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Development of MEMS Capacitive Sensor Using a MOSFET Structure

Development of MEMS Capacitive Sensor Using a MOSFET Structure

Extended Summary 本文は pp.102-107

Development of MEMS Capacitive Sensor Using a MOSFET Structure

Hayato Izumi Non-member (Kansai University, [email protected]) Yohei Matsumoto Non-member (Kansai University, [email protected] u.ac.jp) Seiji Aoyagi Member (Kansai University, [email protected] u.ac.jp) Yusaku Harada Non-member (Kansai University, [email protected] u.ac.jp) Shoso Shingubara Non-member (Kansai University, [email protected]) Minoru Sasaki Member (Toyota Technological Institute, [email protected]) Kazuhiro Hane Member (Tohoku University, [email protected]) Hiroshi Tokunaga Non-member (M. T. C. Corp., [email protected])

Keywords : MOSFET, capacitive sensor, accelerometer, circuit for temperature compensation

The concept of a capacitive MOSFET sensor for detecting voltage change, is proposed (Fig. 4). This circuitry is effective for vertical force applied to its floating gate was already reported by compensating ambient temperature, since two are the authors (Fig. 1). This sensor detects the displacement of the simultaneously suffer almost the same temperature change. The movable gate electrode from changes in drain current, and this performance of this circuitry is confirmed by SPICE simulation. current can be amplified electrically by adding voltage to the gate, The operating point, i.e., the output voltage, is stable irrespective i.e., the MOSFET itself serves as a mechanical sensor structure. of the ambient temperature change (Fig. 5(a)). The output voltage Following this, in the present paper, a practical test device is has comparatively good linearity to the gap length, which would fabricated. A MOSFET is fabricated on a SOI wafer, and the box be effective for practical sensor applications such as an oxide under the gate is removed to release the gate structure (Fig. accelerometer (Figs. 5(b) and (c)) 2). Its perfomance of detecting the applied force is characterized (Fig. 3), confirming that the detected drain current is surely increased in proportion to the applied force. A circuitry, which converts the drain current change to the

Fig. 3. Relationship between applied force and drain current

Fig. 1. Principle of MOSFET sensor

Fig. 2. SEM images of fabricated test devices Fig. 4. Model of circuitry combining two MOSFETs

(a) Operating point change vs. ambient temperature chang (b) Operating point change vs. gap change of MOSFET sensor (c) Relationship between output voltage and gap length Fig. 5. Result of SPICE simulation of detecting circuitry

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Paper

Development of MEMS Capacitive Sensor Using a MOSFET Structure

* Hayato Izumi Non-member * Yohei Matsumoto Non-member * Seiji Aoyagi Member * Yusaku Harada Non-member * Shoso Shingubara Non-member ** Minoru Sasaki Member *** Kazuhiro Hane Member **** Hiroshi Tokunaga Non-member

The concept of a capacitive MOSFET sensor using a SOI wafer for detecting vertical force applied to its floating gate was already reported by the authors. A MOSFET is fabricated on a SOI wafer, and the box oxide under the gate is removed to release the gate structure. This sensor detects the displacement of the movable gate electrode from changes in drain current, and this current can be amplified electrically by adding voltage to the gate, i.e., the MOSFET itself serves as a mechanical sensor structure. Following this, the present paper reports the fabrication of a practical test device and its preliminary characterization. The present paper also proposes a circuitry, which converts the drain current change to the voltage change while compensating the temperature change. The performance of this circuitry is confirmed by SPICE simulation. In accelerometer application, a comparatively heavy proof mass and thin supporting beams are necessary for increasing the sensitivity. For this purpose, a fabrication process of depositing a thick mass structure using electroplating is newly proposed.

