Andrew Frauenglass

Nanoheteroepitaxy for the integration of highly mismatched semiconductor materials

We describe an ongoing study of nanoheteroepitaxy (NHE), the use of nanoscale growth-initiation areas for the integration of highly mismatched semiconductor materials. The concept and theory of NHE is briefly described and is followed by a discussion of the design and fabrication by interferometric lithography of practical sample structures that satisfy the requirements of NHE. Results of NHE growth of GaAs-on-Si and GaN-on-Si are described, following the NHE process from nucleation through to coalescence. Micro-Raman measurements indicate that the strain in partially coalesced NHE GaN-on-Si films is <0.1 GPa.

Defect reduction mechanisms in the nanoheteroepitaxy of GaN on SiC

This article describes defect reduction mechanisms that are active during the growth of GaN by nanoheteroepitaxy on (0001) 6H SiC. Nanoheteroepitaxial (NHE) and planar GaN epitaxial films were grown and compared using transmission electron microscopy, photoluminescence, x-ray diffraction, and time resolved photoluminescence. It was found that in addition to the previously reported defect reduction mechanism that results from the high compliance of nanoscale nuclei, other independent defect reduction mechanisms are also active during NHE including: (i) filtering of substrate defects, (ii) improved coalescence at the nanoscale, and (iii) defect termination at local free surfaces. Also, it was found that the biaxial strain in the GaN film could be significantly reduced by using a “grouped” NHE pattern geometry. Time resolved photoluminescence measurements on NHE GaN samples with this geometry showed a more than tenfold increase in carrier lifetime compared to GaN grown on planar SiC.

Large second-harmonic signal in thermally poled lead glass-silica waveguides

Thermal poling of silica-lead glass-silica waveguides formed by laser ablation of lead glass and e-beam evaporation of the silica cladding is reported. A large nonlinearity localized within the Pb-glass layer was found by scanning the probe laser beam across an angle-polished sample, and a peak second-order nonlinear susceptibility χ(2) as high as 15 pm/V was achieved in the Pb-glass layer. A simple theoretical model based on charge transport in the different materials during poling is proposed to explain complex χ(2) profile. The large third-order nonlinearity of lead glass plays a key role in the generation of the large second-harmonic signal.

Deep-UV immersion interferometric lithography

Liquid immersion lithography (LIL) can extend the resolution of optical lithography well beyond today’s capabilities. The half-pitch limit is given by the well-known formula P=λ/(4/NA), where λ is the optical wavelength and NA=nsin(θ) is the numerical aperture of the exposure device with n the refractive index of the exposure medium. Through the use of exposure media such as purified water (n of 1.44 at 193 nm), it is possible to reduce minimum pitches 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 on a lithography processes. This paper will address issues con-cerning resist chemistry and 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 im-mersion. 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.

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...

Moire interferometric alignment and overlay techniques

Moire alignment and overlay measurement techniques with nm-scale precision are demonstrated. Using 0.47-micrometers pitch gratings, a 1-nm alignment resolution is demonstrated. A novel double-period moire grating is used to provide both coarse (approximately 10 micrometers ) and fine (approximately 1 micrometers ) capture ranges for integration with existing stage positioning systems. A new diffraction-order interferometry technique for nm-precision remote overlay readout, with potential application to latent image structures immediately after exposure, is demonstrated.

Infrared transmission resonances in double-layered, complementary-structure metallic gratings

A double-layered metallic grating (metal-dielectric-metal) with a complementary capacitive (isolated discs) / inductive (connected film with apertures) structure exhibits multiple infrared transmission resonances peaks with up to 70% at wavelength ranges corresponding to local modes for geometric dimensions less than a wavelength. The period, dielectric thickness, refractive index and unit cell size of the periodic structure modulate the local mode positions and amplitudes. The electromagnetic field distribution and energy flow in the structure explain the relation of transmission resonance, local modes, and distributed surface plasma wave modes.

Transport of high‐current electron beams in a racetrack betatron

This paper reports theoretical and experimental work on the transport of space‐charge‐dominated relativistic electron beams in a racetrack betatron with cusp focusing. High‐efficiency propagation of 550‐keV, 280‐A beams through a complex optical system consisting of two 180° arcs with 36 focusing cells and a straight section was observed. More than 90% of the injected beam was captured and transported through the 7‐m system. The experiments resolved several recent problems of beam injection and trapping in the machine and showed that electron beam transport at the space‐charge limit is possible in a curved cusp array despite strong axial variations of focusing forces. We encountered no problems with the transitions between curved and straight focusing sections. The experiments demonstrated good beam containment in a cusp array with mixed focusing cell geometries as long as the axially averaged focusing force was the same for all cells. The ability to use long cells is a critical feasibility issue for othe...

244-nm Imaging interferometric lithography

Imaging interferometric lithography, combining off-axis illumination, multiple exposures covering different regions of spatial frequency space, and pupil plane filters to ensure uniform frequency-space coverage, is a relatively new imaging concept that provides an approach to accessing the fundamental, linear-systems-resolution limits of optics. With an air medium between the lens and the wafer, the highest spatial frequency available with 244-nm exposure tool with a numerical aperture of 0.9 corresponds to a half-pitch of 68-nm. Allowing for ∼10% subbands above this central frequency, this suggests that ∼75-nm half-pitch patterns should be accessible. A 22× reduction imaging interferometric lithography testbed demonstration of printing a non-periodic (arbitrary) 86-nm half-pitch pattern is reported. This result was achieved with a simple chrome-on-glass mask without the use of any mask-based resolution-enhancement techniques such as phase-shift or optical proximity correction. Scaling this result to a 19...