- Email: [email protected]
Ellipsometric sensitivity to halothane vapors of hexamethyldisiloxane plasma polymer films
Sensors and Actuators B 44 (1997) 243 – 247
Ellipsometric sensitivity to halothane vapors of hexamethyldisiloxane plasma polymer films S. Guo a, R. Rochotzki b, I. Lundstro¨m a, H. Arwin a,* a
Laboratory of Applied Physics, Department of Physics and Measurement Technology, Linko¨ping Uni6ersity, S-581 83 Linko¨ping, Sweden b Technische Uni6ersita¨t Chemnitz, Institut fu¨r Physik, 09107 Chemnitz, Germany Received 9 July 1997; accepted 14 July 1997
Abstract A halothane vapor sensor based on hexamethyldisiloxane (HMDSO) plasma polymer films have been studied using an ellipsometric technique. The sensor signals, i.e. the ellipsometric parameters c and D of a HMDSO film change reversibly upon exposure to halothane vapors. The sensor shows very good repeatability and stability. Our study also revealed that the sensitivity and response time depend strongly on the film deposition conditions and a film with a high refractive index has low sensitivity to halothane and longer response time. © 1997 Elsevier Science S.A. Keywords: Ellipsometric sensitivity; Hexamethyldisiloxane plasma polymer films; Refractive index
1. Introduction Various gas-sensitive techniques using polymer films as sensitive layers have been developed. These sensor devices can, e.g. be optical-sensitive (spectral detector [1] and wave guide type [2]) mass-sensitive (quartz microbalance, QBM [3,4], surface acoustic wave [5]), dielectric-sensitive (capacitive transducers [6]) or calorimetric-sensitive (thermopile [7]). Here, we present a new method based on using the ellipsometric response from films of hexamethyldisiloxane (HMDSO) for detection of the anesthetic gas halothane. In the following we refer to this method as the sensor. The principle of this sensor is based on monitoring the changes of the polarization state, i.e. the ellipsometric parameters c and D upon film exposure to halothane. With fast and precise ellipsometric measurements, it is possible to record changes in the ellipsometric parameters caused by the absorption and desorption of gases. This provides a new way to evaluate and control gas delivery to patients. The advantages of this method are rapid response, high sensitivity, good selectivity and good repeatability. Moreover, it allows non-electrical contact between the sensor and the anesthetic vapors which * Corresponding author. Tel.: + 46 13 281215; fax: + 46 13 137568; e-mail: [email protected] 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 2 1 6 - 5
ensures safety during surgery. HMDSO plasma polymerized membranes are highly permeable and permselective sorption material [8]. It is a suitable material for selective sorption of anesthetic vapors [9–11]. In this report, the sensitivity and response time of plasma polymerized HMDSO films to halothane vapors are studied. Their dependence on the refractive index of HMDSO films is presented. The repeatability and stability of the ellipsometric halothane gas sensor are also discussed.
2. Experimental details The films were deposited on silicon substrates in a vacuum chamber using an r.f. glow discharge method as described elsewhere [12]. A series of films with thicknesses from 80 to 120 nm was prepared through plasma chemical gas conversion of the monomer HMDSO at a pressure of 0.1–0.2 mbar and a r.f. power (13.56 MHz) of 30 W. The halothane (CF::CHBrCl) was produced by Trofield Surgicals A.G. (Switzerland) and has a boiling point of 50.2°C and a vapor pressure of 234 mmHg at 20°C. Different concentrations of halothane vapors were obtained by mixing halothane and nitrogen gas at controlled flow ratios through two mass-flow meters. A
244
S. Guo et al. / Sensors and Actuators B 44 (1997) 243–247
temperature controlled water bath was used to obtain halothane gas at different saturated vapor pressures. Gas was supplied to the samples (about 4 mm from the sample) through a tube 5 mm in diameter at a total rate of 100 ml min − 1. The tube length from the organic solvent container to the sample was about 80 cm. The sensor is based on monitoring the changes in the state of polarization, i.e. the changes in the ellipsometric parameters c and D from a sample with a thin polymer film exposed to halothane. These variations, dc and dD, are caused by changes of thickness and refractive index in the polymer films due to absorption and desorption of halothane molecules. A schematic view of the set-up for measurement is shown in Fig. 1. The equipment used consists of a computer-controlled variable angle spectroscopic ellipsometer of the rotating analyzer type (J.A. Woollam) and a