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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 24 — Nov. 26, 2007
  • pp: 16285–16291

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Encapsulation of low-refractive-index SiO2 nanorods by Al2O3 with atomic layer deposition

Sangho S. Kim, Nicholas T. Gabriel, Woo-Bin Song, and Joseph J. Talghader  »View Author Affiliations


Optics Express, Vol. 15, Issue 24, pp. 16285-16291 (2007)
http://dx.doi.org/10.1364/OE.15.016285


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Abstract

Thin films composed of SiO2 nanorods or nanoporous SiO2 (np-SiO2) are attractive for use as a low refractive index material in various types of optical coatings. However, the material properties of these films are unstable because of the high porosity of the films. This is particularly apparent in dry versus humid atmospheres where both the refractive index and coefficient of thermal expansion (CTE) vary dramatically. In this article, we demonstrate that np-SiO2 can be encapsulated by depositing Al2O3 with Atomic Layer Deposition (ALD), stabilizing these properties. In addition, this encapsulation ability is demonstrated successfully in a 4-pair distributed Bragg reflector (DBR) design. It is hoped that this technique will be useful in patterning specific regions of a film for optical and mechanical stability while other portions are ambient-interactive for sensing.

© 2007 Optical Society of America

1. Background

Thin films consisting of SiO2 nanorods or np-SiO2 are of great interest because they tend to have a refractive index lower than the bulk value of about 1.46, which is especially useful in distributed Bragg reflectors [1

J.-Q. Xi, J. Kyu Kim, E. F. Schubert, D. Ye, T.-M. Lu, and S.-Y. Lin, “Very low-refractive-index optical thin films consisting of an array of SiO2 nanorods,” Opt. Lett. 31, 601–603 (2006). [CrossRef] [PubMed]

] and anti-reflection coatings [2

J.-Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S.-Y. Lin, W. Liu, and J. A. Smart, “Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection,” Nature photonics 1, 176–179 (2007).

]. In principle, there is an inverse relationship between film porosity and refractive index because the air trapped between nanorods reduces the composite refractive index of the film. Oblique-angle deposition makes it possible to grow these nanorods due to the self-shadowing effect [1

J.-Q. Xi, J. Kyu Kim, E. F. Schubert, D. Ye, T.-M. Lu, and S.-Y. Lin, “Very low-refractive-index optical thin films consisting of an array of SiO2 nanorods,” Opt. Lett. 31, 601–603 (2006). [CrossRef] [PubMed]

]. A DBR made of the same material can be also realized by this oblique angle deposition [3

M. F. Schubert, J.-W. Xi, J. K. Kim, and E. F. Schubert, “Distributed Bragg reflector consisting of highand low-refractive-index thin film layers made of the same material,” Appl. Phys. Lett. 90, 141115–141117 (2007). [CrossRef]

]. Increasing the porosity, however, also causes the film to interact unfavorably with the external environment so that material properties such as refractive index and coefficient of thermal expansion (CTE) are unstable. For instance, water, with a refractive index of 1.33, can be trapped in the pores and boost the overall refractive index of a thin film, with the effect depending heavily on the level of humidity in the surroundings [4

D. Grosso, C. Boissiere, and C. Sanchez, “Ultralow-dielectric-constant optical thin films built from magnesium oxyfluoride vesicle-like hollow nanoparticles,” Nat. Mater. 6, 572–575 (2007). [CrossRef] [PubMed]

]. In addition to SiO2, there are many other films whose mechanical properties are dependent on the ambient as well, for example, zirconium tungstate thin films [5

Michael S. Sutton and Joseph Talghader, “Zirconium Tungstate (ZrW2O8)-Based Micromachined Negative Thermal-Expansion Thin Films,” J. MEMS 13, 688–695 (2004).

] show different CTE in dry versus wet environments.

In this paper we demonstrate that it is possible to stabilize these material properties to prevent the porous films from interacting with the ambient by coating ALD Al2O3 on top of the unstable dielectric films. This technique will be extremely important not just for mirrors which need to be precisely controlled over temperature, but also for making thermally invariant mirrors [6

W. Liu and J. J. Talghader, “Thermally invariant dielectric coatings for micromirrors,” Appl. Opt. 41 3285–3293 (2002). [CrossRef] [PubMed]

].

