Abstract

The fact that the optical characteristics of thin-film materials are generally different from those of the same materials in bulk form is well known. The differences depend very much on the conditions in which the deposition has been carried out. A good understanding of these differences, their causes, and the influence of deposition parameters is vital if we are to be able to improve coating quality. We have developed two complementary methods with the objective of deriving information on the index of refraction and its variation throughout the thickness of the film. Perceptible optical inhomogeneity is normally present and appreciable inhomogeneity is frequently present in thin films. Such inhomogeneity is usually associated with layer microstructure. The first is a postdeposition technique that makes use of measurements in air of the transmittance and reflectance of the layer under study over a wide wavelength region. The second, in contrast, makes use of in situ measurements, that is measurements made under vacuum and during the actual deposition of the layer. We shall show with the help of several examples that the two methods lead to results that are consistent and demonstrate the existence in deposited materials of an inherent variation of the index of refraction normal to the surface. The thermal sensitivity of the layer properties and their tendency to adsorb atmospheric moisture must be taken into account before the residual differences between the two techniques can be explained.

© 1984 Optical Society of America

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References

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  1. H. K. Pulker, “Characterization of Optical Thin Films,” Appl. Opt. 18, 1969 (1979); H. K. Pulker, G. Paesold, E. Ritter, “Refractive Indices of TiO2 Films Produced by Reactive Evaporation of Various Titanium-Oxygen Phases,” Appl. Opt. 15, 2986 (1976); E. Ritter, “Dielectric Film Materials for Optical Applications,” Phys. Thin Films 8, 1 (1975).
    [CrossRef] [PubMed]
  2. H. M. Liddell, Computer Aided Techniques for the Design of Multilayer Filters (Adam Hilger, London, 1981); F. Abelès, “Methods for Determining Optical Parameters of Thin Films,” Prog. Opt. 2, 250 (1963).
  3. J. P. Borgogno, B. Lazaridès, E. Pelletier, “Automatic Determination of the Optical Constants of Inhomogeneous Thin Films,” Appl. Opt. 21, 4020 (1982).
    [CrossRef] [PubMed]
  4. P. Bousquet, E. Pelletier, “Optical Thin Film Monitoring—Recent Advances and Limitations,” Thin Solid Films 77, 165 (1981); H. A. Macleod, “Monitoring of Optical Coatings,” Appl. Opt. 20, 82 (1981); E. Pelletier, “Monitoring of Optical Thin Films During Deposition in Thin Film Technologies,” Proc. Soc. Photo-Opt. Instrum. Eng. 401, 74 (1983); F. Flory, B. Schmitt, E. Pelletier, H. A. Macleod, “Interpretation of Wide Band Scans of Growing Optical Thin Films in Terms of Layer Microstructure, Proc. Soc. Photo-Opt. Instrum. Eng. 401, 109 (1983).
    [CrossRef] [PubMed]
  5. H. A. Macleod, “Turning Value Monitoring of Narrow-Band All Dielectric Thin Film Optical Filters,” Opt. Acta 19, 1 (1972).
    [CrossRef]
  6. M. Harris, H. A. Macleod, S. Ogura, E. Pelletier, B. Vidal, “The Relationship Between Optical Inhomogeneity and Film Structure,” Thin Solid Films 57, 1973 (1979).
    [CrossRef]

1982

1981

P. Bousquet, E. Pelletier, “Optical Thin Film Monitoring—Recent Advances and Limitations,” Thin Solid Films 77, 165 (1981); H. A. Macleod, “Monitoring of Optical Coatings,” Appl. Opt. 20, 82 (1981); E. Pelletier, “Monitoring of Optical Thin Films During Deposition in Thin Film Technologies,” Proc. Soc. Photo-Opt. Instrum. Eng. 401, 74 (1983); F. Flory, B. Schmitt, E. Pelletier, H. A. Macleod, “Interpretation of Wide Band Scans of Growing Optical Thin Films in Terms of Layer Microstructure, Proc. Soc. Photo-Opt. Instrum. Eng. 401, 109 (1983).
[CrossRef] [PubMed]

1979

1972

H. A. Macleod, “Turning Value Monitoring of Narrow-Band All Dielectric Thin Film Optical Filters,” Opt. Acta 19, 1 (1972).
[CrossRef]

Borgogno, J. P.

Bousquet, P.

P. Bousquet, E. Pelletier, “Optical Thin Film Monitoring—Recent Advances and Limitations,” Thin Solid Films 77, 165 (1981); H. A. Macleod, “Monitoring of Optical Coatings,” Appl. Opt. 20, 82 (1981); E. Pelletier, “Monitoring of Optical Thin Films During Deposition in Thin Film Technologies,” Proc. Soc. Photo-Opt. Instrum. Eng. 401, 74 (1983); F. Flory, B. Schmitt, E. Pelletier, H. A. Macleod, “Interpretation of Wide Band Scans of Growing Optical Thin Films in Terms of Layer Microstructure, Proc. Soc. Photo-Opt. Instrum. Eng. 401, 109 (1983).
[CrossRef] [PubMed]

Harris, M.

M. Harris, H. A. Macleod, S. Ogura, E. Pelletier, B. Vidal, “The Relationship Between Optical Inhomogeneity and Film Structure,” Thin Solid Films 57, 1973 (1979).
[CrossRef]

Lazaridès, B.

Liddell, H. M.

H. M. Liddell, Computer Aided Techniques for the Design of Multilayer Filters (Adam Hilger, London, 1981); F. Abelès, “Methods for Determining Optical Parameters of Thin Films,” Prog. Opt. 2, 250 (1963).

