- Email: [email protected]
Moisture effect on the dielectric and structure of BaTiO3-based devices
Microelectronics Reliability 50 (2010) 887–890
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Moisture effect on the dielectric and structure of BaTiO3-based devices H.Y. Wang, X.M. Xiong, J.X. Zhang * State Key Laboratory of Optoelectronic Materials and Technologies and School of Physics & Engineering, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 1 August 2009 Received in revised form 21 October 2009 Available online 10 February 2010
a b s t r a c t Moisture effect on the dielectric behavior and structure of BaTiO3-based devices were studied systematically. The dielectric measurement mainly concerned the capacitance and loss of specimen as a function of frequency and temperature between 10 and 50 °C in different measuring environments (vacuum, ambient air with and without dryer). A broad peak of capacitance and loss due to the moisture absorption, depending on frequency and temperature rate and corresponding to a dynamic process involving water absorption and de-absorption, was observed in the heating run when the measurement was carried out in ambient air without dryer. It showed that water absorption would change the apparent dielectric properties of devices and a heat treatment at 50 °C for the BaTiO3-based devices was suggested to reduce the moisture absorption effect. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental procedure
Absorbate interactions have been investigated extensively for decades due to the application of materials in device electronics and biomedical applications [1–8]. Barium strontium titanate BaxSr1xTiO3 (BST) [9–12], an important ferroelectric material, has been widely applied in micro-electric devices such as voltage-tunable microwave components and dynamic random access memories. The stability and lifetime of the barium strontium titanate, which are strongly affected by environmental factor, are among the most important concerns. Many types of BaTiO3-based capacitor would encounter the degradation of permittivity, which was closely related to the environmental factor, especially the moisture absorption [13]. Although the effect of moisture on the properties of BST-based devices is recognized, detailed investigation hasn’t been reported. Since BST- or BaTiO3-based devices in subtropics are often used around or below the dew point, moisture effect on the performance of devices should be taken into serious consideration. In the paper, effects of moisture absorption on the dielectric anomalies and the surface structure of Ba0.8Sr0.2TiO3 (BST (80/20)) ceramics were investigated by dielectric spectroscopy and X-ray diffraction. The temperature rate and thermal history dependence of dielectric properties measured in ambient air without dryer, as well as the change of lattice distortion c/a of specimen after immersed into water, were provided.
The poly crystalline BST (80/20) specimen was prepared by the conventional solid-state reaction. Stoichiometric amounts of BaCO3 (>99.5%), SrCO3 (>99.5%) and TiO2 (>99.5%) (Fuchen Inc.) were mixed in ethanol with agate ball for 20 h, then sintered at 1200 °C for 5 h. The crushed, milled and sieved materials were pressed into pellets with a bond of 10% (weight) polyvinyl alcohol (PVA). The pellets were sintered at 1300 °C for 10 h after removing PVA at 600 °C. X-ray diffraction (XRD) with Cu Ka radiation was performed in an X-ray Diffractometer (Rigaku D/MAX 2200 VPC) to examine the phase constitution of the specimen at room temperature and its structural evolution with water absorption. The XRD results were obtained immediately after sintering at 1300 °C, re-annealing at 50 °C in the dielectric measurement and immersed into water for 2 and 14 days. Both sides of the cut and polished samples with the capacitor area of 4.44 mm 4.84 mm are coated with silver electrodes and the thickness of the sample is 1.14 mm. The lattice distortion c/a in different conditions is calculated by the professional software Jade 5.0. The dielectric measurements of capacitance C and loss tg d as a function of temperature are carried out on an Agilent 4284A LCR meter with the measuring frequencies of 1, 10, and 100 kHz respectively. Different temperature rates (0.5, 2, 4 °C/min) are provided by Delta Design 9023 Temperature Chamber where the dryer silica gel was put in. The temporal room temperature is 25 °C and relative humidity is 73–90%, i.e., the environmental dew point is 19–23 °C. 3. Results and discussion
* Corresponding author. Tel.: +86 20 84113672 301; fax: +86 20 84113320. E-mail address: [email protected] (J.X. Zhang). 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.01.004
The X-ray diffraction pattern of the as-sintered BST (80/20) at room temperature is shown in Fig. 1. The pattern indicated that
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H.Y. Wang et al. / Microelectronics Reliability 50 (2010) 887–890
Fig. 1. The X-ray diffraction patterns of as-sintered BST (80/20) specimen at room temperature.
