Sang-Soo Choi

Electrically controllable long-period liquid crystal fiber gratings

An electrically controllable fiber-optic filter for broad-band rejection has been demonstrated by periodically poling a liquid-crystalline core in a hollow-core fiber by means of an external long-period-combed electrode. The periodically poled liquid-crystalline core in a period of 483 /spl mu/m couples the fundamental core mode to the leaky cladding modes. A maximum loss dip of approximately 15-nm bandwidth and 6 dB band rejection has been obtained for a nonpolarized light with a combed 250-V external voltage modulation. The experimental result matches well with a numerical expectation, which has been analyzed by the discretized coupled-mode theory for an anisotropic perturbation.

Measurement method for profiling the residual stress of an optical fiber: detailed analysis of off-focusing and beam-deflection effects

The effects of off-focusing and beam deflection on polarimetric stress measurements of optical fibers are investigated. A simple method for reducing the distortion of the phase retardation caused by unwanted beam deflections in residual stress measurement is introduced. The method is examined numerically by ray-tracing techniques and experimentally by use of hollow silica fibers into which various index-matching liquids have been inserted. An autofocusing technique is introduced. The error in stress measurement reproducibility was determined to be less than 4%. We tested the absolute error in measured stress by applying incremental external tension and determined that it is less than 0.464 MPa.

Macrobend sensor via the use of a hollow-core splice fiber: theory and experiments

The characteristics of the use of a hollow-core splice scheme for macrobend measurements are discussed both theoretically and experimentally. A perturbation theory for the modes of a bent hollow-core fiber and its loss characterization are developed so as to better understand the characteristics of the scheme. The maximal detection range of fabricated sensors with the proposed scheme is experimentally determined to be as large as a few hundred millimeters relative to the radius of curvature. In addition, the numerical estimation of the loss characteristics using the scheme shows modal trends which are in good agreement with experimental data.

Design of illumination system for ArF excimer laser step-and-scanner

In a lithography tool, illumination uniformity on the wafer surface is important, because the variance of intensity on the wafer surface makes it hard to control the line width of the pattern. An illuminating system for ArF excimer laser step- and-scanner has been designed and assessed. The system showed good illumination homogeneity in both of the reticle and pupil plane with a reasonable light transmission efficiency. The goal of design is the uniformity within plus or minus 1% on the reticle surface. In order to achieve the goal, the output beam of the excimer laser with nonuniform intensity distribution was re-shaped by using a beam expander which is composed of 4 cylindrical lenses, and the zoom lens varying the beam size according to the aperture of flys eye lens integrator. The flys eye lens integrator consists of 208 lenses and generates the good homogeneity in the reticle plane. The effective light sources, i.e. the images made by flys eye lenses, were projected onto the entrance pupil of the combined system of projection and relay lenses. The exposure field at reticle plane is 104 X 20 mm2, and is defined as the image of reticle blinder by the 1x relay optics. The designed illumination system showed good performance by simulation and it will be adequate to the ArF excimer laser step-and-scanner under development.

Ultra-fast acoustically tunable all-fiber polarization controller based on a hollow optical fiber

Local deformation of circular HOF induced by traveling acoustic wave results in a versatile control of SOP by varying frequency and voltage of RF signal. Traces of SOP on Poincare sphere are reported for various frequency and RF voltage ranges.

Electrically controllable liquid crystal fiber gratings

A novel controllable fiber optic filter for broadband rejection is demonstrated by periodically poling liquid crystal core in hollow-core fiber by means of an external long-period-patterned electrode. The theory and experimental results are presented.

Electron Beam Damage in the SiN Membrane of an X-Ray Lithography Mask

The damage caused as a result of exposure to an electron beam of 20 to 50 kV acceleration voltage on the SiN membrane of an X-ray mask has been investigated. It determined that the optical and the mechanical properties of this material are modified and may potentially limit its use as a membrane in an X-ray mask structure for high density memory devices of giga bit dynamic random access memory (DRAM) level. In particular, after exposure to the electron beam of a 50 kV acceleration voltage and a dosage of 900 µC/cm2, the optical transmission of the SiN membrane fell by about 17% in the wavelength of 633 nm and the change of the out of plane distortion(OPD) on the 16 ×16 mm2 membrane was observed. The difference in the mechanical deflection before and after exposure to the electron beam of 20 kV to 50 kV acceleration voltage on the membrane area of 800 ×800 µm2 was about 500 A to 200 A which was measured by the α-step (Tencor 200) with the stylus force of 19.6 dyn.

Performance of small-field 193-nm exposure system

A small field ArF excimer laser based exposure tool has been designed and fabricated for the 193 nm lithography process research and exposure tool development. The projection optics based upon Schwartzchild concept has a specification of 3 mm field diameter, X5 reduction ratio, and 0.5 NA. The exposure tool uses an unnarrowed ArF excimer laser as a light source, and uses a flys eye homogenizer to produce a reticle illumination uniformity of < +/- 5% RMS/pulse. The results of preliminary exposure with PMMA resist coincided with simulation and expectation, and advanced imaging tests carrying out for various resists. In this paper, we report the detailed system parameters and characterization data of the small field ArF excimer laser exposure tool and some of advances in 193 nm lithography that have been achieved with the system.

Optimization of electron-beam lithography for super-low-noise HEMTs

This paper reports on electron beam lithography for super low noise HEMT (high electron mobility transistor)s. For the optimization of the electron beam lithography for the HEMTs, the SPACING (spacing between the footprint and the head patterns) and the dosage for the head and the footprint to define 0.1 micrometers T-shaped gates with wide head at 30 kV acceleration voltage were investigated by experiment and Monte Carlo simulation. We also compared the single exposure method with dose split method. The footprint of 1 pixel line is to be 0.1 micrometers and the head size to be larger that 1 micrometers when the dosage of the footprint is 700 (mu) C/cm2, the SPACING is 3 pixels and the head dosage is 60 (mu) C/cm2. Using this optimized technique, a T-shaped gate of 0.1 micrometers level with a high ratio of gate head size to footprint, larger than 10, was obtained, and a AlGaAs/InGaAs/GaAs pseudomorphic HEMT with 0.13 micrometers T-shaped gate was successfully fabricated. The HEMT device exhibited very low noise figures of 0.34 dB and 0.49 dB at 12 GHz and 18 GHz, respectively.

Hollow core fibers and their applications in optical communications

During the last two decades, varieties of optical fibers have been developed for optical communications and sensors. Conventional single mode fibers are massively deployed in optical networks, and fibers to compete the chromatic dispersion have been added in the portfolio of transmission fibers. Furthermore specialty fibers for optical devices such as rare earth doped fibers, photosensitive fibers, attenuation fibers, as well as polarization maintaining fibers have been developed and put into commercialization. All of these fibers, however, do have a common structure, a solid core with a high refractive index and SiO2 cladding. Recently, there have been simultaneous attempts to radically modify the conventional fiber structure by introducing air holes in the waveguides. Holy fibers[1], Omniguide fibers[2], and hollow core fibers[3] have proposed unconventional guiding structures and subsequently demonstrated new applications. Hollow core fiber (HCF), composed of central air hole, GeO2-SiO2 ring core, and SiO2 cladding, has shown excellent compatibility with conventional fibers due to its ability to transform the mode adiabatically, which could provide versatile applications. For example, HCF has been used to guide atom providing optical funnel.[4] In optical communications, on the other hand, HCF has demonstrated new feasibility to improve differential modal delay in Gigabit Ethernet,[5] to compensate chromatic dispersion for a longer span,[6] and to acousto-optically control the polarization of guided modes.[7]