Due March 7, 2002
Chapter 10 + class handout:
1) A chlorine (Cl2) plasma is used for etching aluminum. If the electron density is 1x1011 cm-3, the electron temperature is 3 eV, and the bias voltage applied to the wafer is -100v, please find the maximum chlorine pressure that could be used such that the plasma sheath is collisionless. Assume that the collision cross section for Cl2+ is 3x10-14 cm2.
2) An argon plasma is ignited between to large parallel plates separated by a distance of 10 cm. A very sensitive wafer is to be processed in the plasma; this wafer will be damaged if it is bombarded by ions of more than 15 eV. Find the minimum pressure at which this process can be safely run.
Chapter 11:
3) Chapter 11, Problem 3
4) Beginning with the plasma etch model developed
in class, derive Eq. 11.8 in the text by Campbell. Find the expressions
for "k" and "Ro". Explain how to desensitize a process
to the loading effect.
(Recall for this model that only fluorine atoms are important to the
etch process and the atoms are generated by electron impact dissociation:
CF4 + e -> CF3 + F + e, where the rate constant is
kF = 2e-9 exp(-13/Te) cm3/sec. Atoms
are lost to the pump and by reaction with the wafer to create SiF4(g).)
Chapter 12:
5) A magnetron is used to deposit gold (Au) by sputtering
a 20-cm-diameter target. The target voltage is -400 volts and the
DC power applied to the target is 1 kW. If 50% of the sputtered gold
reaches the wafer, find the deposition rate in um/min. (Gold has a density
of 19.3 grams per cm3). Assume that the Ar plasma generated
by the magnetron is uniform between the target and wafer. Find the
wafer voltage that will halve the net deposition rate of the Au due to
resputtering. Explain why resputtering the wafer using Ar ions is
sometimes important.
Chapter 13:
6) Problem 1. In addition, assume that the sticking probability for B is 0.1 and the sticking probability for A is zero. Find the deposition rate if the density of solid B is 5x1022 atoms per cubic centimeter and the mass of B is 32 a.m.u. per atom. You should assume that this deposition is reaction rate limited (h>>k). Repeat the calculation if T=800 K. Comment on the significance of this result.