Pressure dependence of oxide growth rate on silicon and metal silicides

Surface Science 243 (1991) 127-131 North-Holland

127

Pressure dependence L.J. Terminello, IBM

J.A. Yarmoff

Thomas J. Wcrt.ron Rexurch

Received

of oxide growth rate on silicon and metal silicides

19 July 1990; accepted

I, F.M. d’Heurle

Center, PO Box 218. Yorktown

for publication

3 August

and F.R. McFeely Heights,

NY 10598,

USA

1990

Si 2p photoemission was used to determine the relative oxidation rates for Si(lll), Ru 2Si3 and CoSi,. Samples were thermally oxidized at 750° C with oxygen pressures varying from 1 X 10Kh to 1 Torr. The results confirm that CoSi, and Ru,Sis oxidize faster than Si(lll) at “ hight” pressures, but reveal the surprising feature that the growth of SiO, is faster on Si at pressures below 1 X 10m4 Torr.

1. Introduction When

a structure consisting of a sufficiently of a metal silicide over a silicon substrate is exposed to oxygen at sufficiently high temperature, two interesting results are, with a few exceptions, always obtained. First, metal-free oxide is formed, without consumption of the silicide film, even in cases in which the heat of formation of the metal oxide exceeds that of SiO,. Second the rate of SiO, formation always exceeds that which would be exhibited by the substrate Si alone. The interesting kinetics implied by these observations have prompted numerous studies, articles [1,2] and reviews [3]. Recently, Frampton et al. have employed [4] in situ ellipsometry to make a particularly careful and precise determination of the rates of SiO, formation for a number of metal silicides. In their work the initial oxidation rates, corresponding to the linear rate constants derived from a modified Deal-Grove analysis, were found to be correlated with the free carrier concentrations at the silicide/oxide interface. These as well as most previous experiments were performed at atmospheric pressure, except for example ref. [5] which was concerned with the problem of metal versus thin

film

’ Present address: Department of Physics, fornia, Riverside, CA 92521, USA. 0039-6028/91/$03.50

University

of Cali-

6 1991 - Elsevier Science Publishers

silicon oxidation in TiSi,. In the work we present below, we shall show the existence of an interesting new phenomenon not anticipated before, namely that at low oxygen pressures even the most reactive silicides become apparently impervious to oxidation, and that this effect is not correlated with the surface free carrier concentration or with the interfacial reactivity. In addition to the oxidation of silicide/ silicon structures, we conducted identical oxidation treatments of Si(ll1) substrates at each pressure investigated. This provides a rate standard for oxidation against which the behavior of the silicides could be compared straightforwardly. Our pressure dependent oxidation rate results are to be compared with reports [6,7] that Si itself does not form SiO, at low pressures but instead oxidizes with formation of volatile SiO.

2. Experimental Metal silicide and Si(ll1) substrates were cleaned in ultrahigh vacuum prior to oxidation. The Si(ll1) samples were flashed to 1050°C and then checked with photoemission to determine surface cleanliness. Sample temperatures were measured in situ using an optical pyrometer. The presence of a well defined surface state [8] in the

B.V. (North-Holland)

Si2p photoemission and the absence of any features due to contaminants in the photoenlissi(~ll verified surface cleanliness. Silicide samples were grown via electron beam deposition in another chamber, followed by metal-silicon reaction in a furnace under a flow of purified helium. Like the Si(l11). the silicide samples were cleaned in vacuum by flashing to 95O*C and were checked for contaminants through photoemission. Once clean, the Si and silicide samples were oxidized for 5 min at 750’ C in dry 0,. Oxygen was introduced into the sample oxidation chamber through a leak valve; the pressure was monitored by a C_ 1 X 10 ’ Torr and by a nude ionization gauge for pressures < 1X 1O-4 Torr. The pressure was held fixed during oxidation and was maintained during the cooling of samples to prevent the formation of oxide-void pin holes [9]. Specific pressures ranging from 1 X lo-’ to 1 Torr were used in this experiment and are listed in table 1. Immediately after oxidation and cooling, the samples were transferred under ultrahigh vacuum conditions from the oxidation chamber to the analysis chamber, and the relevant photoe~ssion spectra were obtained. The Si 2p photoemission spectra were measured at the National Synchrotron Light Source Beamline U8B which uses an ellipsoidal mirror analyzer

Table 1 The oxide thickness was computed from eq. and fSIO,= 7.1 A Substrate

P (Tom)

Oxide thickness

Si(lll)

1 x10-” 1x10 j 1 x10-

(I ) using d = 2.1 Average

(A)

(A/min)

1.o

4.6 4.x 11.2 15.3

0.92 0.96 2.24 3.m

CoSi 1

IxlO-h 1x10 i 5x10 i 1 x10-” 1 x10-’ 1 .O

0 0.7 11.2 17.7 21.6 31.1

0 0.14 2.24 3.54 4.32 6.22

Ru LSi 1

1 x lo- h Ixlo-i 1 XlO-4 1 x10-’