the specimen was of tetragonal and the tetragonal distortion c/a was 1.0045 ± 0.0013, being in agreement with the result of about 1.0053 [10]. To investigate the moisture effect on the dielectric properties of the specimen, different measuring environments (vacuum, ambient air with and without dryer) were used. Fig. 2 shows the dielec-
Fig. 2. Capacitance (a) and loss (b) versus temperature at 1 kHz with the rate of , without dryer; 1 °C/min for the specimen under different ambient conditions ( , in vacuum; D, with dryer). The arrows indicate the direction of the temperature variation for each curve.
tric spectrums of BST (80/20) specimens in the temperature range from 10 to 50 °C. Results in vacuum and in ambient air with dryer showed that no abrupt change was observed in its dielectric spectrums, neither on heating nor on cooling. As described in previous investigation [11], BST (80/20) was always in the tetragonal phase in the temperature range from 10 to 50 °C. The results obtained in vacuum and in ambient air with dryer were consistent with previous results in [10,11]. However, in ambient air without dryer, the dielectric spectrums for both cooling and heating runs were much different from each other. Capacitance C and loss tg d exhibited a large broad peak in the heating run while the broad peak disappeared in the cooling run. Similar results were achieved for different bulk samples of BST (80/20). From above results, we can reasonably conclude that different humidity should be responsible for the different behaviors in dielectric spectrums of BST (80/20). In ambient air without dryer, the environmental dew point is 19– 21 °C and the water molecules could be absorbed on the surface of specimen at temperature lower than the dew point. So when BST (80/20) stayed at low temperature of 10 °C for a period in ambient air without dryer, water molecules would prefer to be absorbed or condensed on the surfaces of sample instead of the interspaces between electrode and sample, forming a water film, thus leading to different dielectric behaviors. It should be mentioned that only the absorbed water on the side surface without electrodes would contribute to the dielectric broad peak because the absorbed water molecules on the electrodes was not in the region of two electrodes for the dielectric measurement. In addition, it was worthwhile to mention that when the dielectric measurements were repeatedly carried out from 50 °C after finishing the heating run, no obviously broad peak was observed during cooling. It indicated that the dielectric properties were almost not affected by the environmental moisture after a heat treatment at 50 °C because the absorbed water had almost evaporated from the surface of sample. Further investigations showed that the broad peak of capacitance and loss in the heating run strongly depended on the measuring frequency, the temperature rate and the thermal history, which was clearly indicated in Fig. 3. The peak height increased with the decrease of measuring frequency and sometimes the single loss peak spitted into two at certain frequency. While the heating rate increased, the peak became broader and the peak height decreased, showing that the degree of moisture absorption on the BST specimen was affected by its staying time at low temperatures. XRD was applied to study the structural change of BST (80/20) ceramics due to moisture effect. However, if the content of moisture absorption was very little, it could not be detected by XRD technique. In order to accelerate the experiment and manifest the effect of moisture absorption, we adopted the specimen immersed into water for different time intervals instead of exposing to air with high humidity. Two types of samples immersed into water for 2 and 14 days were adopted. Fig. 4 shows the patterns of XRD of the specimens in different conditions. The XRD results of as-sintered one at 1300 °C and re-annealed one that experienced a heat treatment at 50 °C in the dielectric measurement were used for comparison. For the BST (80/20) after being immersed in water for 14 days, the diffraction peak h1 1 1i shifted to 39.15°, higher than 39.04° of the as-sintered BST (80/20). These XRD results indicated that the lattice distortion c/a of BST (80/20) changed with the absorbed water. For the as-sintered and re-annealed BST (80/20), the tetragonal distortion c/a had almost the same value. While for the BST (80/20) immersed in water for 2 and 14 days, it became to be 1.0055 ± 0.0011 and 1.0084 ± 0.0010 respectively. Very likely, the water molecules had been inserted into the inside sites of samples after immersed into water for 2 and 14 days. For all that it was different from the moisture absorption on the surface of samples in
H.Y. Wang et al. / Microelectronics Reliability 50 (2010) 887–890
889
Fig. 3. Variation of capacitance and loss tg d of the BST (80/20) ceramics as a function of temperature at frequencies in the ambient air without dryer. Data are taken firstly during the cooling run, then the heating run at different temperature ramp, a: 0.5 °C/min; b: 2 °C/min; c: 4 °C/min. The arrows indicate the direction of the temperature variation for each curve.
