Dennis E. Hardy

Outlook for 157-nm resist design

We have measured the transparencies of a number of a candidate resist materials for 157 nm, with an emphasis on determining which chemical platforms would allow resist to be used at maximum thicknesses while meeting requirements for optical density. Assuming an ideal resist optical density of 0.4, our findings show that all existing commercially available resists would need to be < 90 nm thick, whereas specialized hydrocarbon resists could be made approximately 120 nm thick, and new resists based on hydrofluorocarbons, siloxanes, and/or silsesquioxanes could be engineered to be used in thicknesses approaching 200 nm. We also assess the tradeoff between these thicknesses and what current information exists regarding defects as a function of resist thickness.

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.

High-index immersion lithography with second-generation immersion fluids to enable numerical aperatures of 1.55 for cost effective 32-nm half pitches

To identify the most practical and cost-effective technology after water immersion lithography (Gen1) for sub-45 nm half pitches, the semiconductor industry continues to debate the relative merits of water double patterning (feasible, but high cost of ownership), EUV (difficulties with timing and infrastructure issues) and high index immersion lithography (single-exposure optical lithography, needing a suitable high index last lens element [HILLE]). With good progress on the HILLE, high index immersion with numerical apertures of 1.55 or above now seems possible. We continue our work on delivering a commercially-viable high index immersion fluid (Gen2). We have optimized several fluids to meet the required refractive index and absorbance specifications at 193 nm. We are also continuing to examine other property/process requirements relevant to commercial use, such as fluid radiation durability, last lens element contamination and cleaning, resist interactions and profile effects, and particle contamination and prevention. These studies show that both fluid handling issues, as well as active fluid recycling, must be well understood and carefully managed to maintain optimum fluid properties. Low-absorbing third generation immersion fluids, with refractive indices above 1.7 (Gen3), would further expand the resolution of singleexposure 193 nm lithography to below 32 nm half pitch.

Measurement of trace explosive residues in a surrogate operational environment: implications for tactical use of chemical sensing in C-IED operations

A campaign to measure the amount of trace explosive residues in an operational military environment was conducted on May 27–31, 2007, at the National Training Center at Fort Irwin, CA, USA. The objectives of this campaign were to develop the methods needed to collect and analyze samples from tactical military settings, to use the data obtained to determine what the trace explosive signatures suggest about the potential capabilities of chemical-based means to detect IEDs, and, finally, to present a framework whereby a sound understanding of the signature science can be used to guide development of new sensing technologies and sensor concepts of operation. Through our use of combined background and threat signature data, we have performed statistical analyses to estimate upper limits of notional sensor performance that is limited only by the spatial correlation of the signature chemicals to the threats of interest.

Experimental measurements of diffraction for periodic patterns by 193-nm polarized radiation compared to rigorous EMF simulations

Polarization dependent diffraction efficiencies in transmission through gratings on specially designed masks with pitch comparable to the wavelength were measured using an angle-resolved scatterometry apparatus with a 193 nm excimer source. Four masks - two binary, one alternating and one attenuated phase shift mask - were included in the experimental measurements. The validity of models used in present commercially available simulation packages and additional polarization effects were evaluated against the experimental scattering efficiencies.

Long-term 193-nm laser irradiation of thin-film-coated CaF2 in the presence of H2O

The final projection lens element in a 193-nm immersion-based lithographic tool will be in direct contact with water during irradiation. Thus, any lifetime considerations for the lens must include durability data of lens materials and thin films in a water ambient. We have previously shown that uncoated CaF2 is attacked by water in a matter of hours, as manifested by a substantial increase in AFM-measured surface roughness.1 Thus, CaF2 lenses must be protected, possibly by a thin film, and the coatings tested for laser durability in water. To address the above lifetime concerns, we have constructed a marathon laser-irradiation system for testing thin film exposure to water under long-term laser irradiation. Coated substrates are loaded into a custom water cell, made of stainless steel and Teflon parts. Ultrapure water is delivered from a water treatment testbed that includes particle filtration, deionization and degassing stages. In-situ metrology includes 193-nm laser ratiometry, UV spectrophotometry and spectroscopic ellipsometry, all with spatial profiling capabilities. In-situ results are coupled with off-line microscopy, AFM measurements and spatial surface mapping with spectroscopic ellipsometry at multiple incidence angles. A variety of laser-induced changes have been observed, from complete adhesion loss of protective coatings to more subtle changes, such as laser-induced index changes of the thin films or surface roughening. Implications of the study on expected lifetimes of the protective coatings in the system will be discussed.

Optical materials and coatings at 157 nm

We update previously reported results on the absorption of optical materials and coatings for use in 157 nm based optical projection system. New results include the transmissions spectrum of a modified from of fused silica with suitable initial transmission for use as a mask substrate. We also report on a more systematic study of the effects of surface contaminants on optical components at 157 nm. We have modified our vacuum spectrometer to allow in- situ cleaning to enable a closer examination of purging requirements and cleaning procedures.

Angle-resolved scattering measurements of polished surfaces and optical coatings at 157 nm

An angle-resolved scattering detection system has been designed and implemented for use at 157 nm. This tool will enable the optimization of polishing and thin-film deposition, whith an eye towards minimizing small-angle scatter in projection lithography tools. In this test-bed, scattered rays can be collected to 4° from the directional ray of the specularly transmitted beam (corresponding to spatial wavelengths of surface roughness below 2 μm) over a dynamic range of 7 orders of magnitude, and to 0.5° with a dynamic range of 5 orders of magnitude. The angular scattering distributions in CaF2 samples and antireflective coatings are compared. From these results, the impact of scattering on image performance in exposure tools at 157 nm is estimated.

Contamination rates of optical surfaces at 157 nm in the presence of hydrocarbon impurities

Photodeposition rates for ten hydrocarbon species have been measured on CaF2 substrates under 157-nm irradiation in the presence of ppm scale levels of oxygen. The species are representative of hydrocarbon based compounds observed in outgassing studies of common build materials used in 157-nm based lithographic systems. Photodeposition rates have also been measured for a subset of the hydrocarbon species on a MgF2 thin film, six anti-reflective dielectric stacks, and fluorine doped fused silica for comparison with the results on CaF2 substrates. Two contamination processes are observed. One is the formation of an equilibrium layer on the surfaces. The other is a quasi-permanent contamination which is most pronounced at elevated levels of contaminant.

Contamination rates of optical surfaces at 157 nm: impurities outgassed from construction materials and from photoresists

Photo-induced contamination rates on 157-nm optical surfaces have been studied in controlled experiments with contaminants containing fluorocarbon, sulfur and iodine. The compounds investigated represent species generated in controlled outgassing studies of common construction materials and photoresists used in 157 nm steppers. No photocontamination was measured for highly fluorinated alkanes and ethers on an anti-reflective coating, at levels exceeding 10 ppm. Photocontamination with sulfur based compounds was similar to the behavior observed with hydrocarbon based derivatives. Sulfur containing residues, even from oxidized precursors, are fully cleanable in oxygen, with cleaning rates scaling proportionally with the level of oxygen. In contrast, at elevated levels of oxygen, non-volatile iodate complexes can form from iodine based contaminants. Sulfonium salts should therefore be considered over iodonium species in photoacid generators in 157 nm photoresists. In addition to studying these new classes of compounds, cleaning rates of hydrocarbon residues in trace levels of water were also studied.