Ian Laboriante

Graphitization of n-type polycrystalline silicon carbide for on-chip supercapacitor application

Synthesis of silicon carbide-derived carbon films with excellent supercapacitor characteristics is demonstrated by a process that is fully compatible with standard microfabrication technology. NiTi alloy deposited on nitrogen-doped polycrystalline SiC films is shown to result in the growth of a rough, porous, high conductivity, nanocrystalline graphitic carbon film upon rapid thermal annealing to 1050 °C. Electrodes fabricated in this manner exhibit high charge/discharge rates with a time constant of about 0.062 s. Analysis shows that the incorporated nitrogen in the carbon electrode may induce pseudo-capacitance, and the electrodes exhibit the capacitance/area values comparable to those reported on carbon nanotube-based supercapacitors.

Measurement of adhesion forces between polycrystalline silicon surfaces via a MEMS double-clamped beam test structure

An electrostatically actuated double-clamped beam test structure has been designed and fabricated for the quantitative determination of adhesion forces between two contacting polycrystalline silicon (polysilicon) surfaces. The experimental measurements of the beam profile at varied bias and simulations based on finite element methods are combined to evaluate more accurately the adhesion forces experienced between polysilicon surfaces. In particular, the electrostatic force at pull-off is obtained by measuring the pull-off voltage and the beam profile through optical interferometric methods. The adhesion force is then determined from the mechanical restoring force obtained by finite element methods simulations and the calculated electrostatic force. The results show a weak scaling of the adhesion force with the apparent contact area, defined via microfabricated dimples.

Interfacial Adhesion between Rough Surfaces of Polycrystalline Silicon and Its Implications for M/NEMS Technology

An electrostatically actuated double-clamped cantilever beam test structure has been designed and fabricated to determine the adhesion forces between co-planar, impacting polycrystalline silicon (polysilicon) surfaces. To examine the effect of apparent contact area, dimples of varying sizes have been included in the test structure. By measuring the cantilever beam profile, through optical interferometric methods, as a function of applied bias, the force of adhesion has been determined for various device geometries. The results reveal a weak dependence of adhesion on apparent contact area, rather than a linear dependence. Fabrication process artifacts, observed and discussed here, contribute to but do not suffice to explain this observed weak scaling. The results strongly suggest that contact on the micrometer scale between rough, rigid materials such as polysilicon involves only a few asperities.

Micellar block copolymer templated galvanic displacement for epitaxial nanowire device integration

New strategies for catalyst nanoparticle placement in arbitrary patterns and non-planar geometries will likely accelerate the large scale device integration of epitaxial semiconductor nanowires (NWs) grown through the vapor-liquid-solid (VLS) process. Herein we report a technique for rational metal catalyst nanoparticle deposition based on galvanic displacement onto semiconductor surfaces through block copolymer micelle templates. Nanoparticle volumes and areal densities are controlled by the time in the plating solution and by mixing homopolymer with the micelle suspension, respectively. Above a minimal nanoparticle diameter, the mean diameters of epitaxial VLS-grown Si NWs scale directly with mean sizes of the template deposited nanoparticle seeds from which they are grown. The substrate selectivity and conformality of galvanic displacement makes possible two-level micro/nano-patterning in a variety of geometries by applying micellar templates over photolithographically patterned masks; the growth of single sub-50 nm diameter Si NWs in 600–700 nm diameter wells demonstrates a feature size reduction greater than one order of magnitude. Two-point electrical measurements across single NWs or a few NWs epitaxially bridging silicon-on-insulator electrodes after ex situ doping demonstrate the viability of this approach for epitaxial NW device integration.

In situ studies of interfacial contact evolution via a two-axis deflecting cantilever microinstrument

The time-dependent assessment of two contacting polycrystalline silicon surfaces is realized using a microinstrument that allows for in situ surface analysis. The evolution in contact resistance, morphology, and chemistry is probed as a function of contact cycle. Initially, the contact resistance is found to decrease and then increase with impact cycle. Upon prolonged cycling, the fracture of Si grains is observed which grow to form a wear crater. The electrical, morphological, and chemical analyses suggest that the wear of rough polysilicon surfaces due to impact proceeds through three distinct phases, namely plastic deformation of asperities, adhesive wear, and grain fracture.

