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A SEM study of plasma-polymerized hexamethyldisiloxane thin films
Vecuum/volume 48inumber l/pages 47 to 42/1997 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/97 $17.00+.00
PII: SOO42-207X(96)00212-6
A SEM study of plasma-polymerized hexamethyldisiloxane thin films Radeva, Institute Sofia, Bulgaria
E
received
5 June
of Solid State Physics,
Bulgarian
Academy
of Sciences,
72 Tzarigradsko
Chaussee,
1784
1996
Plasma-polymerized layers were obtained from hexamethyldisiloxane (HMDSO) in a parallel plate diode operated at 400 V 50 Hz and at a current density of 3.0 mNcm* with a gas pressure of 133 Pa in a dynamic type system, The polymers were used as humidity sensors and to increase their sensitivity the polymer was modified in an ammonia plasma. An SEM study was made of the polymer layer to fix the effect of the polymerization conditions and modification by the ammonia plasma. It was shown that the polymer layers were uniform, dense and had a good adhesion to the substrate. Copyright 0 1996 Elsevier Science Ltd
introduction The use of plasma-polymerized films as corrosion protective coatings for microelectronic devices is widely appreciated.la There is an interest in the use of these materials as selective layers in sensors’ but the films must have selective characteristics. They must be homogeneous, very dense without pinholes, thermally stable and have a good adhesion to the substrate. Films produced in a glow discharge can fulfill all these requirements.5 Of interest are polymer layers plasma-deposited onto the surface of a quartz resonators, which could be used as a sorptive sensor for measuring the relative humidity in the air.6 As a selective layer for water molecules polymer, obtained from hexamethyldisiloxane (HMDSO), is used.6 However, it has previously been established ’ that under different plasma conditions the films can have different chemical composition and structure. The aim of this work was to study the effect of polymerization conditions of HMDSO: glow discharge duration and modification with ammonia on the polymer surface and morphology. Experimental Plasma polymerization was performed in the plasma reactor system shown in Figure 1. The reaction chamber (l), connected to a vacuum system, had two aluminum electrodes (2) placed horizontally 20 mm above each other. The glow discharge voltage was about 400 V at 50 Hz. The polymer layers were deposited on polished glass substrates covered with aluminum to simulate the electrode of the quartz resonators. The samples were placed on the lower electrode. The gas pressure in the reactor was measured with a vacuummeter type Pirani. The monomer dosage and ammonia vapour used to modify the polymer were controlled by inlet microvalves. The layers were obtained at 3.0mA/cm* and 133 Pa pressure of the monomer gas. The polymer deposition
Figure 1. Plasma reactor system: I-reaction chamber; 2Gelectrodes; 3-sample holder; k-generator; 5-microvalves; &containers for monomer and modificator; ‘I---diffusion pump; I(-vacuum balloon, and 9-rotary pump.
duration varied from 5 to 15 min (the polymer thickness was from 0.5 to 2.0 pm). The exposure of the deposited polymer layer with ammonia vapour was done without monomer under the same plasma conditions for IOmin. The polymerization in a mixture of monomer and ammonia vapours for IO min was also 41
E Radeva: Plasma-polymerized
hexamethyldisiloxane
Figure 2. SEM micrographs of PHMDSO films obtained for (a) 5 min. and (b) 15 min.
Figure 4. SEM micrographs of profiles of (a) PHMDSO films, and of (b) the same polymer treated with ammonia vapour in plasma after deposition.
carried out. The treatment with ammonia was aimed at increasing the humidity selective sorption of the PHMDSO layer. SEM analyses was performed using the JEOL superprobe 733.
