- Email: [email protected]
STW gas sensors using plasma-polymerized allylamine
Thin Solid Films 515 (2007) 4105 – 4110 www.elsevier.com/locate/tsf
STW gas sensors using plasma-polymerized allylamine Hiromi Yatsuda a,⁎, Makoto Nara a , Takashi Kogai a , Hidenobu Aizawa b , Shigeru Kurosawa b a
b
Japan Radio Co., Ltd., 2-1-4 Fukuoka, Fujimino, Saitama 356-0011, Japan National Institute of Advanced Industrial Science and Technology (AIST), 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Available online 6 March 2006
Abstract Gas sensors generally consist of two major components: a gas recognition element which provides the specificity and selectivity of the measurement and a physical transducer which translates the gas absorption or desorption event into electronic signal. In this paper, plasma polymerized allylamine (PPAa) film is used as a gas recognition element and a surface transverse wave (STW) device is used as a physical transducer. It is confirmed that STW sensor devices coated with PPAa films provide high sensitivity for moisture. The STW sensor device with a 63 nm PPAa film provided twenty four times higher sensitivity than that of a non-coated STW device. In addition, the chemical structure of PPAa films is characterized by the FT-IR and the contact angle measurement. © 2006 Elsevier B.V. All rights reserved. Keywords: Plasma-polymerization; Gas sensor; Sensor; STW; Surface transverse wave; Sensitivity; Humidity
1. Introduction Gas sensors generally consist of two major components: a gas recognition element which provides the specificity and selectivity of the measurement and a physical transducer which translates the gas absorption or desorption event into electronic signal. The combination of the gas recognition element and the physical transducer is very important for gas sensors. For the gas recognition elements, plasma-polymerization has been widely used to fabricate ultra thin films from a variety of volatile compounds without chemical solvents (dry process) and catalysts in coating process only by one-step [1]. For the physical transducers elastic devices such as quartz crystal microbalance (QCM) [2] and surface acoustic wave (SAW) devices [3,4] have been widely used. The QCM devices are based on thickness mode vibrations and the SAW devices are based on Rayleigh wave on the surface of the substrate. In these acoustic wave based gas sensors, higher operating frequencies make the sensitivity higher. SAW devices are suitable for higher operating frequencies in contrast of QCMs. On the other hand, band-pass filters or resonators using the surface transverse wave (STW) have been utilized in specific communication equipments [5] as well as SAW devices. The ⁎ Corresponding author. Tel.: +81 49 266 9311; fax: +81 49 266 9131. E-mail address: [email protected] (H. Yatsuda). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.02.041
STW is a shear horizontal (SH) wave which is traveling on quartz wafers and has advantages of high acoustic wave velocity and stable temperature characteristics. Then STW devices are very suitable for higher frequency applications. The STW is one of SH waves such as waves on 36° rotated y-cut LiTaO3 substrates that are widely used for sensor applications [6]. A few studies on STW sensor devices have been published [7,8]. In this paper, a plasma-polymerized allylamine (PPAa) film [9–11] is used as a gas recognition element and an STW device is used as a physical transducer. When the PPAa films on the STW sensor devices detect gases, the oscillation frequency of the oscillator circuit using the STW sensor device can decline. The sensitivity of 240 MHz STW sensor devices coated with PPAa films with different thicknesses has been evaluated using moisture concentration. The chemical structure of a PPAa film has been characterized by the FT-IR and the contact angle measurement. 2. Plasma-polymerization For plasma-polymerization, the plasma-polymerization equipment Model BP-1 of Samco International Co. (Kyoto, Japan) was used [9,12]. The plasma-polymerized films were deposited on one side of the STW devices. The schematic diagram of plasma-polymerization equipment was illustrated
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Needle valve Electrodes
STW
Monomer reservoir Stir bar
Leak valve
Oil bath
Stirrer
Stop valve Matching network
Vacuum Rotary pump gauge
13.56MHz generator Fig. 1. Schematic diagram of plasma-polymerization equipment.
in Fig. 1. Prior to plasma-polymerization, the STW devices were treated by plasma sputtering for 5 s under 100 W of RF power and 100 Pa of He pressure. Vapor pressure of the monomer and the RF power of the glow discharge are two of
the most important controllable parameters in plasma-polymerization. In this experiment, vapor pressure of the monomer and the RF power were settled at 100 Pa and 100 W, respectively. The monomer was vaporized at a constant liquid C=C
N-H stretching
CH 2 stretching
C-H bending
-(CH 2)stretching
C-H bending
N-H bending
Monomer
Plasma-polymerized
-C = =N stretching -C=N stretching 4000
3500
3000
2500
2000
1500
1000
-1
Wavenumbers (cm ) Fig. 2. FT-IR transmission spectrum of allylamine monomer and PPAa film.
