Heng-Chung Chang

A novel stress isolation guard ring design for the improvement of three-axis piezoresistive accelerometer

This study designs and implements a stress isolation guard-ring structure to improve the performances of the existing single proof-mass 3-axis piezoresistive accelerometer. Thus, the environment disturbances (temperature variation and force/deflection transmittance) for such packaged 3-axis piezoresistive accelerometer are significantly reduced. Measurements demonstrate the guard-ring design successfully reduce the false signals (induced by temperature variation and force/displacement transmittance) for one order of magnitude. Moreover, the proposed accelerometer still maintains the advantages of existing design, such as single proof-mass for 3-axis acceleration sensing, and better linearity. Sensitivities of present accelerometer range 0.127∼1.77mV/G/V and non-linearity<1.02%VFSS.

Mechanical force-displacement transduction structure for performance enhancement of CMOS-MEMS pressure sensor

This study implements a mechanical force-displacement transduction structure in Fig.1a using the TSMC 0.18μm 1P6M CMOS process to improve CMOS-MEMS capacitive pressure sensor. The membrane will be deformed by pressure and cause the sensing-gap change between undeformed movable-electrode and fixed-electrode. Feature of this study is CMOS-MEMS deformed membrane and undeformed movable-electrode to enable the parallel-plate gap-closing pressure detection. Thus, the performance of pressure sensor can be improved. In comparison, the design with mechanical force-displacement transduction structure will increase sensitivity for 61% in pressure range (100kPa-60kPa). Moreover, sensitivity (non-linearity) of proposed design changes from 2.1fF/kPa (1.7%) to 1.9fF/kPa (2.5%) as pressure range changed from 100~60kPa to 100~20kPa. However, sensitivity (non-linearity) of existing design significantly drops from 1.3fF/kPa (1.5%) to 1.0fF/kPa (13.7%) as pressure range changed.

Method for sensitivity improvement and optimal design of a piezoresistive pressure sensor

This study reports a systematic approach to achieve optimal design of a piezoresistive sensor under the limitation of fabrication capabilities. The commercial finite element analysis (FEA) software, CoventorWare, is employed to evaluate the candidate designs of a piezoresistive pressure sensor. The Taguchis robust design method is further employed to consider the deviation from manufacturing (e.g. dimensions, alignment, etc.). The full scale span (sensitivity) of the sensors was chosen as quality characteristics. The effects of manufacturing related restrictions were estimated by the noise factor analysis. Then, the optimal design of a sensor, which realizes the optimization of sensor signal output and minimization the influence of the variation of the design factors, is achieved through thoughtful considerations and arrangements. In application, the micro pressure sensor is design and fabricated, and the full scale span of sensor has been improved for 20% (from 78.3mV to 94mV).

A reliable test key for thin film mechanical properties characterization

In this study, a reliable test key using resonant technique to characterize the mechanical properties of thin film materials is reported. The test key consists of micromachined cantilever array that are fabricated using the thin film to be determined. The Youngs modulus, Shear modulus, and Poissons ratio of the thin film are successfully extracted after the natural frequencies of bending and torsional modes are measured. In applications, this technique has been employed to determine the mechanical properties of SiO/sub 2/ film fabricated from bulk micromachining processes. Moreover, the test key has also been used to determine the mechanical properties of poly-silicon layers in MUMPs surface processes.

Development of a CMOS MEMS pressure sensor with a mechanical force-displacement transduction structure

This study presents a capacitive pressure sensor with a mechanical force-displacement transduction structure based on the commercially available standard CMOS process (the TSMC 0.18 μm 1P6M CMOS process). The pressure sensor has a deformable diaphragm to support a movable plate with an embedded sensing electrode. As the diaphragm is deformed by the ambient pressure, the movable plate and its embedded sensing electrode are displaced. Thus, the pressure is detected from the capacitance change between the movable and fixed electrodes. The undeformed movable electrode will increase the effective sensing area between the sensing electrodes, thereby improving the sensitivity. Experimental results show that the proposed pressure sensor with a force-displacement transducer will increase the sensitivity by 126% within the 20 kPa–300 kPa absolute pressure range. Moreover, this study extends the design to add pillars inside the pressure sensor to further increase its sensing area as well as sensitivity. A sensitivity improvement of 117% is also demonstrated for a pressure sensor with an enlarged sensing electrode (the overlap area is increased two fold).

