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Optical properties of thin films
Conference Optical University
properties
of Southampton,
of thin films
UK, 24-26
September
This conference, organized by the Optical Group of the Institute of Physics, was attended by about 120 delegates from 13 countries. There were five sessions, four of which began with review papers by well known authorities in the field. Session 1 was on the Basic Optical Properties of Thin Film Materials; the review by Professor F. Abeles of the University of Paris was followed by 11 papers. Abeles stressed that because the optical properties of a material were effected by its crystalline structure, a knowledge of them is useful for the interpretation of structural properties. The first contributed paper was by J. Richard, L. Penevaire, P. Cheyssac, R. Garrigos, and R. Kofman (Laboratoire d’Electro-optique, Nice) on the ‘Thermo-reflectance of silver fdms’; the technique of measurement was described and results discussed. A second paper from France had the title ‘Collective oscillations of electrons in alkali metal thin films’ (J. C. Payan and D. Roux, Universite de Provence, Marseille). It was suggested that the abnormal transmission properties in these films were the result of such oscillations. Two papers of the session originated from Sweden. L. G. Bernland, 0. Hunderi, and H. P. Myers (Chalmers University, Gothenburg) talked on ‘Optical properties of some quench condensed polyvalent metal films’. Results with aluminium and beryllium indicated that absorption did not follow the free-electron behaviour predicted by the Drude theory. L. Stensland and L. E. Flordal (Optical Research Institute, Stockholm) discussed ‘Optical properties of some metal-dielectric mixture fdms’. They deduced that Maxwell-Garnett theory applies to metaldielectric mixtures with low resultant polarizability, but that the formula by Bottcher is valid for high polarizability mixtures. Of the four papers from the UK, three were mainly experimental studies directly connected with optical properties, the fourth was theoretical. The paper by L. S. Julien, A. J. Taylor, and R. F. Miller (Royal Holloway College, London) had the title ‘Optical constants of gadolinium and terbium films obtained from in situ reflection measurements in ultra-high vacuum’. The importance of optimized reflection ratio measurements was stressed and results given in the photon energy range 1.8-3.1 eV. T. J. Fox, R. P. Howson, and D. C. Emmony (Loughborough University) gave a paper on ‘Optical properties of thin films of antimony’. The results showed that antimony has free carriers which for their optical effects can be represented as holes with relaxation times of 3.1 x 1O-l4 s and a density of 1.7 x 1O-26 inm3. R. Hill (Newcastle-upon-Tyne Polytechnic) spoke on ‘Some optical properties of alloy semiconductor thin films’, giving results for ZnS-Te, CaAs-ZnSe, and GaSb-ZnTe. 0. S. Heavens (University of York) spoke on the ‘Use of lateral waves for the examination of surface films’. The lateral wave flux in a substrate depends on the optical constants of the superstrate film and the theory of
84
1973
this for a film of thickness small compared to wavelength was presented. German workers gave two papers. J. Bodesheim and A. Otto (University of Munich) covered ‘Quantitative measurements of the roughness and roughness spectrum of silver films’ and H. Kostlin, R. Jost, and W. Lens (Philips, Aachen) spoke on ‘The optical and electrical properties of doped In,03 films’. The last paper of the session was contributed by D. Bedeaux and J. Vlieger (Lorentz Institute, Leiden). Talking on ‘Some theoretical aspects of the optical properties of thin ftims’, they gave a phenomenological theory and used it to calculate the transmission, reflection, and absorption of light. Optical Applications of Thin Films was the general title for Session 2. A. Thelen (Balzers, Liechtenstein) began with a review of recent developments in the field. He concentrated on the design of optical filters and dealt with antireflection coatings, high reflectors, neutral beam splitters, dichroic filters (short and long wave pass), monochromatic filters, minus filters, and polarizing ffiters. Practical limitations in filters still lie in the materials available and in production problems. Of the ten following papers, four dealt with films for specific applications: these were planar optical guides (C. W. Pitt and R. J. Stevens, University College, London), proustite parametric oscillators (A. R. Henderson, University of Southampton), antireflection coatings over the range 415-950 nm (G. F. Doughty Barr and Stroud, Glasgow), ultra-violet interferometry (G. H. C. Freeman, National Physical Laboratory). Freeman called for more data on optical constants and for developments of broad band