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Thin plasma-polymerized layers of hexamethyldisiloxane for acoustoelectronic humidity sensors
Sensors and Actuators B 44 (1997) 275 – 278
Thin plasma-polymerized layers of hexamethyldisiloxane for acoustoelectronic humidity sensors Ekaterina Radeva * Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee bl6d., 1784 Sofia, Bulgaria Received 21 July 1997; accepted 21 July 1997
Abstract Plasma-polymerized layers are obtained from hexamethyldisiloxane (HMDSO). Using scanning electron microscopy (SEM), the effect of glow discharge duration and modification with ammonia on polymer surface and morphology is studied. It was shown that polymer layers were uniform and without defects. The effect of some plasma polymerization parameters on the humiditysorptive properties of the polymers are investigated by measuring the mass changes as a result of water sorption. For this purpose, a system consisting of a sensor array of quartz crystal microbalances coated with plasma polymer films is used. The resonant frequency of the coated quartz resonators decreases with increasing relative humidity. The results showed that the increase in the glow discharge current density, gas phase pressure, layer thickness and modification with ammonia in plasma, increases the sensitivity of the polymers to humidity. The study shows that plasma-polymerized HMDSO layers may be used as a hygroscopic element for sensing the humidity of ambient air and therefore, for the development of quartz resonator sensors. © 1997 Elsevier Science S.A. Keywords: Acousticoelectronic; Plasma-polymerized; Resonator sensors
1. Introduction The use of plasma-polymerized films as protective coatings against corrosion for microelectronic devices is widely appreciated [1 – 3]. There is an interest in using these materials as selective layers in chemical sensors [4 – 6]. In order for a film to be useful as a coating for sensors, it must be amorphous, homogeneous, very dense without pinholes, thermally stable, with a good adhesion to substrate, should protect the sensor surface from oxidation and not react with substances in the media. Films produced in a glow discharge can fulfil all these requirements. Some layers, such as glow discharge hexamethyldisiloxane (HMDSO) have the ability to absorb water vapor [7,8]. A system consisting of a sensor array of quartz crystal microbalances coated with plasma polymer films could be used for registering changes in mass of the polymer layers as a result of water molecule sorption. The changes in resonance frequency (Df ) of the quartz resonator relate to the changes in relative humidity (RH) of the ambient air. * Tel.: +359 2 9757431; fax: + 359 2 9753632. 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 2 9 - 3
This relationship is known as the humidity-frequency characteristic (HFC) of the quartz resonator [8]. The aim of the present work was to study the effect of some parameters of the plasma polymerization of HMDSO on the polymer surface and the humidity sorptive properties of the samples prepared.
2. Experimental The reactor for plasma polymerization was in a vacuum chamber. Two aluminum electrodes (80 cm2) were placed horizontally 20 mm above each other. Polymeric layers were obtained by glow discharge alternatively, on both sides of a quartz resonator and glass substrate covered with aluminum placed on the lower electrode. HMDSO was used as a monomer. The glow discharge voltage was about 400 V, 50 Hz. The current density varied from 0.5 to 3.6 mA cm − 2 and the gas phase pressure in the reactor chamber was from 26 to 266 Pa. The layer thickness was estimated from the frequency shift of the quartz resonator before and after polymer deposition [9].
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E. Rade6a / Sensors and Actuators B 44 (1997) 275–278
The modification of the plasma-polymerized layer with ammonia vapor was carried out in the same reactor for 7 min, under the same conditions as the polymer [10]. The treatment with ammonia was aimed at increasing the humidity sorption of the polymers, obtained from HMDSO. Scanning electron microscopy (SEM) analyses were performed by JEOL superprobe 733. Humidity-frequency characteristic (HFC) measuring of the samples prepared was carried out in Mytron climatic test cabinet type KPW (Germany). The quartz resonators with polymer layers were placed in the climatic cabinet and connected to a measuring generator and frequency counter for registration of the frequency changes brought about by changes in the relative humidity inside the cabinet. The data obtained were collected and calculated by personal computer.
