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Unformatted text preview: UNIVERSITY OF CALIFORNIA SANTA BARBARA DEPARTMENT OF CHEMICAL ENGINEERING CHE 142 and ChE 242: Microelectronics Processing Spring 2003 Homework # 5 (Due May 21, 2003) 1. Consider the growth of ZnO posts such as the one shown in figure below. These posts grow in hexagonal form because ZnO crystal structure, wurtzite, has hexagonal symmetry. The post shown below is approximately 300 nm in diameter. Assume that the gas temperature and the substrate temperature are 550 o C. Each post grows mainly by addition of material to its top. (a) If the posts grow at a rate of 1 m/hour and the growth precursor (Zn) has unity sticking probability estimate the partial pressure of the growth precursor in the gas phase. (b) On average, how often (in s-1 ) does a precursor impinge on this surface? This is called the impingement frequency. (c) Consider the observation that the top of this column is very smooth. This could be because the precursor is very mobile on the surface. This observation leads to the conclusion that the diffusion length in between precursor impingement is at least approximately the column diameter perhaps larger. (Think about this statement.. Why should this be the case?) Estimate the activation energy for diffusion of Zn on the surface. 2. Consider the CVD reactor we analyzed in Homework #3 shown again in the sketch below....
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