Figure 3(b). The layer structures are similar. The film surfaces consist of separated spheres with a diameter from 0.5 to 3.0 pm. These particles are not agglomerates. It is likely that several of smaller spherical structure collapse together. The mean particle diameter in Figure 3(a) is l.Opm and those in Figure 3(b) is 2.0 pm. The pictures in Figures 2 and 3 show that the modification
Results and discussion The effect of glow discharge duration on the relief of the polymer surface is presented in Figure 2. The film surfaces deposited on aluminum over perids of 5 min and 15 min consist of small spherical particles. A similar picture has been observed by Thompson.’ The particle diameters of the layer deposited after 5 min (Figure 2(a)) are from 0.05 pm to 4.0 pm. Some of them are agglomerates containing smaller spherical particles. An increase in the deposition duration (and therefore the film thick-
ness) leads to an increase in the number of agglomerates per unit area (Figure 2(b)) and the film surface becomes rougher. Figure 3(a) shows the SEM micrographs of polymer obtained from HMDSO after modification in ammonia plasma. SEM micrographs of the polymer layer formed in plasma containing both HMDSO monomer and ammonia vapours are presented in
in ammonia plasma does not significantly change the polymer surface. The profiles of the polymer layers obtained without and after ammonia plasma treatment are shown in Figure 4(a) and (b), respecfvely. The thickness of the first layer is 1.8pm and of the second 1.Opm. The polymer film obtained from HMDSO is dense without pinholes and defects, homogeneous films with good adhesion to aluminum substrate (Figure 4(a)). The ammonia plasma treatment after polymerization process leads to a rougher film surface structure (Figure 4(b)). Conclusion The SEM analysis shows that plasma polymer films obtained from hexamethyldisiloxane with or without ammonia vapours are uniform, dense, without defects, and have a good adhesion to the substrate. The modification in ammonia plasma does not significantly change the polymer surface. These properties of the polymer layers and their humidity sorption established earlier6 make them suitable as active layers for humidity sensors. References 1. Hirotsu, T.. J. Appl. Polym. Sci., 1979,24, 1957. 2. Yasuda, H., J. Membr. Sci., 1984, 18, 273. 3. Sacher, E., Klemberg-Sapieha, J.E., Schreiber, H.P. and Wertheimer, M.R., J. Appl. Posy& Sk, Appl. Polym. Symp., 1984,38, 163. 4. Wertheimer, M.R., Klembera-SaDieha. J.E. and Schreiber. H.P.. Thin Solid Films, 1984, 115, 109. 5. Hamann, C. and Kampfrath, G., Vacuum, 1984,34, 1053. 6. Radeva, E., Bobev, K. and Spassov, L., Sensors and Actuators B, 1992, 8(l). \ 21. 7. Radeva, E., Tsankov, D., Bobev, K. and Spassov, L., J. Appl. Polym. Sci., 1993, 50, 165. 8. Thompson, L.F. and Smolinski, G., J. Appl. Polym. Sci., 1972, 16, I,
Figure 3. SEM micrographs of (a) PHMDSO films treated with ammonia vapour in plasma after deposition, and of(b) polymer obtained in plasma containing both HMDSO and ammoniacal vapors.
42
1179.
PII: SOO42-207X(96)00212-6
A SEM study of plasma-polymerized hexamethyldisiloxane thin films Radeva, Institute Sofia, Bulgaria
E
received
5 June
of Solid State Physics,
Bulgarian
Academy
of Sciences,
72 Tzarigradsko
Chaussee,
1784
1996
Plasma-polymerized layers were obtained from hexamethyldisiloxane (HMDSO) in a parallel plate diode operated at 400 V 50 Hz and at a current density of 3.0 mNcm* with a gas pressure of 133 Pa in a dynamic type system, The polymers were used as humidity sensors and to increase their sensitivity the polymer was modified in an ammonia plasma. An SEM study was made of the polymer layer to fix the effect of the polymerization conditions and modification by the ammonia plasma. It was shown that the polymer layers were uniform, dense and had a good adhesion to the substrate. Copyright 0 1996 Elsevier Science Ltd
introduction The use of plasma-polymerized films as corrosion protective coatings for microelectronic devices is widely appreciated.la There is an interest in the use of these materials as selective layers in sensors’ but the films must have selective characteristics. They must be homogeneous, very dense without pinholes, thermally stable and have a good adhesion to the substrate. Films produced in a glow discharge can fulfill all these requirements.5 Of interest are polymer layers plasma-deposited onto the surface of a quartz resonators, which could be used as a sorptive sensor for measuring the relative humidity in the air.6 As a selective layer for water molecules polymer, obtained from hexamethyldisiloxane (HMDSO), is used.6 However, it has previously been established ’ that under different plasma conditions the films can have different chemical composition and structure. The aim of this work was to study the effect of polymerization conditions of HMDSO: glow discharge duration and modification with ammonia on the polymer surface and morphology. Experimental Plasma polymerization was performed in the plasma reactor system shown in Figure 1. The reaction chamber (l), connected to a vacuum system, had two aluminum electrodes (2) placed horizontally 20 mm above each other. The glow discharge voltage was about 400 V at 50 Hz. The polymer layers were deposited on polished glass substrates covered with aluminum to simulate the electrode of the quartz resonators. The samples were placed on the lower electrode. The gas pressure in the reactor was measured with a vacuummeter type Pirani. The monomer dosage and ammonia vapour used to modify the polymer were controlled by inlet microvalves. The layers were obtained at 3.0mA/cm* and 133 Pa pressure of the monomer gas. The polymer deposition
Figure 1. Plasma reactor system: I-reaction chamber; 2Gelectrodes; 3-sample holder; k-generator; 5-microvalves; &containers for monomer and modificator; ‘I---diffusion pump; I(-vacuum balloon, and 9-rotary pump.
