AVS2010 Session MN-ThM: Multi-scale Interactions of Materials at the Micro- and Nano-scale

We have fabricated resonant mechanical structures from different membrane materials, including single-atom-thick graphene, and explored there function as sensors and devices. Silicon nitride can be lithographically fabricated to form electromechanical devices, and these structures have been widely studied, often in the form of cantilevers, as resonators and sensors. We have explored different geometries of silicon nitride as resonant mass sensors and also studied a new stress-based resonant device for sensing atmospheric gasses. With advances in processing methods we can now create similar devices from lithographically patterned graphene atomic sheets and monitor their mechanical response by methods similar to those used for surface patterned silicon nitride. The ability to integrate membranes into nanomechanical devices and systems creates the possibility for new sensing modalities and systems.

Nanomechanical resonators operating in vacuum are capable of detecting and weighing single biomolecules, but their poor performance in liquid, a consequence of strong viscous damping, hinders many biological applications. One approach that has been demonstrated to improve the performance of resonant MEMS operating in contact with liquid, encapsulating the liquid within the resonator, had until this work not been extended to devices with effective mass smaller than ~100 ng. Here, we show that the practice of confining liquid within a resonator improves the performance of NEMS with mass as small as 100 pg. We optically actuate and detect the motion of doubly clamped beams containing fluidic channels with height 100 nm, which show quality factors as high as 800 when filled with fluid. We use these devices to measure fluid density, demonstrating a mass responsivity of 100 Hz/fg and a noise equivalent mass of 2 fg. We also demonstrate that the quality factor of the fluid-filled resonators is limited by the fluid, and discuss the physical mechanisms causing the enhanced dissipation. Our analysis suggests methods of improving the mass resolution of fluid-filled resonators and demonstrates their promise for novel biological applications.

Mechanical detection of magnetic resonance opens up exciting possibilities for characterizing soft materials and biomolecules with elemental specificity at nanometer-scale, and potentially atomic-scale, resolution. Achieving atomic-scale resolution requires using cantilevers with a low minimum detectable force at small tip-sample separations and fabricating magnetic tips with only a few nanometers of damage at the leading edge. We address these challenges by 1) fabricating cantilevers with overhanging magnetic tips, 2) protecting the nanomagnet leading edge by atomic layer deposited (ALD) alumina, and 3) characterizing the extent and chemical mechanism of damage by nanometer-resolution electron energy loss spectroscopy (EELS).

After determining by EELS analysis that the nickel magnet leading edge incurred substantial damage during processing, we introduced tantalum and ALD alumina interdiffusion barriers into our forty-two step fabrication process. We demonstrate that these modifications have significantly reduced the damage layer thicknesses. The nanomagnet grain structure, point-by-point relative atomic concentrations at the leading edge, and magnetization are determined by high-resolution transmission electron microscopy (TEM), EELS, and frequency-shift cantilever magnetometry, respectively. We will also detail ongoing work to reduce the number of processing steps after magnet deposition, which could greatly improve magnet yield and quality. Our findings suggest that fabricating a cantilever suitable for single proton detection, while a materials processing challenge, should be possible.

Magnetic Resonance Force Microscopy is a technique which combines the elemental discrimination and three-dimensional imaging of magnetic resonance with the spatial resolution of scanned probe microscopy. Key to this technique is attonewton-sensitivity mechanical oscillators with magnetic particles at the tip.

The aim of this work was to create silicon cantilevers with nickel magnets which extended past the tip of the cantilever body - a design conceived to minimize surface-induced force noise.

The fabrication challenges of this design, including alignment across multiple lithographic modes and silicide prevention, will be covered. As well, characterization data of successfully produced devices will be presented.

The force can be effectively viewed as a reaction of an elastic foundation, which increases the stiffness of the system. The time-varying voltage applied to the electrode results in the modulation of this electrostatic stiffness and consequently in the parametric excitation of the structure. The device is distinguished by a simple single-layer architecture and may exhibit large vibrational amplitudes, which are not limited by the pull-in instability common in close-gap actuators. In contrast to previously reported devices excited by the fringing fields, the force considered here is of distributed character. The reduced order model was built using the Galerkin decomposition with undamped linear modes as base functions and the resulting system of nonlinear differential equations was solved numerically. The electrostatic forces were approximated by means of fitting the results of three-dimensional numerical solution for the electric fields. The devices fabricated from a silicon on insulator (SOI) substrate using deep reactive ion etching (DRIE) based process combined with the critically timed etching were operated in ambient air conditions and the responses were registered by means of Laser Doppler Vibrometry. The experimental resonant curves were consistent with those predicted by the model. Theoretical and preliminary experimental results illustrated the feasibility of the suggested approach.