- Email: [email protected]
Short-circuit measurement by Seebeck current detection of a single thermocouple and its application
Sensors and Actuators A 139 (2007) 104–110
Short-circuit measurement by Seebeck current detection of a single thermocouple and its application Seung Seoup Lee, Mitsuteru Kimura ∗ Tohoku Gakuin University, Faculty of Engineering, 13-1, Chuo-1, Tagajo, Miyagi 985-8537, Japan Received 31 July 2006; received in revised form 16 December 2006; accepted 4 April 2007 Available online 20 April 2007
Abstract A novel temperature-difference measurement method based on Seebeck current detection by a single thermocouple is proposed. Principle of the sensor is based on the measurement of short-circuit of Seebeck current due to the thermoelectric power in a single thermocouple. We focused on the fact that the internal resistance of n2+ -Si corresponding to the short-circuit current can be three or four order decreased while Seebeck coefficient αs decreases only for a factor of three or four. We have made a theoretical model to evaluate the performance of our proposed short-circuit measurement using the generated thermoelectric output power comparison by sensor itself due to infrared (IR) absorption. It is shown that our proposed single thermocouple has a higher thermoelectric power output or equivalent than that of traditional thermopile. A single micro-cantilever thermocouple as for an IR sensor, which is composed of extremely low resistivity (<10−3 cm) materials of n2+ -Si and Al (Si: 1%) metal is developed, and its validity and effectiveness are demonstrated. © 2007 Elsevier B.V. All rights reserved. Keywords: Thermopile; Thermocouple; Temperature-difference measurement; Seebeck current; Infrared (IR) sensor; Cantilever
1. Introduction IR sensors using the thermopile have relatively low responsivity compared to thermistor bolometer and pyroelectric sensors. However, it is widely used and has been developed as IR sensors [1–6], especially for body temperature measurement, because it is a unique sensor, which can measure only the temperature-difference, and can operate with non-bias and has no current-induced noise. Thermopile consists of thermocouple-array structure because each thermocouple itself has a low sensitivity. Lenggenhager and Baltes have proposed thermoelectric infrared (IR) sensors with n-poly/p-polythermopile with relatively high sensitivity of 170 V/W (Seebeck coefficient αs of n+ -poly: −110 V/K; p+ -poly: 195 V/K) [2]. However, their sensors show a low S/N because of very high internal resistance (2.7 M) due to 62 thermocouple-array connected in series with 2.8 m wide conductor-lines to get enough sensitivity. Chen-Hsun et al. have shown that there is a trend of saturation of specific detectivity
∗
Corresponding author. Tel.: +81 22 368 7162; fax: +81 22 368 8535. E-mail address: [email protected] (M. Kimura).
0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.04.045
D* curve, since the increase of responsivity Rv will be compensated by the increase of Johnson noise caused by high internal resistance due to many thermocouple pairs, such as 62 pairs in a thermopile [4]. In spite of its low sensitivity and low S/N, due to its high internal resistance and complicated structure of array, thermopiles or thermocouples are mostly used only for the case in which the temperature-difference measurement is required because it is a unique sensor which can measure only temperature-difference. Also, the yield of thermopile is not good in terms of production, because of its complex structure of array composed of many pairs of thermocouples. Therefore, high sensitive temperature-difference sensor with a simple structure and high S/N has been strongly demanded. Since Seebeck effect was found, the thermocouple had been used as a temperature-difference sensor to measure the opencircuit voltage among consisting different materials. The opencircuit measurement has merits such that it is independent of the electrical resistance and easy to measure, and that thermoelectric voltage is only determined by the pair materials and temperaturedifference to be measured. The author, Kimura, has proposed as a Japanese patent a novel temperature-difference measurement method based on the thermal current detection in the short-circuit of a sin-
S.S. Lee, M. Kimura / Sensors and Actuators A 139 (2007) 104–110
gle thermocouple using imaginary short of the OP amplifier [7]. In this paper, we propose the short-circuit measurement of Seebeck current by a thermocouple [7], and it is evaluated through the simple theoretical model based on the comparison of the generated thermoelectric powers of our proposed sensor and the traditional open-voltage measurement thermopile due to IR absorption. It is theoretically shown that our proposed single thermocouple sensor of extremely low internal resistance has equivalent or higher thermoelectric power output than that of the traditional thermopile sensor. In addition, there has ever been no idea on the direct measurement of the short-circuit Seebeck current using the imaginary short of OP amplifier. In this paper, Seebeck current is measured using this imaginary short of the OP amplifier. However, recently Imran and Bhattacharyya have reported on the characterization of on-chip thin film thermopile based on short-circuit Seebeck current measurement using hot and cold junctions, and heater on-chip [8]. They have used five thermocouples of chromium (Cr) and Titanium (Ti) in their thermopile. Fundamentally the principle is same with our proposal in terms of the detection of Seebeck current. But they had just focused on the experimental characterization of Seebeck current from the thermopile for the application of on-chip electrical microthermoelectric power generators, not for temperature sensor or IR sensor application, even though they have mentioned its possibility of the use of Seebeck current for the temperature sensor application. In this paper, a temperature-difference sensor of microcantilever type single thermocouple is fabricated and its validity as a temperature and IR sensor based on Seebeck current measurement is demonstrated. 2. Principle It is well known that the Seebeck coefficient αs for silicon is approximated as a function of electrical resistivity ρ as shown below [9]: mk ρ ln αs = q ρ0
(1)
where ρ0 ≈ 5 × 10−4 cm, m = 2.6 for silicon (Si) and k is the Boltzmann constant and q the electron charge. Lee et al. have reported the thermopile (17 pairs) made of p and n type Si with higher resistivity of 10 cm, because the higher resistivity makes it have the higher Seebeck coefficient [10]. However, authors have noticed that it is easy to decrease the resistivity three or four orders, which can extremely increase the short-circuit Seebeck current of the thermocouple, because the Seebeck coefficient αs only decreases several times [7]. If we adopt the short-circuit Seebeck current measurement method by a single thermocouple with the n2+ -Si and metal film instead of open-circuit voltage measurement in the traditional thermopile, it is expected that our proposed thermocouple can generate higher thermoelectric power output, Wi , than that, Wv , of a
105
Fig. 1. Structural models of an n-pairs thermopile (a) and a single thermocouple (b) with the same IR sensing area of cantilever.
