ESCA investigations of ion beam effects on surfaces

Journal of Electron Spectroscopy and Related Phenomena, 16 (1979) 183-193 0 Ekevler Sclentlfic Pubhshmg Company, Amsterdam - Prmted m The Netherlands

ESCA INVESTIGATIONS

OF ION BEAM EFFECTS ON SURFACES*

S STORP and R HOLM Bayer AG, Ing Bererch Angew

Physrk, D-5090

Leverkusen

(W Germany)

(First received 7 March 1978, tn final form 14 July 1978)

ABSTRACT ESCA can be used for the mvestlgatlon of ion-beam-induced effects, such as selective sputtermg, lmplantatron of primary ions and surface atoms, changes m crystal structure, decomposltlon and formation of compounds In this paper the followmg questlons are discussed (1) What happens durmg the bombardment of an oxide-to-metal mterface? (2) Does a lower hmlt exist with respect to primary ion dose, current density, mass and energy for the production of Ion-beam-induced effects’ In both cases the possrblllty of, and time needed for, the reconstruction of the mltlal state seem to be important factors

INTRODUCTION

Bombardment of solid surfaces unth ions (usually rare gas ions) 1s used extensively in surface science, for example as excitation m ion surface scattenng spectroscopy (ISS)’ and secondary ion mass spectroscopy (SIMS)2 and also for surface layer removal and depth profile measurements3-6 by SIMS, ISS, ESCA (electron spectroscopy for chemical analyws)’ , AES (Auger electron spectrosc~py)~ and other analytical methods There are also nondestructwe methods for obtammg m-depth mformalzong but these cannot be apphed universally, so that it IS not possible at present to dispense with surface layer removaI by means of ion bombardment Therefore, it 1s all the more unportant to consider what changes are produced m sohd surfaces by xon bombardment and to what extent these changes restnct the mformatlon obtamed by analytical methods In prmclple, such artefacts can be detected by any method of surface analysis, but parkularly well by techniques

*Presented at the InternatIonal SIMS Conference, Munster, W Germany, 1977

184

permlttmg quantitative statements, charactenzatlon of compounds, nondestructive analysis and, wlthm hmlts, the obtammg of m-depth mformatlon by non-destructive means This applies par&ularly to ESCA The most nnportant lnreverslble changes resulting from ion bombardment of the surface of a solid are as follows (1) sputtering, (2) lmplantatlon of bombardmg particles and surface atoms m the lattice of the solid, (3) lattice damage, (4) destruclzon of compounds, and (5) formation of new compounds

EXAMPLES

OF ESCA INVESTIGATIONS

OF ION-INDUCED

EFFECTS

All the aforementioned changes have been mvestlgated by means of ESCA, ESCA spectrometers are of course also coupled with ion guns m order to record depth profiles. Smce ESCA can be used to obtam m-depth mformation by non-destructive means and also readily permits quantltatlve statements, effects due to selective sputtermg can obviously be mvestlgated. Thus, for example, Cu enmchment was observed when bombardmg brass surfaces” and Cr depletion when removmg oxide layers from slamless steelI _ Durmg sputtenng some of the bombardmg ions are implanted m the surface The number of unplanted rare gas ions vmes greatly and depends largely on the system bemg examined We detected no unplantatzon of rare gas ions m such precious metals as Au, Ag and Pd, on the other hand, relatively mtense lmes of implanted rare gas ions were found m metals with passive layers that have low sputtenng ratesI An example of surface atom lmplantatlon m the lattice of a sohd 1s the formation of carbides durmg the removal of contammatlon layers from met&l4 . Jt 1s naturally difficult to detect lattxce damage by analytical techniques smce the composltlon of the specimen remams unchanged However, when ESCA 1s used, certam channellmg effects can also be produced by varying the angle of electron emlsslon relative to the surface plane of the speclmen15 In the case of an S1 smgle crystal, these effects disappear lmmedlately when the specnnen 1s subJected only to mmlmal ion bombardment. The eneraes generally used for ion bombardment range from a few hundred eV to several keV so that the penetration depth of rare gas ions m solids 1s of the order of magmtude l-10nm Thus, most of the transmitted energy 1s absorbed close to the pomt of Impact, I e. within a radxus of less than 10 nm The energy passed on to the lattice atoms mthm the bulk and on the surface can be greater than the bmdmg energy m molecules by several orders of magnitude It 1s therefore great enough to break chemical bonds and form new ones A number of papers have been pubhshed on this subj-ect10,14,16-19

