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SEMI F79-0703 © SEMI 2003 4 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 , …
SEMI F79-0703 © SEMI 2003 3
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
4
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.
SEMI F79-0703 © SEMI 2003 5
8.16 P. B. Henderson, et al. “Cylinder Package Effects
on the Purity of Electronic Specialty Gases.” Solid
State Technology, pg. S5, June 1997.
8.17 H. Kobayashi. “How Gas Panels Affect
Contamination.” Semiconductor International, 81, Sept.
1994.
8.18 A. K. Henning, et al. “Contamination Reduction
Using MEMS-Based, High-Precision Mass Flow
Controllers.” Presented at SEMICON West, SEMI,
1998.
8.19 D. E. Ibbotson, et al. “Plasmaless Dry Etching of
Silicon with Fluorine-Containing Compounds,” J. Appl.
Phys. Vol. 56 (10), pp. 2939–42, 1984.
8.20 D. E. Ibbotson, et al. “Appl. Phys. Lett.” Vol. 44
(12), pp. 1129, 15 June 1984.
8.21 H. F. Winters, et al. “The Etching of Silicon with
XeF
2
Vapor.” J. Appl. Phys. Lett., Vol. 34 (1), pp. 70, 1
January 1979.
8.22 M. J. Vasile, et al. “Reaction of Atomic Fluorine
with Silicon: The Gas Phase Products.” J. Appl. Phys.,
53 (5), 3799, 1982.
8.23 ES and H Manual: Volume 2, Part 14: Chemical,
14.6, The Safe Handling of Fluorine. Environment,
Safety, and Health. UCRL-MA-133867, April 1, 2001
<http://www.llnl.gov/es_and_h/hsm/doc_14.06/doc14-
06.html>.
8.24 M. A. Douglas. “Trench Etch Process for a single-
wafer RIE Dry Etch Reactor.” U.S. Patent 4784720;
Nov. 15, 1988.
8.25 M. A. Douglas. “Trench Etch Process for a single-
wafer RIE Dry Etch Reactor.” U.S. Patent 4855017,
Aug. 8, 1989.
8.26 W. C. Tian, et al. “Comparison of Cl
2
and F-
Based Dry Etching for High Aspect Ratio Si
Microstructures Etched With an Inductively Coupled
Plasma Source.” J. Vac. Sci. Technol. B 18(4), Jul/Aug
2000; 1890.
8.27 J. C. Grigger, et al.. “Effect of Chlorine
Trifluoride and Perchloryl Fluoride on Construction
Materials.” Materials Protection, pp. 53, September
1964.
8.28 Air Products and Chemicals, Inc. “Recommended
Passivation Procedure for Reactive Fluorides.” Pub. #
320-9460, August 1994.
8.29 T. Mebrahtu, et.al. “SEM and XPS
Characterization of the Carbon Steel Surface
Passivation Film in Anhydrous HF Media.” Corrosion
95, The NACE Int’l Annual Conference.
8.30 R. L. Farrar, Jr. “Safe Handling of Chlorine
Trifluoride and the Chemistry of the Chlorine Oxides
and Oxyfluorides.” Union Carbide Nuclear Company,
Oak Ridge, TN, Nov. 11, 1960; K1416.
8.31 A. Zdunek, et.al. “Chlorine Trifluoride(ClF
3
)
Passivation of Stainless Steel Tubing.” Air Liquide,
internal memo.
8.32 A. P. Taylor, et al. “Interaction of ClF
3
with Metal
Alloys and Polymer Gaskets.” Semiconductor
International, 201, July 1999.
8.33 R. Duguid, et.al. “Compatibility Study of a
Waferpure Reactive Micro Matrix Material with SF
6
.”
Application Note, Millipore Corp. 3/25/99;
<http://millispider.millipore.com/micro/miegas/MA019
.htm>.
8.34 Compressed Gas Association, Inc. “Handbook of
Compressed Gases.” Van Nostrand Reinhold, New
York, 1990.
8.35 C. J. Gugliemini, et al. “Properties and Reactivity
of Chlorine Trifluoride.” Semiconductor International,
June 1999.
8.36 Air Products and Chemicals, Inc. “Chlorine
Trifluoride Technical Data, Safety and Handling.”
Pub.# 320-9711.
8.37 Air Products and Chemicals, Inc. “Nitrogen
Trifluoride Reactivity, Environmental and
Toxicological Data.” 1993.
8.38 Air Products and Chemicals, Inc. “Nitrogen
Trifluoride Material Safety Data Sheet.” Pub.# 320-801
Rev 1/96.
8.39 M. Maeno, et al. “Fluorine-Passivated Electroless
Ni-P Films.” Jl.E.C.S., 141
, No. 10, Oct. 1994, 2649.
8.40 A. L. Cabrera, et al. “Surface Analysis of Copper,
Brass and Steel Foils Exposed to Fluorine Containing
Atmospheres.” Jl.Vac. Sci.Tech., A 8 (6); Nov./Dec..
1990, 3988.
8.41 K.Y. Tsao, et al. “Low Pressure Chemical Vapor
Deposition of Tungsten on Polycrystalline and Single-
Crystal Silicon Via the Silicon Reduction.” Jl.E.C.S,
131 (11), Nov. 1984, 2702.
8.42 D. L. Flamm, et al. “The Reaction of Fluorine
Atoms With Silicon.” Jl.Appl.Phys., 52(5), May 1981,
3633.
8.43 J. A. Yarmoff, et al. “Mechanism for Chemical-
Vapor Deposition of Tungsten on Silicon From
Tungsten Hexafluoride.” Jl.Appl.Phys., 63(11), June
1988, 5213.