Secondary ion mass spectrometry

Vacuum/volume

Pergamon

Secondary

45lnumbers 6/7/pages 753 to 772/l 994 Copyright @ 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/94$7.00+.00

ion mass spectrometry

P C Zalm. Philips Research Laboratory,

Prof Holstlaan

I. Introduction

In secondary ion mass spectrometry. henceforth abbreviated to SIMS, use is made of a phenomenon called sputtering. When energetic ( 20. I keV atom- ‘) ions, atoms or molecules impinge upon a solid this inevitably leads to the ejection of particles out of the target from near (- I nm) the point of impact. This form of erosion goes under the name of sputtering. As the liberated species stem from very shallow depth (< I nm), these carry information about the (local instantaneous) composition of the target surface. By continuing bombardment with primary species for a prolonged time an increasingly deeper sputter crater develops. Consequently, the sputtered secondary species will then reflect the composition inside the target (e.g. bulk-rather than surface-propertics). By dynamically registering the number of ejections per unit time. after these have been (preferably uniquely) identified. as a function of time. one thus obtains a (perhaps distorted) depth distribution for those species. This technique is known as sputter-depth profiling. In the case of SIMS the sputtering by primary ions serves two purposes simultaneously, namely to generate emission from the target surface for in-flight analysis and to peel the surface off. In another variation sputtering is used exclusively to expose inner parts of a sample for analysis by some on-target surface-sensitive analytical technique like Auger Electron Spectroscopy (AES). In the next section we will discuss many aspects of sputtering in detail. as it is of crucial importance in the appraisal of any technique that analyses (part of) the sputtered species and/or employs sputter depth profiling. With the prcscnt day high quality measurement instruments the attainable limits ofdetection, resolution etc.. arc largely dictated by the sputtering process itself. As follows from its acronym. in SIMS the secondary species to be analysed are charged. This has the advantage that standard. well-established and reliable. spcctromctric methods can bc applied to discriminate on mass (or rather mass over charge ratio). Given a sufficiently good mass resolution. in principle a perfect mass separation is possible and then a unique identification for all cjectcd elemental and poly-atomic ions is possible. Hence SIMS allows for the dctcction of all elements and their isotopes, a rare feature for surface sensitive analytical techniques. Of course practical restrictions make the true situation less roseate. A considerable disadvantage of SIMS is that often only a minute fraction of the sputtered particles leaves the surface in a charged state. Techniques related to SIMS have been developed to circumvent this problem by post-ionization of the ejected neutrals prior to detection, the so-called Secondary Neutral Mass Spectrometries (SNMS). but with only limited success. Another problem in SIMS lies in the Fact that the primary species are ions. This leads to charge-up of insulating surfaces, thereby creating difficulties that can only be overcome at some sacrifice. Neutral primaries are occasionally used, in Fast Atom Bombardment (FAB)-SIMS. but the drawbacks of low(er) intensity and more

4, 5656AA

Eindhoven,

The Netherlands

difficult focusability!‘positioning generally do not outweigh the advantages over conventional and convenient ion beams. In the opening paragraph of this introduction WChave already implicitly encountered three of the most important forms of practical SIMS. namely : (i) Static SIMS, where the presence of molecules!‘adsorbatcs at the very surface is the subject of analysis. Characteristic fat S-SIMS are a low primary beam density (-pA cm ?. in order to avoid excessive damage!distortion of the surface impurity population) and a large analysed area (-cm?. in order to get reasonable statistics in spit, of the low fuencc). Bombardment energy and ion-type is not too important, since subsurface damage and primary species incorporation are of no concern, and arc chosen at convenience. Because of the low ion flux, even charge-up problems are usually not very severe. (ii) Dynamic SIMS. where the depth distribution of impurities in the - IO nrn-IO Atrn range has the main interest. Characteristic for D-SIMS is a more intense primary beam (-0.1 mA cm ‘) usually well focused to allow for rastcring over a small area (- IO ’ cm’. in order to get a rcasonahle erosion rate comhincd with a flat sputter crater bottom of which only a central part (- IO”/,) actually contributes to the analysis to avoid redcposition off the crater walls). The type and energy of the impinging species are now of foremost importance as during depth profiling previously implanted primaries will be encountered and may influence the secondary ions’ formation and survival probability. Attainable detection limits and depth resolution depend on the sputter yield and the damage or disturbance to the original profile inflicted by bombardment,;erosion. So the primary beam parameters must bc optimized in accordance with the (anticipated) analytical requiremcnts for the particular sample at hand. (iii) Bulk contamination;composition studies. where one aim is to establish the prcsencc of impurities. or set upper limits to their concentration level. in solids. Characteristic here is the strive for maximum secondary species intensity, i.e. high density cnergctic (more or less focused) primary beams (in order to get the highest possible sensitivity). Achievable detection limits arc typically in the ppm to ppb regime, but vary considerably with impurity. A completely different application is found in the fields of geologylastronomy!gcography. Isotopic abundances vary slightly with origin over the earth’s crust and the isotopical composition of extraterrestrial rock may differ significantly from that encountered here. So SIMS may provide a means of locating or assigning a specimen. Clearly the requirements differ somewhat from those in impurity detection and a better primary beam definition is usually required. Imaging SIMS can be considered a particular version of (i). Here the two dimensional distribution of one or more elements over part of the surface (lateral dimensions typically -0. I mm) is probed. This requires either a very finely focused primary ion 753

Secondary

P C Zalm.

ion mass spectrometry

benm or an cxcellcnt prcscrvcs a fairly

position

secondary

prolilc.

is tn principle

data points store

For

straightforward.

computers

per datum.

limiting

lirctor.

samples

this

Imaging

SIMS

and also

Thcrc

industry

largely

restrict

good

devices.

of SIMS

the fundamental SIMS

with

possibilities knowlcdgc,

this

ttchicvcd

with

SIMS

un cxpcrt

Given

operator.

notably

In addition.

in ;1rcas other

of the state-of-the-art dircctcd

It

is

the right

that.

:~rmcd

CUE (not)

bc

and 01

For

advancing,

tnrgets.

rcadcr

intcrmitional

II

is

SIMS

Figure E,

(ii)

on a solid atoms.

and penetrates.

initially

crcatcd

at rest.

which

continuously

in the thus

transrcrablc

energies

;t I‘urthcr

is damped proccsscs nm)

acquire

over

rcqitiring

sufficient

relatively

target small

-

IO

for

fcctly

pairs.

When

sharp

interface

\+ill (locally)

beconic

Lid

directed

the events able.

following

Almost

results Many

all

(-

IL10

blurred

the impact

aspects

\+ill bc

on average.

in

it ma)

bc

In single of stnblc

cl-ystals vacancy

an originally

A and

B this

pcr-

intcrlitcc

bccausc the relocations statistical

of a single techniques,

that arc an average over billions

devol-

this

recoils)

oriented.

of A and H. The

of the basic

of cwxdc

:tnd.

fast

the cascade intcrsccts

somewhat

I

(<

and Icave the target.

nm)

;I few (the

‘inwardly’

mcasuremcnt

region

;md II~WKI~-

cncrgy

conscqucncc

between materials

to an intermixing

the casc~lc

arc I-elocatcd. Mostly

distances

’\

necdcd to

cascade dc\clop-

“I s. Dul-ing

cscapc barrier

atoms

and typically

Finally

at~jms

5 x IO’

c.g.. phonon-assisted

usually le:idb to damage, i.c. the creation

interstitial

I

:irc

and ;I

slower

sonic

in the near-surface

Another

but

IO cV).

through.

‘outwardly’

direction

considcrablc

(x

atoms

the surface

is that

:I r:mdom

754

atom

bccomc sputtered.

opmcnt

recoils

in motion

of progrcssivcly

dissipation

of the target

(urn to surmount Thcq

atoms

cascade. After

wjith target

Fast

have become less than the cncrgy Ltrgcl

typically

some

target

number formed

by energy

mcnt

this

set other

of collisions.

fluctuations

particle

and characteristics

in

arc considcr-

howcvcr.

of individual

01‘ ;~n cilrlicl

pcncratc cascades.

of SIMS

arc

0.5 kcV).

cascade of t:qct

I

) is

E;

yield

and then

)’ litrich

i\

maximum cncrgl

pronoun& light

On11 at \er! of obscr\;tblc

found. numbc~- /I,:

for

hc~\ Icr

angle of incidcncc

to ;I marimum to /cro

ol‘particlc

for

usually

target

is most

prominent

clcmcnt

(Z,)

for light

dcpendcncc.

:i,.

dclined

arot~nd 60

near-glancing

and cncrgq rcilcction

(Z,.

altho~igh

mass

prqicctilc5.

normal. stccpl>

in

and praduall>

(McV).

\\ith cncrg)

r:ithci-

is given

1’ on ion type (i.c. atomic

incrc;lsc”\ with

:i gi\cn beam condition

(I kicld

Micah. c$pccinllq at low cncrs>.

\er]

drops

7 hc barialion

of target

out

information

abo\c but near the onset

more

to the surface

) T;trgct

atoms

(an cxaniplc

to ;I brozi

Ihirl!

slightly

The

predictions.

and atnorphou5

cxtcnsi\,c

5pultcrin,

cncrgy

is in steep incrcasc

(:+. - 90 ) bccausc

incidcncc

(cf. FiStIre

7).

ions. magnitude

The

E, a,), oscillates

wildly

of 1.. for-

II\ a function

spccics.

most

succcssl‘ul

model,

htcnls

from

inlinitc

attempt

2nd wrtainlq Sigmund”.

ccluation

iI5 the product depth-

ion

increasing

:tnd tow;irds

rclativc X0

linear

the most

the total

dcpcndcncc of

I\ bccomcs

(scm-)

impingcb

part of its cncrgy

in a series

increasing

participutc displace

in turn

it shares

particle

;I summ;tr\

number

to /cro again for \cry hish

The

(iii)

and (i.c.

fat- which

that

transport an cncrgctic

and ii good

rcadcr is rcfcrrcd

data I’or polq-crystalhnc

incident

( - 0. I

atoms

in)

with

see also Figure

date.

When

;iccount

bc discussed.

I ), first

The

2. Sputtering

background.

with

\.qstcmatic\

be given.

yield

appcai-s

5muothl>

;I single

General

;lssociatcd

of5omc

complctc

Her-c only

will

total

ion

will

(ix

an imprcszion

the intcrcstcd

of the bi-annual

to

paper cannot

i:, still

profiling.

analysis.

its basic

questions

space. this

more

systematics

in

av:tilablc.

sputtering.

at I‘llmil-

;I specific problem

technique

depth

in SIMS

aspcut

confcrcnccs’.

2.1.

yield

clcincntal

low

As ~t~~tvxcs~-

hoped

01‘ phcnotncna

(r in c ccncral the intcrcstcd

of tcxtboohs’.

trends

dropping

is ;L practic:tl

paper aims

regarding

the SIMS

to the proceedings

‘. rathet

aspects of (D-)SIMS.

the limited

than

rcccntl)

tirst

other’

the prcscnt

and ask roughly information

we will

Fairly

The

a much

by the author’

Total

(i)

scmicon-

research,

bc able to .judpe what

feed the uppropriatc bc complete.

The

limitations.

one will

the

its many pitfalls.

the m:tin

and

fcaturcs, with

in

and instrumental

bttrictics.

is to bc avoided,

the rcadcr

2.2.

but. given the dom-

a\ailablc.

dcscribcs

with

is ;I grainy

well as

in this paper.

bccomc

difrcrcnt

iariring

as

materials

of the many arq rcpctjtion

to ;I hcrlcs rccic~

\arietq

pool-

is the ni;ttn

compounds.

applicutions

to D-SIMS

for depth profiling

of

largc-

The

For

per clcmentnl

cxh:iustivcly, handbook

problem. inipurttics.

ticld.

preparation

and in (commercial) have

the number modern

in this

the

over\ iow on sputterin,

\I hcrc spccimcn

profiling

ourselves

to

to

So WC ct;irt \\ith ;I hricl‘in\cntot-v

Firstly.

manifestations

textbooks

extension

rclatcd

sputtci-ing.

with rcmarkablc

and ccramc

of depth

ductor

real

till- dilute

to setniconductor

exist other

position

Of course no

dircclly

m:tpping

has been applied

samples.

like metal alloys

erosion,

h~~gc, but with

is

in particular-

~ucccss to biological key issue.

bccomcs

that :iccuratclb

8 surface

a full three dimensional

depth to obtain

then rapidly

desk-top

st:Itlstics

two

system

ofejection).

low Huencc is needed. By continuing

type (ii). that is including

nant

ion optical

(i.c. oripin

solid.

to dcscribc Then

of ii ni;ltcrial

and cncrfy-avcragcd

to escape

from

Impinging

ion at the surf:Lce

the surface

to capture

thcsc obs;er\ation\

the niost

coniprchcnsi\c

f-lc II&

;I lincari/ed

the collision

the sputtering pal-ametcr. c~apc and

casc~ic !icld

the

;IS t;irgct

Bolt/mann In ;L random the an&

for ;I target atom

cncrgy atom

tn to

c;tn bc cxprcsscd

comprising

prob;tbilit!

thwr!

dcpositcd

motion

by the

PC Zalm:

I

Secondary

ion mass spectrometry

predominantly involves the first generation recoils. The specific ejection kinematics determine the yield. The minimum incident ion energy required to eject target atoms, the threshold (.I$,,), strongly depends on those particulars and 9,. Available emission channels (possible collision sequences), may become inoperable above certain impact angles. At energies an order of magnitude above Et,,, the yield behaviour approaches that expected in the linear cascade regime again.

6

-z 5 2 5 $ 4

Although various arguments have been put forward to criticise many aspects of the binary collision linear cascade theory, equation (I) provides an amazingly good description over a large portion of (Z,, Z,. E,,9,)space.

/

I

0 0”

30”

60”

90” t?-

Figure 2. Angle-of-incidence dependence of the total sputtering yield for amorphous silicon (thick full curve: 5-10 keV Ar’ ion bombardment as compiled in ref 6 : thin dashed line: 9 keV (+ ) and 12 keV (x ) 0: irrddiation data, from ref 7) and on single crystalline Si( I 1I) tilted around the [21 I] axis [thin full line: 30 keV Ar+ (ref S)].