Fig. 4. The 2h XRD scans at room temperature in the h1 1 1i layer for the BST (80/ 20) specimen: (a) as-sintered, exposure to water for 2 days (b) and 14 days (c), (d) re-annealed at 50 °C in the dielectric measurement.
the dielectric measurement, it still indicated that the water molecules could be absorbed on the samples and the interaction between water molecules and BST (80/20) would result in the increment of the tetragonal distortion c/a, as demonstrated in [3]. Considering the fact that some copolymer films showed a peak in capacitance on heating from 10 to 30 °C in ambient conditions [14–16] but no peak arose in vacuum [3], we can also reasonably propose that moisture effect should be responsible for the dielectric changes. Since water was accompanied by its own dipole moment, in heating run the polar water was absorbed on the BST (80/ 20) specimen at low temperature side and resulted in the increase of capacitance and loss. When experiencing the heat treatment at relatively high temperature, water would evaporate. In the cooling run, no increase of capacitance and loss originating from moisture absorption was observed, i.e., no abrupt change of dielectric properties appeared.
Actually, the observed broad peak of capacitance and loss of BST specimen in the heating run in ambient air without dryer could be a dynamic process involving water absorption and de-absorption. Below the dew point, the process was mainly dominated by water absorption. While higher than the dew point, the process preferred to be dominated by water de-absorption. Although bulk water showed relaxation behaviors in the same temperature and frequency range[17], the formed water film accompanying water absorption and de-absorption would exhibit a distinct frequency dependence just like that of phase transition [11,12], i.e., the lower frequency, the higher capacitance C and loss tg d. For the case of small temperature rate, much more water molecules were absorbed because they had enough time to condense onto the side surface of specimen, thus resulting in a much higher capacitance and loss peak, which were readily shown in Fig. 3. In addition, in different heating run with the same heating rate the value of C and tg d and the shape of the observed peak showed some difference, possibly related to the change of surround environment such as humidity and position of specimen in each measurement. Therefore it was suggested that the dynamic process of water absorption and de-absorption seemed to be complex and sometimes two loss peaks could be observed [18]. Moreover, water molecules could be repeatedly absorbed and removed from BST devices, as described in the dielectric results. A heat treatment at 50 °C would recover its performance even when the devices experienced the dew point. Therefore in application of the micro-electrics devices, heating in moderate temperature before use will be very necessary to reduce the water absorption effect, especially when they are used in subtropics.
4. Conclusions Dielectric response and structural change owing to water absorption were studied on ferroelectric BST (80/20) devices systematically. In the temperature range from 10 and 50 °C a broad capacitance and loss peak resulted from moisture absorption was observed in the heating run when measurement carried out in ambient air without dryer. The peak was strongly affected by the
890
H.Y. Wang et al. / Microelectronics Reliability 50 (2010) 887–890
measuring frequency, the temperature rate and the thermal history, and it could be a dynamic process of water absorption and de-absorption. It indicated that the dielectric properties of BST devices were closely related to the environmental humidity and the moisture effect was a complex and dynamics process. XRD results for the samples immersed into water for 2 and 14 days showed that the diffraction peak of h1 1 1i layer shifted to higher position due to water absorption and the tetragonal distortion c/a increased with the time immersing in water. For the BaTiO3-based devices, a heat treatment at 50 °C was suggested to reduce the moisture absorption effect and ensure the reliability of devices. Acknowledgment The authors gratefully acknowledge the financial support of the National Physics Base at Sun Yat-Sen University (J0630320 and J0730313). References [1] Shiro T. Dielectric properties of water of adsorption on protein crystals. J Polym Sci 1961;62:233–40. [2] Saito Y, Kataoka H, Quartarone E, Mustarelli P. Carrier migration mechanism of physically cross-linked polymer gel electrolytes basedon PVDF membranes. J Phys Chem B 2002;106:7200–4. [3] Jacobson PA, Rosa LG, Othon CM, Kraemer KL, Sorokin AV, Ducharme S, Dowben PA. Water absorption and dielectric changes in crystalline poly(vinylidene fluoride-trifluoroethylene) copolymer films. Appl Phys Lett 2004;84:88–90. [4] Frank B, Frubing P, Pissis P. Water sorption and thermally stimulated depolarization currents in nylon-6. J Polym Sci Part B: Polym Phys 1996;34:1853–60. [5] Laredo E, Hernandez MC. Moisture effect on the low- and high-temperature dielectric relaxations in nylon-6. J Polym Sci Part B: Polym Phys 1997;35:2879–88.
