Relative-Air Humidity Sensing Element Based on a Micromachined Floating Polysilicon Resistor P. Zambrozi Jr., F. L. Della Lucia and F. Fruett School of Electrical and Computer Engineering, University of Campinas Center for Semiconductor Components, University of Campinas Campinas, Brazil e-mail:
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[email protected] Abstract—This paper presents a new type of air-humidity sensing element based on heat transfer mechanisms such as conduction and convection. The sensing element is a single floating resistor of doped polysilicon and operates in two phases: thermal actuation and thermal sensing. As actuator, a convenient biasing current is applied to the resistor leading to self-heating by Joule Effect. As sensor, the resistor biasing current is reduced and the time constant of the cool-down process is measured. This Thermal-Time Constant (TCC) is close related to the relative humidity of the air (%RH) surrounding the floating resistor. Characterization results show that TCC was 16 s, 5.1 s and 1.4 s for 30, 50 and 70 %RH, respectively. The sensing element operated at room temperature and presented a maximum power consumption of 5 mW.
I.
INTRODUCTION
Humidity sensors can be found in a variety of applications, including agriculture, climate control, food storage and domestic appliances. In order to satisfy all applications, microelectronic humidity sensors should provide high sensitivity over a wide range of humidity and temperature, low power consumption and to be compatible with standard IC fabrication technology. Nowadays, there are various types of humidity sensors, employing different hygroscopic materials for the sensing element. However, 75% of the humidity sensors, in the market, are based on the resistive or capacitive techniques [1], despite nonlinear, humidity-resistance characteristics and short life-time problems (especially in harsh environment), are still present.
temperature [8]. The Smetana’s sensor was realized in Low Temperature Cofired Ceramics (LTTC) technology, where presented the power consumption of 3.2 W. In this work, we present a new type of air-humidity sensing element based on heat transfer mechanisms. The sensing element is a single floating resistor, made of doped polycrystalline silicon (polysilicon) and its fabrication is fully compatible with CMOS microelectronic process. II.
PHYSICS PRINCIPLE OF THE SENSOR
In thermal sensors, the input signal, which can belong to any of the six signal domains (thermal, electrical, radiant, mechanical, chemical or magnetic), is transduced into the output signal in two steps: first, the input signal is transduced into a thermal signal, which is then transduced into electrical output signal [9] – with the exception of the temperature sensors, which the signal is transduced in just one step from the thermal to electrical signal domain. In thermal actuators, the electrical inputs signal is transduced into a thermal signal, which is then transduced into any of the six signal domains. In this work, a single polysilicon resistor suspended (floating resistor) was used as both actuator and sensor element. Fig. 1 shows the photograph of the floating resistor.
Besides microelectronic technology, micromachined structures have also been applied for a wide range of sensors [2-5]. Micromachined thermal sensors based on the heat transfer mechanism of conduction, convection (free or forced flow) and radiation, have been fabricated using MEMS technology [6]. MEMS flow sensors for liquids and gases are already successful commercial products [7]. On the order hand, micromachined thermal sensors have still been underexplored as humidity sensors. W. Smetana, developed a humidity sensor based on the difference of thermal conductivity between dry air and water vapor, at high
Figure 1. Photograph of the sensing element
This research is funded by the Brazilian National Council of Scientific and Technological Development – CNPq
978-1-4244-5335-1/09/$26.00 ©2009 IEEE
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IEEE SENSORS 2009 Conference
In the thermal actuator phase, a convenient biasing current is applied to the floating resistor leading to self-heating by Joule Effect. In the sensor configuration, the floating resistor biasing current is reduced and the time constant of cool-down process is measured. This process will depend on the vapor water quantities in the ambient, since the resistor is suspended and cooling will be due to heat transfer mechanisms (thermal conduction and thermal convection). The heat transfer due to thermal radiation will not be considered since the sensor operates in low temperature. III.
b.
FABRICATION OF THE SENSOR
The fabrication steps of the floating resistor sensor are illustrated in Fig. 2. The first step is the deposition of a sacrificial layer of SiO2 over a Silicon wafer. This was accomplished by using a CVD-ECR system and the oxide thickness was of 0.56 μm, as shown in Fig. 2a. In the second step a polysilicon layer was deposited by vertical LPCVD at 800 °C using SiH4 gas diluted in H2. This layer is 1 μm thick and was then implanted with the 3E13 cm-2 Boron dose using 40 kev. Rapid thermal annealing – RTA (1000 °C / 40 s) was used to activate the dopants and reconstruct the crystalline grid, as shown in Fig. 2b. In a third step, the resistor structure was defined by Reactive-Ion Etching – RIE (SF6/CF4/N2; 10:15:20). The etching time was 4 minutes and photolithographic process was performed with the AZ3312 photoresist. After plasma-etching, another implantation, with a high Boron dose (5E15 cm-2) was made, only at the contacts to improve contact resistance characteristics. A second RTA step (1000 °C / 40 s) activates the new dopants. Fig. 2c shows the resistor structure. Next, Aluminum was evaporated and the contacts are defined by lift-off, as shown in Fig 2d. The last step is etching away the oxide layer under the polysilicon using buffered HF solution to create the suspended bridge. Fig. 2e shows the Micromachined Floating Resistor formed. The final dimensions of the resistor are: length of 450 μm, width of 75 μm and thickness of 1 μm. IV.
a.
c.
d.
e. Figure 2. Schematic of the main steps of the floating polysilicon resistor fabrication: (a) SiO2 on silicon wafer, (b) Polysilicon on SiO2 layer, (c) Polysilicon structure resistor, (d) Definition of the region aluminum pads, (e) Etching of sacrificial SiO2 - Floating polysilicon resistor formed.
