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Surface reflectivity of TiN thin films measured by spectral ellipsometry
Surface
200
Surface reflectivity E. Pascual, Unirwrsctat
Received
M.C.
Polo,
de Barcelonu.
1 October
of TIN thin films measured J. Esteve
Departament
1990: accepted
and
E. Bertran I Electr~nrcu,Av.
de Fisica Aplicuda
for publication
23 November
Diugonai
Science 251/252
by spectral
h4.L64
7, E-ilXlQN
(19Yl) 200-203 North-Holland
~~~ipso~etry
Barcchr1o.
sparn
1990
Optical properties of TiN thin films have been measured by spectroscopic ellipsometry in the UV-vtsihle range. TiN is a transition metal nitride showing some singular properties such as hardness. wear resistance and golden color, which are very useful in metallurgical and optical coatings. TIN thin films were prepared by plasma enhanced chemical vapor deposition from a gas mixture of NZ, Hz and TiCl, vapor. The properties of TIN thin films mainly depend on deposition conditions and film stoichiomctrv. In particular, optical properties of TiN thin films can be strongly modulated by technological parameters such as suhstrate temperature. dilution of gases and RF-power. The reflectivity spectra of TiN films were calculated from ellipsometric measuren~ents and were compared to the evaporated and crystalline gold reflectivity spectra. The reflectivity spectra of TIN films show a shape similar to that of gold, with an edge in the visible region. although the TiN films present lower reflecttvity than the gold film.
1. Introduttion Transition metal nitrides and carbides have been studied because of their singular properties such as hardness, wear resistance, low friction coefficient, golden color (TIN) and chemical stability at high temperature. These properties are very useful for applications such as hard coatings on cutting tools [l-3], wear resistance coatings with selective transmittance on optical surfaces [4]. and diffusion barriers in semiconductor electronic devices (51. TIN thin films are probably the most commercially used hard coating. In addition to its mechanical properties, the optical properties of TiN are very useful because, at least in the UV-visible range, they are very similar to those of gold. This feature together with its wear resistance have a particular relevance in some applications as a coating, specially in jewellery and watchmaking industries and in reflective surfaces. From the sixties, thin films of TIN have been 0039-6028,/91/$03.50
,i’ 1991 - Elsevter Science Publishers
obtained by chemical vapor deposition (CVD) from compounds based on Ti and N,, but the main difficulty of this method is that the reaction between the different gases only can be carried out at high temperatures (900”--1100°C) [h]. This feature limited the applications of TiN films to substrates being stable to high temperatures. Nowadays. TIN coatings can he obtained at low temperatures by physical deposition methods as reactive sputtering or ion platting j7). But, it is difficult to cover pieces with intricate shapes by this methods. Attempts to lower the deposition temperature of TiN CVD have been effected by plasma assisted chemical vapor deposition PACVD [2,8]. By this method, the species present in the plasma acquire sufficient activation energy for producing the chemical reactions that originate the film growth. In this way. possible temperature induced changes in the substrates can be avoided. In this paper we have studied the influence of the RF-power, supplied to the plasma. on the
R.V. (North-Holland)
201
E. Pascuai et al. / Optical properties of TiN thin fibs
optical properties of the TiN thin films deposited by PACVD, and we have compared the reflectivity spectra of TiN thin films to the reflectivity spectra of evaporated gold films and crystalline gold.
2. Experimental 2.1. Deposition
zer (PSA) configuration [lO,ll]. Ellipsometry is a technique of optical characterization specially appropriate to analyze solid surfaces and thin films, it is very sensible to surface roughness and very thin overlayers. Ellipsometry measures the angles q and A which are defined in terms of the complex reflectance ratio fi by:
process
TiN thin films were obtained by plasma assisted chemical vapor deposition (PACVD) on slide glass substrates in a reaction chamber [9]. This reactor is a capacitively coupled radio frequency plasma system that operates at 13.56 MHz. The reactor chamber consists of a cylindrical stainless steel vacuum vessel equipped with a chemical proof mechanical pump and feedthroughs for the RF-power supply, the gas supply, the heating system and the temperature and pressure gauges. Within the reactor chamber there are two electrodes: the cathode, in the top side, which is connected to the RF-power generator through an automatic matching network, and the anode, 4 cm below the cathode, which is connected to ground. The RF-power was varied in the range lOO-~0 W. The substrates were normally on the anode and the deposition temperature was kept constant at 550°C by a heating system and was measured with a thermocouple. The gases (N2, H, and TiCl, vapor) are admitted into the chamber through a mixing box, and their fluxes are separately regulated by flowmeters in order to obtain the adequate concentrations. With the purpose of obtaining a constant flow of TiCl, vapor, the TiCl, vessel was accurately maintained at 30°C during the deposition process. The pressure in the reactor was kept constant at 113 Pa and monitored by an absolute capacitive meter. The partial pressures of N, and H, were close to 50 Pa.
