Mordechai Rothschild

Effects of excimer laser irradiation on the transmission, index of refraction, and density of ultraviolet grade fused silica

Radiation‐induced changes in high‐purity fused silica during prolonged irradiation with a pulsed laser at 193 nm have been studied. Radiolytically induced UV absorption bands, an increase in index of refraction, and stress birefringence are observed. The formation mechanisms are analyzed in terms of radiolytic atomic rearrangement of a‐SiO2 initiated by two‐photon absorption. The quantum efficiency for the formation of E’ point defects per pair of absorbed 193 nm photons has been determined to be ∼7.5×10−4; matrix compaction, as high as a few parts in 10−5, is identified as the source of the birefringence and index change. It has been further observed that E’ centers can be photobleached.

Excimer‐laser etching of diamond and hard carbon films by direct writing and optical projection

Excimer‐laser patterning of monocrystalline diamond was performed in the direct writing and in the optical projection modes. The etching mechanism employs combined photochemical/thermal transformation of the initial crystal to graphite followed by sublimation or reaction of the transformed solid. In optical projection, linewidths as narrow as 0.13 μm were etched with patterning possible using single 0.193 μm (wavelength) laser pulses. Etch rates of 2000 A/pulse were achieved. The morphology of the etched features was optimized by introducing gases which reacted chemically with the hot graphitic layer generated in the process of etching. Patterning of diamond‐like carbon thin films was accomplished and the effectiveness of these films as resists for submicrometer‐resolution lithography of semiconductors was demonstrated.

Resolution enhancement of 157 nm lithography by liquid immersion

We present the results of a preliminary feasibility study of liquid immersion lithography at 157 nm. A key enabler has been the identification of a class of commercially available liquids, perfluoropolyethers, with low 157 nm absorbance α157 ∼ 10 cm−1 base 10. With 157 nm index of refraction around 1.36, these liquids could enable lithography at numerical aperture ∼1.25 and thus resolution of 50 nm for k1 = 0.4. We have also performed preliminary studies on the optical, chemical, and physical suitability of these liquids for use in high throughput lithography. We also note that at longer wavelengths, there is a wider selection of transparent immersion liquids. At 193 nm, the most transparent liquid measured, de-ionized water, has α193 = 0.036 cm−1 base 10. Water immersion lithography at 193 nm would enable resolution of 60 nm with k1 = 0.4.

Lithography with 157 nm lasers

Projection photolithography at 157 nm was studied as a possible extension of current 248-nm and planned 193-nm technologies. At 157 nm, lasers are available with ∼8 W average power. Their line width is narrow enough as to enable the use of catadioptric, and maybe all-refractive optics similar to those used at 248 and 193 nm. The practicality of such designs is further enhanced by measurements of calcium fluoride, which show that its absorption is sufficiently small (∼0.004 cm−1) at 157 nm. Binary masks with chromium and chromeless phase shifting masks were fabricated on calcium fluoride as the transparent substrate. Robust photoresists at 157 nm still need to be developed, and they probably will be of the top surface imaging or bilayer type. Indeed, a silylation resist process was shown to have characteristics at 157 nm similar to those at 193 nm. The calcium fluoride based masks were integrated with the silylation process and a home-built, small-field, 0.5-numerical aperture stepper to provide projection...

A Nanoparticle Convective Directed Assembly Process for the Fabrication of Periodic Surface Enhanced Raman Spectroscopy Substrates

