Hardness and Stress of Amorphous Carbon
Films Deposited by Glow Disharge and
Ion Beam Assisting Deposition
F. C. Marques and R.G. Laerda
Institutode Fsia\GlebWataghin",UniversidadeEstadual deCampinas
CaixaPostal6165,13083-970, Campinas,S~aoPaulo,Brazil
Reeived31January,2000;reeivedinnalformon17Marh,2000
Thehardness andstress ofamorphousarbonlmspreparedbyglow disharge andbyion beam
assisting deposition are investigated. Relatively hard and almost stress free amorphous arbon
lmsweredepositedbytheglow disharge tehnique. Onthe otherhand,byusingtheion beam
assistingdeposition,hardlmswerealso obtainedwitha stressofthe sameorderofthosefound
intetrahedralamorphousarbonlms. Astruturalanalysisindiatesthatalllmsareomposed
of a sp 2
-rih network. These results ontradit the urrently aepted onept that both stress
andhardnessareonlyrelatedtotheonentrationofsp 3
sites. Furthermore,thesameresultsalso
indiatethatthesp 2
sitesmayalsoontributetothehardnessofthelms.
I Introdution
Thearbonatomisprobablythemostversatileelement
of nature. Besides its paramount importane in most
livingbeingsandorganiompounds,alargevarietyof
struturesbasedonthearbonatomhavebeen
disov-eredin nature suh asdiamondandgraphiteor
arti-ially developed strutures suh asthose of bukballs,
nanotubes, and diamond-like arbon thin lms [1,2℄.
The ability of arbon to hybridize in sp, sp 2
and sp 3
ongurations is nota ommonharateristi of most
elements. This peuliarity limitsthe establishment of
universal models to explain thelarge varietyof
stru-turesfoundinthosematerials. Forinstane,oneofthe
maindiÆultiesfaedbymanyresearhersisto
under-stand theoriginoftheintrinsistress andhardness of
amorphousarbonlms.
Diamond is one of the hardest materials found in
nature. Its high hardness is attributed to strong
C-C sp 3
bonds in a tetrahedrally oordinated network.
On theother hand, the softnessof graphite is
assoi-atedwithastrutureofplanesformedbylosed-paked
rings of sp 2
-hybridized arbon atoms. In the
prepa-ration of amorphous arbon lms one usually nds a
mixture of sp 2
and sp 3
bonds. Thehardnessof
amor-phousarbonlmshasbeenurrentlyattributedtothe
onentration of sp 3
sites presentin the lms. In
ad-dition, the stress of those lms has also been
assoi-ated withtheonentrationofsp 3
sitesreatedduring
tween hardness and stress with the sp 3
onentration
havebeenestablishedandfrequentlyusedinliterature
[5,6℄. In this paper we show that the explanation for
thestrutural originof the hardnessand the stress of
amorphousarbonlms in termsof theonentration
ofsp 3
bondsis notuniversal. We observed,usingtwo
dierentkindsofdepositionsystems,thatitispossible
topreparehardamorphousarbonlmswithhigh
on-entrationsofsp 2
bonds. Inaddition, wedemonstrate
that itisalso possibletoprepare highly stressedlms
withsmallonentrationsofsp 3
bonds.
II Experimental
Thelmswerepreparedusingtwodierenttehniques,
i.e., glowdisharge (GD)and Ion BeamAssisting
De-position (IBAD). A rstseries of lmswere deposited
on the athode eletrode of a GD system through a
plasmadeomposition of methanegas. All lms were
preparedat room temperature. The biasvoltage was
variedwithintherange-100Vto-1200VandtheCH
4
gaspressurewaskeptonstantat1.0Pa. Aseond
se-riesoflmswaspreparedbyIBADusingtwoKauman
ionsoures. Detaileddesriptionsofthedeposition
sys-temarefoundelsewhere[7℄. Argongaswasintrodued
into both Kauman ion soures for the sputtering of
agraphite targetandfor thesimultaneousassisting of
A set of lms was preparedat 150C Æ
in the rangeof
0to800eVargonionbeamassistingenergy. Thebase
pressureofthehamberwasabout10 5
Pa,andduring
depositionthe pressurewaskeptonstantat 610 2
Pa.
Thethikness of the lms, measuredby a Dektak
prolometer, was about 1 m for the lms prepared
byGDandabout0.1mforthosepreparedbyIBAD.
