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Production of Mg and Al Auger electrons by noble gas ion bombardment of Mg and Al surfaces
Surface Science 58 (1976) 613-617 @ Noxth-Holland Publishing Company
PRODUCTION OF Mg AND A! AUGER ELECTRONS BY NOBLE GAS ION BOMBARDMENT OF Mg AND AI SURFACES Received 4 February 1976; manuscript received in final form 29 Marcl', 1976
Auger electrons from metal atoms on metal surfaces bombarded by argon ions have recently been observed [1,2]. Interest in this phenomenan by surface scientists is prompted by the wide use of noble gas ion beams to produce clean surfaces [3] and for contro!led erosion in depth proFding [4]. For example, the ion-induced Auger signal has been used as an aid in the alignment of ion beams in depth profiling systems [5]. It kzs also been pointed out that since the ion-induced Auger line shape is different from that of the electron-induced line shape and of comparable strength for the case of Mg, simultaneous ion and electron bombardment as used in depth profding can result in a line shape characteristic of neither and thus a source of error [6]. Aside from the original work by Hennequin [ 1], most studies reported thus far have employed argon ions at a fixed energy. Since sputtering by other noble gases is frequently employ ~'d over a wide range of energies, an investigation of thes ~. other cases is of ir.terest. In this lette; we report the relative production efficiency of Mg and A1 Auger electrons by I-le, Ne, Ar, Kr and Xe ion bombardment as a function of ion energy (<3 keV). Some comments on the interpretation of the results in terms of electron promotion arc als~ given. The experimental apparatus was a LEED-Auger system equipped with a~a ion gun and a four-grid retarding potential analyzer op~rated in the usual dN(E)/dE mode. The ion gun was a commerci~l unit wifl, a 200 eV electron bombardment ionization chamber [7]. A quadrupole gas analyzer monitored the purity of the bombarding gas at 8 × 10 -s torr. Both s~gle crystals and eva,.~orated films of metals were used with similar results. The samples were mounted on a Faraday cup just above an apertua: cut to the shape of the sample. The effective incident ion current could therefore be accurately measured at any incident ion gun voltago by u anslating the aperture to the position of the sample by a multiple motion feedthro.tgh on which the cup was mounted. The normat to the sample made an angle o!' 30 ° with the analyzer axis and 60 ° with the ion gun axis. It was tound that chemisorbed oxygen or chlorine on the metal surfaces ~uppressed the ion excited Auger signal sc) that the surfaces were bombarded until electron-excited Auger analysis shuwcd ml~, oxygeu or cldorine signal. Only the results for clean surf ices are reported here. The shape of the ion-excited Auger sigl~al from a given metal was found to be independent of the rare gas and, as pointtd out in ref. [2], quite symmetric. The peak-peak magnitude of the ~,igaals was ,:hus used as a measure of the Auger t Ice6!3
I. Fcrrante,S.K Pepper / l~oduction o~ M ~ ~ d A,~A u ~ i . . e l ~
614
t,,'on current. 1"his was then divi~ed by the measured incident ion current and normslized to the maximum signal strength from either Mg or AI to obtain relative pro. e,u~lion ~fticiCncies for the ~fferent gases o n each metal. In fig. 1 the relative .~,n~ :.:,s of the Mg and AI Auger electron signals induced by Ne, At, Kr and Xe ion b..~ml~,.~ent ale, pre~ented a,~ a function of ion gun voltage. For asiven bombarding species and ion gun voltage, the AI Auger signal was about an order of magnitude smaller than the Mg Auger signal. No signal was found for He + bombardment
1.0
rl
I la~t~jnesium
.9 .8
O Neon 0 Argon Krypton O Xenon
---
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.?--
8 A D
O O c
O AD
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D° J _..
