Gas diffusion barriers on polymers using Al2 O3 atomic layer deposition

Article abstract of DOI:10.1063/1.2168489

Thin films of Al2 O3 grown by atomic layer deposition (ALD) were investigated as gas diffusion barriers on flexible polyethylene naphthalate and Kapton polyimide substrates. Al2 O3 ALD films with thicknesses of 1-26 nm were grown at 100-175 °C. For Al2 O3 ALD films with thicknesses ≥ nm, oxygen transmission rates were below the MOCON instrument test limit of ～5× 10-3 cc m2 day. Applying a more sensitive radioactive tracer method, H2 O -vapor transmission rates of ～1× 10-3 g m2 day were measured for single-sided Al2 O3 ALD films with thicknesses of 26 nm on the polymers. Ultrathin gas diffusion barriers grown by Al2 O3 ALD may enable organic displays and electronics on permeable, flexible polymer substrates.

Full text of DOI:10.1063/1.2168489

Gas diffusion barriers on polymers using Al 2 O 3 atomic layer deposition
M. D. Groner, S. M. George, R. S. McLean, and P. F. Carcia
Citation: Applied Physics Letters 88, 051907 (2006); doi: 10.1063/1.2168489
View online: http://dx.doi.org/10.1063/1.2168489
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/88/5?ver=pdfcov
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APPLIED PHYSICS LETTERS 88, 051907 ͑2006͒
Gas diffusion barriers on polymers using Al2O3 atomic layer deposition
M. D. Groner
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309
S. M. Georgea͒
Department of Chemistry and Biochemistry and Dept. of Chemical and Biological Engineering, University
of Colorado, Boulder, Colorado 80309
R. S. McLean and P. F. Carcia
DuPont Central Research & Development, Wilmington, Delaware 19803
͑Received 6 July 2005; accepted 6 December 2005; published online 31 January 2006͒
Thin films of Al2O3 grown by atomic layer deposition ͑ALD͒ were investigated as gas diffusion
barriers on flexible polyethylene naphthalate and Kapton® polyimide substrates. Al2O3 ALD films
with thicknesses of 1–26 nm were grown at 100–175 °C. For Al2O3 ALD films with thicknesses
ജ5 nm, oxygen transmission rates were below the MOCON instrument test limit of ϳ5
ϫ10−3 cc/m2/day. Applying a more sensitive radioactive tracer method, H2O-vapor transmission
rates of ϳ1ϫ10−3 g/m2/day were measured for single-sided Al2O3 ALD films with thicknesses of
26 nm on the polymers. Ultrathin gas diffusion barriers grown by Al2O3 ALD may enable organic
displays and electronics on permeable, flexible polymer substrates. © 2006 American Institute of
Physics. ͓DOI: 10.1063/1.2168489͔
The development of organic light-emitting diode
͑OLED͒ devices may lead to efficient displays fabricated in-
expensively on flexible substrates.1 One obstacle to this de-
velopment is the facile permeation of O2 and H2O through
the flexible plastic substrates. For most polymers, the trans-
mission rates are generally ജ1 cc/m2/day for O2 and
ജ1 g/m2/day for water.1 The permeability of polymer sub-
strates can be reduced by about two orders of magnitude
using single-layer inorganic coatings.1–3 These levels of bar-
rier improvement are sufficient for applications such as liq-
uid crystal displays and food packaging. However, the re-
quirements for reducing O2 and H2O permeability are orders
of magnitude more demanding for OLEDs.1,2,4,5 Barrier im-
provements of 105–106 are needed to exclude O2 and H2O
that can seriously degrade both the light-emitting polymer
and the water-sensitive Ca or Ba metal cathode in OLEDs.1
A defect-free, continuous thin-film coating of an inor-
ganic material should ideally be impermeable to atmospheric
gases. Unfortunately, most thin films have pinholes and de-
fects caused by the deposition process or substrate imperfec-
tions that compromise the barrier properties. Grain bound-
aries in a barrier film can also present a pathway for facile
permeation. Recent studies have recommended a multilayer
structure as the only practical way to achieve “ultrabarrier”
performance.1,5 Barriers with alternating inorganic/organic
layers with as many as 12 individual layers reportedly ap-
proach the performance needed by OLEDs.5 However, more
recent detailed measurements and modeling of these
multilayer structures attribute the “apparent” low transmis-
sion rate for these multilayers to long lag times and not to
reduction in steady state permeability.4 This recent study also
highlights the need to deposit more perfect single-layer bar-
rier films.
that have been measured for Al2O3 ALD films are consistent
with defect-free films needed for superior permeation
barriers.8,9 High quality Al2O3 films can be deposited by
ALD at temperatures as low as 33 °C.9 These temperatures
are compatible with most thermally fragile plastic substrates.