185

The exrstence of such Ion-induced effects 1s indisputable, particularly when hgh pnmary ion current densltles are used However, whether these effects occur at low pnmary ion current denwtles, and whether or when they can be neglected, are questlons that have not yet been adequately answered This wrll be dealt wrth m more det& below The measurements were made urlth an AEI ES 200 electron spectrometer Expenmental detils have been reported elsewhereI

INVESTIGATIONS

ON OXIDE/METAL

INTERFACES

If ESCA 1s used to mvestlgate oxide layers on metals and the thickness of the oxide layer 1s smaller than the escape depth of the photoelectrons, the oxrde and metal lmes m the ESCA spectrum, separated by the chemical shift, appear close together Thus a fixation of the ongmal state 1s clearly obtamed for a layer thickness of 5-10 nm, and all changes m the oxrdatlon state dunng ion bombardment (changes m oxidation state and thus m lme shLft or lme profile) are due to ion-Induced effects The metal lme serves as a reference for measunng the chemical shift and the oxide/metal mtenslty ratlo as a measure of the correspondmg oxide layer thickness” This sunple thickness evaluation 1s no longer vahd d a direct reduction of oxide to metal occurs dunng the ion bombardment, as has been observed, for example, with oxldes of Br14, Fe’s and Co” On the other hand there was no evidence of a direct reduction of Moo3 powder to MO metal at the rather low prunary ion current densltles applied m Fig 1 Therefore the estlmatlon of the oxide layer thickness from intensity ratios is Justified m the case of MOO, on metallic MO The mtenslty measurements were performed by area integration following subtraction of MO 3d lmes of clean MO metal or Mo(V1) oxide Normally the mtenslty ratio of the lmes pertammg to the various valencles of an element reflect the correspondmg quantity ratios. Of course the precise depth dlstnbulzon of the atoms m different valency states must be known A nonctestructzve m-depth analysts by means of ESCA requires certam assumptions about the structure of the layer. After Ion bombardment this seems to be almost lmposslble because the depth drstnbution of the reaction products 1s largely unknown Therefore no quantltatlve statements will be made. In the followmg examples the observed ion-mduced changes are so evident that a quahtatlve descnptlon of the reduction phenomena 1s sufficient Figure 1 shows the processes occurrmg dunng Ion bombardment of the thm oxide layer on MO metal. The oxide layer was prepared by heatmg m a current of 02 In the mltlal state only the lmes of the hexavalent oxidation state and of the metal substrate appear Lower oxldatron states are not detectable. In view of the lme widths they cannot be completely excluded but then percentage probably lies below 5%

186

oxide (vi)metal nn

3&2

3%~

34x2

ox’dFl 3

MO

3%~

3dw2

3%2

%/2345/2

lnltlal state

Ar+ Ion bombardment (5 ke V)

2OOOsec5xlO-9Acm-2 20 WC 5~10-7~

cm-2

50sec5xlWAcm-2

’x#l

126OeV

Figure 1 Ion-mduced reduction of MOOS on MO (Ar+, 5 keV, normal mcldence)