(1) where & and 3 are the complex reflectances for light polarized parallel (p) and pe~endicul~ (s) to the plane of incidence. In the rotating analyzer ellipsometry the normalized detected intensity i( 6) as a function of the azimuth of analyzer 8 takes the general form [12]: i(B)=l-cos2*cos28+sin2+cosA
sin28, (2)
where B is a function of time B = 8, + wt with w the analyzer angular frequency and $ the initial azimuth. The intensity measurements of the monochromatic beam reflected from the TiN thin film were averaged over several rotations of the analyzer to reduce noise effects. The ellipsometric angles ( ?I!, A) for each wavelength (X) were calculated from a Fourier analysis of the detected light intensity, after substraction of the background signal. Because of TiN thin films were opaque in the visible-UV range with the absorption coefficient a: greater than lo5 cm-‘, the ellipsometric measurements from the reflection on the air-film interface provided info~ation on the surface properties without cont~butions of the bulk and the substrate. The spectral complex dielectric function, Z(X) = e, - ic,, was obtained from the measured complex reflectance ratio @(X) assuming a model of semi-infinite medium [lo]: ~(h)=sin’~,
[ l+tancp 2 a( :;~i:iil].
(3)
2.2. Upticui system Optical measurements in the range 300-800 nm were performed by means of a rotating analyzer ellipsometer (RAE) with polarizer-sample-analy-
where (pO is the angle of incidence, which was fixed at 69.8”. In operation, no compensator was used and the polarizer prism was set to an azimuth of 45”.
E. Pascual et al. /
202
Opticalpropertm of TiN thin films
From the complex reflectances ( Fp, it) for light with oblique incidence on the surface air/TiN, the reflectivity in the plane of incidence ( RP) and the reflectivity in the perpendicular plane (R,) were deduced, obtaining the following expressions: R,=
R,=
-N,
cos ‘po+cos
’
‘P,
N, cos cp,,+ cos q+ cosq-N,coscp, cos q. + Nl cos
’ (4)
’ ‘pl
’
where N, were the complex refractive index which were obtained from P and ‘p, was the value of the refraction angle in the film obtained from ‘pO and
3. Results and discussion The spectral optical parameters (<, N, and n) and the reflectivities (R,, RP) of TIN thin films, obtained by PACVD, were measured by spectroscopic ellipsometry in the spectral range from 300 to 800 nm and at 69.8” of the angle of incidence. The results were compared with those of evaporated gold in the same spectral range. In fig. 1 it can be seen the spectral dependence of the real (E,) and imaginary (E,) part of the
Fig. 1. Spectral dependence for TiN thin film deposited
ooL-L-L!
100
N,.
of the complex dielectric function at RF-power = 300 W. (0) c, and (0) f,.
Fig. 2. R, wavelength
11
200
300
400
I
500
Wavelength
L
600
11
ii
700
800
900
(nm)
and R, reflectivities at 69.X0 incidence versus of evaporated gold (solid lines) compared with those of crystalline gold (dashed lines).
complex dielectric function < characteristic of our TIN thin films. The maximum of E, ( c, > 1) is obtained for values of X = 400 nm and e, presents a gradual decrease to values lower than - 3 for h > 750 nm. In the same wavelength range, e7 presents values between 1.0 and 2.0 with a minimum at A = 475 nm. As can be seen in fig. 2, the measured spectral values of reflectivities R, and R, at q+, = 69.8” on evaporated gold on glass substrates are very close to the reflectivities on crystalline gold at the same angle of incidence, calculated from the existing data of n and k [13]. The reflectivities of the evaporated gold are slight greater than these of crystalline gold in the spectral range from 300 to 500 nm, but for larger wavelengths are smaller than the reflectivities of crystalline gold. The parallel reflectivity (R,) spectra. for ‘p,, = 69.8”. measured on the TIN thin films are presented in fig. 3. It can be seen that the values are lower than reflectivity values of gold (fig. 2) but with a similar spectral shape, presenting the same edge in the visible region. The minimum values vary in the range 0.085 and 0.13 and lie between 370 and 410 nm depending on the sample preparation conditions and probably, the variations
E. Pascual et al. / Optical properties
tion conditions
o.6 3 0.5
*...