gas mixing system. Various concentrations of halothane were obtained by dynamic mixing of halothane vapors and synthetic dry air at various flow ratios through a blender with two mass-flow meters. A water bath was used to keep halothane at different saturated vapor pressure through cooling or heating. Gas was supplied to the samples (about 4 mm from the sample) through a tube 5 mm in diameter at a total rate of 100 ml min − 1 (except for the lowest concentration 0.04%, where the gas flow was 200 ml min − 1). A dynamic scan was used in the measurements to record the changes of ellipsometric parameters c and D upon halothane vapors exposure. The changes of thickness and refractive index were determined by in situ spectroscopic ellipsometry by exposing samples to a constant flow of halothane gas or air. Spectra were taken at three angles of incidence (f =65, 70 and 75°) in the photon energy range 1.25 – 4.5 eV at 0.05 eV intervals. The optical model used in the analysis is a three-phase model with a silicon substrate and a single layer. The measured quantity is the complex reflectance ratio r = Rp/Rs = tan ce iD
(1)
Fig. 2. Typical ellipsometric response of a HMDSO film for stepwise exposure to different concentrations of halothane measured at a photon energy of 3.1 eV.
where Rp (Rs) is the complex reflection coefficient of light polarized parallel (perpendicular) to the plane of incidence, and c and D are the ellipsometric angles. Formally we can express the dependence on all system parameters as r= r(d, Ns, Nf, f, l), where the complex indices of refraction Ns = ns + iks and Nf = nf +ikf of the substrate and the film, respectively, are introduced. To be exact one should also include a thin (2 nm) native oxide between the polymer film and the substrate. However, the oxide optical properties are very similar to those of the polymer film and we therefore neglect the oxide. The polymer layer thicknesses given here are therefore slightly larger than in reality. When a sample is exposed to halothane vapor, d, and Nf will change, and c and D are thus functions of the concentration of halothane vapor. We also assume the substrate properties to be halothane gas independent and c and D therefore only depend on changes in the HMDSO film. All the measurements are taken at room temperature. The ellipsometric data were analyzed with the WVASE software package (J.A. Woollam), which is based on least-squares regression analysis to obtain the unknown fitting parameters and their 90% confidence limits. The procedure is to vary fitting parameters to minimize the differences between the measured and calculated c and D values by using the exact ellipsometric equations [13] in the optical model described above.
3. Results
Fig. 1. A schematic view of the measurement set-up including a computer-controlled variable angle spectroscopic ellipsometer and a gas mixing system.
A typical ellipsometric response of a HMDSO film with refractive index n= 1.4860 to pulses of different concentrations of halothane is shown in Fig. 2. The measurement was done at a photon energy of 3.1 eV at an angle of incidence of 70°. Preliminary studies
S. Guo et al. / Sensors and Actuators B 44 (1997) 243–247
Fig. 3. The thickness of a HMDSO film upon exposure to halothane vapor vs. halothane concentrations at an angle of incidence of 70° and a photon energy of 3.1 eV.
showed high sensitivity under these conditions. The concentration was determined from the gas mixing ratio. The vapor pressure was calculated according to following equation [14]: log P si (mmHg)= A−B/(t(°C) + C)
(2)
where P si is the saturated vapor pressure, t is the temperature and A, B, C are constants. For the calculation of halothane vapor pressure, the values of A, B, C are 6.53534, 915.201 and 200.229, respectively. It is seen that the HMDSO layers possess a strong sorption ability for halothane, good reversibility and short response time. In order to show the repeatability of the sensor, the sample was exposed to four identical gas pulses at each concentration. The reproducibility in terms of the relative standard deviations in dc and dD for halothane exposure was 90.2%, and 91.8% at a concentration of 28%, 9 0.2% and 92.9% at a concentration of 17%, 9 2.5% and 92.1% at a concentration of 9.5%, 9 2.5% and 95.5% at a concentration of 3.5%, 99.6% and 9 9.8% at a concentration of 0.35%, respectively. The lowest concentration which could be detected was around 0.04% at which the signal-to-noise ratio is 1. It should be pointed out that the sensitivity
245
Fig. 4. Typical responses to a pulse of halothane (3.5%) measured at a photon energy of 3.1 eV for two films with different refractive indices.
