University of California s15

EE 147/247A Prof. Pister Fall 2015 Homework Assignment #11 Due by online submission Monday 11/23/2015 (Tuesday at 9am)

1. You have a lithography setup that allows you to pattern 1um thick photoresist into lines that are 1um wide on 2um centers (1um space between them). The photoresist sidewalls are vertical. Draw the cross-section of each of the following processes, all of which start with a bare wafer covered with 1um of SiO2. A. Deposit 0.1um aluminum by evaporation. Pattern the two lines described above in photoresist on the aluminum, and etch using a vertical sidewall RIE. B. Same as in A, but etched using a liquid isotropic etchant with perfect selectivity. C. Same as in A, but with 1um of aluminum. D. Same as in C, but with an aluminum:oxide selectivity of 10:1, and a 50% overetch. E. Pattern the two lines described above in photoresist on the oxide. Deposit 0.5um aluminum by evaporation. Strip resist and sonicate (put in an ultrasonic cleaner). 2. Describe the difference between a sputter deposition system, a sputter etcher, a plasma etcher, a reactive ion etcher, and a deep reactive ion etcher in terms of A. the substrate electrical potential, and how it is applied B. which kind of gases are used C. whether the surface process is primarily chemical or kinetic, or a combination of the two D. whether the process is conformal/directional (dep), isotropic/anisotropic/selective (etch) 3. In the Bosch Deep RIE process, your friend from Stanford says that the vertical etch profiles are due to really fast reactive ions digging straight into the silicon surface. A. Do you agree? B. Is the silicon etching primarily due to chemical etching, or kinetic? How can you tell from looking at the etched cross-section? C. Would you expect the silicon etch to be very selective to other materials, somewhat selective, or not selective? D. What prevents lateral etch due to reactive neutrals? 4. What would happen in an LPCVD tube being fed with silane and oxygen if the process were run at atmospheric pressure instead of low pressure? 5. Go the Berkeley Nanolab web page, and click on the “Lab Manual” link. A. Of the seven sputtering systems, the Randex has the widest variety of targets. A.i. How many different metal and dielectric targets do we have? A.ii. You’ll notice that for many tools the longest section of the user manual is on the vacuum system, often with arcane units. For the Randex the pressure at which the high-vacuum system takes over from the roughing pump is given in section 9.2 in microns of mercury in honor of Evangelista Torricelli. 1 mm of mercury is 1 Torr. At approximately what pressure does the high-vac pump take over, and how low does it need to go before the rest of the process can run? B. Technics C is a simple plasma etcher. If you etch 1000 Anstroms of silicon nitride and want to stop on oxide, how much oxide will be etched if you do a 20% overetch of the nitride? What if the nitride were on bare silicon (how much overetch of silicon)? C. For the STS-2 etcher, section 7.4 shows the cross-section of two different features on the same wafer, both etched for 100 cycles using the DEEP SILICON 1 recipe. C.i. Estimate the etch rates of the two different features. C.ii. Why might they be different? C.iii. How do they compare to the results on page 7 in http://robotics.eecs.berkeley.edu/~pister/147fa14/Resources/BoschProc-STS.pdf 6. Look through the window into the ee143 lab on the 2nd floor of Cory Hall. What thin film systems do you see?