University of California s6

EE 147/247A Prof. Pister Fall 2015 Homework Assignment #12 Due by online submission Wednesday 12/3/2015 (Thursday at 9am)

1. The polyMUMPS design guide recommends an unusual combination of three masks that can be used to etch through the nitride layer and expose the silicon substrate. See for example Fouladi et al. [1]. The masks are DIMPLE, P1P2VIA, and ANCHOR2. In a layout editor working with the polyMUMPS process, you create a new layer called “PIT”. The minimum size of a PIT feature is 30um. When you draw a features on layer PIT, what you get is features on three masks in the final layout: DIMPLE is the same size as PIT. P1P2VIA is 7um smaller than PIT in every direction. ANCHOR2 is 5 um smaller in every direction than P1P2VIA (these recommendations are very conservative). Assuming a a 40um square on PIT is the only feature in your layout A. Draw the resulting cross-section assuming perfect selectivity in every etch. B. Draw the resulting cross-section assuming that there is a 5:1 selectivity in all RIE etches, and that POLY2 has the only significant overetch. Do this in two steps: B.i. what does the cross-section look like right after POLY2 deposition? B.ii. what does the cross-section look like after POLY2 etch (see 1.B-iii !) B.iii. During which etch is the nitride breached? 2. Fouladi et al. seem a little confused on the etch rate of polysilicon in TMAH (which is very similar to KOH). Assuming that the poly etch rate is the same as the (100) etch rate (a good assumption), what would cross-section 1c really look like? 3. Design a piezoresistive force sensitive cantilever using the process in [1]. Estimate the minimum detectable force. You will need to make a lot of design decisions and assumptions! State them clearly. 4. How much does kBT/2 vary over the industrial temperature range, -40 to +85C? 5. Some sensitive experiments are run with their front-end sensors and amplifiers at the temperature of liquid helium (4K). Why? 6. A piezo-resistive accelerometer has a sensitivity of 1mV per gravity. The piezoresistors have a combined effective impedance of 1k. Assuming no other noise sources, what is the noise- equivalent acceleration in a 10kHz bandwidth? 7. Does the power spectral density (in Watts per Hz) of thermal noise in an energy dissipater like a resistor or a viscous damper depend on the resistance value or the damping coefficient? 8. For the resonator that you analyzed in Homework 5, problem 1 A. Calculate the average (RMS) displacement of the spring due to thermal noise. B. Calculate the power spectral density of the thermal noise due to the viscous damping at room temperature. C. Calculate the average thermal noise force on the resonator, FN. in a 1Hz bandwidth. D. What is the average displacement due to thermal noise in a 1Hz bandwidth i) near DC; ii) at resonance; iii) at 10 times the resonant frequency E. Compare your answers in D to A. Are they consistent? 9. Stiction is a problem during release etching (specifically, drying the wafer after a wet release and rinse), as well as during normal device operation when surfaces come into contact. Which of these processes helps prevent stiction during release, operation, or both? Why? A. CO2 critical point drying (also used in decaffeinating coffee, by the way) B. dimples C. XeF2 sacrificial etch D. OTS or DDMS self-assembled monolayer E. vapor HF release etch [1] Fouladi, Bakri-Kassem, Mansour, “Suspended on-chip RF MEMS components fabricated using PolyMUMPS technology”, Can. J. Elect. Comput. Eng. V31N2, 2006.