Quality, Microstructural Refinement and Stability of Atomic-layer-deposited Aluminum Nitride and Aluminum Oxide Films

High-quality, stable ALD films are required in microelectronics when the films are exposed to further processing during device manufacturing, or if the films are exposed to a demanding environment. For example, front-end-of-line processing exposes the deposited materials to high temperatures and aggressive chemicals during process steps such as dopant activation and wafer cleaning. Furthermore, a protective film against humidity and corrosion may need to maintain its structural integrity for the lifetime of the device which can be several years. Therefore, engineering the film quality and understanding the effects of high-temperature processing on thin films are required for the successful integration of the films to a semiconductor device. The goal of this thesis was to study the quality, microstructural refinement, and the stability of ALD AlN and Al2O3 films. The results were divided to the process development of ALD AlN and Al2O3 films, the examination of their microstructural development due to high-temperature thermal treatments, and the resulting stability of the ALD films. Film stability was understood to encompass thermal stability (e.g. oxidation) and chemical stability (ability to resist dissolution and corrosion). Film quality comprised of attributes such as the amount of impurities, stoichiometry, and crystallinity which were characterized for the as-deposited films and after the high-temperature treatments. The emphasis on ALD AlN was in process development. Trimethylaluminum (TMA) -based AlN was amorphous and contained a high amount of hydrogen when deposited at 200 °C. The hydrogen outgassed during high-temperature treatments and the AlN films began to oxidize at and above 800 °C. AlCl3-based AlN films, processed closer to 500 °C, had less impurities and a polycrystalline microstructure as opposed to the TMA-based films deposited at 200 °C. The AlN film residual stress was also tunable in the plasma-enhanced AlCl3 process by adjusting the plasma time of the nitrogen precursor. ALD AlN studied in this thesis and the literature review show promise of the film quality continuously improving. The main challenges are in improving the crystalline quality and minimizing the amount of impurities, such as hydrogen, in the AlN films. The focus on ALD Al2O3 was in understanding the effects of the high-temperature treatments. As-deposited ALD Al2O3 was amorphous and dissolved into wet chemical cleaning solutions. Heat treatments at and above 800 °C crystallized the films. However, high vacuum annealing caused blistering of the alumina films, whereas atmospheres with hydrogen and nitrogen produced crystalline films without blisters. The fully-crystallized alumina films were stable in SC-1 and HF cleaning solutions. The crystallized alumina films are demonstrated to be suitable for technologies such as silicon on insulator. Furthermore, crystallized ALD alumina could be utilized as a protective layer in a variety of applications that withstand the crystallization temperature.