2. Experimental results and discussion

Thin films of np-SiO2 were deposited on 250µm (100) Si wafers by electron beam evaporation, with the substrates at room temperature and mounted at an oblique angle of 60° relative to the source material. The oblique angle is typically defined as an angle between vapor flux and the substrate normal [1

J.-Q. Xi, J. Kyu Kim, E. F. Schubert, D. Ye, T.-M. Lu, and S.-Y. Lin, “Very low-refractive-index optical thin films consisting of an array of SiO2 nanorods,” Opt. Lett. 31, 601–603 (2006). [CrossRef] [PubMed]

], and that is the convention used here. The refractive index versus temperature of the resulting films was measured at 632.8nm by a Gaertner ellipsometer in a clean-room environment with 40% relative humidity (RH). Heating and cooling was performed with a thermoelectric heating element and temperature controller. Wafer curvature versus temperature measurements were performed with Frontier Semiconductor FSM-900TC commercial stress measurement equipment. These measurements were made both in clean-room air at 40%RH, and in a dry N2 ambient with continuous flow rate of roughly 2000 sccm.

Figure 1 shows how measured film properties vary with changes in ambient conditions. Figure 1(a) indicates that the refractive index drastically decreases as the temperature increases. The thermo-optic coefficient (dn/dT) of the np-SiO2 film is almost -3×10-3/°C in the range of 10°C~70°C, over two orders of magnitude larger than the roughly 10-5/°C expected from changes in intrinsic material properties [7

T Toyoda and M Yabe, “The temperature dependence of the refractive indices of fused silica and crystal quartz,” J. Phys. D 16. L97–L100 (1983). [CrossRef]

]. This is most simply explained by the presence of pores that can easily desorb and adsorb the water vapor from the external environment as temperature increases and decreases, respectively. Figures 1(b) and 1(c) show the measured curvature change with respect to temperature in air and N2, respectively. The sign of the slope depends upon the CTE value of the substrate; a slope of zero would indicate the film CTE exactly matches that of the substrate. When the CTE of a thin film is lower than the substrate it curves upward upon heating. Similarly, the substrate curves downward upon heating when film CTE is larger than substrate CTE. Therefore, the positive and negative slopes in Fig. 1(b) and 1(c) occur even though both np-SiO2 CTE values are positive.

Quantitatively, the np-SiO2 film CTE in each case is extracted from the slope of each dataset using a linear, multilayer analysis based on free-plate theory [6

W. Liu and J. J. Talghader, “Thermally invariant dielectric coatings for micromirrors,” Appl. Opt. 41 3285–3293 (2002). [CrossRef] [PubMed]

], with assumptions of substrate and film biaxial modulus. For this particular film, the CTE change between air and N2 is roughly one order of magnitude (0.4ppm/°C to 4.3ppm/°C), which would make it difficult to model and use in a multilayer optical design where thermomechanical performance is also of interest. As with refractive index, the observed CTE instability is most likely caused by adsorption and desorption of water vapor contained in the air ambient by the nanoporous film, changing its properties. In contrast, as the temperature is cycled in an N2 environment any moisture in the film tends to outgas until the film reaches a “steady-state”, with thermal expansion behavior dominated by intrinsic film properties [8

M. T. K. Soh, J. Thurn, J. H. Thomas III, and J. J. Talghader, “Thermally induced stress hysteresis and coefficient of thermal expansion changes in nanoporous SiO2 ,” J. Phys. D 40, 2176–2182 (2007). [CrossRef]

].

Fig. 1. The properties of np-SiO2 change significantly with ambient conditions. Measurements of (a) refractive index versus temperature in the presence of humidity, and wafer curvature change versus temperature (b) in air and (c) in dry N2 indicate that the film adsorbs and desorbs water from the ambient. Extracted np-SiO2 CTE values are indicated on each plot.