In this work, Al2O3 ALD films grown directly on polyethyl-
ene naphthalate ͑PEN͒ and Kapton® substrates are investi-
gated as water and oxygen permeation barriers. Earlier stud-
ies have measured water vapor transmission rates for Al2O3
ALD coated on polyethersulfone ͑PES͒ substrates.3
Al2O3 ALD was performed in a hot-wall ALD flow re-
actor using sequential, self-limiting exposures to trimethyla-
luminum ͑TMA͒ ͑Aldrich͒ and water ͑Fisher HPLC-grade͒.7
The reaction chamber was built to accommodate 4 in. square
PEN and Kapton® substrates. This chamber was 3 in. tall
and 6 in. diameter and was equipped with a loading port.
Substrates were taped to 4-in. Si wafers to achieve single-
sided ALD coatings. Multiple substrates could be coated si-
multaneously by stacking them in a cassette-like arrange-
ment.
Polymer films were rinsed and loaded into the reactor in
a laminar flow hood operating at class 100 cleanroom con-
ditions to minimize particle contamination. Al2O3 ALD
growth temperatures ranged from 100–175 °C. Nitrogen gas
flowed through the reactor at 100 sccm and produced a pres-
sure of ϳ1 Torr. A typical ALD cycle at 120 °C consisted of
a 0.1 s TMA exposure, a 30 s purge, a 0.15 s water exposure,
and another 30 s purge. The substrates were PEN and
Kapton® polyimide with glass transition temperatures of
Tg=126 °C and Tgϳ300 °C, respectively.
Al2O3 ALD film thicknesses on the polymer films were
determined using an n&k 1280 Analyzer metrology system.
This instrument is able to determine film thicknesses on
transparent substrates using transmittance and reflectance
measurements. These results agreed with growth rates for
Al2O3 ALD of 1.2–1.3 Å/cycle that were measured on Si
wafers.7–9 This agreement indicates a rapid nucleation of
Al2O3 on PEN and Kapton®. Rapid nucleation of Al2O3
Atomic layer deposition ͑ALD͒ is a process that can de-
posit smooth, conformal, and pinhole-free films with a nearly
featureless structure.6–8 The excellent dielectric properties
a͒
Electronic mail: steven.george@colorado.edu
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0003-6951/2006/88͑5͒/051907/3/$23.00 88, 051907-1 © 2006 American Institute of Physics
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051907-2
Groner et al.
Appl. Phys. Lett. 88, 051907 ͑2006͒
FIG. 1. Schematic of the HTO water test to measure the water vapor trans-
mission rate. LiCl absorbs the HTO that permeates through the ALD-coated
polymer film.
FIG. 2. Water vapor transmission rate ͑WVTR͒ data for a PEN film with a
26 nm Al2O3 ALD coating. The WVTR averaged 1.1ϫ10−3 g/m2 /day.
ALD has also been demonstrated on other polymers.10,11
Oxygen transmission rates ͑OTRs͒ were measured using
a commercial MOCON OX-TRAN 2/21 instrument with a
detection limit of ϳ5ϫ10−3 cc/m2/day. OTR measurements
at 23 °C and 50% relative humidity were performed on a
series of Al2O3 ALD films with thicknesses of 1, 5, 10, and
26 nm. Growth temperatures were 100 and 125 °C on PEN,
and 125, 150, and 175 °C on Kapton®. OTR measurements
of polymers with an Al2O3 ALD thickness of 1 nm yielded
no improvement over uncoated polymer films with an OTR
of ϳ1 cc/m2/day. Al2O3 ALD films with thicknesses of 5,
10, and 26 nm achieved OTR values below the MOCON test
limit on both PEN and Kapton® at all growth temperatures.
OTR values of Ͻ5ϫ10−3 cc/m2/day have never been
reported for such thin single-layer coatings on polymers.1
These OTR results indicate a very low critical thickness of
between 1 and 5 nm for the onset of useful barrier properties.
In comparison, SiO2 barrier films grown by plasma-enhanced
chemical vapor deposition ͑PECVD͒ on polyethythene
terephthalate ͑PET͒ have a corresponding critical thickness
of 15 nm and a OTR of ϳ0.4 cc/m2/day for thick films
Ͼ100 nm.12 Unfortunately, the MOCON OTR test is not
sensitive enough to measure these low OTR values or to
characterize the effect of parameters such as film thickness
and growth temperature.
