Reaction of chlorine and molybdenum by modulated molecular beam mass spectrometry

322

Surface Science 249 (S991) 322-334 North-Holland

Reaction of chlorine and molybdenum mass spectrometry M. BaIooch aY DR.

Ulander

‘, W.J.

Sietiaus

by modulated molecular beam

a and D.E.

MiIIer

a

Received 18 September 1990, accepted for publication 26 December 1990

The reaction of molecular chlorine with poiycrystalline molybdenum was studied by the modulated molecular beam mass spectrametry technique. Between 300 and 900 K, MoCl, was produced with a reaction probability of about lo-’ on a chlorine-covered surface. At higher temperatures, the chlorine coverage decreased because of three direct reactions on the substrate metal: recombination to form Cl,; direct desorption of atomic chlorine; and a second reaction channel to produce MoCl,. Chlorine cbernisorbed on the metal surface acquired sufficient mobility to move away from the spot illuminated by the incident molecular beam. By 1500 K, the only major processes remaining were chemisorption of incident Cl, with near-unity sticking probability followed by rapid desorption of atomic chlorine. A kinetic model of the reaction, inchrding ah of the component elementary steps, was developed and compared to the data.

1. Introduction Refractory metals are excellent candidate materials for electrical connection lines in microelectronic devices and integrated circuits. Metals such as tungsten and molybdenum are stable at high temperatures and are amenabie to dry-etch patterning. Chlorine is a useful reactant gas for these metals because etching occurs even at room temperature and is also enhanced by ion bomb~dment {l]. This paper describes the basic surface chemistry of thermal etching of molybdenum by molecular chlorine in a mariner similar to that applied to the tungsten-chlorine reaction [2]. In the early work of Rosner and Allendorf [3,4], the reactions of W and MO with atomic and molecular chlorine were studied by resistance heating techniques; the extent of reaction was determined by the change in resistivity of the specimen. They reported chlorination reaction probabilities (the fraction of the incident chlorine molecules that return to the gas phase as part of a metal chloride molecuIe> over a surface temperature range from 400 to 1500 IL Recently, Fischl

and Wess [5] have reported the etching rate of fiIms of these metals in and downstream of a chlorine plasma as functions of atomic chlorine concentration and temperature. Since neither of these studies were mechanistic in nature, we have used the modulated molecular beam teclmique with in situ mass spectrometric detection of reaction products to study the Ma/Cl, reaction,

The basic features of the apparatus are shown in fig. 1. Detailed descriptions are available in refs. [2,6] and only a short summary is presented here. The system consists of three differentiallypumped vacuum chambers separated by orifices. The function of the latter is to form collimated molecular beams from the diffuse fluxes in the upstream chambers which contain a source of molecules. In the source chamber, the reactant gas fIux is generated by effusion of Cl, from a hole in the end of a quartz tube connected to a chlorine cylinder. This flux is square-wave modulated at

0039~6028/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

hf. Balooch et al. / Reaction

of chlorine

frequencies between 20 and 800 Hz by a rotating toothed disk. A small portion of this efflux passes through the orifice leading to the target chamber as a modulated molecular beam and impinges on the target, which is a 15 mm diameter disk of polycrystalline molybdenum. Because of the geometries of the hole in the source tube and the collimating orifice, the beam striking the target has an umbra of full intensity I, about 1.3 mm in diameter and a penumbra extending to a diameter of 4.2 mm over which the intensity falls linearly. The shape function of the incident beam is given by: P

h(r)=

K 1 I P

0

for

r
for

r,
for

r>rp,

u

.n .\\\

323

and molybdenum

where ru and rp are the radii of the umbra and the penumbra, respectively. The molybdenum target is heated by electron bomb~dment and its temperature is measured by an infrared pyrometer. Prior to an experiment, the specimen is heated to 1500 K for 30 min to remove surface oxide. The specimen is then cooled to room temperature and ion-bombarded to remove carbon on the surface. The base pressure in the target chamber is about lop8 Torr. Part of the incident reactant beam is reflected from the surface without significant interaction (other than partial thermal accomodation). The rest is chemisorbed and is ult~ately emitted as products. A small fraction of the reflected chlorine and the reaction products pass in free-molecule flow through an orifice leading to the detection chamber. This chamber houses a quadrupole mass spectrometer whose ionizer has a line-of-sight view of the reaction spot on the surface.

