Highly piezoresistive hybrid MEMS sensors

Sensors and Actuators A 209 (2014) 161–168

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Highly piezoresistive hybrid MEMS sensors D. Thuau a,b,∗ , C. Ayela a , P. Poulin b , I. Dufour a a b

Université de Bordeaux, IMS, UMR 5218, 351 cours de la Libération, 33405 Talence, France Université de Bordeaux, CNRS, Centre de Recherche Paul Pascal, 115 Avenue Schweitzer, 33600 Pessac, France

a r t i c l e

i n f o

Article history: Received 7 October 2013 Received in revised form 23 January 2014 Accepted 24 January 2014 Available online 31 January 2014 Keywords: MEMS Strain sensor CNT/SU-8 nanocomposite

a b s t r a c t A flexible organic piezoresistive micro-electro-mechanical system (MEMS) strain sensor with enhanced sensitivity is presented. The piezoresistive transducers consist in CNT/SU-8 nanocomposites. The electrical resistance changes versus strain of CNT/SU-8 piezoresistive materials have shown linear and non-linear regions at low and high strain levels, respectively. The measurements have been correlated with an existing analytical model showing good agreement. This work also highlights the role of the MEMS design in the enhancement of the final sensing performance of the devices. The gauge factor calculated from the change of resistance as a function of applied strain has been found to reach 200 for optimized sensor geometries, providing highly sensitive piezoresistive strain sensors for flexible electronic applications. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Micro-electro-mechanical systems (MEMS) such as microcantilevers have been demonstrated as promising stress sensors having key advantages such as low power consumption and small size compared to traditional technologies. The fast-growing number of applications of microcantilever with integrated piezoresistive transduction has created a pressing need for device optimizations including choice of materials, design, micro-fabrication, signal to noise ratios and reliability. Examples of main applications are chemical, biological and fluidic sensors [1–7]. Most commonly, metal foils have been used as strain gauges [8]. Nevertheless, these gauges have limited strain measurement sensitivity [8]. Metals strain gauges typically exhibit gauge factor in the range of 0.8–3. Another common type of strain sensor is the piezoresistive single crystal silicon strain sensor compatible with CMOS process and owing high gauge factor up to 135 [9]. However, these sensors are more expensive, more sensitive to temperature changes, and more fragile than foil gauges. In addition, they are not neither suitable for flexible strain sensing applications [10]. A literature survey has revealed that metal particles and carbon black (CB)/SU-8 composites have shown great promise as an alternative option that could measure much larger strains. The combination of CB having interesting electromechanical properties and polymeric material with low Young’s modulus, thus highly deformable for piezoresistive strain sensors has acquired significant attention in the field. For

∗ Corresponding author at: Université de Bordeaux, IMS, UMR 5218, 351 cours de la Libération, 33405 Talence, France. Tel.: +33 0540002709. E-mail address: [email protected] (D. Thuau). 0924-4247/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2014.01.037

instance, piezoresistive CB/SU-8 composites have been reported with gauge factor in the range 15–30 [11–13]. Carbon nanotubes (CNTs) have also been used as conductive filler in polymer composite. CNTs are different from CB or metal particle having a higher Young’s modulus and a larger aspect ratio. Their high aspect ratio allows CNTs to create at low loading a more efficient conductive network within the insulting polymer matrix compared to other common fillers. Although the conductive network may be brittle, this turns out to be an advantage for strain sensing application where, the network configuration is therefore very sensitive to mechanical disturbances resulting in large change of electrical resistance at CNT concentration in the percolation region. The issue with conductive filler based polymer nanocomposite is their high intrinsic electrical resistivity at the percolation threshold, requiring electronics equipment such as lock-in amplifier and/or instrumentation amplifier with low noise and high input impedance to minimize the signal to noise ratio and amplify the electric response. Thus, when piezoresistive nanocomposites are integrated into a MEMS strain sensor as embedded piezoresistors, the design of the device becomes critical. Indeed an optimization of the geometry of the sensors is necessary in order to maximize the measured changes of properties in response to a given stimulus. This work analyzes in detail the piezoresistive responses of CNT/SU-8 nanocomposite materials as a function of applied strain as well as CNT concentration. These measurements have been correlated with existing analytical models describing the electronic transport properties within a nanocomposite matrix. Then, the prepared piezoresistive materials have been integrated into a MEMS strain sensor as piezoresistive transducers. In particular, a new design has been developed in order to improve the sensitivity of the strain sensor. The geometrical approach developed in this

