Towa ds nanome ic esolu ion in mul ilaye dep h p o iling:
a compa a i e s udy o RBS, SIMS, XPS and GDOES
Ramón Escoba Galindo & Raul Gago & Da id Duday &
Ca los Palacio
Abs ac An inc easing amoun o e o is cu en ly being
di ec ed owa ds he de elopmen o new unc ionalized
nanos uc u ed ma e ials (i.e., mul ilaye s and nanocompo-
si es). Using an app op ia e combina ion o composi ion
and mic os uc u e, i is possible o op imize and ailo he
inal p ope ies o he ma e ial o i s inal applica ion. The
analy ical cha ac e iza ion o hese new complex nano-
s uc u es equi es high- esolu ion analy ical echniques
ha a e able o p o ide in o ma ion abou su ace and
dep h composi ion a he nanome ic le el. In his wo k, we
compa a i ely e iew he s a e o he a in ou di e en
dep h-p o iling cha ac e iza ion echniques: Ru he o d
backsca e ing spec oscopy (RBS), seconda y ion mass
spec ome y (SIMS), X- ay pho oelec on spec oscopy
(XPS) and glow discha ge op ical emission spec oscopy
(GDOES). In addi ion, we p edic u u e ends in hese
echniques ega ding imp o emen s in hei dep h esolu-
ions. Subnanome ic esolu ion can now be achie ed in
RBS using magne ic spec ome y sys ems. In SIMS, he
use o o a ing sample holde s and oxygen looding du ing
analysis as well as he op imiza ion o loa ing low-ene gy
ion guns o lowe he impac ene gy o he p ima y ions
imp o es he dep h esolu ion o he echnique. Angle-
esol ed XPS p o ides a e y powe ul and nondes uc i e
echnique o ob aining dep h p o iling and chemical
in o ma ion wi hin he ange o a ew monolaye s. Finally,
he applica ion o ma hema ical ools (decon olu ion
algo i hms and a dep h-p o iling model), pulsed sou ces
and su ace plasma cleaning p ocedu es is expec ed o
g ea ly imp o e GDOES dep h esolu ion.
Keywo ds Dep h p o iling .XPS .SIMS .GDOES .RBS .
Mul ilaye .High esolu ion
In oduc ion
The cu en oad o minia u iza ion (i.e., sub-32 nm
echnologies) demands no el o ad anced analy ical ech-
niques wi h ex emely high dep h esolu ions ha a e
capable o cha ac e izing sample ea u es on he nanoscale
[1–3]. In pa icula , ul a hin laye s and mul ilaye sys ems
a e gaining inc easing in e es because hey can exhibi
no el s uc u al, physical and chemical p ope ies ha di e
om hose o he co esponding bulk ma e ials. Wi h he
dec easing hicknesses o such laye s, ilm cha ac e iza ion
wi h dep h esolu ions a he scale o a ew in e a omic
dis ances mus be achie ed. Fo a single in e ace be ween
wo laye s A and B, he mos common de ini ion o he
dep h esolu ion as ecommended by IUPAC and ASTM
E-42 is Δz, which co esponds o he in e al o he dep h
R. Escoba Galindo
Cen o de Mic o-Análisis de Ma e iales,
Uni e sidad Au ónoma de Mad id,
28049 Mad id, Spain
R. Escoba Galindo (*):R. Gago
Ins i u o de Ciencia de Ma e iales de Mad id,
Consejo Supe io de In es igaciones Cien í icas,
28049 Mad id, Spain
e-mail: [email p o ec ed]
D. Duday
Dépa emen de Science e Analyse des Ma é iaux,
Cen e de Reche che Public Gab iel Lippmann,
41 ue du B ill,
Bel aux 4422, Luxembou g
C. Palacio
Depa amen o de Física Aplicada,
Uni e sidad Au ónoma de Mad id,
Can oblanco 28049 Mad id, Spain
coo dina e za an in e ace o e which he e is a 84% o
16% change in in ensi y [3].
Nowadays, he chemical analysis o hese e y hin ilms
is ca ied ou using ei he des uc i e o nondes uc i e
me hods. Ac ually, o hin ilms abo e 10 nm in hickness,
he me hod mos equen ly applied o ob ain he concen-
a ion dep h p o ile (CDP)— ha is, he composi ion o he
hin ilm as a unc ion o dep h—is ion spu e ing in
combina ion wi h any o he well-known su ace analysis
echniques [4,5]. Among hese, X- ay pho oelec on
spec oscopy (XPS) [6,7], seconda y ion mass spec oscopy
(SIMS) [8,9], and, mo e ecen ly, glow-discha ge op ical
emission spec oscopy (GDOES) [10,11] a e becoming he
mos popula . In addi ion, Ru he o d backsca e ing spec-
oscopy (RBS) is a nondes uc i e echnique ha is
commonly employed o in-dep h composi ional analysis
[12,13]. The me hod ha should be used depends
speci ically on he ac ual analy ical p oblem o be sol ed,
al hough he gene al end is owa ds he simul aneous use o
se e al echniques due o he complemen a y in o ma ion
p o ided. The p incipal mul ilaye sys ems (i.e., ni ide, oxide,
me al, e c.) ha e been ex ensi ely s udied by a combined
analyses, such as XPS-RBS [14–16], RBS-GDOES [17,18],
GDOES-SIMS [19,20], SIMS-RBS [21,22], XPS-GDOES
[23], XPS-SIMS [24,25], XPS-RBS-GDOES [26]o RBS-
SIMS-GDOES [27].