depends strongly on film thickness, film density and wavelength. A considerable improvement in sensitivity is possible to obtain with optimization of the film thickness and density. The responses of HMDSO films to non-chlorinated hydrocarbons such as ethanol, methanol and acetone were also checked. In contrast to halothane, the sensitivity was only 15–30% of the sensitivity to halothane for these vapors. The life time was more than 6 months without significant loss of the sensitivity. The films are thus very stable. Fig. 3 shows a typical relationship between the film thickness and halothane concentrations for a HMDSO sample. The changes of thickness were determined by in situ spectroscopic ellipsometry. It is observed that the thickness increases monotonously with halothane concentration. A corresponding change in the refractive index is also obtained but is not reported here. The dependence of sensitivities on refractive index and the relative change of film thickness dd/d are summarized in Table 1. It is seen that the sensitivity decreases with the refractive index. dc and dD of the film with n= 1.4860 at 3.1 eV are 10–30 times larger than for the film with n = 1.5336 at 3.1 eV. The relative change of the film thickness also decreases with the
Table 1 Ellipsometric sensitivities and thickness changes at two different halothane concentrations for HMDSO films with different refractive indices n at 3.1 eV
1.4860 1.5019 1.5069 1.5145 1.5336 a
d (nm)
79.5 87.0 84.3 119.1 87.7
Dc (°)
dD (°)
dd/d (%)
c = 7%
c = 14%
c =7%
c= 14%
c= 7%
c =14%
1.62 1.60 0.49 0.21a 0.17
3.39 2.90 0.64 0.29a 0.22
6.2 5.2 5.0 4.2a 0.2
13.4 6.9 6.2 5.2a 0.4
3.8 1.4 0.9 0.5 0.04
6.2 1.9 0.7 0.7 0.09
Measured at 4.1 eV instead of 3.1 eV due to a different thickness.
246
S. Guo et al. / Sensors and Actuators B 44 (1997) 243–247
Fig. 5. Spectra of c and D in air and at two different halothane concentrations (5.75 and 35% in dry air) recorded by in situ spectroscopic ellipsometry using an HMDSO film with a thickness of 77.8 nm.
refractive index. The film with the lowest refractive index has the largest relative change of film thickness. In other words, under the impact of the halothane vapor, the less dense film will easily swell and shrink more than the more dense film. The refractive index of HMDSO plasma polymer films depends on the deposition conditions. The refractive index decreases with increasing pressure and decreasing r.f. power [12]. The samples with largest sensitivity, best stability and repeatability in this study were made at a pressure of 0.2 mbar and a r.f. power of 30 W. Fig. 4 shows typical time responses to 3.5% halothane for two different layer refractive indices. It is observed that the film with a low refractive index (n = 1.4860) has a fast response but the dense layer with a high refractive index (n =1.5019) has a slow response which gives a diffusion-like appearance at the beginning of the transients. It takes a longer period of time for halothane molecules to absorb in and desorb from the dense layers. It is seen from Fig. 4 that for the film with lower refractive index, for the rise time to reach the 90% level and the decay time to fall to the 10% level are approximately the same and of the order of 5 s. For the film with higher refractive index, however, the adsorption and desorption rates are different with a rise time of 20 s and a decay time of 30 s. The spectra of c and D with respect to three different concentrations 0, 5.75 and 35% in dry air are recorded by an in situ spectroscopic scan for three samples, as
shown in Fig. 5. It is clearly seen that the sensitivities depend strongly on wavelength. The maximum changes of c and D appear at a wavelength of around 3.1 eV. The maximum sensitivity seems to be obtained at the wavelength close to the optical interference peak which depend on the angle of incidence. Thus, the optimum thickness could be calculated in advance if the wavelength of the light source is known. The film thickness should therefore be carefully selected to optimize the sensitivity in a sensor application. For example, if the light source is a He–Ne laser with a wavelength of 633 nm, the maximum sensitivity could be found at the film thickness about 140 nm at the incidence angle of 70°.
