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Ellipsometric study of a-C: H films
ELSEVIER
Diamond and Related Materials 5 (1996) 1397-1401
D IAMOND AND RELATED MATERIALS
Ellipsometric study of a-C" H films V.A. Tolmachiev, E.A. Konshina All-Russian Research Center "S.I. VaviIov State Optical Institute'; St. Petersburg 199034, Russia Received 26 July 1995; accepted in final form 18 April 1996
Abstract The optical constants, porosity and thickness of amorphous hydrogenated carbon (a-C : H) films were determined by muItipleangle eltipsometry at an optical wavelength of 632.8 nm. The films were produced by a plasma-activated CVD process in a dc glow discharge of acetylene, toluene and octane. The results indicate that the refractive index of the a-C: H films can be changed over the interval 2.35-1.55 by increasing the deposition rate and an appropriate choice of hydrocarbon precursor. A reduction of the refractive index correlates with a decrease in the extinction index in the range 0.3-0.01. The influence of the chemical nature of the hydrocarbon precursor on optical constants in the visible region is found from the results of the ellipsometric measurements. The variations of the experimental data are reviewed in view of the a-C : H structure model proposed previously. The porosity of the a-C : H films has been determined by a new technique based on the ellipsometric measurements. Keywords: Ellipsometry; Hydrocarbons; Refractive index; Porosity
1. Introduction
2. Experimental methods
Application of diamond-like carbon (DLC) films as optical coatings for the visible region of the electromagnetic spectrum is restricted by their insulticient transparency in this region. One method of investigating the optical properties of the film in the visible region is ellipsometry. Previous ellipsometric studies indicate that amorphous hydrogenated carbon (a-C:H) films have a high refractive index and are opaque in the UV-visible range [1,2]. The dependences of the optical constants of a - C : H films on the parameters of the condensation process have been established by ellipsometry [3]. The change in the refractive and extinction indexes of a-C: H films at a wavelength of 632.8 nm with increasing deposition rate of the hydrocarbons has been established by multiple angle of incidence (MAI) ellipsometry [4]. The effective medium approach was used to estimate the void fraction or porosity (the presence of water in the films) [ 5 ]. In this paper we report a comparative analysis of optical constants of a-C: H films produced from different hydrocarbon precursors which was performed using MAI ellipsometry. Two methods of ellipsometry were used: the usual MAI measurements in air and a new technique for determining film porosity [6].
2.1. Preparation conditions for a-C : H films
0925-9635/96/$15.00 © 1996 Elsevier Science S.A. All fights reserved S S D I 0925-9635(96)00551-1
In this work we studied a - C : H films produced from acetylene, toluene and octane by a plasma-activated CVD technique. We used a dc glow discharge for the CVD process [7]. The system was pumped to a pressure of about 10 -s tort before each deposition, and the hydrocarbon vapors were then admitted to the vacuum system. During deposition the pressure was 0.2-4 mTorr. The voltage between the anode and substrate (cathode) was varied from 700 to 1300 V. The films were deposited on polished glass substrates at room temperature. The substrate surface was plasma etched (in oxygen) immediately before deposition of the film.