duration varied from 5 to 15 min (the polymer thickness was from 0.5 to 2.0 pm). The exposure of the deposited polymer layer with ammonia vapour was done without monomer under the same plasma conditions for IOmin. The polymerization in a mixture of monomer and ammonia vapours for IO min was also 41
E Radeva: Plasma-polymerized
hexamethyldisiloxane
Figure 2. SEM micrographs of PHMDSO films obtained for (a) 5 min. and (b) 15 min.
Figure 4. SEM micrographs of profiles of (a) PHMDSO films, and of (b) the same polymer treated with ammonia vapour in plasma after deposition.
carried out. The treatment with ammonia was aimed at increasing the humidity selective sorption of the PHMDSO layer. SEM analyses was performed using the JEOL superprobe 733.
Figure 3(b). The layer structures are similar. The film surfaces consist of separated spheres with a diameter from 0.5 to 3.0 pm. These particles are not agglomerates. It is likely that several of smaller spherical structure collapse together. The mean particle diameter in Figure 3(a) is l.Opm and those in Figure 3(b) is 2.0 pm. The pictures in Figures 2 and 3 show that the modification
Results and discussion The effect of glow discharge duration on the relief of the polymer surface is presented in Figure 2. The film surfaces deposited on aluminum over perids of 5 min and 15 min consist of small spherical particles. A similar picture has been observed by Thompson.’ The particle diameters of the layer deposited after 5 min (Figure 2(a)) are from 0.05 pm to 4.0 pm. Some of them are agglomerates containing smaller spherical particles. An increase in the deposition duration (and therefore the film thick-
ness) leads to an increase in the number of agglomerates per unit area (Figure 2(b)) and the film surface becomes rougher. Figure 3(a) shows the SEM micrographs of polymer obtained from HMDSO after modification in ammonia plasma. SEM micrographs of the polymer layer formed in plasma containing both HMDSO monomer and ammonia vapours are presented in
in ammonia plasma does not significantly change the polymer surface. The profiles of the polymer layers obtained without and after ammonia plasma treatment are shown in Figure 4(a) and (b), respecfvely. The thickness of the first layer is 1.8pm and of the second 1.Opm. The polymer film obtained from HMDSO is dense without pinholes and defects, homogeneous films with good adhesion to aluminum substrate (Figure 4(a)). The ammonia plasma treatment after polymerization process leads to a rougher film surface structure (Figure 4(b)). Conclusion The SEM analysis shows that plasma polymer films obtained from hexamethyldisiloxane with or without ammonia vapours are uniform, dense, without defects, and have a good adhesion to the substrate. The modification in ammonia plasma does not significantly change the polymer surface. These properties of the polymer layers and their humidity sorption established earlier6 make them suitable as active layers for humidity sensors. References 1. Hirotsu, T.. J. Appl. Polym. Sci., 1979,24, 1957. 2. Yasuda, H., J. Membr. Sci., 1984, 18, 273. 3. Sacher, E., Klemberg-Sapieha, J.E., Schreiber, H.P. and Wertheimer, M.R., J. Appl. Posy& Sk, Appl. Polym. Symp., 1984,38, 163. 4. Wertheimer, M.R., Klembera-SaDieha. J.E. and Schreiber. H.P.. Thin Solid Films, 1984, 115, 109. 5. Hamann, C. and Kampfrath, G., Vacuum, 1984,34, 1053. 6. Radeva, E., Bobev, K. and Spassov, L., Sensors and Actuators B, 1992, 8(l). \ 21. 7. Radeva, E., Tsankov, D., Bobev, K. and Spassov, L., J. Appl. Polym. Sci., 1993, 50, 165. 8. Thompson, L.F. and Smolinski, G., J. Appl. Polym. Sci., 1972, 16, I,
Figure 3. SEM micrographs of (a) PHMDSO films treated with ammonia vapour in plasma after deposition, and of(b) polymer obtained in plasma containing both HMDSO and ammoniacal vapors.
42
1179.