College of Engineering Department of Electrical Engineering and Below are your weekly quizzes. You should print out a copy of the quiz and complete it before your lab section. Bring in the completed quiz to lab the week it is due. Week 3--Active Area Definition Week 4--Gate Oxidation Week 5--Poly deposition--will be given in lab after Microlab tour Week 6--Gate Definition Week 7--Source/drain Week 8--Contact cut Week 9--Metallization Week 10--Metal Definition Week 11--Characterization (I) Week 12 MEMS Characterization Last update 11/9/2001 Usage of this site is subject to the U.C. Computer Usage Policy.
Spring Semester, 2000 Week #3 Quiz--Active Area Definition Name Section Date (1) How does spin speed and time affect photoresist application? (2) What is the purpose of the soft bake (before exposure)? (3) What color are the lights in the lithography room, and why? (4) What chemical do we use to define the features in the photoresist? (5) What chemical do we use to define the features in the oxide layer? (6) What chemical do we use to remove the photoresist after we're all done? (7) Will the dark areas on the mask produce islands of oxide or pits in the surrounding oxide? (8) What might happen if we do not leave the wafers in the oxide etchant long enough? (9) Where do we keep the ointment to treat HF burns? (10) What is the purpose of HMDS? A. Franke 8/31/99
Spring Semester, 1999 Week #4 Quiz--Gate Oxidation Name Section Date (1) What is the purpose of the furnace tube TCA step? (2) Why do we run an additional "control" wafer during the oxidation? (3) How fast is the wafer boat pushed into and pulled out of the furnace? (4) Why is this push/pull rate important? (5) Are we performing a wet or dry oxidation? Why? (6) Why do we do an anneal step after oxidation? (7) What do we use the NanoSpec for, and how does it work (briefly)? (7) What do we use the Four-Point Probe for, and how does it work (briefly)? M. Freed 1/20/99
Fall Semester, 1999 Week #6 Quiz--Gate Definition Name Section Date (1) Assuming perfect alignment, please draw a top view of what the overlapping alignment markers will look like for this week's mask and the ACTIVE mask? (2) Order the following steps for this weeks lab(1 for the first step, through 5 for last step): ( ) Etch the oxide ( ) Expose the photoresist ( ) Strip the photoresist ( ) Develop the photoresist ( ) Etch the polysilicon (3) Which of these is what a normal wet-etched film looks like (e.g. an oxide film after a BHF etch)? (top layer=photoresist middle layer=oxide bulk=silicon) ( ) ( ) ( ) (4) What can be done to help all the 2-micron gates survive the steps we will do in the lab this week? (5) Why do exposure times vary with substrate material?
(6) What do we etch the polysilicon with? What is the nominal polysilicon etch rate? Using the nominal etch rate above, assuming 3500Å of poly, and using a 10% overetch, how long should we etch the poly? (7) Why do we do an oxide etch after the polysilicon etch? (8) Show how one of your MOSFETs will look after this week's lab (draw cross section)?
Fall Semester, 1999 Week #7 Quiz--Source/Drain Name Section Date (1) What is the purpose of the spin-on glass (SOG)? (2) Why is the timing of the pre-diffusion step important? (3) What is the dopant atom we are depositing in the source/drain regions? (4) Why do we use a wet oxidation for the intermediate oxide? (5) What is the purpose of the N 2 anneal? (6) If the polysilicon was only 100nm thick instead of 350nm what problems might this create? (7) What are the advantages of using ion implantation instead of spin-on-glass to dope the source and drain? What are some of the disadvantages? (8) After the deposition, when we use the four-point probe, are we measuring sheet resistance or bulk resistivity? (9) Draw a cross-sectional view of what the diffused source and drain regions will look like after this week's lab for a simple transistor (MOSFET), including all other layers and the effects of isotropic etching. Is this a self-aligned process? Explain. (2 points)
Fall Semester, 1999 Week #8 Quiz--Contact Cut Name Section Date (1) What layer will we be cutting through today? (2) What etchant (and what concentration) will we be using? What is the nominal etch-rate? Assuming 1400 Å of the layer and using a 10% overetch, how long (in minutes and seconds) would you need to etch? (3) What are the potential problems if we overetch or underetch? (4) What is the purpose of the NH 4 F in the BHF (i.e. why is there NH 4 F in BHF instead of H 2 O)? (5) Is today's mask (CONT) a bright-field or dark-field mask? (6) What will you be aligning this week's mask to, the ACTV or POLY alignment markers? If we're off by several microns in alignment, how will this affect the performance of your MOSFETs? (7) If the wet etch is done for twice as long as needed, how will this affect the performance of your MOSFETs? (8) Draw the cross-sectional view, and top down view of a MOSFET before and after today's processing (3 points).
