Vivekananda P. Adiga

Are Diamonds a MEMS' Best Friend?

Next-generation military and civilian communication systems will require technologies capable of handling data/ audio, and video simultaneously while supporting multiple RF systems operating in several different frequency bands from the MHz to the GHz range [1]. RF microelectromechani-cal/nanoelectromechanical (MEMS/NEMS) devices, such as resonators and switches, are attractive to industry as they offer a means by which performance can be greatly improved for wireless applications while at the same time potentially reducing overall size and weight as well as manufacturing costs.

Photothermal Self-Oscillation and Laser Cooling of Graphene Optomechanical Systems

By virtue of their low mass and stiffness, atomically thin mechanical resonators are attractive candidates for use in optomechanics. Here, we demonstrate photothermal back-action in a graphene mechanical resonator comprising one end of a Fabry-Perot cavity. As a demonstration of the utility of this effect, we show that a continuous wave laser can be used to cool a graphene vibrational mode or to power a graphene-based tunable frequency oscillator. Owing to graphenes high thermal conductivity and optical absorption, photothermal optomechanics is efficient in graphene and could ultimately enable laser cooling to the quantum ground state or applications such as photonic signal processing.

Dielectric properties of hydrogen-incorporated chemical vapor deposited diamond thin films

Diamond thin films with a broad range of microstructures from a ultrananocrystalline diamond (UNCD) form developed at Argonne National Laboratory to a microcrystalline diamond (MCD) form have been grown with different hydrogen percentages in the Ar∕CH4 gas mixture used in the microwave plasma enhanced chemical vapor deposition (CVD) process. The dielectric properties of the CVD diamond thin films have been studied using impedance and dc measurements on metal-diamond-metal test structures. Close correlations have been observed between the hydrogen content in the bulk of the diamond films, measured by elastic recoil detection (ERD), and their electrical conductivity and capacitance-frequency (C-f) behaviors. Addition of hydrogen gas in the Ar∕CH4 gas mixture used to grow the diamond films appears to have two main effects depending on the film microstructure, namely, (a) in the UNCD films, hydrogen incorporates into the atomically abrupt grain boundaries satisfying sp2 carbon dangling bonds, resulting in inc...

Stress-based resonant volatile gas microsensor operated near the critically buckled state

We describe sensing of chemical vapors from the atmosphere using critically buckled polycrystalline silicon doubly clamped mechanical resonators coated on one side with polymethyl methacrylate (PMMA). Our method of sensing is based on stress-induced resonance frequency shifts through volumetric swelling of the 60 nm thick PMMA layer resulting in altered tension in the beams. The stress change produces shifts in the resonance frequency as large as 150% of the baseline frequency. In order to maximize the sensitivity, we tailor residual stress of the polycrystalline silicon resonators to slightly exceed the critical buckling stress. We incorporate a relatively large gap between the bridge and a substrate to provide optical readout and minimize squeezed film effects. We show that the larger gap results in substantial improvements of the quality factor and frequency stability of our resonators under ambient pressure and temperature conditions compared to previous implementations. These lead to resonance freque...

Modal dependence of dissipation in silicon nitride drum resonators

We have fabricated large (≤400 μm diameter) high tensile stress SiN membrane mechanical resonators and measured the resonant frequency and quality factors (Q) of different modes of oscillation using optical interferometric detection. We observe that the dissipation (Q−1) is limited by clamping loss for pure radial modes, but higher order azimuthal modes are limited by a mechanism which appears to be intrinsic to the material. The observed dissipation is strongly dependent on size of the membrane and mode type. Appropriate choice of size and operating mode allows the selection of optimum resonator designs for applications in mass sensing and optomechanical experiments.

Approaching intrinsic performance in ultra-thin silicon nitride drum resonators

We have fabricated circular silicon nitride drums of varying diameter (20 μm to 1 mm) and thickness (15 nm–75 nm) using electron beam lithography and measured the dissipation (Q−1) of these amorphous silicon nitride resonators using optical interferometric detection. We observe that the dissipation is strongly dependent on mode type for relatively large, thick membranes as predicted by the current models of dissipation due to clamping loss. However, this dependence is drastically reduced for smaller or thinner resonators, with thinner resonators showing higher quality factors, for low order modes. Highest quality factors that can be reached for these thin resonators seems be limited by an intrinsic mechanism and scales linearly with the diameter of the membrane. Our results are promising for mass sensing and optomechanical applications where low mass and high Qs are desirable.

Charging characteristics of ultra-nano-crystalline diamond in RF MEMS capacitive switches

Modifications to a standard capacitive MEMS switch process have been made to allow the incorporation of ultra-nano-crystalline diamond as the switch dielectric. The impact on electromechanical performance is minimal. However, these devices exhibit uniquely different charging characteristics, with charging and discharging time constants 5–6 orders of magnitude quicker than conventional materials. This operation opens the possibility of devices which have no adverse effects of dielectric charging and can be operated near-continuously in the actuated state without significant degradation in reliability.

Graphene metallization of high-stress silicon nitride resonators for electrical integration

High stress stoichiometric silicon nitride resonators, whose quality factors exceed one million, have shown promise for applications in sensing, signal processing, and optomechanics. Yet, electrical integration of the insulating silicon nitride resonators has been challenging, as depositing even a thin layer of metal degrades the quality factor significantly. In this work, we show that graphene used as a conductive coating for Si3N4 membranes reduces the quality factor by less than 30% on average, which is minimal when compared to the effect of conventional metallization layers such as chromium or aluminum. The electrical integration of Si3N4-Graphene (SiNG) heterostructure resonators is demonstrated with electrical readout and electrostatic tuning of the frequency by up to 0.3% per volt. These studies demonstrate the feasibility of hybrid graphene/nitride mechanical resonators in which the electrical properties of graphene are combined with the superior mechanical performance of silicon nitride.

Simultaneous electrical and optical readout of graphene-coated high Q silicon nitride resonators

Resonant mechanics of high quality factor (Q) graphene coated silicon nitride devices have been explored using optical and electrical transduction schemes. With the addition of the graphene layer, we retain the desirable mechanical properties of silicon nitride but utilize the electrical and optical properties of graphene to transduce and tune the resonant motion by both optical and electrical means. By positioning the graphene-on-silicon-nitride drums in a tunable optical cavity, we observe position dependent damping and resonant frequency control of the devices due to optical absorption by graphene.

Thermomechanical stability of ultrananocrystalline diamond

We have measured mechanical stiffness and dissipation in ultrananocrystalline diamond (UNCD) from 63 K to 450 K using microcantilever resonators in a custom ultrahigh vacuum (UHV) atomic force microscope. UNCD exhibits a temperature coefficient of modulus that is found to be extremely low: −26 ppm/K, which is close to the previously measured value of −24 ppm/K for single crystal diamond. The magnitude and the temperature dependence of dissipation are consistent with the behavior of disordered systems. The results indicate that defects, most likely at the grain boundaries, create the dominant contribution to mechanical dissipation. These measurements of modulus and dissipation versus temperature in this temperature range in UNCD establish the nanostructure’s effect on the thermomechanical stability and suggest routes for tailoring these properties.