transmission filters in the range 200- 110 nm. Other papers in this session dealt with various effects produced during film preparation. H. A. Macleod and D. Richmond (Newcastle-upon-Tyne Polytechnic) contributed a paper on ‘Sources of error in the optical monitoring of narrow band all-dielectric thin film optical filters’. Methods for their reduction were described. ‘Optical thickness changes in freshly deposited layers of lead telluride’ was the title of a paper by C. S. Evans, R. H. Hunneman, and J. S. Seeley (University of Reading). A. Thetford (Reading) talked on ‘The effect of absorption on the performance of all-dielectric bandpass interference filters’. He gave simple, reasonable, accurate formulae for finding the peak transmittance, residual reflectance, and half-width of the filters when a small amount of absorption is present in the high index layers. D. J. Hemingway (University of Salford) considered the dependence of scattering in alldielectric filters on wavelength and angle of scattering. Information on filter production was the theme of the last two contributions. E. Pelletier, P. Roche, and L. Bertrand (Universite de Provence, Marseille) were interested in the ‘Limiting bandwidth of interference filters and the
OPTICS AND LASER TECHNOLOGY
APRIL
1974
influence of temperature changes during production’. The paper by R. G. T. Neilson (Rank Precision Industries, Leicester) on a ‘Method for the control of refractive index of oxide films’ was read by N. Bevan. The apparatus used was described and results with particular reference to a three-layer broad band anti-reflection coating were discussed. In Session 3 on Magneto-Optics of Thin Magnetic Films, only two of the three planned papers were given. J. Krizek and K. N. R. Taylor (University of Durham) spoke on ‘Optical properties of rare earth metals in the paramagnetic and magnetically ordered phases’. R. Atkinson and P. H. Lissberger (University of Salford) talking on ‘Ellipsometric determination of the optical and magneto-optical constants of thin fdm MnBi’ discussed a technique involving radiation reflected from the interface between the substrate and the film. A third paper, a substitute for the one in the programme, was presented by W. Hasan, R. B. Inwood, and R. F. Miller (Royal Holloway College) with the title ‘Optimization of the measurement of magneto-optical parameters’. Two papers not related to the subject matter of this session were then given: one by J. C. Lemonnier, M. Priol, and S. Robin (Universite de Rennes) on ‘Optical properties of thin lead films in the energy range 3- 15.5 eV’ and a short contribution from P. Hendra (University of Southampton) on ‘Raman scattering measurements on thin films at electrode-electrolyte interfaces’. From the point of view of a chemist, he described the technique and gave examples of spectra from films some 0.3-0.6 nm thick. The fourth session was on Thin Film Imperfections. H. E. Bennett (Michelson Laboratory, China Lake, California) opened with a review paper entitled ‘The influence of imperfections in thin films on their optical properties’. He listed ten factors which influence the quality of evaporated films: purity of the evaporent, deposition speed, pressure, source type, substrate temperature, source to substrate distance, deposition angle, coating thickness, material of substrate, and substrate roughness. Measurements of imperfections were described in two papers which followed. ‘A study of thin film imperfections by optical methods’ was presented by M. J. Downs and R. J. King (National Physical Laboratory). Commenting that values of the thickness of dielectric films obtained by ellipsometry differ from those obtained by step height methods, King ascribed this to film inhomogeneity. Jean M. Bennett (Michelson Laboratory) described in detail the scanning inteferometer used for measuring the roughness of optical surfaces. W. R. Hunter (Naval Research Laboratory, Washington DC) and G. Hass (Night Vision Laboratory, Virginia) gave a paper on the ‘Thickness of absorbing films necessary to measure thin optical constants using the reflectance versus angle of incidence method’. Hunter said that thicknesses less than that required to give 0.1% transmission could be used. K. W. Raine and P. B. Clapham (NPL) were concerned with ‘Optical absorption in films deposited by electron bombardment sources’. The cause of the absorption is the creation of colour centres, but electron guns employing a magnet do not give this effect. Ch. Kuhle, H. Schlotterer, and F. Schwidefsky (Siemens, Munich) gave a paper on the ‘Influence of deposition parameters on the optical constants of thin silicon films’. In the final paper of this session, J. M. Pearson (University of Salford) spoke on ‘Improvements in falter performance based on structural studies by electron microscopy’.
