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