Gas phase reactions of SiF2 with F2 and Cl2

Volume 122, number

GAS PHASE AC

CHEMICALPHYSICSLETTERS

3

REACTIONS

STANTON.

Aero&ne

Research

OF SiF, W1lI-i

A. FREEDMAN, Inc,

45 Mannrng

Rood

6 Deoemter

1985

F2 AND Cl,

J WORMHOUDT Bdlerrco

MA 01821.

USA

and PP

GASPAR

Recerved

13 Seplember

1985

cm’ Reaclrons of SlF2 rndlcnls have been sludled m a fast-flow syslem Rate conskm~s at 295 K of (47&03)~10-‘~ No cm’ molecule-’ s -’ for SIF,+ Cl, were obtamed molecule-’ s- ’ for the reactIon of SIFT + F2. and (5 1 +_O 6)X1O-‘3 reacuon was observed with O2 and Hz SIF,wz detected by laser-Induced fluorescence and hfellme observations and an excllntlon speclrum are reporled

1. Introduction The chemistry of the srhcon difluonde radical has been studied extensively [ 1,2], and it 1s known to be relatively easy to make and transport [3], yet there have been no studies of its absolute reaction rates Here we present results of the first such study of the reactions of SiF;! with F, and Cl,. SiF2 + F2 + SiF3 + F ,

(1)

SiF, + Cl, += SiFzCl + Cl

(2)

Until the recent pubhcation of studies of SiCl, reactions with several organic compounds [4], there had been no reported rate constant measurements for any of the mgher group WA drhahdes In contrast, the kinetics of halogenated carbenes have received considerable study. In addrtion to the reactrons wrth organic compounds reviewed in ref_ [4], rate constants are available for CF, + F2 [S] as well as for CFBr + F2 and Cl, [6] and for CF, reacting with several other inorgamc molecules [7]. This data base allows compansons of reactiwty between carbon and slllcon

analogs. Our mterest in reactrons of SrFz stems from a 190

program to investigate basic chemical mechanisms involved m plasma etchmg [8,9]. This is a process now receiving wide industrial application in with substrates such as srlicon are etched to form mtegrated circuit patterns. In this process, a glow discharge in a halogen-containmg gas (hke CF4 or Cl,) generates halogen atoms which attack unmasked areas of the substrate, removing matenal in the form of volatile halides In fluorme systems, the final product is SiF,, but some fraction IS thought to desorb as SiF, species of lower fluorinatron [lo]. Such groups are certainly observed on the substrate surface [l I]. Recent chemrcal kinetic modehng of the etching process [9] has sunply assumed that when gaseous SiF2 is produced, it is quickly fluormated in the gas phase to form SiF,. The results presented here permit a quantitative assessment of that assumption _

2. Experimental Measurements were performed in 2.54 cm diameter . room temperature(295 K) flow reactor constructed of Pyrex The halogen reactants, admixed in an excess of helium, were introduced directly into the upstream 0 009-2614/85/S (NorthaolIand

0330 0 Elsevier Science Pubhshers B-V. Physics Pubhshing Divrsion)

Volume

122, number

3

CHEMICAL

PHYSICS

end of the flow tube The S1F2 reactant m argon carrier gas was InJected through a movable quartz tube (0.64 cm outer diameter). The reaction be varied from 0 to 60 cm, corresponding

distance could to a maximum

LETTERS

6 December

1985

the quartz and teflon tubing (as noted by Margrave I31 )Tlus source, although easy to assemble and operate, has two drawbacks_ First, 1t performance 1s sensitive

reaction time of 50 ms at a typical flow velocity of 1200 cm s-l _Reactant and earner gas flow rates were measured directly by thermal conductlnty type mass flowmeters (Tylan) which were calibrated for the gas

to any presence of oxygen and second, the source appears to become exhausted after a penod of tune. Both effects are presumably due to the poisonmg of the s&con surface from which SiF2 IS produced We