VIEW PORT

Fig. 1. Modulated molecular beam apparatus.

I /

,/

ION TITANIUM PU~P+SUBLiMATlON PUMP

324

M. Balooch et al. / Reaction of chlorine and rn~~b~n~rn

The mass spectrometer output signal, which is modulated at the same frequency as the incident reactant beam, is processes by a lock-in amplifier to produce an amplitude and phase angle for each species under each experimental condition. These signals are combined and subjected to various correction factors to yield the apparent reaction probability, <, which is the ratio of the product and reactant flux amplitudes, and the reaction phase lag, q, which is the difference between the phase angles of the same two species. The method for determining e and rp from the mass spectrometer data is outlined in the appendix. These quantities are compared to predictions of theoretical models of the surface reaction.

f = 20 Hz

-3

3. Results Three products have been identified: molecular chlorine from recombined adsorbed Cl; atomic chlorine from direct desorption of adsorbed Cl; and MoCl, from the reaction of adsorbed Cl with the substrate metal. The reaction products were characterized as functions of surface temperature, of modulation frequency, and to a lesser extent, of incident reactant beam intensity. At each combination of experimental variables, the amplitude and phase angle of a product and those of the reactant gas scattered from the surface were measured. The latter measurement was performed at the same modulation frequency and incident beam intensity as that used in measuring the product signals but with the surface temperature at 300 K. At this temperature, 99.9% of the incident reactant Cl, scatters from the surface without reaction. The reflected reactant signal is a measure of the incident beam amplitude and phase angle.

r

f=ZlOHz

o.oooL 3. I. Molecular and atomic chlorine products

300

500

700

I

900

1100 1300 1500 6o

T W)

The information contained in the reflected Cl, signal is twofold. At temperatures at which chemical interactions are small (roughly < 900 K), thermal accom~a~on of the incident beam with the surface increases the speed of the reflected Cl, molecules. As shown in fig. 2, this results in a slight depression of the amplitude of the signal

Fig. 2. Molecular chlorine signal amplitude and phase lag with respect to those from a reflected beam from a room-temperature target at a fixed incident beam intensity of 4 x 10” s-l.

compared to that measured when the target is at room temperature and a reduction of the phase angle compared to the same reference (i.e., a phase

Fig. 3. km

lead rather than a phase lag). As explained in the appendix, the thermal accomodation coefficient of Clz on the chlorine-covered molybdenum surface can be obtained from this ~fo~~o~~ As the temperature is increased above 9W KY;1 ~ro~o~~~d reduction in the rna~t~d~ of the Cl, signal is observed. The phase lead becomes a phase lag beyond 1200 R. This behavior indicates that the observed Cl2 signal contains a contribution due to surface recombination of Cl atoms chemisorbed from the incident beam in addition to Cl, reflected from the surface without interaction, Conservation of mass requires that reduction in the 61, signal should be accompanied by an increase in the signals of other chlotine products, and this is in fact obmed; fig 3 gives the reaction probably and phase lag of the atomic chIorine product (after correction for cracking of molecular chlorine). By 13.30 K, nearly total dissociation of the incident molecular beam occurs aniX the phase lag rapidly approaches zero, which indicates a fast surface decomposition axed desorption, process. Thus, at the highest surface temperatures studied, the fate of the incident Cl, can be accounted for. Between 900 and 131x)z(, however, the chlorine material balance based on observable products does not close. At 1200 I& for ez3rnple, fig. 2

shows that - 90% of the chlorine in the incident beam has been remwed, However, only lt)% is amounts for as atomic chlorine (fig- 3) and less than 1% appears as moly~en~ chlorides, Bxamination of the reacted specimens by scanning electron microscopy suggests the reason for this discrepancy. The molybdenum surface exposed to the Clz beshows considerably roughening and preferential etching of gram boundaries [I]# Following exposure to Cl, at high temperature, evidence of attack is observed on portions of the surface far beyond the confines of the beam spot on the exposed surface of the spe&men. Even the rear face of the sample shows signs of above etching. The loss of CI, signal and the pervasive surface ro~~~~ng are consistent with rapid surface diffusion of adsorbed chlorine atoms following chemisorption. As will be shown later, the processes of Cl atom recombination to produce Cl,, direct desorption of adsorbed Cl, and surface reaction to form molybdenum chlorides do in fact occur at rates that balance the rate of chlorine chemisorption from the incidem beam. However, the products are produced at locations on the specimen whi& cannut be seen by the mass spectrometer, which views o&y the beam spot.