2.2. Ellipsometric measurements All measurements were carried out using an LEF-3m ellipsometer (Russia) at a light wavelength of 632.8 nm. The configuration was polarizer-compensator-sample analyser. A dual-zone null method at angles of incidence ~bof 50 °, 60 ° and 70 ° was used to calculate the ellipsomettic parameters A and ~ with the actual errors of the instrument [8]. The homogenous isotropic layer model was used to calculate the optical constants: refractive
1398
V.A. Tolmachiev, E.A. Konshina/Diamond and Related Materials 5 (1996) 1397-1401
index N, extinction index K and thickness D. The optical constants of the substrates were Ns = 1.515 and K s = 0 . The minimization function F was calculated as the difference between the experimental and calculated values of A and ~/taking into account the measurement errors 5A and 5~ for every value of ¢ (software EDIP-2, PC AT286) [9 ]. If F <~1, the calculated and experimental values of A and ~ are equal, and the selected model and its parameters are suitable. The degree of parameter correlation was estimated in the time error calculation of N, K and D by the technique described in Ref. [91. 2.3. Determination o f porosity
3. Results and discussion
A porous film can be physically modelled as a mixture of three general components: (1) a film framework with refractive indexes NTRU~, (2) voids and (3) substances adsorbed into voids with refractive index Nv. The usual measurements in air yield information about an effective refractive index N~F of a film, when there are atmospheric components in the voids. Porous films can contain water, oxygen, nitrogen, hydrocarbons and other atmospheric components. The higher is the void fraction of film, the more incorrect is NTRU~. Thus, NTRUE is unknown and its direct measurement is impossible. The optical characteristics of such materials systems can be described by the effective media approach (EMA), such as the Lorenz-Lorentz equation: (NEF)2 -- (NTR~) 2 (gv) 2 -- (NTRuE)2 (NEF)2 + 2 = Q (Nv)2 _t_2
The above ellipsometric system allows measurement of the two different values of N~F (if the film is porous): the complex value NEF1 in a vacuum and the complex value NzF2 when water vapour is condensed in the voids. It is assumed that the film thickness is constant for both measurements. The degree of void filling by water is controlled. This technique allows the correct use of the value Nv = 1 (measurement in a vacuum) and Nv = 1.332 (with condensed water) in Eq. (1). We found the two unknown values Q and NTRUE using the system of two Eqs. (1).
(1)
where Q is a void fraction or a porosity. Usually it is assumed that NTRUEis known and N v = 1. However, this is incorrect because NTRUE of real substances may be different. Nv is equal to unity only in a vacuum when all the voids are empty. We measured values of N~F using equipment consisting of a vacuum chamber and a vacuum-gas handling system (VGS) [6]. The chamber is housed with a built-in table for the sample, entrance and exit windows, a load lock, and other items. The VGS incorporates fore pumps, zeolite pumps, vacuum valves, pressure gauges, an electronics unit, an ampoule with the adsorbate etc. This attachment for the ellipsometer makes it possible to monitor samples on any light-reflecting substrate in air and in a vacuum of 0.1 mTorr, as well as under the conditions for condensing gaseous adsorbates. The angle of incidence ¢ is 60 °. The windows of the vacuum chamber were carefully fabricated, selected and inserted into the mounting. Repeated measurements on polished surfaces and on films made it possible to establish that the effects of the entrance and exit windows on the parameters A and ~ do not exceed the statistical mean of their variation over the sample surface: 80 = -t-_1-5 and 8A = + 3-12 min.
The dependence of the optical constants of a - C : H films produced from acetylene, toluene and octane on the deposition rate were investigated at a wavelength of 632.8 nm. The rate of deposition was taken as the ratio of the film thickness D to the time from the beginning of the deposition process to the end. The rate was varied by changing the voltage between the electrodes. Increasing the interelectrode voltage within the interval 700-1300 V leads to an increase in the rate from 0.2 to 2.5A s -1. Figs. 1 and 2 show the dependences of refractive and extinction indexes on the deposition rate. It was found that the refractive index (Fig. 1) and extinction index (Fig. 2) of a-C: H films obtained from acetylene and toluene decrease monotonically as the rate of deposition increases. It should be noted that, for the same interval of deposition rates, the films obtained from acetylene have a high refractive index ( N = 2.35-2.0) and a high extinction index (K=0.3-0.1), while those obtained from toluene have lower values of these coefficients ( N = 1.8-1.6 and K=0.1-0.01). 2.4
2.2
x ._= .~ 2.0 n~ t.8
•
2
i
0
~
1
i
I
1
2
3
Deposition rate, ,~/s
Fig. t. Refractiveindex of a-C:H films obtained from (1) acetylene, (2) toluene and (3) octane as a function of the deposition rate.
V.A. Tolmachiev, E.A. Konshina/Diarnond and Related .Materials 5 (1996) 1397-1401
X
0.1
II
•
._¢ E O
2
O e"
LU
0.01
012 110
'210 2 ;
Deposition rate, ~/s Fig. 2. Extinction index of a-C:H films obtained from (1) acetylene and (2) toluene as a function of the deposition rate.
