Future Extreme Ultraviolet Lithography Research Articles

Protective Freely Hanging Films for Projection Lithography Installations in the Extreme UV Range

Variants of the structures of freely hanging films with a high transmittance coefficient at a wave-length of 13.5 nm, which were developed for use in the projection of extreme ultraviolet (EUV) lithography as protective and filtering elements, are considered. Our attention is focused on the most problematic—in terms of manufacturing and the requirements on their characteristics—ultrathin freely hanging films (pelli-cles), which are installed in front of the mask (photomask) in modern EUV scanners and serve to protect the sur-face of the mask from contamination. The main approaches used to manufacture ultrathin freely hanging large-aperture films are compared. A brief review of the research on the development of high transmittance pellicles at a wavelength of 11.2 nm, which may become an operating wavelength for future EUV lithography, is given.

High power light source for future extreme ultraviolet lithography based on energy-recovery linac free-electron laser

The development of ways to increase the intensity of extreme ultraviolet (EUV) light sources for future EUV lithography is important to realize high throughput fine patterning. The energy-recovery linac (ERL) free-electron laser (FEL), which is an accelerator based light source, is a candidate for this. We clarify the design concept of the ERL-FEL for EUV light sources for future lithography, delivery systems of the FEL light to multiscanners, and future development items of the accelerator technologies and a possibility of the beyond EUV.

Cascade and cluster of correlated reactions as causes of stochastic defects in extreme ultraviolet lithography

Background: Stochastic defects are becoming major concern in the future extreme ultraviolet (EUV) lithography as their probability Pd exponentially increases with decreasing feature size and is highly sensitive to variations in process/mask conditions. Photon shot-noise and discrete/probabilistic nature of materials have been blamed as their causes. Aim: We introduce models for relating Pd to photon and resist statistics under various exposures and material conditions and analyze their impact in future EUV lithography. Approach: Three-dimensional reaction distribution is calculated by a fully coupled Monte Carlo simulation including discrete photon, photoelectron scattering, and resist stochastics. Then probability models predict Pd from statistical data extracted from Monte Carlo results. Results: Stochastic defect generation is enhanced by cascade and/or cluster of correlated reactions among nearby polymers/molecules due to secondary electrons (SE)/acid diffusion and SEs generated along scattered photoelectron trajectories. Pd decreases with increasing reaction density, suppressing effective image blur, and introducing quenchers, where reaction density is limited by SE, photoacid generator, and reaction site. Defect probability increases with decreasing target size for the same k1-factor, while strongly dependent on image slope and defocus. Conclusions: Our analyses suggest that applying EUV lithography to smaller target requires careful material choice, extremely precise process control, and further EUV power enhancement.

Investigation of alternative absorbers for extreme ultraviolet mask blanks

Mask three-dimensional (3D) effects in extreme ultraviolet (EUV) lithography critically influence pattern transfer such as horizontal-vertical bias and/or CD shift. Although the mask 3D effects can be reduced using a thin film absorber, the current Ta-based absorber reaches its thin film limit at >60 nm for both binary and phase-shift type usages. To improve mask 3D effects for future EUV lithography production, the absorber material and film stack should be reconsidered. We investigated two types of thin film structure as a binary type absorber. One is a high-k type absorber. We selected nickel which has been proposed as one of the thin absorber solutions. Nickel enables less than 2% peak reflectivity at 30-nm thickness due to its high absorption. The other is a multilayer (ML)-type absorber. This type of absorber cancels out reflection from each interface, and thus low reflectivity is achieved. In the case of Ta/Si ML absorber, ≤2% reflectivity can be expected at the 39-nm film thickness.

Characterization of a laser-plasma extreme-ultraviolet source using a rotating cryogenic Xe target

A laser-plasma source for extreme-ultraviolet (EUV) light that uses a rotating cryogenic solid-state Xe target has been characterized. We focused on parameters at the wavelength of 13.5 nm with 2% bandwidth required for an EUV lithography source and investigated improvements of the conversion efficiency (CE). With the drum rotating, there was an increase in CE and less fast ions compared with the case for the drum at rest. It is considered that the Xe gas on the target surface can produce optimal-scale plasma, and satellite emission lines in Xe plasma effectively increase the EUV intensity, and the ion number is decreased by the gas curtain effect. The dependence of CE on the laser wavelength, laser energy and intensity also studied. As a result, the maximum CE was 0.9% at 13.5 nm with 2% bandwidth under the optimal condition. By continuous irradiation of a Nd:YAG slab laser at a repetition rate of 320 Hz and an average power of 110 W, the target continuously generated EUV light with an average power of 1 W at 13.5 nm with 2% bandwidth. The achieved performances provide valuable information for the design of a future EUV lithography source.