4. Discussion These results reveal that the plasma HMDSO layer can swell reversibly under the impact of the halothane molecules. This is probably associated with the structure of the layers being formed by polymerization in plasma. It is known that the films formed by plasma polymerization are deposited in networks of highly branched and highly cross-linked segments [15]. The –Si–O–Si– backbone in the HMDSO structure gives the layer stability but probably also an open structure, which enables the polymer to interact with halogenated hydrocarbon molecules. Sensor response up to high concentrations indicates that the saturation level is high in the layer. The used halogenated hydrocarbons also
S. Guo et al. / Sensors and Actuators B 44 (1997) 243–247
have high vapor pressure which enable the molecules to desorb quickly as observed by the rapid response rate in the measurements. The refractive index n is a measure of the dipole density, which is determined by mass density, film structure and composition of the plasma polymer HMDSO films. For the dense layer with a higher refractive index, the ability of the absorption and desorption is lower than for films with lower refractive index. The sensitivity is, most probably, coupled to the degree of cross-linking in the films. In other words, a dense layer provides fewer interaction possibilities. We conclude that HMDSO layers show a strong sorption ability for halothane molecules with good repeatability and stability. Largest ellipsometric sensitivity in c and D can be achieved when an appropriate wavelength is used. The lower refractive index, the higher sensitivity and shorter response times are obtained. We suggest that HMDSO films can be used as an optical sensor material for evaluation and control in anesthetic measurements.
Acknowledgements Financial support was obtained from the Swedish National Board for Technical and Industrial Development.
References [1] W. Nahm, G. Gauglitz, Thin polymer films as sensors for hydrocarbons, GIT Fachz. Lab. 7 (1990) 889.
.
247
[2] G. Gauglitz, J. Ingenhoff, Integrated optical sensors for halogenated and non-halogenated hydrocarbons, Sensors and Actuators B 11 (1993) 207 – 212. [3] A. Kindlund, I. Lundstro¨m, Physical study of quartz crystal sorption detectors, Sensors and Actuators 3 (1982/83) 63–77. [4] M.S. Nieuwenhuizen, A. Venema, Mass-sensitive devices, in: W. Go¨pel, J. Hesse, J.N. Zemel (Eds.), Sensors — A Comprehensive Survey, vol. 2, Part I, VCH, Weinheim, 1991. [5] S.W. Wenzel, R.M. White, Flexural plate-wave gravimetric chemical sensors, Sensors and Actuators A 21 – A23 (1990) 700– 703. [6] H.E. Endres, S. Drost, Optimization of the geometry of gas-sensitive interdigital capacitors, Sensors and Actuators B 4 (1991) 95 – 98. [7] M. Haug, K.D. Schierbaum, H.E. Endres, S. Drost, W. Go¨pel, Controlled selectivity of polysiloxane coating: their use in capacitance sensors, Sensors and Actuators A 32 (1992) 326–332. [8] J. Sakata, M. Yamamoto, I. Tajima, Plasma polymerization of mixed monomer gases, J. Polymer Sci., Part A 26 (1988) 1721– 1731. [9] L. Sodomka, J. Janca, Electrical properties and selective gas sorption of plasma polymerized organosiloxane thin films, Scr. Fac. Sci. Nat. Univ. Purk. Brun. 8 (1988) 217 – 228. [10] J. Sakata, M. Yamamoto, Application of plasma polymerized film to gas separation membranes, J. Appl. Polymer Science: Appl. Polymer Symp. 42 (1988) 339. [11] N. Inagaki, Gas separation membranes plasma-polymerized from mixtures of silanes and fluorocarbons, J. Appl. Polymer Sci.: Appl. Polymer Symp. 42 (1988) 327. [12] R. Rochotzki, M. Arzt, F. Blaschta, E. Kreyßig, H.U. Poll, Optical properties of plasma polymer films (hexamethyldisiloxane), Thin Solid Films 234 (1993) 463 – 467. [13] J.A. Woollam, The Users Manual of Variable Angle Spectroscopic Ellipsometry, 1992, p. 29. [14] J. Gmehling, U. Onken, H.W. Schulte, Vapour-liquid equilibria for the binary systems diethyl ether-halothane (1,1,1-trifluoro-2bromo-2-chloroethane), halothane-methanol, and diethyl ethermethanol, J. Chem. Eng. Data 25 (1980) 29 – 32. [15] H. Nomura, P.W. Kramer, H. Yasuda, Preparation of gas separation membranes by plasma polymerization with fluoro compounds, Thin Solid Films 118 (1984) 187 – 195.
