David B. Heinz

Radio frequency electrical transduction of graphene mechanical resonators

We report radio frequency (rf) electrical readout of graphene mechanical resonators. The mechanical motion is actuated and detected directly by using a vector network analyzer, employing a local gate to minimize parasitic capacitance. A resist-free doubly clamped sample with resonant frequency ∼34 MHz, quality factor ∼10 000 at 77 K, and signal-to-background ratio of over 20 dB is demonstrated. In addition to being over two orders of magnitude faster than the electrical rf mixing method, this technique paves the way for use of graphene in rf devices such as filters and oscillators.

The long path from MEMS resonators to timing products

Research on MEMS Resonators began over 50 years ago. In just the last 10 years, there has been a series of important technological developments, and (finally!) success at commercialization. The presentation will highlight some key milestones along this path, describe some of the critical technology steps, and outline some of the important non-technological events within SiTime - all of these factors contributed to the successful outcome.

Characterization of stiction forces in ultra-clean encapsulated MEMS devices

We show that contact between encapsulated MEMS devices and the bare silicon surrounding sidewalls generally results in a reversible adhesion with a consistent adhesion force. This force is small enough (25 mN) to be overcome by the restoring force of the springs in inertial sensors with resonant frequency above 4 kHz. Therefore, it should be possible to design and build stiction-free inertial sensors in this process - a significant advantage over approaches that rely on deposition, tuning and maintenance of chemical coatings for inertial sensors.

Experimental Investigation Into Stiction Forces and Dynamic Mechanical Anti-Stiction Solutions in Ultra-Clean Encapsulated MEMS Devices

A systematic experimental and theoretical evaluation of stiction between intermittently contacting silicon surfaces in an ultra-clean encapsulation process is presented, evaluating magnitude of stiction forces, the reversible nature of sidewall contact, and repeatability of results. The uniquely stable environment and the lack of native oxide are leveraged to enable reliable collision and contact models, which confirm the nature of the asperity contact. In addition, we demonstrate a series of dynamic mechanical anti-stiction solutions and the mechanisms by which they mitigate stiction. These devices are shown to reduce susceptibility to stiction-related failure by 50%.

Stiction forces and reduction by dynamic contact in ultra-clean encapsulated MEMS devices

We demonstrate the consistent and manageable nature of surface adhesion and stiction forces in MEMS devices fabricated using the high-temperature epitaxial encapsulation process. In this encapsulation process (commercialized by SiTime), there are no chemical anti-stiction films or getters. Data from more than 2000 test structures with more than 80 design variations from three different fabrication runs were gathered in this study. Surprisingly, the adhesion force is shown to be independent of design geometry. The measured adhesion forces (18-25uN) are small enough for inertial sensors. In addition, we demonstrate anti-stiction bump stops with springs for a sliding contact, which reduce the probability of stiction by over 50%.

Capacitive sensor fusion: Co-fabricated X/Y and Z-axis accelerometers, pressure sensor, thermometer

This paper presents a capacitive X/Y and Z-axis accelerometer, pressure sensor, and resonant thermometer, co-fabricated in a single die using an ultra-clean, high-temperature, wafer-scale, production-compatible encapsulation process. This process is free of bond rings and getter areas, thereby reducing overall die size to a minimum. Utilizing a process that allows for nitride and silicon etch stops, we show that it is possible to design high-sensitivity pressure sensors and accelerometers with very little cross-sensitivity. In addition, remaining sensitivities to environmental effects, such as temperature, can be compensated to reveal a suite of on-chip high-performance sensors that is accurate over temperature. All of this is accomplished in a process similar to current high-volume production processes in industry.

Stable charge-biased capacitive resonators with encapsulated switches

A large dc bias voltage (tens of volts) is often useful for the operation of capacitive MEMS devices. Charge-biasing techniques have been demonstrated to be able to replace a bias voltage source by trapping an equivalent charge on an electrically floating electrode. This work presents a charge-biased resonator that uses an electrostatically actuated mechanical switch with a pull-in voltage of 36 V to introduce a voltage-equivalent charge of 10-15V onto a resonant body. The switch and resonator are hermetically sealed in a clean vacuum cavity, within an epitaxial polysilicon encapsulation process (epi-seal). No charge leakage has been observed, even at an elevated temperature of 125°C for weeks.

Transfer function tuning of a broadband shoaling mechanical amplifier near the electrostatic instability

We present a tunable broadband shoaling mechanical amplifier and a method to extend its operation near the electrostatic pull-in instability. The model has been verified experimentally on a vacuum encapsulated silicon MEMS device. We show that by adding an appropriate mechanical compensation spring, the amplifier can be operated near the pull-in instability in a quasi-linear fashion. Furthermore, electrostatic band-pass region and amplification tuning is shown.

Effective quality factor and temperature dependence of self-oscillations in a thermal-piezoresistively pumped resonator

We measure the influence of ambient temperature (from −40 °C to 80 °C) on the threshold current for self-oscillations inathermal-piezoresistively pumped, capacitively sensed resonator. We demonstrate that for our flexural mode self-oscillator, the threshold current decreases with decreasing ambient temperature. We observe the generation of harmonics during self-oscillation with amplitudes that decrease and effective quality factors that increase with each successive mode. Following self-oscillations, we observe that the effective quality factor and amplitude is extremely sensitive to further DC current increases.

Direct comparison of stiction properties of oxide coated polysilicon and smooth single crystal silicon

We directly compare both the process and in-use stiction properties of two identical sets of devices fabricated with both rough oxide-coated polysilicon and smooth, oxide-free single crystal silicon. The polysilicon devices show dramatically reduced process stiction as a result of the surface roughness, enabling higher yield on more compliant structures. In-use stiction is slightly higher for the polysilicon-oxide devices than it is for the single crystal devices. Our ultra-high purity, vacuum encapsulation process allows careful measurement of surface adhesion properties in the absence of environmental variation.