Woo-Tae Park

Review: Semiconductor Piezoresistance for Microsystems

Piezoresistive sensors are among the earliest micromachined silicon devices. The need for smaller, less expensive, higher performance sensors helped drive early micromachining technology, a precursor to microsystems or microelectromechanical systems (MEMS). The effect of stress on doped silicon and germanium has been known since the work of Smith at Bell Laboratories in 1954. Since then, researchers have extensively reported on microscale, piezoresistive strain gauges, pressure sensors, accelerometers, and cantilever force/displacement sensors, including many commercially successful devices. In this paper, we review the history of piezoresistance, its physics and related fabrication techniques. We also discuss electrical noise in piezoresistors, device examples and design considerations, and alternative materials. This paper provides a comprehensive overview of integrated piezoresistor technology with an introduction to the physics of piezoresistivity, process and material selection and design guidance useful to researchers and device engineers.

Long-Term and Accelerated Life Testing of a Novel Single-Wafer Vacuum Encapsulation for MEMS Resonators

We have developed a single-wafer vacuum encapsulation for microelectromechanical systems (MEMS), using a thick (20-mum) polysilicon encapsulation to package micromechanical resonators in a pressure <1 Pa. The encapsulation is robust enough to withstand standard back-end processing steps, such as wafer dicing, die handling, and injection molding of plastic. We have continuously monitored the pressure of encapsulated resonators at ambient temperature for more than 10 000 h and have seen no measurable change of pressure inside the encapsulation. We have subjected packaged resonators to >600 cycles of -50 to 80degC, and no measurable change in cavity pressure was seen. We have also performed accelerated leakage tests by driving hydrogen gas in and out of the encapsulation at elevated temperature. Two results have come from these hydrogen diffusion tests. First, hydrogen diffusion rates through the encapsulation at temperatures 300-400degC have been determined. Second, the package was shown to withstand multiple temperature cycles between room and 300-400degC without showing any adverse affects. The high robustness and stability of the encapsulation can be attributed to the clean, high-temperature environment during the sealing process

Single wafer encapsulation of MEMS devices

Packaging of micro-electro-mechanical systems (MEMS) devices has proven to be costly and complex, and it has been a significant barrier to the commercialization of MEMS. We present a packaging solution applicable to several common MEMS devices, such as inertial sensors and micromechanical resonators. It involves deposition of a 20 /spl mu/m layer of epi-polysilicon over unreleased devices to act as a sealing cap, release of the encapsulated parts via an HF vapor release process, and a final seal of the parts in 7 mbar (700 Pa) vacuum. Two types of accelerometers, piezoresistive and capacitive sensing, were fabricated. Piezoresistive accelerometers with a footprint smaller than 3 mm/sup 2/ had a resolution of 10 /spl mu/g//spl radic/Hz at 250 Hz. Capacitive accelerometers with a 1 mm/sup 2/ footprint had a resolution of 1 mg/spl radic/Hz over its 5 kHz bandwidth. Resonators with a quality factor as high as 14,000 and resonant frequency from 50 kHz to 10 MHz have also been built. More than 100 capacitive accelerometers and 100 resonators were tested, and greater than 90% of the resonators and accelerometers were functional.

Frequency stability of wafer-scale encapsulated MEMS resonators

This paper presents an investigation of the long-term frequency stability of wafer-scale encapsulated silicon MEMS resonators. Two aspects of stability were examined: long-term stability over time and temperature-related hysteresis. Encapsulated resonators were tested over a period of 8,000 hours in constant environmental temperature of 25/spl deg/C /spl plusmn/ 0.1/spl deg/C. No measurable drift, burn-in time, or other changes in resonant frequencies were detected. Another experiment was performed to investigate the stability of the resonators with temperature cycling. The resonant frequency was measured between each cycle for more than 450 temperature cycles from -50/spl deg/C to +80/spl deg/C. Additional data is presented for short-term hysteresis measurements -10/spl deg/C to +80/spl deg/C temperature cycle. No detectable hysteresis was observed in either of the temperature cycle experiments. These series of experiments demonstrate resonant frequency stability of wafer-scale silicon based MEMS resonators.

