A review of MEMS based humidity sensors

Introduction:

Humidity is amount of water vapor present in the air at a give temperature. In general relative humidity is a measured using the sensors it is defined as the ratio of water vapor present in the air to the maximum water the air can hold at a given temperature. The measurement of humidity is essential in numerous applications, to predict weather, humidity calculation is very essential for food storage industry to keep the food fresh, domestic appliances like air conditioning, medical equipment etc. Hence there is need for sensing humidity

Humidity sensors are classified based on the working principle. They are (i) capacitive based (ii) measuring resistance (iii) using optical techniques (iv) gravimetric techniques. Conventional humidity sensors are bulkier and have high operating and manufacturing cost. Therefore there is a necessity for miniaturized humidity sensors. MEMS based humidity sensors can overcome these disadvantages like low cost, bulk manufacturing, small size and low operating cost. In literature already different kinds of MEMS based humidity sensors are available and few of them are commercially available. In general the important parameters to measure a sensor are fast response time, high sensitivity, no hysteresis, linearity in response, low power consumption.

In this report a review of MEMS based humidity sensors is presented. The report focuses on mainly three different kinds of sensing mechanism (i) capacitive (ii) resistive (iii) thermal conductive. In the first section two humidity sensors available in the literature are presented following a thermal conductive based sensing method. In the later section the resistive based humidity sensor is discussed. The report ends with conclusions.

Capacitive based sensing:

Among all the existing humidity sensors capacitive based humidity measurements are used extensively. These sensors work on the principle of measuring the change of the dielectric constant of the thin films in between the electrodes. They are extensively used because the dielectric constant of the thin film polymers varies significantly upon water intake. The main variations in the existing capacitive humidity sensors are the pattern of the electrodes and the sensing material i.e. thin film polymers. Figure shows different kinds of electrode patterns.

Lei Gu et al has shown that the interdigitated electrode pattern is optimum for humidity sensing. Figure shows the layout of the sensor. The fabrication is done in six stages and are presented in Figure . Initially a layer of SiO2 is deposited on the substrate upon this layer polysilicon and SiO2 are deposited. Aluminium which acts as the electrodes is etched onto the SiO2 structures. A passivation layer is coated onto the aluminium electrodes. Polyimide was coated completely on top this surface. Figure shows the sem image of the sensor.

The sensor was tested by connecting to a printed circuit board and keeping it in an environmental chamber and the saturation solution. Figure [] shows the measured capacitance vs relative humidity at 27degrees. Sensitivity is defined as the amount of change of capacitance to change in relative humidity. This can be obtained from the slope of the figure[].

Hysteresis is the delay in response of a given input. To measure hysteresis the sensor is operated from 20 -80% humidity and no significant hysteresis was observed. There was 3% error at 80% which could be reduced by heating the heater of the sensor.

Stability he sensor was also stable

Response speed

Second method for capacitive sensing

Lazarn et al proposed an advanced humidity sensor which detects a change of xx% in the relative humidity. This is very useful in medical respiratory systems. In this sensor instead of using conventional electrode pattern innovative way was utilized so that maximum contact is possible. The main substrate is removed from the electrode and is hanged by support of trusses. Capillary wicking technology is used to pull the polymer in the solution.

The fabrication process is bit complex compared to the previous method. Figure [] shows the steps in the fabrication process. Initially a fabricated CMOS is taken and anisotropic oxide reaction-ion etch (RIE) is performed to expose the silicon substrate and release the structure.

Drop-on-demand ink jet system is used to selectively deposit the polymer in the solution between the electrodes.

This is has a 0.18% sensitivity to the relative humidity.

The applicaitonssss

The measurements

Commercialization blaaahhh

Thermal conductive based sensing:

While the capacitive based humidity sensors exploit the change of dielectric constant, the thermal conductive based sensing measures the change in the thermal conductivity of the medium. Burak okcan et al proposed a thermal conductive based humidity sensor. The basic working principle is two pn junction diodes are placed in such a way that one of them is isolated and the other is exposed to ambience the relative change in the voltage due to change in thermal conductance of the medium is measured and thus the humidity is measured.

To have a significant change in thermal conductivity measurements the pn junction diodes are heated to 250deg cel.

Figure shows a thermal conductive based humidity sensor.

Conclusions:

References: