Marnix J. van der Wiel

High-brightness compact soft x-ray source based on Cherenkov radiation

Cherenkov radiation in the soft x-ray region is generated in narrowband regions at inner-shell absorption edges. Mainly low-Z elements are suitable Cherenkov sources, which emit in a photon energy range from 30 eV to 1 keV and require moderate electron energies up to 25 MeV. Generally, in the soft x-ray region materials are highly absorbing and therefore the Cherenkov radiation theory is discussed for absorbing media. A detailed description includes transition radiation that is generated at the interface when the relativistic electron exits the material. We show that the transition radiation yield equation, when it is adopted for an absorbing medium, includes Cherenkov radiation. Based on this approach it is shown that the spectral intensity of Cherenkov radiation in the soft x-ray region is large compared to transition radiation for moderate electron energies. First measurements of soft x-ray Cherenkov radiation in the water-window spectral region, generated in titanium and vanadium foils, are discussed in detail. The measured spectral and angular distribution of the radiation, and the measured total yield (≈ 10-4 photon per electron) are in agreement with theoretical predictions based on the refractive index data. We show that the brightness that can be achieved using a small electron accelerator is sufficient for practical x-ray microscopy in the water window.

Control of debris production of laser plasma sources with high-average XUV power

Recent investigations on methods to eliminate the contamination of high power laser plasma sources are reviewed. The use of separate methods such as a buffer gas environment or thin tape targets suppresses the debris yield drastically up to a factor of 103. A combination of this with other effective as well as practically convenient methods (use of debris angular characteristics, a rotating shutter, or thin UV filters) provides a truly contamination-free XUV source. As an example of a novel high power source we describe the performance of a 1.5 J KrF laser designed to drive a laser plasma XUV source. The laser was used to generate narrowband XUV radiation at 18 nm. A conversion efficiency of >= 1.4% in a bandwidth of 6% was measured, demonstrating the feasibility of laser plasma sources for applications such as XUV projection lithography, requiring high average power.

Laser Plasma As X-Ray Source For Lithographic Imaging

Experimental work on laser plasma based X-ray lithography is presented, aimed at the development of a compact lithographic work station for VLSI pattern transfer. The results are obtained with a frequency doubled low repetition rate Nd:YAG/glass laser (532 nm, 3.5 J, 14 ns) with a power density at the laser focus of 3.5 x 1012 W/cm2. Spectral and time characteristics of X-ray emission of the laser plasma are shown. From a comparison of resist exposure using synchrotron radiation and radiation from the laser plasma, a conversion efficiency of 7.6 % of laser energy into X-ray energy in the exposure producing 845 eV band is calculated. The laser plasma X-ray source is used to image Si X-ray masks with submicron Au absorber patterns. Experimental X-ray sensitive photoresists, i.e. RAY-PF and a more sensitive formula of this resist, are used to record the structures and are analysed for their imaging properties. With 90 laser pulses 0.5 μm mask structures were faithfully reproduced on RAY-PF X-ray resist using a source-to-wafer distance of 95 mm. Only 35 laser pulses give sufficient intensity for correct exposure of the enhanced sensitivity formula. Under optimized conditions a single laser pulse was found to be sufficient for the replication of submicron patterns.

Laser Induced Tunnel Ionization and Electron Density Evolution in Air

An indigenously built Ti:sapphire laser system delivering 50 mJ, 100 fs pulses at a repetition rate of 10 Hz, is employed to tunnel ionize air at 1 atm pressure and form a plasma channel. The laser is line focused using a cylindrical lens to a spot size of 20 µm×1 mm. A folded wave interferometry is used to deduce the radial electron density profile by measuring the phase shift and employing Abel inversion. The maximum value of chord-integrated electron density and radial electron density are estimated to be approximately equal to 3.0×10 16 cm -2 and 4.0×10 18 cm -3 respectively with density scale length of 20 µm. These results have been theoretically interpreted by developing a unified formalism of tunnel ionization. The experimental results are in reasonable agreement with the theory.

Spectral charateristics of an advanced x-ray generator at the KIPT based on Compton backscattering

An international co-operative project to develop an advanced X-ray source on the basis of Compton backscattering is described. The goal is the re-configuration of the Kharkov Institute of Physics and Technology (KIPT) N-100 storage ring to support the efficient interaction of its electron beam with a high power pulsed-laser cavity. At equilibrium both the electron and X-ray beams’ phase space characteristics are determined by a balance of stochastic photon cooling and emission. In this paper the operating parameters and fundamental spectral and temporal properties of the novel source are summarized and the potential for its development.into an ultra-short pulse source is discussed.

Optimization of excimer laser-induced x-ray sources for soft x-ray projection lithography

We report results on high power excimer lasers and their application to generating laser plasma soft x-ray sources. A conversion efficiency of laser light to monochromatized soft x- ray radiation of 0.7% has been achieved at 13.5 nm (2% BW). Two methods to mitigate the production of plasma debris have been analyzed: tape targets and the use of Kr as a buffer gas. The optimal coating thickness of tape targets coated with Ta has been determined as 1 micrometers . Ta tape targets and the Kr buffer were used in a debris contamination test of 105 pulses and evaluated by the loss in reflectivity of a normal incidence Mo-Si multilayer mirror.