Many dielectric materials made by electron beam evaporation even without oblique-angle deposition show similar ambient dependence [8

M. T. K. Soh, J. Thurn, J. H. Thomas III, and J. J. Talghader, “Thermally induced stress hysteresis and coefficient of thermal expansion changes in nanoporous SiO2 ,” J. Phys. D 40, 2176–2182 (2007). [CrossRef]

]. After numerous attempts to keep the CTE and optical constant stable, we found that ALD Al2O3 works quite well. ALD Al2O3 films were deposited with a Cambridge NanoTech, Inc. Savannah system, using alternating pulses of H2O and trimethylaluminum at a process temperature of 250°C and pressure 1 torr, with N2 as the carrier gas flowing at 20 sccm. Figure 2(a) shows the np-SiO2 optical constant change with temperature when encapsulated by ALD Al2O3. The refractive index for the buried np- SiO2 layer was calculated indirectly based on the Ψ and Δ of the total reflection obtained from ellipsometry, assuming constant film thicknesses and ALD Al2O3 index. The dn/dT of np-SiO2 was found to be 4.3×10-5/°C, which is similar to the intrinsic property of fused silica (about 10-5/°C [7

T Toyoda and M Yabe, “The temperature dependence of the refractive indices of fused silica and crystal quartz,” J. Phys. D 16. L97–L100 (1983). [CrossRef]

]). Since the dn/dT of bulk Al2O3 is also ~10-5/°C [9

Crystran Ltd, optical component material data for sapphire (Al2O3) http://www.crystran.co.uk/products.asp?productid=231

], the exact quantitative result for dn/dT of encapsulated np-SiO2 may be somewhat lower than what we measured, but the overall point remains. The refractive index of np-SiO2 is stabilized by ALD Al2O3.

Fig. 2. (a). The refractive index of np-SiO2 encapsulated ALD Al2O3, plotted on the same scale as Fig. 1(a), shows very little change with temperature. Similarly, the CTE values extracted from measurements of curvature versus temperature indicate a np-SiO2 CTE of (b) 6.1ppm/°C in air, and (c) 6.2ppm/°C in N2, assuming a constant Al2O3 CTE of 3.4ppm/°C.

As shown in Fig. 2(b) and 2(c), ALD Al2O3 can also encapsulate the np-SiO2 in terms of thermomechanical properties. The measured film thicknesses for the system in Fig. 2 are 1302 Å of Al2O3 and 2037 Å of np-SiO2. We determined the refractive index of the Al2O3 to be 1.65 compared to 1.6–1.7 reported in [10

D. Riihela, M. Ritala, R. Matero, and M. Leskela, “Introducing atomic layer epitaxy for the deposition of optical thin films,” Thin Solid Films 289, 250–255 (1996) [CrossRef]

]. The instability of the underlying film CTE between air and N2 has dropped from ~3.9ppm/°C to~0.1ppm/°C when encapsulated, the latter being a much more manageable level of variation for design purposes. These results agree with recent observations that ALD Al2O3 is believed to be a gas diffusion and moisture permeation barrier [11

P. F. Carcia, R. S. McLean, M. H. Reilly, M. D. Groner, and S. M. George, “Ca test of Al2O3 gas diffusion barriers grown by atomic layer deposition on polymers,” Appl. Phys. Lett. 89, 31915–31917 (2006). [CrossRef]

, 12

E. Langereis, M. Creatore, S. B. S. Heil, M. C. M. van de Sanden, and W. M. M. Kessels, “Plasma-assisted atomic layer deposition of Al2O3 moisture permeation barriers on polymers,” Appl. Phys. Lett. 89, 81915–81917 (2006). [CrossRef]

]. While that research was based on encapsulation of polymer materials, our measurements indicate ALD Al2O3 is a promising moisture permeation barrier for low-index np-SiO2.

We developed and fabricated a DBR based on np-SiO2, TiO2 made by electron beam evaporation, and ALD Al2O3, as shown in Table 1. In order to make a DBR mirror with more than 90% reflectance, simulations suggest we must put 8 layers of alternating np-SiO2 and TiO2, with the top TiO2 layer replaced by ALD Al2O3 as a barrier. A simulation of this coating on a silicon substrate is shown in Fig. 3, along with actual measured reflectance; both have a 15° angle of incidence relative to normal. Data were measured on a J.A. Woollam Co. VASE spectroscopic ellipsometer system. The discrepancy between the model and the data possibly comes from deviations in actual thickness and refractive index of the DBR layers. It could be partially due to ALD Al2O3 penetration into the top np-SiO2 layer, since the resulting increase in np-SiO2 refractive index would be expected to induce a similar red-shift in peak reflectance. It is noted that if standard silica was used instead of the reduced-index np-SiO2, simulations indicate that more than 12 layers would be needed to achieve a similar peak reflectance.