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Contact angle [degree]
50
IDT(Au/Ti 130nm)
40λ
Top-View
PPA-film
Side-View
Substrate(Quartz) Fig. 4. STW sensor device.
which can originate from a primary amine, a secondary amine or an imine as well. The deformation vibration of primary amines was observed in both spectra, but was considerably broadened in the spectrum of PPAa, which indicates the presence of alkene groups, or imines. A new band appeared for the polymers treated with plasma at 100 W RF power at 2185 cm− 1, which was associated with the stretching vibration of nitrile (C`N) groups. The multiple absorption peaks at 2970, 2922, and 2855 cm− 1, are due to the stretching vibration of aliphatic C–H groups, which were also detected in the polymer, however, instead of the peak of the polymer at
(a) 0 1 2 3 4 5 6 7 8 9 10 248
0 10 20 30 40 50 245
40
246
247
Insertion Loss [dB]
60
λ = 20 μm
Insertion Loss [dB]
temperature (25 °C), which was maintained by an oil bath, and a controlled vacuum at selected condition. The distance between the electrodes was 6.0 cm and the power of the glow discharge was fixed at selected condition with 13.56 MHz of CW. The apparatus and procedure for determining the deposition amounts of PPAa film have been described previously [13]. Spectroscopic ellipsometer (M-210H, Jasco, Co., Ltd., Tokyo, Japan) was used to determine the thickness of PPAa films. Since it is difficult to evaluate a thin film on FT-IR, we used a thick film for FT-IR evaluation which was deposited for 600 s. The FT-IR (Jasco 610, Tokyo, Japan) was used to investigate the bonding structure of the PPAa films in the wave number ranging from 500 to 4000 cm− 1 while the instrument was purged with dry nitrogen gas. For the deposition, freshly prepared FT-IR-grade KBr pellets were used as substrates to coat plasma-polymerized films. The spectra of the films were typically obtained by averaging 20 scans. The contact angles of pure water on PPAa films deposited glass plates (18 × 18 × 0.15 mm3 of cover glass) were performed using a contact angle meter (model CA-D, Kyowa Interface Science Co., Ltd., Japan) at 25 °C. The contact angle was measured by means of the advanced angle method using glass slides coated with PPAa films. Five measurements at different places were carried out immediately after plasma-polymerization and the average value was calculated for each sample [14]. As shown in Fig. 2, IR spectrum of PPAa (lower) and the monomer (upper) indicates that the monomer has undergone a reorganization during the plasma-polymerization, and amine groups of the monomer were partially transformed into amide, imine, or nitrile functionalities. One can see after comparison that some bands were significantly broadened, some disappeared while new bands also appeared. Double peaks of primary amine N–H stretching vibration at 3500–3300 cm− 1 were well resolved on the spectra of the monomer but a wide absorption band was found on the polymer at 3290 cm− 1,
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(b)
INPUT
OUTPUT
20
10
0 0
20
40
60
80
100
Plasma polymerization time [s] Fig. 3. Contact angles of PPAa films as a function of plasma-polymerization time.
Fig. 5. Frequency characteristics of STW device. (a) Insertion loss characteristics. (b) Impedance characteristics for input and output interdigital transducers.
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0 -500
Amplifier
STW device Frequency shift [ppm]
-1000 -1500 -2000 -2500 -3000 -3500 -4000 -4500
0
10
20
30
40
50
60
70
PPAa-film
Thickness of PPAa-film [nm] Fig. 6. Frequency shift of STW sensor devices as a function of PPAa film thickness.
1452 cm− 1, the deformation of the vibration band has appeared at 1418 cm− 1[15,16]. Fig. 3 shows the contact angles of water observed for PPAa films deposited on cover glasses as a function of polymerization time. The contact angles increase as polymerization time increases until 30 s, and then reach the plateau value. For the polymerization time of 30 s, a PPAa film thickness of about 25 nm was obtained. The increase of contact angle means the film surface gradually became a hydrophobic surface.