0.3 6.0 9.9 1 x.9

0 1.20 1.98 3.78

rate ---

-

prr\:iousl> described 1101. hlonochloma~ic photons i)f Ii0 CL’ \vcre pr~4uceJ h\ ;I 6M toroidal grating tltcttlnfhron-tatr,r [ 1 11 which gawait3 Si 2p photcrelectrons at a kinetic energ? of 17 rV. l-.arlicl studies have shown thib energ! range to be hurfacc and \vcll adapted for high-rcsolutic~n sensitive chemical charactcrilation of the surface region. ~h~~t(~en~issi~~n spectra were proccsaed by- first normalizing the rau spectrum with the photon-flux and then removing the secondary clcctr~~n hachground which includes the inelastic tail usxociatecl v,ith each Si photcrcmisaion peak. The \pt’c‘tra Mere then nun&call\ deconloluted to remo~c the greati! sizp, : \pin orhit c~~n~p~~l~eli( thereby simplifying the appearance of the spectra. The relative intensit\ of each formal oxidation state was then obtained h\ fitting each ccunp~~nent of the >pin w-hit decoupltxl spectra with ;I Voigiian line-shape (i.c. ;I f iaussian cctnvolutrd u-ith 3 Lorentxian dis~ribL~ti~~t1 fu~~ct~~~~~),Areas obtained from the linear least-squares fitting were Lised to calculate the ratio between the Si” ’ component of the oxide (i.e. the fully oxidized portion of the thermal oxide) and the unreucted Si.

3. Kesults Fig. I illustrates the cs~3cc of our resultx for C’oSi 2 oxidation. The top panel compare\ thcl obtained from the rcfercncc Si 2p3 Z spectrum Si( 111 ) iurfacc to that obtained from the C’oSi Z structure after each haa been subjected to oxidation at a preaxurc r)f 1 x 10 ’ Torr a1 7%)” C‘ for 5 min. In addition tcl the ~vell known interfacial suboxide peaks. the spectrum for the Si sample contains 3 prominent peak arising t‘rrwi fullyformed SKI,. By contrast. in the CoSi 2 qectrum this feature is completely absent, tend the spectrum is little altered from that obtained for the clean surface prior to oxidation. A small amount 01 tailing of the Si 2p, Z spectm~n is noticeable. however. as the samples BYW cooled 1~ r(-rom temperature before the 0: supply was turned off: this ma! he entirely attributed to ;I small ~irnoun~ ()I‘ Icm trmperaturc o*?;gen chemisorptic~n. !\t ,111IV,I&tion pressure of 1 Torr, displayed in the bottom panel, the situation is quite rtwxwd. In this figure

L.J. Terminello et al. / Oxide growth raie on SI, Ru$lj I

I

I

I

I

I

I

I

I

thickness

and Co.%,

of the SiO, overlayer

Is,o,/Zsl= A (e(d/‘Scc)~)-

130eV

P = 1 Torr oxbdation

__.-

I 18

I

20

/’

i

\ I 22

via the expression

181:

P = 1x 1O-6 Torr ox,dat,on

hv=

129

‘----___

I

I 24

---______e------

I

I 26

T

-

28

Kinetic Energy(eV1

Fig. 1. This illustrates the spin-orbit deconvoluted, background-subtracted Si2p photoemission spectrum taken on Si(lll) (solid line) and CoSi, (dashed line) that were oxidized Torr (upper panel) and 1 Torr (lower panel). at 1X10-’ Spectra were normalized in each frame by the largest peak. This demonstrates how pressure independent the Si(ll1) oxidation rate is, while CoSi, shows a stronger dependence.

the intensities have been scaled to give equal peak areas for the two SiO, signals. In the Si(ll1) spectrum the contribution from the unoxidized bulk is still clearly visible, however on the CoSi2 structure, this feature is almost completely obscured. This demonstrates the formation of a thicker oxide layer, as would be expected from the high pressure data of Frampton et al. [4]. Using a simple continuum model for the attenuation of the photoemission intensity, the ratio of the intensity of SiO, to bulk Si is related to the

1) ,

(1)

where Zsio is the fully oxidized silicon photointensity, I,,’ is the unreacted Si intensity, A is the 2p photoemission cross section ratio between fully-oxidized Si and unoxidized Si, Is,o, is the mean electron escape depth in the oxide, and d is the oxide thickness. In the limit of a thick oxide this holds true for a fixed photon excitation energy. This expression incorporates the Si photointensity attenuation owing to the oxide, thereby permitting the extraction of the oxide thickness, d, and thus the oxidation rate. While this continuum model of electron attenuation is for thick oxides, we use it to estimate our SiO, thicknesses for comparative purposes. Values for A and lsloz were obtained from table 1 in ref. [8] for the photon energy used in this experiment. Himpsel et al. [8] computed lsioz using eq. (1) starting with an oxide overlayer of known thickness as measured by ellipsometry. The empirical parameter A was calculated from the ratio of the Si2p photointensity of pure SiO, to the photoelectron intensity of pure Si measured at several photon energies. The oxide thicknesses computed using this model have a typical error estimate of k 10% [8]. Differences in 2p photoemission from Si and each silicide may affect Zsi for the silicides and can have several possible sources: from inherent differences in photoelectric cross section (if any), from unequal Si densities in bulk Si and the silicides, and from differences in signal attenuation within the silicides as compared to bulk silicon. A corrected Zsi for each of the silicides was derived from the clean Si and silicide 2p photoelectron intensity obtained at the same photon energy as the oxide spectra. An estimate of the growth rates could then be computed from the observed oxide thickness ratios and thus compared. The results of this analysis are displayed in fig. 2 and listed in table 1. The high pressure rates thus derived compare favorably with those obtained [4] by Frampton et al. when considering that our rates reflect the faster initial stages of oxidation and theirs are averaged over both the