Cast in more tractable. mately be rewritten as

numerical,

form the result can approxi-

where K,, and E,, are scaling constants depending only on target and projectile atomic number (Z) and mass (M) and U,, is the surface escape barrier energy in eV (usually taken equal to the sublimation energy). The reduced nuclear stopping cross-section S,,(t), which comes in through the energy deposition function mentioned before, can be estimated as S,(t)

= +ln(l

+i’)/(i”+(5/383)3x},

whilst the constants

are given, respectively

E,, = (I +M,/M,)Z,Z,(Z:‘3 K,, -

(Z,Z,)““/3

n (3,),

approximated,

+Z:‘s)“‘/32.5

for 0.05 < Z,/Z,

the angular dependence ,1’(9;) = cos

(la)

f’(s)

by

[keV] and

< 5,

is not very accurately

(lb) (fc)

predicted

as

2.3. Crystallinity and target temperature. Another regular exception to the above cited systematics is encountered in the sputtering of single crystalline targets. An example is given in Figure 2. The reason lies in the relatively larger ‘transparency’ for ion incidence along low index crystallographic directions (a reverse argument holds for high index directions) relative to a random (polycrystalline or amorphous) target. Subtle complications arise at low E,, where kinematic effects become important, and at high E,.where eventually Ycryrlil,< Yrsndomfor all orientations. The target temperature normally has little influence on the sputtering behaviour, except when the radiation damage generation becomes impaired. At low temperature damage builds up with accumulated ion dose from isolated Frenkel (i.e. interstitial/vacancy) pairs which may agglomerate into extended defects (dislocation loops, which in turn may interconnect to tangled networks). In semiconductors and other brittle crystalline structures (e.g. the new high - T, superconducting ceramics) this soon leads to complete amorphization over the entire projected range of the incident ions. In metals. however, the distance over which a Frenkel pair may recombine is so large that amorphization never occurs. In fact, prolonged bombardment of originally untextured (i.e. random oriented tine-grain) polycrystalline metal targets often leads to the formation of texture (coarser crystallites with a preferred orientation). At sufficiently high temperature the defect annihilation rate becomes larger than the bombardment-induced produced rate and one observes recrystallization. Whether or not the incident ion beam will heat up the target dangerously can be estimated roughly as follows. Assuming homogeneous irradiation over an arca of diameter u’, large compared to the projectiles penetration depth, and in the absence of radiative losses, the local temperature increment at the surface of a semi-infinite target will be AT = Q/Cd. where Q is the input power (i.e. beam current times acceleration voltage) and C the thermal conductivity in W K ’ m ‘. As most samples are fairly thin (- I mm) and heat conduction to its holder is usually imperfect, this must be considered a conservative estimate (i.e. special caution is in order when it predicts AT - IO K!).

(Id)

for 9, d 60’ only an n = l-2 a weak function of mass ratio and/or energy. The basic assumptions underlying equation (1) are known to break down in two limiting cases : (i) for very high-energy heavy ions (but this well outside the SIMS and depth profiling application range) ; (ii) at very low energies (E,- 0.1-l keV) and or for very light ions (e.g. H+, He+). At very low E,and/or M,, sputtering

2.4. Reactive ions and high fluence effects. So far we have tacitly assumed that the target, except for the erosion, remains unaltered during sputtering. Obviously at sufficiently large fluences the incorporation of the bombarding species will have some effect on the erosion behaviour. Even with chemically inert, noble gas, ions the observed yield varies with increasing dose/eroded depth to the point where an effective steady-state (sub-) surface modification has been reached by the competition of implantation and ejection processes. Sputtering through any interface between 755

P C Zalm

Secondary

dissimilar

spccics in ii target

equilibrium)

erosion

Further

with

like for fluorine

ion (here NC as for oxygen reduced. yield

atoms.

Formation

’ ). Conversely,

or reduction

\olatilc

incidence

reason

scvcrc,

for

loading

whilst

and

of silicon than

this

sputtering

is simple.

At

the bombarding angle

and

usually

steepor

(see again

yield

yield mcasurcmcnts

These limited data and 3 pronounced

applications

whcrc

occasionally. at ;I given

O_

For such simple diatomic

ion at half the cncrgy

monatomic

is not

is used very

E, and 3 is twice

s;~mc angle $. This rule can be gcncralizcd

that

(E,:?)

material. yield

materials.

address multicomponcnt UnIhrtunately or scaling

lies bctwccn

Ihund

us hcrc.

alloys.

Also

compound

on the

as distinct

stoichiometric surface

removal

composition

incidence

and cvcn

bardment

equilibrium

partial

sputtering

exactly

the bulk

the surface ponent

tcmperaturc.

which

problem

in sputter-depth

sensitive

analytical

removal

of tens (to hundreds)

relocation.

resulting

in new compounds.

Then

volatile

spccics arc involved.

halides.

electronic

756

by thermal

prolonged

processes decay) relt~se

In ionic invoked

surlitcc matcriul.

bond breaking possibly

or even dcsorption crystals.

ncutralirc

surface

(of the halogen).

(for

c’vcn

cxamplc.

target

Similar

niatcrials

t,oi-

prcdictx Ihr sinplc

that of random

tar-

and considerabl!

directions.

Consequcntl~.

distribution

pi-olongcd

01‘ orig-

erosion.

the :ingiilar

As ;I conscquencc.

direction

di41ribiition\

ion h~~iiib~~rciiii~iit. cascade theory

When

distribution the sputtered

need ncvcr rcllcct

in the steady-state!

in its simplest

pcakcd

15 the

anisotropic

during

depends

distribution

and

The bchaviour

lattice

In

wllisioil in

from

arc sputter&

fiux in ;I particular

A\ I’or the cnerg thcor\i

linear

is strongly

nict:il\

t’vcn

the angular

distribution.

can dit’f‘cr Ihr each constituent. particle

M hcrc spccilic

fraring

lbi

ha\ y ions at intcr-

(i.c. peaked 111the clircction

alTccts the angular

poly-crystalline

(i.c. amorph-

rhc singular- cniission

inc~clcncc

close-packed

texturing

:I\ f01-

the bulk

The maglitudc

01‘ the

on beam (Z,. E,. :3,) conditions ol‘ejcctcd

atoms;. linear- ca\cadc

I’orni predicts

ti 1’ dE

of L’(E+ 1 /, 15 agarn

\SIlCK

Rclincmcntb $3

I’,,)

j_ planar

Itic

to equation

appi-oath

d Y,‘dE

the niii~inium of equation

f<[l -\,

I

surliicc

cxxp~

(3) bccomc ncccssar)

M,+ ,zil,)‘]. in particular modification

(7) bar-ricr

cnci-g!.

M hen kinetic

cnct--

transfcrablc cncrgy T,,, [ = /:3,21,:1/, :21,. An cmpiricxl Ihr lou E:, an&or

(2) 01’ the form

((E+

I:,,):T,,,)],!(E+

U(,)’

(32)

‘.

when

c.g. the aikali-

by the ion

along

to

considerably

gets. in that the emission

thcorg.

distl-ihution

to

discriminablc

recombination.

differs

ca~adc

random

regime.

atomh.

(i.e. over-cosine)

sputtering

tcmpcraturc.

may rcquii-c

of target

dircctlon

of 11target

(lolal (dilltir-

01‘ the anplc 01‘ incidcncc.

the behaviour.

and target

in the com-

or equilibrium

irt-chpccti\,c

sensitively

~rcllcct

on-tar@

lcads to chetnical

may

spccukir.

ion

cncrgy

;I cosine

cncyics

I’[,[- pcrpcndicular

c_jcctcd clusters

ol‘co~irsc).

targets this seems to bc treason-

the low I-, and or :\I, \puttcring

opposite.

collision

predicts

o\cr-cosine

noi-niJ)

discrepancy

then

linear

M

becomes oxygen

per incident

at Icast for medium

01‘

hom-

become\

the pure clement

mith \ih;it

and

For high impact

stoichiomctry.

is. the

Potatom

now \+c Iwvc‘ only do\-

Until

arc ejcctod

models.

in

yield. This is ;i jcrioua

cmployinp

occurs

bc

That

atoms

ol‘ nanometrcs

either

angle

that this need not apply

Onset

Ion bombardment

Auger

After

may bc caused by various

through

interatomic

cncrgy.

bc reached.

sputtering

profiling

tcchniqucs.

or depletion

mechanisms.

Vollowcd

Note

must

to changes

may still bc, c.g.. enriched

with the lowest clcnicntal

Enrichment

or

surl’ace

and the surtilcc

direction

hccomcs strongI!

01‘ the surflicc

inall!

targets.

/. the total

the

or to cvcccd

poly-crystalline)

multicomponcnt

lies in the non-

type.

01‘ the constituent

composition.

itself.

on ion

must eventually

yields

footing

difticulty

atoms Icading

of surLm

target

or not metal

and insLilators.glasscs~s~ilts

same theoretical

dcpcnding

trends

and

atom Hur. For well-behaved

cncrgics.

ion-induced

classes. The major

particles

well-fLillillcd.

mediate

with MgO)

and

other

ous oi- lint-gr:Lin ably

yields.

The anal! ticul

the sputtered

enhanced

it is not cvcn clear hhcthcr

largct

yield systematics.

~+.\c‘Ilas szvcral

any systcmatics.

general

foi- clcmcntal

~mdci- c~x~pcn 1on ho~iih;~rdmc~iI

Differential

as thcsc arc the most common.

semiconductors

may be treated regarded

targets

.Al,O;

(cuccpt

;L cos”-type time to

siigcyt ;I smooth I-.,. 1 ariation L\ ilh 1at-yl

( I ~ \-) Y,, and I’,, for t’, 2 1’,$. whcrc‘

.I- 1., +

to he coniparablc

41cld (cxucpt

2.6.

ha\c

;ind ~~mc

in B. For oxides. MO,,. the total atom \,icld is commonly

under-cosine

hut the

At this stage it bccomcs

there exist hardly laws.

akin to the rcsultb discuwd

1’ , ,( arc the clcnicnt~il

crystal 2.5. Multicomponent

SC:-

conditions

;~lkalih:llidcs

metal but also. c.g.. SiGc a1loyh. A,H,

scclucnces dctcrtnine

ol‘thc

to the more complex

ions. but this need not concern

alloys.

yields).

2). As

steady-state

nictal

cntial

no

untlcr

hinar)

:j, and %, depcndcncc

bu1 nol in which

is of great

ions for sputtering

in the

Induced

to the non-~toichiomctrl~

Ibr (metal-)oxidcs.

yield)

ungulat

Figure

system

mechanisms.

partitioning

(dcfcct)

been rcportcd

pattern

(poly-atomic)

and cvcn Ni

Total

normal

In the case of oxygen

leads to a much

radiation

mainl

cussed how many

is virtually

Other

cncrfy

also contribute

and diffusion.

can be

species will

there

irradiation.

rcgation

dcpletcd

indication

projectile/target

in SIMS

the sputter

molecular

I‘orni a

in SIMS.

except

corresponding

with

the O+:Si

relevance

frequently ions.

restrictions

cncountercd

The use of molecular common.

7’can

and XII a rough

hence no alteration.

SK later,

practical

only

at glancing

incorporation dependence

P and target

or photon

the mass depcndcnt and:ol-

cnrichcd

for r&c&on that

thcsc

bombardment we will

from CCILI;I-

coiiip~~rublc-rn~is~

x >‘. if T,,P,, is inkolatilc.

implies

saturation

be most

with

from

cascade and thermal

binary

or subtracted

as

when projectile

-(h’f/+h)

the 5 sign in (ii) The

state. A I.,,,LL,. I‘ot-

Y’ (either estimated

cxperimcnts

The < sign in (i) accounts gi\cn.

forms.

T,,P,, :

compound

(ii) A Y,.,,,, -

under electron

surl’acc composition.

or target atom

bc added

IIILIS~

noble pas ions) may bc cstimatcd (i) AZ’ ,c,,l, < +(I,‘/I.

occur

mash inert

to SiO,. the yield is gcncrally

in the steady

yield

from

com-

compound

the magnitude

which

or ‘inert’

or obtained

of a volatile

when an involatilc

ion bombardment

ion will react

to that of a comparahlc

ions on silicon loading

cnhancemcnt

(I)

(non-

leads to an cnh;inccmcnt

ions on silicon.

yield relative

l’rom the ‘physical’ tion

transitory

following

As a coarse rule of thumb

rcacti\e

;I similar

arise when the incoming

the target

of the sputtering

implies

behaviour.

complications

chemically pound.

Ion mass spectrometry

atoms

proccsscs

with to

I an adjustable describe

equation

the

data.

(2;~). however.

on regime,

equation

parameter There

has been adopted is no

Yet equation (‘a),

are found

physical

f‘requentl!

justification

f’or

(2) and. in the direct knockto work

well.

Indeed.

in

P C Zalm

: Secondary ion mass spectrometry

elemental sputtering a maximum in the energy distribution around half the sublimation energy is usually found, as well as an E -’ roll-off at higher ejection energies. Also for alloys equation (2) seems to apply well. For ejected clusters of n target atoms deviations from equation (2) occur at higher ejection energies, because dissociation quenches the clusters’ survival probability. The high-energy roll-off is approximately d Y/dEuE(’ 5n1’2(above about the dissociation energy, corrected for centre-of-mass effects). In passing we note that the energy distribution to be expected when ion-induced decomposition, followed by out diffusion and desorption, occurs is of the Maxwell-Boltzmann type :

d YjdE cc E exp (- E/k T).

