HW#5-2023

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University of Michigan *

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414

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Mechanical Engineering

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Dec 6, 2023

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1 EECS 414 Introduction to MEMS Fall 2023 Reading Assignments ● Class Handouts and Notes, “Bulk Micromachining”, “Surface Micromachining”, “Electroplating Processes”, and “Materials” Homework #5 Total: 220 Points Handed Out: Thursday Sept. 28, 2023 Due: Thursday Oct. 5, 2023 @ 9 pm 1. Which one of the following wafer bonding techniques would require the application of an electric field/voltage to enable the bond: (circle all that apply): 5 points a) Si-Glass anodic bonding b) Wafer bonding using Parylene c) Silicon to silicon bonding using an intermediate evaporated glass film d) Fusion bonding e) Thermocompression bonding f) Eutectic bonding 2. Anodic bonding of a 7740 Pyrex glass wafer to a silicon wafer is accomplished because of which of the following actions during the bonding process (circle all that apply): 5 points a) Electric field uncovers oxygen ions at the surface b) The raised temperature will make the glass soft c) The raised temperature will make the glass conductive d) The high voltage makes the silicon conductive e) The high voltage attracts the two wafers together f) The raised temperature helps oxidize the silicon 3. A surface micromachined structure is made of polycrystalline silicon deposited using APCVD. Which of the following materials could have been used as a sacrificial layer when making this structure (circle all that apply): 5 Points a) Polycrystalline silicon b) Sputtered silicon dioxide c) Thermally evaporated aluminum d) Silicon nitride e) Ion beam deposited gold 4. Select any of the following features that makes surface micromachining a useful process for making MEMS (circle all that apply): 5 Points a) Can be especially used to make very thick and structures b) Features not dependent on the wafer crystal orientation c) It is a high temperature process

2 d) Can be used to make multi-layer structures e) Structures can be made of only a few materials 5. DRIE produces very high aspect ratio structures by utilizing (circle all that apply): 5 points a) A concentration-dependent etch rate b) An orientation dependent etch rate c) A voltage-dependent etch rate d) A temperature dependent etch rate e) None of the above f) All of the above 6. The trench refill process was explained in class and has been used to build several different devices. It provides the following attractive features, (circle all that apply): 5 points a) It allows the fabrication of sensors and circuits on the same chip b) It is performed at very low temperatures c) It is conformal d) It allows the fabrication of tall and narrow structures e) It allows the fabrication of short and fat structures f) It can be easily combined with electroplating 7. This problem deals with the fabrication of a polysilicon cantilever beam suspended over a silicon substrate. A cantilever beam is like a miniature diving board. The beam is supposed to be 1µm thick, 50µm wide, and 2000µm long. It is attached (i.e., anchored) to the substrate through an anchor region that is approximately 30µm by 30µm. The photolithography process is capable of providing minimum feature size of 10µm, and alignment tolerance of 5µm. a. Describe a process (list all the key process steps with any relevant information) that are required to fabricate such a structure; 25 Points b. Draw the layout and the masks required for this structure. Please explain how many masks and whether they need to be clear or dark field (we use only positive photoresist). 15 points 8. This problem deals with the fabrication of the following structure. This structure was shown in the bulk silicon machining technology slides and shows the 3D cross sectional view of a silicon diaphragm pressure sensor. Please note the following about this structure: - The diaphragm is supported by the thick silicon rim that surrounds it and is bonded to a glass substrate. Note that the cross section is provided to show the structure. The diaphragm is 1mm square and is heavily doped using boron. It is ~10µm thick. - The diaphragm is separated from the glass substrate by an air gap of 2µm. - Opposite the diaphragm is a 0.1µm gold electrode deposited on the glass substate. This electrode is also square.

3 - The photolithography process is capable of a minimum feature size of 10µm and an alignment tolerance between two layers of 10µm. a. Describe a process (list all the key process steps with any relevant information) that are required to fabricate such a structure; 40 Points b. Draw the layout and the masks required for this structure. Please explain how many masks and whether they need to be clear or dark field (we use only positive photoresist). 20 points You need to be very specific about the process steps, and show the layout for the mask that you need to use in this process. Explain the specific technologies you need to create the various layers, the etchants you use, the masking materials, and the orientation of your mask patterns and the alignment of the mask patterns with respect to the <110> flat. 9. This problem deals with the fabrication of the structure whose cross-sectional view is shown in the following figure. It consists of a 15µm thick gold metal line that is supported on and spans the length of a diaphragm consisting of ~10µm thick silicon layer + 1µm silicon dioxide. The diaphragm is suspended over a cavity that is shaped like an inverted pyramid with a square base. The silicon wafer is a standard (100), 500µm-thick wafer with a standard <110> flat. The cavity height is approximately 100µm. Note that the empty cavity is completely sealed by the diaphragm and has no access to the external environment. Please describe how you would fabricate this structure. You need to be very specific about the process steps, and show the layout for the mask that you need to use in this process. Explain the specific technologies you need to create the various layers, the etchants you use, the masking

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