Ellipsometric sensitivity to halothane vapors of hexamethyldisiloxane plasma polymer films S. Guo a, R. Rochotzki b, I. Lundstro¨m a, H. Arwin a,* a
Laboratory of Applied Physics, Department of Physics and Measurement Technology, Linko¨ping Uni6ersity, S-581 83 Linko¨ping, Sweden b Technische Uni6ersita¨t Chemnitz, Institut fu¨r Physik, 09107 Chemnitz, Germany Received 9 July 1997; accepted 14 July 1997
Abstract A halothane vapor sensor based on hexamethyldisiloxane (HMDSO) plasma polymer films have been studied using an ellipsometric technique. The sensor signals, i.e. the ellipsometric parameters c and D of a HMDSO film change reversibly upon exposure to halothane vapors. The sensor shows very good repeatability and stability. Our study also revealed that the sensitivity and response time depend strongly on the film deposition conditions and a film with a high refractive index has low sensitivity to halothane and longer response time. © 1997 Elsevier Science S.A. Keywords: Ellipsometric sensitivity; Hexamethyldisiloxane plasma polymer films; Refractive index
1. Introduction Various gas-sensitive techniques using polymer films as sensitive layers have been developed. These sensor devices can, e.g. be optical-sensitive (spectral detector [1] and wave guide type [2]) mass-sensitive (quartz microbalance, QBM [3,4], surface acoustic wave [5]), dielectric-sensitive (capacitive transducers [6]) or calorimetric-sensitive (thermopile [7]). Here, we present a new method based on using the ellipsometric response from films of hexamethyldisiloxane (HMDSO) for detection of the anesthetic gas halothane. In the following we refer to this method as the sensor. The principle of this sensor is based on monitoring the changes of the polarization state, i.e. the ellipsometric parameters c and D upon film exposure to halothane. With fast and precise ellipsometric measurements, it is possible to record changes in the ellipsometric parameters caused by the absorption and desorption of gases. This provides a new way to evaluate and control gas delivery to patients. The advantages of this method are rapid response, high sensitivity, good selectivity and good repeatability. Moreover, it allows non-electrical contact between the sensor and the anesthetic vapors which * Corresponding author. Tel.: + 46 13 281215; fax: + 46 13 137568; e-mail: [email protected] 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 2 1 6 - 5
ensures safety during surgery. HMDSO plasma polymerized membranes are highly permeable and permselective sorption material [8]. It is a suitable material for selective sorption of anesthetic vapors [9–11]. In this report, the sensitivity and response time of plasma polymerized HMDSO films to halothane vapors are studied. Their dependence on the refractive index of HMDSO films is presented. The repeatability and stability of the ellipsometric halothane gas sensor are also discussed.
2. Experimental details The films were deposited on silicon substrates in a vacuum chamber using an r.f. glow discharge method as described elsewhere [12]. A series of films with thicknesses from 80 to 120 nm was prepared through plasma chemical gas conversion of the monomer HMDSO at a pressure of 0.1–0.2 mbar and a r.f. power (13.56 MHz) of 30 W. The halothane (CF::CHBrCl) was produced by Trofield Surgicals A.G. (Switzerland) and has a boiling point of 50.2°C and a vapor pressure of 234 mmHg at 20°C. Different concentrations of halothane vapors were obtained by mixing halothane and nitrogen gas at controlled flow ratios through two mass-flow meters. A
244
S. Guo et al. / Sensors and Actuators B 44 (1997) 243–247
temperature controlled water bath was used to obtain halothane gas at different saturated vapor pressures. Gas was supplied to the samples (about 4 mm from the sample) through a tube 5 mm in diameter at a total rate of 100 ml min − 1. The tube length from the organic solvent container to the sample was about 80 cm. The sensor is based on monitoring the changes in the state of polarization, i.e. the changes in the ellipsometric parameters c and D from a sample with a thin polymer film exposed to halothane. These variations, dc and dD, are caused by changes of thickness and refractive index in the polymer films due to absorption and desorption of halothane molecules. A schematic view of the set-up for measurement is shown in Fig. 1. The equipment used consists of a computer-controlled variable angle spectroscopic ellipsometer of the rotating analyzer type (J.A. Woollam) and a gas mixing system. Various concentrations of halothane were obtained by dynamic mixing of halothane vapors and synthetic dry air at various flow ratios through a blender with two mass-flow meters. A water bath was used to keep halothane at different saturated vapor pressure through cooling or heating. Gas was supplied to the samples (about 4 mm from the sample) through a tube 5 mm in diameter at a total rate of 100 ml min − 1 (except for the lowest concentration 0.04%, where the gas flow was 200 ml min − 1). A dynamic scan was used in the measurements to record the changes of ellipsometric parameters c and D upon halothane vapors exposure. The changes of thickness and refractive index were determined by in situ spectroscopic ellipsometry by exposing samples to a constant flow of halothane gas or air. Spectra were taken at three angles of incidence (f =65, 70 and 75°) in the photon energy range 1.25 – 4.5 eV at 0.05 eV intervals. The optical model used in the analysis is a three-phase model with a silicon substrate and a single layer. The measured quantity is the complex reflectance ratio r = Rp/Rs = tan ce iD
(1)
Fig. 2. Typical ellipsometric response of a HMDSO film for stepwise exposure to different concentrations of halothane measured at a photon energy of 3.1 eV.