Table 1.  Parameters measured for the various layers in the 4-pair DBR mirror
Layer No. (1=bottom)MaterialsThickness (Å)a Refractive index @ 632.8nma
8ALD Al2O3 10861.66b
7SiO2 10981.33c
6TiO2 15691.96d
5SiO2 16201.31c
4TiO2 16271.95d
3SiO2 14361.32c
2TiO2 11082.07d
1SiO2 19261.31c
aAll measurements were performed on dummy wafers with a single film, co-deposited with the DBR mirror layers. For comparison, literature values for refractive index are listed below.
bALD Al2O3 is 1.6–1.7 [10

D. Riihela, M. Ritala, R. Matero, and M. Leskela, “Introducing atomic layer epitaxy for the deposition of optical thin films,” Thin Solid Films 289, 250–255 (1996) [CrossRef]

]
cTiO2 made by e-beam evaporation is 1.8–2.1 from Thin Solid Films 360 96–102 (2000)
dSiO2 made by e-beam evaporation is 1.43–1.49 from Appl. Opt. 44 3006–3012 (2005)
Fig. 3. Simulated and measured reflectance versus wavelength for the encapsulated 4-pair DBR on Si. Both include a 15° angle of incidence relative to normal, since measurement at 0° was not practical. Reasonable agreement with simulation is observed, with expected variations due to some uncertainty in the thickness and refractive index of each layer.

The encapsulation capability of ALD Al2O3 is also confirmed with this DBR mirror, as seen in Fig. 4(a) and 4(b). The measured curvature versus temperature of the DBR coated wafer does not vary based on ambient conditions. We also performed a fit to the experimental data using a multilayer model to extract the CTE of the TiO2 layers in each case. To do this we assume that the CTE of np-SiO2 and ALD Al2O3 remain constant at the values in Table 2, and that all TiO2 layers have identical CTE. This fit indicates that the CTE of the TiO2 films is 3.6ppm/°C in air and 4.0ppm/°C in N2, a difference of only ~0.4ppm/°C. Although it is based on several assumptions, this result gives a sense of the coating stability in a changing ambient. For comparison, we measured the curvature change of a three-layer structure consisting of a silicon wafer with thin films of np-SiO2 and e-beam evaporated TiO2. Assuming that np-SiO2 properties remain constant underneath for simplicity, we found that the difference in TiO2 CTE between air and N2 was about 5 ppm/°C. This suggests the TiO2 would be a poor encapsulant, and could also have a nanoporous structure.

Fig. 4. Curvature measurements versus temperature of the 4-pair DBR on Si, (a) in air and (b) dry N2. The axes are scaled identically, and we observe no significant change due to ambient conditions.
Table 2.  Material properties of the DBR materials used in themomechanical modeling.
MaterialsBiaxial modulus (GPa)CTE (ppm/°C)
ALD Al2O3 247a 3.3–3.4
SiO2 56a 6.1–6.2
TiO2 96 a 3.6–4.0
Si216.7b 2.6c
aNanoindentation was performed on thin films to estimate the biaxial modulus of SiO2, TiO2, and ALD Al2O3
bFrom Mechanics of Materials 23 311 (1996).
cFrom J. Phys. D: Appl. Phys., 14 L163 (1981).

Figure 5(a) shows a cross-sectional scanning electron microscope (SEM) image of the np-SiO2 films showing the nanorod structure, which seem to be ~30–60 nm in diameter. It is noted that the film thickness of this particular sample was roughly 300 nm, somewhat thicker than the others. In other films we had difficulty seeing these nanorods clearly, even though all films exhibited a low refractive index when deposited in this manner. It is believed that porosity plays an important role in lowering its optical constant so that even films without this particular nanorod structure have low refractive index. Figure 5(b) indicates how the ALD Al2O3 deposition process at 250°C may influence the np-SiO2 films. The np-SiO2 remains porous, although it is difficult to see if ALD Al2O3 can penetrate into the pores. One thing that we do notice, however, is that the refractive index of np-SiO2 when encapsulated by ALD Al2O3 (in Fig. 2(a)) is about 0.08 higher than the “dry” np-SiO2 film without encapsulation, near T=60°C in Fig. 1(a), which suggests that ALD Al2O3 may partially penetrate into np-SiO2. Furthermore, the 8-layered DBR can be seen in Fig. 5(c), showing that all of the np-SiO2 films, except perhaps the top np-SiO2 layer (layer 7), appear to maintain a significantly nanoporous structure.

