Electrical Characterization and Modeling of Hexamethyldisiloxane Thin Film Humidity Sensors

Size: px

Start display at page:

Download "Electrical Characterization and Modeling of Hexamethyldisiloxane Thin Film Humidity Sensors"

Amelia Robbins
5 years ago
Views:

1 Journal of Chemical Science and Technology Jan. 014, Vol. 3 Iss. 1, PP Electrical Characterization and Modeling of Hexamethyldisiloxane Thin Film Humidity Sensors Noubeil Guermat *1,, Azzedine Bellel, Salah Sahli 3 and Patrice Raynaud 4 1 Département d Electronique, Faculté de Technologie, Université de M sila, BP.166, Route Ichebilia, M sila 8000 Algérie Laboratoire des Etudes de Matériaux d Electronique pour Applications Médicales (LEMEAMED), Faculté des Sciences de l Ingénieur, Université de Constantine 1, 5000, Algérie. 3 Laboratoire de Microsystèmes et Instrumentation (LMI), Faculté des Science de l Ingénieur, Universitéde Constantine 1, 5000, Algérie. 4 Laboratoire de Génie Electrique UniversitéPaul Sabatier 118, Route de Narbonne 3106 Toulouse-Cedex, France. * g_noubeil@yahoo.fr Abstract- This paper reports the study of humidity-sensitive and electrical properties plasma polymerization of hexamethyldisiloxane (pp-hmdso) thin film based sensors. The humidity sensitive film was deposited by glow discharge at low frequency power (19 khz) in a capacitively coupled parallel plate plasma reactor. The sensor design comprises the interdigitated electrodes and the absorbing layer. The sensor was calibrated in terms of impedance as a function of relative humidity (10% to 95%), using a Frequency Response Analyzer. The signal frequency range was between 10 to 10 7 Hz with amplitude of 3 V. Hexamethyldisiloxane thin film is investigated as humidity sensor. The deposited film sensor exhibited a small hysteresis (% RH) and fast response (8 and 34 s for adsorption and desorption between 35% RH and 95% RH, respectively). The results of an electrochemical impedance spectroscopy (EIS) study performed by measuring the sensor's complex impedance, were analyzed and employed to extract equivalent circuit models. Such models have humidity-dependent element values and their structure has a direct relationship with the sensor physics. The performance of the films and the results of the model simulations are herewith presented and discussed. The HMDSO film showed promising characteristics for humidity sensor development. Keywords- HMDSO; Humidity Sensors; Thin Film; Electrical Characterization; Modeling I. INTRODUCTION Thin film humidity sensors are widely used in many measurement and control applications, including those in automated process control, meteorology, domestic appliances, agriculture, and medical equipment. They can be categorized into capacitive, resistive, mechanical, and oscillating types, based on the sensing principle used. Various materials such as porous ceramics [1-], organic polymers [3], and electrolytes [4] are used as humidity-sensitive materials. Among them, the organic polymers materials have gained the most attention due to their low cost, easy processing and better humidity sensitive characteristic. The humidity sensors based on polymers can be classified as resistive-type and capacitive-type [5] sensors. They have simple fabrication and small hysteresis. The sensing mechanism of these materials is that the adsorption of the surrounding water vapor enhances the surface ionic conductivity of sensing materials. In this paper, we report the electrical characterization of pp-hmdso thin film humidity sensors elaborated by low frequency plasma discharge (19 KHz) realized in our lab, together with the modeling adopted. A. Electrical Characterization and Sensors Performance II. RESULTS The plasma polymerized HMDSO thin films deposited on two-intredigitated aluminum electrodes were used as sensor element and evaluated for humidity detection under an applied voltage of 3 V and signal frequency of 1 khz. The measured result was achieved by HP 484A-model LCZ meter. Fig. 1 shows the measured impedance responses over thin pp-hmdso film in the range of relative humidity (RH) of 10% to 95% at temperature of about 7 C. The impedance of the sensor decreased two orders of magnitude when relative humidity decreased from 30% to 95% on a semi-logarithmic scale. Between 10% and 30 % of RH, the deposited films were found to be insensitive to water vapor. The pp-hmdso film sensor did not show a visible change of the electrical impedance, the value of this later was in order of about Increasing RH beyond 30% gives rise to an abrupt impedance decrease. The value of the electrical impedance decreases significantly until reaching the value of At lower RH, the electrical response is caused by proton hopping between chemisorbed hydroxyl groups. Afterwards, when the amount of physisorbed water molecules starts to increase, the hydronyum ion, H 3 O +, is most likely the charge carrier. Furthermore, pp-hmdso-based sensor showed small hysteresis (%), excellent sensitivity to humidity and wide scale of impedance (Fig. 1), which indicated that the reversible absoption/desorption is easily achieved in this case [5]. The capacitance of the sensor increases with RH increasing, it changes a little at low RH, and changes greatly at high RH (Fig. 3)