(2b)

with Tthc target temperature. Thermal spike or ‘hot spot’ models for sputtering, in which evaporation during cascade development/ life is considered responsible for sputter, yield a form similar to equation (2b) but at much higher ‘temperatures’ (Tbplkc - 103P104K). Thermal distributions have been observed occasionally, e.g. in alkalihalide sputtering at room temperature and in high-energy heavy-ion sputtering of metals. 2.7. Aspects of ion formation/survival. Hitherto we have carefully avoided a discussion of the charge state of the ejected species. Implicitly it was assumed that this was neutral. Indeed, most often the vast majority of sputtered particles are not ionic, but there are notable exceptions. For a better understanding of the mechanisms involved in ion formation it is useful to follow the description by Hagstrum’“. depicted in Figure 3, of charge-transfer and (de-)excitation in the vicinity of a metal surface (but similar events equally apply to semiconductors). Electronic transitions, i.c. tunnelling to and from a particle outside the surface. are limited to very small separation distances (S < 1 nm). For a moving particle the dominant processes arc of the Auger or resonance type because these occur in about 10 I5 s, whilst the probability for radiative transitions is small. owing to the long ( - 10 ’ s) lifetime. The dwell time for even a thermal particle in the near-surface region is far too short in this respect. At the other extreme, at energies of the order of keV amu’. the probability that a bombarding particle will undergo electronic transitions bcforc penetration becomes small. For the kinetic energies

discussed here (i.e. those connected with ejected species) the following charge exchange mechanisms (see also Figure 3) are operative for positive ions : (i) An electron from a bound state (EH) the metal tunnels into the ground state (G) of the ion (transition I); the energy released by this transition may promote a second electron in the metal to an unoccupied state or the vacuum (transition 2) ; this process is known as Auger neutralization. (ii) An electron from the metal may tunnel into a level of the same energy leaving a neutral but excited atom (transition 4) a process called resonance neutralization. (iii) An electron in the metastable (or weakly bound) level M, of the atom may tunnel into an empty state in the metal lcaving an ion (transition 3) ; this is called resonance ionization. (iv) An electron from a bound level in the metal (&) may tunnel into the ground state (G) of the metastable atom (again transition 1) and the released energy is consumed by ejecting the electron out of the metastable state M, of the atom (transition 6); adversely it is also possible that it is this electron which fills the ground state (G), thereby ejecting an electron from the metal (transitions 5 followed by 2); these processes have been christened Auger de-excitation. Note that slightly further away from the metal surface a metastable atom may either radiatively de-excite or Auger self-ionize. Note further that processes (i)--(iv) may occur more than once. From the above it becomes evident that ion formation and survival is a complex and delicate process in which the particular electronic structure (level density etc.), of both target and external atom/ion arc of crucial importance. As the local band structure is composition and impurity dependent and, in addition, the surface is still in a highly disrupted state because of the bombarding particle impact, prediction of the resulting ion formation/survival probability is a virtually impossible task. Nevertheless, many theoretical treatments have been published, some of which indeed do have some virtues although none is considered successful enough to merit discussion hcrc. A more or less reasonable result is

P, (El cc exp { -C(ETx -

for the escape probability of positive ions. Here C is some constant, cp is the target’s Fermi energy or work function and E:X the ionization excitation between the ‘true’ free atom E,,, and the one applicable in the proximity of a (n agitated) surface. As the latter is unknown the former is invariably used in considerations as presented hcrc. Finally I*, is the particle velocity normal to the surface [i.e. cI = (2E/M)“’ x cos 3, with 9, the angle of emission]. For negative ions. a similar expression applies

P_(E)

atom/ ion Figure 3. Neutralization, ionization and (de-)excitation processes occwring in the proximity (S < 1 nm) of a surface (after Hagstrum’“). The symbols are explained in the main text.

v)h f

xexp{-C*(q-,4)/r,],

(3b)

where A is the particle’s electron affinity. The electron affinity trivially replaces the ionization potential for negative, as opposed to positive, ion formation. Equations (3a and b) are conceptually transparent in that they show that energetic ejected particles have a large survival probability, unless emitted at grazing angles, and that the neutralization probability increases when the difference between ionization potential and Fermi energy, or Fermi energy and electron affinity becomes large. The low energy roll-off is not very well described by equation (3) as it is far less steep [namely more like (P,a Vet-’ with a prefactor again governed by the magnitude of (E:x-cp) or (q-A)]. For poly-atomic (cluster) ions or multiply charged ions of 757

Secondary

P C Zalm: course

further

IO” mass spectrometry

complications

arise

and the above bricl‘ sketch

an cvcn more gross oversimplificatiotl Note

finally

that

convolution

of

appropriate

difTcrcntial chose Ibr

survival

but the concepts still

yields

for

charged

the corresponding

improvement

Armed

cannot

bc impro\cd

Finally.

it must bc stressed that oxygen-covering

ing

target will onl)

with

the

protxibilit).

\+ith ;I fuir

for

understand

SIMS

and

01‘21

01‘ sputtering

proccsscs

conscquenccs

that

these

have

LIC c‘xn

alternatively,

arc so typical

tcchniquc

for

aIrno\

practical

ones will

bc

Sensitivity

probability

strongly

at the origin a ditrcrcnt

depends

ofcjcction.

secondary

compositional

tures).

and quantification.

gradients

for

(2

conscq~~~ccs.

is 21 possible

4 will bc largely

vacuum

is rcquircd

during

the mcxuremcnts

on the type

of spccics cjcctcd

impurity

a high

for ;I given

secondary

about

their

relatikc

Au the positive

taminated

bomburdmcnt Ihr barium

clcmcntal

hut it is improbahlc

mass 197, i.e. gold!). the ionized

Under for

fraction

of the order of IO cvcn higher.

strongly

ion

dcpcnds

As

matri\c.

;t consc-

is seen 1.01

not indicate

duriy

intensity

xx~und

usually only

IO ’ or worse to 0. I.

Necdlcss to say this will atfcct the dctcction

cllicicnq

cno~-mously.

The problem

of low) to modest

many casts by the selection can rcndily

bc understood

(3). It is possible achieved tivcly

oxygen

the positive the work

the near-surface

with

0:

(occasionally

3.5 and Section

been adopted

in SIMS

5) or C-s ’

much

solution,

for over

is to use noble

sputtering

yield,

surl‘acc region flooding 758

may

considerably.

bombarding with

primary

the build-up

or Cs+

bombardment.

still

is dctcctcd.

spccics (clusters)

bc of good

service

fat-

‘target combinations).

the

(cxtrcmcly)

secondary is sho\+n

intensltics

0,

Lvith ditfcrcnt

nal’ mass (in aniu)

;I nl;ixs

’

resolution

difficult

Figure

1.

the

hon\ignal

idcntlcal

‘nom~is \er>

in

a silicon

in csc‘c‘ss of 3000

for ;I particular (although

mall-i\

” P’ “‘SiH

at the same time

isotope

01‘ silicon

Ob\iousIk

but with

arc

uhich

(see lnsct

that

clement

On the one hand interl‘crcncc

the proper

polyatomic .A> an c‘\;ln-

unless mabs resolution

examples

&,A;\/

;I n,ass o\cr

sputtering.

ions 01. the pair

shows

more than one isotope nor ;I blessing.

that

‘composition‘

and “As

This caamplc

sclcctinp

just

ion mass spcctruni in

may intcrltii-o,

Famous

indeed.

“‘Si “‘Si “‘0

11, SIMS \\c knoll

arc also cjcctcd during

bardcd

with

principle

In addition

plc the. so-called.

the

rcquirc

of I:iyurc

the cxistcncc i$ neither

ma!

01’

;I curse

bc c\cludcd

h>

rare ones arc to be a\oideti

1

n si

-s .g 2 al F ._

10E

10’

104

10’

In that Yet the

IQ+ 31p+

St++

I/‘60;

102

10’

ion yield cnhanccwith

oxygen

ion reflection of an altered Additional

zC+ ,

gas

angles the increased

with

S:;O'

SIO'

has

noble gas ion ones. A

secondary

obstruct

by 0,’ then

solution

Of course care has to be taken to avoid to the source (e.g. by differential pump-

in combination

9, > 60 ), will

on :I

design

prolonpcd

;I dccadc.

gas ions in combination

ing). Note that at large off-normal angles

than, cg..

at least for positive

flooding of the target. backstream of oxygen

ratio

of these two species make the sources

more maintenance-prone

partial ment.

nature

typicall>

instrumental

rcspcc-

as WC will see in Sub-

time the sources for such ions habe matured rcacti\c:aggrcssivc

c.g. with

This particular

instruments

:I Ill

sccondar!

q. This can bc

accompanies

0

blass interferences. In

3 and equation

(ncgativo)

region

in

ion type. This

function

and cacsium. This naturally

bombardment section

(lowering)

by saturating

can bc allcviatcd

primar!

on the basis of Figure

to enhance

ion yield by raising

sensitivity

of a suitable

SIMS

clcmcnt

limit

spccics is typicall?

I. but it may rango from

is

asset

rate

ions 01.

Ar

a ppm detection

to get any sign4 emitted

an!

to impurities

contaminant

unique

in making

almost

somctimcs

for certain

this

helpful

oxygen

anything

noble gas ion bombardment,

a given

for

and set limit? (although

and:or

dctcct

Aul

in the erosion

(e.g. for Ha con-

intcnsitics

arc such that one can obtain

3.2.

3)!

concentrations

fee

good

good

will bc dctailcd

ion intensity

X and a low one for spccics Y dots

whatsocvcr

‘standards’

a rather

3.9).

ion escape probability

the fact that

and

und the best

: (ii) (this

in Section

level 01‘ ion yield.

against

to this topic)

matrix

reduction

arc ccry can

that :tllO\\

quantization.

charge

struc-

(i) absolute

impossible

calibration

dcvotcd

3.2 and particularly the secondary

qucncc.

implies

multilaycr

namely

virtually

cvory

ions

tyl?c.

for the aforcinentioncd

gas flooding

potcntiall)

ion

or oxygcn-load-

cnvironmcnt

automatically

I ‘!,b) all atfcct the scandal-y

bccomcs

(Section

Also.

cscupc

and c\.cn ;I high

gas ad/absorption

(I />~io,.i quantification

in Section

matrix

(as in GaAs.AIGaAs

This has two immediate hope

on the local electronic

A ditfcrent

contamin:ition

one may

ion

ion yield Ihr a given spccics. Conscqucntlq.

background

impurity

secondary

The

cacsium that

arc inhibitive

The 3.1.

Yet oxygen

the sensitivit!

of primary

for those samples

(i.c. not

Icads to ‘romi:

ppm lcvcl or better tars

in the following.

2.5).

sclcction

hc ctliclcnt

to some cutcnt

(cf. Section

of the characteristics

the

by proper

and this inevitably

At some length the more important

implcmc~itation. discussed

knowledge most

In surface COntainInation

arc a

Basic characteristics of SIMS

easily

the latter.

whcrc the total Rucncc used IS low (S-SIMS)

o\ldalion 3.

is possible \lith

itudics

particles

neutrals

is

apply.

(fol near-

oxygen

the formcl-.

no

lO[

i

20

IJI 40

60

80

100

M/a ______

P C Zalm

: Secondary ion mass spectrometry

in general). On the other hand, more isotopes open up the path to more interfering combinations. Light elements suffer least from interferences (although the detection limit for “‘B in Si is ultimately dctcrmined by the ‘“B+/3”Si’f interference, even though ‘“Si has only 3% natural abundance and multiply-charged ion survival is rare!). At masses above 100 one should (always) implicitly assume (some) mass interference. A modest resolution secondary ion mass spectrum will enable some cross-checks against natural isotopic abundance for the element of interest and when large deviations are observed this generally signals, yet does not identify, mass interference (see also Section 3.9). Background gases in a non-uhv system are typically HZ, N,, CO(,, and O,, sometimes also accompanied by some higher hydrocarbons from pump oils etc. That is, low-mass containing species with usually a fair sticking probability on the sputtered target. Interferences with clusters containing one (or more) of these atoms will be the result. A high mass resolution can be obtained but usually at the expense of sensitivity and often also by sacrificing long-term stabilityircproducibility of the instrument. The one exception is the Time-of-Flight (TOF-) SIMS design. but this approach is limited to very-near surface studies, as will be discussed in Section 5. For selected casts the use of different primary ions and/or secondaries may help in minimizing interference problems (e.g. “As+/SiOz+ is a severe problem with 0: bombardment of silicon but detecting the negative ions “As or 2XSi75As , using Cs+, largely nullifies the problem. A more widely applicable remedy derives from the difference in energy distributions of elemental and cluster ions. As has been argued in Subsection 2.6, the kinetic energy distribution of a cluster ion is more confined to lower energies because the dissociation probability becomes high upon forceful ejection (an example is given in Figure 5). So by restricting the analysis to ions that have been ejected at sufficiently high energy (‘how?’ will be discussed in Section 5) one discriminates efficiently against interfering ‘clusters’. Usually a lower limit of IO-25 cV will do, occasionally a (much) higher value is required. Finally we note that this method is occasionally inoperative because of ionization/neutralization particulars causing an abnormal kinetic energy distribution and for highly asymmetric clusters (e.g. SiH, WO etc.) because there the energy transfer upon ejection is mainly to the heavy partner and hardly any ends up in the centre-of-mass to evoke dissociation. 3.3. Pre-equilibrium effects. As we have seen before (Subsection 2.3) at any interface an altered layer is built up during bombardment, owing to incident particle incorporation. In turn this modifies the erosion rate and the secondary ion survival probability. These effects will be less important for high sphttering yield materials and grazing ion incidence (because of reflection). And of course at lower energies steady-state conditions are reached quicker (in terms of removed target atoms). Yet one should be aware that in this transitory regime the observations must be considered unreliable. An additional problem affecting the analysis in the initial stages of depth profiling lies in the prcsencc of surface contamination by ad/absorbed species. For convenience, e.g. a native oxide, can equally well be regarded as such. Although this contamination may (but need not necessarily) be confined to the very-near surface region ( < I nm), it can be mixed into the target during ion bombardment and eventually exert influcncc to considerably larger depths (see also the next subsection). Aside from the aforementioned remedy, by lowering the inci-

0

50 ejection

100 energy

(eV) -

Figure 5. Ejected kinetic energy distributions of some positive secondary ions emitted when silicon is bombarded by oxygen ions. The data are asmeasured in a Cameca ims 3f, which implicitly acts as a complex prefilter discriminating against high exit-angles and -cncrgies. Yet the larger extension of the distribution for monoelcmental Si+, as compared to the cluster SizO+ becomes very evident. Also it is clear that the detection of clusters, with very asymmetric atom mass of the constituents, cannot be suppressed efficiently by setting a lower limit on the accepted kinetic energies.

dent ion energy and angle of incidence, there is one other cure for the problem of pre-equilibrium uncertainties but it is not a universal one! When one is interested in positive ions, oxygen flooding of the target usually greatly helps in attaining steadystate conditions at eroded depths, a factor of 2-5 less than observed in the absence of oxygen flooding. The magnitude of the improvement does depend on the oxygen affinity of the target material. As with lowering E, and or 3,, oxygen flooding usually leads to a decreased erosion rate but fortunately this is (usually more than) compensated by the gain in positive secondary ion intensity. Combining both methods, reliable depth profiles from a depth of about 2-3 nm onward can be obtained; quite some sensitivity will have to be sacrificed, however. 3.4. Detection/resolution limits and ion bombardment induced mixing. In dynamic SIMS one always has to compromise between attainable detection and resolution limits, This does not hold for static SIMS, where the boundary condition of a virtually unperturbed surface dictates the admissible primary ion fluence. When enough material is available one may depth profile under various conditions to get the best of both worlds, but every so often the analytical problem at hand does not allow for this simple solution. Obviously a good detection limit and a high secondary ion sensitivity are interrelated. In general, the higher the erosion rate the better the sensitivity. So high current, high energy primary ion beams appear ideal. Unfortunately, with the energy not only the sputter yield but also the penetration depth and cascade volume (i.e. the extent of bombardment, induced damage/ relocation/disturbance in the target) go up. All this adversely affects resolution. 759

P C Zalm: Secondary Ion mass spectrometry Following tributing

’ ’ one may distinguish

Hormann

to the attainable depth resolution.

(i) Instrumental by non-uniform prominently.

three sources COP namely

factors. Here erosion inhomogcneities irradiation

The

caused

of the analysed area reaturc most

resolution

deterioration

A:

is lincarl)

redeposition

pro-

portional

to the eroded depth 1. until

importunt

at very large depths where it bccomcs constant. 0h\,1-

ously this type ol‘ problem can. at least in principle, and f’urthcr discussion (ii)

is dcfcrrcd to Subsection

Sample characteristics.

also poly-crystallinity

Intrinsic

bcconics bc avoided

3.6.

surl;icc

roughncsa.

resolution

but

(2)

to AJ-. (?)