.? t;
Co.2 c? 0.1
0.0
-
.. .--- 0 .. / . . -2.
-
.: : .:’ ~ 9. D .. .::::::::::::::::..:
-
t_ 200
300
400
600
500
Wavelength
700
800
(nm)
Fig. 3. Parallel reflectivity R, spectra (300-750 nm) at 69.8’ incidence on TiN thin films obtained at different RF-power values: (a) 400 W. (b) 300 W and (c) 200 W.
could be attributed to differences in film stoichiometry or surface morphology [14]. The perpendicular reflectivity (R,) spectra measured on TiN films are presented in fig. 4, where a minimum of about 0.4 at h = 420 nm is observed for all the samples and R, rises to values over 0.8 for h > 700 nm. By comparing R, and R, in fig. 3 and fig. 4 it can be seen that the shape of R, at X < 450 nm is more sensitive to deposi0.9
r
....
0.8
..**
-
02’ 200
...m.
I
. . ..
. . . . _:.’ .. /. _,..,. . .. _..* ., b
’
’
300
j
400
/
500
Wavelength
c
’
600
I
700
of the TIN films than the shape of
4. Conclusions
..f. .. _.*. .. ..=+. :.. .: _: : ...f. _.. ..I *a* : :I .*..:
K”
203
RP.
b . .._... c -* ... ..: ..f.
-
0.4
,x0.3
of TiN thin films
’
800
(nm)
Fig. 4. Perpendicular reflectivity R, spectra (300-750 nm) at 69.8O incidence on TiN thin films obtained at different RFpower values: (a) 400 W, (b) 300 W and (c) 100 W.
Ellipsometry is a powerful tool for the quantitative analysis of the surface properties of thin films and we demonstrate its usefulness in the case of TIN films. This technique allows a complete optical evaluation of these films when they are going to be used for decorative or heat mirrors purposes where the reflectivity spectrum quantifies their quality. Reflectivities of all the TiN films obtained by PACVD with values of RF-power between 100 and 400 W are found to be similar to that of gold on a wide spectral range, but reflectivity values are lower than those of gold. The spectra present the edge in the visible region ( - 500 nm) characteristic of the gold spectrum. The present optical analysis can be extended to the study of the influence of other growth conditions on the TIN optical surface properties and also to the study of other related materials interesting for their optical surface applications. References VI S.R. Kurtz and R.G. Gordon,
Thin Solid Films 140 (1986) 277. [21 L. Shizhi, Z. Cheng, S. Yulong, H. Wu, X. Yan and Y. Hongshun, Proc. Electrochem. Sot. (1987) p. 1233. 131 D.H. Jang, J.S. Chun and J.G. Kim. J. Vat. Sci. Technol. A 7(l) (1989) 31. and M. Sigrist, Solar 141 L. Roux, J. Hanus, J.C. Fraqois Energy Mater. 7 (1982) 299. B. Lee and E.C. PI N. Kumar. M.G. Fissel, K. Pourrezaei, Douglas, Thin Solid Films 153 (1987) 287. [61 K.H. Habig. J. Vat. Sci. Technol. A 4 (1986) 2832. 171 A.J. Perry, Thin Solid Films 135 (1986) 73. G.J. Vandetop. M. Salmeron and G.A. PI M.R. Hilton, Somorjai, Thin Solid Films 154 (1987) 377. 191 M.C. Polo, J. Esteve and J.L. Morenza. Surf. Coat. Tech., to be published. Azzam and N.M. Bashara. Ellipsometry and UOI R.M.A. Polarized Light (North-Holland, Amsterdam, 1987). [111 D.E. Aspnes and A.A. Studna. Appl. Opt. 14 (1975) 220. WI P.S. Hauge and F.H. Dill, IBM J. Res. Dev. 17 (1973) 472. of Chemistry and Physics, Ed. R.C. P31 CRC Handbook Weast (CRC Press. Boca Raton, FL. 198881989). P41 J.-E. Sundgren, Thin Solid Films 128 (1985) 21.
