G Model SNB-14726; No. of Pages 11 ARTICLE IN PRESS Sensors and Actuators B xxx (2012) xxx–xxx Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Flexible sensing systems based on polysilicon thin film transistors technology Luca Maiolo a , Alessandro Pecora a,∗ , Francesco Maita a , Antonio Minotti a , Emiliano Zampetti a , Simone Pantalei a , Antonella Macagnano a , Andrea Bearzotti a , Davide Ricci b , Guglielmo Fortunato a a b IMM-CNR, Via del Fosso del Cavaliere 100, 00133 Roma, Italy RBCS, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy a r t i c l e i n f o Article history: Available online xxx Keywords: Flexible electronics Sensing systems Polysilicon thin film transistors a b s t r a c t Flexible sensors are gaining increasing interest in a number of applications, including biomedical, food control, domotics and robotics, having very light weight, robustness and low cost. In order to improve signal-to-noise ratio, integration of readout electronics is crucial and several technologies are available for the fabrication of thin film transistors (TFTs) based circuits on flexible substrates. Among these technologies, the low temperature polycrystalline silicon (LTPS) is particularly attractive, since LTPS TFTs show excellent electrical characteristics, good stability and offer the possibility to exploit CMOS architectures. The different aspects for the direct fabrication of LTPS TFTs on polymer substrates are reviewed and the specific fabrication process adopted on ultrathin polyimide substrates is described in some detail. Then, as examples of flexible sensing systems, we present both chemical and physical sensors integrated with LTPS TFTs frontend electronics. The present results can pave the way to advanced flexible sensing systems, where sensors and local signal conditioning circuits can be integrated on the same flexible substrate. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Wearable electronics and pervasive ambient computing are two scenarios where largely distributed flexible sensors networks are expected to be heavily used [1–3]. The growing demand for sharing large stream of data among groups of people in different places at different times is now pushing researchers and scholars toward the creation of individual smart sensors networks, where each person constitutes a node of the web or, on the other hand, toward the development of smart sensing infrastructures, where some information can be concentrated in a place and can be exchanged simply passing through them. The advantages of such a communication philosophy can be numerous in terms of time optimization, costs reduction and more generally in terms of improving the quality of life. Depending on the needs of a particular application, one of the two strategies can be adopted: monitoring the biometric parameters of a large number of patients or a group of soldiers deployed in a battle can be efficiently performed by a personal sensor network, while the control of perishable products or air pollutants can be achieved using, for instance, disposable flexible sensors distributed in a finite place connected by wireless networks [4–6]. Flexible smart sensors can be cheap, light, long-lasting, and even stretchable. They can also be made transparent and extremely thin ∗ Corresponding author. E-mail address: alessandro.pecora@cnr.it (A. Pecora). in order to be easily hidden in specific places such as museum and galleries [7]. Flexible smart sensors can be fully integrated with their front-end electronics, thus leading to flexible sensing systems, or conceived in disposable smart tags, exploiting the so-called multi-foil approach. However, in any case, a suitable flexible electronics must be mounted on these sensing systems to further boost the development of these applications and to push forward toward other emerging markets like robotics, aerospace, automotive, etc. Therefore, new requirements on front-end electronics and sensing systems are mandatory: the device performances must be competitive respect to the properties of conventional applications based on crystalline silicon, in terms of operating frequencies, electrical stability and low power consumption. Today, wafer thinning technology, printing technology or direct fabrication on flexible substrate are ripe manufacturing techniques to provide sensors for bendable electronics [8–11]. Although many different materials can be chosen to fabricate flexible structures, it should be pointed out that many degrees of flexibility exist. We can conveniently distinguish at least three categories: (1) bendable and rollable devices, (2) permanently shaped devices, and (3) elastically stretchable devices [12]. The technique that exploits the thinning of commercial crystalline silicon can be very attractive for the well-known electrical performances and its electrical stability, but it is essentially limited to some particular non-planar surfaces and it is not really rollable, since the regions where the chip is embedded in the substrate is almost unbendable. Moreover, as in case of ultrathin chip integration, the fabrication process 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.10.093 Please cite this article in press as: L. Maiolo, et al., Flexible sensing systems based on polysilicon thin film transistors technology, Sens. Actuators B: Chem. (2012), http://dx.doi.org/10.1016/j.snb.2012.10.093 G Model SNB-14726; No. of Pages 11 2 ARTICLE IN PRESS L. Maiolo et al. / Sensors and Actuators B xxx (2012) xxx–xxx involves a large number of steps. On the other hand, printed technology based on organics can provide, at the moment, devices with low field effect mobility (up to 1 cm2 /V s) and very low working frequencies. Moreover, organic electronics still show severe instability problems related to aging and air exposure. A very attractive technology for flexible electronics application is represented by the low temperature polycrystalline silicon (LTPS) thin film transistors (TFTs) technology, since it combines good electrical characteristics (carrier mobility above 40 cm2 /V s), the possibility to exploit CMOS architectures, excellent mechanical and electrical stability with the compatibility to fabricate devices directly on flexible substrates at minimal changes in the equipment and processes [11]. Indeed, LTPS TFTs based electronics has been recently developed on polymer substrates and applications to flexible and low-cost sensor devices [13,14] and active matrix organic light emitting displays [15,16], have been reported. In this work we will review the fabrication process of LTPS TFTs on polymer substrates and then we will present some applications to flexible chemical and physical sensing systems. In particular, we will describe applications to chemical sensors [13,14], where both capacitive and resistive sensors have been integrated with frontend electronics based on LTPS TFTs. In addition, we will also present some novel results on a flexible pressure sensors based on a piezoelectric capacitor coupled with an LTPS TFT, suitable for tactile sensing in robotics applications. 