Fall Semester, 1999 Week #9 Quiz--Metallization Name Section Date (1) Draw a schematic diagram of the vacuum system, in the detail given in the lab manual. (label all components) (2) Why do we need the "roughing" or mechanical pump? Briefly describe how they work. (3) At what pressure do we switch to the diffusion pump (i.e. at what pressure has "roughing" been completed)? What could happen if we continue to pump using the mechanical pump at this pressure? (4) What does the oil in the diffusion pump do? What is the danger inherent in the diffusion pump oil? What is the purpose of the cooled baffle? (5) What is a cold trap and how does it "pump"? (6) What is the purpose of the HF dip just prior to metallization? (7) Why is low pressure so important for aluminum deposition?
(8) Why do we heat the charges at 40 Amps for ~20 seconds before we evaporate them? (9) Will the metal layer be conformal? Why or why not?
Fall Semester, 1999 Week #10 Quiz--Metal Definition Name Section Date (1) At what temperature is the aluminum etchant kept? Why can't the aluminum etchant be used at room temperature? (2) Why must the wafer be kept moving during the etch? (3) What is the nominal etch rate of the aluminum etchant? Assuming 8000Å of aluminum and using a 10% overetch, how long should we etch? (4) Why does the lithography step require a lower exposure time this week? (5) Why do we do the sintering step? (6) What will happen if the sintering temperature is too high or too low? (7) What is "spiking" and how can it be prevented? (8) What problems might have occurred if we used gold instead of aluminum for our contacts? (9) Why don't we have to use a slow push/pull for the sinter?
Fall Semester, 1999 Week #11 Quiz--Characterization Name Section Date (1) What is Metrics and what is it used for? (2) What is a HP-4145 and what can it measure? (3) How many terminals will be connected to the diodes, capacitors, MOSFETs? What are these terminals for the MOSFET (i.e. the common names)? (4) What will we use to measure C-V? Is this tool available at all probe stations? (5) How many circuits (not devices) are there on your test chip? What are they? (6) How can we sometimes measure a two terminal device with only one probe tip? (hint: these are often vertical devices) (7) Give a brief description of a wafer probe station with schematic diagram.
Fall Semester, 2001 Week #12 Quiz MEMS Processing & Characterization Name Section Date (1) Unlike MOSFET fabrication where we removed PR of METL layer to prepare for sintering, we leave the resist to stay on Al layer for MEMS fabrication. Why is this the case? (2) Etch rate of XeF 2 for <100> single-crystal silicon varies from 2400 to 2900 A/min. To release our cantilevers with 100 micron in their width, what is the shortest expected release time? We assume that XeF 2 etch is perfectly isotropic, and is also perfect in selectivity between silicon and silicon dioxide. (3) If your wafer is not dehydrated enough before you bring it into the XeF 2 chamber, what kind of undesired byproduct is possibly created? (Hint: Recall the etch chemistry of XeF 2, especially when it reacts with moisture, i.e., H 2 O.) (4) What would happen if native oxide film was left on the wafers as it went into the XeF 2 etching step? (5) Once the bimorph is released, is the cantilever bent towards upward or downward? (Thermal expansion coefficients of Al and oxide are 6.5 and 0.55 10-6 /C, respectively.) (6) To measure voltage and current characteristics of the oxide-metal bimorph as it undergoes deflection, which equipment(s) do you need to have?