IEEE SENSORS JOURNAL, VOL. 13, NO. 11, NOVEMBER 2013 4487 A Capacitive Humidity Sensor Suitable for CMOS Integration Nooshin Saeidi, Member, IEEE, Jörg Strutwolf, Amandine Maréchal, Andreas Demosthenous, Senior Member, IEEE, and Nick Donaldson Abstract—This paper describes the design, fabrication, and performance of a thin film humidity sensor fabricated in standard CMOS process, hence it may be combined with an integrated circuit. The sensor is based on a capacitance between interdigi- tated electrodes in the top metal layer and water adsorption in the polyimide layer. The design is optimized by analytical and then finite element models which show that, within the constraint of the CMOS structure, the sensitivity can be no greater than one third of the sensitivity of the polyimide alone. Experimental sensors were fabricated in-house before an improved design was fabricated in a commercial foundry. The different behavior of these sensors, despite their similar designs, leads to an investigation into the effects of fabrication process on the sensor linearity. Characterizing the polyimide film by contact angle, AFM and FTIR revealed that the difference in linearity of the response between the two sensors resulted from different etching techniques employed to pattern the film. Index Terms— Thin film sensors, interdigitated electrodes, CMOS integration, humidity measurement, polyimide film, plasma etching. I. I NTRODUCTION M OISTURE is a major concern for electronic devices implanted in the body or operated in humid envi- ronments. Moisture that has penetrated the device package eventually causes condensation on the active area of the device, corrosion of the structures, performance deterioration and device failure. The most direct way to ensure that the package is functional and remains dry inside, is to measure the internal humidity. Humidity measurement is essential for a wide range of applications including environmental monitoring (e.g. indoor Manuscript received January 27, 2013; revised May 26, 2013; accepted June 10, 2013. Date of publication June 19, 2013; date of current version October 2, 2013. This work was supported by the U.K. Engineering and Physical Science Research Council under Grant EP/F009593/1, the European Commission’s FP7 project NEUWalk under Grant 258654, and the Tyndall National Institute. The associate editor coordinating the review of this paper and approving it for publication was Prof. Massood Zandi Atashbar. N. Saeidi and N. Donaldson are with the Department of Medical Physics and Bioengineering, University College London, London WC1E 6BT, U.K. (e-mail: n.saeidi@ee.ucl.ac.uk; n.donaldson@ucl.ac.uk). J. Strutwolf is with the Institute of Organic Chemistry, Department of Chemistry, University of Tübingen, Tübingen D-72076, Germany (e-mail: joerg.strutwolf@uni-tuebingen.de). A. Maréchal is with the Institute of Structural and Molecular Biol- ogy, University College London, London WC1E 6BT, U.K. (e-mail: a.marechal@ucl.ac.uk). A. Demosthenous is with the Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE, U.K. (e-mail: a.demosthenous@ucl.ac.uk). 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/JSEN.2013.2270105 climate control for domestic or industrial applications), process control (e.g. for high quality chemical products), pharmaceu- tical and biomedical applications (e.g. respiration monitoring systems). This wide-ranging usage has inspired much research and industrial efforts to develop humidity sensors based on different sensing mechanisms. Major mechanisms to sense and measure relative humidity can be classified in three categories: optical, mechanical and electrical (capacitive or resistive). Among the commercial humidity sensors the majority are of the capacitive type. This technique offers low power consump- tion, high output signal amplitude and wide operating range. Also, this type of humidity sensors are less influenced by temperature and require less complicated readout electronics compared to the resistive type. However, the sensitivity is not high with most dielectrics. Capacitive humidity sensors can be realized with a moisture sensing film (dielectric layer) either sandwiched between two parallel plates or deposited on top of interdigitated electrodes (IDEs). Dokmeci et al. [1] demon- strated a capacitive humidity sensor which utilized a thin (120 nm) polyimide layer as the moisture sensing film, placed between two parallel electrodes. The sensor with a capaci- tance of 275 pF resulted in sensitivity of 0.86 pF/%RH [1]. Kang et al. [2] reported a parallel plate capacitive sensor with patterned polyimide layer (2 μm thick) to form multiple columns having diameters of a few microns which allow moisture to diffuse into them circumferentially to decrease the response time. The sensor with a capacitance of 13 pF exhibited a sensitivity of 30.0 fF/%RH [2]. Humidity sensors utilizing thin polyimide sensing layers and interdigitated elec- trode configurations have been developed by several groups [3]–[5]. The sensitivity range was between 5 and 22 fF/%RH for sensors with capacitances between 5 and 8 pF. We report on the design of a humidity sensor with interdig- itated electrodes and polyimide as the sensing material, made in a standard CMOS process so that it may be incorporated on a custom integrated circuit (Fig. 1). Combining the sensor with its read-out circuit improves performance by avoiding external interconnections. Once integrated with electronic devices, the sensor allows for real time monitoring of moisture inside the package while the device is operating. The interdigitated configuration was particularly selected in order to utilize the topmost metal layer in standard IC fabrication as well as the polyimide layer which is available in many fabrication processes as stress relief or passivation layer. The following sections of this paper discuss the design, fabrication and characterization of an interdigitated capacitive 1530-437X © 2013 IEEE