Keywords : MOSFET, capacitive sensor, accelerometer, circuit for temperature compensation

structure(8). This sensor has merits in terms of CMOS 1. Introduction compatibility and fabrication cost/simplicity. Moreover, by using a Capacitive sensors are widely used as pressure sensors(1), SOI wafer having a thick active silicon layer, a thick gate structure accelerometers(2), tactile sensors(3), (4), etc. This type of is made possible. sensor has two electrodes, one is fixed and the other is movable, The principle of the FET sensor, in the case of an n-channel and the detection of the displacement of the movable electrode is metal-oxide-semiconductor field effect transistor (MOSFET), is made observable from changes in electrode charge. To obtain schematically shown in Fig. 1. In this article, this sensor is called a practical sensitivity of MEMS capacitive sensor while keeping its “MOSFET sensor”. A MOSFET forms a by gate, gate small size, adding some amplifying function to the device is one oxide, and silicon under the gate. By applying positive voltage to solution. A smart sensor is well documented, which comprises the gate, electrons accumulate on the silicon surface under the micro-machined mechanical elements and CMOS electrical oxide, which forms a channel or electron path, causing a drain circuits including those fulfilling the amplifying function. current ID . The ID is proportional to the capacitance of the However, if a mechanical element itself can perform the gate oxide layer C , and amplified electrically by the square of amplifying function, it would be better in terms of S/N ratio and VVGT− , where VG is input gate voltage, and VT is threshold spatial efficiency. A capacitive field effective transistor sensor voltage(8). In the MOSFET sensor, the structure has a floating gate (FET sensor) is one of such smart sensors(5)-(7). A FET is a device as a result of the removal of the oxide under the gate. When the used to control current which utilizes the charge accumulating gate is moved by input force, the capacitance is changed, causing effect of gate oxide, which is similar to the function of a capacitor. the change of ID . Therefore, if the gate electrode is made movable by using air as a dielectric material instead of oxide, the FET itself can be used as a capacitive sensor. This sensor can detect the displacement of the movable gate electrode from changes in drain current, and it is noted that this current can be amplified electrically by adding the voltage to its gate. The authors have already reported a FET sensor, which is fabricated by etching away the gate oxide of a standard MOSFET

* Kansai University, 3-3-35, Yamate-cho, Suita, Osaka 564-8680 ** Toyota Technological Institute *** Tohoku University **** M. T. C . Corp. Fig. 1. Principle of MOSFET sensor

© 2008 The Institute of Electrical Engineers of Japan. 102 MEMS Capacitive Sensor Using MOSFET Structure

The present paper reports the fabrication of a practical test confirmed by SPICE simulation. Some degree of mass and device of MOSFET sensor and its priliminary characterization. comparatively thin supporting beams are required for the floating The present paper also proposes a circuitry, which converts the gate structure, especially in application as an accelerometer, in drain current change to the voltage change while compensating order to obtain good sensitivity, since inertia force is proportionate the temperature change. The perfomance of this circuitry is to mass weight. For this purpose, a fabrication porcess of depositing a thick mass structure using electroplating is newly proposed. 2. Fabrication and Characterization of Test Device 2.1 Fabrication Fabrication of a test device is the preliminary task, in order to confirm the basic measurement principle of the proposed MOSFET sensor. This device has a thick bridge structure made of an active layer of SOI wafer, which is used as the floating gate as shown in Fig. 2. This structure is fabricated by partially removing the oxide under the pillar structure. During the fabrication process of such MOSFET sensors having a bulk gate of comparative thickness, spin-coating photoresist is not applicable due to the large step height of the gate structure on wafer surface. To overcome this problem, spray coating method of photoresist is employed, the detail of which was reported in the reference (8). SEM images of fabricated devices are shown in Fig. 3. It can be seen that the SOI’s box layer of 2 µm is successfully etched away by BHF wet etching process in 28 hours. 2.2 Characterization The bridge of the test device, i.e., Fig. 2. Schematic view of test device of MOSFET the floating gate structure, is applied force vertically by touching a sensor probe of the probe station preliminarily in advance of developing

(a) Overview (b) Side view (c) Enlargement of floating gap Fig. 3. SEM images of fabricated test devices

Fig. 5. Relationship between rotational angle and Fig. 4. Experimental setup using probe station applied force

