D. K. Thomas

Diffusive Dynamics of Nanoparticles in Arrays of Nanoposts

The diffusive dynamics of dilute dispersions of nanoparticles of diameter 200-400 nm were studied in microfabricated arrays of nanoposts using differential dynamic microscopy and single particle tracking. Posts of diameter 500 nm and height 10 μm were spaced by 1.2-10 μm on a square lattice. As the spacing between posts was decreased, the dynamics of the nanoparticles slowed. Moreover, the dynamics at all length scales were best represented by a stretched exponential rather than a simple exponential. Both the relative diffusivity and the stretching exponent decreased linearly with increased confinement and, equivalently, with decreased void volume. The slowing of the overall diffusive dynamics and the broadening distribution of nanoparticle displacements with increased confinement are consistent with the onset of dynamic heterogeneity and the approach to vitrification.

Development and Fabrication of Nanoporous Silicon-based Bioreactors within a Microfluidic Chip

Multi-scale lithography and cryogenic deep reactive ion etching techniques were used to create ensembles of nanoporous, picolitre volume, reaction vessels within a microfluidic system. The fabrication of these vessels is described and how this process can be used to tailor vessel porosity by controlling the width of slits that constitute the vessel pores is demonstrated. Control of pore size allows the containment of nucleic acids and enzymes that are the foundation of biochemical reaction systems, while allowing smaller reaction constituents to traverse the container membrane and continuously supply the reaction. In this work, a 5.4 kb DNA plasmid was retained within the reaction vessels and labeled under microfluidic control with ethidium bromide as an initial proof-of-principle. Subsequently, a coupled enzyme reaction, in which glucose oxidase (GOX) and horseradish peroxidase (HRP) were contained and fed with a substrate solution of glucose and Amplex Red to produce fluorescent resorufin, was carried out under microfluidic control and monitored using fluorescent microscopy. The fabrication techniques presented are broadly applicable and can be adapted to produce devices in which a variety of high aspect ratio, nanoporous silicon structures can be integrated within a microfluidic network. The devices shown here are amenable to being scaled in number and organized to implement more complex reaction systems for applications in sensing and actuation as well as fundamental studies of biological reaction systems.

Effects of irradiation temperature and dose on exfoliation of H+-implanted silicon carbide

H+ implantation of SiC is the basis for a thin-film transfer process, which when combined with oxidation and hydrophilic wafer bonding, can be exploited to produce silicon carbide-on-insulator material useful as a wide-band-gap semiconductor. This thin-film transfer process has been successfully applied to Si to produce a commercial silicon-on-insulator material. The efficacy of hydrogen to produce thin-film separation was studied by investigation of H+-induced exfoliation in implanted SiC. Results showed that the onset and degree of exfoliation of SiC depends initially upon the concentration of implanted H+. However, the dose dependence of exfoliation exhibits a rather marked retrograde behavior. The degree of exfoliation eventually starts to decrease with increasing ion dose until exfoliation is completely suppressed. This behavior is attributed to a competition between the positive effects of hydrogen on exfoliation and the negative effects of ion-induced damage. Experiments were done to isolate the ef...

Damage accumulation during high-dose, O+ implantation in Si

High‐dose O+ implantation of Si between 450 and 500 keV is investigated to better understand the mechanisms responsible for ion‐induced growth of damage, especially in the top Si layer ahead of the region where a buried oxide forms. Two distinct states are identified in this Si layer over an extended range of fluence (≥1018 cm−2): a low‐density defect state and a high‐density one. These states are observed at all irradiation temperatures, including ambient temperature. The transition between the states is rather abrupt with the onset at a high fluence, which decreases with decreasing temperature. The existence of the low‐density state offers a possibility of forming dislocation‐free silicon‐on‐insulator wafers, even for ambient temperature irradiations. A processing method for achieving such wafers is discussed.