1021

74 s m

z ._ z ;: : & 0

1020

10’9 1O18

AZ 01‘

rate. The accompanying dctcrioration

is proportional

t i

:

and/or multiple phases (i.c. compositional

difl’crcnccs ;ICI-ass the target) all introduce local variation the mean erosion

1022

:

01‘

and dcpcnds on depth like

1’ ’ or z dcpcnding on grain SIYC. Clearly

problems

can hardly bc circumvcntcd. it’ at all. Additional

of this type

details will hc

provided in the next subsection. (iii) Particle~solid

interaction particulars.

For the sake ol‘co11-

\,enicncc only ion beam mixing (i.e. boiiibardiiient-induce~l

rclo-

cation) will bc considered under this heading. although one could argue that sputter yield diffcrcnccs bctwccn grains (cithcr owing to their

crystallographic

orientation

or to their

belong here rather than in category (ii). Further we will encounter an uncommonly rclaled to the prqjcctilc:targct prcscnt discussion.

ofall

is ulti-

concern

hcrc.

bc it inexact. simple model ol‘ion

stems li-om Liau

ion bombardment

redistribution

phcnomcnon

;I topic avoided in the

this will be our exclusive

A conceptually transparent. incident

nasty distortion

chemistry,

Since the attainable depth resolution

mately limited by mixing,

bcnm mixing

composition)

in Subsection 3.9

(71 rrl ‘-. It is ass~~mcd that the

lcads to ;I complctc and unilhrm

particles within the collision

R,, incrcascs with impact cncrgy (approximatcl! 10 E,:,’ ’

co+‘1 (at not too glancing a,). As similar

picture arises.

impacts and furthermore

Si and

gc~~cously.Tho implications of this model will bc olucidatcd Ihr the

of a dilute

delta (or monatomic plant) impurity distribution

embedded at depth I,, in ;I target profiled with SIMS. erosion front comes within the ‘disruption no mixing

depth‘ R,,

occurs, but once a depth z,,-

suddenly

all impurity

surlilcc.

The

abruptly maximum

R,, has been rcuchcd

atoms arc miwd

corresponding

increases

from

(or

many dccadcs hi&r.

up to the

backuardly

secondary

zero

Bcforc the

ion

signal

Upon

intcnsitk

Icwl)

background continuing

to

dccrcascs cxponcntially

bornh;lrd-

with a characteristic

length equal to R,, (after conversion

I‘rom time-to-depth

dccuq etc.. of

the raM’ data. cf. Section 4).

concentration

depth I,,,‘,, > R,,).

From

C’, the loss of ;I layer of thickness

(that is : cxtcndsome starting d: and mixing-

in of an undopcd layer dr into the disrupted depth zone Icads to a IX\+ average concentration equation dC(z)/dz

c’[l -d:;R,,].

so the ditfcrcntial

= - C’(z)/R,, holds. leading to the said expon-

cntial bchaviour. Experimentally.

(cf. Figure

many authors

jcctcd rungc R,, of the incident correlation

trailing usually

edges arc

frcqucntly

6). Since the disruption rcplacc R,, by the pro-

ions. Surprisingly

olicn a close

between this R,, and the characteristic

decay Icngth is

observed (whcrc it is clearly advantageous that R,, cannot be

to make delta-

(in. :ilmost

.4jidc

cucluzi\cl).

l‘roni their tcchnologic;ll

:11-ccutrcmcly intcrcsting

li)r

depth

Not only do these provide khc ultimate

test in depth profiling.

hut ;IISO the analytical ~rcsponsc f’unction

oI‘thc instrument

and nicasurcnicnt

used

rc\,calcd Ihl- that particular under those conditions.

impurit!

combination

collcctcd under identical circunist:inccs

and

use the cuperimentall!

l’or the delta to dccon\olutc

from the deformed result

dis~rihution

cniploycd i5

condition5

matrix

One can then

the true depth

for an unknown

sample

“.

Strangely enough, a finite rising slope is always cxpcrimcntall) observed with SIMS. sharp

even on deltas that are allegedly infinitely

according to their

m~inuliictui-cl-s

and other

clcctron mitt-oscop!,

analytical

pho~olumin-

csccncc :md clcctrical charactcri/~lli~~n~: all, adniittcdl?. ably lucking concentration sions).

the sensitivity

of SIMS

for ;I low-lcvcl

or averaging over extremely

Furthcrmorc.

small lntcral dimcn-

;I nearly exponential bchaviour too. even for impurity a linitc thickness initial

invai-iimpurity

the leading cdgc very ~rcquently exhibits

(cl‘. I;igurc

6). This

with the simple complctc-redistribution exponential

observed in depth profiling depth is ill-defined

applications.

tcchniqucs (like transmisGon

In I‘uct this applies to any confined distribution ing to some maximum

importance sucli structures

obscr\cd prolilc ;I

mcnt. cndlcss forMard mixing takes place and the pcrccivcd conccntration

Ibr selcctcd impurities

I II V scmicoiiciLictors).

profiling

ofthat I:I~CI

cdgc in a profile is less straight-

I’or-ward. In rcccnt years it has bccomc possible doped samples

cast

’ ‘, ;II~ apparently consistent and trustworthy

different approach

In addition. this is &ken to extend to equal depth Ihr all individual that erosion proceeds pcrfcctly homo-

dcpcndcnccs have been

predicted f’or ion beam-induced broadening on the basis of ;I

The behaviour of’ the &ding

cascade volume.

proportionall!

at not too high E,) and dccrcascs with impact angle like

layers with

is apparcntlq in C‘O~~IKISI model. which predicts an

linear rise. In s~~ppot-t is the observation

that the ~II;II-ac-

teristic rise Icngth is usttally liLr smaller than the decay Icngth and dcpcnds only weakly on R,, (R,,) and the beam paramctcrs (II,.&). Of course the oversimplification of that model can, at least partly. bc blamed Ihr its shortcomings. exist,

In rculity

thcrc is always some roughness

no pcrlict

interfaces

(if only steps etc.). l-‘ur-

measured very prcciscly and has to be estimated from theoretical

thermorc.

cxprcssions

interatomic dist:mccs in depth. That this is u llct is proven by i/r

760

which alloy, for ;I Ihir degree 01‘ li-ecdom!). In turn

erosion

is not homogeneous but spreads over a few

P C Zalm: Secondary

ion mass spectrometry

situ scanning tunnelling microscopy images with (near-) atomic resolution of individual ion impact craters”. Such studies. in addition, reveal that the dimensions of those craters increase only weakly with E, and that the roughness only slowly becomes more pronounced at higher fluences. The latter is presumably due to preferential sputtering of minute protrusions, thereby effectively smoothing the surface. The most exhaustive (semi-) theoretical treatment to datelh, which comprises all omissions of and circumvents all objections to the complete-redistribution approach, comes surprisingly enough to very much the same conclusions. That is, for a true impurity delta-distribution, the near-exponential leading edge is largely dictated by initial roughness/collection statistics and the exponential trailing edge predicted and observed reflects the mixing processes. That is, the latter depends sensitively on primary beam parameters (E,. N, and, to a lesser extent, Z,) and target properties (Z, and the minimum displacement energy). In addition to the so-called ballistic relocation process, radiation induced or enhanced migration may occasionally play an important role. These can be mediated by a variety of mechanisms c.g. involving crystal defects, chemical gradients etc. (see also Subsection 3.9). The prediction of the occurrence and magnitude of such (rare) events is virtually impossible, as knowledge is still incomplete. Therefore, this section will be closed by just mentioning some empirically observed trends and lower limits for the exponential rise, 1, and decay (&) lengths. One may roughly add all contributions to the attainable resolution statistically, i.e.

In a similar way. although mathematically not strictly sound, one could cast the characteristic exponential length i in a form like equation (4). In practice it then turns out for near perfect brittle materials, which amorphize quickly under ion bombardment, that, up to eroded depths of about 0.5-1.0 pm at best, intrinsic (i.e. probe depth and natural minimal interface sharpness [steps etc!]) and mixing terms dominate. For low energy (E, < 3 keV) and grazing angle (9, > 60”) ion incidence the first term dictates i,. for which values as low as 0.4 nm have been reported. and mixing becomes important only at higher energies. For /I, the mixing term dominates. It depends on the beam parameters very approximately, like i.,,, z C E,“* cos 9, (for the full-width-half-maximum a proportionality to E, has also been reported!). The prefactor C is, in first order, not very sensitive to the primary ion type at sufficiently off-normal 9, ( > 45”), except at higher E, (> 5 keV) and for low E, in combination with perpendicular incidence when there is a chemical reaction between ion and target species. Its magnitude is determined by the matrix type but not so much by the particulars of the impurity profiled. The lowest values reported for i, are about I nm (for E, -’ 1-2 keV, 9, z 60”). At E, x 5 keV and 9, -45” a value of 5-8 nm appears to be reasonable. At sufficiently large eroded depths (>0.5 nm for semiconductors, single crystalline ceramic films, etc., but only - 0. I pm for metals and other ductile materials) the depth dependent terms (roughening, erosion inhomogeneity and preferential texturing) determine the resolution. Then the broadening of features becomes less asymmetric, than in the mixing regime where i,, < i, to <
rate. Unfortunately, in some manifestations of broadening by physico-chemical gradients, the time scale for migration is too swift to observe such a dependence (cf. Subsection 3.9). 3.5. Topography development. Under ion bombardment the surfaces of poly-crystalline metals generally become very rough very quickly. The reason for this lies in the considerable differences in sputter yield with crystal orientation (cf. Figure 2) and the fact that metals do not amorphize under irradiation but rather texture (i.e. preferential ‘growth’ of favoured crystallite orientations takes place during erosion). Texturing is somewhat retarded at (near) normal incidence but still so severe that, even by starting from a mirror-like surface, the relative unevenness AZ/-_ of the crater bottom at a given depth I is typically of the order of lo-50%! Practical metal surfaces often already initially exhibit a considerable manufacturing induced roughness (micro-scratches from polishing, undulations from lapping or rolling). This seems to render depth profiling of metal targets a virtually useless exercise and suggests that good quality results can only be obtained on single crystals. Some improvement is, however, already obtained when use is made of oxygen or nitrogen flooding of the samples to convert the eroding surface to an oxide or nitride which is less sensitive to topography development ; but by far the most promising approach is to apply sample rotation at a few (> 0.5) rpm (ref 17). This improves the depth resolution dramatically. It is not always simple to incorporate a rotation stage, notably not in instruments with a high extraction field (i.e. with the sample at a high potential; we discuss such design aspects in Section 5). When no rotation option is available, meaningful SIMS depth profiling is largely restricted to brittle materials. Unfortunately, roughness is not the exclusive domain of metallic targets. Any dust particulate may act as an ‘etch mask’ during (the early stages of) ion erosion and leave a protrusion of the receding surface. Decomposition during bombardment and agglomeration of one of the constituents into droplets on the surface sometimes takes place (e.g. with In from InP as recombining P,(?,, molecules desorb) and has similar consequences. But also semiconductors, like Si and GaAs (ref l8), and insulators, like CaF, and CaCO,, can develop a ripple-like surface topography after erosion to depths of around 330.3 pm with 0’ ions incident at angles between 15 and 45”. 3.6. Redeposition off crater walls (and surround). When depth profiling, one likes the secondary ions to originate from one welldefined position below the starting surface at any stage during erosion. A prerequisite for this is a homogeneous erosion over the area analysed. In principle an infinite (at least sufficiently large) diameter uniform primary ion beam would do, except for the fact that this cannot be realized and also would imply a single analysis per sample. In practice a focused beam is rastcred over a fairly small area (- 0.1 mm’). With a sufficiently large number of scan lines this will give rise to a constant flux density over the majority of the area scanned, even with a spatially nonuniform (e.g. Gaussian-shaped) current distribution in the beam. Precautions should be taken to avoid line pairing and rastering should be swift enough to prevent trench formation, but this does not constitute a substantial technological problem. A much more serious problem is associated with off-normal incidence. This leads to focus variations over the rastered area and consequently to erosion inhomogeneities. In addition, in instruments that employ high extraction fields (cf. Section 5) the pro761

P C Zalm:

Secondary Ion mass spectrometry Charge-up of the sample may occur and lhis

106

25keV 5x 10’5”B/cm2

t

in Si(100)

the impact parameters of the primary well

as

those

of

consequently

the

in turn

species (energy

cjccted

will

atfcct

beam (E,, $. position).

;1s

distributions):

it has to bc avoided whencvcr possible.

Simple

rcmcdies like ;I \cry low primary ion current or s~umpleheating to

100%

iniprovc

80%

its conductivity

sensitivio

arc unatlracli\c

;Ind dilt‘usion-intillced

two I:lil-l!

for rcabons 01‘ loss 01‘

profile

uidclq applicable solulions.

alteration.

Thcrc

;tIc

namcl!

: (A) C‘o:itiny the curlitce with ;I thin ( - 30 nm) mc’tallic I;iyei-. combined with negative primary ions (often 0 ) for analysing positive secondary ions. In principle. ncgativc primary ions induct negative charge-up; libcratcd

but during ion bombardment

;I mo(:~l. The

cncrgy distribution

of these sccond:lq

~!p~call~ peaks at ;I I‘cw cV and onl) enerfics

ion-irradiated rather

than escape, or clcctrom the nict;illized

cralcr rastcrcd ;irc:i energy may cause a deformation This

and rounding

of the cra(ur

the plane spanned by the beam and the surface normal.

situation

occurs precisely

when one tries

to achieve ~hc

(H)

Exposing

ions tlclcctcd

;lrc cjectcd Cram) is smaller

than

raster G/c.

the

is ncccssary to avoid secondaries Iibcratcd fIron the cralct

walls contributing

to the analysis.

about the distribution from

Ihe bottom

sonlcwllat

of redeposition owing

(cf. Figure from

Although

this

is only

to li,llow

concentration

dcl‘ormed Finally.

analyscd area has LO be

w;~lls onto the crater

ejection ;I minute

pattern fraction,

distributions

b! cvcn such tract

bottom.

of spultercd the abilit)

dynamically

that the IOU Impurity

species. SIMS

of

over m:lny

lcvcl part is scvcrcl)

amounts.

bc noted that

it should

than those coming

;I (Lcry)

inevitably

tion 01‘ the emitted species dots not end

LIP

I;irpc I‘ILLC-

in the dclcctor but

ralhcr will bc dcpositcd on inner parts of the instrument. thcrc

these can bc rcspullercd

reflected primary arca. Or

particles

they may nol

and return

stick

tribution

to the irradiated

cfficicnlly

chamber

way back to their

their

and/or

dcsorb

extraction

plates.

origin.

In theory

this

primary

the targcl continuously

(space charge blow-up

finite opening angle ofthc

detector,

rringc

(rrom

utllircd.

Thermal

becomes impossible

with a given set of measurement

Icm in insulaling

st> cncyctic

ion.\ ;lrc

used in ~hc abscncc 01

lx beams

arc prcl’ct-I-cd. i\t LOCJ

surl’acc chat-pc: compensation

to electron penetration.

Also.

the

niultilaycr

sumples.

or r;is-

del’ocusing

t’inall!

