Rotational and vibrational excitation of sputtered molecules

Nuclear Instruments and Methods 170 (1980) 351-355 © North-Holland Publishing Company

351

R O T A T I O N A L AND V I B R A T I O N A L EXCITATION OF SPUTTERED MOLECULES K.J. SNOWDON * and R.J. MACDONALD **

Department of Electrical Engineering, University of Saffbrd, Salford M5 4WT, England The population distribution of the rotational states A 2 A of CH created in both gas phase collisions and sputtering of an adsorbed butane layer on Si is found to depend on the projectile in a way which is not consistent with the predictions of either thermodynamic or random cascade models of excited sputtered particle creation. The process leading to the ejection of these electronically excited molecules is therefore governed by a direct projectile-adsorbate interaction. The vibrational excitation of the CH A 2A state is sensitive to the collision environment, whether gas phase or adsorbate layer. This also appears to be true for the vibrational excitation of the CO+ A2II state produced during the sputtering of molecularly adsorbed CO from a variety of metallic substrates.

1. Introduction The excitation and ionization of atomic and molecular species sputtered from solid surfaces has been studied in some detail in recent years. To date, however, no direct evidence exists which conclusively demonstrates the relative importance o f direct (projectile)-(surface atom) collisions in producing the observed excitation or ionization. An attempt by Snowdon et al. [1,2] to investigate this question for the case of sputtered excited metal atoms from a metal substrate by comparing gas-phase and sputtering excitation for the same collision partners proved inconclusive. The relative excited level populations were found to be experimentally indistinguishable, and indeed were for many levels indistinguishable from that seen in an equilibrium plasma. For the range o f excitation energies of valence electrons o f neutral atoms, it was not difficult to show that this was to be expected theoretically [3]. Such experiments are, therefore, unlikely to provide us with an answer, unless perhaps a p r o j e c t i l e - t a r g e t combination is selected where either the ( p r o j e c t i l e ) - ( t a r g e t atom) or (target a t o m ) - ( t a r g e t atom) gas phase collision exhibits a strong resonance. The excitation of molecules, however, to electronic vibrational and rotational states above the ground state involves (during collision) energy trans* Present address: Max-Planck-lnstitut fiir Plasmaphysik, 8046 Garching bei Miinchen, W. Germany. ** Visiting Lecturer from: Department of Physics, Australian National University, Canberra, A.C.T., Australia.

fer to the available electronic and "mechanical" degrees o f freedom. It was thought that this more complex collision situation may show greater sensitivity to the collision partners and collision environment than did atomic excitation. This was indeed found, and here we examine possible interpretations o f the observed dependences.

2. Apparatus The accelerator, target chambers and detection facilities used in these experiments are conventional, so only a brief outline is provided here. A heavy ion accelerator provided mass analysed beams of Kr ÷, Aft, and Ne ÷ ions at energies and currents o f 21 keV, 35 # A (110 /aA/cm2); 50 keV, 10/2A (50/2A/cm2); and 43 keV, 35 /aA (110 /2A/cm 2) respectively. Studies of the CH A 2 A - X 2 [ I system were performed in a target chamber whose base pressure was 10 -6 Torr, while those o f the CO ÷ comet tail system used a UHV chamber with a base pressure of 10 -1° Torr. The optical geometry was identical for both target chambers, with the optical axis of the 1/3 m monochromator placed perpendicular to the primary ion beam, and the entrance slit aligned perpendicular to the plane containing the ion beam and optical axis. Photon counting techniques were employed with the number o f photons counted at a particular wavelength setting being recorded by a multichannel analyser operated in multiscaling mode. Channel advance was affected by a digital current integrator, which also controlled the setting o f the wavelength drum of the monochromator via a stepping motor. In VIII. SPUTTERING

352

K.J. Snowdon, R.J. MacDonald/Excitation of sputtered molecules

these

experiments,

one

channel

corresponds

to

1.04 A.

3. Experiments The projectile (species and energy) dependence of the population of the A2A state of the radical CH produced in both gas phase collisions and during sputtering of a butane layer on silicon is shown in fig. 1. Our earlier study [4] of the dissociative excitation of CH in gas phase collisions identified spectra of the N~ first negative system, arising from a nitrogen impurity in the target gas. This accounts for part of the feature seen in fig. lc below 4280 A. The other part of this feature, also seen in fig. la (labelled X) remained unidentified [5] but appears to be associated with a transition of CH or CH ÷. Neither the presence of these features, nor their projectile or environment dependences, affect the conclusions here, since we are concerned only with the remaining

i (e)

