Tip Surface Area Research Articles

Sea of Polymer Pillars Electrical and Optical Chip I/O Interconnections for Gigascale Integration

Optical chip-to-chip communication is a promising technology that can mitigate some of the performance short-comings of electrical interconnections, especially bandwidth. Moreover, future high-performance chips are projected to drain hundreds of amperes of supply current. To this end, it is important to develop a high-density and high-performance integrated electrical and optical chip I/O interconnection technology. We describe sea of polymer pillars (or polymer pins), which enables the simultaneous batch fabrication of electrical and optical I/O interconnections at the wafer-level. The electrical and optical I/O interconnections are designed to be laterally compliant to minimize the stresses on the die's low-k dielectric as well as to maintain optical alignment between the coefficient of thermal expansion (CTE)-mismatched board and die during thermal cycling. We demonstrate the fabrication and mechanical performance of various size and aspect ratio electrical and optical polymer pillars. We also describe methods of fabricating polymer pillars with nonflat tip surface area for optical interconnection.

EFFECTS OF ROOT-ZONE TEMPERATURE ON THE ROOT DEVELOPMENT AND NUTRIENT UPTAKE OF LACTUCA SATIVA L. “PANAMA” GROWN IN AN AEROPONIC SYSTEM IN THE TROPICS

Lactuca sativa L. cv. Panama is a temperate plant, but can be grown in the tropics by only subjecting its roots to 20°C while its aerial portions are exposed to the hot, fluctuating temperatures in the greenhouse. This study showed that in Lactuca sativa L. cv. Panama grown at 20°C root-zone temperature (RZT) in the tropics, the roots were longer with a greater number of root tips and total root surface area, and smaller average root diameter as compared with those of ambient RZT (A-RZT) plants. In plants transferred from 20°C to A-RZT (20°C ⇒ A-RZT) when they were 3 weeks old, rate of increase in root length was decreased as was root tip number and surface area; root thickening was increased. The reverse was observed for plants transferred from A-RZT to 20°C-RZT (A ⇒ 20°C-RZT). Mineral nutrients such as NO3 −, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), copper (Cu), iron (Fe), magnesium (Mg), manganese (Mn), and zinc (Zn) present in the plant shoot and root tissues were also determined. Generally, it was found that 20°C-RZT plants had higher leaf N and P concentrations on the basis of per unit dry weight compared with plants grown at A-RZT. Results also showed that total shoot and root NO3 −, K, Ca, Cu, Fe, Mg, Mn, and Zn accumulation of 20°C-RZT plants were more than A-RZT plants. 20°C ⇒ A-RZT plants suffered from a reduction of total mineral accumulation, and Ar ⇒ 20°C-RZT plants increased in total mineral accumulation. Relationships between RZT, root growth, and mineral composition of the experimental plants are discussed.

Apertureless near-field optical microscopy via local second-harmonic generation.

We describe an apertureless scanning near-field optical microscope (SNOM) based on the local second-harmonic generation enhancement resulting from an electromagnetic interaction between a probe tip and a surface. The imaging mechanisms of such apertureless second-harmonic SNOM are numerically studied. The technique allows one to achieve strongly confined sources of second-harmonic light at the probe tip apex and/or surface area under the tip. First experimental realization of this technique has been carried out using a silver-coated fibre tip as a probe. The experiments reveal a strong influence of the tip-surface interaction as well as polarization of the excitation light on images obtained with apertureless second-harmonic SNOM. The technique can be useful for studying the localized electromagnetic excitations on surfaces as well as for visualization of lateral variations of linear and nonlinear optical properties of surfaces.

Molecular dynamics simulation of single and repeated indentation

Atomic-scale material responses of dynamic metal-like substrates due to single and repeated indentation by metal-like or covalent rigid tips are interpreted in light of three-dimensional molecular dynamics simulation results. Single-indentation results for a face-centered-cubic metal substrate indented by a relatively blunt tip of the same material and a sharper pyramidal tip of a covalent material are compared to elucidate the effects of interfacial atomic potential and tip shape on the deformation behavior. Force hysteresis occurs due to inelastic deformation and heating of the substrate. The abrupt decrease of the repulsive force during penetration is attributed to permanent deformation. The formation of a connective neck during unloading observed only in the indentations involving the metal tip is associated with the stronger interatomic forces and larger tip surface area in these simulations. The rapid decrease of the attractive force during retraction of the tip is associated with the reconstruction of the elongated neck. Results for different metallic substrates repeatedly indented by a rigid covalent tip up to a fixed maximum depth or maximum normal force are presented in order to reveal the evolution of deformation and heating in the substrate with indentation cycles. Repeated indentation gives rise to behaviors resembling cyclic hardening and softening observed at the macroscale.

