Changes in optical transmittance and surface morphology of AlN thin films exposed to atmosphere
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Changes in optical transmittance and surface morphology of AlN thin films exposed to atmosphere Yoshihisa Watanabe, Yoshifumi Sakuragi, Yoshiki Amamoto, and Yoshikazu Nakamura Department of Materials Science and Engineering, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239, Japan (Received 4 June 1997; accepted 4 October 1997)
Aluminum nitride (AlN) thin films have been prepared by the ion-beam assisted deposition (IBAD) method, and the influence of exposure to different atmosphere on optical transmittance and surface morphology has been studied. AlN films have been prepared with the nitrogen ion beam energy of 0.1, 0.2, or 1.5 keV. Synthesized films have been exposed to the following conditions: (i) laboratory air (RT and 40–60% RH), (ii) saturated humidity air (RT and 80–90% RH), and (iii) elevated temperature air (100 ±C and 10– 20% RH). Optical transmission spectrum in the wavelength region from 190 to 2200 nm has been measured by a UV-visible spectrometer every week. Surface morphology of the films has been observed with an optical microscope (OM), and phase identification has been performed by thin film x-ray diffraction (TFXRD). The optical transmittance has not changed drastically after exposure both to the laboratory air and the saturated humidity air for 60 weeks and after exposure to the elevated temperature air for 48 weeks. Observations by OM showed that round-shaped substances were formed on the film surfaces after exposure to the atmosphere, and the size of the substances on the film surface exposed to saturated humidity air is much larger than those on the surface exposed to other atmosphere. The results of TFXRD revealed that the AlN diffraction peaks have gradually decreased with exposure time, but any new phase due to reaction products has not been detected for the samples exposed to the laboratory air, the saturated humidity air, or the elevated temperature air. From the present results, it is concluded that the IBAD AlN films can be applied in low humidity air without losing high transparency up to 60 weeks, and the films prepared with 1.5 keV ion beam show better durability than the films prepared with 0.1 or 0.2 keV ion beam for exposure to the saturated humidity air.
I. INTRODUCTION
Aluminum nitride (AlN) possesses some outstanding properties, such as high hardness (18–24 GPa),1 high thermal conductivity (200 WymK),2 and high electrical resistivity (1011 –1013 Vm).3 In addition, the close value of thermal expansion coefficient to that of Si (4– 5 3 1026 K21 )3 makes AlN an attractive material in the microelectronics field. Owing to these unique properties, AlN films have been reported as an excellent candidate for (i) silicon-on-insulator (SOI) materials,4 (ii) metalinsulator-semiconductor (MIS) materials,5 and (iii) cap layers for GaAs substrate.5 Furthermore, AlN films are highly transparent in the visible wavelength region, and thus they have potential application for optical coating materials.6 AlN thin films have been
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