2. Low temperature polysilicon TFT technology for flexible electronics As already mentioned, among the different TFT technologies, LTPS offers best device performance as well as the possibility to provide CMOS circuits. Fabrication of LTPS TFTs on polymeric substrates has been accomplished following two main routes: (1) standard fabrication of LTPS TFTs on glass substrates followed by a transfer process of the devices on the polymeric substrate; and (2) direct fabrication of the devices on the polymeric substrate. The first approach has been originally proposed by Shimoda and co-workers at Seiko EPSON, who developed the surface-free technology by laser annealing (SUFTLA) that enables the transfer of thin-film devices from an original substrate to another substrate by using laser irradiation [17,18]. The transfer process techniques are attractive because the technical barriers for the fabrication of complex circuits are low, as conventional processes are applied for device fabrication. In addition, no new mass production equipment or substrate handling equipment has to be developed and installed, at least for the display applications. However, two major issues limit the transfer process approach: the cost, which will always be higher than for devices fabricated on glass, because of the additional transfer process, and the yield of the transfer processes. Direct fabrication onto the polymer substrate itself is certainly more attractive, but it does present major issues, related to the handling and the dimensional instability of the polymeric substrate at typical processing temperatures and the severe temperature limitations in the fabrication process, due to the low maximum processing temperatures, TM , of polymeric substrates. In general, polymers have coefficient of thermal expansion, CTE, larger than those of inorganic materials used for the TFT fabrication [19]. As a result, when depositing an inorganic layer on the polymeric substrate at T > room temperature, due to the CTE mismatch, a strain will build up in the system at room temperature. Consequently, the deposited inorganic films will be subjected to a compressive stress while the substrate will be under tensile stress, causing the structure to distort into a cylindrical shape. The curvature in the polymeric substrates can clearly worsen the handling issues during processing. Furthermore, the polymeric materials show a tendency to shrink at typical processing temperatures. To maintain dimensional stability throughout the TFT fabrication process, they need to be pre-shrunk, prior to device processing, by annealing them at the intended processing temperature [20,21]. Considering the shrinkage rates of the most commonly used polymeric substrates [21] and the required alignment tolerance in the photolithographic process (typically of the order of 3 ␮m), preshrunk cycles lasting ∼10–100 h are needed to achieve a sufficient degree of stability. In spite of the described difficulties to fabricate directly on polymer substrates, LTPS TFTs have been demonstrated on several polymer substrates. Depending upon the glass transition temperatures, Tg , for polymer substrates [22], we can roughly divide the polymer substrates into two groups: those having an ultra-low maximum processing temperature TM < 250 ◦ C, including polyethersulphone (PES), polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and polycarbonate (PC), and those with 250 ◦ C < TM < 350 ◦ C, including polyarylite (PAR), polynorbonene (PNB) and polyimide (PI). Therefore, the direct fabrication of LTPS TFTs can be also divided into two main strategies: development of ultra-low temperature (<250 ◦ C) processes and adapting existing processes to reasonably low temperatures (250–350 ◦ C). Several groups have followed the challenging target to use lowcost polymer substrates, in spite of the more stringent limitations on the maximum processing temperatures [23–26]. However, some processing steps had to be completely modified, compared to conventional LTPS TFT processing on glass, including the use of sputtered a-Si as precursor or the source–drain contact formation. The use of polymer substrates with 250 ◦ C < TM < 350 ◦ C appears more attractive for several reasons: (i) it is possible to use many of the conventional processes adopted for LTPS TFT fabrication with minor modifications; (ii) key materials, such as dielectrics, which are fundamentally poorer at low temperatures (<250 ◦ C), show adequate performance in the T-range 250–350 ◦ C; and (iii) ULTPS TFT performance are generally inferior to those fabricated at T > 250 ◦ C, as key fabrication steps, such as device hydrogenation, are problematic at very low temperatures [20]. Therefore, more recent activities have focused on the use of PAR [20,27] and PI [11,20,28–32] and devices with performance comparable to those fabricated on glass have been demonstrated [30,32]. To solve many of the handling and dimensional stability issues associated with the use of polymeric substrates it has been recently proposed by Philips Res. Lab [33] the so-called Electronics on Plastic by Laser Release (EPLaR) process. In the EPLaR process, PI is spin-coated onto glass or quartz substrates and laser released after device fabrication, by irradiating the PI through the substrate. This process has been also applied by the CEA-LETI group for the fabrication of p-channel LTPS TFTs on PI [30,31]. We recently developed an LTPS process, inspired to the EPLaR process, where PI is simply peeled off from the substrate, thus eliminating the laser release step [11,29]. The direct fabrication process using spin-coated polyimide presents several advantages: PI can stand up to 350 ◦ C, it shows a high chemical resistance and can be detached, even mechanically, from a rigid holder without affecting the overlying devices. The direct fabrication of polysilicon TFTs on polymeric substrates implies many challenges and suitable solutions must be adopted to obtain high-performance sensing system with a yield close to industry needing. In the next paragraphs we will discuss in some detail those proposed for the three main fabrications steps: active layer crystallization; low-temperature deposition of high quality gate dielectrics; formation of source/drain contacts. 