Leonard E. Klebanoff

Modeling radiation-induced carbon contamination of extreme ultraviolet optics

A comprehensive model of radiation-induced carbon contamination of extreme ultraviolet (EUV) optics is presented. The mathematical model describes the key processes that contribute to the deposition of a carbon film on a multilayer optic when the optic is exposed to EUV radiation in the presence of residual hydrocarbons. These processes include the transport of residual hydrocarbons to the irradiated area, molecular diffusion across the optic surface, and the subsequent dissociation or “cracking” of the hydrocarbon by both direct EUV ionization and secondary electron excitation. Model predictions of carbon growth are compared to measurements taken on optics exposed to EUV in the presence of residual hydrocarbons. Model estimates of hydrocarbon film growth under various conditions of hydrocarbon partial pressures and EUV power demonstrate the sensitivity of film growth to varying operating conditions. Both the model and experimental data indicate that the predominant cause of hydrocarbon dissociation is bo...

EUV Engineering Test Stand

The Engineering Test Stand (ETS) is an EUV laboratory lithography tool. The purpose of the ETS is to demonstrate EUV full-field imaging and provide data required to support production-tool development. The ETS is configured to separate the imaging system and stages from the illumination system. Environmental conditions can be controlled independently in the two modules to maximize EUV throughput and environmental control. A source of 13.4 nm radiation is provided by a laser plasma source in which a YAG laser beam is focused onto a xenon-cluster target. A condenser system, comprised of multilayer-coated mirrors and grazing-incidence mirrors, collects the EUV radiation and directs it onto a reflecting reticle. A four-mirror, ring-field optical system, having a numerical aperture of 0.1, projects a 4x-reduction image onto the wafer plane. This design corresponds to a resolution of 70 nm at a k1 of 0.52. The ETS is designed to produce full- field images in step-and-scan mode using vacuum-compatible, one-dimension-long-travel magnetically levitated stages for both reticle and wafer. Reticle protection is incorporated into the ETS design. This paper provides a system overview of the ETS design and specifications.

System integration and performance of the EUV engineering test stand

The Engineering Test Stand (ETS) is a developmental lithography tool designed to demonstrate full-field EUV imaging and provide data for commercial-tool development. In the first phase of integration, currently in progress, the ETS is configured using a developmental projection system, while fabrication of an improved projection system proceeds in parallel. The optics in the second projection system have been fabricated to tighter specifications for improved resolution and reduced flare. The projection system is a 4-mirror, 4x-reduction, ring-field design having a numeral aperture of 0.1, which supports 70 nm resolution at a k1 of 0.52. The illuminator produces 13.4 nm radiation from a laser-produced plasma, directs the radiation onto an arc-shaped field of view, and provides an effective fill factor at the pupil plane of 0.7. The ETS is designed for full-field images in step-and-scan mode using vacuum-compatible, magnetically levitated, scanning stages. This paper describes system performance observed during the first phase of integration, including static resist images of 100 nm isolated and dense features.

First lithographic results from the extreme ultraviolet Engineering Test Stand

The extreme ultraviolet (EUV) Engineering Test Stand (ETS) is a step-and-scan lithography tool that operates at a wavelength of 13.4 nm. It has been developed to demonstrate full-field EUV imaging and acquire system learning for equipment manufacturers to develop commercial tools. The initial integration of the tool is being carried out using a developmental set of projection optics, while a second, higher-quality, projection optics is being assembled and characterized in a parallel effort. We present here the first lithographic results from the ETS, which include both static and scanned resist images of 100 nm dense and isolated features throughout the ring field of the projection optics. Accurate lithographic models have been developed and compared with the experimental results.

Verification studies of thermophoretic protection for extreme ultraviolet masksa)

A “thermophoretic pellicle” has been proposed as an alternative to the traditional organic pellicle as a means of protecting extreme ultraviolet (EUV) lithographic photomasks from particle contamination. The thermophoretic pellicle protects a mask from particles by exploiting the thermophoretic force, which is exerted on a particle by a surrounding gas in which a temperature gradient exists. Two critical requirements of the thermophoretic pellicle are: (1) the mask is kept warmer than its surroundings and (2) the surrounding gas pressure is kept sufficiently high to enable thermophoretic protection. Experiments are presented which verify the viability of thermophoretic protection for EUV masks. In these experiments, wafers are exposed to a monodisperse, polystyrene-latex-sphere aerosol under carefully controlled experimental conditions. Robust thermophoretic protection is observed over a wide range of argon gas pressures (50–1600mTorr or 6.66–213Pa), particle sizes (65–300nm), and temperature gradients (2...

Use of molecular oxygen to reduce EUV-induced carbon contamination of optics

Carbon deposition and removal experiments on Mo/Si multilayer mirror (MLM) samples were performed using extreme ultraviolet (EUV) light on Beamline 12.0.1.2 of the Advanced Light Source, Lawrence Berkeley National Laboratory (LBNL). Carbon (C) was deposited onto Mo/Si multilayer mirror (MLM) samples when hydrocarbon vapors where intentionally introduced into the MLM test chamber in the presence of EUV at 13.44 nm (92.3eV). The carbon deposits so formed were removed by molecular oxygen + EUV. The MLM reflectivities and photoemission were measured in-situ during these carbon deposition and cleaning procedures. Auger Electron Spectroscopy (AES) sputter-through profiling of the samples was performed after experimental runs to help determine C layer thickness and the near-surface compositional-depth profiles of all samples studied. EUV powers were varied from ~0.2mW/mm2 to 3mW/mm2(at 13.44 nm) during both deposition and cleaning experiments and the oxygen pressure ranged from ~5x10-5 to 5x10-4 Torr during the cleaning experiments. C deposition rates as high as ~8nm/hr were observed, while cleaning rates as high as ~5nm/hr could be achieved when the highest oxygen pressure were used. A limited set of experiments involving intentional oxygen-only exposure of the MLM samples showed that slow oxidation of the MLM surface could occur.