traditional open-circuit thermopile. Each subscript ‘v’ and ‘i’ correspond to voltage measurement (thermopile) and current measurement (single thermocouple), respectively. In Fig. 1, the theoretical model of the cantilever type thermal sensors is shown. One is a model of the thermopile composed of n-pairs of thermocouples (a), and the other is that of a single thermocouple (b). Both (a) and (b) have the same area (a × b) of IR absorption region, and is assigned the same temperature gradient T due to the absorption of incident IR power P0 . And they have different resistivity ρv and ρi . Internal resistance rv and ri of sensors are proportional to their resistivity ρv and ρi , respectively. In the thermopile (a), following equations are obtained: the total thermoelectric voltage Ev = nev = nαv T; internal resistance rv = nr0v (r0v : resistance of each thermocouple); generated power Wv = Ev2 /rv = nα2v T 2 /r0v . In the single thermocouple (b), following equations are also obtained: for γ = αv /αi and ρi /ρv = 10−f , the thermoelectric voltage Ei = ei = αi T; internal resistance ri = r0v × 10−f /n; generated power Wi = Ei2 /ri = α2i T 2 /ri = nα2v T 2 × 10f /γ 2 r0v . Therefore, it is derived the generated thermoelectric power ratio M of Wi and Wv by following equation: M=
10f Wi = 2 . Wv γ
(2)
From Eq. (2), we can see that the ratio of the thermoelectric power by each sensor is only the function of the Seebeck coefficient and the resistivity. For f = 3, M will be 91.5 because of γ = 3.3, and for f = 3.5, M will be about 110 because of γ = 5.3. For example, it is seen that the generated thermoelectric power Wi by a single thermocouple with extremely low resistivity semiconductor, such as n2+ -Si thin film of ρi = 3 × 10−3 cm is expected to be 110 times higher than that of the thermopile with n-pairs thermocouples made of relatively high resistivity Si, such as ρv = 10 cm (f = 3.5 determined by ρi and ρv . And we can see that M will be 1 for f = 0 (ρv = ρi in the case of γ = 1. It means that both n-pairs thermopile and our proposed single thermocouple have the same generated thermoelectric power as long as they are made of same materials, including the resistivity, i.e they are equivalent. However, since a pair of thermocouple of the thermopile requires line and space, the internal resistance will be
106
S.S. Lee, M. Kimura / Sensors and Actuators A 139 (2007) 104–110
Two diode-thermistors, Th.A and Th.B, made of pn junction diode proposed by the author are prepared to measure the absolute temperature at each point A and B, corresponding to the junction portions of the calibration thermocouple, due to temperature gradient generated by the buried micro-heater formed on the same substrate [11]. 3.1. Fabrication process and cantilever sensors on-chip
Fig. 2. Measuring circuit for short-circuit current of a single thermocouple using imaginary short of an OP amplifier.
larger than an ideal n-pairs thermopile configuration shown in Fig. 1(a). We can use the imaginary short of the OP amplifier to measure the short-circuit Seebeck current of the single thermocouple sensor as shown in Fig. 2. The combination of the single thermocouple sensor and the OP amplifier will compose an active IR sensor. 3. Fabrication and measurement Thermocouple sensors of cantilevers, which is made of highdoped n2+ -Si using SOI (1 0 0)-oriented silicon-on-insulator (SOI) wafer with 6.5 m thick, slightly n-doped silicon (1.0–3.0 cm), a 1.0 m thick silicon buried dioxide intermediate layer (SOI layer) and a 400 m thick base, are fabricated to evaluate the generated thermoelectric power ouput by a sensor itself and the sensitivity for IR absorption between our proposed thermocouple sensor and the traditional thermopile sensor. Fig. 3 shows the basic concept of the micro-thermocouple cantilever sensors for IR detection, in which one thermocouple sensor, one calibration thermocouple, two pn diodes as a diode thermistor (as absolute temperature sensors to measure the temperatue of the substrate) and micro-heater are formed on the same SOI substrate. The calibration thermocouple is to measure Seebeck coefficient αs for calibration of sensor thermocouple. Heater is used to apply the temperature gradient to the calibration thermocouple in the SOI substrate.
Our proposed single thermocouple of the cantilever type as IR sensor is composed of double layers of n2+ -Si and Al (Si: 1%) metal. Fig. 4 shows the fabrication process of cantilever type thermocouple IR sensor, pn diode thermistors and heater on chip shown in Fig. 3. After initial cleaning, the low resistivity n2+ -Si layer (<10−3 cm) is created by thermal diffusion using phosphorous oxide coating dopant (OCD)-coating diffusion source at 1100 ◦ C for 3 h (Fig. 4, step 1). After the lithography of thermal silicon oxide film (about 0.5 m thick) to open windows for pn junction diode thermistors (Fig. 4, step 2), thermal diffusion for p+ junction of pn diode, n+ junction for metal connection in heavily diffused n2+ -Si layer and p+ layer for heater resistor are carried out using same diffusion source of OCD (Fig. 4, step 3). The thickness of n2+ -Si diffused layer (average concentration l × l020 cm−3 layer) is estimated to be about 2 m thick. The Al (Si: 1%) metal (about 0.3 m thick) of pair material in a single thermocouple is deposited by RF sputtering and patterned using etching process (Fig. 4, step 4). The double layer of n2+ -Si and Al (Si: 1%) metal is isolated via the thermal SiO2 film (about 0.5 m thick) formed on the SOI layer. The backside cavity to release the cantilever from the SOI substrate is formed using deep reactive ion etching (RIE), and finally the gold black film for IR absorption is evaporated under the low vacuum (Fig. 4, step 5). Fig. 5 shows the fabricated micro single thermocouple cantilever sensors for IR detection, in which one is sensor thermocouple and the other is calibration thermocouple. And two pn diodes as a diode thermistor (absolute temperature sensor), and micro-heater are formed on the same SOI substrate. Two micro-cantilever sensors have the dimensions of 200 m wide and two different 1200 m long and 1600 m long and 7.0 m thick (including 1.0 m thick BOX layer). Two diode-thermistors, Th.A and Th.B, made of pn junction diode are prepared to measure the absolute temperature and temperature-difference due to heat generated by the buried micro-heater at each point A and B, corresponding to the junction portions of the calibration thermocouple, formed on the same substrate. 3.2. Measurements and its results
Fig. 3. Sketch of the basic concept of a single micro thermocouple cantilever sensor by novel temperature-difference measurement method.
