S. R. J. Brueck

Experimental Demonstration of Near-Infrared Negative-Index Metamaterials

Metal-based negative refractive-index materials have been extensively studied in the microwave region. However, negative-index metamaterials have not been realized at near-IR or visible frequencies due to difficulties of fabrication and to the generally poor optical properties of metals at these wavelengths. In this Letter, we report the first fabrication and experimental verification of a transversely structured metal-dielectric-metal multilayer exhibiting a negative refractive index around 2 microm. Both the amplitude and the phase of the transmission and reflection were measured experimentally, and are in good agreement with a rigorous coupled wave analysis.

Optical negative-index bulk metamaterials consisting of 2D perforated metal-dielectric stacks

Numerical simulations of a near-infrared negative-index metamaterial (NIM) slab consisting of multiple layers of perforated metal-dielectric stacks exhibiting a small imaginary part of the index over the wavelength range for negative refraction are presented. A consistent effective index is obtained using both scattering matrix and modal analysis approaches. Backward phase propagation is verified by calculation of fields inside the metamaterial. The NIM figure of merit, [ -Re(n)/Im(n) ], for these structures is improved by ~ 10x compared with previous reports, establishing a new approach to thick, low-loss metamaterials at infrared and optical frequencies.

Four-color laser white illuminant demonstrating high color-rendering quality

Solid-state lighting is currently based on light-emitting diodes (LEDs) and phosphors. Solid-state lighting based on lasers would offer significant advantages including high potential efficiencies at high current densities. Light emitted from lasers, however, has a much narrower spectral linewidth than light emitted from LEDs or phosphors. Therefore it is a common belief that white light produced by a set of lasers of different colors would not be of high enough quality for general illumination. We tested this belief experimentally, and found the opposite to be true. This result paves the way for the use of lasers in solid-state lighting.

Secondary ion mass spectrometry study of space-charge formation in thermally poled fused silica

Applying a dc electric field across a fused silica sample at elevated temperatures followed by cooling the sample with the field applied (thermal poling) leads to a second-order nonlinearity that has been linked to the formation of a space-charge region in bulk glass. The first microscopic information on the extent of the space-charge region and its behavior with poling time is reported using secondary ion mass spectrometry to monitor the distribution of charged impurities. Lithium and sodium ions are observed to form depletion regions. Potassium and sodium ions as well as a hydrogenated species appear to be injected from the surface. The extent of the space-charge region evolves approximately logarithmically with poling time well after the nonlinearity as measured by second-harmonic generation has been established. The evolution of the space charge region can be qualitatively understood by an ion-exchange model that allows interaction of two ionic carriers with vastly different mobilities.

Integration of block copolymer directed assembly with 193 immersion lithography

An integration scheme of block copolymer directed assembly with 193 nm immersion lithography is presented. It is experimentally shown that a thin silicon nitride film can be used as an antireflective coating (ARC). With such an ARC, directed assembly of a block copolymer (BCP) to triple the feature density of a chemical pattern was demonstrated. A high quality of assembly was obtained over a large area, and pattern transfer feasibility was illustrated. The integration of feature density multiplication via directed assembly of a BCP with 193 nm immersion lithography provided a pattern quality that was comparable with existing double patterning techniques, suggesting that the process could be a promising candidate for extending the use of current 193 immersion lithography tools to higher pattern densities.

Morphological evolution and strain relaxation of Ge islands grown on chemically oxidized Si(100) by molecular-beam epitaxy

We have previously demonstrated that high-quality Ge can be grown on Si by the touchdown process, where chemically oxidized Si is exposed to a Ge molecular beam. The causes of strain relaxation in the Ge epilayer were also proposed and discussed. Herein, we present a detailed analysis on the morphological evolution and strain relaxation of nanoscale Ge islands on SiO2-covered Si in order to identify the mechanisms by which the high-quality epilayer forms. During the touchdown, the Ge seeds are anchored to the underlying Si. This immobility of Ge islands gives rise to a unique bimodal size distribution during coarsening. Three events are observed during coalescence: (1) merging of two small (<10nm) islands largely driven by surface diffusion, (2) merging of a small island and a big island (∼50nm), and (3) merging of two big islands. The coalescence of two small islands is characterized by the formation of twins or stacking faults at the two merging fronts. In contrast, no stacking fault or grain boundary r...