At an ion current density of 5 x lo-’ A cme2 and after the apphcatlon of an ion dose sufflclent to remove fractions of -a monolayer (Fig 1, r@t), changes m the MO spectrum are already detectable the lines become broader, the mmuna between the lines disappear, and gradually new maxima appear which can be assigned to tetravalent molybdenum m partrcular (for detarls see ref 14) Vutually no change is detected If one compares the mtenslty ratro of the metal lmes to the oxide lmes as a whole m the uutlal state with the correspondmg mtenslty ratio dunng the fust stages of ion bombardment This means that only part of the contamrnatlon was removed and that oxide layer removal had not yet taken place Yet the lower oxide layers were changed considerably, depending on the penetratron depth of the pnmary Ions Reduction of the oxides 1s not necessarily connected wrth release of oxygen mto the vacuum Since the oxygen can remam m mterstltlal sites,

187

the decrease m 0 1s intensity does not parallel the reduction process but rather the actual oxide layer removal. Furthermore, oxygen 1s transported to lower layers by recoil implantation*3V21n 22 If the metal adlommg the oxide IS then reached, this process, too, results m the formation of lowvalency oxrdes smce the oxygen supply is hmrted. With Moo3 on MO the net rate of reduction clearly depends on the ion current density. After the application of the same dose with a lower ion current density (Fig 1, left) and higher exposure tune the reduction effect is much less marked. The ion-mduced formation of oxides havmg lower valencres than those existing m the mltlal state (after oxidation by the ambient air or m an oxygen atmosphere) was observed for so very many metals that it IS probably applicable without exception. However, m some instances relatively high primary ion current densities had to be used to make the reduction effects visible’4 This seems to contrast wrth the results of expenments conducted by Kun et al 16, who were the first to observe ion-mduced reduction phenomena by means of ESCA but found that these no longer occur when a certain free energy of oxide formation 1s exceeded These authors performed their measurements on compact oxides, whereas we used thin oxrde layers on metals. In the latter case the formation of low-valency oxides 1s mcreased owing to recoil unplantatron, but before transfer of an oxygen atom from the oxide layer to the metal a metal/oxygen bond must first be broken, 1 e the oxide has to be reduced. A simple explanation for the dependency on the pnmary ion current density14923 IS that different recovery times are needed for restoration of the ongmal state. Accordmgly, permanent reduction occurs when a molecule IS hit a second trme before the oxygen separation caused by the first hit has had time to recover It should be noted that m the case of compact oxides the oxygen separated from the metal can return to the metal at any time This takes place more or less rapidly depending on affinity. However, m the bombardment of thin oxide layers the separated oxygen may become chemically bonded It either diffuses through to the metal substrate or is even implanted m the metal owmg to recoil processes. In both cases adequate quantities of oxygen are no longer avsulable for the recovery processes. This manifests itself m the very apparent occurrence of low-valency oxides. The hmrt of free energy for oxide for-matron specified by Kun et al M, above which decomposition can no longer be observed, is nevertheless quite Justified. we have mterpreted this llmlt not in the sense that firm bonds cannot be broken by means of ion bombardment but as a measure of the metal/oxygen affinity above which, at a given ion current density, the recovery processes take place so rapidly that, unless the oxygen forms other bonds, no reduction phenomena can be detected by means of ESCA Thus, it is not contradictory to the statement of Kim et al-l6 that no decomposition is observed with compact SIOz, Ta205, etc, whereas our measurements l4 have shown that It IS clearly

188

Sl 2p oxide (IV)

SI 2p

metal

oxide (IV) metal

,\,

rz

A y

1 PA cm-* lOpA cm-*

40 set 1 PAcm-2 +60 set lOpAcm-2

fl

ElpAcm-2

I

1385

I

1390

*

eV EIM

I

I

1385

1390

Figure 2 Ion-Induced reduction of SlOz on S1 (Ar+, 5 keV, normal mcadence)

detectable at the oxide/metal mterface (Fig 2). These facts must also be taken mto consideration when the oxide/metal mterface 1s reached after removmg thick oxide layers, Statements on the valencles ongmally exlstmg at the m&face are very problematical after Ion bombardment, even d for a long time no decomposltlon IS observed when the compact oxide 1s bombarded In the case of MoOa on MO the cntxcJ plnmary Ion current density for Ar+ Ion bombardment 1s lower than 5 x IO” Acme2 (Fig l), 1.e It lies w&m a range which has unti now been considered SW harmless m most cases. Reduction occurs durmg bombardment not only with Ar+ but also with hghter rare gases such as Ne, It 1s even observed with He (Fig 3) However, the cntlcal pnmary ion current density increases, correspondmg to the decrease m transfer of momentum, it 1s approximately 5 x lo* A cmW2 for He