In his pape we will e iew he cu en s a us o each o
hese ou cha ac e iza ion echniques, as well as he mos
ecen de elopmen s ega ding imp o emen s in he dep h
esolu ions o hese echniques. Rele an examples o hei
applica ion o nanome e -scale mul ilaye analysis will be
p esen ed.
X- ay pho oelec on spec oscopy (XPS)
Among he ins umen al echniques used o he chemical
analysis o solids, X- ay pho oelec on spec oscopy (XPS)
is one o he mos app op ia e o examining su aces. I is
based on Eins ein’s quan um in e p e a ion o he pho o-
elec ic e ec . This in e p e a ion assumes ha he ene gy
o he emi ed pho oelec ons can be calcula ed as he
di e ence be ween he inciden pho on ene gy and he binding
ene gy (BE) o he elec ons in he a oms o he sample. The
mode n XPS echnique was o igina ed by Siegbahn and
cowo ke s a Uppsala Uni e si y in Sweden in he mid-1950s.
They de eloped a high- esolu ion spec ome e o de e min-
ing he binding ene gies o he a omic shells o he less igh ly
bound o bi als. As a esul o his p og am i was disco e ed
ha he BEs o he co e elec ons we e dependen on he
chemical en i onmen ; ha is, he chemical shi s in XPS
e lec changes in chemical bonding o oxida ion s a e and can
he e o e be used o ollow such changes on su aces. XPS is
sensi i e (0.01 monolaye s o mos ma e ials p oduce a
de ec able signal), quan i a i e, and signals can be measu ed
o all elemen s excep H and He. S ic ly speaking, XPS is a
su ace echnique, bu i can also be used o analyze a laye o
adso bed ma e ial in o de o see bo h he su ace and he
adso ba e, since only he elec ons c ea ed in a zone
cha ac e ized by a dep h lcosθ(called he escape dep h)
om he su ace escape o con ibu e o he XPS spec um.
He e lis he a enua ion leng h o he emi ed pho oelec on
and θis he angle o emission wi h espec o he su ace
no mal. Values o llie in he ange 0.5−2.5 nm, and his
phenomenon makes XPS a su ace-sensi i e echnique, as
s a ed be o e.
Nowadays, when employed o dep h-p o iling analysis,
XPS is mos equen ly used in combina ion wi h ion
spu e ing [28,29]. An example o dep h p o iling wi h
XPS is depic ed in Fig. 1. The ul ima e dep h esolu ion
depends on ins umen al ac o s, he induced su ace
oughness, a omic mixing, enhanced induced di usion
and he in o ma ion dep h o he su ace analysis echnique
used. Howe e , applying some e inemen s can lead o
eliable esul s, wi h dep h esolu ions as good as 1–2nm
[30–32]. Expe imen al CDPs a e usually ex ac ed om he
in ensi y I
i
( ) ha is cha ac e is ic o a sample componen i
as a unc ion o he e osion ime . Howe e , he desi ed
esul is he concen a ion c
i
(z) o each componen as a
unc ion o he dep h z. The p oblem is hen he
quan i ica ion o he expe imen al in o ma ion o ge he
desi ed in o ma ion. This quan i a i e e alua ion o measu ed
p o iles in ol es he calib a ion o he hickness scale ( ha is,
he con e sion o he spu e ing ime in o he spu e ed dep h)
and he calib a ion o he concen a ion scale ( ha is, he
con e sion o he elemen al signal in ensi y in o he elemen al
concen a ion, and he co ec ion/decon olu ion o he e ec s
ha modi y he dep h esolu ion). The p ocedu e used o
pe o m he quan i a i e analysis is shown schema ically in
Fig. 2.
In mos cases, he ime →dep h and in ensi y →
concen a ion ela ions a e p ac ically linea and can easily
be ob ained om app op ia e spu e ing a es and elemen al
sensi i i y ac o s, espec i ely [30,31]. E ec s ha modi y
he dep h esolu ion can be co ec ed o using he so-called
esolu ion unc ion g(z′−z) along wi h Eq. 1(below) in
o de o ob ain he eal concen a ion c
i
(z) using he
app op ia e decon olu ion p ocedu es [33,34]:
c0
iz0
ðÞ¼
Z
1
1
ciðzÞgz
0zðÞdz ð1Þ
The esolu ion unc ion can be modeled ei he expe i-
men ally o ia heo e ical models ha ake in o accoun all
o he abo emen ioned ac o s ha modi y he dep h
esolu ion. In ac , o he in o ma ion dep h o he su ace
2726 R. Escoba Galindo e al.
analysis echnique used, he esolu ion unc ion is p ope ly
ep esen ed by Eq. 2:
gzz0
ðÞ¼exp zz0
licos q
 ð2Þ
whe e l
i
cosθis he a enua ion leng h o he emi ed
pho oelec ons, and θis he ake-o angle wi h espec o
he su ace no mal [33].