3. Results and discussion
3.1. Effects of glow discharge duration and ammoniacal plasma treatment on the surface and morphology of the polymer The effect of glow discharge duration on the polymer surface is presented in Fig. 1. The surfaces consisted of small spherical particles with diameters between 0.05 and 4 mm. Some of them were agglomerates. The increase in the deposition time led to an increase in the number of agglomerates per unit area (Fig. 1b) and the surface became rougher. The surface of the polymer obtained from HMDSO after modification in ammonia plasma is shown in Fig. 2a. The particles were not agglomerated and the mean particle diameter was 1 mm. As shown by Figs. 1 and 2, the modification in ammonia plasma did not significantly change the polymer surface. The profile of the polymer layer after modification in ammonia plasma was shown in Fig. 2b. The polymer films were dense without pinholes and defects, homogeneous, with good adhesion to aluminum substrate, observed in micrographs and tested by scotch.
3.2. Effects of glow discharge current density and gas phase pressure on the polymer humidity-sorpti6e properties The HFCs of the quartz resonators with polymer layers obtained at different glow discharge current densities were measured. The gas phase pressure and layer thickness were constant — 106 Pa and 0.2 mm, respectively. The humidity sensitivity (Cc ) of the HFCs, which was the frequency change of the resonator per 1% change in RH, was calculated. The increase in the current density (I) resulted in an increase in the humidity sensitivity (Fig. 3a).
Fig. 1. SEM micrographs of PHMDSO films obtained for (a) 5 min and (b) 15 min.
E. Rade6a / Sensors and Actuators B 44 (1997) 275–278
277
Fig. 3. Humidity sensitivity (Cc ) dependence of (a) glow discharge current density and (b) gas phase pressure.
The HFCs of samples obtained at different gas phase pressure (P) were measured, all other experimental conditions being held constant, i.e. 1.5 mA cm − 2 current density and 0.2 mm layer thickness. The humidity sensitivity of the layer increased with increasing gas phase pressure (Fig. 3b).
3.3. Effects of layer thickness on the polymer humidity-sorpti6e properties The HFCs of the samples with different layer thickness (h) were measured. The glow discharge current
Fig. 2. SEM micrographs of (a) the surface and (b) the profile, of PHMDSO films treated with ammonia in plasma after deposition.
Fig. 4. Humidity sensitivity (Cc ) relationship with layer thickness.
E. Rade6a / Sensors and Actuators B 44 (1997) 275–278
278
Table 1 Humidity sensitivity of plasma polymer layers obtained from HMDSO with and without modification in ammonia plasma Current density (mA cm−2)
1.5 1.5 3.1
Gas phase pressure (Pa)
106 266 266
density was 3.6 mA cm − 2 and the gas phase pressure— 266 Pa. It was established that the humidity sensitivity (Cc ) increased with thickness increase (Fig. 4).
3.4. Effects of ammoniancal plasma treatment of the polymer layer on the humidity-sorpti6e properties The humidity sensitivity of HMDSO layers obtained in plasma and treated with ammonia at the same conditions was higher than that of plasma-polymerized HMDSO (PHMDSO) layers without further treatment (Table 1).
4. Conclusions The SEM analysis shows that plasma polymers obtained from HMDSO have stable parameters as coatings —density, homogeneous without defects. The increase in the glow discharge current density, the gas phase pressure during plasma polymerization and the layer thickness, results in an increase in the humidity sensitivity of the polymers, obtained from HMDSO. Modification with ammonia vapors in plasma after the polymerization of HMDSO leads to an increase in the humidity sorption of the polymer layers. The results for plasma HMDSO polymers and plasma modification with ammonia show that such layers may be used as hygroscopic elements for sensing the humidity of ambient air and therefore for the development of quartz resonator sensors.
References [1] H. Yasuda, Plasma polymerization for protective coatings and composite membranes, J. Membr. Sci. 18 (1984) 273–284.
.