mixtures used. The only exceptlon was chlorine, whose flow rate was directly measured durmg the experiment by divermg its flow mto a known volume and measurmg the pressure increase with tune. The flow

have developed an alternate source based on the disproportionation reaction of S1,Fs to produce S1F2 and S1F4. Si,F, at a flow rate of 1 X 10e3 see s-l, entramed m argon carrier gas (2.4 see s-l flow), was

tube pressure was measured with a capacitance manometer (MKS). Rate measurements were made under pseudofirst-order conditions where the halogen reactant was

sent through the same quartz reaction tube (without the s&con) at approximately 1100 K. Tunable diode laser absorption radical indicated

m great excess. Reaction times were varied by changmg the inJector position at fmed total flow velocity and pressure_ First-order decays of the minor reactant

K was required to produce SiF, but no apparent temperature dependence between 1000-1500 K was observed_ The S1,F, was prepared by warming a

species (SiFZ) were measured using laser-induced fluorescence detection_ The output of a frequencydoubled nitrogen-laser-pumped dye laser (Molectron DL-14) was focused mto a cross (purged with helium) situated at the end of the flow tube The induced fluorescence was monitored with a Hamamatsu R821 photomultlplier tube whose signal was processed by a gated integrator (SRS 250) mterfaced to an IBM XT computer through a Data Translation 2818 (12 bit) A-D board The fluorescence signal was averaged over 100 laser pulses, accounting for non-fluorescent background signal and for pulse-to-pulse fluctuations in laser mtensity.

mixture cordmg

Two methods of production of the S1F, radical were used The first techmque, developed by Margrave’s group [3], involves passing silicon tertrafluoride (99.99% pure) diluted m an mert gas through a heated quartz tube which has been packed with silicon lumps (99.999% pure) In these experiments, argon was added as a dduent at a flow rate of 3 O-3.2 see s-1 while the SiF, flow rate was set at 5 X 10-3 see s-l_ The temperature of the quartz reactor was maintained at 1350-1450 K as measured with a chromel-alumel thermocouple The major products of this source have been established by mass spectrometric analysis [3] to be SiF2 and S1F4 SiF2, in sufficient quantity to perform the expenments, could be transported through teflon tubmg to the main flow reactor, although deposits of perfluorosllane polymer could be seen on

measurements [12] of the SiF, a mmimum temperature of =Z1000

of Si2C16 and anhydrous zinc fluonde, to the method of Schumb and Gamble

ac[ 13!_

Purification was accomplished by trap-to-trap distflation in a vacuum line Punty of greater than 96% was established by the absence of detectable unpunt1es in a gas chromatogram Thus source was not prone to either large polymer production or great sensitlvlty to the existence

of oxygen

m the line.

3 Results 3 1 LaFer excitation spectnm The laser excltatlon

spectrum

of SfiZ of SiF,

x 1B,

c

z 1A, band IS shown 1n fig. L This spectrum was generated by scanning the grating and doubling crystal of the dye laser and collecting the non-dispersed induced fluorescence of the S1F2 radical. The observed band structure IS m accordance with the original absorption spectra by Kharma, Besenbruch and Margrave [ 141 using the SlF4/Si hot reactor source. There has been some controversy over the identity of the species producing an emission spec:rum (induced by a microwave discharge of SiF4) in the same spectral region. Wang et al [15] claimed that the emission spectrum corresponded to the SSE, species- But a recent analysis of all the extant data by Griffith and Mathews [I 61 indicates that all observations in both absorption and 191

