Gas Diffusion Barriers on Polymers Using Multilayers Fabricated by

Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, DuPont Central Research and Development, ... Stretchable Se...
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J. Phys. Chem. C 2008, 112, 4573-4580

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Gas Diffusion Barriers on Polymers Using Multilayers Fabricated by Al2O3 and Rapid SiO2 Atomic Layer Deposition Arrelaine A. Dameron,†,| Stephen D. Davidson,§ Beau B. Burton,† Peter F. Carcia,‡ R. Scott McLean,‡ and Steven M. George*,†,§ Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309, DuPont Central Research and DeVelopment, Wilmington, Delaware 19803, and Department of Chemical and Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed: August 28, 2007; In Final Form: NoVember 21, 2007

Single Al2O3 atomic layer deposition (ALD) films on polymers have demonstrated excellent gas diffusion barrier properties. Further improvements can be achieved using multilayers of Al2O3 ALD layers with other inorganic layers. In this study, multilayers of Al2O3 ALD and rapid SiO2 ALD were grown on Kapton and heat-stabilized polyethylene naphthalate substrates. Transmission rates for tritium through the films were measured using the radioactive HTO tracer method. Comparison with previous Ca tests and tritium exchange experiments with alcohols indicated that the tritium in HTO may diffuse as either molecular HTO or atomic tritium. Assuming that the tritium diffuses only as HTO, single Al2O3 ALD films reduced the effective water vapor transmission rate (WVTR) to ∼1 × 10-3 g/m2/day. When the Al2O3 ALD film was directly exposed to the saturated H2O/HTO vapor pressure, the barrier properties deteriorated markedly after 4-5 days. Al2O3 ALD barriers that were not subjected to direct H2O/HTO exposure indefinitely maintained low tritium transmission rates (TTRs). Rapid SiO2 ALD layers deposited on the Al2O3 ALD layer improved the diffusion barrier properties and protected the Al2O3 ALD layers from H2O corrosion. The effective WVTR also reduced to ∼1 × 10-4 g/m2/day for the Al2O3/SiO2 ALD bilayer. Multilayers of Al2O3/SiO2 bilayers initially further reduced the TTRs. Two Al2O3/SiO2 bilayers reduced the effective WVTR to ∼5 × 10-5 g/m2/day. Multilayers composed of >2 Al2O3/SiO2 bilayers displayed degraded performance and effective WVTRs that were comparable with the single Al2O3 ALD film. These multilayer barriers may have cracked during handling and mounting as a result of brittleness at larger thicknesses. The barrier improvement observed for one Al2O3/SiO2 bilayer and two Al2O3/SiO2 bilayers could not be explained using laminate theory. The improvement suggests that the rapid SiO2 ALD layer successfully closed pinhole defects in the Al2O3 ALD layer.

I. Introduction Organic light emitting devices (OLEDs) have great potential for full-color flat-panel display technology. A major obstacle for the development of flexible OLEDs (FOLEDs) on polymers is the high permeability of H2O and O2 through the polymer substrates.1,2 These gases can oxidize the low work function alkali-metal cathode in the OLEDs.3,4 Inorganic thin film gas diffusion barriers can significantly reduce the H2O and O2 gas permeability. However, these inorganic films are not currently able to meet the challenge of a water vapor transmission rate (WVTR) of 10 000 h.2,5,6 Atomic layer deposition (ALD) techniques have recently been employed to deposit inorganic films on polymers. The excellent properties of ALD films resulting from sequential, self-limiting surface reactions should produce films that are effective gas diffusion barriers. Electrical measurements of Al2O3 ALD films indicate that the films are nearly pinhole-free.7 Transmission * Corresponding author. E-mail: [email protected]. † Department of Chemistry and Biochemistry, University of Colorado. ‡ DuPont Central Research and Development. § Department of Chemical and Biological Engineering, University of Colorado. | Current address: Dynamic Organic Light, 2410 Trade Center Ave., Longmont, CO 80503.