Macleod, H. A.

M. Harris, H. A. Macleod, S. Ogura, E. Pelletier, B. Vidal, “The Relationship Between Optical Inhomogeneity and Film Structure,” Thin Solid Films 57, 1973 (1979).
[CrossRef]

H. A. Macleod, “Turning Value Monitoring of Narrow-Band All Dielectric Thin Film Optical Filters,” Opt. Acta 19, 1 (1972).
[CrossRef]

Ogura, S.

M. Harris, H. A. Macleod, S. Ogura, E. Pelletier, B. Vidal, “The Relationship Between Optical Inhomogeneity and Film Structure,” Thin Solid Films 57, 1973 (1979).
[CrossRef]

Pelletier, E.

J. P. Borgogno, B. Lazaridès, E. Pelletier, “Automatic Determination of the Optical Constants of Inhomogeneous Thin Films,” Appl. Opt. 21, 4020 (1982).
[CrossRef] [PubMed]

P. Bousquet, E. Pelletier, “Optical Thin Film Monitoring—Recent Advances and Limitations,” Thin Solid Films 77, 165 (1981); H. A. Macleod, “Monitoring of Optical Coatings,” Appl. Opt. 20, 82 (1981); E. Pelletier, “Monitoring of Optical Thin Films During Deposition in Thin Film Technologies,” Proc. Soc. Photo-Opt. Instrum. Eng. 401, 74 (1983); F. Flory, B. Schmitt, E. Pelletier, H. A. Macleod, “Interpretation of Wide Band Scans of Growing Optical Thin Films in Terms of Layer Microstructure, Proc. Soc. Photo-Opt. Instrum. Eng. 401, 109 (1983).
[CrossRef] [PubMed]

M. Harris, H. A. Macleod, S. Ogura, E. Pelletier, B. Vidal, “The Relationship Between Optical Inhomogeneity and Film Structure,” Thin Solid Films 57, 1973 (1979).
[CrossRef]

Pulker, H. K.

Vidal, B.

M. Harris, H. A. Macleod, S. Ogura, E. Pelletier, B. Vidal, “The Relationship Between Optical Inhomogeneity and Film Structure,” Thin Solid Films 57, 1973 (1979).
[CrossRef]

Appl. Opt.

Opt. Acta

H. A. Macleod, “Turning Value Monitoring of Narrow-Band All Dielectric Thin Film Optical Filters,” Opt. Acta 19, 1 (1972).
[CrossRef]

Thin Solid Films

M. Harris, H. A. Macleod, S. Ogura, E. Pelletier, B. Vidal, “The Relationship Between Optical Inhomogeneity and Film Structure,” Thin Solid Films 57, 1973 (1979).
[CrossRef]

P. Bousquet, E. Pelletier, “Optical Thin Film Monitoring—Recent Advances and Limitations,” Thin Solid Films 77, 165 (1981); H. A. Macleod, “Monitoring of Optical Coatings,” Appl. Opt. 20, 82 (1981); E. Pelletier, “Monitoring of Optical Thin Films During Deposition in Thin Film Technologies,” Proc. Soc. Photo-Opt. Instrum. Eng. 401, 74 (1983); F. Flory, B. Schmitt, E. Pelletier, H. A. Macleod, “Interpretation of Wide Band Scans of Growing Optical Thin Films in Terms of Layer Microstructure, Proc. Soc. Photo-Opt. Instrum. Eng. 401, 109 (1983).
[CrossRef] [PubMed]

Other

H. M. Liddell, Computer Aided Techniques for the Design of Multilayer Filters (Adam Hilger, London, 1981); F. Abelès, “Methods for Determining Optical Parameters of Thin Films,” Prog. Opt. 2, 250 (1963).

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Figures (9)

Fig. 1
Fig. 1

Model chosen to represent the inhomogeneous layer between media of indices ns (substrate) and na (air). The index of the layer varies from ni at the inner surface to no at the outer. Inhomogeneity is given by Δn/ n ¯ which is negative for the index decreasing linearly from substrate to outer surface.

Fig. 2
Fig. 2

Three-dimensional plot representing expected development *of the transmittance over the 400–1000-nm spectral region during the deposition of a layer of titanium oxide 6λ0/4 thick with λ0 = 550 nm.

Fig. 3
Fig. 3

Refractive index of titanium oxide layers depends strongly on the deposition conditions, see the number near each curve. The values of n ¯ as a function of wavelength are deduced from reflectance and transmittance measured in air.

Fig. 4
Fig. 4

Homogeneity defects of the layers referred to in Fig. 3: Δn = ninsidenoutside is plotted as a function of wavelength; the index systematically decreases from the substrate.

Fig. 5
Fig. 5

Refractive index n of titanium dioxide layers of Fig. 3 calculated from in situ measurements during deposition.

Fig. 6
Fig. 6

Inhomogeneity Δn of titanium dioxide layers of Fig. 3 calculated from in situ measurements during deposition; the index systematically decreases from the substrate.

Fig. 7
Fig. 7

Differences between the values of the refractive indices measured in air (Fig. 3) and those deduced from in situ measurements (Fig. 5).

Fig. 8
Fig. 8

Measured transmittance of the titanium oxide layer (Ref. 6 of Fig. 3): 1, measured in situ at the end of the deposition; 2, measured in air, three days after deposition; 3, calculated after Harris et al.6

Fig. 9
Fig. 9

Measured transmittance of the titanium oxide layer (Ref. 20 of Fig. 3): 1, measured in situ at the end of the deposition; 2, measured in air, three days after deposition; 3, calculated after Harris et al.6

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