01‘ rhc ion irr:idiated

;irc;i ib usualI>

ncccxsarv.

this.

3.X. Signal intensities and sampling. The intcnsil! current)

COW

beam,

As a conscqucncc it

fact

le-ad to problems

thal

the primary

u,hcn examining

beam is insul:llors.

(secondary 1011

! c;In formally

hc

IF)

where .I,, is the primary ;iIc;i.

ion currcnl

Y Ihc total sputtering 5 i&i.

density.

instrunicnl‘s

transmission

I‘unction.

for education ~LII-poscs.

cucepl p;trticular

( Y. r,). sonic

by the operator. signals

and

vcrsionj

iiiaii~ ~LII-e

is

proper.

ullcrl!

cxisl. but

bc;ml (.I,,.

II

I,,,.

(T) dcpendcnt quantities and I\\()

of which

Knouin

and 7‘ the

(5)

One may distinguish

target [C‘,]) and instrument mixed ones

probability

Equation

cho~cc i\ not c\cn scientifically

scrvcs ~hc purpose.

IL,, the analyscd

[<‘,I the impurit> conccnlralion.

2, Ihc secondary ion‘s l’ormation surviv;lI

alloy, some nIc;isurc of control

f (‘roni cupcricncc whal mqniLudc

01’

to cvpcct cnablcs the expert IISCI- to m:rkc an cducakci

guess

of what paramctcrs to choose I)r- ;in oplimuni

result

in ;I given problem

intrinsically

or

customer

depth profile

ions sign:il(s)

time. Usually The

inipuril!

/, = ./,,.-I,.,, F.[C’:jx, 7:

ondur!

conditions

dilulc.

written ;IS

This

(targcl

I, 01’ ;I parlicu1;ir.

than six (and often cvcn only Ihur to live) decades.

762

licld\.

tcring over an :ire;i iii csccss

equation (5). In ;I SIMS

charged may

positi\c prima’>

mhcn

beam current must he tuned to each nc~ target ma~crial. ;I proh-

to follow any profile over a dynamic concentration range of more

problems.

and

clcctrons c;tn onl!

bccomcs incomplctc owing

spon-

fields on the cxtructor.

design prohibit

Charging

( - 0.1 I

;~n c-gun) \\orks rcasonablq well in the

( > I 2 kcV). houcvcl-.

high cnel-g)

~isclcss.

Practical

of the primary

etc.) of instrument

3.7.

;I

to ;I Ilu\ 01‘ thermal

kc\‘) clcctrons

t:irgct

beam defection,

focusing optics, detectors etc.), from the target vicinity. considerations

OI

e.g. by oxygen bleed

could bc suppressed by removing ~hc surround walls,

From

by cncrgctic secondarlcs

taneously, which in turn may be stimulated. in, and tind

Occasionally

in order to minimiLc the problem

the crater

to the angular

decades implios

7). The

( - IO-20’/0)

snlallcl-

since these carry information

at depths shallower

beam

0C the spullcr

;I hot tilament) or ;I beam ol‘encr~et~c

high extraction

This

irim surrounding5

return

primary

clcctrons (from

rcquircmcnt.

of the scanned target part where the

Creed by stray

aid in nc~~i--ncutrali/ation.

abscncc ol‘ any target bias

A second demand ix that the ticld of \ICM (i.e. the

targzl bcc~omc’\ posili\cl! either the clcclrons

diaphragm can rcplacc Ihc coating.

highest deplh resolution. Having a flat crater bottom is a ncccssary. hul insufficient. analysed area, that portion

awal

and land cl\cMhere. As soon as the

and -cxposcd insul:Lting

spccich from

bottomin

electron\

;I minute I‘mction rccci\c\

charged to more than ;I lb\4 \ol(s.

more glancing angle-of-irlcidcnce Mith trcduccd impacl

arc l‘rom

in excess of IO -15 cV. so the electrons can drift

I’rom the point of inip;ic(

grcssivcly

electrons

:~lmost an\ tarSct and relati\clq cfticlcntl!

from

situation. ill-defined.

provided WC will

the intensity

an;llytic;Il

the I;itlcr

is nol

come back to

ol‘ one (or more) XC-

is rccordcd as ;I t‘unction ofclapscd erosion

dctcctor pulses al a given secondary mass-to-charsc

ratio arc inlcgratcd during ;L Gme interval A/,. convcrtcd to countsper-second (c s

‘) by division.

:ind assigned to ;I point in lime

P C Zalm:

Secondary ion mass spectrometry

(often the midpoint of that interval). After a time delay At?, the analysis returns to that particular setting. The delay accounts for the time spent at all other mass-to-charge ratios monitored and includes intermediate waiting times for instrument response, whenever appropriate. The product Z,x At, determines statistics (hence sensitivity, attainable detection limit and dynamic range) per data point and the product J,Y(At, +Atz), the total material removal (in at cm -‘) between successive points in the sampling process. In the next section the conversion to depth and concentration will be discussed. Even when only a single type of impurity is the subject of investigation. one prefers to keep track of a second matrix-related species as well. The (lack of) variation in the signal intensity of a majority element is often used as a check on the measurement conditions’ stability during data collection. Also, it allows for reproducibility assessment in successive trials. Finally. relative calibrations against a matrix signal are often convenient.

2OOkeV 1~10'~ %u/cm2

depthipm) -

Figure 8. Depth profile of a copper

3.9. Miscellaneous. A few aspects need brief mentioning. These could not properly be incorporated in one of the previous subsections, so have been gathered here. This does not imply any relation, however! Different isotopes of an element do not entirely have identical ion formation!survival probabilities. The reason for this lies in the effectively slightly different velocity distribution [cf equations (2) and (3)] accompanying a given energy distribution. The effect can usually be ignored. but must be taken into account when one tries to study isotopic abundance differences, e.g. in geological matter. For quantification one may simply use tabulated abundances to correct the data for obtaining the total impurity conccntration. The error made is small compared to others. We have been rather jubilant about the role of oxygen flooding in SIMS, its positive role in enhancing the positive secondary ion yield, in minimizing the pre-equilibrium regime and in reducing surface topography development. There are, however, some negative points that put the advantages in a proper perspective, namely : -the addition of relatively light mass atoms to the target potentially increases the possibility of mass interference with heavy secondaries of interest (cf. Figure 4) ; -the oxidation reduces the erosion rate and according to some theories (and experiments) the enhanced ion flux needed to profile to a given depth leads to an increase in broadening by mixing, but the reverse may equally well apply” ; -matrix ef’fects ‘may be reduced in fdvourable cases for multilayer samples but, owing to large oxygen rctcntion differences between material results. may equally well be changing for the worse ; -the formation of an insulating oxidized top layer on the target may lead to charging effects, which in turn may cause impurities to (field) migrate away from the surface and move to the oxide/bulk interface (Figure 8). The latter constitutes the most devastating manifestation of a SIMS artefact known to date. causing redistribution of impurities over microns. Luckily it is quite rare” ; but even in the presence of an electric field, the chemical gradient can cause migration of mobile impurities. Here the lesser affinity of oxygen, as compared to the matrix atoms, provides a ‘thermodynamic’ driving force which invokes site exchange between minority and majority species. This amounts effectively to diffusion of the former into the bulk. Similar processes may accompany nitridation of the

implantation in silicon measured using 0: primary ions incident al 45 off-normal such that the surface is not fully oxidized (a) and at perpendicular incidence where an SiOl layer forms (b). This demonstrates the dramatic effect of (local) sample charging leading to a redistribution over microns. Note that the signal intensity is normalized at the peak: in reality the segregating species gives an order of magnitude lower count rate.

surface. That form of profile distortion is. however, far less severe than the one depicted in Figure 8. Detection limits of several substances will be affected strongly by their presence as a component of the residual gases in the instrument. This typically includes hydrogen. carbon. nitrogen and oxygen, which may stem from cracked pump oil or become introduced during sample exchange via a load lock. It is simple to summarise the problem. It follows from kinetic gas theory that the contaminant gas arrival rate to any surface equals P/,,/(nMkT) - 3.5 x 1O”‘P [at cm ’ s ‘1 where P is the partial pressure in torr of the contaminant, M its mass in amu and T the temperature in K; the - sign holds for M -30 at room temperature. Of course the sticking probability on the target is less than unity, although not always as low as one would hope ( d 10 ‘) because radiation damage makes the surface more reactive. Now with a primary beam current density J,, of practically 0.01-0.5 mA cm-’ and a total sputtering yield Y of typically - I10 at ion-‘. it follows that the background gas arrival to target atom departure rate ratio is of the order of IO-’ at a pressure of IO-’ torr. This already implies a detection limit of only 10 ppm and a dynamic range of a sub-percent impurity of a few decades at best. For oxygen in silicon, the combination of a low sputtering yield and a high sticking probability make the situation considerably worse. Modern integrated circuit technology favours continuously decreasing lateral dimensions. For on-chip SIMS analysis (c.g. to locate or examine failures) this would require a reduction in rastered area size. This conflicts with reasonable detection limits and/or depth resolution as we will show here. Consider an analysed area of I x 1 pm’. A reasonable sampling period should be such that no more than 10 nm of material is removed in between data points (depth dimensions become smaller too). At a typical target density of 5 x lo*’ at cm-’ (coincidentally that of crystalline Si) this leaves 5 x IO"ejected atoms per data point on the depth profile curve. A reasonably dopant or impurity level is always (well) below 0.1% and typical ionization efficiencies will 763

P C Zaln?:

ion mass spectrometry

Secondary

bc of the order of IO ‘. This leaves LIS with only too dccadcs dynamical range for a spccics with a peak concentralion 5 x IO”

SIMS or similar techniques are facing a very difficult

task indeed. Also. st~~aii

01

cm ;. Cicarig. cvcn with signal cnhancetnent by post-

ionization, ia

of

as

it has recently been shown”

that raster sizes

singic inlcgratcd circuit componcn~ ma) inlroducc

ii

[‘cry nasty artcfacts in ones profiic when the irradiated not cmbeddcd in an identical matrix

(i.e. protrudes

rounded by a diffcrcnt tnatcriai like SKI,

part ih

ot- is sur-

used for planarization).

lo be to learn to model important technological processes so well, using iargcr arca SIMS results

The logical road thus appears for testing predictions

in great detail. that future extrapolations

into the r-calm of the submicron

can

he

trusted.

It has been shown that prc-equilibrium intcrfcrcnccs

can

distort

effects and. e.g., mass

;I depth profiic. Rcmcdics arc not

possibic and. when applicable. always titnc consuming. tempting to apply background subtraction procedures to ‘improve’ the data. This labelled scientifically

fraudulent,

be that you cheat yourself

alway

So it is

and front correction

cannot itnmcdiatcly

bc

but it dots border on it! It may

welt in doing so. 8s the somewhat

as

pathological cxampic of Figure

9 dcmonslratcs.

This

dots not

imply that all conccivabic tricks arc to be condcmncd. When one background

contribution

suspects

;I

rcsiduai

gases. the fact that

LI~OII

shrinking

stemming

the impurity

from

signal dots

adsorbed not rise

the rastcrcd arca whilst the matrix intensity

allows one to apply a subtraclion

doci.

correction.

Note that this onl) improves the detection limit rctiably by ;I factor of three 01. so as the differcncc bctwccn two large numbers has a relativciy iargc error associated to it. A second tolerable manipulation of data is the combining or the ‘better’ parts of scbcral depth protilcs taken on the same sampic (c.g. when the tirst

measurement

suffcrcd from a slightly

sample Irans-

inferior

vacuum following

fcr. spoiling the near surface part. whcrcas the second one showed

deteriorated

rcsoiution

area misalignment). drop-out correction

at larger depths owing to some anaiysed

Finally. there can be no great objection to and occasionally to smoothing the profile

when the profile was accidentally ovcrsamplcd.

4. Quantification

of depth protiles

4.1.

General remarks. In this scct~on wc \+iii discuss quantification 01‘ the raw SIMS data. i.c. conversion of (timc-dcpcn-

dent) signal intensity ;I malrix

I,(r) for ;I given ionic spccics i c_jcctcdfrom

M inlo ;I conccnLt-ation (depth distribulion)

the corresponding

impurity

necessarily related directly. it i\ olicn

C’,(Z) foi

cicmcn( (2. Note that (2 and

i arc nc~~

For example. with c‘s‘ primary

ad~antagcous to monilor

ions

the “As“Si

cluster l’oi arbcnic impurities in silicon. L\hilst for tint in indium phosphidc (ItiP) CLL’IIthe positive “‘Zii”‘C’5 5ccnndarl ctubtct- ion give\ good rcsuits. In the iattcr cast one dctccts only one isolopc. ;I common situation,

so that ;t l‘urthcr

abundance correction

ncccssary 10 arrive at the lolai clemcnl concenlration. ciuuntification assumption

methods apply to diiutc impurities.

for which Ihc

of ;I iincar dcpcndcncc bctw:ccn signal intcnsit!

conccnlration

is \,atid. t:rom

lhc discussion

ti>rmation and sur\ ivai (Subsection

2.7

and

14

all

Aimosl

of sccondar!

and ion

I’igurc 3) it Ihllo\+ \

th;ll this assumption starts 10 break down a~ conccnrration lc\els above about impurity

bccomc charged

Lcry

cscapc.

In addirion.

neat- (he poinl hcncc will

large and

impossible. intcnsily

I ‘!c,. since then the chances of finding ;I scconti

;i(om

This

(or

of emission might)

of ;t first oiic

affcc~ its

s~tcccssll~i

dots not imply that quantification

bccomcs

only more tedious. as WC wilt see in Subsection in the cast of ;i depth profiic.

v:iriation

4.3.

the tcmporai 5ignal

is 01‘ importance and a conversion

from lime

lo depth is ncedcd. For the sake ofconvcnicncc NC will start with this enc. 4.2.

Time-to-depth conversion. In ;I homopcncous

tc)-depth conversion is rclativcl)

conditions and ignoring prc-cquiiibrium

cffccts. one ma) assume

that the erosion rate is constant in time (d: dr = ciplc one can then c~~icuialc il dirccti) (alai

~ampic timc-

easy. Gi\,cn stabic primary hc:um cons().

beam current /,,/(A, lotal sputter yicid >‘at ion

density

,Y at cm

in

prin-

from rastcrcd area ,I cm’.

I via d:!dr = Y/:(A!V).

’ anti targcl

linfortunatelq.