Humidity sensitivity (Hz % RH−1) With NH3
Without NH3
8.9 11.5 12.5
4.5 8.0 10.9
[2] E. Sacher, J.E. Klemberg-Sapieha, H.P. Schreiber, M.R. Wertheimer, Moisture barrier properties of plasma-polymerized hexamethyldisiloxane, J. Appl. Polym. Sci. 38 (1984) 163–171. [3] M.R. Wertheimer, J.E. Klemberg-Sapieha, H.P. Schreiber, Advanced in basic and applied aspects of microwave plasma polymerization, Thin Solid Films 115 (1984) 109 – 124. [4] I. Sugimoto, M. Nakamura, H. Kuwano, Molecular sensing using plasma polymer thin-film probes, Sensors and Actuators B 10 (1993) 117 – 122. [5] M. Nakamura, I. Sugimoto, H. Kuwano, R. Lemos, Chemical sensing by analysing dynamics of plasma polymer film-coated sensors, Sensors and Actuators B 20 (1994) 231 – 237. [6] Jay W. Grate, M. Klusty, R. Andrew McGill, M. Abraham, G. Whiting, J. Andomian-Haftvan, The predominant role of swelling-induced modules changes of the sorbent phase in determining the responses of polymer-coated surface acoustic wave vapor sensors, Anal. Chem. 64 (6) (1992) 21 – 25. [7] C. Hamman, G. Kampfrath, Glow discharge polymeric films: preparation, structure, properties and applications, Vacuum 34 (12) (1984) 1053 – 1059. [8] E. Radeva, K. Bobev, L. Spassov, Study and application of glow discharge polymer layers as humidity sensors, Sensors and Actuators B 8 (1992) 21 – 25. [9] V.V. Malov, Piezoresonancnie Datchiki, Energoatomizdat M., 1989, p. 68 (in Russian). [10] E. Radeva, D. Tsankov, K. Bobev, L. Spassov, Fourier transform infrared analysis of hexamethyldisiloxane layers obtained in low-frequency glow discharge, J. Appl. Polym. Sci. 50 (1993) 165 – 171.
Biographies Ekaterina Rade6a was born in 1954 in Isperih, Bulgaria. In 1977 she received the MS degree in chemistry from Sofia University. Since 1982 she has been with the Institute of Solid State Physics of the Bulgarian Academy of Sciences. She has been working on glow discharge polymer layers: synthesis, structure, properties and application as sensitive coatings for chemical sensors.
Thin plasma-polymerized layers of hexamethyldisiloxane for acoustoelectronic humidity sensors Ekaterina Radeva * Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee bl6d., 1784 Sofia, Bulgaria Received 21 July 1997; accepted 21 July 1997
Abstract Plasma-polymerized layers are obtained from hexamethyldisiloxane (HMDSO). Using scanning electron microscopy (SEM), the effect of glow discharge duration and modification with ammonia on polymer surface and morphology is studied. It was shown that polymer layers were uniform and without defects. The effect of some plasma polymerization parameters on the humiditysorptive properties of the polymers are investigated by measuring the mass changes as a result of water sorption. For this purpose, a system consisting of a sensor array of quartz crystal microbalances coated with plasma polymer films is used. The resonant frequency of the coated quartz resonators decreases with increasing relative humidity. The results showed that the increase in the glow discharge current density, gas phase pressure, layer thickness and modification with ammonia in plasma, increases the sensitivity of the polymers to humidity. The study shows that plasma-polymerized HMDSO layers may be used as a hygroscopic element for sensing the humidity of ambient air and therefore, for the development of quartz resonator sensors. © 1997 Elsevier Science S.A. Keywords: Acousticoelectronic; Plasma-polymerized; Resonator sensors
1. Introduction The use of plasma-polymerized films as protective coatings against corrosion for microelectronic devices is widely appreciated [1 – 3]. There is an interest in using these materials as selective layers in chemical sensors [4 – 6]. In order for a film to be useful as a coating for sensors, it must be amorphous, homogeneous, very dense without pinholes, thermally stable, with a good adhesion to substrate, should protect the sensor surface from oxidation and not react with substances in the media. Films produced in a glow discharge can fulfil all these requirements. Some layers, such as glow discharge hexamethyldisiloxane (HMDSO) have the ability to absorb water vapor [7,8]. A system consisting of a sensor array of quartz crystal microbalances coated with plasma polymer films could be used for registering changes in mass of the polymer layers as a result of water molecule sorption. The changes in resonance frequency (Df ) of the quartz resonator relate to the changes in relative humidity (RH) of the ambient air. * Tel.: +359 2 9757431; fax: + 359 2 9753632. 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 2 9 - 3
This relationship is known as the humidity-frequency characteristic (HFC) of the quartz resonator [8]. The aim of the present work was to study the effect of some parameters of the plasma polymerization of HMDSO on the polymer surface and the humidity sorptive properties of the samples prepared.
