Chem. Mater. 1995, 7, 683-687
683
Thermal Decomposition of N F 3 by Ti, Si, and Sn Powders Elizabeth Vileno,? Michael K. LeClair,* Steven L. Suib,*y'J?§ Michael B. Cutlip,+ Francis S. Galasso,? and Steven J. Hardwickl U-60, Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060; Department of Chemical Engineering, University of Connecticut, Storrs, Connecticut 06269-3060; Institute of Material Sciences, University of Connecticut, Storrs, Connecticut 06269-3060; and Novapure Corp., 7 Commerce Drive, Danbury, Connecticut 06810 Received September 16, 1994. Revised Manuscript Received December 22, 1994@ The thermal decomposition of NF3 by Ti, Si, and S n powders was investigated using gas chromatography, X-ray photoelectron spectroscopy, BET surface areas, and X-ray powder diffraction. The decomposition of NF3 using Ti powder starts at 160 "C, with 299.8% conversion occurring a t 325 "C. Using Si powder, NF3 starts to decompose at 250 "C, with >99.8% conversion at 500 "C. S n powder starts to decompose NF3 at 160 "C; however, complete conversion could not be obtained due to the low melting point of tin. In all cases, nitrogen and the corresponding metal fluorides were the only major products from the reaction. XPS revealed no nitrogen on the metal surface of the reacted powders, but metal fluoride species were observed. The XRD pattern for the reacted Ti showed crystalline TiF3 a s well as Ti metal. The reacted Si and S n patterns showed no new peaks. BET measurements revealed increases in surface area after reaction of Si and Ti with NF3. From these studies, it has been concluded that of the three powders, Ti powder is the best getter for NF3.
Introduction The development and use of plasmas that reduce contamination and damage and enhance reaction selectivity have become much more important in the plasma-processing area as the sophistication of microcircuit technology increases. NF3 has been found to be an excellent choice for etching and cleaning ~ i l i c o n , l - ~ t u n g ~ t e n ,and ~ tungsten ~ i l i c i d e ,as ~ well as other materials. NF3 is also used to clean reactors6 and reactor chamber^.^ Commonly diluted with Ar, NF3 can also be mixed with oxygen to increase the oxidation rate of silicon8 or with hydrogen to improve the etching selectivity of Si02 with respect to Si.9 The past seven years has seen an escalation in the use of NF3. With this came concerns regarding its toxicity (the threshold limit value is 10 ppm)1° and its
* To whom correspondence should be addressed.
' Department of Chemistry.
* Department of Chemical Engineering.
Institute of Material Sciences. Novapure Corp. Abstract published in Advance ACS Abstracts, February 1, 1995. (1) Barkanic, J.; Hardy, T.; Shay, R. H.; Fukushima, H. Off-gas analysis and disposal of NFs in plasma processing. Air Products and Chemicals, Inc., Allentown, P A Daido Sanso KK, Osaka, Japan, 1984. (2) Woytek, A.; Lileck, J. T.; Barkanic, J. A. Solid State Technol. 