Diode-thermistor is a thermistor-like sensor based on the temperature dependence of forward current of pn junction at constant forward voltage Vf (e.g. Vf = 0.55 V). In Fig. 6, the characteristics of diode current I (plotted in logarithm) are shown as a function of reciprocal temperature 1/T for various fixed forward bias voltage Vf of Th.A. Logarithm of current I at a constant
S.S. Lee, M. Kimura / Sensors and Actuators A 139 (2007) 104–110
107
Fig. 4. Fabrication process of a micro single thermocouple cantilever sensor on chip.
voltage is theoretically proportional to the reciprocal temperature 1/T. We can see from this Fig. 6 that a constant biased diode behaves like a thermistor and has a very high sensitivity as an absolute temperature sensor. The actual Seebeck coefficient αs of the heavily phosphorous diffused n2+ -Si in the SOI layer is determined by applying the temperature gradient to strip specimens (not using device-chip), which are 20 mm long and 2 mm wide and cut away from the SOI wafer with the uniformly phosphorous diffused n2+ -Si layer. In Fig. 7, the measurement set-up for the Seebeck coefficient αs of the heavily phosphorous diffused n2+ -Si is shown. Two Ohmic electrodes on SOI strip specimen of the phosphorous diffused n2+ -Si bar-chip are formed using Al (Si: 1%) with the distance L of 10 mm. We set the distance D between two copper plates to measure the temperature-difference so as to be the same as the distance L of two electrodes in actual experiments. The heater and the copper plate just below the heater are thermally connected in the actual set-up shown in Fig. 7. We consider that the temperature within the copper plate will be uniform. Since the distance D between two copper plates is set so as to be the same distance as L between two Ohmic electrodes in the actual experiments, the temperature-difference T measured by
the thermocouples between two copper plates will be the same as the temperature-difference between two Ohmic electrodes formed on the n2+ -Si bar-chip. In the actual experiments, the measurement set-up shown in Fig. 7 for Seebeck coefficient is covered with a small plastic hood to block the undesirable air flow. Fig. 8 shows the measured characteristics on thermoelectric voltage V of the phosphorous diffused n2+ -Si bar-chip versus temperature-difference T. Measurements of the Seebeck coefficient αs are repeatedly carried out. We can see that the error of plots of the data is within only a few percentage. Ignoring the Seebeck coefficient and the thermoelectric voltage generated in the Al metal acting as one conductor of the thermocouple composed of the phosphorous diffused n2+ -Si and the Al metal, the Seebeck coefficient αs of the phosphorous diffused n2+ -Si barchip is determined from the gradient of the linear characteristics shown in Fig. 8 to be about −160 V/K. We can see that the measured Seebeck coefficient αs for our specimen is higher in spite of its high concentration than that (−110 V/K) of n+ poly by Baltes et al. We think it is attributed that our specimen is made of single crystal, while their material is poly-Si in which the mobility of carriers is low.
108
S.S. Lee, M. Kimura / Sensors and Actuators A 139 (2007) 104–110
Fig. 7. Set-up for Seebeck coefficient αs measurement of the n2+ -Si chip.
Fig. 5. Micrograph of sensor chip: (a) top view (whole on-chip) and (b) two fabricated micro-cantilever type IR sensors (magnified).
In Fig. 9, it is shown that the output signal of our proposed single thermocouple cantilever type (longer one, 1600 m long) IR sensor, which is irradiated by chopped IR light from 500 K black body furnace at the received power of 16.4 W. In this experiment, we have used the circuits shown in Fig. 2 to measure
Fig. 6. Relationship between diode current I and reciprocal temperature 1/T of diode thermistor.
Fig. 8. Characteristics of thermoelectric voltage V of the phosphorous diffused n2+ -Si chip vs. temperature-difference T.
Fig. 9. Output signal of the current measurement type single thermocouple (cantilever) irradiated by chopped infrared light from 500 K black body (input power P0 : 16.4 W).
