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A novel means of fabricating microporous structures for the dielectric layers of capacitive pressure sensor
Microelectronic Engineering 179 (2017) 60–66
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
A novel means of fabricating microporous structures for the dielectric layers of capacitive pressure sensor Joon Il Yoon, Kyo Sang Choi, Sung Pil Chang ⁎ Department of Electronic Engineering, Inha University, 100 Inha-ro, Nam-gu, Incheon 22212, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 February 2017 Received in revised form 4 April 2017 Accepted 24 April 2017 Available online 28 April 2017 Keywords: Flexible Capacitive pressure sensor PDMS Micro-pores Dielectric layer 3D printer
a b s t r a c t The authors developed a novel method for fabricating microporous dielectric layers on capacitive pressure sensors using a mixture of sugar and PDMS. In order to obtain different layer morphologies, a mold was used to control morphology during 3D printing. The microporous structures of the sensors produced showed enhanced deformability and large changes in dielectric constant. In addition, sensors showed high sensitivities from 0.00832 to 0.01097 kPa−1 at low pressure (~10 kPa), and from 0.06285 to 0.51285 kPa−1 at medium pressure (10–100 kPa). Maximum hysteresis was 6.49%, stabilities were 0.1726 at 1 kPa and 0.1809 at 50 kPa, and response time was 200 ms. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Flexible pressure sensors are attracting attention due to their wide potential applications, for example, for electronic skin (e-skin) systems [3–6], flexible touch screens [7,8], robotics [9–11], health care [13], and for diagnostic purposes [14,16], which require tactile interactions with devices. For practical purposes, pressure sensors should cover the pressure range 0–10 kPa to 10–100 kPa [14]. In particular, there is huge demand for more accurate, delicate pressure sensors and high sensitivity in the low-pressure region. Three types of tactile pressure sensing sensors are commonly available, that is, piezoresistive [17–20], piezoelectric [12,25,26], and capacitive sensors [21–24]. The capacitive-type has several advantages over the other two, such as, less susceptibility to noise, low sensitivity to temperature and humidity changes, and low power consumption [27,28]. For capacitive-type pressure sensors, sensitivity depends on the capacitance change induced by a given applied pressure. Furthermore, capacitance is proportional to the dielectric constant and the overlapping surface of two electrodes and is inversely proportional to the thickness of the dielectric layer. To change capacitance characteristics, the thickness of the dielectric layer is commonly changed because the dielectric constant and overlapping surface of electrodes are fixed variables in most cases. Recently, researchers have focused on the use of micro-
structured dielectric materials to improve the sensitivities of capacitive-type pressure sensor at low-pressure, and as a result, several different structural types of dielectric materials have been introduced, such as, micro-pyramids [1] and nano-needles [2]. However, although such systems have achieved high sensitivity at low-pressure, those described to date require complicated fabrication processes and operate effectively only in the low pressure range. In this paper, we describe a capacitive pressure sensor incorporating a microporous dielectric film as a dielectric layer that functions over the entire tactile pressure range (0–100 kPa) with high sensitivity and reliability. The microporous dielectric film was fabricated using a mixture of sugar and polydimethylsiloxane. In order to obtain diverse film morphologies, a mold was used to control morphology of dielectric layers using 3D printing. The microporous dielectric films produced showed large dielectric constant changes as compared with non-porous dielectric materials and were more flexible and more sensitive. To control numbers of micro-pores, the amount of sugar in polydimethylsiloxane/sugar mixes was changed. Under applied pressure, the micropores in the dielectric layer were closed and opened without evidence of viscoelastic behavior on applying pressure. Furthermore, the sensors produced had low hysteresis characteristics. 2. Fabrication 2.1. Fabrication of microporous dielectric films
⁎ Corresponding author. E-mail address: [email protected] (S.P. Chang).
http://dx.doi.org/10.1016/j.mee.2017.04.028 0167-9317/© 2017 Elsevier B.V. All rights reserved.
The fabrication process of the dielectric layer is shown schematically in Fig. 1. Briefly, prepolymer type of polydimethylsiloxane (PDMS;
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Fig. 1. Fabrication process of the microporous dielectric film.
Sylgard 184 Dow Corning Corp.) was mixed with sugar (Fig. 1a). PDMS was chosen for the main film component because of its low elastic modulus and high thermal stability, and the sugar was used to produce microporous structures in the PDMS. Different porosities were obtained by changing the volume ratio of sugar (vn) in PDMS solution (Table 1). In order to obtain desired film morphologies, we used a 3D printed mold [29]. The size and shape of the mold were determined by considering sensor characteristics, and molds were fabricated by 3D printing (FineBot×420, TPC) with a PLA filament as source. The mold for of the microporous dielectric film in this capacitive pressure sensor had a hollow rectangular shape of dimensions 10 mm × 10 mm × 8 mm. The mixture of PDMS solution and sugar was poured into the 3D printed mold (Fig. 1b), and mixture was cured for different times on a hotplate at 110 °C (Fig. 2). After curing the material was removed from the mold (Fig. 1c), and then dipped into water several times to completely dissolve the sugar and form pores (Fig. 1d). A schematic of the fabrication process is shown in Fig. 1e.