Impact of geometry on thermoelastic dissipation in micromechanical resonant beams

Thermoelastic dissipation (TED) is analyzed for complex geometries of micromechanical resonators, demonstrating the impact of resonator design (i.e., slots machined into flexural beams) on TED-limited quality factor. Zener first described TED for simple beams in 1937. This work extends beyond simple beams into arbitrary geometries, verifying simulations that completely capture the coupled physics that occur. Novel geometries of slots engineered at specific locations within the flexural resonator beams are utilized. These slots drastically affect the thermal-mechanical coupling and have an impact on the quality factor, providing resonators with quality factors higher than those predicted by simple Zener theory. The ideal location for maximum impact of slots is determined to be in regions of high strain. We have demonstrated the ability to predict and control the quality factor of micromechanical resonators limited by thermoelastic dissipation. This enables tuning of the quality factor by structure design without the need to scale its size, thus allowing for enhanced design optimization

Measurement system for low force and small displacement contacts

To support the continued miniaturization of electrical contacts in multichip systems, three-dimensional (3-D) systems, wafer probe cards, and MEMS relays, there is a need for combined measurements of electrical and mechanical phenomena during contact formation. We have carried out a study of electrical contacts in the nN-mN force range for future generation probe cards and novel electronic packaging. One critical phenomenon in the contact formation process is nm-scale deformation of the material layers. To directly study this contact displacement, we have designed a measurement system comprised of a piezoresistive cantilever and an optical interferometer. Together, this system simultaneously measures contact resistance (mOhm to kOhm), force (nN to mN), and displacement (nm-/spl mu/m). These measurements allow the first direct observation of contact mechanical behavior in this important application range. These measurements show that asperities at the contact surface dominate the behavior of the contacts, causing deviations from the Hertzian model of elastic contacts. This paper describes the design and construction of this apparatus, and the operation in a contact mechanics experiment.

A CMOS Rectifier With a Cross-Coupled Latched Comparator for Wireless Power Transfer in Biomedical Applications

A highly efficient rectifier for wireless power transfer in biomedical implant applications is implemented using 0.18-m CMOS technology. The proposed rectifier with active nMOS and pMOS diodes employs a four-input common-gate-type capacitively cross-coupled latched comparator to control the reverse leakage current in order to maximize the power conversion efficiency (PCE) of the rectifier. The designed rectifier achieves a maximum measured PCE of 81.9% at 13.56 MHz under conditions of a low 1.5-Vpp RF input signal with a 1- k output load resistance and occupies 0.009 mm2 of core die area.

Gate-All-Around Junctionless Nanowire MOSFET With Improved Low-Frequency Noise Behavior

We present n-type gate-all-around (GAA) junctionless nanowire field-effect transistor (JL-NWFET) along with low-frequency noise (LFN) with respect to channel doping and the gate bias voltage. Irrespective of doping level in the channel, which is the same as that of source/drain, the JL-NWFET shows approximately five orders of magnitude lower spectral noise than the inversion-mode counterpart. LFN in JL-NWFET is also found less sensitive to gate bias voltage and to the frequency. The superior LFN behavior in GAA JL-NWFET is attributed to the conduction of carriers inside the uniformly doped nanowire channel. JL-NWFET-based sensing elements can thus be suitable in physical transducers to maximize the detection limits.

Optimization of NEMS pressure sensors with a multilayered diaphragm using silicon nanowires as piezoresistive sensing elements

A pressure sensor with a 200 µm diaphragm using silicon nanowires (SiNWs) as a piezoresistive sensing element is developed and optimized. The SiNWs are embedded in a multilayered diaphragm structure comprising silicon nitride (SiNx) and silicon oxide (SiO2). Optimizations were performed on both SiNWs and the diaphragm structure. The diaphragm with a 1.2 µm SiNx layer is considered to be an optimized design in terms of small initial central deflection (0.1 µm), relatively high sensitivity (0.6% psi−1) and good linearity within our measurement range.

Encapsulated submillimeter piezoresistive accelerometers

While micromachined accelerometers are widely available and used in various applications, some biomedical applications require extremely small dimensions (<mm) or mass (<mg) that cannot be fulfilled with commercially available accelerometers. In this work, we present a fully packaged piezoresistive accelerometer that has the smallest dimension (0.034mm/sup 3/) ever published. We achieve miniaturization by using a film encapsulation technique with a thick epitaxial polysilicon layer. This packaging technique enables the dimensions of the die to be only tens of microns larger than the micromechanical structure. We have fabricated accelerometers as small as 0.034mm/sup 3/ (387/spl mu/m/spl times/387 /spl mu/m/spl times/230/spl mu/m) with noise floor of 0.25mg//spl radic/Hz. These ultra-miniature motion sensors have potential opening up new frontiers in biomedical science and engineering.