2.1. Major fabrication issues and solutions Although PI can stand relatively high temperatures (up to 350 ◦ C), the temperatures associated with solid-phase crystallization of amorphous Si (a-Si) (>550 ◦ C) are much higher than the Please cite this article in press as: L. Maiolo, et al., Flexible sensing systems based on polysilicon thin film transistors technology, Sens. Actuators B: Chem. (2012), http://dx.doi.org/10.1016/j.snb.2012.10.093 G Model SNB-14726; No. of Pages 11 ARTICLE IN PRESS L. Maiolo et al. / Sensors and Actuators B xxx (2012) xxx–xxx maximum processing temperature of polymer substrates. Therefore excimer laser crystallization remains the only process able to convert the amorphous silicon (a-Si) precursor into polysilicon [11,15,16] with a thermal budget compatible with PI substrates. In fact, in pulsed excimer laser annealing, typically performed at a wavelength of 308 nm (XeCl) with pulse duration of 30 ns, the ultra-fast heating of the near surface region induces melting of the Si layer, while the temperature rises by no more than a few hundred degrees in depth in the sample. Actually, it has been shown by using phase field simulations that the temperature at the barrier layer/PI interface can exceed Tg of PI, while still no degradation of the substrate was observed [29]. This is presumably due to the very short dwell time at T > TM so that the limited thermal budget is insufficient to cause damage to the polymer substrate. In order to prevent substrate damage by excessive heating, barrier layer of dielectric films must be engineered in order to minimize the thermal budget that the PI has to stand. Generally, low temperature silicon oxide and silicon nitrite films deposited with remote PECVD techniques can provide a good barrier layer with a very low mechanical stress [11,34,35]. Another issue to consider in the crystallization of the a-Si into polysilicon, is related to the hydrogen content of the precursor. Indeed, when a-Si is irradiated by excimer laser, the hydrogenated amorphous silicon (a-Si:H) can undergo severe morphologic and structural modifications, due to the nucleation and growth of volume defects, formed by the agglomeration of H2 and the subsequent out-diffusion, leaving cavities in the Si layer. Unfortunately, amorphous silicon films deposited at low temperature (<300 ◦ C) by PECVD show a rather high hydrogen content (around 10 at.% in films deposited at 250 ◦ C and increases for decreasing deposition temperatures [36]). To overcome these problems, two different strategies can be adopted: (1) the use of a-Si precursor film deposited by RFsputtering; (2) the laser de-hydrogenation/crystallization process. In the case of RF-sputtered a-Si films, although the H content can be substantially suppressed, the films normally contain an appreciable amount of the sputtering gas (typically Ar) to levels of the order of 3–5 at.% [37], thus shifting the problem from the H-removal to the sputtering gas removal. The laser de-hydrogenation/crystallization approach allows to use a-Si:H deposited by PECVD, which remains the most attractive solution as precursor, being a well-established technology with mass production equipment available. In general, in the laser de-hydrogenation/crystallization process the a-Si:H film is irradiated with increasing energy densities, in order to progressively heat the film to promote de-hydrogenation, while for higher energy densities the film is crystallized via the melt/resolidification process [11,20,29–32]. Different methodologies of laser de-hydrogenation/crystallization have been adopted, including multi-shot irradiation with a top-hat beam at increasing energy densities [16,38,39]; scanning a semi-Gaussian beam, thus irradiating the same area with an increasing energy density as the beam is moved forward [11,29]; by using multi-shot and multiple laser pass at increasing energies, either using a multishot (20 shots/point) first laser pass at low intensity (230 mJ/cm2 ), with a laser beam having a ramped leading edge for controlled release of the hydrogen, followed by a second multi-shot (20 shots/point) laser pass at 320 mJ/cm2 [20] or by using a multi-shot (10 shots/point) multi-pass process where the starting energy density was 100 mJ/cm2 and the final 210 mJ/cm2 incremented in step of 10 mJ/cm2 [40]. Most of these techniques, however, have been shown to require very high shot densities [20,39–42] and the crystallized material can show the inclusion of some voids and defects in the grains [38], limiting the performance of the polysilicon TFTs [38]. In order to reduce the shot density of the laser process, we have developed a combined low-temperature (300 ◦ C) thermal pre-treatments with laser dehydrogenation/crystallization process [11,29]. We showed that 3 weakly bonded H can be evolved even at such low annealing temperatures (300 ◦ C), as supported by ERDA and FTIR analysis of the samples [29], facilitating the laser dehydrogenation/crystallization process and allowing a reduced shot density. In addition to the active layer, the other critical step to fabricate high performance TFTs on polymer substrates is the gate dielectric. Indeed, previous works [43] have shown that SiO2 is fundamentally poorer, when deposited at low temperature, both electrically and with respect to defects, such as pinholes, causing a major issue for yield. Moreover, device performance seems to be inferior due to higher interface state densities and key processes for improving it, such as device hydrogenation, are problematic at low temperatures. Therefore, to develop LTPS TFTs on polymer substrates there has been not only a strong effort in optimizing SiO2 PECVD at low deposition temperatures [24,25] but also alternative deposition techniques have been investigated. In particular, high quality SiO2 films can be deposited at very low temperatures by high density plasma techniques, such as electron cyclotron resonance (ECR) PECVD [17,18,34] and inductively coupled plasma (ICP) PECVD [44,26,45]. ECR-PECVD oxides have been shown to have excellent electrical properties, to provide very good interface with polysilicon and to allow very low temperature deposition. Low interface state density can be achieved even using relatively low-temperature annealing (200 ◦ C) [34,46]. Another critical process in LTPS TFTs technology on polymer substrates is the formation of source–drain contacts, that is normally obtained by using ion-implantation, auto-registered to the gate electrode, followed by dopant activation, which can be obtained by either thermal [47] or laser [44,26,45] annealing. While conventional ion implantation or ion-shower doping techniques can be easily applied when using polymeric substrates [20,30–32,44,26,45], both dopant activation processes are problematic. In fact, thermal annealing requires temperatures (typically T > 450 ◦ C [47]) well above the polymer TM , while laser activation, which takes place after active layer island formation, implies UV irradiation of the polymer substrate. Since most of the polymer substrates have a strong absorption at 308 nm [27], UV irradiation can produce damage of the polymer and/or delamination of the films. To overcome this obstacle, several solutions to prevent the polymer substrate irradiation have been proposed, including the use of quarter-wavelength Bragg reflector [28] or absorbing barrier layer (a-Si) sandwiched between buffer oxide films [27,30,31]. A substantial simplification in the device fabrication can be achieved if a non-self-aligned architecture is adopted. In this case, ion doping of the source and drain is performed prior laser annealing and, using a single laser process, both a-Si crystallization and doping activation can be obtained [20,23,40]. Of course the process simplification implies a loss in device performance, due to the increased input capacitance, related to the gate/source and gate/drain overlap capacitances. In order to further simplify the fabrication process, laser doping technique has been also proposed to eliminate the ion-implantation step. In the laser doping technique, a film containing dopants is deposited on the under-laying Si layer and dopants are driven-in and activated through a meltregrowth process by using energy densities just above the melting threshold, so that the dopants can diffuse in the molten region [11,24,25,29]. 