S.S. Lee, M. Kimura / Sensors and Actuators A 139 (2007) 104–110
Seebeck current. The feedback resistance Rf is chosen to be 100 K for making it to be same or equivalent internal resistance like that of the traditional thermopile. The internal resistance, rs , of fabricated cantilever thermocouple was 38 . We suppose that all area of the cantilever serves as an IR sensing area (3.2 × l0−3 cm2 ) covered with gold black. However, it obviously seemed that less IR absorption occurred in the cantilever by its inherent bending which happened due to its bimetallic structure after the release from SOI substrate. The thermal time constant τ of a prototype cantilever sensor is observed to be about 23 ms from the step response. 4. Evaluation and discussion We compare the performance of the single thermocouple of cantilever type in terms of the generated thermoelectric power output with that of a commercial thermopile composed of n2+ polysilicon/Al film (fabricated by I company in Japan) with internal resistance rv = 65 K, responsivity Rs = 15 V/W and thermal time constant τ = 15 ms. Since our single thermocouple at received IR power P0 of 16.4 W has an output voltage Vo of about 16 mV as saturated value from the response shown in Fig. 9, we can calculate the generated power Wi = 0.973 × l0−12 W taking account of the feedback resistance Rf = 100 K and internal resistance ri = 38 . And the commercial thermopile has output voltage of Vv = Rs × Pi = 2.46 × 10−4 V for the same P0 of 16.4 W. Therefore, we can see the generated thermoelectric power output Wv by this commercial thermopile itself is 0.931 × l0−12 W taking account of its internal resistance rv = 65 K. After all, we obtain the generated thermoelectric power comparison value, M = Wi /Wv = 1.04. It means that the performance of our single thermocouple is equivalent to the commercial excellent thermopile. However, our proposed sensor has great advantages, which is able to reduce dramatically the fabrication process and the production cost owing to its simple design (only single pair of thermocouple) of sensor structure with ensuring same performance and less material cost etc. Moreover, it is expected that the performance of our prototype thermocouple can be further improved by deeper n2+ -diffused SOI layer or thinner cantilever with full impurity-diffusion depth to reduce thermal loss and better absorption layer. For example, the generated power Wi will be 5 times larger than the prototype thermocouple with 2 m thick n2+ -diffused layer in the 10 m thick SOI layer by fully n2+ -diffused SOI layer of 10 m thick, because the internal resistance of the thermocouple will be expected to be 1/5 of that of the prototype one. This will result in 5 times larger M value than that of prototype one. The thinner cantilever as an infrared absorption region will contribute to the larger temperature increase T for the same incident infrared absorption, and this will also contribute to the performance improvement.
109
demonstrated that the single thermocouple is superior or equivalent to the traditional thermopile through the theoretical model and the experiment. The authors believe that our proposed Seebeck current measurement thermocouple sensor combined with the OP amplifier (as an active sensor) will be a promised one to be used mostly instead of traditional thermopiles in thermal measurement systems, such as IR sensor, uncooled IR imager and flow sensor etc. because of its simple structure, high sensitivity and micro size. The single thermocouple sensor as a temperature-difference sensor will be more useful when it is combined with the diode thermistor, fabricated on the same Si chip, as an absolute temperature sensor proposed by the author [11], because all of these sensors and circuits are full compatible to CMOS technologies. Acknowledgements This work has been funded by the project of Intelligent Cluster. The authors are grateful to Dr. Katsuto Oyama for his considerable assistance of thermal experiments. References [1] P.M. Sarro, H. Yashiro, A.W. Van Herwaarden, S. Middelhoek, An integrated thermal infrared sensing array, Sens. Actuators 14 (1988) 191–201. [2] R. Lenggenhager, H. Baltes, Improved thermoelectric infrared sensor using double poly CMOS technology, in: International Conference on Solid-State Sensors and Actuators (Transducers 1993), Yokohama, Japan, June 7–10, 1993, pp. 1008–1011. [3] M. Hirota, S. Morita, A high speed thermoelectric infrared sensor by CMOS technology, in: Technical Digest of the 14th Sensor Symposium, Kawasaki, Japan, June 4–5, 1996, pp. 125–128. [4] C.-H. Du, Z. Lin, C. Lee, Design and characterization of CMOS compatible thermopile using front-side bulk micromachining, in: Technical Digest of the 17th Sensor Symposium, Kawasaki, Japan, May 30–31, 2000, pp. 165–168. [5] J. Schieferdecker, R. Quad, E. Holzenk¨ampfer, M. Schulze, Infrared thermopile sensors with high sensitivity and very low temperature coefficient, Sens. Actuators A 46–47 (1995) 422–427. [6] T. Eick, A. Berger, D. Behrendt, U. Dillner, E. Kessler, An alternative method for measuring the responsivity of thermopile infrared sensors, in: Sensor 2005 Proceedings, vol.1, N¨urnberg, Germany, May 10–12, 2005, pp. 109–114. [7] M. Kimura, JP. Application No. 2004-026247 (JP publication No. 2005221238). [8] M. Imran, A. Bhattacharyya, Characterization of the closed circuit response of an on-chip thin film thermopile, Sens. Actuators A 132 (2006) 487–498. [9] A.W. Van Herwaarden, P.M. Sarro, Thermal sensors based on the Seebeck effect, Sens. Actuators A 10 (1986) 321–346. [10] Y.T. Lee, H. Takao, M. Ishida, Fabrication of the high sensitive infrared sensor using SOI (silicon-on insulator) structure, in: Proceedings of the 20th Sensor Symposium, Tokyo, Japan, July 23–24, 2003, pp. 195–198. [11] M. Kimura, K. Toshima, Thermistor-like pn junction temperature-Sensor with variable sensitivity and its combination with a micro-air-bridge heater, Sens. Actuators A 108 (2003) 239–243.
Biographies 5. Conclusions Novel temperature-difference measurement method based on short-circuit Seebeck current measurement of a single thermocouple with extremely low resistivity is proposed, and it is
Seung Seoup Lee received the BS and MS degrees in mechanical enginnering from Hanyang University, Seoul, Korea in 1987 and 1989, respectively. And he received PhD degree in mechatonics engineering from Tohoku University, Sendai, Japan, in 2000. From 2001 to 2004, he was engaged in developments of micro interferometer system using micro optical waveguide, micro gripper
110
S.S. Lee, M. Kimura / Sensors and Actuators A 139 (2007) 104–110
and piezoresistive AFM cantilever for using at cryogenic temperature. He is currently working on physical microsensors based on semiconductor devices and MEMS as a researcher at Tohoku Gakuin University, Tagajo, Japan. Mitsuteru Kimura was born in Akita, Japan in 1942. He received the BS degree in electronics from University of Electro-Communications in 1967, and the MS
and PhD degrees in electronics from Tohoku University in 1969 and 1974, respectively. He joined Tohoku Gakuin University in 1974, and currently he is a professor. He is currently engaged in research on semiconductor devices, optical devices and sensor devices.
