- Email: [email protected]
In situ determination of the optical constants of titanium surface films formed in higher voltage anodization
ucelrochim~caAera. Vol 37. No Fnnted 111 Great Bntmn
00134666/92 SS00 + 0 00 Q 1992 Fwplon Press Ltd
I I, pp 2099-2100,1992
SHORT COMMUNICATION IN SITU DETERMINATION OF THE OPTICAL CONSTANTS OF TITANIUM SURFACE FILMS FORMED IN HIGHER VOLTAGE ANODIZATION LJ D ARSOV and A T
EFFCEMOVA
Faculty of Technology and Metallurgy, Umverslty “Cynl and Metody” 91000 SkopJe, Yugoslavui (Recerved 14 November 1991, zn rewed form 27 January 1992) Abstract-The opt& constants and thickness of anodlc oxide films formed on htamum surface, dunng the polanzatlon, have been determmed m 0 5 M H,SO., by VI szru elhpsometnc measurements The anodlc titanium films are formed m the voltage repon from 0 to 100 V The posslhhty of determmmg the real part film refractive index without direct utdlzatlon of the metal substrate refractive index has been shown Key worcis elhpsometry, titanium oxide films, anodization, optical constants
EXPERIMENTAL
NOMENCLATURE
A
A, = n,(l -
1 K,) fi, = PI&l - 1 K2) fi, = n,(l - 1 K,)
relative phase change relative amplitude attenuation refractive index (real part) extmctlon mdex (lmagmary part) complex refractive index of medium complex refractive index of film complex refractive index of metal substrate
film thickness angle of incidence change of phase of the beam crossing the film wavelength
Electrodes
Tl discs (U T 40 Ugme Kuhlmen), 2 5 cm m dlameter were cut from annealed sheet, mechanically polished by 600 grade emery paper and then electropohshed[4] A Pt gnd was used as a counter electrode and a saturated calomel electrode (see) as a reference electrode, for lower anode potential Optical-electrolytic cell
The optical-electrolytic cell was a quartz tube placed m an air thermostat at a temperature of 20 + 1°C Solutron
INTRODUCTION For the determtnatton of the exact value of refractive index and thickness of a film, grown on a metallic substrate, it IS necessary to have a good preliminary knowledge of the refractive index of the metal substrate The Qrect determmatlon of the refractive index of the metal substrate from the measured elhpsometnc values A and $, without any assumption about the nature of the surface film, has been rather unsuccessful m the past In the literature there 1s a very large discrepancy m the reported values of n2 (2 00-2 53) and ~2 (O-O 318) for anodlc oxide films on Tl surface[l-31 This dlscrepancy can be explained by application of various methods, vanous manners of the surface preparation and utlhzmg various equations for calculations which very often cause significant errors In our knowledge there have been no systematic studies of Tl anodlc films refractive index, by elhpsometry, especially for higher voltage up to 100 V
Analytical grade reagent of H,SO, (Merck) was used for the preparation of solution m tnply dlstdled water Apparatus and experrmental procedure
The elhpsometnc measurements were camed out by means of a Thm Film Elhpsometer type 43603-200 Rudolph Research at wavelength of 546 1 nm and mcldence angle of 70°C The electrodes were anodized at various voltage for a penod of time of 30 s The potentlostatlcally anodlc oxidation was performed every 0 5 V by successively increasing the potential from 0 to 10 V using Wenkmg potentiostat type 121 For higher voltage, anodic oxldatlon was performed every 2 5 V also by successively increasing the applied voltage from 10-100 V utlhzmg the high supply Drusch 5140 When the workmg electrode was anodized at voltage higher than 2V, after 30s the voltage was rapidly dummshed and was kept at a constant value of 2 V (small anodlc current flowmg durmg the measurements) The elhpsometnc
Short Commumcatlon
2100
measurements control
were performed under potentlostatlc
RESULTS
AND DISCUSSION
For a three component system (medmm, oxide film, metal substrate) the expenmentally measured A and $ would be related as complex function with the followmg parameters tan $ exp(l A) = f(i,, li,, ii,, d, rp, A)
(1)
The refractive mdzes of the immersed medium li, most simply can be determined by Abbe’s refractometer For 0 5 M H,SO, m ambient temperature of 2O”C, ri,=1337 (l-10) ze n,=l337 and K,=O These values are m good agreement with values of n, and K, gven in the literature for 0 5 M H,SO,[5] The refractive indices of the metal substrate ii, were taken from our previous elhpsometnc measurements when the electropohshed TI surfaces were cathodltally polanzed m a solution of 0 5 M H,SO,[6,7] The computed values for clean metal subtrate were &=294(1-12171)[6,7] The refractive indices of anodlc films covered metal substrate were determined by expenmentally measured A and $ as a function of applied voltage, Fig 1 The curve for A has three inflection points (a, b and c) on three drops from which two for A = 0 or A = 360” are large and bnskly decrease and one for A = 180” 1s relatively weak The mflectlon points of these three drops on the $ curve correspond to two peaks (maximums a and c) and one peak (mmlmum b) The peaks a and c which