3. Smart sensing system fabrication on polyimide In this section, we will summarize some key technological steps, implying merging of the LTPS based electronics on polyimide with sensing devices, to obtain flexible sensing systems. In particular we will describe: the process to achieve ultra-thin PI substrates; the detailed fabrication process of LTPS TFTs on PI substrates; the Please cite this article in press as: L. Maiolo, et al., Flexible sensing systems based on polysilicon thin film transistors technology, Sens. Actuators B: Chem. (2012), http://dx.doi.org/10.1016/j.snb.2012.10.093 ARTICLE IN PRESS G Model SNB-14726; No. of Pages 11 4 L. Maiolo et al. / Sensors and Actuators B xxx (2012) xxx–xxx Table 1 Cured polyimide properties. PI2611 Density (g/cm3 ) at 25 ◦ C Tensile stress (MPa) Modulus (GPa) Elongation (%) Glass transition temperature (◦ C) Decomposition temperature (◦ C) Coefficient of thermal expansion (ppm/◦ C) Dielectric constant at 1 kHz 1.08 350 8.5 100 360 620 3 2.9 integration of both chemical and physical sensors with a polysilicon readout circuitry to proper manage and amplify the output signals. 3.1. How to get ultra-thin polyimide substrates Among different polyimides available in the market, PI2600 series has been intensively used for a huge number of different applications, reaching thickness as low as 4 ␮m [48,49]. In particular, PI 2611 is a low moisture, low stress, high glass transition temperature (>350 ◦ C) and high modulus of elasticity (8.5 GPa) polyimide from HD Microsystems designed for MEMS applications such as semiconductor and packaging dielectric as well as substrate material (see Table 1). Synthesis of polyimides is achieved by adding a dianhydride and a diamine into a dipolar aprotic solvent (like N,N-dimethylacetamide or N-methylpyrrolidinone) which rapidly forms poly(amic acid) at room temperatures. This precursor of polyimide can be easily stored, shipped or used to form thin films, coatings and fibers. Conversion of poly(amic acid)s to the designated polyimides is most commonly performed by thermal imidization. For wafer level manufacturing, this involves spin-coating of the precursor onto the wafer, which specifies the thickness of the layer, a prebake at modest temperature (∼120 ◦ C) to drive the solvent partly out of the layer, which makes it more sticky, and a curing step at high temperature (∼350 ◦ C) in nitrogen atmosphere [48]. This polyimide is not transparent in the visible range, but it has a light yellow color for a thickness of few microns. The PI could be patterned using dry etching techniques as well as TMAH etching (for example: developers MF 319 and AZ 300). Film thicknesses in the range 4–8 ␮m can be obtained in a single coat with good uniformity by varying spin speeds in the range 1000–5000 rpm. To improve adhesion between PI2611 and the following inorganic layers, oxygen or ammonia plasma can be performed in order to chemically activate the polymer surface. The cured polyimide substrate can withstand metal sputtering such as Cr, Au and Al without substantial damage. the n+ film from the channel regions using a selective wet etching. After a furnace annealing at 350 ◦ C in N2 atmosphere for 10 h, in order to partially remove the hydrogen weakly bonded in the amorphous film, the samples were irradiated in vacuum by XeCl Excimer laser ( = 308 nm) by using a triangular line shape energy profile (≈250 mJ/cm2 as maximum laser energy) in overlapping multi-shot, scanning the whole surface area of the sample. This technique permit to remove the remaining hydrogen in the a-Si layer with the advancing low-energy density leading edge of the beam, while crystallization takes place at higher energy densities at the top of the beam. It should be pointed out that, concomitantly to the crystallization of the channel region, dopants, present in the source and drain regions, are also activated. After definition of the active layer island, the gate oxide was deposited (80 nm thick) at room temperature by ECR-PECVD. Then, via-holes were opened by wet etching and aluminum + 1%Si was deposited, to form source, drain and gate electrodes. Finally, to passivate Si-dangling bonds at the semiconductor–insulator interface, post-annealing treatments at 350 ◦ C in forming gas atmosphere for 30 minutes was performed. When the device fabrication was completed, the PI layer was mechanically released from the rigid carrier, which can be reused for a new fabrication process. It could be noted that, due to the low mechanical stress of the deposited films, no bending problems of the freestanding PI film were observed. In Fig. 2 the transfer and the output characteristics of LTPS TFTs on PI are shown and the main parameters are: leakage current below 10 pA, Ion /Ioff > 106 , threshold voltage Vt = 7 V, field effect 3.2. LTPS thin film transistor fabrication In this paragraph we report the details of the fabrication process that we developed to integrate n-channel LTPS TFTs directly on ultra-thin polyimide substrates (<10 ␮m) [11,13]. The devices were fabricated adopting a non-self-aligned architecture in order to simplify the device processing and avoid the use of the costly ion implantation technique for source/drain contacts formation (see Fig. 1). First, PI HD-Microsystem 2611 is spin-coated onto 3′ ’ thermally oxidized Si–wafer and cured at a maximum temperature of 350 ◦ C. Before starting deposition of active layers, we grow on PI a barrier layer consisting of a stack of silicon nitride and silicon dioxide layers by using ECR-PECVD system. Silicon nitride film is deposited at 300 ◦ C, 50 nm thick, from a mixture of SiH4 , NH3 and He gases. Subsequently, a silicon oxide film of about 400 nm is deposited at 200 ◦ C. After the deposition of the barrier layer, a film of hydrogenated amorphous silicon (a-Si:H), 40 nm thick, is deposited by PECVD at a temperature of 300 ◦ C. Then, a highly doped a-Si:H PECVD layer (25 nm thick) is deposited by PECVD at a low temperature, using a SiH4 + PH3 (1%) gas mixture. Source and drain regions were defined by photolithography, removing Fig. 1. Schematic of the main fabrication steps of LTPS TFTs on polymer substrates. Please cite this article in press as: L. Maiolo, et al., Flexible sensing systems based on polysilicon thin film transistors technology, Sens. Actuators B: Chem. (2012), http://dx.doi.org/10.1016/j.snb.2012.10.093 G Model SNB-14726; No. of Pages 11 ARTICLE IN PRESS L. Maiolo et al. / Sensors and Actuators B xxx (2012) xxx–xxx Fig. 2. (a) Transfer and (b) output characteristics for a LTPS TFT with L = 10 ␮m, W = 10 ␮m fabricated on PI. mobility 50 cm2 /V s and subthreshold slope 0.9 V/dec. The device electrical characteristics were also measured under compressive and tensile strains at four different bending conditions (7.5, 5, 3 and 1.3 cm bending radius) and no relevant effects on transfer characteristics were observed for both strain condition cases [11]. 