Piezoresistive SU-8 Cantilever With Fe(III)Porphyrin ... - IEEE Xplore

IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 11, NO. 4, JULY 2012

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Piezoresistive SU-8 Cantilever With Fe(III)Porphyrin Coating for CO Sensing C. Vijaya Bhaskar Reddy, Mrunal A. Khaderbad, Sahir Gandhi, Manoj Kandpal, Sheetal Patil, K. Narasaiah Chetty, K. Govinda Rajulu, P. C. K. Chary, M. Ravikanth, and V. Ramgopal Rao, Senior Member, IEEE

Abstract—Carbon monoxide detection is required for various healthcare, environmental, and engineering applications. In this paper, 5,10,15,20-tetra (4,5-dimethoxyphenyl)-21H,23Hporphyrin iron(III) chloride (Fe(III)porphyrin) coated on a piezoresistive SU-8 microcantilever has been used as a CO sensor. Rapid detection of CO down to 2 ppm has been observed with aforementioned sensors. Cantilevers without Fe(III)porphyrin have not responded to CO exposure. Fe(III)porphyrin-coated cantilever selectivity toward CO has been analyzed by measuring the sensor response to various gases such as N2 , CO2 , O2 , ethanolamine, N2O, and moisture. The sensor has exhibited a fast response and recovery times and is fully recoverable after repeated exposures. Index Terms—Cantilevers, carbon monoxide, iron porphyrin, piezoresistance, sensor.

I. INTRODUCTION ONSIDERING its omnipresence, carbon monoxide (CO) is one of the most harmful compounds for human beings. CO can jeopardize oxygen transport in blood and it is one of the most important gases to be detected for gas sensor-based fire detection [1] applications. The efficiency of fuel combustion in combustion engines, power plants, fuel cells, and automobiles can be monitored by quantifying CO emission. This provides information not only for feedback control of combustion processes, but also to indicate various fire and health hazards [2]. This demand has stimulated research to realize low-power sen-

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Manuscript received January 6, 2012; accepted February 24, 2012. Date of publication March 12, 2012; date of current version July 11, 2012. This project is supported by the Department of Information Technology, Ministry of Communication and Information Technology, Government of India under the Indian Nanoelectronics Users Program (INUP) at Indian Institute of Technology Bombay (IIT Bombay). The review of this paper was arranged by Associate Editor J. Li. C. V. B. Reddy is with the Department of Mechanical Engineering, Srikalahasteeswara Institute of Technology, Srikalahasthi 517640, India (e-mail: [email protected]). M. A. Khaderbad, S. Gandhi, M. Kandpal, and V. Ramgopal Rao are with the Centre of Excellence in Nanoelectronics, Indian Institute of Technology Bombay, Mumbai 400076, India (e-mail: [email protected]; [email protected]; [email protected]; [email protected]). S. Patil is with the Nanosniff Technologies Pvt. Ltd., Indian Institute of Technology Bombay, Mumbai 400076, India (e-mail: [email protected]). K. N. Chetty and K. G. Rajulu are with the Department of Mechanical Engineering, Jawaharlal Nehru Technological University, Anantapur 515002, India (e-mail: [email protected]; [email protected]). P. C. K. Chary is with the Department of Mechanical Engineering, Sree Vidyanikethan Engineering College, Tirupathi 517501, India (e-mail: [email protected]). M. Ravikanth is with the Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400076, India (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNANO.2012.2190619

