emission of positive oxygen ions from bombardment … · 2019-12-20 · abstract emission of...

EMISSION OF POSITIVE OXYGEN IONS FROM BOMBARDMENT OF

ADSORBATE-COVERED METAL SURFACES

MICHAEL GLENN KAURIN

YALE UNIVERSITY

1989

A b s tra c t

E m is s io n o f P o s it iv e O x y g e n Io n s fr o m Io n B o m b a rd m e n t o f A d so rb a te -C o v e re d M e ta l S u r fa ces

M ichael G lenn K aurin Yale U niversity

1 9 8 9

D uring ion bom bardm ent of m etal surfaces, collision cascades can

resu lt In the em ission of sputtered secondary ions. Recent

experim ents, however, have suggested th a t the em ission of positive

ions of electronegative adsorbates can resu lt from electronic processes

ra th e r th an from processes involving elastic collisions. This

dissertation presents the results o f experim ents studying the emission

o f positive oxygen ions from oxygen- and carbon-m onoxide-covered

tran sitio n m etal surfaces during bom bardm ent by 2 5 -2 5 0 keV ions of

neon, argon, and krypton.

The systems studied m ay be grouped in to four categories. For a

nickel substrate w ith adsorbed oxygen, the em ission of positive oxygen

ions proceeds through collision cascades. For titan iu m and niobium

w ith adsorbed oxygen, the emission of positive oxygen ions is

proportional to the prim ary ion velocity, consistent w ith emission

from electronic processes; for a given p rim ary ion velocity, the oxygen

ion yield is independent of prim ary ion species. For substrates of

m olybdenum and tungsten, the oxygen yield is proportional to prim ary

ion velocity, b u t the yield also depends on the p rim ary ion species for

a given prim ary ion velocity in a m anner th a t is consistent w ith

em ission resu lting from electronic processes. For these two groups,

except for titan iu m , the yields during neon ion bom bardm ent do not

extrapolate (assum ing linearity w ith p rim ary Ion velocity) to a nonzero

value a t zero beam velocity. The m agnitude of the oxygen ion yields

from these targets is not consistent w ith th a t expected i f the em ission

were induced by secondary electrons em itted d uring the ion

bom bardm ent. The emission o f positive oxygen ions from carbon

m onoxide adsorbed onto nickel surfaces is not a sim ple function of

prim ary ion velocity_and species, although the em ission certain ly does

not resu lt from collision cascades. F inally , the em ission from carbon

m onoxide adsorbed onto pallad ium surfaces resembles th a t for the

oxidized surfaces (excepting n ickel).

E m is s io n o f P o s i t iv e O x y g e n Io n s fr o m Io n B o m b a rd m en t o f A d s o ib a te -C o v e re d M e ta l S u r fa c e s

A D issertation

Presented to the Faculty o f the G raduate School

of

Yale U niversity

in Candidacy for the Degree of

Doctor of Philosophy

by

M ichael G lenn K aurin

Decem ber 1989

ACKNOWLEDGMENTS

My family and Debbie Burns have always encouraged my endeavours and

given me moral support and love. Debbie especially has tolerated the behavior

of a graduate student who spends long boms with a noisy accelerator.

My thesis advisors, Robert Weller (who got me started in this field) and Peter

Parker, provided guidance, encouragement, and merciless comments on my prose.

I also thank the other members of my defense committee for their comments on

this thesis: Subir Sachdev, Partha Chowdhuiy, Vic Henrich, and Tom Tombrello

(outside reader).

These experiments would have been impossible without the help of the other

students in the research group. Kevin Hubbard, Laurie Baumel, Patty Blauner,

and John O’Connor assisted with advice, labor, and crisis control. Also, Patty

taught me the ropes of secondary ion mass spectrometry.

The clerical and technical staff at WNSL made research much easier. John

MacKay and Richard Hyder (accelerator engineers), as well as Charles Gingell

(electronics engineer) lent their expertise, especially during the cleaning of the

ion implanter. Tom Barker and Dick Wagner (electronics technicians), Ray

Comeau, Joe Cimino, Tom Leonard, Al Jeddry, and Alan Ouelette (machinists)

built and maintained much of our equipment. Dee Berenda made some of the

drawings in this thesis. The office staff, Mary Ann Schulz, Rita Bonito, Karen

DeFelice, and Lisa Close, kept the laboratory running smoothly. D. Allan

Bromley, laboratory director, provided support for our efforts. Finally, Sara

iii

Batter has been very helpful to all physics graduate students.

The other denizens of WNSL have made life more pleasant and interesting.

In particular, I thank my fellow students for friendship and assistance: Paul

Cottle, Paul Magnus, Heping Li, Dan Blumenthal, Michael Smith, Steve Rugari,

Joe Germani, Zheping Zhao, and Pat Ennis.

T A B L E O F C O N T E N T S

Acknow ledgm ents .................................................................................................. ii

Table of C onten ts .................................................................................................... iv

C H A P T E R O N E : IN T R O D U C T IO N ............................................................ 1

1.1. G eneral In tro du ctio n ......................................................................... 1

1.2. Emission of Secondary Ions...................................................................3

1.2.1. Sputtering Theory..................................................................... 4

1 .2 .2 . Ionization Th eo ry ................................................................7

1.3. Positive Ions of Electronegative E lem en ts ...............................11

1.4. Secondary Electron Emission............................................................. 13

1.5. Desorption Induced by Electronic Transitions...........................15

1.5.1. Menzel-Gomer-Redhead Model............................................16

1 .5 .2 . Knotek-Feibelm an M odel................................................. 16

1.6. O utline of E xperim ents.................................................................... 17

Table and Figures........................................................................................ 2 0

C H A P T E R T W O : E X P E R IM E N T A L A P P A R A T U S ..........................3 2

2 .1 . H ardw are...............................................................................................3 2

2 .2 . Analyzers and E lectronics..............................................................3 5

2 .2 .1 . Secondary Ion Mass Spectrom etry................................3 5

2 .2 .2 . Secondary E lectron Energy A nalysis............................ 3 8

Figures.............................................................................................................4 0

C H A P T E R T H R E E : E X P E R IM E N T A L P R O C E D U R E ..................... 4 9

3 .1 . Target P reparation ...................................................................4 9

3 .2 . Secondary Ion Yield M easurem ents.......................................... 5 0

iv

3 .2 .1 . Procedure..............................................................................5 0

3 .2 .2 . Corrections for System atic E rro rs ............................. 5 2

3 .3 . Secondary Electron Yields and Energy D istributions 5 3

Figures..............................................................................................................55

C H A P T E R F O U R R E S U L T S A N D D IS C U S S IO N .................................. 5 7

4.1. O2 Adsorption onto Tl, Nb, Mo, and W ............................................ 5 7

4 .1 .1 . 0 + E m ission.........................................................................59

4.1.2. Ne+Bombardment.................................................................. 6 2

4 .1 .3 . Secondary Electron Energy D is trib u tio n s ................ 6 5

4 .2 . O /N i Targ ets ....................................................................................... 6 9

4 .3 . CO A dsorption.................................................................................... 7 0

4 .3 .1 . CO+ E m ission........................................................................7 0

4 .3 .2 . N i+ E m ission.........................................................................71

4 .3 .3 . 0 + Em ission..........................................................................71

Tables and Figures.......................................................................................7 4

C H A P IE R F IV E : C O N C LU S IO N S ..................................................................... 9 3

5.1. Summary...................................................................................................9 3

5.2. Further Experiments.............................................................................. 9 4

Bibliography...................................................................................................................9 8

v

C H A P T E R O N E IN T R O D U C T IO N

1.1. G e n e ra l In t r o d u c t io n

A fast ion strik ing a solid surface loses its k inetic energy by

im p artin g m om entum to target atoms and by exciting target electrons.

This m ix tu re o f atom ic m otion, electronic excitation, and projectile

im p lan ta tio n can drastically change the properties o f the surface by

the expulsion o f target atom s from the surface ("sputtering") or by the

form ation o f a new compound. Studies o f the yield and excitation (or

ionization) o f the ejected atom s, in clud ing th e ir dependences on the

characteristics o f the projectile and the targ et surface, can disclose

useful in form ation about the surface and the events occurring during

the in teraction o f the projectile w ith th e target. Since the surface of a

solid is w here m uch of the solid's in teraction w ith the rest of the

world takes place, and ion bom bardm ent o f solids occurs both

n a tu ra lly and artific ia lly, th is inform ation can be a valuable contribution

to various fields of science and technology.

Bom bardm ent-induced m odification o f a surface can be an

im p o rtan t and useful technological and scientific process, or an

undesirable side-effect. The im p lantation o f nitrogen into the surface

of an a rtific ia l h ip jo in t can increase the resistance of the surface to

m echanical w ear and chem ical attack fP i85], b u t the im pact of plasm a

ions against th e walls of fusion reactors can erode the walls and

contam inate th e plasm a w ith atoms sputtered from the w alls [Be81].

Ion im p lan ta tio n is used for precise doping o f semiconductors, b u t

th is im p lan ta tio n also damages the solid [B e81, P i85). Large organic

m olecules m ay be sprayed onto a surface, th en sputtered off, allowing

1

2

mass analysis o f the m olecule and its m ajor constituents ITo83a,

H u87]. In nature, erosion o f surfaces by ion bom bardm ent occurs on

the moons of Ju p iter, w here solid gases are exposed to bom bardm ent

by ions trapped in the p lanet's m agnetic field [To82,83a, Jo81J.

Fo r studies o f sputtering phenom ena, sputtered atom s m ay be post

ionized by lasers for analysis using electrostatic fields and tlm e-of-

flig h t techniques, or they m ay be collected on a foil for la ter analysis by

R utherford backscattering [An81, Th87J. The study o f the variation of

the sputtering yields w ith such param eters as the energy and angle of

incidence of the projectile, as w ell as the study of the yields of

secondary ions, can provide inform ation about the processes w hich

lead to sputtering. The relative ease w ith w hich ions can be analyzed

has m ade the study of secondary ions popular, despite the sensitivity

of the ionization probabilities to the chem ical state o f the surface (e.g.

oxidation) [B179, W i77 ,79]. Moreover, secondary ion mass

spectrom etry (SIMS) is a com m on, sensitive technique used to study

the atom ic composition of surfaces as w ell as the adsorption and

bonding of molecules onto surfaces [B e73,75, W i82al. The

investigation of secondary ions is also im portant in itself, since the

processes of ionization are no t fu lly understood.

The experim ental w ork presented in th is dissertation focussed on

the em ission of secondary ions during bom bardm ent o f adsorbate-

covered m etal surfaces by projectile ions having a varie ty o f energies

and masses. In particu lar, we investigated the apparent role played by

the bom bardm ent-induced electronic excitation in the em ission of

positive ions of electronegative adsorbate atom s, as w ell as the

varia tio n o f th is emission w ith the target m etal and the nature o f the

3

adsorbate-substrate bond. For m etals, attention in the past has

generally focussed on m etal-ion em ission produced by m om entum

transfer during atom ic collisions. Also, the study of positive secondary

ions of electronegative atom s is fa irly recent.

The rem ainder o f th is chapter contains a presentation o f the basic

concepts o f sputtering, secondary Ion em ission, and ion em ission

stim ulated by electron bom bardm ent, in addition to the im m ediate

m otivation for and an outline of our experim ents. Chapters 2 and 3

contain the details of the experim ental apparatus and procedure,

w hile chapter 4 is a discussion of the results o f the experim ents.

Finally , chapter 5 presents conclusions and suggestions for fu rth er

research.

1 .2 . E m is s io n o f S e c o n d a ry Io n s

Sputtered ions generally are ejected from the surface by an elastic

collision w ith e ither the incident ion or another target atom . The

decay of any excited state of the sputtered particle and the electronic

in teractions between the particle and the surface then determ ine the

ionization state o f the sputtered particle. Therefore, th is section firs t

outlines the basic theory of sputtering by atom ic collision cascades,

then presents various m echanism s th at have been proposed for the

ionization of sputtered atom s. Sputtering caused by the

bom bardm ent-induced electronic excitation o f in su la ting solids w ill

also be discussed.

4

1 .2 .1 . S p u tte r in g T h e o r y

The basic sputtering theory, set forth t y Sigm und, is described in

[S i81a]; more recent developm ents are reviewed in [Si87J. In th is

theory, Sigmund assumed th a t the num ber of target atom s set into

m otion by the im pact o f the projectile ion is sm all enough th a t m oving

atom s never collide w ith one another (linear approxim ation).

However, they do collide w ith stationary atom s, setting them into

m otion so th at these m ay in tu rn collide w ith other stationary atom s.

The resu lting cascade of m oving target atoms, term ed "collision

cascade", is shown schem atically in fig. 1.1a. This lin ear cascade,

assum ed to be isotropic (i.e. the direction of m otion after a collision is

random ), is described by the lin ear Boltzm ann transport equation.

Atom s are considered to be sputtered i f they cross the surface w ith

kin etic energy greater th an the p lan ar surface barrier Uo (typically

about 5 eV). Then, solution o f the Boltzm ann equation for the

distribu tio n of moving atom s produces the differential sputtering yield

d3y E iSn(E) — ----- 7— — Icos0il ,

d E id 2D i (E i + Uo)3‘m

w here E is the energy of the projectile, Sn(E) its nuclear stopping

power, and E i, ©1. and d2O i are the energy, angle of em ission w ith

respect to the surface norm al, and d ifferential solid angle o f the

sputtered atom. The param eter m is related to the scattering

potentia l used; often, m =0 Is used.

There are two features o f the sputtering yield th a t w ill be pertinen t

fo r la te r discussion. F irst, th e yield is proportional to the nuclear

stopping power, shown schem atically in fig. 1.1b. The m axim um of the

5

nuclear stopping power occurs when the reduced energy £ is 0 .5 ,

w h ere

e = — — — ----------— - ■ Em i + m 2 Z i - Z 2 -e2

(Z and m are the atom ic num ber and atom ic mass; 1 and 2 refer to

p ro jec tilean d target; E is in eV). The screening length a is

0 .885 apa “ (Z i2/ 3 + Z 2 2/ 3) l / 2 ’

where ao is the B ohr radius. The scale of fig. 1.1b can be estim ated by

reference to table 1. 1, w hich presents the projectile energy and

stopping power for the m axim um in the nuclear stopping power curve.

(Figure 1.1b also shows the electronic stopping power Se, w hile

table 1.1 presents the position of the m axim um of Se and values of Se

a t the m axim a of Sn and Se.) Second, the energy d istribu tion of

sputtered particles is broad (decreases as E*2) and has a m axim um at

low energy,

F - ° °max - 2 ( l - m ) '

These two features are verified by experim ental resu lts (see [An81]

and [M a84] for com pilations of the data).

The sputtering yield form ula we used for com parison w ith our data

is the sem i-em pirical form ula proposed by M atsunam i e t a L [M a84,

Ya83J. This form ula takes in to account the energy lost by the

projectile to electronic excitation of the target and the enhanced

6

back-scattering of light projectile ions. The sputtering yield is

calculated as

_ 0 .4 2 .q - Q .K Sn(e) / / W * f 8^ ' U SH + 0 .3 5 U s s.(e)| P T E ) ) ■

w here sn(e) and se(£) are Lindhard's reduced nuclear and electronic

stopping powers in term s of the reduced energy £ [L i61,63]. a *, Q,

and Erh are em pirical param eters (determ ined fo r each beam -target

com bination), w hile Us is the sublim ation energy of the solid (used to

approxim ate the surface b arrie r Uo). In [M a84], it is shown th a t th is

form ula provides a good description of experim ental sputtering yields.

