Table Of ContentAvailable online at www.sciencedirect.com
ThinSolidFilms516(2008)4036–4039
www.elsevier.com/locate/tsf
Letter
Protection of polymer from atomic-oxygen erosion using Al O atomic layer
2 3
deposition coatings
⁎
Russell Coopera, Hari P. Upadhyayaa, Timothy K. Minton a, , Michael R. Berman b,
Xiaohua Duc, Steven M. Georgec
a DepartmentofChemistryandBiochemistry,MontanaStateUniversity,Bozeman,MT59717,USA
bAirForceOfficeofScientificResearch,875N.RandolphStreet,Arlington,VA22203,USA
c DepartmentsofChemistryandBiochemistryandChemicalandBiologicalEngineering,UniversityofColorado,Boulder,CO80309,USA
Received8April2007;receivedinrevisedform2July2007;accepted13July2007
Availableonline21July2007
Abstract
ThinfilmsofAl O grownusingatomiclayerdeposition(ALD)techniquescanprotectpolymersfromerosionbyoxygenatoms.Toquantify
2 3
thisprotection,polyimidesubstrateswiththesamechemicalrepeatunitasKapton®wereappliedtoquartzcrystalmicrobalance(QCM)sensors.
Al O ALDfilmswithvaryingthicknessesweregrownonthepolyimidesubstrates.TheALD-coatedpolyimidematerialswerethenexposedtoa
2 3
hyperthermal atomic-oxygen beam. The mass loss versus oxygen-atom exposure time was measured in situ by the QCM. Al O ALD film
2 3
thicknesses of ∼35Å werefoundto protect the polymer fromerosion.
©2007Elsevier B.V. All rightsreserved.
Keywords:Polymers;Polyimide;Ätomiclayerdeposition;Al2O3;Atomicoxygen;Erosion;Quartzcrystalmicrobalance
Spacecraft in low Earth orbit collide with ambient oxygen However,handlingandthermalcyclingofthesethickcoatingsare
atomswithrelativevelocitiesof∼7.4kms−1andfluxesinthe likelytoproducecracksinthecoatingsandexposethesubstrateto
range of 1013 to 1015 atoms cm−2 s−1. These high-energy potentialO-atomattack.Atomicoxygenthatreachesthesubstrate
collisions cause oxidation and erosion of materials. Organic throughdefectsmayerodelargecavitiesunderneaththecoatingin
polymers used on the external surfaces of spacecraft as thevicinityofthedefect[6,7].
structuralandthermal-controlmaterialsareparticularlyvulner- ThePECVDcoatingtechniquealsorequiresline-of-sightto
abletoatomicoxygen[1–4].Coatingsofinorganicoxides,such thesubstrate.Thisrequirementmaybeimpracticalforshadowed
as SiO , have been used to provide protection from atomic or high-aspect-ratio structures. An approach that is an
2
oxygen. These coatings are generally applied by plasma improvement over PECVD is to attach silicon-containing
enhanced chemical vapor deposition(PECVD). moieties to the surface through a photochemical reaction. The
PECVD is known to produce films that tend to be columnar modifiedsurfacemaythenreactwithatomicoxygentoforma
withalargedefectdensity.Becauseofthelargenumberofdefects, continuous layer of SiO . Although this technique has
2
relatively thick coatings have been used to reduce the defect demonstratedaonehundred-foldreductioninerosionyieldfor
density.Kapton®polyimidesurfacesthathaveSiO coatingswitha flat samples, there are still line-of-sight limitations when the
2
1300Åthicknesshavebeenshowntoexhibiterosionyieldsaslow techniqueisappliedtonon-planarsurfaces[8].
as∼0.1–0.2%oftheerosionyieldofunprotectedKapton®[5,6]. In addition to protecting existing polymers with inorganic
coatings, inorganic/organic copolymers are being developed
thattakeadvantageofthereactionoftheinorganiccomponent
⁎
Correspondingauthor.