200
Surface reflectivity E. Pascual, Unirwrsctat
Received
M.C.
Polo,
de Barcelonu.
1 October
of TIN thin films measured J. Esteve
Departament
1990: accepted
and
E. Bertran I Electr~nrcu,Av.
de Fisica Aplicuda
for publication
23 November
Diugonai
Science 251/252
by spectral
h4.L64
7, E-ilXlQN
(19Yl) 200-203 North-Holland
~~~ipso~etry
Barcchr1o.
sparn
1990
Optical properties of TiN thin films have been measured by spectroscopic ellipsometry in the UV-vtsihle range. TiN is a transition metal nitride showing some singular properties such as hardness. wear resistance and golden color, which are very useful in metallurgical and optical coatings. TIN thin films were prepared by plasma enhanced chemical vapor deposition from a gas mixture of NZ, Hz and TiCl, vapor. The properties of TIN thin films mainly depend on deposition conditions and film stoichiomctrv. In particular, optical properties of TiN thin films can be strongly modulated by technological parameters such as suhstrate temperature. dilution of gases and RF-power. The reflectivity spectra of TiN films were calculated from ellipsometric measuren~ents and were compared to the evaporated and crystalline gold reflectivity spectra. The reflectivity spectra of TIN films show a shape similar to that of gold, with an edge in the visible region. although the TiN films present lower reflecttvity than the gold film.
1. Introduttion Transition metal nitrides and carbides have been studied because of their singular properties such as hardness, wear resistance, low friction coefficient, golden color (TIN) and chemical stability at high temperature. These properties are very useful for applications such as hard coatings on cutting tools [l-3], wear resistance coatings with selective transmittance on optical surfaces [4]. and diffusion barriers in semiconductor electronic devices (51. TIN thin films are probably the most commercially used hard coating. In addition to its mechanical properties, the optical properties of TiN are very useful because, at least in the UV-visible range, they are very similar to those of gold. This feature together with its wear resistance have a particular relevance in some applications as a coating, specially in jewellery and watchmaking industries and in reflective surfaces. From the sixties, thin films of TIN have been 0039-6028,/91/$03.50
,i’ 1991 - Elsevter Science Publishers
obtained by chemical vapor deposition (CVD) from compounds based on Ti and N,, but the main difficulty of this method is that the reaction between the different gases only can be carried out at high temperatures (900”--1100°C) [h]. This feature limited the applications of TiN films to substrates being stable to high temperatures. Nowadays. TIN coatings can he obtained at low temperatures by physical deposition methods as reactive sputtering or ion platting j7). But, it is difficult to cover pieces with intricate shapes by this methods. Attempts to lower the deposition temperature of TiN CVD have been effected by plasma assisted chemical vapor deposition PACVD [2,8]. By this method, the species present in the plasma acquire sufficient activation energy for producing the chemical reactions that originate the film growth. In this way. possible temperature induced changes in the substrates can be avoided. In this paper we have studied the influence of the RF-power, supplied to the plasma. on the
R.V. (North-Holland)
201
E. Pascuai et al. / Optical properties of TiN thin fibs
optical properties of the TiN thin films deposited by PACVD, and we have compared the reflectivity spectra of TiN thin films to the reflectivity spectra of evaporated gold films and crystalline gold.
2. Experimental 2.1. Deposition
zer (PSA) configuration [lO,ll]. Ellipsometry is a technique of optical characterization specially appropriate to analyze solid surfaces and thin films, it is very sensible to surface roughness and very thin overlayers. Ellipsometry measures the angles q and A which are defined in terms of the complex reflectance ratio fi by:
process
TiN thin films were obtained by plasma assisted chemical vapor deposition (PACVD) on slide glass substrates in a reaction chamber [9]. This reactor is a capacitively coupled radio frequency plasma system that operates at 13.56 MHz. The reactor chamber consists of a cylindrical stainless steel vacuum vessel equipped with a chemical proof mechanical pump and feedthroughs for the RF-power supply, the gas supply, the heating system and the temperature and pressure gauges. Within the reactor chamber there are two electrodes: the cathode, in the top side, which is connected to the RF-power generator through an automatic matching network, and the anode, 4 cm below the cathode, which is connected to ground. The RF-power was varied in the range lOO-~0 W. The substrates were normally on the anode and the deposition temperature was kept constant at 550°C by a heating system and was measured with a thermocouple. The gases (N2, H, and TiCl, vapor) are admitted into the chamber through a mixing box, and their fluxes are separately regulated by flowmeters in order to obtain the adequate concentrations. With the purpose of obtaining a constant flow of TiCl, vapor, the TiCl, vessel was accurately maintained at 30°C during the deposition process. The pressure in the reactor was kept constant at 113 Pa and monitored by an absolute capacitive meter. The partial pressures of N, and H, were close to 50 Pa.