electron microscope images of the Al2O3 ALD films also show excellent conformality.8 Al2O3 ALD has been employed to encapsulate OLEDs with reasonable success.9-11 An Al2O3 ALD capping layer has improved the performance of organic solar cells12 and polymer-based transistors.13 Plasma-assisted Al2O3 ALD has also recently demonstrated effective gas diffusion barrier properties on polymers for deposition performed at room temperature.14 We have explored thermal Al2O3 ALD films as gas diffusion barriers. Our recent work using the radioactive HTO tracer method has shown that permeation rates through a single Al2O3 ALD layer decrease with increasing thicknesses until an optimal thickness is reached. A single Al2O3 ALD layer with a thickness of >10 nm on Kapton or heat-stabilized polyethylene naphthalate (HSPEN) polymer substrates can reduce the water vapor transmission rate to ∼1 × 10-3 g/m2/day at room temperature.15 Additional measurements using the Ca test method have demonstrated that the WVTR is 1.7 × 10-5 g/m2/day at 38 °C.16 Extrapolations indicate that the WVTR may be as low as 6 × 10-6 g/m2/day at room temperature.16 To understand the differences between the HTO tracer and Ca tests, we have explored tritium delivery from either HTO or alcohols with a tritiated hydroxyl group in this paper. The similarity between the different tritium sources at low tritium transmission rates

10.1021/jp076866+ CCC: $40.75 © 2008 American Chemical Society Published on Web 03/05/2008

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(TTRs) suggests that tritium can permeate the ALD-coated polymer films either by HTO diffusion or atomic tritium diffusion. Although the low WVTRs produced by a single ultrathin Al2O3 ALD film are very promising, additional improvements are needed to reach and exceed FOLED requirements. Multilayer films can be employed to reduce further the WVTR.17 This paper has targeted Al2O3/SiO2 bilayers because Al2O3 and SiO2 are both utilized extensively as gas diffusion barriers on polymers. Depending on the applicable mechanism of gas diffusion, multilayers of Al2O3/SiO2 bilayers may provide significant improvement. Laminate theory would predict a total permeability of 1/PT ) ∑ 1/PL where the PL values are the permeabilities in the individual layers.17 The permeability may be reduced more substantially if the individual layers in the multilayer act to seal the defects in the underlying layers. The mechanisms of gas diffusion in multilayer films have been discussed in an excellent review.17 The Al2O3 and SiO2 layers in the multilayer films are both grown using ALD methods. ALD is based on sequential, selflimiting surface reactions and deposits conformal films with atomic-level thickness control.18 Al2O3 ALD film growth using Al(CH3)3 (trimethylaluminum (TMA)) and water has been well characterized and proceeds according to two self-limiting surface reactions19-21

(A) AlOH* + Al(CH3)3 f AlOAl(CH3)2* + CH4 (1) (B) AlCH3* + H2O f AlOH* + CH4

(2)

where the asterisks denote the surface species. One AB reaction cycle typically produces a 1.2 Å thick layer of Al2O3 ALD.20,21 Previous studies have demonstrated that Al2O3 ALD can be conducted at low temperatures and can be performed on polymers.20,22 The SiO2 layers are grown using rapid SiO2 ALD.23 Rapid SiO2 ALD uses various silanol precursors to deposit SiO2 layers with thicknesses up to ∼120 Å in only one silanol exposure.23 Although the mechanism of rapid SiO2 ALD is not completely understood, this unique SiO2 ALD requires a catalyst on the surface. This catalyst can be in the form of aluminum from TMA. In this study, rapid SiO2 ALD was performed using tris(tert-pentoxy)silanol and TMA. The SiO2 ALD is believed to occur via siloxane polymerization.23 First, TMA binds to available hydroxylated sites. Subsequently, the silanol reacts at the Al centers to release CH4:

(A) SiOH* + Al(CH3)3 f SiOAl(CH3)2* + CH4

(3)

(B1) SiOAlCH3* + (CH3CH2C(CH3)2O)3SiOH f SiOAlOSi(OC(CH3)2CH2CH3)3* + CH4 (4) Additional silanol precursors then can insert at the Al centers and release tert-pentanol:

(B2) SiOAlOSi(OC(CH3)2CH2CH3)3* + (CH3CH2C(CH3)2O)3SiOH f SiOAlOSi(OC(CH3)2CH2CH3)2-O-Si(OC (CH3)2CH2CH3)3* + CH3CH2C(CH3)2OH (5) The polymerization reaction is believed to proceed as long as the silanol can diffuse to the Al catalyst.23 Crosslinking reactions between the siloxane chains are in competition with the silanol diffusion. First, the tert-pentoxy

ligands eliminate isopentylene and leave behind hydroxyl groups:

