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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 1, Iss. 1 — May. 1, 2011
  • pp: 101–107
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Formation of antireflective nanostructures on melamine and N,N´-di (1-naphthyl)-N,N´-diphenyl benzidine (NPB)

U. Schulz, C. Präfke, P. Munzert, C. Gödeker, and N. Kaiser  »View Author Affiliations


Optical Materials Express, Vol. 1, Issue 1, pp. 101-107 (2011)
http://dx.doi.org/10.1364/OME.1.000101


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Abstract

Plasma-etched nanostructures are useful to provide antireflective properties to glass and plastic substrates. Organic compounds were deposited on glass substrates by thermal evaporation and etched by plasma emitted from an ion plasma source. A self-organized formation of surface structures takes place during etching of a layer of 1,3,5-Triazine-2,4,6-triamine (melamine). On the other hand, the surface of N,N´-di(1-naphthyl)-N,N´-diphenyl benzidine (NPB) remained smooth after etching. In this case, the structure formation was initiated by depositing a thin oxide layer on to the organic layer prior to the etching step. For both materials the etching process can be tailored to achieve antireflective properties over a desired wavelength range.

© 2011 OSA

1. Introduction

Commonly, interference coatings are used to reduce the Fresnel-reflections of optical lenses [1

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

,2

2. 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]

]. As an alternative, nanostructures with sub-wavelength size, as found on the eye of the night-flying moth (“moth-eye structures”), can provide antireflective (AR) properties [3

3. P. B. Clapham and M. C. Hutley, “Reduction of lens reflexion by the “Moth Eye” principle,” Nature 244(5414), 281–282 (1973). [CrossRef]

]. The basic principle is to mix the substrate material with air on a sub-wavelength scale in such a way that the effective refractive index of the surface layer is reduced. In an ideal case the modified surface shows a gradual fall of the effective refractive index from the substrate to air [4

4. M. Chen, H. C. Chang, A. S. P. Chang, S.-Y. Lin, J.-Q. Xi, and E. F. Schubert, “Design of optical path for wide-angle gradient-index antireflection coatings,” Appl. Opt. 46(26), 6533–6538 (2007). [CrossRef] [PubMed]

]. In comparison to common interference coatings, gradient index surfaces are normally less sensitive to the angle of light incidence [5

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

].

2. Methods

The chemical composition of melamine and NPB is shown in Fig. 1
Fig. 1 Chemical structure and melting temperature of N,N´-di(1-naphthyl)-N,N´-diphenyl benzidine (NPB) and 2,4,6-triamino-1,3,5-triazine (melamine).
. Melamine was chosen because it is an easy to handle, inexpensive compound [11

11. M. Irimia-Vladu, N. Marjanovic, M. Bodea, G. Hernandez-Sosa, A. M. Ramil, R. Schwödiauer, S. Bauer, N. S. Sariciftci, and F. Nüesch, “Small-molecule vacuum processed melamine-C60, organic field-effect transistors,” Org. Electron. 10(3), 408–415 (2009). [CrossRef]

]. Melamine possesses very good fire and abrasion resistance, and excellent chemical inertness.Thin films of melamine are commonly used as gate dielectrics in organic field effect transistors [12

12. S. Jahromi and U. Moosheimer, “Oxygen barrier coatings based on supramolecular assembly of melamine,” Macromolecules 33(20), 7582–7587 (2000). [CrossRef]

]. NPB is well known as an evaporable layer material for organic light emitting diodes (OLEDs) [13

13. S. A. Van Slyke, C. H. Chen, and C. W. Tang, “Organic electrolumi- nescent devices with improved stability,” Appl. Phys. Lett. 69(15), 2160–2162 (1996). [CrossRef]

]. The thin films were prepared in a BAK 640 high vacuum chamber (Balzers). Two thermal evaporators (CreaPhys GmbH) were used for evaporation of the organic substances. These consist of conic alumina crucibles heated by a filament. In addition, the vacuum chamber is equipped with an electron beam source.

