June 1, 1999
A combination of leading-edge optical lithographic techniques and novel photoresist materials has enabled researchers at Lucent Technologies-Bell Labs (Murray Hill, NJ) to fabricate a functional electronic device with features as small as 80 nm. The device-a flash-memory cell containing an 80 x 160-nm floating gate-is made in silicon. Extending optical lithography to these smaller feature sizes is a primary goal of researchers around the world because it forestalls the eventual costly switch to

Optical lithography attains 80-nm features

A combination of leading-edge optical lithographic techniques and novel photoresist materials has enabled researchers at Lucent Technologies-Bell Labs (Murray Hill, NJ) to fabricate a functional electronic device with features as small as 80 nm. The device-a flash-memory cell containing an 80 x 160-nm floating gate-is made in silicon. Extending optical lithography to these smaller feature sizes is a primary goal of researchers around the world because it forestalls the eventual costly switch to another form of lithography.

The memory cell was made using 193-nm light, a projection lens with a 0.6 numerical aperture, and a phase-shift mask. The lens pupil illumination fill was 0.3 for the 80-nm feature and 0.7 for the surrounding larger features. Fabrication of the smallest feature was made possible by two materials developed at Bell Labs-a new resist based on cyclo-olefin maleic anhydride chemistry and a light-absorbing material layered between the resist and the silicon wafer. The technology can be put to immediate use in developing communications-related integrated circuits, according to Mark Pinto, chief technical officer. See

Diode-pumped passively modelocked Nd:YAG laser achieves 10-W average power

Researchers at the Swiss Federal Institute of Technology and the Centre Suisse d`Electronique et de Microtechnique-Zurich, together with others from Time-Bandwidth Products Inc. (all Zurich, Switzerland) and Lightwave Electronics (Mountain View, CA), have developed a diode-pumped passively modelocked Nd:YAG laser that emits 10.7 W of average power and 7.8 kW of peak power in a diffraction-limited beam. External frequency doubling in a single pass through a potassium titanyl phosphate (KTP) crystal yields 3.2-W average power at 532 nm; subsequent frequency doubling with beta barium borate (BBO) produces 120 mW of ultraviolet output.

The laser-cavity design, which should be scalable to higher powers, consists of four mirrors, including a semiconductor saturable-absorber mirror (SESAM), a Brewster plate, and a direct-coupled-pump laser head with two 20-W diode bars. Two curved mirrors control the mode radii in the laser head and on the SESAM, which, combined with the relatively small laser mode in the gain medium, promotes stable self-starting modelocking without any Q-switching instabilities. The direct-coupled pump arrangement eliminates problems typically associated with side-pumping. Contact G. J. Spühler at [email protected].

Metal-organic chemical-vapor deposition grows cubic-phase gallium nitride LED

Despite industry doubts that satisfactory metastable cubic-phase gallium nitride (GaN) can be grown on a gallium arsenide (GaAs) substrate, researchers at the National Research Center for Optoelectronic Technology and the Institute of Semiconductors (both Beijing, China) have made a blue-output light-emitting diode (LED) based on this approach. Most existing blue LEDs and laser diodes are based on hexagonal-phase GaN, which is the most stable phase. Metastable cubic-phase GaN does not exist in nature but is predicted to have higher carrier mobility, easier p-type doping, and a narrower energy bandgap.

The team grew the cubic-phase GaN layered structures on silicon-doped GaAs substrates by metal-organic vapor-phase epitaxy. The submicron-size grains were free from threading dislocations or stacking faults with good optical quality. The researchers say the structure is promising both for blue-LED and laser-diode applications, with a cost comparable to other GaAs-based LEDs. If this structure can be perfected, a key benefit would be an improved far-field pattern because the mirrors necessary for lasing can be easily cleaved from a cubic GaN-on-GaAs laser diode. Contact Hui Yang at [email protected].

Ultrafast photoexcitation may produce ultrafast memories

Ultrafast laser pulses have been used to investigate magnetic interactions by researchers at Brown University (Providence, RI) and the IBM Almaden Research Center (San Jose, CA); the work may eventually lead to ultrafast data-storage devices. By using subpicosecond laser pulses to photoexcite the ferromagnetic/antiferromagnetic (FM/AF) interfaces of thin nickel iron/nickel oxygen (NiFe/NiO) bilayer films, the researchers have reversed the polarity in groups of atoms in as little as 100 ps. The fast switching comes from a rapid but large laser-induced modulation in the unidirectional exchange bias field and coherent magnetization rotation in the permalloy film.

Currently the researchers are using the technique to study the basic physics involved in the collective process of flipping the moments of many thousands of atoms simultaneously. Eventually, Arto Nurmikko at Brown and his colleagues at Brown and IBM hope to surpass the practical speed limits of current magnetic-data-storage media, which switch an order of magnitude slower than the photoexcited permalloy. Contact Arto Nurmikko at Arto [email protected].

Separating layers could allow long-wavelength avalanche photodiodes

Photodetectors for wavelengths up to and beyond 2 µm are useful for optical sensing applications. Current detectors based on indium gallium arsenide (InGaAs) exhibit large dark currents and are unsuitable for high-electric-field devices such as avalanche photodiodes (APDs). Now researchers at Princeton University (Princeton, NJ) have developed a technique for making APDs theoretically capable of detection to 2.1 µm.