1984, 72, 172-5. (3) Delfino, M.; Chung, B. C.; Tsai, W.; Salimian, S.; Favreau, D. P.; Merchant, S. M. J. Appl. Phys. 1992, 72, 3718-25. (4) Hirase, I.; Petitjean, M. European Patent EP 382,986, 1990. (5) Lee, R.; Terry, F. L. J . Vac. Sci. Technol. B 1991, 9, 2747-51. (6) Bruno, G.; Capezzuto, P.; Cicala, G.; Manodoro, P. J. Vac. Sci. Technol. A 1994, 12, 690-8. (7) Langan, J. G.; Felker, B. S. In Studies of the reaction of nitrogen trifluoride/argon and hexafluoroethane/oxygen plasmas with anodized aluminum surfaces using x-ray photoelectron spectroscopy. Proc. Electrochem. SOC.1992. 92-18. 135-44. (8) Morita, M.; Kubo, T.; Ishihara, T.; Hirose, M. Appl. Phys. Lett. 1984,45, 1312-14. (9) Yokoyama, S.;Yamakage, Y.; Hirose, M. Appl. Phys. Lett. 1986, 47,389-91. 8
@
environmental effects as a greenhouse gas. Current scrubbers for NF3 operate at high temperatures (130800 "C) and are expensive. Several systems are in use or have been proposed. Much of the NF3 in the United States is abated from waste-gas streams by incineration, with or without hydrocarbon fuels. This results in the production of HF and NO,'s, which in turn must be abated. Also in use are activated carbon scrubbers, which can accidentally release the trapped NF3 after adsorption, or decompose to volatile fluorocarbons.lI2J1 Some of the more recently reported gettering systems involve the thermal decompositon of NF3 using metals, supported metals, oxides, and other systems. For example, the following systems have been patented: BN at 250-260 0C;12J3Si3N4 at 2200 "C;14 Cu and Ni nonoxide compounds supported on activated carbon at 100300 "C;15 and metal oxides and activated carbon at 200600 "C.16 Pure metal and metalloid systems have also been patented. Ti was used at temperatures 2200 O C . 1 7 Si, B, W, Mo, V, Se, Te, Ge, and/or their non-oxide compounds also removed NF3 at temperatures between 200 and 800 O C . 1 8 (10) Woytek, A. J. In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Grayson, M., Eckroth, D., Eds.; Wiley: New York, 1980; Vol. 10, p 768-72. (11) Aramaki, M.; Nakagawa, S.; Nakano, H.; Ichimaru, H.; Tainake, M. Ger. Offen. DE 4,002,642, 1990. (12) Iwanaga, T.; Harada, I.; Aritsuka, M. Jpn. Kokai Tokkyo Koho JP 02,245,223, 1990. (13) Iwanaga, T.; Harada, I.; Aritsuka, M. Jpn. Kokai Tokkyo Koho J P 02,265,6205, 1990. (14) Kashiwada, K.; Hasumoto, T.; Torisu, J.; Konishi, M. Jpn. Kokai Tokkyo Koho J P 04,225,818, 1992. (15) Kubo, M.; Nakagawa, S. Jpn. Kokai Tokkyo Koho JP 05,192,538. 1993.
0897-475619512807-0683$09.00/0 0 1995 American Chemical Society
Vileno et al.
684 Chem. Mater., Vol. 7, No. 4, 1995
TEMPERATURE CONTQOLLEFI
BO
1-
60
-
40
-
20
-
loo
$ c
.-0
E0
.z
T I
HP
I
I
GC
I
s 11
BUBBLE
Figure 1. Diagram of reaction line.
However, little research into the basic chemistry of how the decomposition occurs has been published. This knowledge would be very useful for choosing an optimum getter. NF3 is a strong oxidizer but quite stable a t ambient pressure and temperature. Its thermal reactivity is probably due to its equilibrium with F: NF3 NF2 F, which becomes significant at 300 "C. This research is concerned with the thermal decomposition of NF3 using Si, Ti, and Sn powders. X-ray photoelectron spectroscopy, scanning electron microscopy, surface area measurements, and X-ray diffraction were used to help explain the decomposition process, with the ultimate goal of finding the metal or metalloid powder that can decompose NF3 a t the lowest possible temperature without producing toxic species.