(a)
Insertion Loss [dB]
4 6 8 10
coated 55nm
non-coated
Fig. 8. A feedback type oscillator circuit using STW sensor device.
3. STW sensor device coated with PPAa films The STW sensor device configuration is shown in Fig. 4. The gold metallized input and output interdigital transducers are placed on quartz crystal because the gold metallization can provide high chemical stability. The STW which is based on SH waves has the advantage of that its propagation loss is less affected with heavy electrode materials such as gold. The chip is 4.5× 1.5× 0.35 mm in size. The electrode finger pitch is 10 μm and the center frequency of the STW device is around 240 MHz. The frequency characteristics of the STW device are shown in Fig. 5. An insertion loss of about 6 dB and a good band-pass response were obtained. Allylamine was plasma-polymerized onto the STW devices. Different thicknesses of PPAa films were obtained with different plasmapolymerization times. The frequency shifts of STW devices coated with PPAa films as a function of the PPAa film thickness is shown in Fig. 6. In a range of 10 to 60 nm thicknesses, the changes of the insertion loss of STW devices were within 1 dB and a frequency shift ratio of 0.016 MHz/nm was obtained. The frequency Path A
12 14 245
246
airtight container
Frequency counter
MFC
247
Frequency [MHz]
(b)
INPUT
OUTPUT
N2
H2O Water bath 35ºC
STW sensor Humidity sensor
MFC Path B Fig. 7. Frequency characteristics of STW devices before and after coated with 55 nm PPAa film. (a) Insertion loss characteristics. (b) Impedance characteristics for input and output interdigital transducers.
Thermostatic chamber 35ºC
Reference
Fig. 9. STW sensor device measurement system.
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80
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40
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0 0
Time [min]
30
60
90
120
150
180
210
240
Time [min] Fig. 10. Frequency changes of STW sensor devices. Fig. 12. Normalized frequency changes of STW sensor devices.
Gas sensitivities for the STW sensor devices coated with PPAa films were evaluated using moisture. The STW sensor device measurement system is shown in Fig. 9. The STW sensor devices are placed in an airtight container which is placed in a thermostatic chamber with 35 °C and a beaker with water in a water-bath with 35 °C. Dry nitrogen gas flows into two paths. One path flows to the beaker with water in a water-bath with 35 °C and then humid nitrogen gas from the beaker flows into the airtight container. The other path flows into the airtight container and then it makes to 11.5% RH -200 non coated
-180
Frequency change [ppm]
16nm -160
35nm
-140
63nm
-120
100
-1400
90 -1200 80 -1000
70 60
-800
50 -600
40
Humidity [%RH]
3.1. Sensitivity for moisture
dilute humidity in the airtight container. The humidity over the sensor devices can be changed as the dilution flow rate changes. The oscillation frequencies of the oscillator circuit using the STW sensor devices were measured by a frequency counter as the dilution flow rate was changed in an interval of 10 min. The oscillation frequency changes of the STW sensor devices coated with PPAa with 63, 35 and 16 nm and non-coated STW devices are shown in Fig. 10 as well as a commercialized humidity sensor output. It was confirmed that the frequencies of STW sensor devices were changed with the humidity changes. The thicker PPAa films made the higher frequency changes for STW sensor devices.
Frequency change [ppm]
responses of STW sensor devices before and after plasmapolymerization with a 55 nm thickness are shown in Fig. 7. A feedback type oscillator circuit was designed using the STW devices as shown in Fig. 8 and it is easy to get STW frequency changes from the oscillator circuit.
30
-400
-100
20 -200
-80
10
-60 0
-40
0
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10
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20
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Experimental number [times]
-20 0 0
30
60
90
120
150
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210
240
non coated
16nm
63nm
Humidity
35nm
Time [min] Fig. 11. Frequency changes of STW sensor devices.
Fig. 13. Frequency changes of STW sensor devices for repeated gas adsorption and desorption test.
