Jordan M. Berg

Studies on surface wettability of poly(dimethyl) siloxane (PDMS) and glass under oxygen-plasma treatment and correlation with bond strength

An issue in microfabrication of the fluidic channels in glass/poly (dimethyl siloxane) (PDMS) is the absence of a well-defined study of the bonding strength between the surfaces making up these channels. Although most of the research papers mention the use of oxygen plasma for developing chemical (siloxane) bonds between the participating surfaces, yet they only define a certain set of parameters, tailored to a specific setup. An important requirement of all the microfluidics/biosensors industry is the development of a general regime, which defines a systematic method of gauging the bond strength between the participating surfaces in advance by correlation to a common parameter. This enhances the reliability of the devices and also gives a structured approach to its future large-scale manufacturing. In this paper, we explore the possibility of the existence of a common scale, which can be used to gauge bond strength between various surfaces. We find that the changes in wettability of surfaces owing to various levels of plasma exposure can be a useful parameter to gauge the bond strength. We obtained a good correlation between contact angle of deionized water (a direct measure of wettability) on the PDMS and glass surfaces based on various dosages or oxygen plasma treatment. The exposure was done first in an inductively coupled high-density (ICP) plasma system and then in plasma enhanced chemical vapor deposition (PECVD) system. This was followed by the measurement of bond strength by use or the standardized blister test.

Almost-global tracking of simple mechanical systems on a general class of Lie Groups

We present a general intrinsic tracking controller design for fully-actuated simple mechanical systems, when the configuration space is one of a general class of Lie groups. We show that if a suitable error function can he found, then a general smooth and hounded reference trajectory may be tracked asymptotically from almost every initial condition, with locally exponential convergence. Such functions may be shown to exist on any compact Lie group, or on any product of a compact Lie group and R/sup n/. In the case of compact Lie groups, we show that the full-state feedback law composed with an exponentially convergent velocity estimator, also converges globally for almost every initial tracking error. We explicitly compute these controllers on SO(3), and simulate their performance for the axisymmetric top problem.

Control of an Electrostatic Microelectromechanical System Using Static and Dynamic Output Feedback

This paper examines control strategies for electrostatically actuated microelectromechanical systems (MEMS), with the goals of using feasible measurements to eliminate the pull-in bifurcation, robustly stabilize any desired operating point in the capacitive gap, decrease settling time, and reduce overshoot. We show that input-output linearization, passivity-based design, and the theory of port-controlled Hamiltonian systems lead naturally to static output feedback of device charge. This formalizes and extends previously reported results from the MEMS literature. Further analysis suggests that significantly improving transient behavior in lightly damped MEMS requires dynamic estimation of electrode velocity. We implement output-feedback control using a reduced-order nonlinear observer. Simulations predict greatly improved transient behavior, and large reductions in control voltage.

Self-heating study of an AlGaN∕GaN-based heterostructure field-effect transistor using ultraviolet micro-Raman scattering

We report micro-Raman studies of self-heating in an AlGaN∕GaN heterostructure field-effect transistor using below (visible 488.0nm) and near (UV 363.8nm) GaN band-gap excitation. The shallow penetration depth of the UV light allows us to measure temperature rise (ΔT) in the two-dimensional electron gas (2DEG) region of the device between drain and source. Visible light gives the average ΔT in the GaN layer, and that of the SiC substrate, at the same lateral position. Combined, we depth profile the self-heating. Measured ΔT in the 2DEG is consistently over twice the average GaN-layer value. Electrical and thermal transport properties are simulated. We identify a hotspot, located at the gate edge in the 2DEG, as the prevailing factor in the self-heating.

Thin-Film Thermal Conductivity Measurement Using Microelectrothermal Test Structures and Finite-Element-Model-Based Data Analysis

We present a new method for measuring thermal conductivities of films with nanoscale thickness. The method combines a micro electrothermal test structure with a finite-element- based data analysis procedure. The test device consists of two serpentine nickel structures, which serve as resistive heaters and resistance temperature detectors, on top of the sample. The sample is supported by a silicon nitride membrane. Analytical solution of the heat flow is infeasible, making interpretation of the data difficult. To address this, we use a finite-element model of the test structure and apply nonlinear least-squares estimation to extract the desired material parameter values. The approach permits simultaneous extraction of multiple parameters. We demonstrate our technique by simultaneously obtaining the thermal conductivity of a 280 mum x 80 mum x 140 nm thick aluminum sample and the 360 mum x 160 mum x 180 nm thick silicon nitride support membrane. The thermal conductivity measured for the silicon nitride thin film is 2.1 W/mK, which is in agreement with reported values for films of this thickness. The thermal conductivity of the Al thin film is found to be 94 W/mK, which is significantly lower than reported bulk values and consistent both with reported trends for thin metallic films and with values that were obtained using electrical resistivity measurements and the Wiedemann-Franz law.