Flow injection system for the scanning tunneling microscope

A flow injection scanning tunneling microscopy (FISTM) system is described and characterized which permits observation of surface chemical processes. This modification to a standard scanning tunneling microscope consists of a peristaltic pump attached via a manual HPLC-type flow injector to a constant-volume sample cell. Images recorded as fluids continuously exchange in the cell are reported. Evidence supports that analytes are transported into the tip region through a stagnant fluid domain by diffusion from the adjacent, circulating bulk solution. Electrochemical measurements of the exposed tip surface area and rates of diffusion of analytes are presented and interpreted. Erosion of contaminants on a gold surface exposed to hydrogen peroxide was observed using FISTM, and these results are presented.

Field emission characteristics of diamond coated silicon field emitters

Single crystal silicon field emitters have been modified by surface deposition of diamond using bias-enhanced microwave plasma chemical vapor deposition. Polycrystalline diamond with a high nucleation density (1010/cm2) and small grain size (<20 nm) was achieved on silicon field emitters. Field emission from these diamond coated emitters exhibited significant enhancement both in total emission current and stability compared to pure silicon emitters. A large effective emitting area comparable to the tip surface area was obtained from a Fowler–Nordheim analysis. The effective work function of the polycrystalline diamond coated emitter surface was found to be larger than that of a pure silicon emitter surface.

Electron emission from diamond coated silicon field emitters

Polycrystalline diamond thin films have been formed on single crystal silicon field emitters using bias-enhanced nucleation in a microwave plasma chemical vapor deposition system. A diamond nucleation density greater than 1010/cm2 with small grain sizes (<25 nm) was achieved on the surfaces of silicon emitters with nanometer scale curvature. Field emission from these diamond coated silicon emitters exhibited significant enhancement compared to the pure Si emitters both in total emission current and stability. Using a Fowler–Nordheim analysis a very large effective emitting area of nearly 10−11 cm2 was obtained from the diamond coated Si emitters compared to that of uncoated Si emitters (10−16 cm2). This area was found to be comparable to the entire tip surface area.

Transscleral Iontophoresis of Foscarnet

Current local treatments of cytomegalovirus retinopathy may result in serious intraocular complications. Using an animal model, we investigated transscleral iontophoresis as a technique for delivery of foscarnet to the vitreous. Using a probe tip surface area of 0.19 mm2, a current of 1 mA, and a duration of ten minutes, transscleral iontophoresis of 0.5 ml of a 24-mg/ml foscarnet solution was administered to 72 normal rabbits. Vitreous aspiration was performed at 12 intervals (15 minutes, 30 minutes, and one, two, four, eight, 16, 24, 32, 40, 48, and 60 hours) after iontophoresis, and samples were analyzed by high-performance liquid chromatography to determine the vitreous pharmacokinetics of foscarnet. A peak foscarnet concentration of 200 +/- 31 microM (mean +/- standard deviation) was attained four hours after iontophoresis and was well below the concentration reported to cause retinal toxicity. Therapeutic levels were maintained until 60 hours after iontophoresis. The elimination half-life was approximately 24 hours. No toxic effects to anterior chamber structures were observed by biomicroscopy. Transscleral iontophoresis of foscarnet may provide an effective and safe technique for local treatment of cytomegalovirus retinopathy in patients with acquired immunodeficiency syndrome.

