UNIVERSITY OF CALIFORNIA SANTA BARBARA
DEPARTMENT OF CHEMICAL ENGINEERING
CHE 142 and ChE 242: Microelectronics Processing
Spring 2003 Homework # 7
(Due June 11, 2003)
1)
In a plasma reactor Cl
2
is completely dissociated into Cl to etch crystalline Si.
Assume that the etch
product is exclusively SiCl
4
and that every Cl that hits the surface leads to addition of a Cl to a
surface Si atom.
What is the maximum etch rate that can be obtained at 5 mTorr.
Assume that the
gas temperature is 300 K.
Average molecular weight of Cl is 35.5 amu.
2)
In a plasma etching process, a stable gas such as SF
6
is dissociated into reactive radicals such as F
using electron bombardment in a discharge.
The locations on the wafer where we do not want
etched are protected by a mask (typically a polymer, metal or oxide).
Locations where we want to
pattern by etching are exposed to F from the plasma.
Reactions are complicated but for the
purposes of this problem they may be simplified to the following overall reaction.
Solid Si is
transformed into gaseous SiF
4
, which is then pumped out of the reactor.
)
g
(
2
)
g
(
4
)
g
(
2
)
g
(
6
)
s
(
SO
2
SiF
3
O
2
SF
2
Si
3
+
→
+
+
Using this method, lines and trenches and various complicated shapes can be etched into Si.
a) In industry, most people speak of etching rate in Si etch depth per unit time; for example in
micrometers/min or microns/min (
µ
m/min).
Show that the mass rate of Si removal from the
wafer surface is given by
ρ
RA
m
removal
=
&
where R is the etching rate in etched depth per unit time, A is the area exposed to the plasma
and
ρ
is the density of silicon, 2.328 g/cm
3
.
b) In a plasma etching reactor SF
6
and O
2
are fed at 40 sccm (std cm
3
/min) each.
Calculate the
maximum possible etch rate that can be achieved for a 325 cm
2
wafer area.
c)
What is the smallest size (pumping speed) turbo pump required for this process.
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 Fall '09
 Ceweb
 Chemical Engineering, Electron, Ion source, etch rate, Cl2

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