The organic layers were deposited at a base pressure of 5x10−6 mbar and a deposition rate of 0.2 nm/s. Substrates were fused silica, B270 glass and silicon. Process control of the deposition rate and film thickness was performed using a quartz oscillator.The etching process was carried out in an APS904 vacuum deposition chamber (Leybold-Optics) equipped with an advanced plasma source (APS) [14

14. 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]

]. In the case of NPB, a thin layer of TiO2 was deposited by electron beam evaporation immediately prior to etching [10

10. 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), 13108–13113 (2007). [CrossRef] [PubMed]

]. The etch step was carried out by applying the plasma emitted from the APS which is a direct current (DC) plasma source. Oxygen (30 sccm) was used as the reactive gas and partly ionized by the argon (15 sccm) plasma emitted directly from the source. The typical pressure while running the plasma source was 2x10−4 mbar. The ions were accelerated by a self-bias voltage to impinge on the substrate with a maximum energy of 80 eV (at bias voltage 80 V) or 110 eV (at bias 110 V). In some cases a SiO2 layer with a thickness ranging from 25 nm to 45 nm was deposited with a deposition rate of 1.0 nm/s by ion-assisted deposition after etching in the same vacuum process.

The etching was controlled by in situ broadband monitoring (350 nm – 800 nm) of the sample’s transmission (OptiMon) [15

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]

]. Ex-situ optical characterization of samples was carried out using a Lambda 900 spectrophotometer (Perkin Elmer). The refractive index n and extinction coefficient k were derived from transmission and reflection spectra of the thin films deposited on fused silica by applying OptiChar software [16

16. OptiChar software, http://www.optilayer.com

]. IR transmission spectra of organic films deposited on silicon were measured using a Varian 3100 FT-IR spectrometer over the range 500–4000 cm–1. The structured surfaces were visualized using a Zeiss SIGMA SEM.3.

3. Results and Discussion

3.1 Structure formation on melamine-layers

Monoclinic melamine crystals start sublimating at a temperature of about 220°C in a vacuum of 5x10-6 mbar. Typically the colorless and transparent layers show a scatter loss of about to 3% to 4% at 500 nm, which is caused by the columnar growth structure of the thin film (Fig. 2a
Fig. 2 SEM images of 200 nm melamine thin film on glass (a) prior to etching and after etching for (b) 60 s, (c) 160 s and (d) 200 s.
). The refractive index of melamine is 1.81 at 500 nm and 1.75 at 800 nm [17

17. U. Schulz, C. Präfke, C. Gödeker, N. Kaiser, and A. Tünnermann, “Plasma-etched organic layers for antireflection purposes,” Appl. Opt. 50(9), C31–C35 (2011). [CrossRef]

].

A condition for a subsequent structure formation is an adequate etch rate. The layer thickness of homogeneous layers can normally be derived from simulation of the spectral curves in the visible range. In the case of melamine a structure formation occur during etching and the layers after etching appear not homogeneous. Therefore, the change of layer thickness during etching was determined by measuring the absorption bands in the infrared (IR) region (for IR band assignment see [13

13. S. A. Van Slyke, C. H. Chen, and C. W. Tang, “Organic electrolumi- nescent devices with improved stability,” Appl. Phys. Lett. 69(15), 2160–2162 (1996). [CrossRef]

]). The absorbance calculated from the transmission spectra of the thin films deposited on silicon decreases linearly with the layer thickness according to the Lambert–Beer law (Fig. 3
Fig. 3 Left: detail of FTIR absorbance spectra of melamine measured before and after etching and, right: absorbance values at 3130 cm-1 (band of interacting NH groups [13]) as a function of etch time and the evaluation of the etch rate.
). An average etching rate for melamine of 1.3 ± 0.1 nm/s for an APS bias voltage of 110 V was determined.