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I 1 ! I 1 I. 1 I I J 1000 [?00 la00 1000 1800 2000 ~0(~ 2400 2000 ~00 .t000 Ion gun voltage
J. Ferrante, S. V. P~'pper/ Production of Mg and AI Auger electrons I.
615
0
(b) Aluminum 0 0 A D
[]
Neon Argon Krypton Xenon
0
6 0 A O
I::3
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6
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-
o 6b
800 100o 1200 1~00 16001800 Zo00 2200 2d00 2600 2800 3000 Ion gu r voltage {b}
Fig. 1. !~elative efficiency for Mg and A1 Auger electron production by Ne. Ar, Kr and Xe ion bombar,:lment as a function of inn gun voltage: (a) Mg, (b) AI.
under any of the bombardment cordifion:; employed laerein. The signal strengths were hadependent of temperature (<~ 300°C) and independent of ion dose ( t 0 1 2 1016 ion/cm2). Thus, implanted noble gas atoms we,.-, probably not a factor in deterr~ining the resalts. In {:on~idering these results, we first note that at io~. gun voltages typically used
616
J. Fcrrante, S. V. Pepper / Production o f Mg and AI Auger electrons
:
for sputter cleaning, the Auger production efficiencies diftered by no more than a factor of 2 among the different gases (with the exception of He+) on a given metal. Thus, any of tb~se gases used for sputter cleaning or depth profiling can also be used for ien b~am alignment 'and all,provide Mg ~or AlAUger s i g n ~ thatlean interfere with the Mgor A1 electr0n-exeited signals when depth profiling Mg or AI. No Auger signal was generated by He+b0mbardment, but its sputiering yield is too low to be useful in depth prof'ding applications.AU the signal strengths increased with increasing ion gun voltage with no maximtun exhibited. This is in contrast to the recent observation of a maximum in the signal strength of neon Auger electrons resulting from Ne + bombardment of Mg and AI [8]. Note, however, that the small shape difference between the Mg and AI curves indicates that the Ar-Mg and NeMg curves are bending over and appear to be approaching a maximura in the Auger production efficiency while all the A1 curves are still rising rapidiy. It is alse observed that the apparent threshold energy for the AI signal is higher than that for the Mg signal. The patterns of results observed are determined by the mechanism for inner shell vacancy productiot~, which is considered to be that of atom-atom collisions leading to electron promotion [1,9]. The difference between the :esults from Mg and AI (threshold energy, energy dependence and relative signal strength) are undoubtedly due to the fact that the AI 2p orbital lies deeper in energy and closer to the nucleus th~.n the Mg 2p orbital. Thus, a higher approach velocity is required by AI relative to idg to reduce the internuclear distance with the collisi,m partner to the poir.t wl~~re electron promotion can occur. At present it is not clear whether the collision partner is mother metal atom produced by a knock-on by the nob'e gas projectile (s!/ inlet, it metal-met,d collision) or whether the colli:~ionpartner is the noble ga~ pr%ccfile itself (asymnv.~tric collision) [1 ]. LJnf,srtunately, en interpretation of the ska?e of the energy dcFendenc,: curve:, in tcrms of specific models [ I0] and of the relr live orderfng with the diffe;ent noble gases to clarit) this point is hindered by th." fact tl-.at the electron bombardment ( ' 2 0 0 eV) ion source in the ion gun [71 Fro.:aees multiple charged ion~ as well as singly charged ions. The ion arrival rate (,L, .xined by a measurement of the incident effe,, ti:'e current is thus overestimated, anJ the signal strength per incident ion is actually somewhat higher. On the other hand, the multiply charged ion ~ attain higher kir~etic en~ergy in the ion gun than do singly charged iotas and therefore have hinter Auger electron production efficiency (see fig. 1) ;o that the signal strength for a sing!y charged beam is actually somewb-t lower. The effects of multiple ionization thus tend to compensate for each oti~er, but tile experimentally observed ener~ y dependence and the relative efficiency ef the different gas spec;es or a given nmtal are probab,y different from that ~,:,.