Frederic P. Stratton

A MEMS-based quartz resonator technology for GHz applications

We report on the development of a new MEMS quartz resonator technology that allows for the processing and integration of VHF to UHF high-Q oscillators and filters with high-speed silicon or III-V electronics. The paper describes the successful demonstration of new wafer bonding and dry plasma etching processes that make quartz-MEMS technology possible. We present impedance, Q, and temperature sensitivity data along with comparison to 3D harmonic and thermal analysis of VHF-UHF resonators. We also show Coventor simulation data of our first two- and three-pole monolithic crystal filter designs as well as a filter array layout which facilitates integration with front-end RF electronics and switches. Finally, we demonstrate a mechanical tuning technique for our resonators utilizing focused-ion-beam (FIB) technology.

A new tunneling-based sensor for inertial rotation rate measurements

Micro-electro-mechanical (MEM) technology promises to significantly reduce the size, weight, and cost of a variety of sensor systems. For vehicular and tactical-grade inertial navigation systems, high-performance MEM gyroscopes are required with 1 to 100°/h resolution and stability. To date, this goal has proven difficult to achieve in manufacturing for many of the previous approaches using Coriolis-based devices due, in part, to the need to precisely tune the drive and sense resonant frequencies. We have designed, fabricated, and tested a new highly miniaturized tunneling-based sensor that employs the high displacement sensitivity of quantum tunneling to obtain the desired resolution without the need for precise mechanical frequency matching. Our first tested devices with 300-μm-long cantilevers have demonstrated 27°/h/√Hz noise floors. Measurements indicate that this number can be reduced to near the thermal noise floor of 3°/h/√Hz when a closed loop servo, operating at the devices oscillation frequency, is implemented around the sensor.

Anelastic Creep Phenomena in Thin Metal Plated Cantilevers for MEMS

During the past several years, we have developed high displacement sensitivity tunneling accelerometers using surface micromachining and metal electroplating techniques. These devices consist of a Au tunneling tip fabricated below a 1-2 μm thick metal cantilever beam of electroplated Ni or Au. A thin film of e-beam evaporated Au on the underside of the cantilever serves as the tunneling counter electrode. In operation, a 100mV bias is applied across the tunneling gap. A larger turn-on voltage is also applied between the cantilever and a control electrode, located on the substrate, to deflect the cantilever and maintain a constant tunneling current of 1 or 10 nA. Typical deflections of the end of 100 μm-long and 250 μm-long cantilevers are 0.5μm during operation. We have observed that the turn-on voltage decreases over time for most devices with a larger drop observed for the Au cantilevers. In all cases, the initial decay of the turn-on voltage was almost completely recoverable after the device was turned off for 24 hrs. This decay was not found to be strongly dependent on the magnitude of the tunneling current, but could be significantly reduced by pre-stressing the cantilever before operation. Finally, a vacuum anneal at 100°C influences the measured temperature dependence of the turn-on voltage. The observed effects appear to be consistent with fatigue and creep phenomena in the cantilevers. These effects are reversible at room temperature and are dependent on the stress and temperature history of the devices. A comparison is made between metal plated and all-Si structures.

Optimized DRIE etching of ultra-small quartz resonators

Current manufacturing technology for quartz resonators does not provide a straightforward path for reducing the size and thereby increasing the frequency of operation into the UHF range. Using MEMS processing techniques and a commercial deep reactive ion etching (DRIE) tool, we are developing new techniques that may provide the ability to integrate large numbers of high performance filters onto a single chip for future handheld programmable communication systems.

Nonlinear UHF quartz MEMS oscillator with phase noise reduction

Stable local oscillators with low phase noise are extremely important elements in high performance communication and navigation systems. We present the development of compact UHF-band frequency sources capable of maintaining low phase noise for handheld portable systems. We also explored nonlinearity in MEMS resonators and attempted to use nonlinear dynamics to enhance phase noise performance. Using the quartz MEMS technology, we have thus far demonstrated a 635 MHz oscillator with -112 dBc/Hz phase noise at 1 kHz offset frequency. The controlled oscillation of this nonlinear Duffing resonator in a closed-loop system with improved phase noise is described.

Optimizing UHF quartz MEMs resonators for high thermal stability

A 1 GHz AT-cut quartz thickness shear mode resonator is modeled for the first time with thermally induced bonding stresses and their effect on the device frequency-temperature (f-T) characteristic. Without the details of the bonding configuration, modeling indicates the f-T characteristic slightly rotates as a function of the change in stiffness of a simplified absorbing mount. However, if details of the bonding configuration are included, our modeling predicts the potential for a significant distortion in the f-T curve. High or varying stress over temperature in the device active region is found to lead to an undesirable increase in the f-T slope. The origin of the active region stress can be varied, but in practice it frequently originates from a temperature dependent bonding stress, or from fabrication steps such as metal depositions. In this paper we highlight the magnitude of the thermal stress effect on the f-T curve, and offer design methods that mitigate the thermally induced bonding stress by de-coupling the active resonator area from high stress regions of the quartz device.

MEMS-based quartz oscillators and filters for on-chip integration

We report on the development of a new microelectronicmechanical system (MEMS)-based quartz resonator technology that allows for the processing and integration of VHF to UHF high-Q oscillators and filters with high-speed silicon or III-V electronics. This paper describes the first demonstration of prototype oscillators and filters using this newly developed technology. We present impedance, Q, and temperature sensitivity data of UHF resonators along with phase noise and Allan deviation measurements. Our first 2-pole filter data showing low insertion loss are also presented. Finally, the results of power handling measurements are described for applications where high levels of background signals are present.

UHF quartz MEMS oscillators for dynamics-based system enhancements

Processes for fabricating full wafers of UHF quartz MEMS oscillators bonded to Si have been developed at HRL over the past several years. These devices have shown state-of-the-art noise and stability along with extremely small vacuum packaged die size of less than 3 mm. An interesting by-product of the high frequency, small size, and wafer-scale fabrication of these devices is that several novel dynamics-based enhancements can be considered. These include the use of nonlinear dynamics for reducing oscillator phase noise at CMOS capable voltages and co-integration with more complex structures for sensing vibration and serving as a local timing reference for reducing thermally-induced sensor drifts. Several of these novel concepts made possible by wafer-scale MEMS-based processing will be reviewed.

Next Generation Quartz Oscillators and Filters for VHF-UHF Systems

We report on the development of a new MEMS-based quartz resonator technology that allows for the processing and integration of VHF to UHF high-Q oscillators and filters with high-speed silicon or III-V electronics. This paper describes the first demonstration of prototype oscillators and filters using this newly developed technology. We present impedance, Q, and temperature sensitivity data of UHF resonators along with phase noise and Allan deviation measurements. Our first 2-pole filter data showing low insertion loss is also presented. Finally, the results of power handling measurements are described for applications where high levels of background signals are present

MEMS-Based UHF Monolithic Crystal Filters for Integrated RF Circuits

We report our work in developing microelectromechanical systems (MEMS)-based Ultra High Frequency (UHF) AT-cut quartz monolithic crystal filters operating between 350 and 400 MHz for integration with Si electronics for highly compact Radio Frequency (RF) front-end electronics. Our narrow bandwidth (0.2%) high Q filters have measured insertion losses of -2 dB with temperature stability of roughly 50 ppm over a temperature range of 10°- 80°C. Wafer-level optical metrology and ion milling techniques have been developed to provide enhanced accuracy of the filter center frequency and resonator parameters for optimized performance and improved yields.