sors, which are compact and work in real-world conditions [3]. Various sensing elements comprising of nanomaterials to improve the selectivity and sensitivity of detection have been examined for this purpose. The usability of copper chloride (CuCl) as a selective CO sensor in the presence of 50 vol% hydrogen has been demonstrated for proton-exchange membrane fuel cell applications. However, the CO sensing performance of CuCl films is very sensitive to the synthesis and fabrication methods [4]. CO gas sensors based on gold (Au)-doped tin oxide (SnO2 ) have shown efficient sensing in the 80–210 ◦ C temperature range [5], [6]. Pthalocyanine nickel(II)-coated piezoelectric crystal has also been investigated as a CO sensor [7], [8]. Other types of sensors for CO detection include electrochemical sensors, thermoelectric, colorimetric detectors, and infrared detectors [9]–[12]. It is well known that molecules with a central metal atom surrounded by neutral or charged ligands have been employed for active gas sensing, where vacant sites or weakly bound coligand interact with the metal ion. In this interaction, the gaseous molecule to be detected becomes a co-ligand either by occupying free coordination site or by displacing other ligands. Colorimetric sensor based on the interaction between CO and porphyrin films has been explored for the estimation of the biological damage due to CO exposure [13]. Radhakrishnan et al., have used polypyrrole functionalized with 5,10,15,20-tetraphenyl21H, 23H-porphyrin iron(III) chloride (PPy–FeTPPCl) as an active sensing material for CO gas. It was observed that semiconducting PPy–FeTPPCl’s resistance was drastically increased upon CO exposure [14], [15]. In this study, we demonstrate highly sensitive CO detection using 5,10,15,20-tetra(4,5-dimethoxyphenyl)-21H,23Hporphyrin iron(III) chloride (Fe(III)porphyrin)-coated SU-8 nanocomposite microcantilevers with integrated piezoresistive readout. As compared to conventional Si-based cantilevers, polymer (SU-8 as a structural layer)-based cantilevers provide higher sensitivity and use low cost fabrication techniques such as spin coating, evaporation and wet etching etc. [16], [17]. A typical (Fe(III)[T(4,5(OCH3 )2 P)P]Cl)-CO interaction modifies surface stress on the cantilever when exposed to the CO gas. The change in surface stress results in a resistance change in the integrated piezoresistor, thus enabling electrical detection. II. EXPERIMENTATION The polymer cantilevers for the fabrication of CO sensor were fabricated using the following process [17], [18]. Nanocomposite-based polymer microcantilevers of the dimensions, 200, 50, and ∼3.5 μm, length, width, and thickness,

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Fig. 1. Process sequence for polymer composite microcantilevers. (a) Silicon dioxide as a sacrificial layer. (b) First layer of SU-8. (c) Cr/Au for contacts. (d) SU-8/CB composite layer. (e) Encapsulating SU-8. (f) Thick SU-8 die base. (g) Release of cantilever die from the substrate.

Fig. 2. (a) Schematic of the porphyrin molecule and SEM images of the released nanocomposite microcantilever devices. (b) Change in resistance (ΔR/R) as a function of deflection.

respectively, are used in this study in order to improve the surface stress sensitivity, mechanical stability, and packaging compatibility of the sensors. Nanocomposite, based on a dispersion of carbon black nanoparticles in SU-8, a nonconductive, negative tone photoresist, is used as a piezoresistive layer in these polymer cantilevers. The SU-8/carbon black nanocomposite is obtained by homogeneously mixing the carbon black powder in the photosensitive SU-8 resin. The lower Young’s modulus of SU-8 compared to Si and the higher strain sensitivity of SU8/carbon black nanocomposite provides these devices the required sensitivity to detect CO down to the ppm sensitivity. The nanocomposite polymer microcantilever fabrication process sequence is illustrated in Fig. 1. The fabrication starts with RCA cleaning of silicon substrate with 500-nm thermally grown silicon dioxide as a sacrificial layer. SU-8 structural layer (SU8 2000.5, Microchem, MI) was spin coated and pre-exposure baked at 70 ◦ C and 90 ◦ C for optimized timings with a slow ramp up and ramp down to room temperature. To transfer the microcantilever pattern [see Layer-1, Fig. 1(b)], the samples were exposed to UV light using Karl Suss MJB3 mask aligner and subjected to a post exposure bake cycle, development and rinsed with Iso Propyl Alcohol (IPA). A thin layer of Cr/Au (10 nm/200 nm) was deposited by sputtering, and the contact pads were patterned using PPR photolithography with the corresponding mask [see Fig. 1(c)]. The Cr–Au layer was wet etched in respective enchants. To obtain an electrically conductive and a strain sensitive layer, SU-8/CB nanocomposite was prepared by dispersing the carbon black of 8–9 Vol.% in SU-8. The nanocomposite was spin coated and subsequently patterned using mask for layer 3 [see Fig. 1(d)] by UV lithography, followed by additional ultrasonic cleaning step in IPA. This strain