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The frequency change of STW sensor devices coated with a 63 nm PPAa film was twenty four times higher than a non-coated STW device. The bigger magnification can be expected for an STW sensor devices coated with the thicker PPAa film. On the other hand, for STW sensor devices the frequency changes in desorption were somewhat weak. The frequency changes in adsorption and desorption are shown in Fig. 11 for STW sensor devices coated with PPAs films and non-coated STW device. The test procedure is the following: dry nitrogen gas for dilution flows into the measurement chamber for 90 min in order to achieve a very low humidity (path A: 0 SCCM, path B: 500 SCCM), the path A flows in order to achieve a humidity of 11.5% (path A: 100 SCCM, path B: 500 SCCM), the humidity is kept at 11.5% in 60 min (path A: 100 SCCM, path B: 500 SCCM), the path A is closed in order to achieve a very low humidity (path A: 0 SCCM, path B: 500 SCCM). Normalized frequency changes of STW sensor devices coated with different PPAa films are re-shown in Fig. 12 corresponding to Fig. 11. It is noted that adsorption and desorption behavior of STW sensor devices cannot be affected PPAa film thickness. 3.2. Repeatability of gas adsorption and desorption for STW sensor devices In order to evaluate repeatability of gas adsorption and desorption for STW sensor devices coated with PPAa films, a repeated gas adsorption and desorption test was performed. A low humidity for 90 min (path A: 0 SCCM, path B: 500 SCCM) and a high humidity for 60 min (path A: 250 SCCM, path B: 50 SCCM) are repeated twenty times. The test result is shown in Fig. 13. It is found that the humidity in the measurement chamber was not stable in the repeated test as shown in the commercialized humidity sensor output. However, the STW sensor devices coated with different PPAa film thicknesses show good results corresponding to the humidity changes. 4. Conclusion PPAa film was evaluated as a recognition element of STW gas sensor devices. The 240 MHz STW sensor devices coated with PPAa films provide a high sensitivity for moisture. The thicker PPAa film made the higher sensitivity and the STW
sensor device coated with a 63 nm PPAa film provided a magnification of twenty four in contrast to non-coated STW devices. The durability of STW sensor devices coated with PPAa films was confirmed with a repeated gas adsorption and desorption test. The PPAa films are suitable for recognition elements of STW gas sensor devices. In addition, the STW devices can be effective for evaluation of very thin plasma-polymerized films. References [1] H. Yasuda, Plasma Polymerization, Chaps. 1, 2 and 6, Academic, New York, 1985. [2] M. Nakazawa, M. Kuroiwa, Y. Shimamoto, H. Iwahashi, S. Wakamoto, IEEE Freq. Control Symp., Proc. (2000) 124. [3] A.D. Amico, A. Palma, E. Verona, IEEE Ultrason. Symp., Proc. (1982) 308. [4] M. Rapp, J. Reibel, U. Stahl, S. Stier, A. Voigt, IEEE Freq. Control Symp., Proc. (1998) 621. [5] H. Yatsuda, H. Iijima, K. Yabe, O. Iijima, IEEE Ultrason. Symp., Proc. (2002) 11. [6] S. Shiokawa, J. Kondoh, JJAP Ser. 43 (5B) (2004) 2799. [7] I.D. Avramov, S. Kurosawa, M. Rapp, P. Krawczak, E.I. Radeva, IEEE Trans. Microwave Theor. Tech. 49 (4) (2001) 827. [8] I.D. Avramov, M. Rapp, S. Kurosawa, P. Krawczak, E.I. Radeva, IEEE Sens. J. 2 (3) (2002) 150. [9] S. Kurosawa, T. Hirokawa, K. Kashima, H. Aizawa, D.S. Han, Y. Yoshimi, Y. Okada, K. Yase, J. Miyake, M. Yoshimoto, J. Hilborn, Thin Solid Films 374 (2000) 262. [10] Q. Chen, R. Forch, W. Knoll, Chem. Mater. 16 (2004) 614. [11] P. Hamerli, T. Weigel, T. Groth, D. Paul, Biomaterials 24 (2003) 3989. [12] S. Kurosawa, B. Atthoff, H. Aizawa, J. Hilborn, Thin Solid Films 457 (2004) 26. [13] S. Kurosawa, H. Aizawa, J. Miyake, M. Yoshimoto, J. Hilborn, Z.A. Talib, Thin Solid Films 407 (2002) 1. [14] H. Aizawa, S. Kurosawa, K. Kobayashi, K. Kashima, T. Hirokawa, Y. Yoshimi, M. Yoshimoto, T. Hirotsu, J. Miyake, H. Tanaka, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 12 (2000) 49. [15] V. Krishnamurthy, I.L. Kamel, J. Polym. Sci., A, Polym. Chem. 27 (1989) 1211. [16] F. Fally, C. Doneux, J. Riga, J.J. Verbist, J. Appl. Polym. Sci. 56 (4) (1995) 567.