189

MO

oxide (VI) metal

Ii-7

+a/2

3%2

343/2

34512

( a 1 Imtral state

(bll00~,5rcIO-~

AcK2

.&-I

1250

1260

ev

Fqure 3 Ion-induced reduction of Moo3

EXAMPLES OF ION-INDUCED CORmNT DENSITIES

on MO (He+, 5 keV, normal mcrdence)

REACTIONS

AT VERY

LOW PRIMARY

ION

The destructwe effect of ion bombardment on organic compounds IS at lea&t equal to &at on metal oxides. An aromatic rmg 18 detectable m the ESCA spectrum for a typ1ca.l shake-up satelhte m the C 1s rangeM. The top part of Q. 4 shows the conditions for p~lystyrene~~. The satelhte has

190

-&-I,-

\’ 0 Inhal state

2Osec 8x10 7 A cm-2 Ar+, 5 keV, 90”

100 set 10-S A cm 2 Ar +, 5 keV, 90”

I

1195

Figure 4

I

1200

c Ek,n [eVl

Ion-Induced decornposltlon of polystyrene (Ar+, 5 keV, normal mcldence)

already lsappeared after bnef ion bombardment at relatively low current density, 1 e. an aromatic nng 1s no longer detectable m the upper monolayers of the specnnen (Fig, 4, bottom part) The benzene nng 1s one of the most stable basic units m organic chemistry , If It can be destroyed so easily, it * must be assumed that all orgamc compounds can be changed rapldly and easily by means of ion bombardment. This can be demonstrated very well with zmc phenyl sulphmate (Fig 5), a substance which can be kept for years m an unchanged condltlon under norm& laboratory conditions. During Ar* bombardment, however, complete destruction takes place vMhin a few seconds rf a pnmary ion current density of only 5 x 10e6 Acme2 IS used. the tetravalent sulphur 1s reduced to its blvalent state (as, mcldentally, was also observed m the case of sulphltes),

191 0

0

-~__O--Zn-o_&_

0\-

c

s 2P

0\Zn Auger

IS

A --lc_._& mtt1.4 state

Sal

x10

Ekl”

1315

1320

+

1325

ev

1195

1200

ev

4m .

Eiwl +

Qm

Flgure 5 Ion-Induced changes m zinc phenyl sulphmate (A?,

sso ev

6 keV, normal mcldence)

the Auger spectrum for Zn mdicates a change m this element’s bmdmg state, and the satellite m the C 1s spectrum disappears, suggesting destruction of the phenyl residue. In such cases the posslbllities of recovery, i.e. restoration of the ongmal state, are vlrtuaily negligible. Consequently, no lower prunary ron current density hmit can be expected. On the contrary, smce every hit may produce ureversibie changes, it will be only a matter of the dose apphed until the damage IS detectable by means of ESCA Figure 5 shows that this IS m fact the case. The same effects were detected at 1,000s and 5 x 10V8 Acm’2 ,andat10sand5x10-6Acm-2. A further example of this phenomenon IS the bombardment of NaN02 (Fig 6) As a result of oxygen release the formation of NO< ions can be observed, whrlst the reduced mtrogen escapes in the gaseous form. The decisive factor rs that these two processes prevent reorganization formmg nitnte ions. Thus, m this case too, no lower pnmary ion current dens&y lrmit can be expected, at current densities of only 5 x X0-’ AcmV2 a sunple

rdlal state NaN02

2000 set 5 1 O-9 A cm-p, AP,

5 ke V

200 set 5 1 O-8 A cm-*, Ar+, 5 ke V

1075

1060

I

1085eV

*

&I”

Figure 6. Ion-induced changes m NaNG2 (Ar+, 6 keV, normal mcldence)

accumulation of the destruction can be observed. At the same time the nltr@ quantaty does not mcrease mdefinitely smce it 1s constantly bemg removed or reduced by Ion bombardment.

