Klaus Bergmann

Compact soft x-ray microscope using a gas-discharge light source

We report on a soft x-ray microscope using a gas-discharge plasma with pseudo spark-like electrode geometry as a light source. The source produces a radiant intensity of 4 x 10(13) photons/(sr pulse) for the 2.88 nm emission line of helium-like nitrogen. At a demonstrated 1 kHz repetition rate a brilliance of 4.3 x 10(9) photons/(microm2 sr s) is obtained for the 2.88 nm line. Ray-tracing simulations show that, employing an adequate grazing incidence collector, a photon flux of 1 x 10(7) photons/(microm2 s) can be achieved with the current source. The applicability of the presented pinch plasma concept to soft x-ray microscopy is demonstrated in a proof-of-principle experiment.

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.

Optimization of a gas discharge plasma source for extreme ultraviolet interference lithography at a wavelength of 11 nm

In this work, we report about the optimization of the spectral emission characteristic of a gas discharge plasma source for high-resolution extreme ultraviolet (EUV) interference lithography based on achromatic Talbot self-imaging. The working parameters of the source are optimized to achieve a required narrowband emission spectrum and to fulfill the necessary coherence and intensity requirements. The intense 4f-4d transitions around 11 nm in a highly ionized (Xe8+–Xe12+) xenon plasma are chosen to provide the working wavelength. This allows us to increase the available radiation intensity in comparison with an in-band EUV xenon emission at 13.5 nm and opens up the possibility to strongly suppress the influence of the 5p-4d transitions at wavelengths between 12 and 16 nm utilizing a significant difference in conditions for optical thickness between 4f-4d and 5p-4d transitions. The effect is achieved by using the admixture of argon to the pinch plasma, which allows keeping the plasma parameters approximate...

Scalability limits of Talbot lithography with plasma-based extreme ultraviolet sources

Abstract. Lithography has been faced with a challenge to bring resolution down to the 10-nm level. One of the promising approaches for such ultra-high-resolution printing is self-imaging Talbot lithography with extreme ultraviolet (EUV) radiation. However, as the size of structures on the mask approaches the wavelength of the radiation, diffraction influence needs to be evaluated precisely to estimate the achievable resolution and quality of the patterns. Here, the results of finite-difference time-domain simulations of the diffraction on EUV transmission masks in dependence to the period (pitch) of the mask are presented with the aim to determine the resolution that can be realistically achieved with the EUV Talbot lithography. The modeled experimental setup is utilizing partially coherent EUV radiation with the wavelength of 10.9 nm from Xe/Ar discharge plasma EUV source and Ni/Nb-based amplitude transmission mask. The results demonstrate that the method can be used to produce patterns with resolution down to 7.5-nm half-pitch with 2∶1 mask demagnification utilizing achromatic Talbot effect and transverse electric (TE)-polarized light.

EUV microscopy for defect inspection by dark-field mapping and zone plate zooming

A laboratory scale EUV microscope is presented to be used, e.g., in future inspection of EUV masks and mask blanks in extreme ultraviolet lithography. The system can be operated in bright and dark field mode. For defect inspection purpose the dark field mode is preferred, with increased contrast and sensitivity of the system to small structures. The characteristics of the used Schwarzschild objective as imaging component such as large object field and moderate magnification become advantageous for high process speeds, whereas the detector pixel size (13 μm) does not give the spatial resolution in principle possible with the imaging optics. The presence of a defect causes a spot (with otherwise dark background) to appear on the detector. As necessary it can be zoomed in with the help of a second magnification step. For this purpose we suggest to employ a zone plate adapted to the system. The methods feasibility is demonstrated by means of experiments on test structures and the apparatus is characterized with regard to design parameters for commercial systems.

Physical properties of the HCT EUV source

The paper describes the physical properties and recent technical advances of the hollow cathode triggered pinch device (HCT) for the generation of EUV radiation. In previous publications we have demonstrated continuous operation of the untriggered device at 1 kHz in pure Xe. The newer generations operate with a triggering facility which allows a wider parameter space under which stable operation is possible. Repetition frequencies of up to 4 kHz could be demonstrated. Many of the experiments are performed in repetitive bursts of variable lengths and spacing. This allows also to demonstrate that there is only little transient behavior upon switching on and off the source. Conversion efficiencies into the 2 percent frequency band around 13.5 nm are about 0.4 percent in 2p, comparable to the values reported from other groups. Another important parameter is the size of the light emitting region. Here we have studied the influence of electrode geometry and flow properties on the size, to find a best match to the requirements of the collection optics. A major problem for the design of a complete wafer illumination system is the out-of-band portion of the radiation. Especially the DUV fraction of the source spectrum is a concern because it is also reflected to some extend by the Mo-Si multilayer mirrors. We show that the source has a low overall non-EUV part of the emission. In particular, it is demonstrated that there is very little DUV coming out of the usable source volume, well below the specified level.