MoCl’ and MaClt are the only metal-containing ions detected. Both ions exhibit the same dependence on temperature and have the same phase angles, indicating that they originate from the same parent species. In the absence of any evidence of higher cuorides, the sole gaseous rne~~-~~~~ product of the ma&ion is MoC12, However% since MoCIf has the highest intensity signal, ah measn~men~s of the et&ring portion of the reaction were performed on it. Correction using the observed fragmentation pattern yields a measure of the emission rate of neutral MoC1, from the surface. Fig. 4 shows the apparent reaction probability and phase lag as functions of temperature at a constant incident beam intensity and modulaticm frequeneiies from 20 to WO Hz. The apparen& reaction ~r~bab~ty increases slowly with increasing t~~~t~re up tu - ?JOQR Thereafter, a

326

M. Balooch et al. / Reaction

of chlorine and molybdenum

rapid drop occurs and the signal passes through a sharp minimum at 1200-1300 K. At higher temperatures, the signal recovers its low-temperature value. The large phase lags of the Cl, signal seen in fig. 2 indicate that a dominant reaction at high temperature is recombination of, adsorbed Cl atoms to produce Cl,. The presence of a nonlinear I

L

1

7016

2

I

I

5

10”

‘0

Beam Intensity, lo (Molecules/cm*-s) Temperature 15001000 10-2 I I

(K)

Fig. 5. Variation of the MoCl, reaction product incident reactant beam intensity.

300

500

I-

/

f=20Hz

r,

_60

vector

with

6-‘5

10-J ;

'

/

step in the reaction mechanism should affect all other steps that depend on the concentration of adsorbed Cl. In particular, MoCl, should be so influenced and should exhibit nonlinear behavior with respect to the intensity of the incident Cl, beam. Fig. 5 shows that this behavior is indeed observed.

' 80

f = 210 Hz -60 . l

. l

z

.

o o

0

2v 1 o-46

-40 . 0

‘:

-20

= F

8

n

20 10

14I

18 1

22

26 /

30 1

340

104/T (K-l) Fig. 4. Reaction

product vector for MoCl, at an incident intensity of 4 X lOI cm-* s-‘.

beam

4. Reaction model 4. I. Low temperature

reaction

Although the principal chloride of molybdenum produced by reaction of the metal with atmospheric-pressure gas is MoCl,, the trichloride is produced at lower Cl, pressures [7]. The present experiments were conducted at even lower equivalent Cl, pressures (- 10e4 Torr), so observation of a reaction product with a Cl/M0 ratio lower than those found at near-atmospheric pressure is not surprising; it is a consequence of a type of Le Chatelier’s principle: the Cl/M0 ratio of the principal reaction product decreases as the Cl, pressure is reduced, provided that the product is reasonably stable and volatile. A product such as MoCl, for example, would probably not be produced no matter how low the Cl, pressure. MoCl s disproportionates into the dichloride and the tetrachloride. MoCl, is reported to be nonvolatile (at least at room temperature) but to dissolve in alcohol in the form of the hexamer (MoCl,),

]71.

321

M. Balooch er al. / Reaction of chlorine and molybdenum

During reaction at low temperature in the molecular beam apparatus, it is likely that the metal surface is covered by a ti~tly-bound chlorine-containing scale, possibly in the form of polymerized MoCl z units. Modulated ion beams studies of this reaction show evidence of such a scale [l]. At low temperatures, fig. 2 shows that nearly all of the incident chlorine is reflected from the surface. Fig. 4 shows that a small fraction ( -zz0.003) reacts to form volatile MoCl,. The reaction probability and phase lag of this product relative to the incident Cl, beam are only slightly temperature-dependent. The phase lag, on the other hand, shows a distinct dependence on modulation frequency, increasing from < 10 o at 20 Hz to 60’ at 800 Hz. The mechanism proposed for the low temperature reaction is as follows. Incident chlorine dissociatively adsorbs on the chlorine overlayer according to:

impact, instead forming TaFz as the principal ion 181. However, the sequence represented by eqs. (3a) and (3b) is deemed to be the more likely of the two because the likelihood of complete fragmentation of MoCl, be electron bombardment is small. In either of the above two possibilities for volatile product formation, the data require that the surface reaction be first order with respect to the intensity of the incident Cl, flux. This leads to description of the kinetics of this step by a simple linear or pseudo-linear reaction with rate constant designated by k,. To implement this model, the surface balance. for the concentration of the Cl* species, designated by n*, is written as: dn* dt