Fo hin ilms wi h hicknesses below 10 nm, he
changes ha occu du ing ion bomba dmen be o e a s eady
s a e is es ablished du ing he spu e ing p ocess make his
app oach comple ely inaccu a e, and al e na i e me hods
mus be used. Such me hods, o example angle- esol ed
X- ay pho oelec on spec oscopy (ARXPS), a e nonde-
s uc i e [32,35,36]. Since chemical in o ma ion is e y
apidly des oyed by dep h-p o iling me hods ha in ol e
spu e ing, angle- esol ed me hods a e also ad an ageous
o p ese ing chemical s a e in o ma ion. Figu e 3shows a
schema ic diag am o he ARXPS expe imen al se -up. In
his igu e, ls ands o he a enua ion leng h o he emi ed
pho oelec ons. I spec a a e eco ded a di e en emission
angles θ, he pho oelec on escape dep h, lcosθ, can be
a ied in such a way ha he dep h p obed is a ied, so he
se o angula measu emen s will con ain in o ma ion on
he CDP o each species. The in ensi y o he emi ed
pho oelec ons co esponding o a pa icula XPS band o a
speci ic elemen is gi en by Eq. 3:
IiqðÞ¼ I0
i
licos q
Z
1
0
ciðxÞexp x
licos q

dx ð3Þ
whe e c
i
(x) is he concen a ion o elemen ia a dep h x,
assumed o be uni o m a he su ace, l
i
is he a enua ion
leng h o he pho oelec ons om his elemen , θis he
ake-o angle wi h espec o he su ace no mal, and I
i
0
is a
cons an ha depends on ins umen al ac o s, angula
asymme y and pho oioniza ion c oss-sec ion. Equa ion 3
is an in eg al equa ion o he i s kind and ep esen s a
p o o ype o ill-posed p oblems [35]. Two di e en
app oaches ha e been used o ex ac he concen a ion
dep h p o iles, c(x): simple pa ame ic models ha make
some assump ion abou he o m o he dep h p o ile (see
he bo om panel o Fig. 3), and gene al algo i hms ha use
egula iza ion in addi ion o singula alue decomposi ion
(SVD) echniques [35] wi hou any p io hypo hesis o he
shape o he concen a ion dep h p o ile. In gene al, hese
me hods display common limi a ions imposed by he low
I
i( ) i
c' (z')
calib a ion o he
hickness scale
calib a ion o he
concen a ion scale
ci(z)
decon olu ion o
bomba ding e ec s
Fig. 2 Scheme o he dep h-p o ile quan i ica ion p ocedu e
Fig. 1 XPS concen a ion–
dep h p o iling da a om a (le )
C (N)/C(DLC) nano-
mul ilaye ed coa ing ( aken om
[28] and ep oduced wi h he
pe mission o Else ie ) and
( igh ) a 15 nm Si/15 nm Al
mul ilaye ed sys em ( aken om
[29] and ep oduced wi h he
pe mission o Else ie )
A
S
020406080
IA/ I
S(a.u.)
ake-o an
g
le (0)
c(x)=
1 i 0 x/λ1
0 o he wise
Sample
No mal o he
su ace
Analyze
λ θ
cos
θ
λ
≥≥
Fig. 3 Top: schema ic diag am o he ARXPS expe imen al se -up.
Bo om: schema ic diag am o a s ep-like dep h p o ile o a pu e
subs a e S, co e ed by a laye A, o hickness d=land cons an
composi ion. The simula ed in ensi y a io I
a
/I
s
as a unc ion o he
ake-o angle is also shown ( igh -hand side). Taken om [35] and
ep oduced wi h he pe mission o Else ie
Towa ds nanome ic esolu ion in mul ilaye dep h p o iling: a compa a i e s udy o RBS, SIMS, XPS and GDOES 2727
in o ma ion con en o ARXPS measu emen s, wi h he
dep h esolu ion Δzlimi ed o abou 0.8z[32]. Al hough
his is a poo e ac ional dep h esolu ion han spu e
dep h p o iling allows in he dep h ange o ~10–500 nm, i
is be e han he spu e dep h p o iling in he nea -su ace
(0–5 nm) egion accessible o ARXPS. Anyway, he wo s
alue o he dep h esolu ion in his hickness in e al is
abou 4 nm, as can be obse ed in he le -hand panel o
Fig. 4. This igu e shows he dep h p o iles calcula ed using
he expe imen al da a collec ed o an ul a hin laye
p epa ed by he unde po en ial deposi ion (upd) o sil e
on polyc ys alline pla inum using he in e sion p ocedu e
desc ibed abo e [35]. The egula iza ion algo i hm p oduces
a easonable es ima e o he concen a ion dep h p o ile
whe e wo laye s can be obse ed: a su ace ca bon laye
~1.6 nm hick, ollowed by a complex laye con aining S, O,
and Ag wi h a hickness o ~1 nm. Figu e 4( igh -hand
panel) shows ARXPS esul s o a na u ally oxidized Al
laye [36]. Acco ding o he esul s shown in Fig. 3,an
inc ease in he signal in ensi y (o , equi alen ly, he
concen a ion) wi h inc easing ake-o angle indica es ha
he co esponding species is nea e o he su ace han
species ha display he opposi e beha io . The e o e,
ARXPS o Fig. 4( igh -hand panel) indica es ha C
con amina ion co e s he na u ally oxidized Al laye [36].