2. Experimental The reactor for plasma polymerization was in a vacuum chamber. Two aluminum electrodes (80 cm2) were placed horizontally 20 mm above each other. Polymeric layers were obtained by glow discharge alternatively, on both sides of a quartz resonator and glass substrate covered with aluminum placed on the lower electrode. HMDSO was used as a monomer. The glow discharge voltage was about 400 V, 50 Hz. The current density varied from 0.5 to 3.6 mA cm − 2 and the gas phase pressure in the reactor chamber was from 26 to 266 Pa. The layer thickness was estimated from the frequency shift of the quartz resonator before and after polymer deposition [9].
276
E. Rade6a / Sensors and Actuators B 44 (1997) 275–278
The modification of the plasma-polymerized layer with ammonia vapor was carried out in the same reactor for 7 min, under the same conditions as the polymer [10]. The treatment with ammonia was aimed at increasing the humidity sorption of the polymers, obtained from HMDSO. Scanning electron microscopy (SEM) analyses were performed by JEOL superprobe 733. Humidity-frequency characteristic (HFC) measuring of the samples prepared was carried out in Mytron climatic test cabinet type KPW (Germany). The quartz resonators with polymer layers were placed in the climatic cabinet and connected to a measuring generator and frequency counter for registration of the frequency changes brought about by changes in the relative humidity inside the cabinet. The data obtained were collected and calculated by personal computer.
3. Results and discussion
3.1. Effects of glow discharge duration and ammoniacal plasma treatment on the surface and morphology of the polymer The effect of glow discharge duration on the polymer surface is presented in Fig. 1. The surfaces consisted of small spherical particles with diameters between 0.05 and 4 mm. Some of them were agglomerates. The increase in the deposition time led to an increase in the number of agglomerates per unit area (Fig. 1b) and the surface became rougher. The surface of the polymer obtained from HMDSO after modification in ammonia plasma is shown in Fig. 2a. The particles were not agglomerated and the mean particle diameter was 1 mm. As shown by Figs. 1 and 2, the modification in ammonia plasma did not significantly change the polymer surface. The profile of the polymer layer after modification in ammonia plasma was shown in Fig. 2b. The polymer films were dense without pinholes and defects, homogeneous, with good adhesion to aluminum substrate, observed in micrographs and tested by scotch.
3.2. Effects of glow discharge current density and gas phase pressure on the polymer humidity-sorpti6e properties The HFCs of the quartz resonators with polymer layers obtained at different glow discharge current densities were measured. The gas phase pressure and layer thickness were constant — 106 Pa and 0.2 mm, respectively. The humidity sensitivity (Cc ) of the HFCs, which was the frequency change of the resonator per 1% change in RH, was calculated. The increase in the current density (I) resulted in an increase in the humidity sensitivity (Fig. 3a).
Fig. 1. SEM micrographs of PHMDSO films obtained for (a) 5 min and (b) 15 min.
E. Rade6a / Sensors and Actuators B 44 (1997) 275–278
277
Fig. 3. Humidity sensitivity (Cc ) dependence of (a) glow discharge current density and (b) gas phase pressure.
The HFCs of samples obtained at different gas phase pressure (P) were measured, all other experimental conditions being held constant, i.e. 1.5 mA cm − 2 current density and 0.2 mm layer thickness. The humidity sensitivity of the layer increased with increasing gas phase pressure (Fig. 3b).
3.3. Effects of layer thickness on the polymer humidity-sorpti6e properties The HFCs of the samples with different layer thickness (h) were measured. The glow discharge current
Fig. 2. SEM micrographs of (a) the surface and (b) the profile, of PHMDSO films treated with ammonia in plasma after deposition.