STW gas sensors using plasma-polymerized allylamine Hiromi Yatsuda a,⁎, Makoto Nara a , Takashi Kogai a , Hidenobu Aizawa b , Shigeru Kurosawa b a
b
Japan Radio Co., Ltd., 2-1-4 Fukuoka, Fujimino, Saitama 356-0011, Japan National Institute of Advanced Industrial Science and Technology (AIST), 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Available online 6 March 2006
Abstract Gas sensors generally consist of two major components: a gas recognition element which provides the specificity and selectivity of the measurement and a physical transducer which translates the gas absorption or desorption event into electronic signal. In this paper, plasma polymerized allylamine (PPAa) film is used as a gas recognition element and a surface transverse wave (STW) device is used as a physical transducer. It is confirmed that STW sensor devices coated with PPAa films provide high sensitivity for moisture. The STW sensor device with a 63 nm PPAa film provided twenty four times higher sensitivity than that of a non-coated STW device. In addition, the chemical structure of PPAa films is characterized by the FT-IR and the contact angle measurement. © 2006 Elsevier B.V. All rights reserved. Keywords: Plasma-polymerization; Gas sensor; Sensor; STW; Surface transverse wave; Sensitivity; Humidity
1. Introduction Gas sensors generally consist of two major components: a gas recognition element which provides the specificity and selectivity of the measurement and a physical transducer which translates the gas absorption or desorption event into electronic signal. The combination of the gas recognition element and the physical transducer is very important for gas sensors. For the gas recognition elements, plasma-polymerization has been widely used to fabricate ultra thin films from a variety of volatile compounds without chemical solvents (dry process) and catalysts in coating process only by one-step [1]. For the physical transducers elastic devices such as quartz crystal microbalance (QCM) [2] and surface acoustic wave (SAW) devices [3,4] have been widely used. The QCM devices are based on thickness mode vibrations and the SAW devices are based on Rayleigh wave on the surface of the substrate. In these acoustic wave based gas sensors, higher operating frequencies make the sensitivity higher. SAW devices are suitable for higher operating frequencies in contrast of QCMs. On the other hand, band-pass filters or resonators using the surface transverse wave (STW) have been utilized in specific communication equipments [5] as well as SAW devices. The ⁎ Corresponding author. Tel.: +81 49 266 9311; fax: +81 49 266 9131. E-mail address: [email protected] (H. Yatsuda). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.02.041
STW is a shear horizontal (SH) wave which is traveling on quartz wafers and has advantages of high acoustic wave velocity and stable temperature characteristics. Then STW devices are very suitable for higher frequency applications. The STW is one of SH waves such as waves on 36° rotated y-cut LiTaO3 substrates that are widely used for sensor applications [6]. A few studies on STW sensor devices have been published [7,8]. In this paper, a plasma-polymerized allylamine (PPAa) film [9–11] is used as a gas recognition element and an STW device is used as a physical transducer. When the PPAa films on the STW sensor devices detect gases, the oscillation frequency of the oscillator circuit using the STW sensor device can decline. The sensitivity of 240 MHz STW sensor devices coated with PPAa films with different thicknesses has been evaluated using moisture concentration. The chemical structure of a PPAa film has been characterized by the FT-IR and the contact angle measurement. 2. Plasma-polymerization For plasma-polymerization, the plasma-polymerization equipment Model BP-1 of Samco International Co. (Kyoto, Japan) was used [9,12]. The plasma-polymerized films were deposited on one side of the STW devices. The schematic diagram of plasma-polymerization equipment was illustrated
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Needle valve Electrodes
STW
Monomer reservoir Stir bar
Leak valve
Oil bath
Stirrer
Stop valve Matching network
Vacuum Rotary pump gauge
13.56MHz generator Fig. 1. Schematic diagram of plasma-polymerization equipment.
in Fig. 1. Prior to plasma-polymerization, the STW devices were treated by plasma sputtering for 5 s under 100 W of RF power and 100 Pa of He pressure. Vapor pressure of the monomer and the RF power of the glow discharge are two of
the most important controllable parameters in plasma-polymerization. In this experiment, vapor pressure of the monomer and the RF power were settled at 100 Pa and 100 W, respectively. The monomer was vaporized at a constant liquid C=C
N-H stretching
CH 2 stretching
C-H bending
-(CH 2)stretching
C-H bending
N-H bending
Monomer
Plasma-polymerized
-C = =N stretching -C=N stretching 4000
3500
3000
2500
2000
1500
1000
-1
Wavenumbers (cm ) Fig. 2. FT-IR transmission spectrum of allylamine monomer and PPAa film.