EXPERIMENTAL RESULTS
A. Temperature characterization of the floating resistor Fig. 3 shows the resistance versus temperature. This test was performed in a climactic chamber (THERMOTRON – 3800), where the temperature changes from 15 °C to 55 °C, with steps of 5 °C. A multimeter (AGILENT – 34401A) was used to measure the resistance values of the floating resistor. The result can be fitted by the following equation:
Rsen(T) = 78637,77 - 341,76T + 0,93T 2
Figure 3. Resistance vs. Temperature
(1)
where, Rsen(T) is the resistance of the floating resistor and T the temperature. Fig. 4 shows the TCR (Temperature Coefficient of Resistance) of the floating resistor versus temperature. The TCR changes from – 0.425 to – 0.380 %/°C in the range between 15 °C and 55 °C. Figure 4. TCR floating resistor
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B. Themal Actuator Phase In order to configure the floating resistor as a thermal actuator, we applied a convenient biasing current, leading its self-heating by Joule effect. An amplifier circuit on inverting configuration was used to characterize the thermal actuator. Fig. 5 shows the schematic circuit.
cooling process depends on the heat transfer mechanisms (conduction and convection). In thermal conduction mechanism, the cooling process is due to the heat transfer through neighboring molecules. In this case, the heat transfer takes place by two ways: due to physical contact of the floating resistor with its mechanical support and, due to the expose of the floating resistor with the ambient. The thermal conduction, due to physical contact, will not have a great influence on the cooling process, because the floating resistor is fixed only at the extremities. On the order hand, the thermal conduction due to exposure with the ambient will have a great influence on the cooling process, because of the large area of the floating resistor with the ambient. The equation that describes the heat transfer by conduction mechanism is given by:
Q = −kA
Figure 5. Schematic circuit for characterization of the sensing element.
The resistance of the floating resistor (Rsen) is obtained by measuring the output voltage (Vout) as a function of the input voltage (Vin) and is given by the following equation: Rsen = −
Vout 10k Vin
(2)
Fig. 6 shows the resistance of the floating resistor as a function of the input voltage. The floating resistor temperature is obtained by (1) – right axis. The test was performed until the floating resistor temperature heats up to approximately 50 °C. In this case, the input voltage was 2.8 V and maximum power consumption was 5 mW.
∂T ∂x
(3)
where, Q is the heat flux; k the thermal conductivity, A the transversal area of the floating resistor and ∂T/∂x the temperature gradient. Recently, P. T. Tsilingiris presented a study about the variation of the thermophysical properties of the air (density, viscosity, thermal conductivity, specific heat capacity, thermal diffusivity and Prandtl number) as a function of the temperature and relative humidity of the air [10]. In thermal convection mechanism, the cooling process occurs due to the movement of the air from hot regions to cool regions – when the gas is heated-up, its density reduces in relation the density of cool regions – resulting in a convection current. In this case, the convection mechanism dominates the cooling process. The equation that describes this mechanism is given by:
Q = hA(T − T∞ )
(4)
where, Q is the heat flux; h is convective heat transfer coefficient, A the transversal area of the resistor and (T - T∞) the temperature difference between the floating resistor and the ambient.
Figure 6. Resistance of the sensing element versus input voltage
During thermal actuator phase, the input voltage was fixed in 2.8 V during 3 minutes. C. Thermal Sensor Phase The characterization of the floating resistor, Rsen, operating as a humidity sensor, is done by measuring the Thermal Time Constant (TCC) during the cooling process. In this case, the
In order to perform the floating resistor during the thermal sensor phase, the input voltage was fixed in 0.5 V during 10 minutes and the cooling process was monitored for three different humidities (30, 50 and 70 %RH) at room temperature. The relative humidity and temperature were controlled using a climatic chamber (THERMOTRON – 3800). Fig. 7 shows the dependence of the floating resistor, in the cooling process, for different humidities. Based on the capacitor charge/discharge theory, we define TTC as a time required for the floating resistor to reduce the temperature in 37% (from 50 °C to 31.5 °C, in this case). Fig. 8 shows the temperature time profile (cooling process) for 30, 50 and 70 %RH where TTC is measured as 16 s, 5.1 s and 1.4 s, respectively.
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the air surround the floating resistor. Characterization results presented TTC of 16 s, 5.1 s and 1.4 s for 30, 50 and 70 %RH, respectively. This floating resistor operates at room temperature and its maximum power consumption was of 5mW. ACKNOWLEDGMENT Authors acknowledge the Center for Semiconductor Components (CCS – Unicamp) staff for the help in preparation of the samples. CAPES and the Brazilian National Research Council – CNPq, under Microelectronic National Program – PNM, National Institute of the Science and Technology of Micro and Nanoelectronic System – NAMITEC and Universal Project n° 481412/2008-5 for financial support. REFERENCES Figure 7. Cooling process of the sensor
[1]
Figure 8. Thermal Time Constant – TTC measurements
V. CONCLUSION A new type of air-humidity sensing element was developed based on heat transfer mechanisms. The single floating resistor made of doped polycrystalline silicon (polysilicon) was fabricated using standard CMOS microelectronic processes. We found that the Thermal Time Constant (TTC) is closed related to the relative humidity of
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