Volume 122. number 3

CHEMICAL

2214

222

2

223

PHYSICS

0

EXCITATION Fig 1 Laser excltatlon vectrum absorption experiments

of the A + X bands of SiFl

emrssron are consistent with the assignment of the SrF;? A +X band. We attempted to record an excitation spectrum of SiF3 in this spectral region by driving the SrF, + F2 reactron to completion and probing the SIF, product with the laser. No spectrum was observed, indicating that there are no strong SrF3 spectral bands in the 220-24-O nm region which are accessible at room temperature from thz ground electronic state Spectrally unresolved lifet’me measurements on the SiFz fluorescence induced at 22 1 S run (04-O +OOO) mdrcated the existence of more than one hfetrme. The fastest fluorescence (shorter than 60 ns)accounted for approxrmately 80% of the total signal. An intermedrate hfetime on the order of 150 ns was recorded as was an extremely long hfetime of several mrcroseconds The long hfetime behaviour of the decay was qtute sensitive to the total pressure of either argon or helium, indicating the existence of a very efficient relaxation process AU kmetrc measurements were taken at the 221 6 nm (040 + 000) peak whrch gave the strongest absolute signal, using an integrator gate width of 60 ns. 3.2

Rate constant Typical

192

for SiFz + X2 {X= F, C7)

pseudo-first-order

decays of SiF2 radical

LETTERS

223

6 Dea?mber

2246

B

WAVELENGTH The arrows indicate

0

1985

2254

(nm) the band pounons

10

20

Reoctlon

reported

10

Dlslonce

in ref_ [ 141 from

40

so

60

km)

F= 2 PseudcGrst-order decays for SlFz reacting cess Fz at 10 Torr total pressure

with ex-

m excess fluorine reagent at 10 Torr total pressure are shown m frg 2. The lmearity of these decays over more than two orders of magnitude hnhcates the correctness of the pseudo-first-order assumption Similar

Volume 122, number 3

CHEMICAL PHYSICS LETTERS

decay plots were measured for 2 6 and 5 0 Torr total pressure Least-squares fits to these data generate first-order rate constants which are simply the slopes of the semilog plots after adJustment from distance to time scale. The true secondarder rate constant is extracted from fig. 3 which plots the first-order rate constants, corrected for diffusion as lscussed below, versus fluorine concentration_ A study of the SiF2 + Cl, reaction was made at 2 5 Torr total pressure and its results are also shown in fig 3 The rate constants (m cm3 molecule-l s-l) at 295 K derived from the above results are. k(SlF,

+ F2) = (4.7 f 0.3) X lo-l3

k(SlF,

+ Cl,) = (5.1 f 0.6) X lo-l3

No reactlon of SIF, W&I either 0, This Imphes an upper bound of 2 ecule-l s-l for the rate constant Correctrons for both axial and wall removal were made with the by Brown [ 171 Complete details

600

I

500

_

or H2 was observed cm3 molfor these reactions radial diffusion and procedure outlined are @ven elsewhere X lo-l7

1985

[ 181 The tifunon coefficient for SiF2 m AI was estimated as 0.095 + 0.015 cm2 s-1 at 1 atrr , by scaling values for a number of other systems for changes in molecular weight, size, and dipole moment. The uncertainty limits m the diffuzron coefficient gave rise to uncertainties in the derived rate constants of 1.8%. Systematic errors in flow velocity, temperature, and pressure are assigned a total uncertainty of 5.4%. The total uncetiamty lirmts expressed above are the square root of the sum of the squares of these vstematic uncertainties and an estimate of random errors. FOI the latter, we used the external consistency standard deviation [ 191, derived from the (internal conmtency) standard deviations of the slopes from the leastsquares fits shown in fig. 3. Since the experiments used the movable source configuration, wall losses of SiF2 measured m the absence of reactant gases were subtracted from the observed rate constants before plotting m fig 3. In our case, the small non-zero intercepts are due to changes in the wall removal rate. As noted before, the “Margrave” source, used in studying the reaction with