2 Journal of Chemical Science and Technology Jan. 014, Vol. 3 Iss. 1, PP B. Complex Impedance Fig.1 Humidity impedance characteristics of pp-hmdso based sensor Impedance spectroscopy is a powerful technique to understand the conduction mechanisms of humidity sensors. Therefore, the impedance plot was adopted to elucidate the transport process by ions in the conduction mechanism of the pp-hmdso films. Complex impedance plotting techniques can help in building the equivalent circuit model and then analysing the mechanism of the humidity sensing material. It is known that a semicircle in an impedance spectroscopy plot represents a resistor R in parallel with a capacitor C. The angular frequency of the peak of the semicircle ωp is such that ωprc = 1. The value of the resistor and capacitor with the frequency determine whether semicircle is closed or not. The impedance measurements were carried out in the frequency range of 300 Hz to 800 khz at humidities of 10% to 95% RH, an AC voltage of 3 V. The typical complex impedance spectra (Nyquist plots) of the pp-hmdso film at different humidities are shown in Fig.. The electrode geometry and thickness were constant for all measurements; the only variable parameter in our measurements was the frequency signal. Zre and Zim are the real and imaginary parts of the complex impedance, respectively. It was observed that at low humidity levels, the Nyquist plot describes an arc with a very large curvature and the semicircle is not closed. This radius decreases with increasing RH due to the effect of the physisorbed water layer on sensor surface [6]. The decrease in the impedance is due to the increase in electrical conductivity by H 3 O + and H + ions in the dielectric film. From the Nyquist plot, it can be understood that the sensor contains a parallel combination of R and C. It is seen that at low frequency, impedance is purely resistive and at high frequency it is purely capacitive in nature. At low humidity, the resistance is very high then decreases when the RH increases and becomes comparable to capacitive reactance [6]. In other words, at low RH (< 40% RH), only a small amount of water is adsorbed, and the response of the sensor resistance to RH was mostly caused by the structure change in the film [7]. When RH increased (50% RH), the inclined semicircle appeared. Many authors [6-9] have explained that it is due to a kind of polarization and it can be modeled by an equivalent circuit of parallel resistor and capacitor which agrees with the result of Fig. 3. Fig. The complex impedance plots of HMDSO sensor at different RH

Journal of Chemical Science and Technology Jan. 014, Vol. 3 Iss. 1, PP. 13-17 C. Response Time Fig.

3 Journal of Chemical Science and Technology Jan. 014, Vol. 3 Iss. 1, PP C. Response Time Fig. 3 The relationship curve between capacitance and relative humidity Response and recovery behavior is one of the significant features for estimating the performance of the humidity sensors. The response time was measured by quickly moving the humidity sensor from relatively dry environment (35% RH) to the other chamber in equilibrium at 90% RH and the recovery time was recorded in the opposite way. The time taken by the sensor to achieve 90% of the total impedance change is defined as the response time for humidification and the recovery time for desiccation. Fig. 4 shows the response recovery property of the pp-hmdso humidity sensor. It is found that the sensor exhibits a fast response and recovery time, the response and recovery times was in the order of 8 s and 34 s, respectively, which are among the best results reported for resistive type humidity sensor [10-11]. The faster response time to humidity of PPHMDSO sensor might be due to regular morphology and suitable thickness of the sensing layer. Films deposited by low frequency plasma from pure HMDSO have been reported to be homogeneous, without pinholes and defects [1]. This is convenient for efficient absorbing and desorbing of water molecules. The response time associated with the absorption process is shorter than that associated with the desorption process. This asymmetry in diffusion of water inside the polymer film is a characteristic property of most humidity sensors, where kinetics of desorption of water molecules from the pores are slower than their absorption [8]. D. Morphological Analysis Fig. 4 Response-recovery properties of pp- HMDSO sensor The SEM micrograph of HMDSO sample is presented in Fig. 5. A uniform growth of the film is deposited with the presence of microvoids through the whole surface of the pp-hmdso layer. Fig. 5 SEM micrographs of pp-hmdso

