Til angle con ol o nanocolumns g own by glancing angle spu e ing
a a iable a gon p essu es
J. M. Ga cía-Ma ín,1,a兲R. Al a ez,2P. Rome o-Gómez,2A. Cebollada,1and A. Palme o2
1IMM-Ins i u o de Mic oelec ónica de Mad id (CNM-CSIC), Isaac New on 8, 28760 T es Can os,
Mad id, Spain
2Ins i u o de Ciencia de Ma e iales de Se illa, (CSIC-Uni e sidad de Se illa), Ame ico Vespucio 49,
41092 Se ille, Spain
共Recei ed 20 July 2010; accep ed 7 Oc obe 2010; published online 25 Oc obe 2010兲
We show ha he il angle o nanos uc u es ob ained by glancing angle spu e ing is inely uned
by selec ing he adequa e a gon p essu e. A low p essu es, a ballis ic deposi ion egime domina es,
yielding high di ec ional a oms ha o m il ed nanocolumns. High p essu es lead o a di usi e
egime which gi es ise o e ical columna g ow h. Mon e Ca lo simula ions ep oduce he
expe imen al esul s indica ing ha he loss o di ec ionali y o he spu e ed pa icles in he gas
phase, oge he wi h he sel -shadowing mechanism a he su ace, a e he main p ocesses
esponsible o he de elopmen o he columns. © 2010 Ame ican Ins i u e o Physics.
关doi:10.1063/1.3506502兴
Glancing angle deposi ion 共GLAD兲is a e sa ile and
powe ul echnique o ob ain nanos uc u es in la ge a eas
and wi h a high a ie y o mo phologies by exploi ing a omic
shadowing e ec s du ing physical apo deposi ion.1–6E en
hough e-beam o esis i ely hea ed e apo a ion sou ces a e
commonly used o his pu pose due o hei high di ec ion-
ali y, spu e ing has also been used o ob ain nanos uc u ed
sys ems by GLAD.7–14 In his case, he spu e ing gas p es-
su e, Pg, is ob iously a c ucial pa ame e in he o ma ion o
he columna s uc u es, ying o be minimized in o de o
educe he numbe o collisions o he spu e ed a oms in he
gas phase and he e o e o inc ease he di ec ionali y o he
spu e ed ma e ial owa d he subs a e.
Pgde e mines he mean ee pa h, ␭, o he apo lux
ha lea es he a ge in hei way o he subs a e. Since his
magni ude oughly anges om 1 cm o 1 m be ween 10−2
and 10−4 mba , i is ob ious ha in a s anda d size deposi-
ion sys em, a ying he A p essu e be ween hese anges
will allow uning he mean ee pa h o he spu e ed a oms
be ween he di usi e 共mul iple collision兲and ballis ic 共col-
lisionless兲 egimes, he e o e modi ying he deg ee o di ec-
ionali y and as a consequence he e ec i e shadowing e -
ec s cha ac e is ic o he GLAD echnique. Fo his, he use
o a hea y a om wi h espec o A , o example Au, is con-
enien since i allows ex ending Pg o la ge alues wi h
espec o ligh e species, which would lose hei di ec ion-
ali y a lowe p essu es.