I is impo an o poin ou ha he dep h esolu ion in
ARXPS is limi ed by he signal- o-noise a io, no he
numbe o emission angles o which da a a e acqui ed.
Table 1compa es he main ea u es o ARXPS o hose o
s anda d XPS dep h p o iling.
Finally, i should be no ed ha dep h p obed by XPS can
be also a ied using a unable-ene gy exci a ion sou ce,
such as synch o on adia ion (SR), since he pho oelec on
escape dep h depends on he pho oelec on kine ic ene gy
[37]. XPS spec a om di e en dep hs can be ob ained by
a ying exci a ion ene gy. This echnique can p o ide he
dep h p o ile nondes uc i ely, jus like he ake-o angle
a ia ion echnique. Howe e , only a ew s udies ha e used
his app oach, p obably due o he need o a la ge-scale
ins alla ion o ca y ou he expe imen s and he ma hema ical
p oblems in ol ed in eco e ing he CDP; also, he ac ional
dep h esolu ion ha can be ob ained is simila o ha
achie ed wi h ARXPS measu emen s.
Seconda y ion mass spec ome y (SIMS)
Seconda y ion mass spec ome y is based upon spu e ing
a ew a omic laye s om he su ace o he sample h ough
“p ima y ion”bomba dmen . A p ima y ion impac igge s
a cascade o a omic collisions ha induce he emission o
a oms and a omic clus e s. Al hough mos o he a oms and
molecules emo ed om he sample by he in e ac ion o
he p ima y beam and he sample su ace a e neu al, some
pe cen age o hem a e ionized. These “seconda y ions”a e
cha ac e is ic o he composi ion o he analyzed a ea. I he
p ima y beam is composed o posi i ely cha ged ions, he
esul ing ioniza ion a o s he p oduc ion o nega i e ions,
and p ima y beams o nega i e ions a o he gene a ion o
posi i e ions. These ions a e hen accele a ed, sepa a ed
acco ding o hei mass, and analyzed by a mass spec ome e .
Finally, an image con aining quan i a i e in o ma ion is
ob ained o a selec ed mass. In [8], Benningho en e al. de ail
he basic concep s, ins umen al aspec s and applica ions o
SIMS.
Con en ional SIMS sys ems equipped wi h ion beam
dep h p o iling can achie e nanome e esolu ion a la ge
dep hs, oge he wi h a high le el o de ec ion and
quan i ica ion (below 1 ppm) [38,39]. Ne e heless, such
nanome ic dep h esolu ion is only ob ained when using
p ima y ion ene gies in he sub-keV ange [40]. Howe e ,
he ad an ages o lowe ing he impac ene gy a e comp o-
mised by no only signi ican su ace oughening bu also a
dec ease in spu e ing yield (p opo ional o he numbe o
emi ed/de ec ed pa icles). The dep h esolu ion is limi ed
o a high ion lux, since ene ge ic in e ac ions o he
0 2 4 6 8 10 12 14
0
10
20
30
40
50
60
70
80
Ag
S
O
C
c(x)[%]
dep h (nm)
Fig. 4 Le : ARXPS dep h p o iles calcula ed using he expe imen al
da a collec ed o all species de ec ed in an ul a hin laye o Ag deposi ed
elec ochemically on P using he gene al in e sion algo i hm desc ibed in
[35]. Righ : esul s o ARXPS in es iga ions o a na i e Al oxide sample
using a model o a C con amina ion/Al oxide/Al subs a e mul ilaye .
Taken om [36] and ep oduced wi h he pe mission o Else ie
2728 R. Escoba Galindo e al.
p ima y ion wi h he sample lead o p o ile dis o ions due
o he p ima y inco po a ion o he inciden p ojec iles and
he damage induced by he collision cascades. Di e en
app oaches a e now being de eloped wi h he aim o
achie ing he ul ima e dep h esolu ion. Vande o s ecen ly
e iewed hese di e en app oaches in o de o assess i he
SIMS me hod will be able o ul il he needs o indus ial
semiconduc o analysis [41].
