Rainer Lebert

Soft x-ray emission of laser-produced plasmas using a low-debris cryogenic nitrogen target

Laser-produced nitrogen plasmas (LPPs) are intense narrow-band emitters in the “water window” spectral range which is suited for x-ray microscopy of biological samples. Frozen gases as target materials are predicted to have low debris precipitated onto laser or x-ray optics arranged inside the vacuum chamber. Gaseous nitrogen is frozen onto a metal surface at a temperature of 16 K. The solid nitrogen is illuminated with nanosecond pulsed laser radiation with intensities of 1.3×1013 W/cm2 and the emission characteristics of laser-produced nitrogen plasmas are investigated with a calibrated imaging spectrograph. The x-ray emission in the water window region is compared to the radiation of a LPP with a solid boron nitride target at the same laser parameters.

Laser-produced lithium plasma as a narrow-band extended ultraviolet radiation source for photoelectron spectroscopy

Extended ultraviolet (EUV) emission characteristics of a laser-produced lithium plasma are determined with regard to the requirements of x-ray photoelectron spectroscopy. The main features of interest are spectral distribution, photon flux, bandwidth, source size, and emission duration. Laser-produced lithium plasmas are characterized as emitters of intense narrow-band EUV radiation. It can be estimated that the lithium Lyman-alpha line emission in combination with an ellipsoidal silicon/molybdenum multilayer mirror is a suitable EUV source for an x-ray photoelectron spectroscopy microscope with a 50-meV energy resolution and a 10-mum lateral resolution.

Narrowband laser produced extreme ultraviolet sources adapted to silicon/molybdenum multilayer optics

The extreme ultraviolet radiation emitted from a plasma generated by a pulsed Nd:yttrium aluminum garnet laser is investigated around 13 nm wavelength for several low Z elements (lithium, nitrogen, oxygen, fluorine). A narrowband EUV source can be designed by using the narrowband line emission of low Z elements in combination with the broadband reflection characteristic of silicon/molybdenum (Si/Mo) multilayer mirrors. Experimental results are discussed within a theoretical model, which allows a deduction of an optimization criterion for a maximum conversion efficiency. The Lyman-α line of hydrogenlike lithium ions fulfills the demands for high intense, free-standing narrowband emission at the long wavelength side of the silicon absorption L edge.

Comparison of laser produced and gas discharge based EUV sources for different applications

Pulsed plasmas, e.g., laser produced plasma (LPP) and gas discharge based pinch plasmas are known as intense sources of soft x-ray and extreme ultraviolet (EUV) radiation in the wavelength interval of about 1 nm to beyond 50nm. They can be generated in compact, laboratory scale devices. Both schemes exhibit similar source characteristics concerning, e.g., wavelength region, bandwidth, source size, emitted radiation per pulse or average emitted power when operated at similar parameters. By exploiting their technological degrees of freedom their source characteristics can be matched to the special demands of a given application like EUV-lithography or metrology, x-ray microscopy, XPS or other x-ray analytics. In this work laser produced and gas discharge based plasmas are compared with respect to their source characteristics. The comparison concerns to a laser produced plasma using a commercial 1 J Nd:YAG laser with different target concepts (liquid, cryogenic or solid target) and a gas discharge based EUV sources with electrical pulse energies of about 1 J. The same absolute calibrated diagnostics used for both plasmas allows for a direct comparison.

CALIBRATION OF CHARGE COUPLED DEVICES AND A PINHOLE TRANSMISSION GRATING TO BE USED AS ELEMENTS OF A SOFT X-RAY SPECTROGRAPH

The development of new types of sophisticated soft x-ray sources requires the knowledge of their emission characteristics such as photon flux, spectral distribution, and size of the radiation source. Calibrated spectrographs for the soft x-ray region are needed to determine these properties. The components of a soft x-ray spectrograph consisting of a pinhole gold transmission grating and a charge coupled device (CCD) camera are calibrated at the radiometry laboratory of the Physikalisch-Technische Bundesanstalt using the synchrotron radiation facility BESSY. Two different kinds of CCD-based photon detectors (one thinned and back illuminated, one coated with a phosphorous layer) are compared with regard to their sensitivities in the spectral range between 50 eV and 1.7 keV. The results obtained for the thinned CCD are compared with theoretical calculations of the sensitivity.