Frequency scaling in a hollow-cathode-triggered pinch plasma as radiation source in the extreme ultraviolet

The hollow-cathode triggered discharge extreme ultraviolet source is based on the same principle as pseudospark switches. The electrode geometry consists of a planar anode and cathode with central opposing holes, the one on the cathode side being connected to the hollow cathode. Radiation is generated by magnetic compression of the working gas under high-current operation. Essential for the operation is that the pressure and voltage are chosen to be on the left side of the Paschen curve to insure insulation of the gap between the electrodes. However, this insulation of the electrode system needs to be reinstalled after breakdown. Typical recovery times of a xenon-based system are down to 100 /spl mu/s, depending on the electrode geometry. It will be shown that the decay of the electron density in the hollow cathode is the limiting process. Investigation of the recovery mechanism has led to a design that allows operation above 4 kHz which is close to the required frequency for extreme ultraviolet lithography.

Comparison of different source concepts for EUVL

There are some candidates discussed as potential high power EUV sources for EUV lithography. Laser produced and discharge produced plasmas are most promising. In principle, the most efficient steady state plasma can be well defined with respect to plasma temperature and density. However, the conversion efficiency of a practical EUV source is mainly determined by the efficiency of plasma generation and heating and by the dynamics during the emitting phase. The different approaches to achieve the most efficient EUV source imply different heating mechanisms, different plasma geometries, different plasma densities and different time scales. Moreover, each approach has individual technical aspects that influence the efficiency and the technical chances of realization. A general approach for comparing different EUV source concepts is presented based on a discussion of fundamental aspects of the plasma generation and based on technical issues.

EUV sources for the alpha-tools

In this paper, we report on the recent progress of the Philips Extreme UV source. The Philips source concept is based on a discharge plasma ignited in a Sn vapor plume that is ablated by a laser pulse. Using rotating electrodes covered with a regenerating tin surface, the problems of electrode erosion and power scaling are fundamentally solved. Most of the work of the past year has been dedicated to develop a lamp system which is operating very reliably and stable under full scanner remote control. Topics addressed were the development of the scanner interface, a dose control system, thermo-mechanical design, positional stability of the source, tin handling, and many more. The resulting EUV source-the Philips NovaTin(R) source-can operate at more than 10kW electrical input power and delivers 200W in-band EUV into 2π continuously. The source is very small, so nearly 100% of the EUV radiation can be collected within etendue limits. The lamp system is fully automated and can operate unattended under full scanner remote control. 500 Million shots of continuous operation without interruption have been realized, electrode lifetime is at least 2 Billion shots. Three sources are currently being prepared, two of them will be integrated into the first EUV Alpha Demonstration tools of ASML. The debris problem was reduced to a level which is well acceptable for scanner operation. First, a considerable reduction of the Sn emission of the source has been realized. The debris mitigation system is based on a two-step concept using a foil trap based stage and a chemical cleaning stage. Both steps were improved considerably. A collector lifetime of 1 Billion shots is achieved, after this operating time a cleaning would be applied. The cleaning step has been verified to work with tolerable Sn residues. From the experimental results, a total collector lifetime of more than 10 Billion shots can be expected.

Generation of circularly polarized radiation from a compact plasma-based extreme ultraviolet light source for tabletop X-ray magnetic circular dichroism studies

Generation of circularly polarized light in the extreme ultraviolet (EUV) spectral region (about 25 eV-250 eV) is highly desirable for applications in spectroscopy and microscopy but very challenging to achieve in a small-scale laboratory. We present a compact apparatus for generation of linearly and circularly polarized EUV radiation from a gas-discharge plasma light source between 50 eV and 70 eV photon energy. In this spectral range, the 3p absorption edges of Fe (54 eV), Co (60 eV), and Ni (67 eV) offer a high magnetic contrast often employed for magneto-optical and electron spectroscopy as well as for magnetic imaging. We simulated and designed an instrument for generation of linearly and circularly polarized EUV radiation and performed polarimetric measurements of the degree of linear and circular polarization. Furthermore, we demonstrate first measurements of the X-ray magnetic circular dichroism at the Co 3p absorption edge with a plasma-based EUV light source. Our approach opens the door for laboratory-based, element-selective spectroscopy of magnetic materials and spectro-microscopy of ferromagnetic domains.