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Fig. 1. Preparation steps of nanocomposite thin films.

work consists in a U-shaped cantilever beam where the piezoresistors have been placed at the anchor of the cantilever, region where the strain is maximum when a force is applied at the tip of the beam, and then linked with a thin gold layer along the U-shaped of the cantilever beam. As a result, the impedance of the piezoresistor is sufficiently small so that its resistance, and resistance changes, can be accurately measured.

2. Piezoresistive thin films 2.1. Nanocomposite preparation CNTs used in this work have been supplied by using Graphistrength Epoxy Master batch pellets from Arkema. The pellets consist in 25 wt% of MWNTs made via catalytic chemical vapor deposition (CCVD) dispersed in an epoxy type matrix (Fig. 1a). The nanocomposite has been prepared by mixing the SU-8 epoxy photoresist with the pellets using a high shear mixer, Silverson L4RT at 5000 rpm for 60 min in an ice bath (Fig. 1b). Subsequently, the obtained solution (Fig. 1c) has been spin coated on a sheet of 100 ␮m thick polyethylene terephthalate (PET) (Fig. 1d) and soft baked at 95 ◦ C for 2 min. Then, the thin film has been cross-linked by exposure to UV light combined with a post-exposure bake step at 65 ◦ C for 1 min and 95 ◦ C for 3 min followed by final hard bake of 150 ◦ C for 15 min. Finally, the thin film has been cut into dog-bone shaped specimens using a Graphtec CE-5000 (Fig. 1e). The dimensions of the specimens were 10 mm long, 1 mm wide and 110 ␮m thick. The piezoresistive responses of the CNTs/SU-8 samples have been characterized using a tensile test machine of the type Zwick Roell; meanwhile the electrical resistance of the thin films has been measured with a two-point probe measurement set-up in dry air at ambient temperature. Electrical connections have been made by placing silver paste onto the two ends of the thin films connected

to copper electrodes. Since the typical resistance of these samples is on the order of 103 –107 , wire contact resistance can be considered negligible, thereby deeming a four-point probe technique unnecessary. The electrical conductivity of the thin films as a function of the sample’s geometrical dimensions has been investigated by measuring and averaging the electrical resistance of four samples for each CNT wt% concentration. An electrometer/high resistance meter (Keithley 6517A) has been used to measure the electrical conductivity of these films. A variation of the post-processed thin films resistivity of about 10% has been classically observed probably due to the quality of the CNT dispersion. 2.2. Characterization 2.2.1. Electrical properties The electrical conductivity of CNT/SU-8 nanocomposites has been seen to increase by about 14 orders of magnitude as the CNT concentration increases from 0 to 4 wt% (Fig. 2). The electrical conductivity shows an abrupt increase where electrical conductivities change from 5.0 × 10−9 to 1.1 × 10−3 S cm−1 between 0.1 and 1 wt% CNT concentration. The electrical conductivity of the composite can be considered to follow a percolation theory whereby a critical concentration of CNTs exists at which a conductive path is created in the polymer matrix, causing the polymer to change from being an insulator to conductor. The percolation threshold has been deduced by fitting the data in Fig. 2 with a scaling relationship [14]:  = k( − c )

−t

(1)

where k is a constant, t is the critical conductivity exponent calculated to be 1.54 and  and c are the volume and critical volume fraction defined as 0.6 ± 0.2 wt% of the conductive phase respectively.

D. Thuau et al. / Sensors and Actuators A 209 (2014) 161–168

Fig. 2. Electrical conductivity of CNT/SU-8 nanocomposite as a function of CNT wt% concentration.