[6] Laredo E, Grimau M, Sánchez F, Bello A. Water absorption effect on the dynamic properties of nylon-6 by dielectric spectroscopy. Macromolecules 2003;36:9840–50. [7] Castela AS, Simões AM. An impedance model for the estimation of water absorption in organic coatings. Part I: a linear dielectric mixture equation. Corros Sci 2003;45:1631–46. [8] Castela AS, Simões AM. An impedance model for the estimation of water absorption in organic coatings. Part II: a complex equation of mixture. Corros Sci 2002;45:1647–60. [9] Szymczak L, Ujma Z, Hañderek J, Kapusta J. Sintering effects on dielectric properties of (Ba,Sr)TiO3 ceramics. Ceram Int 2004;30:1003–8. [10] Fu CL, Yang CR, Chen H, Chen HW, Wang YX, Hu LY. Microstructure and dielectric properties of BaxSr1xTiO3 ceramics. Mater Sci Eng B 2005;119:185–8. [11] Frayssignes H, Cheng BL, Fantozzi G, Button TW. Phase transformation in BST ceramics investigated by internal friction measurements. J Eur Ceram Soc 2005;25:3203–6. [12] Abdelkefi H, Khemakhem H, Vüélu G, Carru JC, van der Mühll R. Dielectric properties and ferroelectric phase transitions in Bax Sr1x TiO3 solid solution. J Alloys Compd 2005;399:1–6. [13] Daniel DN, Michael P, Lloyd IK, Ganesan S. Moisture induced degradation of multilayer ceramic capacitors. Microelectron Reliab 2006;46:400–8. [14] Choi J, Dowben PA, Pebley S, Bune A, Ducharme S, Fridkin VM, Palto SP, Petukhova N. Changes in metallicity and electronic structure across the surface ferroelectric transition of ultrathin crystalline poly(vinylidene fluoridetrifluoroethylene) copolymers. Phys Rev Lett 1998;80:1328–31. [15] Blinov LM, Fridkin VM, Palto SP, Bune AV, Dowben PA, Ducharme S. Twodimensional ferroelectrics. Phys Usp 2000;43:243–67. [16] Bune AV, Fridkin VM, Ducharme S, Blinov LM, Palto SP, Sorokin AV. Twodimensional ferroelectric films. Nature 1998;391:874–7. [17] Sengwa RJ, Sankhla S. Characterization of solvent effect on low-frequency dielectric dispersion and relaxation behaviour of ethylene glycol oligomers. J Mol Liquid 2008;141:73–93. [18] Actually, the evolution of absorbed water is often observed in our daily life such as the case that a cup is taken out of icebox. While we take a cup out of an icebox, we will at first observe water vapor around the cup surface followed by the formation of water film. Then slowly water droplets will appear on the cup and become gradually bigger. Sometimes, the droplets will roll down. In our dielectric measurement, two loss peaks were sometimes observed when the temperature rate was small (e.g. 0.5 °C/min), which is possibly related to the evolution of absorbed water film after a long time interval at low temperature.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Moisture effect on the dielectric and structure of BaTiO3-based devices H.Y. Wang, X.M. Xiong, J.X. Zhang * State Key Laboratory of Optoelectronic Materials and Technologies and School of Physics & Engineering, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 1 August 2009 Received in revised form 21 October 2009 Available online 10 February 2010
a b s t r a c t Moisture effect on the dielectric behavior and structure of BaTiO3-based devices were studied systematically. The dielectric measurement mainly concerned the capacitance and loss of specimen as a function of frequency and temperature between 10 and 50 °C in different measuring environments (vacuum, ambient air with and without dryer). A broad peak of capacitance and loss due to the moisture absorption, depending on frequency and temperature rate and corresponding to a dynamic process involving water absorption and de-absorption, was observed in the heating run when the measurement was carried out in ambient air without dryer. It showed that water absorption would change the apparent dielectric properties of devices and a heat treatment at 50 °C for the BaTiO3-based devices was suggested to reduce the moisture absorption effect. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental procedure
