Effect of the Surface Nanomorphology on the Growth of Al Whiskers Formed by Glancing Angle Deposition on a High Temperat
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Effect of the Surface Nanomorphology on the Growth of Al Whiskers Formed by Glancing Angle Deposition on a High Temperature Substrate Kenji Hamachi, Motofumi Suzuki, Kaoru Nakajima, and Kenji Kimura Department of Micro Engineering, Kyoto University, Kyoto, 606-8501, Japan ABSTRACT We have investigated the effect of the substrate-surface morphology on the growth of Al whiskers grown by high temperature glancing angle deposition (HT-GLAD). Before the HT-GLAD of Al at 390 °C, the morphology of the substrate was systematically modified by depositing nanocolumnar SiO2 layer of thickness between 0 and 100 nm on the flat SiO2 layer. Aluminum whiskers with the width of ≈ 100 nm and the length ≤ 8 µm are found on all the samples. The number of short whiskers, which can be grown from very small nuclei, depends strongly on the thickness of the SiO2 nanocolumnar layer and shows the maximum at SiO2 thickness of 20 nm. On the other hand, the number of long whiskers, which requires extraordinary amount of Al than that deposited on the side surface of the whiskers, is almost independent of SiO2 thickness. These facts suggest that the surface roughness of the substrate plays an important role in the nucleation of the whiskers and that there are some transport processes of Al, which are insensitive to the surface morphology. INTRODUCTION
Recently we have found that unusual Al whiskers grown by glancing angle deposition on high temperature substrate (HT-GLAD) [1, 2]. When Al is deposited on heated surface-oxidized Si substrate at a glancing angle, single crystalline Al whiskers with thickness of 30–500 nm and length up to about 10 µm grow. Since the whiskers grow only under the condition that the angle of incidence of the vapor flux is larger than 80°, the oblique incidence of the vapor plays an important role in whisker formation [1]. The critical temperature of the formation of the Al whiskers is between 180 °C and 290 °C [2]. Thus, the HT-GLAD is essential for the formation of whiskers. However, the details of the growth mechanism of whiskers formed by vacuum evaporation technique have not been clear. In the conventional growth model of the whiskers from their vapor phase[3, 4], adatoms diffuse over the whisker sides and are incorporated at the top of the whisker. By assuming that all atoms impinging upon the whiskers are incorporated into whiskers and that their radius does not change during growth, the length of the whiskers l depends exponentially on the amount of the deposition as follows [1, 4], ⎛ 2d tan α ⎞ ⎟, ⎝ πr ⎠
l = l0 exp⎜
(1)
where l0 is a initial length of the nucleus, d is amount of the material deposited on the surface in average thickness, α is a deposition angle measured from surface normal, and r is a radius of the whiskers. Although the growth of many whiskers grown by HT-GLAD can be understood based on this simple model [1], a also significant number of whiskers much longer than the predicted are also observed. This suggests that alternative transport processes such as extremely enhanced surf
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