Chao-Lin Cheng

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.

Determining the thermal expansion coefficient of thin films for a CMOS MEMS process using test cantilevers

Many standard CMOS processes, provided by existing foundries, are available. These standard CMOS processes, with stacking of various metal and dielectric layers, have been extensively applied in integrated circuits as well as micro-electromechanical systems (MEMS). It is of importance to determine the material properties of the metal and dielectric films to predict the performance and reliability of micro devices. This study employs an existing approach to determine the coefficients of thermal expansion (CTEs) of metal and dielectric films for standard CMOS processes. Test cantilevers with different stacking of metal and dielectric layers for standard CMOS processes have been designed and implemented. The CTEs of standard CMOS films can be determined from measurements of the out-of-plane thermal deformations of the test cantilevers. To demonstrate the feasibility of the present approach, thin films prepared by the Taiwan Semiconductor Manufacture Company 0.35 μm 2P4M CMOS process are characterized. Eight test cantilevers with different stacking of CMOS layers and an auxiliary Si cantilever on a SOI wafer are fabricated. The equivalent elastic moduli and CTEs of the CMOS thin films including the metal and dielectric layers are determined, respectively, from the resonant frequency and static thermal deformation of the test cantilevers. Moreover, thermal deformations of cantilevers with stacked layers different to those of the test beams have been employed to verify the measured CTEs and elastic moduli.

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.

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).

Performance improvement of CMOS-MEMS Pirani vacuum gauge with hollow heater design

A novel heater-with-holes design is developed to realize a CMOS-MEMS Pirani vacuum gauge. The Pirani vacuum gauge is a thermal-based pressure sensor consisting of heater and heat-sink units. The heat is conducted from heater to heat-sink through gas whose thermal conductivity changed with pressure. The proposed design has the following merits: (1) The holes on heater increase the thermal resistance and then improve the efficiency of heat transfer; (2) The heat-sink mesas are added to compensate the lost active area between heater and heat sink; (3) Easily integrate with other packaged CMOS-MEMS devices for pressure monitoring [1]. This study demonstrates two heaters with different hole-designs for concept proven. The devices have total die size of 206μm×82μm and 0.53μm sensing gap. Preliminary results show that the gauges have sensing range of 0.8-200torr and 0.2-200torr with sensitivity of 6.99×103(K/W)/torr and 8.15×103(K/W)/torr. The power dissipations are respectively 179.43μW and 127.59μW. In comparison, the conventional gauge containing heater w/o holes has sensing range 1-200torr with sensitivity of 0.91×103(K/W)/torr and power dissipation of 1117.34μW.

Development of a CMOS-MEMS RF-aerogel-based capacitive humidity sensor

This study presents the design, implementation and characterization of a high-sensitivity capacitive humidity sensor. TSMC 0.18μm CMOS process was used followed by in-house maskless post-processing and pneumatic dispensing of precursors of aerogels to form a vertical parallel-plate (VPP) topology for capacitive sensing. A new kind of sensitive polymer, resorcinol-formaldehyde (RF) organic aerogel, was prepared by the sol-gel method and supercritical fluid (SCF) drying. The low-density RF-aerogels synthesized by this method exhibit high surface areas, high porosities, and mesoporosity, which are beneficial to moisture diffusion and sensing reaction. Despite the sensor response to moisture is non-linear, a minimum sensitivity up to 0.571% capacitance change per percent relative humidity (RH), is achieved. Further measurements show that a response time of 19s, maximum hysteresis of 1.1%RH and a 48-hr signal drift of 0.75%RH have been obtained.

Sensitivity improvement for CMOS-MEMS capacitive pressure sensor using double deformarle diaphragms with trenches

This study presents a novel capacitive pressure sensor implemented by TSMC 0.18μm 1P6M CMOS process. Features of this design is to exploit double deformable sensing diaphragms to enhance the sensitivity of capacitive pressure sensor (Fig. 1a). Moreover, the sensing diaphragms with trenches can further improve the sensitivity due to stiffness reduction. The sensitivity of designed pressure sensor with trenches on double deformable electrodes is 0.26fF/kPa (within the absolute pressure range of 20kPa∼110kPa). The sensitivity is improved for 2.9-fold as compared with the single diaphragm reference design in Fig.1b.

Wireless magnetostrictive type inductive sensing CMOS-MEMS pressure sensors

This study demonstrates the wireless magnetostrictive type inductive sensing CMOS-MEMS pressure sensors using the TSMC 0.18μm 1P6M process. Features of this study are: (1) High sensitivity pressure sensors are realized based on the inverse-magnetostrictive sensing principle; (2) metal and dielectric layers of CMOS process are employed to form the magnetic coil and sensing diaphragm respectively; (3) additional magnetostrictive CoFeB film was deposited and patterned by the shadow sputtering; (4) wireless sensing is available by the external reading coil. Experiments show sensors with gauge-factor ranging 480~1100 are achieved, and wireless sensing capability is also demonstrated.

Micro devices integration with stretchable spring embedded in long PDMS-fiber for flexible electronics

This study presents a PDMS (Polydimethylsiloxane) fiber integrated with multi devices scheme using stretchable electroplating copper spring. Each device was located on the node and embedded in PDMS-fiber. Thus, devices are mechanically connected by PDMS-fiber and electrically connected by inner stretchable spring. Thus, large-area and flexible applications can be achieved. Advantages of this approach: (1) length magnification by stretchable spring; (2) thicker stretchable spring embedded in PDMS provides well mechanical and electrical characteristics; (3) node acts as a hub for devices implementation and integration; (4) partially stretched spring could reduce the resistance variation by external loads. A 6.2cm PDMS-fiber with sensors and LED is implemented using a 2.4cm node-spring components fabricated on 4-inch wafer. PDMS-fiber longer than 30cm can be achieved using different spring design.

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.