Integrated Temperature and Humidity Sensor Based MEMS
Integrated Temperature and Humidity Sensor Based on MEMS Zhen Fang, Zhan Zhao, Yu Wu, Bojun Zhang and Yizhong Wang State Key Laboratory of Transducer Technology Institute of Electronics,Chinese Academy of Sciences Beijing, China, 100080
[email protected] Abstract - MEMS-based integrated temperature and humidity sensor has the advantages of low weight, small size, low cost, easy integrity and is a highly advanced apparatus of identity, reliability, easy production. The feasibility of thermal conductivity method used to measure humidity and structure of the integrated temperature and humidity sensor are analyzed in this paper, and provides the measuring methods in detail. We mainly focus on measuring result of the humidity part, and verify the excellent performance of the integrated temperature and humidity sensor.
sensor is very fast response and high sensitivity, and the sensitivity comes to 2% RH in room temperature and 1 atm pressure. The micro-air-bridge heater is heated up to above 350℃ within 50 ms by application of a constant current of 10mA, and only atmosphere in the region adjacent to microair-bridge will be quickly heated up to this temperature and then reaches the thermal equilibrium state. The output voltage of micro-air-bridge is in inversely proportional to relative humidity change.
Index Terms - MEMS integrated; Temperature and humidity sensor; Micro-air-bridge.
II. PRINCIPLE OF INTEGRATED TEMPERATURE AND HUMIDITY SENSOR In different ambient temperatures, resistors have different resistances as shown in equation (1). (1) R t＝R 0（1+αt＋βt 2＋…） We only need the former three orders and omit the latter orders. α,β act as constants in a certain temperature scope. We can measure the ambient temperature by measurement of resistance change. The thermal conductivityλmix of a mixed gas consisting of n components is expressed by the Sutherland-Wassiljewa equation (2)as follows [4]:
I. INTRODUCTION Humidity and temperature are among the most frequently measured physical characteristics in measurement science. Nowadays the measurement of temperature can be detected with a satisfactory accuracy, while the measurement of the water vapor content in gaseous atmosphere, i.e. hygrometry, appears much complex and complicated. In view that water vapor is a concomitant component of the air, its measurement plays an important role in various practical measurements. Today, different humidity sensors based on MEMS take advantage of miscellaneous principles and different technical design. Its wide applications require that it have the following characteristics: (1) good sensitivity over a wide range of humidity, (2) a short response time, (3) small hysteresis, (4) good repetition, (5) good durability and long work life, (6) good resistance against contaminants, (7) ability to eliminate the environmental temperature influence. MEMS-based humidity sensors can not only achieve advantages abovementioned but also be beneficial to batch fabrication and reduction of cost. State-of-the-art miniaturized humidity sensors can be classified in numerous principles, i.e. capacitive-, resistive-, hygrometric-, gravimetric- and opticaltechniques [1]. We have designed and fabricated a new MEMS-based integrated temperature and humidity sensor with very rapid response time and good sensitivity in this paper. The new sensor uses the effect of heat-conductivity change and the effect of conductivity change to measure humidity and temperature respectively [2] [3]. In view of the easiness of measurement of temperature, we do not illuminate the principles here. Measurement of relative humidity by this
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λi
n
λmix = ∑ i =1
n
∑
1+
(2)
Aij ( Xi / Xj )
j =1, j ≠ i
Where λi and Xi are the thermal conductivity and molar fraction of the ith gas component, respectively, and Aij is a coupling coefficient between the ith and jth component gases. Aij is given by the Lindsay-Bromley approximation(3) as follows [4]:
⎧ ⎡ ⎛µ 1 ⎪⎪ Aij = ⎨1 + ⎢⎜ i ⎜µ ⎢ 4⎪ ⎢⎝ j ⎣ ⎩⎪
2
⎞⎛ M j ⎟⎜ ⎟⎜ M ⎠⎝ i
⎞ ⎟⎟ ⎠
3 4
1 ⎫ ⎤2 ⎪ T + S i ⎥ ⎪ T + S ij ⎬ T + S j ⎥ ⎪ T + Si ⎦⎥ ⎪ ⎭
(3)
Where μi is the viscosity of the ith component gas under 1 atm and Si is the Sutherland constant of the ith component gas given by Si = 1.47Tb (Tb=boiling temperature).Sij is approximately given by Sij = (Si Sj) 1/2 and Sij = 0.733 (Si Sj) 1/2 for a gas consisting of non-polar gas components and a gas containing polar gas components, respectively. By calculations of the Sutherland-Wassiljewa equation, we found that the thermal conductivity λmix in humid air has
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Proceedings of 2004 International Conference on Information Acquisition the following characteristics, as shown in fig.1: it is less than that in dry air at a temperature below about 100℃; it is insensitive to humidity in the temperature range of 100℃-200 ℃ because of similar thermal conductivity between the humid air and dry air; it becomes larger than that in dry air above 350 ℃.
Thermal Conductivity λmix (W/mK)
Fig.3 Schematic diagram of integrated temperature and humidity sensor with two identical micro-air-bridge: cross-sectional view.
Fig.4 SEM micrograph of micro-air-bridge: top view.
Molar Fraction x (H2O)
Fig.3 is an SEM micrograph of integrated temperature and humidity sensor with two identical micro-air-bridges. The left micro-air-bridge is used to be measurement of temperature and the right one to be measurement of humidity. An SEM micrograph of micro-air-bridge is shown in Fig.4.
