EE665 Homework Problem Set #1 (total 110 points)
Problem 1. (Lecture 1, CMOS Technology) – 5 points
This is the first step of CMOS process flow as we learned in lecture 1. This step is for
LOCOS formation process. We have 40nm pad oxide and 80nm nitride. Explain the
purpose of pad oxide in this process in terms of wafer stress. What will happen if you do
not have pad oxide?
Problem 2. (Lecture 1, CMOS Technology) – 5 points
At the beginning of the semiconductor technology, Ge had widely used. However, Si
wafer dominates integrated circuits. Give a reason why semiconductor integrated circuits
have used Si instead of Ge?
Problem 3. (Lecture 1, CMOS Technology) – 5 points
Explain the difference between LOCOS process and STI process? Do you think LOCOS
process is suitable for short channel device? Why?
Problem 4. (Lecture 2, Semiconductor Manufacturing) – 5 points
Calculate APF for HCP structure.
Problem 5. (Lecture 2, Semiconductor Manufacturing) – 5 points
The table shown above is for oxidation-reduction reactions. For RCA clean, H2O2 is used
as a strong oxidant. However, O3 is more strong oxidant, but limited to several applications.
Give comments why H2O2 is widely used, not O3.
Problem 6. (Lecture 2, Crystal Growth) – 10 points
A Czochralski crystal is pulled from a melt containing 5 x 1016cm-3 boron and 1015 cm-3
phosphorus. Initially the crystal will be P type but as it is pulled, more and more
phosphorus will build up in the liquid because of segregation. At some point the crystal
will become N type. Assuming kO = 0.32 for phosphorus and 0.8 for boron, calculate
the distance along the pulled crystal at which the transition from P to N type takes place.
Problem 7. (Lecture 2, Crystal Growth) – 10 points
Suppose your company was in the business of producing silicon wafers for the
semiconductor industry by the CZ growth process. Suppose you had to produce the
maximum number of wafers per boule that met a fairly tight resistivity specification.
a). Would you prefer to grow N type or P type crystals? Why? (5 points)
b). What dopant would you use in growing N-type crystals? What dopant would you use
in growing P type crystals? Explain your answer (5 points)
Problem 8. (Lecture 3, Lithography) – 10 points
There are no clear choices for lithography systems beyond optical projection tools based
on 193-nm ArF eximer lasers. One possibility is an optical projection system using a 157nm F2 excimer laser.
a). Assuming a numerical aperture of 0.7 and k1 = 0.8, what is the expected resolution of
such a system using a first order estimate of resolution? (4 points)
b). Actual projections for such systems suggest that they might be capable of resolving
features suitable for the 2009 0.07 µm generation. Suggest three approaches to actually
achieving this resolution with these systems. (2 points for each approach)
Problem 9. (Lecture 3, Lithography) – 5 points
Lithography often has to be done over underlying topography on a silicon chip. This can
result in variations in the resist thickness as the underlying topography goes up and down.
This can sometimes cause some parts of the photoresist image to be underexposed and/or
other regions to be overexposed. Explain in terms of the chemistry of the resist exposure
process why these underexposure and overexposure problems occur.
Problem 10. (Lecture 3, Lithography) – 10 points (5 points each)
EUV (Extreme Ultraviolet Lithography) is now used for short channel devices.
a) The NA(numerical aperature, ~0.25NA) of EUV is worse than ArF NA(~1.35NA),
which means worse resolution. However, the resolution of EUV is superior to ArF
lithography process. Can you explain why?
b) The EUV systems use reflective optics instead of refractive optics. Explain why.
Problem 11. (Lecture 5, Thermal oxidation) – 15 points (5 points each)
Draw Energy band diagram for case a), b) and c). Case c is for high frequency only.
Problem 12. (Lecture 5, Thermal oxidation) – 10 points (5 points each)
Suppose you have wafers with <100> surface crystal orientation and no initial oxide, and
you want to oxidize these wafers using wet O2 at 1 Atm. The parabolic regime starts to
dominate when
of the parabolic term equals
of the linear term. Using
equation (6.28) in the text for the limiting forms of the linear parabolic growth:
Equating these equations gives x0 = A/2 when the parabolic regime starts to dominate.
Calculate the thickness and time at which parabolic regime starts to dominate at oxidation
temperatures of (i) 900°C and (ii) 1100°C. For both temperatures, what
is the maximum oxidation rate
and what is
when the parabolic regime starts
to dominate? Use the Deal-Grove model for the oxidation kinetics. (Hint : use the
following equation and table 6-2.)
Problem 13. (Lecture 5, Thermal oxidation) – 5 points
Why is steam oxidation more rapid than dry O2 oxidation?
Problem 14. (Lecture 5, Thermal oxidation) – 10 points (5 points each)
A <100> silicon wafer is covered by an SiO2 film 0.3 μm thick. (use the table 6-2 and
figure 6-18 in the textbook.)
a. What is the time required to increase the thickness by 0.5 μm by oxidation in H2O at
1150˚C?
b. Repeat for oxidation in dry O2 at 1150˚C.