The films obtained from octane under the same conditions have the lowest values of N (1.55-1.6) and K <0.01, and their refractive index at a wavelength of 632.8 um is not dependent on the deposition rate (Fig. 1). Fig. 3 shows the data for the deposition rate and the absorption coefficient at a wavelength of 632.8 nm of films obtained from octane as a function of the interelectrode voltage. The alteration of voltage at a pressure of 0.5-0.8 mTorr causes an increase in the deposition rate. It should be a
1399
noted that the absorption coefficient is independent of the voltage in this case. Fig. 4 shows the data for the deposition rate and the absorption coefficient at a wavelength of 632.8 nm of films obtained from toluene as function of the ion current of the glow discharge. The voltage was constant at 1000 V and the pressure was altered from 0.5 to 4 retort. As the ion current increased from 1 to 4 mA, the deposition rate increased from 1 to 2.5]t s -1. It is obvious that there is a correlation between the deposition rate and the absorption at 632.8 nm of the a-C :H films obtained from toluene. The absorption of a-C:H films at the visible region of spectrum is due to re-re* electronic transitions in r~-bonded clusters with a common conjugated system of multiple bonds. The model of the structure of the a-C: H films obtained from acetylene was proposed by Baranov and Konshina [ 10]. It interprets the observed anomalies in the Raman spectrum upon variation of the wavelength of the exciting light. The model is a randomly oriented network containing two-, three- and four-functional junctions. They connect long conjugation chains of the polyene and polyine type of different lengths and polycycles with different numbers of aromatic rings. A change in chain length or ring number causes a change in the
o~ 2"51
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Interelectrode voltage, V Fig. 3. (a) Deposition rate and (b) absorption coefficient of a-C:H films obtained from octane as a function of the interelectrode voltage (pressure, 0.5-0.8 retort; ion current 2-12 mA).
<
V
3
1
2
3
4
Ion current, mA Fig. 4. (a) Deposition rate and (b) absorption coefficient of a-C:H films obtained from toluene as a function of the ion current (pressure, 0.5-4 retort; interelectrode voltage, 1000 V).
V.A. Tohnachiev, E. A, I£onshina/Diamondand Related Materials 5 (1996) 1397-1401
1400
energy of rc-~* electronic transitions. Decreasing the length of the conjugated chains of multiple bonds or the size of the re-clusters leads to an increase in the energy of the rc-rc* transition in this system. The maximum of the absorption band corresponding to this transition will be shifted to shorter wavelengths. Therefore the variation in the absorption of a-C: H films at 632.8 nm with the deposition rate (Fig. 2) may be attributable to a change in the size distribution of the zt-clusters. Increasing the deposition rate of the hydrocarbons tends to decrease the size of the ~-clusters. The absence of a dependence of absorption at 632.8 nm on the deposition rate in a-C:H films obtained from octane may be associated with structural differences compared with films obtained from acetylene and toluene. Thus we conclude that the films produced from acetylene and toluene contain larger ~-clusters than the films produced from octane. We relate the observed changes in the optical constants of a-C:H films to the structural changes due not only to the ldnetics of condensation of the hydrocarbons in the plasma but also to the influence of the chemical nature of the hydrocarbon precursor on the condensation process. We studied the porosity of the a-C:H films by ellipsometry. The results of the calculation of N and K in air, in a vacuum and in the presence of condensed water are given in Table 1 which also lists the values of NTRUE, KTaUE and Q. It can be seen that the values of N for the porous films (2, 4 and 5) are increased when the pores are filled with condensed water; however, there was an insignificant increase in the values of K. NTRu~ and Q were calculated using Eq. (1). The greater the difference
between NEF1 and NnF2, the higher is the value of Q. Porous structures have been found only in a-C:H films obtained from toluene and octane, and have a refractive index N<~1.6. The films obtained at a voltage 800 V (1 and 3 in Table 1) had porosities below 0.5% and can be considered as non-porous. The data of the porosity of a-C:H films obtained from toluene and octane are plotted as a function of the interelectrode voltage in Fig. 5. The film thicknesses ranged from 50 to 140 rim. Increasing the voltage from 800 to 1300 V causes the porosity to increase from 0.5% to 4.8%. These results can be explained by an increase in the internal compres-
hi
# P o 12. 1
' 800
'
I o O0
'
1
0
'
l nterelectrode voltage, V Fig. 5. Porosity of a-C: H films of thickness 50-140 nm obtained from octane and toluene under a pressure 0.5 mTorr as a function of the interelectrode voltage.
