Abstract

The residual reflectance obtained for a broad wavelength range depends mainly on the refractive index of the last layer. Using interference layer stacks composed of naturally available low- and high-index materials, the residual reflection for a broad range cannot be adjusted below a certain limit. However, nanostructured (gradient) and porous layers are effective media with a refractive index lower than that of natural materials. Results demonstrate that an interference layer stack combined with a structured layer as the last layer yields better antireflection properties owing to the low effective index of the structure.

©2009 Optical Society of America

1. Introduction

The generation of antireflective (AR) properties is a basic requirement for optical surfaces. The effects of low-index treatments and coatings on antireflection have been known since the first experiments by Joseph Fraunhofer in 1817. AR coatings have been applied in optical instruments for more than 70 years. Super wideband AR coatings (ratio of the upper wavelength limit λU to the lower wavelength limit λL of g = 2 and higher) are required for optical parts used over wide incidence ranges. These coatings are also useful in providing antireflection over an enhanced visible range for sharp curved substrates. The development of super wideband AR coatings, especially for low-index substrates, remains a challenge because of the limited availability of low-index thin-film materials and the technological difficulties in producing complex multilayers. The aim of the present study was to combine different strategies to obtain better antireflective properties. Better antireflection over a broad spectral range was demonstrated theoretically and in practice by combining interference multilayers with a low-index gradient layer on the top. This paper discusses suitable designs and the problems associated with practical implementation.

2. Methods

An interference stack was deposited by electron beam evaporation of SiO2 and Ta2O5. PMMA granules (7N, Evonik) were dissolved and spin-coated on top of the interference layer. Coating and etching processes were carried out in an APS904 vacuum deposition chamber (Leybold-Optics) equipped with an advanced plasma source [1]. Oxygen used as the reactive gas was partly ionized by the argon plasma emitted from the plasma source. Argon and oxygen ions were accelerated by a self-bias voltage to impinge on the substrate with energy of up to approximately 120 eV. For the etching step the bias voltage was kept at 80 V. A Phillips XL40 scanning electron microscope (SEM) was used to visualize the nanostructures. A thin Au layer (<10 nm) was deposited to achieve electrical conductivity. Optical characterization of the samples was carried out using a Lambda 900 spectrophotometer (Perkin Elmer). All designs were evaluated using OptiLayer software, Version 5.22.

3. Theoretical considerations

The theory of AR coatings has been widely discussed in the literature and is not repeated here in detail. With respect to the problem of antireflection of low-index substrates over a broad wavelength range, there is broad consensus on the following limitations and properties [24]:

  • a) Broader wavelength ranges normally require coatings of greater physical thickness;
  • b) The lowest residual reflectance that can be obtained depends on the refractive indices (ratio of maximum to minimum refractive index) of the materials used in the interference stack;
  • c) The refractive index of the last low-index layer has a dominant influence on the residual reflectance obtained for a given (broad) bandwidth; and
  • d) For a given design and materials, the average residual reflectance increases with the bandwidth.

Therefore, realization of broadband AR coatings with very low residual reflectance is limited by the optical properties of the layer materials.

The following examples were calculated for a substrate index of ns = 1.52 and for high-index material H with nH = 2.35 and low-index material L with nL = 1.46. These conditions correspond to the typical thin-film materials TiO2 (H) and SiO2 (L) deposited on glass or plastic without consideration of dispersion and absorption. Tikhonravov et al. recently derived a formula to estimate the minimum average residual reflectance for a defined spectral range and a given pair of materials [5]. According to this formula an AR coating for the spectral region 400–1200 nm (g = 3) can exhibit average reflectance not less than approximately 0.5–0.6% and needs at least 16 layers with a total optical thickness of 1.4 μm. Figure 1 (design 1) shows an optimized design consisting of 19 layers arranged in two so-called cycles or clusters. In fact, the calculated residual reflectance is slightly greater than 0.6%. This value is markedly lower if a layer with refractive index of 1.22 is applied as the last layer instead of the L material. Design 2 in Fig. 1 shows the layer stack after such a modification followed by new refinement. The system can be simplified further by removing the second cluster of the design without negatively affecting the optical properties (design 3).

 figure: Fig. 1

Fig. 1 Reflectance and refractive-index profile for AR coatings designed for the spectral region from 400 to 1200 nm with layer materials L (n = 1.46) and H (n = 1.35) in design 1 and with an additional low-index last layer (n = 1.22) in designs 2 and 3.

