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SEMI F79-0703 © SEMI 2003 3 6.8 Fluorine and Fl uorides 6.8.1 It is documented t hat fluori ne (F 2 ) and atomic fluorine, F, will attack silicon at room temperature (19, 22, 26, 42, 49). 6.8.2 Chen (49) used fluorine ga…
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a halogenated hydrocarbon, is a radical specie of
fluorine, chlorine or other halogen. A low pressure
plasma is the typical method of creating this radical. In
addition, Winters (78) points out that for an
“unassisted’ etching reaction to take place adsorption
onto the silicon surface must be followed by four well
defined steps. Should any of the steps not occur, or be
interrupted, etching will not proceed.
6.4.2 Two of the most widely used halogenated
hydrocarbons are CF
4
and C
4
F
8
. The Matheson Gas
Book (79) says this about the chemical properties of
CF
4
, “...Carbon tetrafluoride is extremely stable,
reacting only slightly even at the temperature of a
carbon arc...”. About C
4
F
8
, “...Octafluorocyclobutane
is extremely stable. It is unreactive with other materials
under ordinary conditions. At high temperatures
(600°C), it dissociates to form carbon and carbon
tetrafluoride and some toxic compounds...”
6.5 Hydrides
6.5.1 References (1, 2, 3, 4, 5, 64, 65, 66) are cited
which indicate these gases do not react with silicon
under the prescribed conditions.
6.6 Halogens, other than fluorine
6.6.1 The halogens represent a special class of
materials. Molecular or atomic chlorine, iodine or
bromine have been reported not to etch silicon at
temperatures below 200° C or without the assistance of
a plasma (19, 26, 50, 55, 56, 63). Schwartz (74) makes
the comment. “Although Cl
2
chemisorbs spontaneously
on silicon, (in a plasma) at low temperatures the
reaction between Si and Cl does not proceed to the
formation of SiCl
4
. That is, silicon does not etch in Cl
2
except at elevated temperature.” He goes on to note:
“...except in the case of highly doped silicon, etching
occurs at low temperatures in the reactive ion etching
system not because of the ease of Cl attachment but
because partially reacted silicon can be sputtered
readily.” Schwartz then reports the observation that
there was no perceptible etching after thirty minutes in
a Cl
2
plasma when there was no cathode bias on the
wafer; in contrast to 4.5 microns of removal when the
bias was applied.
6.6.2 Flamm (55) published in 1981 and titled “The
Design of Plasma Etchants”, states, ”...Chlorine
containing halocarbon feed gases are frequently used
instead of Cl
2
because they are not hazardous (before
reaction), are noncorrosive, and they decompose in the
plasma to generate radicals (in addition to chlorine
atoms) which induce some desired effects.” This
statement applies to a plasma based system; Flamm is
pointing out the difficulty of etching silicon with just
Cl
2
or Br
2
in a plasma. He points out that Wang and
Maydan (from a private communication in 1979) have
etched silicon in a pure Cl
2
plasma at pressures below
0.05 torr.
6.6.3 Flamm goes on to say that, “...It seems that
chlorine and bromine atoms do not spontaneously etch
SiO
2
or undoped single crystal silicon but do etch some
forms of polycrystalline silicon and heavily n-doped
single crystal and polycrystalline silicon...”
6.6.4 A conclusion here is that even with a plasma Cl
2
and Br
2
require very specialized conditions to etch
silicon.
6.7 Halides, Other Than Fluorides
6.7.1 Gaseous mixtures of HCl, HBr or HI will not etch
silicon at the prescribed conditions (1, 2, 3, 4, 5, 62).
However, mild pitting of the silicon may be observed in
HBr or HI under aqueous conditions (5). In laboratory
testing (10) no reaction of the silicon was noted with
50% aqueous HBr after 45 days immersion.