(1) where & and 3 are the complex reflectances for light polarized parallel (p) and pe~endicul~ (s) to the plane of incidence. In the rotating analyzer ellipsometry the normalized detected intensity i( 6) as a function of the azimuth of analyzer 8 takes the general form [12]: i(B)=l-cos2*cos28+sin2+cosA
sin28, (2)
where B is a function of time B = 8, + wt with w the analyzer angular frequency and $ the initial azimuth. The intensity measurements of the monochromatic beam reflected from the TiN thin film were averaged over several rotations of the analyzer to reduce noise effects. The ellipsometric angles ( ?I!, A) for each wavelength (X) were calculated from a Fourier analysis of the detected light intensity, after substraction of the background signal. Because of TiN thin films were opaque in the visible-UV range with the absorption coefficient a: greater than lo5 cm-‘, the ellipsometric measurements from the reflection on the air-film interface provided info~ation on the surface properties without cont~butions of the bulk and the substrate. The spectral complex dielectric function, Z(X) = e, - ic,, was obtained from the measured complex reflectance ratio @(X) assuming a model of semi-infinite medium [lo]: ~(h)=sin’~,
[ l+tancp 2 a( :;~i:iil].
(3)
2.2. Upticui system Optical measurements in the range 300-800 nm were performed by means of a rotating analyzer ellipsometer (RAE) with polarizer-sample-analy-
where (pO is the angle of incidence, which was fixed at 69.8”. In operation, no compensator was used and the polarizer prism was set to an azimuth of 45”.
E. Pascual et al. /
202
Opticalpropertm of TiN thin films
From the complex reflectances ( Fp, it) for light with oblique incidence on the surface air/TiN, the reflectivity in the plane of incidence ( RP) and the reflectivity in the perpendicular plane (R,) were deduced, obtaining the following expressions: R,=
R,=
-N,
cos ‘po+cos
’
‘P,
N, cos cp,,+ cos q+ cosq-N,coscp, cos q. + Nl cos
’ (4)
’ ‘pl
’
where N, were the complex refractive index which were obtained from P and ‘p, was the value of the refraction angle in the film obtained from ‘pO and
3. Results and discussion The spectral optical parameters (<, N, and n) and the reflectivities (R,, RP) of TIN thin films, obtained by PACVD, were measured by spectroscopic ellipsometry in the spectral range from 300 to 800 nm and at 69.8” of the angle of incidence. The results were compared with those of evaporated gold in the same spectral range. In fig. 1 it can be seen the spectral dependence of the real (E,) and imaginary (E,) part of the
Fig. 1. Spectral dependence for TiN thin film deposited
ooL-L-L!
100
N,.
of the complex dielectric function at RF-power = 300 W. (0) c, and (0) f,.
Fig. 2. R, wavelength
11
200
300
400
I
500
Wavelength
L
600
11
ii
700
800
900
(nm)
and R, reflectivities at 69.X0 incidence versus of evaporated gold (solid lines) compared with those of crystalline gold (dashed lines).
complex dielectric function < characteristic of our TIN thin films. The maximum of E, ( c, > 1) is obtained for values of X = 400 nm and e, presents a gradual decrease to values lower than - 3 for h > 750 nm. In the same wavelength range, e7 presents values between 1.0 and 2.0 with a minimum at A = 475 nm. As can be seen in fig. 2, the measured spectral values of reflectivities R, and R, at q+, = 69.8” on evaporated gold on glass substrates are very close to the reflectivities on crystalline gold at the same angle of incidence, calculated from the existing data of n and k [13]. The reflectivities of the evaporated gold are slight greater than these of crystalline gold in the spectral range from 300 to 500 nm, but for larger wavelengths are smaller than the reflectivities of crystalline gold. The parallel reflectivity (R,) spectra. for ‘p,, = 69.8”. measured on the TIN thin films are presented in fig. 3. It can be seen that the values are lower than reflectivity values of gold (fig. 2) but with a similar spectral shape, presenting the same edge in the visible region. The minimum values vary in the range 0.085 and 0.13 and lie between 370 and 410 nm depending on the sample preparation conditions and probably, the variations
E. Pascual et al. / Optical properties
tion conditions
o.6 3 0.5
*...
