Reprinted from
REGULAR PAPER
Convection-Based Tilt Sensor with Minimized Temperature Fluctuation Ju Chan Choi and Seong Ho Kong
Jpn. J. Appl. Phys. 50 (2011) 06GM13
# 2011 The Japan Society of Applied Physics
Person-to-person distribution (up to 10 persons) by the author only. Not permitted for publication for institutional repositories or on personal Web sites.
REGULAR PAPER
Japanese Journal of Applied Physics 50 (2011) 06GM13 DOI: 10.1143/JJAP.50.06GM13
Convection-Based Tilt Sensor with Minimized Temperature Fluctuation Ju Chan Choi and Seong Ho Kong School of Electrical Engineering and Computer Science, Kyungpook National University, Daegu 702-701, Korea Received November 30, 2010; revised January 9, 2011; accepted January 19, 2011; published online June 20, 2011 This paper presents a simple and novel method for minimizing the performance dependence of a convection-based tilt sensor on environmental temperature fluctuation. Previously reported convective tilt sensors show considerable fluctuation in output voltage because of various external thermal effects. Therefore, it is necessary to have a complex circuit to compensate the external temperature fluctuation or an additional cautious package to reduce the thermal effect. In this paper, a convective tilt sensor that inhibits the environmental thermal effect without an additional circuit or package is discussed. An integrated thin-film-based thermoelectric device is utilized not only for heating an encapsulated air medium with a symmetric temperature profile when it is balanced, but also for minimizing external thermal effects. The hot junction of the thermoelectric device operates as a heater and the cold one keeps the temperature difference between two junctions constant regardless of the external thermal environment. The proposed tilt sensor showed a fairly linear output voltage within the inclination range of 90 on two axes and even exhibited a consistent output voltage under various external temperature conditions. The fluctuation rate of the proposed tilt sensor was kept within less than 2% of the output voltage; this was even the case for a temperature range of 25 to 60 C. # 2011 The Japan Society of Applied Physics
1. Introduction
Temperature Sensor
Tilt sensing is becoming a quick need for many applications in aerospace, automobiles, entertainment, computer peripherals, health sciences and so on.1,2) The development of lowpower and low-cost sensor would further broaden the scope of those applications. Various types of tilt sensors based on magnet,3,4) electrolyte,5–7) piezo,8) light,9) convection10,11) are previously reported in many papers. Compared to other approaches,3–9) the convection based tilt sensors10–13) have an advantage of considerably large detecting angle. Moreover, it is simple and inexpensive to fabricate, and gives more reliable and durable performance than other types. In recent years, some efforts have been made by our laboratory to realize convection-based tilt sensors for many applications, including those in automotive, environmental monitoring, and industrial process control fields.12,13) The previously reported tilt sensor covers a measurement range of 90 on two axes with excellent linearity and symmetric sensitivity.12) Moreover, the previously reported experiments have been carried out with different cavity volumes and gas pressures in order to identify the optimal operation conditions for the sensor.13) One of the main drawbacks of the previously reported convective tilt sensor is its fluctuation in output voltage due to the external thermal effect. Therefore, a complex circuit that would compensate for the temperature fluctuation or an additional cautious packaging method for minimizing the effect is necessary.13) In this paper, a simple and novel method for minimizing the temperature fluctuation of convection-based tilt sensors with thermoelectric device is reported. In order to maintain a constant temperature distribution regardless of external thermal conditions, chrome- and nickel-based thermoelectric devices were integrated in the air chamber. The integrated thermoelectric device generated an adequate temperature distribution for stable air convection. Low-cost mass production of the proposed tilt sensor is feasible and its performance can be further improved by optimizing the structure and materials of the thermoelectric device and minimizing its thermal conduction.
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Temperature Sensor
Heater
Temperature Sensor
Cooler
Temperature Sensor
Heater
Cooler
θ
θ
variable external environment
ΒΤ
ΑΤ
variable external environment
ΒΤ
Hot ΑΤ − ΒΤ = ΔT ΔT = unstable
ΒΤ
Cold
(a)
ΑΤ
Hot ΑΤ − ΒΤ = ΔT ΔT = stable
ΒΤ
Cold
(b)
Fig. 1. (Color online) Structure and operation mechanism of the previously reported tilt sensor and proposed tilt sensor.