3.3. Resistive and capacitive sensors fabrication on polyimide LTPS TFTs based flexible electronics can provide a proper sensor signal amplification and conditioning, but in order to further reduce the fabrication cost, minimize the device dimensions and 5 maintaining a high degree of system mechanical flexibility, readout circuit interfaces must be integrated on flexible substrate together with sensors. The simplest kind of flexible sensor is a resistive one and it can be formed by an interdigitated metal electrodes (IDE) covered with a specific chemical interactive material (CIM) that can be eventually nanostructured in order to enhance the sensitivity of the device [50]. The resistive metal structure is directly fabricated above the buffer layer, made by silicon nitride and silicon oxide stack deposited at low temperature close to the island of the LTPS TFTs that forms the readout circuitry (see Fig. 3a). All these devices are usually encapsulated with another polyimide layer avoiding any other stress that could bend the device, once detached from the rigid carrier. In our resistive sensors aluminum, gold and chromium are used as underlying electrodes and a blend of polyaniline (PANi) mixed with different polymers such as polyvinilpyrrolydone (PVP), polyethylene oxide (PEO), poly(methylmethacrylate) (PMMA) and polystyrene (PS) were used to fabricate nanofibers deposited by electrospinning technique [14,51]. The fabricated transducer has dimensions w = 2000 ␮m, d = 200 ␮m and h = 1500 Å. The distance between two parallel fingers is 200 ␮m and there are 4 fingers per electrode. The sensor works since it can transduce the chemo-physical reactions, occurring between analyte and sensing material, into conductance variations. Such a structure can be easily controlled and read with an active Wheatstone micro-bridge configuration (see Fig. 3b) that converts the resistance variation into a voltage signal [50] and that is able to prevent undesirable sensor fluctuations due to temperature and/or power supply changes. A reference structure is placed side by side the sensor and it can consist of another IDE completely isolated from the analyte. To fabricate capacitive sensors, the IDE structure can be also used, but to avoid electric field dispersion through the air, the thickness of the interdigitated metal has to reach several microns, thus compromising the integrity of the metal itself that tends to cracks more easily under bending stress. To overcome this problem a nonplanar structure can be chosen. The simplest one is a structure consisting of two parallel overlapping metal plates separated by a dielectric material. The upper metal plate is typically patterned in lines or in squares allowing the absorption of chemical species in the sensing dielectric (see Fig. 4). In this way a certain amount of the dielectric surface is in direct contact with the analyte [51]. Several circuit configurations can be adopted in order to interface a flexible capacitive sensor using different conversion techniques or strategies: switched capacitors, sigma–delta converter, amplitude modulation technique, oscillating circuits, etc. A simple circuit interface based on ring oscillator (RO) directly integrated on PI can be successfully adopted. The RO circuit consists of a Fig. 3. (a) Schematic of the whole stack of materials for the definition of the resistive sensor integrated with LTPS TFTs; (b) Schematic of the active Wheatstone micro-bridge configuration. In this simple circuit the RSENS variation produces VAB shift and the temperature effects are reduced by the presence of RSREF . Please cite this article in press as: L. Maiolo, et al., Flexible sensing systems based on polysilicon thin film transistors technology, Sens. Actuators B: Chem. (2012), http://dx.doi.org/10.1016/j.snb.2012.10.093 G Model SNB-14726; No. of Pages 11 ARTICLE IN PRESS L. Maiolo et al. / Sensors and Actuators B xxx (2012) xxx–xxx 6 Fig. 4. (a) Schematic of the capacitive flexible sensor connected to the readout interface, consisting of a ring oscillator based on LTPS TFTs. (b) A detail of the square shape of the upper electrode of the capacitive sensor, that allows the analyte diffusion. cascade of an odd number of identical inverter gates. The output of the last inverter was connected to the input of the first one. Since the circuit has only one stable operating point in DC mode (the logic inverter threshold voltage), it is inherently unstable. The ring oscillator circuit converts the sensor capacitance variations into the oscillation frequency shift, FOUT . For the definition of piezoelectric capacitive sensors based on PVDF and copolymers, the fabrication process is a little more complicated since it involves additional steps needed to enhance the piezoelectric properties of the film. First, a metal bilayer, typically aluminum/gold or aluminum/chromium, is evaporated and patterned defining the bottom electrode. In this way, the structure is connected to the main polysilicon TFT in an extended gate configuration (see Fig. 5). Then, the sensing polymer is deposited by spin coating and cured at 120 ◦ C for 2 h reaching a thickness of about 2 ␮m. The upper electrode, made by a chromium layer 60 nm thick is subsequently evaporated and patterned and the PVDF is etched from the substrate using a sacrificial mask of aluminum. The sensing element must be poled up to 200 V to properly align the dipoles embedded in the PVDF matrix, heating the device up to 85 ◦ C. At the end of this process we obtain the piezoelectric flexible sensor shown in Fig. 5. 