In his pape , we ha e s udied he in luence ha A gas
p essu e has on he columna g ow h o gold nanos uc u es
ob ained by GLAD magne on spu e ing, and we ha e com-
pa ed he ob ained mo phologies wi h heo e ical simula-
ions. The Au apo lux is p oduced by magne on spu e -
ing o a 3.8 cm diame e gold a ge using a gon as spu e
gas. The base p essu e o he chambe is in he mid
10−9 mba ange and he 2 cm2subs a e o ul asonically
cleaned Si共100兲is placed a L=19 cm om he a ge , and
il ed 85° wi h espec o i s no mal. Pgwas a ied om
1.5⫻10−3 o 4⫻10−2 mba , wi h he powe and he deposi-
ion ime being kep cons an a 100 W and 1800 s, espec-
i ely. We ha e checked ha he ilm empe a u e du ing
he spu e ing p ocess was always below 350 K. Es ima ing
he c oss-sec ion o an elas ic sca e ing o a Au a om on
an A a om by he geome ical c oss-sec ion 共
␴
g=3.25
⫻10−19 m2兲, he a io L/␭can be calcula ed as L/␭⬃800
⫻pg共wi h pg he A p essu e in milliba and using a gas
empe a u e o 600 K兲, which de e mines he numbe o col-
lisions expe ienced by he Au a om un il i is deposi ed.15 In
his way, he quan i y ⌶=L/
␯
␭, wi h
␯
being he a e age
numbe o collisions equi ed o he malize he spu e ed a -
oms, es ima es he he maliza ion deg ee o he deposi ion
lux. Since o Au a oms wi h ini ial kine ic ene gy o 5 eV in
A he alue o
␯
is
␯
=12 共see Re . 15 o mo e de ails兲, he
alue o L/
␯
␭in ou condi ions anges om ⌶共pg=1.5
⫻10−3 mba 兲⬃0.1 o ⌶共pg=4⫻10−2 mba 兲⬃2.7. In his
way, when ⌶ⱖ1共i.e., o deposi ion p essu es abo e
⬃0.015 mba 兲, mos o he Au a oms in he gaseous phase
a e he malized wi h he backg ound gas, and he e o e hey
each he subs a e wi h an iso opic eloci y dis ibu ion
unc ion.15
Figu e 1shows c oss-sec ional scanning elec on mic o-
g aph images o he deposi ed samples illus a ing he e ec
o a ying he A p essu e du ing he deposi ion. Figu e 1共a兲
co esponds o a sample g own wi h 1.5⫻10−3 mba o A .
Wi h his p essu e and he geome ical pa ame e s p e iously
men ioned, he spu e ed Au a oms expe ience in a e age
abou one collision in hei way o he subs a e, so he depo-
si ion akes place in he so-called low p essu e long h ow
spu e ing egime ha inc eases he collima ion wi h espec
o con en ional spu e ing.16,17 Due o he sel -shadowing e -
ec and he low ada om di usion compa ed o he Au a oms
a i al ime o he used deposi ion condi ions, nanos uc-
u es ha a e elonga ed along he lux di ec ion a e o med.
The column il angle,
␤
, is always smalle han he deposi-
ion angle 共85°兲, wi h
␤
=62° in his case. In Figs. 1共b兲–1共d兲,
we ind simila columna s uc u es whose il angle de-
c eases wi h he alue o he deposi ion p essu e:
␤
=38°
o 1⫻10−2 mba 共⌶⬃0.67兲,
␤
=16° o 2.5⫻10−2 mba
a兲Elec onic mail: [email p o ec ed].
APPLIED PHYSICS LETTERS 97, 173103 共2010兲
0003-6951/2010/97共17兲/173103/3/$30.00 © 2010 Ame ican Ins i u e o Physics97, 173103-1
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共⌶⬃1.69兲, and inally he columns g ow pe pendicula o
he subs a e o 4⫻10−2 mba 共⌶⬃2.7兲.
The ela ion be ween he il angle and he incidence
angle has been heo e ically s udied o many di e en ma-
e ials and unde se e al deposi ion condi ions. Nieuwenhui-
zen and Haans a18 ound a phenomenological ela ion be-
ween he column il angle,
␤
, and he deposi ion angle,
␣
,
as an共
␤
兲=0.5 an共
␣
兲, ha o e es ima es he o me o in-
c easingly oblique deposi ion 共 o
␣
=85° i gi es
␤
=80°兲.