(C) -OSiOC(CH3)2CH2CH3* f -OSiOH* + H2CdCCH3CH2CH3 (6) The hydroxyl groups can subsequently react with other hydroxyl groups to yield H2O and cross-linking Si-O-Si bonds. Once the polymerization reaction stops, TMA is redeposited on the SiO2 surface to repeat the rapid SiO2 ALD reaction sequence. Our recent studies of rapid SiO2 ALD indicate that the silanol precursors can form a multilayer on the underlying surface. The formation of a multilayer of silanol monomers may be significant because this multilayer may be able to form easily in defects and pinholes through capillary condensation. This preferential condensation in defects and pinholes may help to seal the defects and pinholes and reduce the gas permeability much more than that expected from laminate theory. The growth of rapid SiO2 ALD films using tris(tert-pentoxy) silanol and the mechanism of this growth will be the topic of another paper.24 This paper will measure the TTRs through bilayers of Al2O3/ SiO2 on Kapton and HSPEN substrates using the radioactive HTO tracer method. HTO measurements will compare single Al2O3 layers and Al2O3/SiO2 bilayers as a function of the initial Al2O3 ALD film thickness. Additional experiments will explore the corrosive effect of H2O at saturation vapor pressure on the Al2O3 ALD layer and the effect of SiO2 ALD as a protection layer. Subsequently, the HTO measurements will be performed for various numbers of Al2O3/SiO2 bilayers on the Kapton and HSPEN polymer substrates. These experiments provide guidance for the continued improvement of ALD gas diffusion barriers on polymers. II. Experimental Section A. ALD Film Growth. The multilayer ALD films were grown on both Kapton and heat-stabilized polyethylene naphthalate. The Kapton was type 200EZ from DuPont and was 50 µm thick. The HSPEN was type Teonix Q65 from DupontTeijin and was 125 µm thick. Prior to deposition, the smooth sides of the polymer films were steam-cleaned in solvent vapor, dried, and stored under class 100 clean room conditions. The films were taped with the smooth side up to base-bath-cleaned glass slides during deposition. The glass slide support was removed before the TTR measurements using the radioactive HTO tracer method. All ALD films were deposited in a ∼4.7 L hot-wall viscous flow reactor.15,25 To control particle contamination, this ALD reactor was housed inside an enclosure that was controlled by a laminar flow hood operating under class 100 clean room conditions. The samples on glass supports were loaded into the 6 in. diameter by 6 in. main chamber under class 100 conditions. Multiple samples were fabricated simultaneously by loading the samples in the ALD reactor in a stacked fashion. Gaps between the samples allowed for adequate gas flow. The reactor was backed by a mechanical pump that was isolated from the chamber by a gate valve. The reactor was heated from the outside with heaters (Watlow) that were operated with a PID controller (Eurotherm). All films were deposited at a reactor temperature of 175 °C. The Al2O3 ALD was grown using sequential exposures of TMA (97%, Sigma Aldrich) and water (Optima Grade, Fischer) by methods described previously.20 The Al2O3 ALD films were fabricated by alternating the entrainment of the TMA and H2O precursors into the carrier gas under viscous flow.25 The Al2O3