2.0

1 wt% CNT-SU/8 4 wt% CNT-SU/8 7 wt% CNT-SU/8

1.0

2.2.2. Piezoresistivity The sensitivity of a piezoresistive material is defined by its gauge factor (GF) and can be expressed as: GF =

- Z im

0.5

Z real

Z real

0.0 0.0

0.5

1.0

[15,16]. These results revealed the influence of the CNT concentration on the electrical impedance parameters of the piezoresistive thin films. Here, we report the use of CNT/SU-8 material as piezoresistive transducer in a strain sensor; therefore it is important to ensure that only the resistance of the CNT/SU-8 transducer changes due to strain with limited influence of the capacitance variations. These measurements can be helpful to tune the resistance or capacitance component of the material in order to optimize the sensor performances. The Zreal versus −Zim curves plotted in Fig. 3 for different CNT/SU-8 thin films correspond to an equivalent resistor–capacitor (R–C) parallel circuit describing the insulting SU-8 epoxy resin loaded with conductive CNTs. The electrical circuit model described as a resistor and a capacitance in parallel (Fig. 4a) has been verified throughout the simulation of a Nyquist diagram based on its impedance relationship (Fig. 4b) and intrinsic parameters. Simulation has been performed in order to extract the Rp and Cp parameters of a 4 wt% CNT-SU/8 nanocomposite. The correlation between simulation and measurement is presented in Fig. 4c. From this graph, Rp and Cp have been defined to be 550 k and 2 pF, respectively.

Increase Cp

Decrease Rp

- Z im

-Z im (MΩ)

1.5

1.5

2.0

2.5

163

3.0

Z real (MΩ) Fig. 3. Nyquist diagram of 1, 4 and 7 wt% CNT/SU-8 samples.

Then, further electrical characterizations of CNT/SU-8 thin films with various CNT concentrations have been performed by running electrical impedance measurements. Fig. 3 shows the impedance of different thin films as a function of frequency plotted in a Nyquist diagram form (Zreal /−Zim ) in the frequency range 1 kHz–1 MHz. It can be seen that the typical semi-circle curve, corresponding to a constant capacitance in parallel to a constant resistance, is obtained for specimen containing high CNT concentration, whereas only the onset of the semi-circle has been observed for samples containing less CNTs indicating an increase of the capacitance component

R/R ε

(2)

where R is the initial resistance of the thin film at rest, R the change of resistance and ε the strain. In order to investigate the sensitivity of the conformable CNT/SU-8 strain sensor, a series of tensile stress load pattern has been undertaken to thin films with different initial resistances under the same experimental conditions. The measurements have been carried out in an air and humidity controlled room where the temperature was set at 23 ± 0.5 ◦ C and the humidity level at 50 ± 2%. Fig. 5 shows the relative change of resistance as a function of applied strain for five thin films containing various CNT concentrations. The resistance of all the samples increases as the applied strain increases. This is due to the alteration of the network of carbon nanotubes under mechanical deformation resulting in an increase in the resistivity. Particularly, this change of resistivity is associated with the modification of contact arrangements and the tunneling distance between carbon nanotubes. However, it is worth noting the difference of sample’s responses depending on their CNT concentration. In fact, two distinct behaviors can be observed from the measurements: thin films containing a CNT concentration well above the

Fig. 4. (a) Equivalent circuit model, (b) impedance relationship and (c) simulation and measurement of the Nyquist diagram of a 4 wt% CNT/SU-8 nanocomposite.

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Fig. 5. Relative variation of resistance as a function of applied strain for different CNT concentrations.

percolation region show a linear (4 wt% CNT) or quasi-linear (2 wt% CNT) piezoresistive response; whereas the piezoresistive response of the thin films containing CNT around the percolation threshold presents a non-linearity with an apparent change of regime as function of strain for 0.4, 0.8 and 1 wt% CNT concentration. The piezoresistive response of these samples is linear up to a critical strain level specific to each CNT concentration and afterwards increases exponentially above it. The critical strain of 0.4, 0.8 and 1 wt% CNT/SU-8 nanocomposite have been defined around 2 wt% of strain level. Here, we define the strain values below the critical strain as region 1, while strain values above the critical strain belongs to region 2. Fig. 6 shows the gauge factor, (GF), defined as the ‘local slope’ of the curves in Fig. 5, of different CNT/SU-8 composites in both low and high strain regions namely region 1 and 2 respectively. As expected, the highest piezoresistive sensitivity of the nanocomposite thin films has been achieved for sample containing a CNT concentration just above the percolation threshold [17]. 0.8 wt% CNT/SU-8 thin films have shown gauge factor of approximately 100 for a strain level of 3.0% (see inset of Fig. 6). As the CNT concentration moves away from the percolation threshold the gauge factors have been found to decrease drastically as shown 160