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H. Yatsuda et al. / Thin Solid Films 515 (2007) 4105–4110
Contact angle [degree]
50
IDT(Au/Ti 130nm)
40λ
Top-View
PPA-film
Side-View
Substrate(Quartz) Fig. 4. STW sensor device.
which can originate from a primary amine, a secondary amine or an imine as well. The deformation vibration of primary amines was observed in both spectra, but was considerably broadened in the spectrum of PPAa, which indicates the presence of alkene groups, or imines. A new band appeared for the polymers treated with plasma at 100 W RF power at 2185 cm− 1, which was associated with the stretching vibration of nitrile (C`N) groups. The multiple absorption peaks at 2970, 2922, and 2855 cm− 1, are due to the stretching vibration of aliphatic C–H groups, which were also detected in the polymer, however, instead of the peak of the polymer at
(a) 0 1 2 3 4 5 6 7 8 9 10 248
0 10 20 30 40 50 245
40
246
247
Insertion Loss [dB]
60
λ = 20 μm
Insertion Loss [dB]
temperature (25 °C), which was maintained by an oil bath, and a controlled vacuum at selected condition. The distance between the electrodes was 6.0 cm and the power of the glow discharge was fixed at selected condition with 13.56 MHz of CW. The apparatus and procedure for determining the deposition amounts of PPAa film have been described previously [13]. Spectroscopic ellipsometer (M-210H, Jasco, Co., Ltd., Tokyo, Japan) was used to determine the thickness of PPAa films. Since it is difficult to evaluate a thin film on FT-IR, we used a thick film for FT-IR evaluation which was deposited for 600 s. The FT-IR (Jasco 610, Tokyo, Japan) was used to investigate the bonding structure of the PPAa films in the wave number ranging from 500 to 4000 cm− 1 while the instrument was purged with dry nitrogen gas. For the deposition, freshly prepared FT-IR-grade KBr pellets were used as substrates to coat plasma-polymerized films. The spectra of the films were typically obtained by averaging 20 scans. The contact angles of pure water on PPAa films deposited glass plates (18 × 18 × 0.15 mm3 of cover glass) were performed using a contact angle meter (model CA-D, Kyowa Interface Science Co., Ltd., Japan) at 25 °C. The contact angle was measured by means of the advanced angle method using glass slides coated with PPAa films. Five measurements at different places were carried out immediately after plasma-polymerization and the average value was calculated for each sample [14]. As shown in Fig. 2, IR spectrum of PPAa (lower) and the monomer (upper) indicates that the monomer has undergone a reorganization during the plasma-polymerization, and amine groups of the monomer were partially transformed into amide, imine, or nitrile functionalities. One can see after comparison that some bands were significantly broadened, some disappeared while new bands also appeared. Double peaks of primary amine N–H stretching vibration at 3500–3300 cm− 1 were well resolved on the spectra of the monomer but a wide absorption band was found on the polymer at 3290 cm− 1,
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(b)
INPUT
OUTPUT
20
10
0 0
20
40
60
80
100
Plasma polymerization time [s] Fig. 3. Contact angles of PPAa films as a function of plasma-polymerization time.
Fig. 5. Frequency characteristics of STW device. (a) Insertion loss characteristics. (b) Impedance characteristics for input and output interdigital transducers.
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0 -500
Amplifier
STW device Frequency shift [ppm]
-1000 -1500 -2000 -2500 -3000 -3500 -4000 -4500
0
10
20
30
40
50
60
70
PPAa-film
Thickness of PPAa-film [nm] Fig. 6. Frequency shift of STW sensor devices as a function of PPAa film thickness.
1452 cm− 1, the deformation of the vibration band has appeared at 1418 cm− 1[15,16]. Fig. 3 shows the contact angles of water observed for PPAa films deposited on cover glasses as a function of polymerization time. The contact angles increase as polymerization time increases until 30 s, and then reach the plateau value. For the polymerization time of 30 s, a PPAa film thickness of about 25 nm was obtained. The increase of contact angle means the film surface gradually became a hydrophobic surface.