4. Sensing system characterization Once fabricated and detached from the rigid carrier, the flexible sensing systems, employing both capacitive and resistive sensors, have to be properly interfaced with external buses, using different techniques (wire bonding, flip-chip bonding, etc.). The LTPS electronics, embedded in these sensing systems were monitored before and after the operation of detachment and interfacing showing the same functional behavior for bending radii up to 0.5 cm. With this technology we fabricated chemical flexible sensing systems to detect volatile organic compounds (VOCs, e.g. ethanol, propanol, methanol, lactic acid, etc.), gases (e.g. NH3 ) and humidity [13,14,50]. We also tested physical flexible sensors to register dynamic pressure changes and temperature variations [52]. In case of chemical sensors, the measurements were carried out at room temperature using a MKS 247 mass flow controller, where N2 gas (carrier) was mixed with both increasing concentration of NH3 by a cylinder at known concentration or controlled percentages of selected VOCs. For humidity sensors evaluation the tests were done at a temperature of 25 ± 0.5 ◦ C in a test chamber, made of a water repellent and chemical resistant material. Dynamic variations of the relative humidity were obtained by introducing into the chamber a stream of dry nitrogen mixed with an another nitrogen stream loaded with deionized water vapor in the proper ratios. In case of pressure flexible sensors, the device was pressed by a vibration generator or a shaker against a commercial force probe and the signal was monitored through an oscilloscope in order to record the sensor behavior for different applied stimuli at increasing frequencies (up to 600 Hz); at lower working frequency a digital electrometer/voltmeter Keithley 6517A was used. 4.1. Flexible chemical sensors responses As already mentioned, we used both capacitive and resistive flexible gas sensors, exploiting an integrated LTPS based electronics on ultra-thin polyimide substrates as readout interface. We performed tests holding the sensor in a fixed position and by analyzing its variations at different bending radii. In case of humidity sensor, we found best sensitivity for the capacitive sensor based on benzocyclobutene (BCB), as sensing material, connected to a LTPS RO circuit. As can be seen in Fig. 6a and b the behavior of the humidity sensor is very similar to a commercial one in terms of signal amplitude and recovery time. In the case of ammonia sensor we monitored the relative resistance variation for three different polymer fibers deposited by electrospinning on the interdigitated structures (PANI/PS, PANI/PVP, PANI/PEO). A similar sensor device, not exposed to the NH3 , was fabricated close to the first sensor in order to compensate temperature variation. In Fig. 7 the sensor response curves are Fig. 5. (a) Cross section of the flexible pressure sensor, based on the piezoelectric capacitance connected to a LTPS TFT, using the extended gate configuration. (b) Image of a flexible pressure sensor, employing a circular piezoelectric capacitive sensor with a diameter of 1 mm, integrated with a LTPS TFT on polyimide. PVDF-TrFE was used as sensing material and acted also as passivation layer of the LTPS electronics. Please cite this article in press as: L. Maiolo, et al., Flexible sensing systems based on polysilicon thin film transistors technology, Sens. Actuators B: Chem. (2012), http://dx.doi.org/10.1016/j.snb.2012.10.093 G Model SNB-14726; No. of Pages 11 ARTICLE IN PRESS L. Maiolo et al. / Sensors and Actuators B xxx (2012) xxx–xxx 7 Fig. 6. (a) Dynamic capacitance variation of the capacitive humidity sensor, based on BCB, exposed to relative humidity steps varying from 35 to 70% showing the reproducibility of the sensor. The comparison with the output signal of the commercial device (HIH3602C) shows a similar behavior of the two sensors; (b) frequency variations of the humidity sensing system, including the capacitive BCB sensor integrated with a ring-oscillator based on LTPS TFTs, for increasing concentrations of H2 O. Fig. 7. The relative resistance variation due to increasing concentrations of ammonia diluted in N2 . Three different types of nanofibers (PANI/PS, PANI/PVP, PANI/PEO) were tested as sensing materials. reported. Probably the different ranges of sensitivity and saturation are due to differences in fibers’ structure and electrical properties. In the case of VOCs detection, we investigated the response of different sensing polymeric materials, such as BCB, PTFE and PMMA, that are widely used in the microelectronic fabrication processes and have a relative low cost: in fact, these films are easily available on the market or can be deposited exploiting spincoating procedures. Also in this case, we employed the capacitive sensor architecture shown in Fig. 4, with LTPS RO circuits as readout interface. We checked the sensing system behavior for four alcohols (ethanol, methanol, 1-propanol and 1-butanol), monitoring the frequencies variations for increasing analyte concentration. The variations of the analyte concentration were obtained by mixing two fluxes at different ratio: a dry flux of nitrogen and a flux of nitrogen loaded with the analytes, by means of a bubbler. Fig. 8. Responses of sensing system, based on capacitive sensors connected to a ring-oscillator, to different alcohols partial pressure for different materials used for the capacitive sensors: (a) BCB; (b) PMMA; (c) PTFE. In (d) an example of the dynamic responses of the BCB, PMMA and PTFE based sensing systems, reporting the frequency shifts (FOUT ) toward partial pressure variations of alcohols reaching 6240 Pa for ethanol, 563 Pa for 1-butanol, 1848 Pa for 1-propanol. Please cite this article in press as: L. Maiolo, et al., Flexible sensing systems based on polysilicon thin film transistors technology, Sens. Actuators B: Chem. (2012), http://dx.doi.org/10.1016/j.snb.2012.10.093 G Model SNB-14726; No. of Pages 11 8 ARTICLE IN PRESS L. Maiolo et al. / Sensors and Actuators B xxx (2012) xxx–xxx Fig. 9. Schematic of the tactile sensor common-source biasing configuration, with the two external resistances (Rbias , RLoad ) and two supplied voltages (Vb , V+ ). The fluxes were controlled and supplied into measure chamber by a mass flow controller, the total flux was maintained constant at 200 sccm. The saturated vapor pressure of each component has been calculated by the Antoine equation at 22 ◦ C, obtaining the following values: ethanol 6538 Pa, methanol 14,351 Pa, 1-propanol 2310 Pa and 1-butanol 704 Pa [13]. The measurements of the dynamic response were performed varying the VOCs partial pressure each 700 s and reaching 6240 Pa for ethanol, 563 Pa for 1-butanol, 1848 Pa for 1-propanol as maximum values (see Fig. 8d). The characteristics showed similar behaviors, in terms of response times and trend, both in the adsorption and in the desorption processes. The overall performances, in terms of relative frequency shifts (FOUT × 100/F0 ) calculated at different alcohols vapor pressures and for the different sensing materials, are shown in Fig. 8a–c. The measured response point out a specific behavior of each sensing material respect to the four alcohols examined. In fact, while from the PTFE response it is not possible to discriminate between ethanol and methanol, we observed different sensitivities (calculated as slope of the response curves) toward 1-butanol and 1-propanol. Moreover, PMMA and BCB exhibit higher relative frequency spreads and a non-linear behavior. Sensitivity values of a single sensing unit were evaluated [13] and, in the case of PMMA, the detection limit was estimated to be around 240 ppm for methanol and 620 ppm for ethanol. These sensitivities are an order of magnitude lower than conventional sensorial systems but are perfectly comparable with other flexible sensorial systems [53,54]. By analyzing all data concerning the responses toward VOCs using standard multivariate data analysis techniques, we showed that, a sensing system, including sensors based on the three differ- Fig. 10. Charge variation of the flexible piezoelectric capacitor, based on PVDF-TrFE, for increasing applied forces. Fig. 11. Flexible pressure sensor output signal for a sinusoidal stimulus of 1 N at a frequency of 300 Hz with a preload force of 350 gr. The tactile sensor was biased at a drain voltage (V+ ) of 10 V and a gate voltage (Vb ) of 7.5 V, using RLoad = 250 k and Rbias = 2 M, exploiting a common-source biasing configuration. ent examined materials, allow the discrimination of methanol from ethanol [13]. 