0.8

140

0.6

Region 2

Region 1 0.0 0.00

60

0.01

0.02

0.03

Strain,ε AtRegion 1.5 % stra 1 in level AtRegion 3.0 % strain 2 level

40 20 0 0

1

with the subscript, 0, referring to the position at rest (ε = 0). The tunneling resistance between nanotubes varies exponentially as a function of the separation between the nanotubes. Slight changes of the contact geometry can therefore result in large variations of resistance. The behavior presently observed as well as the existence of two regimes in the response to mechanical loading has been reported by several groups in related materials [19–21]. While the origin of the two regimes remains debated the present behavior has been modeled by considering geometrical deformation of the materials. The analytical model used in this work is based on the model developed by Simmons [22] and later endorsed by Sheng et al. [23]. Accordingly, the resistivity of a two-phase composite system (where one phase is conductive, CNTs and the other insulating, SU-8) deformed at low strain level is expected to be quasi linear and modeled by the term (2ε + ε2 ) in Eq. (4) [20,24]. At higher strain level (non-linear region), the relative resistivity variation is dominated by the tunneling resistance variation; therefore the total change of resistance can be expressed as [20]:

˛=

0.2

80

2

3

(3)

(4)

with

0.4

100

R − Rd0 Rt0 Rt − Rt0 R − R0 ≈ d + · R0 Rd0 Rd0 Rt0

Rt0 (2˛d0 ε) R − R0 = 2ε + ε2 + [e − 1] R0 Rd0

ΔR/R

Gauge factor, GF

120

in Fig. 6. Therefore, samples containing of 0.8 wt% CNT have been chosen for further investigations on a piezoresistive fitting model as well as repeatability measurements as a function of time. It is worth noting the large discrepancy in the measured values of the gauge factors for CNT/SU-8 composite containing a CNT concentration close to the percolation region. This is assumed to come from the fact that at such CNT wt% concentration slight change in the CNT network (mainly due to the quality of dispersion) from one sample to the other can have drastic change in the piezoresistive behavior due to the high sensitivity to applied strain (Fig. 6). The piezoresistive behavior of CNT network involves a complicated interplay of different mechanisms that include the change of resistance of the matrix due to dimensional changes, Rd , the tunneling resistance, Rt , the resistances of the intertube contacts and the intrinsic piezoresistivity of the nanotubes [18]. The fact that the sensitivity to strain is enhanced near the percolation threshold suggests that the piezoresistive behavior is essentially governed by changes of the nanotube connectivity resulting in large variation of the tunneling resistance, Rt . Assuming that Rd0 is much larger than Rt0 , and also that the piezoresistivity of the nanotubes is negligible for the present level of stress, thus, the total resistance change can be defined as:

4

CNT concentration (wt%) Fig. 6. Gauge factors of CNT/SU-8 composites as a function of CNT concentrations at 1.5% and 3.0% of applied strains (inset: relative change of resistance versus applied strain of 0.8 wt% CNT/SU-8 nanocomposite).

2  2mˇ h

(5)

where ε is the strain, m the mass of an electron, h the Planck’s constant, d0 the tunneling distance and ˇ the tunneling barrier. Fig. 7 illustrates the fitting model with the experimental data. The best fit to the data of a 0.8 wt% CNT-SU-8 sample has been obtained with d0 = 10.37 nm and Rd0 ≈ 3000 Rt0 assuming that the tunneling barrier, ˇ, is approximately 5 eV [25]. In addition, the gauge factor has been calculated from the derivative of the fit (slope of the fit) and found to be 100 ± 25 which is comparable with the gauge factor value obtained from the measurement. It is worth noting that the model presented is simplified as it assumes mean values for interparticle distance and do not take into consideration defect on CNTs surface or the wavy shape of some CNTs as well as the formation or breakup of CNT contacts. Nevertheless, the good fit obtained highlights the comprehensive behavior of

