HW#3-F2023

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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, “Review of Standard Microfabrication Technologies”, and “Silicon Etching”. Homework #3 Total: 180 Points Handed Out: Thursday Sept. 14, 2023 Due: Thursday Sept. 21, 2023 @ 9 pm 1. You would like to make a 15-μm thick silicon diaphragm using p+ silicon etch stop. What combination of processes would you choose among the following? 5 Points a) A short-time predep using a solid boron source followed by a long drive-in b) A long-time predep followed by a short drive-in c) A long-time predep from a liquid source (POCl3, phosphorus oxychloride) followed by a short drive-in d) Ion implantation of BF2 atoms with a high dose followed by a rapid thermal annealing e) None of the above 2. In DRIE (Deep Reactive Ion Etching), select all the correct options: 5 Points a) The etch rate is temperature dependent b) High aspect ratio etching can be maintained all the way through the wafer. c) The sidewall of etch profile results in scalloping due to repeated polymer deposition and etching for sidewall protection. d) Photoresist can be used as a masking material . 3. Please calculate the angle between a (111) plane and a (100) plane. Show your work. 10 Points 4. In silicon wet etching, select all the correct answers: 5 Points a) The wet etch process is always anisotropic. b) The etch rate of (100) plane is faster because of higher concentration of silicon in this plane. c) The etch rate of (111) plane is slower than that of other planes in some wet etchants. d) The etch rate is increased if the etch solution is heated. 5. Dry plasma etching often does not provide as good a selectivity as standard wet etching technique. However, we often use dry etching in many applications. Name at least three reasons why? 5 points

2 6. Can a single-crystal silicon layer be grown/deposited on top of a silicon oxide layer? Explain why or why not? 5 points 7. Which of the following chemicals are routinely used to etch silicon through specific crystal planes (circle all that apply): 5 points a) BHF b) KOH c) TMAH d) HCl e) Hot phosphoric acid f) HF+HNO 3 +Acitic 8. Does BHF etch silicon or polysilicon? 5 points 9. This problem has 6 parts (a-f) and deals with the processing of a (100) silicon wafer that is 500µm thick and is covered with silicon nitride all around. The nitride is removed from the circle area (diameter of 2µm) shown in the middle of the wafer. The wafer is now subjected to a number of processing steps as described below, and you are asked to draw the cross section of the device along the A-A’ line after each of the process steps. I only need the cross section and not the top view , but if you want you can also provide the top view. You should show any specific dimensions or angles that you think are important for each step . 55 Points Total <110> (100) Si Wafer Silicon Nitride y x (100) Si Wafer 2µm A A’

3 a) The wafer is first etched in a mixture of HF-Nitric-Acetic (HNA, 1:8:3) for 25 minutes. The etch rate is about 10μm/minute in this etchant. The silicon nitride mask does not get etched during this process. Draw the cross section of the wafer along A-A’ after this step. Please provide a very short description of why the cross section looks as it does. I just want you to show how you arrived at this answer. Please show all the important dimensions and angles that you think I need to know. 10 points b) The wafer is now etched from the front side in DRIE for 100 minutes, with an etch rate of 3μm/minute. The nitride mask is not etched in this step. Draw the cross section of the wafer along A-A’ after this step. Please provide a very short description of why the cross section looks as it does. I just want you to show how you arrived at this answer. Please show all the important dimensions and angles that you think I need to know. 10 points c) The wafer is now cleaned and then subjected to an ion implantation and annealing step that will create a boron doped region of concentration 5x10 20 cm -3 in some areas. The nitride mask will step boron from getting into silicon wherever there is a nitride mask. Draw the cross section of the wafer along A-A’ after this step (please make sure that the boron- doped region is also highlighted). Please provide a very short description of why the cross section looks as it does. I just want you to show how you arrived at this answer. Please show all the important dimensions and angles that you think I need to know. 10 points d) The wafer is now cleaned and placed in an LPCVD furnace and a 2μm thick layer of silicon oxide is deposited on it. Draw the cross section of the wafer along A-A’ after this step. Please provide a very short description of why the cross section looks as it does. I just want you to show how you arrived at this answer. Please show all the important dimensions and angles. 5 points e) The wafer is now etched for 30 minutes in BHF which etches the silicon oxide at a rate of 0.1μm/minute. Assume the BHF etch does not attack either the silicon nitride or the silicon. Draw the cross section of the wafer along A-A’ after this step. Please provide a very short description of why the cross section looks as it does. I just want you to show how you arrived at this answer. Please show all the important dimensions and angles. 10 points f) The silicon nitride layer is now removed from all of the wafer surfaces, and the wafer is etched in EDP for a long time. Draw the cross section of any parts remaining along A-A’ after this step. Please provide a very short description of why the cross section looks as it does. I just want you to show how you arrived at this answer. Please show all the important dimensions and angles. 10 points

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4 10. This problem deals with the device whose cross section is shown in the following figure. You are asked to draw the layout (meaning the pattern of the masks needed to realize this device), and to show the main process steps required to make this device. 50 points total a- First, assume that in the device cross section shown below the patterns, whose cross section you see, are all squares. The dimensions for each square side are shown on the figure. Assume that this process requires an alignment tolerance of 5µm. Please show how many masks are required for this device. Note that I have also shown the thickness for each layer. 10 points b- Show the layout (shape of the patterns) for this complete device. Please show the layout as in the example that I had shown at the end of the Microfabrication slides for the simple MOS transistor. Clearly label each masking layer and show any dimensions and also show whether each mask layer is clear field or dark field (assume that you use positive photoresist for all photolithographic process steps). 20 points c- Please provide the major process steps that are required to fabricate this device. For example, state which technique you use to deposit the nitride. You can again use the example at the end of Microfabrication section as a guide when you write the process sequence. 20 points 11. This problem deals with the sensor structure whose cross-section is shown below. The processing of this sensor is complete except for the last etching steps. The various layers used in this device are labeled and are distinguished with different shades. The polysilicon material used is moderately doped with phosphorus. The contact holes which provide contact between the various metal layers and the underlying polysilicon layers that have a square shape and are generally smaller than the layers they contact below them. This means that for example the polysilicon layer below the contact holes is a square. In fact, you can assume that all the features whose cross section is shown in this figure are squares when you look at them from

5 the top of the wafer. The side of this square is aligned along the [110] direction. The wafer is a (100) lightly-doped silicon wafer. 30 points total The wafer is now passed on to an inexperienced individual who will have to complete the processing of the wafer, and decides to expose the wafer to the following etchants. Answer the following questions. a- In order to finish the processing of this sensor, the wafer is first etched in a solution of EDP for about an hour. Explain what happens to the wafer after this step is completed? 5 points b- After this EDP etch, the wafer is etched in a solution of BHF for about an hour. Explain what layers are etched after this step is completed? Show the new cross section. 10 points c- After this BHF etch, the wafer is again etched in EDP for several hours. This completes the processing of the device. Draw a cross section of the device after this step and clearly mark all the layers that remain after all of these etching steps are completed. 15 points