Chris P. Tigges

Toward the development of miniaturized imaging systems for detection of pre-cancer

In this paper, we describe the progress toward the development of miniaturized imaging systems with applications in medical imaging, and specifically, detection of pre-cancer. The focus of the article is a miniature, optical-sectioning, fluorescence microscope. The miniature microscope is constructed from lithographically printed optics and assembled using a bulk micro-machined silicon microoptical table. Optical elements have been printed in a negative tone hybrid glass to a maximum depth of 59 /spl mu/m and an rms surface roughness between 10-45 nm, fulfilling the requirements of the miniature microscope. Test optical elements have been assembled using silicon-spring equipped mounting slots. The design of silicon springs is presented in this paper. Optical elements can be assembled within the tolerances of an NA=0.4 miniature microscope objective, confirming the concept of simple, zero-alignment assembly.

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.

Simulation of the athermal coarsening of composites structured by a uniaxial field

We report the results of a computer simulation of the evolution of structure in a two component fluid consisting of a liquid phase and a dispersed colloidal phase subjected to a biaxial field. A biaxial field, such as a rotating field, can induce the organization of polarizable particles into two-dimensional sheets, in contrast with the essentially one-dimensional columns formed in a uniaxial field. Our primary objective is to explore the kinetics of coarsening, the emergence of structure, and the anisotropy in materials properties. Using an efficient, linear-N simulation method we report studies of systems of N=10 000 particles over the concentration range of φ=10–50 vol %. We present a variety of methods of characterizing the structures that emerge, including the two dimensional pair correlation, velocity correlations, microcrystallinity, optical attenuation, dipolar interaction energy, conductivity, and permittivity. The anisotropies that we compute are generally inverted relative to those found in mat...

Planar microoptomechanical waveguide switches

Planar micromechanical waveguide switches based on lateral deflection of a cantilever beam are presented. Two material systems have been used: a GaAs-AlGaAs structure with integrated waveguides and a silicon-on-insulator (SOI), with postprocessed polymeric waveguides. The switches are characterized by low actuation voltage (3-20 V), short switching times (32-200 /spl mu/s), and low crosstalk (< -30 dB).

THERMAL COARSENING OF UNIAXIAL AND BIAXIAL FIELD-STRUCTURED COMPOSITES

When a suspension of colloidal particles is subjected to a strong electric or magnetic field, the induced dipolar interactions will cause the particles to form organized structures, provided a sufficient permittivity or permeability mismatch exists, respectively, between the particles and the suspending liquid. A uniaxial field will produce uniaxial structures, and a biaxial field, such as a rotating field, will produce biaxial structures, and either of these structures can be pinned by polymerizing the continuous phase to produce field-structured composites. We have previously reported on the coarsening of field-structured composites in the absence of thermal effects, i.e., Brownian motion. Athermal simulations are primarily valid in describing the deep quenches that occur when the induced dipolar interactions between particles greatly exceed kBT. However, deep quenches can lead to kinetic structures that are far from equilibrium. By introducing Brownian motion we have shown that structures with signific...

Flux flow microelectronics

Flux-flow-based devices such as the superconducting flux flow transistor and magnetically controlled long junctions have been made from thin films of TlCaBaCuO and YBaCuO. The devices are based on the magnetic control of flux flow in their respective structures: a long junction or an array of weak links. The equivalent circuits of the two devices are similar: a low-impedance input control line, an output impedance of 3-20 Omega , and an active current-controlled element. The long junctions have tended to be slower, to have lower gain, and to be somewhat less noisy than their counterparts. Circuits such as narrowband and distributed amplifiers (50-GHz bandwidths, noise figures <3 dB), phase shifters (continuous with <2-dB loss, 4-40 GHz), logic gates (2-3-ps gate delays). and memories made using these devices are compared and analyzed in terms of performance.<<ETX>>

YBa2Cu3O7 nanobridges fabricated by direct‐write electron beam lithography

A direct method for nondamaging, nanometer‐scale patterning of high Tc superconductor thin films is presented. We have fabricated superconducting nanobridges in high‐quality, epitaxial thin‐film YBa2Cu3O7 (YBCO) by combining direct‐write electron beam lithography and an improved aqueous etchant. Weak links with both length and width dimensions less than 20 nm have exhibited critical currents at 77 K of 4–20 μA and IcRn products of 10–100 μV which compare favorably with results for other YBCO junction technologies. We have used this technique in the fabrication of a shock‐wave pulse former as an initial demonstration of its applicability to monolithic superconductive electronics.

S-parameter measurements and microwave applications of superconducting flux flow transistors

Microwave two-port S-parameter measurements and modeling of superconducting flux flow transistors are presented. The transistors, based on the magnetic control of flux flow in any array of high temperature superconducting weak links, exhibit significant available power gain at microwave frequencies (over 20 dB at 7-10 GHz in some devices). The input impedance is largely inductive while the output impedance is both resistive and inductive. It is shown that the characteristics of these devices are useful in numerous applications including matched amplifiers, phase shifters and active impedance convertors. >

GaAs-based microelectromechanical waveguide switch

We describe a 1/spl times/2 waveguide switch which is also a cantilever, fabricated in GaAs-based materials. This switch can be cascaded into 1/spl times/N structure. The layout and layer cross section of the waveguide are shown schematically. Actuation is accomplished by electrostatic means, by application of bias between the movable waveguide and static electrodes. This results in 4 /spl mu/m motion of the cantilevered waveguide in the plane of the wafer. The waveguide consists of 4 /spl mu/m thick GaAs/AlGaAs layer, while the release layer is composed of 2 /spl mu/m of Al/sub 0.7/Ga/sub 0.3/As. Metal contacts are deposited on a planar substrate prior to waveguide definition. Then 3 /spl mu/m wide waveguide is defined by RIBE. Photoresist is defined on the areas to be protected against release and sacrificial layer is removed by a HF-based wet etch. After photoresist removal, devices are sublimation dried. Fabrication issues, such as choice of materials, release chemistries and their implications are further discussed. Also, further details of device performance are given.

Tl‐Ca‐Ba‐Cu‐O step‐edge Josephson junctions

There has been considerable progress in the development of nonhysteretic Josephson junction microelectronic technologies for YBaCuO. Such a technology for Tl‐Ca‐Ba‐Cu‐O junctions would be interesting because of the higher operating temperatures and the very different grain boundary structures in the different cuprate superconductors. We have successfully made step‐edge junctions with TlCaBaCuO grown on ion‐milled LaAlO3. Nonhysteretic grain boundary‐based junctions have been demonstrated with critical current‐normal state resistance products exceeding 5 mV at 77 K, critical current densities ranging from 500 to 25 000 A/cm2, and critical current versus field profiles suggesting very uniform junctions. Yield has exceeded 70% on over 250 junctions tested and operation has been demonstrated to 100 K.