where Rp (Rs) is the complex reflection coefficient of light polarized parallel (perpendicular) to the plane of incidence, and c and D are the ellipsometric angles. Formally we can express the dependence on all system parameters as r= r(d, Ns, Nf, f, l), where the complex indices of refraction Ns = ns + iks and Nf = nf +ikf of the substrate and the film, respectively, are introduced. To be exact one should also include a thin (2 nm) native oxide between the polymer film and the substrate. However, the oxide optical properties are very similar to those of the polymer film and we therefore neglect the oxide. The polymer layer thicknesses given here are therefore slightly larger than in reality. When a sample is exposed to halothane vapor, d, and Nf will change, and c and D are thus functions of the concentration of halothane vapor. We also assume the substrate properties to be halothane gas independent and c and D therefore only depend on changes in the HMDSO film. All the measurements are taken at room temperature. The ellipsometric data were analyzed with the WVASE software package (J.A. Woollam), which is based on least-squares regression analysis to obtain the unknown fitting parameters and their 90% confidence limits. The procedure is to vary fitting parameters to minimize the differences between the measured and calculated c and D values by using the exact ellipsometric equations [13] in the optical model described above.
3. Results
Fig. 1. A schematic view of the measurement set-up including a computer-controlled variable angle spectroscopic ellipsometer and a gas mixing system.
A typical ellipsometric response of a HMDSO film with refractive index n= 1.4860 to pulses of different concentrations of halothane is shown in Fig. 2. The measurement was done at a photon energy of 3.1 eV at an angle of incidence of 70°. Preliminary studies
S. Guo et al. / Sensors and Actuators B 44 (1997) 243–247
Fig. 3. The thickness of a HMDSO film upon exposure to halothane vapor vs. halothane concentrations at an angle of incidence of 70° and a photon energy of 3.1 eV.
showed high sensitivity under these conditions. The concentration was determined from the gas mixing ratio. The vapor pressure was calculated according to following equation [14]: log P si (mmHg)= A−B/(t(°C) + C)
(2)
where P si is the saturated vapor pressure, t is the temperature and A, B, C are constants. For the calculation of halothane vapor pressure, the values of A, B, C are 6.53534, 915.201 and 200.229, respectively. It is seen that the HMDSO layers possess a strong sorption ability for halothane, good reversibility and short response time. In order to show the repeatability of the sensor, the sample was exposed to four identical gas pulses at each concentration. The reproducibility in terms of the relative standard deviations in dc and dD for halothane exposure was 90.2%, and 91.8% at a concentration of 28%, 9 0.2% and 92.9% at a concentration of 17%, 9 2.5% and 92.1% at a concentration of 9.5%, 9 2.5% and 95.5% at a concentration of 3.5%, 99.6% and 9 9.8% at a concentration of 0.35%, respectively. The lowest concentration which could be detected was around 0.04% at which the signal-to-noise ratio is 1. It should be pointed out that the sensitivity
245
Fig. 4. Typical responses to a pulse of halothane (3.5%) measured at a photon energy of 3.1 eV for two films with different refractive indices.
