J. T. Hastings

Fabrication and characterization of narrow-band Bragg-reflection filters in silicon-on-insulator ridge waveguides

We describe the design, fabrication and measurement of an integrated-optical Bragg grating filter, operating at a freespace wavelength of 1543 nm, based upon a silicon-on-insulator (SOI) ridge waveguide. The measured spectral response for a 4-mm long grating has a bandwidth of 15 GHz (0.12 nm), and shows good agreement with theoretical predictions.

Dual-mode surface-plasmon-resonance sensors using angular interrogation.

Surface-plasmon-resonance (SPR) sensors are widely used in biological, chemical, medical, and environmental sensing. SPR sensors supporting two surface-plasmon modes can differentiate surface binding interactions from bulk index changes at a single sensing location. We present a new approach to dual-mode SPR sensing that offers improved differentiation between surface and bulk effects. By using an angular interrogation, both long- and short-range surface plasmons are simultaneously excited at the same location and wavelength but at different angles. Initial experiments indicate that angular interrogation offers at least a factor of 3.6 improvement in surface and bulk cross-sensitivity compared to wavelength-interrogated dual-mode SPR sensors.

Optical waveguides with apodized sidewall gratings via spatial-phase-locked electron-beam lithography

We describe a technique to fabricate Bragg gratings in the sides of optical waveguides using a single lithographic step. This technique is particularly suited to the apodized gratings required for add/drop filters in dense-wavelength-division multiplexing. Apodization minimizes cross talk between channels and improves the filter response. Silicon-on-insulator rib waveguides with both uniform and apodized gratings were fabricated using direct-write spatial-phase-locked electron-beam lithography (SPLEBL). This approach combines SPLEBL’s pattern-placement accuracy with the flexibility of direct-write device prototyping. The resulting grating-based devices exhibited substantially reduced side-lobe levels.

Optimal self-referenced sensing using long- and short- range surface plasmons

Dual-mode surface-plasmon resonance (SPR) sensors use both long- and short- range surface plasmon waves to differentiate surface binding interactions from interfering bulk effects. We have optimized the design of these sensors for minimum surface limit of detection (LOD) using a Cramer-Rao lower bound for spectral shift estimation. Despite trade-offs between resonance width, minimum reflectivity, and sensitivity for the two modes, a range of reasonable design parameters provides nearly optimal performance. Experimental verification using biotin-streptavidin binding as a model system reveals that sensitivity and LOD for dual-mode sensors remains competitive with single-mode sensors while compensating for bulk effects.

Nanometer-level stitching in raster-scanning electron-beam lithography using spatial-phase locking

Pattern-placement inaccuracy is a persistent problem in scanning-electron-beam lithography (SEBL) despite the high-resolution obtained in SEBL systems. Pattern-placement errors stem from a variety of environmental and system variations; however, the fundamental issue is the open-loop nature of the system, i.e., the beam location on the substrate is not monitored during exposure. In contrast, spatial-phase-locked electron-beam lithography (SPLEBL) provides closed-loop control of the beam position by monitoring the signal from a fiducial grid on the substrate. By detecting the spatial phase of the grid signal one can estimate the beam position within a small fraction of the grid period. We have implemented SPLEBL by adding real-time signal processing, feedback control, and raster-scan exposure capability to an inexpensive SEBL system. Using a 246 nm period, electron-transparent fiducial grid that covers the entire substrate, we have exposed patterns that exhibit global placement accuracy with respect to the...

Electron-beam-induced deposition of gold from aqueous solutions

Focused electron-beam-induced deposition (EBID) using bulk liquid precursors is a novel approach to nanofabrication that has shown improvements in purity compared to EBID with gas-phase precursors. Here we report the first EBID of gold using bulk liquid precursors. We study the differences in gold deposited from three different aqueous solutions containing chloroauric acid (HAuCl(4)), sodium tetrachloroaurate (NaAuCl(4)), and the disulfitoaurate complex ([Au(SO(3))(2)](3-)). We also examine the dependence of threshold dose upon precursor concentration and demonstrate high-resolution patterning with a pitch as small as 50 nm. Finally, we show that the purity of gold deposited using these liquid precursors is significantly improved in comparison with deposits from metal-organic gaseous precursors.

Liquid-precursor electron-beam-induced deposition of Pt nanostructures: dose, proximity, resolution.

While electron-beam-induced deposition (EBID) from various gaseous precursors has been known and studied for decades, EBID from bulk liquid precursors is very much in its infancy and the following is only the second report on this technique. Here we present liquid-precursor (LP-)EBID of platinum (Pt) nanostructures from a dilute aqueous solution of chloroplatinic acid (H(2)PtCl(6)). We investigate how the lateral size of Pt nanoparticles (NPs) varies with charge dose, and how already deposited Pt NPs are affected by the subsequent deposition of their neighbors (proximity effect). We also demonstrate LP-EBID of dense arrays of small Pt dots (60 nm pitch, 30 nm diameter) and thin Pt lines (60 nm pitch, 25 nm width), which compare favorably with the typical resolution of resist-based electron-beam lithography.

Spatially selective melting and evaporation of nanosized gold particles

We have developed an atomic force microscope-tip-based concept to pattern metallic nanoparticles on substrates. This new process has the potential to control the assembly of nanometer sized particles by combining their unique optical and thermophysical properties and is a flexible and low energy method of patterning at the nanoscale. The proof of concept is detailed by preliminary experimental work showing selective melting and evaporation of groups of 50 and 100 nm gold spherical particles.

Two-dimensional spatial-phase-locked electron-beam lithography via sparse sampling

We report a new mode of spatial-phase-locked electron-beam lithography based on alignment of each e-beam deflection field to a fiducial grid on the substrate. Before exposing the pattern in a given field, the fiducial grid is sparsely sampled with the electron beam at a subexposure dose. These samples form a two-dimensional moire pattern that is analyzed to calculate field shift, scale, rotation, nonorthogonality, and trapezoidal distortion. Experimental verification of the approach was carried out with a scintillating fiducial grid quenched by interference lithography. Despite a poor signal-to-noise ratio, we achieved sub-beamstep field-stitching and pattern-placement accuracy.

Liquid Phase Electron-Beam-Induced Deposition on Bulk Substrates Using Environmental Scanning Electron Microscopy

The introduction of gases, such as water vapor, into an environmental scanning electron microscope is common practice to assist in the imaging of insulating or biological materials. However, this capability may also be exploited to introduce, or form, liquid phase precursors for electron-beam-induced deposition. In this work, the authors report the deposition of silver (Ag) and copper (Cu) structures using two different cell-less in situ deposition methods--the first involving the in situ hydration of solid precursors and the second involving the insertion of liquid droplets using a capillary style liquid injection system. Critically, the inclusion of surfactants is shown to drastically improve pattern replication without diminishing the purity of the metal deposits. Surfactants are estimated to reduce the droplet contact angle to below ~10°.