(a)
Insertion Loss [dB]
4 6 8 10
coated 55nm
non-coated
Fig. 8. A feedback type oscillator circuit using STW sensor device.
3. STW sensor device coated with PPAa films The STW sensor device configuration is shown in Fig. 4. The gold metallized input and output interdigital transducers are placed on quartz crystal because the gold metallization can provide high chemical stability. The STW which is based on SH waves has the advantage of that its propagation loss is less affected with heavy electrode materials such as gold. The chip is 4.5× 1.5× 0.35 mm in size. The electrode finger pitch is 10 μm and the center frequency of the STW device is around 240 MHz. The frequency characteristics of the STW device are shown in Fig. 5. An insertion loss of about 6 dB and a good band-pass response were obtained. Allylamine was plasma-polymerized onto the STW devices. Different thicknesses of PPAa films were obtained with different plasmapolymerization times. The frequency shifts of STW devices coated with PPAa films as a function of the PPAa film thickness is shown in Fig. 6. In a range of 10 to 60 nm thicknesses, the changes of the insertion loss of STW devices were within 1 dB and a frequency shift ratio of 0.016 MHz/nm was obtained. The frequency Path A
12 14 245
246
airtight container
Frequency counter
MFC
247
Frequency [MHz]
(b)
INPUT
OUTPUT
N2
H2O Water bath 35ºC
STW sensor Humidity sensor
MFC Path B Fig. 7. Frequency characteristics of STW devices before and after coated with 55 nm PPAa film. (a) Insertion loss characteristics. (b) Impedance characteristics for input and output interdigital transducers.
Thermostatic chamber 35ºC
Reference
Fig. 9. STW sensor device measurement system.
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Time [min] Fig. 10. Frequency changes of STW sensor devices. Fig. 12. Normalized frequency changes of STW sensor devices.
Gas sensitivities for the STW sensor devices coated with PPAa films were evaluated using moisture. The STW sensor device measurement system is shown in Fig. 9. The STW sensor devices are placed in an airtight container which is placed in a thermostatic chamber with 35 °C and a beaker with water in a water-bath with 35 °C. Dry nitrogen gas flows into two paths. One path flows to the beaker with water in a water-bath with 35 °C and then humid nitrogen gas from the beaker flows into the airtight container. The other path flows into the airtight container and then it makes to 11.5% RH -200 non coated
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Frequency change [ppm]
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3.1. Sensitivity for moisture
dilute humidity in the airtight container. The humidity over the sensor devices can be changed as the dilution flow rate changes. The oscillation frequencies of the oscillator circuit using the STW sensor devices were measured by a frequency counter as the dilution flow rate was changed in an interval of 10 min. The oscillation frequency changes of the STW sensor devices coated with PPAa with 63, 35 and 16 nm and non-coated STW devices are shown in Fig. 10 as well as a commercialized humidity sensor output. It was confirmed that the frequencies of STW sensor devices were changed with the humidity changes. The thicker PPAa films made the higher frequency changes for STW sensor devices.
Frequency change [ppm]
responses of STW sensor devices before and after plasmapolymerization with a 55 nm thickness are shown in Fig. 7. A feedback type oscillator circuit was designed using the STW devices as shown in Fig. 8 and it is easy to get STW frequency changes from the oscillator circuit.
30
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35nm
Time [min] Fig. 11. Frequency changes of STW sensor devices.
Fig. 13. Frequency changes of STW sensor devices for repeated gas adsorption and desorption test.