.? t;
Co.2 c? 0.1
0.0
-
.. .--- 0 .. / . . -2.
-
.: : .:’ ~ 9. D .. .::::::::::::::::..:
-
t_ 200
300
400
600
500
Wavelength
700
800
(nm)
Fig. 3. Parallel reflectivity R, spectra (300-750 nm) at 69.8’ incidence on TiN thin films obtained at different RF-power values: (a) 400 W. (b) 300 W and (c) 200 W.
could be attributed to differences in film stoichiometry or surface morphology [14]. The perpendicular reflectivity (R,) spectra measured on TiN films are presented in fig. 4, where a minimum of about 0.4 at h = 420 nm is observed for all the samples and R, rises to values over 0.8 for h > 700 nm. By comparing R, and R, in fig. 3 and fig. 4 it can be seen that the shape of R, at X < 450 nm is more sensitive to deposi0.9
r
....
0.8
..**
-
02’ 200
...m.
I
. . ..
. . . . _:.’ .. /. _,..,. . .. _..* ., b
’
’
300
j
400
/
500
Wavelength
c
’
600
I
700
of the TIN films than the shape of
4. Conclusions
..f. .. _.*. .. ..=+. :.. .: _: : ...f. _.. ..I *a* : :I .*..:
K”
203
RP.
b . .._... c -* ... ..: ..f.
-
0.4
,x0.3
of TiN thin films
’
800
(nm)
Fig. 4. Perpendicular reflectivity R, spectra (300-750 nm) at 69.8O incidence on TiN thin films obtained at different RFpower values: (a) 400 W, (b) 300 W and (c) 100 W.
Ellipsometry is a powerful tool for the quantitative analysis of the surface properties of thin films and we demonstrate its usefulness in the case of TIN films. This technique allows a complete optical evaluation of these films when they are going to be used for decorative or heat mirrors purposes where the reflectivity spectrum quantifies their quality. Reflectivities of all the TiN films obtained by PACVD with values of RF-power between 100 and 400 W are found to be similar to that of gold on a wide spectral range, but reflectivity values are lower than those of gold. The spectra present the edge in the visible region ( - 500 nm) characteristic of the gold spectrum. The present optical analysis can be extended to the study of the influence of other growth conditions on the TIN optical surface properties and also to the study of other related materials interesting for their optical surface applications. References VI S.R. Kurtz and R.G. Gordon,
Thin Solid Films 140 (1986) 277. [21 L. Shizhi, Z. Cheng, S. Yulong, H. Wu, X. Yan and Y. Hongshun, Proc. Electrochem. Sot. (1987) p. 1233. 131 D.H. Jang, J.S. Chun and J.G. Kim. J. Vat. Sci. Technol. A 7(l) (1989) 31. and M. Sigrist, Solar 141 L. Roux, J. Hanus, J.C. Fraqois Energy Mater. 7 (1982) 299. B. Lee and E.C. PI N. Kumar. M.G. Fissel, K. Pourrezaei, Douglas, Thin Solid Films 153 (1987) 287. [61 K.H. Habig. J. Vat. Sci. Technol. A 4 (1986) 2832. 171 A.J. Perry, Thin Solid Films 135 (1986) 73. G.J. Vandetop. M. Salmeron and G.A. PI M.R. Hilton, Somorjai, Thin Solid Films 154 (1987) 377. 191 M.C. Polo, J. Esteve and J.L. Morenza. Surf. Coat. Tech., to be published. Azzam and N.M. Bashara. Ellipsometry and UOI R.M.A. Polarized Light (North-Holland, Amsterdam, 1987). [111 D.E. Aspnes and A.A. Studna. Appl. Opt. 14 (1975) 220. WI P.S. Hauge and F.H. Dill, IBM J. Res. Dev. 17 (1973) 472. of Chemistry and Physics, Ed. R.C. P31 CRC Handbook Weast (CRC Press. Boca Raton, FL. 198881989). P41 J.-E. Sundgren, Thin Solid Films 128 (1985) 21.