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ABSTRACT. In this work we report on a self-standing parallel- plate capacitive humidity sensor on a thin dry photoresist film with integrated resistive temperature ...

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SELF-STANDING PRINTED HUMIDITY SENSOR WITH THERMO-CALIBRATION AND INTEGRATED HEATER A. Vásquez Quintero*, F. Molina-Lopez, G. Mattana, D. Briand and N.F. de Rooij Ecole Polytechnique Fédérale de Lausanne (EPFL), STI IMT SAMLAB, Neuchâtel SWITZERLAND ABSTRACT

DESIGN AND FABRICATION

In this work we report on a self-standing parallelplate capacitive humidity sensor on a thin dry photoresist film with integrated resistive temperature sensor and heater on each electrode, respectively. The simple concept proposes dry photoresist film as substrate but also as dielectric humidity sensing layer. The characteristics of the temperature sensor (TCR ~900 ppmK-1), resistive heater (0.65 °C/mW) and humidity sensor (response time ~230 seconds, sensitivity ~23 fF / 1%R.H) were measured and are reported. FEM simulations were used to visualize the temperature distribution generated by the integrated heater.

The printed capacitive humidity sensor was designed using the parallel-plates (PP) configuration [7]. Top and bottom electrodes were designed and printed to be used as a resistive temperature sensor and resistive heater, respectively. The complete structure is depicted in Figure 1, highlighting in black the top electrode as the meandershaped resistive temperature sensor and in dark-gray the bottom electrode as the resistive heater with the same shape. A self-standing layer of dry photoresist film PerMX3014® (14 µm-thick) from DuPont® was used as dielectric and as humidity sensing layer. The top and bottom electrodes were inkjet-printed (Dimatx DMP2800™ printer) directly on the PerMX3014 surface using the silver-based nanoparticles ink SunTronics Jet EMD506 from SunChemical (20% of solid content).

KEYWORDS Capacitive humidity sensor, temperature sensor, integrated heater, dry photoresist film, inkjet printing.

Top electrode / Temp. sensor

INTRODUCTION Advances in printed humidity sensors on polymeric substrates are being recently presented in literature to achieve large-area manufacturing, light-weight, mechanical conformability and potentially low-cost devices [1-3]. Capacitive humidity sensors transduce the moisture-induced dielectric changes in the sensing layer to changes in capacitance electrically detected. However, previous designs are generally based on in-plane electrodes and relatively thick polymeric substrates in contact with the sensing layer, which reduces the sensor response time due to an increment of the gas diffusion path [4]. We propose here to eliminate the substrate influence by designing a thin self-standing membrane acting as both substrate and sensing layer in-between outof-plane electrodes. The dry photoresist film is considered as base material due to its intrinsic relatively good moisture absorption, high surface homogeneity, patternability, laminability and relatively small thickness. The photolithography 3D-patterning of the film around the electrodes and the possibility of self-standing configurations enhance the moisture diffusion by increasing the area exposed to the environment, potentially leading to a quicker response [5, 6]. Additionally, this work proves the inkjet printability of silver-based nanoparticles ink on both sides of selfstanding dry photoresist film. The integrated heater could reduce the dry photoresist film partition coefficient when heated up, leading to a more linear response, reduced hysteresis effects and sensor response, and reset the sensing layer by desorption of residual trapped moisture [6]. The sensor design presented here enables a simple fabrication of relatively fast printed and polymeric humidity sensors with integrated thermal functionalities in a single structure.

978-1-4673-5983-2/13/$31.00 ©2013 IEEE

Bottom electrode / Heater

Figure 1: Design of the parallel-plate humidity sensor, showing the temperature resistive sensor (top electrode in black) and the resistive heater (bottom electrode in gray). In order to obtain the self-standing device on PerMX3014 the following process was performed: after the bottom protection sheet was peeled-off from the dry photoresist stack, the latter was laminated (at 85 °C) to a 125 µm-thick polyethylene terephthalate (PET – Melinex® ST506 from DuPont) frame, as shown in Figure 2a. After the post-lamination bake (5 minutes at 85 °C) the laminated stack was patterned using standard photolithography by exposing it to ultra-violet (UV) light (200 mJ.cm-2) using a transparency mask with the desired meander-like design. After the post-exposure bake (10 minutes at 60 °C) the exposed stack was immersed in a pure bath of PGMEA (Propylene glycol methyl ether acetate) for 2 minutes. The unexposed area was etched away due to the negative-tone nature of PerMX3014, as shown in Figure 2b. Finally, the patterned stack was cleaned with deionized (DI) water, dried with nitrogen and hard-baked in a convection oven at 120 °C for 1 hour. Since the PerMX3014 becomes flexible after the hardbake [8] it is possible to remove its top protection sheet

