MEMS Based Humidity Sensor with Integration of Temperature Sensor

MEMS Based Humidity Sensor with Integration of Temperature Sensor

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Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 5 (2018) 10728–10737

www.materialstoday.com/proceedings

ILAFM2016

MEMS Based Humidity Sensor with Integration of Temperature Sensor Abdurrashid Shuaibu Hassana*, Vimala Julieta ,C. Joshua Amrith Rajb a

Department of Instrumentation And Control Eng., SRM University, Chennai, 603203, India. b Department of Mechanical Eng., BITS Pilani, Hyderabad campus

Abstract Humidity and temperature are one of the basic weather parameters that play a significant role in human life and its environment. This study utilizes finite element analysis for the optimization of temperature sensor The temperature sensor utilizes platinum RTD with a meander shape as a sensing element and parallel plate capacitive humidity sensor with array of hole at the upper electrode made of aluminum was design to operate in the range of -70℃ to 70℃, 0% to 100% respectively. The humidity sensor gives a higher capacitance value of 120pF and sensitivity of 22pF/%RH. Moreover, the average temperature coefficient of resistances was found 5 × 10 . The results were compared based on analytical method and simulation in COMSOL Multiphysics. In future, atmospheric pressure sensor and wind velocity will be integrated with the above-mentioned sensor design. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Second International Conference on Large Area Flexible

Microelectronics (ILAFM 2016): Wearable Electronics, December 20th–22nd, 2016. Keywords: Capacitive Humidity Sensor, Temperature sensor, COMSOL, Finite Element Method.

1. Introduction Globally, as the expectation for excellent quality of human life increases, monitoring of weather parameter are brought into consideration, however, the utilization of high accuracy sensor have been incorporated for environmental monitoring, particularly, humidity is a vital aspect to consider in the weather forecast.

* Corresponding author. Tel.: +2348063629350, +919940449418 E-mail address: [email protected], [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Second International Conference on Large Area Flexible Microelectronics (ILAFM 2016): Wearable Electronics, December 20th–22nd, 2016.

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A humidity sensor has a verse applications ranging from environmental monitoring, automation control system, incubators, sterilizers, biological products, medical equipment and so forth. To overcome this challenges and demand for batch production of micro - sensing requirement in modern industries and human need, such sensor are commonly fabricated using MEMS technology [1-2]. General, humidity sensor required a vapor absorbent film as sensing material [3-6]. The sensing material changes when exposed to water vapor which determined the value of relative humidity. For this application, humidity sensor requires a prescribed range such as temperature effect, high sensitivity, and response time and so on, though a capacitive humidity sensor has the above advantages. Nowadays, humidity sensor are classified into two section namely, resistive and capacitive humidity sensor. Though, the most frequently used is capacitive type. This is because it offers high sensitivity, low power consumption, fast response time, low fabrication cost, high output signal amplitude and wide operating range. However, when compared to resistive type, it requires less influenced of temperature with minimal complicated read out electronics. Capacitive humidity sensor can be achieved with a moisture sensing film (dielectric) either sandwiched between two parallel electrode or deposited on top of interdigited electrodes (IDEs). Several researches has been carried out to tackle this problem, for example, For example, Hussain and Rittersma reported that increasing the porous size can improve sensor response time effectively [8]; lung – tai chen et al. Present a novel capacitive humidity sensor with integration of platinum RTD based sensor. The study comprises of freestanding cantilever structure with thermal compensation mechanism. The humidity sensor varies from 65%RH to 85%RH with a response time of 0.9s. The humidity sensor presents convergence measurement characteristics for an ambient temperature range 0–50℃ [9]. Chen – yen and Gwo –Bin Lee presented a capacitive based humidity sensor with interdigited RTD based sensor. There design and fabricate a freestanding cantilever with polyimide coated on top of the nitride layer with platinum RTD as temperature compensation in order to avoid signal drift as a results of changes in the ambient temperature was study. So, the output of heating and sensing filament calibration of the resistance signal produces by the sensor which varies proportionally with the ambient humidity and therefore produces a satisfactory working performance of the humidity sensor. But this left the whole top surface subjected to the environmental condition and had an option for sensing layer protection. [10]. Dokmeci et al. [11] also presented a highly sensitive polyimide capacitive humidity sensor. A submicron – thick (120nm) polyimide layer as is used as sensing film which is sandwiched between two parallel electrodes. The measured capacitance is 275pF resulting a sensitivity of 0.86pF/%RH. The aim of the research work is to design and simulate a polyimide - based capacitive humidity sensor and a meander shape temperature sensor and the same time checking the performances with better optimization and possibilities for the integration onto a single chip as multisensor. The remaining issue including fabrication and characterization are left for future work. 2. Theoretical Background 2.1. Capacitive Humidity Sensor Design. The fig. 1 show the structure of parallel plate capacitive humidity sensor with polyimide sandwiched between the topmost electrode and the bottom electrode which exhibit wide operating range and good sensitivity. The upper electrode comprises an array of hole which allows vapor to penetrate into the sensitive layer, the vapor content spread over the sensor, polymer absorbs the analyte. The absorbed vapor molecules diffuse in the polyimide and change its permittivity.