Gas Diffusion Barriers on Polymers was deposited using a 0.25 s TMA exposure time and a 0.15 s H2O exposure time. The exposures were controlled using pneumatic valves and corresponded to ∼70-100 mTorr precursor pressure transients above the baseline pressure. Between each TMA and H2O exposure, the reactor was purged for 60 s. Ultrahigh purity N2 (99.999%, Airgas) was used as both the carrier gas and the purge gas. The N2 was flowed through the reactor at 100 sccm. This flow resulted in base pressures between 900 mTorr and 1.4 Torr depending on the particular mechanical pump. Tris(tert-pentoxy)silanol (99.99%, Sigma-Aldrich) was degassed with freeze-pump-thaw cycles prior to use. Contrary to the Al2O3 ALD that was always performed using a N2 carrier gas, the rapid SiO2 ALD was performed in a semi-static mode. First, the N2 flow was turned off and the reactor was pumped out for 80 s. Subsequently, the gate valve was closed and TMA was introduced into the reactor for 0.20 s, resulting in a ∼350 mTorr pressure transient. After a 3 s TMA exposure time, the gate valve was opened and the TMA was pumped from the reactor for 15 s. Subsequently, the reactor was purged using a N2 flow for 120 s. This procedure completed the first half cycle of the rapid SiO2 ALD reaction. The N2 flow was then shut off, and the reactor was pumped out for 80 s. The gate valve was subsequently closed, and the silanol was introduced for 1.5 s, resulting in a 1 Torr pressure increase. After a 5 s exposure time, the gate valve was opened and the silanol was pumped out for 45 s. The gate valve was then closed again before repeating this silanol exposure sequence. The silanol was exposed for a total of 10 times. TMA was exposed only once before the first of these 10 silanol exposures. Subsequently, the N2 flow was resumed and the reactor was purged for 120 s to complete the second half cycle of the rapid SiO2 ALD reaction. Prior to deposition of rapid SiO2 ALD on bare polymer samples, a thin 5 AB cycle Al2O3 ALD seed layer was grown on the polymers. The rapid SiO2 ALD then was performed on this thin Al2O3 seed layer as described above. For rapid SiO2 ALD layers grown on the Al2O3 ALD films, an additional TMA exposure is not needed because the Al2O3 ALD film can catalyze the rapid SiO2 growth. Multilayers were fabricated by alternating the growth of Al2O3 ALD and rapid SiO2 ALD. In all cases, the ALD films were deposited immediately following the deposition of the previous ALD layer without exposure to the ambient environment. B. X-ray Reflectivity Analysis. The thicknesses of each layer in the ALD multilayer films were determined by X-ray reflectivity (XRR) analysis. These multilayer films were simultaneously deposited on the polymers and boron-doped p-type Si(100) substrates (Silicon Valley Microelectronics, Inc.). The XRR analysis was performed on the films on the Si(100) wafers using a Bede D1 high-resolution X-ray diffractometer from Bede Scientific, Inc. The Bede D1 diffractometer was equipped with a Cu KR X-ray tube with a wavelength of 1.54 Å. A filament current of 40 mA and a voltage of 40 kV were used for the measurements. For each sample, a ω-2θ scan was performed using 10 s per step acquisition time and a 15 arcsec step size from 300 to 7000 arcsec. The XRR data was fit using the REFS fitting software from Bede Scientific, Inc., to extract independently the thickness, density, and roughness of each material in the multilayer samples. C. Tritium Transmission Rate (TTR) Measurement. The barrier properties of the ALD-coated polymer films were tested using the radioactive HTO tracer method.15,26,27 HTO is the

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Figure 1. Schematic depicting the HTO test apparatus with the ALDcoated polymer film in position in the UP orientation.

source of tritium that can diffuse through the films either as molecular HTO or possibly as tritium atoms. To make the TTR measurements, the ALD-coated polymer films were sealed onto a stainless-steel vessel using a Viton O-ring.15 The barrier film separated the top and bottom of the vessel. The Viton O-ring defined a 41 cm2 test area on the ALD-coated polymer film. A schematic of the experimental setup with an ALD-coated polymer film in position is shown in Figure 1. All testing was performed at room temperature and atmospheric pressure. At the beginning of the test, a 50 µL droplet of 1 mCi/mL HTO (Perkin-Elmer Lifesciences) was placed in the bottom half of the vessel. After sealing of the vessel, the humidity on the HTO source side equilibrated to 100% relative humidity (RH). The HTO could not move to the top half of the vessel without passing through the ALD-coated polymer film. A scintillation vial containing ∼80 mg of LiCl (Aldrich) was suspended in the top half of the vessel.15 This vial was then removed periodically from the vessel without disturbing the ALD-coated polymer film. The LiCl absorbed the tritium contained in the HTO that had permeated through the polymer film. The LiCl also absorbed residual H2O from the walls of the vessel or H2O from the laboratory air. The vial containing the LiCl salt was exchanged every 1-3 days with a fresh vial containing new LiCl salt. The LiCl salt containing the tritium was then dissolved in 1 mL of deionized water and 3 mL of scintillation cocktail (Ultima gold LLT, Perkin-Elmer Lifesciences).15 The tritium decays were counted twice using a Packard 1600 TR scintillation counter. The TTRs and the WVTRs were calculated from the average of two measurements. The determination of the WVTRs assumed that all the tritium diffused through the ALD-coated polymer as molecular HTO. Each measured WVTR or TTR value was an average WVTR or TTR for the time duration since the last measurement. In the figures, this average is shown as a constant rate between the two time points. As a result, the graphs of WVTR or TTR versus time have a discrete appearance with steps in the data. These steps are the result of the time intervals in the data collection. The high or low detection limits were determined experimentally with either no polymer film or an impermeable Al foil, respectively. These detection limits are inherent to the scintillation counter, and they do not scale with sample area. The resulting high and low WVTR limits were 2.93 and 5.87 × 10-6 g/m2/day, respectively. The uncoated Kapton and HSPEN films were also tested for comparison with the ALDcoated films. The average WVTRs were 0.74 and 0.18 g/m2/ day, for the uncoated Kapton and HSPEN films, respectively.