Nanoscale spatial phase modulation of GaN on a V-grooved Si Substrate-cubic phase GaN on Si(001) for monolithic integration

Nanoscale spatial phase modulation of GaN grown on a 355-nm period array of V-grooves fabricated in a Si(001) substrate is reported. Orientation-dependent selective nucleation of GaN in metal-organic vapor phase epitaxy begins from the opposing Si{111} sidewalls and rapidly fills each V-groove. At the initial stages of growth, the GaN deposited on the sidewalls has hexagonal phase with the c-axis normal to the Si{111}. As the growth continues, the filling of the V-groove over these misaligned hexagonal phase regions results in a transition to a cubic phase with its principal crystal axes parallel to those of the Si substrate. In a cross-sectional view perpendicular to the grooves, the defected hexagonal phase region and the clean cubic phase region above it form a boundary at the inside of each V-groove which is parallel to the Si{111} sidewalls. The GaN surface is almost planarized for only 75-nm deposition and is parallel to the original [001] plane of the Si substrate. The GaN clearly exhibits nanoscale spatial phase modulation with a periodic separation of hexagonal and cubic crystal structures across the groove direction for 600-nm deposition, implying a possibility of cubic phase GaN on an isolated single V-groove fabricated in a Si(001) substrate for monolithic integration. The structural/optical properties and stress measurements of this phase-modulated GaN grown on a nanoscale faceted Si surface are presented.

Selective growth and associated faceting and lateral overgrowth of GaAs on a nanoscale limited area bounded by a SiO2 mask in molecular beam epitaxy

We report homoepitaxial selective growth of GaAs on a 350 nm period two-dimensional SiO2-patterned substrate by molecular beam epitaxy. Ga atoms are largely desorbed from a SiO2 surface at high growth temperature (≳615u200a°C) when the Ga flux is about 0.1 monolayer/s. Under these conditions, a GaAs epilayer selectively grown in circular openings on GaAs(100) with a diameter of about 120–200 nm bounded by a 40 nm thick SiO2 mask shows faceting over its entire surface for 100 nm thick deposition. Lateral growth associated with faceting over the SiO2 mask in 〈100〉 is observed.

Imaging capabilities of resist in deep ultraviolet liquid immersion interferometric lithography

Liquid immersion lithography (LIL) extends the resolution of optical lithography to meet industry demands into the next decade. Through the use of exposure media such as purified water (n of 1.44 at 193nm), it is possible to reduce minimum pitches compared with traditional air/vacuum exposures media by a factor of as much as 44%—a full technology node. Beyond this simple observation, there is a good deal of work necessary to fully understand the impact of LIL immersion lithography on a lithography processes. This article addresses the impact of water immersion on the imaging capabilities of different resist formulations. All resists were evaluated by imaging dense line-space structures at a 65-nm half-pitch both in air and with water immersion. Studies of dense 65-nm lines made by immersion imaging in HPLC grade water with controlled variations in resist components were performed. Significant differences were observed and will be discussed.

Fabrication of 22nm half-pitch silicon lines by single-exposure self-aligned spatial-frequency doubling

The relentless progression of semiconductor technology to smaller feature sizes will likely soon outstrip the theoretical linear system limits of today’s optical lithography tools (a half-pitch of λ∕4n or 34nm with a 193nm wavelength source and water immersion). We demonstrate a self-aligned process involving only a single lithographic exposure followed by spatial-frequency doubling that results a half-scaling of the original pattern and have achieved a 22nm half-pitch pattern with 193nm water immersion. A lithographic pitch of 89nm was realized with a 193nm ArF-excimer laser source and de-ionized-water immersion interferometric lithography. A self-aligned spatial-frequency doubling technique, taking advantage of the well-known anisotropic etching of silicon by KOH, was used to affect the frequency doubling. A protective layer (metal) was deposited parallel to the (110) direction of a (100) silicon wafer and the sample was immersed in an appropriate KOH solution, resulting in a series of 44.5nm opening wi...