with atomic oxygen to form a passivating oxide layer. This
E-mailaddresses:[email protected](T.K.Minton),
protection strategy has the advantage that the oxide layer can
[email protected](M.R.Berman),[email protected]
(S.M.George). reformafterthedegradationoftheinitialoxidelayer.Anearlier
0040-6090/$-seefrontmatter©2007ElsevierB.V.Allrightsreserved.
doi:10.1016/j.tsf.2007.07.150
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02 JUL 2007 2. REPORT TYPE 00-00-2007 to 00-00-2007
4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Protection of polymer from atomic-oxygen erosion using Al2O3 atomic
5b. GRANT NUMBER
layer deposition coatings
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6. AUTHOR(S) 5d. PROJECT NUMBER
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Biochemistry,Boulder,CO,80309
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R.Cooperetal./ThinSolidFilms516(2008)4036–4039 4037
class of self-passivating polymers was based on phosphine Aluminum oxide (Al O ) ALD coatings with various
2 3
oxide [9]. More recently, copolymers that include polyoligo- thicknesses were applied to the polyimide substrates using
meric silsesquioxane have shown great promise as a replace- sequential, self-limiting exposures to trimethylaluminum
mentforKapton® [10].However,thesepolymersexhibitfinite (TMA) (Aldrich) and water (Fisher HPLC-grade). The Al O
2 3
erosion by atomic oxygen and may experience damage under ALDwasperformedat177°Cinaviscousflow,hot-wallALD
vacuum ultraviolet (VUV) radiation inspace. flow reactor [25]. The TMA and H O yield Al O ALD
2 2 3
Earlierstudiesindicatethatanultrathinandflexibleinorganic accordingtothefollowingtworeactions[18,19]:
oxide coating may provide an effective barrier. Exposure of a
⁎ ⁎
siliconsurfacetoabeamofhyperthermalatomicoxygenleadsto ðAÞAl–OH þAlðCH Þ →Al–O–AlðCH Þ þCH ð1Þ
thegrowthofanSiO filmwithaterminalthicknessof∼50Å 3 3 3 2 4
2
[11–13]. The native thickness of aluminum oxide (Al O ) on ⁎ ⁎
2 3 ðBÞAl–CH þH O→Al–OH þCH ð2Þ
aluminum has also been measured to be only 30–40 Å [14]. 3 2 4
Anecdotalevidencefromspaceflightexperimentsexiststhatan
wheretheasterisksdenotethesurfacespecies.Whenperformed
Al2O3 coating with a nominal thickness of 50 Å protects a in an ABAB… reaction sequence, these sequential reactions
reactivesubstrate [15].Ifsuchthin andflexible oxide coatings
produce linear, atomic-layer-controlled Al O growth. The
2 3
can be applied to polymer substrates without line-of-sight
steady-state Al O ALD growth rate after nucleation is
2 3
restrictions and without defects, then they should be able to ∼1.2 Å per AB cycle [19,26]. The polyimide substrates were
protectthesubstrateevenafterhandlingandthermalcycling.
coated with Al O ALD films resulting from 8, 13, 17, 21, 25
2 3
Chemicalvapordeposition(CVD)isawellcharacterizedthin
and50ABcycles,respectively.Thereactantexposuresequence
film deposition method that does not depend on line-of-sight.
wasTMA(1.0s),purge(20s),H O(1.0s),andpurge(10s).The
2
However, CVD methods typically require elevated substrate
polyimide surfaces were coated without any prior surface
temperatures togrow the desired films. Avariation on CVD is
treatmentorfunctionalization.
