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2. Microelectronic devices are fabricated by forming layers of thin films onto a scon wafer. FIGURE Q2 shows a description of chemical vapour deposition to form slicone thin film The chemical action is occurred on the surface of im and given as below The surface reaction is camied out at 100 Pa at 900 K and the distance from the difuser to the im surface is 6 cm SiH, vapour and Ha ges mixture To vacuum SIH Ha Heated plate FIGURE 02 a. State TWO (2) more assumptions for above system 4 marks] b. Formulate the mass flux distribution for SiHs using shell balance echnique 8 marks c In a production of circular silicon wafer, two dfferent dameters are manufactured which are 12 and 8 am, respectively. Evaluate the initial depostion rate of Si in g/min for circular wafer of both diameters. The initial gas mixture at steady state contans 20 m00% of SiHa Given; Molecular weight Si 28 g/molS 32 g/mol Difusivity of StH4 in miture 4.06 10 cms Total molar concentration, c 4.188 10molicm 8 marks a) How long does it take for the layer of solid substance to be dssolved completely? [5 marks
engineering   Chemical-Engineering   
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Microckctronic devices ure fabricated by foming many layers of tlin films ontoa silicon waser. Each film has uniquc cheical and electrical properties. For example, a thin film of solid crystalline silicon (Si), vhen doped with the appropriate elements-for example, boron or silicon has semiconductor properties. Silicon thin films are commonly fomed by the chemical vapor eposition, or CVD, of silane vapor (SiH4) onto the surface of the wafer. The chemieal reaction is SiH4(g) → Si(s) + 2H2 (g) This surface reaction is usually carried out at very low pressure (100 Pa) and high temperature (900 K). In many CVD reactors, the gas phase over the Si film is not mixed. Furthermore, at high temperatures, the surface reaction is very rapid Consequently the molecular dffusion of the SiH, vapor to the surface often controls the rate of Si film formation. Consider the very simphfied CVD ractor shown in Figure 25.5. A mixture α silane and hydrogen gas flows into the ractor. A diffuser provides a quiescent gis space over the growing Si film. Develop a differential model for this proess, including statements of assumptions and boundary cond tions. SiH, vapor + H2 gas Ditfuser H2(9 Solid Si thin fim Quiescent gas Heated plate Figure 25.5 Chemical vapor deposition of a siicon thin film from silane gas. SH, (g)→ Si (s) + 2 H2 (g) The silane in the feed gas serves as the source for mass transfer, whereas the Si film serves as the sink for silane mss tansfer. In contrast the fomation ofHz at the Si film surface serves as the source for H mass transfer, whereas the feed gas serves as the sink for H2 mass transfer. The physical system possesses rectilinear geometry. and the major assumptions for model developmcnt are listed here. (1) The reaction occurs only at the surface of growing Si thin film. Consequently, there is no homogeneous reaction of silane within the diffusion zone, so that RA 0. In this context, the surface reaction is the sink for silane mass transfer. (2) The gas space in th diffusion zone is not externally mixed, so that molecular diffusion dominates. (3) The feed gas provides silane in high excess relative to that consumed by reaction, so the silane concentration in the gas space at the diffusion-zone boundiry is constant. (4) The flux of silane is one-dimensional along z. as the source and sink for silane mass transfer are aligned at the boundaries along the z direction. (5) The thickness of the Si film is very thin relative to δ, the diffusion path length along the z direction. Therefore, 8 is essentially constant. 6) The mass-transfer process within the diffusion zone is at steady state The assumptions are used to reduce the general fons of the differential cquation for mass trunsfer and Ficksflux equation. The general differential equation for mass transfer in terms of recilinear coordnates is For steady-state one-dimensional flux along the z dircction with no homogencous chemical reaction (RA 0), the gencral differential equation for mass transfer reduces dz which shows tht the flux is constant along the zdircction. As the iffusion flux is with respect to only onc dimension, the partial derivative becomes an ordinary derivative. Ficks flux equation for the one-dimensional flux sikine through a biniry mixture in

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