Short-circuit measurement by Seebeck current detection of a single thermocouple and its application Seung Seoup Lee, Mitsuteru Kimura ∗ Tohoku Gakuin University, Faculty of Engineering, 13-1, Chuo-1, Tagajo, Miyagi 985-8537, Japan Received 31 July 2006; received in revised form 16 December 2006; accepted 4 April 2007 Available online 20 April 2007
Abstract A novel temperature-difference measurement method based on Seebeck current detection by a single thermocouple is proposed. Principle of the sensor is based on the measurement of short-circuit of Seebeck current due to the thermoelectric power in a single thermocouple. We focused on the fact that the internal resistance of n2+ -Si corresponding to the short-circuit current can be three or four order decreased while Seebeck coefficient αs decreases only for a factor of three or four. We have made a theoretical model to evaluate the performance of our proposed short-circuit measurement using the generated thermoelectric output power comparison by sensor itself due to infrared (IR) absorption. It is shown that our proposed single thermocouple has a higher thermoelectric power output or equivalent than that of traditional thermopile. A single micro-cantilever thermocouple as for an IR sensor, which is composed of extremely low resistivity (<10−3 cm) materials of n2+ -Si and Al (Si: 1%) metal is developed, and its validity and effectiveness are demonstrated. © 2007 Elsevier B.V. All rights reserved. Keywords: Thermopile; Thermocouple; Temperature-difference measurement; Seebeck current; Infrared (IR) sensor; Cantilever
1. Introduction IR sensors using the thermopile have relatively low responsivity compared to thermistor bolometer and pyroelectric sensors. However, it is widely used and has been developed as IR sensors [1–6], especially for body temperature measurement, because it is a unique sensor, which can measure only the temperature-difference, and can operate with non-bias and has no current-induced noise. Thermopile consists of thermocouple-array structure because each thermocouple itself has a low sensitivity. Lenggenhager and Baltes have proposed thermoelectric infrared (IR) sensors with n-poly/p-polythermopile with relatively high sensitivity of 170 V/W (Seebeck coefficient αs of n+ -poly: −110 V/K; p+ -poly: 195 V/K) [2]. However, their sensors show a low S/N because of very high internal resistance (2.7 M) due to 62 thermocouple-array connected in series with 2.8 m wide conductor-lines to get enough sensitivity. Chen-Hsun et al. have shown that there is a trend of saturation of specific detectivity
∗
Corresponding author. Tel.: +81 22 368 7162; fax: +81 22 368 8535. E-mail address: [email protected] (M. Kimura).
0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.04.045
D* curve, since the increase of responsivity Rv will be compensated by the increase of Johnson noise caused by high internal resistance due to many thermocouple pairs, such as 62 pairs in a thermopile [4]. In spite of its low sensitivity and low S/N, due to its high internal resistance and complicated structure of array, thermopiles or thermocouples are mostly used only for the case in which the temperature-difference measurement is required because it is a unique sensor which can measure only temperature-difference. Also, the yield of thermopile is not good in terms of production, because of its complex structure of array composed of many pairs of thermocouples. Therefore, high sensitive temperature-difference sensor with a simple structure and high S/N has been strongly demanded. Since Seebeck effect was found, the thermocouple had been used as a temperature-difference sensor to measure the opencircuit voltage among consisting different materials. The opencircuit measurement has merits such that it is independent of the electrical resistance and easy to measure, and that thermoelectric voltage is only determined by the pair materials and temperaturedifference to be measured. The author, Kimura, has proposed as a Japanese patent a novel temperature-difference measurement method based on the thermal current detection in the short-circuit of a sin-
S.S. Lee, M. Kimura / Sensors and Actuators A 139 (2007) 104–110
gle thermocouple using imaginary short of the OP amplifier [7]. In this paper, we propose the short-circuit measurement of Seebeck current by a thermocouple [7], and it is evaluated through the simple theoretical model based on the comparison of the generated thermoelectric powers of our proposed sensor and the traditional open-voltage measurement thermopile due to IR absorption. It is theoretically shown that our proposed single thermocouple sensor of extremely low internal resistance has equivalent or higher thermoelectric power output than that of the traditional thermopile sensor. In addition, there has ever been no idea on the direct measurement of the short-circuit Seebeck current using the imaginary short of OP amplifier. In this paper, Seebeck current is measured using this imaginary short of the OP amplifier. However, recently Imran and Bhattacharyya have reported on the characterization of on-chip thin film thermopile based on short-circuit Seebeck current measurement using hot and cold junctions, and heater on-chip [8]. They have used five thermocouples of chromium (Cr) and Titanium (Ti) in their thermopile. Fundamentally the principle is same with our proposal in terms of the detection of Seebeck current. But they had just focused on the experimental characterization of Seebeck current from the thermopile for the application of on-chip electrical microthermoelectric power generators, not for temperature sensor or IR sensor application, even though they have mentioned its possibility of the use of Seebeck current for the temperature sensor application. In this paper, a temperature-difference sensor of microcantilever type single thermocouple is fabricated and its validity as a temperature and IR sensor based on Seebeck current measurement is demonstrated. 2. Principle It is well known that the Seebeck coefficient αs for silicon is approximated as a function of electrical resistivity ρ as shown below [9]: mk ρ ln αs = q ρ0
(1)
where ρ0 ≈ 5 × 10−4 cm, m = 2.6 for silicon (Si) and k is the Boltzmann constant and q the electron charge. Lee et al. have reported the thermopile (17 pairs) made of p and n type Si with higher resistivity of 10 cm, because the higher resistivity makes it have the higher Seebeck coefficient [10]. However, authors have noticed that it is easy to decrease the resistivity three or four orders, which can extremely increase the short-circuit Seebeck current of the thermocouple, because the Seebeck coefficient αs only decreases several times [7]. If we adopt the short-circuit Seebeck current measurement method by a single thermocouple with the n2+ -Si and metal film instead of open-circuit voltage measurement in the traditional thermopile, it is expected that our proposed thermocouple can generate higher thermoelectric power output, Wi , than that, Wv , of a
105
Fig. 1. Structural models of an n-pairs thermopile (a) and a single thermocouple (b) with the same IR sensing area of cantilever.