atomiclayerdeposition(ALD).ALDcanusuallyemploymore
ThecoatedQCMsensorswereexposedtoabeamcontaining
reactiveprecursorgasesthanCVDandachievelowerdeposition
hyperthermal atomic oxygen. Mass loss was measured in situ
temperatures. ALD techniques can also produce continuous,
versustimeusingaMaxtekQCMhead(ModelDSH-200)anda
Angstrom-level-controlled,anddefect-freefilms[16].ALDisa
Model RQCM data collection system. The QCM head accom-
gas-phasemethodbasedontwosequential,self-limitingsurface
modatedtwodiscs,allowingeachatomic-oxygenexposuretobe
reactions[17].Eachsurfacereactionallowsonly∼1monolayer
conducted with an Al O -coated polyimide test sample and a
2 3
ofdeposition.
polyimide control sample without an Al O coating. For each
2 3
Al O ALD is a particularly robust and well-defined ALD
2 3 exposure,theatomic-oxygenfluxwasdeterminedfromthemass-
system [18,19]. Al O ALD films can be pinhole-free as
2 3 lossrateofthepolyimidecontrolsample.Theoxygen-atomflux
demonstratedbyelectricalmeasurements[20],andtheycanbe
did not vary by more than a few percent from one exposure to
depositedonpolymersubstrates[21,22].Excellentgasdiffusion
another.Testandcontrolsampleswerealwaysplacedinthesame
barrier properties are observed for Al O ALD films on
2 3 locationstoensureconsistencybetweenthevariousexposures.
polyethylene naphthalate (PEN) and Kapton® polymer sub-
A pictorial diagram of the atomic-oxygen source and
strates [23,24]. Consequently, Al O ALD should provide an
2 3 experimental apparatus is shown in Fig. 1. This apparatus has
idealmodelsystemforthesystematicinvestigationofthin-film
coatingstoprotectapolymerfromhyperthermalatomicoxygen.
Quartz crystal microbalance (QCM) sensors were used to
studytheefficacyofthinAl O ALDfilmstoprotectapolymer.
2 3
Polyimidesubstrateswerepreparedon0.5-inch-diameterQCM
disc sensors. The precursor to the polyimide was Pyralin®
PI5878G from HD Microsystems. This polyamic acid as pur-
chased was dissolved in 1-methyl-2-pyrrolidinone. The polya-
mic acid was diluted and then spin-coated onto gold-coated
QCM sensors. Before spin-coating, the QCM sensors were
modifiedbyapplyinganadhesionpromoter(HDMicrosystems
VM651) and heating the substrates in an air oven to 80 °C to
removeanyresidualwater.Afterspin-coating,thesampleswere
allowed to dry in air under a watch glass for two days. The
sampleswerethenplacedinavacuumfurnace(CenturionVPM)
andcuredwiththefollowingschedule:2hat50°C,2hat80°C,
1 h at 200 °C, and 2 h at 270 °C. The chemical repeat unit of
theresultingpolyimideisequivalenttothatofDuPontKapton®.
ThepolyimidelayersontheQCMdiscsensorsweredetermined
Fig.1.Experimental apparatususedtomeasurethemasslossof spin-coated
by profilometry to have thicknesses in the range of 800–
polyimidelayersonQCMsensorsduringexposuretoahyperthermalatomic-
1000nm. oxygenbeam.
4038 R.Cooperetal./ThinSolidFilms516(2008)4036–4039
Theerosionofpolymersubstratesbyanoxygenplasmainthe
vicinity of coating defects has previously been used to make
defectsmorevisibletoSEMandopticalinspection[30].Defects
intheALDcoatingswouldbeexpectedtobesimilarlyamplified
by exposure to the hyperthermal O-atom beam. Scanning
electron microscope (SEM) images of the exposed surfaces
showed erosion that was consistent with the mass loss
measurements. The SEM images also demonstrated the
sensitivity of the QCM to identify mass loss from erosion at
tinydefectsonthesurface.