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Transducers 2013, Barcelona, SPAIN, 16-20 June 2013

(which was kept during whole process) preserving the integrity of the film. After subsequent oxygen plasma treatments on each PerMX3014 surface, the printing process was performed in two phases. First, the top electrode was inkjet-printed through the PET frame, validating the use of the non-contact technique (Figure 2c). Second, the bottom electrode was inkjet-printed on the opposite surface, as shown in Figure 2d. Finally, both silver-based nano-particles electrodes were simultaneously sintered to compensate thermal mismatch stresses, in a convection oven (2 hours at 130 °C). The complete stacked device was attached to a rigid printed circuit board (PCB) for testing purposes.

b)

The commercially available software COMSOL Multiphysics 4.3a was used for the thermal finite element modeling (FEM) of the device presented in this work. The model was performed in the 3D space dimension and it combines the heat transfer in solids and joule heating, in order to simulate the temperature distribution generated by the heater. The convection coefficient was found as follows: first a curve of resistance (from the temperature sensor) versus power applied to the heater was measured; next, at each applied power the temperature was calculated using the calibrated temperature sensor (using its temperature coefficient of resistance); finally, by sweeping the convection coefficient in COMSOL, the simulated temperatures at the top were compared to the measurements until a value of 70 W/(m2.K) was found to give a good fitting between them. Since dry film photoresist is an epoxy-based material very similar chemically and mechanically to SU-8, its thermal conductivity (0.2 W/(m.K)) was used as property in the FEM simulations [9]. Figure 4a shows the temperature gradient on the sensor area when 80 mW are applied to the heater (bottom electrode). At this power, it was found that the temperature generated by the heater is around 93 °C and concentrated at the center of the design. Figure 4b shows the temperature profile at the temperature sensor versus the power applied. The temperature simulations are useful to visualize the temperature distribution through the dry film thickness and along the electrodes. This model could be used to optimize the heater design to improve the temperature distribution which in turn would improve desorption.

PET 175 µm Dry film 14 µm

Lamination

a)

FINITE ELEMENT MODELING

Patterning 180 µm 420 µm

Inkjet c) t ≈300 nm

Inkjet

ρ=50μΩ.cm t≈300 nm

d)

Temp. sensor Heater

Figure 2: Process flow of the capacitive humidity sensor, a) Lamination of PerMX3014 on PET frame, b) Photolithography patterning of PerMX3014, c) Inkjet printing of top electrode (temperature sensor), d) Inkjet printing of bottom electrode (resistive heater). Figure 3 presents different viewing angles of the processed devices. Figure 3a shows the patterned PerMX3014, while Figure 3b shows the printed device after sintering of the metal ink, showing mechanical deformation due to thermal mismatch stresses. Figure 3c shows a close-up view of the printed top electrode. Finally, Figure 3d emphasized the flexibility nature of the self-standing device attached to the PET frame.

a)

a)

b)

b) 1 mm

c)

2.3 mW 9.1 mW 20.4 mW 36 mW 55.5 mW 79.08 mW 107.3 mW 139.7 mW

1 mm

d)

200 µm

5 mm

Figure 3: a), b) Top view of the patterned dry foil and printed humidity sensor, respectively; c) Close-up on the temperature sensor; d) Picture of the dry photoresistbased humidity sensor.

Figure 4: a) Temperature gradient of printed device FEM model after applying (80 mW) to the heater; b) Temperature at the top electrode versus applied power.