4576 J. Phys. Chem. C, Vol. 112, No. 12, 2008 For the HTO tests, the ALD coating on the polymer film was oriented in one of two ways. In one orientation, the ALD coating faced the HTO source and the water was required to pass first through the ALD coating. This orientation is called a DOWN orientation. The ALD coating could also be oriented facing the LiCl salt and away from the HTO vapor pressure. In this case, the water passes first through the polymer film. This orientation is called a UP orientation. D. Mechanism of Tritium Diffusion. The exact mechanism of tritium (3H) travel through the ALD-coated polymer film is not completely understood. Previous results show a discrepancy between the WVTRs obtained by the radioactive HTO tracer method and the Ca test for the same samples.15,16 The Ca test measured WVTRs that were lower than the rates obtained by the HTO tracer method.16 The Ca test measures the transmission of O species since the Ca is oxidized to either CaO or Ca(OH)2.28,29 The HTO tracer method measures the transmission of tritium. Tritium may diffuse through the polymer film as molecular HTO or may travel as atomic 3H. This atomic migration may be the result of H/T exchange30 and tritium atomic diffusion.31,32 To understand the mechanism of tritium diffusion, additional experiments were performed to determine if tritium can migrate through the barrier films by other mechanisms in addition to molecular HTO diffusion. Tritiated linear alcohols were prepared by isotope exchange. These alcohols were larger in size and have different chemical properties than water. Consequently, the alcohols should diffuse through the polymer barrier films at different rates than water. However, if the 3H atoms are capable of exchanging with surface hydrogens on the hydroxyl groups of the Al2O3 and SiO2 surfaces, then the 3H atoms may diffuse through the metal oxide films as tritium atoms independent of the source of the tritium. Anhydrous n-propanol, anhydrous n-hexanol, sodium chloride, and molecular sieve with a pore diameter of 3 Å were obtained from Sigma-Aldrich. These materials were all used as received to determine the tritium diffusion mechanism. Tritiated n-propanol (ProOT) and tritiated n-hexanol (HexOT) were prepared by isotope exchange. In a small vial, 0.5 mL of 1 mCi/ mL HTO was added to 0.5 mL of the alcohol. The miscible mixture was vigorously shaken for 5 min. Sodium chloride was then added to the mixture until reaching saturation. The mixture was vigorously shaken again for an additional 5 min. The salt produced a two-phase mixture. The alcohol was removed from the vial and placed into a separate vial containing a few grains of 3 Å molecular sieve. The alcohol was dried a second time over molecular sieve and was stored frozen with a third portion of molecular sieve. The tritium partitioned between the molar quantities of hydroxyl hydrogens in the alcohol and water in the original 0.5 mL volumes. The resulting tritiated alcohol concentrations were 8.1 mCi/mol or 0.10 mCi/mL for ProOT and 8.5 mCi/mol or 0.07 mCi/mL for HexOT. The HTO tests were then performed as described above using 50 µL of tritiated alcohol instead of HTO. Al2O3 ALD films with thicknesses of 0, 2.5, 5, 10, and 26 nm were grown on HSPEN films. The polymer films were then tested simultaneously with HTO, ProOT, and HexOT. III. Results and Discussion A. Single Al2O3 ALD Layers and Al2O3/SiO2 Bilayers. Figure 2 shows the TTRs and effective WVTRs versus time for two 26 nm Al2O3 ALD films on Kapton at room temperature. The sample with the Al2O3 ALD layer oriented away from the HTO vapor pressure (UP) displayed a TTR of 4.2 × 10-11 (

Dameron et al.

Figure 2. TTRs and effective WVTR versus time for Al2O3 ALD films with thicknesses of 26 nm on Kapton films. UP and DOWN refer to the orientations away from and in direct contact with the saturated water vapor, respectively.

