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CERAC Coating Materials News

Volume 8 - Issue 2
June, 1998

Coating Materials for UV Wavelengths

Introduction

There is continued importance in the development of coatings that operate in high energy-density environments for anti-reflection, beamsplitting, and high-reflecting applications. In the past decade, technology was developed for the Nd-YAG laser at 1064 nm and its harmonics in fields including nuclear fusion, range gauging, and medical surgery. The wavelength region of emphasis has shifted to the UV where excimer lasers and other lasers are finding more applications in eye surgery, industrial manufacturing, and data storage.

UV-laser coatings and materials have applications in fluorescence stimulation in bio-medical applications and, more recently, in photolithography of ever-decreasing dimensions in microprocessor manufacturing. Typical wavelengths include 355 nm and 266 nm (third and fourth harmonics of Nd-YAG), 351 nm (XeF), 308 (XeCl), 248 nm (KrF), 193 nm (ArF). The gas lasers are called excimer lasers. The shorter wavelengths have application in deep UV microlithography for generating feature sizes <0.25 µm for data storage and microelectronics fabrication. ArF is finding application in cornea surgery (photorefractive keratectomy).

Pulse widths for excimer lasers vary from >30 ns to sub-ns with powers exceeding 500 MW, and repetition rates approaching 1 kHz. The Laser-Induced Damage Threshold (LIDT) of a coating is a measure of the survivability of a coating from repeated exposure to the laser energy. LIDT of a thin film coating is dependent on many parameters, including the characteristics of the laser pulse, the substrate composition, cleanliness, and polish, and the coating's composition, microstructure, and design. Coating micro-defects, inclusions, and thermal conductivity play limiting roles in LIDT.

Damage mechanisms vary with pulse width and repetition rate. In this article, we are concerned with coating composition, design, and morphology, which in turn are functions of material combinations, deposition technique, and material preparation. Because of the obvious complexity of the subject of laser damage, we can only summarize some of the recent research. The reader is referred to additional publications for further details.

Materials Useful for Excimer Wavelengths

A critical parameter for materials used in UV coatings is low absorption. Absorption caused by impurities or departures from stoichiometric composition will decrease the LIDT because localized heating will disrupt the coating physically and chemically, and accelerate the damage in a cascading process. The coating materials must have very low transition metal impurity levels, and the coating chamber and process must not introduce impurities or produce incomplete oxidation. Carbon has high UV absorption and is a common impurity that can be introduced in many ways, often in the cleaning procedure.

High LIDT is dependent on layer number because of the possibility of absorption induced by sub-stoichiometric composition, or by the number of interfaces between the high- and low-index layers and their layer microstructure and morphology. The layer count for high reflector and beamsplitter designs is inversely determined by the ratio of the high to low refractive indices. The lowest-index transparent materials available have an index range of 1.5 to 1.4 at wavelengths 300 nm to <200 nm. The few candidate high-index materials that are non-absorbing down to 200 nm are oxide compounds. Oxide compounds must be completely oxidized to be absorption free and failure to achieve this is one of the main causes for low LIDT. Table 1 lists typical coating materials.

Table 1.
Materials for Wavelengths Shorter Than 300 nm

Material Refractive Index Lower Wavel. Limit (nm)
MgF2 1.40 - 1.45 <200
AlF3 1.40 - 1.45 ~200
CaF2 1.45 - 1.50 <200
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.

Material Combinations

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.

Table 2.
Examples of UV Laser Coatings

 
Wavel. (nm) Coating LIDT (J/cm2) Comment
355 Sc2O3 / SiO2 3.6 HR on BK-7 [1]
248 Al2O3/SiO2 16 HR on BK-7 [2]
248 Sc2O3/SiO2 >6 AR [3]
308 ZrO2/Y2O3 >7 HR on fused silica [4]

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 [7].

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 [7]. 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 [4] 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.

Conclusion

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.

 

References

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
CERAC, inc.
P.O. Box 1178 | Milwaukee, WI 53201
Phone: 414-289-9800 | FAX: 414-289-9805
e-mail: marketing@cerac.com

Samuel Pellicori, Principal Contributor
Pellicori Optical Consulting
P.O. Box 60723 | Santa Barbara, CA 93160
Phone/FAX: 805-682-1922
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