sensitive resistive layer is then encapsulated by 1.6 μm of SU-8 (2002) that is spin coated and photolithographically patterned using mask for layer 4 [see Fig. 1(e)]. Finally to form an anchor [see Fig. 1(f)] for the cantilevers, a 180-μm thick SU-8 was defined by spin coating and patterning of SU-8 (2100). The devices were released by wet etching the silicon dioxide layer in the buffered hydrofluoric acid approximately for 30 min. The arrays of released SU-8 nanocomposite microcantilever chips were rinsed in DI water, isopropyl alcohol and allowed to dry. Fig. 2(a) shows the SEM images of the released microcantilever devices. The microcantilever chips were characterized electromechanically to demonstrate the piezoresistive behavior. The tip of the microcantilever was deflected with a calibrated micromanipulator needle from Suss Microtech with simultaneous measurement of resistance using Keithley 4200 source measuring unit. The change in resistance (ΔR/R) as a function of deflection is given in Fig. 2(b), the calculated deflection sensitivity is 1.1 ppm/nm [18], which is higher compared to the polymer microcantilevers with Au as the strain gauge. The piezoresistance value of the fabricated cantilevers was measured to be 500 kΩ. The (Fe(III)[T(4,5(OCH3 )2 P)P]Cl) was dissolved in IPA solution (1 mg in 20 mL of solution) and the selected cantilevers were drop coated using a microdispenser. Further, the backside of the cantilever is coated with gold using the Nordiko sputter machine at a base pressure of 1.0 × 10−5 mbar and sputter pressure of 2.6 × 10−3 mbar to avoid the interaction of porphyrins with the CO vapors on the bottom side thus enhancing the system’s electrical response. The cantilever is then mounted onto a printed circuit board (PCB) and electrical connections were made between the cantilever and the preexisting contact leads on the PCB using

REDDY et al.: PIEZORESISTIVE SU-8 CANTILEVER WITH FE(III)PORPHYRIN COATING FOR CO SENSING

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Fig. 3. (a) Cantilever mounted on PCB and flow cell arrangement for gas sensing. (b) Experimental setup.

conductive silver epoxy (1:1) which included a 80 ◦ C heat treatment for curing purposes. The cantilever is then enclosed in a flow cell made of Teflon and sealed for further measurements. This standard flow cell has an inlet and an outlet for the gas flow as shown in Fig. 3(a). The experimental setup for sensor calibration used is shown in Fig. 3(b). CO and the carrier gas (N2 ) were allowed to flow out of the cylinders through flow controllers. The flow cell was connected to a gas-mixing chamber as inlet and the flow cell outlet was connected to a pump (not shown). The cantilever sensor is connected to a Wheatstone’s bridge and the dc voltage was recorded using the ADS123X TI board (Texas Instrument Board). The wheatstone bridge consists of four arms of which one arm is cantilever and the remaining three arms have potentiometers. Initially, the bridge is balanced by matching R4 with cantilever and R1 with R2. As the change in the resistance ΔR due to strain in the cantilever is very small, in orders of few ohms in 100 kΩ, nonlinearity error of the bridge is negligible. The output of the bridge is fed to one of the three input channels of ADC. The strain on the piezoresistive layer of the nanoelectromechanical cantilever results in the deflection sensitivity, which is an important performance parameter. The relative change in the resistance (ΔR) with respect to the fixed arm of the bridge is determined by the deflection sensitivity. By means of the Wheatstone bridge the change in the resistance is measured in terms of voltage. Sensitivity calculation of the current system is based on the change in output voltage for the corresponding resistance change in one of the arms of the bridge. N2 purge was carried out before the start of the experiment. Fe(III)porphyrin-CO interaction causes the cantilever to deflect, which changes the resistance of cantilever [14]. The change in the resistance was measured as a voltage change in the output [17]–[20]. III. RESULTS AND DISCUSSION Fig. 4(a) shows the response of a bare SU-8/CB cantilever for alternating cycles of CO and N2 gases at 500 sccm flow