1’ i\ ~cidom known or can he cst~matcd [c,g, \ i;l equation (I )I \ulIicicntl~ accuratct! LOtrust rcyuits bcttcr than ui(hin ;I fac(otol‘:tbout two. This is not good cnouph for most applications. :I far more uscfui cmpiricai approach is lo dctcrmine the actual

crater depth rl aficr Icrminalion 01‘ Lhc mcasurcmcnl al I,,,,, and take the erosion rate as the ratio of both quantities (d:,‘dr = t//r,,,,,). Then the conversion from time-to-depth is simple (Z = r/f /_J and straightforward. When available. ;I !icld value can advantageously be used to roughly &mate the erosion rate foi cross-check (reliability) purposes. Dctcrmination of the crater depth can be done by. c.g.. optical intcrferometry of- other highly sophisticated meuns. In pracllcc ;I simple profilomctcr, ;I niicromcchanicai stylus transduccr. will do cyuaily well. Its readout accuracy is aircady limited to sonic 2% and ils absolute accuracy definitely no better than 5’%,. yet other errors than thcsc dctcrminc attainubic precision. We mention :

0

200

400

600

800

1000

depth (nm) -

Figure 9. Depth profile of a carbon implantation in silicon dctcrmincd in poor wcuum ( _ IO-’ torr) of ;I contaminated surface (thin dashed line) and in uhv ( - 5 x IO ’ tom) on a clean surface (thick solid line). The thin full Iinc shows an attempt to subtract the background in the former and to correct for the surfxe contamination. to dcmonstratc the hazards 01 such an approach. 764

matrix density (amorphous silicon may have ;I density 01‘ only X0’!,,,01‘thal of singic crystalline silicon. )cl depth is depth so the aosion rate in terms of u real density is \+a! OR) : prc-equilibrium cKccts (s\+ciiing by primary spccics incurporation and non-linear erosion affect the conversion, as shown scanning errors or primary beam distortion, xc Pigurc IO) ; ct-osion inhomogcneitics (uncvcnncss of crater hotton~ h! scanning errors or primary beam distortion. see Figure IO) : surface roughening (see also Figure IO) :

P C Zalm:

Secondary

ton mass spectrometry

dzl dt t

swelling

Figure IO. (a) Schematic variation of the erosion rate as a function of time. This particular form is somewhat exaggerated but otherwise typical for Si with a thin (native) oxide overlayer (low sputtering yield!) bombarded at sufficiently off-normal angles with energetic 0; ions (so that steady-state erosion conditions do not set in rapidly and that no full oxide is formed). (b) Schematic representation of the errors made in depth assignment by assuming a constant erosion rate in (a). Usually the effects are not as strong as suggested here. (c) Pathological example of the depth allocation problems associated with a deep crater made in a target that develops considerable roughness and had an originally undulated surface (thick solid line). In addition, erosion inhomogeneity (thin line) has taken place and, also, the sample has been insufficiently displaced after fine tuning the instrument so that overlap with the previous crater occurs.

-surface position dctcrmination (tither undulations present or primary beam halo-induced near crater-edge distortion influence the zero-point assessment. again Figure 10) ; -analysed area location (has not necessarily coincided with the centre of the sputter crater during data collection). A (few) marker layer(s) at given depth(s) is greatly advantageous in establishing a reasonably accurate depth scale. External calibration against RBS or other techniques is clearly helpful and gauge samples can be combined with very stable primary beam conditions to allow for superior relative depth standards. Such an approach is almost essential whenever very shallow depths are probed, since most profilometers can hardly be used when craters are (significantly) less deep than 100 nm (if only for the problem of locating the irradiated area with the human eye!). Then thicker gauge depth profiles are unavoidable at the expense of loss of precision. In the regime where safe operation of profilometry is possible (typically depths of -0. I-IO pm, good quality primary beams and decent, flat targets) absolute errors of 10% and relative errors of 5% are routinely obtainable, when one is sufficiently careful, over most of the depth distribution. For multilayered structures (possible) differences in the erosion

rate per layer necessitate termination of subsequent measurements at each successive interface and determination of all crater depths to obtain a more or less reliable estimate for dz/dt per layer. As uncertainties increase, owing to equilibration problems occurring at every interface, subtraction of quantities of similar magnitude (crater depths after termination at subsequent interfaces) and possible fluctuations in bombardment conditions (during the effectively prolonged measurement), the reliability usually deteriorates by at least a factor of two over that obtained for homogeneous samples. In the case of compositional gradients in majority target elements the situation is even worse, because here the erosion rate may vary continuously in an often unpredictable manner. Here only elaborate computer-aided deconvolutions can help. but the precision these appear to convey must be thoroughly mistrusted. 4.3. Intensity-to-concentration conversion. Whereas one can occasionally calculate the erosion rate to within a factor of two or better, the sensitivity effectively evades such a treatment. Even in ultra-high vacuum for any impurity in any elemental target bombarded with noble gas ions no theory to date can come up with numbers that are trustworthy within an order of magnitude. For practical problems under realistic measurement conditions, this implies that such an approach is useless and that one has to rely either on previously established knowledge obtained from calibration against standards or use such standards directly for comparison in the same measurement cycle. Both approaches will be discussed next. 4.3.1. Absolute standards and relative sensitivity factors. Equation (5), or similar ones, enable a very powerful solution to the intensity-to-depth conversion problem. A gauge sample ofmatrix m with a known (i.e. independently established) bulk concentration [C,]’ of impurity i can be used directly to set a concentration scale for the unknown sample. By measuring the steady-state signal I: emerging from this sample under completely identical and stable experimental conditions as those with which the unknown sample is profiled WC have, according to equation (5),

and we already learned how to do the time-to-depth calibration to complete the result. A simple extension of this method allows us to examine different isotopes of the same impurity on the same footing by inserting in the appropriate (natural) abundances. The loss of accuracy is very small unless mass interferences creep in. The rather strict requirement of identical. stable measurement conditions can be relaxed somewhat by the following approach. As stated before, whenever possible one includes monitoring of a matrix-related secondary ion m quasi-simultaneously with the impurities under investigation. Then, by using the ratio of impurity and matrix secondary ion intensity signals Z,/I”,, and combining this ratio for standard/gauge and unknown in a way similar to that adopted in equation (6a) one arrives at

[C,(t)1= [C,l+(~;(t)/~m(t>)/(~~/~~>. The slight disadvantage of than compensated for by fluctuations are neutralized be it that these still affect

(6b)

introducing some extra noise. is more the gain in robustness. Erosion rate (since divided out) by this approach, the depth assignment; but the added 765

P C Zalm: Secondary Ion mass spectrometry flexibility is that one may change the rastered and/or analyscd area or current density in between the measurements of gauge and unknown samples and again the impurity isotope bag of tricks may be opened. It is possible to take this whole scheme ycl one slcp further. By monitoring lhc same type of matrix and impurity sccondurics (i.c. both arc singly-charged monatomic ions. coincidentally lhc optimal choice) the cxpecled similarity in energy distribution and angular cjcction pattern allows for the change of (some 01‘) the transmission characteristics of the instrument in-between measurcmcnts without influencing the outcome ofthc calibration too much. The reliability will obviously bc aR‘cclcd advcrsclq. The magnitude of this deterioration is hard to predict bccausc of the cmihsion particulars involved. Yet it is this Iask, cnormousl~ adaptable. schcmc that finally opens the way to not mcasul-ing the st;lndard anymore in con_junction with the unknown in ;I single experimental run, Rather, one can collect the so-calkd Relative Sensitivity Factor (RSF) [c’,]‘l~,;/: on one day for :I particular set of measurcmcnt conditions and use it on any othc~ day for an approximately similar inslrumcnt setting. Note Ihat the RSF‘ is in principle gauge concentration indcpcndcnt in the dilute limit. Note further that various RSF dctinitions arc cu-rcntly in USC,either specifying prcciscly the isotopes involved fol matrix and impurity or already including natural abundances. Tables of RSFs exist for spccitic instruments and curtain primal-> beam energies. Typical variatibns in RSFs dctcrmined in one laboratory lit around 20%. but using RSFs obtained clscwhcrc by others one may at best estimate accuracies at about 50”/;1. Convcrscly, equations (6n). (6b) with concurrcnt measurements on gauge and unknown(s) allow routinely for a 20”% concentration scale accuracy. Hitherto wc have conccntratcd on Ihc virtues of tixcd bulk impurity concentration standards, bu[ thcrc arc obvious disadvanlagcs that must bc mentioned. Maximum precision i\ obraincd under identical mcasurcmcnt conditions and for similar concentration levels in standard and unknown. Any compromise to this basic rule rcduccs the reliability of Ihc comparison, Unfortunately. often it is not possible to choose the most dcsirablc matrix secondary ion bccausc of its huge signal intensity: ot one must accept Ihc introduction of an cwtra error source 1~) measuring the matrix spccics on ;I Faraday cup and the impurit) one on an clcctron multiplier. But by far the largest drawback iz Ihat one dots not learn anything about attainable dclcction limits in using uniformly doped samples. That is, given that the unknown impurity contamination level after calibration is found as constant and say two orders of magnitude Iowcr than in the standard spccimcn. how do WC distinguish bctbvccn u true background doping. an intcrfcring spccics or ;I condition-dicl~ttcd detection limit? Obviously. by doing more than one cxpcrimcnl with altered measurement conditions some of the answers can be provided but it will be a tiresome job. An alternative will bc discussed below. But first one more item deserves briel addressing. As already stated before. matrix effects affect the sensitivity (severely) for high impurity concentrations (> I % level). Yet the above calibration scheme can even be applied there when certain precautions are taken. For example the aluminum content in an AI,Ga / ,As sample can bc obtained by comparing the AI+!Ga ’ secondary ions intensity ratio against that obtained from two samples with known Al concentration by linear intcrpolation, provided x lies in between the values of both gauges, but the error is quite large. Superior is the dcccrmination of a 766

calibration

curve using Acvcral AlGaAs stnndwds ;~nd fitting the ’ ‘Ci;, intensity ratio dcpcndcncc on ,II contcnl lo $omc slmplc malhematical function \s hich is then used 10 dclcl.mint .I. Thih RSF-function can of course bc \lorcci for later tl\c undcl- near-identical circumbtanccs. obscr\cd

Al

1.3.2. Implantation standards for calibration. 11 gattp impIanLition of impurit\ i in nialrix 11, \\itli ;I gi\cn cnerg) E’ and io ;I well-dcfncd tlucncc (11 is an idcal aid in concentration c;IIIbration”. One $impl) dctcrminch. under idcntlcal mca~urcmcnt conditions ;15 ((0 bc) u\cd li)r the unkno\l n spccimcn. the dcptl~ distribution of the implan~cd zamplc. No\4 Ihc hcnhitivil! lilclot-. dctincd lb!.

is cvaluatcd. Note that the integral is actualI! ;I sum Gncc I (/) is onI> a\ailablc LIS;I dihcrctc \crlc’;. No\\ .\‘. C’;INbc u\cd ;I\ ;kn in\trunicnt and circumstance 5pccilic absolute \cnsili\ il! I’;lctor Lhat linearly relates delcctor coun(\ for inipuril) i in malri* 111IO ~1real dcnsily. From its dctinition it follows that Sl is indcpcndent of erosion rate. Incorporation of the time-to-depth convcrGon i\ straightforward (fat ;I homogcncous sample. c.9. C‘,(z) = S;l,(r)l,,,,, tl and : = r/t /,,,,,). Nole that RSF-s C;II~ hc% dcrivcd from implanlutions as ncll. A ~cond

advantage

of [his approach

is that during

the gattgc

sample mcaxurcmcnl one gclh ;I feeling fat- the attainable dctc~.tion limit. Since implantations only cxctcnd to 8 linitc depth the impurily concentration must drop to zero. which is cilhcr 5 6 orders of magnitude below the peak intensity (cl’. Subsection 3.6) or the dctcction limit. When and if the result iz unsatisfactory. cnunlcrliic;lstIrcs (changes of conditions) can bc applied. Al\o one has ;I few on-lint rule ol‘thumb cstimalcs a\ailablc that allo\\ li,r ;1n immcdiatc l\irhoul

h;t\ing

clcnicn~al

check on ~\cral

I;irgcl\

coincides

bvilh

the

pwI\

poG(lon

Ihr imnicdiatc

(~1‘ lhc

calculalor

tlcptlr

and allo\\

rate. ‘Phc pca!i conccnlra(ion

prolilc. a:, aboul 0.3 (1,’ AR,, \+ here the range stra,,
two or 50. Ewn mining

whcthcr

problem

a Gaussian

- 25”11 i

\j hich almo\l

iniplnnL1tion

~)II ;I pockcl

asscssmcnl o(‘croxion

1~) assuming

,4t Icasl I‘01

(accut-;Lq

range R,, 01‘ an implant.

Thcsc can bc c\aluatcd

prolilc.

anal>si~ paramclcr\ analysis.

coarse anal> tical c7;Ltm;llc\

’ for [he projcctcd

cxisl/

impor(;lnt

to L\\ail for a later olI-lint

implantarion

Ihc50 approximate

tigurc\

ot- not the adopted

(i.c. the conditions

hand. S;IIIK

the peak

;IS the one‘ (cxpcctcd)

the pcnctraCon

in the unknown:

not very

important!

Further.

us well 10 implantation

can bc made to equation secondary

ion signal

hc ~Ix)LI~

gauges.

about

Finally,

isotopes

slight

lo Lhal 01‘ ;I marriz

but the gain in precision

al the

bc close lo the mwl

but hcrc these demands

all rcmarks

loo

sample and likc\\ixc

should

(6~) b) lirst normalizing

intcnsit)

gaufc5

li,l- the problem

should

in the unkno\+n

depth in the standard

depth

integration;

L’. 0;

concentration

important just

Ihr Implvnlalion

to sslcct particular

I’rcTcrably

in dctcr-

to the anal) (ical

will result in ;I \atijl’actor!

chosen)

;In>\\ccI-! ;\s \+ith ;I conxl;inl lcvcl \Llndal-d. iI i5 ;id\antagcous

ma> aj\is;l

approach

arc

apply

cxicnsion$ rhc impuril> one prior

is small cxccpt

I<,

in patho--

logical casts. When

gauge implan~a(ion

and unknoun

~amplc(~) at-c mc;t-

P C Zalm: Secondary

ion mass spectrometry

concurrently, an accuracy of S-IO% in a real density scale can be obtained. The final concentration scale accuracy attained depends, in addition of course, on the quality of the absolute depth scale allocation but can be as good as l&15% overall. Ultimate precision (0.5-l% in relative depth and concentration scales) can be achieved for a series of unknowns when the calibration implantation is done in those samples themselvesZ4. A prerequisite is a suitable. rare. isotope which is not always available (e.g. not for P or As impurities); but whenever possible, in addition to the total fluence, the shape of the gauge implantation distribution can be used to establish a relative depth scale. As this shape must bc identical in all samples, one may merge them with help of a computer by minimizing the (relative) difference of any two profiles. The resulting correction to the depth scale of each individual sample is then fed back into the raw data for all other elements measured quasi-simultaneously with the gauge. This proccdurc is tedious but rewarding.