More sensitive water vapor transmission rates ͑WVTRs͒
were obtained using tritiated water ͑HTO͒ as a radioactive
tracer.13,14 The HTO test chamber is shown in Fig. 1. A drop-
let of HTO ͑Perkin Elmer, 1 mCi/ml͒ was placed on the
bottom flange. This tritiated water with 0.6 parts per million
HTO created a 100% relative humidity environment on the
upstream side of the Al2O3-coated polymer film. The barrier
film to be tested was clamped between a Viton® o-ring and a
reducing flange. The Al2O3 ALD film was on the down-
stream side of the polymer film. A scintillation vial contain-
ing ϳ20 grains of LiCl was suspended in the top part of the
chamber on the downstream side. The hygroscopic LiCl ab-
sorbs the HTO and water permeating through the polymer
film as well as the background water in the chamber.
once per day for several weeks until the transmission rate
stabilized at a constant value.
The detection limit of the HTO test was confirmed by
testing an impermeable aluminum foil. The resulting tritium
decay counts were at the background level for the scintilla-
tion counter. These results were consistent with a WVTR
detection limit of ϳ1ϫ10−6 g/m2/day. This HTO test detec-
tion limit is over 1000 times more sensitive than the detec-
tion limit of the MOCON PERMATRAN-W instrument.
WVTR measurements were performed on a series of
Al2O3 ALD films with thicknesses of 2.5, 5, 10, and 26 nm.
Growth temperatures were 100 and 120 °C on PEN, and
100, 120, 150, and 175 °C on Kapton®. WVTR measure-
ments of Al2O3 ALD-coated polymers with a thickness of
2.5 nm yielded no improvement over uncoated polymer films
that had a WVTR of ϳ1 g/m2/day. Al2O3 ALD films with
thicknesses of 5 nm yielded rates about an order of magni-
tude lower than the uncoated polymers. Al2O3 ALD films
with a thickness of 10 nm achieved a WVTR of ϳ2
ϫ10−3 g/m2/day. Al2O3 ALD films with a thickness of
26 nm obtained transmission rates of ϳ1ϫ10−3 g/m2/day
on both PEN and Kapton® at all growth temperatures.
A plot of the water vapor transmission rate versus time
for a 26 nm thick Al2O3 film is shown in Fig. 2. Each hori-
zontal line represents the average WVTR over the time pe-
riod covered by the line. The time periods varied from ap-
proximately one to four days. The average WVTR for this
film is 1.1ϫ10−3 g/m2/day. A summary of all the WVTR
measurements versus Al2O3 ALD thickness is shown in Fig.
3.
Comparing these results to SiO2 PECVD barriers on
PET,12 the critical thickness was 5 nm for Al2O3 ALD versus
25 nm for SiO2 PECVD. The thick film WVTR was ϳ1
ϫ10−3 g/m2/day at 26 nm for Al2O3 ALD versus
0.2 g/m2/day at Ͼ100 nm for SiO2 PECVD. The results are
far superior for the Al2O3 ALD films. Recently, another
study measured a WVTR of ϳ6ϫ10−2 g/m2/day for 30 nm
thick Al2O3 ALD films coated on both sides of a PES
substrate.3 The WVTR values for the single Al2O3 ALD
films with thicknesses of 10 and 26 nm in this study are over
an order of magnitude lower than the reported WVTR values
on PES.
The vial was periodically removed and replaced with a
new vial. The LiCl-containing HTO was dissolved in 1 ml of
water and 3 ml of PerkinElmer Ultima Gold LLT scintilla-
tion cocktail. A Packard 1600TR scintillation counter was
then used to count the tritium decays and to calculate the
The barrier properties of an Al2O3 ALD film with a
thickness of 10 nm were also evaluated using the “Ca
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HTO transmission rates. Measurements were taken usually
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051907-3
Groner et al.
Appl. Phys. Lett. 88, 051907 ͑2006͒
ing. This lack of degradation in barrier performance indicates
that the Al2O3 ALD films have not cracked after bending.
In summary, flexible gas diffusion barriers with a WVTR
of ϳ1ϫ10−3 g/m2/day were achieved using ultrathin Al2O3
ALD coatings with thicknesses of 26 nm on PEN and
Kapton® substrates. These results are exceptional for such
thin barrier layers. This Al2O3 ALD barrier technology is an
important step forward to enable a broad class of flexible
organic electronics on polymers.
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