpass filters.
The issue of coating durability in the face of high fluences has been an ongoing concern and the subject of much research. At the recent 30th Boulder Damage Symposium meeting (Sept. 1998; Boulder, CO), Acton Research Corp. (ARC) engineers reported on a coating development effort that has yielded DUV coatings with damage thresholds as high as 1.29 J/cm2 at 193 nm.
High-reflection coatings (back to top)
Most notable among the work by the ARC group is a series of dielectric multilayer coatings consisting of alternating stacks of low- and medium-index fluoride and selected low-absorption oxide materials. The total number of layers in a high-reflection stack typically ranges from 35 to 45, depending on the design wavelength. Optimized for turning mirrors, dielectric multilayer coatings designed for 193 nm provide 94% reflectance at a 45° angle of incidence (see Figure 1).
In testing, the coatings have withstood exposure to 1.14 J/cm2, 18-ns pulses for 10,000 shots with no detectable damage; such fluences are typically experienced by beam delivery optics. At fluences of 30 to 40 mJ/cm2, more typical of projection optics, the coatings demonstrated lifetimes in excess of 7 billion pulses without signification performance degradation. The coatings are designed for beam-turning and delivery optics.
Broadband coatings (back to top)
The optical design of many lithographic systems includes reflective optical components that are used in non-collimated light. Such components require optical coatings that maintain high reflectance over a rather wide range of incident angles. This is accomplished by designing the coating to have a wide wavelength region of maximum reflectance centered at the design wavelength. Many analytical instrument system optics, such as those used in spectrometers, also require broadband reflectance. Conventional mirrors frequently incorporate an aluminum (Al) base layer to enhance performance at longer wavelengths, but the material introduces a high absorption at DUV wavelengths that degrades absorbency.
By covering the Al base layer with a single overcoat of magnesium fluoride (MgF2), the ARC group has developed a family of broadband coatings optimized for the DUV spectral region. With these designs, designers obtained mirror reflectances between 88% to 90% at 193 nm, and 84% to 86% at 157 nm (see Figure 2).
ARC has also developed high-reflectance coatings for 157 nm. In preliminary tests at fluences on the order of 5 mJ/cm2, coatings have withstood more than 200-million pulses without damage.
Antireflection coatings (back to top)
At 157 nm and 193 nm, only fused silica, calcium fluoride, and MgF2 offer sufficiently low absorption to function as refractive components. The material still requires antireflection coatings to optimize throughput. Again, the design issue is robustness in the face of high fluences, as well as long-lifetime at low fluences.
The group has developed antireflection coatings for CaF2 optics designed for use at normal incidence and optimized for long-duration, low-fluence 193 nm applications (see Figure 3). After exposure to 15 mJ/cm2 at 400 Hz for a total of 1 billion pulses, the coatings displayed no significant change in optical performance and only slight physical discoloration. The thin films reduce reflectance from between 4% and 5% per surface to less than 0.2% to 0.5%. Similar films designed for use at 157 nm have survived 50 million pulses at 3 mJ/cm2.
Future work will include high-angle (70 to 75°) anti-reflective coatings for use at 157 nm and 193 nm, as well as polarizing optics for these wavelengths.
References
Glen Callahan and Bruce Flint, "Characteristics of Deep UV Optics at 193 nm and 157 nm", Proc. SPIE 3578, paper #3578-72.