Thehardnesswasobtainedfrom ananoindenterusing
aset of dierentloads in the0.5mN to 90 mNrange.
The Raman spetrosopywas obtained at room
tem-perature with a T64000 Jobin Ybon spetrometer in
thebaksatteringonguration. Stressmeasurements
were taken from lms deposited on4250:4 mm 3
-Sibars,usingthebendingbeammethodtodetermine
theradiusofurvatureofthelm/substrateomposite
[8℄. Thestress wasthenalulatedusing themodied
Stoney'sequation[9,10℄:
=[E
s =(1
s )℄t
2
s =6t
f
(1=R 1=R
o )
where E, , andt are theYoung;smodulus, Poisson's
ratio,andthikness,respetively. Thesubsriptssand
f refertothesubstrateandthelm,respetively. 1=R
o
and1=Raretheurvaturesbeforeandafterthelm
de-position.
III Results
Fig. 1displaysthedepositionrate ofthetwoseriesof
lms prepared by glow disharge and IBAD. Observe
thatthedepositionrateofthelmspreparedbyGDis
muhhigherthanthat ofthelmspreparedbyIBAD.
Theinreaseofthedepositionratewiththebiasvoltage
ofthelmspreparedbyGDisattributedtoaninrease
of thedeompositionrate ofthemethanegas. Onthe
otherhand,intheIBADtehniquethedepositionrate
isdeterminedbythetarget-sputteringyieldandbythe
assistingionenergy. Eventhoughtheuxofsputtered
speiesfromthegraphitetargetwaskeptonstant,the
assisting ions play an important role for determining
thedepositionratesinetheysputterthegrowinglm.
Hene,astheassistingionenergyinreasesthe
deposi-tionrateredues.
Hardamorphousarbonlmsareusually obtained
at very low deposition rates, typially at about 0.01
nm/s[11,12℄. However,Fig. 2showsthatintheaseof
glowdisharge,itispossibletoahievehighdeposition
rates and also relatively high hardness. This resultis
veryimport for industrial appliation and it may
de-pendonthedeposition systemaswellasonits
geome-try. Ontheotherhand,eventhoughthelmsprepared
tionratesareverylow,whihhinderstheiruseinmany
industrial appliations.
Figure 1. Deposition rates of amorphous arbon lms as
afuntionofthebiasvoltage (forlmspreparedglow
dis-harge)and theassistingenergy(forlmsprepared byion
beam assisting deposition). Observe the dierene inthe
saleofbothseries.
Figure2.Hardnessversusdepositionrateofamorphous
ar-bonlmspreparedbyglowdisharge,depositedatdierent
biasvoltages.
Fig. 3 shows the ompressive intrinsi stress as
a funtion of the bias (GD) and the assisting energy
of the same order of those typially obtained in hard
amorphous arbon lms (2-3 GPa). For higher bias
voltagethe stressdereasessubstantiallyreahing
val-uesaslowas0.5GPa.Thestressofthelmsdeposited
by the IBADinreases sharply forargon assisting
en-ergy upto 100eV,and it varies slightlyfor energyin
the 100eVto 650eVrange. Inaddition,the absolute
valueofthestressisofthesameorderofthosereported
for lmswith high onentrationsof sp 3
bonds (ta-C)
[5,13℄. Eventhoughthestressesofbothseries oflms
are verydierent, thehardnessof thoselmsis about
the same, Fig. 4. This gure shows that the
hard-nesshangesjustaboutafatoroftwowhilethestress
diersbymorethanoneorder ofmagnitude.
Figure 3. Stress of amorphous arbon lmsas a funtion
of the biasvoltage (for lms prepared by glow disharge)
and the assisting energy(for lms prepared by ion beam
assistingdeposition).
Alllmsreportedinthis workareharaterizedby
havingahighonentrationofsp 2
hybridizationas
in-diated by thestrutural analysis of theRaman
spe-trosopy, Fig. 5. This gure shows typial Raman
spetraoftwolmsdepositedbyGDandIBAD.They
learlyshowthedisorderband(D)at1340m 1
,and
thegraphiteband(G)at1530m 1
. Theratiobetween
theareasofthesetwobands, I
d =I
g
,indiatesthe
pres-eneofsp 2
sitesinthelmstruture[14℄. Fig. 6shows
the dependene of theI
d =I
g
on boththe bias voltage
(GD) and the assistingenergy (IBAD). Forlms
pre-paredbyGD,oneobservesaninreaseoftheI
d =I
g ratio
asthebiasinreases,whihindiatesaninreaseofthe
onentrationof thesp 2
sites. Although notrendhad
of theassisting energy, theyalso have ahigh value of
I
d =I
g
ratioofabout2whihalsoindiatesa
graphiti-likestruture. Theresultsofbothseriesontrastswith
thelow valuesof theI
d =I
g
ratio (0.1) foundin high
tetrahedralamorphousarbonlms[15℄.