2.2. Fabrication of a capacitive pressure sensor using the microporous dielectric film The fabrication process is presented schematically in Fig. 3. To increase sensor flexibility, a polyethylene terephthalate (PET) was used as a substrate due to its high mechanical flexibility and low roughness [14]. The electrodes (dimension 150 nm × 10 × 10 mm2) on the inner sides of PET substrates were made of indium-tin-oxide (ITO) and the thickness of the PET substrate was 150 μm and then PDMS was then spun coated at a thickness of 1 μm on surface of ITO electrodes at the rate of 3000 rpm (Fig. 3a and c). After the microporous dielectric film was placed between the two PET substrates (Fig. 3b), the ensemble was cured in conventional oven for 30 min at 100 °C (Fig. 3d).
Table 1 Dielectric films produced using different volume ratios of sugar to PDMS. Sample vn
Volume of sugar in the mixture (ml)
Volume of PDMS in the mixture (ml)
Volume (3 ml) ratio between sugar and PDMS
Percentage of sugar in the mixture (%)
v0 v2 v4 v6 v8 v10
0 2 2.4 2.57 2.66 2.73
3 1 0.6 0.43 0.34 0.27
0:1 2:1 4:1 6:1 8:1 10:1
0 66.67 80 85.71 88.89 90.9
Fig. 2. Curing times of sugar/PDMS mixes with different sugar ratios.
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Fig. 3. Schematic diagram of the process used to fabricate a capacitive pressure sensor using the microporous dielectric film.
Fig. 4 showed a photomicrograph of a fabricated capacitive pressure sensor with the microporous dielectric film. 3. Results and discussion 3.1. Properties of the microporous dielectric films produced 3.1.1. Porosities of the microporous dielectric films The cross-sections of four different samples (v4, v6, v8, v10) as shown in Table 1 were characterized by High Resolution Scanning Electron Microscopy (HR-SEM; Su8010, Hitachi) as shown in Fig. 5. Porosities ‘Φ’were calculated from the densities of the microporous dielectric layer ρb and the solid dielectric layer ρt ,as follows [15]. Φ ¼ 1−
ρb ρt
ð1Þ
where ρb and ρt are densities of the films before and after sugar dissolution, respectively. The porosities of four different microporous films are shown in Table 2. Film porosities calculated using FE-SEM images were almost proportional to sugar/PDMS volume ratios. 3.1.2. Elastic moduli of microporous dielectric films A universal testing machine (UTM QRUTS-S105, QURO) with a 1 kN load cell was employed to measure the elastic moduli of microporous
dielectric films, and compression testing was performed to obtain stable responses at a constant velocity of 0.5 mm/s. Compressive stress-strain curves are shown in Fig. 6; gradients were used to determine elastic moduli (Table 3). The elastic modulus of a porous dielectric film is much lower than that of a non-porous film and the elastic modulus of the sample v10 is nearly eleven times lower than the sample v0, which means the sample v10 is much more ductile than sample v0. As the porosity of the samples increases the elastic modulus decreases as shown in Table 3. The v2 sample did not have a porous structure because the amount of sugar is too small to make porous structures. Above the sugar ratios of the sample v10, the dielectric films could not sustain the film structures with micro-pores. 3.2. Characterization of sensor response The test setup used to determine sensor response is shown in Fig. 7. Pressure was applied using a force gauge (DS20-500N, IMADA) and a motorized stand (MR-PP200, Mirae Science). A cylindrical non-conductive compressor of diameter 30 mm was used to produce uniform sensor deformation. The motorized stand stabilized the test assembly to allow stable responses to be obtained at a speed of 10 mm/min. The capacitance of each pressure sensor was measured using an LCR meter (4263B LCR meter, Agilent) at a frequency of 100 kHz. A computer was used to record sensor capacitances.
Fig. 4. Photomicrographs of a fabricated capacitive pressure sensor: (a) Top view of a fabricated sensor, (b) photographs demonstrating the flexibility of the sensor
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Table 3 The elastic moduli of samples.
Fig. 5. HR-SEM images of four different microporous dielectric films produced at sugar/ PDMS volume ratios of (a) v4, (b) v6, (c) v8, (d) v10.
3.3. Sensitivity
v0
v4
v6
v8
v10
5.10122 MPa
2.25962 MPa
1.33309 MPa
0.53199 MPa
0.45487 MPa
Sensitivities (S = δ(ΔC/C0)/δP) in the low pressure range from 0 to 10 kPa and in the medium pressure range from 10 to 100 kPa are presented in Table 4 for various values of vn. As shown in Table 4, higher dielectric porosity yielded higher sensitivity. For a pressure sensor v10, high sensitivity (0.51285 kPa− 1) was achieved in low-pressure regime, and this was nearly 8.15 times higher than that of sensor with a non-porous dielectric layer (v0). This highly sensitive pressure-sensing characteristic presumably resulted from two factors: substantial compressibility of the dielectric layer and replacement of air in pores by PDMS during compression. The volume fraction of micro-pores was enough to create marked deformation of the porous dielectric layer as compared with a non-porous dielectric layer, which resulted in a significant change in capacitance. The dielectric constant of air (εr ∼ 1) was replaced with that of PDMS (εr ∼ 2.5) during compression, and this increase in effective dielectric constant and narrowing of the distance between electrodes contributed
To examine sensitivities, changes of relative capacitances (ΔC/C0) of v0, v4, v6, v8, v10 sensors versus initial capacitance (C0) (under zero pressure) were plotted as a function of applied pressure ranging in the range 0 to 300 kPa (Fig. 8). The capacitance changes shown by pressure sensors with micropores were larger than those with no pores. Capacitances showed rapid initial increases and saturation at high-pressure, possibly because micro-pores in the dielectric material were closed by the applied pressure.