= 2r)*Z,g(t)

- 2k,n*,

MoCl,(surf)%MoCl,(g),

(3a)

where I,, is the steady incident beam intensity and g( t ) represents the square-wave modulation function (g = 1 during the “on” period and g = 0 during the “off) part of the cycle). n* is the sticking probability of Cl, on the chlorine-covered metal. In describing the kinetics of the volatile MO product formation, the dichloride is assumed to be the gaseous product. The time-dependent quantities in eq. (5) are written as two-term Fourier expansions, including a steady-state term and the fundamental-mode term:

2Cl*(ads)

(3b)

g(r)

Cl, -+ 2c1*,

(2)

where Cl * denotes chlorine adsorbed on top of the existing strongly bound Cl adlayer. Subsequent etching of the metal may occur by one of two reaction sequences. In the first possibility, Cl* induces evaporation of MoCl, units in the scale and then reforms the scale by reacting with molybdenum atoms from the substrate:

+ MO -+ MoCl,(surf).

= *(l+

A second possible surface reaction involves production of the trichloride by the reaction: Cl* -t M~l~(surf)

--, MoCl,(g),

(4a)

followed by reformation of the scale by: 2Cl* + MO --, MoCl 2 (surf).

(4’$

Although the reaction product is the trichloride, it may not be detected by the mass spectrometer because of complete fragmentation by the 70 eV electrons in the ionizer: MoCl, + e -+ MoCl:

+ Cl + 2e.

There is precedent for the above process. TaF& for example, produces no parent ion upon electron

g, eaWlri),

(6a) (6b)

In these equations, i = m and g, is the coefficient of the fundamental mode of the Fourier expansion of the modulated incident beam gating function g(t), The modulation frequency is f Hz. n,* is the steady-state component of the surface concentration n * and ii * is its fundamental mode coefficient. It is a complex number. Substituting eqs. (6a) and (6b) into eq. (5) and collecting the time-independent term yields:

rl”Z* “0*=-g-, 2

(7)

while the fund~en~-mode

terms yield:

The fundamental mode coefficient of the MoCl, emission rate from the surface is fNnCI, = k,fi*. Dividing this quantity by the fundamental mode coefficient of the incident reactant beam intensity (& iO) yields the reaction product vector for production of MoCl,:

Identifying the apparent reaction probability f M&l* and the product phase lag with respect to the reflected Cl 2 beam, $MoC,2yields: ‘M&l,

=

(10)

,l + (v;;k2)2,1'2'

f(Hz) Fig. 7. Experimental sticking probability of CIz on a chbrinecovered surface according to eq. (10). Conditions as in fig. 6,

and

To illustrate the method of deducing reaction parameters from the apparent reaction probability

-

i-7

-~-----

---T---/

-

1

I

__._._J

4

l_______l_i_____

Fig. 6. Frequency dependence of the MoCSI, phase lag at 385 K with respect to the reflected Cl, beam at a surface temperature of 300 K at an incident beam intensity of 4 X 1Ol6 cm-’ sci.

and phase lag of the MO& signal, the data from fig. 4 at 285 K are plotted according to eqs. (10) and (II) in Figs. 6 and 7. The slope of the line in fig. 6 gives a rate constant k, = 1500 k 1000 S-I and fig. 7 gives q * = (2.2 f 0.8) x 10s3. The temperature dependence of the rate constant and somewhat more accurate values of the sticking probability are obtained by fitting the data from fig. 4 at other temperatures < 900 K.