Figure4. Hardnessversusstressofseveralamorphous
ar-bonlmspreparedbyglowdishargeandionbeamassisting
deposition.
Figure5.TypialRamanspetraofamorphousarbonlms
preparedbyglowdishargeandion beamassisting
deposi-tion.
The density of thelms is in the 1.5 - 2.25g/m 2
range, Fig. 7. Within experimental error, there is no
in the 100-600 eV range. These densities are smaller
thanthosereportedfortetrahedrallyoordinated
amor-phous arbon lms (ta-C), with density of about 3
g/m 3
[16℄. It is worth notingthat someof thelms
prepared byIBAD have astress of the same order of
those found in ta-C lms(typiallyof theorder of 10
GPa), see Fig. 8. However, thedensity ofthese lms
ismuhhigherthanthedensityofthelmsdeveloped
in this work. This result is very dierent from what
hasbeenreportedintheliterature,i.e.,lmswithvery
highstresshavehighdensity.
Figure 6. Raman, I
d
=Ig,ratio of amorphous arbon lms
prepared by glow disharge and ion beam assisting
depo-sition as a funtion of the bias voltage and the assisting
energy,respetively.
IV Disussions
Foramorphousarbonlms,therearesomeintriguing
resultsobtainedinourworkthathallengethevalidity
ofsometheoretialmodelsreportedintheliterature. 1)
thelmslearlyhaveastruture withhigh
onentra-tionsofsp 2
hybridization,yettheyarerelativelyhard;
2) It was possible to prepare suh kind of lms with
verylowstressaswellaslmswithveryhighstress;3)
lms with high hardness and low stress ould also be
preparedwith relativelyhigh deposition rates. Let us
addressthose features.
First, it is important to stress that our lms are
haraterized by having a very high onentration of
2
oordinated amorphous arbon lms. TheRaman
re-sultsareimportanttodeterminethisharateristi. As
itiswellknow,theRamansatteringhasbeenusedto
identifythepreseneofsp 2
andsp 3
hybridization. Even
thoughthis tehniqueis notthe best wayfor
measur-ingthesp 2
/sp 3
ratio,ithasbeenusedasaqualitative
measureofthedegreeofgraphitizationofthelms. As
weobservedinFigs. 5and6,alllmsareharaterized
byarelativelyhighI
d =I
g
ratio,indiatingthatthe
on-entration of sp 2
sites is veryhigh. These results are
also supported by the small band gap of those lms,
whihistypiallysmallerthan1.0eV,andalsobythe
relativelysmalldensityoflessthan2.3g/m 3
[17,18℄.
Itiswellaeptedthatthehardnessofhighly
tetra-hedrallyoordinatedarbonlmsisduetothepresene
ofsp 3
bonds. However,inthisworkwedevelopedlms
withhighonentrationsofsp 2
sitesbutwithrelatively
highhardness(15-30GPa). Thisresultontraditsthe
urrentlyaeptedoneptthat thehardnessof
amor-phous arbonlmsdependsonly ontheonentration
of sp 3
sites,evenfor lmswith high onentrationsof
sp 2
sites. Therefore, we propose that the sp 2
sites, if
theyareappropriatelydistributed,mayalsoontribute
tothehardnessofmaterialssuhasrosslinked[19,20℄
or urved graphiti sheet strutures [21℄. It is
impor-tant to stress this ontribution, sine graphite has a
verysmallhardness. However,itshardnessomesfrom
anorganizednetwork,whihallowsaneasydisloation
of parallel planarlayersand hene reduingthe
hard-ness of the material. An appropriated distributionof
thearbonringsanbedevelopedinawaytoeliminate
this problemand providearigidnetwork. Oneshould
bearin mindthatthesp 2
bondisinfat strongerthan
thesp 3
bond,soitisnotstrangetoassumethat
some-howthesp 2
sitesouldalsoontributetothehardness
ofthelms.