Atmospheric pressure gas discharges for surface treatment

Dielectric barrier discharges (also called silent discharges) driven by high voltage at frequencies from 50 Hz to several kilohertz have been used for decades in plasma chemical processing of gases. In this paper the new concept of direct dielectric barrier discharges for surface treatment in the range of several hundred kilohertz is introduced, where the material being treated is used as one of the electrodes. Without absorbing components in between the discharge and the surface, high frequency electric fields, energetic electrons, UV light and reactive radicals are efficiently combined for plasma chemical surface treatment. Direct barrier discharges allow for continuous high speed processes in open-electrode configurations without expensive vacuum equipment.

Investigations on the transition between column and micropinch mode of plasma focus operation

X‐ray emission from pinch plasma devices with pinch currents ranging from 200 to 400 kA operated with pure high‐Z gases are investigated with temporal spatial and spectral resolution. If operated using elements Z<18 (e.g., nitrogen or neon) K‐shell emission is observed from column‐like volumes several 100 μm in diameter and several mm in length (bulk or column mode). For Z≳18 (e.g., krypton or xenon) emission with hν≳1 keV is only observed from micropinches. For argon (Z=18) both modes of operation can be observed. The occurrence of a specific mode depends on the initial gas pressure. In this paper the transition regime between column and micropinches is investigated with particular regard to argon. A criterion is proposed to decide whether column mode or micropinch mode is expected.

A multi-kilohertz pinch plasma radiation source for extreme ultraviolet lithography

A pinch plasma source in the extreme ultraviolet is presented where the special design of the electrodes leads to advantages concerning low erosive operation and an effective coupling of the electrically stored energy to the electrode system. Most promising results of the source parameters with respect to the demands for extreme ultraviolet lithography are achieved when operating with xenon. Intense emission around 11 nm and 13 nm is observed. The plasma column has a diameter of less than 500 μm when viewed from the axial direction. The electrode design allows for an accessible solid angle of around π sr. The shot-to-shot stability is better than 4% (rms). A maximum output of 0.8 mJ/(sr 2% bw) at 13.5 nm has been observed with an input pulse energy of 2 J. Operation at a repetition rate of 1 kHz and an electrical input power of 2 kW has been demonstrated with an average emitted power of around 0.3 W/(sr 2% bw). Approaches of power scaling into the range which is desired for EUVL will be discussed.

Pinch plasmas as intense EUV sources for laboratory applications

Compact pinch plasma devices are intense sources of pulsed EUV radiation with output energies of several joules per pulse in single lines. Their spectrum peaks in a wavelength range where conventional x-ray tubes provide poor intensity. With correct optimization, both continuous radiation or line radiation with λ/Δλ > 1000 can be produced for broadband and narrowband applications, respectively. Because of their low cost and their compact size, pinch plasmas seem well suited to supplement research activities based on synchrotron radiation.In this paper, pinch plasma sources developed for x-ray lithography and x-ray microscopy are described. Their emission characteristics are optimized with regard to specific requirements given by the particular application and are compared to laser produced plasmas. The lithography source is compatible with the electron storage ring printing process with respect to its spectrum and enables full-depth exposures in 1-μm-thick 60 mJ cm-2 sensitivity resist at resolution below 0.2 μm within 10 minutes. The source for microscopy applications enables flash imaging of biological specimens with suboptical resolution (0.1–0.2 μm) at nanosecond exposure times. In addition, the averaged plasma parameters meet the requirements for an EUV laser medium. The nitrogen 2–3 transition is especially promising for achieving amplified spontaneous emission of hydrogen-like or helium-like ions excited by three-body or charge exchange recombination.

Triggering a radial multichannel pseudospark switch using electrons emitted from material with high dielectric constant

The electron emission from a field emission trigger unit was matched to the requirements for the simultaneous ignition of all channels in a radial multichannel pseudospark switch for a large parameter range of the pseudospark discharge. In such trigger unit the electrons are extracted from the surface of a high-e material by field emission after applying a pulsed voltage. Electron emission occurs in a pressure range from 1–50 Pa, where the pseudospark discharge is operated. The access to low working gas pressures (<10 Pa) using such trigger unit allows for achieving high hold-off voltages. The ignition of the pseudospark discharge was investigated by means of high speed photography and by a parallel observation of the electron beam of a single discharge channel, which occurs in hollow-cathode phase.