Fig. 5. Cross-sectional SEM images of (a) a np-SiO2 film, indicating the nanorod structure, (b) a np-SiO2 film encapsulated by ALD Al2O3, and (c) the 8-layered DBR discussed above.

3. Conclusion

We successfully demonstrate that the optical and thermomechanical properties of a SiO2 nanorod array or np-SiO2 can be stabilized by encapsulation with ALD Al2O3, while maintaining the very useful reduction in refractive index to ~1.33. The dn/dT of encapsulated np-SiO2 is only 4.3×10-5/°C in a humid environment, which is much less than the -3×10-3/°C observed in the exposed film. Similarly, the encapsulated np-SiO2 CTE difference between air and dry N2 was only 0.1 ppm/°C, compared to a shift of 3.9 ppm/°C in the bare film. The successful demonstration of this encapsulation ability is also done with a 4-pair DBR design, showing very little curvature dependence on ambient gas, while maintaining about 90% reflectance. This technique could be useful to stabilize entire coatings, or in patterning specific regions of a film for optical and mechanical stability.

Acknowledgments

The authors acknowledge Terry Brough, Brad Tiffany, and Bob Hafner for useful discussions. This work was financially supported by the AFOSR (Contract FA9550-05-1-0399). Parts of this work were carried out in the Minnesota Characterization Facility and Nanofabrication Center which receive partial support from NSF through the NNIN program.

References and links

1.

J.-Q. Xi, J. Kyu Kim, E. F. Schubert, D. Ye, T.-M. Lu, and S.-Y. Lin, “Very low-refractive-index optical thin films consisting of an array of SiO2 nanorods,” Opt. Lett. 31, 601–603 (2006). [CrossRef] [PubMed]

2.

J.-Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S.-Y. Lin, W. Liu, and J. A. Smart, “Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection,” Nature photonics 1, 176–179 (2007).

3.

M. F. Schubert, J.-W. Xi, J. K. Kim, and E. F. Schubert, “Distributed Bragg reflector consisting of highand low-refractive-index thin film layers made of the same material,” Appl. Phys. Lett. 90, 141115–141117 (2007). [CrossRef]

4.

D. Grosso, C. Boissiere, and C. Sanchez, “Ultralow-dielectric-constant optical thin films built from magnesium oxyfluoride vesicle-like hollow nanoparticles,” Nat. Mater. 6, 572–575 (2007). [CrossRef] [PubMed]

5.

Michael S. Sutton and Joseph Talghader, “Zirconium Tungstate (ZrW2O8)-Based Micromachined Negative Thermal-Expansion Thin Films,” J. MEMS 13, 688–695 (2004).

6.

W. Liu and J. J. Talghader, “Thermally invariant dielectric coatings for micromirrors,” Appl. Opt. 41 3285–3293 (2002). [CrossRef] [PubMed]

7.

T Toyoda and M Yabe, “The temperature dependence of the refractive indices of fused silica and crystal quartz,” J. Phys. D 16. L97–L100 (1983). [CrossRef]

8.

M. T. K. Soh, J. Thurn, J. H. Thomas III, and J. J. Talghader, “Thermally induced stress hysteresis and coefficient of thermal expansion changes in nanoporous SiO2 ,” J. Phys. D 40, 2176–2182 (2007). [CrossRef]

9.

Crystran Ltd, optical component material data for sapphire (Al2O3) http://www.crystran.co.uk/products.asp?productid=231

10.

D. Riihela, M. Ritala, R. Matero, and M. Leskela, “Introducing atomic layer epitaxy for the deposition of optical thin films,” Thin Solid Films 289, 250–255 (1996) [CrossRef]

11.