Seond, it has been laimed that the hardness of
amorphous arbon lms is related to the stress sine
thesameproessofsub-implantationhasbeenusedto
explainbothstressandhardnessthroughtheformation
of sp 3
sites [3-5℄. Weintend to showherethat this is
notalwaysvalid. Fig. 7showsaplotofhardness
ver-sus stress for a seleted set of lms preparedby both
tehniques. It is amazing to note that lms prepared
withastressaslowas0.5GPahavehardnessofabout
the same order of that of lms whih have stress of
morethanoneorderof magnitudehigher. It is
impor-tantto rememberthatlmswithastressofaboutthe
same order ofthose obtainedfor ta-C were developed
byIBAD.Inotherwords,weshowthatitisalso
possi-bletopreparehighlystressedandhardlmswithhigh
onentrations of sp 2
Figure7. Densityofamorphousarbonlmsasafuntion
of the biasvoltage (for lms prepared by glow disharge)
and the assisting energy(for lms prepared by ion beam
assistingdeposition).
Figure8. Densityversusintrinsistressofamorphous
ar-bonlmspreparedbyglowdishargeandionbeamassisting
deposition. Aseriesoftetrahedrallyoordinatedamorphous
arbon(ta-C)lmsisalsoinludedfor omparison[16℄.
and the subsequentarbon implantation ompat the
lms,generatinghighompressivestressinadensesp 2
struture[18℄. Thisresultbringsbakthedebateonthe
explanation for the origin of the stress in amorphous
arbonlms. Theyshowthattherearenounique
rela-3
addition,thepreseneofhydrogen,asintheaseofthe
lmsdeposited by glow disharge,must also betaken
intoaount,sinetheyareimportantfordetermining
thestrutureofthelms. Amongother ontributions,
hydrogen ould relieve the stress and hinder the
for-mationof large lusters. Hene, with theinuene of
hydrogen,itisreasonableto assumethatitis possible
to obtain lms with the same sp 2
/sp 3
ratio but with
verydierentproperties,suh ashardnessandstress.
Finally, in Fig. 2weobservedthat it waspossible
to prepare lms at high deposition rates with
reason-ably high hardness and low stress. These results are
of extreme importane for industrial appliation.
Re-ently, we have also developed good quality lms by
theglowdishargetehniquewithanevenhigher
depo-sitionrate(0.7nm/s)usinghighermethanegas
pres-sureand bias voltage [22℄. It is verylikelythat these
dataanstillbeimprovedbyusing moreappropriated
onditions. Forinstane,someredutionofstressould
be ahieved by theintroduing of nitrogen and boron
intothearbonnetwork[23,24℄. Thisproedure ould
beusedtogetherwiththeonditionadoptedhereto
im-provethe lmproperties, espeially forlmsprepared
athighbias.
In summary, the study of amorphous arbon lms
hasemphasizedmainlytheahievementofhighly
tetra-hedrallyoordinatedarbonlms. Usually,theselms
haveahardnessnearthatofdiamondbuttheyarevery
highly stressed and deposited at very low deposition
rate. Hene, these lms are not good andidates for
most appliations, sine they an not be prepared at
an appropriate thikness beause of their low
deposi-tionrateandhigh stress. Inthis workweshowthat it
ispossibletopreparelmswitharelativelyhigh
hard-ness,lowstressandhighdepositionrate. Eventhough
theirhardnessisnotashighasthosereportedforta-C
lms, the lms preparedby GD are hard enough for
manyindustrial appliations.
V Conlusion
In this work we demonstrated that it is possible to
prepareamorphousarbonlmsbytheglowdisharge
tehniquewith alowstress, arelativelyhighhardness
andahigh depositionrate. These lmsaregood
an-didates for industrial appliations. They are sp 2
-rih
withastruturethatsomehowprovideshighhardness,
suh asrosslinkedorurvedgraphiti sheets. Onthe
otherhand,weouldalsopreparehighlystressedlms
with similar harateristisusing the ion beam
demon-hardnessandsp 3
hybridization. Suh typeof relations
reported in the literature may apply to some spei
asesbut theyannotbegeneralized.
Aknowledgment
Theauthorsare indebt toF. Alvarezand P.
Ham-merforfruitfuldisussions,toF.L.FreireJr. forRBS
measurementsandtoC.M.Lepienskifor
nanoindenta-tionmeasurements.
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