Fig. 1 Theoretical thermal conductivityλmix of a mixed gas of pure air and (H2O) gas (molar fraction x ) as a function of x at various temperature.
III. FABRICATION OF SENSORS Fig.2 is a schematic diagram of the integrated temperature and humidity sensor with two identical micro-air-bridges [2] [3]. A square cavity (600µm ∗ 600µm in area, 10µm in depth) is anisotropic etched on a (100) Si wafer surface. A thin Si3N4 film (2µm) is deposited by sputtering on a (100) Si wafer, then a Pt thin film about 0.1µm thick is deposited and shaped by sputter-etching as a zigzag pattern of 15µm wide stripes and 15µm spacing for use as a heating and temperature-sensing element [5]. Using Si3N4 as covering mask, micro-air-bridge (300µm ∗ 300µm in area, 2µm in thickness) suspended by four narrow 50µm-wide beams are formed by etched from the back of the Si wafer (see Fig.4). The structure is shown in Fig.2.
IV. EXPERIMENTAL RESULTS
Output Voltage (V)
The electrical resistance R of the Pt micro-air-bridge is about 300Ω at room temperature. When current flow across the Pt micro-air-bridge, Joule power consumption is expressed
Fig.5 Time dependence of output voltage V across the micro-air-bridge when current is 10 mA Time (m sec).
Fig.2 Schematic diagram of integrated temperature and humidity sensor with two identical micro-air-bridge: cross-sectional view.
by P= I2 R, the temperature T of the micro-air-bridge is drastically increased, at that moment the power dissipation is
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Integrated Temperature and Humidity Sensor Based MEMS
Sensitivity of Relation Humidity (mV/%)
increased. When the Pt micro-air-bridge is in equilibrium, output voltage V across the micro-air-bridge comes to balance. It is confirmed that the temperature T of the microair-bridge increases almost linearly with P and the response time (the time taken to reach the final value nearly) is less than 40 ms in air, as shown in Fig.5. Fig.6 shows the relations between output voltage and relative humidity at 25℃, I = 4mA and 1 atm. We can see from this figure that output voltage is nearly a constant at various relative humidity. When the micro-air-bridge reaches
Output Voltage (V)
Micro-air-bridge Current I (mA) Fig.8 Relationship between input current of the micro-air-bridge sensitivity for relative humidity Rh.
Figure.7 shows that the output voltage is decreased linearly with relative humidity increased because the equilibrium temperature is a highly sensitive to the humidity such as above 350℃ (demonstrated at 410℃, as shown in Fig.7) by application of a constant current of 10mA. We can directly measure the relative humidity in room temperature and pressure from this chart. We measured the humidity sensitivity as a function of the micro-air-bridge current I as shown in Fig.8. We can see this curve crosses the zero-sensitivity line for the relative humidity at current from 1mA to 5mA, corresponding to a micro-airbridge temperature of below 200 ℃ , With the current increased above 5mA, sensitivity is increased too.The relative humidity sensitivity is about 0.6mV/% when current is 10mA, but the current can not increase infinitely, The Pt micro-air-bridge will melt when current is above 12mA.
Relative Humidity Rh (%) Fig.6 Relation between output voltage and relative humidity at 25℃, I=4mA and 1atm.
equilibrium by application of a constant current of 4mA, the equilibrium temperature is insensitive to the humidity in the temperature range 100℃-200℃ (demonstrated at 140℃, as shown in Fig.6), because of similar thermal conductivity at various relative humidity.
V. SUMMARY AND CONCLUSIONS
Output Voltage (V)
A new integrated temperature and humidity sensor based on MEMS is proposed and demonstrated. Since micro-air-bridge in this new means has an operating temperature of above 350℃, the surface of its sensing area is cleaned by the burning out of dust, oil, etc. thus guaranteeing the long life and stable sensitivity of the humidity part. This measurement has quick response and good sensitivity. Measurement of relative humidity is prone to environmental influence. Some characteristic, such as how to protect it from the ambient temperature influence, humidity detection in low temperature (below 0 ℃ ) and high temperature (above 100℃),etc. are still under further research.
Relative Humidity Rh (%) Fig.7 Relation between output voltage and relative humidity at 25℃, I=10mA and 1atm.
ACKNOWLEDGMENT The author would like to thank my instructor Mr .Zhan Zhao professor and my co-worker Yu Wu, Bojun Zhang and YiZHong Wang for device preparation, measurements and
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Proceedings of 2004 International Conference on Information Acquisition [3] M. Kimura, “A new method to measure the absolute-humidity independently of the ambient temperature,” Sensors and Actuators B, pp. 156-160,33(1996). [4] K. Makita, “Viscosity and thermal conductivity,” Baifukan, Tokyo, 1975, Ch.5, pp70-153. [5] Z. Fang, Y. Wu, Z. Zhao, “ Design an integrated temperature and humidity sensor based MEMS,” Measurement&Control Technology, Vol.23, pp.10-11,Mar 2004.
useful discussions, also to thank Li Wang for helping in the device fabrication. REFERENCES [1] M.Rittersma, “Recent achievements in miniaturized humidity sensors: a review of transduction techniques,” Sensors and actuators A, pp.196210,96 (2002). [2] M. Kimura, etal, “Application of the air-bridge microheater to gas detection, ” Sensors and Actuators B, pp.857-860,24-25 (1995).
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