I 0

261~ -

,

6 I)ecemlxr

3 0

tom

0

10

IDrr

P

0

-

[Fz],

IO ” GIII-~

Fig 3 First-order reactlon rate constants

versus halogen concentiation

(a) SIFT + Fz; (b) SIFT + Cl2

193

Volume 122, number 3

CHEMICAL PHYSICS LETTERS

F2, produces polymer which apparently passlvates the Pyrex wall Indeed, after a few mmutes of passlvation, no wall loss of S1F2 could be measured Introduction of F2 into the system apparently removes tius pass1vatlon dunng the rate constant measurements From the mtercept III fig. 3, the wall loss 1s 22 s-l The inert nature of the passivated wall is restorted w1tb.m tens of seconds after removal of the F2 The Si2F6 dlsproport1onat1on source, used to study the reaction with Cl,, does exhlb1t some wall removal rate (m keepmg with its apparant low production of polymer) in the absence of Cl,, measured as about 20 s-l This was subtracted from the points 1n fig 3, which shows a residual Intercept of 14 s-l In any case, the effect of non-zero Intercepts on the measured rate constants is minimal in these studies

4. Discussion The results reported here are summanzed 1n table L For comparison, this table also contains avdable values for analogous CF2 reactions Perhaps most strrkmg is the sunllanty between CF, and SlF, The faster rate for SiF, + F2 over CF, + F2 may correlate with the ordering of reaction exotherrmcitles [21], which are 100 and 87 kcaljmole respectively, or may reflect Ihe larger polarlzability of S1F2. The rate constants for SIFZ react1on with F2 and Cl2 are seen to be very similar Although a drrect comparison with a value for CF, + Cl, is unavdable, the rate constant for CFBr + Cl2 has been reported, with a 300 K value of (1.5 i0 6)X lo-l3 cm3 molecule-l s-l [6] This shows a s&con to carb’on reactivity ratio sunilar to that seen for reactions with F,. We observed no react1on of S1F2 with 0, or H2 The upper limits reported in table 1 are the slowest rate constants observable under our conditions This lack of react1on IS not due to unfavorable energetlcs, given that exothermlcities for formation of SiOF, and HSF2 are 31 and approximately 50 kcal/mole, respectively However, CF, + O2 also has a 50 kcal/ mole exotherrmcity, and 1s known to be extremely slow [20] As another example, only a very slow upper limit has been obtained for CHF + 0, [22] Theoretical studies of carbene and sllylene insertion into H2 show a dramat1c increase m barner height with fluorme substitution [23] 194

6 December 1985

Table 1 Rate

COIIS~~~S

Reactant

for reactions of S& x-(SC)

(4.7 (5 1 t2 x <2 x

F2

Cl2 02 Hz

(this work) and CFI a) WW

* 0.3) x

lo-13

_+0 6) x 10-13 10-l’ 10-l’

8.3 x lo-l4 b) -

=2 x 10-2Oc)

a) Rate constants in umts of cm3 molecule-’

b) Ref_

[s].

‘=I Ref

-

s-l

_

[20].

We assume the products of the SiF2 + F2 reaction to be SiF3 and F. Formation of S1F4 would be much more exotherrmc (282.5 kcal/mole), and any product S1F4 would require third-body stablllzatlon before It decomposed mto SiF2 and F,. However, as seen in fig 3, the reaction rate was not influenced by the carrier gas pressure. The exotherm1c1ty of the reaction to form SiF3 1s sufficient to account for a broad visible chcmllumineazzr e contmuum which has been previously observed in plasma etching studies [24,25] Using a 0 25 m monochlomator, we obtamed spectra of this emission from the reactlon zone, noting some superimposed band structure which has not yet been identified To assess the slgnlficance of these results to the understanding of plasma etching mechanisms, we examine the assumption that desorblng S1F, is quickly fluorinated m the gas phase, so that the effect on the overall product yield 1s the same as that of desorbed SiF,. Using the S1F2 + F2 rate coefficient and F2 concentrations (from the modehng of ref. [9]) 1n the 1Ol3 to 1014 cm-3 range Bves a half-time to SiF, formation by this process of from 0 2 to 0.02 s. In addition, our prellmlnary observations of S1F2 + F + M support very similar half-times due to this reaction Here, the lower efficiency of the three-body process 1s balanced by the higher fluonne atom concentratlons [9] These half-times are short enough that only SiF4 would be expected far from the substrate (as is observed) However, they are long enough that S1F2 may partrclpate in gas-phase chemistry near the silicon surface. Therefore, 1ts fluonnation reactions should be included in a complete model of the chemical kmetics of plasma etching.