5. Commercially available equipment 5.1. Common features. In the following two subsections we will very briefly and inexactly discuss the two most widespread commercially available (dynamic) SIMS instruments. This choice does not imply any preference or customer advice whatsoever. Simply, these two offer nice conceptual contrasts that illustrate much of contemporary design ; but first we will treat some genera1 features, common to any good system. As this is not a chapter on engineering, a detailed presentation including ion optics etc., is carefully avoided. Any SIMS machine can be described as a combination of the following elements : (a) one (or more) primary ion source(s) ; (b) a primary beam selector (more precisely, a purifying energy and/or mass filter) ; (c) focus and deflection stage(s) ; (d) target chamber with loadlock and holder ; (e) a secondary ion energy discriminator; (f) the mass analyser ; (g) the detector (assembly) ; (h) data storage and manipulation facilities. The primary species used in SIMS are largely restricted to noble gas (mostly Art, because argon is cheapest whilst xenon has distractingly many isotopes and helium is too light [direct knock-on! ; cf. Subsection 2.21) 0: or 0 -, Cs+ and Ga+ ions, although occasionally results with N t and I& have been reported as well. Noble gas ion sources are user friendly because they do not age quickly. Many varieties exist (with specific features like high current or low energy spread), including the hot filament, electron impact ionization type. The much more reactive oxygen can only be used in sources operating at or near room temperature, employing gas discharges. The so-called duoplasmatron combines long life with good energy definition and allows for both 0: and O- extraction. Older caesium sources used evaporation and electron impact ionization of the elemental, highly reactive, metal. These work well but have to bc cleaned in an inert gas (Ar, NJ flooded glove-box and are sensitive to sudden vacuum breakdown failures. More modern ones are based on the thermal decomposition of Cs containing compounds (chromates), that are far less sensitive to air, moisture etc., a significant improvement. Gallium is mostly used in liquid metal ion sources which work on the principle of field extraction from the top of a

sharp needle wetted by the (liquified) metal. This point source allows for excellent focusability (beam diameters below 0.1 pm!) at high currents. Use of such sources is limited to very high lateral resolution applications, a mode not normally called for (see also Subsections 5.3 and 5.4.2). Also the high ion energy accompanying these virtues does not allow for simultaneous good depth resolution. Finally, it should be mentioned that the current drawn from any source is limited by the extraction voltage (according to the Child-Langmuir law I rc Y”‘) and that space charge blowup during beam transport too may limit large on-target currents at low impact energy, unless deceleration just in front of the sample is applied. The primary beam selector has to bc reasonably good when more than one ion source is coupled to the same beam line, so magnetic filtering is preferred. For a single source with a dominant, well-defined emission in terms of mass and energy spreadthis necessitates uncontaminated feeding material&an Ex B Wicn velocity filter of modest quality suffices. Excellent primary ion optics is a prerequisite for focusing the beam onto and rastering it over the target. As we will see in the next subsection, imaging SIMS depends crucially on the quality of this stage in certain instruments. Ultra-high vacuum in the target chamber is important in reducing mass interferences and contamination involving ad/absorbed residual gases. This necessitates a load lock to avoid venting during sample insertion and turbomolecular or cryopumps (ion-getter pumps arc prohibited owing to the need for occasional oxygen bleed-in). Also, differential pumping of the source section is required. The secondary ion energy analyser is important in reducing cross-talk/background contributions from reflected (neutralized) primary species, post-ionized secondaries desorbing or liberated from the walls of the system etc. It is usually of the electrostatic type. coarse but robust and reliable. It allows for positioning of the detector out of the direct line of sight of the sample. The mass analyser is either of the electric quadrupole or magnetic sector type, each with its particular advantages and drawbacks. The detector can be an electron multiplier or a Faraday cup whilst for imaging applications a channel plate with fluorescent screen and a (video) camera is also used. Multipliers saturate at high count rate (2 10h c s ‘) and their response degrades in time, although rejuvenation is possible. Faraday cups are insensitive and unreliable at low count rate but do not saturate at IO”’ c ss’. Finally, the data storage/manipulation facility may be as primitive as an oscilloscope or as sophisticated as a (big) computer. The latter is often utilized to control (part of) the instrument settings and for off-line data conversion, correction, display etc. 5.2. Quadrupole based instruments. The quadrupole mass analyser is a highly successful device for residual gas composition determination in uhv systems. It has also found widespread application in SIMS instruments. The principle is outlined in Figure 1 I. Residual gas atoms are ionized by electron impact and accelerated from thermal (- 0.03 eV) to some 5-10 eV energy, to be injected parallel to the rods into the quadrupole electric field region. Opposite cylinders are equally powered by a dc (V,) and ac (V, cos cot) potential, but the voltages change sign for adjacent rods. By solving the equation of motion, neglecting fringe fields, it follows that a stable oscillatory trajectory develops for a certain mass (over charge ratio) region only and provided V, > 6 I’,. The mass scales linearly with V, and ideally the mass resolution is approximately given by M/AMcr (1 - 6 I’,,/ V,) ‘. Transmission 767

P C Zalm

Secondary

ion mass spectrometry

M"

b

g

Vact

vt

energy

target

ctficicncy and rclativc portional.

Other

mass

resolution

important

are almost

parameters

arc

the

inversel)

pro-

CtTcctivc

ticld

radius r,,. or rather the distance travcllcd by the ion within ol‘ I’(,. and the rod length L (mass .M II

rf cycle in units whilst

practical resolution

design by varying

scan over the transmitted of a detector

AU x

the voltages

behind

,

within

the tolcrablc recording

a

limit‘;

of the mxs

problems

associated

with

the use ol’ quadrupolcs

In

the broad

energy distributions

especially towards

the high-6

of sputter-ejected

tail, detcrloratc

and atTccctline shape significantly

mass resolution

ions may be scattcrcd on

the rods and enter the detector or cause additional of particles

ions.

:

(ii) retlcctcd/backscattcrcd primary desorption

that can also contribute

sputtering,’

to a spurious

or background signal.

stage of Figure quite narrow

I I) is csscntial.

but as this

has to provide :I

pass window it does advcrscly atfcct transmission ovcr\icw

An exccllcnt

of the design aspects. when

practically implementing quadrupolcs in SIMS by Wittmaack“.

Figure

Here the secondary

analyscrs. is gi\,cn

13 shows his design for the secondary

ion detection stage for the Atomika

Dynamic In-Depth

ions arc acccleratcd towards

Analyscr.

the cntrancc

aperture of a parallel plate capacitor energy analyscr. entering the quadrupolc, field

Prior

to

the secondary ions arc retarded in the

lens. By scanning the target bias. keeping

or an immersion

all other potentials fixed. cncrgy spectra can bc rccordcd with :I resolution

of about

Other important

I cV at best. features of the Atomika

(i) A carousel Ibr storage

of up to

samples which rcduccs transfer

2

Deflection

I2

instrument (in earlier

operations

of the velocity-analysed

elf the target normal, just

arc

:

versions

depth at ;I given energy. So tilted ;I sample tilting

rclcxcs.

(iii)

sample holder5 and. in Iatcl

stage. have been added to regain thcxc

Ah the targcl ih c\trcmclq acccssiblc ir is eah! to build in

gun.

fol- charge-compensation

analyscd. Also.

The near-perpendicular

;I limit

materials

al-c

to sample si/.c. a virtus routine

monitoring

xi/c Si

of intefralcd

circui[

procc\h 5tcp5 on full

\I afcrh! onI> day lo obtain laterally

The

rcsolvcd information.

comes under the n;imc ‘imaging’.

with

which

;L quads-upolc

basal

in\(rumcnt ih b> making uxc of the focus in combination with the scanning. By recording the signal intensity

for a given species. as

;I function

beam in the rastcrcd

of the position

arca on the target.

of the primary

a mapping

can be made. This

location is dctincd \ia the potential (ramp) dcllcction

plates in the primary

trcsolution

is limited

column.

Of c’oursc ~hc attainable

by the focusability

\vhich is best at high cncrgq an&or

instantancou\

[cd to the .\- and .I’-

ol‘thc

primary

lo\+ current.

ion hcam.

For the standard

ion sources i-csolution is 1ypically limited cfYccti\clq to some IO /cm or thcroabouts,

i.c. already mcdiocl-c to awful for stale-of-

the-art intugratcd-circtlit mcntioncd intcnsc

liquid

technology dimensions.

metal ion sources.

kvcll-lhcuscd

beam.

the sub-micron

as sketched

trudc-oEf

which

nxlm

in Subsection

in signal

intensity

With the aforc-

cnablc

use 01‘ an

is opcncd.

3.9. pertaining preservation,

01‘

to the

inilucncc

of such sources adccrsely.

the applicability

and thus facilitates 5.3.

primary

(re-)neutral(-izcd)

species. ovcl component.

incidcncc helps to build up a high surface

concentration of, e.g., oxygen during 0:

L\hen insulating

thcrc is hardly

rhal has Icd to application in semi- and fL11ly-;lLltBmiltL’d

Iatcral,dcpth 5)

in front of the sample which scp-

arates the ions from a possible

bombardment and is

bcncficial from a point of view of retarding surface topography 76%

only partly otisct the drawback\

ad\antagcs

IoMcr erosion rate (see c.g. Figure 2) and a larger pcnctration

course arguments

maintenance of uhv during operation. (ii)

;I

usually

So ;I secondary ion cncrgy analyrcr (which replaces the ionization

clficicncy.

of

other surface analytical techniques. as well as an electron flood

SIMS ;lre : (i)

dc\clopmcnt. These

lust options.

spectrum. Obvious

immersion lens (QMS housing)

one

mass can bc made and the rcsponsc

the dcvicc cnnblcs

drift space

filter

I ‘,,,:co’r:.

So for a glvcn

E,,,,,,,,,,,,~f,,‘L’).

IL;,

VO

(Double

focusing)

magnetic

sector

field

B pcrpcndicular

circular

trajectory

to the direction with

a

(p, = c/vB) and the centrifugal I’ = r>zv,c/B. For

;I fixed velocity

clearly

mass

enables

v enters

of motion I’ such

radius

When

instruments.

particle \vith mass 1~2.charge q and velocity

it will

that

;I

;I magnchc

the

t’ollo\k ;I

Lorent/

(I-; = .%Iv’:r) forces balance, i.c.

separation

or kinetic cncrpy in

this

relation

a magnetic sector

ticld

assembly of fixed dimensions by varying the magnetic induction. Unfortunately, sputtered ions have a considerable energy spread which would deteriorate mass resolution. In a series arrangement of an electrostatic and a magnetic sector field, however, the energy dispersion of the former can exactly compensate that of the latter, so that such a combination will in first order only have the mass-dispersive properties of the magnetic sector. Here the electrostatic sector analyser is of the toroidal type (i.e. two plates, curved in the plane of deflection as well as often perpendicular to it, to which a potential difference is applied ; for a so-called cylindrical or spherical condenser the two plates are segments of concentric cylinders or spheres, respectively, but other possibilities exist; the design is such that usually deflection over 90 is effectuated). Here a mass m, charge q, particle follows a circular trajectory on which the centrifugal and electric field (EF = qA V/AR, with A V the potential difference across the plates and AR their separation) forces balance. By careful choice of the design parameters (radii, voltages, etc.) the resulting relation can be made such as to achieve energy dispersion compensation. We will not elaborate on the rather complex ion optics involved but rather discuss the practical implementation, depicted in Figure 13, as part of a whole Cameca IMS 3/4f instrument for SIMS analysis. For ion-optical details the interested reader is referred to a rather intractable paper (in French) by its inventor Slodzian” One immediately recognises the standard (optional) dual caesiurn/oxygen ion sources with (magnetic sector) beam selector and focus lenses/deflection plates in the primary column. Uncommon is the target assembly, detailed (not to scale) in the inset of Figure 13, where the sample is made part of an electrostatic parallel plate primary beam deflector/decelerator which also serves as a

~ :

projection lenses & deflectors

n

lock

chamber

4.5 kV

Figure 13. Principle of the Cameca ims 3/4/5f secondary ion microscope/ spectrometer. Ions from either a Cs+ or a duoplasmatron ion source (DIS) pass, after extraction, a primary selection magnet (PSM) and are focused and rastered onto the sample. The secondary ions are accelerated away from the 4.5 kV biased target towards the grounded immersion lens which is part of the secondary ion transfer optics to enter the electrostatic sector analyser (ESA) through an entrance slit (S,,). The ESA is powered such that ions with the nominal energy of 4.5 keV are deflected over 90.. After passing the energy slit (S,), which defines the transmitted part of the ejected kinetic energy spectrum, the secondaries are focused into the magnetic sector analyser (MSA). Mass resolution is defined by the exit slit (S,,,) width. Finally deflection and projection into the detectors, either a photomultiplier (PM) or a Faraday cup (FC), or onto a channel plate fluorescent screen (C/FS) takes place. The insert shows, not to scale, the secondary ion extraction/primary beam retardation/target holder assembly for the particular combination of positive primary and secondary ions.

secondary ion extractor. The target potential is high (nominally k 4.5 keV, depending on the sign of the charge of the secondary ions to be analysed) and the opposite electrode (the emission accelerator or immersion lens) is grounded. As the primary beam comes in at a nominal off-normal angle of 9, = 30” and will be decelerated or accelerated towards the target, depending on whether positive or negative primary ions are used and what charge type of secondary ions will be analysed, the actual impact energy E, and angle of incidence 9, are simultaneously affected. Solving the equation of motion, the primary ion impinges with _t 4.5 keV, as all practical primary species an energy E1 = EextrautlOn used are single charged only, and an angle give as sin !& = sin 9,. (f&,/E,) ’ = (E,,,,/4E,)“‘. This implies that at lower E,, e.g. for an 0: beam, and looking at positive secondaries, 9, becomes progressively more glancing until (nominally at 1.5 keV, but practically slightly lower) the beam no longer reaches the target. The advantage of this approach is a rapid improvement in depth resolution by lowering the energy, but clear disadvantages are a loss of primary species incorporation and hence sensitivity and a deterioration of on-target focusability and sputter crater definition. In the plane spanned by the incoming beam direction and the target surface normal, the etched pit becomes very rounded at low E, while it remains more rectangular in shape in the other lateral direction. This gives problems in analysed area allocation, depth determination and, eventually, depth resolution. Yet on brittle targets, like GaAs. at shallow depth (go.5 pm) results as good as or superior to the best resolution attainable in an Atomika can be obtained and only at larger depths (> 1.O pm) do the disadvantages of grazing incidence low-energy beams become fully apparent. The sacrifice in convenience, and occasionally in performance, is more than paid back by the two extra possibilities a high extraction field offers. First of all the secondary ion collection efficiency is greatly enhanced, so sensitivity is relatively high. Another highly interesting feature is that the immersion lens is made part of a secondary ion optical transfer system that preserves positional identity. That is, throughout the instrument the secondary ions follow a path to the detector that uniquely depends on their origin of ejection. So the Cameca IMS 3/4f can be used as an ion microscope which projects the analysed area onto a channel plate/fluorescent screen detector in a magnified image. The quality of the image (magnification, resolution, brightness etc) depend on the abberations in the ion optical system but in no way on the primary beam focus. Lateral rcsolution down to 1 pm can be obtained without great difficulty and, even with a poorly focused beam, a well-defined analysed area is guaranteed. Returning to the description of the Cameca IMS 3/4f, following the transfer optics the secondary ions are energy analysed by a spherical 90 toroidal condenser. This is designed such that initial separated rays stemming from different parts of the target irrespective of mass or ejection angle are focused onto the same cross-over point, save for a dispersion in energy. Nominally, one axis corresponds to an energy of 4.5 keV whilst higher (lower) kinetic energies follow a less (more) curved trajectory. This has two advantages, namely the suppression of energetic (reflected/ backscattered) primary ions, as well as the possibility to select an energy window (nominally &-130 eV wide and one of the reasons of the high transmission) on the secondary ions kinetic energy emission distribution by mechanically operable slits. By changing its lateral position a lower (or upper) limit can be set on transferable energies, whilst by closing them the width (energy 769

P C Zalm

Secondary

spread)

eon mass spectrometry

is rcduccd.

ion optical Finally.