For m etal targets, such as those used in our experim ents,

bom bardm ent-induced electronic excitations In the b u lk decay too

quickly to produce appreciable sputtering by electronic processes.

This is not necessarily true, however, for targets th a t are insulators

[To83b]. E lectronic excitations in frozen gases can survive long

enough to m igrate to the surface, there to decay and cause sputtering;

th is m echanism is responsible for some o f the sputtering from the

moons of J u p ite r [To83a, Sc87]. The decay o f a defect produced in

alkali-halides can result in a replacem ent sequence w hich, on

reaching the surface, also can produce sputtering [To83b]. T h at

desorption by these mechanism s is produced by electronic excitation

ra th er th an by m om entum transfer during atom ic collision cascades is

underscored by the observation th a t these m echanism s also operate

during bom bardm ent of insulating solids by electrons and photons

[B r85, To83c]. F inally , Tom brello and W atson [To83d, Wa85J have

proposed th a t the electronic excitation produced by M eV Ion

7

bom bardm ent o f dielectric solids (such as U F4 and A I2O 3) can change

the lattice electrostatic p o ten tia l so th a t la ttice atom s are expelled

from the solid.

The sputtering o f organic molecules from in su la ting surfaces also

results from electronic processes. Using fast fission fragm ents as

projectiles, researchers have found th a t the sputtering yield Is related

to the electronic stopping power; secondary electrons m ay cause the

sputtering (H u 87, To83aJ.

1 .2 .2 . Io n iz a t io n T h e o r y

Models of the ionization o f atom s sputtered from m etal surfaces

m ust explain several features o f the experim ental results (for reviews

of experim ent and theory, see (B179, W i77 ,79 ,8 2b , Y u 86a,b]). The

secondary ion yields generally follow the sputtering yields as functions

o f projectile energy, except for m ultiply-charged ions produced in

energetic collisions during the early stages o f the collision cascade.

The yields of positive ions from a common m atrix are proportional to

e*]/ E, where I is the ionization potential of the sputtered atom and E is

an experim ental param eter (the in terpretation of w hich varies from

m odel to m odel). O xidation of the target surface can enhance the

positive ion yield by several orders o f m agnitude; enhancem ent of the

negative ion yield is observed w hen cesium is deposited onto the

target surface. A decrease in the target's w ork function usually

increases the positive ion yie ld , and an increase in w ork function

u sually decreases the yie ld . The sign of the change in yield expected

from the change in w ork function , however, often is the opposite o f

8

the observed change. In p articu lar, oxidation of the surface can

enhance the yield o f positive ions w hile increasing the w ork function.

W e w ill consider three basic types o f ionization models: those

invoking energetic collisions, excitation o f and electronic in teraction

w ith the surface, and breaking of m olecular bonds. This discussion

w ill not be a complete review of ion ization theory, b u t ra th er an

overview o f concepts w hich com m only appear in these models.

A su ffic ien tly energetic collision can create a hole in a core

electronic level o f an atom by Fano-Lichten prom otion of th a t level

[Ba72, Fa65]. Th is m echanism is illu s tra ted in fig. 1.2a, w hich shows

the correlation between the atom ic levels for the separated atoms in a

collision (between A r and Cu) and the atom ic levels for the "unified"

atom (Ag) th a t is the no-separation lim it o f the collision. Levels th a t

cross one another can be coupled by the m otion of the colliding atom s

towards each other (radial coupling) or by th eir rotation around each

other (angular coupling), allow ing an electron to be promoted to a

higher-energy level (in itia lly em pty). The electron in itia lly in the 4 f

level (derived from the A r 2p level in fig. 1.2a), w hich rises steeply and

is coupled to m any other levels, is p articu larly prone to prom otion.

This would leave a hole in the 2p level o f the A r atom (in general, the

hole w ill be on the lighter atom ).

According to the kinetic m odel o f ionization proposed by Joyes

[Jo73J, such a core hole in a sputtered atom w ill undergo Auger decay,

in w hich a valence electron fills the hole and another electron is

ejected (see fig. 1.2b) [Ba82, Be83]. (This Auger decay of atom X is

denoted X (C W ), indicating core level C filled b y valence electron V ,

w ith an other valence electron V being em itted.) The sputtered atom

9

is thus ionized; if several A uger electrons have been em itted in the

decay, the atom is m u ltip ly ionized. Since th 's m echanism requires a

collision energetic enough for prom otion of the 2p level, the ejected

ion has fa irly high kinetic energy. Also, the ion yield increases w ith

energy ra th er th an follow ing the nuclear stopping power (see [B187b,c]

fo r representative experim ental results). __

According to other m odels, ionization is produced by the electronic

in teractions between a sputtered atom and the surface th a t it is

leaving. In the surface excitation model proposed by W illiam s [W i79],

the electrons near the sputtering event have been excited by the

collision cascade; th e ir energy d istribu tion is calculated using Ferm i-

D irac statistics. The resonant electron exchange between the surface

and the departing particle is assumed to be so strong th a t the

probab ility th a t a level in the sputtered atom is em pty is equal to the

probab ility th a t the surface electronic level a t the same energy is also

e m p ty The ionization probab ility is then e ^ / ^ e , where AE is the

difference between the m etal w ork function and the energy of the

atom ic level in question w hen the ion is fa r enough from the surface

th a t the electron exchange ceases, and T e is the tem perature of the

excited electrons.

Several w orkers [B179, W i79 , S r81] have m odelled the interactions

between the sputtered atom and the surface by using perturbation

theory or tunnelling theory. In general, the m otion of the sputtered

atom m eans th a t the H am ilton ian of the system is tim e-dependent;

th is m ixes atom ic levels w ith surface levels. The survival probability of

an ion a t large distances from the surface m ay be calculated from th is

perturbation . Figure 1.3a shows schem atically w h at is found for one

10

such m odel. As the Ion leaves the surface, the electronic interactions

w ith the surface change the w idths and energies of its en erg / levels.

The ionization probab ility is determ ined by the distance Zc a t w hich

the ionized level crosses the Ferm i level of the m etal; if Zc is large

enough, electron tunnelling into the ionized level w ill not occur and

the ion w ill survive. These models (including the surface excitation

m odel) are therefore successful a t explaining the dependence of the

ion ization prob ab ility on ionization energy and target w ork function.

The fin a l m odel to be discussed is the bond-breaking m odel, w hich,

based on the Landau-Zener model o f m olecular dissociation, treats

ionization as the resu lt of the break-up o f a m olecule composed o f the

sputtered atom and its neighbor on the surface [Yu87]. The level-

crossing w hich occurs during the dissociation is illustrated in fig. 1.3b.

The tran sitio ns th a t occur a t the crossing-point o f the ionic energy

curve M ++X* (in itia l ground-state) w ith the neutra l energy curve

M °+ X ° (dissociated ground-state) determ ine w hether the sputtered

particle rem ains an ion. The ionization probability is then calculated

as

-Z7th' 2

p+ = exp v ia l

w here H i2 is the transition m atrix elem ent, v the velocity of the

sputtered ion, Rc the position of the crossing, and I a I the absolute

value o f the difference between the derivatives o f the two energy

curves as functions of the separation distance R. The relevance of the

ion ization po ten tia l. I, of the sputtered ion for th is model is th a t I-A

(where A is the electron a ffin ity of the neighboring atom ) is the

reparation between the energy curves for large R. The bond-breaking

m odel is w ell-suited for explaining the enhancem ent o f positive ion

yields by oxidation of the target, since an oxidized surface is in an

ionic state; then the breaking of ionic bonds influences the secondary

ion yields more than does the variation o f the w ork function.

1 .3 . P o s i t iv e Io n s o f E le c t r o n e g a t iv e E le m e n ts

Recent studies of ion bom bardm ent o f electronegative atoms

adsorbed onto m etal surfaces have found unexpectedly large yields of

positive secondary ions from the adsorbates. Th is contradicts the

usual resu lt (discussed in the previous section) th a t the ionization

probab ility decreases w ith increasing ionization potential; positive

electronegative ions should be easily neutralized. Moreover, the

secondary ion energy distributions and the dependence of the ion

yields on projectile energy indicate th a t these ions do not originate

from collision cascades. For instance, W illiam s [W i81,831 found th at

the energy d istribution of the F+ em itted from fluorinated silicon

du ring bom bardm ent by 8 keV Ar+ did not resemble the energy

d istrib u tio n of the sputtered S i+, b u t ra th er th a t o f the F+ em itted

d u rin g electron bom bardm ent (see fig. 1.4a). Also, the dependence of

the F * yield on incident ion energy resembled th a t of the S i(L W )

A uger electron yield more than th a t of the S i+ yield (see fig. 1.4b). The

k in etic m echanism of Joyes is ru led out in th is case by the low energy

o f the m axim um of the F+ energy d istribution . Therefore, W illiam s

proposed th a t a S i(L W ) Auger electron was creating a core hole in a

fluorine atom ; the Auger decay o f th is hole stripped enough electrons

12

from the fluorine atom to leave it positively ionized. The expulsion of

the F + from the surface then resulted from the reversal o f the

Coulom bic forces acting on it.

O 'Connor e t a L [B185, O c83,85,86a,b ] bom barded m etal surfaces

w ith ions having M eV energies. They observed large yields o f

electronegative ions w hich did not follow the m etal ion yields as

functions of incident ion energy. A n exam ple of th e ir results appears

in fig. 1.5, w hich also dem onstrates th a t the yields of 0+ and C l+ are as

large as the yields of Y+ for bom bardm ent o f y ttriu m targets.

Reference to fig. 1.1b and table 1.1 shows th a t the electronic stopping

power is m uch larger th an the nuclear stopping power for the beam

energies used by O ’Connor.

The im m ediate predecessors of our experim ents were the

experim ents perform ed by B launer and W eller [B186,87a,b,c], who

studied the emission of 0 + from oxidized alum inum and vanadium

surfaces during bom bardm ent by 2 5 -2 5 0 keV noble ions. The purpose

of those experim ents was to study the secondary ion em ission for a

wide range of nuclear and electronic stopping powers. O f p a rtic u la r

in terest here is the relative strength of the electronic stopping power

as compared to the nuclear stopping power; the ratios of the form er to

the la tter, shown in fig. 1.6 for Ne+, Ar+, and Kr+ beam s, cover a large

range of values for the projectile energies used.

An exam ple o f the dependence of the m etal and m etal-d im er ion

yields on projectile energy appears in fig. 1.7 (showing the yields of V +

and V 2+ observed by B launer), along w ith the sputtering yields

calculated using the form alism of M atsunam i e t a L (solid lines) [M a84].

The variation of the m etal Ion yields w ith projectile energy are

13

consistent w ith ejection produced by collision cascades. The 0 + yields

from V and Al (shown as functions of beam velocity in fig. 1.8),

however, do not follow the sputtering yields b u t increase lin early w ith

projectile velocity (w ith the exception of He+. bom bardm ent). Indeed,

excluding the data for He+ bom bardm ent the yields of 0 + for the V

target are collinear functions o f velocity, independent o f beam species.

Noting the lin ear dependence of the electronic stopping power Se on

the projectile velocity v, B lau ner divided the 0 + yields by dSe/d v ,

w hich is projectile-dependent, to remove the dependence on

projectile species expected if the em ission o f 0 + depended d irectly on

energy deposited into electronic excitation. The resu lt is shown in

fig. 1.9. The A l data for d ifferent beams are now closer to fa lling on a

common curve; th is is not true for the V data. Nevertheless, because

of the lin earity in beam velocity o f the 0 + yields, B launer proposed th at

the 0 + emission was induced by electronic processes; in p articu lar,

desorption stim ulated by secondary electrons was suggested, sim ilar

to the m odel of W illiam s for the ion-induced desorption o f F+

(m echanism s of electron-stim ulated desorption w ill be fu rth e r

discussed in section 1.5).

1 .4 . S e c o n d a ry E le c t r o n E m is s io n

Since secondary electrons appear to be im plicated in the ion-

bom bardm ent-induced desorption of positive ions of electronegative

atom s, a b rie f discussion o f secondary electron em ission is in order.

Sum m aries o f the experim ental results m ay be found in [Be82,

H a8 1 ,88 ]. For the range o f projectile velocities used in our w ork

(2 -1 4 x lO 7 cm /s), the secondary electron yield is proportional to

14

velocity, as shown by fig. 1 .10. In general, the electron yield follows

the electronic stopping power as a function o f projectile velocity

(compare fig. 1 .10 w ith fig. 1.1b). The energy d is trib u tion peaks a t

around 1 eV, then decreases w ith electron energy E as E _n, w ith

1.5 £ n £ 3 .0 . The high-energy ta il often contains features from the

em ission of Auger electrons by the k in etic m echanism discussed in

section 1 .2 .2 [Ba82, Be821.

The theory of secondary electron em ission resem bles th a t of

sputtering by collision cascades [Sc80, Si81bJ. The incident ion

produces an electron collision cascade th a t is described by the lin ear

B oltzm ann equation, w ith the equation being com plicated by the

collisions of moving electrons w ith (stationary) la ttice nuclei. It is,

therefore, not surprising th a t the theoretical electron yield is found to

be proportional to the electronic stopping power, in agreem ent w ith

the experim ental results.

U ndhard showed th a t the electronic stopping power for low

projectile velocities is proportioned to th e projectile velocity IU 54.61J.

Q ualitatively, th is result m ay be understood by considering the

reference fram e of the m oving ion [L i54], in w hich electrons lose

energy by scattering off of the ion, s im ila r to the m echanism of

electrical resistance in solids. The proportionality o f the energy loss

to velocity is then equivalent to O hm ’s law (velocity is proportional to

force, hence to energy loss (As76I). Q u antitatively , an expression for

the stopping power is found by using quantum -m echanical

perturbation theory to solve for the F o u rier transform o f the dielectric

constant o f the electron gas in the solid; th is describes the response

of the electron gas to the moving ion, allow ing calculation of the

15

energy d ra in on the Ion. The resu lt for the electronic stopping power

Se is, in u n its of eV-A2,

where E (in keV) and m i are the energy and atom ic m ass of the

incident ion, and Z i and Z2 are the projectile and target atom ic

num bers. Since the square-root factor is proportional to the projectile

velocity, the dependence o f the stopping power (as a lin ear function of

velocity) on projectile species is contained in the atom ic-num ber

factor (henceforth referred to as the "Lindhard factor").

1.5 . D e s o r p t io n In d u c e d b y E le c t r o n ic T ra n s it io n s

D esorption induced by electronic transitions (D IET) m ay be

produced by bom bardm ent of a solid w ith electrons (ESD: electron-

stim ulated desorption) or photons (PSD). ESD w ill be particu larly

relevant for our discussion. Because the mass of an electron is sm all

relative to the mass of an atom ic nucleus, incident electrons carry

relatively little m om entum , and the transfer o f energy from an

electron to a target nucleus is inefficient. Therefore, an incident

electron w ill not produce an atom ic collision cascade; desorbed ions

m ust have received th e ir m om entum from electronic processes. The

desorption of ions from m etals also requires a m echanism to allow the

ions to escape n eutra lizatio n by electrons from the surface.

16

1 .5 .1 . M e n z e l-G o m e r -R e d h e a d M o d e l

The classic m odel o f E SD , proposed by M enzel, Gomer, and

Redhead (MGR) [M e64, Re64; see also Go831, is illu strated in

fig. 1. 11a, w hich shows potential energy curves for an adsorbate atom

A bound to a m etal atom M . A n excitation of the valence electrons

produces a Franck-C ondon transition from the ground-state binding _

curve M +A to the anti-bonding curve M-+A+. I f the departing adsorbate

escapes reneutralization , it desorbs as A+. As fig. 1.11a shows, the

io n ’s energy d istribu tio n is then the reflection of the ground-state

spatial d istribu tio n through the excited potentia l curve. I f the ion is

reneutralized only after bu ild ing up sufficient kinetic energy, it

escapes as A0; otherwise, the adsorbate atom is recaptured by the

surface.