OPTICS AND LASER TECHNOLOGY
APRIL
1974
The last session was on Techniques of Measurement of Optical Constants. H. A. Macleod (Newcastle-upon-Tyne Polytechnic) gave the review paper and confined his remarks mainly to isotropic media. The values of optical constants are required because they give information on material structure and are necessary for filter design. Macleod discussed the accuracy with which the constants should be determined. Papers which described methods of measurement then followed. F. R. Gfeller and C. W. Pitt (University College, London) presented ‘Measurement techniques for optical guide materials’ in which a method for the simultaneous measurement of refractive index and thickness of thin optically transparent dielectric films with low loss was described. Two papers were concerned with computation methods. ‘Automatic evaluation of optical constants and thickness of thin films with application to thin dielectric films’ was the title of one. In this E. Pelletier, P. Roche, and B. Vidal (Universitd de Provence) described the principle of a completely automatic spectrophotometer covering the range 200-700 nm, which was coupled to a computer. ‘The theoretical determination of optical constants of thin films’ was presented by Heather Liddell (Queen Mary College, London). A method based on the assumption that the refractive index obeys a Sellmeier dispersion formula was described. M. S. Shalaan and A. J. Taylor (Royal Holloway College, London) talked on the ‘Interferometric determination of the extinction coefficient of highly reflecting metallic films’. The method is based on the multiple beam interference fringes produced by an air wedge as used by Tolansky. The only paper in the conference entirely devoted to instrumentation was that by C. C. Matheson and J. G. Wright (University of East Anglia) on ‘A high precision polychromatic automatic ellipsometer’. He described the instrument as being automatic, versatile, fast, precise, accurate, and easy to use. From the remarks of delegates, the conference as a whole was successful - both in its coverage and in the gathering together of workers in the subject. There was no shortage of material which unfortunately meant there was a shortage of time. Delegates left the conference with the feeling that there was much to be done and much being done in this field. H. G. Jerrard
Bound Volumes Why not have your back copies of OPTICS AND LASER TECHNOLOGY bound in an attractive hardback cover. We will bind a year’s issues (6 journals plus index) for a cost of f6. This includes postage in the UK, but overseas postage will be extra. Please send all orders, together with remittance to Brian Roylance, OPTICS AND LASER TECHNOLOGY 32 High Street, Guildford, Surrey, UK
85
properties
of Southampton,
of thin films
UK, 24-26
September
This conference, organized by the Optical Group of the Institute of Physics, was attended by about 120 delegates from 13 countries. There were five sessions, four of which began with review papers by well known authorities in the field. Session 1 was on the Basic Optical Properties of Thin Film Materials; the review by Professor F. Abeles of the University of Paris was followed by 11 papers. Abeles stressed that because the optical properties of a material were effected by its crystalline structure, a knowledge of them is useful for the interpretation of structural properties. The first contributed paper was by J. Richard, L. Penevaire, P. Cheyssac, R. Garrigos, and R. Kofman (Laboratoire d’Electro-optique, Nice) on the ‘Thermo-reflectance of silver fdms’; the technique of measurement was described and results discussed. A second paper from France had the title ‘Collective oscillations of electrons in alkali metal thin films’ (J. C. Payan and D. Roux, Universite de Provence, Marseille). It was suggested that the abnormal transmission properties in these films were the result of such oscillations. Two papers of the session originated from Sweden. L. G. Bernland, 0. Hunderi, and H. P. Myers (Chalmers University, Gothenburg) talked on ‘Optical properties of some quench condensed polyvalent metal films’. Results with aluminium and beryllium indicated that absorption did not follow the free-electron behaviour predicted by the Drude theory. L. Stensland and L. E. Flordal (Optical Research Institute, Stockholm) discussed ‘Optical properties of some metal-dielectric mixture fdms’. They deduced that Maxwell-Garnett theory applies to metaldielectric mixtures with low resultant polarizability, but that the formula by Bottcher is valid for high polarizability mixtures. Of the four papers from the UK, three were mainly experimental studies directly connected with optical properties, the fourth was theoretical. The paper by L. S. Julien, A. J. Taylor, and R. F. Miller (Royal Holloway College, London) had the title ‘Optical constants of gadolinium and terbium films obtained from in situ reflection measurements in ultra-high vacuum’. The importance of optimized reflection ratio measurements