Fig. 4. (a) Response of a bare SU-8/CB polymer composite microcantilever for consecutive cycles of CO and N2 . (b) Response of a Fe(III)porphyrin-coated microcantilever for consecutive cycles of CO and N2 .

rate. It can be clearly seen that the cantilevers did not respond to either of the gasses. Similarly, the cantilevers functionalized with Fe(III)porphyrin were exposed to alternating cycles of CO and N2 (500 SCCM) and the response is as shown in Fig. 4(b). From the figure, it is clear that there is an abrupt increase in the sensor’s response due to CO adsorption. A voltage output of ∼80 mV was observed on each alternating cycle, which shows the recovery and repeatability of the sensor. The sensor response and recovery times have been measured to be 1 and 2 s. This clearly demonstrates that only porphyrin functionalized microcantilevers respond to CO gas, while the bare cantilevers do not respond to the CO exposure. The overall time responses demonstrate that Fe(III)porphyrin-coated films show a very good response to CO at room temperature and ambient conditions. The Fe(III)porphyrin functionalized cantilevers were then exposed to various concentrations of CO ranging from 7 to 70 sccm and the response was plotted as shown in the Fig. 5. The porphyrin functionalized sensor was also tested for its selectivity by exposing it to various other gases such as CO2 , O2 , N2 0, and ethanolamine (300 sccm). Fig. 6 shows the response indicating that the Fe(III)porphyrin functionalized cantilevers did not respond to the other gases. Output response of the same cantilever for 300 sccm CO exposure was around 12 mV, depicting the high selectivity of the sensor toward CO. Further, Fe(III)porphyrin and CO binding was analyzed by Fourier transform infrared spectroscopy (FTIR). The FTIR spectra of Fe(III)porphyrin coated on Si before and after CO exposure is shown in Fig. 7. In the spectrum, an IR peak is observed

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firms that the Fe(III)porphyrin reported in this paper is efficient in CO binding and the functionalized cantilevers are suitable for CO sensing applications. Moreover, porphyrin molecules are known to be stable in solution as well as in solid-state form, for the long-term operation of these sensors [22], [23]. In addition, moisture effect on SU8 devices can be overcome with differential measurements and through postfabrication processes involving hard baking. Such baking for longer times is known to increase the polymer’s cross-linking density, which, in turn, decreases the sensitivity of the material to the environmental humidity [24], [25]. Fig. 5. Response of porphyrin functionalized SU-8/CB microcantilevers for different CO flow rates.

IV. CONCLUSION In conclusion, microcantilever-based CO sensor has been fabricated based on Fe(III)porphyrin-coated SU-8/CB cantilever. Experimental results indicate that the Fe(III)porphyrin-coated cantilevers have a very high sensitivity toward CO as compared to bare SU-8 cantilevers. In addition, Fe(III)porphyrin-coated cantilever’s selectivity toward CO was compared by measuring the response with gases such as N2 , CO2 , O2 , ethanolamine, N2 O, and moisture. ACKNOWLEDGMENT The authors would like to thank Prof. P. Mathur, Department of Chemistry, IIT Bombay, for CO facility.

Fig. 6. Response of porphyrin functionalized SU-8/CB microcantilevers for different gases.

Fig. 7. FTIR spectroscopy of Fe(III)porphyrin on Si before and after CO exposure.

around 1820 cm−1 after CO exposure, which is because of the metal–CO binding frequency. This indicates that CO interacts with the central metal ion of porphyrin, by forming a coordinate covalent bond with Fe with its lone pair of electrons [14], [21]. Moreover, microcantilevers coated with free base tetra phenyl porphyrin (H2 TPP) and zinc(II)tetra phenyl porphyrin (Zn(II)TPP) were tested for CO selectivity. These cantilevers did not show any sensing behavior toward CO. This is because of the lack of CO binding site (no metal) in freebase porphyrin and group 12 metals such as zinc. In Zn(II)TPP, Zn does not bind to CO as its d-orbitals are not really available. This con-

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C. Vijaya Bhaskar Reddy received the B.E. degree from S. V. University, Tirupathi, and the Masters degree from J.N.T.University, Hyderabad, India. He is currently working towards the Ph.D. degree with the J.N.T University, Anantapur, India. He is presently working in SKIT Engineering college Srikalahasthi, India as an Assistant Professor. His research interests are automotive sensors systems, bioinstrumentation, nanofabrication, thin and thick film sensor systems.