2.5 × 10-11 mol T/m2/day and an average effective WVTR of 1.2 × 10-3 ( 7.0 × 10-4 g/m2/day. This effective WVTR was constant throughout the duration of the experiment. The sample with the Al2O3 ALD layer placed in direct contact with the HTO vapor pressure (DOWN) initially displayed an effective WVTR of ∼1 × 10-3 g/m2/day. However, the WVTR then started increasing after 130 h. The film failed after 160 h and resulted in an effective WVTR slightly below the effective WVTR measured for the Kapton films without Al2O3 ALD coatings. The DOWN samples were in contact with 100% RH HTO vapor for the duration of the test. Water exposure on Al2O3 can lead to the corrosion and reconstruction of the Al2O3 film.33,34 This degradation may lead to the formation of pinholes in the Al2O3 ALD film and result in barrier failure. A number of measurements with the Al2O3 ALD layer in the DOWN position showed failure times between 120 and 180 h. This range in failure times likely results from the variations in testing temperatures at room temperature and in the kinetics of aluminum hydrate formation and Al2O3 film restructuring. On the basis of the earlier literature on alumina degradation, Al2O3 ALD film failure is not expected at H2O RHs e40-50%.33,34 To protect the Al2O3 ALD layer, rapid SiO2 ALD was deposited on the Al2O3 ALD layer using five cycles of rapid SiO2 ALD. These samples had a reproducible SiO2 thickness of 59 ( 2.5 nm. Rapid SiO2 ALD films were also deposited using three cycles of rapid SiO2 ALD. These films generated similar effective WVTR values. However, the SiO2 ALD film thicknesses were less reproducible and ranged from 22 to 30 nm. Under the reaction conditions, the nucleation regime for the rapid SiO2 ALD is approximately four cycles. The thickness deposited by each successive SiO2 ALD cycle increases gradually until the rapid SiO2 ALD growth rate reaches ∼12.5 nm/cycle for >four cycles at 175 °C. Each cycle also increases the roughness of the SiO2 ALD film. The ∼60 nm thick SiO2 films had average surface roughnesses of ∼2.0 nm. Figure 3 shows the TTRs and effective WVTRs versus time for representative films consisting of ∼60 nm of rapid SiO2 ALD deposited on top of 26 nm Al2O3 ALD barrier films. The rapid SiO2 ALD layer prevented the barriers from failure for at least 1000 h of testing regardless of sample orientation. For the DOWN samples, the rapid SiO2 ALD film provides a material barrier between the water vapor and the Al2O3 ALD layer. The additional SiO2 ALD also dramatically improves the effective WVTRs. The average TTRs were 1.2 × 10-11 ( 1.15 × 10-11 mol T/m2/day and 2.7 × 10-12 ( 1.2 × 10-12 mol T/m2/day

Gas Diffusion Barriers on Polymers

Figure 3. TTRs and effective WVTR versus time for Al2O3/SiO2 bilayers composed of 26 nm Al2O3 ALD and ∼60 nm rapid SiO2 ALD on Kapton films in the UP and DOWN orientations.

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Figure 5. TTRs and effective WVTR versus Al2O3 ALD thickness on HSPEN films for Al2O3 ALD films by themselves and for Al2O3 ALD films with ∼60 nm rapid SiO2 ALD films in the UP and DOWN orientations. The data points are located at Al2O3 ALD thicknesses of 0, 2.5, 5, 10, and 26 nm and have been shifted slightly for clarity.

enhanced chemical vapor deposition on poly(ethylene terephthalate).35-37 Figure 4 also illustrates that there was no observable difference between the UP and DOWN samples for the SiO2 ALD films on Kapton. According to laminate theory, the permeation rate for a multilayer can be derived from the permeation rates in each layer.17 For the Al2O3/SiO2 bilayer, laminate theory would predict a total permeability, PT, defined by

1 1 1 ) + PT PAl2O3 PSiO2 Figure 4. TTRs and effective WVTR versus time for rapid SiO2 ALD films with thicknesses of ∼60 nm on Kapton films in the UP and DOWN orientations.