~ch ,,vould ~esult from a pure ion beam. Ot the gases that generated metal Auger elec,,rons, neon has the lowes~ probability for multiple ionization [11] in the ion gun so the :esuit',lg energy Jependence is expected to be least affected. Although ;.',,~ present results are valid for typical :,..,n gun configurations, further work with
J. Femnte,
the monoenergetic
S. V. Pepper /Production
of Mg and Al Auger electrons
617
singly charged ions is required to clarify the mechanism for
irm, shell vacancy production. and Stephen V. PEPPER
John FEiiRANTE
NASA, Lewis Reseurch Center, Cleveland,Ohio 4413.5, USA
References [l] J.F. Hennequin, J. Phvs. (Paris) 29 (1968) 1053; J.F. Hennequin and P. Viaris de Lesegno, Surface Sci. 42 (1974) 50. [2] LT. Grant’et al., f. Varuum Sci. Technol. 12 (1975) 481. [3] H.E. Farnsworth et al., J. App1. Phys. 29 (1958) 1150. 141 P.W.Palmberg, J. Vacuum Sci. Technol. 9 (1972) 160. [5] R.W. Springer et al., Rev. Sci. Instr. 45 (1974) 1113. [6] M.P. Hooker and J.T. Grani, Surface Sci. 51 (1975) 328. 171 Varian Associates Ion Gun Model 981-2043, Varian Associates Power Suppl:: Model 981-2046. (8) J. Ferrante and S.V. Pepper, Surface Sci. 57 (1976) 420. [9] For a review see J.D. Garcia, Rev. Mod. Phys. 45 (1973) 111. [ 101 RJ. Fortner ct al., Phys. Rev. 1ES (1969) 164. [ 111 H.S.W. Massey, E.H.S. Rurhop and H.B. Gilbody. Electronic cna, 2nd ed. (Clarendon. Oxf’ord. 1969) Vol. 1, table 3.2.
and Ionic Impact Phcnom-
PRODUCTION OF Mg AND A! AUGER ELECTRONS BY NOBLE GAS ION BOMBARDMENT OF Mg AND AI SURFACES Received 4 February 1976; manuscript received in final form 29 Marcl', 1976
Auger electrons from metal atoms on metal surfaces bombarded by argon ions have recently been observed [1,2]. Interest in this phenomenan by surface scientists is prompted by the wide use of noble gas ion beams to produce clean surfaces [3] and for contro!led erosion in depth proFding [4]. For example, the ion-induced Auger signal has been used as an aid in the alignment of ion beams in depth profiling systems [5]. It kzs also been pointed out that since the ion-induced Auger line shape is different from that of the electron-induced line shape and of comparable strength for the case of Mg, simultaneous ion and electron bombardment as used in depth profding can result in a line shape characteristic of neither and thus a source of error [6]. Aside from the original work by Hennequin [ 1], most studies reported thus far have employed argon ions at a fixed energy. Since sputtering by other noble gases is frequently employ ~'d over a wide range of energies, an investigation of thes ~. other cases is of ir.terest. In this lette; we report the relative production efficiency of Mg and A1 Auger electrons by I-le, Ne, Ar, Kr and Xe ion bombardment as a function of ion energy (<3 keV). Some comments on the interpretation of the results in terms of electron promotion arc als~ given. The experimental apparatus was a LEED-Auger system equipped with a~a ion gun and a four-grid retarding potential analyzer op~rated in the usual dN(E)/dE mode. The ion gun was a commerci~l unit wifl, a 200 eV electron bombardment ionization chamber [7]. A quadrupole gas analyzer monitored the purity of the bombarding gas at 8 × 10 -s torr. Both s~gle crystals and eva,.