The growth of a fine grained structure begins shortly after etching (Fig. 2b). The self-organized nanostructure is gradually destroying or overlapping the columnar growth structure (Fig. 2c, d). A maximal structure depth of about 130 nm was reached after about 200 s when applying 110 V bias. The transmission spectra of a 270 nm layer, as deposited on B270 glass before and after different etching times, are shown in Fig. 4
Fig. 4 Transmission T and total losses (100- R + T) of a 300 nm melamine thin film before etching and after 100s etching (dashed lines) and after 200 s etching (dotted lines). The transmission of the uncoated B270 glass is shown for comparison.
. The scatter loss (100%-R-T) decreases with etch time, as estimated. The minimum of the reflectance in the range from 350 nm to about 770 nm is adjustable by using different etching parameters (bias voltage, etch time), as shown in Fig. 5
Fig. 5 Reflectance of etched melamine layers on glass including 30 nm silica top layer (without back side reflectance).The reflectance of the uncoated B270 glass is shown for comparison.
.

3.2 Structure formation on NPB-layers

5. Conclusion

Antireflective nanostructures have been obtained on both materials. On melamine, a structure grows in a self-organized way so no artificial mask was used. The anisotropic etching is probably caused by a passivation layer formed in an initial chemical reaction of the melamine in the plasma zone. The structure formation starts if ions penetrate the passivation. However, the etch rate is probably dominated by the chemical degradation of the organic material rather than by physical etching (ion bombardment). The structure depth on melamine is about 110 nm to 130 nm (Fig. 8
Fig. 8 SEM images of structured melamine (a) and NPB (b) on silicon, viewing angle 45°.
). The structured zone consists of small needles which exhibit only a modest and gradual change of filling factor from the bulk to the ambient.

NPB layers show isotropic etching. A structure formation can be initiated by a thin oxide layer, which probably operates like a passivation layer. It is surprising and not completely understood yet that the etch rate is nearly the same, independent of whether an initial layer was applied or not. Hence the layer does not act as an etching mask during the entire process. Probably the initial layer is etched away after a certain time but the nanostructure remains stable afterwards and can be transferred into the depth until the substrate is reached. However, the thickness of the structured zone never exceeded about 80 nm. The structure is characterized by holes like a sponge, as shown in Fig. 8.

For application on glass lenses, melamine seems to be more suitable than NPB. The essential advantage is the considerable structure depth. Thin films of silica deposited on top of the structures are helpful to improve the chemical and mechanical stabilities of organic structured surfaces in both cases. In addition, a notable broadening of the antireflective performance was observed for NPB. It is assumed by considering interference effects that the antireflective performance of both structured layers can be further improved by using at least one underlying oxide layer [17

17. U. Schulz, C. Präfke, C. Gödeker, N. Kaiser, and A. Tünnermann, “Plasma-etched organic layers for antireflection purposes,” Appl. Opt. 50(9), C31–C35 (2011). [CrossRef]

].

The reflection of light not only causes a loss in intensity but also generates ghost images and stray light which reduce the image quality in an optical system. The vapor deposition and etching of an organic layer on glass offers a new possibility to achieve homogeneous antireflective properties especially on curved lenses. On inclined areas of a lens a vacuum deposited layer will occur thinner but also the etch rate is lower than on a horizontal area. The problem of the disturbance of antireflective interference stacks on curved lenses caused by different layer thicknesses on top in on the inclined surfaces can be overcome in this way. A reduction of ghost images and stray light can be estimated therefore by applying the method for imaging systems.

References and links

1.

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

2.

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]

3.

P. B. Clapham and M. C. Hutley, “Reduction of lens reflexion by the “Moth Eye” principle,” Nature 244(5414), 281–282 (1973). [CrossRef]

4.

M. Chen, H. C. Chang, A. S. P. Chang, S.-Y. Lin, J.-Q. Xi, and E. F. Schubert, “Design of optical path for wide-angle gradient-index antireflection coatings,” Appl. Opt. 46(26), 6533–6538 (2007). [CrossRef] [PubMed]

5.

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

6.

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]

7.

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]

8.

G. Wu, Z. Denga, B. Fanb, D. Zhoub, and F. Zhang, “A novel route to control refractive index of sol-gel derived nano-porous silica films used as broadband antireflective coatings,” Mater. Sci. Eng. B 78(2-3), 135–139 (2000). [CrossRef]

9.

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).

10.

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), 13108–13113 (2007). [CrossRef] [PubMed]

11.