As the surface temperature exceeds - 900 K, the strongly-bound chlorine overlayer on the substrate begins to break up and bare metal is exposed. A second channel involving direct reaction of surface Cl with the substrate metal rather than via a chlorine overlayer is opened. Sticking of the incident Cl, molecules on the bare metal surface is much more efficient than on the chlorine-covered layer present at low temperatures. The complete conversion of incident Cl, to gaseous Cl at 1500 K (fig. 3) indicates that the sticking probability on the bare metal, q, is close to unity. It is assumed to be independent of tem-

perature, so the chemisorption reaction on the metal surface, Cl,(g) -+ 2c1, is dependent only on the fraction of the total surface area that is bare metal. For simplicity, a linear dependence is assumed. The remaining elementary chemical steps on the surface are: desorption:

Cl * Cl(g),

031

which is an observed reaction product, and recombination :

x1 -

C&(g),

ff4

which, although not directly observable because of the large quantity of other products, is inferred by the observed nonlinearity of the MoCl, product at high temperature. The rate constants for the above reactions are denoted by k, and k,, respectively. The second channel to produce the volatile dichloride is broken into a slow step: Cl + MO -+ MoCl,

(15a1

with rate constant k, followed by a fast step: Cl + M&l--,

M&3,(g).

Fig. 8. Specimen geometry and incident reactant beam in&msity profile: (a) actual specimen; fb) equivalent geometry with the same surface area as the front and rear faces of the actual specimen.

matter flow. The simplified geometry, shown in fig. Sb, has a radius A = 10.5 mm. The balance on chemisorbed chlorine on the bare portion of the metal surface ~~n~~tratio~ = fi):

ffW

This combination gives a better fit to the data than the bimolecular reaction 2CX + MO + MoC!l,(g). The latter is equivalent to equilibrium for eq. (Via) followed by slow step represented by eq. (15b). At temperatures exceeding 900 II, surface mobility of the chlorine adlayer is sufficient to spread Cl beyond the confines of the beam spot. This depresses the MoCl, signal by removing the lieu of reaction (15) from observation by the mass spectrometer. As the temperature is further increased, the second reaction channel to form MoCl, becomes important. This pathway has a higher activation energy than does surface diffusion of Cl adatoms and so produces reaction before beam-spreading on the surface occurs; the product signal therefore begins to increase. In order to model this aspect of the overall process with a minimum of rna~~rn~tic~ complexity, the actual geometry of the specimen (rear and front faces, fig_ ga) is replaces by a single circular surface of the same total area as the actual surface but with a periphery that is impervious to

- 2k,n2 - k,n - 2kp.

06)

The function h(r) is the beam shape profile shown in fig. 8 and given by eq. (1). N is the maximum of available sites for chlorine adsorption on the molybdenum surface. In the simplified geometry of fig. 8b, the boundary conditions for eq. (16) are:

The above balance on Cl is a~ornp~~ by an equivalent surface balance on Cl *, which is given by a modified form of eq. (5): an*

at

= 2~j*l,h(r)g(t)$

- 2k,n*;.

(18)

The last term on the right hand side of eq. (18) represents MoCl, production by the low-temperature reaction channel analyzed in the preceding section. It continues as long as strongly bound Cl is present on the surface, but in the high-temperature regime, its rate depends on the surface cover-

M. Bafooch et al. / Reaction of chlorine and molybdenum

330

age by Cl. This is the reason for the presence of the surface coverage n/N in the two terms on the right hand side of eq. (18). The above surface balance equations are solved by the two-term Fourier expansion technique utilizing eqs. (6a) and (6b) and the analogous expansion of n(r): n = n, + n e*+.

(19)

Subs~tut~on of the Fourier expansions into eqs. (16) and (1%) and collection of the steady state terms and the coefficients of the fundamental mode terms yields two ordinary differential equations: D,p%,,+qz&)(l-

component of the total rate at which reactant molecules strike the target: f,,r = 2G(-fgt)~%+dr = ~z*&(r,z+r,r,+r,z).

