Coating Materials News Vol 8 Issue 2
Coating Materials for UV Wavelengths
|Material||Refractive Index||Lower Wavel. Limit (nm)|
|MgF2||1.40 - 1.45||<>|
|AlF3||1.40 - 1.45||~200|
|CaF2||1.45 - 1.50||<>|
|SiO2||1.45 - 1.55||200|
|LaF3||1.60 - 1.65||250|
|CeF3||1.65 - 1.75||~250|
|Al2O3||1.70 - 1.80||200|
|Sc2O3||1.90 - 2.05||~300|
|Y2O3||2.0 - 2.1||~250|
|HfO2||2.3 - 2.4||~250|
|ZrO2||2.3 - 2.4||~300|
The value of the refractive index is dependent on the deposition method because the packing density, composition, and morphological type (amorphous or polycrystalline) are determined by deposition energy, pressure (partial pressure of oxygen for oxides), deposition rate, and substrate temperature.
The highest index ratios are produced by alternating fluoride compound layers and Group IIIB and IVB oxide compound layers. Not all combinations are stable under high energy or varied temperature conditions. Stress, mismatch in thermal expansion coefficients, and chemical differences are among the causes for incompatibility. Alternating oxides and fluorides is problematic in deposition because a partial pressure of oxygen must be present to oxidize the oxide compound, but, simultaneously, oxygen incorporation or substitution in the fluoride layer must be avoided to maintain low optical absorption.
With these and other precautions to accommodate, researchers have succeeded in developing coatings for deep UV optics. Table 2 lists some of the more successful combinations and their reported LIDTs. The testing laser parameters, coating design and deposition technique are described in the references.
Examples of UV Laser Coatings
|Wavel. (nm)||Coating||LIDT (J/cm2)||Comment|
|355||Sc2O3 / SiO2||3.6||HR on BK-7 |
|248||Al2O3/SiO2||16||HR on BK-7 |
|308||ZrO2/Y2O3||>7||HR on fused silica |
Advantages of Mixture-Composite Films in UV Laser Coatings
Some of the refractory oxide compounds suffer from inhomogeneity through their layer thickness, zirconia being one of the most problematic. The optical consequence is varying refractive index with depth and variable index with exposure to moist air, thereby compromising the performance of the coating design. Accompanying this property are physical consequences such as weaker microstructure or higher stress. It has been found that making solid solutions with glass-forming additives improves homogeneity and layer toughness, and other properties. The admixing can be done by co-evaporation from independent sources, but this requires twice the equipment and control. Co-sputtering is often used to produce new properties, but special targets can be expensive. A more reproducible technique is to use solid solutions of the two materials. Examples of such materials that have been researched include: ZrO2-Y2O3, HfO2-Y2O3, TiO2-ZrO2, CeO2-CeF3, and ZnS-CeF3. Among the refractory oxide compounds, Al2O3, In2O3, MgO, Sc2O3, TiO2, Y2O3, and ZrO2, high mutual solubility is possible [5,6]. The mixtures exhibit new and often improved physical and optical properties because of their greater packing densities and stabilized structure forms. For example, the MgO-ZrO2mixture produces a very tough, temperature-resistant ceramic and is used in high temperature wear resistant applications .
An extensive study of the MgO-ZrO2 mixture system for optical applications was reported in which the deposition parameters were varied, and those of greatest influence on film properties were identified . For this work, CERAC ZrO2-MgO solid solution (Z 1078) was used. This mixture consists of roughly 10% MgO. The researchers investigated the optical constants, index homogeneity, surface topography (scatter), and composition as functions of oxygen pressure, substrate temperature, and deposition rate. Of these parameters, the dependence of index on rate (1-3 Å/s) was small. Index homogeneity was a strong function of substrate temperature. The highest index was obtained at 167°C, but was relatively inhomogeneous. The best homogeneity was obtained at 125°C.
Oxygen pressure was the strongest influence on refractive index, extinction coefficient, packing density, and scatter. The highest indices and lowest extinction coefficients were obtained when the oxygen pressure was below ~7.5E-05 torr. Higher pressures caused superstoichiometric layers with lower packing densities and greater absorption. At 300 nm wavelength, k was <5e-05 and="" n="" ~2.1.="" this="" makes="" the="" material="" useable="" for="" uv="" laser="" applications.="" similarly="" the="" surface="" roughness="" was="" lowest="" at="" the="" lower="" pressures.="" it="" was="" concluded="" that="" sufficient="" oxygen="" is="" provided="" by="" the="" compounds="" without="" additional="" oxygen="" being="" required.="">
The researchers used the pressure dependence to their advantage and succeeded in producing multilayer interference structures by modulating the oxygen pressure. In a single deposition run, they were able to vary n from 1.59 to 2.1 by modulating the oxygen pressure between 6E-04 torr and 1.5E-06 torr.
Other workers  found that the LIDT for ZrO2-Y2O3 as the high-index material is about three times greater than for pure ZrO2. The optimum mixture was 4:1 by weight, a substrate temperature of 200°C, and oxygen pressure 5E-05 torr. They also found that a high index composite made from HfO2 -Y2O3 exhibited sufficiently low absorption to be useable in the 248 nm laser, but provided no test data.
Composite materials consisting of solid solutions have several advantages over single, "pure" materials. Improved optical and mechanical properties can be had with the simplicity and facility of a single evaporation source, rather than multiple sources. The technology developed for the demanding laser applications can also be applied to more common coatings such as ophthalmic and camera lenses.
1. S. Tamura, S. Kimura, Y. Sato, H. Yoshida, K. Yoshida, Thin Solid Films, 228 (1993), 222.
2. N. Kaiser, H Ulig, U. B. Schallenberg, B. Anton, U. Kaiser, K. Mann, E. Eva, Thin solid Films, 260 (1995), 86.
3. F. Rainer, W. H. Lowdermilk, D. Milam, T. Tuttle-Hart, T. L. Lichtenstein, C. K. Carniglia, Appl. Opt., V21, No. 20 (1982), 3685.
4. Jin Tainfeng, Yuan Youxin and Xu Juan, Proc. SPIE.
5. CERAC Coating Material News, V5, I1, Jan-Mar 1995.
6. CERAC Coating Material News, V7, I2, April-June 1997
7. N. K. Sahoo and A. P. Shapiro, Appl. Opt. 37 No. 4, (1998), 698.
Dr. Mitchell C. Colton, Editor
P.O. Box 1178 | Milwaukee, WI 53201
Phone: 414-289-9800 | FAX: 414-289-9805
Samuel Pellicori, Principal Contributor
Pellicori Optical Consulting
P.O. Box 60723 | Santa Barbara, CA 93160
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