Download Full Size | PPT Slide | PDF

Surface structures with features of sub-wavelength size are useful for realizing antireflective properties. Periodically or stochastically arranged structures exhibit a gradual increase in effective index from the substrate side to the ambient medium (air). Early theoretical papers approximate sub-wavelength gratings as effective thin-film stacks and use especially the effective media theory (EMT) to describe its optical properties [68]. Sub-wavelength surface structures are typically less sensitive to the angle of incidence of light than homogeneous multilayer stacks [9]. Technologies to produce suitable structures include lithographic procedures, microphase separation of polymer mixtures and etching processes [1012]. AR structures have to consist of features smaller than the wavelength concerned, but the depth should be at least a significant fraction of the wavelength. Typical procedures generally deliver aspect ratios (depth divided by the period of the structure) of approximately 1–2 and work well for the visible spectral range. Even plasma-etched PMMA structures with a larger aspect ratio (approx. 3–4) can reduce the reflectance to <1% only for a limited spectral range of 400–800 nm (Fig. 2 ) [13].

 figure: Fig. 2

Fig. 2 SEM morphology of a plasma-etched AR structure on PMMA and reflection before and after etching (without sample backside).

Download Full Size | PPT Slide | PDF

Combinations of suitable interference stacks with low-index effective media should be able to reduce the residual reflectance in any case. The first step in the design process is to approximate an experimental achieved inhomogeneous layer or sub-wavelength structure as effective thin-film stack. Then an underlying layer stack consisting of available H and L materials can be adapted to achieve low reflectance over a broad wavelength range and/or for a wide incidence range.

4. Experimental results

In a preliminary experiment, a PMMA sub-wavelength structure was used. The optical behavior of PMMA as a function of the etching parameters has already been investigated [13,14]. The structure shown in Fig. 2 was self-organized within approximately 500 s of etching. During this process approximately 350 nm of pure PMMA was completely removed. The composition of the remaining structured surface was modeled as shown in Fig. 3 . Design 4 considers the structured PMMA with a total thickness of 250 nm approximated by a step-down arrangement of four single layers. The first layer is PMMA itself with n = 1.49. The underlying layer stack was obtained by needle optimization with a target reflectance of 0.1% for the spectral range 400–1200 nm. The thickness and composition of the structured layer were held constant during design optimization. The dispersion of experimental Ta2O5 (H) and SiO2 (L) thin films was considered in this case. The total thickness of the combined coating is approximately 500 nm. An average residual reflectance of <0.2% is theoretically attainable on B270 glass.

 figure: Fig. 3

Fig. 3 Reflectance and refractive index profile for design 4 (calculated and experimental data)

Download Full Size | PPT Slide | PDF

The basic layer stack, consisting of seven layers of SiO2 and Ta2O5 to a total thickness of approximately 250 nm, was deposited onto glass. The PMMA layer of 600 nm in thickness was deposited on top of this sample before etching. The etching process was controlled by an optical monitoring system (OptiMon [15]) that allows in situ measurement of sample transmission for the wavelength range 350–920 nm. Etching was stopped when the transmission reached a peak.