Charging and discharging behavior in dielectric-coated MEMS electrodes probed by Kelvin probe force microscopy

The charging and discharging behavior of silicon dioxide, silicon nitride and aluminum nitride dielectric coatings on microfabricated aluminum electrodes in response to an applied voltage, thermal treatment, operating environment and monolayer coating have been investigated through Kelvin probe force microscopy (KPFM) techniques. Correlated results from surface potential measurements and finite element simulations demonstrate the existence of capacitive coupling between the KPFM probe tip assembly and the device sample which give rise to as much as 20–40% difference between the applied bias and the measured surface potential. Surface charge mobility on the three material systems has been differentiated focusing on the influence of bulk and surface water and the relevant physicochemical properties. The merits and limitations of proposed schemes for mitigating the effects of dielectric charging, including thermal treatment and monolayer coating, are presented.

A finite element technique for accurate determination of interfacial adhesion force in MEMS using electrostatic actuation

This paper reports on accurate analysis of adhesion force between polysilicon–polysilicon surfaces in micro-/nanoelectromechanical systems (M/NEMS). The measurement is carried out using double-clamped beams. Electrostatic actuation and structural restoring force are exploited to respectively initiate and terminate the contact between the two surfaces under investigation. The adhesion force is obtained by balancing the electrostatic and mechanical forces acting on the beam just before the separation of the two surfaces. Different finite element models are developed to simulate the coupled-field multiphysics problem. The effects of fringing field in the electrostatic domain and geometric nonlinearity and residual stress in the structural domain are taken into consideration. Moreover, the beam stiffness is directly obtained for the case of combined loading (electrostatic and adhesion). Therefore, the overall electrostatic and structural forces used to extract the actual adhesion force from measured data are determined with high accuracy leading to accurate values for the adhesion force. The finite element simulations presented in this paper are not limited to adhesion force measurement and can be used to design or characterize electrostatically actuated devices such as MEM tunable capacitors and micromirrors, RF switches and M/NEM relays.

Anodic oxidation of polycrystalline 3C-silicon carbide thin films during MEMS operation

In this paper, we provide the first report of corrosion occurring at the anode of polycrystalline 3C-silicon carbide MEMS electrodes under high relative humidity and applied voltage. This unusual phenomenon is determined to be electrochemical in nature. Electrode pair and canary wire corrosion behavior are investigated to yield a detailed evaluation of the stability and oxidation rate during corrosion. The effects of film stress on anodic oxidation are discussed, and suggestions to prevent damage due to corrosion are presented.

Suppression of wear in cyclically loaded polycrystalline silicon MEMS via a thin silicon carbide coating

The surfaces of a microfabricated polysilicon test structure, designed to measure adhesion forces in MEMS, were modified with a thin (∼50 nm) silicon carbide coating to take advantage of the outstanding tribological, electrical, and chemical properties of polycrystalline SiC (polySiC). Adhesion forces in polySiC-coated interfaces as a function of apparent area of contact have been determined quantitatively and compared to those in uncoated polycrystalline silicon (polySi) contacts. A detailed study of changes in physico-chemical surface properties after >100 billion contact cycles is also presented, highlighting suppression of wear upon SiC coating.

2-axis MEMS deflecting cantilever microinstrument for in-situ reliability studies

Growing interest in assessing the reliability of microelectromechanical systems has created a need for the development of microinstruments to interrogate MEMS behavior under diverse conditions. This paper describes the design and testing of a microinstrument for in-situ reliability studies of contacting surfaces in MEMS. Time-dependent changes in the structure, chemistry and electrical properties of polycrystalline silicon (poly-silicon) impacting surfaces are observed, and possible wear mechanisms are proposed.