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N.Cheung, Spring 99 UNIVERSITY OF CALIFORNIA College of Engineering Department of Electrical Engineering and Computer Sciences
EECS143 Midterm Exam #
Family Name _______________________ First name___________________________
Signature______________________________________________________________
Instructions: DO ALL WORK ON EXAM PAGES This is a 90-minute exam (open reader materials and homework).
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 (30 points)________________
Problem 2 (20 points)_______________
Problem 3 (25 points) ________________
Problem 4 (25 points) ________________
TOTAL (100 points) __________________
Problem 1 Oxidation (30 points total)
( a) (15 points) A lightly doped Si wafer was processed by some unknown IC processing steps. After these unknown steps, you performed a thermal oxidation experiment [fixed temperature and fixed oxidizing ambient] with this wafer and observed the following results: Oxidation Time SiO2 Thickness 0 hour 0 1 hour 2000 Å 4 hours 2500 Å
Are the following conjectures TRUE or FALSE? You have to give brief explanations to justify your answers.
Conjecture 1 : The processed Si wafer was oxidized first to an oxide thicknes of 100 Å and then have the oxide dissolved in HF.
Conjecture 2 : The processed Si wafer has a highly doped surface layer ( doping > 10^19 /cm^3 ) which is less than 1000 Å thick.
Conjecture 3 : The processed Si wafer has a thin layer of poly-Si layer on top surface.
Problem2 Ion Implantation (20 points total)
(a) (10 points) Self-aligned source and drain of a MOSFET is formed by ion implantation of phosphorus. If the gate has a tapered sidewall with angle θ , indicate in the following table whether the electrical channel length L will increase or decrease or no change ( ↑= increase, ↓ = decrease, 0 = no change) when one of the parameters changes while the others remain constant.
Parameter Electrical Channel Length L
Implant Dose ↑
Substrate conc. NB ↑
Sidewall Angle θ ↑
Gate material changed from poly-Si to Tungsten
Implant ions changed from Phosphorus to Arsenic (same energy)
(b) (5 points) Explain why we need an additional annealing step at >900C after implantation of dopants.
(c) (5 points) We would like to form ultra-shallow junctions, discuss two methods used in IC processing to minimize the ion channeling effect
SiO
n+ n+
SiO
p substrate
Problem 3 Diffusion (25 points total) (A) (15 points) Boron is diffused into a Si substrate having a background phosphorus concentration of 1016 /cm^3. The measured junction depth (xj) is 0.7 μm and the sheet resistance is 5 Ω/square.
Since we do not have information about the boron depth profile, we make up two approximations:
Profile 1: The profile is an erfc function with (Dt)1/2^ =0.1μm Profile 2: The profile is a rectangular profile with constant concentration from surface to xj.
Calculate the surface concentration of boron for both profiles. The hole mobility can be taken as constant ( = 60 cm^2 /V-s) for all depths. Based on the surface concentration values, which profile is a better approximation?
(b) During the initial stage of diffusion, the doped region is intrinsic ( n = p) so we won’t expect to see any high concentration diffusion effect. However, Boron diffuses faster than As and the near surface region becomes highly n-doped and the deeper region becomes p-doped. Since the net carrier concentration can still be higher than ni. we will start to observed high concentration diffusion effects in
both regions.
Problem 4 Lithography (25 points total) (a) (15 points) Two diagonal alignment marks A1 and A2 along the x-axis near the edge of a wafer with 4-inch diameter have overlay errors (x 1 , y 1 ) and (x 2 , y 2 ) respectively.
x 1 , x 2 , y 1 and y 2 are defined as + along the positive x and y directions.
(x 1 , y 1 ) and (x 2 , y 2 ) are measured to be (+0.2, +0.2) um and (+0.4, +0.4) um.Calculate numerical values
of : (i) Run in/out misalignment.
(ii) Rotational misalignment.
(iii) Translational misalignment.
(B) For a particular lithography process based on projection printing, the minimum resolution (lm) is 0.5um and the depth of focus (DOF) is 1um. By placing a smaller aperture over the projection lens, the numerical aperture (NA) is reduced by a factor of 2, calculate the new values of lm and DOF.