+
Experimental Section Materials. NF3 (1%by mole, balance He) was purchased from Air Products and Chemicals in Allentown, PA. The following powders were used as received and came from the following sources: Si, 99.99%, 325 mesh, Atlantic Equipment Co.; Ti, 99.5%, 325 mesh, from Cerac, Milwaukee, WI; Sn, 99.8%, 325 mesh, Johnson Matthey Catalog Co., Ward Hill,
MA. Reaction Process. A schematic for the reaction line is shown in Figure 1. A typical reaction process proceeds in the following manner. A reactor, made of standard '18 in. Inconel tubing, was plugged at one end with glass wool (both inert to NF3 a t the temperatures used). Inconel is a nickel based alloy of the following composition: 70% Ni, 25% Cu, and 5% Fe. Approximately 0.250 g of metal powder was placed inside the vertical reactor. Ultrahigh purity grade helium gas was used to flush the system. Then nitrogen trifluoride (l%, balance He) was passed over the metal powder a t a flow rate of 7.5 m u m i n . This represents a residence time for the reaction of approximately 0.2 s. A tube furnace was used to heat the reactor, the temperature being controlled by a thermocouple placed inside the reactor, just above the bed of metal powder. The temperature was raised in increments, and for each temperature studied, two or three samples of exhaust gases were injected directly into a Hewlett-Packard 5890 Series I1 gas chromatograph. The GC is equipped with a thermal conductivity detector and an H P 3396 Series I1 integrator. A Haysep D packed column was used to separate the gases, and all peaks were well resolved. The system can detect NF3 to less than 0.002 mol % in the exhaust gas. The percent conversion of NF3 was calculated based on an average of NF3 counts obtained by bypassing the reactor. Sample Characterization. A Scintag XDS 2000 X-ray powder diffractometer was used to identify and characterize (18) Aramaki, M.; Sakaguchi,H.; Suenaga, T.; Kobayash, Y. Jpn. Kokai Tokkyo Koho JP 61,204,025, 1986.
o 100
200
300
400
Temperature (C)
Figure 2. Conversion of NF3 using titanium. the reacted and unreacted materials. Cu Ka radiation was used as the X-ray source, with a beam current and voltage of 45 mA and 40 kV, respectively. All patterns were compared to a JCPDS database. Scanning electron microscopy was done using a n Amray 1810-D instrument. Surface area measurements were done by BET, using a n Omnisorp lOOCX instrument with Nz adsorption. X-ray photoelectron spectroscopy was done using a LeyboldHeraeus LHS 10 X-ray photoelectron spectrometer equipped with a n EA-10 hemispherical analyzer. The source used was Mg Ka X-rays, with a beam current and voltage of 10 mA and 13 kV, respectively. The base pressure in the analysis chamber was Torr. Survey scans were collected in a constant relative resolution mode with a retardation ratio of 3.0. Detailed scans were collected in a constant-resolution mode in which high-resolution spectra were obtained using a pass energy of 50 eV, and low-resolution spectra using a pass energy of 150 eV. The instrument was calibrated by setting the Cu 2 ~ 3 1 peak 2 at 932.7 i 0.1 eV and the Au 4f7,~peak a t 75.1 & 0.1 eV. The adventitious C Is peak was set a t 284.8 eV for charge correction purposes. Peaks were resolved using the Proctor LOGAFIT program. The binding energies are generally reproducible within i O . 1 eV, and atomic percents have a n estimated error of 5%. Quantitative analysis was conducted using sensitivity factors provided by the manufacturer. XPS was done on reacted samples after they were exposed to air. All samples were mounted in a n Al boat; no Al was detected during analyses.