'l =`1"4 I

features of the spectrum, all of which are well resolved from the above. The line-like feature at 4324 A in fig. 1 is the Q branch of the (2,2) band of the CH (A2A-X2II) system [6]. The rest of the spectrum (apart from the H 7 4340 A line) corresponds to the overlap of the different branches of the (0,0), (1,1) and (2,2) vibrational transitions [5], convoluted with the instrumental resolution function (8 A fwhm). With this identification we notice a great sensitivity of the vibrational level population distribution to the projectile (species and/or energy) and parent molecule environment (whether gas phase or adsorbed). The rotational level population distribution however, shows a strong projectile, but a weak parent molecule environment, dependence. The above mentioned band overlap makes the CH A2A-X2IJ transition a poor example to study in detail the environment dependence of the vibrational level excitation mechanism. The CO ÷ A 2 I I - X 2 E comet tail system [5], however, has clearly separated (4,0), (3,0), (2,0) and

J(b) ~1-o.24

¢) ~= ,1.4

f d ) '~ = 0.32

60C 0.25

40C ._~ o

30(3

o

a. 2OC

dd T T

,oo

4300

4400

4300

4400 wov¢lcncjt h

4300

4400

4300

440C

(~}

Fig. 1. Spectra produced by (a) 21 keV Kr ÷ b o m b a r d m e n t of but a ne under multiple collision conditions (6 × 10 .4 Torr) and (b)

adsorbed (at a pressure during bombardment of 1.5 X 10.4 Torr) on device grade silicon, normal incidence bombardment; (c) as for (a) except projectile was 43 keV Ne÷ and (d) as for (b) except projectile was 43 keV Ne÷. The factors ,7 are used for scaling, and allow the ordinate scale to be converted to a measured photon counts per analyser channel scale. Band heads for the CH Q(0,0) and Q(2,2) transitions are marked, as is the H7 line. The feature X and the N~ impurity spectra observed in the gas phase collision are discussed in the text.

K.J. Snowdon, R.J. MacDonaM / Excitation of sputtered molecules

Table 1 Photon yields in the peaks of the (3,0) and (2,0) bands of the CO+ A2rI-X2z system normalized to that in the (2,0) band for the gas phase collision and sputtering from several substrates (monochromator resolution, 30 A fwhm; primary beam, 50 keV Ar+; incidence angle of primary beam to surface normal, 20o ; CO, B.O.C. spectroscopically pure). Substrate

S(3,0)/S(2,0)

Ni A1 Si None

0.81 0.69 0.77 0.80

± 0.02 ± 0.01 ± 0.02 ± 0.02

(1,0) bands. Substrates of A1 and Ni (99.999% purity, polycrystalline) and device grade Si were used, since these have no known atomic or ionic emission lines [6] in the region of the peaks of either the (3,0) or (2,0) bands. Second order diffraction peaks are no problem since the quantum efficiency of our system is very poor below 2500 A. At a monochromator resolution of 30 )~ fwhm no difference in rotational level population distribution for either the (3,0) or (2,0) vibrational transitions was observed for the sputtering of CO* from these substrates, or for the gas phase collision. It was, therefore, possible to study the environment dependence of the vibrational excitation of the CO + comet tail system by simply measuring the photon yields at the peak of the distributions. The photon yields in the (3,0) band, measured as described above, are compared with that observed for the (2,0) band in table 1, from which it would appear, based particularly on the result for adsorption on AI, that an environment dependence for the vibrational excitation of CO* to the A211 state does sometimes exist. We wish to qualify this conclusion, however, since with A1 (unlike Ni and Si) a structureless non-zero signal was observed at about 4270 A (3,0) during the sputtering of A1 without the presence of CO in the vacuum chamber. This signal may arise from substrate luminescence [7] or stray light. If this substrate related signal shows an enhancement in the presence of CO, our subtraction of the yield at the chamber base pressure will not be an adequate correction.