P. F. Carcia, R. S. McLean, M. H. Reilly, M. D. Groner, and S. M. George, “Ca test of Al2O3 gas diffusion barriers grown by atomic layer deposition on polymers,” Appl. Phys. Lett. 89, 31915–31917 (2006). [CrossRef]

12.

E. Langereis, M. Creatore, S. B. S. Heil, M. C. M. van de Sanden, and W. M. M. Kessels, “Plasma-assisted atomic layer deposition of Al2O3 moisture permeation barriers on polymers,” Appl. Phys. Lett. 89, 81915–81917 (2006). [CrossRef]

OCIS Codes
(120.6810) Instrumentation, measurement, and metrology : Thermal effects
(160.5320) Materials : Photorefractive materials
(160.6840) Materials : Thermo-optical materials
(230.1480) Optical devices : Bragg reflectors
(220.4241) Optical design and fabrication : Nanostructure fabrication

ToC Category:
Materials

History
Original Manuscript: September 4, 2007
Revised Manuscript: October 31, 2007
Manuscript Accepted: November 2, 2007
Published: November 21, 2007

Citation
Sangho S. Kim, Nicholas T. Gabriel, Woo-Bin Song, and Joseph J. Talghader, "Encapsulation of low-refractive-index SiO2 nanorods by Al2O3 with atomic layer deposition," Opt. Express 15, 16285-16291 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-24-16285


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References

  1. J.-Q. Xi, J. Kyu Kim, E. F. Schubert, D. Ye, T.-M. Lu, and S.-Y. Lin, "Very low-refractive-index optical thin films consisting of an array of SiO2 nanorods," Opt. Lett. 31, 601-603 (2006). [CrossRef] [PubMed]
  2. J.-Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S.-Y. Lin, W. Liu, and J. A. Smart, "Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection," Nature photonics 1, 176-179 (2007).
  3. M. F. Schubert, J.-W. Xi, J. K. Kim, and E. F. Schubert, "Distributed Bragg reflector consisting of high- and low-refractive-index thin film layers made of the same material," Appl. Phys. Lett. 90,141115-141117 (2007). [CrossRef]
  4. D. Grosso, C. Boissiere, and C. Sanchez, "Ultralow-dielectric-constant optical thin films built from magnesium oxyfluoride vesicle-like hollow nanoparticles," Nat. Mater. 6,572-575 (2007). [CrossRef] [PubMed]
  5. Michael S. Sutton, Joseph Talghader, "Zirconium Tungstate (ZrW2O8)-Based Micromachined Negative Thermal-Expansion Thin Films," J. MEMS 13, 688-695 (2004).
  6. W. Liu and J. J. Talghader, "Thermally invariant dielectric coatings for micromirrors," Appl. Opt. 413285-3293 (2002). [CrossRef] [PubMed]
  7. T Toyoda and M Yabe, "The temperature dependence of the refractive indices of fused silica and crystal quartz," J. Phys. D 16.L97-L100 (1983). [CrossRef]
  8. M. T. K. Soh, J. Thurn, J. H. ThomasIII, and J. J. Talghader, "Thermally induced stress hysteresis and co-efficient of thermal expansion changes in nanoporous SiO2," J. Phys. D 40,2176-2182 (2007). [CrossRef]
  9. Crystran Ltd, optical component material data for sapphire (Al2O3) http://www.crystran.co.uk/products.asp?productid=231
  10. D. Riihela, M. Ritala, R. Matero, and M. Leskela, "Introducing atomic layer epitaxy for the deposition of optical thin films," Thin Solid Films 289,250-255 (1996) [CrossRef]
  11. P. F. Carcia, R. S. McLean, M. H. Reilly, M. D. Groner, S. M. George, "Ca test of Al2O3 gas diffusion barriers grown by atomic layer deposition on polymers," Appl. Phys. Lett. 89, 31915-31917 (2006). [CrossRef]
  12. E. Langereis, M. Creatore, S. B. S. Heil, M. C. M. van de Sanden, and W. M. M. Kessels, "Plasma-assisted atomic layer deposition of Al2O3 moisture permeation barriers on polymers," Appl. Phys. Lett. 89, 81915-81917 (2006). [CrossRef]

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