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Fall 2003 UNIVERSITY OF CALIFORNIA College of Engineering Department of Electrical Engineering and Computer Sciences
EE143 Midterm Exam #
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Signature______________________________________________________________
Make sure the exam paper has 5 pages + 3 pages of data for reference
Instructions: DO ALL WORK ON EXAM PAGES This is a 90-minute exam (4 sheets of 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.
Problem 1 (25 points)________________
Problem 2 (25 points)_______________
Problem 3 (25 points) ________________
Problem 4 (25 points) ________________
TOTAL (100 points) __________________
Problem 1 Simple Transistor structure and Process Sequence (25 points total) Figures below (taken from your Jaeger textbook) show the top view and cross-section of a MOSFET along the line A-A’.
(a) (5 points) Sketch the cross-section of the device along the line B-B’ [Hint: the drain, source , and gate interconnects show in top view are all aluminum]. Label all important features.
(b) (5 points) Sketch the cross-section of the device along the line C-C’. Label all important features.
(c) (10 points) How many lithography steps are needed to fabricate this device? Describe the functions of each masking step.
(d) (5 points) Can the polysilicon gate be replaced with an aluminum gate in this process sequence? Explain why/ why not.
A A’
B
B’
C
C’
A A’
B
B’
C
C’
Problem 3 Ion Implantation (25 points total)
Phosphorus ions (P+) are implanted into a p-type silicon wafer (NB =10 16 /cm^3 ). The Gaussian
concentration profile versus depth x is plotted below.
( i ) (3 points) What is the energy of the P+ ion?
(ii ) (5 points) What is the implantation dose?
(iii ) (7 points) Use the Irvin Curves to calculate the sheet resistance of the implanted layer. Show clearly
the steps you use to derive your answer [ No credit will be given for answers using the 1/ (q μ.φ) approximation]
(v) (5 points) To avoid intentional ion channeling, one usually tilt the Si wafer by 7 degrees. However, a small channeling tail still occurs. Explain the origin of this small channeling tail.
(vi) (5 points) After implantation of dopants, it is typically followed by a 900 oC annealing step. Why do we need this annealing step?
x
C(x)
x=0 0.2um 0.4um
N B = 10 16 /cm^3
C max =?
x
C(x)
x=0 0.2um 0.4um
N B = 10 16 /cm^3
C max =?
Problem 4 Diffusion and Sheet Resistance (25 points total) (a ) (10 points) A boron predeposition diffusion step is performed on a Si wafer uniformly doped with 10 15 As/cm^3
The boron diffusion conditions are : Diffusion Temperature = 950 oC Diffusion time = 600 seconds
Boron diffusion constant = 6.6 x E-15 cm^2 /sec
Boron solid solubility = 3.9 × E20/cm^3 Diffusion profile = erfc function
Use the Irvin curves to calculate the sheet resistance of the boron diffused layer. Show all calculations.
(b ) (5 points) Assume singly-charged positive vacancy is the only diffusion mechanism for Boron diffusion in Si. Write down the expression for the diffusion constant which include high concentration diffusion effects.
( c) (5 points) An old diffusion process design gives a half-gaussian phosphorus drive-in profile. Is it possible to duplicate an identical doping profile by using arsenic diffusion (assuming there is no high concentration diffusion effect)? Explain your answer.
( d) (5 points) Why is the transient enhanced diffusion mechanism only “transient? Also, explain why this effect is desirable / not desirable for shallow junction formation.
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
Irvin Curves
