A Capacitive Humidity Sensor Suitable for CMOS Integration (original) (raw)
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fett.tu-sofia.bg
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PurposeTo establish an accurate and sensitive method to characterize the moisture content of a particular environment.Design/methodology/approachThis paper proposes a relatively simple humidity sensor design consisting of electrodes on a suitable substrate coated with a polyimide material. The changes in relative humidity are denoted by a corresponding change in the polyimide material's electrical resistance profile. The design proposed in this work can be microfabricated and integrated with electronic circuitry. This sensor can be fabricated on alumina or silicon substrates. The electrode material can be made up of nickel, gold or aluminum and the thickness of the electrodes ranges typically between 0.2 and 0.3 μm. The sensor consists of an active sensing layer on top of a set of electrodes. The design of the electrodes can be configured for both resistive and capacitive sensing.FindingsThe polyimide material's ohmic resistance changes significantly with humidity variations...
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Nowadays humidity sensors are used in every industry, such as in food, pharmaceuticals [1], medicine, [2] and agriculture [3, 4]. These commercial humidity sensors are quite expensive, complicated in operation, and have low sensitivity and stability because of the materials used as a sensing element [5]. It is difficult to maintain their operational cost, power losses, sensitivity and stability [4]. Therefore, it is essential for a sensor to have high sensitivity and stability, low cost, small hysteresis, wide linear range, simple operation, short response, and short recovery time [5, 6]. Electrical conduction is affected significantly by dipoles of water molecules, which makes it important for researchers to investigate the magnitude of change in impedance and capacitance of the samples with respect to varying relative humidity. The investigation becomes more important for composite materials due to the contribution of properties by two or more ingredients. Changing the constitutin...
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This paper presents a low cost, easy to fabricate, thin-film sensor. We used three different methods to make the thin film using two different base (film) materials: carbon and graphite. We briefly summarize the method and outcome of multi-layer thin film fabrication using spray coating, spin-coating, and drop-casting. The quality of the thin films is compared, and electrical resistance is characterized. The fabricated thin film is used to measure humidity and is characterized against a commercial humidity sensor (DHT22). The experimental details and results of humidity measurement are discussed. We were able to achieve resistance range of 5k-1.5k ohms for 40-80 % RH, respectively. Plans to further optimize the sensitivity of the sensor, in future is discussed.
Fabrication and Characterization of a CMOS-MEMS Humidity Sensor
This paper reports on the fabrication and characterization of a Complementary Metal Oxide Semiconductor-Microelectromechanical System (CMOS-MEMS) device with embedded microheater operated at relatively elevated temperatures (40 °C to 80 °C) for the purpose of relative humidity measurement. The sensing principle is based on the change in amplitude of the device due to adsorption or desorption of humidity on the active material layer of titanium dioxide (TiO2) nanoparticles deposited on the moving plate, which results in changes in the mass of the device. The sensor has been designed and fabricated through a standard 0.35 µm CMOS process technology and post-CMOS micromachining technique has been successfully implemented to release the MEMS structures. The sensor is operated in the dynamic mode using electrothermal actuation and the output signal measured using a piezoresistive (PZR) sensor connected in a Wheatstone bridge circuit. The output voltage of the humidity sensor increases from 0.585 mV to 30.580 mV as the humidity increases from 35% RH to 95% RH. The output voltage is found to be linear from 0.585 mV to 3.250 mV as the humidity increased from 35% RH to 60% RH, with sensitivity of 0.107 mV/% RH; and again linear from 3.250 mV to 30.580 mV as the humidity level increases from 60% RH to 95% RH, with higher sensitivity of 0.781 mV/% RH. On the other hand, the sensitivity of the humidity sensor increases linearly from 0.102 mV/% RH to 0.501 mV/% RH with increase in the temperature from 40 °C to 80 °C and a maximum hysteresis of 0.87% RH is found at a relative humidity of 80%. The sensitivity is also frequency dependent, increasing from 0.500 mV/% RH at 2 Hz to reach a maximum value of 1.634 mV/% RH at a frequency of 12 Hz, then decreasing to 1.110 mV/% RH at a frequency of 20 Hz. Finally, the CMOS-MEMS humidity sensor showed comparable response, recovery, and repeatability of measurements in three cycles as compared to a standard sensor that directly measures humidity in % RH.
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In this work we show the design and fabrication process of a read-out interface based on polysilicon TFTs technology integrated with capacitive humidity sensor. All the devices were fabricated on a thin flexible polyimide substrate (8 µ m) according to a low temperature polysilicon (LTPS) fabrication process (<350°C). We proposed as read-out interface a three stages ring oscillator circuit, employed to convert the sensor capacitance variation into output frequency change. The sensor capacitor is a three layer interdigitated capacitor (IDC). The transducers layout has been optimized by means of 3D Finite Element simulations in order to increase the response speed, considering the diffusion of the water molecules into the chemical interactive material (CIM). In particular we have compared two different sensor structures, one with the upper metal shaped in fingers 20 µ m wide (IDC1) and an alternative structure (IDC2) with the upper metal divided in squares 15 µ m wide. We have chos...
The humidity sensing characteristics of different sensing materials are important properties in order to monitor different products or events in a wide range of industrial sectors, research and development laboratories as well as daily life. The primary aim of this study is to compare the sensing characteristics, including impedance or resistance, capacitance, hysteresis, recovery and response times, and stability with respect to relative humidity, frequency, and temperature, of different materials. Various materials, including ceramics, semiconductors, and polymers, used for sensing relative humidity have been reviewed. Correlations of the different electrical characteristics of different doped sensor materials as the most unique feature of a material have been noted. The electrical properties of different sensor materials are found to change significantly with the morphological changes, doping concentration of different materials and film thickness of the substrate. Various applications and scopes are pointed out in the review article. We extensively reviewed almost all main kinds of relative humidity sensors and how their electrical characteristics vary with different doping concentrations, film thickness and basic sensing materials. Based on statistical tests, the zinc oxide-based sensing material is best for humidity sensor design since it shows extremely low hysteresis loss, minimum response and recovery times and excellent stability.
Polymer-electrolyte-film-based humidity sensor with integrated signal conditioner
2005 12th IEEE International Conference on Electronics, Circuits and Systems, 2005
This paper describes a humidity sensor, whose sensing element is an ionic polymer film the resistance of which is measured by a dedicated signal conditioning circuit implemented in 0.35um CMOS. In addition to providing excitation and measurement of the film resistance, the implemented signal conditioning circuit detects the temperature of the sensing film by way of an on-chip temperature sensor in order to perform the necessary compensation for the influence of the temperature upon the measured relative humidity. The sensing polymer film is 10.4 J..lm thick deposited on glass by casting using a solution of P(EO-EPI)84-16+NaI on top of the interdigitated gold electrodes. The signal conditioning circuit excites the sensing element with a 50% duty-cycle, 1KHz, 700nA, square wave. Samples of the sensor have been measured in the range 25 to 85% RH, varying the temperature from 10 to 50°C. At 55%RH/30°C the mean value of resistance per square of the measured samples is 16.9k OlD.