Absorbate interactions have been investigated extensively for decades due to the application of materials in device electronics and biomedical applications [1–8]. Barium strontium titanate BaxSr1xTiO3 (BST) [9–12], an important ferroelectric material, has been widely applied in micro-electric devices such as voltage-tunable microwave components and dynamic random access memories. The stability and lifetime of the barium strontium titanate, which are strongly affected by environmental factor, are among the most important concerns. Many types of BaTiO3-based capacitor would encounter the degradation of permittivity, which was closely related to the environmental factor, especially the moisture absorption [13]. Although the effect of moisture on the properties of BST-based devices is recognized, detailed investigation hasn’t been reported. Since BST- or BaTiO3-based devices in subtropics are often used around or below the dew point, moisture effect on the performance of devices should be taken into serious consideration. In the paper, effects of moisture absorption on the dielectric anomalies and the surface structure of Ba0.8Sr0.2TiO3 (BST (80/20)) ceramics were investigated by dielectric spectroscopy and X-ray diffraction. The temperature rate and thermal history dependence of dielectric properties measured in ambient air without dryer, as well as the change of lattice distortion c/a of specimen after immersed into water, were provided.
The poly crystalline BST (80/20) specimen was prepared by the conventional solid-state reaction. Stoichiometric amounts of BaCO3 (>99.5%), SrCO3 (>99.5%) and TiO2 (>99.5%) (Fuchen Inc.) were mixed in ethanol with agate ball for 20 h, then sintered at 1200 °C for 5 h. The crushed, milled and sieved materials were pressed into pellets with a bond of 10% (weight) polyvinyl alcohol (PVA). The pellets were sintered at 1300 °C for 10 h after removing PVA at 600 °C. X-ray diffraction (XRD) with Cu Ka radiation was performed in an X-ray Diffractometer (Rigaku D/MAX 2200 VPC) to examine the phase constitution of the specimen at room temperature and its structural evolution with water absorption. The XRD results were obtained immediately after sintering at 1300 °C, re-annealing at 50 °C in the dielectric measurement and immersed into water for 2 and 14 days. Both sides of the cut and polished samples with the capacitor area of 4.44 mm 4.84 mm are coated with silver electrodes and the thickness of the sample is 1.14 mm. The lattice distortion c/a in different conditions is calculated by the professional software Jade 5.0. The dielectric measurements of capacitance C and loss tg d as a function of temperature are carried out on an Agilent 4284A LCR meter with the measuring frequencies of 1, 10, and 100 kHz respectively. Different temperature rates (0.5, 2, 4 °C/min) are provided by Delta Design 9023 Temperature Chamber where the dryer silica gel was put in. The temporal room temperature is 25 °C and relative humidity is 73–90%, i.e., the environmental dew point is 19–23 °C. 3. Results and discussion
* Corresponding author. Tel.: +86 20 84113672 301; fax: +86 20 84113320. E-mail address: [email protected] (J.X. Zhang). 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.01.004
The X-ray diffraction pattern of the as-sintered BST (80/20) at room temperature is shown in Fig. 1. The pattern indicated that
888
H.Y. Wang et al. / Microelectronics Reliability 50 (2010) 887–890
Fig. 1. The X-ray diffraction patterns of as-sintered BST (80/20) specimen at room temperature.
the specimen was of tetragonal and the tetragonal distortion c/a was 1.0045 ± 0.0013, being in agreement with the result of about 1.0053 [10]. To investigate the moisture effect on the dielectric properties of the specimen, different measuring environments (vacuum, ambient air with and without dryer) were used. Fig. 2 shows the dielec-
Fig. 2. Capacitance (a) and loss (b) versus temperature at 1 kHz with the rate of , without dryer; 1 °C/min for the specimen under different ambient conditions ( , in vacuum; D, with dryer). The arrows indicate the direction of the temperature variation for each curve.