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H. Yatsuda et al. / Thin Solid Films 515 (2007) 4105–4110
The frequency change of STW sensor devices coated with a 63 nm PPAa film was twenty four times higher than a non-coated STW device. The bigger magnification can be expected for an STW sensor devices coated with the thicker PPAa film. On the other hand, for STW sensor devices the frequency changes in desorption were somewhat weak. The frequency changes in adsorption and desorption are shown in Fig. 11 for STW sensor devices coated with PPAs films and non-coated STW device. The test procedure is the following: dry nitrogen gas for dilution flows into the measurement chamber for 90 min in order to achieve a very low humidity (path A: 0 SCCM, path B: 500 SCCM), the path A flows in order to achieve a humidity of 11.5% (path A: 100 SCCM, path B: 500 SCCM), the humidity is kept at 11.5% in 60 min (path A: 100 SCCM, path B: 500 SCCM), the path A is closed in order to achieve a very low humidity (path A: 0 SCCM, path B: 500 SCCM). Normalized frequency changes of STW sensor devices coated with different PPAa films are re-shown in Fig. 12 corresponding to Fig. 11. It is noted that adsorption and desorption behavior of STW sensor devices cannot be affected PPAa film thickness. 3.2. Repeatability of gas adsorption and desorption for STW sensor devices In order to evaluate repeatability of gas adsorption and desorption for STW sensor devices coated with PPAa films, a repeated gas adsorption and desorption test was performed. A low humidity for 90 min (path A: 0 SCCM, path B: 500 SCCM) and a high humidity for 60 min (path A: 250 SCCM, path B: 50 SCCM) are repeated twenty times. The test result is shown in Fig. 13. It is found that the humidity in the measurement chamber was not stable in the repeated test as shown in the commercialized humidity sensor output. However, the STW sensor devices coated with different PPAa film thicknesses show good results corresponding to the humidity changes. 4. Conclusion PPAa film was evaluated as a recognition element of STW gas sensor devices. The 240 MHz STW sensor devices coated with PPAa films provide a high sensitivity for moisture. The thicker PPAa film made the higher sensitivity and the STW
sensor device coated with a 63 nm PPAa film provided a magnification of twenty four in contrast to non-coated STW devices. The durability of STW sensor devices coated with PPAa films was confirmed with a repeated gas adsorption and desorption test. The PPAa films are suitable for recognition elements of STW gas sensor devices. In addition, the STW devices can be effective for evaluation of very thin plasma-polymerized films. References [1] H. Yasuda, Plasma Polymerization, Chaps. 1, 2 and 6, Academic, New York, 1985. [2] M. Nakazawa, M. Kuroiwa, Y. Shimamoto, H. Iwahashi, S. Wakamoto, IEEE Freq. Control Symp., Proc. (2000) 124. [3] A.D. Amico, A. Palma, E. Verona, IEEE Ultrason. Symp., Proc. (1982) 308. [4] M. Rapp, J. Reibel, U. Stahl, S. Stier, A. Voigt, IEEE Freq. Control Symp., Proc. (1998) 621. [5] H. Yatsuda, H. Iijima, K. Yabe, O. Iijima, IEEE Ultrason. Symp., Proc. (2002) 11. [6] S. Shiokawa, J. Kondoh, JJAP Ser. 43 (5B) (2004) 2799. [7] I.D. Avramov, S. Kurosawa, M. Rapp, P. Krawczak, E.I. Radeva, IEEE Trans. Microwave Theor. Tech. 49 (4) (2001) 827. [8] I.D. Avramov, M. Rapp, S. Kurosawa, P. Krawczak, E.I. Radeva, IEEE Sens. J. 2 (3) (2002) 150. [9] S. Kurosawa, T. Hirokawa, K. Kashima, H. Aizawa, D.S. Han, Y. Yoshimi, Y. Okada, K. Yase, J. Miyake, M. Yoshimoto, J. Hilborn, Thin Solid Films 374 (2000) 262. [10] Q. Chen, R. Forch, W. Knoll, Chem. Mater. 16 (2004) 614. [11] P. Hamerli, T. Weigel, T. Groth, D. Paul, Biomaterials 24 (2003) 3989. [12] S. Kurosawa, B. Atthoff, H. Aizawa, J. Hilborn, Thin Solid Films 457 (2004) 26. [13] S. Kurosawa, H. Aizawa, J. Miyake, M. Yoshimoto, J. Hilborn, Z.A. Talib, Thin Solid Films 407 (2002) 1. [14] H. Aizawa, S. Kurosawa, K. Kobayashi, K. Kashima, T. Hirokawa, Y. Yoshimi, M. Yoshimoto, T. Hirotsu, J. Miyake, H. Tanaka, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 12 (2000) 49. [15] V. Krishnamurthy, I.L. Kamel, J. Polym. Sci., A, Polym. Chem. 27 (1989) 1211. [16] F. Fally, C. Doneux, J. Riga, J.J. Verbist, J. Appl. Polym. Sci. 56 (4) (1995) 567.