Table 2 Calculated porosities of microporous dielectric layer. Sample Volume ratio between vn sugar and PDMS v4 v6 v8 v10
4:1 6:1 8:1 10:1
Density after dissolution ρb (g/cm3)
Density before dissolution ρt (g/cm3)
Porosity (%)
0.34 0.25 0.20 0.15
1.66 1.54 1.49 1.41
79.5 83.8 86.4 89.3
Fig. 6. Compressive stress-strain curve of five different vn samples.
Fig. 7. Measurement setup for the capacitive pressure sensor with microporous dielectric films.
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Fig. 9. Schematic illustrations of the capacitive pressure sensor (a) without a porous dielectric layers, (b) with micro porous dielectric layers.
Fig. 8. Plots of relative capacitance versus applied pressure at five different vn values.
to capacitance changes. Fig. 9 illustrates schematically the operations of capacitive pressure sensors without or with micro pores. Unlike the non-porous structure shown in Fig. 9a, increased deformation and effective dielectric constant result in a large capacitance change for the capacitive pressure sensor with a porous dielectric layer (Fig. 9b). Furthermore, the sensitivities of the capacitive pressure sensors with porous dielectric layers in the medium pressure region were lower than those in low-pressure region. In the medium pressure region, most of the micro-pores may have been closed and the elastic resistance of the PDMS may have been increased. Thus, the increased elastic resistance of the PDMS and micro-pores closure probably caused the sensor to saturate at high applied pressures.
Table 4 The relationship between sensor sensitivity and vn.
Sensitivity low pressure 0–10 kPa (kPa−1) Sensitivity medium pressure 10–100 kPa (kPa−1)
v0
v4
v6
v8
v10
0.06285
0.42857
0.47142
0.49285
0.51285
0.00832
0.01063
0.01073
0.01119
0.01097 Fig. 10. Relative capacitance changes were measured for samples v0, v4, v6, v8, v10 by increasing or decreasing applied pressures.
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3.4. Hysteresis The hysteresis properties of five sensors samples (v0, v4, v6 v8, v10) were also evaluated. Relative capacitance changes were recorded on
Fig. 12. Dynamic responses of the v0, v4, v6, v8, samples.
increasing and decreasing applied pressure. The results obtained are shown in Fig. 10. Maximum hysteresis errors for samples v0, v4, v6, v8, v10 were 6.49%, 4.05%, 3.76%, 3.26% and 3.11%, respectively. As demonstrated in Fig. 9, sensors showed negligible hysteresis effect and excellent repeatability.
3.5. Sensor stabilities To investigate long-term stabilities, sensors were subjected to 100 compression/release cycles using a minimum pressure of 1 kPa and a maximum pressure of 50 kPa to check sensor reliability at low and medium pressures. The stabilities of the sensors containing different dielectric layers are shown in Fig. 11. Variations in relative capacitance ranged from 0.1726 to 0.2827 at 1 kPa and from 0.1809 to 0.8472 at 50 kPa, indicating no significant changes in capacitances and considerable sensor stabilities.
3.6. Dynamic responses The dynamic responses of v0, v4, v6, v8, v10 samples were also investigated (Fig. 12). Three different pressures (1, 10, and 100 kPa) were applied for 10 s to determine dynamic responses at low and medium pressures. Relative capacitance changes shown by sensors with microporous dielectric films were considerably larger than those of sensor with non-porous dielectric film (v0). However, responses time were similar, and the average response time over rising and falling times was ~200 ms. Table 5 provides a summary of the results obtained.
Table 5 Summary of the results obtained for the five samples.
Fig. 11. Stabilities of (a) v0 (b) v4, v8 (c) v6, v10 sensors subjected to 100 compression/ release cycles.