4 Journal of Chemical Science and Technology Jan. 014, Vol. 3 Iss. 1, PP A. Moisture Diffusion Modeling in HMDSO Films III. MODELING The device concept consists of a resistive-type humidity sensor based on a thin polymerized HMDSO films deposited on two-intredigitated aluminum electrodes. A sketch of the top view and the cross section of the sensor are depicted in Fig. 6. Impedance H O (H +, OH -, H 3 O + ) x = 0 x = L E + Humidity sensitive film Al electrode d Fig. 6 Schematic of the theoretical model In order to compare the impedance of the conventional and the high-speed structures, equations governing the transient capacitance for each structure have been derived. In this analysis, it has been assumed that no moisture is present inside the film in its initial state and that the diffusion constant is independent of moisture concentration. The diffusion kinetics is assumed to obey Fick s law [13]. Assuming that the upper electrode is transparent to moisture, the transient moisture concentration distribution inside the film is derived by solving the one-dimensional diffusion equation. C t x, t Cx, t Where C(x, is the moisture concentration and D is the diffusion coefficient of the moisture. D The diffusion and kinetic parameters of the sensor are evaluated by correlating its experimental transient response-time data according to the transient-state model using the explicit finite difference method. The solution of Eq. (1) is expressed in the form of difference equations as: x D. t D. t D. t C( x, t C( x x, (1 ) C( x, C( x x, () x x x With initial conditions of C(0, = Cs and a boundary condition of C(L, = 0, where Cs is the surface moisture concentration. B. Time Comparison The response time of a humidity sensor can be improved either by selecting a hygroscopic material with a high moisture diffusion coefficient or by changing the dimensions of the moisture sensing film. However, once the material has been specified, the response can only be enhanced by modifying the geometry of the film. The moisture diffusion modeling presented in this section assumes the hygroscopic material to be HMDSO and specifies a diffusion coefficient of cm /sec (Fig. 7). The results of the model simulations show an excellent agreement with measured data [5], and the model features offer a deeper knowledge of either the device behavior or the changes that may be operated to improve the sensor's response. (1) Fig. 7 Simulated and experimental responses of sensor

5 Journal of Chemical Science and Technology Jan. 014, Vol. 3 Iss. 1, PP IV. CONCLUSIONS Due to the great interest in humidity sensors for their applications in electronic control systems, we have developed and investigated HMDSO thin films. The humidity sensor based on pp-hmdso thin film exhibited good electrical response to relative humidities from the range of 10% to 95 % of RH with small hysteresis of about %. The Cole-Cole plots of the complex impedance of the HMDSO layer in different relative humidity show a tendency that the shapes of the curves change from a semicircle to a line with the increasing of RH. An analysis of an equivalent circuit of parallel resistor and capacitor and the complex impedance leads us to the following explanation: at low frequency the impedance is purely resistive and for high frequency it is purely capacitive in nature. The results of the model simulations show an excellent agreement with measured data and the model features offer a deeper knowledge of either the device behavior or the changes that may be operated to improve the sensor s response. ACKNOWLEDGMENT This work was supported by the Algerian-French cooperation. REFERENCES [1] L. Wu, C. C. Wu, and J. C. Her, Ni (AI, Fe) O 4 -TiO ceramic humidity sensors, Journal of Materials Science, vol. 6, pp , [] N. Yamazoe, and Y. Shimizu, Humidity sensors: principles and applications, Sensor. Actuat. vol. 10, pp , [3] N. Guermat, A. Bellel, S. Sahli, Y. Segui, and P. Raynaud, Electrical and structural characterisation of plasma-polymerized TEOS thin, M. J. Condensed Matter, pp , 010. [4] A.T. Ramaprasad, and Vijayalakshmi Rao, Chitin polyaniline blend as humidity sensor, Sensors and Actuators B, vol. 148, pp , 010. [5] N. Guermat, A. Bellel, S. Sahli, Y. Segui, and P. Raynaud, Thin plasma-polymerized layers of hexamethyldisiloxane for humidity sensor development, Thin Solid Films, vol. 517, pp , 009. [6] K. P. Biju, and M. K. Jain, Effect of crystallization on humidity sensing properties of sol gel derived nanocrystalline TiO thin films, Thin Solid Films, vol. 516, pp , 008. [7] P.G. Su, and W.Y. Tsai, Humidity sensing and electrical properties of a composite material of nano-sized SiO and poly(- acrylamido--methylpropane sulfonate), Sensors and Actuators, B, vol. 100, pp , 004. [8] C.P.L. Rubinger, C.R. Martins, M.-A. De Paoli, and R.M. Rubinger, Sulfonated polystyrene polymer humidity sensor: Synthesis and characterization, Sensors and Actuators, B, vol. 13, pp. 4-49, 007. [9] J. Wang, B. K. Xu, S. P. Ruan, and S. P. Wang, Preparation and electrical properties of humidity sensing films of BaTiO 3 /polystrene sulfonic sodium, Materials Chemistry and Physics, vol. 78, pp , 003. [10] Z. Yao, and M. Yang, Sens. Actuators B, vol. 117, p. 93, 006. [11] Y. Li, Y. Chen, C. Zhang, T. Xue, and M. Yang, Sens. Actuators B, vol. 15, p. 131, 007. [1] E. Radeva, Sens. Actuators B Chem. vol. 44, p. 75, [13] A. Tetelin and C. Pellet, Modeling and Optimization of a Fast Response Capacitive Humidity Sensor, IEEE Sensors Journal, vol. 6, iss. 3, pp ,

Highly Sensitive and Stable Humidity Nanosensors based on LiCl Doped

Supporting Information for: Highly Sensitive and Stable Humidity Nanosensors based on LiCl Doped TiO 2 Electrospun Nanofibers Zhenyu Li 1, Hongnan Zhang 1, Wei Zheng 1, Wei Wang 1, Huimin Huang 1, Ce Wang