4.2. Flexible pressure sensors Flexible pressure sensors were successfully tested by using the electromechanical equipment that consists mainly of a shaker which can generates random dynamic forces up to 18 N with frequency in the range 2 Hz to 18 kHz. The force generated by the shaker is measured by a piezoelectric load cell (PCB Fig. 12. (a) Flexible pressure sensor signal amplitude for increasing applied voltage of the incoming stimulus at a frequency of 300 Hz with a preload force of 350 gr; (b) pressure sensor signal amplitude for a stimulus of 2 N at increasing frequencies with a preload force of 350 gr. Please cite this article in press as: L. Maiolo, et al., Flexible sensing systems based on polysilicon thin film transistors technology, Sens. Actuators B: Chem. (2012), http://dx.doi.org/10.1016/j.snb.2012.10.093 G Model SNB-14726; No. of Pages 11 ARTICLE IN PRESS L. Maiolo et al. / Sensors and Actuators B xxx (2012) xxx–xxx Piezotronics), which can move along the z-axis. The devices were inserted between the commercial sensor probe and the shaker; during the mechanical measurements a protective PDMS rubber, 300 ␮m thick, was used to cover the sensor in order to avoid possible damages to the PVDF based structure. To perform the tests, we connected the flexible pressure sensor in a common-source amplifier configuration using two external resistances (Rbias , RLoad ) and two supplied voltages (Vb , V+ ) (see Fig. 9). First, we tested the response of the piezoelectric capacitor for increasing applied force, which was found linear, as shown in Fig. 10, and a piezoelectric coefficient d33 of 26.7 pC/N was extracted. Then, we analyzed the tactile sensor output by applying a sinusoidal force of 1 N at 300 Hz: the recorded signal is reported in Fig. 11, showing a response of 44 mV. In Fig. 12a the sensor output signal is reported for increasing input force amplitude up to 4 N at an incoming frequency of 300 Hz and a linear behavior of the device response was observed. We also tested how the sensor output changed for different working frequencies in the range 30–1200 Hz for an incoming sinusoidal stimulus of 2 N (see Fig. 12b). The high-pass behavior is due to the “series” RBias –CPVDF (leakage current through the gate oxide can be neglected). We preferred a gate-biased configuration over a floating-gate one (that has an almost flat frequency response) in order to reach the optimization of the quiescent point (and amplification) of the common-source amplifier. The correct sensor operation was confirmed by simulations (not shown) performed with a SPICE model of the sensor, which included an equivalent circuit of the lossy piezoelectric capacitor, in qualitative agreement with the experimental data. 5. Conclusions The increasing demand of flexible sensors in a number of sectors, including biomedical, food control, domotics and robotics, as well as the need to improve their signal-to-noise ratio, has stimulated a strong activity on the integration of sensors with readout electronics directly on flexible substrates. Among the different technologies available for the fabrication of TFTs based circuits on flexible substrates, the LTPS technology is particularly attractive, since LTPS TFTs show excellent electrical characteristics, good stability and offer the possibility to exploit CMOS architectures. In this work we have reviewed the different aspects for the direct fabrication of LTPS TFTs on polymer substrates. Then, as examples of flexible sensing systems, we have shown both chemical and physical sensors integrated with LTPS TFTs frontend electronics. In particular, we fabricated and tested gas sensing systems based on resistive sensors, exploiting nanofibers of PANI mixed with different polymers, and integrated with active microbridge, and sensing systems based on capacitive sensors, using polymeric materials such as PMMA, PTFE and BCB, and integrated with ringoscillator circuits based on LTPS TFTs. The sensing systems based on the three capacitive sensors was able to detect different species, such as humidity and VOCs (ethanol, methanol, 1-propanol and 1-butanol) and, by using standard multivariate data analysis, discrimination of methanol from ethanol was shown to be possible. These results could be relevant in food applications and encourage the further development of the sensorial systems based on capacitive flexible sensor technology. We also presented new results for a flexible pressure sensor, where a piezoelectric capacitance, based on PVDF-TrFE, is directly connected to the gate of an LTPS TFT in an extended gate configuration. The device response was tested for different frequencies and different applied forces and showed excellent performance, portending the fabrication of a more sophisticated prototype of artificial skin for robotics applications. 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Metta, Piezoelectric flexible tactile sensor based on poly-silicon TFT for humanoid robots, in: International TFT Conference (ITC2012), 30–31 January, Lisbon, Portugal, 2012. [53] S.J. Kim, Flexible alcohol vapor sensors using multiple spraycoated SWNTs on PES substrates, Journal of the Korean Physical Society 54 (5 (Pt. 1)) (2009) 1779–1783. [54] C. Wongchoosuk, A. Wisitsoraat, A. Tuantranont, T. Kerdcharoen, Portable electronic nose based on carbon nanotube-SnO2 gas sensors and its application for detection of methanol contamination in whiskeys, Sensors and Actuators B 147 (2010) 392–399. Biographies Luca Maiolo received the Master degree in Physics in 2003 and the Ph.D. on “Realization and characterization of polysilicon based electronic devices on flexible substrates for electronics on plastic” in 2008, both from “Università degli studi di RomaTre”. In the biennium 2004–2005 he was a research assistant at Institute for Photonics and Nanotechnologies, working on fabrication and electrical characterization of polycrystalline thin film transistors for large area electronics applications. In 2009 he joined to Robotics, Brain and Cognitive Sciences Department of Italian Institute of Technology in Genova, as post-doctoral fellow, where he developed and tested tactile flexible sensors for humanoid robot. Currently, he works as researcher at the Institute for Microelectronics and Microsystems of the National Research Council (CNR-IMM). His activity is mainly focused on fabrication and characterization of electronic circuits and smart sensors integrated on ultrathin