We have discussed how important layer nano-structure is in determining the hardness, coherence, and stress properties of a coating layer - properties that are required to increase the mechanical durability of a surface [see: Engineering Hard Durable Coatings" in CMN V18 I2, June 2008) and references 1]. A film layer structured as nano-crystalline units containing open pores, often in the form of columns, is permeable to the deep infusion of water vapor. When exposed to moist atmospheres, diffused water reacts to form hydrated surfaces thereby reducing the bond strengths that existed before contamination. The result is softening of the layer and reduction of mechanical strength. In some cases, detrimental changes in internal stress level are created. Deposition processes and material preparation are developed with the specific goal of growing dense nano-structure with low intrinsic stress. In principle, film layers that possesses these characteristics and are insoluble should resist the effects of water on their optical and mechanical properties. In fact, studies have revealed the influence of water on reducing the wear resistance of oxides and ceramics (see references in 1). Exposure reduces the hardness and increases the fracture resistance of such coatings and makes them susceptible to plastic deformation when the film is scratched [2].

Testing of the individual layers revealed their chemomechanical effects. Greater indenter displacement is experienced for the TiO2 layer after 24 hours soaking compared to a dry layer. Low loading <10 mN) again revealed the creation of a soft layer of reacted material whose hardness is reduced from ~10 GPa to ~9 GPa. The ZnO layer experienced a similar indenter displacement, but a larger reduction in hardness with load. The SnO2 layer showed little effect. SnO2 is used to protect glass bottles from scratching even after washing, so is known to retain its hardness properties. TiO2 and ZnO layers, in contrast, do not retain their mechanical properties after extended exposure to water. The results of the published study suggest that other oxides or denser layer nano-structures are needed to prevent water-induced softening of layers intended for surface protection.

Titania is a favorite material for visible-range coatings because of its high refractive index and because many dozens of deposition-related papers have appeared that teach the optimum processes for producing good films with reasonable ease. Titania is mostly used in AR coatings for ophthalmic, instrument panel, and architectural window coatings. Past CMN issues have discussed this material, its starting material forms and proper pre-evaporation conditioning, and its optical properties.

Al doped ZnO (AZO) is becoming popular as a transparent conducting oxide (TCO) replacement for ITO. We have discussed the economic and transparency advantages of AZO previously [4 ]. Applications of this TCO include the front transparent window and contact on thin-film amorphous silicon and CuInGaSe (CIGS) solar cells. AZO is also used in gas sensing and surface acoustic wave (SAW) devices and UV emitters. The favored deposition process is DC reactive magnetron sputtering of Al:doped ZnO targets, and Al2O3/ZnO targets are used with RF sputtering. In the recent report reviewed here, a target 99.9% pure Zn alloyed with 2% (m/m) Al was used [5]. The researchers deposited films using O2 gas flow and target voltage settings that occur within the hysteresis transition region, O2 flows 20-45 sccm and voltage drop 325 – 600 V. To overcome the instability of such operation, plasma emission was used to control the gas flow by monitoring the metal emission line. A set of transparency and resistivity values was determined at different voltage settings and nearly constant power. The table below lists some examples of transparency (transmittance average from 450 – 500 nm) and resistance. Close examination shows large variations in resistance occur with small changes in power and voltage, suggesting a strong relationship with voltage. Sample A25 has a sheet resistance of 7 W /sq for a thickness of 260 nm, yet the P and V values are not substantially different compared to the other tests, making this result exceptional. Perhaps another sensitive parameter or two must have been encountered to achieve this low R. The resistance of the other high transmittance samples ranges from 300 to 900 W /sq for thicknesses 500 – 600 nm.

Spectroscopic ellipsometry (SE) modeling to determine the absorption coefficients of these samples suggested a positive correlation relating smaller absorption with lower resistance. The other conclusion that can be drawn is that oxygen and voltage deposition parameters for producing low sheet resistances with AZO are very critical; and from experience, we can conclude that this sensitivity is greater than that for ITO deposition.

Obtaining resistivity and transmission >80% with AZO (and ZnO-based materials in general) that is as low as that obtained with ITO is proving to be difficult, independent of the deposition technique used. A recent paper demonstrates the advantages of the sputtering technique variation, high power pulsed magnetron sputtering (HPPMS), to achieve 37 W /sq sheet resistance with 150 nm thick AZO films on 200° C substrates. The best obtained using reactive DC magnetron sputtering (RDCMS) was 50 W /sq [6]. ITO films of thickness ~500 nm deposited on 300° C substrates by both HPPMS and RDCMS gave about the same resistance, <3 W /sq, and ~85% transmission. On unheated substrates, the resistivity is 10 X higher. Process control based on plasma emission was able to stabilize the discharge in the transition mode, and this seems to be the process needed to achieve the minimum resistivity for AZO and similar compositions. Voltage and O2 flow rate have large influences on film roughness (scatter) and resistivity, and the reader is encouraged to consult the paper for details.

CERAC is developing AZO target compositions with varied Al doping concentrations to produce films with lower sheet resistance, and parallel efforts are being directed toward deposition process optimization. Stay tuned to these pages for future reports.

1. CMN: V18 Issue 2 (March 2008); V17, Issue 3 (Sept 2007); V16 Issue 2 (June 2006); V15 Issue 2 (June 2005); V14, Issue 3 (Sept 2004).

2. Krishna J. Belde and S. J. Bull, "Chemomechanical effects in optical coating systems", Thin Solid Films, 515, 859 (2006).

3. His-Chao Chen, Kuan-Shiang Lee, and Cheng-Chung Lee, "Annealing dependence of residual stress and optical properties of TiO2 thin film deposited by different deposition methods", Applied Optics, V47, no 13, C284 (2008).

4. CMN: V18 Issue 1 (Jan 2008).

5. A. Nemeth, Cs Major, M. Fried, Z. Labadi, and I. Barsony, "Spectroscopic ellipsometry of transparent conductive ZnO layers for CIGS solar cell applications", Thin Solid Films, 516, 7016 (2008).

6. V. Sittinger, F. Ruske, W. Werner, C. Jacobs, B. Szyszka, and D. J. Christie, "High power pulsed magnetron sputtering of transparent conducting oxides", Thin solid Films 516, 5847 (2008).