Following

system

link

this spectrometer

produces

on the plane of the mass The

C’ameca IMS

instrument.

Its

fications

in the VG

(which some

when

SIMS

philosophy

has

adapted ment

type

Further,

parallel

on systems

most

with

analytical

recent

When

VG

type ol

one or the laltcl

ofl’er (scm-)

add-

Instruments).

is the restriction

tricks

like

oxygen bleed-in or cuesium

yield

(defined

divided

as Ihe

by the total

logical

casts,

dominantly

particles

removed

in

the ground

and matrix ctficicnlly in this

direction.

positive

ions

but only neutral

mcnt-based

clcctron

but a very much gas discharge load-lock

into

hombardmcnt

by ions

which

typically

from

;I separate the plasma

During

ioni&.

stage.

less matrix high

Ihis

Mass

advantages

SIMS.

owing

for pas-phase able side suppressed

pnssagc.

scheme

via

the

arc offset

then two

or to Ihat

cjcctcd

spccics

in the oppobice

of them

bccomc

in a quad-

Ihllows

advantages.

namcl>

and the possibility

lo\Y-bombarding-cncrgq sensilivity

comparctl

[or cmittcd

spccics (i.e. larger

matrix-effccls

a larger with

dwell times

their

arc low, especially SNMS

to

particles

cross-section

arc definitely

intensities.

01‘

option.

unlivournot

totally

Ibr impurities systems

have

for a few years now and have so far failed to

make a lasting

impression.

to date cannot

compete may

The

fraction

collisions

been on the market

770

has

by a low

ranges

gas dizcharsc

kcV).

an aperture

lrajcctory-altering

and dynamic

They

gun.

also implies

Also

;I lo

argon)

which

ct’iecls).

through

cfliciencics

for low velocily

plasma.

lila-

is employed.

inserted

acccptancc angle

because of the high background

strong.

ion

a (large)

dctcction

as in con-

it can bc sub.jectcd

( - 0.3

(and dctcction)

whole

way

Occasionally

is

Hcrc (moslly

energy)

to the small

the prel‘crcncc

in the ionizing

the

the neutral

is to USC‘ ;I high-frcqucncy

sample

to cscapc through

resolution

These

and

(higher

dependent

depth

the same

option

from

atlcmpth

Ihe resulting

and

take

Apparently

the instruments

in the areas where SIMS the advantage

over

SIMS

54.2. through

(modcratcly)

(SNMS) impact

present,

ioniration.

to ;I virtual rcquircd.

but

il. This

Employing

atoms

i\

tun-

clcmcnl

c.jcctcd in ;I pa--

state).

ionir.ation

state but this

(~1‘;~second identical

arc

and :I

slate and ;I

ionizes

detect complctclq

in the ground

mulliphoron

lhc

first

is immcdiatcly

photon followed

one which Icad\ to ionization. then

both

approaches

the

arc still

or not commercialization

Time-of-flight

secondaries

here.

have low energies

selection The

have proven

of the neutrals

chamber.

01‘

are used

has led to many

At

and whether

can be cnhanccd

these neutrals

cluadrupolcs).

bombardment

a plasma

ilsclf.

and mosll>

uhcn

lo il mctasluhlc

bctwccn

stale and those

photons

proces\

can

bc \cr!

in the l;tboratory

slagc

will ever occur is doubtful.

bc

as neutrals

sensitivity

The

may

surface

in much

more efficient

lypc

majority

spcctromctry

environment.

the useful

that

fluxes

resonanl

most

Ncl : YACi

:

~ubsequcntl~

one C;LII in principle

non-rcxonant

atoms

the atom

frcclucncy

or

cllicicnt

a few patho-

an oxide

trcatmcnt

with

Ihr

cxcilcs

of3JO

ionization

makes

c’xclmcr cncrgetic

namely

c\cn dialinguish

cxcitcd

H~igc

shre\\d

of a certain

Except

by electron

(mostly

with

the (baa0

notion

mass

titular

howc\er.

with

reasonably

exist.

multiphoton

(and

b> absorption

slate.

two schemes

are analyscd

SIMS

(ravcrsc

This

ions

from

principle

arc post-ioni&

vcntional

rupole

So

and deser\c ;I brief

sputtered

low.

ions

Ica\,c the target

in the analysis.

sccondarics

wall.

remains

metal

Even

ion bombardment,

in the cxcitcd

state.

ions.

of ejected

effects can be rcduccd

succcs~ful In

yield)

e.g. like

cmitlcd

to sputtered

number

lhc so-called

cvcitcs limitations

both

for elcclron is ~~suallq no1

is belo\\ mos;t

absorption.

schcmcs

photon

photon

(even ;I \vavelcn$h

which

ionization

second one of diflercnt

(17) In

the fundamental

provide

Two

able dye lascrb.

5.4.

01‘ SIMS

which

specific

5 eV.

to

thcrc is reluctance

1s an altcrnativc

01‘ a single

too IOM cncrpy

to only

Mulliphoton

(a) A lirhl

01‘ instr.Li-

photons

Absorption

accessible

reigns

power.

with

because ol’thc

dense flux.

based microprobes

companics

enough.

laser\.

com-

it less ucll

this

methods.

clcments

improvements

to combine

quadruple

Other approaches. 54.1. Detection of neutrals. One of

its established

Augcl-

range but while

depth profiling

ionization

thresholds.

103OS

The

makes

automation

1Hcilitics.

Several

uscl‘~d.

variety).

tens of percent concentration

nm corresponds

that circum\cnts

howcvcr. (full)

of that type (c.g. Ribcr.

arc modi-

and the Kralos

although

is deemed necessary.

arc probably

quests

plate

impac(

llcxihlc

analytical

assembly

toward

to

Spectroscopic

to surrender

image

been copied

it is not possible

with other

Iwo options

( 3 0.1%) Electron In-llight

70s

system.

tasks.

hccn directed

settings.

cquipmcnt

of Ihe

ion optical

to routine

have largely

IX

largclcxtractor

ol‘ the problems

plicatcd secondary

;I ~LIIIIIC~

analyscr.

slits.

Instruments

has a difrcrcnt

stage

mass

a very compact and cxtrcmcly

suited

design

prism

a real (mass-dispersed)

selection

34f’is

idcally

divcrsificd.

the energy sclcction

is to the magnetic

available

is traditionally

in the sub-

problems

static SIMS.

01‘ nominally

the dispersion have long

in flight stood

The

idea of separation

the same energy

but diflcrent

01

masses

time is already quite old. Practical

in the \\a> o1‘this

realization.

but in

P C Zalm:

Secondary

ion mass spectrometry

last five years instruments have been built that perform beyond even the boldest imagination”. The field of static SIMS has benefited from these developments. A very nice example of a Time-of-Flight (TOF) SIMS instrument is schematically reproduced in Figure 14, but other solutions exist and have

the

broadly the same basic features. The reason uniformity in design ties in with the underlying and restrictions, which include : (i) extraction

or post-acceleration

for this (relative) physical demands

of the liberated

ions to some

cJ% I (-

~=23

Si -0-)” !

CH3

1.5

1.0

0.5

0.0 3000

4000

5000 mass

[emu]

cJ%

I

n=23:

Ag’ + (CH,),SiO-(-

Si -

0 -)23-Si(CH,),

I

n=23: Theor.

“TOSS [amu]

moss[ml”]

Figure 15. Positive ion mass spectrum of a monolayer of polydimethylsiloxane adsorbed onto an Ag film evaporated onto Si. Below the part around M/e = 1971 is expanded to show the agreement between observed (Exp.) and calculated (Theor.) isotope ratio for the peaks due to the particular neutral polymer molecules with n = 23 clustered to Ag’ ions. Data courtesy of Dr H van der Wel, Phillips Research Laboratory, Eindhoven, The Netherlands. 771

P C Za/ln.

Secondary

2 3 IicV in order ion optical

(ii) ;I

to inipro\#c collection

tlisht

path

Icngth

comhin;ltion

oi

tinic

(ii)

implies

and

the tinic

ircsolution

niLis

~11 hc ol‘thc

14. the pi-imar> parallel-plate

3

\\ IliCll

\clocit!

hunchcr ions

which

to their

position.

thus

he

secondary

high

thq

c,\+In~

final

temporal

to

al-c

corrcctcd

less

cwnbincti

with

cncq!

) li)cuh~n,

anti

b! 211

Icns l\lr bcani trxnspot-t

niii-i-w

01‘2 30 mm

consisting

gap dcfincd

by grids

01‘ cW,II

I77

ctfwti\c

tlight

Total

cnct

01‘ their journe!

to yx)unci

I~> ;I

rrcgistr;ition

scintillator

;ind

total

range

and

accumul;lting

arc‘

an111 can

Other

iii

the

the

routinely

approaches.

xcondar~

primary

dcpictcd I\\0

hnom

(although

high

in

clLlstclui

p~-~xx~sc~. printing‘

lllal

pertain Ci0od

772

TrLle

~‘loch

ixx~lution Including

l’or iii;15scs

01’ \I ~ 50

ion

and

clLliLiitilicatioii same and.

Ilight

on

n101-c

;I silicon

pat-cnt

peak\

tbc \~ould

(hi\

I’riirly

fur awa)

nxitLircct.

e.g.

l‘rclni

pol~n~~~ delidc\rro\

the

impact

by phonon-assi\tccl

complicated

but

1.1.agmclltatioii

i5 ;I LISCI‘LII cvcn

I’hc

suh~lralc.

lxd~~~~cr- niolcct~lc~ ;Lbotlt

molecule)

III the to \cr!

bccaL~sc

occasionally.

01‘

nwdcd

cxtcnciin?

fraymcnt\

typical

one

Ill;Issc\ arc

01‘ SLICII I:II-gc

i\ very or

iii

the

b\ irs invc‘n-

time*.

inl’ormatlon

imp;Lct

has I‘LIII~

than

lllC c\xct

hy litting

cjcction

niagnct

inli-riol-

L~LIc to coniplctc g\c

sector ;unalyscr

bLincliing

oIlI>

adsorbatc ;II-c

rccognilion

to the

boat-cl.

time

as l‘ull \+idth

I5 sho\\s ;I spcxtrLim

The

qualitative

.TIIc \\ith ;I

wn~crlcr

commcrclall/cd

a>

is improved

;I polymc~-

cascade

fi.c.

IO

nieasurcd

;l

al \lioi-t

thcb arc cniittcd

llic

the

i\ cotiplcd

twpwti\cl!.

(slightI>)

is no\\

hc due to direct

Kathcr.

and

IS siniplc

spectrum

cannot al’tcr

arc

Figur-c

distribution,

them.

/one

this

~‘1

:\t

combination.

an clcctrostatic

palh

prcl*crahly

to All r\g

i+cipht nitcl!

Ihr

IU;ISK~

/lx. 6000

include

14. \\hich

calibration

trcfion).

IhO

:WA:2;1.

Ilighl

the precision

IL)\\ m;Iss

pcah\

which

ii pcahs.

and tlic signal

memory

ol’-WO

scclion\.

in Figure M\/l;is\

tars.

angle

hc ohtalncd.

ions’ bwni

;I

rcllccliny

I\ 2 ni

ion pulse and

rc’solutions

in cxcc55

al hall‘ maximum.

200

;I 60 nini

: the rcllection

lime-to-di$tal

I .25 ns anti

mass

the Izoi-

I~lllS I\ lrcall/ed

I~hotomultiplicl.

is with the pi-iniary

all uncertaintit‘s.

at ;I target

ion\ ;It-c posl-accclcl’alcci

ol‘;i

an

the

according

L\ith ;I t\\o-stage

gap :Lncl

plate

consisls

fl~st bulYcr nxmor~ ~4nchi-oni/ation

;1\i;11

\+hcn

isochronou5

conibincd

the wcontiai-)

syslcm

;iii

to 3 hcV.

in the rcflcction

and dctccted by ;I channel

IO I\cV

I‘I-oiii

I~!

c,jcction.

(’ 1% Il~cL!ss;II‘\.

path

;111d L!\ll

cncrgies

t;ollow,ing

transparenq

;I

clip\

sLiKcr5

i\ lkcl

tranwiission.

retarding

)

\a!-! in?

p2tl1

in arriving

I no. ;I l‘ull

Thi\

[or

pulse sprd

than

:\II\ onI\

01’ ion5

different

(i.c. annular

Fin/cl

organic

contaminants

quantitative

‘lingcl-paltcrn~

aid

results

of surfaces.

nmnolnycrs finds

surllcc

modification

that

the instrument

im~~glng-SIMS-l;lcilit!. cnvisagcci.

ol‘thc

at \lightl)

arc cxtractcd’post-~~ccclcr~ited

resolution

on

TOF-SIMS

the

u Iiich

to ctlil’t

hunch

’ cL 4n

width.

( - IO kc\’

\rldth.

the

high-\OIt;yc

so that

The

ions

Illa\\

which

I-cduccd

bum wlcctoi-.

pulw

stage

;I second

arc at its ccntrc.

c;iii

that

dif~wmx~

to

ion

bcani that will.

on

At

01‘

orJe~- 01‘ I n\!

cncrg!

I jl\ high-voltxgc dcp”ld

uw

01‘ the ordct-

o,f the clctcctoi-

hc transniittcd.

;i\i;il

times

~LIISC

during

exit an&!lcs

flight

primary

in ;I 90

li>l-

cn\ironmcnt.

bunch ol‘( s IO’) ion7 out clflhc

dit.

to cnahlo

the

In the c;isi‘ 01‘l‘isLLrc

po~~c‘rcd

;I few mctcrs

or only

(i) and

clcctron~ca

i\ dcllcctcti

illlow

jts. C‘onsequcntly.

characteristics

ircgi5tratiun

anti

self-assembled

111 ;I lahorator~

01‘ . \ ‘Zl[amu]

cticicncv

:

manipulation

the insti-unicnt

‘Tbc

Ion mass spectrometry

etc..

increasingly studies. 01‘ Figure

c\cn

LangmuirmHlodgctt

haw

ken

successful As it has

application wry

I4 could

films.

rcportcd rccentlq

and.

been shown

bc transf’ormcd

more widcsprcad

also.

in chemical

application

”

into

;~n

can

bc