traditional open-circuit thermopile. Each subscript ‘v’ and ‘i’ correspond to voltage measurement (thermopile) and current measurement (single thermocouple), respectively. In Fig. 1, the theoretical model of the cantilever type thermal sensors is shown. One is a model of the thermopile composed of n-pairs of thermocouples (a), and the other is that of a single thermocouple (b). Both (a) and (b) have the same area (a × b) of IR absorption region, and is assigned the same temperature gradient T due to the absorption of incident IR power P0 . And they have different resistivity ρv and ρi . Internal resistance rv and ri of sensors are proportional to their resistivity ρv and ρi , respectively. In the thermopile (a), following equations are obtained: the total thermoelectric voltage Ev = nev = nαv T; internal resistance rv = nr0v (r0v : resistance of each thermocouple); generated power Wv = Ev2 /rv = nα2v T 2 /r0v . In the single thermocouple (b), following equations are also obtained: for γ = αv /αi and ρi /ρv = 10−f , the thermoelectric voltage Ei = ei = αi T; internal resistance ri = r0v × 10−f /n; generated power Wi = Ei2 /ri = α2i T 2 /ri = nα2v T 2 × 10f /γ 2 r0v . Therefore, it is derived the generated thermoelectric power ratio M of Wi and Wv by following equation: M=
10f Wi = 2 . Wv γ
(2)
From Eq. (2), we can see that the ratio of the thermoelectric power by each sensor is only the function of the Seebeck coefficient and the resistivity. For f = 3, M will be 91.5 because of γ = 3.3, and for f = 3.5, M will be about 110 because of γ = 5.3. For example, it is seen that the generated thermoelectric power Wi by a single thermocouple with extremely low resistivity semiconductor, such as n2+ -Si thin film of ρi = 3 × 10−3 cm is expected to be 110 times higher than that of the thermopile with n-pairs thermocouples made of relatively high resistivity Si, such as ρv = 10 cm (f = 3.5 determined by ρi and ρv . And we can see that M will be 1 for f = 0 (ρv = ρi in the case of γ = 1. It means that both n-pairs thermopile and our proposed single thermocouple have the same generated thermoelectric power as long as they are made of same materials, including the resistivity, i.e they are equivalent. However, since a pair of thermocouple of the thermopile requires line and space, the internal resistance will be
106
S.S. Lee, M. Kimura / Sensors and Actuators A 139 (2007) 104–110
Two diode-thermistors, Th.A and Th.B, made of pn junction diode proposed by the author are prepared to measure the absolute temperature at each point A and B, corresponding to the junction portions of the calibration thermocouple, due to temperature gradient generated by the buried micro-heater formed on the same substrate [11]. 3.1. Fabrication process and cantilever sensors on-chip
Fig. 2. Measuring circuit for short-circuit current of a single thermocouple using imaginary short of an OP amplifier.
larger than an ideal n-pairs thermopile configuration shown in Fig. 1(a). We can use the imaginary short of the OP amplifier to measure the short-circuit Seebeck current of the single thermocouple sensor as shown in Fig. 2. The combination of the single thermocouple sensor and the OP amplifier will compose an active IR sensor. 3. Fabrication and measurement Thermocouple sensors of cantilevers, which is made of highdoped n2+ -Si using SOI (1 0 0)-oriented silicon-on-insulator (SOI) wafer with 6.5 m thick, slightly n-doped silicon (1.0–3.0 cm), a 1.0 m thick silicon buried dioxide intermediate layer (SOI layer) and a 400 m thick base, are fabricated to evaluate the generated thermoelectric power ouput by a sensor itself and the sensitivity for IR absorption between our proposed thermocouple sensor and the traditional thermopile sensor. Fig. 3 shows the basic concept of the micro-thermocouple cantilever sensors for IR detection, in which one thermocouple sensor, one calibration thermocouple, two pn diodes as a diode thermistor (as absolute temperature sensors to measure the temperatue of the substrate) and micro-heater are formed on the same SOI substrate. The calibration thermocouple is to measure Seebeck coefficient αs for calibration of sensor thermocouple. Heater is used to apply the temperature gradient to the calibration thermocouple in the SOI substrate.
Our proposed single thermocouple of the cantilever type as IR sensor is composed of double layers of n2+ -Si and Al (Si: 1%) metal. Fig. 4 shows the fabrication process of cantilever type thermocouple IR sensor, pn diode thermistors and heater on chip shown in Fig. 3. After initial cleaning, the low resistivity n2+ -Si layer (<10−3 cm) is created by thermal diffusion using phosphorous oxide coating dopant (OCD)-coating diffusion source at 1100 ◦ C for 3 h (Fig. 4, step 1). After the lithography of thermal silicon oxide film (about 0.5 m thick) to open windows for pn junction diode thermistors (Fig. 4, step 2), thermal diffusion for p+ junction of pn diode, n+ junction for metal connection in heavily diffused n2+ -Si layer and p+ layer for heater resistor are carried out using same diffusion source of OCD (Fig. 4, step 3). The thickness of n2+ -Si diffused layer (average concentration l × l020 cm−3 layer) is estimated to be about 2 m thick. The Al (Si: 1%) metal (about 0.3 m thick) of pair material in a single thermocouple is deposited by RF sputtering and patterned using etching process (Fig. 4, step 4). The double layer of n2+ -Si and Al (Si: 1%) metal is isolated via the thermal SiO2 film (about 0.5 m thick) formed on the SOI layer. The backside cavity to release the cantilever from the SOI substrate is formed using deep reactive ion etching (RIE), and finally the gold black film for IR absorption is evaporated under the low vacuum (Fig. 4, step 5). Fig. 5 shows the fabricated micro single thermocouple cantilever sensors for IR detection, in which one is sensor thermocouple and the other is calibration thermocouple. And two pn diodes as a diode thermistor (absolute temperature sensor), and micro-heater are formed on the same SOI substrate. Two micro-cantilever sensors have the dimensions of 200 m wide and two different 1200 m long and 1600 m long and 7.0 m thick (including 1.0 m thick BOX layer). Two diode-thermistors, Th.A and Th.B, made of pn junction diode are prepared to measure the absolute temperature and temperature-difference due to heat generated by the buried micro-heater at each point A and B, corresponding to the junction portions of the calibration thermocouple, formed on the same substrate. 3.2. Measurements and its results
Fig. 3. Sketch of the basic concept of a single micro thermocouple cantilever sensor by novel temperature-difference measurement method.