1 .5 .2 . K n o te k -F e ib e lm a n M o d e l

The M G R m odel is not adequate to explain the observed ESD of O *

from m etal oxides. Since the oxygen is ionized as O '2 in the oxide, a

sim ple valence excitation cannot cause its desorption as 0 +.

Therefore, K notek and Feibelm an (KF) [Fe78, K n78,79] proposed a

m echanism , illu strated in fig. 1. 11b, w hich was the basis for the

m echanism proposed by W illiam s (section 1.3). The desorption begins

w ith the creation by the incident electron (or photon) o f a core-hole

in the m etal atom . In m axim ally valent oxides, the m etal atom has

given a ll o f its valence electrons to the oxygen atom . Then the core-

hole m ust be filled by an electron from the oxygen atom (inter-atom ic

A uger decay), resu lting in the ejection of other oxygen electrons and

leaving the oxygen atom ionized as 0 +. W ith the reversal o f the

17

Coulom bic forces, the 0 + is repelled from the surface and so desorbs.

(A core-hole in an oxygen atom lacks sufficient energy to produce

desorption of 0 + by an In tra -atom ic Auger decay m echanism , such as

th a t proposed by W illiam s for ion-induced F+ desorption.) The

desorbing 0 + can escape reneutralization since the filling of the holes

in Its valence shell by m etal electrons is slowed by the correlation

between the holes [C i81, Fe81]. Thus, the KF theo iy predicts 0 +

desorption from m axim ally va len t systems bu t not from nonm axim ally

valent systems.

This prediction is not com pletely fu lfilled , since ESD o f 0 + is

observed for systems th a t are not m axim ally valent. For instance,

researchers have observed ESD of 0 + from NiO fGe84, N i81] as w ell as

ESD o f F+ from m etal fluorides [Wo81). Indeed, the yield of 0 + from

T i02 th a t has been reduced by ion bom bardm ent is larger th an the

yield of 0 + from undam aged TiC>2 [Ku85]. ESD of 0 + also is observed

from covalent systems, w hich also are not m axim ally valent. For

instance, CO adsorbs m olecularly onto m etals such as Ni, w ith the

carbon atom bound directly to the m etal and the oxygen atom bound

only to the carbon atom; the oxygen atom is thus bound covalently

fBa77, W 086. Za88). ESD o f 0 + is observed, however, from CO

adsorbed onto Ni (M a76, R a83]. Extensions of the KF theory propose

th a t such desorption can be caused by excitations beyond the sim ple

creation of a core-hole fR a83].

1 .6 . O u t lin e o f E x p e r im e n ts

W e investigated the dependence on projectile velocity o f the 0 +

secondary ion yields, I(0 +), during ion bom bardm ent of transition

18

m etal surfaces w ith separately adsorbed O 2 and CO. The incident ion

beams used were 2 5 -25 0 keV Ne4, Ar+, and Kr+. As fig. 1.6 shows, our

beam energies ranged from the regime dom inated by nuclear stopping

to th a t w here electronic stopping becomes im portant, providing a

good range for determ ining w hether the 0 + em ission is better

correlated w ith the nuclear stopping pow er or w ith the electronic

stopping power.

The adsorbate/substrate com binations used were chosen to provide

a varie ty o f in itia l environm ents for the oxygen. Two types of oxidized

m etal surfaces were used. The firs t group o f m etals oxidized fn . Mo,

Nb, and W ) is located near V in the periodic table; like V , a ll these

m etals can form m axim ally valent oxides, although researchers

disagree on the actual valency of the oxide form ed [ e . g . [L174] and

[Ca871). These m etals were investigated to determ ine w hether the

pro jectile-independent proportionality o f I(0 +) to projectile velocity,

found by B launer for oxidized V , is a general phenom enon for

tran sitio n m etals or the resu lt o f a coincidental cancellation of factors.

For these targets, as well as for a ll other targets, we compared any

dependence of I(0 +) on beam species to the dependences of the

electronic stopping power and the secondary electron yields on beam

species, to determ ine the role of electronic excitations or secondary

electrons in the emission of 0 +.

The other oxidized m etal investigated was N i. As discussed above,

some w orkers have observed ESD of 0+ from N iO , although Ni does

not form a m axim ally valent oxide. Com parison o f the dependence of

1(0 *) on projectile velocity for th is target w ith th e dependences for

bom bardm ent o f the m axim ally valent oxides could then help us

1 9

determ ine the Im portance o f the KF m echanism for lon-lnduced 0 +

desorption.

We also m easured the secondaiy-electron energy d istribu tion for

ion bom bardm ent o f oxidized V . If ESD produced by secondary

electrons is active during ion bom bardm ent, com bining th is energy

distribution w ith the,know n ESD cross-sections as functions of

electron energy should allow approxim ation of the contribution of

secondary-electron stim ulated desorption to our observed 0+ yields. If

the estim ated contribution were to be significantly larger or sm aller

than the actual yields of 0 +, then the case for secondaiy-electron

stim ulated desorption would be weakened. The energy distributions

m ay also indicate w hether a significant role is played by Auger

electrons in stim ulating the desorption of 0 +, as W illiam s found for the

ion-induced desorption of F + from Si.

F inally , we investigated the 0 + em ission induced by ion

bom bardm ent o f Ni and Pd surfaces w ith adsorbed CO. For both

m etals, the adsorption of CO is m olecular, w ith the oxygen atom bound

covalently to the carbon atom [Ba77J. The observed ESD of 0 + from

m etals w ith adsorbed CO, in contradiction of the m axim al-valency

requirem ent of the KF theory, m akes these systems interesting. Also,

by com parison of the dependence on projectile velocity of the ion-

induced 0 + yields for these covalent systems w ith the dependences for

the ionic systems, we m ay determ ine the effect th a t the type of

bonding of the oxygen has on the 0 + em ission. We also looked for

evidence of ion-induced electron-stim ulated desorption of CO+, since

CO+ desorption is observed during ESD experim ents [Cr83].

20

Table 1.1

Sam ple stopping powers and energies of m axim a (see fig. 1.1), for

Ne+, Ar+, and K r+ projectiles incident on T i and W , showing: Energy

E(e = 1 /2 ) a t w hich nuclear stopping is a m axim um , w here E is the

reduced energy; nuclear stopping power Sn and electronic stopping

power Se a t the m axim um in the nuclear stopping curve; energy

Emax(Se) a t w hich electronic stopping is m axim um ; m axim um

electronic stopping power Se(Emax)- A ll stopping powers are given in

term s of the energy loss per layer of target m ateria l.

Nuclear Stopping Power Electronic Stopping PowerE(E = 1/2) Sn(E = 1/2) Se(C = 1/2) EnaxtSe) Se(Emax)

T itan iu m

Ne+: 17 keVAr*s 43 keV Ki+: 149 keVTungsten

Ne+; 59 keV Ar+i 123 keV Kr+: 318 keV

92 eV/layer 237 eV/layer 588 eV/layer




11 HeV 47 MeV 248 MeV

11 MeV 47 MeV 248 MeV


1460 eV/layer 3718 eV/layer 10,542 eV/layer

21

Figure 1.1

a) A prim ary ion strikes a solid surface, producing a collision cascade

th a t can resu lt in the sputtering of target atom s from the firs t few

m onolayers o f the target (from [B187c]).

b) Stopping power d E /d x o f a projectile ion in a solid as a function of

projectile energy E . Exam ples of the locations of and the stopping

power a t the m axim a in the curve are given in table 1.1. For our

experim ents, the projectile energies were in the nuclear-stopping

and L indh ard -S ch arff electronic-stopping regim es (from [S i81a]).

( a )

C o l l i s i o n C a s c a d e M o d e l

S p u t t e r e d

M a t e r i a l

P r i m a r y

I o n9 § ® ®

9 0 0

T a r g e t

( b )

2 2

Figure 1.2

a) Exam ple of a Fano-Lichten correlation diagram , showing the

correlation between the atom ic levels for colliding atom s of A r and

C u (right-hand side) and those for the "unified" Ag atom (left-hand

side). Solid, dashed, and dotted lines represent a, n , and 5 states

(from [B a72]).

b) Illu s tra tio n o f Auger decay. In the top panel, a core electron o f an

atom is rem oved, perhaps through Fano-Lichten prom otion. Then,

as shown in the m iddle panel, a valence electron fills the core-hole

w hile another valence electron is em itted. The bottom panel shows

the fin a l state: the core-hole has been filled, and two (or more)

valence electrons have been removed.

Afl Cu + Ar

( a )

2 3

Figure 1.3

a) Change in the energy E a of an electronic level o f an atom (on the

right) th a t is leaving a m etal surface (on the left). Because o f the

electronic interactions between the surface and the atom , E a and the

level w id th 2A both depend on the distance z from the surface. The

ionization probability is determ ined by the distance Z q a t w hich the

atom ic level crosses the Ferm i level of the solid (from |Yu86b]).

b) M olecular potential energy curves, as functions o f separation

between atoms M and X , for the bond-breaking m odel o f ionization.

Curve (ii) is the in itia l ground-state, in w h ich both atom s are ionized.

Curve (i) is the dissociated ground-state, in w hich both atom s have

been neutralized. The interactions between the curves a t the

crossing po int Rc can result in the dissociated state being th a t for

w hich the atom s are ionized (from [Yu87]).

a)

VACUUM LEVEL

Ea(ao) =-I

b)

DISTANCE

2 4

Figure 1.4

a) Left-hand side: Energy distributions of secondary S i+ and F+

em itted during ion bom bardm ent of fluorinated silicon. R ight-hand

side: Energy d istribu tion of F+ em itted during electron

bom bardm ent of fluorinated silicon (from [Wi811).

b) Y ields of secondary ions and S i(L W ) Auger electrons for 8 keV A r*

bom bardm ent of fluorinated silicon (from W i81]).

a)

KINETIC ENERGY (eV) KINETIC ENERGY (eV)

b)

PRIMARY ION ENERGY (keV)

2 5

Figure 1.5

Secondary ion yields as functions o f projectile energy for M eV ion

bom bardm ent o f y ttriu m (from (Oc86bJ).

k5 '

uiQ

(0)

» •

10 :

10*g »*•o

:(d)

V)

p kS7!oo

79Br—’Y Y + YIELD

i l l

Y0+ YIELD

< t t I

10* 107 BEAM ENERGY <«V)

' i

2 »0# Vtnt-2

Kb)0+ YIELD

1 "mJOO .J

to- H 1 -

»•

-

:ic)Cl+ YIELD

-. . . . _ I

I

-t

-

2 6

Figure 1.6

R atio o f the electronic stopping power to the nuclear stopping power

for 1 0 -1 0 0 0 keV Ne+, A r+, and Kr+ prim ary beam s. Stopping powers

are calculated using the form alism of Lindhard [Ma841.

I I ' I "T T

5 0 100 2 0 0 5 0 0

B E A M E N E R G Y ( k e V )

r r |

1 0 0 0

2 7

Figure 1.7

Yields o f m etal secondary ions from ion bom bardm ent o f clean

vanadium surfaces (the labels on the legend are reversed from w hat

they should be). The solid lines represent the sputtering yields

calculated according to the form alism of M atsunam i e t a L and scaled to

equal the V + yields at a beam energy of 2 0 0 keV (from [B187c]).

Coun

ts / I

ncid

ent

Ion

(10

)V4 Yields from Clean Vanadium

Energy (keV)

2 8

Figure 1.8

a) Yields of secondary O * em itted during ion bom bardm ent o f oxidized

vanadium surfaces, as functions o f projectile velocity. For all

projectile species except H e+, the data po ints are collinear, and the

extrapolation of the data passes through the origin (from [B187c]).

b) Yields of secondary 0 + em ittted d u ring ion bom bardm ent of

oxidized a lu m inum surfaces, as functions o f projectile velocity (from

IB187c]).

0+

Yie

ld

(10

*8co

un

ts

/ in

cid

en

t io

n)

5 0

+ yi

eld

(1

0'7

co

un

ts

/ in

cid

en

t io

n)

a ) Vanadiua

8V e l o c i t y (1 0 c m / s e c )

V e l o c i t y (1 0 c m / s e c )

2 9

Figure 1.9D ata from fig. 1 .8 , divided by dSe/d v , w here Se is Lindhard's electronic

stopping pow er (w hich is lin ear in projectile velocity v). The solid

lines represent em pirical electronic stopping powers scaled by the

same factors (from [B187c]).

Vanadium TargetI

▲ H e +

□ N e +

■ A r+

A K r+

V e lo c i t y (1 0 c m / s e c )

100

V e l o c i t y (1 0 c m / s e c )

3 0

Figure 1.10

Secondary electron yield per incident ion, y , as a function of projectile

velocity v for ion bom bardm ent of m etal surfaces. The upper portion

o f the figure indicates the energies (slanted lines) corresponding to

the velocities on the horizontal axis of the low er p a rt of the figure for

various projectiles (as labelled on the left side o f the upper part of the

figure). For our experim ents, projectile velocities were in the range

2 -1 4 x lO 7 c m /s , w h ich Is in the linear regim e (from [Be82J).

Ar bitr or y

units

log)f

31

Figure 1.11

a) Illu s tra tio n of the M enzel-G om er-Redhead m odel of electron-

stim ulated desorption, showing the potential energy curve for an

adsorbate A on a m etal M before (M + A) and after (M* + A +) a Franck-

Condon transition of a valence electron. The in itia l adsorbate wave-

function \y i, reflected through the excited potential curve, gives the

energy distribution o f the desorbing adsorbate, w hich has m inim um

k inetic energy T min (from [G o83]).

b) Illu stratio n of the K notek-Feibelm an m echanism o f electron-

stim ulated desorption from m axim ally valent m etal oxides. A core

hole on the m etal atom m ust be filled by in ter-atom ic A uger decay

involving electrons from the oxygen atom , since the m etal atom has

no valence electrons. Enough electrons m ay be lost by the oxygen

atom th a t it becomes positively ionized. Desorption of O * then

results from the reversal o f the Coulombic forces acting on the

oxygen atom (from [K n79]).

a)

b )

Auger Electrons

CHAPTER TWO EXPERIMENTAL APPARATUS

2 .1 . Hardware

The 30 0 kV Cockcroft-W alton accelerator used to produce the ion

beam s for these experim ents is shown schem atically in fig. 2 .1 . Neon,

argon, or krypton gas was adm itted to the hot-cathode ion source

th rough a needle valve and ionized by electrons em itted from the hot

filam ent. The ion source could be operated either w ith o r w ith o u t the

creation o f a plasm a discharge. In the form er case ("lighted" source),

the accelerator produced beam s w ith large currents (about 4 0 |iA on

target), used for sputter-cleaning the targets. In the la tte r case

("unlighted" source), the beam curren ts were 1 0 0 -1 0 0 0 tim es sm aller

th a n w ith the lighted source. Therefore, to avoid sign ificant alteration

o f the target surface during th e secondary ion yield m easurem ents, the

p rim ary ion beams used for those m easurem ents were produced by an

unlighted source. These beam s were more stable, and th e ir currents

easier to control, than those produced by a lighted source.