tric spectrums of BST (80/20) specimens in the temperature range from 10 to 50 °C. Results in vacuum and in ambient air with dryer showed that no abrupt change was observed in its dielectric spectrums, neither on heating nor on cooling. As described in previous investigation [11], BST (80/20) was always in the tetragonal phase in the temperature range from 10 to 50 °C. The results obtained in vacuum and in ambient air with dryer were consistent with previous results in [10,11]. However, in ambient air without dryer, the dielectric spectrums for both cooling and heating runs were much different from each other. Capacitance C and loss tg d exhibited a large broad peak in the heating run while the broad peak disappeared in the cooling run. Similar results were achieved for different bulk samples of BST (80/20). From above results, we can reasonably conclude that different humidity should be responsible for the different behaviors in dielectric spectrums of BST (80/20). In ambient air without dryer, the environmental dew point is 19– 21 °C and the water molecules could be absorbed on the surface of specimen at temperature lower than the dew point. So when BST (80/20) stayed at low temperature of 10 °C for a period in ambient air without dryer, water molecules would prefer to be absorbed or condensed on the surfaces of sample instead of the interspaces between electrode and sample, forming a water film, thus leading to different dielectric behaviors. It should be mentioned that only the absorbed water on the side surface without electrodes would contribute to the dielectric broad peak because the absorbed water molecules on the electrodes was not in the region of two electrodes for the dielectric measurement. In addition, it was worthwhile to mention that when the dielectric measurements were repeatedly carried out from 50 °C after finishing the heating run, no obviously broad peak was observed during cooling. It indicated that the dielectric properties were almost not affected by the environmental moisture after a heat treatment at 50 °C because the absorbed water had almost evaporated from the surface of sample. Further investigations showed that the broad peak of capacitance and loss in the heating run strongly depended on the measuring frequency, the temperature rate and the thermal history, which was clearly indicated in Fig. 3. The peak height increased with the decrease of measuring frequency and sometimes the single loss peak spitted into two at certain frequency. While the heating rate increased, the peak became broader and the peak height decreased, showing that the degree of moisture absorption on the BST specimen was affected by its staying time at low temperatures. XRD was applied to study the structural change of BST (80/20) ceramics due to moisture effect. However, if the content of moisture absorption was very little, it could not be detected by XRD technique. In order to accelerate the experiment and manifest the effect of moisture absorption, we adopted the specimen immersed into water for different time intervals instead of exposing to air with high humidity. Two types of samples immersed into water for 2 and 14 days were adopted. Fig. 4 shows the patterns of XRD of the specimens in different conditions. The XRD results of as-sintered one at 1300 °C and re-annealed one that experienced a heat treatment at 50 °C in the dielectric measurement were used for comparison. For the BST (80/20) after being immersed in water for 14 days, the diffraction peak h1 1 1i shifted to 39.15°, higher than 39.04° of the as-sintered BST (80/20). These XRD results indicated that the lattice distortion c/a of BST (80/20) changed with the absorbed water. For the as-sintered and re-annealed BST (80/20), the tetragonal distortion c/a had almost the same value. While for the BST (80/20) immersed in water for 2 and 14 days, it became to be 1.0055 ± 0.0011 and 1.0084 ± 0.0010 respectively. Very likely, the water molecules had been inserted into the inside sites of samples after immersed into water for 2 and 14 days. For all that it was different from the moisture absorption on the surface of samples in
H.Y. Wang et al. / Microelectronics Reliability 50 (2010) 887–890
889
Fig. 3. Variation of capacitance and loss tg d of the BST (80/20) ceramics as a function of temperature at frequencies in the ambient air without dryer. Data are taken firstly during the cooling run, then the heating run at different temperature ramp, a: 0.5 °C/min; b: 2 °C/min; c: 4 °C/min. The arrows indicate the direction of the temperature variation for each curve.