On he o he hand, Tai e al.19 assumed ballis ic deposi ion
and shadow e ec s leading o he ela ion 2⫻sin共
␣
−
␤
兲=1
−cos共
␣
兲, ha o
␣
=85° gi es
␤
=58°, i.e., close o ou
expe imen al alue. Some o he models ha e in oduced ad-
di ional pa ame e s depending on he deposi ion a e o he
di usi i y.20,21 Howe e , none o hem a e sui able o ep o-
ducing he expe imen al e olu ion illus a ed in Fig. 1: his
has mo i a ed us o de elop a model ha pe mi s he calcu-
la ion o
␤
as a unc ion o he deposi ion p essu e. The
be e ag eemen wi h he model o Tai e al. o he lowes
p essu e case gi es clues abou he impo ance o he su ace
shadowing. On he o he hand, he abo emen ioned model
does no conside any modi ica ion o he inciden angle dis-
ibu ion unc ion o he deposi ion pa icles due o sca e ing
p ocesses in he gaseous phase, hus no deposi ion p essu e
dependence is aken in o accoun . In o de o unde s and he
in luence o Pgon he de elopmen o he hin ilm nano-
s uc u e, we ha e sol ed a Mon e Ca lo ballis ic model us-
ing di e se calcula ed inciden angle dis ibu ion unc ions in
a ious condi ions.
We conside he deposi ion o Au a oms on a wo-
dimensional subs a e ha de ines he x-y plane, whe eas he
z axis is de ined by he di ec ion pe pendicula o he sub-
s a e. The h ee-dimensional space is di ided in o a NL
⫻NL⫻NHg id and each a om mo es owa d he subs a e
om an ini ial andom posi ion ollowing he di ec ion de-
ined by he sphe ical angles
␪
and
␸
, whe e
␪
苸关0,
␲
/2兲is
he pola angle 共
␪
=0 is he di ec ion no mal o he subs a e兲
and
␸
苸关0,2
␲
兲is he azimu hal angle. The mo emen o he
pa icle con inues, assuming pe iodic bounda y condi ions,
un il i hi s he su ace, whe e i s icks. The angles
␪
and
␸
a e andomly calcula ed by de ining an inciden angle dis i-
bu ion unc ion pe uni ime and uni su ace, I共⍀兲, wi h
d⍀=sin
␪
d
␪
d
␸
being he di e en ial solid angle.22,23 We
ha e no in oduced su ace di usion no deso p ion mecha-
nisms, due o he low empe a u e o he ilm du ing g ow h.
In o de o es ima e he magni ude I共⍀兲as a unc ion o
Pgwe ha e employed he SIMTRA code wi h pa ame e s ha
deal wi h he sys em geome y 共 il angle o he subs a e,
a ge size, a ge -sample dis ance, dimensions o he eac o 兲
and he deposi ion condi ions 共Pg, gas empe a u e, na u e o
he spu e ed species兲.24 The in e ac ion po en ial be ween
he Au and he A a om was conside ed a Moliè e ype, and
h ee possible alues o he spa ially a e aged gas empe a-
u e we e conside ed: 500, 600, and 700 K. In gene al, gas
empe a u e is no cons an along he plasma discha ge; he
hea ing o gas pa icles caused by he collisions wi h he
spu e ed pa icles in oduce impo an gas empe a u e g a-
dien s, wi h he ca hode empe a u e depending in a g ea
deal on he cooling e iciency. In his way, nume ical and
expe imen al es ima ions ha e ob ained ca hode empe a-
u es a ound and abo e 700 K, whe eas he ilm empe a u e
emains below 400 K.15,25 Fu he mo e, elas ic collisions be-
ween spu e ed and gas a oms hea up he gas specially a a
dis ance o abou ␭, whe e empe a u es a abo e he ca h-
ode empe a u e can be ound.26,27 Once he unc ion I共⍀兲is
calcula ed, he model is sol ed o alues o NLup o 2000
共in o de o a oid ini e size e ec s in he solu ions兲and
NH=500.
Resul s o he model o a gas empe a u e o 600 K and
o he same deposi ion p essu es as in Fig. 1appea in Fig.
2. The e, we show a columna s uc u e wi h il angle de-
pending on Pg: o Pg=1.5⫻10−3 mba 关Fig. 2共a兲兴we ind
il ed columns which become wide o inc easing alues o
he ilm hickness. Fo Pg=1⫻10−2 mba 关Fig. 2共b兲兴, he
column il angle dec eases, which becomes mo e e iden
o Pg=2.5⫻10−2 mba 关Fig. 2共c兲兴. Finally o Pg=4
⫻10−2 mba 关Fig. 2共d兲兴, columns g ow pe pendicula o he
subs a e, which is cohe en wi h an iso opic inciden angle
dis ibu ion unc ion o he deposi ion pa icles. Til angle as
a unc ion o ⌶appea s in Fig. 3 o di e en alues o he
spa ially a e aged gas empe a u e in he discha ge; model
esul s ma ch qui e well wi h he expe imen al alues o
␤
.