2. Device Structure
Figure 1 shows the structure and operation mechanism of the previously reported tilt sensor and the proposed tilt sensor. Figure 1(a) shows the functional mechanism of the previously reported tilt sensor. When the sensor is balanced without tilting, a symmetric temperature profile can be formed; this is detected by the temperature sensors located at the same distance from the heating element. When a tilting motion is applied, an asymmetric temperature distribution occurs between sensors on the same axis, owing to thermal convection. Air convection occurs owing to a temperature difference caused by heating of the central part. This type of tilt sensor has no cooling function in the chamber. Therefore, an external environmental temperature fluctuation affects the air convection in the chamber. This kind of irregular convection change causes an output fluctuation of tilt sensors exposed to environments where the temperature varies. Figure 1(b) shows the functional mechanism of the proposed tilt sensor in this paper. This sensor works on the principle of minimizing the temperature fluctuation caused
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# 2011 The Japan Society of Applied Physics
Person-to-person distribution (up to 10 persons) by the author only. Not permitted for publication for institutional repositories or on personal Web sites.
Jpn. J. Appl. Phys. 50 (2011) 06GM13
J. C. Choi and S. H. Kong
Temperature sensor Peltier device
Cold junction
oxidation
Cr deposition
oxide etching
Ni deposition
Hot junction Fig. 2. (Color online) Structure of the proposed tilt sensor.
silicon etching
by the external thermal environment. The only difference from previously mentioned tilt sensor is that the temperature is controlled in the chamber by newly added cooling parts — thin-film-based thermoelectric devices. The cold junction of the thermoelectric device keeps the temperature difference with the hot junction constant, regardless of the variable external environment. This constant temperature difference brings stable air convection in the sealed chamber, resulting in a tolerant tilt sensor. Figure 2 shows the structure of proposed tilt sensor. Four heating parts were prepared on the central area of dielectric membrane and four cooling parts were located at the same distance around the heating parts, as parts of the thermoelectric device. The thermoelectric device, presented in this paper, has been adopted as the heating and cooling parts because it can synchronistically generate heating and cooling effects. Finally, four Ni temperature sensors were fabricated around the heating area of the membrane with the same distance for tilting measurement on two axes.
Fig. 3. (Color online) Fabrication process of the bottom substrate.
Top substrate
Bottom substrate
Fig. 4. (Color online) Fabricated tilt sensor.
3. Fabrication
A
A
3.1 Bottom substrate
The bottom substrate consists of a microheater, coolers, and temperature sensors. The heater and coolers are made of an integrated thin-film-based thermoelectric device. This thermoelectric device is appropriate for the proposed tilt sensor because the cooling is synchronized with the heating operation. The hot junction of the thermoelectric device operates as a heater and the cold junction keeps the temperature difference with the hot junction constant, regardless of the external environment. Chrome and nickel were used to fabricate the thermoelectric device owing to their high efficiency, simple process, and low production costs. The temperature sensor used in the tilt sensor was fabricated using nickel, because of the high temperature coefficient of resistance of nickel. Figure 3 shows the fabrication process of the bottom substrate. Silicon oxide was grown by wet oxidation, and the oxide membrane was fabricated by backside silicon etching for thermal isolation. After fabrication of the membrane, the thin-film-based thermoelectric device and temperature sensor were fabricated through two lift-off processes. Nickel temperature sensors were deposited by e-beam evaporation on the membrane. It was possible to fabricate the tilt sensor by simple fabrication sequences because the same material — nickel — was to be used for both the thermoelectric device and the temperature sensors. Finally, the bottom substrate was encapsulated with the prepared top substrate using epoxy bonding.
B C B B C C A
A
A: Temperature sensor (Nickel) B: Chrome, C: Nickel Fig. 5. (Color online) Microscopic image of the proposed tilt sensor showing the positions of the thermoelectric device and temperature sensors.
3.2 Top substrate
The top substrate provides a hermetic air cavity for free convection. The fabrication sequence began with thermal oxidation to grow a 500-nm-thick oxide layer. The oxide layer was patterned using a buffered oxide etchant at room temperature. After formation of the oxide pattern, a 150m-deep silicon cavity was etched by deep reactive ion etching (D-RIE). Finally, another thick oxide layer was grown in a wet oxidation furnace to reduce the thermal environmental impact on the convection of the proposed tilt sensor. Figure 4 shows the fabricated tilt sensor, while Fig. 5 shows the microscopic image of the sensor showing the position of the thermoelectric device and temperature sensors.
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# 2011 The Japan Society of Applied Physics
Person-to-person distribution (up to 10 persons) by the author only. Not permitted for publication for institutional repositories or on personal Web sites.
Jpn. J. Appl. Phys. 50 (2011) 06GM13
J. C. Choi and S. H. Kong temperature
temperature
distance
distance
θ
temperature sensor A
θ
temperature sensor B
Fig. 8. (Color online) Measurement method for the proposed tilt sensor.
Fig. 6. (Color online) Fundamental characteristic result of the fabricated thermoelectric device on the proposed tilt sensor.
Heating current: 30 mA Sensing current: 10 mA
Fig. 9. (Color online) Two-axis measurement result of the proposed tilt
sensor.
