Thomas Wilhein

Investigation of soft X-ray emission from a gas puff target irradiated with a Nd:YAG laser

Abstract Experiments investigating soft X-ray emission in the 1–22 nm wavelength range from plasmas produced using a gas puff target irradiated with a Nd:YAG laser are described. Gas puff targets were created by pulsed injection of high-density gas through a nozzle. The use of the gas puff targets eliminates the production of debris associated with solid targets. Laser pulses of either 0.9 ns or 10 ns time duration with energies up to 0.7 J were used to produce plasmas. X-ray emissions from laser-produced gas puff plasmas were characterized for various gases. The spectral measurements of the soft X-ray emission were performed with the use of grating spectrographs equipped with the back-illuminated CCD camera. The source size was measured using the Fresnel zone plate imaging system and the grating spectrograph equipped with a slit placed perpendicularly to the dispersion direction. The obtained results would allow to develop an efficient and debrisless laser-produced X-ray source to be useful for applications in various fields.

Off-axis reflection zone plate for quantitative soft x-ray source characterization

A compact system for high-resolution spectroscopy and quantitative photon flux and brilliance measurements of pulsed soft x-ray sources is described. The calibrated system combines a novel elliptical off-axis reflection zone plate with charge-coupled device detection for simultaneous spectral and spatial measurements. Experiments on a water-window droplet-target laser-plasma source demonstrate λ/Δλ⩾1000 spectral resolution and absolute flux and brilliance measurements.

Scaling-up a liquid water jet laser plasma source to high average power for Extreme Ultraviolet Lithography

In this article we describe a laser plasma source for Extreme Ultraviolet Lithography (EUVL) based on a liquid water jet target. Although jet targets are known for some time now, no attempts have been made to prove the functionality of the target under conditions similar to an EUVL production-line facility, that means illumination with high average power laser systems (in the multi-kW regime) at repetition rates in the kHz region. Such systems are currently under development. We used the MBI-burst laser to simulate these extreme illumination conditions. We examined the hydrodynamic stability of the target as a function of the laser repetition rate at different average laser powers (0.6kW and 5kW per burst). Additionally, the dependence of the conversion efficiency on pulse duration in the range from 30ps to 3ns was investigated. From our results one can conclude parameters for future design of driver lasers for EUVL systems.

Normal-incidence condenser mirror arrangement for compact water-window x-ray microscopy

We demonstrate a water-window condenser arrangement for transmission x-ray microscopy based on table-top sources. A spherical normal-incidence multilayer mirror is used to focus and monochromatize water-window x-ray emission from a high-brightness droplet-target laser-plasma source. The condenser arrangement is compact, has high collection efficiency, and is easy to align. The maximum normal- incidence reflectivity at the desired wavelength, (lambda) equals 3.37 nm, was determined to be up to 3 percent. The potential for compact water-window transmission x-ray microscopy using the condenser arrangement is discussed.

X-ray microscopy with high-resolution zone plates: recent developments

In order to expand the applications of x-ray microscopy, developments in the fields of zone plate technology, specimen preparation and imaging techniques have been made. A new cross- linked polymer chain electron beam resist allows us to record zone plate pattern down to 19 nm outermost zone width. High resolution zone plates in germanium with outermost zone widths down to 19 nm have been developed. In addition, phase zone plates in nickel down to 30 nm zone width have been made by electroplating. In order to enhance the image contrast for weak absorbing objects, the phase contrast method for x-ray microscopy was developed and implemented on the Gottingen x-ray microscope at BESSY. The effects of x ray absorption on the structure of biological specimen limits the maximum applicable radiation dose and therefore the achievable signal to noise ratio for an artifact-free x-ray image. To improve the stability especially of biological specimen, a cryogenic object chamber has been developed and tested. It turns out that at the operating temperature T less than or equal to 130 K unfixed biological specimen can be exposed to a radiation dose of 109 - 1010 Gy without any observable structural changes. A multiple-angle viewing stage allows us to take stereoscopic images with the x-ray microscope, giving a 3D-impression of the object. As an example for the applications of x-ray microscopy in biology, erythrocytes infected by malaria parasite have been examined. Studies of the aggregation of hematite by sodium sulfate gives an example for the application of x-ray microscopy in the field of colloid research.