Fig. 4. Humidity sensitivity (Cc ) relationship with layer thickness.
E. Rade6a / Sensors and Actuators B 44 (1997) 275–278
278
Table 1 Humidity sensitivity of plasma polymer layers obtained from HMDSO with and without modification in ammonia plasma Current density (mA cm−2)
1.5 1.5 3.1
Gas phase pressure (Pa)
106 266 266
density was 3.6 mA cm − 2 and the gas phase pressure— 266 Pa. It was established that the humidity sensitivity (Cc ) increased with thickness increase (Fig. 4).
3.4. Effects of ammoniancal plasma treatment of the polymer layer on the humidity-sorpti6e properties The humidity sensitivity of HMDSO layers obtained in plasma and treated with ammonia at the same conditions was higher than that of plasma-polymerized HMDSO (PHMDSO) layers without further treatment (Table 1).
4. Conclusions The SEM analysis shows that plasma polymers obtained from HMDSO have stable parameters as coatings —density, homogeneous without defects. The increase in the glow discharge current density, the gas phase pressure during plasma polymerization and the layer thickness, results in an increase in the humidity sensitivity of the polymers, obtained from HMDSO. Modification with ammonia vapors in plasma after the polymerization of HMDSO leads to an increase in the humidity sorption of the polymer layers. The results for plasma HMDSO polymers and plasma modification with ammonia show that such layers may be used as hygroscopic elements for sensing the humidity of ambient air and therefore for the development of quartz resonator sensors.
References [1] H. Yasuda, Plasma polymerization for protective coatings and composite membranes, J. Membr. Sci. 18 (1984) 273–284.
.
Humidity sensitivity (Hz % RH−1) With NH3
Without NH3
8.9 11.5 12.5
4.5 8.0 10.9
[2] E. Sacher, J.E. Klemberg-Sapieha, H.P. Schreiber, M.R. Wertheimer, Moisture barrier properties of plasma-polymerized hexamethyldisiloxane, J. Appl. Polym. Sci. 38 (1984) 163–171. [3] M.R. Wertheimer, J.E. Klemberg-Sapieha, H.P. Schreiber, Advanced in basic and applied aspects of microwave plasma polymerization, Thin Solid Films 115 (1984) 109 – 124. [4] I. Sugimoto, M. Nakamura, H. Kuwano, Molecular sensing using plasma polymer thin-film probes, Sensors and Actuators B 10 (1993) 117 – 122. [5] M. Nakamura, I. Sugimoto, H. Kuwano, R. Lemos, Chemical sensing by analysing dynamics of plasma polymer film-coated sensors, Sensors and Actuators B 20 (1994) 231 – 237. [6] Jay W. Grate, M. Klusty, R. Andrew McGill, M. Abraham, G. Whiting, J. Andomian-Haftvan, The predominant role of swelling-induced modules changes of the sorbent phase in determining the responses of polymer-coated surface acoustic wave vapor sensors, Anal. Chem. 64 (6) (1992) 21 – 25. [7] C. Hamman, G. Kampfrath, Glow discharge polymeric films: preparation, structure, properties and applications, Vacuum 34 (12) (1984) 1053 – 1059. [8] E. Radeva, K. Bobev, L. Spassov, Study and application of glow discharge polymer layers as humidity sensors, Sensors and Actuators B 8 (1992) 21 – 25. [9] V.V. Malov, Piezoresonancnie Datchiki, Energoatomizdat M., 1989, p. 68 (in Russian). [10] E. Radeva, D. Tsankov, K. Bobev, L. Spassov, Fourier transform infrared analysis of hexamethyldisiloxane layers obtained in low-frequency glow discharge, J. Appl. Polym. Sci. 50 (1993) 165 – 171.
Biographies Ekaterina Rade6a was born in 1954 in Isperih, Bulgaria. In 1977 she received the MS degree in chemistry from Sofia University. Since 1982 she has been with the Institute of Solid State Physics of the Bulgarian Academy of Sciences. She has been working on glow discharge polymer layers: synthesis, structure, properties and application as sensitive coatings for chemical sensors.