(24)

The fundamental mode of the flux of atomic chlorine desorbed from the surface is: & = Zal”k,Er

dr,

(25)

and the fundamental mode of the flux of MoCl, emitted from the surface is:

S)-2k,,li,-k,n,

- 2k,n,=O,

(26)

(20)

For the atomic chlorine reaction product, the theoretical reaction product vector is defined by: - kd& - 2k,%=2=fSi,

(21)

where v2 = r- 'd/dr( r d/dr). The boundary conditions for eqs. (20) and (21) are given by eq. (17). The Cl* balance (eq. (18)) yields two algebraic equations: * _

no-

v*zoh(r) 2k

(22)

2

and s*

- Eq*loh(r)/2k,](no/N)g, _

(no/N) + (vf/k2Y

*

(23)

Eqs. (20)-(23) are solved nume~cally for a set of rate constants and a surface diffusion coefficient. To compare the theoretical result with the experimental data, the fundamental modes of the product and reflected reactant fluxes are formed from the solutions for ?i and Z *. These fundamental-mode fluxes result from emission from all parts of the surface viewed by the mass spectrometer. The geometry of the target-collimator-mass spectrometer combination (fig. 1) is such that the ionizer is exposed to a region on the surface which is appro~mately the same as the area illu~nated by the incident Cl, beam. The reference fundamental-mode flux to which all other fluxes are compared is the first Fourier

f cl e- WC1 = f,/21,,

.

(27)

The factor of two in the deno~nator limits the maximum apparent reaction probability to unity. The theoretical reaction product vector for the MoCl, gaseous species is defined by: ‘MoCIZ

-i+ eMoCI,

= A40C,z/freff

(28)

These model-based reaction product vectors are compared to the experimental values determined from eq. (A.2) in the appendix. For comparison with the experimental data, eqs. (AS)-(A.7) of the appendix are used to produce theoretical predictions of the ordinates of fig. 2. 4.3, Fitting the model to the data The theoretical model contains nine parameters which are obtained by fitting to the data contained in figs. 2-5. Some of the parameters are easily obtained by examination of the data under limiting conditions. The bare surface sticking probability Q, for example, is close to unity because of the essentially complete Cl, dissociation observed in fig. 3 at 1500 K. Based on previous experience with chose-Mets reaction systems studied by the same method, temperature-independent dissociative sticking probabilities are the

norm, although, with the exception of tungsten 121, they are usually not as large as unity [9,IO]. The beak a~m~ation coefficient of Cl, on the Cl-covered MO surface is the sole parameter responsible for the decrease of the reflected chlorine signal amplitude ratio and its phase lead seen in fig. 2 at low temperature. Finally, the rate constant k, and the sticking probability of Cl, on the Cl-covered MO surface are the onIy parameters that affect the MoCl, reaction product vector at temperat~~ less than 900 K. Tbe remaining two rate constants (k, and k,), the surface diffusion coefficient (DJ, and the satiation density of Cl on the surface (N) must be determined by comparing the fulI solution to the data at intermediate temperatures in figs. 2-5. Fitting is performed by a Monte Carlo technique. Parameter values guessed .and the mean-square deviation of the model predictions from the ensemble of the data is computed. Since the data are complex numbers (i.e., each point possesses an amplitude and a phase), there is no straightforward method of combining global error measures for each into a single goodness-of-fit number. This problem is solved by an arbitrary weighting of the mean error of the ~~~c~jo~~~fference between the experimental and theoretical apparent reaction probabilities and the mean deviation of the ubsoltlfe difference in the corresponding phase lags. Other techniques are possible [ll]. The parameters yielding the best fit of the model to the data are listed in table 1 and the model predictions are shown as the curves in figs.

Table 1 Parameters of the Mop& Parameter

N

reaction model

~exponential factor 8 X10’4cm-2 1.0 3 x10-3 1.3 X 107cm2/s 5 x lo-2cmz/s 6 x10’2 s-1 3 x10= s-l 25x10’s-1 0.3

Activation enW%Y (kcal/mol)

16 54 $8 61 0.4

Tabie 2 &sorption

rate constants of atomk chhrine 011 ~~ybd~~

Surface

Activation energy (kcai/mol)

Pre-exponential factor (s-1 x10-‘3)

Ref.

(110) (100) (111) (100) Polycr

Q4

2 5 3 3 0.6

[I21 WI [I21 113) This wo&

92

95 94 58

2-5. The fit of the model to the data is quite good. In particular, the sharp minima and maxima in the MoCl, apparent reaction probab~ty and phase lag in fig. 4 is adequately reproduced. The nonintuitive effect of decreasing the MoCl, reaction probability and phase lag by increasing the incident beam intensity is properly matched, If the surface reaction to produce MoCl, were second order in the concentration of adsorbed Cl, increasing the beam intensity should have increased, not decreased, the apparent reaction probability (fig. 5). The reason for the opposite effect is that this reaction step is first order in the surface Cl concentration (eq. (1Sa)) while the r~mbination reaction is second order (eq. (13)). Therefore, an increase in surface Cl concentration caused by increasing the incident beam intensity enhances the recombination reaction more than the MoCI, production reaction.