Figure 3 shows the reflectance obtained for the combined layer system (basic layer stack + PMMA gradient layer) in comparison to calculated data. Average residual reflection of <0.35% was obtained in the designated spectral range. The differences between the calculated and the experimental data result mainly from thickness deviation of the PMMA layer and the etching step itself. For the basic layer stack random thickness errors in the order of 1-2 nm have to be taken into account. Figure 4 shows the transmission data under light of normal incidence and an angle of 45° (average of TE and TM polarization, backside removed by calculation). It is evident that the gradient layer has a favorable effect on the transmission obtained for light of oblique incidence. Scattering and absorption losses for the combined system were below the detectable limit.

 figure: Fig. 4

Fig. 4 calculated and measured transmittance for design 4 under light at normal incidence and an angle of 45° (average polarization, without uncoated sample backside).

Download Full Size | PPT Slide | PDF

5. Conclusion

It has been demonstrated that an interference layer stack with a sub-wavelength structured layer as the last layer is useful in reducing the residual reflectance of antireflection coatings for wide spectral ranges owing to the low effective index of the last layer. An inhomogeneous last layer used in this context does not need defined optical properties despite its generally low effective index. A reasonable approach is to produce and characterize such a layer in a preliminary step and then to adapt a suitable interference stack.

Further investigations are focused on processes to achieve suitable low-index layers. Direct vacuum evaporation of organic materials that can be plasma-etched is promising in this context. This is a precondition to produce broadband AR-coatings of the new type in a single vacuum process. The production of structured layers with higher mechanical resistance remains the main challenge for future developments.

Acknowledgements

This research was supported by the Bundesministerium für Wissenschaft und Forschung (BMBF) under contract number 13N9160.

References and links

1. S. Pongratz and A. Zöller, “Plasma ion assisted deposition: A promising technique for optical coatings,” J. Vac. Sci. Technol. A 10(4), 1897–1904 (1992). [CrossRef]  

2. A. Macleod, Thin-Film Optical Filters, 3rd edition (Institute of Physics Publishing, 2001).

3. P. G. Verly, J. A. Dobrowolski, and R. R. Willey, “Fourier-transform method for the design of wideband anti-reflection coatings,” Appl. Opt. 31(19), 3836–3846 (1992). [CrossRef]   [PubMed]  

4. R. Willey, “Predicting achievable design performance of broadband antireflection coatings,” Appl. Opt. 32(28), 5447–5451 (1993). [CrossRef]   [PubMed]  

5. A. V. Tikhonravov, M. K. Trubetskov, T. V. Amotchkina, and J. A. Dobrowolski, “Estimation of the average residual reflectance of broadband antireflection coatings,” Appl. Opt. 47(13), C124–C130 (2008). [CrossRef]   [PubMed]  

6. M. Minot, “The angular reflectance of single-layer gradient refractive-index films,” J. Opt. Soc. Am. 67(8), 1046–1050 (1977). [CrossRef]  

7. W. H. Southwell, “Pyramid-array surface-relief structures producing antireflection index matching on optical surfaces,” J. Opt. Soc. Am. A 8(3), 549–553 (1991). [CrossRef]  

8. D. H. Raguin and G. M. Morris, “Antireflection structured surfaces for the infrared spectral region,” Appl. Opt. 32(7), 1154–1167 (1993). [CrossRef]   [PubMed]  

9. J. A. Dobrowolski, D. Poitras, P. Ma, H. Vakil, and M. Acree, “Toward perfect antireflection coatings: numerical investigation,” Appl. Opt. 41(16), 3075–3083 (2002). [CrossRef]   [PubMed]  

10. A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, “Subwavelength-structured antireflective surfaces on glass,” Thin Solid Films 351(1-2), 73–78 (1999). [CrossRef]  

11. S. Walheim, E. Schäffer, J. Mlynek, and U. Steiner, “Nanophase-separated polymer films as high-performance antireflection coatings,” Science 283(5401), 520–522 (1999). [CrossRef]   [PubMed]  

12. A. Kaless, P. Munzert, U. Schulz, and N. Kaiser, “Nano-motheye antireflection pattern by plasma treatment of polymers,” Surf. Coat. Tech. 20, 58–61 (2004).