Diode-thermistor is a thermistor-like sensor based on the temperature dependence of forward current of pn junction at constant forward voltage Vf (e.g. Vf = 0.55 V). In Fig. 6, the characteristics of diode current I (plotted in logarithm) are shown as a function of reciprocal temperature 1/T for various fixed forward bias voltage Vf of Th.A. Logarithm of current I at a constant
S.S. Lee, M. Kimura / Sensors and Actuators A 139 (2007) 104–110
107
Fig. 4. Fabrication process of a micro single thermocouple cantilever sensor on chip.
voltage is theoretically proportional to the reciprocal temperature 1/T. We can see from this Fig. 6 that a constant biased diode behaves like a thermistor and has a very high sensitivity as an absolute temperature sensor. The actual Seebeck coefficient αs of the heavily phosphorous diffused n2+ -Si in the SOI layer is determined by applying the temperature gradient to strip specimens (not using device-chip), which are 20 mm long and 2 mm wide and cut away from the SOI wafer with the uniformly phosphorous diffused n2+ -Si layer. In Fig. 7, the measurement set-up for the Seebeck coefficient αs of the heavily phosphorous diffused n2+ -Si is shown. Two Ohmic electrodes on SOI strip specimen of the phosphorous diffused n2+ -Si bar-chip are formed using Al (Si: 1%) with the distance L of 10 mm. We set the distance D between two copper plates to measure the temperature-difference so as to be the same as the distance L of two electrodes in actual experiments. The heater and the copper plate just below the heater are thermally connected in the actual set-up shown in Fig. 7. We consider that the temperature within the copper plate will be uniform. Since the distance D between two copper plates is set so as to be the same distance as L between two Ohmic electrodes in the actual experiments, the temperature-difference T measured by
the thermocouples between two copper plates will be the same as the temperature-difference between two Ohmic electrodes formed on the n2+ -Si bar-chip. In the actual experiments, the measurement set-up shown in Fig. 7 for Seebeck coefficient is covered with a small plastic hood to block the undesirable air flow. Fig. 8 shows the measured characteristics on thermoelectric voltage V of the phosphorous diffused n2+ -Si bar-chip versus temperature-difference T. Measurements of the Seebeck coefficient αs are repeatedly carried out. We can see that the error of plots of the data is within only a few percentage. Ignoring the Seebeck coefficient and the thermoelectric voltage generated in the Al metal acting as one conductor of the thermocouple composed of the phosphorous diffused n2+ -Si and the Al metal, the Seebeck coefficient αs of the phosphorous diffused n2+ -Si barchip is determined from the gradient of the linear characteristics shown in Fig. 8 to be about −160 V/K. We can see that the measured Seebeck coefficient αs for our specimen is higher in spite of its high concentration than that (−110 V/K) of n+ poly by Baltes et al. We think it is attributed that our specimen is made of single crystal, while their material is poly-Si in which the mobility of carriers is low.
108
S.S. Lee, M. Kimura / Sensors and Actuators A 139 (2007) 104–110
Fig. 7. Set-up for Seebeck coefficient αs measurement of the n2+ -Si chip.
Fig. 5. Micrograph of sensor chip: (a) top view (whole on-chip) and (b) two fabricated micro-cantilever type IR sensors (magnified).
In Fig. 9, it is shown that the output signal of our proposed single thermocouple cantilever type (longer one, 1600 m long) IR sensor, which is irradiated by chopped IR light from 500 K black body furnace at the received power of 16.4 W. In this experiment, we have used the circuits shown in Fig. 2 to measure
Fig. 6. Relationship between diode current I and reciprocal temperature 1/T of diode thermistor.
Fig. 8. Characteristics of thermoelectric voltage V of the phosphorous diffused n2+ -Si chip vs. temperature-difference T.
Fig. 9. Output signal of the current measurement type single thermocouple (cantilever) irradiated by chopped infrared light from 500 K black body (input power P0 : 16.4 W).
