Many important commercial applications use ultraviolet (UV) lasers, including microlithography, marking and micromachining, fiber Bragg grating (FBG) fabrication, and medical procedures such as photorefractive keratectomy. One of the technical challenges encountered in the development of these applications has been the volume production of reliable, high-performance, laser-grade UV optics.
The UV spectral region begins around 400 nm and goes down to about 1 nm. As wavelength decreases, different substrate materials, coating materials, production methodologies, and cleaning techniques must be used. Thus, in a discussion of UV optics, it is useful to break this spectrum into several different bands.
In the region from 300 nm to 400 nm, the most commonly used material for external cavity optics is fused silica, which possesses good chemical and physical characteristics for polishing. These optics are coated using the same materials and thin-film coating techniques employed in the visible spectrum. This includes the use of oxide coating materials such as hafnium oxide (HfO2) and aluminum oxide (Al2O3), which offer a high refractive index, high damage resistance, and excellent mechanical (durability) characteristics. For low index layers, manufacturers also turn to fluoride materials such as magnesium fluoride (MgF2), which are generally softer and more hygroscopic.
The biggest problem in this spectral region derives from the fact that as design wavelength shrinks, coating response narrows. For example, a typical quarter-wave stack high reflector has a bandwidth of about ±10%. At 512 nm, this translates into a total usable bandwidth of about 100 nm, while at 308 nm, it is only 60 nm. Therefore, as design wavelength decreases, it becomes increasingly important to center the coating on the nominal wavelength. This requires that layer thicknesses be more tightly controlled, and makes coating chamber distribution effects more pronounced. The same logic applies to substrate preparation: An optic with /10 flatness at 308 nm is more than twice as flat as a /10 optic at 633 nm, and is therefore more difficult to produce.
From about 240 nm to 300 nm, many oxide coating materials can still be used. In the case of substrates, however, systems must contain more than fused silica to achieve optimal performance. For color correction, in particular, designers use calcium fluoride and MgF2. These are crystalline materials that are anisotropic, hygroscopic, and very prone to edge chipping and fracturing.
Many manufacturers have developed their own proprietary techniques for working with crystalline materials. At Alpine Research Optics, we have had the most success with pad polishing, as opposed to conventional pitch laps. However, probably the most important step we have taken is simply to environmentally isolate the crystalline material polishing area to prevent small airborne particulates from our glass- and silica-polishing operations from settling on the optics and causing scratches. Fine debris must also be continuously evacuated from within the crystalline material production area.
Absorption precludes the use of fused silica for transmissive optics below 193 nm and virtually all oxide coating materials below about 220 nm. Because optics manufacturers are restricted to using low-index fluoride coating materials, typical coatings must incorporate more layers than at longer wavelengths. This is because the number of layers required to achieve a given level of coating performance (such as the peak reflectivity in a quarter-wave stack high reflector) increases as the index difference between coating layers drops. Reliably producing coatings with larger layer counts presents practical challenges with fluoride materials, however, because of their physical characteristics. In particular, these films tend to be more brittle and prone to crazing. Producing durable coatings for wavelengths below 220 nm thus requires precise monitoring, a well-characterized process, and tight control of all deposition parameters, such as temperature, pressure, layer counts, and materials.
Contamination also becomes increasingly important at shorter wavelengths because most materials absorb strongly at UV wavelengths. When working below 250 nm, for example, contamination concerns necessitate the use of a dry pump system that utilizes no hydrocarbon oils whatsoever.
At 157 nm, contamination becomes the overriding issue. In fact, absorption is so strong at this wavelength that even a monolayer of surface contamination (oil, water, or oxygen) can cause losses of up to 15%. Furthermore, traditional cleaning methods and substrate heating are not sufficient to completely remove impurities on this scale. In response, some manufacturers of deep UV optics have developed proprietary cleaning methods. For example, we use a two-stage cleaning process. The first stage involves traditional cleaning using a methanol wipe with high-quality lens tissue and nanograde methanol. The second stage is a form of reactive cleaning performed in a sealed container that is continuously flushed with an ultrapure combination of inert gas and oxygen. The component is irradiated with either a deep UV laser or light from a deep UV discharge lamp. The reactive combination of energetic photons and oxygen removes most types of surface contamination. The oxidized and vaporized contamination is then flushed away by the gas flow. The two-stage method yields a typical surface transmission of better than 99.5%.
Driven by the needs of several commercially important applications, as well as the availability of more powerful laser sources, deep UV optics remains an area of very active development. Ongoing advances in both substrate preparation and thin-film coating technology should continue to yield optics that can withstand higher fluencies and larger pulse counts while operating at shorter wavelengths.
(By Wayne Pantley and David Collier, Alpine Research Optics, OEmagazine, October, 2001)
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