Development of low-cost poly(vinyldifluoride) sensor for low-pressure application B.P. Mahale 1 , D. Bodas 2 , S.A. Gangal 1 1 Department of Electronic Science, University of Pune, Ganeshkhind Road, Pune 411 007, India 2 Centre for Nanobioscience, Agharkar Research Institute, GG Agarkar Road, Pune 411 004, India E-mail: dhananjay.bodas@gmail.com Published in Micro & Nano Letters; Received on 24th February 2011; Revised on 11th April 2011 The development of a poly(vinyldifluoride) (PVdF)-based dynamic pressure sensor for low-pressure application is reported. b-phase PVdF film of thickness 10 mm is fabricated using the spin coating method with thermally evaporated Al electrodes (200 nm) on both sides of the film. The film is polled and packaged in a poly(dimethyl siloxane) block. The exposed area of the pressure sensor is ≏500 mm in diameter. A signal conditioning circuit is designed to amplify the signal and a NI DAQ card with LabVIEW software is used to acquire the signal on a PC. The dynamic pressure response of the sensor is recorded which shows linearity in detection. Pressure measured by the sensor is in the range of 10–150 kPa. 1. Introduction: In some pneumatic and hydraulic applications, there is a requirement of sensing the change in pressure at the lower range ≏100 kPa. To develop such a type of sensor, better sensitivity and linear response are essential parameters with wide working temperature range, faster response time, good chemical and thermal stability and low cost. Considering these requirements, poly(vinyldifluoride) (PVdF) was chosen for development of a sensor. PVdF is the most popular semi-crystalline polymer because of its piezoelectric property. In addition, its flexibility, chemical stability, biocompatibility, low acoustic impedance, high bandwidth, ease of fabrication, low weight and low cost have attracted most of the researchers in the field of sensors and actuators. PVdF is being used in the field of SONAR, biomedicine, acoustics, pneumatic and hydraulic systems, MEMS and bioMEMS-based applications. For many years PZT and quartz have been widely used in piezoelectric application areas, but recently PVdF has gained popularity in the field of sensors and actuators because of its versatility [1–7]. Researchers have developed a PVdF-based sensor for different applications. For example, Shirinov and Schomburg [1] have developed a pressure sensor using a PVdF film packaged in PVdF material; Gonza ´lez-Moran et al. [2] have reported the PVdF pressure sensor for biomedical applications; and Yi and Liang [3] have modelled and successfully experimented with the PVdF-based deformation and motion sensor. The important property for the use of PVdF as a sensor is its piezoelectric property, which is based on b-phase content in the film [7, 8]. In b-phase PVdF, the chains are packed in the unit cell in such a way that the dipoles associated with individual molecules are parallel to one another, leading to the non-zero dipole moment of the crystal [9]. Stretching of the PVdF film is essential to generate b phases in the film. Sencadas et al. [7] and Sajkiewicz et al. [9] have developed b-phase PVdF using mechan- ical deformation. Davis et al. [10] have reported development of b- phase in PVdF by high electric polling. Ma et al. [11] have studied the formation of b-phase in PVdF by using mixtures of polar sol- vents. Gregorio and Cestari [12] have studied the effect of crystal- lisation temperature on phase in PVdF. The present Letter reports an easier fabrication process; spin coating, for producing b-phase PVdF film as the sensing element [8]. Spin coating provides mechanical stretching of the film, overcom- ing limitations, viz. expensive instrumentation, tedious procedures and so on, over commercial methods of stretching. The developed sensor has a sensing area smaller than that reported in [1] and the packaging of film was carried out using poly(dimethyl siloxane) (PDMS). The development of the sensor includes preparation of the sensing element, metallisation, polling, packaging and testing of the sensor, which are discussed in the following paragraphs. 2. Materials and methods 2.1. Film fabrication: PVdF in powder form (Sigma-Aldrich Co.) was procured and used to form the film. Polar solvent N-methyl pyrolidone (NMP) (Central Drug House, New Delhi) was used to dissolve the PVdF. A solution of 20 wt% PVdF in NMP was pre- pared by mixing the 1 g PVdF powder in 4 ml NMP and heating at 608C with continuous stirring for 45 min. Spinning speed and time were optimised to obtain desired thickness and b-phase in the film. Stretched PVdF film was spin coated using the completely dissolved PVdF solution at 1000– 2200 rpm for 40 s on the silicon substrates. A single wafer spin processor WS-400E-6NPP-LITE (Laurel Technologies Corporation) was used for spin coating. Spin coated films were annealed at 608C for 20 min to evaporate the solution and thereby cure the film. Annealing temperature higher than 708C reduces the b-phase content in the film; hence temperature was optimised to 608C. Films were peeled off from the substrate for further processing. 2.2. Metallisation: 200 nm-thick Al electrodes were deposited on both sides of the film covering the entire area by physical vapour deposition technique for making contacts on the film. 2.3. Polling: Polling of the film was carried out by applying an elec- tric field of 80 MV/m across the electrodes at 1108C for 3 h and then at room temperature for 1 h. Polling ensures the permanent align- ment of dipoles in the direction of the applied field [7]. 2.4. Packaging: PDMS is an elastomer having good chemical and thermal stability [13]. It was used as a packaging material to host the PVdF pressure sensor. To cast the PDMS in a desired sensor shape two cylindrical moulds were fabricated in acrylic for the upper and lower parts of the sensor. A two-component PDMS (Sylgard 184 from Dow Corning Chemicals Ltd) was mixed in a 10:1 (base curing agent) ratio, degassed, poured in both the moulds and cured at 808C for 2 h. The diameter of both moulds was 6 mm. PDMS was casted using 0.5 and 1 mm diameter wire and sealed together with the PVdF film sandwiched. To ensure appropriate connectivity between the sensor and the signal con- ditioning unit, Al tape as electrical contact was used on both sides of the PVdF film. Sealing was done by pressing two PDMS with PVdF and contact assembly together, by applying liquid PDMS on the joint and then curing the PDMS at 808C for 2 h. Fig. 1 shows the schematic of the sensor fabricated using PDMS as packa- ging material (cross-section and top view). Application of pressure on the PVdF film generates voltage on the electrodes due to the piezoelectric effect. This signal was transmitted to the signal con- ditioning unit through electric contacts for further measurements. 540 Micro & Nano Letters, 2011, Vol. 6, Iss. 7, pp. 540–542 & The Institution of Engineering and Technology 2011 doi: 10.1049/mnl.2011.0082