4. Discussion The simplest explanation for the dependence of the position of the major peak in the CH spectrum

353

around 4300 A on projectile, and its independence on parent molecule environment, is that the excitation of the CH A2A state occurs during a collision of the projectile ion with the subsequently observed CH radical. This conclusion is reinforced by a related study [4] which demonstrated that the rotational excitation of this state is independent of the parent molecule from which the radical was derived. An alternative model for the excitation of sputtered molecules has, however, been suggested [8], and we must examine here whether it too provides a plausible explanation of these results. Before we can discuss in detail the various possible sputtering molecular excitation mechanisms, however, we must examine both the initial state of the molecule before collision, and the possible dynamical collision processes for both the gas phase and sputtering collision. We shall assume that for the sputtering of CH, the CH radical exists on the surface at the time of the collision leading to its ejection as part of the parent butane molecule (or its dissociation fragments) which we adsorb from the vapour state. Similarly we assume that the CO+ molecules we observe existed on the surface as molecularly adsorbed CO. This latter assumption is supported by several studies for Ni and Si at room temperature [9,10], and may be valid for the adsorption state of CO on A1 [11]. Since we have conducted these experiments under dynamic adsorption conditions, the effect of fragments from the parent molecule (created during sputtering) on the observed yield has been minimized. Recent computer simulations [12] have indicated that in the sputtering of adsorbate layers of strongly bound molecules such as CO, the CO molecules can molecularly eject, and do so predominantly. Thus the molecule we observe exists at the time of the collision leading to its ejection, just as it does in the gas phase collision. In gas phase collisions, all impact parameters are allowable, although it is difficult to estimate what range of impact parameters are the most efficient in producing electronic vibrational and rotational states of molecular ions such as CO*, or dissociative excitation of CH from hydrocarbons. At submonolayer adsorbate coverages, collisions between either the projectile ions or substrate atoms and the adsorbate molecules can occur over all impact parameters. For desorption or sputtering to occur, the binding energy of the molecule (or fragment) to the substrate (or substrate plus parent molecule) must be overcome. VIII. SPUTTERING

354

K.J. Snowdon, R.J. MacDonaM /Excitation of sputtered molecules

For molecular adsorption, the molecular bond energy dominates the molecule-substrate binding energy [12], and we shall assume that the effect of the latter on the final electronic vibrational and rotational energy distributions is small. The gas-phase and normal incidence sputtering situations differ significantly, however, in that in the latter case, molecules (or substrate atoms) cannot be ejected from the surface by a direct projectile-molecule (or atom) collision. Thus any excited CH molecule we observe, if ejected following a (projectile)(butane molecule) collision, has undergone further collision with the substrate or adsorbate layer. The only other mechanism involving the projectile is if the latter is reflected off sub-surface atoms and re-emerges from the substrate surface. This is unlikely to be an important desorption mechanism, especially in the case of Kr ÷ bombardment of Si. The other mechanisms which could lead to desorption involve the collision cascade [13] and possible thermal spike [14] initiated by the transfer of projectile energy to the substrate atoms. The random cascade contribution to the desorption should, according to theory [13], only show a projectile dependence in the total yield of desorbed molecules, since the energy distribution within the cascade is projectile independent. Random cascade induced desorption cannot, therefore, explain the change in rotational energy distribution seen between figs. lb and ld. An extrapolation of the results of Thompson and Walker [15] for P÷ and P~ bombardment of Si would suggest the exis.tence of a small spike component in the sputtering of Si by 21 keV Kr ÷. Such a component would not be expected for the other (projectile)-(solid target) combinations we have used. According to Kelly [14], the temperature of a thermal spike increases with projectile ion mass, and decreases with projectile energy. Thus, assuming for a moment that thermal spikes exis~ in Si under both 43 keV Ne ÷ and 21 keV Kr ÷ bombardment, the spike temperature under Kr ÷ bombardment will be the greater. The CH 4300 A rotational distribution indeed peaks at higher rotational energies under Kr ÷ bombardment. A comparison of this spectrum, however, with that computed by Beenakker et al. [16] for the (dominant) (0,0) band suggests that the rotational level population distribution is strongly non-Boltzmann in character, and as such, cannot have a temperature ascribed to it. Further, Thomas and De Koning [8] have found that the population of these rotational states under 10 keV Kr ÷ bombardment is Boltzmann-

like, with an effective temperature of around 4500 K. This is similar to that which describes our distribution obtained under Ne ÷ bombardment, and contradicts the predictions of thermal spike theory outlined above. It has been argued that the apparent temperatures which describe relative secondary ion yields and sputtered atom excited level population distributions may reflect an electron spike temperature [17]. The equilibration of energy between these electrons and "mechanical" excited modes of molecules in the electron or (even) atom spike lifetime of up to 10 -11 s [18], however, faces formidable difficulties.