Results Titanium Powder. Titanium powder decomposed NF3 a t 160 "C, with >99.8% conversion at 325 "C. No loss of reactivity was noted when the sample was held at a constant temperature. For example, at 240 "C there is a consistent 32% conversion of NF3. The initial conversion temperature of NF3 was lowered considerably (from 225 to 160 "C)when the titanium was first treated with NF3 a t 360 "C for 30 min. See Figure 2 for an example of NF3 conversion by Ti. N20 was detected in trace amounts at temperatures over 350 "C. The XRD pattern of the reacted titanium powder clearly shows evidence for the existence of crystalline TiF3 (see Figure 3), as well as titanium metal. Crystallite sizes were calculated from line broadening data using the Scherrer equation to be about 38 nm before reaction and 36 nm after. SEM revealed no changes in morphology or particle size. The powder was comprised of round and long, smooth-shaped particles, most of which were between 10 and 50 pm in size. BET
Chem. Mater., Vol. 7, No. 4, 1995 685
Thermal Decomposition of NF3
Table 1. X-ray Photoelectron Spectroscopy Results for Ti Powder, As Received and after Reaction with NFs" as received reacted binding atomic binding atomic element transition enerm (eV) % enerm (eV) % 462.3 16b 458.5 13b Ti 2P3/2 687.8 48 ndc nd F 1s 533.0 25 530.1 50 0 1s 532.3 8 C 1s 284.8 29 284.8 11 ~~
~
a The as received powder and the reacted powder were both charge corrected 0.8 eV. 2~312and 2pm peaks were combined to determine atomic percent. Not detected.
1
I
480
1
o
,
470 0
460 0 Binding Energy / e V
450.0
Figure 3. Ti 2~112and 2~312peaks of reacted Ti powder using a pass energy of 150 eV.
25x103
-
1
5
Sipowder
Si/CVD
Temperature (C)
Figure 6. Conversion of NF3 using silicon.
Two Theta, degrees
Figure 4. X-ray diffraction pattern for the reacted Ti powder. The starred peaks are from TiF3, the unstarred peaks are from
Ti.
measurements revealed a surface area of 10 m2/gbefore and 82 m2/g after reaction. However, 10 m2/g surface area is at the instrument's limits; it should be noted that the surface area increased after reaction. There was a color change from silver-blackto a yellowish color, and a strong odor of greasy soot was noted when the material was removed from the reactor. XPS survey spectra revealed that there was oxygen, carbon, and titanium on the surface of the unreacted powder. After reaction with NF3, fluorine was also observed and no nitrogen was seen. Detailed spectra of the unreacted titanium powder were charge corrected by 0.79 eV. The Ti 2~312peak was centered at 458.5 eV, just where Ti02 is expected, indicating a completely oxidized surface. No shakeup peaks were seen. The ratio of Ti:O was 1:4. The XPS of the reacted powder was charge corrected by 0.95 eV. The Ti 213312 peak showed a shift to a higher binding energy of 462.3 eV and was accompanied by shakeup peaks (see Figure 4). The Ti:F ratio was calculated to be 1:3. Table 1 gives the complete results of the X P S analysis. Silicon Powder. Si powder decomposed NF3 a t 250 "C, with complete (>99.8%) decomposition occurring at 500 "C. Loss of activity was noted when the temperature was held constant, i.e., the conversion of NF3 decreased at specific temperatures. For example, at 400 "C conversion dropped from 46% t o 36% in 5 min. Pre-
treatment with NF3 did not lower the lowest temperature of reaction with the silicon. Conversion results can be found in Figure 5. NO, production, primarily NO and N20, was observed at temperatures above 400 "C. The XRD patterns before and after the reaction are almost identical, the exception being that the material after treatment appeared more crystalline. There was a visible difference in the peak widths at half-maximum, which were narrower after NF3 reaction than before the reaction. Crystallite sizes were calculated to be approximately 30 nm before reaction and 53 nm after reaction. A solid product was found in the arm of the NaOH trap and was determined by XRD to contain Na2SiFs. There was no color change or odor detected. The SEM revealed no change in the morphology of the rough, irregularly shaped particles. Before reaction the particle size ranged from 1 to 40 pm. However, several larger particles, about twice the size of the largest unreacted particles, were seen in the reacted material. BET measurements showed an increase in the surface area from 66 f 2 to 150 f 2 m2/g. X P S results are summarized in Table 2. The survey spectrum of the as-received material shows the existence of silicon, oxygen, and carbon. After reaction, the survey shows fluorine also, and no nitrogen is seen. The detailed spectra of the as-received material did not need charge correction. The Si 2p transition of the unreacted powder has two peaks, one at 100.3 eV, which is assigned to elemental Si, the other at 104.4, which is due to oxidized surface silicon. The ratio of the oxidized silicon to oxygen is 1:2.3, and the ratio of oxidized silicon to elemental silicon is 1.0:1.5. After reaction with NF3 the detailed spectra were charge corrected by 0.24 eV. There were still two Si 2p transitions, one a t 100.4 eV,
686 Chem. Mater., Vol. 7, No. 4, 1995
Vileno et al.