Fig. 3 shows SEM imagesfor exposed surfaces with Al O
2 3
ALD films after 17, 21 and 50 AB cycles for atomic oxygen
exposuretimesof500min.Theleftandrightcolumnsconsistof
images collected with low (×20) and high (×1000) magnifica-
Fig.2.Masschangeofpolyimidesubstratescoatedwithvaryingnumbersof tion, respectively. The number and size of defects decrease as
Al2O3ALDABcyclesversusexposuretimetoatomicoxygen. the number of AB cycles increases. The surface after 17 AB
cycles exhibits numerous large round defects that appear as
been described in detail previously [1,10,27,28]. Sample regions where the polymer substrate has eroded significantly.
exposures were performed with a pulsed beam operating at a Thesurfaceafter21ABcycleshasmanysmalldefectsscattered
repetitionrateof2Hz.Thepulsedbeamcontainedhyperthermal overthesurface.Thesurfaceafter50ABcyclesdoesnotshow
O(3P)atomsandO moleculesthatweregeneratedwithalaser- anydefects that can berelatedto theatomic-oxygen exposure.
2
breakdown source [28,29]. Average translational energies for The Al O ALD films after 8, 13, 17 and 21 ABcycles are
2 3
atomic and molecular oxygen were ∼5.4 and ∼9.3 eV, poorprotectingbarriersagainstattackbyoxygenatoms.Earlier
respectively.Theenergywidths(fullwidthathalfmaximum)of QCM and X-ray fluorescence measurements revealed that
the atomic and molecular oxygen components were ∼2.0 and approximately 10–20 AB cycles are required to nucleate a
∼5.3 eV, respectively. The average mole fraction of atomic continuous Al O film on various polymer substrates [22,31].
2 3
oxygeninthebeamwas∼90%. Prior to nucleation, the ALD reactants are observed by QCM
ThehyperthermalO-atombeamwasdirectedatthepolymer measurementstoabsorbintothepolymer[22].Afternucleation,
samples on QCM sensors that were positioned about 40 cm anAl O ALDfilmbarriercontinuouslycoversthepolymerand
2 3
fromtheapexoftheconicalnozzle.Masslossasafunctionof blocksanyreactantabsorptionintothepolymer.
time for both the test and control samples was recorded for at
least 500 min. This time corresponds to 60,000 pulses of the
hyperthermalbeamforeachatomic-oxygenexposure.Basedon
the erosion yield of a Kapton® H polyimide [27], the atomic-
oxygen flux was ∼2×1015 O atoms cm−2 pulse−1.
Mass loss results for Al O ALD film thicknesses resulting
2 3
from8,13,17,21,25and50ABcyclesareshowninFig.2.The
uncoated polyimide control sample eroded away completely
duringeveryexperiment.Thelinearmasslossrateforthecontrol
samplewasconsistentwithalossof600μgcm−2after500min.
Polyimide samples with Al O ALD films after 8 and 13 AB
2 3
cycleserodedbefore500minofexposure.Thesamplewithan
Al O ALDfilmafter17ABcycles showedarapid changeof
2 3
slopeafter300minofexposureandprobablywouldhaveeroded
completelyiftheexposurehadsignificantlyexceeded500min.
The samples with Al O ALD films after 21 and 25 AB
2 3
cyclesshowedaverygraduallossofmassduringtheexposure.
Themasslossforthesampleafter25ABcyclesisonlyslightly
higherthanthemasslossexpectedwhenoxygenatomsremove
residualcontaminationfromthesurface.Thepolyimidesample
coated with an Al O ALD film after 50 AB cycles did not
2 3
exhibitanymeasurablemasschangeaftermorethan500minof
exposure. This Al O ALD film presumably has no defects or
2 3
pinholes where O atoms could attack the polyimide substrate.
The measurements suggest a threshold of ≥25 AB cycles for
Fig.3.SEMimagesofpolyimidesubstratescoatedwith17,21,and50Al O
Al O ALD coatings to protect a polymer substrate from 2 3
2 3 ALDABcyclesafterexposuretoatomicoxygen.Theleftandrightcolumns
erosion by oxygen atoms with ∼5 eVof translational energy. displayimagescollectedwith×20and×1000magnification,respectively.
R.Cooperetal./ThinSolidFilms516(2008)4036–4039 4039
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2 3
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2 3
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2 3
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