Iontophoresis of Methylene Blue for Gonioscopic Pulsed Dye Laser Sclerostomy

Iontophoresis of methylene blue using pipette tip probes is an effective means of scleral staining in preparation for gonioscopic pulsed dye laser sclerostomy. The effects of the surface area of the pipette tip and of the duration of iontophoresis on the concentration and distribution of stain were investigated in rabbit eyes. Iontophoresis of 1% methylene blue was done using currents from 0.2 to 2.0 mAmp, pipette tip probes with surface areas from 0.02 to 7.1 mm2, and durations of iontophoresis from 0.5 to 10 minutes. Optimum iontophoresis parameters for delivering methylene blue to the eye were a current of 0.4 mA, a probe tip surface area of 0.19 mm2, and a duration of five minutes. The maximal scleral stain concentration, approximately 0.4%, was achieved using these optimal parameters. Microscopic analysis of frozen tissue sections demonstrated the stain to have penetrated the full thickness of the sclera after five minutes. Tissue samples were analyzed for dye 0.5, 2, 5, 12, and 24 hours after iontophoresis was completed. Decreased dye concentration by over 50% within 2 hours and complete disappearance of dye within 24 hours were demonstrated. Methylene blue concentration thresholds for laser ablation were also examined; the threshold of ablation of methylene blue was 0.0625% when an energy level of 75 mJ was used. We propose that iontophoresis of methylene blue is a useful adjunct to pulsed dye laser sclerostomy formation.

Iontophoresis of Reactive Black 5 for Pulsed Dye Laser Sclerostomy

Ab interno pulsed dye laser sclerostomy uses a gonioscopic approach to form a limbal fistula for the treatment of glaucoma. This procedure requires a full thickness penetration of stain in sclera for adequate absorption of the visible light energy. Iontophoresis is a technique using an electrical current to noninvasively deliver Reactive Black 5 (RB5) stain into sclera. This project determined the stability of RB5 stain as well as the optimal parameters for iontophoresis (probe tip surface area, current, and duration) in a rabbit model. RB5 stain was stable over time (72 hr) as well as after exposure to extreme heat (120 degrees C), scleral constituents (namely collagen), high concentrations of oxidants (1.5% H2O2), and laser light energy. Ideal parameters for iontophoresis included a probe tip surface area between 0.1 and 0.7 mm2, a current of 0.5 mA, and a duration of 5 min. The maximum concentration of RB5 stain achieved in sclera was 0.15%. The threshold of ablation for RB5 using an energy of 250 mJ was 0.001%. Iontophoresis can effectively deliver RB5 stain into sclera and may be a viable adjunct to ab interno pulsed dye laser sclerostomy procedures in the eye.

Polypyrrole nanoplating on HOPG utilizing an STM tip (biosensor fabrication)

The authors describe the plating of polypyrrole utilizing STM tips as the plating probe. Pyrrole is oxidized in electrolyte solution to form a polymer that can switch between conducting and insulating states. The combination of small effective tip surface area, small tunneling gap and a short applied-pulse duration results in nanoscale size features of polypyrrole plated on highly oriented pyrolytic graphite. It is shown that tip-induced deposition can proceed only above a certain threshold pulse potential value. Polypyrrole was deposited from two types of electrolyte solution: one containing supporting electrolyte and the monomer and the other containing, in addition, 1000 units ml-1 of the enzyme glucose oxidase. The reason for studying the deposition in the solution containing the enzyme is to simulate the conditions of fabricating an enzyme-immobilized biosensor. The results presented for pyrrole/glucose oxidase deposit agree with earlier macroscale data for this biosensor.

Behaviorally measured refractory periods are lengthened by reducing electrode tip exposure or raising current.

Small tip exposures and high currents allow the stimulation of high-threshold axons. These studies were conducted to measure the effects of current and tip surface area on stimulation parameters required to produce a constant rate of self-stimulation of lateral hypothalamic sites, or of circling when stimulating medial brain stem sites. Thresholds for circling decreased when tip surface area was decreased to 0.01 mm2. Refractory period curves for both circling and self-stimulation were shifted to slightly longer intrapair intervals when tip surface area was decreased or current was increased. This suggests that long refractory period axons make larger contributions when current density is increased. Chronaxies were not lengthened by small tip exposures but were reduced when electrodes were placed far lateral to the circling substrate.

An evaluation of long-term stimulation thresholds by measurement of chronic strength duration curve.

Successful long-term cardiac pacemaker therapy requires both reliable generator output and stable, attainable chronic stimulation thresholds. Strength duration curves, which display cardiac stimulation thresholds as a function of impulse duration, provide needed data to evaluate more thoroughly acute and chronic lead thresholds. We measured chronic strength duration curves at the time of generator replacement in 47 patients with three different types of lead systems of variable geometry and surface area and compared these curves with acute strength duration curves measured at the time of initial implantation in 19 patients. We found that strength duration curves has a characteristic shape; however, chronic strength duration curves were shifted upward and to the right of acute strength duration curves, regardless of electrode tip surface area or geometric design. Our data suggest that variable pulse width exceeding 1 ms may not significantly reduce stimulation thresholds.