for the UP and DOWN samples, respectively. The average effective WVTRs were 3.5 × 10-4 ( 3.3 × 10-4 g/m2/day and 7.8 × 10-5 ( 3.5 × 10-5 g/m2/day for the UP and DOWN samples, respectively. The SiO2 ALD film is expected to protect the Al2O3 ALD film because SiO2 does not form a hydrate with water. Thermochemical calculations using HSC Chemistry (Outokumpu Research Oy, Pori, Finland) indicate that the reaction SiO2 + 2H2O f Si(OH)4 is endothermic and has a large positive Gibbs free energy of ∆G ) +67 kcal/mol at 100 °C. In contrast, the reaction Al2O3 + 3H2O f 2Al(OH)3 is exothermic and has a much smaller positive Gibbs free energy of ∆G ) +6 kcal/mol at 100 °C. The Al2O3 ALD films may also be more able to form a hydrate since the Al2O3 ALD film is amorphous and has a lower density than various crystalline forms of Al2O3.20,21 To understand the effect of the SiO2 ALD layer on the Al2O3 ALD layer, TTRs and effective WVTRs were measured for the SiO2 ALD layers by themselves. Figure 4 shows plots of the TTRs and WVTRs versus time for two representative ∼60 nm SiO2 ALD films on Kapton. The rapid SiO2 ALD films by themselves make poor permeation barriers. All the ∼60 nm SiO2 samples yielded TTRs of ∼5 × 10-9 mol T/m2/day and effective WVTRs of ∼ 1 × 10-1 g/m2/day. These effective WVTRs are better than or comparable with previously reported WVTR values measured for silicon oxide layers deposited by plasma-

(7)

The effective WVTRs are PAl2O3 ) 1 × 10-3 g/m2/day and PSiO2 ) 1 × 10-1 g/m2/day. On the basis of eq 7, the total effective WVTR for the bilayer is expected to be PT ) 9.9 × 10-4 g/m2/ day. In contrast, the measured effective WVTR is PT ) 1 × 10-4 g/m2/day. This disagreement argues that the permeability is not occurring through homogeneous films and laminate theory is not applicable. Figure 5 shows the TTRs and effective WVTRs for various thicknesses of Al2O3 ALD on HSPEN. In agreement with earlier reports on the effectiveness of Al2O3 ALD gas diffusion barriers,15 the effective WVTR is reduced significantly at Al2O3 ALD thicknesses >5 nm. The effective WVTR then levels off at a value of ∼1 × 10-3 g/m2/day. This behavior is known to be consistent with gas permeability through pinhole defects in barrier films.35 This indication of pinhole defects and the disagreement of the Al2O3/SiO2 bilayer results with laminate theory argue that the rapid SiO2 ALD is filling pinhole defects in the Al2O3 ALD layer. Figure 5 also shows the TTRs and effective WVTRs for Al2O3/SiO2 bilayers that have different Al2O3 ALD film thicknesses and an additional ∼60 nm rapid SiO2 ALD film. The additional rapid SiO2 ALD layer on the Al2O3 ALD layer improves the TTRs and effective WVTRs for all Al2O3 ALD layer thicknesses. The effect of the rapid SiO2 ALD layer was less for thin Al2O3 ALD films e5 nm that were marginal diffusion barriers. The Ca test performed at DuPont16 also confirmed higher WVTRs for the rapid SiO2 ALD layer on Al2O3 ALD films that were 2.5 and 5.0 nm thick. The rapid SiO2 ALD layer reduced the permeabilities approximately an order of magnitude for the thicker Al2O3 ALD films. The Al2O3/

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Figure 6. TTRs and effective WVTR versus number of Al2O3/SiO2 bilayers on Kapton in the UP and DOWN orientations for bilayers with thicknesses of 26 nm Al2O3 ALD and ∼60 nm rapid SiO2 ALD. Except for the point at 0.5 bilayer, the data points are located at integral numbers of bilayers and have been shifted slightly for a clearer presentation.