Designs of planar sensing inductor on inverse-magnetostrictive type pressure sensor

This study presents a pressure sensor design which consisted of the planar coil and CoFeB magnetic films. As the Si diaphragm deformed by pressure load, the magnetostriction effect of magnetic film will cause the permeability change of CoFeB. Thus, the permeability change as well as the pressure load can be detected by the inductance difference of the planar inductor. The proposed magnetostrictive pressure sensor with planar sensing inductor had been implemented and tested in the previous work. In this study, the design of planar sensing inductor is further investigated and reported. Based on the design modification of planar sensing inductor, such as coil turns or the geometry of magnetic film, the sensitivity could be improved successfully. Preliminary measurements demonstrate a high gauge factor of near 850 can be achieved through the inverse-magnetostrictive type pressure sensor.

Study and characterization of plastic encapsulated package for a three-axis piezoresistive acceleromete with guard-ring structure

This study reports the performance of a three-axis piezoresisitvie accelerometer with a guard-ring structure after plastic packaging. The accelerometers, with and without guard-ring structure, are capped with glass substrate to form the glass/Si/glass sandwich and then encapsulated in plastic package. The testing results on these packaged accelerometers have shown that the guard-ring structure successfully suppresses the performance shift caused by plastic package for one order of magnitude. The glass/Si/glass sandwich structure adds air damping to the accelerometer, hence its resonant frequency is slightly reduced (less than 0.5%). Moreover, the guard-ring structure has relatively large stiffness and is considered as a rigid body, and its influence on the sensor dynamics can be ignored. In conclusion, the guard ring structure significantly reduces the performances variation of packaged sensor. Thus, the inexpensive plastic encapsulated package for accelerometers can be implemented on the real products.

Piezoresistive pressure sensor with Ladder shape design of piezoresistor

In this study, a novel piezoresistor (PZR) design to improve the sensitivity of the piezoresistive type pressure sensor is reported. In this design, the PZR combined with various doping concentrations and doping depths is proposed and implemented. According to the design concept, the combined PZR which includes multiple piezoresistance coefficients would be arranged in a unique shape, just like a ladder. Theoretically, the sensitivity improvement could boost up to 15% based on this PZR design, even under the conditions of membrane edge offset due to fabrication processes. As a result, the sensitivity of the pressure sensor which adopted the Ladder shape PZR design is 0.058 mV/V/kPa. Moreover, the doping profile of the Ladder shape PZR is also extracted by SIMS. The serial measurements successfully prove the feasibility of this design concept. In the future, the presented PZR design can be further extended to various piezoresistive sensors, such as accelerometer, gyro, etc.

A Novel 3-DOF Micromanipulator

In this study, a stiff HARM (high aspect ratio micromachining) manipulator is reported. Since the micromanipulator is made of single crystal silicon, it has superior mechanical properties. The micromanipulator is consisted of a position stage and a robot arm. Moreover, the robot arm is monolithically integrated with the position stage through the micromachining fabrication processes. Hence, its in-plane and out-of-plane positions are precisely controlled by two comb drive actuators and a vertical comb, respectively. In applications, the robotarm, which has three degrees of freedom (DOF), can be exploited as a micromanipulator.Copyright

A novel polymer filled CMOS-MEMS inductive-type tactile sensor with wireless sensing capability

This study presents a novel wireless inductive type CMOS-MEMS tactile sensing unit composed of underneath sensing coil and deformable polymer layer. The advantages of the proposed tactile sensing unit are as follows, (1) No released sensing diaphragm on the sensing unit. Thus, the residual stress caused by CMOS-MEMS process can be avoided. (2) The sensing coil underneath is protected by the above polymer layer, so the sensing unit would not be damaged under applied load. (3) Wireless sensing capability due to the magnetic coupling. (4) Sensing range can be modulated by varying the stiffness of the polymer (e.g. changing the ratio of pre-polymer and curing agent). This tactile-sensing unit is implemented using TSMC 0.18μm 1P6M CMOS process, in-house post-CMOS releasing, and polymer filling. Experiments show the tactile sensor has the sensitivity of 0.02%/mN within the sensing range of 0–80mN, and the wireless sensing ability is also demonstrated.