Relevance of mask-roughness-induced printed line-edge roughness in recent and future extreme-ultraviolet lithography tests.

The control of line-edge roughness (LER) of features printed in photoresist poses significant challenges to next-generation lithography techniques such as extreme-ultraviolet (EUV) lithography. Achieving adequately low LER levels requires accurate resist characterization as well as the ability to separate resist effects from other potential contributors to LER. One potentially significant contributor to LER arises from roughness on the mask coupling to speckle in the aerial image and consequently to LER in the printed image. Here I numerically study mask surface roughness and phase roughness to resist LER coupling both as a function of illumination coherence and defocus. Moreover, the potential consequences of this mask effect for recent EUV lithography experiments is studied through direct comparison with experimental through-focus printing data collected at a variety of coherence settings. Finally, the effect that mask roughness will play in upcoming 0.3-numerical-aperture resist testing is considered.

Theoretical efficiency analysis of a condenser-embedded grating-based spectral purity filter for EUV lithography

Being based on reflective optics, extreme ultraviolet (EUV) lithography systems are, in principle, relatively immune to chromatic errors. However, illumination bandwidth control is still required for EUV lithography. For example, appreciable amounts of UV power, combined with resist sensitivity to this wavelength band, would decrease image contrast. Also, appreciable amounts of IR power would place unacceptable thermal loads on the projection optics. A practical method for spectral filtering, widely used in short-wavelength synchrotron applications, is the grating monochromator. Here we present the theoretical performance analysis of a grating-based spectral purity filter integrated into an EUV lithography condenser system. Although the specific examples presented here are geared towards a specific condenser design, it should be noted that the methods described are generally applicable to a variety of condenser designs as might be found in future EUV lithography systems.

Development of a laser-produced plasma source at 13.5 nm for the French extreme ultraviolet lithography test bench

Within PREUVE, our group developed an EUV source that will meet α-tool specifications in 2002. With this aim, a laser-produced plasma source was built that uses a dense and confined xenon jet target. This device assures high conversion efficiency and low debris flux which are necessary for future extreme ultraviolet lithography. Our technical solution is based on a specific target injector design and the use of well adapted nozzle materials. After injection, the xenon is recycled and purified to reach a low cost of operation. Our ELSA source will allow 2 years full operation of the BEL lithography test bench.

Reflectance enhancement in the extreme ultraviolet and soft x rays by means of multilayers with more than two materials

Sub-quarterwave multilayer coatings with more than two different materials are shown to provide a reflectance enhancement compared with the standard two-material multilayer coatings when reflectance is limited by material absorption. A remarkable reflectance enhancement is obtained when the materials in the multilayer are moderately absorbing. A simple rule based on the material optical constants is provided to select the most suitable materials for the multilayer and to arrange the materials in the correct sequence in order to obtain the highest possible reflectance. It is shown that sub-quarterwave multilayers generalize the concept of multilayers, of which the standard two-material multilayers are a particular case. Various examples illustrate the benefit of sub-quarter-wave multilayer coatings for highest reflectance in the extreme ultraviolet. Applications for sub-quarterwave multilayer coatings are envisaged for astronomy in the extreme ultraviolet (EUV) and soft x rays and also for future EUY lithography.

Optimization of EUV radiation yield from laser-produced plasma

We optimize the conversion of laser energy into extreme ultraviolet (EUV) radiation by tailoring the laser parameters for a laser-produced plasma generated from 20 μm diameter water droplets. It is shown that mass-limited targets require careful adaption of laser-pulse energy and laser-pulse duration separately, rather than laser intensity, which seems to be adequate for bulk targets. The optimal pulse duration scales with the droplet radius, and the optimal pulse energy with the droplet volume. With optimized parameters, we obtain a conversion efficiency of 0.23% in 4π and 2.5% bandwidth for 13 nm radiation, the future EUV lithography light.