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(a) Without force (b) Applied force of 3.5 gf Fig. 6. Example curves of I-V characteristics

charts, and arranged in the form of the applied force vs. the drain current, the result of which is shown in Fig. 7. Looking at this

figure, the detected ID is surely increased in proportion to the applied force, of which sensitivity is approximately 0.2 mA/gf in

case that VG is set to 10 V. In this experiment, the S/N ratio was

over 1000 (the level is less than 0.1% for the detected ID change). This good ratio is mainly based on the good precision of the measuring device, i.e., the semiconductor parameter analyzer, of which precision is better than 10 nA. Substituting a simple and cheap measuring device to this analyzer is discussed in the next chapter. 3. Circuitry for Temperature Compensation In practical applications, it is better to convert the drain current change to the voltage change, since the voltage is easily processed Fig. 7. Relationship between applied force and drain by popular measuring devices such as an oscilloscope, or by a current (CPU) via A/D converter. For this purpose, a circuitry combining the MOSFET sensor (the gap length between the gate and the substrate surface is variable) and the practical sensors such as accelerometers, pressure sensors, etc. reference MOSFET (the gap is fixed) is proposed herein, as shown

The probe tip is moved vertically by controlling the rotational in Fig. 8. The output voltage is defined as the drain voltage VD angle of a knob in a probe manipulator. The photograph of an at the operating point, which is the intersection of the I-V curves experimental setup is shown in Fig. 4. of the two MOSFETs, as shown in Fig. 9(a). In advance, the relationship between this angle and the applied For example, in accelerometer application, it is difficult to force is calibrated using a load cell, the specification of which is judge whether the detected voltage change is due to the as follows: the max. load is 2 N, the displacement at max. load is 4 change or the ambient temperature change, especially µm, and the linearity is 0.5%. The evaluated conversion rate is when the acceleration is rather small. To address this problem, it is approximately 1.8 × 10-2 gf/deg, as shown in Fig. 5. necessary to compensate the temperature change. The proposed After that, the floating gate of the test device is applied force by circuitry is effective for this purpose, since two MOSFETs are pushing the probe by changing the rotational angle of the knob. simultaneously suffer almost the same temperature change. The size of the used test device is as follows: the gate width The operation of this circuitry is simulated using SPICE (equivalent to the bridge length) is 1000 µm, the gate length software (LT-SPICE, this is free downloadable software). The (equivalent to the bridge width) is 100 µm, and the gap between result of the operating point change with respect to the the gate and the channel is 1.8 µm (the original thickness of box temperature change is shown in Fig. 9(a). In this simulation, the layer is 2 µm, which is covered by the oxide of 0.2 µm in order to VG is fixed to 10 V, and the gap is fixed to 0.2 µm. Looking at protect the channel surface). this figure, it is confirmed that the operating point, i.e., the output Under each force condition, the I-V characteristics, i.e., the voltage, is stable irrespective of the ambient temperature change. relationship between the drain voltage VD and the drain current Another simulation was carried out, in which the gap of the

ID at several gate voltage VG , is evaluated by a semiconductor MOSFET sensor is varied from 1 nm (almost zero) to 1000 nm (1 parameter analyzer (Agilent 4155C), an example of which are µm), while that of the reference MOSFET is fixed to 1 µm, under shown in Fig. 6. the room temperature of 25°C. The result is shown in Fig. 9(b). By

The ID at VD =10 V is employed as the representative drain using the data in this figure, the relationship between the gap current. This value is extracted from obtained I-V characteristics length and the output voltage is arranged, as shown in Fig. 9(c).