Fig. 7. (Color online) Infrared thermal image and temperature
distribution of the proposed tilt sensor with an operation current of 30 mA.
4. Experiments 4.1 Fundamental experiment result
Figure 6 shows a fundamental characteristic result of the fabricated thermoelectric device on the proposed tilt sensor. Temperatures of heating area and cooling area were measured in this experiment. Ideally, these two areas should operate separately, but the temperatures of both areas increased. Joule heating appeared to have occurred in the cooling part of the fabricated thin-film-type thermoelectric device. In addition, heat conduction between both areas through the substrates was an ancillary cause of this experimental result. Nevertheless, a temperature difference between both areas was detected. This result shows that the cooling part of the thin-film-type thermoelectric device functions properly, even though heat conduction and Joule heating affect the temperature increase. The largest temperature difference between both areas was detected at an operation current of 30 mA. This current value is the most preferable operation source current because the high temperature difference leads to high sensitivity of the fabricated tilt sensor.12) Figure 7 shows the infrared thermal image and temperature distribution of the proposed tilt sensor with an operation current of 30 mA. This result reveals that the proposed tilt
sensor operates well under a proper temperature distribution for stable air convection. Figure 8 shows the measurement method for the proposed tilt sensor. A positive angle is defined for a clockwise inclination. When the tilt sensor experiences a clockwise inclination, measurement is performed on temperature sensor A located on the right-hand side of the heating area, i.e., the cold part, in a range of 0 to 90 . With the same mechanism, temperature sensor B, located on the left side, takes measurements in the range of 0 to 90 for counterclockwise tilting. As a result, the proposed tilt sensor can measure tilting motion in the range of 90 to +90 on two axes by combining the outputs of four temperature sensors. With inclination, each temperature sensor experiences a change in resistance owing to the convection-induced temperature change. The resistance change of the temperature sensor corresponding to a given inclination angle is measured as the output voltage by Ohm’s law. The fabricated tilt sensor exhibits quite linear and symmetric characteristics on two axes using four temperature sensors over the entire range of 90 , as shown in Fig. 9. 4.2 Effect of external temperature on sensitivity
Figure 10 shows the output result as a function of environmental temperature. The experiment was performed in various thermal environments. The proposed tilt sensor shows consistent output results despite the different external temperatures. The fluctuation rate of the proposed tilt sensor
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# 2011 The Japan Society of Applied Physics
Person-to-person distribution (up to 10 persons) by the author only. Not permitted for publication for institutional repositories or on personal Web sites.
Jpn. J. Appl. Phys. 50 (2011) 06GM13
J. C. Choi and S. H. Kong
performance.14–17) Moreover, different kinds of high-performance microcoolers can be used for the proposed tilt sensor to improve sensing efficiency.18–20) 6. Conclusions
A convection-based tilt sensor that employs a thermoelectric device to minimize temperature fluctuation was proposed. In order to maintain a constant temperature distribution regardless of external thermal conditions, chrome- and nickel-based thermoelectric devices were integrated in the air chamber. The integrated thermoelectric device generated an adequate temperature distribution for stable air convection. The proposed tilt sensor showed a fairly linear output voltage within the inclination range of 90 on two axes and a consistent output voltage despite varying external temperature conditions. Low-cost mass production of the proposed tilt sensor is feasible and its performance can be further improved by optimizing the structure and materials of the thermoelectric device and minimizing its thermal conduction.
Heating current: 30 mA Sensing current: 10 mA
Fig. 10. (Color online) Output result as a function of environmental temperature.
Acknowledgement
This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (2010-0001884).
1) R. Dai, R. B. Stein, B. J. Andrews, K. B. James, and M. Wieler: IEEE
Heating current: 30 mA Sensing current: 10 mA
2) 3) 4) 5)
Fig. 11. (Color online) Output result as a function of external temperature higher than the operation temperature.
6) 7)
remains within less than 2% of the output voltage, even in the temperature range of 25 to 60 C. However, the proposed tilt sensor does not operate at an external temperature higher than its operation temperature, as shown in Fig. 11. This is because the entire cavity of the proposed tilt sensor is overheated by the external temperature and free convection does not occur in the cavity. 5. Discussion
A thin-film-based thermoelectric device can be used to minimize the temperature fluctuation of the convectionbased tilt sensor due to external temperature changes. The performance of the proposed tilt sensor can be further improved by optimizing structural design characteristics such as thickness, length, distance, and structure for ensuring reduced Joule heating and thermal conduction.14) A different material combination of thermoelectric device for achieving higher thermoelectric efficiency would also improve device
8) 9) 10)
11) 12) 13) 14) 15) 16) 17) 18) 19) 20)
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# 2011 The Japan Society of Applied Physics