A fter extraction from the source by the firs t elem ent o f the E inzel

lens and acceleration by the po ten tia l gradient in the accelerating

colum n, the ion beam was focussed by an electrostatic quadrupole lens

(norm ally needed for a lighted source only) and m om entum -analyzed

by a 30° bending m agnet. For a given magnetic field and accelerator

term inal voltage, only those beam ions w ith a p articu lar value of V m /q

(m = m ass, q = charge) w ould be deflected into the u ltra h ig h vacuum

(UHV) beam line. For the heaviest beam ion used, kryp ton , the

analyzing m agnet could resolve ions separated in mass by 1 u . The

vacuum in the accelerator co lum n and the bending m agnet, norm ally

32

33

5 -15 x lO -7 to rr, was m aintained by an oil d iffusion pum p w ith a cold

trap.

The U H V beam line is shown in fig. 2 .2 . The d ifferential pum ping

necessary to achieve - 1 0 '10 to rr in the target cham ber w ith a ll valves

open between the the cham ber and the accelerator was achieved by an

in line cold trap and two ion pum ps. A b eam view er and a Faraday cup

a t the firs t U H V pum ping station were used to m onitor the beam

during the tu n in g o f the ion source. A second set o f electrostatic

quadrupoles supplied m ost of the beam focussing, w hile a set of

deflection plates rastered the beam to create a uniform beam spot.

The size of the beam spot on the target was defined by a collim ator

located ju s t before the target cham ber; a second collim ator, slightly

larger th an the firs t and electrically biased a t -1 8 0 V , prevented

secondary electrons em itted from the firs t co llim ator from h ittin g the

target. The collim ators were m ounted on a lin ear m anipulator,

allowing collim ators w ith various sizes to be inserted into the beam.

The firs t co llim ator had an 0 .2 5 inch diam eter hole used for the

sputter-cleaning beam s, and an 0 .5 m m hole for the beams for the

yield m easurem ents.

The target ladder, shown in fig. 2 .3a , held the fo il targets, as w ell as

a Faraday cup used w hen positioning the ladder relative to the prim ary

ion beam and m easuring beam current. The target ladder was

attached to a H un ting ton P M -600 m anipu lator, w hich could move the

targets in a ll three lin ear dimensions and rotate them through 360°.

Figure 2 .3b shows the set-up of the target cham ber for the

secondary ion y ie ld m easurem ents. The gas bottles were filled w ith

99 .99% pure CO or O2 (Alfa Products). The gases were adm itted to

34

the target cham ber through separate leak valves (G ranville-Phillips)

th a t allowed precise control over the p artia l pressure of the gas in the

target cham ber. The target ladder was surrounded by a Faraday cage,

w hich could be electrically connected to the target ladder, th a t

captured secondary electrons em itted from the target to im prove the

m easurem ents of beam cu rren t. The Faraday cage had openings for_

the prim ary ion beam and the quadrupole mass analyzer.

The mass analysis o f the secondary ions was performed by a

quadrupole mass analyzer (U the Technology Inc., model 100C) th a t

was equipped w ith an energy p re -filte r (Kratos Analytic Instrum ents).

The quadrupole axis was a t an angle o f 30° w ith respect to the prim ary

ion beam and perpendicular to the target surface. The position of the

target, chosen to m axim ize the height and resolution of the secondary

ion mass peaks, was about 5 cm from the front of the pre-filter.

The set-up of the target cham ber for the m easurem ents of the

secondary electron energy d is trib u tion is shown in fig. 2.3c.

G eom etrical constraints precluded the use of the Faraday cage in th is

case. The secondary electrons were energy-analyzed by a cylindrical

m irro r analyzer (P erkin -E lm er Physical E lectronics, PH I 10 -155).

The analyzer axis was perpendicu lar to the prim ary ion beam; the

ta rget norm al was a t an angle o f 55° w ith respect to the beam . The

beam spot was large relative to the field o f view of the analyzer, so the

energy resolution of these m easurem ents was not optim al [0186].

A fter venting of the target cham ber to change targets or analyzers,

th e cham ber was first roughed ou t by tw o sorption pumps

consecutively, then opened to an ion pum p that also contained a

titan iu m sublim ator. B aking the cham ber for 1.5 days a t 120° C

35

produced a cham ber pressure of about 2 x 1 0 *10 to rr. W hen the target

cham ber was open to the accelerai or w ith the Ion source unlighted,

the pressure rem ained below 3 x lO *10 to rr. A t th is pressure, the tim e

for a m onolayer of residual gas to form on the target surface was more

th an two hours [Ro82],

2 .2 . A nalyzers and E lectro n ics

2 . 2 .1 . Secon d ary Ion M ass Sp ectrom etry

The theory of quadrupole mass analyzers has been presented by

Dawson [D a76]. The ideal quadrupole structure, shown in fig. 2.4 ,

consists o f four electrodes w ith hyperbolic cross-sections; for easier

construction, electrodes w ith circular cross-sections are usually used.

A djacent electrodes are oppositely charged, w ith the voltage

difference between ad jacent electrodes being an oscillating function of

tim e:

O 0 = U - V cos(cot).

Then the equations of m otion for an ion of mass m and charge q are

^ + OJ - V COS(O)t)) x = 0

( u ' v cos^ y = °*

w here 2ro is the separation between opposite electrodes, and was

2 M H z for our quadrupole. These equations are exam ples of the

M ath ieu equation.

In the x-d irection , a ll ion trajectories would be stable if only the

tim e-independent com ponent o f the electric field w ere present. The

36

oscillating component o f the electric field destabilizes the trajectories

of sufficiently light ions, since only light ions can respond quickly

enough to the reversals in the direction o f the force. In the y -

direction, the tim e-independent com ponent would destabilize a ll

trajectories, b u t the trajectories of sufficiently light ions are stabilized

by the oscillating component. Thus, only those ions lig h t enough to

respond to the oscillating com ponent in the y-direction b u t heavy

enough to not respond to the oscillating com ponent in the x-d irection

w ill have trajectories stable enough to m ake it through the analyzer

w ith o u t h ittin g the electrodes. Exam ples o f "successful" trajectories

appear in fig. 2 .5 . Because the analyzer has fin ite length, some ions

w ith diverging trajectories, such as those shown in fig. 2 .5 a and b, can

actua lly pass through the analyzer w ithout h ittin g the electrodes.

C alculations show th at the mass resolution of the analyzer is

proportional to U /V , w hile, for a given resolution, m /q for the ions

th a t are allowed through the analyzer is proportional to U (or V ). For

our experim ents, the resolution (fu ll-w id th a t half-m axim um ) was

0.6 u.

Consideration of fig. 2 .5 (particu larly parts a and c) shows th a t the

filte rin g action of the quadrupole increases as the ion experiences

more cycles of the oscillating electric field. Thus, the mass o f a high-

energy ion, w hich spends little tim e in the analyzer, is not w ell

resolved. In addition, the background in the mass spectrum can be

decreased by preventing negative ions and n eu tra l atoms from

entering the quadrupole. Therefore, a cy lind rical energy p re -filte r,

shown in fig. 2 .6 a , was attached to the entrance of the quadrupole

(B187cl. W ith the pre-filter voltages set as shown in fig. 2 .6a, ions of

37

energy 4 -6 eV were deflected in to the quadrupole; the relative

transm ission as a function o f ion energy is indicated by fig. 2 .6b

[B187c]. A central stop prevented neutral atom s from entering the

quadrupole, and the voltages on the p re -filte r prevented negative ions

from entering. The electrical filters attached to the voltage cables

w ere necessary to_elim inate high-frequency electrical p ick -u p from

the quadrupole rods [B187cl.

A schem atic o f the electronics for com puter control o f the

quadrupole and for the counting of secondary ions appears in fig. 2 .7 .

The quadrupole rod RF voltages were produced by the R F generator,

w hich was controlled by the U T I 100C quadrupole controller. The

m /q value for the ions th a t were allowed through the spectrom eter

(0 -30 0 u /e ) was proportional to the voltage (0 -10 V) a t the E xternal

Program In p u t o f the controller. This voltage was established by the

DAC output of the PD P-1 1 /2 3 + com puter and modified by a biased

am plifier to allow coverage o f the mass region of in terest. The DAC

output voltage was proportional to the curren t channel num ber of the

m ulti-channel analyzer (C anberra series 40) operated in m ultichannel

scalar mode under com puter control to store incom ing counts; thus,

there was a linear relationship between the secondary ion m ass and

the num ber of the MCA channel used to store the counts obtained for

th a t particular mass.

A pulse counted by the M CA ultim ately originated w ith an analyzed

ion h ittin g the channeltron (Galileo); the pulse thus produced was

shaped by the fast-tim ing pream plifier (ORTEC 9305) and am plifier-

discrim inator (ORTEC 9302 ). The fast d iscrim inator (ORTEC 417)

then converted the pulse to one suitable for in p u t to the M CA.

38

The com puter stepped through program m ed ranges of channels

("full " spectrum ) or single channels located a t the peaks o f in terest

("spike" spectrum ) for a preset num ber o f sweeps, dw elling on each

channel for a preset num ber o f in terru p ts generated by the voltage-to-

frequency converter. The frequency o f in terru p ts (0-1 kH z) was

proportional to the 0 -1 0 V voltage level sent to the converter by the

picoam m eter (Keithley 410A). Th is voltage was in tu rn proportional

to the m easured beam current. Thus, counts were accum ulated in

each channel (or, a t each mass) for a pre-determ ined am ount o f beam

charge incident on the target.

2 .2 .2 . Secondary E lectron E nergy A n alysis

The theory o f the cylindrical m irro r analyzer (CMA) used to

m easure the secondary electron energy d istributions appears in [Sa67,

Za66]. The CM A is shown schem atically in fig. 2 .8 . The electric field

between the two coaxial cylindrical electrodes is proportional to 1 /r ,

where r is the radius from the axis o f the C M A The equation of

m otion for electrons in the CM A is thend2r U 0 _

m dt2 + “T ~ ° ’

w h ere

IT - eVpu ° _ ln (b /a ) *

Here, Vp is the potential difference between the electrodes, and a and

b are the rad ii o f the in ner and outer electrodes. The energy o f the

electrons passed by the analyzer is determ ined by the angle 0o

between the electron's in itia l velocity and the axis o f the analyzer, and

by the physical dim ensions of the analyzer. For 0q equal to 42° 18.5 ',

39

the first-order angular aberration of the electron trajectories vanishes,

and the analyzed electrons are focussed; therefore, CMA's are usually

designed to use this angle. Since the resolution (A E /E ) o f the CM A is

independent o f energy, the m easured spectrum is actua lly EN(E),

where N(E) is the true energy spectrum of the electrons [Sa67J.

The electronics for controlling the CM A and counting analyzed

secondary electrons appear schem atically in fig. 2 .9 . The set-up is

essentially the same as th a t shown previously for secondary ion mass

analysis (fig. 2 .7 ). Norm ally, electron energy spectra (from a CMA) are

m easured by using lock-in am plification to m easure the derivative o f

the secondary electron cu rren t as a function of electron energy. In

our case, however, the low p rim ary-io n beam cu rren t desired m ade

pulse-counting necessary. Therefore, the electron m u ltip lie r, w hich

was connected to the lock-in am plifier, was capacitively coupled to the

fast-tim ing electronics. The voltage level in p u t to the Analog In p u t of

the Auger controller (PHI 500A) w as proportional to the outer-

cylinder voltage of the CMA. Thus, the channel num ber of the M CA

was lin early related to the energy of the electrons th a t were allowed by

the CM A to reach the electron m u ltip lie r.

40

Figure 2.1

Schem atic o f the 3 0 0 k V Cockcroft-W alton accelerator and analyzing

m agnet. (From [B187c])

WNSL 300 kV Cockcroft-Walton

Accelerator

Hot Cathode Ion Source

AcceleratingColumn

AnalyzingMagnet

gpooKxscapg!

Einzel Lens

Electrostatic - Quadrupole

Lens

Differentially Pumped

Beam Line

41

Figure 2.2

The U H V beam line, includ ing the target cham ber. The beam enters

the cold trap after passing through the analyzing m agnet. (From

[B187cD

UHV B eam lin e

42

Figure 2.3

a) Target holder, including a Faraday cup (for m easuring beam

current) and a quartz viewer. The foil target is held onto the ladder

by th in m etal strips w hich are bolted to the ladder. (From [B187c])

b) Schem atic of the target cham ber for secondaiy ion yield

m easurem ents (top view).

c) Schem atic of the target cham ber for secondary electron energy

m easurem ents (top view).

T a rg et

Faraday Can

Beam

Mass S p e c tr o m e te r

Target Holder

(a )

FaradayCup

C

Target

QuartzViewer

43

Figure 2.4

Electric quadrupole mass analyzer. Opposite electrodes are separated

by 2ro, w h ile the electric po ten tia l between adjacent electrodes is d>o-

In th is figure, the electrodes have the (ideal) hyperbolic cross-section.

(From [D a76])

44

Figure 2.5

Exam ples of trajectories o f ions w hich pass through the quadrupole

mass analyzer. Shown is the am plitude u (in either the x-direction or

the y-direction) as a function of the num ber o f quadrupole voltage r f

periods (£ /x ) experienced by the ion . (From [Da76])

Amplitude (/(orbit rory units)* # A A.R

Amplitude utorbrtrory units)

—X--

45

Figure 2.6

a) The energy p re -filte r on the quadrupole used for our

m easurem ents, shown in cross-section. The p re -filte r has

cylindrical sym m etry. The focus p late voltage was supplied by the

U T I controller; the other voltages w ere supplied by external power

supplies.

b) Num bers of Na+ and K+ ions (produced by a heated ion source)

passed by the quadrupole and p re -filte r for the p re-filte r voltages

used for our ion yields m easurem ents, as functions of the energy of

the ions, (from fB187c])

(a ) F ocusP la t e

I'PJ?M tb.-

•al111

PP

Q uadrupole

ijypii s s I S liii!s b Hi

I'lli i

R e f le c t o r

/

< H H0.1 [IF

F ro n t ' G rid

C e n tr a lS to p

H H >0.1 [IF

-2 5 V 2 .9 V -5 V

( b )

46

Figure 2.7

Schem atic of the electronics used for control o f the quadrupole mass

spectrom eter and the collection of secondary ions. The cone voltage

for the channeltron (-2 .8 kV) was supplied by the U TI controller.

SecondaryIons

Targetz

p

QuadrupoleRods

Channeltron

y-2. 8 kV

RF Generator

H V W -1 MQ

QuadrupoleControllerUTI 100-C

External Program Input

Voltage-to-FrequencyConverter

so Q

Fast-Counting Preamplifier

ORTBC 9305

I 90 Q

Amplifier-Discriminator

ORTBC 9302Discriminator Output

T 90 Q

Filter H>0.2 pP

> 1 KQ92 Q

BiasedAmplifier

I

ADC in, EXT AMP

Multi-ChannelAnalyzerCanberra

Series 40Computer I/O

Integrator DAC Output I/O PortInterrupt

PDP 1 1 / 2 3 +

47

Figure 2.8

Schem atic of a cylindrical m irro r analyzer (in cross-section), showing

the focussing property o f the CMA about the angle 0o (42° 18.5'). Here,

a and b are the in ner and outer cylinder rad ii from the axis o f the

CMA. V p, the voltage on the outer cylinder, is supplied by the Auger

controller and determ ines the energy of the electrons w hich reach

the electron m u ltip lie r.

48

Figure 2.9

Schem atic o f the electronics for the control o f the cylindrical m irror

analyzer and the collection of secondary electrons.