Fig.1 The Structure of Humidity Sensor with Array of Hole at the Top Electrode

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The change in the dielectric permittivity, in turn, the change in Capacitances values. The polyimide is widely used as sensing material due its good sensing characteristics and gold is used as the electrode because of its good thermal conductivity. Consider a theoretical model for the sensor behaviour upon water vapour absorption. The device was modelled as a parallel-plate capacitor using the expression.

=

(1)

Where ε Is the dielectric permittivity of polymer, Is the permittivity of free space, the distance between the of the polyimide film changes when it electrodes, A is the area of the upper electrode. The dielectric constant absorbs water molecules. The dielectric constant of water vapor in polyimide can be modelled using the Looyenga’s given in equation [2]: = [ (∛(

) − ∛( ) + ∛(

)]

(2)

and Where γ is the volume fraction of water in the film, and polyimide respectively. ( − )+ . × = . .{ + . ×

are the dielelctric constants of water and dry ( −

) }

(3)

Where, T is the temperature in Kelvin. shibata et al. described the formula for obtaining the fractional volume of water in film γ =

∅( )

()

=

%

(4)

Is the maximal fractional volume at initial temperature , ∅ (T) represents the temperature dependence of the vapor absorption coefficient, while φ(T) represent the temperature dependence of dielectric permittivity of water and the catalytic effect. Although polyimide expansion upon water vapor absorption also results in a change in capacitance, this expansion is disregarded because the sides of the channel are fixed by trusses and the volume expansion coefficient for polyimide is relatively small, only 60-75 ppm/%RH [12]. This physical interpretation indicates that the Fickian model is applicable to the diffusion process, γ is calculated using the equation in the literature [13]. .

Dielectric Constant of PI

= [ .

(∛

.

−∛ . ) + ∛ . ]

(5)

4.6 4.4 4.2 4 3.8 3.6 3.4 3.2 3 0

10

20

30

40

50

60

70

80

90 100 110

Relative Humidty (RH%) Figure 2: Relationship between dielectric constant of polyimide and relative humidity