was stressed and results given in the photon energy range 1.8-3.1 eV. T. J. Fox, R. P. Howson, and D. C. Emmony (Loughborough University) gave a paper on ‘Optical properties of thin films of antimony’. The results showed that antimony has free carriers which for their optical effects can be represented as holes with relaxation times of 3.1 x 1O-l4 s and a density of 1.7 x 1O-26 inm3. R. Hill (Newcastle-upon-Tyne Polytechnic) spoke on ‘Some optical properties of alloy semiconductor thin films’, giving results for ZnS-Te, CaAs-ZnSe, and GaSb-ZnTe. 0. S. Heavens (University of York) spoke on the ‘Use of lateral waves for the examination of surface films’. The lateral wave flux in a substrate depends on the optical constants of the superstrate film and the theory of
84
1973
this for a film of thickness small compared to wavelength was presented. German workers gave two papers. J. Bodesheim and A. Otto (University of Munich) covered ‘Quantitative measurements of the roughness and roughness spectrum of silver films’ and H. Kostlin, R. Jost, and W. Lens (Philips, Aachen) spoke on ‘The optical and electrical properties of doped In,03 films’. The last paper of the session was contributed by D. Bedeaux and J. Vlieger (Lorentz Institute, Leiden). Talking on ‘Some theoretical aspects of the optical properties of thin ftims’, they gave a phenomenological theory and used it to calculate the transmission, reflection, and absorption of light. Optical Applications of Thin Films was the general title for Session 2. A. Thelen (Balzers, Liechtenstein) began with a review of recent developments in the field. He concentrated on the design of optical filters and dealt with antireflection coatings, high reflectors, neutral beam splitters, dichroic filters (short and long wave pass), monochromatic filters, minus filters, and polarizing ffiters. Practical limitations in filters still lie in the materials available and in production problems. Of the ten following papers, four dealt with films for specific applications: these were planar optical guides (C. W. Pitt and R. J. Stevens, University College, London), proustite parametric oscillators (A. R. Henderson, University of Southampton), antireflection coatings over the range 415-950 nm (G. F. Doughty Barr and Stroud, Glasgow), ultra-violet interferometry (G. H. C. Freeman, National Physical Laboratory). Freeman called for more data on optical constants and for developments of broad band transmission filters in the range 200- 110 nm. Other papers in this session dealt with various effects produced during film preparation. H. A. Macleod and D. Richmond (Newcastle-upon-Tyne Polytechnic) contributed a paper on ‘Sources of error in the optical monitoring of narrow band all-dielectric thin film optical filters’. Methods for their reduction were described. ‘Optical thickness changes in freshly deposited layers of lead telluride’ was the title of a paper by C. S. Evans, R. H. Hunneman, and J. S. Seeley (University of Reading). A. Thetford (Reading) talked on ‘The effect of absorption on the performance of all-dielectric bandpass interference filters’. He gave simple, reasonable, accurate formulae for finding the peak transmittance, residual reflectance, and half-width of the filters when a small amount of absorption is present in the high index layers. D. J. Hemingway (University of Salford) considered the dependence of scattering in alldielectric filters on wavelength and angle of scattering. Information on filter production was the theme of the last two contributions. E. Pelletier, P. Roche, and L. Bertrand (Universite de Provence, Marseille) were interested in the ‘Limiting bandwidth of interference filters and the
OPTICS AND LASER TECHNOLOGY
APRIL
1974
influence of temperature changes during production’. The paper by R. G. T. Neilson (Rank Precision Industries, Leicester) on a ‘Method for the control of refractive index of oxide films’ was read by N. Bevan. The apparatus used was described and results with particular reference to a three-layer broad band anti-reflection coating were discussed. In Session 3 on Magneto-Optics of Thin Magnetic Films, only two of the three planned papers were given. J. Krizek and K. N. R. Taylor (University of Durham) spoke on ‘Optical properties of rare earth metals in the paramagnetic and magnetically ordered phases’. R. Atkinson and P. H. Lissberger (University of Salford) talking on ‘Ellipsometric determination of the optical and magneto-optical constants of thin fdm MnBi’ discussed a technique involving radiation reflected from the interface between the substrate and the film. A third paper, a substitute for the one in the programme, was presented by W. Hasan, R. B. Inwood, and R. F. Miller (Royal Holloway College) with the title ‘Optimization of the measurement of magneto-optical parameters’. Two papers not related to the subject matter of this session were then given: one by J. C. Lemonnier, M. Priol, and S. Robin (Universite de Rennes) on ‘Optical properties of thin lead films in the