The pre-exponential factor for atomic chlorine desorption (kd in table 1) is in the range expected for the ~bration~ frequency of single atoms bound to the surface. The MO-Cl binding energies and pre-exponential factors from this study and earlier work using single-crysta1 molybdenum specimens [12,13] are compared in table 2. The present results using polycrystalline MO show a considerably lower binding energy than that on single crystals. The pre-exponential factor of the recombination rate constant k given in table 1 is in the upper range of such parameters for other bimolecular surface r~mbination steps jf4,15f. It is close

332

h4. B&o& et at. / Reachon ofchlorineand mo~~enum

to that expected from the collison frequency of a two-dimensional ideal gas. The thermal accomodation coefficient LYof Cl, on the Cl-covered MO surface of 0.3 is approximately three times larger that of Cl, on GaAs [16]. In addition, EYappears to be temperature-independent. The saturation Cl adatom density on the metal (N= 8 x lOI cme2) is the same as that found on GaAs /16]. The reaction probability for production of MoCl, at high temperature is in good agreement with the earlier work of Rosner and Allendorf [3]. At low temperatures, however, the reaction probability measured in the present work is considerably greater than theirs. Since the background pressure in the apparatus used by Rosner and Allendorf was orders of magnitude higher than in our vacuum system, the discrepancy at low temperatures is probably due to impurities on the surface of the specimens in ref. [3] that impeded etching. The reaction probability measured by Rosner and Allendorf with atomic chlorine as the reactant gas agrees reasonably well with that measured in our system [ 11. On the other hand, the low-temperature reaction probability measured in the present work (- 2 x 10d3) is a factor of 40 larger than deduced from the etch rate measurements of Fischl and Hess [S], assuming that MoCl, is the etch product. The opposite behavior is expected because they used atomic chlorine as the gaseous reactant whereas the present study used molecular chlorine. In addition, Fischl and Hess found no etching of MO by chlorine atoms below 100°C. At about that temperature, the reaction probability increased from near zero to about 7 X 10-5. The data in fig. 4 end at 110°C and there is no sign of a drastic drop of reactivity. Another experiment at 25 * C [l] showed a reaction probability of 5 X 10T4 at room temperature, which is consistent with extrapolation of the data in fig. 4 for f= 20 Hz. Fischl and Hess [5] interpreted the lack of etching at room temperature to a protective surface layer, which, on the basis of earlier observation [7], they took to be a polymer of MoCI,. However, it is more likely that the low reaction probabilities they observed are due to surface contamination. This may have been due to the lack of in situ

surface-cleaning procedures available in their apparatus. A similar discrepancy was observed in the etching of tungsten by chlorine by the two methods [2]. The activation energy for surface diffusion of Cl on polycrystalline MO (table 1) is intermediate between that of the essentially athermal low-temperature reaction channel for etching and the highly activated reactions probability seen in fig. 4. The pre-exponential factor for the surface diffusion coefficient given in table 1 is extremely large compared to the usual values of - 0.01 cm’/s for a normal site-to-site hopping process. The large value of the pre-exponential factor 0, is consistent with a two-dimensional ideal gas with a mean free path on the order of the grain size. Bonzel [17] has developed a model for this type of surface diffusion, and pre-exponential factors approaching lo7 cm2/s have been observed on ceramics [la] as well as metals /17]. The distribution of reaction products in chlorine-metal reactions has often been found to follow the predictions of the quasi-equi~b~um model. This model, which was originally proposed to explain the products of metal oxidation [19,20] is also applicable to metal-halogen reactions [21,22]. However, application of this model requires knowledge of the gas phase thermochemistry of the metal halides. Unfortunately, no data on the stability of MoCl,(g) are available, so the presence of this chloride in the etch products cannot be compared to the quasi-equilibrium model.