flexible substrates. Alessandro Pecora received the Laurea degree in Physics in 1990 from University of Rome “La Sapienza”, with a dissertation about solid-surface electron spectroscopy. In May 1990 he joined the Institute for Photonics and Nanotechnologies (IFN) of National Council of Research in Rome working up to 2000 as researcher with several fixed term contracts in the fields of fabrication of amorphous and polycrystalline thin film transistors, characterization and their main application as sensors and switching element in pixel for active matrix liquid crystal displays. In 2001 he was appointed permanent researcher at IFN and in 2008 he moved at the Institute for Microelectronics and Microsystems in Rome. Here, he is currently responsible of the activity of devices for large area electronics based on polycrystalline silicon thin film transistors (p-Si TFTs). In particular his activity is focused the following themes: (1) develop of fabrication techniques of p-Si TFTs on flexible substrates at low process temperatures; (2) study of electronic transport properties as high electric field related phenomena, off-current, noise performances, etc.; (3) characterization of Plasma Chemical Vapour Deposition of gate dielectrics for TFTs; (4) develop of physical and chemical sensors with readout electronic circuits based on p-Si TFTs. He is author of about 70 journals papers, 50 conference proceedings papers and is also co-inventor of 4 patents. Francesco Maita was born in Rome, Italy in 1984. He received Master degree in Electronics Engineering from “Università di Roma Tor Vergata” in 2009. Currently he is a Ph.D. student on “Engineering Of sensorial and learning systems” and his studies are focused on the development of electronics on flexible substrate. Antonio Minotti is currently working at CNR-IMM in Rome as technician. He received first level degree in Material Science in 2002 from “Università degli studi di Roma Tre”. His research activities have been focused mainly on MEMS systems, C-MUT (Capacitive Micromachined Ultrasonic Transducers) and recently on microfabrication processes and thin-film depositions on flexible substrates. Emiliano Zampetti received the Master degree in Electronics Engineering in 2002 and the Ph.D. on “Engineering of sensorial and learning systems” in 2007 from “Università degli studi di Roma Tor Vergata”. Currently he works as researcher (nonpermanent position) at the Institute for Microelectronics and Microsystems of the National Research Council (CNR-IMM). His research interests are concerned with the design and development of the electronic circuits for Electronic Nose Systems, electronic readout circuits for gas sensor, electrospinning apparatus for nanotechnology applications and bio-electronic noise in cells and neurons. Simone Pantalei received the Master degree in Electronics Engineering in 2002, and the Ph.D. on “Engineering of sensorial and learning systems” in 2007 from “Università degli studi di Roma Tor Vergata”. Currently he works as researcher (non-permanent position) at Institute for Microelectronics and Microsystems of the National Research Council (CNR-IMM). His research interest include the enhancement of the sensing capabilities of the Quartz Crystal Microbalances, design of Multichannel Quartz Crystal Microbalance, finite elements analysis applied to gas sensing systems. Antonella Macagnano is Research Scientist at the Institute of Microelectronics and Microsystems (IMM) of CNR (since 2001). She graduated in Biological Sciences at the University of Lecce, Lecce, Italy, in 1993 and obtained professional degree as Biologist, in 1994. Mainly, her research activities have concerned the study, the design, the characterization and the optimization of chemical (metallo-porphyrins, cavitands, metallo-oligomers and nanostructured polymers) and biological membranes (oligopeptydes, bio-polymers) for selective interactions with both gases and volatile organic compounds. She has been involved in several International and National Please cite this article in press as: L. Maiolo, et al., Flexible sensing systems based on polysilicon thin film transistors technology, Sens. Actuators B: Chem. (2012), http://dx.doi.org/10.1016/j.snb.2012.10.093 G Model SNB-14726; No. of Pages 11 ARTICLE IN PRESS L. Maiolo et al. / Sensors and Actuators B xxx (2012) xxx–xxx research projects focused on design, improvement and implementation of sensor devices for environment, health and agri-food industry. Andrea Bearzotti was born in Udine, Italy on 19 October 1960. He received the Italian Laurea degree in Physics from the “Università degli studi di Roma La Sapienza” in February 27, 1986 with a thesis titled “M.O.S. structure with palladium gate as hydrogen and oxygen sensor”. He then joined the “Solid State Electronic Institute” of the C.N.R. on September 1, 1988 where he was working in the field of solid-state sensors. From February 1998 he’s working at the Microelectronics and Microsystems Institute (I.M.M.–C.N.R.). His present interest is focused on the development of mesostructured materials for chemical sensors. Davide Ricci received his degree in Physics (1989) and Ph.D. in Electronic Engineering (1993) at the University of Genoa. Since 1996 holds a permanent position at the University of Genoa. He is currently Team Leader at the Robotics Brain and Cognitive Sciences Department of the Italian Institute of Technology and supervises the Soft Materials Design Laboratory. His research focus is on the exploitation of nanomaterials and the development of new technologies for brain computer interfaces 11 (high-density, low-impedance, conformable in vivo electrode arrays), robotics (artificial skin and muscles) and tissue engineering (prosthetics) applications. He has co-authored over 50 journal articles and several book chapters. Guglielmo Fortunato is research director and is responsible of the “Devices for large area electronics” Unit at IMM-CNR. He has been a visiting scientist at the Tokyo Institute of Technology (1983) and at GEC-Hirst Research Center (1985–1986). His main scientific activity is on the physics and technology of inorganic (amorphous, microand polycrystalline silicon) and organic (pentacene) thin film transistors. Recently he focused on flexible electronics, application of excimer laser annealing for polysilicon PS-nTFTs and shallow junction formation. He has been responsible of several National (9) and European (6) Research contracts and also of industrial research contracts with ST-Microelectronics, Philips, THALES, GEC-Marconi. He was parttime professor of Semiconductor Device Physics at the Roma III University between 2001 and 2006. He has been co-organiser of several E-MRS Symposia and has been recently Chairman of the Third International Thin Film Transistor Conference (Rome, 24–25 January 2007). Please cite this article in press as: L. Maiolo, et al., Flexible sensing systems based on polysilicon thin film transistors technology, Sens. Actuators B: Chem. (2012), http://dx.doi.org/10.1016/j.snb.2012.10.093