Target

CylindricalMirrorAnalyzer

Auger System ControllerPHI 500-A

Analog Input

Picoammeter Keithley 410A

Voltage-to-FrequencyConverter

BiasedAmplifier

ISO kQ

2 pP

Fast-CountingPreamplifier

ORTBC 9305 1 50 Q

Amplifier-Oiscriminator

ORTBC 9302Discriminator Output

30 O

FastDiscriminator

ORTBC 417POS Output

ADC in, BIT AMP

Multi-ChannelAnalyzerCanberra

Series 40Computer I/O

Integrator DAC Output I/O PortInterrupt

PDP 1 1 / 2 3 +

3 .1 . Target Preparation

The targets used in these experim ents were 0 .1 2 7 -0 .2 5 m m th ick

polyciystalline foils (Alfa Products) o f T l, Nb, Mo, W , N i, and Pd, w ith

the m inim um target p u rity being 99.95% . Before being m ounted on

the target ladder, the targets were degreased: soaked in w arm w ater

and Alconox for 15 m inutes, rinsed in tap w ater, soaked in Freon T F

fo r 15 m inutes, rinsed in d istilled w ater, rinsed in isopropyl alcohol,

and h o t-a ir dried.

In vacuum , the targets were sputter-cleaned by 2 0 0 keV Ar+ to

remove surface contam inants. Before the cleaning, fu ll secondary-ion

mass spectra of the in itia l targets were taken for calibration of the

m ass-scale of the M CA and for comparison to fu ll spectra taken after

the sputter-cleaning. A m ass spectrum of an in itia l target o f titan iu m

appears in fig. 3 .1a , showing the presence of Na, K, and T iO on the

target surface. The 4 0 -6 0 |iA ion beams used for sputter-cleaning

w ere rastered and collim ated to form a 1 cm2 beam spot. The total

beam charge incident on the target during the cleaning was

0 .4 -1 .0 Coulom b, w hich was sufficient to remove 2 -15 x lO 3 m onolayers

o f target m ateria l (estim ated from the sem i-em pirical sputtering yield

found in fMa841). The sputter-cleaning was continued u n til

secondaiy-ion mass spectra showed th a t further cleaning would not

significantly reduce the level o f surface contam inants. A mass

spectrum of clean titan iu m appears in fig. 3.1b, showing the

elim ination of Na+ and K+, the reduction of the am ount o f T iO +, and

the observation of T i2+ (which indicates a clean target surface [B187a]).

CHAPTER THREEEXPERIMENTAL PROCEDURE

49

50

A fter the com pletion of the sputter-cleaning, the targets were

exposed to the adsorbate gas a t a pressure of lx lO -6 to rr for an am ount

of tim e chosen to allow saturation of the surface (determ ined by

alternating adsorption w ith the taking of mass spectra). For O 2

adsorption, th is tim e was 1000 s [B187c], w h ile for CO adsorption it

was 2 0 0 0 s. Throughout the .secondary ion yield m easurem ents, a

residual pressure of 5 x l0 *8 to rr o f the adsorbate gas was m aintained in

the target cham ber. This residual gas could then adsorb onto the

surface during the m easurem ents to replace sputtered adsorbate

atom s or molecules. A t the start o f each day, the target was again

sputter-cleaned (for about 3 0 m inutes), and the adsorption was

repeated .

A fter the adsorption, another fu ll mass spectrum was taken for each

target. Sam ple mass spectra appear in fig. 3 .2 for O2 adsorbed onto T i

(denoted O /T i) and for CO adsorbed onto Ni (C O /N i). For O 2

adsorption, the spectra generally consisted o f M + and M O+, w ith some

0 +, N a+, Fe+, and complex m etal-oxide ions. For CO adsorption, the

spectra consisted of M+, 0 +, Na+, and MCO+, in addition to a barely

m easurable am ount of CO+. Particularly im portan t is the absence of

M O + or MC+ in the mass spectra for CO adsorption, which shows th a t

the adsorption is indeed m olecular rather th an dissociative [Ba77J.

3 .2 . Secon dary Ion Y ield M easurem ents

3 .2 .1 . Procedure

The procedure for the secondary ion yie ld m easurem ents was

essentially th a t used by B launer and W eller [B187c]. A fter the target

surfaces were prepared by sputter-cleaning and adsorption, the

51

secondary Ion yields were m easured during bom bardm ent o f the

targets by beam s of 2 5 -2 5 0 keV Ne+, Ar+, and Kr+. The p rim ary ion

beams, rastered and collim ated to form a beam spot o f 0 .0 9 cm 2, were

incident on the target a t an angle of 30° from the target norm al.

Significant damage to the target surface during the yield

m easurem ents was avoided by the use of low beam currents

(0 .5 -1 .0 nA) for short tim es (5 nC incident charge per M C A channel,

or about 5 -7 m inutes to tal per spectrum ). The am ount o f target

m ateria l removed during the m easurem ent o f a mass spectrum was

less th an 0 .0 2 m onolayer [M a84], qualifying these m easurem ents as

static S IM S (Be73J.

The secondary ion yield m easurem ents were repeated several tim es

for each beam energy (not consecutively). M easurem ents using

2 0 0 keV Ar+ beam s were made throughout each ru n to verify the

constancy of the surface and to provide the basis for norm alizing the

data for day-to-day variations in the experim ental conditions.

The mass spectra taken for the yield m easurem ents consisted of

the counts m easured a t a single channel corresponding to the peak of

each interesting mass, as determ ined from the fu ll mass spectra. (For

the m easurem ents of the 0 + yields from bom bardm ent of C O /N i and

C O /P d , 3 channels were used.) O ther, off-peak channels were

included in each sweep to m onitor the background (found to be

negligible). Each spectrum included 10-20 sweeps of 10 -20 channels;

the num ber of sweeps was chosen so th a t the statistical un certa in ty o f

the num ber of counts in the 0 + peak was 1-2% .

52

3 .2 .2 . C orrections for S y stem a tic Errors

The m easured secondary ion yields were corrected for three

sources of system atic error in the m easurem ent o f the incident beam

cu rren t. Since counts were collected for a specified am ount o f beam

charge incident on the target, incorrect cu rren t m easurem ents

resulted in system atic errors in the num ber o f counts per incident ion.

Two of the sources of error were a background or leakage cu rren t

Ic (about -1 0 pA) from the Faraday cage, and an increase in m easured

beam cu rren t th a t was proportional to the increase in the quadrupole

rod voltage (about 5 -1 0 pA m axim um ). The la tte r error varied w ith

M C A channel num ber; le ttin g Iv be the increase observed for the

highest channel num ber used for the mass spectrum , then the

increase for a p articu la r channel was ch- Iv , w here ch was the ratio of

the p a rticu la r channel num ber to the highest channel num ber. These

two sources of error were m easured im m ediately before and after the

collection of each mass spectrum .

The th ird source o f system atic error resulted from the escape o f

secondary electrons to the quadrupole p re -filte r. These electrons

were not captured by the Faraday cage and thus constituted a

contribution Ip to the m easured beam current. Th is error was

corrected for by m aking separate cu rren t m easurem ents w hich

included the p re -filte r (along w ith the target and the Faraday cage) in

the curren t-in tegration c ircu it. M easurem ents o f th e beam curren t

w ith the Faraday cup m ounted on the target ladder verified the

accuracy of these m easurem ents. Therefore, it w as assum ed th a t the

beam cu rren t thus m easured was the actual beam cu rren t I. Moreover,

it was assum ed th a t the fraction of secondary electrons h ittin g the

53

pre-filte r, proportional to Ip/I, w as a constant for a given com bination

o f beam species, beam energy, and target m etal, and th a t the Ip/I th u s

m easured could, therefore, be used in the correction of the secondary

ion yield data.

The curren t In during collection o f counts in to a p articu la r channel

was

I j l — I + Ip + I q + c h * Iy .

Therefore, the correction factor k for the secondary ion yield per

incident beam ion was

(If the m easured beam curren t In were larger th an the actual beam

cu rren t 7, then counts were accum ulated for too short o f a tim e; thus,

the yield per incident ion was increased to com pensate.) In practice,

the currents recorded for a given m ass spectrum were 7C, 7p, and the

beam cu rren t Im m easured a t the s ta rt o f each spectrum (ch = 0):

Im = I + Ip + Ic •

Thus, the correction factor was actually calculated as

Im + ch'Iy I + Ip

K = I m - Ic ‘ I

3 .3 . S econd ary E lectron Y ields and E n ergy D istrib u tion s

The secondary electron yields were estim ated for each ru n by also

m easuring the curren t It for the target alone (w ithout the Faraday cage

in the c ircu it to collect secondary electrons). Then the secondary

electron yield per incident ion was

54

(These yields were also corrected for the sources o f system atic error

discussed above.) The secondary electron yields w ere m easured as

functions of beam velocity for com parison w ith the yields of secondary

0 + ions.

Secondary electron energy d istribu tions were m easured w ith the

cy lindrical m irro r analyzer (CMA); beam s of 5 0 -2 0 0 keV A r* were used

as projectiles, w ith O /V as the target. F u ll spectra were taken,

covering the energy range 0 -2 0 0 0 eV, w ith several sweeps per

spectrum . The focussing properties of a CM A are such th a t the energy

reso lution A E /E is independent o f electron energy E [Sa671, so th a t

the range AE of electron energies contributing to the counts in a

p a rtic u la r M CA channel increased w ith electron energy. Therefore,

th e d a ta were divided by th e secondaiy-electron energy to produce

the fin a l spectra presented in the following chapter.

55

Figure 3.1

a) F u ll secondaiy-ion mass spectrum (mass range 0 -2 0 0 u) for

2 0 0 keV A r* bom barding a T i target as in itia lly p u t into the target

cham ber (after degreasing), for to tal incident charge o f 0 .6 nC per

M C A channel.

b) F u ll secondary-ion mass spectrum (0 -200 u) for 2 0 0 keV At*

bom barding sputter-cleaned T i (about 1400 m onolayers removed

during the cleaning), for to tal Incident charge of 1 .0 nC per M CA

channel.

a) In itia l'll

Channel Number

b) Sputter-cleaned Ti

IOOO 1500 2000 2500

Chonnel Number

5000 5500 4000

56

Figure 3.2


2 0 0 keV Ar+ bom barding T i after sputter-cleaning followed by

exposure to 1000 L o f O2 (1 L = 10 '6 to rrs ); to ta l in cident charge

was 0 .2 nC per M CA channel.

b) F u ll secondary-ion mass spectrum (0 -20 0 u) for 2 0 0 keV Ar+

bom barding Ni after sputter-cleaning followed by exposure to 2000 L

o f CO; total incident charge was 0.1 nC per M CA channel.

56

Figure 3.2


2 0 0 keV Ar+ bom barding T i after sputter-cleaning followed by

exposure to 1000 L of O2 (1 L = 10‘6 to rr s); to tal incident charge

was 0 .2 nC per M CA channel.

b) F u ll secondary-ion mass spectrum (0 -200 u) for 2 0 0 keV Ar+

bom barding Ni after sputter-cleaning followed by exposure to 2000 L

of CO; total incident charge was 0.1 nC per M CA channel.

a) o m

500 1 0 0 0 1500 2000 2500

Chonnel Number5000 5500 4000

b) CO/Ni

1400

1500 2 0 0 0 5000 4000Chonnel Number

4 . 1 / O2 A dsorption o n to Ti, Nb, Mo, and W

As an example o f the dependence of the m etal secondary ion yields

on the energy o f the incident beam , fig. 4.1 shows the yields of M o+

from Ne+, Arty and Kr+ bom bardm ent o f O /M o . (All secondary ion

yields presented In th is chapter have been corrected fo r the

system atic errors discussed in section 3 .2 .) The dependence on beam

energy of a ll other m etal and m etal-oxide ion yields, fo r a ll targets

studied, resembled th a t o f M o+. Figure 4.1 also shows the to ta l Mo

sputtering yields, calculated using the sem i-em pirical form alism of

M atsunam i et aL [M a84] and arb itra rily scaled to equal the m easured

secondary ion yields a t 2 0 0 keV. The em pirical scaling factors S,

tabulated in table 4.1 for a ll adsorbate/substrate systems studied, are

th e products of two unm easured quantities:

S = T • 1+ ,

where the transm ission factor T includes the angu lar acceptance and

transm ission of the mass spectrom eter, as w ell as the efficiency of the

channeltron, w hile I+ is the ionization probability of a sputtered atom.

Benninghoven [Be75] found I+ to be 0 .4 for T i+ and 0 .0 3 5 for Wty when

these num bers are com bined w ith the decrease in the transm ission of

a quadrupole mass spectrom eter w ith increasing ion m ass [D a76,

Ut79J. T is found to be o f the order 10*6-1 0 *5.

The scaling factors decrease slightly from Ne+ to Krty th is decrease

could resu lt from the com bination of the choice o f 2 0 0 keV as the

standard energy and the difference between the beam -energy

dependence of the secondary ion yields and th a t o f the sputtering

CHAPTER FOURRESULTS AND DISCUSSION

57

58

yields. The difference in the dependence on beam energy between

the emission of secondary ions and sputtered atom s could indicate

that, for the lower beam energies and the heavier beam ions, the

collision cascade was too dense to be adequately described by the

linear collision cascade theory; th is w ould have resulted in a secondary

ion yield th a t was larger than th a t predicted by the lin ear theory

[Si81a].

Figures 4 .2 and 4 .3 show the yields o f secondary O * ions em itted

during ion bom bardm ent o f oxidized T i, Nb, M o, and W targets. The

O * yields Increase lin early w ith beam velocity; the param eters from

linear fits to the data are presented in tab le 4 .2 . Com parison of these

figures w ith fig. 4.1 shows th a t for these targets there is a clear

difference between the dependence of the m eta l ion yields on beam

energy and th a t o f the oxygen ion yields: th is resu lt is consistent w ith

the results obtained by B launer and W eller fo r bom bardm ent of

oxidized A l and V (B187a,b,c].

Figure 4 .4 shows the yields of secondary electrons em itted during

ion bom bardm ent of O /M o and C O /N i. These yields also increase

linearly w ith beam velocity, as expected from the experim ental and

theoretical resu lts discussed in section 1 .4 . The param eters from

linear fits to the secondary electron yields as functions of beam

velocity appear in table 4 .2 . The discussion in section 4 .1 .1 w ill center

on com parisons o f the beam -velocity dependence o f the O * yields w ith

th a t of the secondary electron yields and th a t o f the electronic

stopping power; the dependences of the la tte r tw o are expected to be

sim ilar. I f a connection exists between electronic excitation and 0+

em ission [B187a], one m ight expect a correlation between the O *

59

yields and either the secondary electron yields or the electronic

stopping power (or both).

4 . 1 . 1 . O* E m ission

As shown in fig. 4 .2 and tab le 4 .2 , the O * yields from ion

bom bardm ent of O /T l and O /N b are collinear for a ll beam s and are

stric tly proportional to beam velocity, w ith the exception o f the yields

from Ne+ bom bardm ent o f O /N b . Th is resu lt agrees w ith the resu lt of

B lau ner and W eller for ion bom bardm ent of O /V [B187al. Therefore,

the lack of explicit dependence on beam species probably does not

resu lt from a coincidental cancellation o f factors or a pecu liarity o f any

of these systems. For Ne+ bom bardm ent o f O /N b , the y-in tercept

found from a linear fit to th e d a ta is significantly d ifferent from zero;

th is apparent offset of the 0 + yie lds w ill be discussed fu rth er in

section 4 .1 .2 .