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According to above equations, the relation between and temperature can be determined. Fig. 2 shows the dielectric constant of polyimide as a function of relative humidity at T=298K 3. Geometry, Modeling And Simulation Approach The model is simulated using Comsol/Multiphysics software which is an interactive engineering and physics tools that perform equation based on modelling in a visual interface. This software allows the modelling and simulation of physical phenomena in such as to be easy to implement. It also predict the base capacitance and sensitivity of the sensor 3.1 Geometry Modelling Figure 2 shows the build-up geometry in built in Comsol. The model comprises of a substrate, upper and lower electrode made of gold and a sensing layer with dielectric constant. We used the Electrostatics part in AC/DC of MEMS module in Comsol Multiphysics software. Specifically, the approach was built in 3D electrostatics problem by first creating a 2D geometry using the array tools and then extrudes it to a 3D geometry, perform Mesh analysis and compute the capacitance using the Electrostatics application mode port boundary conditions. The area of the sensor is (40 × 40 . A voltage of difference ( ) of volts is applied to the excitation electrode (i.e. the positive terminal electrode) and ground the sensing electrode (negative terminal electrode). The silicon is chosen as substrate material with dielectric constant 11.69. Finite element analysis is used to evaluate the electric potential distribution, capacitance and the total electrical energy stored. 3.2 Physics Consider the electrostatics model in the AC/DC module solves the poisson’s equation to calculate the electric potential distribution across the field, the capacitance is computed by calculating the electrical energy across the sensing layer. − ∙(

)=

is the permittivity of the vacuum, Where V is the terminal voltage applied across the sensing layer, dielectric permittivity of the sensing layer and is the space charge density

(6) the

Under static condition =−

(7)

=

(8)

But, ∁ = / where Q is the charge and V is the voltage across the capacitor. The amount of energy required to charge a capacitor equal to the energy applied to the electrostatic field, given by

=

(9)

The energy in the electrostatic module can be easily calculated in COMSOL software. Finite element meshing calculate = ( . ) Ω (10) Where D is the electric flux density and the field is the field intensity.

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3.3 Computation.

Fig. 3 Three Dimensional view of the Parallel Plate Capacitive Humidity Sensor with an Array at the Top of the Electrode

4. Temperature Sensor The sensing and heating element used in the sensor was designed using thin film Platinum resistance temperature detector due to its ability to long term stability, elevated resistivity, accuracy and contamination free. 4.1 Principle. The operating principle of resistance temperature detectors (RTDs) is the is usually obtained by changes in the resistance with respect to change temperature, which implies that change in resistance corresponds to the changes in temperature of the film. A suitable temperature sensing material is chosen based on the characteristics of low resistivity. This is as results of their resistance are relatively low magnitude and linearity of response of the electrical resistance to temperature in the measurement of interest. The stability of the film under the operating condition is also considered. The pt RTD comprises of two gold electrode used as a bonding pad on both side inorder to minimize the effect of voltage drop due to the probe leads. In real life model, the relationship between temperatures – resistance of an RTD are approximated by the equation known as the calendar-van dusen equation which give a very precise results. The equation for the linear temperature dependence of the electrical resistances is described as = Where

( +

+

+…)

are the constants terms which depend absolutely upon the material

(11)

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4.2 Geometry Model The fig. 4 describes the two -dimensional (2D) structure of the proposed temperature sensor which comprises of snake-like platinum resistance and gold bonding at both of two ends.

Fig. 4 One-Dimensional Model Geometry for the Temperature Sensor

4.3 Material And Properties The temperature sensor is made of platinum as heating and sensing material with properties as mentioned in the Table I Table1. Material properties of platinum Parameter

Values

Young’s modulus Electrical conductivity(k) Coefficient Of thermal expansion(α)

168e9 [Pa]

Relative permittivity

6.9

Poisson’s ratio Density(ρ)

71.6 [W/(m*K)] 8.80e-6[1/K] 0.38 21450 [Kg/

]

4.4 Simulation Result Inorder to enable the flow of current through the sensing layer, 0.5v and 0volt is applied to the positive and negative terminal of both ends respectively. The temperature sensor is simulated using joule heating and thermal expansion in COMSOL. This study presents the corresponding electric potential distribution and temperature distribution when applied over the sensing layer. As shown in fig.4 we observed that the response of the sensor increases with changes in ambient temperature.

