Amorphous or nanocrystalline AlN thin films formed from AlN: H
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This work describes the formation of stoichiometric AIN films by single ion-beam sputtering of Al, using an ionized N 2 (75%) + H 2 (25%) mixture, onto substrates heated to 200 °C or above. The role of substrate temperature on film composition and properties is followed in the substrate temperature range between ambient and 250 °C. Infrared spectra of freshly prepared and 2 month old (aged in air) films demonstrate that substrate heating significantly affects the chemical nature of the resulting films. SEM and STM data, combined with IR and UV-visible spectral results, indicate that films formed at a substrate temperature of 5^200 CC are very smooth and highly resistant to attack by atmospheric gases. X-ray diffraction data show no diffraction peaks, indicating that the film is either amorphous or crystalline on a scale of less than 4 nm.
I. INTRODUCTION Aluminum nitride has a large band gap (6 eV), 1 a high density (3.3 g/cm 3 ), large thermal conductivity [260 W/(m-K)], a small thermal expansion coefficient (4.6 X 10~ 6 /C), and a very large volume resistivity.2 Aluminum nitride is a hard material, with a bulk hardness similar to quartz, about 2 X 103 kg/mm 2 . 3 Thin films of freshly prepared AIN and AIN: H are even harder, with the hydrogenated film being about as hard as alumina.4 AIN is also chemically stable to attack by atmospheric gases at temperatures less than about 700 °C.5"8 These excellent physical and chemical properties have stirred considerable interest in practical applications of aluminum nitrides. AIN has received attention from the electronic industry because of the need for reliable high voltage and high power devices. AIN is also an attractive material to the optics industry for both dielectric and protective coatings. Other applications include integrated circuit packaging, ignition modules, rf/microwave packages, heat sinks, cutting tools, and laser diode heat spreaders.2 Most of the work on AIN film growth to date has resulted in films that are crystalline and preferentially oriented relative to the substrate surface, although changes in deposition conditions can change which axis is preferred.9"14 While oriented crystalline films are preferred for piezoelectric devices, they are not always the most desirable form for thin film applications. For example, Aita and Tait have suggested that nanocrystalline AIN films might serve as excellent corrosion inhibitors.15 Aita's films were made by rf sputtering and were characterized as a mixture of crystallites of about 10 nm imbedded in regions of poorly defined crystal structure. Hasegawa and co-workers have suggested that an amorphous form of AIN might have superior properties J. Mater. Res., Vol. 9, No. 6, Jun 1994
for electronic applications since a mismatch between substrate and AIN lattice constants would not result in high levels of strain.16 They used CVD to form their films and found that the addition of hydrogen to the reactive gas flow gave films with much better density and lower oxygen content. Mazur and co-workers found that single
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