correspond to A = 0 and A = 360” are high and sharp while the peak b for A = 180” is weak and broad The change m value of A for 360” 1s provoked by film thickness formation which corresponds to a voltage difference of 57V between points a and c The refractive index A2 and thickness d of anodlc oxide films can be determined most simply by fitting the expenmentally measured points A and $ (gven m Fig 1) wth theoretical A-$ curves for fixed values of n, = 1 337 and ir, = 2 94( 1 - 1 2 17 1) The fitting procedure was performed by a specially prepared computer programme m which A, was searched for gven values of d m increasing dlrectlon The refractive indices of ii, = 2 38( 1 - 0 006 1) were determined The computed curve showed that the value for film thlckness difference of d, = 135 nm corresponds to voltage difference. of 57 V between points a and c Taking into account that the film thickness depends linearly on applied anodlc voltage[l, 81 and there is the relation d = ct V where o! is the coefficient of film thickness grown per volt, from computed d, = 135 nm and expenmentally determined voltage difference of 57 V, the value of a can be determined by a = 135/57=237nm V-’ According to Fresnel’s equation the phase change 6 1s
The expenmentally determmed points a and c m Fig 1 correspond to the maximal value of S = 180”
E/
v
Rg 1 Dependence of expenmentally measured A (a) and $
(b) on anode voltage m equation (2) This value 1s caused by a film thickness difference of dc = 135 nm If we suppose that ~~ = 0, from equation (2) we can approximately calculate n2 without knowing m advance the refractive index of the metal substrate For 6 = 180” and dc = 135 nm the calculated value of n2 1s 2 381 This value 1s very close to the value obtained by the fittmg procedure Comparing the values of n2 = 2 38 computed by the fittmg procedure with that of n2 = 2 381 calculated by equation (2) we have succeeded m determining very precisely the refractive indices and thickness of anodlc oxide films covered T1 substrate It are also evident that the previous determmed refractive mdlces A3= 2 94(1 - 1 217 1) of metal substrate are quite reasonable REFERENCES C Dear and S Leach. J electrochem Sot 125. 1032 (1978) G Blondeau, M Froehcher, M Froment and A Hugot Le-Gaff, Thm Sohd Fdms 42 147 (1977) R Menard, J Opt Sot Am- 52, 427 (i962) LJ Arsov, M Froehcher, M Froment and A Hugot Le-Goff, J Chun Phys 3, 275 (1975) Handbook of Chemufry and Physu, 56th Edn CRC Press, Boca Raton (1976) LJ Arsov, Electrochlm Acta 30, 1645 (1985) A Prusl and LJ Arsov, Cow &I 33, 153 (1992) LJ Arsov, C Komann and J Pheth, J electrochem Sot 138, 2964 (1991)
00134666/92 SS00 + 0 00 Q 1992 Fwplon Press Ltd
I I, pp 2099-2100,1992
SHORT COMMUNICATION IN SITU DETERMINATION OF THE OPTICAL CONSTANTS OF TITANIUM SURFACE FILMS FORMED IN HIGHER VOLTAGE ANODIZATION LJ D ARSOV and A T
EFFCEMOVA
Faculty of Technology and Metallurgy, Umverslty “Cynl and Metody” 91000 SkopJe, Yugoslavui (Recerved 14 November 1991, zn rewed form 27 January 1992) Abstract-The opt& constants and thickness of anodlc oxide films formed on htamum surface, dunng the polanzatlon, have been determmed m 0 5 M H,SO., by VI szru elhpsometnc measurements The anodlc titanium films are formed m the voltage repon from 0 to 100 V The posslhhty of determmmg the real part film refractive index without direct utdlzatlon of the metal substrate refractive index has been shown Key worcis elhpsometry, titanium oxide films, anodization, optical constants
EXPERIMENTAL
NOMENCLATURE
A
A, = n,(l -
1 K,) fi, = PI&l - 1 K2) fi, = n,(l - 1 K,)
relative phase change relative amplitude attenuation refractive index (real part) extmctlon mdex (lmagmary part) complex refractive index of medium complex refractive index of film complex refractive index of metal substrate
film thickness angle of incidence change of phase of the beam crossing the film wavelength
Electrodes
Tl discs (U T 40 Ugme Kuhlmen), 2 5 cm m dlameter were cut from annealed sheet, mechanically polished by 600 grade emery paper and then electropohshed[4] A Pt gnd was used as a counter electrode and a saturated calomel electrode (see) as a reference electrode, for lower anode potential Optical-electrolytic cell
The optical-electrolytic cell was a quartz tube placed m an air thermostat at a temperature of 20 + 1°C Solutron
INTRODUCTION For the determtnatton of the exact value of refractive index and thickness of a film, grown on a metallic substrate, it IS necessary to have a good preliminary knowledge of the refractive index of the metal substrate The Qrect determmatlon of the refractive index of the metal substrate from the measured elhpsometnc values A and $, without any assumption about the nature of the surface film, has been rather unsuccessful m the past In the literature there 1s a very large discrepancy m the reported values of n2 (2 00-2 53) and ~2 (O-O 318) for anodlc oxide films on Tl surface[l-31 This dlscrepancy can be explained by application of various methods, vanous manners of the surface preparation and utlhzmg various equations for calculations which very often cause significant errors In our knowledge there have been no systematic studies of Tl anodlc films refractive index, by elhpsometry, especially for higher voltage up to 100 V