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3. Piezoresistive MEMS sensor

0.8

ΔR/R

3.1. Design optimization

Experimental data Fit

0.6

0.4

0.2

0

165

0

0.01

0.02

0.03

Strain , ε Fig. 7. Fitting model and experimental resistance change as a function of strain for a 0.8 wt% CNT/SU-8 thin film.

piezoresistive mechanism occurring in a CNT/SU-8 piezoresistive nanocomposite. 2.2.3. Reliability The reproducibility of the piezoresistive response of CNT/SU-8 composites has been characterized by applying saw-tooth tensilerelaxation cycle load patterns to a 0.8 wt% CNT/SU-8 sample as shown in Fig. 8. The change of resistance has been measured over 10 cycles at low and high strain levels as a function of time. Low strain level corresponds to strain oscillations in region 1 between 0.2% and 0.55% (Fig. 8a) while high strain level refers to strain oscillations in the range 3–3.7%, corresponding to region 2 (Fig. 8b). The resistance increases when tensile stress is applied and decreases under stress relaxation. The film resistivity has been shown to be repeatable and reversible over the 10 cycles for both strain levels indicating that the composite experiences elastic deformations. By using Eq. (2), the gauge factor of a 0.8 wt% CNT/SU-8 has been measured to be approximately 8 in region 1 and 60 in region 2. In agreement with the static measurements presented in (Fig. 5), larger variations of resistances have been obtained at higher strain level (region 2). These results open potential application of CNT/SU-8 composite material as highly sensitive strain gauges. In particular, their integration as piezoresistive transducer into MEMS devices has attracted our interest for the development of a sensitive strain sensor.

Geometrical dimensions of both cantilever and piezoresistor play a critical role in the sensitivity of MEMS sensors [26]. Although the stress–strain is uniform in a thin film it is not the case for cantilever strain sensor. The gauge factor of a strain sensor can be optimized by the geometry of the sensor and not only the material. On the one side a common way to increase the sensitivity of a sensor is to decrease the cantilever thickness but on the other too thin cantilever beams are brittle and harder to handle. On the contrary, optimization of the piezoresistor’s position and dimensions can highly enhance the sensors’ sensitivity while retaining the cantilever’s mechanical properties. High piezoresistive responses have been obtained for piezoresistors placed at the clamped ends of the cantilever beam with a length of approximately 2/5 of the cantilever length [26]. This is due to the fact that in a cantilever the stress–strain is not uniform resulting in part of the beam highly strained (at the anchors) and others regions unstrained (tip end). Moreover, the piezoresistors have to be thin and located on the surface of the cantilever as the stress within the beam increases linearly, from the beam ‘neutral axis’ to the top and bottom face. Fig. 9 illustrates the two designs that have been realized. Initially, a bimorph cantilever made of a SU-8 layer covered with a thin piezoresistive CNT/SU-8 layer has been fabricated as shown in Fig. 9a and c. The design geometry has been previously reported by Seena et al obtaining CB/SU-8 nanocomposite microcantilever with a gauge factor of 90 [6]. Nevertheless, in our case the sensitivity of this MEMS design has not shown a large improvement in the sensitivity compared to traditional metal foils strain gauges. Actually, the sensitivity is higher at CNT concentration in the percolation region; however at such concentration the initial resistance of the piezoresistive material is large. This results in low signal to noise ratio and prevents large relative resistance changes, R/R. Therefore, we propose a MEMS design as shown in Fig. 9b enhancing surface stress for optimizing sensing responses. Thus, a second batch of MEMS devices has been fabricated using an optimized geometry where the piezoresistors have been shortened and located at the anchor (concentrated stress region) of the cantilever while the ending part of the U-shaped piezoresistor has been patterned by gold (Fig. 9d). Gold has been mainly chosen due to its low resistivity reducing the electrical noise level in the piezoresistor. As a consequence the initial resistance of the piezoresistor, R, is reduced and the relative resistance change, R/R, optimized.

Fig. 8. R/R response of a 0.8 wt% CNT/SU-8 thin film for tensile/compressive strain oscillations between (a) 0.2–0.55% and (b) 3–3.7%.