Table 1 Optical constants, thickness and porosity of the a-C: H films Sample number
Hydrocarbon precursor
N, K and D (nm)
NTaUE and Krr~u~
Q (%)
1.615 0.024
<0.5
1.641 0.028
1.1
1.585 0.033
<0.5
I n air
In a vacuum
With condensed water
Toluene
1.615 0.024 102.2
1.615 0.024
Toluene
1.634 0.026 13.62
1.632 0.024
Octane
1.585 0.033 47.6
1.586 0.033
1.545 0.007 83.5
1.539 0.005
1.546 0.008
1.550 0.010
1,7
Octane
1.595 0.033 131.0
1.579 0.035
1.600 0.037
1.614 0.038
4.8
Octane
1.637 0.026
V.A. Tolmachiev, E.A. Konshina/ Diamond and Related Materials 5 (1996) 1397-1401
sive tension, which leads to the formation of microdefects in the films and spontaneous cracking. Thus it is reasonable to assume that the origin of the pores in a - C : H films is the variation of the packing density and the formation of microcracks. The a - C : H films studied in this work were generally non-porous, were chemically stable and had secure protective properties.
1401
examined were non-porous. The origin of the pores in the a - C : H films is associated with the packing density of the structure of the a - C : H films and the possibility of spontaneous cracking because of internal compressive tensions.
References 4. Conclusion An ellipsometric investigation of a - C : H films produced from acetylene, toluene and octane, including the determination of the optical constants, the thickness and the porosity, has been carried out. The absorption at 632.8 nm can be reduced not only by increasing the deposition rate but also by a suitable choice of the precursor for a-C : H production. However, the refractive index decreases simultaneously. Optical constants in the visible region of the spectrum give indirect information about the structure of a - C : H thin solid films. Low absorption at 632.8 m a y be related to the absence of large re-clusters in the structure. Most of the films
[17 A.A. Khan, D. Mathine and I.A. Woollam, Phys. Rev., 28 (1983), 7229. [2] E. Pascual, C. Serra, J. Esteve and E. Bertran, Surf. Coat. Teehnot., 47 (1991), 263. [3] C. Serra, E. Pascual, F. Maass and J. Esteve, Surf. Coat. TechnoL, 47 (1991), 87. [4] E.A. Konshina and V.A. Tolmachiev, Tech. Phys. USSR, 40 (1995), 97. [5] S. Orzeszko, B.N. De, P.G. Snyder, LA. ~'oollam,J.3. Ponch and S.A. Alterovitz, J. Chem. Phys., 84 (1%7~, 1496. [6] V.A. Tolmachiev, M.A. Okatov and E.F. Matsoian, Soy. J. Opt. Technol., 60 (t993), 327. [7] A.V. Balakov and E.A. Konshina, Soy. J. Opt. Teehnol., 49 (1983), 591. [8] O.S. Dron', T.V. Leonova and M.V. Sokolova, Soy. J. Opt. Technol., 56 (1989), 4. [9] O.S. Dron' and V.A. Tolmachiev,Y. Opt. Technol., 61 (1994), 50i. ['10] A.V. Baranov and E.A. Konshina, Opt. Spectrosc., 65 (1988), 506.