A i s app oach (EXLE-SIMS) in ol es using ex emely
low bomba dmen ene gies as low as 100 eV. New p ima y
ion columns called loa ing low-ene gy ion guns (FLIG)
ha e been de eloped, as shown in Fig. 5, as a eplacemen
o classical ion columns, in o de o dec ease he ion beam
ene gy down o 100 eV while keeping ch oma ic abe a ions
a a low le el in he newes gene a ion o EXLE-SIMS
echniques [42]. Se e al eams ha e al eady achie ed a dep h
esolu ion o 1–1.2 nm by using p ima y ion ene gies o
100–550 eV [41,43,44]. This made i possible, o example,
o be e analyze in e aces in o ganic–ino ganic mul ilaye s
(as shown in he le -hand panel in Fig. 6) o polyme sola
cells [43]. Mo eo e , in [45], nanome e dep h esolu ion was
also ob ained o a 50×(16 monolaye s
28
Si/10 monolaye s
30
Si) iso opic supe la ice sys em (see he igh -hand panel in
Fig. 6) using a 1 keV Cs p ima y beam. The na u e o he
p ima y ions used has an impo an e ec on he dep h
esolu ion. Fo example, he esul s ob ained wi h cesium
(Cs) ions a e no as good as hose achie ed wi h oxygen (O)
ions, which gi e a much la ge decay leng h, e en o
Table 1 Compa ison o he main ea u es o he XPS and ARXPS con igu a ions
XPS + ion spu e ing ARXPS
P obing pa icle Pho ons Pho ons
De ec ed pa icle Elec ons Elec ons
Spu e ing Ion beam (noble gas) <10 keV No
A omic mixing? Yes No
Sampling dep h (nm) 1–10 0.3–3
Dep h esolu ion (nm) 1–2 nm (in he bes cases)
(wi h o a ing samples)
~2–4nm
(in he wo s cases)
La e al esolu ion (μm) ~5 ~5
Max. dep h analysis (nm) 100–1000 10
De ec ion limi 0.01 a om% 0.01 a om%
Elemen al ange All excep H, He All, excep H, He
Reliable quan i ica ion (elemen al composi ion)? Yes bu des oyed du ing spu e ing Yes
Des uc i e? Yes No
Chemis y in o ma ion? No, des oyed du ing spu e ing Yes
Vacuum (mba ) 10
−10
10
−10
Decon olu ion needed? Yes (di icul o de e mine
he esolu ion unc ion)
Yes (a enua ion leng h unc ion
as esolu ion unc ion)
Analysis ime Slow (hou s) Mode a e (minu es)
Fig. 5 Schema ic d awing (le ) and pho og aph ( igh ) o he FLIG ins alled on he spu e deposi ion chambe o he p o o ype S o ing Ma e
ins umen . Taken om [42] and ep oduced wi h he pe mission o Else ie
Towa ds nanome ic esolu ion in mul ilaye dep h p o iling: a compa a i e s udy o RBS, SIMS, XPS and GDOES 2729

ene gies as low as 150 eV [41]. The bes dep h esolu ion
ob ained o ge manium (Ge) p o iles in a hin (2.5 nm) SiGe
laye g own in a Si subs a e we e achie ed wi h a 100 eV
O
+
p ima y ion beam and an impac angle o 57°. In heo y,
e en lowe p ima y ion ene gies will allow u he dec eases
ha will imp o e he dep h esolu ion. Un o una ely, below
100 eV he spu e ing e iciency (e osion a e) dec eases
qui e apidly, and bo h he ime need o de ec a su icien
numbe o ions and he ime equi ed o analysis apidly
inc ease. The de ec ion limi also dec eases as he p ima y
ion ene gy is lowe ed. Fo example, in he case o bo on
de ec ion in silicon, he de ec ion limi wo sens om
0.001 ppm a 10 keV o 0.1 ppm a 500 eV. Ins umen al
de elopmen s ha enable imp o ed p ima y beam cu en s in
he 50–200 eV ange a e p esen ly being a ge ed o imp o e
he spu e ing pe o mance o he 1 nm/min le el [41], bu
his is a huge challenge. In some cases, he use o oxygen (o
some o he elemen s wi h a low hea o sublima ion)
looding a he same ime as p ima y ion spu e ing can
inc ease he nega i e seconda y ion yield, hus compensa ing
o he lowe speed o spu e ing a low p ima y ion ene gies
[46,47]. Sample holde o a ion can also be use ul in
condi ions whe e high oughness is c ea ed in he bo om o
he analysis c a e , and especially when analysis a la ge
dep hs (mic ome e ange) is needed. In such cases, o a ing
he sample holde dec eases o supp esses oughening in he
analyzed a ea, allowing o be e dep h esolu ion. Finally, i
appea s ha he dep h- esolu ion limi s o EXLE-SIMS a e
close o being eached o he semiconduc o indus y, bu o
o he applica ions—such as s udies o he oxida ion o he
wea o hin ilms— his could become a e y in e es ing ool
o s udying he i s s eps in deg ada ion.
A second app oach uses clus e beams ins ead o ions
beams in o de o imp o e he dep h esolu ion. I was shown
ha he use o clus e beams (C60, I -based clus e s) yields no
ad an age in e ms o dep h esolu ion, as he o al ene gy
appea s o be mo e in luen ial han he ene gy pe cons i uen
a om, and he educ ion in he o al ene gy is es ic ed by he
onse o clus e deposi ion [48]. Howe e , u he explo a ion
o hese clus e s may lead o mo e in e es ing esul s.
