Large Proof Mass Research Articles

A dicing free SOI process for MEMS devices

This paper presents a full wafer, dicing free, dry release process for MEMS silicon-on-insulator (SOI) sensors and actuators. The developed process is particularly useful for inertial sensors that benefit from a large proof mass, for example accelerometers and gyroscopes. It involves consecutive front and backside deep reactive ion etching (DRIE) of the substrate to define the device features, release holes, and trenches. This is followed by hydrofluoric acid vapor phase etching (HF VPE) to release the proof mass and the handle wafer underneath to allow vertical displacements of the proof mass. The release process also allows the devices to be detached from each other and the substrate without the need of an extra dicing step that may damage the delicate device features or create debris. In the work described here, the process is demonstrated for the full wafer release of a high performance accelerometer with a large proof mass measuring 4x7mm^2. The sensor was successfully fabricated with a yield of over 95%.

Wafer Level Fabrication of Vibrational Energy Harvesters using Bulk PZT Sheets

This paper presents a complete wafer level microfabrication process for the production of unimorph MEMS energy harvesters based on bulk PZT. Recently, piezoelectric harvesters designed to take advantage of the high quality piezoelectric properties of bulk PZT have been fabricated using local placement and bonding of individual PZT pieces and large proof masses at the chip level. With the process presented here, 16 piezoelectric energy harvesters have been fabricated in parallel at the wafer level by bonding a single bulk PZT sheet to a silicon wafer and processing the wafer with standard microfabrication techniques including an electrodeposition step to deposit a thick (> 200 mu m) nickel proof mass. The fabricated harvesters are able to generate an output power of 116 mu W at an acceleration of 1 g and a resonant frequency of 203 Hz. (C) 2012 Elsevier Ltd....Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o.

A Low-Cost CMOS-MEMS Piezoresistive Accelerometer with Large Proof Mass

This paper reports a low-cost, high-sensitivity CMOS-MEMS piezoresistive accelerometer with large proof mass. In the device fabricated using ON Semiconductor 0.5 μm CMOS technology, an inherent CMOS polysilicon thin film is utilized as the piezoresistive sensing material. A full Wheatstone bridge was constructed through easy wiring allowed by the three metal layers in the 0.5 μm CMOS technology. The device fabrication process consisted of a standard CMOS process for sensor configuration, and a deep reactive ion etching (DRIE) based post-CMOS microfabrication for MEMS structure release. A bulk single-crystal silicon (SCS) substrate is included in the proof mass to increase sensor sensitivity. In device design and analysis, the self heating of the polysilicon piezoresistors and its effect to the sensor performance is also discussed. With a low operating power of 1.5 mW, the accelerometer demonstrates a sensitivity of 0.077 mV/g prior to any amplification. Dynamic tests have been conducted with a high-end commercial calibrating accelerometer as reference.

A High-Resolution Silicon-on-Glass $Z$ Axis Gyroscope Operating at Atmospheric Pressure

This paper describes a high-resolution silicon-on-glass z axis gyroscope operating at atmospheric pressure. The mechanical structure is designed in such a way that it exhibits low cross coupling between drive and sense mode of less than 0.5% simulated using finite-element method and 1.35% verified by experimental measurements. Due to a symmetrically designed structure, the specified bandwidth can be maintained despite of fabrication imperfections. The fabrication process flow is based on a combination of silicon on glass bonding and deep reactive ion etching which results in a large proof mass and capacitances. A closed loop self-oscillation drive interface is used to resonate the gyroscope in the drive mode, which reaches steady-state after 150 ms. Using area-varying capacitors, large quality factors of 217 and 97 for drive and sense mode, respectively, were achieved operating at atmospheric pressure. A low drive voltage, with a 1 Vpeak-peak AC drive amplitude and 10 V DC bias was used to excite the drive mode. The measured scale factor was 10.7 mV/°/s in a range of ±300°/s with a R 2-nonlinearity of 0.12%. The noise equivalent angular rate is 0.0015°/s/Hz1/2 (=5.4°/h/Hz1/2) in a 50 Hz bandwidth. The measured SNR was 34 dB at an angular rate input signal with an amplitude of 12.5°/s and a frequency of 10 Hz. Without any active temperature control, zero bias stability of 1°/s was achieved for long-term measurements over six hours and 0.3°/s for short-term measurements over 120 seconds (1-?).

A full wafer dicing free dry release process for MEMS devices

This paper presents a full wafer, dicing free, dry release process using hydrofluoric acid (HF) vapour phase etching (VPE) for MEMS sensors and actuators fabricated using silicon on insulator (SOI) wafers. It is particularly beneficial to MEMS sensors whose performance benefits from a large proof mass, for example accelerometers and gyroscopes. Such a fabrication method was first proposed by Overstolz et al. where the wafer level release steps for a tilting platform measuring 2×2 mm2 were presented Overstolz et al. (2004) . In the work described here, the process is extended to the full wafer release of an accelerometer with a large proof mass measuring 4×7 mm2. The sensor was successfully fabricated with a yield of over 95%.