M. Irimia-Vladu, N. Marjanovic, M. Bodea, G. Hernandez-Sosa, A. M. Ramil, R. Schwödiauer, S. Bauer, N. S. Sariciftci, and F. Nüesch, “Small-molecule vacuum processed melamine-C60, organic field-effect transistors,” Org. Electron. 10(3), 408–415 (2009). [CrossRef]

12.

S. Jahromi and U. Moosheimer, “Oxygen barrier coatings based on supramolecular assembly of melamine,” Macromolecules 33(20), 7582–7587 (2000). [CrossRef]

13.

S. A. Van Slyke, C. H. Chen, and C. W. Tang, “Organic electrolumi- nescent devices with improved stability,” Appl. Phys. Lett. 69(15), 2160–2162 (1996). [CrossRef]

14.

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]

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]

16.

OptiChar software, http://www.optilayer.com

17.

U. Schulz, C. Präfke, C. Gödeker, N. Kaiser, and A. Tünnermann, “Plasma-etched organic layers for antireflection purposes,” Appl. Opt. 50(9), C31–C35 (2011). [CrossRef]

OCIS Codes
(160.4890) Materials : Organic materials
(310.1210) Thin films : Antireflection coatings
(310.6628) Thin films : Subwavelength structures, nanostructures

ToC Category:
Metamaterials

History
Original Manuscript: January 26, 2011
Revised Manuscript: February 16, 2011
Manuscript Accepted: February 17, 2011
Published: April 22, 2011

Citation
U. Schulz, C. Präfke, P. Munzert, C. Gödeker, and N. Kaiser, "Formation of antireflective nanostructures on melamine and N,N´-di (1-naphthyl)-N,N´-diphenyl benzidine (NPB)," Opt. Mater. Express 1, 101-107 (2011)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-1-1-101


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References

  1. A. Macleod, Thin-Film Optical Filters, 3rd edition (Institute of Physics Publishing, 2001).
  2. 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]
  3. P. B. Clapham and M. C. Hutley, “Reduction of lens reflexion by the “Moth Eye” principle,” Nature 244(5414), 281–282 (1973). [CrossRef]
  4. M. Chen, H. C. Chang, A. S. P. Chang, S.-Y. Lin, J.-Q. Xi, and E. F. Schubert, “Design of optical path for wide-angle gradient-index antireflection coatings,” Appl. Opt. 46(26), 6533–6538 (2007). [CrossRef] [PubMed]
  5. M. Minot, “The angular reflectance of single-layer gradient refractive-index films,” J. Opt. Soc. Am. 67(8), 1046–1050 (1977). [CrossRef]
  6. 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]
  7. 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]
  8. G. Wu, Z. Denga, B. Fanb, D. Zhoub, and F. Zhang, “A novel route to control refractive index of sol-gel derived nano-porous silica films used as broadband antireflective coatings,” Mater. Sci. Eng. B 78(2-3), 135–139 (2000). [CrossRef]
  9. 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).
  10. 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), 13108–13113 (2007). [CrossRef] [PubMed]
  11. M. Irimia-Vladu, N. Marjanovic, M. Bodea, G. Hernandez-Sosa, A. M. Ramil, R. Schwödiauer, S. Bauer, N. S. Sariciftci, and F. Nüesch, “Small-molecule vacuum processed melamine-C60, organic field-effect transistors,” Org. Electron. 10(3), 408–415 (2009). [CrossRef]
  12. S. Jahromi and U. Moosheimer, “Oxygen barrier coatings based on supramolecular assembly of melamine,” Macromolecules 33(20), 7582–7587 (2000). [CrossRef]
  13. S. A. Van Slyke, C. H. Chen, and C. W. Tang, “Organic electrolumi- nescent devices with improved stability,” Appl. Phys. Lett. 69(15), 2160–2162 (1996). [CrossRef]
  14. 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]
  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]
  16. OptiChar software, http://www.optilayer.com
  17. U. Schulz, C. Präfke, C. Gödeker, N. Kaiser, and A. Tünnermann, “Plasma-etched organic layers for antireflection purposes,” Appl. Opt. 50(9), C31–C35 (2011). [CrossRef]

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