I

I

I

I

Presure dependenceof

,,. I' I'

oxldatlon /' /'

i 10)

10-O

lo->

10“

10“

10'

10'

1

10

Oxidaton presure (Tom 0,)

Fig. 2. The pressure dependent oxidation rate for Si, Ru,Si, and CoSiz is illustrated. The samples were thermally oxidized in 0, at 750 o C for 5 min. The ratio of normalized photoemission intensities for SiO, to Si provides the basis for estimating the oxide layer thickness thereby enabling the average oxidation rate to be computed.

initial oxidation regime.

and the slower, thick-oxide

growth

4. Discussion Fig. 2 illustrates two interesting points. First, the transition from low-pressure to high pressure behavior is quite abrupt. It would appear that if a monolayer or two of SiO, can be formed, converting the surface reaction into an interfacial reaction. the high pressure limit is qualitatively reached. Second, this transition occurs at lower pressures for Ru,Si, than it does for CoSi?. It has been reported [3] by d’Heurle et al. that in the high pressure limit the more metallic silicides like CoSi 2 exhibit a much faster oxidation rate than Si, while silicides with less metallic character (e.g. Ru2Si3) oxidize at a rate commensurate with Si. This has been attributed to the availability of electrons at the Fermi energy. Also, the catalytic effect of the metal atom has been indicated [3].

Although either of these may explain ordering ol the (high pressure) rates of the interfacial reaction. it clearly does not fully describe the rates of SiO, formation that we observe at low oxygen pressure. Two possible mechanisms could he operative to explain our observations. The first is that the silicide surfaces could be extremelv inert towards molecular oxygen, and the IOU pressure observations would simply be a matter of different sticking coefficients at the different surfaces. This is somewhat difficult to accept. however. The L‘irht step in the oxidation process is essentially a charge transfer from the surface to the 0, molecule. It is difficult to imagine why this process should be any different at a surface than at an oxide covered interface. In addition, if the relatively low rates of initial reactivity are due to low sticking prohability we would be forced from our data to conclude that this sticking coefficient is smaller for a metallic surface than for an SiO, surface. which seems exceedingly odd. Finally. in the case 01 small sticking coefficients, one might expect to observe a region in which the growth rate might be small. but would vary approximately linearly with exposure. Instead we see a sharp transition between no oxide growth and rapid growth. All of these factors lead us to favor a second possible explanation that the oxidation reaction is still in fact proceeding below a certain critical pressure, but the nascent oxide is volatilized (as SiO) as rapidly as it is formed. Effects such as this have indeed been observed [6.7] for Si oxidation. The evolution of the volitile oxidation species would then continue until a critical pressure was reached at which the rate of formation of the nascent oxide exceeded its rate of volatilization. At the point at which monolayer coverage was achieved (at least locally) further reaction would then take place below the surface. There the overlying matrix could serve to trap the nascent SiO for sufficient time as to allow for its further reaction to form stable SiO,. The ordering of oxidation rate onsets in fig. 2 implies that Si bonding to a single oxygen does far more to weaken the bonds to the rest of the lattice in a silicide compound than in the corresponding case of pure Si. This may be substantiated if we consider that upon oxidation of a silicide such as

L.J. Terminello et al. / Oxide growth rate on Si, Ru,Si,

CoSi,, the Si atom would pick up a significant positive charge. While the bonding in the silicide is overwhelmingly covalent and shows little charge transfer, the binding of the nearest neighbor Co atoms to a positively charged ligand might very well be much less favorable than in the case of residual Si coordination. In conclusion, we have shown that at sufficiently low oxidation pressures no SiO, is formed on the surfaces of CoSi 2 and Ru,Si,. We suggest that this lack of apparent reactivity may be due to the volatiliztion of the nascent product. An attempt to detect the nascent SiO in the gas phase, as has been done [6,7] for Si oxidation, could serve to test this hypothesis.

Acknowledgements We would like to thank F.J. Himpsel for helpful discussions and C. Costas, J. Yurkas and A. Marx for technical assistance with the experiment. We are grateful to A. Taleb-Ibrahimi for exploratory work performed on WSi,. This work was conducted at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the Department of Energy (Division of Materials Sciences and Division of Chem-

and CoSi,

ical Sciences of Basic Energy Sciences) tract No. DE-AC02-76CH0016.

131

under Con-

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