Sensitivity 0–10 kPa (kPa−1) Sensitivity 10–100 kPa (kPa−1) Hysteresis Stability (1 kPa) Stability (50 kPa) Response time
v0
v4
v6
v8
v10
0.06285
0.42857
0.47142
0.49285
0.51285
0.00832
0.01063
0.01073
0.01119
0.01097
6.49% 0.2827 0.2847 200 ms
4.05% 0.2527 0.2232 200 ms
3.76%, 0.2012 0.1994 200 ms
3.26% 0.1761 0.1704 200 ms
3.11% 0.1726 0.1809 200 ms
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4. Conclusion In the present study, a capacitive pressure sensor was developed using a microporous dielectric film. A 3D printed mold was used to obtain microporous dielectric films of required size and morphology and film porosity was easily controlled by adjusting the amount of sugar in sugar/PDMS mixtures. The responses of sensors containing five different microporous dielectric layers (v0, v4, v6, v8, v10) were characterized. As the porosity of microporous dielectric layers in the capacitive pressure sensors increases the sensitivities of the capacitive pressure sensors have been increased. And highest sensitivity (0.5612 kPa− 1) among the sensors with five different dielectric layers was obtained for the v10 sample due to the increased deformation and increased effective dielectric constant. Acknowledgments This work was supported by Inha University Research Grant No. 55869-01 (2017). References [1] G. Schwartz, B.C.-K. Tee, J. Mei, A.L. Appleton, D.H. Kim, H. Wang, et al., Flexible polymer transistors with high pressure sensitivity for application in electronic skin and health monitoring, Nat. Commun. 4 (2013) 1859. [2] J. Kim, T.N. Ng, W.S. Kim, Highly sensitive tactile sensors integrated with organic transistors, Appl. Phys. Lett. 101 (2012) 103308. [3] S.C.B. Mannsfeld, B.C.-K. Tee, R.M. Stoltenberg, C.V.H.-H. Chen, S. Barman, B.V.O. Muir, et al., Highly sensitive flexible pressure sensors withmicrostructured rubber dielectric layers, Nat. Mater. 9 (2010) 859–864. [4] S. Yao, Y. Zhu, Wearable multifunctional sensors using printed stretchable conductors made of silver nanowires, Nano 6 (2014) 2345–2352. [5] M.L. Hammock, A. Chortos, B.C.K. Tee, J.B.H. Tok, Z. Bao, 25th anniversary article: the evolution of electronic skin (ESkin): a brief history, design considerations, and recent progress, Adv. Mater. 25 (2013) 5997–6038. [6] X. Wang, L. Dong, H. Zhang, R. Yu, C. Pan, Z.L. Wang, Recent progress in electronic skin, Adv. Sci. 2 (2015) 1500169. [7] D.J. Lipomi, M. Vosgueritchian, B.C. Tee, S.L. Hellstrom, J.A. Lee, C.H. Fox, Z. Bao, Skinlike pressure and strain sensors based on transparent elastic films of carbon nanotubes, Nat. Nanotechnol. 6 (2011) 788–792. [8] F.R. Fan, L. Lin, G. Zhu, W. Wu, R. Zhang, Z.L. Wang, Transparent triboelectric nanogenerators and self-powered pressure sensors based on micropatterned plastic films, Nano Lett. 12 (2012) 3109–3114. [9] D. Trivedi, C.D. Rahn, W.M. Kier, I.D. Walker, Soft robotics: biological inspiration, state of the art, and future research, Appl. Bion. Biomech. 5 (2008) 99–117. [10] R. Pfeifer, M. Lungarella, F. Iida, The challenges ahead for bio-inspired ‘soft’ robotics, Commun. ACM 55 (2012) 76–87 (ACS Applied Materials & Interfaces).
[11] S. Bauer, S. Bauer-Gogonea, I. Graz, M. Kaltenbrunner, C. Keplinger, R. Schwodiauer, 25th anniversary article: a soft future: from robots and sensor skin to energy harvesters, Adv. Mater. 26 (2014) 149–161. [12] J. Kim, M. Lee, H.J. Shim, R. Ghaffari, H.R. Cho, D. Son, Y.H. Jung, M. Soh, C. Choi, S. Jung, K. Chu, D. Jeon, S.T. Lee, J.H. Kim, S.H. Choi, T. Hyeon, D.H. Kim, Stretchable silicon nanoribbon electronics for skin prosthesis, Nat. Commun. 5 (2014) 5747. [13] C.M. Boutry, A. Nguyen, Q.O. Lawal, A. Chortos, S. Rondeau-Gagne, Z. Bao, A sensitive and biodegradable pressure sensor array for cardiovascular monitoring, Adv. Mater. 27 (2015) 6954–6961. [14] Y. Zang, F. Zhang, C.-a. Di, D. Zhu, Advances of flexible pressure sensors toward artificial intelligence and health care applications, Mater. Horiz. 2 (2015) 140–156. [15] K. Yoshimur, K. Nakano, Y. Hishikawa, Flexible tactile sensor materials based on carbon microcoil/silicone rubber porous composites, Compos. Sci. Technol. 123 (2016) 241–249. [16] C. Dagdeviren, Y. Su, P. Joe, R. Yona, Y. Liu, Y.-S. Kim, et al., Conformable amplified lead zirconate titanate sensors with enhanced piezoelectric response for cutaneous pressure monitoring, Nat. Commun. 5 (2014) 4496. [17] S. Jung, J.H. Kim, J. Kim, S. Choi, J. Lee, I. Park, et al., Reverse-micelle-induced porous pressure-sensitive rubber for wearable human–machine interfaces, Adv. Mater. 26 (2014) 4825–4830. [18] S. Chun, Y. Kim, H. Jung, W. Park, A flexible graphene touch sensor in the general human touch range, Appl. Phys. Lett. 105 (2014) 041907. [19] C.-L. Choong, M.-B. Shim, B.-S. Lee, S. Jeon, D.-S. Ko, T.-H. Kang, et al., Highly stretchable resistive pressure sensors using a conductive elastomeric composite on a micropyramid array, Adv. Mater. 26 (2014) 3451–3458. [20] B.R. Burg, T. Helbling, C. Hierold, D. Poulikakos, Piezoresistive pressure sensors with parallel integration of individual single-walled carbon nanotubes, J. Appl. Phys. 109 (2011) 064310. [21] H.-K. Lee, S.-I. Chang, E. Yoon, A flexible polymer tactile sensor: fabrication and modular expandability for large area deployment, J. Microelectromech. Syst. 15 (2006) 1681–1686. [22] K.F. Lei, K.-F. Lee, M.-Y. Lee, A flexible PDMS capacitive tactile sensor with adjustable measurement range for plantar pressure measurement, Microsyst. Technol. 20 (2014) 1351–1358. [23] K.F. Lei, K.-F. Lee, M.