13. U. Schulz, P. Munzert, R. Leitel, I. Wendling, N. Kaiser, and A. Tünnermann, “Antireflection of transparent polymers by advanced plasma etching procedures,” Opt. Express 15(20), 13108–13111 (2007). [CrossRef]   [PubMed]  

14. R. Leitel, U. Schulz, N. Kaiser, and A. Tünnermann, “Stochastic subwavelength structures on poly(methyl methacrylate) surfaces for antireflection generated by plasma treatment,” Appl. Opt. 47(13), C143–C146 (2008). [CrossRef]   [PubMed]  

15. S. Wilbrandt, O. Stenzel, N. Kaiser, M. K. Trubetskov, and A. V. Tikhonravov, “In situ optical characterization and reengineering of interference coatings,” Appl. Opt. 47(13), C49–C54 (2008). [CrossRef]   [PubMed]  

References

  • View by:

  1. S. Pongratz and A. Zöller, “Plasma ion assisted deposition: A promising technique for optical coatings,” J. Vac. Sci. Technol. A 10(4), 1897–1904 (1992).
    [Crossref]
  2. A. Macleod, Thin-Film Optical Filters, 3rd edition (Institute of Physics Publishing, 2001).
  3. P. G. Verly, J. A. Dobrowolski, and R. R. Willey, “Fourier-transform method for the design of wideband anti-reflection coatings,” Appl. Opt. 31(19), 3836–3846 (1992).
    [Crossref] [PubMed]
  4. R. Willey, “Predicting achievable design performance of broadband antireflection coatings,” Appl. Opt. 32(28), 5447–5451 (1993).
    [Crossref] [PubMed]
  5. A. V. Tikhonravov, M. K. Trubetskov, T. V. Amotchkina, and J. A. Dobrowolski, “Estimation of the average residual reflectance of broadband antireflection coatings,” Appl. Opt. 47(13), C124–C130 (2008).
    [Crossref] [PubMed]
  6. M. Minot, “The angular reflectance of single-layer gradient refractive-index films,” J. Opt. Soc. Am. 67(8), 1046–1050 (1977).
    [Crossref]
  7. W. H. Southwell, “Pyramid-array surface-relief structures producing antireflection index matching on optical surfaces,” J. Opt. Soc. Am. A 8(3), 549–553 (1991).
    [Crossref]
  8. D. H. Raguin and G. M. Morris, “Antireflection structured surfaces for the infrared spectral region,” Appl. Opt. 32(7), 1154–1167 (1993).
    [Crossref] [PubMed]
  9. J. A. Dobrowolski, D. Poitras, P. Ma, H. Vakil, and M. Acree, “Toward perfect antireflection coatings: numerical investigation,” Appl. Opt. 41(16), 3075–3083 (2002).
    [Crossref] [PubMed]
  10. A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, “Subwavelength-structured antireflective surfaces on glass,” Thin Solid Films 351(1-2), 73–78 (1999).
    [Crossref]
  11. S. Walheim, E. Schäffer, J. Mlynek, and U. Steiner, “Nanophase-separated polymer films as high-performance antireflection coatings,” Science 283(5401), 520–522 (1999).
    [Crossref] [PubMed]
  12. A. Kaless, P. Munzert, U. Schulz, and N. Kaiser, “Nano-motheye antireflection pattern by plasma treatment of polymers,” Surf. Coat. Tech. 20, 58–61 (2004).
  13. U. Schulz, P. Munzert, R. Leitel, I. Wendling, N. Kaiser, and A. Tünnermann, “Antireflection of transparent polymers by advanced plasma etching procedures,” Opt. Express 15(20), 13108–13111 (2007).
    [Crossref] [PubMed]
  14. R. Leitel, U. Schulz, N. Kaiser, and A. Tünnermann, “Stochastic subwavelength structures on poly(methyl methacrylate) surfaces for antireflection generated by plasma treatment,” Appl. Opt. 47(13), C143–C146 (2008).
    [Crossref] [PubMed]
  15. S. Wilbrandt, O. Stenzel, N. Kaiser, M. K. Trubetskov, and A. V. Tikhonravov, “In situ optical characterization and reengineering of interference coatings,” Appl. Opt. 47(13), C49–C54 (2008).
    [Crossref] [PubMed]

