143f2010-m2-soln

143f2010-m2-soln - Fall 2010 UNIVERSITY OF CALIFORNIA...

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1 Fall 2010 UNIVERSITY OF CALIFORNIA College of Engineering Department of Electrical Engineering and Computer Sciences EE143 Midterm Exam #2 Family Name _______________________ First name_________________SID___________________ Signature________________ Solution________________ Make sure the exam paper has 9 pages total (including cover page) Instructions: DO ALL WORK ON EXAM PAGES This is a 90-minute exam (4 sheets of notes allowed) Grading: Please be concise with your answers. For answers requiring explanation, adding sketches can be very effective. To obtain full credit, show correct units and algebraic sign. Numerical answers orders of magnitude off will receive no partial credit. Problem 1 (25 points)________________ Problem 2 (30 points)_______________ Problem 3 (20 points) ________________ Problem 4 (25 points) ________________ TOTAL (100 points) __________________
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2 Problem 1 Thin-Film Deposition (25 points) (a) Poly-Si is deposited by CVD using SiH4 gas. There is a transition temperature T transition between the mass-transfer limited and surface-reaction limited regimes. (i) (3 points) How will you define the transition temperature using the CVD deposition rate model parameters? Show your reasoning. Grove Model for CVD: Deposition Rate = [1/ (1/ k s +1/ h G )] ( N g / N solid ) with k s (T) = k so exp [ -E A /kT] and h G (T) = D o T 3/2 / P The transition temperature can be defined as the temperature where k s (T) = h G (T) (ii) (4 points) Suppose we maintain all CVD conditions the same except the gas flow velocity is reduced . Sketch a new deposition rate curve versus 1/T in the above figure. [No credit will be given without a brief explanation] EXPLANATION- k s does not change with flow rate and h G decreases with slower flow velocity ( U). (iii) (3 points) Describe the mass depletion problem due to gas inlet in CVD and the approach to minimize this effect. Recation surface close to gas inlet will consume chemical reactants, hence diluting the gas concentration. Reaction surfaces further from the inlet will have a lower deposition rate. One approach to minimize rate nonuniformity is to use a distributed gas inlet system ( e.g. shower heads). (iv) (3 points) Do you think deposition rate of Atomic Layer Deposition (ALD) will depend on the gas flow velocity? Explain why or why not. No. ALD rate is determined by the cycle time for Gas-A absorption/purge/Gas-B absorption/purge. Dependence of absorption time on gas flow velocity ( h G ) is minimal.
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3 (b) Sputtering Deposition Why is sputtering deposition advantageous to prepare compound thin films (e.g. Al-2%Cu) ? Compared with Evaporation (3 points) - Compound evaporation needs separate evaporation sources and precise independent control of each evaporation flux. Sputtering deposition only needs a sputtering target of the Al-2% Cu composition.
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143f2010-m2-soln - Fall 2010 UNIVERSITY OF CALIFORNIA...

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