Complete surface exfoliation of 4H–SiC by H+- and Si+-coimplantation

Implantation of 4H–SiC with 1H+ and 28Si+ ions followed by annealing is shown to result in complete ejection or exfoliation of the implanted layer. This is in contrast to H+-only implantation where only partial exfoliation of the surface can be achieved. The mechanisms of this process and its dependence on implantation conditions are discussed. It is shown that amorphization of the surface region during Si+ irradiation is a necessary condition to produce this effect, and that it depends critically upon the thickness of the amorphous layer. Stress, induced by bulk recrystallization of the amorphized layer, acts as an additional driving force for H+ induced exfoliation causing the surface layer to separate completely at a depth near the end-of-range of the H+ ions. The morphologies of the newly exposed surfaces are studied by profilometry measurements and atomic force microscopy.

Cryogenic Etching of Silicon: An Alternative Method for Fabrication of Vertical Microcantilever Master Molds

This paper examines the use of deep reactive ion etching of silicon with fluorine high-density plasmas at cryogenic temperatures to produce silicon master molds for vertical microcantilever arrays used for controlling substrate stiffness for culturing living cells. The resultant profiles achieved depend on the rate of deposition and etching of an SiOxFy polymer, which serves as a passivation layer on the sidewalls of the etched structures in relation to areas that have not been passivated with the polymer. We look at how optimal tuning of two parameters, the O2 flow rate and the capacitively coupled plasma power, determine the etch profile. All other pertinent parameters are kept constant. We examine the etch profiles produced using electron-beam resist as the main etch mask, with holes having diameters of 750 nm, 1 ¿m , and 2 ¿m.

Technique to suppress dislocation formation during high-dose oxygen implantation of Si

Damage accumulation during high‐dose oxygen implantation of Si to form a silicon‐on‐insulator material can deleteriously affect the quality of the material. In particular, dislocations formed in the superficial silicon layer are difficult to anneal, requiring temperatures near the melting point of Si to reduce their density to acceptable levels. A technique to suppress the formation of these dislocations during irradiation is presented. The success of this technique lies in its ability to interact with vacancy‐type defects within the superficial layer whose accumulation precedes dislocation formation. A Si+ self‐ion beam is used as a spatially specific tool to introduce Si atoms into the vicinity of these precursor defects prior to the onset of dislocation growth. The interaction of this beam with the precursor defects is shown to be effective in suppressing dislocation formation during subsequent O+ implantation.

Control of diffusion of implanted boron in preamorphized Si: Elimination of interstitial defects at the amorphous-crystal interface

Transient-enhanced diffusion (TED) during thermal annealing of ion-implanted B in Si is well established and attributed to the ion-induced, excess interstitials. On the other hand, the mechanism to account for TED of B in preamorphized (PA) Si remains unclear. Enhanced diffusion of the B persists in regrown layers even though the ion-induced interstitial defects responsible for TED in B+-only implanted Si are eliminated following regrowth. To test the hypothesis that TED in PA Si results from the “excess” interstitial-type defects below the amorphous-crystalline (a-c) interface, a buried PA layer has been recrystallized from the surface inward to the SiO2 interface of silicon-on-insulator material to eliminate all possible sources of excess interstitials. The effect on B diffusion and the role of the residual interstitial-type defects will be discussed.

Photonic Crystal Slab and Waveguide Design for Biological Detection

In this work we have designed, fabricated, and tested a photonic crystal slab (PCS) with a line defect waveguide for the detection and identification of pathogenic DNA. A PCS is constructed by fabricating a material with 2-dimensional dielectric periodicity sandwiched between two semi-infinite cladding regions of lower effective index [1]. In order to uniquely identify pathogens critical to medical and homeland defense applications, the PCS was functionalized with a single stranded probe molecule providing highly specific binding for the target DNA. Integrated microfluidic channels provide delivery of the pathogen DNA resulting in hybridization and binding in the PCS holes. The binding event changes the refractive index of the PCS which results in a measurable change in the transmitted power. We will discuss design parameters and the suite of modeling tools used to optimize the PCS, defect waveguide, and coupling devices. An overview of the fabrication methods and tools will be provided and we will also report preliminary experimental results.

Design Considerations for Ag Ion Implanted Waveguides in Glass

The material properties and their impact upon design and structure of planar and channel waveguides is presented. Comparison to other silver ion exchange techniques will be discussed.