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Mrunal A. Khaderbad received the M.Tech. degree from VNIT, Nagpur and the M.Phil. from the University of Cambridge, U.K. Currently, he is pursuing the Ph.D. degree in Centre of Excellence in Nanoelectronics, Electrical Engineering Department, IIT Bombay, India. He was a Visiting Researcher at the Energy Research Institute at the NTU (Eri@N), Nanyang Technological University, Singapore. His research areas are Cu/low-k interconnects, NEMS, Sensors, and graphene electronics.

Sahir Gandhi is working toward the Ph.D. degree at Imperial College London, U.K. advised by Dr. Danny O.Hare and Prof. Martyn Boutelle at the Bioengineering department. He is a postgraduate in biomedical engineering from Imperial College London. He worked at IIT-B as a Sr. Research Assistant on Lab-on-Chip devices.

Manoj Kandpal received the M.Tech degree in materials science and engineering from the Indian Institute of Technology (IIT) Kharagpur, India, in 2008. He is currently a Ph.D. student in the Department of Electrical Engineering, IIT Bombay, India. His research interests includes piezoelectric nanocomposite based devices and chemical sensors.

Sheetal Patil received the Ph.D. degree from the University of Pune, India, in 2004. She has worked as a Research Scientist and Research Faculty, at Department of Electrical Engineering, University of South Florida, Tampa and Bio-MEMS Group, Department of Mechnical Engg., University of Maryland, College Park, respectively. She has published more than 25 peer reviewed journal papers in the fields of Microfabrication and Chem-Bio sensors. She is currently working as a ‘Lead Manager (R&D)’ in ‘NanoSniff Tech. Pvt. Ltd.’, a company incubated at IIT-Bombay, Mumbai, India.

K. Narasaiah Chetty graduated in mechanical engineering from S.V. University Tirupathi, India and received the doctoral degree from Indian Institute of technology Madras. After the Ph.D. degree he joined as an Assistant Professor at J.N.T. University, Anantapur, India. He is a life member in Indian Society Technical Education, Solar Energy Society of India and Indian Society of Heat and Mass Transfer.

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K. Govinda Rajulu received the doctoral degree from the Indian Institute of Technology, Roorkee, India, in 1993. After the Ph.D. degree he joined as an Assistant Professor at J. N. T. University, Anantapur, India. He is Fellow of Institutional of Engineers, INDIA, holds Charted Engineer (INDIA) Certificate and Life member of Indian Society for Technical Education.

M. Ravikanth M. Ravikanth was born in Andhra Pradesh, in 1966. He received the B.Sc. and M.Sc. degrees from Osmania University, Hyderabad, india and the Ph.D. degree from the Indian Institute of Technology, Kanpur, India, in 1994. After his postdoctoral stay in USA and Japan, he joined as a faculty at Indian Institute of Technology, Bombay, where he is currently a full Professor. His current research interest includes porphyrin and related macrocycles, and boron dipyrromethenes.

P. C. K. Chary received the B.Tech, M.E., Ph.D., and MISTE degrees. He is working as a Principal for Sree Vidyanikethan Engineering College, Tirupati, India. He did his Ph.D. in CAD/CAM from Sri Venkateswara University, Tirupati, India. He has published 22 papers so far at National and International level. His research interests are Intelligent CAD/CAM Systems, MEMS, FlexibleManufacturing Systems and Applications of Computers in Manufacturing.

V. Ramgopal Rao (M’98–SM’02) is an Institute Chair Professor in the Department of Electrical Engineering, IIT Bombay. He has over 300 publications in the area of Electron Devices & Nanoelectronics in refereed international journals and conference proceedings and has 16 patents issued or pending.