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a,

b

Fig. 5: Comsol Simulation Results for the temperature sensor. a.) Electric potential distribution over the meander shape b.) Temperature distribution of the sensor 5. Result And Discussion A parallel plate capacitive humidity sensors that utilize a polyimide layer between a two layer with an array of holes was demonstrated in this designed. Having designed a proper a proper model, the study conducted a systemic approach for investigating the performance of the sensor at a different distance between the holes (i.e. inter-hole distance, ) as shown in figure 6

Sensing Capacitance (pF)

140 120 100

C (pF) at 25µm C(pF) at 15µm C(pF) at 5µm

80 60 40 20 0 0

10

20

30

40

50

60

70

80

90

100

Relative Humidity(%) Figure 6: Performance analysis for the variations of capacitance with relative humidity as a function of inter hole distance (

= 25μm, 15μm and 5μm) in Comsol Multiphysics software

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It was observed that the contact area of the water vapor increases by increasing the diameter of the holes as well as decreasing the inter hole distance (

). Therefore, the sensor capacitance is increase due to increased dielectric gives higher capacitances values

constant in the capacitance of the sensor. From the plot is observed that 5μm of

of about 120pF. The response of the humidity sensor increases linearly with relative humidity in a range of 0100%RH which gives an optimal sensitivity of 28pF per %RH. Figure 7 show relationship between the signal response and the ambient temperature of the platinum RTD temperature sensor, we observed that the resistance of the sensor increase linearly with increase in input temperature for the specified ranges. The average TCR (temperature coefficient of resistance) value is found to be TCR = 5 × 10 C

.

4 Resistance (KΩ) Linear (Resistance…

Resistance (KΩ)

3 2 1 0 -1 -2 -3 -80

-60

-40

-20

0

20

40

60

80

Temperature ( C) Figure 7: Analytical characteristics of resistances versus temperature

0.1 Temperature(ᵒC)

0.08 ΔV/V

0.06 0.04 0.02 0 -80

-60

-40

-20

0

20

40

60

80

Temperature (ᵒC) Figure 8: Temperature Coefficient Resistances (TCR) test for RTD ( TCR = 5 × 10 C )

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However, in fig. 8, by applying the curve - fitting equation of the simulated output voltage, the relationship between the ambient temperature of the environment (℃) and the measured resistance (kΩ) is found to be: = 2000 + 0.0042 Where T is the ambient temperature in degree Celsius and S is the intensity (∆V⁄V). Note that only a small current is allowed to pass through the resistor of the heating and sensing element in order to avoid self- heating effect 6. Conclusion We presented a design and simulation of MEMS based humidity sensor with an incorporation of a temperature sensor. The properties of the parallel plate capacitor were simulated in Comsol 5.2a for different polyimide thickness, the distance between the hole

and the underlying material. It was found that the highest sensitivity is

achieved by increasing the diameter of the hole and making the inter-hole distance as small as possible with a narrow spacing as possible. The pt RTDs temperature sensor has been designed and analyzed using both numerical and finite element method (fem) analysis in Comsol. The meander-shaped Pt was chosen inorder to improve the resistivity of the material as well the TCR. This model result linear sensor with high sensitivity with a temperature ranges70℃ to 60℃. The future works will into designing, fabricating and applying interdigitated structure with better optimization for radiosonde application Acknowledgemets The authors would like to acknowledge the entire Staff of the Department of Instrumentation And Control Engineering and National MEMS Design Center (NMDC), SRM University Chennai, India References [1]

