HW_4-2007-Solutions - EECS 414 Introduction to MEMS Reading...

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HW#4 1 9/29/07 EECS 414 Introduction to MEMS Fall 2007 Reading Assignments Class Handouts and Notes: “Silicon Wet Etching”, “Wafer Bonding”, “Bulk Si Micromachining”, and “Surface Micromachining.” Homework #4 Total: 350 Points Handed Out: Friday Sept. 28, 2007 Due: Friday Oct. 5, 2007 1. This problem deals with wet etching a patterned silicon wafer shown in the figure below. A (100) silicon wafer with a thickness of 400 µ m is oxidized and the oxide is patterned on the front side with the specific pattern shown in the top view. The cross sectional view of the wafer is also shown (this is a diagonal cross sectional view); please note that the cross sectional view does not show any oxide on the sides and the bottom of the wafer since these will not be involved in this etching problem. The wafer is now dropped in EDP and etched. Please assume the following for the rest of this problem: The etch rate of the {100} planes in this EDP etchant is 1 µ m/minute . The etchant does not attack/etch the {111} planes at all, and etches all the other planes at the same rate as {100} planes. The EDP etchant does not attack the masking oxide at all. Please answer the following parts for this problem: Part a: The wafer is etched in EDP for 20 minutes (assume that only the top is exposed to the etchant and that the sides and the bottom are not etching). Please show (draw) ONLY the top view of the wafer after 20 minutes of etching. Assume that the oxide layer is transparent and that you can see through this layer. In answering this part, please make sure that all the planes are labeled and any relevant dimensions or angles are clearly shown on the figure that you draw. 30 points Part b: The wafer is now etched for an additional 115 minutes (this brings up the total etch time to 135 minutes including that in part a) in EDP. Please show (draw) the top view ONLY of the wafer after this second etching step. Assume that the oxide layer is transparent and that you can see through this layer. In answering this part, please make sure that all the planes are labeled and any relevant dimensions or angles are clearly shown on the figure that you draw. 20 points
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HW#4 2 9/29/07
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HW#4 3 9/29/07 Part a Solutions: In this part, the etch proceeds for only 20 minutes. The 45° edges of the opening aligned 45* to the flat are along the <100> and therefore the etch will proceed under the mask opening along these edges and will undercut for 20 µ m as shown. The other four edges of the opening which were parallel or perpendicular to the <110> flat are along <110> directions and therefore as soon as the etch starts the four {111} planes along these four edges get exposed. As the etch proceeds, these four planes limit the etch to the sides, as shown. Note that the bottom of the etched opening is flat and looks as shown.
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HW#4 4 9/29/07 Part b Solutions: In this part, we etch for 135 minutes total (including part a), which means that the square formed by the four {111} planes will get etched out under the masking oxide. However, we do not quite form the inverted pyramid since to do this we need to etch deeper. Therefore, the bottom of the
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HW_4-2007-Solutions - EECS 414 Introduction to MEMS Reading...

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