Diamond and Related Materials 5 (1996) 1397-1401
D IAMOND AND RELATED MATERIALS
Ellipsometric study of a-C" H films V.A. Tolmachiev, E.A. Konshina All-Russian Research Center "S.I. VaviIov State Optical Institute'; St. Petersburg 199034, Russia Received 26 July 1995; accepted in final form 18 April 1996
Abstract The optical constants, porosity and thickness of amorphous hydrogenated carbon (a-C : H) films were determined by muItipleangle eltipsometry at an optical wavelength of 632.8 nm. The films were produced by a plasma-activated CVD process in a dc glow discharge of acetylene, toluene and octane. The results indicate that the refractive index of the a-C: H films can be changed over the interval 2.35-1.55 by increasing the deposition rate and an appropriate choice of hydrocarbon precursor. A reduction of the refractive index correlates with a decrease in the extinction index in the range 0.3-0.01. The influence of the chemical nature of the hydrocarbon precursor on optical constants in the visible region is found from the results of the ellipsometric measurements. The variations of the experimental data are reviewed in view of the a-C : H structure model proposed previously. The porosity of the a-C : H films has been determined by a new technique based on the ellipsometric measurements. Keywords: Ellipsometry; Hydrocarbons; Refractive index; Porosity
1. Introduction
2. Experimental methods
Application of diamond-like carbon (DLC) films as optical coatings for the visible region of the electromagnetic spectrum is restricted by their insulticient transparency in this region. One method of investigating the optical properties of the film in the visible region is ellipsometry. Previous ellipsometric studies indicate that amorphous hydrogenated carbon (a-C:H) films have a high refractive index and are opaque in the UV-visible range [1,2]. The dependences of the optical constants of a - C : H films on the parameters of the condensation process have been established by ellipsometry [3]. The change in the refractive and extinction indexes of a-C: H films at a wavelength of 632.8 nm with increasing deposition rate of the hydrocarbons has been established by multiple angle of incidence (MAI) ellipsometry [4]. The effective medium approach was used to estimate the void fraction or porosity (the presence of water in the films) [ 5 ]. In this paper we report a comparative analysis of optical constants of a-C: H films produced from different hydrocarbon precursors which was performed using MAI ellipsometry. Two methods of ellipsometry were used: the usual MAI measurements in air and a new technique for determining film porosity [6].
2.1. Preparation conditions for a-C : H films
0925-9635/96/$15.00 © 1996 Elsevier Science S.A. All fights reserved S S D I 0925-9635(96)00551-1
In this work we studied a - C : H films produced from acetylene, toluene and octane by a plasma-activated CVD technique. We used a dc glow discharge for the CVD process [7]. The system was pumped to a pressure of about 10 -s tort before each deposition, and the hydrocarbon vapors were then admitted to the vacuum system. During deposition the pressure was 0.2-4 mTorr. The voltage between the anode and substrate (cathode) was varied from 700 to 1300 V. The films were deposited on polished glass substrates at room temperature. The substrate surface was plasma etched (in oxygen) immediately before deposition of the film.