Table 2. X-ray Photoelectron Spectroscopy Results for Si Powder, As Received and after Reaction with NFsa
Table 3. X-ray Photoelectron Spectroscopy Results for Sn Powder. As Received and after Reaction with NFs
as received reacted binding atomic binding atomic element transition energy (eV) % energy (eV) 9%
as received reacted binding atomic binding atomic element transition energy (eV) C9' energy (eV) %
Si
2P
F
Is
0
1s
C
1s
100.3 104.4 ndb 533.7 535.7 284.8
29 19 ndb 40 4 8
100.4 105.3 685.9 689.1 531.8 534.3 536.9 284.8
=0.2 a0.7 4 5 28 38 8 16
No charge correction was needed for the as received powder. The reacted material was corrected 0.24 eV. Not detected.
30
1
Temperature (C) Figure 6. Conversion of NF3 using tin. and another, a very broad peak a t 105.3 eV. The peak a t 105.3 eV is assigned to fluorinated silicon. The fluorinated silicon to elemental silicon ratio was found to be 1:0.33. The total atomic percentage of silicon on the surface is less than 1%, with oxygen making up 74%, carbon 16%, and fluorine 9%. Tin Powder. Tin powder reacted with NF3 a t 130 "C, but complete conversion was never achieved because the reaction was shut down a t 218 "C, below tin's low melting point of 231.8 "C. The powder was visibly sintered when it was removed from the reactor. The NFBconversion was erratic, with 9.0% conversion at 130 "C but none at 157 "C. Maximum conversion occurred a t 204 "C (26%), and the conversion dropped to 15% a t 218 "C. Pretreatment with NF3 did not lower the lowest temperature of reaction. See Figure 6 for the conversion results of an example run. The tin XRD patterns before and after reaction show a single-phase pattern for white tin (P-tin). SEM shows an approximate doubling of the particle size from an average size of 10 to 20 pm, with smooth spherical morphology before and after reaction. The powder changed from gray to olive green in color. XPS results for tin are summarized in Table 3. The survey spectrum for the as received material shows tin, oxygen, and carbon on the surface. Detailed scans did not need charge correction. The unreacted Sn 3d512 transition was at 487.1 eV. The Sn M4N45N45 Auger peak was also collected, and the Auger parameter, a, was calculated to be 917.3 eV. The Sn:O ratio was 1:2.2. After reaction with NF3, the survey scan showed fluorine, in addition to tin, oxygen, and carbon, on the surface. The detail scans were charge corrected 2.1 eV.
Sn F
3dwn Is
487.1 ndb
27 ndb
0
1s
C
1s
531.0 532.3 284.7
44 17 12
486.9 684.6 686.7 530.8 532.4 284.8
19 5 2
30 15 29
No charge correction was needed on the as received powder. The reacted powder was charge corrected 1.995eV. Not detected. a
The binding energy of the Sn 3d512 transition decreased to 486.9 eV, and the Auger parameter, a, was calculated to be 918.7 eV. The Sn:O ratio stayed basically the same at 1:2.4: the Sn:F ratio was 1:0.4.