S.S. Lee, M. Kimura / Sensors and Actuators A 139 (2007) 104–110
Seebeck current. The feedback resistance Rf is chosen to be 100 K for making it to be same or equivalent internal resistance like that of the traditional thermopile. The internal resistance, rs , of fabricated cantilever thermocouple was 38 . We suppose that all area of the cantilever serves as an IR sensing area (3.2 × l0−3 cm2 ) covered with gold black. However, it obviously seemed that less IR absorption occurred in the cantilever by its inherent bending which happened due to its bimetallic structure after the release from SOI substrate. The thermal time constant τ of a prototype cantilever sensor is observed to be about 23 ms from the step response. 4. Evaluation and discussion We compare the performance of the single thermocouple of cantilever type in terms of the generated thermoelectric power output with that of a commercial thermopile composed of n2+ polysilicon/Al film (fabricated by I company in Japan) with internal resistance rv = 65 K, responsivity Rs = 15 V/W and thermal time constant τ = 15 ms. Since our single thermocouple at received IR power P0 of 16.4 W has an output voltage Vo of about 16 mV as saturated value from the response shown in Fig. 9, we can calculate the generated power Wi = 0.973 × l0−12 W taking account of the feedback resistance Rf = 100 K and internal resistance ri = 38 . And the commercial thermopile has output voltage of Vv = Rs × Pi = 2.46 × 10−4 V for the same P0 of 16.4 W. Therefore, we can see the generated thermoelectric power output Wv by this commercial thermopile itself is 0.931 × l0−12 W taking account of its internal resistance rv = 65 K. After all, we obtain the generated thermoelectric power comparison value, M = Wi /Wv = 1.04. It means that the performance of our single thermocouple is equivalent to the commercial excellent thermopile. However, our proposed sensor has great advantages, which is able to reduce dramatically the fabrication process and the production cost owing to its simple design (only single pair of thermocouple) of sensor structure with ensuring same performance and less material cost etc. Moreover, it is expected that the performance of our prototype thermocouple can be further improved by deeper n2+ -diffused SOI layer or thinner cantilever with full impurity-diffusion depth to reduce thermal loss and better absorption layer. For example, the generated power Wi will be 5 times larger than the prototype thermocouple with 2 m thick n2+ -diffused layer in the 10 m thick SOI layer by fully n2+ -diffused SOI layer of 10 m thick, because the internal resistance of the thermocouple will be expected to be 1/5 of that of the prototype one. This will result in 5 times larger M value than that of prototype one. The thinner cantilever as an infrared absorption region will contribute to the larger temperature increase T for the same incident infrared absorption, and this will also contribute to the performance improvement.
109
demonstrated that the single thermocouple is superior or equivalent to the traditional thermopile through the theoretical model and the experiment. The authors believe that our proposed Seebeck current measurement thermocouple sensor combined with the OP amplifier (as an active sensor) will be a promised one to be used mostly instead of traditional thermopiles in thermal measurement systems, such as IR sensor, uncooled IR imager and flow sensor etc. because of its simple structure, high sensitivity and micro size. The single thermocouple sensor as a temperature-difference sensor will be more useful when it is combined with the diode thermistor, fabricated on the same Si chip, as an absolute temperature sensor proposed by the author [11], because all of these sensors and circuits are full compatible to CMOS technologies. Acknowledgements This work has been funded by the project of Intelligent Cluster. The authors are grateful to Dr. Katsuto Oyama for his considerable assistance of thermal experiments. References [1] P.M. Sarro, H. Yashiro, A.W. Van Herwaarden, S. Middelhoek, An integrated thermal infrared sensing array, Sens. Actuators 14 (1988) 191–201. [2] R. Lenggenhager, H. Baltes, Improved thermoelectric infrared sensor using double poly CMOS technology, in: International Conference on Solid-State Sensors and Actuators (Transducers 1993), Yokohama, Japan, June 7–10, 1993, pp. 1008–1011. [3] M. Hirota, S. Morita, A high speed thermoelectric infrared sensor by CMOS technology, in: Technical Digest of the 14th Sensor Symposium, Kawasaki, Japan, June 4–5, 1996, pp. 125–128. [4] C.-H. Du, Z. Lin, C. Lee, Design and characterization of CMOS compatible thermopile using front-side bulk micromachining, in: Technical Digest of the 17th Sensor Symposium, Kawasaki, Japan, May 30–31, 2000, pp. 165–168. [5] J. Schieferdecker, R. Quad, E. Holzenk¨ampfer, M. Schulze, Infrared thermopile sensors with high sensitivity and very low temperature coefficient, Sens. Actuators A 46–47 (1995) 422–427. [6] T. Eick, A. Berger, D. Behrendt, U. Dillner, E. Kessler, An alternative method for measuring the responsivity of thermopile infrared sensors, in: Sensor 2005 Proceedings, vol.1, N¨urnberg, Germany, May 10–12, 2005, pp. 109–114. [7] M. Kimura, JP. Application No. 2004-026247 (JP publication No. 2005221238). [8] M. Imran, A. Bhattacharyya, Characterization of the closed circuit response of an on-chip thin film thermopile, Sens. Actuators A 132 (2006) 487–498. [9] A.W. Van Herwaarden, P.M. Sarro, Thermal sensors based on the Seebeck effect, Sens. Actuators A 10 (1986) 321–346. [10] Y.T. Lee, H. Takao, M. Ishida, Fabrication of the high sensitive infrared sensor using SOI (silicon-on insulator) structure, in: Proceedings of the 20th Sensor Symposium, Tokyo, Japan, July 23–24, 2003, pp. 195–198. [11] M. Kimura, K. Toshima, Thermistor-like pn junction temperature-Sensor with variable sensitivity and its combination with a micro-air-bridge heater, Sens. Actuators A 108 (2003) 239–243.
Biographies 5. Conclusions Novel temperature-difference measurement method based on short-circuit Seebeck current measurement of a single thermocouple with extremely low resistivity is proposed, and it is
Seung Seoup Lee received the BS and MS degrees in mechanical enginnering from Hanyang University, Seoul, Korea in 1987 and 1989, respectively. And he received PhD degree in mechatonics engineering from Tohoku University, Sendai, Japan, in 2000. From 2001 to 2004, he was engaged in developments of micro interferometer system using micro optical waveguide, micro gripper
110
S.S. Lee, M. Kimura / Sensors and Actuators A 139 (2007) 104–110
and piezoresistive AFM cantilever for using at cryogenic temperature. He is currently working on physical microsensors based on semiconductor devices and MEMS as a researcher at Tohoku Gakuin University, Tagajo, Japan. Mitsuteru Kimura was born in Akita, Japan in 1942. He received the BS degree in electronics from University of Electro-Communications in 1967, and the MS
and PhD degrees in electronics from Tohoku University in 1969 and 1974, respectively. He joined Tohoku Gakuin University in 1974, and currently he is a professor. He is currently engaged in research on semiconductor devices, optical devices and sensor devices.