Optical setup for tabletop soft X-ray microscopy using electrical discharge sources

We report on an optical setup for compact full-field transmission microscopy developed for imaging with laboratory scale X-ray sources such as gas discharge plasmas. The optical setup consists of a reflective condenser and a zone plate objective imaging in the water window region (wavelength 2.3 – 4.4 nm). The microscope is able to visualize object structures in the sub-40 nm region. The radiation for the microscope is generated by a gas discharge in nitrogen with an input power of 10 kW and few Watt/(2πsr) narrowband line emission. A gold coated axially symmetrical elliptical mirror is used to collect the radiation and forms a focused spot on the sample. Having a diameter of several 100 μm the spot is larger than the utilized imaging field, which leads to uniform illumination of the object. The mirror can be adjusted by a 5–axes piezo driven translation stage to optimize the flux in the object plane. The He-α line emission (2.88 nm) is selected of the nitrogen emission spectrum by means of a titanium thin film filter providing monochromatic sample illumination. A micro zone plate generates a magnified image detected by a back illuminated TE-cooled CCD camera. At high repetition rates the source is suitable for imaging test objects like Siemens stars, latex spheres and diatoms with magnifications of 1000 at exposure times in the range of a few seconds.

Spatial characterization of the focus produced by an EUV Schwarzschild objective

Schwarzschild objectives are used in the EUV spectral range because of their large aperture, high mechanical stability and excellent achromaticity. The large aperture results in a small, theoretically diffraction limited focus diameter with ideal values of below 200 nm with the current configuration. We employed a zone plate with matched numerical aperture (0.19) to image the focus onto an X-ray CCD camera. Emission from high harmonic generation and a liquid-jet laser-plasma were used as light sources. Images at magnifications of about 150-fold were acquired and focus diameters of 300 nm (FWHM) were observed.

Soft x-ray and EUV emission from cryogenic liquid jets irradiated with fs, ps, and ns laser pulses

Cryogenic liquid jets of either nitrogen or argon of up to 30 micrometer in diameter were exposed to intense laser fields with pulse durations between 70 fs and 3 ns leading to intensities of 1016 W cm-2 and 1013 W cm-2, respectively. The emission of extreme UV light and soft X-rays investigated by means of an absolutely calibrated soft X-ray spectrograph shows the characteristic lines of highly ionized nitrogen and argon atoms. For nitrogen the emitted photon flux at the longer pulse length was several orders of magnitude higher than for 70 fs pulses whereas for argon pulse durations around one ps lead to the highest conversion efficiency (CE) from laser to soft X-ray radiation.

Emission from a gas puff target irradiated with a Nd:YAG laser for EUV and x-ray lithography

Soft x-ray and extreme-UV emissions form plasmas produced using a gas puff target irradiated with a Nd:YAG laser has been investigated. The use of the gas puff targets, create by pulsed injection of high-density gas through a nozzle, eliminates the production of debris associated with solid targets. Laser pulses of either 0.9 ns or 10 ns time duration with energies up to 0.7 J were used to produce plasmas. Emissions in the 1-22 nm wavelength range from laser-produced gas puff plasma were characterized for various gases. The spectral measurements were performed with the use of grating spectrographs equipped with the back- illuminated CCD camera. The source sizes was measured using the Fresnel zone plate imaging system and the grating spectrograph equipped with a slit placed perpendicularly to the dispersion direction. The obtained results would allow to develop an efficient and debrisless laser-produced radiation source for applications in proximity x-ray lithography and extreme UV lithography technologies.

Thinned back-illuminated CCD for x-ray microscopy

X-ray microscopy requires image detectors for soft x-rays (2.4 nm to 4.5 nm wavelength) with high detective quantum efficiency for a low radiation dosage applied to the sample. A thinned backside illuminated CCD has been attached to the Gottingen x-ray microscope, which is installed at the BESSY electron storage ring in Berlin. The CCD was a commercially available device with 1024 by 1024 pixels (each 24 micrometers square) without the anti-reflecting coating, which is applied to the standard device. First experiments performed at the primarily used x- ray wavelength of 2.4 nm show a considerable reduction of exposure time compared to the previously used photographic emulsion. This greatly reduces the radiation dose applied to the sample specimen. There was no degradation in performance of the CCD detected after one week of operation.