6. Conclusion The only possible mechanism for explaining the production of electronic vibrationatty and rotationally excited CH radicals during the normal incidence sputtering of adsorbate layers of butane on Si appears, by elimination, to be projectile-radical collisions leading to dissociation of the parent butane molecule. Thermodynamic or random cascade models of excited sputtered molecule production are not consistent with the observations. A comparison of the rotational level population distributions observed in sputtering and gas phase collisions implies that the influence on this distribution of further deflecting collisions of the radical with either the substrate or adsorbate layer (necessary for ejection at the geometry employed) is small. This is not so for vibrational excitation. A possible sensitivity of the vibrational excitation to the A2II state of sputtered CO* to the substrate upon which CO was adsorbed was observed. Taglauer et al. [19] found that for ion bombardment below 2 keV, the desorption process is governed by direct projectile-adsorbate impact phenomena. We have found that for ion energies an order of magnitude higher, the process leading to ejection of electronically excited molecules is similarly governed by a direct projectile-adsorbate interaction. The theoretical possibility that this conclusion at least partially extends to the excitation and ionization mechanisms of particles sputtered from more general substrates deserves further study. One of us (K.J.S.) wishes to thank Dr. E.W. Thomas for valuable discussions.

K.J. Snowdon, R.J. MacDonald/Excitation of sputtered molecules

References [11 K.J. Snowdon, G. Carter, D.G. Armour, B. Andresen and E. Veje, Rad. Eft. Lett. 43 (1979) 201. [2l K.J. Snowdon, G. Carter, D.G. Armour, B. Andresen and E. Veje, Surface Sci. 90 (1979) 429. [3] K.J. Snowdon, B. Andresen and E. Veje, Rad. Elf. Lett. 43 (1979) 205. [4] K.J. Snowdon, R.J. MacDonald and E. Veje, J. Phys. B; Atom, Mol. Phys., in press. [5] R.W.B. Pearce and A.G. Gaydon, in The identification of molecular spectra, 4th ed. (Chapman and Hall, 1976) p. 90. [6] A.N. Zaidel, V.K. Prokof'ev and S.M. Raiskii, Tables of spectrum lines (Pergamon Press, New York, 196 I). [7] M. Zivitz and E.W. Thomas, Phys. Rev. B 13 (1976) 2747. [8] G.E. Thomas and B.R. de Koning, Chem. Phys. Lett. 55 (1978) 418.

355

[9] R.W. Joyner, Surface Sci. 63 (1977) 291. [10] R.E. Kirby and D. Lichtman, Surface Sci. 41 (1974) 447. [ 11] J.J. Pireaux, J. Ghijsen, J. Verbist and R. Caudano, Jap. J. Appl. Phys. V17 (1978) Suppl. 1 7 - 2 , p. 264. [12] B.J. Garrison, N. Winograd and D.E. Harrison, Jr. J. Vac. Sci. Technol. 16 (1979) 789. [13] P. Sigmund, Rev. Roum. Phys. 17 (1972) 823, 1079. [14] R. Kelly, Rad. Effects 32 (1977) 91. [15] D.A. Thompson and R.S. Walker, Rad. Effects 36 (1978) 91. [16] C.I.M, Beenakker, P.J.F. Verbeek, G.R. Mohlmann and F.J. deHeer, J. Quant, Spectr. Radiat. Trans. 15 (1975) 333. [17] C.J. Good-Zamin, M.T. Shehata, D.B. Squires and R. Kelly, Rad. Effects 35 (1978) 139. [18] P. Sigmund, Appl. Phys. Lett. 25 (1974) 169. [19] E. Taglauer, U. Beitat and W. Heiland, Nucl. Instr. and Meth. 149 (1978) 605.

VIII. SPUTTERING