Hongwei Qu

A Monolithic CMOS-MEMS 3-Axis Accelerometer With a Low-Noise, Low-Power Dual-Chopper Amplifier

This paper reports a monolithically integrated CMOS-MEMS three-axis capacitive accelerometer with a single proof mass. An improved DRIE post-CMOS MEMS process has been developed, which provides robust single-crystal silicon (SCS) structures in all three axes and greatly reduces undercut of comb fingers. The sensing electrodes are also composed of the thick SCS layer, resulting in high resolution and large sensing capacitance. Due to the high wiring flexibility provided by the fabrication process, fully differential capacitive sensing and common-centroid configurations are realized in all three axes. A low-noise, low- power dual-chopper amplifier is designed for each axis, which consumes only 1 mW power. With 44.5 dB on-chip amplification, the measured sensitivities of x-, y-, and z-axis accelerometers are 520 mV/g, 460 mV/g, and 320 mV/g, respectively, which can be tuned by simply changing the amplitude of the modulation signal. Accordingly, the overall noise floors of the x-, y-, and z-axis are 12 mug/radicHz , 14 mug/radicHz, and 110 mug/radicHz, respectively, when tested at around 200 Hz.

Process Development for CMOS-MEMS Sensors With Robust Electrically Isolated Bulk Silicon Microstructures

This paper presents a deep reactive-ion etching (DRIE)-based post-CMOS micromachining process that provides robust electrically isolated single-crystal silicon (SCS) microstructures for integrated inertial sensors. Several process issues arise from previously reported three-axis CMOS microelectromechanical system (MEMS) accelerometers, including sidewall contaminations of SCS microstructures in plasma etch and a severe silicon undercut caused by overheating of suspended microstructures. Solutions to these issues have been found and are discussed in detail in this paper. In particular, a lumped-element model is developed to estimate the temperature rise on suspended microstructures in a silicon DRIE process. Based on the thermal modeling and experiments, a thick photoresist layer has been used as a thermal path to avoid the severe silicon undercut. The sidewall contamination problem is also eliminated using the modified CMOS-MEMS process. A three-axis accelerometer with a low-noise, low-power on-chip amplifier has been successfully fabricated using the new process. Footing effect was observed on the backside of the sensor microstructure, but it has little effect on the structural integrity and sensitivity of the sensor.

A Low-Power Low-Noise Dual-Chopper Amplifier for Capacitive CMOS-MEMS Accelerometers

This paper reports a novel dual-chopper amplifier (DCA) and its application to monolithic complementary metal-oxide semiconductor-microelectromechanical systems accelerometers. The DCA design minimizes the power consumption and noise by chopping the sensing signals at two clocks. The first clock is a high frequency for removing the flicker noise while the second clock is a significantly lower frequency to keep the unit gain bandwidth low. A monolithic three-axis accelerometer integrated with the DCA on the same chip has been successfully fabricated using a post-CMOS micromachining process. The measured noise floors are 40 μ g/√Hz in the x - and y -axis and 130 μ g/√Hz in the z -axis, and the power consumption is about 1 mW per axis.

A multi-degree-of-freedom micromirror utilizing inverted-series-connected bimorph actuators

An ove lm ulti-degree-of-freedom electrothermal micromirror design that uses thermal inverted-series-connected (ISC) bimorph actuators is presented. The ISC bimorph actuators eliminate problems observed in previous designs that use single bimorph actuators. The micromirror can operate in one-dimensional (1D) piston-mode and two-dimensional (2D) tilt-mode. Analytical models for the piston-mode and tilt-mode actuations are shown and are compared to FEM simulation results. The device was fabricated using the AMI 1.5 µ mC MOS (complimentary metal-oxide semiconductor) process followed by a post-CMOS micromachining process for device release. The displacement versus temperature of the micromirror was measured experimentally over a range of temperatures and compared to analytical and FEM simulation results .E xperimental results showed that the micromirror displaced by 56 µ ma t an applied temperature 150 ◦ C. The integrated polysilicon heaters were open-circuited during the post-CMOS microfabrication. The failure was caused by pinholes in the metal layers that allowed etchants to attack the polysilicon layer during post-CMOS microfabrication.

A single-crystal silicon 3-axis CMOS-MEMS accelerometer

The paper presents a single-crystal silicon (SCS)-based, integrated 3-axis accelerometer fabricated using a post-CMOS micromachining process. This new CMOS-MEMS process provides monolithic integration of electronics and SCS microstructures, and electrical isolation of silicon. By employing a unique vertical sensing mechanism, 3-axis acceleration sensing is achieved with a single proof mass. The symmetric structures and fully differential configuration of all the sensing electrodes can greatly reduce the cross coupling among the 3 axes. By sacrificing one interconnect metal layer, the silicon undercut of the sensing comb fingers is minimized, resulting in much higher sensitivity with the same device footprint. A wet Al etching process was also developed to remove the top Al layer without attacking the Al layers exposed from the sidewalls of the multilayer Al/oxide stacks. A two-stage, open-loop, continuous time chopper stabilized amplifier is integrated on the chip.