~orated films of metals were used with similar results. The samples were mounted on a Faraday cup just above an apertua: cut to the shape of the sample. The effective incident ion current could therefore be accurately measured at any incident ion gun voltago by u anslating the aperture to the position of the sample by a multiple motion feedthro.tgh on which the cup was mounted. The normat to the sample made an angle o!' 30 ° with the analyzer axis and 60 ° with the ion gun axis. It was tound that chemisorbed oxygen or chlorine on the metal surfaces ~uppressed the ion excited Auger signal sc) that the surfaces were bombarded until electron-excited Auger analysis shuwcd ml~, oxygeu or cldorine signal. Only the results for clean surf ices are reported here. The shape of the ion-excited Auger sigl~al from a given metal was found to be independent of the rare gas and, as pointtd out in ref. [2], quite symmetric. The peak-peak magnitude of the ~,igaals was ,:hus used as a measure of the Auger t Ice6!3
I. Fcrrante,S.K Pepper / l~oduction o~ M ~ ~ d A,~A u ~ i . . e l ~
614
t,,'on current. 1"his was then divi~ed by the measured incident ion current and normslized to the maximum signal strength from either Mg or AI to obtain relative pro. e,u~lion ~fticiCncies for the ~fferent gases o n each metal. In fig. 1 the relative .~,n~ :.:,s of the Mg and AI Auger electron signals induced by Ne, At, Kr and Xe ion b..~ml~,.~ent ale, pre~ented a,~ a function of ion gun voltage. For asiven bombarding species and ion gun voltage, the AI Auger signal was about an order of magnitude smaller than the Mg Auger signal. No signal was found for He + bombardment
1.0
rl
I la~t~jnesium
.9 .8
O Neon 0 Argon Krypton O Xenon
---
0 8 0 o 0
.?--
8 A D
O O c
O AD
0 []
D
OO D OO
AD
D° J _..
AD AD
O]
AD
OO ~
O0 ,2
AD
O0
CO
- -
0 O OC
0 o
8~
A
D
,A D
AD AD A D
AD
I 1 ! I 1 I. 1 I I J 1000 [?00 la00 1000 1800 2000 ~0(~ 2400 2000 ~00 .t000 Ion gun voltage
J. Ferrante, S. V. P~'pper/ Production of Mg and AI Auger electrons I.
615
0
(b) Aluminum 0 0 A D
[]
Neon Argon Krypton Xenon
0
6 0 A O
I::3
O C.
6
0
D
[]
-E
O A
[]
r..
0 .4
D
0 D
O0 A 1:3 0 A
E~
D
O
.~ [] 0 .~
D
0 A
Q,}
r-
D
D A
/3
D O
DO A D © ,D [] A D
o~O
.1
-
o 6b
800 100o 1200 1~00 16001800 Zo00 2200 2d00 2600 2800 3000 Ion gu r voltage {b}
Fig. 1. !~elative efficiency for Mg and A1 Auger electron production by Ne. Ar, Kr and Xe ion bombar,:lment as a function of inn gun voltage: (a) Mg, (b) AI.
under any of the bombardment cordifion:; employed laerein. The signal strengths were hadependent of temperature (<~ 300°C) and independent of ion dose ( t 0 1 2 1016 ion/cm2). Thus, implanted noble gas atoms we,.-, probably not a factor in deterr~ining the resalts. In {:on~idering these results, we first note that at io~. gun voltages typically used
616
J. Fcrrante, S. V. Pepper / Production o f Mg and AI Auger electrons
:
for sputter cleaning, the Auger production efficiencies diftered by no more than a factor of 2 among the different gases (with the exception of He+) on a given metal. Thus, any of tb~se gases used for sputter cleaning or depth profiling can also be used for ien b~am alignment 'and all,provide Mg ~or AlAUger s i g n ~ thatlean interfere with the Mgor A1 electr0n-exeited signals when depth profiling Mg or AI. No Auger signal was generated by He+b0mbardment, but its sputiering yield is too low to be useful in depth prof'ding applications.AU the signal strengths increased with increasing ion gun voltage with no maximtun exhibited. This is in contrast to the recent observation of a maximum in the signal strength of neon Auger electrons resulting from Ne + bombardment of Mg and AI [8]. Note, however, that the small shape difference