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

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References

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  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]

2007

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]

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]

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]

2006

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]

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]

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]

2004

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

2002

1996

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]

1983

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]

Appl. Opt.

Appl. Phys. Lett.

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]

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]

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]

J. MEMS

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

J. Phys. D

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]

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]

Nat. Mater.

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]

Opt. Lett.

Thin Solid Films

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]

Other

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).

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

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Figures (5)

Fig. 1.
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.

Fig. 2.
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.

Fig. 3.
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.

Fig. 4.
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.

Fig. 5.
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.

Tables (2)

Tables Icon

Table 1. Parameters measured for the various layers in the 4-pair DBR mirror

Tables Icon

Table 2. Material properties of the DBR materials used in themomechanical modeling.

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