Analytical grade reagent of H,SO, (Merck) was used for the preparation of solution m tnply dlstdled water Apparatus and experrmental procedure
The elhpsometnc measurements were camed out by means of a Thm Film Elhpsometer type 43603-200 Rudolph Research at wavelength of 546 1 nm and mcldence angle of 70°C The electrodes were anodized at various voltage for a penod of time of 30 s The potentlostatlcally anodlc oxidation was performed every 0 5 V by successively increasing the potential from 0 to 10 V using Wenkmg potentiostat type 121 For higher voltage, anodic oxldatlon was performed every 2 5 V also by successively increasing the applied voltage from 10-100 V utlhzmg the high supply Drusch 5140 When the workmg electrode was anodized at voltage higher than 2V, after 30s the voltage was rapidly dummshed and was kept at a constant value of 2 V (small anodlc current flowmg durmg the measurements) The elhpsometnc
Short Commumcatlon
2100
measurements control
were performed under potentlostatlc
RESULTS
AND DISCUSSION
For a three component system (medmm, oxide film, metal substrate) the expenmentally measured A and $ would be related as complex function with the followmg parameters tan $ exp(l A) = f(i,, li,, ii,, d, rp, A)
(1)
The refractive mdzes of the immersed medium li, most simply can be determined by Abbe’s refractometer For 0 5 M H,SO, m ambient temperature of 2O”C, ri,=1337 (l-10) ze n,=l337 and K,=O These values are m good agreement with values of n, and K, gven in the literature for 0 5 M H,SO,[5] The refractive indices of the metal substrate ii, were taken from our previous elhpsometnc measurements when the electropohshed TI surfaces were cathodltally polanzed m a solution of 0 5 M H,SO,[6,7] The computed values for clean metal subtrate were &=294(1-12171)[6,7] The refractive indices of anodlc films covered metal substrate were determined by expenmentally measured A and $ as a function of applied voltage, Fig 1 The curve for A has three inflection points (a, b and c) on three drops from which two for A = 0 or A = 360” are large and bnskly decrease and one for A = 180” 1s relatively weak The mflectlon points of these three drops on the $ curve correspond to two peaks (maximums a and c) and one peak (mmlmum b) The peaks a and c which correspond to A = 0 and A = 360” are high and sharp while the peak b for A = 180” is weak and broad The change m value of A for 360” 1s provoked by film thickness formation which corresponds to a voltage difference of 57V between points a and c The refractive index A2 and thickness d of anodlc oxide films can be determined most simply by fitting the expenmentally measured points A and $ (gven m Fig 1) wth theoretical A-$ curves for fixed values of n, = 1 337 and ir, = 2 94( 1 - 1 2 17 1) The fitting procedure was performed by a specially prepared computer programme m which A, was searched for gven values of d m increasing dlrectlon The refractive indices of ii, = 2 38( 1 - 0 006 1) were determined The computed curve showed that the value for film thlckness difference of d, = 135 nm corresponds to voltage difference. of 57 V between points a and c Taking into account that the film thickness depends linearly on applied anodlc voltage[l, 81 and there is the relation d = ct V where o! is the coefficient of film thickness grown per volt, from computed d, = 135 nm and expenmentally determined voltage difference of 57 V, the value of a can be determined by a = 135/57=237nm V-’ According to Fresnel’s equation the phase change 6 1s
The expenmentally determmed points a and c m Fig 1 correspond to the maximal value of S = 180”
E/
v
Rg 1 Dependence of expenmentally measured A (a) and $
(b) on anode voltage m equation (2) This value 1s caused by a film thickness difference of dc = 135 nm If we suppose that ~~ = 0, from equation (2) we can approximately calculate n2 without knowing m advance the refractive index of the metal substrate For 6 = 180” and dc = 135 nm the calculated value of n2 1s 2 381 This value 1s very close to the value obtained by the fittmg procedure Comparing the values of n2 = 2 38 computed by the fittmg procedure with that of n2 = 2 381 calculated by equation (2) we have succeeded m determining very precisely the refractive indices and thickness of anodlc oxide films covered T1 substrate It are also evident that the previous determmed refractive mdlces A3= 2 94(1 - 1 217 1) of metal substrate are quite reasonable REFERENCES C Dear and S Leach. J electrochem Sot 125. 1032 (1978) G Blondeau, M Froehcher, M Froment and A Hugot Le-Gaff, Thm Sohd Fdms 42 147 (1977) R Menard, J Opt Sot Am- 52, 427 (i962) LJ Arsov, M Froehcher, M Froment and A Hugot Le-Goff, J Chun Phys 3, 275 (1975) Handbook of Chemufry and Physu, 56th Edn CRC Press, Boca Raton (1976) LJ Arsov, Electrochlm Acta 30, 1645 (1985) A Prusl and LJ Arsov, Cow &I 33, 153 (1992) LJ Arsov, C Komann and J Pheth, J electrochem Sot 138, 2964 (1991)