Mechanical Properties of AlN Thin Films Prepared by Ion Beam Assisted Deposition
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Mechanical Properties of AlN Thin Films Prepared by Ion Beam Assisted Deposition Shuichi Miyabe, Toshiyuki Okawa, Nobuaki Kitazawa, Yoshihisa Watanabe and Yoshikazu Nakamura National Defense Academy, Department of Materials Science and Engineering, Yokosuka, Kanagawa 239-8686, Japan ABSTRACT
Aluminum nitride (AlN) thin films were prepared by ion-beam assisted deposition method, and the influence of the nitrogen ion beam energy on their microstructure and mechanical properties was studied by changing the ion beam energy from 0.1 to 1.5 keV. Films prepared with a low-energy ion beam show a columnar structure, while films prepared with a high-energy ion beam show a granular structure. The film hardness is found to decrease with increasing nitrogen ion beam energy. It is also found that the film hardness does not change drastically after annealing in nitrogen atmosphere at 500 °C, yielding the residual stress relaxation. It is proposed that the film hardness is dependent on the film microstructure, which can be controlled with the nitrogen ion beam energy, rather than the residual stress in the films. INTRODUCTION Ion beam assisted deposition (IBAD) is a film deposition technique combining evaporation of metals and simultaneous irradiation with an ion beam. The IBAD method has the advantages of low temperature synthesis due to using an energetic ion beam and well defined and independent control of ion bombardment parameters, such as ion beam current, ion beam energy, ion species, and incident angle to substrates [1]. Taking the advantages mentioned above, many researchers have tried to prepare thin films with desired microstructure and properties suitable for their purposes [1]. The present authors’ group has studied microstructure, surface morphology and optical properties of aluminum nitride (AlN) films prepared with IBAD [2-6]. AlN films have attracted attention because they have potential applications for optical protective coatings, insulating materials in integrated circuits and surface acoustic wave devices [7-9]. From previous works, it has been found that the crystallographic structure changes from the c-axis oriented state to a randomized state in dependence on the energy of the nitrogen ion irradiation [4] and the refractive index decreases with the ion beam energy [5]. These results show that film microstructure depends on the ion beam energy during synthesis and the difference in microstructures affects the film optical properties. Therefore, it is attractive to study the relation between film microstructure and mechanical properties, because AlN films can be applied for hard coating materials. This paper describes the relation between the microstructure and mechanical properties of AlN films prepared with different nitrogen ion beam energies using the IBAD method.
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EXPERIMENTAL
Detailed descriptions about film preparation with the IBAD method have been given in the literature [2-6]. The most important deposition parameters can be summarized as follows; (1) raw materials, 99.999% pure nitro
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