6.7.2 One reason for the lack of reactivity of silicon
with chlorine, bromine or iodine halides is the low
vapor pressure of the reaction product. SiCl
4
has the
lowest boiling point at 57° C; SiBr
4
boils at 154° C and
SiI
4
at 286° C. On the other hand, the boiling point of
SiF
4
is –86° C. Practical advantages of this low vapor
pressure and non-reactivity were demonstrated by
Texas Instruments in their patents (24, 25) on RIE
etching; controlled additions of HBr, HI, BCl
3
, among
other gases, led to passivating films of SiBr
4
, etc. on the
silicon side walls and allowed vertical wall etching to
be achieved.
6.7.3 The moisture content of the gas stream can be a
critical factor in the corrosion resistance of most
materials. Typical stainless steel passivity is strongly
dependent upon the amount of H
2
O present, declining
sharply as water content increases above 1 ppm in the
presence of halogens or halides (6, 9, 10, 72). Paciej
(72) reported that 0.1 ppm moisture in HCl does not
attack 316L stainless, while 200 ppm moisture in HCl
can significantly corrode the surface of this material.
Fine (9) points out similar behavior for 316L in HBr at
0.5 ppm and 100 ppm moisture over the course of 10
days.
6.7.4 However, silicon corrosion resistance is virtually
unaffected by moisture content up to and including
aqueous solutions. The “RCA clean” (67) and
conventional wet etches (60, 68, 69) are obvious
examples of this.
6.7.5 Silicon based halides, such as SiCl
4
, SiH
2
Cl
2
, etc.
have not been reported to react with silicon under the
prescribed conditions. (55)
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6.8 Fluorine and Fluorides
6.8.1 It is documented that fluorine (F
2
) and atomic
fluorine, F, will attack silicon at room temperature (19,
22, 26, 42, 49).
6.8.2 Chen (49) used fluorine gas (F
2
) at room
temperature on freshly cleaned silicon; he measured an
etch rate of about 100A° /min at 2.3 torr and 5 sccm of
F
2
flowing. He also noted a two to three hour
incubation period before the etching started; the
authors speculate about the causes for this incubation
period.
6.8.3 Flamm (42) and Vasile (22) authored several
papers detailing the mechanisms involved in atomic
fluorine reacting with silicon. Flamm’s data produced
the following relation for the etch rate of silicon by
atomic fluorine.
6.8.4 R(Si) = 2.91*10
-12
*n
F
T
1/2
e-
E
etch
/kT
where E
etch
is given as 0.108 eV. At n
F
= 2.9*10
15
atoms/cc , the
etch rate is reported to be about 3,000 A° /min at 25° C.
6.8.5 Flamm used an RF discharge of at least 3.8 watts
to produce F from F
2
. Vasile used thermal dissociation
of F
2
to produce F; at 800° C he measured 47%
dissociation; below 650° C , negligible.
6.8.6 Flamm (55) also reports a similar relation for the
etch rate of SiO
2
by atomic fluorine: R(SiO
2
) =
6.14*10
-13
*n
F
T
1/2
e-
E
etch
/kT
where E
etch
is given as
0.163 eV.
6.8.7 Flamm (55) states “...Fluorine atoms react
spontaneously with all forms of silicon, SiO
2
and silicon
nitride to form volatile products.”
6.8.8 Ibbotson (19) examined potential etching
processes which required no external energy sources.
“Silicon is rapidly etched by the gas-phase halogen
fluorides ClF
3
, BrF
3
, BrF
5
, and IF
5
, in analogy to XeF
2
etching silicon..... By contrast, ClF and Groups III and
V fluorides such as NF
3
, BF
3
, PF
3
and PF
5
do not
spontaneously etch either Si or SiO
2
under the same
experimental conditions.”
6.8.9 Winters (21) and Ibbotson (20) report that XeF
2
does not etch SiO
2
. Regardless, silicon, with or without
additional oxide, is not recommended for use with F
2
,
XeF
2
, ClF
3
and the other fluorine containing inter-
halogens which may spontaneously decompose to
atomic fluorine at temperatures below 200° C.