104 IEEJ Trans. SM, Vol.128, No.3, 2008 MEMS Capacitive Sensor Using MOSFET Structure

Looking at this figure, it is proven that the output voltage of the proposed circuitry has comparatively good linearity to the gap length, which would be effective for practical sensor applications such as an accelerometer. Two adjacent MOSFET sensors in the same fabricated die are employed, and one is used for the sensor and another is used for the reference. The floating gate of the sensor is pushed by a probe, and it is confirmed that the output voltage is surely changed at the present. The quantitative investigation in detail is the projected work. 4. Fabrication Process Using Electroplating It was already simulated by the authors that the high sensitivity of 0.5 pF/g, where g is gravitational acceleration, could be Fig. 8. Model of circuitry combining two MOSFETs achieved by using a thick proof mass of 100 µm and thin beams of 2 µm(8). In inclination sensor application detecting static acceleration, for example, the change in the capacitance due to 0.01° inclination change would be 0.5 pF/g×g×sin0.01°=0.1 fF, the electrical detection of which is possible(9). In the current process, a SOI wafer having a thick active layer of silicon, the thickness of which is over 100 µm, is necessary for obtaining the thick floating gate structure(8). In practical accelerometer application, the supporting beams are preferable to be as thin as possible for lowering the stiffness and obtaining good sensitivity. Thus, the active layer must be partially thinned at the beam parts, which requires DRIE process with rather difficult time-etching technique for achieving the precise and reproducible beam thickness. To address this problem, a fabrication process of depositing a thick mass structure using electroplating is proposed in this article. The process flow is shown in Fig. 10. This process uses a SOI wafer having an active layer of popular (a) Operating point change vs. ambient temperature change thickness of several microns, which is thin enough for forming the beam parts. Some degree of mass can be achieved by electroplating thick nickel. The practical fabrication based on this concept is ongoing. 5. Conclusions The present paper investigates a capacitive MOSFET sensor, which detects the displacement of the movable gate electrode from changes in drain current, and this current can be amplified electrically by adding voltage to the gate, i.e., the MOSFET itself serves as a mechanical sensor structure. To briefly summarize: 1) a practical test device of a capacitive MOSFET sensor is fabricated. Its performance is preliminary characterized, which confirms that the drain current is increased in proportion to the applied force, of which sensitivity is approximately 0.2 mA/gf. 2) A circuitry (b) Operating point change vs. gap change of MOSFET sensor combining two MOSFETs is proposed, which converts the drain current change to the voltage change, while compensating the temperature change. The effectiveness of this circuitry is confirmed by SPICE simulation, which exhibits the stability of the output voltage irrespective of ambient temperature and exhibits the good linearity of the output voltage to the gap length between the floating gate and the substrate surface. 3) In order to achieve a comparatively heavy proof mass and thin supporting beams, a fabrication process of depositing a thick mass structure using electroplating is proposed. (c) Relationship between output voltage and gap length Acknowledgments Fig. 9. Result of SPICE simulation of detecting This work was mainly supported by Ministry of Education, circuitry Culture, Sports, Science and Technology (MEXT) KAKENHI

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References

(1) C. S. Sander, J. W. Knutti, and J. D. Meindl : “A Monolithic Capacitive with Pulse-Period Output”, IEEE Trans. Electron Devices, Vol.ED-27, No.5, pp.927-930 (1980) (2) G. Kovacs : “Micromachined Sourcebook”, McGraw-Hill, pp.232-237 (1998) (3) K. Suzuki, K. Najafi, and K. D.Wise : “A 1024-Element High-Performance Silicon Tactile Imager”, IEEE Trans. Electron Devices, Vol.37, No.8, pp.1852-1860 (1990) (4) X. W. Li, B. L. Yang, Y. Y. Wu, and H. Y. Lin : “Determination of Atropine in Injection with β-Cyclodextrin Modified Sensitive Field Effective Transistor Sensor”, Sensors, No.5, pp.604-612 (2005) (5) H. C Nathanson, W. E. Newell, R. A. Wickstrom, and J. Davis : “The Resonant Gate Transistor”, IEEE Trans. Electron Devices, Vol.ED-14, No.3, pp.117-133 (1967) (6) A. Weinert, M. Berggren, and G. I. Andersson : “A Low Impedance Sensing Technique for Vibrating Structures”, Proc. Transducers’99, CDROM 2P1_3 (1999) (7) S. Buschnakowski, A. Bertz, W. Brauer, S. Heinz, R. Schuberth, G. Ebest, and T. Gessner : “Development and Characterisation of a High Asect Ratio Vertical FET Sensor for Motion Detection”, Proc. Transducers’03, pp.1391-1394 (2003) (8) S. Aoyagi, Y. Matsui, K. Makihira, H. Tokunaga, M. Sasaki, and K. Hane : “Fabrication of MOSFET Capacitive Sensor using Spray Coating Method”, IEEJ Tran. SM, Vol.127, No.3, pp.153-159 (2007) (9) S. Aoyagi, D. Yoshikawa, Y. Isono, and Y. C. Tai : “Development of a Capacitive Accelerometer Using Parylene (Part 2) –Measurement Principle Using Fringe Electrical Field and Characterization-, IEEJ Trans. SM, Vol.127, No.6. pp.321-327 (2007)