energy range 3- 15.5 eV’ and a short contribution from P. Hendra (University of Southampton) on ‘Raman scattering measurements on thin films at electrode-electrolyte interfaces’. From the point of view of a chemist, he described the technique and gave examples of spectra from films some 0.3-0.6 nm thick. The fourth session was on Thin Film Imperfections. H. E. Bennett (Michelson Laboratory, China Lake, California) opened with a review paper entitled ‘The influence of imperfections in thin films on their optical properties’. He listed ten factors which influence the quality of evaporated films: purity of the evaporent, deposition speed, pressure, source type, substrate temperature, source to substrate distance, deposition angle, coating thickness, material of substrate, and substrate roughness. Measurements of imperfections were described in two papers which followed. ‘A study of thin film imperfections by optical methods’ was presented by M. J. Downs and R. J. King (National Physical Laboratory). Commenting that values of the thickness of dielectric films obtained by ellipsometry differ from those obtained by step height methods, King ascribed this to film inhomogeneity. Jean M. Bennett (Michelson Laboratory) described in detail the scanning inteferometer used for measuring the roughness of optical surfaces. W. R. Hunter (Naval Research Laboratory, Washington DC) and G. Hass (Night Vision Laboratory, Virginia) gave a paper on the ‘Thickness of absorbing films necessary to measure thin optical constants using the reflectance versus angle of incidence method’. Hunter said that thicknesses less than that required to give 0.1% transmission could be used. K. W. Raine and P. B. Clapham (NPL) were concerned with ‘Optical absorption in films deposited by electron bombardment sources’. The cause of the absorption is the creation of colour centres, but electron guns employing a magnet do not give this effect. Ch. Kuhle, H. Schlotterer, and F. Schwidefsky (Siemens, Munich) gave a paper on the ‘Influence of deposition parameters on the optical constants of thin silicon films’. In the final paper of this session, J. M. Pearson (University of Salford) spoke on ‘Improvements in falter performance based on structural studies by electron microscopy’.
OPTICS AND LASER TECHNOLOGY
APRIL
1974
The last session was on Techniques of Measurement of Optical Constants. H. A. Macleod (Newcastle-upon-Tyne Polytechnic) gave the review paper and confined his remarks mainly to isotropic media. The values of optical constants are required because they give information on material structure and are necessary for filter design. Macleod discussed the accuracy with which the constants should be determined. Papers which described methods of measurement then followed. F. R. Gfeller and C. W. Pitt (University College, London) presented ‘Measurement techniques for optical guide materials’ in which a method for the simultaneous measurement of refractive index and thickness of thin optically transparent dielectric films with low loss was described. Two papers were concerned with computation methods. ‘Automatic evaluation of optical constants and thickness of thin films with application to thin dielectric films’ was the title of one. In this E. Pelletier, P. Roche, and B. Vidal (Universitd de Provence) described the principle of a completely automatic spectrophotometer covering the range 200-700 nm, which was coupled to a computer. ‘The theoretical determination of optical constants of thin films’ was presented by Heather Liddell (Queen Mary College, London). A method based on the assumption that the refractive index obeys a Sellmeier dispersion formula was described. M. S. Shalaan and A. J. Taylor (Royal Holloway College, London) talked on the ‘Interferometric determination of the extinction coefficient of highly reflecting metallic films’. The method is based on the multiple beam interference fringes produced by an air wedge as used by Tolansky. The only paper in the conference entirely devoted to instrumentation was that by C. C. Matheson and J. G. Wright (University of East Anglia) on ‘A high precision polychromatic automatic ellipsometer’. He described the instrument as being automatic, versatile, fast, precise, accurate, and easy to use. From the remarks of delegates, the conference as a whole was successful - both in its coverage and in the gathering together of workers in the subject. There was no shortage of material which unfortunately meant there was a shortage of time. Delegates left the conference with the feeling that there was much to be done and much being done in this field. H. G. Jerrard
Bound Volumes Why not have your back copies of OPTICS AND LASER TECHNOLOGY bound in an attractive hardback cover. We will bind a year’s issues (6 journals plus index) for a cost of f6. This includes postage in the UK, but overseas postage will be extra. Please send all orders, together with remittance to Brian Roylance, OPTICS AND LASER TECHNOLOGY 32 High Street, Guildford, Surrey, UK
85