SiO2 bilayers follow the same trend as the single Al2O3 layers. An asymptotic minimum value is reached at a thickness >10 nm. Previous studies have utilized rapid SiO2 ALD to fill pores in porous structures.38 The rapid SiO2 ALD deposited on the Al2O3 ALD barrier layer may also be successfully filling pinhole defects in the Al2O3 ALD layer. These pinhole defects are not inherent to the extremely continuous and conformal Al2O3 ALD. We believe the pinhole defects result from small impurity particles on the polymer surface during Al2O3 ALD. The measured permeabilities were much higher when the polymer samples were exposed to normal laboratory air and not loaded into the ALD reactor under class 100 clean room conditions. The effectiveness of rapid SiO2 ALD may result from capillary condensation of a silanol multilayer in the pinhole defect because of hydrogen bonding interactions. This silanol multilayer is believed to lead to enhanced rapid SiO2 ALD. Figure 5 also demonstrates the orientation effects of the Al2O3/SiO2 bilayer films. The UP and DOWN configurations yielded permeabilities that were very similar. None of the Al2O3/ SiO2 bilayer films failed when in direct contact with saturated HTO vapor pressure. In addition, the Al2O3/SiO2 bilayer films in the DOWN configuration displayed permeabilities that were slightly lower than the Al2O3/SiO2 bilayer films in the UP configuration for the 10 nm Al2O3 ALD films. This behavior is puzzling and may suggest that hydrate formation may help to close pinhole defects. However, the permeabilities were within experimental error for the UP and DOWN configurations for the 26 nm Al2O3 ALD film. B. Multilayers of Al2O3/SiO2 Bilayers. To evaluate the performance of more than one Al2O3/SiO2 bilayer film, multilayers of Al2O3/SiO2 bilayers were fabricated on Kapton. Each bilayer consisted of 26 nm of Al2O3 ALD and ∼60 nm of rapid SiO2 ALD. The average TTRs and effective WVTRs for these films are shown in Figure 6. The average effective WVTR value for a 26 nm Al2O3 ALD film (value shown at half of a bilayer) on Kapton is also given for reference. Multilayers of Al2O3/ SiO2 bilayers with one and two bilayers decreased the TTR and effective WVTR. The two Al2O3/SiO2 bilayer film had average effective WVTRs of 1.2 × 10-4 ( 1 × 10-4 g/m2/day and 4.2 × 10-5 ( 2.6 × 10-5 g/m2/day for the UP and DOWN orientations, respectively. These effective WVTRs are lower than the WVTRs for the one Al2O3/SiO2 bilayer film.

Dameron et al.

Figure 7. TTRs versus time for Al2O3 films with a thickness of 26 nm on HSPEN measured using HTO, tritiated n-propanol (ProOT), and tritiated n-hexanol (HexOT).

Figure 6 also shows that the multilayers of three and four Al2O3/SiO2 bilayers did not lead to lower TTRs and effective WRTRs. Instead, the permeabilities for these multilayers were no lower than the permeabilities for a single 26 nm Al2O3 ALD film. The increase in the permeabilities may be related to film stress and film cracking.2,39,40 The multilayers of three and four Al2O3/SiO2 bilayers have fairly large thicknesses of 258 and 344 nm, respectively. These thicknesses may produce film strain during bending that is larger than the critical strain for film cracking. The strain in the Al2O3/SiO2 bilayer film is dependent on film thickness and is inversely dependent on the radius of curvature of film bending.2,41 The total thicknesses of 258 and 344 nm may be too large to prevent film cracking during the handling and mounting of the ALD-coated polymer films. There have been previous reports of the critical thickness for inorganic films. Above the critical thicknesses, the films are inflexible and crack. The critical strain for cracking has been reported to be in the range of 1.2-2.0% for thin inorganic films such as SiO2 and ITO.2,39,40 The critical thicknesses are determined by the linear dependence of strain on film thickness and the inverse dependence of strain on radius of curvature.2,41 Incorporation of flexible organic materials between inorganic bilayers may reduce the strain in the inorganic films and allow multiple multilayers to be employed to fabricate a very low permeability barrier.17 This approach has been employed in the Barix Al2O3/polymer multilayers utilized by Vitex.5 The critical strain in the inorganic layer in model inorganic/polymer multilayers has been calculated to determine the optimum film thickness for the polymer layer.42 For a three-layer inorganic/ polymer/inorganic multilayer, the maximum critical strain before cracking was achieved when the polymer layer had a thickness of 25-75% of the inorganic layer thickness.42 C. Tritium Diffusion Mechanism as Molecular HTO or Atomic Tritium. Tritiated alcohols were used to determine if tritium can diffuse through the ALD-coated polymers as atoms. Figure 7 compares the TTRs for 26 nm Al2O3 ALD films on HSPEN using HTO, ProOT, and HexOT as the tritium sources. The measured TTRs are all comparable independent of tritium source. Figure 8 also shows that the various tritium sources yield similar TTRs versus Al2O3 ALD film thickness. The TTRs measured for ProOT deviate very little from the TTRs measured for HTO. All the data points are nearly within experimental

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Figure 9. Illustration of H/T exchange and atomic tritium diffusion on Al2O3 ALD surface via hydroxyl groups. The adjacent polymer film is omitted for clarity.

Figure 8. TTRs versus Al2O3 film thickness on HSPEN measured using HTO, ProOT, and HexOT. The data points are located at Al2O3 ALD thicknesses of 0, 2.5, 5, 10, and 26 nm and have been shifted slightly for clarity.

error, and the results for the 26 nm Al2O3 ALD films almost directly overlap. There is greater variation for the HexOT measurements. The films with Al2O3 ALD thicknesses