The lin ear dependence o f the 0 + yields on beam velocity for ion

bom bardm ent o f O /V , O/Ti, and O /N b suggests th a t electronic

processes are active. However, one m ight then expect an explic it

dependence on beam species, since the secondary electron yields and

the electronic stopping power depend on beam species as w ell as on

beam velocity. For instance, th e slope o f the secondary electron yields

as a function of beam velocity fo r bom bardm ent of O /T l increases by a

factor o f 3 from Ne+ to Kr+; as expected from the discussion in section

1.4, th is increase is s im ilar to th a t of the Lindhard stopping power

factor (0 .332 for Ne+, 0 .8 0 5 for K r+). Th is increase, however, is not

reflected in the 0 + yields from bom bardm ent of O /T l. Also, i f the 0+

yields scale as the electronic excitation , then division of th e yields by

60

the Lindhard stopping power factor should cause the data for different

beams to fa ll on a common line. C learly, th is w ill not occur for O /T i;

fig. 4 .5 shows th a t th is scaling also does not occur for O /N b . We can

also compare the 0 + yields directly to the secondary electron yields by

com puting the ratios of the slopes determ ined by linear fits to the two

sets o f data. A lthough these ratios are consistent w ith one another for

bom bardm ent o f O /N b , th is consistency is n o t as compelling as th a t

found for 0 /M o (discussed below) and resu lts, to some extent, from

the large statistical uncerta in ty in the ratios.

Treating Ne+ bom bardm ent o f O /N b as a special case, it therefore

seems reasonable to group O /N b , O /T l, and O /V together as systems

for w hich the 0 + yields are proportional to beam velocity, w ith no

additional dependence on beam species. T h is suggests th at, for these

targets, the 0 + em ission does not arise from a m echanism , such as

desorption induced by secondary electrons, w h ich depends in a

sim ple m anner on the to tal energy deposited in to electronic

excitation. (It m u st be noted, however, th a t th is sim ple argum ent

assumes th a t the fraction of secondary electrons w ith sufficient energy

to in itia te desorption is a constant for a ll beam species and beam

velocities.) M oreover, in section 4 .1 .3 it w ill be shown th a t the

m agnitude of the 0 + yields is not consistent w ith desorption induced

by secondary electrons.

The lite ra tu re on the oxidation of tran sitio n m eta l surfaces contains

few determ inations o f the stoichiom etry o f th e oxide formed for

exposures s im ilar to those used in this study. M oreover, those

determ inations have often produced conflicting results; for instance,

the oxide form ed on T l has been found to be T IO by one group [Ro84)

61

and T IO 2 by another [Ca87]. Therefore, since we did not attem pt to

characterize our oxides, it cannot be certain th a t the oxides were, in

fact, m axim ally valent. Such a characterization could shed lig h t on the

m echanism responsible for the dependence of the 0 + yields on beam

velocity for O /N b , O /T I, and O /V . A finding th a t the oxides were not

m axim ally valent would relate our studies to those of ESD of 0 + from

beam -reduced TiC>2 (discussed in section 1.5.2).

Figure 4 .3 , along w ith consideration o f the lin ear fit param eters

presented in table 4 .2 , dem onstrates th a t the 0 + yields from O /M o and

O /W , as functions of beam velocity, do depend explicitly on beam

species. Th is dependence is com pared w ith th a t of the electronic

stopping power in fig. 4 .6 , w hich shows the 0 + yields divided by the

Lindhard factor, and in table 4 .2 , w hich presents the slopes o f the

scaled 0 + yields and the ratios of the 0 + slopes to the secondary

electron slopes. For both targets, the scaled yields coincide for the

Ar+ and Kr+ beam s. A lthough the scaled yields for Ne+ bom bardm ent

of O /M o are higher than those for the Ar+ and Kr+ beam s, the scaled

slopes and the ratios of the slopes show th a t the 0 + yields for Ne+

bom bardm ent o f O /M o do scale w ith the electronic excitation. Th is is

not the case for Ne+ bom bardm ent of O /W ; moreover, here the Ne+

data are offset by a relatively sm all am ount compared to th a t found for

O /M o and O /N b , although the offset is s till significantly d ifferent from

zero.

Therefore, for a ll beam species bom barding O /M o and O /W , w ith

the exception of Ne+ bom barding O /W , the 0 + yields have a com ponent

whose dependence on beam species is consistent w ith the suggestion

[B187a] th a t the 0 + emission is caused by secondary electrons or by

62

some other form of electronic excitation. This is in contrast to the

resultsffor O /V , O /T l, and O /N b . Figure 4 .7 shows the relevant portion

of the periodic table and sum m arizes the behavior o f the 0 + yields.

A pparently, the details o f the 0 + emission from a m etal surface are

influenced by some property th a t varies w ith the colum n of the

periodic table of the m etal substrate, such_as the electronic structure

o f the m etal or the m etal oxide. Given the uncerta in ty about the

valency of the oxides, it is possible that the relevant property could be

w hether a m axim ally valen t oxide is actually formed.

4 .1 .2 . Ne+ B om bardm ent

For Ne+ bom bardm ent o f O /N b . O /M o, and O /W , the 0 + yields are

offset from being s tric tly proportional to beam velocity; th is is in

contrast to the behavior o f the 0 + yields for a ll o ther b eam /targ et

com binations discussed th u s far. Systematic error seems to be an

u n like ly explanation of the offsets, since, although they were

reproducible, they were no t observed for Ne+ bom bardm ent of O /V

and O /T I (if there were a problem associated w ith the use o f Ne+

beam s, it should have resulted in offsets for all targets). I f the linear

fits to the 0 + yields hold tru e for lower beam velocities th an used in

th is study, w ith an extrapolated nonzero yield a t zero beam velocity,

then the additional em ission o f 0 + may be related to the potential

energy o f the incident Ne+. If, however, the 0 + yields go to zero for

the low er beam velocities, as suggested by the data fo r the lowest

beam energy used here, th en a kinetic m echanism (s im ilar to th at

discussed in section 1 .2 .2 fo r the emission of m ultip ly-charged m etal

63

ions) could be im plicated, e.g. one Invoking a direct collision between

a backscattered Ne+ io?' and an adsorbed oxygen atom.

The secondary electron yields during Ne+ bom bardm ent are also

offset, as shown in table 4 .2 . Following the hypothesis th a t secondary

electrons can cause 0 + emission, it is reasonable to look for a

connection between the O * offset and th e secondary electron offset.

This connection could take one o f two form s. F irst, the Ne+

bom bardm ent could have produced an additional source of secondary

electrons whose yield did not increase lin early w ith the beam velocity,

either by neutralization of the Ne+ or by some kinetic m echanism such

as electron prom otion (section 1 .2 .2 ). These additional electrons then

could have caused the O * emission by th e same ESD mechanism

responsible for the velocity-proportional em ission. In th is case, one

w ould expect the offsets of the O * yields for different targets to scale

as the O * yield per secondary electron found for velocity-proportional

emission; th a t is.

b(Q+) _ m (Q+)b(e*) “ m (e -)

where m and b are the slope and y-in tercept. However, the ratio o f

the left-hand side to the right-hand side is not consistent w ith u n ity

for any target (4 .7 ± 1 .7 for Nb, 2 .6 ± 0 .9 for M o, and 2 .7 ± 0 .6 for W ).

(It is conceivable, however, th a t the additional electrons had such an

energy d istribu tio n that they were more efficient a t producing O *

desorption th a n the bu lk of the secondary electrons.)

The second type of connection between the O * offset and the

secondary electron offset would be th a t th e additional secondary

electrons w ere an incidental by-product o f some other process th a t

64

produced the extra 0 + em ission. For instance, the 0 + em ission could

have been produced by a k in etic m echanism sim ilar to th a t discussed

in section 1.2.2 for the em ission of m u ltip ly charged m etal ions, or by

Auger neutralization of the incident Ne+ by an oxygen valence electron,

leading to the stripping of the other valence electrons. In the form er

case. one_would expect the 0 + emission to go to zero fo r zero beam

velocity, w hile for the la tte r case th a t is not necessarily tru e . In either

case, the em itted Auger electrons would be the observed additional

secondary electrons.

I f the additional electrons w ere the result of, ra th e r th an the cause

of, the additional emission o f 0 +, the ratios of the 0 + offset to the

secondary electron offset should be independent o f the target. These

ratios are (in un its of 10*7 0 +/e lectro n ) 2 .7 ± 0 .8 for Nb, 2 .6 ± 0 .8 for

M o, and 0 .6 ± 0 .1 for W . The apparent consistency between Nb and

M o m ay be encouraging. F u rth er study of the low-energy behavior of

the 0 + yields during Ne+ bom bardm ent is clearly needed.

A ny speculation about the existence of a m echanism for 0 + em ission

th a t is unique to Ne+ bom bardm ent m ust be tem pered by

consideration of the results from bom bardm ent o f O /T i and O /V .

A lthough the secondary electron yields are offset for both Ne+ and Ar+

bom bardm ent of O /T i, the 0 + yields are not offset for any beam .

Blauner's results for bom bardm ent of O /V also show no signs o f an

offset of the 0 + yields IB187a,c]. Therefore, the suggestion th a t Ne+

bom bardm ent produces an ad d itional source of secondary electrons

w hich then cause 0 + em ission is not supported, since for O /n the

additional secondary electrons w ere observed, b u t the add itional 0 +

em ission was not observed. Also, one would expect th a t a m echanism

65

for 0 + em ission invoking a direct in teraction betw een the incident ion

and the oxygen atom would operate for V and T l as wel] as for Nb, M o,

and W.

4 .1 .3 . S econ d ary E lectron E nergy D istrib u tion s

Figure 4 .8 shows the secondary electron energy d istribution

m easured for 2 0 0 keV A r+ bom bardm ent o f O /V . S im ilar d istributions

were m easured fo r incident Ar+ beams having o ther energies. The

broad feature a t around 200 eV is a Doppler-broadened A r Auger peak

IBe82].

I f secondary electrons were causing the em ission of 0 + by electron-

stim ulated desorption, then the cross-section a (0 +) for ion-induced

emission should be

o (0 +) = J Ye(Ee) OESD(Ee) dE e .

where Ye(Ee) is the secondary electron energy d istribu tion and

OESD(Ee) is the cross-section for ESD of 0 + by an electron w ith energy

E e. The electron energy distribution in fig. 4 .8 w as used to estim ate

the 0 + yields expected from ESD induced by secondary electrons. An

exam ple of the dependence of the ESD cross-section for the em ission

of 0 + on the electron energy is shown in fig. 4 .9 fFe78]. For the sim ple

calculations in th is section, the ESD yield per electron, Y e s d . was

approxim ated as increasing linearly from threshold a t 3 0 eV to a

m axim um , Ymax. a t 9 0 eV, rem aining a t Ymax for larger electron

energies.

The expected y ie ld of 0 + desorbed by secondary electrons was then

calculated as

66

Y<0») = ^ . T . ^ g . Xel ;i i e

both sum m ations were over electron energy. Here, ye is the to ta l

secondary electron yield determ ined from the ion beam cu rren t'

m easurem ents, and Ye is the secondary electron d istribu tio n shown in

fig. 4 .8 . A lthough the transm ission of the mass spectrom eter, T , was

estim ated to be of the order 10*5 fo rT l+ (section 4 .1 ), th e TI+ energy

d istribu tion has a m axim um a t about 10 eV [Sn78], com pared to the

4 -6 eV range o f the energy p re -filte r used for our m easurem ents. In

contrast, approxim ating the energy d istribu tion of the em itted O * by a

G aussian distribution , w ith the w id th and the position of the m axim um

being those found by W eng for E SD from O/Tl (fig. 4 .10) [We81J, shows

th a t about one-half of the 0 + w ould have energies w ith in the range of

the p re -filte r. It would be expected, therefore, th a t T was ac tu a lly

b etter th an 10*5 for the O * ions; for th is calculation, T w as estim ated

as 10-4.

The resu lt o f the calculation was th a t the expected yield o f 0 + from

secondary-electron-induced desorption during 2 0 0 keV Ar+

bom bardm ent of O /V would be

Y (0 + ) = 0 :5 3 • T • Ymax :

w ith Ymax being typically o f the order 10*6-1 0*5 0 +/e lec tro n [K n84,

R e64), then the expected O * yield w ould be about 10*10 0 +/io n . This

is m uch sm aller than the observed 0 + yield from O /V , about

2.5x10*7 0 +/io n [B187a], by a factor o f 2 5 0 0 . A lthough th is calculation

w as crude and the conditions for m easuring Ye were not optim al, the

hypothesis of secondary-electron-induced emission o f 0 + appears to be

inconsistent w ith the m agnitude o f the observed 0 + yields. It m ust be

67

noted, however, th a t th is calculation does not apply to emission

produced by other form s o f electronic excitation. The calculation

would be im proved by using the same p re -filte r and quadrupole mass

spectrom eter fo r bo th ion - and electron-bom bardm ent experim ents,

thereby elim inating T from the calculations and obtaining an accurate

value of Ymax for the specific target surfaces studied. More accurate

m easurem ents o f Ye(Ee) for these targets are also needed.

The yield and the energy o f Auger electrons produced by ion

bom bardm ent depend on the beam -target com bination [Ba82. Be82].

Therefore, it was o f in terest to estim ate the contribution of the A r

Auger electrons to th e calculated 0 + yield from secondary electrons in

order to determ ine w heth er p a rt o f the m easured varia tio n of the 0 +

yield w ith beam species could have resulted from variations In the

Auger electron y ie ld . The background of secondary electrons (shown

in fig. 4 .8 ) was estim ated from the energy d istrib u tio n on both sides of

the Auger feature and subtracted from the electron energy

distribu tion , leaving only the A r Auger feature. Th is feature was used

for the energy d is trib u tio n Ye in a second set of calculations performed

in the m anner described above. It was found th a t the fraction of the

calculated 0 + yield th a t was caused by A r Auger electrons ranged from

6% for 50 keV beam s to 15% for 200 keV beam s. Since th is fraction

was less than or equal to the scatter in the 0 + yield data, it is difficult

to ascribe any significan t effect to it for Ar+ beam s. However, sim ilar

m easurem ents and calcu lations should be m ade for Ne+ and Kr+

beam s, since the y ie ld induced by Auger electrons could be greater for

those beam s, o r th e ir A uger electrons could be m ore effective a t

producing desorption.

6 8

Since the energy p re -filte r on the quadrupole mass spectrom eter

sam pled only a narrow range of 0 + energies, a change In the 0 + energy

d istribu tio n w ith beam energy or beam species could have produced

system atic errors in the variations o f the 0 + yields w ith beam velocity

and beam species. Figure 4 .11 shows th a t the shape of the energy

d is trib u tio n o f the secondary electrons depends on beam energy

[Be82], w h ile fig. 4 .1 0 shows th a t the shape and position o f the 0 +

energy d istribu tion (for ESD) can change w ith electron energy [W e81).

Therefore, a change in the 0 + energy d istribu tion w ith ion beam

velocity and beam species is possible. Calculations com bining Weng's

0 + energy d istribu tion [W e81] w ith the m easured secondary electron

energy d istributions for several ion beam energies showed th a t the

fraction o f the em itted 0 + ions th a t had energies in the energy range

of the p re -filte r was independent o f beam energy for Ar+

bom bardm ent o f O /V . It was desirable, however, to calculate w hether

the lim ited energy range o f the p re -filte r w as introducing system atic

error for other beam -target com binations. Therefore, assum ing th a t

the ta il o f the secondary electron energy d istribu tion decreased w ith

increasing electron energy E as E*n for 1.5 £ n £ 3 .0 [H a87], we

calculated the effect of variations o f n on the estim ated 0 + yield. (The

assum ed behavior of the ESD cross-section was the same as th a t used

above.) The m axim um change in the calculated 0 + yield was less than

4% over th is range of n. Given th a t the actual range of n m ay not have

been th is large for our experim ents, th is resu lt sets an upper lim it on

the system atic error introduced by the energy range of the p re-filte r.