Fig. 6 Le : SIMS dep h p o iles ob ained o a P3HT-P4VP:PCBM/
PEDOT:PSS ac i e laye wi h 17% PCBM (equi alen o C17)
deposi ed on an oxidized silicon wa e , whe e
28
Si (c osses),
12
C
(squa es),
16
O( iangles) and
12
C
14
N(ci cles) we e p obed. Inse :
12
C
14
N signal no malized by he
12
C signal (polyme ma ix) o
a enua e possible ma ix e ec s. The expe imen s epo ed he e we e
ca ied ou in ul ahigh acuum (UHV) using a Cs
+
p ima y ion sou ce
wi h an impac ene gy o 500 eV and nega i e ion de ec ion o
op imize he seconda y de ec ion. The
12
C SIMS signal was used o
iden i y he polyme laye , while he
12
C
14
N SIMS signal was chosen
o p obe P4VP. Taken om [43] and ep oduced wi h he pe mission
o Wiley. Righ : SIMS dep h p o iles o a
28
Si and b
30
Si iso opes in a
50×(16 monolaye s
28
Si/10 monolaye s
30
Si) supe la ice sys em. The
expe imen s we e pe o med using a 1 keV Cs
+
ion beam a 45°.
Taken om [45] and ep oduced wi h he pe mission o Else ie
2730 R. Escoba Galindo e al.
New app oaches ha aim o mee he highe dep h-
esolu ion a ge s, such as ze o-ene gy SIMS o a om-p obe
omog aphy, elimina e he use o a p ima y ion beam. In he
ze o-ene gy SIMS concep he ion beam is comple ely
absen [49,50]. Localized ma e ial emo al is ins ead based
on he adso p ion o eac i e gases a he sample su ace
and he c ea ion o a ola ile compound. Selec i e ma e ial
emo al is achie ed using an elec on beam, which locally
s imula es a chemical eac ion be ween he adso bed
species and he ma ix elemen s. Theo e ically his p ocess
has monolaye dep h esolu ion, as se by he monolaye
adso p ion and deso p ion p ocesses, and a high spa ial
esolu ion (se by he spo size o he elec on beam, which
could be as small as a ew nanome e s), and i is
quan i a i e, as he deso bed species a e non esonan ly
ionized using a lase beam. Howe e , his me hod will end
o be applied o s anda d o e y homogeneous samples, a
leas o he nex ew yea s, because many de iciencies can
occu , such as p e e en ial e ching and/o ou pla ing,
nonplana e osion, incomple e ioniza ion, e c.
A second me hod ha does no use a p ima y ion beam
is a om-p obe omog aphy [51–53]. He e, a om emo al is
induced by applying a e y high (pulsed) elec ical ield.
The emi ed a oms a e ully ionized ( he e is no ma ix
e ec o conside du ing quan i ica ion) and de ec ed in a
spa ially esol ed ime-o - ligh de ec o . Due o he op ical
ion magni ica ions in ol ed, spa ial esolu ion is on he
o de o <0.5 nm. As he a oms a e emo ed one by one,
he dep h esolu ion is on he same o de as his. Clea ly,
he excellen h ee-dimensional esolu ion makes he me hod
much mo e e sa ile han egula SIMS, in pa icula o he
analysis o he e ogeneous samples such as nanocomposi e
coa ings. Complica ions and limi a ions a ise om he need
o special sample p epa a ion ( he o ma ion o a needle-
shaped specimen wi h a op adius o abou 50 nm, which
ypically equi es ex ensi e ocused ion beam milling), sample
conduc i i y equi emen s ha enable he p opaga ion o he
high- ol age pulse ( his equi emen is now pa ially elaxed
due o he addi ion o lase -assis ed e osion), he small ield o
iew (<100 nm), which limi s he o al numbe o a oms
analyzed and hus he achie able sensi i i y (>10
18
a /cm
3
),
and he ex ensi e (3D) da a econs uc ion needed. In
p ac ice, sample b eakage (due o he la ge mechanical
s esses induced by he s ong elec ical ields) is an impo an
mechanism ha limi s he ou ine applica ion o he a om
p obe. Finally, a e y signi ican ad an age o he a om p obe
ela i e o SIMS is i s 3D esolu ion a he subnanome ic
scale, which enables he e ogeneous samples and/o ough
in e aces o be analyzed wi hou any se ious loss in dep h
esolu ion [54].
In conclusion, se e al app oaches wi h imp o ed
dep h esolu ion a e being de eloped, wi h he mos
p omising being ze o-ene gy SIMS and a om-p obe
omog aphy, which a ge he sub-nm ange, bu hese
wo app oaches a e much mo e complex o use han
EXLE-SIMS. Table 2p esen s a compa ison be ween he
igu es o me i o ul alow-ene gy SIMS and hose o a
con en ional sys em.
Glow-discha ge op ical emission spec oscopy (GDOES)
GDOES is a ela i ely new su ace analysis echnique wi h
he ad an ages o mode a e acuum equi emen s and e y
high spu e ing a es ( ypically >1 µm min
−1
), which enable
he CDP o be ob ained in a ew minu es wi h a dep h
esolu ion compa able o mo e s anda d echniques, such as
AES, XPS o SIMS [4,55–57]. Du ing GDOES expe imen s,
he samples a e spu e ed by A
+
ions and accele a ed neu al
species wi h e y low ene gies (<50 eV). The spu e ed
a oms a e hen exci ed by he plasma and de-exci ed by
emi ing pho ons wi h a cha ac e is ic wa eleng h, enabling
elemen disc imina ion. These pho ons a e di ac ed by a
di ac ion g a ing and collec ed by pho omul iplie (PM)
de ec o s posi ioned in a Rowland ci cle [10,11]. Fu he mo e,
he use o a adio equency ( ) sou ce o spu e ing ex ends
he applica ion o GDOES o he s udy o insula o s used
bo h as coa ings and as subs a es [58–60]. In he las ew
yea s, GDOES has been applied ex ensi ely o he s udy o
mul ilaye dep h p o iling [17,18,61–65], al hough a se ious
loss o esolu ion wi h dep h du ing GDOES analysis has
been epo ed. The majo ac o s con ibu ing o his loss o
esolu ion a e ela ed o he high e osion a e, sample hea ing,
ion-induced su ace oughening [66,67], and in pa icula ,
he c a e geome y du ing he spu e ing p ocess, which
esul s in he mixing o he consecu i e laye s in a mul ilaye
sys em when he indi idual laye hicknesses each down o
he nanome e scale [68–72].