2.2. Ellipsometric measurements All measurements were carried out using an LEF-3m ellipsometer (Russia) at a light wavelength of 632.8 nm. The configuration was polarizer-compensator-sample analyser. A dual-zone null method at angles of incidence ~bof 50 °, 60 ° and 70 ° was used to calculate the ellipsomettic parameters A and ~ with the actual errors of the instrument [8]. The homogenous isotropic layer model was used to calculate the optical constants: refractive
1398
V.A. Tolmachiev, E.A. Konshina/Diamond and Related Materials 5 (1996) 1397-1401
index N, extinction index K and thickness D. The optical constants of the substrates were Ns = 1.515 and K s = 0 . The minimization function F was calculated as the difference between the experimental and calculated values of A and ~/taking into account the measurement errors 5A and 5~ for every value of ¢ (software EDIP-2, PC AT286) [9 ]. If F <~1, the calculated and experimental values of A and ~ are equal, and the selected model and its parameters are suitable. The degree of parameter correlation was estimated in the time error calculation of N, K and D by the technique described in Ref. [91. 2.3. Determination o f porosity
3. Results and discussion
A porous film can be physically modelled as a mixture of three general components: (1) a film framework with refractive indexes NTRU~, (2) voids and (3) substances adsorbed into voids with refractive index Nv. The usual measurements in air yield information about an effective refractive index N~F of a film, when there are atmospheric components in the voids. Porous films can contain water, oxygen, nitrogen, hydrocarbons and other atmospheric components. The higher is the void fraction of film, the more incorrect is NTRU~. Thus, NTRUE is unknown and its direct measurement is impossible. The optical characteristics of such materials systems can be described by the effective media approach (EMA), such as the Lorenz-Lorentz equation: (NEF)2 -- (NTR~) 2 (gv) 2 -- (NTRuE)2 (NEF)2 + 2 = Q (Nv)2 _t_2
The above ellipsometric system allows measurement of the two different values of N~F (if the film is porous): the complex value NEF1 in a vacuum and the complex value NzF2 when water vapour is condensed in the voids. It is assumed that the film thickness is constant for both measurements. The degree of void filling by water is controlled. This technique allows the correct use of the value Nv = 1 (measurement in a vacuum) and Nv = 1.332 (with condensed water) in Eq. (1). We found the two unknown values Q and NTRUE using the system of two Eqs. (1).
(1)
where Q is a void fraction or a porosity. Usually it is assumed that NTRUEis known and N v = 1. However, this is incorrect because NTRUE of real substances may be different. Nv is equal to unity only in a vacuum when all the voids are empty. We measured values of N~F using equipment consisting of a vacuum chamber and a vacuum-gas handling system (VGS) [6]. The chamber is housed with a built-in table for the sample, entrance and exit windows, a load lock, and other items. The VGS incorporates fore pumps, zeolite pumps, vacuum valves, pressure gauges, an electronics unit, an ampoule with the adsorbate etc. This attachment for the ellipsometer makes it possible to monitor samples on any light-reflecting substrate in air and in a vacuum of 0.1 mTorr, as well as under the conditions for condensing gaseous adsorbates. The angle of incidence ¢ is 60 °. The windows of the vacuum chamber were carefully fabricated, selected and inserted into the mounting. Repeated measurements on polished surfaces and on films made it possible to establish that the effects of the entrance and exit windows on the parameters A and ~ do not exceed the statistical mean of their variation over the sample surface: 80 = -t-_1-5 and 8A = + 3-12 min.
The dependence of the optical constants of a - C : H films produced from acetylene, toluene and octane on the deposition rate were investigated at a wavelength of 632.8 nm. The rate of deposition was taken as the ratio of the film thickness D to the time from the beginning of the deposition process to the end. The rate was varied by changing the voltage between the electrodes. Increasing the interelectrode voltage within the interval 700-1300 V leads to an increase in the rate from 0.2 to 2.5A s -1. Figs. 1 and 2 show the dependences of refractive and extinction indexes on the deposition rate. It was found that the refractive index (Fig. 1) and extinction index (Fig. 2) of a-C: H films obtained from acetylene and toluene decrease monotonically as the rate of deposition increases. It should be noted that, for the same interval of deposition rates, the films obtained from acetylene have a high refractive index ( N = 2.35-2.0) and a high extinction index (K=0.3-0.1), while those obtained from toluene have lower values of these coefficients ( N = 1.8-1.6 and K=0.1-0.01). 2.4
2.2
x ._= .~ 2.0 n~ t.8
•
2
i
0
~
1
i
I
1
2
3
Deposition rate, ,~/s
Fig. t. Refractiveindex of a-C:H films obtained from (1) acetylene, (2) toluene and (3) octane as a function of the deposition rate.
V.A. Tolmachiev, E.A. Konshina/Diarnond and Related .Materials 5 (1996) 1397-1401
X
0.1
II
•
._¢ E O
2
O e"
LU
0.01
012 110
'210 2 ;
Deposition rate, ~/s Fig. 2. Extinction index of a-C:H films obtained from (1) acetylene and (2) toluene as a function of the deposition rate.
