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Development of MEMS Capacitive Sensor Using a MOSFET Structure

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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 MOSFETs 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 －7－ 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), biosensors(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 capacitor 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 電学論 E，128 巻 3 号，2008 年 103 (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 noise 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.
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