Atomic hydrogen cleaning of EUV multilayer optics

Recent studies have been conducted to investigate the use of atomic hydrogen as an in-situ contamination removal method for EUV optics. In these experiments, a commercial source was used to produce atomic hydrogen by thermal dissociation of molecular hydrogen using a hot filament. Samples for these experiments consisted of silicon wafers coated with sputtered carbon, Mo/Si optics with EUV-induced carbon, and bare Si-capped and Ru-B4C-capped Mo/Si optics. Samples were exposed to an atomic hydrogen source at a distance of 200 - 500 mm downstream and angles between 0-90° with respect to the source. Carbon removal rates and optic oxidation rates were measured using Auger electron spectroscopy depth profiling. In addition, at-wavelength peak reflectance (13.4 nm) was measured using the EUV reflectometer at the Advanced Light Source. Data from these experiments show carbon removal rates up to 20 Å/hr for sputtered carbon and 40 Å/hr for EUV deposited carbon at a distance of 200 mm downstream. The cleaning rate was also observed to be a strong function of distance and angular position. Experiments have also shown that the carbon etch rate can be increased by a factor of 4 by channeling atomic hydrogen through quartz tubes in order to direct the atomic hydrogen to the optic surface. Atomic hydrogen exposures of bare optic samples show a small risk in reflectivity degradation after extended periods. Extended exposures (up to 20 hours) of bare Si-capped Mo/Si optics show a 1.2% loss (absolute) in reflectivity while the Ru-B4C-capped Mo/Si optics show a loss on the order of 0.5%. In order to investigate the source of this reflectivity degradation, optic samples were exposed to atomic deuterium and analyzed using low energy ion scattering direct recoil spectroscopy to determine any reactions of the hydrogen with the multilayer stack. Overall, the results show that the risk of over-etching with atomic hydrogen is much less than previous studies using RF discharge cleaning while providing cleaning rates suitable for EUV lithography operations.

Radio-frequency discharge cleaning of silicon-capped Mo/Si multilayer extreme ultraviolet optics

Remote oxygen and hydrogen radio-frequency (rf) discharge cleaning experiments have been performed to explore their potential for cleaning carbon-contaminated extreme ultraviolet optics. The samples consisted of silicon wafers coated with 100 A sputtered carbon, as well as bare Mo/Si multilayer mirrors (Si terminated). The samples were exposed for 3 h to rf plasma discharges at 100, 200, and 300 W. The carbon removal and surface oxidation rates were evaluated using sputter through depth profiling Auger spectroscopy. Reflectivity changes and surface roughness measurements were performed using at-wavelength reflectometry (13.4 nm) and atomic force microscopy, respectively. Data show that excited rf O2 consistently removes carbon at a rate approximately six times faster than excited rf H2 for a given discharge power and pressure. rf O2 also induces loss of reflectivity that is related to the growth of SiO2 on the optic surface. rf H2 shows a much lower oxidation rate of the optic surface. In spite of the low...

Radiation-induced protective carbon coating for extreme ultraviolet optics

A technique is described that uses radiation and a gas-phase species to produce a protective carbon coating on extreme ultraviolet (EUV) optics. A specific example is given in which a ∼5 A carbon coating is deposited on EUV Mo/Si optics via coexposure to radiation (EUV photons, electrons) and ethanol vapor. Auger electron spectroscopy, sputter Auger depth profiling, and EUV reflectivity measurements are presented that suggest a carbon coating that is substantially void free and protects the optic from water-induced oxidation at the water partial pressures used in the tests (∼2×10−7 Torr). The coating is also resistant to atmospheric degradation, and to gasification by the combination of electrons and molecular oxygen. The protective coating reduces the relative reflectivity (ΔR/R0) of an optic by ∼0.5%.

Initial results from the EUV engineering test stand

The Engineering Test Stand (ETS) is an EUV lithography tool designed to demonstrate full-field EUV imaging and provide data required to accelerate production-tool development. Early lithographic results and progress on continuing functional upgrades are presented and discussed. In the ETS a source of 13.4 nm radiation is provided by a laser plasma source in which a Nd:YAG laser beam is focused onto a xenon- cluster target. A condenser system, comprised of multilayer-coated and grazing incidence mirrors, collects the EUV radiation and directs it onto a reflecting reticle. The resulting EUV illumination at the reticle and pupil has been measured and meets requirements for acquisition of first images. Tool setup experiments have been completed using a developmental projection system with (lambda) /14 wavefront error (WFE), while the assembly and alignment of the final projection system with (lambda) /24 WFE progresses in parallel. These experiments included identification of best focus at the central field point and characterization of imaging performance in static imaging mode. A small amount of astigmatism was observed and corrected in situ, as is routinely done in advanced optical lithographic tools. Pitch and roll corrections were made to achieve focus throughout the arc-shaped field of view. Scan parameters were identified by printing dense features with varying amounts of magnification and skew correction. Through-focus scanned imaging results, showing 100 nm isolated and dense features, will be presented. Phase 2 implementation goals for the ETS will also be discussed.