-Y. Lee, Development of a flexible PDMS capacitive pressure sensor for plantar pressure measurement, Microelectron. Eng. 99 (2012) 1–5. [24] J.A. Dobrzynska, M.A.M. Gijs, Polymer-based flexible capacitive sensor for threeaxial force measurements, J. Micromech. Microeng. 23 (2013) 015009. [25] C. Dagdeviren, Y. Su, P. Joe, R. Yona, Y. Liu, Y.-S. Kim, Y. Huang, A.R. Damadoran, J. Xia, L.W. Martin, Y. Huang, J.A. Rogers, Conformable Amplified Lead Zirconate Titanate Sensors with Enhanced Piezoelectric Response for Cutaneous Pressure Monitoring, Nat. Commun. (2014) 5–4496. [26] C. Pan, L. Dong, G. Zhu, S. Niu, R. Yu, Q. Yang, Y. Liu, Z.L. Wang, High-resolution electroluminescent imaging of pressure distribution using a piezoelectric nanowire LED Array, Nat. Photonics 7 (2013) 752–758. [27] S.-P. Chang, M.G. Allen, Demonstration for integrating capacitive pressure sensors with read-out circuitry on stainless steel substrate, Sensors Actuators A Phys. 116 (2004) 195–204. [28] F. He, Q.-A. Huang, M. Qin, A silicon directly bonded capacitive absolute pressure sensor, Sensors Actuators A Phys. 135 (2007) 507–514. [29] X. Wang, T. Li, J. Adams, J. Yang, Transparent stretchable, carbon-nanotube-inlaid conductors enabled by standard replication technology for capacitive pressure, strain and touch sensors, J. Mater. Chem. A1 (2013) 3580–3586.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
A novel means of fabricating microporous structures for the dielectric layers of capacitive pressure sensor Joon Il Yoon, Kyo Sang Choi, Sung Pil Chang ⁎ Department of Electronic Engineering, Inha University, 100 Inha-ro, Nam-gu, Incheon 22212, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 February 2017 Received in revised form 4 April 2017 Accepted 24 April 2017 Available online 28 April 2017 Keywords: Flexible Capacitive pressure sensor PDMS Micro-pores Dielectric layer 3D printer
a b s t r a c t The authors developed a novel method for fabricating microporous dielectric layers on capacitive pressure sensors using a mixture of sugar and PDMS. In order to obtain different layer morphologies, a mold was used to control morphology during 3D printing. The microporous structures of the sensors produced showed enhanced deformability and large changes in dielectric constant. In addition, sensors showed high sensitivities from 0.00832 to 0.01097 kPa−1 at low pressure (~10 kPa), and from 0.06285 to 0.51285 kPa−1 at medium pressure (10–100 kPa). Maximum hysteresis was 6.49%, stabilities were 0.1726 at 1 kPa and 0.1809 at 50 kPa, and response time was 200 ms. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Flexible pressure sensors are attracting attention due to their wide potential applications, for example, for electronic skin (e-skin) systems [3–6], flexible touch screens [7,8], robotics [9–11], health care [13], and for diagnostic purposes [14,16], which require tactile interactions with devices. For practical purposes, pressure sensors should cover the pressure range 0–10 kPa to 10–100 kPa [14]. In particular, there is huge demand for more accurate, delicate pressure sensors and high sensitivity in the low-pressure region. Three types of tactile pressure sensing sensors are commonly available, that is, piezoresistive [17–20], piezoelectric [12,25,26], and capacitive sensors [21–24]. The capacitive-type has several advantages over the other two, such as, less susceptibility to noise, low sensitivity to temperature and humidity changes, and low power consumption [27,28]. For capacitive-type pressure sensors, sensitivity depends on the capacitance change induced by a given applied pressure. Furthermore, capacitance is proportional to the dielectric constant and the overlapping surface of two electrodes and is inversely proportional to the thickness of the dielectric layer. To change capacitance characteristics, the thickness of the dielectric layer is commonly changed because the dielectric constant and overlapping surface of electrodes are fixed variables in most cases. Recently, researchers have focused on the use of micro-
structured dielectric materials to improve the sensitivities of capacitive-type pressure sensor at low-pressure, and as a result, several different structural types of dielectric materials have been introduced, such as, micro-pyramids [1] and nano-needles [2]. However, although such systems have achieved high sensitivity at low-pressure, those described to date require complicated fabrication processes and operate effectively only in the low pressure range. In this paper, we describe a capacitive pressure sensor incorporating a microporous dielectric film as a dielectric layer that functions over the entire tactile pressure range (0–100 kPa) with high sensitivity and reliability. The microporous dielectric film was fabricated using a mixture of sugar and polydimethylsiloxane. In order to obtain diverse film morphologies, a mold was used to control morphology of dielectric layers using 3D printing. The microporous dielectric films produced showed large dielectric constant changes as compared with non-porous dielectric materials and were more flexible and more sensitive. To control numbers of micro-pores, the amount of sugar in polydimethylsiloxane/sugar mixes was changed. Under applied pressure, the micropores in the dielectric layer were closed and opened without evidence of viscoelastic behavior on applying pressure. Furthermore, the sensors produced had low hysteresis characteristics. 2. Fabrication 2.1. Fabrication of microporous dielectric films
⁎ Corresponding author. E-mail address: [email protected] (S.P. Chang).