A two-stage discrete peristaltic micropump

We demonstrate a discrete, two-stage peristaltic micropump for applications in microfluidics. Prototypes are fabricated in polydimethylsiloxane (PDMS) with water as the working fluid. Off-wafer compressed nitrogen gas provides the actuation energy. The device may be operated in three- or two-stage modes for direct comparison. We show that two-stage pumps have comparable flow rates to the three-stage counterparts, and produce ∼2/3 the static head. Our results suggest that two-stage pumps may be a viable choice under low backpressure conditions where available on-chip area or the number of external connections is limited.

Ignition studies of Al/Fe2O3 energetic nanocomposites

We prepare energetic nanocomposites, which undergo an exothermic reaction when ignited at moderate temperature. The nanocomposites are a mixture of Al fuel and Fe2O3 oxidizer where Fe2O3 is in the form of an array of nanowires embedded in the thin Al film. We achieve a very high packing density of the nanocomposites, precise control of oxidizer–fuel sizes at the nanoscale level, and direct contact between oxidizer and fuel. We find that the flame temperature does not depend on ignition temperature.

Self-heating in a GaN based heterostructure field effect transistor: Ultraviolet and visible Raman measurements and simulations

We report direct self-heating measurements for AlGaN∕GaN heterostructure field effect transistor grown on SiC. Measurements are carried out using micro-Raman scattering excited by above band gap ultraviolet and below band gap visible laser light. Ultraviolet excitation probes the GaN near the AlGaN∕GaN interface region of the device where the two-dimensional electron gas carries the source-drain current. The visible excitation probes the entire ∼1μm thick GaN layer and the SiC substrate near the interface with GaN. These results thus provide a measure of the average temperature throughout the GaN and of the substrate. Results are backed by combined electrical and thermal simulations. We find that the immediate hot spot region of the device, at the edge of the gate electrode, rises by up to ∼240°C over ambient under the most aggressive drive conditions examined.

Direct measurement of thermal conductivity of aluminum nanowires

A nanofabricated electrothermal test structure is reported for directly measuring the thermal conductivity of aluminum nanowires near room temperature. Interdigitated nanowires perturb an otherwise symmetric heater-sensor structure analogous to an electrical bridge circuit. Nanowires studied are 100 nm thick with 75, 100, and 150 nm widths. Finite element simulation accounts for complex device geometry. Thermal conductivity and electrical resistivity vary significantly with nanowire dimensions. Electron transport equation models which adequately describe the resistivity data consistently underestimate the thermal conductivity. Incorporating a phonon contribution of ∼21 W/m K to the total thermal conductivity is found to accurately describe the measured values.

Cell Detachment Model for an Antibody-Based Microfluidic Cancer Screening System

We consider cells bound to the floor of a microfluidic channel and present a model of their flow‐induced detachment. We approximate hydrodynamic force and cell elastic response using static finite‐element simulation of a single cell. Detachment is assumed to occur when hydrodynamic and adhesive forces are roughly equal. The result is extended to multiple cells at the device level using a sigmoidal curve fit. The model is applied to a microfluidic cancer‐screening device that discriminates between normal epithelial cells and cells infected with human papillomavirus (HPV), on the basis of increased expression of the transmembrane protein α6 integrin in the latter. Here, the cells to be tested are bound to a microchannel floor coated with anti α6 integrin antibodies. In an appropriate flow rate range, normal cells are washed away while HPV‐infected cells remain bound. The model allows interpolation between data points to choose the optimal flow rate and provides insight into interaction of cell mechanical properties and the flow‐induced detachment mechanism. Notably, the results suggest a significant influence of cell elastic response on detachment.