Hayato Izumi (Non-member) received the BE and ME degrees in industrial engineering from Kansai University, Osaka, Japan in 2004, and 2006, respectively. He is currently a graduate student of the Systems Management Engineering Department at Kansai University for pursuing the PhD degree. His current research interest is MEMS, with an emphasis on FET sensor, microneedle imitating mosquito, biomedical systems such as a trace collection system, etc.

Yohei Matsumoto (Non-member) will be received the BE degree in industrial engineering from Kansai University, Osaka, Japan, in 2008. He is currently a undergraduate student of the Mechanical Engineering Department at Kansai University. His current research interest is MEMS, with an emphasis on FET sensor.

Seiji Aoyagi (Member) received the BE, ME, and PhD degrees in precision machinery engineering from the Fig. 10. Fabrication process using electroplating University of Tokyo, Tokyo, Japan, in 1986, 1988, and 1994, respectively. From 1988 to 1995, he was with the Mechanical System Engineering (17656090). This work was also partially supported by Department at Kanazawa University, Kanazawa, “High-Tech Research Center” Project for Private Universities: Japan as a Research Associate and an Associate Matching Fund Subsidy from MEXT, 2005-2009, and by the Professor. He is currently a Full Professor of the Kansai University Special Research Fund, 2006 and 2007. Mechanical Engineering Department at Kansai University, Osaka, Japan. (Manuscript received Sept. 3, 2007) His current research interests are , Mechatronics, and MEMS.

106 IEEJ Trans. SM, Vol.128, No.3, 2008 MEMS Capacitive Sensor Using MOSFET Structure

Yusaku Harada (Non-member) received the BE degree in Kazuhiro Hane (Member) received the PhD degree in electrical and Mechanical engineering from Kansai University, from the University of Osaka, Japan, in 2006. He is currently a graduate Nagoya, Aichi, Japan, in 1983. In 1983 he joined the student of the Mechanical Engineering Department University of Nagoya as a Research Associate, at Kansai University. His current research interest is where he was Associated Professor in 1990. From nanotechnology using electroplating. 1985 to 1986, he joined National Research Council of Canada as a visiting researcher. From 1994 to present, he is a Full Professor of Tohoku University, Miyagi, Japan. Shoso Shingubara (Non-member) received the PhD degree in applied from Tokyo Institute of Technology, Tokyo, Hiroshi Tokunaga (Non-member) received the BE degree in electrical Japan, in 1985. From 1990 to 2005, he was with the engineering from Shibaura Institute of Technology, Faculty of Engineering at Hiroshima University, as Tokyo, Japan in 1969. In 1971 he joined Fujitsu an Assistant Professor and an Associated Professor. Laboratories Ltd., Kanagawa, Japan, as a researcher. Now he is a Full Professor of the Mechanical In 1975 he moved to the electronic device business Engineering Department at Kansai University, headquarters of Fujitsu Ltd. In 2002 he moved to Osaka, Japan. His current research interests are Fujitsu Media Devices Ltd., where he retired on in nanotechnology, LSI, and electro- and electroless-plating. 2004. In 2005 he established M.T.C. Ltd. as the representative director president. He is engaged in research/development Minoru Sasaki (Member) received the PhD degree in electrical and and sale of MEMS sensor devices. electronic engineering from the University of Nagoya, Aichi, Japan, in 1995. In the same year, he was a member of Japan Society for the Promotion of Science and a Postdoctoral Fellow. From 1996 to 2007, he was with the Faculty of Engineering at Tohoku University, Miyagi, Japan as an Assistant Professor and an Associated Professor. In 2007 he joined the Mechanical Systems Department at Toyota Technological Institute, Aichi, Japan as a Full Professor.

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