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Fig. 9. (a) and (b) Schematic and (c) and (d) optical image of the two MEMS designs fabricated.

3.2. Fabrication process The fabrication process of the piezoresistive strain sensor is illustrated in Fig. 10. The SU-8 based MEMS with integrated piezoresistive transducers have been fabricated on a silicon substrate, which has only been used to handle the chips during the processing. Initially, a thin layer of Omnicoat® (an adhesion promoter/release layer specifically produced to reduce adhesion between SU-8 and silicon, provided by Microchem Ltd.) has been deposited as sacrificial layer (Fig. 10a). Then, 100 nm of gold (Au) has been evaporated and patterned by chemical etching in order to create the electrodes, connections to the gauges and Au arms at the tip end of the cantilever beam (Fig. 10b). Afterwards, the CNT/SU-8 nanocomposite solution has been spin-coated and patterned by photolithography in order to create the 5 ␮m thick piezoresistive gauges at the anchor of the cantilever beam (Fig. 10c). At relatively high CNT concentration the composite film is not transparent

anymore; therefore an optimization of the standard photolithography process is required. In the case of a 2 wt% CNT/SU-8 composite, the spin coating time has been kept the same as standard procedure; however the acceleration has been increased from 500 to 1000 rpm/s due to higher viscosity. The soft baking step has been increased from 2 to 3 min since solvent evaporation is slowed down by the presence of CNTs. The temperature has been kept the same as standard procedure (95 ◦ C). UV exposure dose, which depends mainly on the composite layer thickness, has been increased from 100 mJ cm−2 to 200 mJ cm−2 due to the presence of the CNTs. The post exposure bake temperature has been increased from 95 ◦ C to 120 ◦ C and the development time has been increased from 1 to 5 min. The final hard bake has been done as standard procedure (150 ◦ C for 15 min). The next steps have consisted in the spin-coating and patterning by photolithography of two successive layers of SU-8; a 30 ␮m thick for the fabrication of the cantilever beam and a 100 ␮m thick for the support of the structure (Fig. 10d

Fig. 10. Fabrication process flow of the hybrid piezoresistive U-shaped MEMS strain sensor.

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167

Fig. 11. (a) Deflection profile and (b) optical profilometry images of U-shaped cantilevers subject to applied forces.

and e). Finally, the free-standing SU-8 cantilevers have been released by chemical etching of the Omnicoat® sacrificial layer in MF319, provided by Microhem (Fig. 10f) [27]. 3.3. Characterization of the MEMS strain sensor The piezoresistive sensitivity of the two sets of fabricated devices has been electromechanically characterized. The experiment has consisted to bend the cantilever beam by applying a force at the cantilever’s tip from a MicroBot probe (Imina Technologies SA) meanwhile measuring the impedance evolution of the piezoresistor to evaluate the change of resistance and capacitance. Fig. 11a shows the evolution of the deflection profile of a U-shaped cantilever of 250 ␮m long, 30 ␮m thick, with arms of 50 ␮m wide, exposed to various applied forces. Optical profilometry images (Veeco NT 9080) of the U-shaped cantilever at rest and bent under an applied force corresponding to a tip-end cantilever beam deflection, ı, of 18 ␮m are presented in Fig. 11b. The strain, ε that experiences the piezoresistor located at the top surface of the two clamped arms of the U-shaped beam can be calculated from the following equation: ε=

3hı 2L2

(6)

with h the thickness of the cantilever, ı the deflection at the tip end of the beam and L the length of the cantilever. Consequently, the change of resistance in response to beam deflection has been measured in order to investigate the piezoresistive behavior of the nanocomposite strain gauges. In agreement with the piezoresistive thin film measurements presented in the previous section, the sensor sensitivity has shown a strong dependence on both the initial resistance of the strain gauges, R, and the applied strain, ε. The deflection–responsive resistance changes of the two designs fabricated, meaning a bimorph SU-8 layer covered with CNT/SU-8 layer U-shaped cantilever (design 1) and a nanocomposite strain gauge placed at the clamped end of a SU-8 cantilever linked with a thin gold layer configuration (design 2) have been characterized and their performances compared. According to the equivalent electrical model developed for the CNT/SU-8 thin films (Fig. 4a), the influence of the change of capacitance (about 2%) has been evaluated and ensured to be negligible compared to the change of resistance on the total material impedance. The authors emphasize the divergence on the measured resistivity between the macroscale thin films and the integrated strain gauges of the MEMS sensors. Although higher sensitivity has been obtained for thin films containing 0.8 wt% CNT, the most sensitive MEMS strain gauges have been achieved using 2 wt% CNT. The