A high performance Σ-Δ readout circuitry for μg resolution microaccelerometers

This paper reports a second order electromechanical sigma-delta readout for micro-g resolution accelerometers in addition to other high-sensitivity capacitive microsensors with large base capacitance. The chip implements a switched-capacitor readout front-end and an oversampled sigma-delta modulator, and hence can be used for both open-loop analog readout and closed-loop control and readout with direct digital output. The readout circuit has more than 115 dB dynamic range and can resolve less than 3 aF/√Hz. Also this IC includes start-up circuit and feedback mechanism for closed-loop control of the accelerometer with a single 5 V supply in a ±4 g range. Together with the accelerometer, bandwidth of the overall system is limited with the sensor resonance frequency (1.53 kHz) in the open-loop mode. However in closed loop mode, oversampling of the acceleration data increases the bandwidth of the system up to few hundred kilohertz which is limited with the cut-off frequency of the low-pass filter placed at the output of the system. The start-up circuit allows rebalancing of a thick silicon proof mass with the limited 5 V supply after system start from power down or in the case of over-range input acceleration. The readout chip has been combined with a Silicon-On-Glass lateral accelerometer, which has a high sensitivity of 1.88 pF/g with large proof mass and long finger structures. A digital filtration and decimation circuitry is also implemented to signal process the output bit stream of the readout circuit. The complete module consumes 16 mW from a ±2.5 V supply and has an adjustable sensitivity up to 8 V/g with a noise level of 4.8 μg/?Hz in open-loop.

An in-plane low-noise accelerometer fabricated with an improved process flow

We present a bulk micromachined in-plane capacitive accelerometer fabricated with an improved process flow, by etching only one-fifth of the wafer thickness at the back of the silicon while forming the bar-structure electrode for the sensing capacitor. The improved flow greatly lowers the footing effect during deep reactive ion etching (DRIE), and increases the proof mass by 54% compared to the traditional way, resulting in both improved device quality and a higher yield rate. Acceleration in the X direction is sensed capacitively by varying the overlapped area of a differential capacitor pair, which eliminates the nonlinear behavior by fixing the parallel-plate gap. The damping coefficient of the sensing motion is low due to the slide-film damping. A large proof mass is made using DRIE, which also ensures that dimensions of the spring beams in the Y and Z directions can be made large to lower cross axis coupling and increase the pull-in voltage. The theoretical Brownian noise floor is 0.47 µg/Hz1/2 at room temperature and atmospheric pressure. The tested frequency response of a prototype complies with the low damping design scheme. Output data for input acceleration from −1 g to 1 g are recorded by a digital multimeter and show very good linearity. The tested random bias of the prototype is 130 µg at an averaging time of around 6 s.

Process flow improvement on a bulk micromachined gyroscope

An introduction of a bulk micromachined frame gyroscope and the efforts made to lower the footing effect during deep reactive ion etching (DRIE) is featured. The gyroscope is driven into oscillation electrostatically and senses rate signal by the varying overlapped area of the electrodes, which eliminates the nonlinear behaviour of sensing the varying gap. Damping factors for the driving and sensing modes are low due to the slide-film damping, resulting in high quality factors in the atmosphere. A large proofmass can be made by DRIE, which also ensures a large pull-in voltage. The footing effect during fabrication has been greatly reduced by the improved process flow, which also increases the proof mass by 50%. Thus device quality and yield rate are both improved. Functional results of an improved prototype are given.

A novel out-of-plane MEMS tunneling accelerometer

A novel out-of-plane MEMS tunneling accelerometer is designed, fabricated and characterized. ICP process, which can generate large proof mass, is used for the first time to fabricate an out-of-plane tunneling accelerometer. To reduce the high Hooke's constant in out-of-plane direction of the ICP etching structure, the beams and the proof mass are dry-etched by double mask ICP process, which reduce the Hooke's constant, thereby enhance the sensitivity of the device. The tunneling current of open loop is tested in the air by HP4145B semiconductor analyzer, which verifies the presence of tunneling current and shows the needed driving voltage. The close-loop test shows sensitivity of 811 mV/g for feedback voltage (the sensitivity of actuate voltage is about 200 mV/g) and nonlinearity of 1% in the measurement range of 0 to +1 g. The spectrum measurement is performed in general environment and the resolution of the device is 0.5 mg/rtHz (1.25–100 Hz), which shows better low frequency property.

An all-silicon single-wafer micro-g accelerometer with a combined surface and bulk micromachining process.

This paper reports an all-silicon fully symmetrical z-axis micro-g accelerometer that is fabricated on a single-silicon wafer using a combined surface and bulk fabrication process. The microaccelerometer has high device sensitivity, low noise, and low/controllable damping that are the key factors for attaining micro g and sub-micro g resolution in capacitive accelerometers. The microfabrication process produces a large proof mass by using the whole wafer thickness and a large sense capacitance by utilizing a thin sacrificial layer. The sense/feedback electrodes are formed by a deposited 2-3 microns polysilicon film with embedded 25-35 microns-thick vertical stiffeners. These electrodes, while thin, are made very stiff by the thick embedded stiffeners so that force rebalancing of the proof mass becomes possible. The polysilicon electrodes are patterned to create damping holes. The microaccelerometers are batch-fabricated, packaged, and tested successfully. A device with a 2-mm x 1-mm proof mass and a full bridge support has a measured sensitivity of 2 pF/g. The measured sensitivity of a 4-mm x 1-mm accelerometer with a cantilever support is 19.4 pF/g. The calculated noise floor of these devices at atmosphere are 0.23 micro g/sqrt(Hz) and 0.16 micro g/sqrt(Hz), respectively.