6.8.10 As quoted from Ibbotson’s article above,
excluded from this list of reactive gases are compounds
such as NF
3
, BF
3
, PF
3
and PF
5
which have been
reported not to etch Si or SiO
2
without the assistance of
a plasma or RF source (19) or require temperatures
above 200° C (1, 35, 37, 38).
6.8.11 The ionized fluorine atom, F
-
, is reported to not
react with silicon under the prescribed conditions (1, 2,
3, 4, 5, 19, 64, 65, 66). Further evidence of this is the
fact that adding H
2
to fluorine containing plasmas
reduces the etch rate (55). The assumed mechanism is
the removal of F atoms as active species.
6.8.12 Other fluoride compounds, such as SF
6
, which
do not spontaneously decompose to atomic fluorine
below 200° C are not a source of reaction (33, 34, 52,
53, 55).
6.8.13 WF
6
is a special case. Numerous articles (41
,43, 44, 45, 46, 47) have been published on the CVD of
tungsten films from WF
6
. Two primary mechanisms
are cited; the reduction of WF
6
by silicon and the
thermal decomposition in the presence of H
2
. As stated
in Yarmoff’s paper (43):
“The dissociative chemisorption of WF
6
on Si (111)
was found to be complete, even at room temperature.
The reaction is self-poisoning at room temperature,
however as the fluorine liberated from WF
6
ties up the
active Si sites responsible for the dissociation.”
6.8.14 Tsao (41) reports that at 410° C only 200A° of
W will deposit from WF
6
onto single crystal silicon
which has an oxide thickness between 5 and 15 A° .
C.A. van der Jeugel (46) describes the effect of doping
levels on the self-limiting growth of tungsten films. It
is documented (44, 45, 47) that WF
6
will not deposit on
SiO
2
without the addition of hydrogen or some other
initiation mechanism.
6.8.15 Using the selective deposition characteristic of
WF
6
in a device, Fleming, et.al. have patented (48) a
technique for producing wear resistant coatings of
tungsten. Their invention requires a “clean” surface
and a temperature higher than 200° C, preferably around
450° C; exposure to WF
6
then results in a self-
terminating film of 5–50 nm.
6.8.15.1 Zdunek (80) reported on electrochemical
based work which indicated that unprotected silicon
surfaces exhibited “no degradation” after exposure to
WF
6
for 5 days at 80° C. A similar result was observed
for Cl
2
gas.
6.8.16 As a point of interest, metals such as stainless
steel, aluminum and nickel develop corrosion resistant
coatings of the respective fluoride when exposed to
atomic fluorine during the proper passivation procedure
(14, 23, 27, 28, 29, 30, 31, 32, 35, 39, 40).
6.8.17 Titanium, molybdenum, tungsten, brass and
columbium have been reported to be unacceptable
when exposed to gases such as ClF
3
, which decompose
to atomic fluorine (27, 32).
SEMI F79-0703 © SEMI 2003
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In summary, it is recommended that neither silicon,
silicon dioxide nor silicon nitride be in the wetted
path when fluorine, F
2
, or the halogen fluorides,
ClF
3
, BrF
3
, BrF
5
, and IF
5
, plus XeF
2
are in use.
6.9 Organo-metallic and Siloxanes
6.9.1 References (1, 2, 3, 4, 5, 64, 65, 66) are cited
which indicate these compounds do not react with
silicon under the prescribed conditions.
6.10 Oxygen and Oxides and Sulfides
6.10.1 References (1, 2, 3, 4, 5, 64, 65, 66) are cited
which indicate these gases do not react with silicon
under the prescribed conditions.
6.11 Nitrogen and Nitrogen Compounds
6.11.1 References (1, 2, 3, 4, 5, 64, 65, 66) are cited
which indicate these gases do not react with silicon
under the prescribed conditions.