http://dx.doi.org/10.1016/j.mee.2017.04.028 0167-9317/© 2017 Elsevier B.V. All rights reserved.
The fabrication process of the dielectric layer is shown schematically in Fig. 1. Briefly, prepolymer type of polydimethylsiloxane (PDMS;
J.I. Yoon et al. / Microelectronic Engineering 179 (2017) 60–66
61
Fig. 1. Fabrication process of the microporous dielectric film.
Sylgard 184 Dow Corning Corp.) was mixed with sugar (Fig. 1a). PDMS was chosen for the main film component because of its low elastic modulus and high thermal stability, and the sugar was used to produce microporous structures in the PDMS. Different porosities were obtained by changing the volume ratio of sugar (vn) in PDMS solution (Table 1). In order to obtain desired film morphologies, we used a 3D printed mold [29]. The size and shape of the mold were determined by considering sensor characteristics, and molds were fabricated by 3D printing (FineBot×420, TPC) with a PLA filament as source. The mold for of the microporous dielectric film in this capacitive pressure sensor had a hollow rectangular shape of dimensions 10 mm × 10 mm × 8 mm. The mixture of PDMS solution and sugar was poured into the 3D printed mold (Fig. 1b), and mixture was cured for different times on a hotplate at 110 °C (Fig. 2). After curing the material was removed from the mold (Fig. 1c), and then dipped into water several times to completely dissolve the sugar and form pores (Fig. 1d). A schematic of the fabrication process is shown in Fig. 1e.
2.2. Fabrication of a capacitive pressure sensor using the microporous dielectric film The fabrication process is presented schematically in Fig. 3. To increase sensor flexibility, a polyethylene terephthalate (PET) was used as a substrate due to its high mechanical flexibility and low roughness [14]. The electrodes (dimension 150 nm × 10 × 10 mm2) on the inner sides of PET substrates were made of indium-tin-oxide (ITO) and the thickness of the PET substrate was 150 μm and then PDMS was then spun coated at a thickness of 1 μm on surface of ITO electrodes at the rate of 3000 rpm (Fig. 3a and c). After the microporous dielectric film was placed between the two PET substrates (Fig. 3b), the ensemble was cured in conventional oven for 30 min at 100 °C (Fig. 3d).
Table 1 Dielectric films produced using different volume ratios of sugar to PDMS. Sample vn
Volume of sugar in the mixture (ml)
Volume of PDMS in the mixture (ml)
Volume (3 ml) ratio between sugar and PDMS
Percentage of sugar in the mixture (%)
v0 v2 v4 v6 v8 v10
0 2 2.4 2.57 2.66 2.73
3 1 0.6 0.43 0.34 0.27
0:1 2:1 4:1 6:1 8:1 10:1
0 66.67 80 85.71 88.89 90.9
Fig. 2. Curing times of sugar/PDMS mixes with different sugar ratios.
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Fig. 3. Schematic diagram of the process used to fabricate a capacitive pressure sensor using the microporous dielectric film.
Fig. 4 showed a photomicrograph of a fabricated capacitive pressure sensor with the microporous dielectric film. 3. Results and discussion 3.1. Properties of the microporous dielectric films produced 3.1.1. Porosities of the microporous dielectric films The cross-sections of four different samples (v4, v6, v8, v10) as shown in Table 1 were characterized by High Resolution Scanning Electron Microscopy (HR-SEM; Su8010, Hitachi) as shown in Fig. 5. Porosities ‘Φ’were calculated from the densities of the microporous dielectric layer ρb and the solid dielectric layer ρt ,as follows [15]. Φ ¼ 1−
ρb ρt
ð1Þ
where ρb and ρt are densities of the films before and after sugar dissolution, respectively. The porosities of four different microporous films are shown in Table 2. Film porosities calculated using FE-SEM images were almost proportional to sugar/PDMS volume ratios. 3.1.2. Elastic moduli of microporous dielectric films A universal testing machine (UTM QRUTS-S105, QURO) with a 1 kN load cell was employed to measure the elastic moduli of microporous
dielectric films, and compression testing was performed to obtain stable responses at a constant velocity of 0.5 mm/s. Compressive stress-strain curves are shown in Fig. 6; gradients were used to determine elastic moduli (Table 3). The elastic modulus of a porous dielectric film is much lower than that of a non-porous film and the elastic modulus of the sample v10 is nearly eleven times lower than the sample v0, which means the sample v10 is much more ductile than sample v0. As the porosity of the samples increases the elastic modulus decreases as shown in Table 3. The v2 sample did not have a porous structure because the amount of sugar is too small to make porous structures. Above the sugar ratios of the sample v10, the dielectric films could not sustain the film structures with micro-pores. 3.2. Characterization of sensor response The test setup used to determine sensor response is shown in Fig. 7. Pressure was applied using a force gauge (DS20-500N, IMADA) and a motorized stand (MR-PP200, Mirae Science). A cylindrical non-conductive compressor of diameter 30 mm was used to produce uniform sensor deformation. The motorized stand stabilized the test assembly to allow stable responses to be obtained at a speed of 10 mm/min. The capacitance of each pressure sensor was measured using an LCR meter (4263B LCR meter, Agilent) at a frequency of 100 kHz. A computer was used to record sensor capacitances.
