Chemical vapor deposition of aluminum nitride thin films
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David M. Hoffman Department of Chemistry, University of Houston, Houston, Texas 77204 (Received 15 August 1991; accepted 6 March 1992)
The atmospheric pressure chemical vapor deposition of aluminum nitride coatings from hexakis(dimethylamido)dialuminum, A12(N(CH3)2)6, and ammonia precursors is reported. The films were characterized by ellipsometry, transmission electron microscopy, x-ray photoelectron spectroscopy, Rutherford backscattering, and forward recoil spectrometry. The films were deposited at 100-500 °C with growth rates up to 1500 A/min. The films showed good adhesion to silicon, glass, and quartz substrates and were chemically inert. Rutherford backscattering analysis revealed that the N/Al ratio was 1.15 ± 0.05 for films deposited at 100-200 °C and 1.05 ± 0.05 for those deposited at 300-500 °C. Films deposited at 100-200 °C had refractive indexes in the range 1.65-1.80 whereas indexes for films deposited at 300-400 °C were 1.86-2.04. The films were transparent in the visible region. The optical bandgap varied from 5.0 eV for films deposited at 100 °C to 5.77 eV for those deposited at 500 °C. Films deposited at 100-200 °C were amorphous whereas those deposited at 300-500 °C were poly crystalline.
I. INTRODUCTION Aluminum nitride (A1N) is a material that has potential applications in microelectronic and optical devices.1 It has a large direct bandgap (6.2 eV), high thermal conductivity (up to 320 W-m^-K" 1 ), low thermal expansion coefficient, and high decomposition temperature (2400 °C) and is transparent in the visible and nearinfrared regions. 2 ^ These properties make it potentially useful as a dielectric and passivation layer in microelectronic devices and coating material in optoelectronic devices.5-6 A1N films also have applications in surface acoustic wave devices due to their high surface acoustic wave velocity and piezoelectric character.7'8 Thin films of aluminum nitride have been deposited by chemical9 14 and physical vapor deposition techniques.7'8'15 Precursor mixtures for chemical vapor deposition (CVD) have included aluminum halides and ammonia9'10 and aluminum alkyls and ammonia.11 The temperatures required for the CVD processes employing these precursors were greater than 800 °C. Recently there has been a considerable effort to lower the temperature of A1N deposition in order to accommodate thermally sensitive substrates. For example, hydrazine (N 2 H 4 ) was reacted with A1(CH3)3 to give films at 575 °C. 12 Other workers have concentrated on single-source routes. Interrante and co-workers used the cyclic compound [(CH3)2A1NH2]3 to deposit films of A1N at 400-800 °C with deposition rates of 5-25 A / J. Mater. Res., Vol. 7, No. 7, Jul 1992 http://journals.cambridge.org
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min, 613 and Gladfelter and co-workers employed the cyclic azide complexes [R2A1N3]3 (R = CH 3 , C 2 H 5 ) to produce coatings at substrate temperatures of only 4 0 0 600 °C. 14 The latter films were amorphous as deposited and at 500 °C the deposition rate was 760 A/h. The temperatures of deposit
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