Discussion Titanium Getters. Titanium powder decomposed NF3 at temperatures as low as 160 "C, with complete conversion at 325 "C, which held steady for 2 h, and exhibited the lowest temperature for complete conversion of the three powders studied. The Ti was not passivated by the NF3 in contrast to when the Sn powder was used. Our studies have also shown that Ti02 can convert NF3 30% a t 300 " C , implying that oxygen contamination would not be a problem for conversion at these temperatures, which did occur when the Si powder was used. Conversion using the oxide produces primarily Nz, NzO, and NO, which explains the appearance of NzO at higher temperatures. Both X P S and XRD results support the formation of TiF3 on the surface of the Ti powder. Considering that the characterized material was still active when the reaction was terminated and that TiF3 does not melt until 1200 "C, we postulate that TiF3 is formed before TiF4 and remains on the surface until it is futher fluorinated to TiF4. The fluorination probably occurs via a n equilibrium of NF3 with F. The lowest conversion temperature can be lowered if the Ti powder has been first pretreated with NF3. When Ti is treated with NF3 a t 360 "C first and then cooled to room temperature and the reaction started again, the initial activity occurs around 160 "C, in contrast to 225 "C for untreated Ti powder. Either the surface is roughed up by the NF3, exposing more surface area, or more active sites are exposed. It is postulated that the surface of the reacted Ti is roughed up because of different rates of reaction between oxygenated titanium and elemental titanium and that this provides more active sites. This is also supported by the increase in surface area seen from the BET measurements. Si Getters. Si powder required the highest temperatures of all the metals tested to decompose NF3, not reacting until 250 "C, and complete conversion occurring a t 500 "C. The material showed deactivation after no more than 5 min at higher temperatures, while conversion at lower temperatures held steady for longer time (10-20 min) but was decreased with time. Pretreatment of Si powder with NF3 did not lower the lowest temperature a t which conversion occurred, as with Ti. XRD and SEM results both indicate the presence of some larger particles but not extreme sintering that could account for the decline in activity. From the
Thermal Decomposition of NF3 results of the XPS experiments, it can be seen that the Si surface is quite dirty, with silicon accounting for less than 1%of the surface atoms. It is assumed that SiF4 (mp -90.2 "C) was formed in the reactor, at elemental Si sites and converted to Na2SiF6 in the NaOH trap. While X P S shows evidence for Si-F bond formation, there is so little Si on the surface of the reacted powder that the XRD patterns of the trap arm residue of NanSiF6 and the treated material support the theory that SiF4 is quickly formed and leaves an etched Si surface so that more fluorination can occur, but at deeper, less accessible sites. Sites are less accessible because the exposed elemental Si sites are more active than oxygenated Si. Our studies have shown that Si02, in many high and low surface area forms, are not reactive to NF3 until 450 "C.19 As the elemental Si sites react with NF3, the Si is etched away, and the active sites become increasingly mass transfer limited, being surrounded by oxygenated sites. The extremely dirty surface of the Si after exposure to air and the more than doubling of the surface area support the postulate that the surface was extremely rough, exposing more surface for adhesion of oxygen, carbon, and fluorine. It has been shown that clean (meaning not contaminated by surface oxygen) elemental Si can decompose NF3 at temperatures as low as 200 "C, with complete conversion a t 345 0C.20 Oxygen is definitely a poison or inhibitor for this reaction. Sn Getters. Tin powder decomposed NF3 at the relatively low temperature of 130 "C. However, the powder was passivated very quickly, and steady state was never achieved. SEM confirms sintering of the powder, with an approximate doubling of the size of the particles. The results from the X P S experiments do not shed much light, as the Sn 3dm peaks did not change position, being of a higher binding energy than SnO, and the Sn:F ratio is 1:0.4 after reaction. The change in color from gray-white of tin t o