Fu he mo e, esul ing cu es o e lap in he whole s udied
ange no ma e he gas empe a u e, indica ing ha he pa-
ame e de ining he il angle is ⌶. In o de o s udy i his
esul can be gene alized, in Fig. 3we ha e also included
calcula ed esul s o he spu e ing o Ag and Cu in A ,
conside ing wo di e en gas empe a u es 共500 and 700 K兲
and di e en alues o L. In hese cases
␯
=6.6 o Ag and
␯
=3.6 o Cu, which a e cohe en wi h he lowe masses o
Ag and Cu in compa ison wi h ha o Au 共see Re . 15 o
mo e de ails兲. Figu e 3shows ha all he calcula ed da a
o e lap, indica ing ha ou esul s a e gene al i espec i e o
he gas empe a u e, he dis ance ca hode- ilm o he compo-
si ion o he ilm.
FIG. 1. 共Colo online兲Field enhanced scanning elec on mic og aph images
o samples p epa ed by GLAD spu e ing a 85° o -no mal wi h di e en
A p essu e: 共a兲1.5⫻10−3 mba , 共b兲1⫻10−2 mba , 共c兲2.5⫻10−2 mba ,
and 共d兲4⫻10−2 mba .
173103-2 Ga cía-Ma ín e al. Appl. Phys. Le . 97, 173103 共2010兲
Au ho complimen a y copy. Redis ibu ion subjec o AIP license o copy igh , see h p://apl.aip.o g/apl/copy igh .jsp
In summa y, we ha e shown how he A p essu e in
GLAD spu e ed Au s uc u es s ongly in luences he deg ee
o di ec ionali y o he impinging Au a oms, and as a conse-
quence, he mo phology and inclina ion o he esul ing
nanocolumns. The inclina ion o hese columns wi h espec
o he subs a e no mal changes om ze o o high A p es-
su es, whe e he Au a oms a e he malized in he gas phase,
o 62° o low A p essu es, whe e he Au ajec o ies a e
mainly ballis ic. The de eloped heo e ical model suppo s
hese expe imen al indings and explains ha he main p o-
cesses esponsible o he o ma ion o he nanos uc u e o
hese ilms a e he sel -shadowing mechanisms a he su ace
and he collisional p ocesses o he spu e ed pa icles in he
gas phase. This is a uni e sal p ocess, heo e ically obse ed
also o o he spu e ed ma e ials and gas empe a u es.
Financial suppo om he Spanish MICINN 共G an
Nos. CSD 2008-00023, MAT 2008-06765-C02-01/NAN,
MAT 2007-65764, and PIE 200960I132兲, Comunidad de
Mad id 共G an No. S2009/MAT-1726兲, Jun a de Andalucía
共G an Nos. TEP2275 and P07-FQM-03298兲and Eu opean
Commission 共G an No. NMP3-SL-2008-214107兲is ac-
knowledged. R. Ál a ez acknowledges he JAE P og am o
CSIC.
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FIG. 2. 共Colo online兲Solu ions o he model using he di e en inciden
angle dis ibu ion unc ion o a ious deposi ion p essu es and a gas em-
pe a u e o 600 K: 共a兲1.5⫻10−3 mba , 共b兲1⫻10−2 mba , 共c兲2.5
⫻10−2 mba , and 共d兲4⫻10−2 mba .
FIG. 3. Resul o he calcula ion o he il angle as a unc ion o he pa-
ame e ⌶=L/
␯
␭ o Au, Ag, and Cu o di e en gas empe a u es and
alues o L, along wi h he expe imen al da a o Au. A nume ical i o he
calcula ed da a has also been included as a isual guide 共solid line兲.
173103-3 Ga cía-Ma ín e al. Appl. Phys. Le . 97, 173103 共2010兲
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