Materials with high indium content can be made on indium phosphate substrates but are compressively strained, which limits how many strained quantum wells can be grown and, therefore, the quantum efficiency. The Princeton group offset the compressive strain of the InGaAs with another layer of equal but opposite strain. The resulting device had a 2.8-?m-thick absorption region with 100 quantum wells made up of more than 600 epitaxial layers-single-pass, unity-gain quantum efficiency was 65% at 1.9 ?m. Primary dark currents were 5 nA at 33 V with responsivities at 1.54 µm of 45 A/W at 100-nW optical power. The researchers say the cutoff wavelength and response speed could be improved with smaller bandgap barrier layers for strain compensation; the theoretical wavelength limit using their strain compensation technique is 2.1 µm. Contact J. Christopher Dries at [email protected].

X-ray beam binds to curved silicon surface

Researchers at Harvard University (Cambridge, MA) and the Rowland Institute for Science (Cambridge, MA) have used photonic-bandgap effects stemming from the crystalline array of atoms in silicon (Si) to bind a beam of 17.5-keV (0.07-nm) x-rays to the outside surface of a solid silicon waveguide curved in a 15-cm radius. The 2-cm-long waveguide was fabricated in the form of a flat triangle so that a displacement of one corner relative to its clamped opposite side resulted in a constant radius of curvature. In addition to the guided beam, other diffracted beams appeared, some following the silicon lattice planes before exiting the crystal.

The researchers attribute the variety of effects to an interaction between the evanescent tails of x-ray whispering-gallery modes with the underlying crystal lattice. Potential applications include cavities for x-ray lasers, optical elements for x-ray interferometers, lithography, and materials science. The researchers suggest that analogous events at optical wavelengths will occur in artificially prepared photonic crystals. Contact Jene Golovchenko at [email protected].

Ionic self-assembly monolayer process grows nanometer-scale mirrors on optical fiber

Growing mirrors on the ends of optical fibers to create Fabry-Perot interferometers allows researchers to produce sensors. Vapor deposition can grow such devices, but film thickness is difficult to control. Instead, researchers at the Universidad Publica de Navarra (Pamplona, Spain) and Virginia Polytechnic Institute (Blacksburg, VA) have used an ionic self-assembly monolayer process to grow such coatings to a desired thickness.

By dipping an optical fiber alternately into solutions of cationic and anionic polymers, with each pair of treatments creating a bilayer that behaved as a homogeneous optical medium, the researchers were able to make a Fabry-Perot etalon containing up to 120 bilayers before errors crept in. They speculated that conditions in the solution changed over extended periods of processing, causing layers of differing thicknesses to be deposited. The group built up a total of 210 bilayers with an average thickness of 4.75 nm but did not determine the maximum possible number. Contact Yanjing Liu at [email protected].

Gas-phase ultraviolet frequency converter produces 186 nm without phase-matching

Researchers at Stanford University (Palo Alto, CA) demonstrated gas-phase frequency conversion without phase-matching. Using a pulsed atomic-lead vapor-phase vacuum converter, they converted 233 to 186 nm with unity photon-conversion efficiency. Up to 300 µJ of 186-nm output was produced, with a peak power of 25 kW. The researchers believe the technique should allow extension of gas-phase coherent sources to spectral regions that are well beyond the transparency regions of crystalline media.

The technique applies two strong pulsed-laser fields that are two-photon resonant with a Raman transition of lead atoms. A 283-nm probe laser and a 406-nm coupling laser adiabatically drive all the atoms of the ensemble into a maximally coherent superposition of the ground and metastable states. Once there, atoms are decoupled from the applied optical fields. The probe and coupling lasers can thus maintain wave vectors equal to their vacuum values and propagate without loss or distortion. This permits substantial resonant enhancement of the four-wave-mixing nonlinear optical susceptibilities that govern generation of the upconverted radiation. When the atoms are driven at maximal coherence, 100% of the input photons can be upconverted within a single coherence length, obviating the need to phase-match the propagating beams. Contact Andrew J. Merriam at [email protected].

Thin-film transistors approach AMLCD performance needs

Researchers at Seoul National University (Seoul, Korea) have devised a simple method of excimer-laser-induced recrystallization that increases the grain size in polysilicon thin-film transistors (poly-Si TFTs) by a factor of five and may facilitate production of active-matrix liquid-crystal displays (AMLCDs). Laser energy density plays a critical role in the grain size and in-grain defect density obtained through melting of amorphous silicon (a-Si) and subsequent solidification into poly-Si crystals. The process is normally limited by the need to maintain an energy level in the superlateral-growth (SLG) regime that maximizes grain size but is difficult to maintain reliably.

According to a report at the annual symposium of the Society for Information Display (SID `99; May 1999; San Jose, CA), the team has avoided the energy-density limitation by forming silicon nucleation seeds within a 5000-Å-thick aluminum layer, which allowed formation of large poly-Si grains using full-melt laser energy densities that exceeded the critical SLG regime, but that could be reliably maintained. The researchers fabricated TFTs using the new method and observed electrical conductivity and transfer characteristics commensurate with the larger grain size. Contact Jae-Hong Jeon at [email protected].

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