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Unformatted text preview: 1 EECS 414 Introduction to MEMS Fall 2007 Reading Assignments Class Handouts and Notes, Bulk Micromachining, Surface Micromachining, and Wafer Bonding. Homework #5 Total: 240 Points Handed Out: Friday October 5, 2007 Due: Friday October 12, 2007 1. This question deals with DRIE. a) Explain what scalloping is using a diagram and description. b) Explain what DRIE lag is and why it happens. c) Why does DRIE provide such a large aspect-ratio etching capability? Briefly explain how this works? d) Is DRIE concentration-dependent? Justify briefly. 10 points Solution: a) Scalloping is what happens on the sidewalls of trenches etched using DRIE. The diagram on the left below shows such a trench with a close-up of the scalloping. The diagrams on the right show in 6 steps how scalloping occurs: 1. Start with a masked wafer 2. The polymer step deposits polymer on all surfaces 3. The etch step is an anisotropic etch process in RIE that mostly etches the bottom of the trench, and the uncovered lower sidewalls. 4. The polymer step again deposits polymer on all surfaces 5. The etch step makes the trench deeper, and also etches the new lower part of the sidewalls. Since the previous lower sidewalls are protected by the polymer, they are not etched anymore. Therefore, another rounded part of the sidewalls is formed. 6. The third polymer step just continues the sequence, depositing a layer of polymer. 2 b) DRIE lag is the reduction in trench depth as the trench opening gets smaller, for the same amount of etch and polymer steps. The diagram below shows four different depths due to four different opening sizes. DRIE lag occurs because a smaller opening reduces the amount of etch gas that can reach the bottom of the trench, resulting in a lower etch rate. c) A two-step process is used. In the first step a polymer is deposited to protect the sidewalls, and in the next step the Si at the bottom of a trench is etched and the sidewall is protected because of the directional nature of the RIE etch. This process continues many times to get the very deep and directional etches we see. d) The etch rate does change, slightly, as a function of doping, but very little. So, for all practical purposes DRIE is not concentration dependent and does not provide a good etch stop. 2. The (111) planes in a silicon wafer etch much slower than the (110) and (100) planes when exposed to wet etchants that exhibit orientation dependence. Briefly explain why? 5 points The atoms on the (111) surface are bound to three other Si atoms below, and only to one above them, so it is much harder to etch these atoms than those on the (100) surface because these atoms are typically bound below to 2 and above to 2 other atoms....
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