A 1mW Dual-Chopper Amplifier for a 50-/spl mu/g/spl radic/Hz Monolithic CMOS-MEMS Capacitive Accelerometer

This paper reports a novel dual-chopper amplifier (DCA) for CMOS-MEMS capacitive accelerometers. A DCA prototype integrated with a single-axis accelerometer has been fabricated using TSMC 0.35 mum CMOS process. The DCA achieves a 16nV/radicHz input-referred noise at 20 Hz with a power dissipation of only 1 mW. The measured accelerometer noise floor is 50 mug/radicHz down to 5 Hz

Design and fabrication of electro-thermally activated micro gripper with large tip opening and holding force

This paper reports on the design, fabrication, and characterization of a distinctive MEMS gripper electro-thermally driven jointly by a new metallic V-shape actuator (VSA), and a set of modified Guckel U-shape actuators (mUSA). The modification of the angle between the hot and cold arms in the mUSA facilitates desired unidirectional in-plane displacement and thus increases the opening of the gripper. This unique configuration distinguishes this MEMS gripper from other similar devices in the capability of generation of larger tip displacement and greater holding force. Tip opening of ∼173 µm and holding force of ∼5 mN have been measured at a low operating voltage of 1 V with consuming power of 0.85 W. MetalMUMPs is employed to fabricate the device. Electroplated nickel is used as the structural material. The metallic structure allows a low operating voltage and low overall power consumption.

Electro-Thermal MEMS Switch With Latching Mechanism: Design and Characterization

This paper presents the design, fabrication, and characterization of a proprietary metallic microelectromechanical systems switch with unique latching and switching mechanisms that are driven electro-thermally by a set of V-shaped actuators (VSA) and modified Guckel U-shaped actuators. These distinctive actuators are specifically designed and uniquely integrated to provide desired large in-plane traveling distance and force for the switch contact and latching mechanisms. Due to the structural symmetry of the VSA, the out-of-plane displacements are minimized in the entire system. The latching mechanism reduces the total power consumption of the switch while providing a large mechanical contact force for a reliable switching function. This switch with latching capability only consumes power at the time of changing states. MetalMUMPs is employed to fabricate the device, in which electroplated nickel is used as the structural material. Using the metallic structure allows a low operating voltage. At 1.0 V with an electrical power of ~ 0.67 W, each nickel 4-arm VSA generates ~ 13.5-μm displacement and ~ 8-mN force.

Displacment amplification and latching mechanism using V-shape actuators in design of electro-thermal MEMS switches

This paper reports two unique mechanisms using “V-shape” electro-thermal actuators to realize large displacement and reliable contact in MEMS switches. The first mechanism is a novel displacement amplification system that enables a large in-plane traveling distance for closing the gap between switch contacts. The structure features a movable end and pivot joints connecting to the amplification beam that can result in smaller actuating force distinguishing this mechanism from others. The second mechanism involves a unique latching system to provide a large lateral force for reliable contact. In both mechanisms, out-of-plane displacements are minimized due to the structural symmetry of the “V-shape” actuators. The devices are fabricated using PolyMUMPS process. At 1 V driving voltage for 2 pairs of 3-arm V-shape actuators, the displacement amplification system provides a gain of ∼6.5 while consuming 2×3.8 mW power. Each 4-arm V-shape latching actuator generates ∼2.7 µm displacement and ∼240 µN contact force with a power consumption of ∼2.2 mW.

Multiferroic oxide composites: synthesis, characterisation and applications

Abstract The nature of mechanical strain mediated electromagnetic coupling in multiferroic composites has been studied extensively in recent years. This review is on composites with ferromagnetic or ferrimagnetic oxides and ferroelectrics. Systems studied so far include samples with spinel ferrites, hexagonal ferrites or lanthanum manganites for the ferromagnetic phase and barium titanate, lead zirconate titanate (PZT), lead magnesium niobate–lead titanate (PMN-PT) or lead zinc niobate–lead titanate (PZN-PT) for the ferroelectric phase. Bilayer and multilayer heterostructures, bulk composites, core shell nanoparticles and core shell nanotubes and nanowires were investigated for their response to magnetic fields, termed direct magnetoelectric effect (DME). Several systems show a giant low frequency DME and resonance enhancement at bending and electromechanical resonance. The response of the composites to an electric field, called converse ME effect, is found to be strong in several ferrite–ferroelectric composites. The potential for use of the composites for pico-Tesla magnetic sensors and high frequency electric field tunable ferrite signal processing devices are also addressed in this review.