Among he di e en s a egies ha a e cu en ly being
employed o imp o e he dep h esolu ion o GDOES dep h
p o iling, we will ocus he e on: i) he use o pulsed
sou ces; ii) he applica ion o decon olu ion algo i hms and
he modeling o mul ilaye p o iles, and; iii) he u iliza ion
o plasma cleaning p ocedu es p io o analysis.
The use o pulsed sou ces in GDOES analysis (pulsed
glow discha ge, o PGD) is based on he pionee ing wo k
o Winches e and Ma cus [73]. Pulses o a sho du a ion
(10 μs o 1 ms) wi h a ious possible epe i ion a es (du y
cycles) and high ins an aneous powe s (up o hund eds o
wa s) can he e o e be gene a ed. I has been epo ed in he
li e a u e [74–76] ha such pulses p oduce an enhancemen
in he emission yield, esul ing in inc eased sensi i i y and
lowe de ec ion limi s oge he wi h simpli ied calib a ion
cu es due o he educed sel -abso p ion o he emission
lines. Mo eo e , pulsed glow discha ges cause a lowe
a e age powe o be applied han in DC mode, and so hey
Towa ds nanome ic esolu ion in mul ilaye dep h p o iling: a compa a i e s udy o RBS, SIMS, XPS and GDOES 2731
a e ideal o he analysis o hea -sensi i e ma e ials such as
glasses, polyme s and low- empe a u e me als. I is wo h
no ing he e he excellen e iew on pulsed glow discha ges
ha was ecen ly published by Belengue and coau ho s [77].
The applica ion o pulsed GDOES o he dep h-p o iling
analysis o ma e ials has a isen due o he abili y o
con ol bo h he la ness o he c a e and he spu e ing
a e using he pulsed sou ce pa ame e s (i.e., equency
and wid h). The e o e, hinne su ace laye s can be
p ope ly esol ed using PGD, as shown in he ini ial
wo ks o Yang and Oxley [78,79] o 10 nm Cu coa ings
on s eel. Mo e ecen ly he e ha e been published
examples o he applica ion o PGD o he analysis o
lead zi cona e i ana e (PZT) hin ilms (which ind
signi ican use in mic oelec onic and mic oelec ome-
chanical sys ems) [80] and hin- ilm sola cells based on
Cu(In,Ga)Se
2
and Cu(In,Ga)S
2
mul ilaye s [81].
As exp essed abo e in Eq. 2, an expe imen ally ob ained
CDP is a con olu ion o he ue dep h p o ile and he
esponse unc ion o he expe imen al se -up. In he
pa icula case o a GDOES dep h p o ile, he esponse
unc ion is de e mined by he spu e ing c a e shape. The
c a e shape can change ma kedly when he in e aces
be ween di e en ma e ials a e a e sed in a mul ilaye
dep h p o ile. The e o e, ma hema ically desc ibing he
esponse unc ion in GDOES analysis is no a i ial ask.
Oswald and coau ho s [82] we e he i s o a emp o
apply decon olu ion p ocedu es o GDOES dep h p o iles
(al hough he me hod was ini ially designed o SIMS
analysis). Based on his ini ial wo k, P äßle and coau ho s
[83] de eloped an i e a i e decon olu ion algo i hm whe e—
using he calib a ed mass– ime p o ile, he pa ial densi ies o
he sample cons i uen s and he measu ed inal shape o he
spu e ing c a e as inpu da a— he dep h p o iles a e
imp o ed by u ilizing in o ma ion on c a e o ma ion. In
Fig. 7(le -hand panel), an example o he success ul
applica ion o such a decon olu ion p ocedu e o a Cu/C Ni
mul ilaye sys em is p esen ed. Yea s la e , in he excellen
e iew by Winches e and Payling [10], i was w i en ha
his success “makes decon olu ion a eal and exci ing
possibili y o GDOES.”Ne e heless, mo e han a decade
la e , decon olu ion p ocedu es a e ye o be in eg a ed in o
any comme cially a ailable GDOES so wa e. In an al e na-
i e app oach, Escoba and Albella [84] ecen ly epo ed on
a simple model o in e p e ing and p edic ing he dep h
p o iles o pe iodic mul ilaye s uc u es o wo elemen s,
whe e he indi idual laye hicknesses a e in he ange o 10–
100 nm (i.e., wi hin he o de o magni ude o oughness
induced when dep h p o iling using GDOES echniques).