The films obtained from octane under the same conditions have the lowest values of N (1.55-1.6) and K <0.01, and their refractive index at a wavelength of 632.8 um is not dependent on the deposition rate (Fig. 1). Fig. 3 shows the data for the deposition rate and the absorption coefficient at a wavelength of 632.8 nm of films obtained from octane as a function of the interelectrode voltage. The alteration of voltage at a pressure of 0.5-0.8 mTorr causes an increase in the deposition rate. It should be a
1399
noted that the absorption coefficient is independent of the voltage in this case. Fig. 4 shows the data for the deposition rate and the absorption coefficient at a wavelength of 632.8 nm of films obtained from toluene as function of the ion current of the glow discharge. The voltage was constant at 1000 V and the pressure was altered from 0.5 to 4 retort. As the ion current increased from 1 to 4 mA, the deposition rate increased from 1 to 2.5]t s -1. It is obvious that there is a correlation between the deposition rate and the absorption at 632.8 nm of the a-C :H films obtained from toluene. The absorption of a-C:H films at the visible region of spectrum is due to re-re* electronic transitions in r~-bonded clusters with a common conjugated system of multiple bonds. The model of the structure of the a-C: H films obtained from acetylene was proposed by Baranov and Konshina [ 10]. It interprets the observed anomalies in the Raman spectrum upon variation of the wavelength of the exciting light. The model is a randomly oriented network containing two-, three- and four-functional junctions. They connect long conjugation chains of the polyene and polyine type of different lengths and polycycles with different numbers of aromatic rings. A change in chain length or ring number causes a change in the
o~ 2"51
d
2.0
¢-. O
0
O D.. (1)
0
o~
® D
800
'7
E
10
1.5
1000
1.0
1200
4
I
r
1
2
,
i
3
r
i
4
b E
0
V
O
t(D
O
© 0 o (..-
0 0
0
0
,m
,m
g
O (/} ..O10
<
(3. 0 .Q
~- 10
3 ,
i
800
r
i
1000
,
1200
Interelectrode voltage, V Fig. 3. (a) Deposition rate and (b) absorption coefficient of a-C:H films obtained from octane as a function of the interelectrode voltage (pressure, 0.5-0.8 retort; ion current 2-12 mA).
<
V
3
1
2
3
4
Ion current, mA Fig. 4. (a) Deposition rate and (b) absorption coefficient of a-C:H films obtained from toluene as a function of the ion current (pressure, 0.5-4 retort; interelectrode voltage, 1000 V).
V.A. Tohnachiev, E. A, I£onshina/Diamondand Related Materials 5 (1996) 1397-1401
1400
energy of rc-~* electronic transitions. Decreasing the length of the conjugated chains of multiple bonds or the size of the re-clusters leads to an increase in the energy of the rc-rc* transition in this system. The maximum of the absorption band corresponding to this transition will be shifted to shorter wavelengths. Therefore the variation in the absorption of a-C: H films at 632.8 nm with the deposition rate (Fig. 2) may be attributable to a change in the size distribution of the zt-clusters. Increasing the deposition rate of the hydrocarbons tends to decrease the size of the ~-clusters. The absence of a dependence of absorption at 632.8 nm on the deposition rate in a-C:H films obtained from octane may be associated with structural differences compared with films obtained from acetylene and toluene. Thus we conclude that the films produced from acetylene and toluene contain larger ~-clusters than the films produced from octane. We relate the observed changes in the optical constants of a-C:H films to the structural changes due not only to the ldnetics of condensation of the hydrocarbons in the plasma but also to the influence of the chemical nature of the hydrocarbon precursor on the condensation process. We studied the porosity of the a-C:H films by ellipsometry. The results of the calculation of N and K in air, in a vacuum and in the presence of condensed water are given in Table 1 which also lists the values of NTRUE, KTaUE and Q. It can be seen that the values of N for the porous films (2, 4 and 5) are increased when the pores are filled with condensed water; however, there was an insignificant increase in the values of K. NTRu~ and Q were calculated using Eq. (1). The greater the difference
between NEF1 and NnF2, the higher is the value of Q. Porous structures have been found only in a-C:H films obtained from toluene and octane, and have a refractive index N<~1.6. The films obtained at a voltage 800 V (1 and 3 in Table 1) had porosities below 0.5% and can be considered as non-porous. The data of the porosity of a-C:H films obtained from toluene and octane are plotted as a function of the interelectrode voltage in Fig. 5. The film thicknesses ranged from 50 to 140 rim. Increasing the voltage from 800 to 1300 V causes the porosity to increase from 0.5% to 4.8%. These results can be explained by an increase in the internal compres-
hi
# P o 12. 1
' 800
'
I o O0
'
1
0
'
l nterelectrode voltage, V Fig. 5. Porosity of a-C: H films of thickness 50-140 nm obtained from octane and toluene under a pressure 0.5 mTorr as a function of the interelectrode voltage.