2008 (3)

2007 (1)

2004 (1)

A. Kaless, P. Munzert, U. Schulz, and N. Kaiser, “Nano-motheye antireflection pattern by plasma treatment of polymers,” Surf. Coat. Tech. 20, 58–61 (2004).

2002 (1)

1999 (2)

A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, “Subwavelength-structured antireflective surfaces on glass,” Thin Solid Films 351(1-2), 73–78 (1999).
[Crossref]

S. Walheim, E. Schäffer, J. Mlynek, and U. Steiner, “Nanophase-separated polymer films as high-performance antireflection coatings,” Science 283(5401), 520–522 (1999).
[Crossref] [PubMed]

1993 (2)

1992 (2)

S. Pongratz and A. Zöller, “Plasma ion assisted deposition: A promising technique for optical coatings,” J. Vac. Sci. Technol. A 10(4), 1897–1904 (1992).
[Crossref]

P. G. Verly, J. A. Dobrowolski, and R. R. Willey, “Fourier-transform method for the design of wideband anti-reflection coatings,” Appl. Opt. 31(19), 3836–3846 (1992).
[Crossref] [PubMed]

1991 (1)

1977 (1)

Acree, M.

Amotchkina, T. V.

Bläsi, B.

A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, “Subwavelength-structured antireflective surfaces on glass,” Thin Solid Films 351(1-2), 73–78 (1999).
[Crossref]

Dobrowolski, J. A.

Döll, W.

A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, “Subwavelength-structured antireflective surfaces on glass,” Thin Solid Films 351(1-2), 73–78 (1999).
[Crossref]

Dreibholz, J.

A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, “Subwavelength-structured antireflective surfaces on glass,” Thin Solid Films 351(1-2), 73–78 (1999).
[Crossref]

Glaubitt, W.

A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, “Subwavelength-structured antireflective surfaces on glass,” Thin Solid Films 351(1-2), 73–78 (1999).
[Crossref]

Gombert, A.

A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, “Subwavelength-structured antireflective surfaces on glass,” Thin Solid Films 351(1-2), 73–78 (1999).
[Crossref]

Heinzel, A.

A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, “Subwavelength-structured antireflective surfaces on glass,” Thin Solid Films 351(1-2), 73–78 (1999).
[Crossref]

Kaiser, N.

Kaless, A.

A. Kaless, P. Munzert, U. Schulz, and N. Kaiser, “Nano-motheye antireflection pattern by plasma treatment of polymers,” Surf. Coat. Tech. 20, 58–61 (2004).

Leitel, R.

Ma, P.

Minot, M.

Mlynek, J.

S. Walheim, E. Schäffer, J. Mlynek, and U. Steiner, “Nanophase-separated polymer films as high-performance antireflection coatings,” Science 283(5401), 520–522 (1999).
[Crossref] [PubMed]

Morris, G. M.

Munzert, P.

U. Schulz, P. Munzert, R. Leitel, I. Wendling, N. Kaiser, and A. Tünnermann, “Antireflection of transparent polymers by advanced plasma etching procedures,” Opt. Express 15(20), 13108–13111 (2007).
[Crossref] [PubMed]

A. Kaless, P. Munzert, U. Schulz, and N. Kaiser, “Nano-motheye antireflection pattern by plasma treatment of polymers,” Surf. Coat. Tech. 20, 58–61 (2004).

Poitras, D.

Pongratz, S.

S. Pongratz and A. Zöller, “Plasma ion assisted deposition: A promising technique for optical coatings,” J. Vac. Sci. Technol. A 10(4), 1897–1904 (1992).
[Crossref]

Raguin, D. H.