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ENVIRONMENTAL CHARACTERIZATION

Upsweep -1 TCR = 0.000918 K 2 R = 0.99981

Normalized resistance (%)

8

The thermal coefficient of resistance (TCR) of the temperature sensor was measured between -20 °C and 80 °C in order to calibrate the sensor and characterize its linearity. The environment was controlled in the climatic chamber SH-661 from Espec at 40% R.H. The resistance values were taken from 4-probes measurements using an Agilent 34411A multi-meter. Additionally, the measurement was performed for the up- and down-sweep mode, in order to characterize its hysteresis. The performance of the resistive heater was characterized by measuring the resistance variation of the temperature sensor while varying the power applied to its terminals and calculating the corresponding temperature with the calibrated TCR information previously measured. The relative changes in capacitance, induced by changes in relative humidity (R.H.), were measured between 40% and 80% of R.H. The capacitance values were recorded using an LCR meter (Agilent E4980A) at 100k Hz and compared to the readout of a commercial sensor. Additionally, the temperature of the chamber was modified from 15 °C to 45 °C in order to decrease the partition coefficient of the substrate and increase the operation range and linearity of the humidity sensor. The dynamic response to humidity, such as response time and stabilization, was measured at a custom-made setup using a relatively small chamber and well controlled R.H. levels. The tests were performed at 10%, 30%, 50% and 70% R.H. (15 minutes each step at room temperature) for three consecutive complete cycles. Finally, the functionality of the integrated resistive heater was proven using the dual measuring device (temperature and humidity). While maintaining constant environment conditions (23 °C / 50% R.H.) the heater was turned-on for fix duration (240 seconds) and different applied powers (from 9.2 mW to 36 mW). The jouleheated temperature generated was estimated with the TCR calibration of the temperature sensor, while the capacitance was following the changes in the dielectric constant of the dry film.

6 4 2

Downsweep -1 TCR = 0.000915 K 2 R = 0.99963

0 -2 -20

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Figure 5: Up-, down-sweep TCR characterization of the temperature sensor at 40% R.H. Normalized resistance Linear fit Temperature

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80 70 60

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Sensitivity = 0.65 °C/mW

0 -10

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Figure 6: Normalized resistance of temperature sensor and estimated temp. versus applied power at the heater. Figure 7 shows the humidity sensor curves (nominal capacitance ~16 pF at 25%.R.H.) for different R.H. levels and at different temperatures, comparing them to a commercial sensor. It is noted that the capacitance increases with the temperature allowing a more stable measurement and faster response time at higher R.H. values. The sensor behavior could be improved by increasing the perforation fraction of the substrate and/or the porosity of the metal electrodes, to allow a faster access of the moisture to the sensing layer. The latter could be achieved by decreasing the size of the device and optimizing the design, which will be presented in further communications.

RESULTS The up-sweep (squares) and down-sweep (circles) TCR curves for the temperature sensor are shown in Figure 5. The curve shows the normalized resistance (at 0 °C) at different temperatures. Both curves were shown to be linear with a TCR of 918 ppmK-1 and 915 ppmK-1 for the up- and down-sweep curves, respectively. Nominal values for the temperature sensor are desired to be relatively high to achieve higher sensitivities. The nominal value can be tuned by changing the ink type, curing conditions and thickness (number layers). The temperature change in the structure, induced by the resistive heater at different applied powers, was estimated by the temperature sensor, as shown in Figure 6. The left axis shows the normalized resistance (at 0 °C) while the right axis shows its corresponding temperatures (see Figure 5 for TCR data). The resistance of the temperature sensor increases linearly with the power as well as the estimated temperature with a sensitivity of 0.65 °C/mW.

18.5

Capacitance (pF)

80 % R.H.

15 °C 25 °C 45 °C

18.0

Commercial sensor

60 % R.H.

60 % R.H.

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40 % R.H.

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Figure 7: Capacitance characterization at different temperatures compared to a commercial sensor. Figure 8 shows the dynamic humidity characterization of three full cycles at different R.H.

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levels, showing a response time of ~230 s (τ: 63%) and a sensitivity of ~23 fF / 1%R.H. Additionally, it is noted that after the first cycle the sensor returns to the baseline set at 10% R.H. 16.0

Capacitance (pF)

15.6 50 15.4 15.2 30

15.0 14.8

Humidity (% R.H.)