an olive-green is perhaps the greatest indicator of when tin reacts. This is probably due to a fluorine shield, making the reaction diffusion controlled, but in a different manner than occurs with Si. The melting point of SnFz and SnF4 are both higher than the melting point of tin, and it is reasonable to assume that the surface became covered with fluorine and inaccessible to NF3, except at increasingly higher temperatures. Overview. In the general reaction of a metal with NF3, Na, and metal fluorides are produced. As one reviewer pointed out, there are references to the production of N2F4 when NF3 is passed over heated metHowever, there was no indication of N2F4 in our studies. The column used to separate the product gases should not trap or decompose N2F4, and the time range at which N2F4 should appear is clear of all peaks. All of the powders used have negative values of Gibbs free energy for the reaction with NF3 and are shown in Table 4 along with the appropriate fluoride formed. Also in Table 4 is a summary of the lowest conversion and (19)Vileno, E.; LeClair, M. K.; Suib, S. L.; Cutlip, M. Manuscript in preparation. (20) Shen, L.; Xiao, Y.; Vileno, E.; Ma, Y.; Suib, S. L.; Galasso, F. S.; Freihaut, J. D.; Hardwick, S. J. In Nano-size silicon whiskers produced by chemical vapor deposition: Active getters for NF,. Materials Research Society Proceedings; San Francisco, 1994. (21) Fluck, E., Merlet, P., Eds. Gmelin Handbook of Inorganic Chemistry; Springer-Verlag: Berlin, Germany, 1986;Supplement Vol. 4.
Chem. Mater., Vol. 7, No. 4, 1995 687 Table 4. Thermodynamic and Conversion Temperature Data
powder Ti Si Sn
AG",/mol of NF3 decomposed (kJ/mol of NF3) -1046.5 (TiF4) -1059.5 (TiFd -1089 (SiF4)-925.2 (SnF2)
lowest temp of conversion ("C) 160 250 130"
99.8% conversion ("C) 325 500 not obtained
" Indicates erratic, nonreproducible behavior. Table 5. Surface Area Measurements and Particle Size
powder Ti Si Sn
surface area by BET (m2/g) as received reacted 82 10 150 66 NIA N/A
particle size by SEM (ccm) as received reacted 10-50 10-50 1-80 1-40 20 10
complete conversion temperatures. NF3 etches the surface of the powders, leaching away the more reactive phase and leaving a rougher, higher surface area material. In the case of Ti, in which the oxygenated material is also active, this enhances the powder's ability to decompose NF3. However, in the case of Si, whose oxygenated surface is more resistant to NF3, the powder must be protected from oxygen contact to avoid limiting the active sites. Table 5 summarizes the surface areas calculated by BET, and the particle sizes as seen by SEM. All of the studies were performed with He as the diluent gas. As one of the reviewers noted, these results could be expected to be quite different if either Ar or N2 was used as the diluent and purge gas. Conclusion A n optimum getter for NF3 would have the following properties. The system would not release toxic products; NF3 would be completely decomposed at low temperatures; conversion of NF3 would not be diffusion controlled or passivated by fluorine; and the corresponding oxide would be reactive to NF3 in the same temperature range as that of the metal. The metal systems studied here all convert NF3 to N2 and the corresponding fluoride, which can then be trapped by an alkaline solution (as in the case of TiF4 and SiF4) if necessary. Forming a solid fluoride on the metal surface creates the problem of fluoride passivation, requiring increasingly higher temperatures to maintain conversion, as occurred with Sn powder. Therefore, a metal which forms a volatile fluoride is preferable. For ease in handling, the oxide of the powder should also be reactive to NF3 (as is TiOz), therefore allowing conversion even with oxygen on the surface, and avoiding diffusion constraints and/or handling problems. The slight difference in reactivity of Ti and Ti02 probably enhances the reaction by roughening up the surface and exposing more active sites. With Si, the difference in reactivity of NF3 with elemental and oxygenated Si led to the protection and inhibited activity of active sites, making the reaction diffusion controlled. Acknowledgment. Support for this research was from the Environmental Protection Agency administered through the Pollution Prevention Research and Development Center of the Environmental Research Institute at the University of Connecticut. CM9404374
