- Improving Coatings: Techniques, Processes, and Testing
The durability of thin films for optical as well as mechanical applications is always a prime concern to the user and the coater. We have approached this subject several times and from different perspectives in CMN. We have discussed how materials, deposition techniques and processes influence the mechanical properties that produce wear resistance such as hardness, adhesion, and strength, and the commonly used methods for evaluating these properties. Similarly, the starting material composition and deposition process parameters determine the optical properties of index, absorption, wavelength stability, etc. In this issue, we discuss work done with ternary compound mixtures toward improving zirconia film layers. Finally, we include a brief discussion on roll coating to mass produce optical coatings for many applications.
- Example: Study of an Optical Coating with Electrical and Mechanical Requirements
- Ternary Composites Produce Stable Oxide Coatings
In these issues of CMN, we have often extolled the optical, mechanical, and stability advantages that mixed materials provide over pure compounds . Such materials include fluoride and oxide compound mixtures that are evaporated or sputtered from solid solutions of the composite starting materials. The general properties achieved in the deposited film layer are controlled crystallinity and better homogeneity, which are responsible for uniform and greater index value (higher packing density), lower internal stress, greater surface smoothness, greater stability to environmental stresses, etc. The presence of the second component (“dopant”) results in modifying the film microstructure, specifically discouraging micro-crystal growth or columnar structure.
- Zirconia is a material that has received a lot of attention because its films exhibit hardness, high temperature durability, and high refractive index. However, in its pure starting state, it is difficult to evaporate or sputter and produce films with consistent index, high packing density, and low stress. A crucial requirement for modern optical film technology is to have wavelength-stable coatings, i.e., coatings that do not shift as a function of environmental humidity (due to void filling when exposed to moisture). As is the case with many oxide and fluoride compounds, very high substrate temperatures (>200°C) are required to build high packing density, but then the film stress is increased. When a solid solution is made with one of the many possible glass-forming oxides (see ref. 2 for examples), improved mechanical and optical properties, including laser damage resistance, are achieved. Two-compound mixtures produce partial stabilization of the crystalline nature, often containing two phases, and therefore not truly amorphous films. Previous researchers reported that ZrO2 could be stabilized to a cubic phase by adding MgO, and that by adding Al2O3 to ZrO2 a tetragonal phase is established . Improved mechanical properties are generally observed. Little work has been done on ternary composites until a recent publication  that reports a thorough study of the MgO-Al2O3-ZrO2 solid solution material (CERAC M-1126).
- Sahoo and Shapiro  observed that when both Al2O3 and MgO are present, the two phases above disappear and an amorphous film results. Their study demonstrates that the ZrO2-composite films exhibit nearly invariant wavelength shifts with humidity exposure, very low stress, high optical transparency between at least 2000 nm and 300 nm (substrate cut-off), and good mechanical properties. They caution that the material must be slowly and thoroughly melted to prevent it from growing out of the crucible during e-beam heating.
- Sahoo and Shapiro studied the optical, compositional, and scattering properties as functions of oxygen pressure, deposition rate, and substrate temperature. The general dependencies found were as follows. Even at ambient, the films showed sufficiently high packing density that no air-to-vacuum shift was observed. Index homogeneity was good at both ambient and the maximum temperature of 237°C, but in between these temperatures, there is apparently a phase change that reduces both index and absorption. The oxygen pressure was <5 e-05="" mbar="" (base)="" and="" the="" rate="" was="" 4="" å/s.="">
- With a substrate temperature of 162°C and 5 E-05 mbar oxygen pressure, the highest indices (~1.85) were obtained at deposition rates between 8 and 10 Å/s. Extinction coefficient was low at rates <10 å/s.="">
- Oxygen pressure has a strong influence on refractive index and extinction coefficient. The best values for 162°C substrate and 8 Å/s conditions were obtained at pressures of 5 E-05 to 10 E-05 mbar (3.7-7.5 E-05 Torr). Similarly surface roughness was low. The interesting observation was the relationship between zirconium content in the films and oxygen pressure. Zirconium content increased as pressure increased above ~6 E-04 mbar, but simultaneously index decreased, while the other components remained somewhat constant with pressure. So a good starting range for the parameters seems to be: substrate temperature 125 - 180°C, pressure 5 E-05 mbar, rate 8 Å/s. In summarizing Sahoo and Shapiro’s work, we related the preferred parameters, but advise the user to experiment with the material and to establish optimum parameters and procedures in his own evaporation system (good advice for starting with any unfamiliar evaporation materials).
- Sputter Deposition Roll Coating
Large area, high volume thin film coatings are produced by roll-to-roll web coating for many optical and electrical applications. The most common substrate is PET film, which is then laminated to glass. Large glass sheets can also be continuously sputter coated. Typical products are architectural and automotive windows, coatings (AR and EMI shielding) for CRT and flat panel displays, transparent conductors for touch panels, reflectors for lighting, flexible circuit boards, etc. We summarize some items of interest from a recent paper that traces the development of roll coater sputtering and reviews some of the developments .
- The mainstay sputter technique for many years has been DC planar magnetron sputtering. High deposition rates are achieved for metals, but the process is not suitable for dielectrics because the oxidative reaction required to produce oxide compound films causes target oxidation and system arcing and consequently instability and inefficiency. The development of alternate sputtering from two cathodes operating in the AC mode at mid-kHz frequencies overcame the oxidation and arcing problems.
- In the case of oxide film production, the problem of composition control was severe because the process is basically an unstable one where a stable operating point exists on a steep curve transitioning between metal-rich and oxidized target regions. To achieve efficient, high rate sputtering, it was necessary to invent methods for sensitive monitoring of the conditions in the plasma environment and rapid-response process control. Two methods are now used: monitor the intensity of specific plasma emission lines of the metallic species, or monitor the partial pressure of oxygen in the coating region. The first method is implemented by using the signal intensity from the metal line emission to control the oxygen flow. Since oxygen consumption is determined by the oxidation rate of the metal atoms in the plasma and on the target surface and the evacuation speed of the pumps, the control is complicated. The second method measures the partial pressure of oxygen and uses this signal to control target power. A complication present is that oxygen consumption is not uniform throughout the chamber because reaction, pumping, and flow rates vary locally. Consequently, multiple sensors must be used.
- Roll coating technology for mass producing functional and decorative coatings has advanced rapidly in a short period, and there is no end to improvement possibilities in sight.
1. CMN V2 No. 2, April-June 1996; CMN V6 No. 1, Jan.-Mar 1996; V1, No. 3, Jul.-Sept. 1991.
2. CMN V8 No. 3 (1998).
3. C. M. Gilmore, C. Quinn, S. B. Quadri, C. R. Gosset, and E. F. Skelton, J. Vac. Sci. Techol., A5, 2085 (1987).
4. N. K. Sahoo and A. P. Shapiro, Appl. Opt. V37, 8043 (1998).
5. W-F Wu and B-S. Chiou, Applied Surface Science 115, 96 (1997).
6. CERAC stock nos.: ITO targets: SS-242, SS-245; Si targets: SS-352.
7. J. B. Fenn, W. C. Kittler, D. Lievens, R. Ludwig, G. Phillips, and A. Taylor, SVC 41st Tech. Conf. Proc., 463, April 1998.