6. Conclusions The moiecular beam mass spectrometric investigation of the reaction of molecular chlorine and polycrystalline molybdenum has shown that the sole products are atomic chlorine and the dichloride of molybdenum. Recombination of surfaceadsorbed chlorine is also an important step in the overall mechanism of the reaction. MoCl, is produced by two reaction channels. The first, which is dominant at low temperatures, involves reaction of the incident reactant gas beam with a chlorinecovered MO surface. The second channel has a higher activation energy than the first and appears

at temperatures at which the Cl coat&S on the surface has been cleaned off by the unset of desorrow and surface effusion of atomic chlorine at about 900 K. Surface diffusion of adsorbed CI is observable only in molecular beam systems in which the reactant gas is delivered to the metal in a locahzed spot. In the temperature range at which Cl desorption occurs, a small fraction of the surf&c-adsorbed chlorine undergoes recombination to produce Cl,. A model of the reaction produces reasonable values for the rate constants cluuacterizing the ~lern~~t~ steps in the mechanism.

This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the US Department of Energy under Contract No, DE-AC03 76EWHJ98 and by the US Department of Energy, Lawrence Livermore National Laboramry under contract No. ~-7~~-~~~~_

The reation product vector is the conjugate pair consisting of the apparent reaction probability and reaction phase lag that is obtained for a fixed set of experimental conditions. The lock-in detector measures the amplitude S and the phase angle @ of the periodic signal from the mass spectrometer tuned to the mass of the products species. This signamis compared to the upside S,, and the phase angle $& of Cl, scattered from the target at room temperature_ In order to convert these signal chamcteristics to apparent reaction probabilities and phase lags for comparison with theoretical models of the surface reactions, several corrections are necessary, These are contained in the formula:

where j is the ~~da~nt~ mode of tile fhrx of the species in questiun emitted from the surface+

Since the mass spectrometer is a density~se~sitiQe detectors the flux must be divided by the mean speed n” of the mulecuIes emitted from the surface. The later is proportional to dm, where M is the molecular weight of the species and T* is its translational temperature. For a gaseous species produced by a surface reaction, T * = T, th surface temperature. For Cl, reflected from a room-temperature target, T * = 300 K. The functions A&u*) and +Ju*) account for amplitude attenuation and phase shift due to spreadiig of a Maxwelki gas of meau speed u* during transit from the target tu the mass spectrometer These functions are ~buIated in ref. P31. Dividing eq, fA.I) for the product species at temperature T by the equivalent equation for chlorine reflected from the room-temperature target yields the experimental reaction product vector;

where:

Frtr each product relative to CI,, the folIow~~g ratios must be estimated: the ionization cross section (6) and ~eu~~-t~ion ~ra~en~~oa pattern (a) of the species detected for the 70 eV electrons in the ionizer of the mass spectrometer; the efficiencies of ion extraction and transmission in the mass spectrometer (7); and the secondary-electron emission coefficient of the electron multiplier used to convert the mass-analyzed ion current to an electronic signal (y). For the atomic chlorine reaction product, the fragmentation of Cl, neutrals to Cl: in the ionizer was determined by anslyziug the direct beam with an in-line mass spectrometer. This quantity was determined by this method to

be 0.70. The additivity rule was used to estimate a value of 0.5 for the ratio of the total ionization cross section of Ci atoms to that of Cl, molecules. The secondary electron multiplier emission coefficient was assumed to be proportional to the square root of the mass of the ion. Ion extraction and transmission coefficients were obtained from the calibration data provided by the manufacturer of the mass spectrometer. The quantity

is the reaction product vector for product p. The experimental values of this quantity are obtained by dividing the left hand side of eq, (A.2) by the correction factors on the right hand side. The signal from molecular chlorine detected at high temperature contains contribution from this species reflected from the surface and from recombination of surface-adsorbed Cl, ‘The translational temperature of the former is determined by the thermal accomodation coefficient cw: T* = 300 + cr(T-

3001,

iA.

whereas the latter is emitted at the temperature of the surface, T. The measured Cl, signal amplitude referenced to its value for a room-temperature target and the phase lag of the reflected Cl, signal from a high-temperature target (S) relative to that from a room-temperature target (S,,) are contained in the equation:

fA.5) Since the masses of the two species compared are the same and only their temperatures are different, the mean speeds have been expressed in terms of the square roots of the temperatures of each. The f~nd~ent~ modes of the scattered and recombined Cl, fluxes, j,,, and f,, respectively, are

calculated from the reaction model by:

(A-6) (A.71

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