Sensors and Actuators B 221 (2015) 357364Contents lists available at ScienceDirectSensors and Actuators B: Chemicaljo ur nal home page: micro interdigitated thin film metal oxide capacitive sensorfor measuring moisture in the range of 175625 ppmTarikul Islama, A.T. Nimalb, Upendra Mittalb,1, M.U. SharmabaElectrical Engineering Department, F/O Engg. & Technology, Jamia Millia Islamia (University), Jamia Nagar, New Delhi 110025, IndiabSolid State Physics Laboratory, Timarpur, Delhi 110054, Indiaa r t i c l e i n f oArticle history:Received 23 February 2015Received in revised form 31 May 2015Accepted 16 June 2015Available online 26 June 2015Keywords:Interdigitated electrodeSolgel thin filmCapacitive moisture sensorElectrical characteristicsa b s t r a c tIn this paper we present the development of a nanostructured hyper thin film sensor for the measurementof humidity in ppm level using the micro sized interdigitated electrode capacitive structure. The sensorelectrode has been fabricated using the optical lithography technique on the oxide sensing film depositedon the quartz substrate. The nanoporous metal oxide thin film of ?-Al2O3has been fabricated by simplesolgel drop coating method. The electrical characteristics of the sensor have been determined in themoisture range from 175 to 625 ppm by Agilent 4294A impedance analyzer at 1 kHz, 50 kHz and100 kHzrespectively. The sensor has excellent yield ratio because of complete avoidance of shorting problem ofthe electrode. The response time is very fast (Tr= 12 s) in comparison to the conventional commercialparallel plate gold electrode capacitive thin film aluminum oxide sensor. The proposed sensor can beuseful for measuring moisture dispersion inside electronic gadgets and packages where the performanceof the devices is deteriorated due to moisture entrapment. 2015 Published by Elsevier B.V.1. IntroductionMoisture measurement in ppm level below 1000 ppm is still anotorious challenging issue 1. There are numerous applicationswhere moisture measurement in ppm or ppb level is required2. Most of the sensors found application in RH range 4,5. Largenumbers of research articles on humidity sensors are publishedin various journals but the literature of moisture measurementin ppm level is 3,6. The moisture sensors for industrial appli-cations must have good sensitivity, low response time, goodreproducibility, small hysteresis, long lifespan, resistance againstcontaminants, negligible temperature dependence and low cost7. The sensitivity, resolution and stability are very importantcharacteristics for humidity sensors used in certain special gasesapplications 2,4. Yield ratio, which represents the success of thesensor fabrication, and is another important parameter whichcan significantly affect the overall cost of the sensor is rarelymentioned in the literature. The alumina thin film sensor is used asan absolute sensor and it is suitable for large dynamic range mois-ture measurement 7. Among different technologies employedCorresponding author. Tel.: +91 011 26982574; fax: +91 011 26982651.E-mail address: tislamjmi.ac.in (T. Islam).1PhD Research scholar, Department of Electrical Engineering, Jamia Millia Islamia(University).for moisture measurement in trace level 8,9, the parallel platecapacitive alumina sensor offers low cost solution, but the cost ofsuch sensor is still high in ppm range. A single sensor costs around10001200 US dollar for ppm range measurement whereas a RHsensor is available for only 2030 US dollar or even less 3.Because of the disadvantages of all practical moisture measure-ment systems, there is an urgent need to design and fabricate asensor which can work in corrosive and contaminating gaseousenvironment is sensitive and fast enough to sudden and dras-tic changes of moisture, requires minimum maintenance and isavailable at low cost with relatively long calibration period. Toobtain a low cost sensor for trace level moisture measurement, sev-eral works utilizing solgel aluminum oxide thin film have beenrecently reported in literature 1519. The salient response char-acteristics of the aluminum oxide sensors available in commercialdew point meters and in literature are given in Table 1. Most ofthe sensors employ parallel plate capacitive structure with costlygold electrode. From the table we observed that most of the paral-lel plate capacitive sensors have high response and recovery time1012,1518, high hysteresis 12,13 and there is no data for yieldratio. A very fast and long term stable alumina moisture sensor hasbeen reported in 2 but the device has very small sensitivity from0 to 1000 ppm moisture and there is no data about the yield ratio.Also in 2,12,13 response time is determined over a wide humid-ity range. Even if the nanostructure sensing layer of high surfacearea in thin film form may offer high sensitivity, resolution, fasthttp:/dx.doi.org/10.1016/j.snb.2015.06.1010925-4005/ 2015 Published by Elsevier B.V.358 T. Islam et al. / Sensors and Actuat