Fig. 4. The 2h XRD scans at room temperature in the h1 1 1i layer for the BST (80/ 20) specimen: (a) as-sintered, exposure to water for 2 days (b) and 14 days (c), (d) re-annealed at 50 °C in the dielectric measurement.
the dielectric measurement, it still indicated that the water molecules could be absorbed on the samples and the interaction between water molecules and BST (80/20) would result in the increment of the tetragonal distortion c/a, as demonstrated in [3]. Considering the fact that some copolymer films showed a peak in capacitance on heating from 10 to 30 °C in ambient conditions [14–16] but no peak arose in vacuum [3], we can also reasonably propose that moisture effect should be responsible for the dielectric changes. Since water was accompanied by its own dipole moment, in heating run the polar water was absorbed on the BST (80/ 20) specimen at low temperature side and resulted in the increase of capacitance and loss. When experiencing the heat treatment at relatively high temperature, water would evaporate. In the cooling run, no increase of capacitance and loss originating from moisture absorption was observed, i.e., no abrupt change of dielectric properties appeared.
Actually, the observed broad peak of capacitance and loss of BST specimen in the heating run in ambient air without dryer could be a dynamic process involving water absorption and de-absorption. Below the dew point, the process was mainly dominated by water absorption. While higher than the dew point, the process preferred to be dominated by water de-absorption. Although bulk water showed relaxation behaviors in the same temperature and frequency range[17], the formed water film accompanying water absorption and de-absorption would exhibit a distinct frequency dependence just like that of phase transition [11,12], i.e., the lower frequency, the higher capacitance C and loss tg d. For the case of small temperature rate, much more water molecules were absorbed because they had enough time to condense onto the side surface of specimen, thus resulting in a much higher capacitance and loss peak, which were readily shown in Fig. 3. In addition, in different heating run with the same heating rate the value of C and tg d and the shape of the observed peak showed some difference, possibly related to the change of surround environment such as humidity and position of specimen in each measurement. Therefore it was suggested that the dynamic process of water absorption and de-absorption seemed to be complex and sometimes two loss peaks could be observed [18]. Moreover, water molecules could be repeatedly absorbed and removed from BST devices, as described in the dielectric results. A heat treatment at 50 °C would recover its performance even when the devices experienced the dew point. Therefore in application of the micro-electrics devices, heating in moderate temperature before use will be very necessary to reduce the water absorption effect, especially when they are used in subtropics.
4. Conclusions Dielectric response and structural change owing to water absorption were studied on ferroelectric BST (80/20) devices systematically. In the temperature range from 10 and 50 °C a broad capacitance and loss peak resulted from moisture absorption was observed in the heating run when measurement carried out in ambient air without dryer. The peak was strongly affected by the
890
H.Y. Wang et al. / Microelectronics Reliability 50 (2010) 887–890
measuring frequency, the temperature rate and the thermal history, and it could be a dynamic process of water absorption and de-absorption. It indicated that the dielectric properties of BST devices were closely related to the environmental humidity and the moisture effect was a complex and dynamics process. XRD results for the samples immersed into water for 2 and 14 days showed that the diffraction peak of h1 1 1i layer shifted to higher position due to water absorption and the tetragonal distortion c/a increased with the time immersing in water. For the BaTiO3-based devices, a heat treatment at 50 °C was suggested to reduce the moisture absorption effect and ensure the reliability of devices. Acknowledgment The authors gratefully acknowledge the financial support of the National Physics Base at Sun Yat-Sen University (J0630320 and J0730313). References [1] Shiro T. Dielectric properties of water of adsorption on protein crystals. J Polym Sci 1961;62:233–40. [2] Saito Y, Kataoka H, Quartarone E, Mustarelli P. Carrier migration mechanism of physically cross-linked polymer gel electrolytes basedon PVDF membranes. J Phys Chem B 2002;106:7200–4. [3] Jacobson PA, Rosa LG, Othon CM, Kraemer KL, Sorokin AV, Ducharme S, Dowben PA. Water absorption and dielectric changes in crystalline poly(vinylidene fluoride-trifluoroethylene) copolymer films. Appl Phys Lett 2004;84:88–90. [4] Frank B, Frubing P, Pissis P. Water sorption and thermally stimulated depolarization currents in nylon-6. J Polym Sci Part B: Polym Phys 1996;34:1853–60. [5] Laredo E, Hernandez MC. Moisture effect on the low- and high-temperature dielectric relaxations in nylon-6. J Polym Sci Part B: Polym Phys 1997;35:2879–88.
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