Since the scatter In the 0 + yield data w as about 15%, the system atic

error in troduced by the p re -filte r w as negligible.

69

4 .2 . O /N l Targets

Figure 4 .1 2 shows the yields of 0 + and N i+ from ion bom bardm ent

of oxidized nickel (the O * yields have been scaled to equal the N i+

yields a t 2 0 0 keV). A lthough the 0 + yields are not exactly proportional

to the N i+ yields as a function of beam velocity, Jh e dependences of the

yields on beam velocity are very sim ilar; the yields certain ly do not

increase lin early w ith increasing beam velocity, u n like those for the

other oxidized targets discussed thus far. Therefore, it appears that

the em ission of O * during ion bom bardm ent of O /N i results from

collision cascades ra th e r than from electronic processes.

Th is resu lt, however, m ay not be inconsistent w ith the observation

of ESD of 0 + from O /N i [Ge84, M a76, N i81]. G erritsen [Ge84] found

the to ta l E SD yield o f oxygen atoms from O /N i to be about

2x1 O'8 atom s/electron. Since, typically, less th an 10% of the total

ESD yield is ionized (K n84], this im plies an O * yield o f less than

2 x l0 *9 0 +/e lectro n , w hich is a factor of 10*3 sm aller th an the ESD

yields o f O * from m axim ally valent m etal oxides. It w ould be expected,

therefore, th a t any electronically-induced em ission of O * during ion

bom bardm ent o f O /N i would be sm aller than th a t from O /T i by a

s im ila r factor. Com parison of the observed O * from ion bom bardm ent

o f the two targets, however, shows th at the yields from O /N i

(presum ably from collision cascades) are sm aller th an those from O/Tl

by a factor of only 10*2. Th is crude calculation suggests th a t, for ion

bom bardm ent o f O /N i, the yields of any O * desorption induced by

electronic processes m ay be too sm all to be observable above the 0+

em ission from collision cascades.

70

4 .3 . CO A dsorption

4 .3 .1 . CO* E m ission

Figure 4 .13a shows the yields of CO + and N i+ from At*

bom bardm ent of C O /N i, w ith the C O * yields scaled to equal the Ni+

yields a t 2 0 0 keV. Figure 4 .13bshow s the equivalent data for A r*

bom bardm ent o f C O /P d . The s im ilarity between the behavior o f the

C O * yields as functions of beam velocity and th a t of the m etal ion yields

shows th a t the emission of CO+ from C O /N i and C O /P d is probably

produced by collision cascades. A lthough Craig has observed ESD of

CO + from C O /N i [Cr83], he does not present the absolute m agnitudes

of the ion yields, so th a t the expected yield o f CO+ em ission induced

by secondary electrons cannot be estim ated. Craig does report,

however, th a t the C O * yield was 0 .4 tim es the O * yield [Cr83], w hile

our ion-induced CO+ yield was only 0.1 tim es our ion-induced O * yield.

It m ay be significant th a t our targets were exposed to 20 00 L

(1 L = 10*6 to rr x s) o f CO, in contrast to the 3 L exposure used by Craig.

Also, C raig used 400 eV electrons in his experim ents; fig. 4 .8 shows

th a t very few of the secondary electrons em itted during ion

bom bardm ent would have such an energy. It is possible, therefore,

th a t, for C raig’s electrons, the ratio of the ESD cross-section for CO +

em ission to the cross-section for O * em ission was d ifferent from the

ra tio for the secondary electrons em itted during ion bom bardm ent,

w hich w ould explain the discrepancy in the ratio of the C O * yields to

the O * yields between our experim ents and C raig’s experim ents.

71

4 .3 .2 . N i* E m ission

Figure 4 .14 shows the yields of N i+ for A t* bom bardm ent o f O /N i

and C O /N i; the yields for O2 adsorption have been scaled to equal

those for CO adsorption a t 2 0 0 keV. The dependence of the N i+ yields

on beam energy is sim ilar for the two adsorbates, w ith the yields being

approxim ately p ro p o rtio n a l to the sputtering yield. The m easured Ni+

yields from the CO-adsorbed target are larger than those from the O 2-

adsorbed target by more th an an order o f m agnitude (see table 4 .1 ).

This extra enhancem ent o f the N i+ yield by adsorption of CO has been

observed previously by W inograd (Wi82cJ. Since the N I-C O bond is not

highly ionic [W i82b, Za88], th is enhancem ent cannot be easily

explained by the bond-breaking model often used to explain the

enhancem ent of m etal ion yields by O2 adsorption (section 1 .2 .2).

C learly, the effects of adsorption on secondary ion em ission are not

fu lly understood.

4 .3 .3 . O* E m ission

The yields of O * from ion bom bardm ent of C O /N i and C O /P d are

shown in fig. 4 .15 . In some respects, the data resemble those for the

oxidized targets, excluding O /N i; the yields for CO adsorption certa in ly

do not follow the sputtering yields. For both C O /N i and C O /P d , the O *

yields from K r* bom bardm ent increase w ith increasing beam velocity,

s im ilar to the behavior found for the oxidized targets. This is also true

for At* bom bardm ent of C O /P d ; however, the 0 + yields from Ar+

bom bardm ent of C O /N i exh ib it a plateau, or even a m axim um , for the

larger beam velocities. The O * yields from Ne+ bom bardm ent o f C O /N i

increase slightly w ith beam velocity for the lower beam velocities, b u t

72

they decrease w ith increasing beam velocity for higher beam

velocities. The 0 + yields are essentially independent o f beam velocity

for Ne+ bom bardm ent o f C O /P d . Thus, there are significant

differences between the results for CO adsorption and those for O2

adsorption. Com parison of the secondary electron yields, as a function

o f beam velocity, from ion bom bardm ent o f O /M o w ith those for C O /N i

(both shown in fig. 4 .4 ) shows th a t the differences in the behavior o f

the 0 + yields between O 2 adsorption and CO adsorption cannot arise

from any differences in the behavior o f the secondary electron

em ission between the two adsorbates.

For C O /N i, the 0 + yields from Ar+ and Ne+ bom bardm ent appear to

lie on a common curve as functions of beam velocity, b u t the yields

from Kr+ bom bardm ent are twice as large. In a check for any

influence o f beam -induced electronic excitation, fig. 4 .1 6 a shows the

0 + yields divided by L indhard ’s electronic stopping power constant.

The scaled yields for Ar+ and Kr+ bom bardm ent fa ll on a common

curve, w hile the scaled Ne+ yields are tw ice as large. If, as suggested

in section 4 .1 .2 , there exists a m echanism for 0 + em ission th a t is

peculiar to Ne+ bom bardm ent, it is conceivable th a t such a m echanism

is operating for the C O /N i system also. Nevertheless, there is no clear

system atic behavior o f the 0 + yields as functions of beam velocity and

beam species for ion bom bardm ent o f C O /N i. A lthough the same

m echanism for 0 + desorption m ay be active for both O2- and CO-

adsorbed targets for the lower beam velocities, the results for the

higher beam velocities suggest the presence of e ith er a d ifferent

m echanism or an additional m echanism in the case o f C O /N i.

73

Com parison o f the scale o f fig. 4 .1 5 a w ith the scale o f fig. 4 .1 2a

(considering the norm alization o f the 0 + yields fo r the la tter) reveals

th a t the 0 + yields from ion bom bardm ent o f C O /N i are a factor of 4 -1 0

greater th an those from O /N i. M adden, however, found the ESD yields

of 0 + from O /N i to be m uch larger than those from C O /N i [M a76].

Both th is discrepancy and the observation of the large enhancem ent of

the N i+ yield by CO adsorption suggest th a t the effects o f CO

adsorption on ionization probabilities m erit fu rth e r study.

The 0 + yields from ion bom bardm ent o f C O /P d (fig. 4 .15b ) do not

fa ll on a common curve for the different beam species. D ivision by

L lndhard ’s electronic stopping power constant (fig. 4 .16b ) im proves

the situation m arginally, a t best. The behavior o f the 0 + yields as a

function of beam velocity for the Ne+ and Ar+ beam s differs from th a t

found for the C O /N i targets. In fact, the behavior o f the 0 + yields from

ion bom bardm ent o f C O /P d resembles the proportionality to beam

velocity found for the 0 2 -adsorbate systems (excluding O /N i), although

the yields fo r Ne+ bom bardm ent o f C O /P d exhib it less dependence on

beam velocity th an do those for Ne+ bom bardm ent o f the oxidized

targets. Therefore, it appears th a t the desorption of 0 + during ion

bom bardm ent o f CO adsorbed onto m etal surfaces is sensitive to the

substrate m etal.

74

Table 4.1

Ratios of m easured secondary m etal ion yields to calculated sputtering

yields [M a84] for 2 0 0 keV noble ion bom bardm ent o f adsorbate-

covered transition m eta l surfaces. These ratios are the product of the

transm ission and angu lar acceptance of the quadrupole, the efficiency

of the channeltron, and the ionization probability of the sputtered

m etal atom (none o f w h ich where m easured separately).

BeamTarget Ne+ Ar+ Kr+

O/Ti 12 XlO"6 8.3 xlO-6 6.3 XlO"6O/Nb 2.3 XlO-7 1.5 xlO-7 1.4 xlO"7O/Mo ©

•V

O ioHX 7.6 xlO-7 3.9 XlO"7O/VI 2 . 1 X10-8 1.3 XlO-8 1.3 XlO-8O/Ni 5.6 X10-8 4.4 XlO-8CO/Ni 6.4 XlO-7 9.0 XlO*7 8 . 0 xlO-7CO/Pd 4.1 XlO"8 2 . 8 XlO*8 2 . 8 XlO-8

75

Table 4.2

Slopes and y-lntercepts computed by least-squares fits to the 0 + yields

and the secondary electron yields as linear functions of beam velocity,

for 2 5 -2 5 0 keV noble ion bom bardm ent o f oxidized tran sition m etal

surfaces. 0 + yields are in un its o f 1 0 '7 per incident ion, electron yields

are per Incident ion, and velocity is in un its o f 107 c m /s .

Target

T i

Nb

Mo

3 earn

0 * Io n s

S lo p e Y - In t e r c e p t

S e co n d a ry E le c t r o n s

S lo p e Y - I n t e r c e p t

Ne7 0 . 4 0 2± 0 . 0 5 1

0 . 4 8 2± 0 . 4 9 3

0 . 1 8 8± 0 . 0 2 9

1 . 4 7± 0 . 0 2 4

A r -1 0 . 4 4 2± 0 . 0 5 3

0 . 4 0 3± 0 . 2 6 9

0 . 4 0 4± 0 . 0 4 5

0 . 7 4 9± 0 . 1 7 9

H r 1 0 . 4 6 1± 0 . 0 5 0

0 . 0 2 6± 0 . 1 8 6

0 . 6 1 2± 0 . 0 7 3

- 0 . 4 8 4± 0 . 3 3 8

Ne-1

A r+

0 . 1 5 3± 0 . 0 2 6

0 . 4 0 6± 0 . 0 5 0

2 . 0 5± 0 . 3 1

- 0 . 1 2 6±0 . 2 2 1

0 . 2 6 9± 0 . 0 2 3

0 . 5 8 1± 0 . 0 7 3

0 . 7 6 3± 0 . 1 4 7

- 0 . 3 5 0± 0 . 3 1 4

Kr"1 0 . 4 1 4± 0 . 0 2 9

0 . 2 7 7± 0 . 1 5 0

0 . 6 8 3± 0 . 0 4 0

- 1 . 0 7±0 . 2 2

Nei 0 . 2 5 8± 0 . 0 3 0

1 . 4 6± 0 . 3 0

0 . 2 5 8± 0 . 0 1 9

0 . 5 5 6±0 . 1 2 2

A r-1 0 . 4 5 8± 0 . 0 3 2

- 0 . 2 5 8±0 . 1 2 0

0 . 4 3 8± 0 . 0 2 9

- 0 . 1 9 0± 0 . 1 0 8

K r + 0 . 6 2 4± 0 . 0 4 2

0 . 0 6 2± 0 . 1 8 6

0 . 5 9 0± 0 . 0 3 6

- 0 . 9 8 9± 0 . 1 6 5

Ne1 0 . 0 4 9 1± 0 . 0 0 4

0 . 3 8 8± 0 . 0 4 9

0 . 2 1 6±0 . 0 1 0

0 . 6 3 5± 0 . 0 9 0

Ar"1 0 . 1 8 4±0 . 0 1 1

- 0 . 2 4 3± 0 . 0 5 6

0 . 4 2 40 . 0 2 0

- 0 . 5 2 4± 0 . 0 8 8

K r i 0 . 2 7 5± 0 . 0 3 4

0 . 0 0 4± 0 . 1 5 2

0 . 4 6 7± 0 . 0 4 7

- 1 . 4 9±0 . 2 0

C

76

Table 4.3

Ratios of the computed slopes for the 0 + yields to the slopes for the

secondary electron yields (table 4 .2 ) and to the beam -dependent factor

in Lindhard's electronic stopping power. U nits for the firs t colum n

are 1 0 '7 0 + per secondary electron; un its for the second colum n are

arb itrary.

Target

Ti

Nb

Mo

Beam 0+ slope 0+ slopeelectron slope Lindhard factor

Ne+ 2.14 1.21±0.43 ±0.15

Ar+ 1.09 0.857±0.18 ±0.103

Kr+ 0.753 0.573±0.121 ±0.062

Ne+ 0.569 0.702±0.108 ±0.119

Ar+ 0.699 1.13±0.123 ±0.14

Kr+ 0.606 0.689±0.055 ±0.048

Ne+ 1.00 1.20±0.14 ±0.14

Ar+ 1.05 1.30±0.10 ±0.09

Kr+ 1.06 1.05±0.10 ±0.07

Ne+ 0.227 0.351±0.022 ±0.030

Ar+ 0.434 0.767±0.033 ±0.046

Kr+ 0.589 0.641±0.094 ±0.079

77

Figure 4.1

M easured yields of M o+ (open circles), and Mo sputtering yields

(closed circles) calculated as in [M a84] and norm alized to the

m easured Mo+ yields a t 20 0 keV (see table 4 .1 for norm alization

factors), for bom bardm ent o f O /M o by 2 5 -2 5 0 keV noble gas ions:

a) Ne+ bom bardm ent: b) Ar+ bom bardm ent; c) Kr+ bom bardm ent.

1 0

8

6

4

2

0

12 “ ■

f

— 0

c

o o

o

—1—

o•

M o + y ie ldM o sp u tte r y ie ld (est.)

• 1

1»<

>2

( a )

iI

• 8 . 1

L ! ?

5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0B e a m E n e r g y ( k e V )

4 0

3 0

2 0

1 0

0

V V

K

O O

O 1

-------1-------

1

( b )

/

)

1 <

©5 • !

e\ • [

5 • i>

1 8

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0

B e a m E n e r g y ( k e V )

3 0

2 5

2 0

1 5

10

T "o

o o

o

T

o

o

- 1—

( c )

o

5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0


Figure 4.2

0 + yields per incident ion from 2 5 -2 5 0 keV Ne+, Ar+, and Kr+

bom bardm ent of: a) O /T i; b) O /N b .

tN'© 4

3

2

1

0

5

‘ (a )i 1 ■■ ■

J O / T ia

___ j— j _____• •

__ □ ___m

I j L • I | •

m

• A

• «

- I _ □ N e +

• A r +

A K r +

1 . T " - 1

« r aAA A

- - - 4 — i ’0

GO

io

u• fH

+O

6'

5 ‘

4

3

2

1

0

8 10 12 14 16

B e a m V e l o c i t y ( 1 0 ^ c m / s )

(b ) O / N b

B□

□

B

T "B

g

□□

A j •

A

P N e +• A r +

A K r +

0 2 4 6 8 10 12 14 16

B e a m V e l o c i t y ( l ( r c m / s )

Figure 4.3

0 + yields per irc id e n t ion from 2 5 -2 5 0 keV Ne+, Ar+, and Kr+

bom bardm ent of: a) O /M o; b) O /W .