Surface enhanced Raman scattering was discovered over 30 years ago, when it was noted that the usually weak molecular Raman scattering cross section was increased by orders of magnitude in the vicinity of metal surfaces. [ 1 , 2 ] Despite the early reports of single-molecule detection, [ 3 ] the promise of the technique as the basis for portable chemical sensors has not been fully realized. The reason for this gap between the science and the engineering of SERS lies in the formidable nanofabrication challenges it poses, which is the need to prepare large numbers of very small yet highly controlled “hot spots” as the sensing device. The need for “hot spots” arises because the SERS enhancement is composed of an electromagnetic effect and a chemical (or resonance) enhancement, with the electromagnetic effect being responsible for the majority of the enhancement. The “hot spots” are the manifestations of this fi eld enhancement, occurring only for select plasmonic materials. [ 4 ] Modeling studies of the electromagnetic effect indicate that dimerized plasmonic metal structures offer a signifi cantly higher electromagnetic fi eld enhancement than isolated structures, with the maximum fi elds occurring in the gap between the structures. [ 5 , 6 ] The fi eld enhancement dependence on the gap size is highly nonlinear: it becomes signifi cant when the gap is less than ∼ 10 nm, and then rises steeply with decreasing gap size. The importance of the small gap size on signal strength has been confi rmed experimentally. [ 7 , 8 ] While the SERS effect may be very high in a localized volume of the order of a few nm 3 , the probability that target analyte molecules adsorb there is very small. [ 9 ] Consequently, practical SERS-based sensors require the engineering of a very large number of such “hot spots” over areas that are at least a few mm 2 . Small interparticle spacing with a large number of metal particles has been realized for SERS experiments performed in nanoparticle solutions, which have been “activated” by electrolyte-induced aggregation. [ 10 ] However, solution-based

Critical issues in 157 nm lithography

Projection lithography at 157 nm is a candidate technology for the 100–70 nm generations, and possibly beyond. It would provide an evolutionary extension to the current primary photolithographic processes and components: excimer lasers, refractive optics, and transmissive masks. This article presents data on the transmission of optical materials at 157 nm, the performance of optical coatings, the issues that must be faced by photomasks, and the considerations related to engineering resists at this wavelength.

22-nm immersion interference lithography

Immersion interference lithography was used to pattern gratings with 22-nm half pitch. This ultrahigh resolution was made possible by using 157-nm light, a sapphire coupling prism with index 2.09, and a 30-nm-thick immersion fluid with index 1.82. The thickness was controlled precisely by spin-casting the fluid rather than through mechanical means. The photoresist was a diluted version of a 193-nm material, which had a 157-nm index of 1.74. An analysis of the trade-off between fluid index, absorption coefficient, gap size and throughput indicated that, among the currently available materials, employing a high-index but absorbing fluid is preferable to using a highly transparent but low-index immersion media.

Periodically poled BaMgF 4 for ultraviolet frequency generation

Ferroelectric domain inversion has been demonstrated in BaMgF(4) . Transparency has been measured to <140nm, and no change in transmission was measured under 157-nm irradiation for >1.1x10(9) shots at 2mJ/cm(2) per pulse. First-order quasi-phase-matched generation of 157 nm is predicted by use of grating periods as long as 1.5mum. This material should permit shorter-wavelength chi((2)) frequency-mixing processes than with any other crystalline material.

Three-dimensional metallodielectric photonic crystals exhibiting resonant infrared stop bands

Using standard microelectronic techniques, we have fabricated arrays of infrared metallodielectric photonic crystals (IR MDPCs) on silicon substrates. The metallic “atoms” are located on a three-dimensional (100)-oriented face-centered-cubic lattice. Resonant stop-band characteristics have been measured with rejection levels of up to 20 dB and widths of up to 83% of the center frequency. We demonstrate structures with stop bands across the midinfrared wavelength range from 2 to 12 μm. Angular studies of the photonic stop bands show an insensitivity to incident angle for some of the structures. The IR MDPC results are compared with measurements made on microwave-scale MDPC structures to help in understanding the infrared results.

Electrical, crystallographic, and optical properties of ArF laser modified diamond surfaces

Pulses of 193 nm radiation from an ArF laser with energies exceeding 0.5 J/cm2 have been shown to modify 40–60 nm thick layers of {100} and {110} oriented diamond surfaces. These layers exhibit highly anisotropic electrical and optical properties which have principal in‐plane axes along the 〈110〉 directions. The minimum resistance is (4–10)×10−4 Ω cm, and minimum in the optical transmittance and maximum in the reflectance occur when the electric field vector of the incident polarized light is aligned along the low resistance direction. Transmission electron microscopy indicates that the modified layer primarily consists of unidentified graphite‐like carbon phases embedded in diamond. The first‐order electron diffraction spots correspond to lattice spacings of 0.123, 0.305, and 0.334 nm. The modified layer is stable at 1800 °C, forms ohmic contacts to type IIb diamond, and supports epitaxial diamond growth.