The model is based on he assump ion ha he su ace
oughening p oduced by he ion bomba dmen gi es ise o
he pa ial mixing o he laye s and hei in e aces, leading
o a smoo hing o he o he wise ab up p o iles. Fi ing he
model o he expe imen al p o iles o a se o samples made
o al e na ing Ti and C laye s, wi h hicknesses a ying o e
a wide ange, allows he deg ada ion cons an b(p e-
exponen ial ac o ) o be ob ained o each ma e ial. The
alues esul ing om he i ing p ocedu e co ela e wi h he
e osion a es o Ti and C , and suppo he idea ha he laye
b oadening o each ma e ial is de e mined by i s e osion a e.
The deg ee o adjus men can be conside ed o be qui e
accep able (see he igh -hand panel o Fig. 7) o mos o he
samples in es iga ed, aking in o accoun he app oxima ions
in ol ed.
Table 2 Compa ison o he cha ac e is ics o SIMS and EXLE-SIMS
SIMS EXLE-SIMS
P obing pa icle Ions Ions
De ec ed pa icle Ions (m/q) Ions (m/q)
Spu e ing Ion beam <10 Ke Ion beam <100 eV
A om mixing? Yes Yes
Sampling dep h (nm) 5 10
Dep h esolu ion (nm) 5 1
La e al esolu ion (nm) 100 100
De ec ion limi (ppm) 10
−3
–10 10
−1
–10
Elemen al ange All All
Calib a ion me hod Complex (need s anda ds) Complex (need s anda ds)
Des uc i e? Yes Yes
Iso ope in o ma ion? Yes (+) Yes (+)
C a e e ec ? Yes (−) Yes (−)
C a e diame e (mm) <1 < 0.5
Vacuum (mba ) 10
−10
10
−10
Analysis ime Slow (hou s) Slow (hou s)
2732 R. Escoba Galindo e al.
Finally, i is clea ha in o de o be able o analyze
ul a hin laye s nea he su ace using GDOES, apid
s abiliza ion o he plasma a he s a o he spu e ing
p ocess is equi ed. The echnique whe e a sac i icial
sample ( ypically a monoc ys alline silicon wa e [85]) is
employed o emo e he con aminan s (ca bon, oxygen,
hyd ogen) om he inne walls o he anode p io o he
analysis o he sample has been ex ensi ely applied by
GDOES use s. Molchan e al. ha e ecen ly p oposed [86]
ha his p ocedu e can be imp o ed by using a low-ene gy
plasma (<5 W) o gen ly emo e con aminan om he
su ace o he sample. In hei wo k hey demons a e how
such a ea men is below he spu e ing h eshold and does
no cause any damage o he analyzed specimen. Figu e 8
shows he la ge educ ion in su ace con aminan s ( en old
o he ca bon in ensi y) achie ed when he plasma cleaning
p ocedu e is applied o an elec opolished aluminum
sample. Mo eo e , no e how he o al plasma esponse
(Fi) s abilizes mo e apidly. This plasma cleaning me hod
has been implemen ed o ou ine su ace analysis in he
la es e sion o he QUANTUM so wa e om Ho iba
Jobin Y on ins umen s.
In conclusion, since GDOES is a ela i ely no el
echnique o ul a hin dep h p o iling, he e is mo e oom
o imp o e i s esolu ion. The use o so plasma cleaning
p io o spu e ing using pulsed discha ges, and he
applica ion o decon olu ion p ocedu es a e wa ds, will
su ely u n GDOES in o an ideal echnique o subnanome e
dep h p o iling in e ms o i s ease o use and he accu acy o
he esul s i yields. Table 3compa es he expec ed bene i s o
an op imized GDOES (OPT-GDOES) o he ypical ea u es
o s anda d GDOES analysis.
Fig. 7 Le : composi ion dep h
p o ile o 100 nm hick Cu/C Ni
mul ilaye s on Si: aa e
decon olu ion wi h a cons an
c a e shape ( he esul is e y
simila o he uncon olu ed
p o ile); ba e decon olu ion
wi h i e a ion o de e mine he
une en c a e shape. Taken om
[83] and ep oduced wi h he
pe mission o Else ie . Righ :
a he GDOES expe imen al
dep h p o ile o a 10×(70 nm
Ti/150 nm C ) mul ilaye ; b he
simula ed dep h p o ile ob ained
using he nominal indi idual
hicknesses om (a) and he
deg ada ion cons an shown in
Table 1o [84]. Rep oduced
wi h he pe mission o Else ie
Fig. 8 GD OES dep h p o ile o an elec opolished aluminum sample,
ob ained a 750 Pa and 35 W, a e no plasma cleaning (le ) and a e
spu e ing he silicon wa e o 1 min and hen cleaning he specimen
a 750 Pa and 3 W o 1 min ( igh ). No e he educ ion in
con aminan s ( en old o ca bon) in he igh -hand dep h p o ile.
Taken om [86] and ep oduced wi h he pe mission o Else ie
Towa ds nanome ic esolu ion in mul ilaye dep h p o iling: a compa a i e s udy o RBS, SIMS, XPS and GDOES 2733
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