Rose, K.

A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, “Subwavelength-structured antireflective surfaces on glass,” Thin Solid Films 351(1-2), 73–78 (1999).
[Crossref]

Schäffer, E.

S. Walheim, E. Schäffer, J. Mlynek, and U. Steiner, “Nanophase-separated polymer films as high-performance antireflection coatings,” Science 283(5401), 520–522 (1999).
[Crossref] [PubMed]

Schulz, U.

Southwell, W. H.

Sporn, D.

A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, “Subwavelength-structured antireflective surfaces on glass,” Thin Solid Films 351(1-2), 73–78 (1999).
[Crossref]

Steiner, U.

S. Walheim, E. Schäffer, J. Mlynek, and U. Steiner, “Nanophase-separated polymer films as high-performance antireflection coatings,” Science 283(5401), 520–522 (1999).
[Crossref] [PubMed]

Stenzel, O.

Tikhonravov, A. V.

Trubetskov, M. K.

Tünnermann, A.

Vakil, H.

Verly, P. G.

Walheim, S.

S. Walheim, E. Schäffer, J. Mlynek, and U. Steiner, “Nanophase-separated polymer films as high-performance antireflection coatings,” Science 283(5401), 520–522 (1999).
[Crossref] [PubMed]

Wendling, I.

Wilbrandt, S.

Willey, R.

Willey, R. R.

Wittwer, V.

A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, “Subwavelength-structured antireflective surfaces on glass,” Thin Solid Films 351(1-2), 73–78 (1999).
[Crossref]

Zöller, A.

S. Pongratz and A. Zöller, “Plasma ion assisted deposition: A promising technique for optical coatings,” J. Vac. Sci. Technol. A 10(4), 1897–1904 (1992).
[Crossref]

Appl. Opt. (7)

J. Opt. Soc. Am. (1)

J. Opt. Soc. Am. A (1)

J. Vac. Sci. Technol. A (1)

S. Pongratz and A. Zöller, “Plasma ion assisted deposition: A promising technique for optical coatings,” J. Vac. Sci. Technol. A 10(4), 1897–1904 (1992).
[Crossref]

Opt. Express (1)

Science (1)

S. Walheim, E. Schäffer, J. Mlynek, and U. Steiner, “Nanophase-separated polymer films as high-performance antireflection coatings,” Science 283(5401), 520–522 (1999).
[Crossref] [PubMed]

Surf. Coat. Tech. (1)

A. Kaless, P. Munzert, U. Schulz, and N. Kaiser, “Nano-motheye antireflection pattern by plasma treatment of polymers,” Surf. Coat. Tech. 20, 58–61 (2004).

Thin Solid Films (1)

A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, “Subwavelength-structured antireflective surfaces on glass,” Thin Solid Films 351(1-2), 73–78 (1999).
[Crossref]

Other (1)

A. Macleod, Thin-Film Optical Filters, 3rd edition (Institute of Physics Publishing, 2001).

Cited By

Optica participates in Crossref's Cited-By Linking service. Citing articles from Optica Publishing Group journals and other participating publishers are listed here.

Alert me when this article is cited.


Figures (4)

Fig. 1
Fig. 1 Reflectance and refractive-index profile for AR coatings designed for the spectral region from 400 to 1200 nm with layer materials L (n = 1.46) and H (n = 1.35) in design 1 and with an additional low-index last layer (n = 1.22) in designs 2 and 3.
Fig. 2
Fig. 2 SEM morphology of a plasma-etched AR structure on PMMA and reflection before and after etching (without sample backside).
Fig. 3
Fig. 3 Reflectance and refractive index profile for design 4 (calculated and experimental data)
Fig. 4
Fig. 4 calculated and measured transmittance for design 4 under light at normal incidence and an angle of 45° (average polarization, without uncoated sample backside).

Metrics