Fig. 12. Change of resistance of the 2 wt% CNT/SU-8 piezoresistor integrated on both MEMS designs as a function of applied strain.

gauge factor of a 0.8 wt% CNT/SU-8 has been found to be less than 10 without significant influence of the cantilever design. Furthermore, the small dimension of integrated piezoresistors in MEMS devices has higher intrinsic resistance making the measurements of 0.4 and 0.8 wt% CNT/SU-8 thin films difficult (resistance in the range of few hundreds M to G). This discrepancy could come from the reproducibility of the CNT dispersion deposited by spin-coating and micro-patterned by photolithography and the micro-fabrication process of the MEMS. The piezoresistive sensitivity of the two sensors designs containing 2 wt% of CNT have been plotted into one diagram as shown in Fig. 12 for comparative purposes. It can be seen that the gauge factors of both designs at low strain (region 1) are similar. At 1.5% of applied strain level, the gauge factors of designs 1 and 2 are 8 and 10 respectively. Nevertheless, the enhancement of the sensitivity is clearly visible for larger applied strain (region 2) with the second MEMS design geometry. At 4% of applied strain level the gauge factor jumped from 20 to 200 between designs 1 and 2, respectively. At higher CNT concentration the gauge factors have been seen to drop drastically as previously observed for bulk material characterization. 4. Conclusion In summary, we have presented the development of a flexible SU-8 MEMS strain sensor with integrated CNT/SU-8

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nanocomposite piezoresistive transducer. Initially, the piezoresistive CNT/SU-8 materials used as strain gauges in the MEMS devices have been prepared with enhanced piezoresistive properties obtained for sample containing a CNT concentration around the percolation region with measured gauge factor of about 100. In addition, the reproducibility and reliability of the piezoresistive response have been investigated by running cycles of tensile and relaxation load to the samples. These measurements have shown no significant hysteresis. The electromechanical responses of the films as a function of applied strain and CNT concentration have been correlated with an existing piezoresistive mechanism occurring within the nanocomposite where the tunneling effect is responsible for the larger change of resistance. Measurements and theoretical model have shown good agreement. Then, CNT/SU-8 nanocomposite thin films have been integrated into a flexible organic MEMS cantilever as piezoresistive transducers. Attention has been focused on the optimization of the MEMS design in order to improve the relative change of resistance of the piezoresistors. A U-shaped cantilever beam with CNT/SU-8 piezoresistors located at the two clamped arms linked with a thin layer of gold along the U-shaped top surface of the cantilever has been chosen as optimized geometry. This cantilever geometry has led to the enhancement of the devices sensitivity by a factor 10 at 4% of applied strain level, compared to the standard design. These results allow the development of simple, highly sensitive (GF up to 200 at 4% of applied strain) piezoresistive strain sensors based on flexible and stretchable materials fabricated at low cost via printing methods. Acknowledgements This research work was carried out in the framework of the LabEx AMADEuS, ANR-10-LABX-0042-AMADEUS, and was supported by a state grant handled by the National Agency of Research in respect to the program of Excellence Initiative IdEx Bordeaux, Grant N◦ ANR-10-IDEX-0003-02. References [1] I. Ellern, A. Venkatasubramanian, J.-H. Lee, P. Hesketh, V. Stavila, A. Robinson, M. Allendorf, HKUST-1 coated piezoresistive microcantilever array for volatile organic compound sensing, Micro Nano Lett. 8 (11) (2013) 766–769. [2] L. Nicu, T. Alava, T. Leichle, D. Saya, J.-B. Pourciel, F. Mathieu, C. Soyer, D. Remiens, C. Ayelaand, K. Haupt, Integrative technology-based approach of microelectromechanical systems (MEMS) for biosensing applications, in: 34th Annual International Conference of the IEEE EMBS, San Diego, CA, USA, 2012. [3] Q. Zhang, W. Ruan, H. Wang, Y. Zhou, Z. Wang, L. Liu, A self-bended piezoresistive microcantilever flow sensor for low flow rate measurement, Sens. Actuators A: Phys. 158 (2) (2010) 273–279. [4] S.M. Yang, T.I. Yin, Design and analysis of piezoresistive microcantilever for surface stress measurement in biochemical sensor, Sens. Actuators B: Chem. 120 (2) (2007) 736–744. [5] L. Armitage Beardslee, A.M. Addous, S. Heinrich, F. Josse, I. Dufour, O. Brand, Thermal excitation and piezoresistive detection of cantilever in-plane resonance modes for sensing applications, J. Microelectromech. Syst. 19 (4) (2010) 1015. [6] V. Seena, A. Fernandes, P. Pant, S. Mukherji, V.R. Rao, Polymer nanocomposite nanomechanical cantilever sensors: material characterization, device development and application in explosive vapour detection, Nanotechnology 22 (2011) 295501. [7] A. Boisen, S. Dohn, S.S. Keller, S. Schmid, M. Tenje, Cantilever-like micromechanical sensors, Rep. Prog. Phys. 74 (2011) 1–31. [8] A.L. Window, Strain Sensor Technology, 2nd ed., Elsevier Applied Science, New York, NY, 1992. [9] V. Kaajakari, Practical MEMS, Small Gear Publishing, Louisiana Tech University, 2009. [10] S. Blazewicz, B. Patalita, P. Touzain, Study of piezoresistance effect in carbon fibers, Carbon 35 (1997) 1613–1618. [11] L. Gammelgaard, P.A. Rasmussen, M. Calleja, P. Vettiger, A. Boisen, Microfabricated photoplastic cantilever with integrated photoplastic/carbon based piezoresistive strain sensor, Appl. Phys. Lett. 88 (2006) 113508–113510.