Volume 122, number 3

CHEMICAL PHYSICS LETTERS

Acknowledgement

6 December 1985

[ 1 l] F R McFeely. J F Morar, N-D. Shinn, G. Landgren and F J. Himpsel,

This

work was supported

Foundation

under Grant No

by the Natlonal Science CPE-8306198.

Drs J A. Sliver and M S Z&miser

We thank

for helpful

slons We gratefully acknowledge the technical sistance of W. Goodwin and P Saconne

discusas-

References

r11 J

L Margrave and P W. Wilson, Accounts Chem Res 4 (1971) 145. [21 T.L Hwang, Y.M. Pai and C.S Lm, J. Am. Chem Sot 102 (1980) 751 131 P L. Tunms, R.A. Kent, T C. EhIert and J-L Margrave,

J Am. Chem. Sot. 87 (1965) 2824.

I41 I Safxik, BP. Ruzslcska, A Jodhan, 0 P Strauxz and T.N. Bell, Chem. Phys Letters 113 (1985) 71 [51 C Seeger, G. Rotzoll, A Lubbert and K. Schugerl, Intern

J. Chem.

Kinetics

14 (1962)

457.

I61 J R Purdy and B A. Thrush, Chem. Phys Letters 80 (1981) 11. 171 D S Y Hsu, M E Umstead and M C Lm, ACS Symp Ser 66 (1978) 128. 181 J.W Cobum and H F. Wmters, Ann. Rev. Mat. Sci 13 (1983) 91. [91 D. Edelson and D.L. Flamm, J. Appl Phys Phys 56 (1984) 1522 1101 H.F Winters and FA. Houle, J. Appl. Phys 54 (1983) 1218

Phys

Rev

B30

(1984)

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[ 121 J Wormhoudt,

A. Stanton and J Silver, in Materials Research Society. Symposmm Proceedmgs, VoL 38. Plasma Synthesis and Etchmg of Electromc Materials, eds R P-H. Chang and B. Abeles, p 91

1131 W.C. Schumb and E-L Gamble, J. Am Chem Sot 54 (1932) 583 [14] YM Khanna, G. Besenbruch and J.L Margrave, J Chem Phys 46 (1967) 2310. [ 151 J.L -F. Wang, C-N Krlshnan and J.L Marpave. J_ Mol. Spectry. 48 (1973) 346. [ 161 W-B Griffith and C-W. Mathews, J. Mol Spectry , submitted for publication_ [17] R L. Brown, J. Res Nat! Bur Std 83 (1978) 1 [18] J A. Silver, J. Chem. Phys 81 (1984) 5125. [ 191 R J. Cvetanonc, D.L Smdeton and G Paraskevopoulous, J. Phys Chem. 83 (1979) 50 [20] W J-R Tyerman, Tranr Faraday Sot 65 (1969) 163. [21] M.J.S. Dewar, The molecular orbital theory of organic chemisey (McGraw-Hill, New York, 1969) p 284 [22] G Hancock, Abstract of Paper Il,8th International Symposium on Gas Kme!ics, Umversity of Nottmgham (15-20 July 1984) [23] C Sosa and H B. Schlegel, J. Am Chem. Sot. 106 (1984) 5847. [24] J A. Mucha, V M Donnelly, D L Flamm and L M Webb, J. Phys Chem 85 (1981) 3529 1251 V M Donnelly and D L. Flamm, J. Appl Phys 51 (19Kl)

5273.

195