Randall L. Kubena

A new miniaturized surface micromachined tunneling accelerometer

The authors have fabricated a new class of miniaturized surface micromachined tunneling accelerometers. The accelerometer structures are fabricated on the surface of a single silicon wafer and consist of a single cantilevered beam with electrostatic deflection electrodes and tunneling tip underneath. The noise level resolutions in air of 100-/spl mu/m and 250-/spl mu/m-long cantilever devices are 8.3/spl times/10/sup -4/ g/Hz/sup 1/2/ and 8.5/spl times/10/sup -5/ g/Hz/sup 1/2/ at 500 respectively. The devices are operated in a force rebalance feedback mode using a low noise automatic servocontrol circuit, providing a dynamic range of over 10/sup 4/ g. This new accelerometer technology provides devices with extremely high sensitivity, high bandwidth and wide dynamic range, in an ultracompact, low-cost package that is easily integrated with silicon control electronics.

Current‐density profiles for a Ga+ ion microprobe and their lithographic implications

A 50‐keV focused Ga+ beam was used to expose dots in a negative‐acting bilevel resist structure in an exposure range between 1 μs/pixel and 4 s/pixel. The radii of the resist dots were used to determine the current‐density profiles of the Ga+ ion microprobe down to six orders of magnitude below the peak versus system magnification and ion source current. The effect of the beam profile on the limiting pitch for densely packed resist dot arrays was modeled and compared to experimental results.

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.

Dot lithography for zero‐dimensional quantum wells using focused ion beams

A 50‐keV focused Ga+ beam formed in a two‐lens microprobe column with prefinal lens deflection was used to expose dot arrays in a negative acting bilevel resist. Dot arrays 600 μm×600 μm with 600‐A‐diam resist posts on 0.6 μm centers (incorporating 1024×1024 dots) were fabricated with ion exposure times of 18 s. By reducing the beam dwell time by a factor of 2, roughly 300‐A‐diam posts were fabricated. Since the ions stop in the bottom resist layer and do not enter the substrate, the optical properties of underlying material should not be altered by damage from the exposure process.

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.

Bilevel Polysiloxane Resist For Ion-Beam And Electron-Beam Lithography

A series of polysiloxane-based negative resists will be described which are applicable to both electron-beam and ion-beam lithography. Compositional changes have been made to nary the Fisitivitg to 20 keV electrons from 40 to 180 μC/cm2 . A sensitivity of 9.4 tiC/cm (2.5 x 1012 ions/cm2 ) has been obtained with 150 keV silicon ions for the 40 iiC/cm E-beam resist. The resists have high glass transition temperaWes and re effective top layers of a bilevel resist system. A thin polysiloxane layer (400Å to 1000Å) makes an effective mask layer over 1 to 2 μm AZ type leveling layer. After the polysiloxane pattern was developed, the images were transferred to the AZ layer by reactive ion etching with oxygen. By properly adjusting the etching parameters, extremely high anisotropy and excellent selectivity (25:1 ratio between bottom and top layers) were achieved. Using this polysiloxane-topped bilevel system, good quality images have been prepared using both electron-beam lithography (EBL) and focused ion-beam lithography (FIBL). With a 600 -700w thick polysiloxane layer over 1 pm of AZ1370 we have produced a 6000 wide gap with parallel side walls in a 2 pm pitchimage by EBL. The same process has also demonstrated high quality images over 5000A steps. For FIBL a resist system consisting of 400w of polysiloxane over 1 pm AZ1370 was used with a focused 50 keV gallium ion beam2 Resist lines 0.12 pm wide and 1 μm high were produced with an exposure of 8 x 1012 ions/cm2.

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.