Thin film coatings for the spectral region shortward of the visible range require special considerations with regard to materials. Wavelengths shorter than 400 nm, the ultraviolet wavelength region, have many uses in science and technology. Depending on the specific discipline, the small wavelengths or high energies of UV have application in fluorescence diagnostics, germicidal equipment, corneal surgery, high-energy astronomy and physics, gas species detection, lithography of circuits, and other laser optics. Deep UV lithography requires performance at the ArF excimer wavelength 193 nm and the F2 wavelength 157 nm. Absorption, surface and bulk morphology defects, impurities, and microstructure are properties that determine the limits of performance, and in the case of laser optics, resistance to damage. These properties are more critical at the higher energies (>3 eV) and smaller wavelengths of UV for both bulk and thin film materials. In the case of absorption, many materials have energy gaps near or greater than 3 eV, and thus are absorbing. This is the case for most oxide compounds, silica and alumina being the most common exceptions. AR and filter coating designs require a high- and low-index combination. The high index would normally be supplied by an oxide compound; the low index by silica or a fluoride compound. Generally the high-index oxide compounds begin to absorb as wavelengths approach 300 nm. Some, like titania absorb as long as 450 nm. Therefore, when a thin-film coating design is needed for use at wavelengths below ~350 nm, the number of candidate oxides is quite limited.

We shall examine the available thin-film materials and their preferred deposition processes in this issue. Past issues [1, 2] have discussed several UV materials. The first referenced issue listed the candidate high- and low-index materials usable below 300 nm wavelength including the optical and mechanical advantages of mixtures or composite materials; the second discussed how e-beam deposition of MgF2 produces superior low damage films in excimer laser applications. New information is continually being published that CMN reviews for its readers, for example, the June Applied Optics contains some interesting papers presented at the 8th Topical Meeting on Optical Interference Coatings held 15-20 July 2001.

The bandgap energy is the energy needed for an electron to transition from a material’s valence band to its conduction band, causing absorption of light. The corresponding extinction coefficient, k and its increase at shorter wavelengths, imposes a limit to the useful transmission range of the thin film material. We assume that the practical maximum k value (acceptable limit depends on application and total thickness of the absorbing layers) is ~0.07 where the transmittance of a QW optical thickness would be reduced by 10%, and list the approximate wavelength limits for some UV materials in Table 1. These were derived from models that were fitted to ellipsometric [3] and transmittance data [4].

The deposition technique also influences the performance of UV coatings, especially those used with excimer lasers. With the support of the European commission, a group of European researchers studied the properties of MgF2 and LaF3 to wavelengths as short as 120 nm [5]. These materials present a promising layer pair for coatings at or shorter than 193 nm wavelength. The refractive indices at 193 nm are for MgF2 1.42 – 1.44 and for LaF3 1.67 – 1.70. Deposition techniques: resistance heated (RH) and e-beam (EB) heated evaporation and ion-beam sputtering (IBS) were compared for micro-structural and optical properties, specifically index, stress, and scatter. In agreement with previous studies that reported higher damage thresholds for RH deposited MgF2 [2, and Protopapa, et al, J. Vac. Sci. Technol., A 19(2), Mar/Apr 2001], the thermal processes produce superior films compared to IBS. IBS films of both MgF2 and LaF3 possess 2 times higher stress and higher UV absorption due to incomplete stoichiometry, but are smoother. The IBS microstructure is more uniform, consisting of smaller grains of uniform size distribution throughout the depth of the layer. RH and EB films grow with a columnar form whose grain size increases toward the surface of the film. The less compact, larger structure scatters more than the IBS deposition. The higher energy of IBS can also result in a reduction of the fluorine content, leading to higher UV absorption. Replacement with adventitious oxygen has been observed to also increase UV absorption. The lowest scatter was observed with EB MgF2 on super polished CaF2 substrates, but MgF2 presented the highest scatter when deposited on super polished fused silica substrates. The same tendency is observed for the LaF3 films. EB layers exhibited the lowest scatter in spite of their roughness tendency to be greater than that of the IBS depositions. In fact, no simple relationship exists between roughness and total backscatter because of the complex nature of the growth process of thin films which includes such factors as the nature of the chemical bond between substrate and film, interface layers, microstructure variations through the film layer depth, adatom and surface energies etc. The authors of the work conclude that IBS films exhibit the lowest surface roughness but high backscatter, highest refractive index, high stress, and homogeneous microstructure compared to films deposited by thermal evaporation techniques. Higher laser damage thresholds are predicted for the thermal processes than for IBS depositions of these fluorides.

UV materials must have very low levels of transition metal impurities or other elements that absorb in the UV. Levels must be below 0.5% and in some cases below 0.1% by weight. A different paper [6] demonstrated the importance of eliminating contamination from the ion source structure when depositing UV films. Construction materials containing Ni, Cr, Mo, W, and Fe caused significant absorption below 300 nm wavelength, while Ti did not. Ti has a lower sputtering rate than the other metals. This precaution also applies to the container (liner) used in EB evaporation. A straying beam striking the liner can contaminate the deposition. The metal grids and neutralizer filament wire in ion-beam sources can be sputtered in ion-beam assisted sources causing UV absorption. Finally, carbon from the coating chamber is an ubiquitous contaminant.

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1. CMN V 8, Issue 2 April-June 1998.

2. CMN V11, Issue 4, Dec. 2001.

3. Bernhard von Blankenhagen, Diana Tonova, and Jens Ullman, Appl. Optics. V41, No. 16, 3137 (2002).

4. Shunsuke Niisaks, et al, ibid pg. 3242.

5. Detlev Ristau, et al, ibid, 3196.

6. Traci R. Jensen, John Warren, and Robert L. Johnson, Jr., ibid pg. 3205.

7. Bernd Heinz, Vacuum & Thin Film V2, No. 10 (1999).

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