Gary Yama

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.

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

Electrical and Thermal Conduction in Atomic Layer Deposition Nanobridges Down to 7 nm Thickness

While the literature is rich with data for the electrical behavior of nanotransistors based on semiconductor nanowires and carbon nanotubes, few data are available for ultrascaled metal interconnects that will be demanded by these devices. Atomic layer deposition (ALD), which uses a sequence of self-limiting surface reactions to achieve high-quality nanolayers, provides an unique opportunity to study the limits of electrical and thermal conduction in metal interconnects. This work measures and interprets the electrical and thermal conductivities of free-standing platinum films of thickness 7.3, 9.8, and 12.1 nm in the temperature range from 50 to 320 K. Conductivity data for the 7.3 nm bridge are reduced by 77.8% (electrical) and 66.3% (thermal) compared to bulk values due to electron scattering at material and grain boundaries. The measurement results indicate that the contribution of phonon conduction is significant in the total thermal conductivity of the ALD films.

Investigation of energy loss mechanisms in micromechanical resonators

Micromechanical resonators with resonant frequencies from 500 kHz to 10 MHz were built and examined for several energy loss mechanisms. Thermoelastic damping, clamping loss and air damping were considered. The devices were shown to be limited by thermoelastic damping, providing experimental verification of this phenomenon at the microscale. Resonators with scaled dimensions also matched well with scaling theory of damping at a given pressure. An energy loss mechanism other than thermoelastic dissipation, most likely clamping loss, was shown to be dominant for resonators whose ratio of length to width was less than 10:1. The devices were fabricated using a single-wafer encapsulation process.

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.

A study of electrostatic force nonlinearities in resonant microstructures

This letter investigates the nature of amplitude frequency (A-f) dependence caused by nonlinearities in the parallel plate electrostatic transduced in resonant microstructures. We present analytical and experimental evidences that the A-f nonlinearities are practically always dominated by third order nonlinear terms. For an electrostatically unbalanced system, we show that the bias voltage at which second and third order nonlinearities have equal impact on A-f dependence corresponds to ∼90% of the dc pull-in voltage.

Si-SiO2 Composite MEMS Resonators in CMOS Compatible Wafer-scale Thin-Film Encapsulation

Si-SiO2 composite resonators in wafer-scale thin-film encapsulation are demonstrated. These resonators are fabricated in hermetic wafer-scale thin-film encapsulation, which enables mass production with high yield even after harsh post processes. Also this encapsulation provides potential for integration of frequency references with CMOS circuitry. The encapsulated composite resonators exhibit tunable temperature dependence of resonant frequency with a high quality factor of much larger than 10,000. Up to 40times improvement in temperature stability was demonstrated.

Scaling of amplitude-frequency-dependence nonlinearities in electrostatically transduced microresonators

This paper studies the scaling of nonlinearities in double-ended-tuning-fork microresonators. We find that the increase in resonant frequency associated with beam length reduction strongly improves current handling. For example, shortening the beams by a factor of 5 results in 20- and 100-fold increases in resonant frequency and sustainable signal current, respectively. Using the nonlinear models and scaling rules outlined in this work, we present considerations for optimization of the resonant structure and its electrostatic gap size.

Temperature Compensation of a MEMS Resonator Using Quality Factor as a Thermometer

Silicon MEMS resonators have great potential for on-chip high frequency signal applications. This paper compares methods of sensing the temperature of an encapsulated silicon MEMS resonator and using this temperature measurement to stabilize the temperature, and hence the resonant frequency, of the resonator. The use of external Pt RTDs, integrated Si thermistors, and the use the Quality factor (Q) of the resonator are explored. Use of the Q as a temperature sensor is explored in detail as it is a nearly ideal temperature sensing method. Characterisations of the temperature sensors and preliminary temperature control results are presented.