depends strongly on film thickness, film density and wavelength. A considerable improvement in sensitivity is possible to obtain with optimization of the film thickness and density. The responses of HMDSO films to non-chlorinated hydrocarbons such as ethanol, methanol and acetone were also checked. In contrast to halothane, the sensitivity was only 15–30% of the sensitivity to halothane for these vapors. The life time was more than 6 months without significant loss of the sensitivity. The films are thus very stable. Fig. 3 shows a typical relationship between the film thickness and halothane concentrations for a HMDSO sample. The changes of thickness were determined by in situ spectroscopic ellipsometry. It is observed that the thickness increases monotonously with halothane concentration. A corresponding change in the refractive index is also obtained but is not reported here. The dependence of sensitivities on refractive index and the relative change of film thickness dd/d are summarized in Table 1. It is seen that the sensitivity decreases with the refractive index. dc and dD of the film with n= 1.4860 at 3.1 eV are 10–30 times larger than for the film with n = 1.5336 at 3.1 eV. The relative change of the film thickness also decreases with the
Table 1 Ellipsometric sensitivities and thickness changes at two different halothane concentrations for HMDSO films with different refractive indices n at 3.1 eV
1.4860 1.5019 1.5069 1.5145 1.5336 a
d (nm)
79.5 87.0 84.3 119.1 87.7
Dc (°)
dD (°)
dd/d (%)
c = 7%
c = 14%
c =7%
c= 14%
c= 7%
c =14%
1.62 1.60 0.49 0.21a 0.17
3.39 2.90 0.64 0.29a 0.22
6.2 5.2 5.0 4.2a 0.2
13.4 6.9 6.2 5.2a 0.4
3.8 1.4 0.9 0.5 0.04
6.2 1.9 0.7 0.7 0.09
Measured at 4.1 eV instead of 3.1 eV due to a different thickness.
246
S. Guo et al. / Sensors and Actuators B 44 (1997) 243–247
Fig. 5. Spectra of c and D in air and at two different halothane concentrations (5.75 and 35% in dry air) recorded by in situ spectroscopic ellipsometry using an HMDSO film with a thickness of 77.8 nm.
refractive index. The film with the lowest refractive index has the largest relative change of film thickness. In other words, under the impact of the halothane vapor, the less dense film will easily swell and shrink more than the more dense film. The refractive index of HMDSO plasma polymer films depends on the deposition conditions. The refractive index decreases with increasing pressure and decreasing r.f. power [12]. The samples with largest sensitivity, best stability and repeatability in this study were made at a pressure of 0.2 mbar and a r.f. power of 30 W. Fig. 4 shows typical time responses to 3.5% halothane for two different layer refractive indices. It is observed that the film with a low refractive index (n = 1.4860) has a fast response but the dense layer with a high refractive index (n =1.5019) has a slow response which gives a diffusion-like appearance at the beginning of the transients. It takes a longer period of time for halothane molecules to absorb in and desorb from the dense layers. It is seen from Fig. 4 that for the film with lower refractive index, for the rise time to reach the 90% level and the decay time to fall to the 10% level are approximately the same and of the order of 5 s. For the film with higher refractive index, however, the adsorption and desorption rates are different with a rise time of 20 s and a decay time of 30 s. The spectra of c and D with respect to three different concentrations 0, 5.75 and 35% in dry air are recorded by an in situ spectroscopic scan for three samples, as
shown in Fig. 5. It is clearly seen that the sensitivities depend strongly on wavelength. The maximum changes of c and D appear at a wavelength of around 3.1 eV. The maximum sensitivity seems to be obtained at the wavelength close to the optical interference peak which depend on the angle of incidence. Thus, the optimum thickness could be calculated in advance if the wavelength of the light source is known. The film thickness should therefore be carefully selected to optimize the sensitivity in a sensor application. For example, if the light source is a He–Ne laser with a wavelength of 633 nm, the maximum sensitivity could be found at the film thickness about 140 nm at the incidence angle of 70°.