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Biographies Damien Thuau received his M.Sc. degree in Nanotechnology and Microsystems from Heriot Watt University, Scotland, in 2007. In the same year, he joined the School of Engineering, at the University of Edinburgh, where he obtained his Ph.D. degree Microsystems and Microfabrication in 2011. Afterwards, he continued his research activity as a post-doctoral researcher at the University of Bordeaux, France. He is currently working on polymer micro electro-mechanical systems (MEMS) with particular focus on piezoresistive transduction made of carbon nanotube based nanocomposite for sensor applications. Cédric Ayela was born in Tarbes (France) in 1981. He received the engineer degree in electronics from the national institute of applied sciences in Toulouse and a Ph.D. degree in 2007 on piezoelectric silicon-based micromembranes for the label-free detection of biological molecules at the Laboratory for Analysis and Architecture of Systems (LAAS-CNRS) in Toulouse. He recently integrated (2009) the French national center for scientific research (CNRS) as a full researcher at IMS laboratory in Bordeaux (France). He is concentrating his researches on developing the emerging field of organic MEMS: fabrication, characterization, and application as bio/chemical sensors. Philippe Poulin is CNRS researcher at the Centre de Recherche Paul Pascal in Bordeaux, France. His fields of interest include soft condensed matter, nanostructured and functional materials, carbon and composites. He obtained a Ph.D. degree in Physical Chemistry from the University of Bordeaux in 1995. Then he undertook postdoctoral research on liquid crystal emulsions at the University of Pennsylvania, USA before taking up his CNRS position in 1998. P. Poulin is currently working on the physical chemistry of carbon nanotubes and graphene and on their behavior in complex fluids and polymers. Isabelle Dufour graduated from Ecole Normale Supérieure de Cachan in 1990 and received the Ph.D. and H.D.R. degrees in engineering science from the University of Paris-Sud, Orsay, France, in 1993 and 2000, respectively. She was a CNRS research fellow from 1994 to 2007, first in Cachan working on the modeling of electrostatic actuators (micromotors, micropumps) and then after 2000 in Bordeaux working on microcantilever-based chemical sensors. She is currently Professor of electrical engineering at the University of Bordeaux and her research interests are in the areas of microcantilever-based sensors for chemical detection, rheological measurements and material characterization.