This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
This Small Business Innovation Research (SBIR) Phase II project seeks to develop a turnkey sputter deposition system to provide low temperature thin film deposition of transparent conductive oxide (TCO) materials. A laboratory scale film deposition system using a closed-field magnetron sputtering with RF plasma enhancement was demonstrated. The thin films were grown at lower temperatures than most competing processes. One key advantage of the deposition process developed is its ability to produce TCOs without the need for post-treatments (to achieve both good resistivity and transparency) thereby simplifying the process compatible for high-volume processing of large flat polymeric substrates. The project will demonstrate the process compatible with alternative TCO materials and with photovoltaic applications.
The broader impact/commercial potential from this technology will be innovations in photovoltaic (PV) technology. The ability to tap solar energy more efficiently will lead to major breakthroughs for many devices. For years, silicon (Si) solar cells have been the backbone of the solar industry using monocrystalline Si substrate with multiple layers of p-n junction diodes. However, one of the main limiting factors is the shortage of silicon for PV applications as it competes with the existing requirements in the semiconductor industry. Many different PV alternatives are in active development which utilizes TCO materials to provide the conductive anode, cathode, or both. Thin film solar cells provide a good alternative to Si-based solar cells as long as the fabrication cost can be reduced. Thin film solar cells use layers of semiconducting materials with little micrometer thickness and deposited on glass, stainless steel or flexible substrates. One cost-effective method to produce PV devices is through the use of polymers. However, the current device performance of polymer-based PV devices is low but can further be improved by fabricating metal oxide semiconductors embedded on the polymer-based device structure. Thus, this technology will be cost-competitive if the fabrication of TCO thin films are proven they can be done on large-area flexible substrates at lower temperatures.
The Kurt J. Lesker Company (KJLC®), founded 1954 in Pittsburgh PA, is a dynamic, innovative, and growing manufacturer/distributor of high vacuum and ultra-high vacuum systems, chambers, pumps, components, thin film sources, and pure materials. The Process Equipment Division (PED) is a leading designer/manufacturer of advanced single-chamber and cluster-chamber, computer controlled, thin film deposition systems. In addition, PED offers a wide range of circular (Torus®) and linear magnetron sputter deposition sources for R&D and large-area production applications. Transparent Conductive Oxides (TCO) is an important class of metal oxide semiconductor materials used in photovoltaic (PV) and display (FPD, OLED) device technology. KJLC has objectives to continue to develop thin film deposition equipment for the cost effective deposition of TCO’s. This project focused on the development of a system as well as process optimization to achieve improved TCO film properties at low-temperature (< 100°C) consistent with the established goal criteria using RF plasma enhancement without the need for high-temperature post-deposition annealing. The glass transition temperature of PET (100°C) was set as an approximate upper limit for the TCO process window. Metal oxide semiconductors such as zinc oxide (ZnO) can well replace the existing indium-based TCOs due to the depleting supply of indium in the world market. Thin ZnO films can be deposited via sputtering techniques with high optical transmission and low resistivity. Doped ZnO is used as a TCO window layer and has been identified to achieve high efficiency in thin film solar cells using copper-indium-gallium selenide (CIGS) as an absorber layer. Thus, there is a need to further investigate sputtering processes that can produce high-quality ZnO films that are scalable to large-area substrates at low temperatures. The specific TCO material investigated during this project was AZO. Based on published results for AZO, it was determined that resistivity < 1 x 10-3 W×cm, sheet resistance < 10 W/sq and optical transmission > 80% were goal criteria for this research. The KJLC designed Linear a-tool platform incorporated 18" wide closed-field DC magnetron sputtering cathodes with supplemental oxygen and RF plasma enhancement capable of high-volume processing, dynamic deposition rates of 38 nm·m/min, of large-area, 12" x 12", substrates. The Linear a-tool platform is designed to be modular with a low profile and is fully computer controlled. In particular, significant effort was applied to the development of a viable and cost effective RF plasma source design. AZO process testing on the KJLC a-tool commenced during year 2. Similar to ITO results, AZO film properties responded to supplemental RF plasma at low-temperatures. It was found that by adjusting the process parameters of cathode power, RF coil power, and supplemental O2 the resulting deposited film properties, resistivity and transmission, could be optimized. Optimal process conditions were established and film properties obtained (e.g., R @ 1.5 mW×cm, Rs @ 44 W/sq and %T @ 83%) at low-temperature (< 100°C) that were consistent with results reported in the literature for low-temperature DC magnetron sputtered films with no post-deposition heat treatment on glass substrates. However, despite significant efforts to optimize the system configuration as well as the deposition processes little improvement in AZO film properties (~10 to 15%) was achieved by the addition of supplemental RF plasma. Complete details of this project are summarized in the full report.