between the Mg and AI curves indicates that the Ar-Mg and NeMg curves are bending over and appear to be approaching a maximura in the Auger production efficiency while all the A1 curves are still rising rapidiy. It is alse observed that the apparent threshold energy for the AI signal is higher than that for the Mg signal. The patterns of results observed are determined by the mechanism for inner shell vacancy productiot~, which is considered to be that of atom-atom collisions leading to electron promotion [1,9]. The difference between the :esults from Mg and AI (threshold energy, energy dependence and relative signal strength) are undoubtedly due to the fact that the AI 2p orbital lies deeper in energy and closer to the nucleus th~.n the Mg 2p orbital. Thus, a higher approach velocity is required by AI relative to idg to reduce the internuclear distance with the collisi,m partner to the poir.t wl~~re electron promotion can occur. At present it is not clear whether the collision partner is mother metal atom produced by a knock-on by the nob'e gas projectile (s!/ inlet, it metal-met,d collision) or whether the colli:~ionpartner is the noble ga~ pr%ccfile itself (asymnv.~tric collision) [1 ]. LJnf,srtunately, en interpretation of the ska?e of the energy dcFendenc,: curve:, in tcrms of specific models [ I0] and of the relr live orderfng with the diffe;ent noble gases to clarit) this point is hindered by th." fact tl-.at the electron bombardment ( ' 2 0 0 eV) ion source in the ion gun [71 Fro.:aees multiple charged ion~ as well as singly charged ions. The ion arrival rate (,L, .xined by a measurement of the incident effe,, ti:'e current is thus overestimated, anJ the signal strength per incident ion is actually somewhat higher. On the other hand, the multiply charged ion ~ attain higher kir~etic en~ergy in the ion gun than do singly charged iotas and therefore have hinter Auger electron production efficiency (see fig. 1) ;o that the signal strength for a sing!y charged beam is actually somewb-t lower. The effects of multiple ionization thus tend to compensate for each oti~er, but tile experimentally observed ener~ y dependence and the relative efficiency ef the different gas spec;es or a given nmtal are probab,y different from that ~,:,.~ch ,,vould ~esult from a pure ion beam. Ot the gases that generated metal Auger elec,,rons, neon has the lowes~ probability for multiple ionization [11] in the ion gun so the :esuit',lg energy Jependence is expected to be least affected. Although ;.',,~ present results are valid for typical :,..,n gun configurations, further work with
J. Femnte,
the monoenergetic
S. V. Pepper /Production
of Mg and Al Auger electrons
617
singly charged ions is required to clarify the mechanism for
irm, shell vacancy production. and Stephen V. PEPPER
John FEiiRANTE
NASA, Lewis Reseurch Center, Cleveland,Ohio 4413.5, USA
References [l] J.F. Hennequin, J. Phvs. (Paris) 29 (1968) 1053; J.F. Hennequin and P. Viaris de Lesegno, Surface Sci. 42 (1974) 50. [2] LT. Grant’et al., f. Varuum Sci. Technol. 12 (1975) 481. [3] H.E. Farnsworth et al., J. App1. Phys. 29 (1958) 1150. 141 P.W.Palmberg, J. Vacuum Sci. Technol. 9 (1972) 160. [5] R.W. Springer et al., Rev. Sci. Instr. 45 (1974) 1113. [6] M.P. Hooker and J.T. Grani, Surface Sci. 51 (1975) 328. 171 Varian Associates Ion Gun Model 981-2043, Varian Associates Power Suppl:: Model 981-2046. (8) J. Ferrante and S.V. Pepper, Surface Sci. 57 (1976) 420. [9] For a review see J.D. Garcia, Rev. Mod. Phys. 45 (1973) 111. [ 101 RJ. Fortner ct al., Phys. Rev. 1ES (1969) 164. [ 111 H.S.W. Massey, E.H.S. Rurhop and H.B. Gilbody. Electronic cna, 2nd ed. (Clarendon. Oxf’ord. 1969) Vol. 1, table 3.2.
and Ionic Impact Phcnom-