6.12 Acids
6.12.1 Aqueous HF is the only single acid reported to
etch silicon (70). Combinations of fluoride ions in
solution with an oxidizing acid such as HNO
3
in an
aqueous solution will etch silicon at practical rates (5,
54, 60, 64, 65, 66, 67, 68, 69).
6.12.2 Madou (70) has an excellent review of the
mechanisms required for aqueous based etching; he
cites Hu and Kerr (72) to report an etch rate of
0.3° A/min. for n-type, 2 ohm-cm (111) silicon in a 48%
HF solution at 25° C. Madou points out the strong
dependence of etch rate in aqueous solutions on doping
level, light conditions and relative potential.
NOTE 1: Most bases will etch silicon and silicon dioxide; the
oxide is etched very slowly.
6.13 Other
6.13.1 As currently designated these are materials in
SEMI E52-0302 not encountered in mass flow
controllers under typical conditions. The manufacturer
should be consulted.
7 Cautions and Warnings
7.1 The reader is specifically requested to consult the
MSDS and the manufacturer’s recommended practices
of any gas or liquid prior to use and to follow the
suggestions prescribed.
7.1.1 All other appropriate safety procedures should be
followed as well.
8 Related Documents
8.1 K. Williams and R. Muller. “Etch Rates for
Micromachining Processing.” IEEE J.
Microelectromech. Syst., Vol. 5, No. 4, pp. 256-269,
Dec. 1996.
8.2 H. Jansen, et al. “A Survey on the Reactive Ion
Etching of Silicon in Microtechnology.” J. Micromech.
Microeng. Vol. 6, pp. 14–28, 1996.
8.3 G. Eriksen and K. Dyrbye. “Protective Coatings in
Harsh Environments.” J. Micromech. Microeng. Vol.6,
pp. 55–57, 1996.
8.4 H. B. Pogge, ed. “Electronic Materials Chemistry.”
Marcel Dekker, NY, 1996.
8.5 P. Walker and W. Tarn, ed. “CRC Handbook of
Metal Etchants.” CRC Press, Boca Raton, FL, 1991.
8.6 E. Flaherty, et al. “Reducing the Effects of
Moisture in Semiconductor Gas Systems.” Solid State
Technology, pg. 69, July 1987.
8.7 S. M. Fine, et al. “Optimizing the UHP Gas
Distribution System for a Plasma Etch Tool.” Solid
State Technology, pg. 69, March 1996.
8.8 M. George, et al. “Minimizing System
Contamination Potential from Gas Handling.”
Semiconductor International.
8.9 S. M. Fine, et al. “The Role of Moisture in the
Corrosion of HBr Gas Distribution Systems.” Jl.E.C.S.,
142; No. 4; 1269, April 1995.
8.10 P. M. Bhadha, et al. “Joule-Thomson Expansion
and Corrosion in HCl Systems.” Solid State
Technology, pg. S3, July 1992.
8.11 T. Ohmi. “Corrosion-free Cr
2
O
3
Passivated Gas
Tubing System for Specialty Gases.” Solid State
Technology, pg. S18, October 1995.
8.12 G. Bitko, et al. “Analytical Techniques for
Examining Reliability and Failure Mechanisms of
Barrier Coating Encapsulated Silicon Pressure Sensors
Exposed to Harsh Media.” Proc. SPIE, 2882
; 248–258,
1996.
8.13 O. Hallberg, et al. “Recent Humidity
Accelerations, A Base for Testing Standards.” Proc.
Quality and Reliability International Conf., 7
, 169–180,
1991.
8.14 M. A. George, et al. “Compatibility of 316L
Stainless Steels and Tungsten Hexafluoride.” presented
at: Tungsten and Other Refractory Metals for ULSI
Applications, Dallas, TX, October 1990.
8.15 S. Lau, et al. “Performance of Diaphragm Valves
in Chlorine.” Solid State Technology, pg. S21, June
1997.