70

Humidity Capacitance

15.8

This work presents the concept, fabrication and characterization of a self-standing parallel-plate capacitive humidity sensor on a thin dry photoresist film with integrated temperature sensor and resistive heater. Along the fabrication process the self-standing patternability of the dry photoresist film was proven, which enhances the moisture diffusion by increasing the area exposed to the environment. Additionally, the inkjet printing of silverbased ink was proven with good conductivity and wettability. It should be noted that the self-standing device could be transferred by lamination techniques to different kinds of substrates. The temperature sensor TCR was proven to be 900 ppmK-1, while the relation between applied power at the heater and temperature at the sensor was shown to be 0.65 °C/mW. The humidity sensor presented a response time and sensitivity of 230 s and 23 fF/1%R.H, respectively, which could be further improved by increasing the perforation fraction of the dry film and/or the porosity of the metal electrodes.

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Figure 8: Dynamic humidity characterization at different R.H. levels and room temperature. Figure 9 shows the effect of the joule-heated generated temperature gradient on the capacitance when measuring at constant humidity in the climatic chamber. The experiment was performed for three different powers (9.2, 20.4 and 36 mW) with durations of 240 s each. It is noted that the capacitance drops 1.5, 2.6 and 3.2% when the heater is turned-on, respectively. The sensor returns always to the baseline when the heater if turned-off. The right axis of Figure 9 shows the calculated %R.H. change due to the temperature increment. The observed capacitance decrement is due to the desorption of water molecules caused by the generated heat at the sensing layer. It is noted that as the power increases the reset-time decreases, (36, 33 and 15 s, respectively) as well as the capacitance (from 50% to 39, 27 and 25.5%, respectively, thus, reaching a bigger delta capacitance in a shorter time.

REFERENCES [1] T. Unander, H.E. Nilsson, “Characterization of printed moisture sensors in packaging surveillance applications”, IEEE Sens. J., 9 922–28, 2009. [2] E. Starke, A. Turke, M. Krause, W. J. Fischer, “Flexible polymer humidity sensor fabricated by inkjet printing”, Transducers ‘11 conference, Beijing, June 5-9, 2011, pp. 1152-1155. [3] F. Molina-Lopez, D. Briand, N.F. de Rooij, Sens. & Act. B, vol. 166-167, 2012, pp. 212-222. [4] F. Molina-Lopez, A. Vásquez Quintero, G. Mattana, D. Briand, N.F. de Rooij, “Large-area compatible fabrication and encapsulation of inkjet-printed humidity sensors on flexible foil with integrated thermal compensation”, J. Micromech. Microeng., 23, 025012, 11pp, 2013. [5] U. Kang, K. Wise, “A high-speed capacitive humidity sensor with on-chip thermal reset”, IEEE Tran. Elec. Dev., 47, 4, 2000. [6] U. Kang, K. Wise, “A high-speed capacitive humidity sensor”, IEEE Solid-state Sens. and Act. workshop, June S.C. 1998, pp. 183-186. [7] F. Molina-Lopez, D. Briand, N.F. de Rooij, M. Smolander, IEEE sens. 2012, Taipei, 2012. [8] A. Vásquez Quintero, D. Briand, N.F de Rooij, “Effect of low-temperature processing on dry film photoresist properties for flexible electronics”, J. Polym. Sci., Part B: Polym. Phy., 51, 668-679, 2013. [9] W. Sun, H. Chien, M. Huang, T. Chang, D. Yao, “A novel method for measuring thick film thermal conductivity”, IEEE Nano/micro Engi. Mol. Sys., Xiamen, Jan. 20-23, 2010, pp.1052-1056.

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-1

40 Power = 9.2 mW Temp. = 25 °C τ(63%) = 36 s

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Power = 36 mW Temp. = 40 °C τ(63%) = 15 s Power = 20.4 mW Temp. = 30 °C τ(63%) = 33 s

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Normalized capacitance (%)

This work is funded by the EU FP7 project FlexSmell, a Marie Curie Initial Training Network (ITN), No. 238454.

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Baseline = 23 °C / 50% R.H.

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ACKNOWLEDGEMENTS

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Time (minutes)

Figure 9: Thermal desorption of water molecules at constant R.H. level using the integrated heater. This experiment corroborates the potential functionality of the heater as sensor-reset in-between measurements, either to increase the reset time or switch between different analytes. However, the time stabilization of the generated heat is currently under investigation and will be presented in further communications.

CONTACT *A. Vásquez Quintero, tel: +41-32-7205028; [email protected]

CONCLUSION

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