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C. Nguyen March 18, 2010
INSTRUCTIONS
- Read all of the instructions and all of the questions before beginning the exam.
- There are 4 problems on this midterm exam, totaling 100 points. The tentative credit for each part is given to help you allocate your time accordingly. You have a total of 1 hour and 30 minutes to finish this exam. Be careful not to spend all your time on any one part.
- This is a closed book exam, except you can have one two-sided 8.5″×11″ sheet of notes.
- Unless otherwise noted on a particular problem, you must show your work (in the space pro- vided plus the backs of the pages) for all problems to receive full credit; simply providing an- swers will result in only partial or no credit, even if the answers are correct. If you require extra space beyond what is provided, be sure to turn in any material that is required to support your solutions. Note that there are extra pages at the end of this exam. Do not use any attached pages until you have exhausted the pages contained in this exam.
- Turn in the entire exam, including this cover sheet.
- Put your name on every page of this exam, as well as on any additional material that you submit.
- Be sure to provide units where necessary.
Signature : ___________________________________________
Problem 1: / 29 points
Problem 2: / 33 points
Problem 3: / 20 points
Problem 4: / 18 points
Total: / 100 points
C. Nguyen March 18, 2010
Problem 1.
Please provide short written answers to the questions that follow.
(a) Would you expect the resolution of lithography over a metal layer to be better or worse than over a silicon dioxide layer? Why?
(b) Would you expect oxidation to be faster or slower over <111> silicon when compared with <100> silicon? Why?
(c) You’ve just done a standard photoresist spin & bake, lithography, and etch step using a per- fectly anisotropic RIE. However, the sidewalls of the etched film are not straight. What is most likely causing this? What can you do to fix this in your next fabrication run?
Total 29 points
EE 143
C. Nguy
Problem
T photoresi etch is 10 and that t the struct and (c) cross-sec wafer sur
en
m 2. The cross-se ist pattern on 00% anisotro the etch rate ture to the le 11 minutes ction. Also, f rfaces were b
Total ction of a n top is show opic; that the e of silicon ( eft of the do of etching. for each cas before any e
l 33 points
MIDTERM
March 18
polysilicon- wn. The patt e selectivity and polysilic tted line afte Be sure to se use dotted etching.
M EXAM
-oxide-silico tern will be of silicon (a con) is 0.1 μ er (a) 2 minu o specify re d lines to ind
NAME __
on substrate etched via R and polysilic μm/min. Car utes of etchi elevant angle dicate where
___________
sandwich RIE. You can con):SiO 2 :ph refully draw ing; (b) 5 mi es and dim e the origina
Mo
__________
structure w n assume tha hotoresist = 2 cross-sectio inutes of etc ensions for al photoresis
ore work sp
_____
with a at the 2:1:1; ons of ching; each st and
ace next page.
C. Nguyen March 18, 2010
Problem 2. (continued)
More work space next page.
C. Nguyen March 18, 2010
Problem 3.
The structure shown below has gone through the following process steps: i) Start with a bare 〈 100 〉 silicon wafer. ii) Grow 300 nm of oxide. iii) Deposit 100 nm of Si 3 N 4. iv) Lithography: Mask 1. v) Etch 100 nm of Si 3 N 4 and m nm of SiO 2. vi) Remove PR. vii) Wet oxidation @ 1000o^ C for t minutes. Determine t and m.
Total 20 points
SiO 2
Si 3 N (^4)
0.3 (^) μm
0.05 (^) μm
0.2 μm Silicon
More work space next page.
C. Nguyen March 18, 2010
Problem 3. (continued)
C. Nguyen March 18, 2010
Problem 4. (continued) xvii) Oxidize a bit (10nm) and anneal at 1050o^ C to activate dopants and drive-in diffusion. xviii) LPCVD PSG: target = 1μm. xix) Reflow PSG (& a little bit of diffusion) at 950o^ C. xx) Lithography: Mask III (contact hole). xxi) Dry/wet etch SiO 2 down to n+ S/D regions. xxii) Sputter Al: target = 500nm. xxiii) Lithography: Mask IV (metal). xxiv) Dry etch Al to clear the field areas. Here’s where things start to differ from standard LOCOS-isolated NMOS. xxv) Lithography: Mask V (release). xxvi) Dip in hydrofluoric acid for 10 minutes. xxvii) Remove PR.
Instructions: Draw the cross-section along the A-A′ axis at the end of this process, giving important vertical and lateral dimensions.
More work space next page.
C. Nguyen March 18, 2010
Extra Page.