Table 1 Optical constants, thickness and porosity of the a-C: H films Sample number
Hydrocarbon precursor
N, K and D (nm)
NTaUE and Krr~u~
Q (%)
1.615 0.024
<0.5
1.641 0.028
1.1
1.585 0.033
<0.5
I n air
In a vacuum
With condensed water
Toluene
1.615 0.024 102.2
1.615 0.024
Toluene
1.634 0.026 13.62
1.632 0.024
Octane
1.585 0.033 47.6
1.586 0.033
1.545 0.007 83.5
1.539 0.005
1.546 0.008
1.550 0.010
1,7
Octane
1.595 0.033 131.0
1.579 0.035
1.600 0.037
1.614 0.038
4.8
Octane
1.637 0.026
V.A. Tolmachiev, E.A. Konshina/ Diamond and Related Materials 5 (1996) 1397-1401
sive tension, which leads to the formation of microdefects in the films and spontaneous cracking. Thus it is reasonable to assume that the origin of the pores in a - C : H films is the variation of the packing density and the formation of microcracks. The a - C : H films studied in this work were generally non-porous, were chemically stable and had secure protective properties.
1401
examined were non-porous. The origin of the pores in the a - C : H films is associated with the packing density of the structure of the a - C : H films and the possibility of spontaneous cracking because of internal compressive tensions.
References 4. Conclusion An ellipsometric investigation of a - C : H films produced from acetylene, toluene and octane, including the determination of the optical constants, the thickness and the porosity, has been carried out. The absorption at 632.8 nm can be reduced not only by increasing the deposition rate but also by a suitable choice of the precursor for a-C : H production. However, the refractive index decreases simultaneously. Optical constants in the visible region of the spectrum give indirect information about the structure of a - C : H thin solid films. Low absorption at 632.8 m a y be related to the absence of large re-clusters in the structure. Most of the films
[17 A.A. Khan, D. Mathine and I.A. Woollam, Phys. Rev., 28 (1983), 7229. [2] E. Pascual, C. Serra, J. Esteve and E. Bertran, Surf. Coat. Teehnot., 47 (1991), 263. [3] C. Serra, E. Pascual, F. Maass and J. Esteve, Surf. Coat. TechnoL, 47 (1991), 87. [4] E.A. Konshina and V.A. Tolmachiev, Tech. Phys. USSR, 40 (1995), 97. [5] S. Orzeszko, B.N. De, P.G. Snyder, LA. ~'oollam,J.3. Ponch and S.A. Alterovitz, J. Chem. Phys., 84 (1%7~, 1496. [6] V.A. Tolmachiev, M.A. Okatov and E.F. Matsoian, Soy. J. Opt. Technol., 60 (t993), 327. [7] A.V. Balakov and E.A. Konshina, Soy. J. Opt. Teehnol., 49 (1983), 591. [8] O.S. Dron', T.V. Leonova and M.V. Sokolova, Soy. J. Opt. Technol., 56 (1989), 4. [9] O.S. Dron' and V.A. Tolmachiev,Y. Opt. Technol., 61 (1994), 50i. ['10] A.V. Baranov and E.A. Konshina, Opt. Spectrosc., 65 (1988), 506.