CONCLUDI.biG

REMARKS

The last -ample (Fig. 6 bottom) shows that not only reduction phenomena but also haer valencles are detectable. This USa further mdlcation that the separated oxygen ls readily avLutable for all types of chemical reactions. The reactions which actually occur are determmed by the energy conditions m the system m question. In the case of compact metallic oxides it IS highiy probable that the onginal state ti be restored. If the meti lies

193

wAhm the range of dlffuslon paths or recorl Implantation, at least some of the oxygen will react with the metal The probability mcreases v&h mcreasmg oxygen release owing to ion bombardment. Thus a cnl~al mmunum pnmary ion current density becomes plausible. However, if the reaction products are more stable than the ongmal matinal or if the ongmal state cannot be restored for other reasons (e.g. escape of the nrtrogen from the reduced nltnte), the effects ovlll snnply accumulate so that they can be easily detected after applymg an adequate dose. Therefore, special attention ~IU have to be paid m future to the recovery processes d an answer 1s to be found to the question whether or not ion-mduced effects falsify analysis results.

REFERENCES

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

D J Ball, T M Buck, D MacNalr and G H Wheatly, Surf Se1 , 30 (1972) 69 A Bennmghoven, Surf Scr, 36 (1973) 427 A Bennmghoven and S Storp, 2 Angew Phys , 31 (1971) 31 F Schulz, K Wlttmaack and J. Maul, Rudzat Eff, 18 (1973) 211 J P Coad and J G Cunnmgham, J Electron Spectrosc Relaf Phenom , 3 (1974) 435 P Braun and W Farber, Surf Scr, 47 (1975) 57 K Slegbahn et al, ESCA - Atomac. molecular and solui state structure studred by meuns of electron spectroscopy, Almqwst and W&sells, Uppsala, 1967 C C Chang, Surf Scr s 25 (1971) 53 R Holm and S Storp, Vuk Tech, 26 (1976) 41, 73 Phys. Usp, 14 (1971) 242 L A Abroyan,Sov R Holm and S Storp, Symposium on Electron Spectroscopy, Uppsala, 1977,Phys Scr ,16 (1977) 442 S Storp and R Holm, Surf Scr, 68 (1977) 10 R A Molme and A G Cullts, AppZ Phye Lett, 26 (1976) 561 R Holm and S Storp, AppZ Phys, 12 j1977) 101 N E Emckson, Symposmm on Electron Spectroscopy, Uppsala, 1977 K S IClm, W E Baltmger, J W Amy and N Wmograd, J Electron Spectrosc Relat Phenom , 6 (1974) 351 H M Nagulb and R Kelly, Radrat Eff, 25 (1976) 1 C R Brundle, T J Chuang and K Wandelt, Surf Scr ,68 (1977) 459 T J Chuang, C R Brundle and D. W Rice, Surf Sex, 59 (1976) 413 R Holm, Vuk Tech, 23 (1974) 208 K Wlttmaack and P Blank, Appl Phye Lett ,31(1977) 21 R Kelly and J B Sanders, Nucl Instrum Methods, 132 (1976) 336 G Mulier, submitted to AppZ Phys D T Clark, D B Adams, A Ddks, J Peehng and H R Thomas, d EZectron Spectrosc ReZat Phenom, 8 (1976) 51 R Holm, L MorbItzer and S Storp, Kunststoffe (German Plastics), 67 (1977) 717