Fig. 4. Photomicrographs of a fabricated capacitive pressure sensor: (a) Top view of a fabricated sensor, (b) photographs demonstrating the flexibility of the sensor
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Table 3 The elastic moduli of samples.
Fig. 5. HR-SEM images of four different microporous dielectric films produced at sugar/ PDMS volume ratios of (a) v4, (b) v6, (c) v8, (d) v10.
3.3. Sensitivity
v0
v4
v6
v8
v10
5.10122 MPa
2.25962 MPa
1.33309 MPa
0.53199 MPa
0.45487 MPa
Sensitivities (S = δ(ΔC/C0)/δP) in the low pressure range from 0 to 10 kPa and in the medium pressure range from 10 to 100 kPa are presented in Table 4 for various values of vn. As shown in Table 4, higher dielectric porosity yielded higher sensitivity. For a pressure sensor v10, high sensitivity (0.51285 kPa− 1) was achieved in low-pressure regime, and this was nearly 8.15 times higher than that of sensor with a non-porous dielectric layer (v0). This highly sensitive pressure-sensing characteristic presumably resulted from two factors: substantial compressibility of the dielectric layer and replacement of air in pores by PDMS during compression. The volume fraction of micro-pores was enough to create marked deformation of the porous dielectric layer as compared with a non-porous dielectric layer, which resulted in a significant change in capacitance. The dielectric constant of air (εr ∼ 1) was replaced with that of PDMS (εr ∼ 2.5) during compression, and this increase in effective dielectric constant and narrowing of the distance between electrodes contributed
To examine sensitivities, changes of relative capacitances (ΔC/C0) of v0, v4, v6, v8, v10 sensors versus initial capacitance (C0) (under zero pressure) were plotted as a function of applied pressure ranging in the range 0 to 300 kPa (Fig. 8). The capacitance changes shown by pressure sensors with micropores were larger than those with no pores. Capacitances showed rapid initial increases and saturation at high-pressure, possibly because micro-pores in the dielectric material were closed by the applied pressure.
Table 2 Calculated porosities of microporous dielectric layer. Sample Volume ratio between vn sugar and PDMS v4 v6 v8 v10
4:1 6:1 8:1 10:1
Density after dissolution ρb (g/cm3)
Density before dissolution ρt (g/cm3)
Porosity (%)
0.34 0.25 0.20 0.15
1.66 1.54 1.49 1.41
79.5 83.8 86.4 89.3
Fig. 6. Compressive stress-strain curve of five different vn samples.
Fig. 7. Measurement setup for the capacitive pressure sensor with microporous dielectric films.
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Fig. 9. Schematic illustrations of the capacitive pressure sensor (a) without a porous dielectric layers, (b) with micro porous dielectric layers.
Fig. 8. Plots of relative capacitance versus applied pressure at five different vn values.
to capacitance changes. Fig. 9 illustrates schematically the operations of capacitive pressure sensors without or with micro pores. Unlike the non-porous structure shown in Fig. 9a, increased deformation and effective dielectric constant result in a large capacitance change for the capacitive pressure sensor with a porous dielectric layer (Fig. 9b). Furthermore, the sensitivities of the capacitive pressure sensors with porous dielectric layers in the medium pressure region were lower than those in low-pressure region. In the medium pressure region, most of the micro-pores may have been closed and the elastic resistance of the PDMS may have been increased. Thus, the increased elastic resistance of the PDMS and micro-pores closure probably caused the sensor to saturate at high applied pressures.
Table 4 The relationship between sensor sensitivity and vn.
Sensitivity low pressure 0–10 kPa (kPa−1) Sensitivity medium pressure 10–100 kPa (kPa−1)
v0
v4
v6
v8
v10
0.06285
0.42857
0.47142
0.49285
0.51285
0.00832
0.01063
0.01073
0.01119
0.01097 Fig. 10. Relative capacitance changes were measured for samples v0, v4, v6, v8, v10 by increasing or decreasing applied pressures.
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3.4. Hysteresis The hysteresis properties of five sensors samples (v0, v4, v6 v8, v10) were also evaluated. Relative capacitance changes were recorded on
Fig. 12. Dynamic responses of the v0, v4, v6, v8, samples.
increasing and decreasing applied pressure. The results obtained are shown in Fig. 10. Maximum hysteresis errors for samples v0, v4, v6, v8, v10 were 6.49%, 4.05%, 3.76%, 3.26% and 3.11%, respectively. As demonstrated in Fig. 9, sensors showed negligible hysteresis effect and excellent repeatability.