4. Discussion These results reveal that the plasma HMDSO layer can swell reversibly under the impact of the halothane molecules. This is probably associated with the structure of the layers being formed by polymerization in plasma. It is known that the films formed by plasma polymerization are deposited in networks of highly branched and highly cross-linked segments [15]. The –Si–O–Si– backbone in the HMDSO structure gives the layer stability but probably also an open structure, which enables the polymer to interact with halogenated hydrocarbon molecules. Sensor response up to high concentrations indicates that the saturation level is high in the layer. The used halogenated hydrocarbons also
S. Guo et al. / Sensors and Actuators B 44 (1997) 243–247
have high vapor pressure which enable the molecules to desorb quickly as observed by the rapid response rate in the measurements. The refractive index n is a measure of the dipole density, which is determined by mass density, film structure and composition of the plasma polymer HMDSO films. For the dense layer with a higher refractive index, the ability of the absorption and desorption is lower than for films with lower refractive index. The sensitivity is, most probably, coupled to the degree of cross-linking in the films. In other words, a dense layer provides fewer interaction possibilities. We conclude that HMDSO layers show a strong sorption ability for halothane molecules with good repeatability and stability. Largest ellipsometric sensitivity in c and D can be achieved when an appropriate wavelength is used. The lower refractive index, the higher sensitivity and shorter response times are obtained. We suggest that HMDSO films can be used as an optical sensor material for evaluation and control in anesthetic measurements.
Acknowledgements Financial support was obtained from the Swedish National Board for Technical and Industrial Development.
References [1] W. Nahm, G. Gauglitz, Thin polymer films as sensors for hydrocarbons, GIT Fachz. Lab. 7 (1990) 889.
.
247
[2] G. Gauglitz, J. Ingenhoff, Integrated optical sensors for halogenated and non-halogenated hydrocarbons, Sensors and Actuators B 11 (1993) 207 – 212. [3] A. Kindlund, I. Lundstro¨m, Physical study of quartz crystal sorption detectors, Sensors and Actuators 3 (1982/83) 63–77. [4] M.S. Nieuwenhuizen, A. Venema, Mass-sensitive devices, in: W. Go¨pel, J. Hesse, J.N. Zemel (Eds.), Sensors — A Comprehensive Survey, vol. 2, Part I, VCH, Weinheim, 1991. [5] S.W. Wenzel, R.M. White, Flexural plate-wave gravimetric chemical sensors, Sensors and Actuators A 21 – A23 (1990) 700– 703. [6] H.E. Endres, S. Drost, Optimization of the geometry of gas-sensitive interdigital capacitors, Sensors and Actuators B 4 (1991) 95 – 98. [7] M. Haug, K.D. Schierbaum, H.E. Endres, S. Drost, W. Go¨pel, Controlled selectivity of polysiloxane coating: their use in capacitance sensors, Sensors and Actuators A 32 (1992) 326–332. [8] J. Sakata, M. Yamamoto, I. Tajima, Plasma polymerization of mixed monomer gases, J. Polymer Sci., Part A 26 (1988) 1721– 1731. [9] L. Sodomka, J. Janca, Electrical properties and selective gas sorption of plasma polymerized organosiloxane thin films, Scr. Fac. Sci. Nat. Univ. Purk. Brun. 8 (1988) 217 – 228. [10] J. Sakata, M. Yamamoto, Application of plasma polymerized film to gas separation membranes, J. Appl. Polymer Science: Appl. Polymer Symp. 42 (1988) 339. [11] N. Inagaki, Gas separation membranes plasma-polymerized from mixtures of silanes and fluorocarbons, J. Appl. Polymer Sci.: Appl. Polymer Symp. 42 (1988) 327. [12] R. Rochotzki, M. Arzt, F. Blaschta, E. Kreyßig, H.U. Poll, Optical properties of plasma polymer films (hexamethyldisiloxane), Thin Solid Films 234 (1993) 463 – 467. [13] J.A. Woollam, The Users Manual of Variable Angle Spectroscopic Ellipsometry, 1992, p. 29. [14] J. Gmehling, U. Onken, H.W. Schulte, Vapour-liquid equilibria for the binary systems diethyl ether-halothane (1,1,1-trifluoro-2bromo-2-chloroethane), halothane-methanol, and diethyl ethermethanol, J. Chem. Eng. Data 25 (1980) 29 – 32. [15] H. Nomura, P.W. Kramer, H. Yasuda, Preparation of gas separation membranes by plasma polymerization with fluoro compounds, Thin Solid Films 118 (1984) 187 – 195.