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Fall 2004 UNIVERSITY OF CALIFORNIA College of Engineering Department of Electrical Engineering and Computer Sciences
EE143 Midterm Exam #
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Instructions: DO ALL WORK ON EXAM PAGES This is a 90-minute exam (4 sheets of HANDWRITTEN notes allowed)
Grading:
- To obtain full credit, show correct units and algebraic sign in
answers. Numerical answers which are orders of magnitude off
will receive no partial credit.
- For answers requiring explanations, adding sketches can be very
effective.
Problem 1 (30 points)________________
Problem 2 (30 points)_______________
Problem 3 (40 points) ________________
TOTAL (100 points) __________________
Problem 1 Processing Steps and Simple Process Sequence (30 points total)
In class, we discussed a simple process flow to fabricate a hinge plate which can create an out-of-plane motion.
(i ) (5 points) Starting with a blanket Si wafer, how many photolithography steps are used to fabricate this device. For each lithography step used, briefly describe its purpose.
(ii ) (5 points) How many chemical vapor deposition (CVD) steps are used to fabricate this device. For each CVD step used, briefly describe its purpose.
(iii ) (5 points) How many thin-film etching steps are used to fabricate this device. For each etching step used, briefly describe its purpose.
Cross-sectional viewsCross-sectional viewsCross-sectional views
Problem 2 Thermal oxidation (30 points total)
(a ) You are given the following oxidation data for (100) Si : (1) B and B/A values for dry oxidation; and (2) the oxide thickness chart versus oxidation time for wet oxidation (initial oxide thickness = 0).
(i) (10 points) Starting with a blanket (100) Si wafer, how long does it take to grow a 0.2μm-thick SiO 2 with dry-oxygen at 1100 oC?
(ii) (10 points) The oxidized wafer in part (i) (with 0.2 μm oxide) is put back into furnace for wet oxidation at 1000 oC. How long will it take to grow an additional 0.3μm oxide?
(b)(5 points) The Deal-Grove model for thermal oxidation will underestimate the oxide growth rate for thin oxides. How does one modify the model to describe the faster initial growth rate?
(c) (5 points) By adding a few percent of HCl to oxygen will increase the oxidation rate. Explain.
Oxide thickness in
μ
m
B (μm^2 /hr) B/A (μm/hr)
Oxidation
Temp ( o^ C)
B (μm^2 /hr) B/A (μm/hr)
Oxidation
Temp ( o^ C)
Rate constants for dry oxidation
<100> Si
Oxide thickness in
μ
m
Oxide thickness in
μ
m
B (μm^2 /hr) B/A (μm/hr)
Oxidation
Temp ( o^ C)
B (μm^2 /hr) B/A (μm/hr)
Oxidation
Temp ( o^ C)
Rate constants for dry oxidation
<100> Si
Problem 3 Ion Implantation (40 points total)
The following schematic shows the process sequence to form a planarized SiO 2 trench by etching Si substrate, thermal oxidation, and followed by boron implantation.
(a) (5 points) For a SiO 2 trench thickness of 0.6 μm, calculate the required etched Si depth ∆.
(b) Let us consider the region underneath the SiO2 trench. We will assume the SiO 2 has identical ion stopping properties as Si , and the Si 3 N 4 is infinitely thin. The boron (B+) implantation (dose =2 × 1013 /cm^2 ) was performed such that only 50% of the implanted boron dose is incorporated inside the Si (the remaining dose is inside the SiO2).
(i) (5 points) Find out the required B+^ ion energy.
(ii)(10 points) The n-Si substrate has a background doping concentration NB = 1 × 1015 /cm^3 .Calculate the junction depth xj2 ( Surface of SiO2 is defined as x = 0)
Si etching
Thermal
oxidation
- Boron 0.6 μ m implantation
n-type Si substrate
Si 3 N 4
Boron implantation
n-Si
n-Si
∆ μm
x j
x j
n-Si
depth x
x = 0
SiO 2
SiO 2
Si etching
Thermal
oxidation
- Boron 0.6 μ m implantation
n-type Si substrate
Si 3 N 4
Boron implantation
n-Si
n-Si
∆ μm
x j
x j
n-Si
depth x
x = 0
SiO 2
SiO 2
Information which may be useful 1 μm = 10 -4^ cm = 1000nm = 10^4 Å Electron charge q= 1.6 × 10 -19 coulombs; Boltzmann constant k = 8.62 × 10 -5^ eV/K ni of Si = 3.69 × 1016 × T 3/2^ exp [- 0.605eV/kT] cm-
z
0.
0.
0.
0.
0.
0.
0.
1
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.
erfc(z)
exp(-z^2)
10 100 1000
100
1000
10000
∆ Rp =185.34201 +6.5308 E -0.01745 E^2 +2.098e-5 E^3 -8.884e-9 E^4
Rp =51.051+32.60883 E -0.03837 E
2
+3.758e-5 E
3
-1.433e-8 E
4
∆ Rp
Rp
B
into Si
Proj ected Range & Straggle in Angstrom
Ion Energy E in keV
10 100 1000
10
100
1000
10000
∆ Rp =24.39576+4.93641 E -0.00697 E^2 +5.858e-6 E^3 -2.024e-9 E^4
Rp =-7.14745 +12.33417 E +0.00323 E^2 -8.086e-6 E^3 +3.766e-9 E^4
∆ Rp
Rp
P
into Si
Proj ected Range & Straggle in Angstrom
Ion Energy E in keV