3.5. Sensor stabilities To investigate long-term stabilities, sensors were subjected to 100 compression/release cycles using a minimum pressure of 1 kPa and a maximum pressure of 50 kPa to check sensor reliability at low and medium pressures. The stabilities of the sensors containing different dielectric layers are shown in Fig. 11. Variations in relative capacitance ranged from 0.1726 to 0.2827 at 1 kPa and from 0.1809 to 0.8472 at 50 kPa, indicating no significant changes in capacitances and considerable sensor stabilities.
3.6. Dynamic responses The dynamic responses of v0, v4, v6, v8, v10 samples were also investigated (Fig. 12). Three different pressures (1, 10, and 100 kPa) were applied for 10 s to determine dynamic responses at low and medium pressures. Relative capacitance changes shown by sensors with microporous dielectric films were considerably larger than those of sensor with non-porous dielectric film (v0). However, responses time were similar, and the average response time over rising and falling times was ~200 ms. Table 5 provides a summary of the results obtained.
Table 5 Summary of the results obtained for the five samples.
Fig. 11. Stabilities of (a) v0 (b) v4, v8 (c) v6, v10 sensors subjected to 100 compression/ release cycles.
Sensitivity 0–10 kPa (kPa−1) Sensitivity 10–100 kPa (kPa−1) Hysteresis Stability (1 kPa) Stability (50 kPa) Response time
v0
v4
v6
v8
v10
0.06285
0.42857
0.47142
0.49285
0.51285
0.00832
0.01063
0.01073
0.01119
0.01097
6.49% 0.2827 0.2847 200 ms
4.05% 0.2527 0.2232 200 ms
3.76%, 0.2012 0.1994 200 ms
3.26% 0.1761 0.1704 200 ms
3.11% 0.1726 0.1809 200 ms
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4. Conclusion In the present study, a capacitive pressure sensor was developed using a microporous dielectric film. A 3D printed mold was used to obtain microporous dielectric films of required size and morphology and film porosity was easily controlled by adjusting the amount of sugar in sugar/PDMS mixtures. The responses of sensors containing five different microporous dielectric layers (v0, v4, v6, v8, v10) were characterized. As the porosity of microporous dielectric layers in the capacitive pressure sensors increases the sensitivities of the capacitive pressure sensors have been increased. And highest sensitivity (0.5612 kPa− 1) among the sensors with five different dielectric layers was obtained for the v10 sample due to the increased deformation and increased effective dielectric constant. Acknowledgments This work was supported by Inha University Research Grant No. 55869-01 (2017). References [1] G. Schwartz, B.C.-K. Tee, J. Mei, A.L. Appleton, D.H. Kim, H. Wang, et al., Flexible polymer transistors with high pressure sensitivity for application in electronic skin and health monitoring, Nat. Commun. 4 (2013) 1859. [2] J. Kim, T.N. Ng, W.S. Kim, Highly sensitive tactile sensors integrated with organic transistors, Appl. Phys. Lett. 101 (2012) 103308. [3] S.C.B. Mannsfeld, B.C.-K. Tee, R.M. Stoltenberg, C.V.H.-H. Chen, S. Barman, B.V.O. Muir, et al., Highly sensitive flexible pressure sensors withmicrostructured rubber dielectric layers, Nat. Mater. 9 (2010) 859–864. [4] S. Yao, Y. Zhu, Wearable multifunctional sensors using printed stretchable conductors made of silver nanowires, Nano 6 (2014) 2345–2352. [5] M.L. Hammock, A. Chortos, B.C.K. Tee, J.B.H. Tok, Z. Bao, 25th anniversary article: the evolution of electronic skin (ESkin): a brief history, design considerations, and recent progress, Adv. Mater. 25 (2013) 5997–6038. [6] X. Wang, L. Dong, H. Zhang, R. Yu, C. Pan, Z.L. Wang, Recent progress in electronic skin, Adv. Sci. 2 (2015) 1500169. [7] D.J. Lipomi, M. Vosgueritchian, B.C. Tee, S.L. Hellstrom, J.A. Lee, C.H. Fox, Z. Bao, Skinlike pressure and strain sensors based on transparent elastic films of carbon nanotubes, Nat. Nanotechnol. 6 (2011) 788–792. [8] F.R. Fan, L. Lin, G. Zhu, W. Wu, R. Zhang, Z.L. Wang, Transparent triboelectric nanogenerators and self-powered pressure sensors based on micropatterned plastic films, Nano Lett. 12 (2012) 3109–3114. [9] D. Trivedi, C.D. Rahn, W.M. Kier, I.D. Walker, Soft robotics: biological inspiration, state of the art, and future research, Appl. Bion. Biomech. 5 (2008) 99–117. [10] R. Pfeifer, M. Lungarella, F. Iida, The challenges ahead for bio-inspired ‘soft’ robotics, Commun. ACM 55 (2012) 76–87 (ACS Applied Materials & Interfaces).
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