Beam

V

elo

city

(1

07

cm

/s)

0 + Y i e l d C M T 7 / i o n )O O »“ * ^o In o Cn o tn

O

K>

Os

00

»—io

MK>

»—»

i—»O s

ot» O

t > >

C O M t t > ^

o m m

> • □

S i ? ? + + +

CD -

□ □ □

m □

J ____

0 + Y i e l d < l ( r 7 / i o n )O M | O W i t i O i O \ S ] » «__ ■ _I ■ » I - 1

H >

1 >

« a m >o t >

i c £ »

2O

K >

[> • □

ff 2 r £ + + +

c o

P #□ □

□ ID □ -J

CD j __

I________

80

Figure 4.4

Secondary electron yields m easured for 2 5 -2 5 0 keV Ne+, Ar+, and K r

bom bardm ent of: a) O /M o ; b) C O /N i.

Beam

V

elo

city

(1

O'

cm

/s)

E l e c t r o n Y i e l d ( / i o n )

O ro CO 4*

O

4*

Os

00

o

>—1 ro

>-»4*►-»Os

•C £ >

B » t >

J > >

- s r ,

1

no

+ + +

• O D •- m

□ n n — I

_ □ □ _

Beam

V

elo

city

(10

cm

/s)

E l e c t r o n Y i e l d ( / i o n )O »-* IO W 4* Ol

O

ro

4*

Os

00

i—»o

1—1 ro

H-»4^

Os

w >&

( C l

t t » —r m r ft> >

M X >

P>

so

t > • □

2 ? 2 r? + + +

« •m —

- □ □ □ -

□ □

81

Figure 4.5

Yields of O * from ion bom bardm ent of O /N b , divided by the beam -

dependent factor in Lindhard's electronic stopping power to remove

any dependence on electronic excitation.

Sc

ale

d

O +

Yie

ld30

2 0

1 0

0

0

IO / N b

B□

a * , a

□

0

□□

□□ B □□ B□ □

4 6 8 10 12 14B e a m V e l o c i t y ( 1 0 ' c m / s )

□ N e +

• A r +

A K r +

16

82

Figure 4.6

Yields of 0 +, divided by the beam -dependent factor in Lindhard's

electronic stopping power to remove any dependence on electronic

excitation, from ion bom bardm ent of: a) O /M o ; b) O /W .

Beam

V

elo

city

(1

0^

c

m/s

)

S c a l e d O + Y i e l d •—iO N> On 00 O

O

to

ON

00

t-aO

»—* to

»—»4*

►—»ON

cr

t> O

4 > D

irsrs

•fp1

- m n•

—

□ -

□ _+ + +

1____

U 11 \

□ o

□ □

1

S c a l e d O + Y i e l d

8 3

Figure 4.7

Periodic table o f the elem ents, showing the transition m etals and

sum m arizing the behavior o f 1(0 +) as a function of beam velocity v (« v:

s tric t proportionality to velocity; « Se: proportionality to electronic

stopping power; cascade: proportionality to sputtering yield from

collision cascade).

S c T ioc V

Voc V

C r M n F e C o N iC ascade

C u Z n

Y Z r N boc V

M ooc Se

T c R u R h P d A g C d

L a H f T a Wo c S e

R e O s I r P t A u H g

8 4

Figure 4.8

Secondary electron energy d istribu tion m easured for bom bardm ent of

O /V by 200 keV Ar+. The dashed line shows the background of "true"

secondary electrons under the Auger feature as extrapolated from the

d istrib u tion on either side of the Auger feature.

ELEC

TRON

YI

ELD

(arb

. un

its)

85

Figure 4.9

Behavior o f the ESD cross-section for desorption of O * from O /W as a

function of incident electron energy. The solid line is the O * yield,

w hile the dashed line (solid dots) is the to ta l O yie ld (from [Fe78J).

De

so

rpti

on

C

ross

S

ec

tio

n

86

Figure 4.10

Location o f the m axim um (upper panel) and h a lf-w ld th (lower panel)

of the energy d istribu tion of O * desorbed from O /W by electron

bom bardm ent (from [W e81]).

EOC

HALF

-WID

TH

(#V)

0

EDC

PEAK

EN

ERGY

(«

V)

4o

1 0 0 2 0 0 3 0 0

ELECTRON ENERGY («V)

8 7

Figure 4.11

Secondary electron energy distributions for bom bardm ent o f M o by

Ar+. Triangles: 2 keV; crosses: 5 keV; fu ll circles: 10 keV; open

circles: 15 keV (from [Be82]).

88

Figure 4.12

Yields of N i+ and O * (norm alized to the N i+ yield a t 2 0 0 keV) from

bom bardm ent o f O /N i by 2 5 -2 5 0 keV noble gas ions: a) Ar+

bom bardm ent (0 + yields m ultip lied by 32 .5 ); b) H r* bom bardm ent (O *

yields m ultip lied by 15.1).

Ion

Y

ield

(H

P

/io

n)

Ion

y

ield

(K

T7

/io

n)

4

3

2

1

00 2 4 6 8 10 12


7

6

5

4

3

2

1

00 1 2 3 4 5 6 7 8


1

( b )■

IK r + ■

8 3 a 2 ■o

8 °■

•s •

o3.

+2

O

■ 0

1 ■

l :

( a )

A r +

i

8o

■ ■

e8 o

- B

89

Figure 4.13

Secondary ion yields from bom bardm ent by 2 5 -2 5 0 keV Ar+: a) N i+

and CO+ from C O /N i (CO+ yields m ultip lied by 324); b) Pd+ and CO+

from C O /P d (CO+ yields m ultip lied by 12.8).

Ion

Y

ield

(H

P

/io

n)

Ion

y

ield

(1

CT

7 /i

on

) 120

100

8 0

6 0

4 0

20

00 2 4 6 8 10 12


6

5

4

3

2

1

00 2 4 6 8 10 12


( b )* 1

■1

■I”" — '1

P d +C O + (s c a le d ) ■

__________ ■■

o

o

1 ■

- § c

■o

j Bu ■c5 8 § I S -

a

1 1 1

( a )i

■1

■ N i +O C O + (s c a le d )

1■

00

B 1 o

■ g ■o I3 ■ 0

8 S f \ Ql o g -

i . . .

90

Figure 4.14

Yields o f N i+ from 2 5 -2 5 0 keV Ar+ bom bardm ent o f O /N i and C O /N i

(yields from O /N i m u ltip lied by 12.5).

Ni+

Y

ield

(I

O"

7 /

ion

)

1 2 0

1 0 0

80

60

40

2 0

00 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0


91

Figure 4.15

Yields o f 0 + from 2 5 -2 5 0 keV Ne+, Arty and H r* bom bardm ent of:

a) C O /N i; b) CO /Pd.

Yie

ld

(HT

7 /

ion

) Q

+ Y

ield

(1

0*

7 /

ion

) 1.2

1.0

0.8

0.6

0.4

0.2

0.0

■ (a )— ■— 1 1 | -

C O / N i £ A

1 1

. ■

> •

0

2? £

?

+ +

+

■ £ a- A □

• ----° A 2 . 3 * P .

-A 8 c

■w w

£ •□

0 6 8 10 12 14 16


1.2

1.0

0.8

0.6

0.4

0.2+O o.o

- ( b ) C O /P d " T 1

5 a

I

•

-/I

t

-□ A 1

w CE

“*

EI B B

1 J-

•9&

a N e +

• A r + .

A K i +

---- 1---- 1 1 | ■“------- 1— — i— -----i— — i— — i-----0 2 4 6 8 10 12 14 16


Figure 4.16

Yields of 0 +, divided by the beam -dependent factor in L indh ard ’s

electronic stopping power, for 2 5 -2 5 0 keV Ne+, Arty and Kr+

bom bardm ent of: a) C O /N i; b) C O /P d.

Sc

ale

d

O +

Yie

ld

Sc

ale

d

O +

Yie

ld1

( a )i

. □ -

□ 1 1

1 □

□ □

□

CO —

0 I ' □ N e +

- • A r +

A K r +C O / N i

. . e ] ' l ‘

A ® .1

V 0

1 □ —• 1

&

i

a

— 1—0 2 4 6 8 10 12 14 16


4

3

2

1

00 2 4 6 8 10 12 14 16


CHA P T E R FIVECONCLUSIONS

5 .1 . Sum m ary

The em ission of 0 + secondary ions during ion bom bardm ent of

adsorbate-covered m etal surfaces does not resu lt from collision

cascades, except for the target O /N i. The other adsorbate/substrate

systems studied can be divided in to three categories. For the group

consisting of O /V , O /T i, and O /N b , the yields of 0 + increase lin early

w ith incident ion velocity, w hich suggests th a t electronic processes

are active. However, the expected scaling of the yields for d ifferent

beam species on the basis o f the electronic stopping power and the

secondary electron yields is absent. The second group consists of

O /M o and O /W . For these targets, the emission of 0 + can be more

directly related to electronic processes; the scaling of the yields for

differen t beam species resem bles th a t o f the electronic stopping

power and the secondary electron yield. The em ission o f 0 + from

targets in these two groups behaves the same for m etals in the same

colum n o f the periodic table. For these targets, however, the

m agnitude of the 0 + yields is larger by a factor of about 2 5 00 than th a t

expected for desorption stim ulated by secondary electrons.

Nevertheless, since the yields o f 0 + from O /N i resu lt only from

collision cascades, the behavior o f the yields does appear to depend on

w hether the system can be m axim ally valent, as expected from the

theory of ESD. This would m ake it d ifficu lt to propose a m echanism

th a t does not invoke some form o f electronic excitation.

93

94

For O /N b , O /M o , and O /W targets, the em ission o f O * during Ne+

bom bardm ent appears to have a com ponent in addition to the velocity-

proportional com ponent. The m echanism for th is additional emission

is not known, nor is it understood w hy such a m echanism would not

also operate for O /V and O /T l.

The th ird group of systems studied consists o f C O /N i and C O /P d.

Fo r these targets, the O * yields are proportional to beam velocity for

some beam species, b u t not for all; in the case of C O /N i, the data

exh ib it a m axim um ra th er th an increase m onotonically w ith beam

velocity. No evidence was found for Ion-induced electronically-

stim ulated desorption of C O *.

5.2 . Further Experiments

There are fu rth e r experim ents th a t should be perform ed to

enhance our understanding o f ion-induced O * em ission. The m etal

oxide systems clearly require fu rth er study to determ ine w hich

properties o f the m etal or m etal oxide affect the details of the

dependence o f the 0 + em ission on beam velocity and beam species. In

p articu la r, other (potentially) m axim ally valen t m etal oxides located

n ear T i in the periodic table (such as Z r, Ta . and Re) should be

exam ined to verify the apparent correlation o f the behavior o f the 0 +

em ission w ith the colum n in the periodic table. Moreover, the

electronic structure o f the oxidized m etal surfaces should be studied

to determ ine w hether m axim ally valent oxides are actu a lly formed, as

w ell as w hether the behavior o f the O * em ission correlates w ith any

other varia tio n in the electronic properties o f the system s.

95

Systems know n to be nonm axim aUy valent should also be fu rther

studied. The decrease in icn-induced ESD of 0 + from other oxidized

m etals between V and Ni could be m easured to determ ine w hether

the yields decrease gradually as one moves to the right in the periodic

table or decrease suddenly w hen one begins to use nonm axim ally

valent oxides. The ion-induced em ission of F+ and C1+ from fluorinated

and chlorinated transitio n -m etal surfaces should also be studied, since

these systems generally are not m axim ally valent.

Thus fa r, the only evidence th a t electronic processes are

responsible for the ion-induced emission of 0 + is th a t the 0 + yields

increase lin early w ith beam velocity, along w ith the scaling of the

yields w ith electronic excitation in some cases. M easurem ents of the

energy d istribu tio n of the 0 + could provide fu rth er evidence, as the

discussion in section 1.3 of the em ission of F+ from fluorinated silicon

shows. The signature of desorption induced by electronic processes

would be a narrow energy d istribution w ith a m axim um a t low energy,

w hile a d istribu tion w ith a m axim um a t high energy would im plicate

processes involving back-scattered prim ary ions. Since the offset of

the 0 + yields during Ne+ bom bardm ent o f some of the targets could be

related to back-scattered Ne+, the energy distributions of 0 + desorbed

during Ne+ bom bardm ent could help to explain these offsets.

There are o ther interesting prim ary beam s th a t could be used.

Bom bardm ent by Ne+ beams w ith lower energy than used in th is study

should be exam ined. I f the emission o f 0 + were to continue to

decrease lin early w ith decreasing beam velocity, w ith an extrapolated

nonzero yield a t zero beam velocity, th is would suggest th a t the

additional em ission of O'*- is caused by some m echanism th a t is related

96

to the potentia l energy of the Incident Ne+. If, however, the 0 + yield

w ere to become a nonlinear function o f beam velocity and go to zero a t

zero beam velocity, then a m echanism invoking backscattered Ne+

could be im plicated. Also, bom bardm ent by lighter ions, such as He+,

could yie ld interesting in form ation on th is phenom enon.

S tudy of the ESD of 0 + from the oxidized m etal surfaces used in

th is study would be useful for two reasons. F irst, such studies would

allow the m easurem ent o f the E SD cross-sections as functions of

electron energy for the specific systems studied under ion

bom bardm ent. This would allow a more accurate calculation of the

in teg ra l o f the product of the secondary electron d istribu tion and the

E S D cross-sections for these systems. Second, using the same mass

spectrom eter for both ion- and electron-beam studies would allow one

to determ ine the absolute ratios o f the ion-induced yields to the ESD

yields, w hich would elim inate the problem s associated w ith the lack of

in form ation about the transm ission of the mass spectrom eter when

calcu lating the expected yield induced by secondary electrons.

O th er possible experim ents should concentrate on the em ission of

secondary electrons. M easurem ents of the electron energy

distribu tion , and any Auger electron em ission, are needed for targets

other th an O /V . In fact, it m ight be possible to m easure coincidences

between the emission o f 0 + and the emission of Auger electrons, if

indeed the emission of 0 + is related to an Auger-induced process.

Such coincidence m easurem ents could be particu larly useful for

determ ining the m echanism behind the offset o f the 0 + yields during

Ne+ bom bardm ent, p articu larly i f Auger neutralization o f the incident

Ne+ is involved.

97

The CO-adsorbate systems also require fu rth er study by

experim ents using other ion beams and other m etal substrates to

better m ap out the behavior of the O * em ission. The use of targets of

CO adsorbed onto such m etals as T i (at low tem peratures, to avoid

dissociation) w ould allow separation of the adsorbate-induced effects

from the substrate-induced effects. Again, the em ission of secondary

electrons should also be exam ined, and experim ents using prim ary

electron beam s should be perform ed, in order to determ ine w hether

the m agnitude o f the O * yields is consistent w ith desorption induced

by secondary electrons.

A s76

Ba72

Ba77

Ba82

B e73

B e75

Be81

B e82

B179

B185

B186

B187a

B187b

B187c

B r8 5

Ca87

C185

C r8 3

An81

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