R. Fenner and E. Zdankiewicz, “Micromachined water vapor sensors:A review of sensing technologies,” IEEE Sensors J., vol. 1, no. 4, pp. 309–317, 2001. [2] Matko, V.; Donlagic, D. Sensor for high-air-humidity measurement. IEEE Trans. Instrum. Meas. 1996, 45, 561–563. [3] C. Lee and G. Lee, “Micro-machine based humidity sensors with integrated temperature sensors for signal drift compensation,” J. Micromech. Microeng., vol. 13, pp. 620–627, 2003 [4] Kyo Sang Choi, et aal “A highly sensitive humidity sensor with a novel hole array structure using a polyimide sensing layer”. RSC Adv., 2014, 4, 32075.Royal college of chemistry. [5] G. Rabilloud, High – performance polymers: Chemistry and applications, vol. 3, paris, france: Editions Technip, 2000 [6] Y. Yorozu, M. Hirano, K. Oka, and Y. Tagawa, “Electron spectroscopy studies on magneto-optical media and plastic substrate interface,” IEEE Transl. J. Magn. Japan, vol. 2, pp. 740-741, August 1987 [Digests 9th Annual Conf. Magnetics Japan, p. 301, 1982]. [7] Das, S.M. Hossain, S. Chakraborty, U. Gangopadhyay, et al., Capacitive type humidity sensor based on porous silicon, Proc. SPIE – Int. Soc. Opt. Eng. 3975(2000) 707–710. [8] Rittersma, Z.M. Recent achievements in miniaturised humidity sensors—A review of transduction techniques. Sens. Actuators A-Phys. 2002, 96, 196–210. [9] Lung-Tai Chena, Chia-Yen Leeb, Wood-Hi Chenga, MEMS-based humidity sensor with integrated temperature compensation mechanism, Sens. Actuators A:Phys. 147 (2008) 522–528. [10] Chia Yen Leea and Gwo-Ben Leeb , “MEMS – based humidity sensors with integrated temperature sensors for signal drift compensation”, Dept. of Engineering Science, National Cheng Kung, Taiwan, 2003 IEEE. [11] M. Dokmeci, K. Najafi, “A high-sensitivity polyimide capacitive relative humidity sensor for monitoring anodically bonded hermetic micropackages,” IEEE Journal of Microelectromechanical Systems, Vol. 10, pp. 197-204.

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[12] H. Shibata et al., A digital Hygrometer using a polyimide Film Relative Humidity Sensor, IEEE Transactions on Instrumentation and Measurement, vol. 45, No. 2, April 1996 [13] Lei Gu, Qing-An Huang And Ming Qin “A Novel Capacitive-Type Humidity Sensor Using CMOS Fabrication Technology”, Science Direct, Sensors And Actuators B 99 , 491–498, Dec 2003. [14] Z.M. Rittersma, A. Splinter, A. Bodecker, Novel surface-micromachined capacitive porous silicon humidity sensor, Sens. Actuators 68 (2000)210–217. [15] Ching-Liang Dai, A capacitive humidity sensor integrated with micro heater and ring oscillator circuit fabricated by CMOS–MEMS technique, Sens. Actuators B:Chem. 122 (2007) 375–380. [16] Kang, U.S.; Wise, K.D. A high-speed capacitive humidity sensor with on-chip thermal reset. IEEE Trans. Electron. Devices 2000, 4, 702– 710. [17] A.Vimala Juliet, Experimental analysis of thermoelectric generator using solar energy, 2nd IEEE International Conference on Smart Structures and Systems, ICSSS 2014; Saveetha Engineering College Chennai; India; Article number 7006197, Pages 67-71, 9 October 2014 [18] Ching-Liang Dai, A capacitive humidity sensor integrated with micro heater andring oscillator circuit fabricated by CMOS–MEMS technique, Sens. Actuators B:Chem. 122 (2007) 375–380. [19] Ashir Kumar sen and Jeff Darabi, “Modeling And Optimization of a microscale capacitive humidity sensor for HVAC Application. IEEE SENSORS JOURNAL, VOL. 8, NO. 4, APRIL 2008.P. 333-340. [20] Nathan S. Lazarus, “CMOS – MEMS chemresistive and chemiresistive chemical sensor system.” PhD research work, Carnegie Mellon University, Pittsburgh, April, 2012 [21] Subhashini & Vimala Juliet, “Micro Cantilever CO2 Gas Sensor Based on Mass”, Applied Mechanics and Materials, 766-767, 2015