In this contribution, we report about nanoscale imaging using a laser produced plasma source based laboratory transmission X-ray microscope (LTXM) in the water window. The highly brilliant soft X-ray radiation of the LTXM is provided by a laser-produced nitrogen plasma source focused by a multilayer condenser mirror to the sample. An objective zone plate maps the magnified image of the sample on the super resolution camera. This camera employs a deep cooled soft-X-ray CCD imaging sensor sandwiched with a xy piezo stage to allow subpixel displacements of the detector. The camera is read out using a very low noise electronics platform, also directing low µm shifts of the sensor between subsequent image acquisitions. Finally an algorithm computes a high resolution image from the individual shifted low-resolution image frames.
High-harmonic-seeded x-ray laser became an important issue in x-ray laser development due to the possibility to obtain a
highly coherent and polarized soft x-ray source. We performed theoretical investigations into amplification of high
harmonic pulses in an x-ray lasing medium by using a model based on Maxwell-Bloch equations. From the theoretical
works, we analyze characteristics of energy extraction and temporal profile of output pulse. In addition, preliminary
experimental results and ongoing experiments related the harmonic-seeded x-ray lasers are reported.
Laboratory based X-ray lasers (XRL) exhibit a broad application potential in material sciences, imaging, spectroscopy and laser plasma diagnostics if two main issues are solved: a stable, well defined output of the system and a high repetition rate for fast data acquisition. During the last few years using the grazing incidence pumping (GRIP) scheme an pump energy level as low as 1 J was demonstrated for saturated XRL operation. This pump energy could be provided in principle even by commercially available Ti:Sa laser systems. However, the repetition rate of these systems is limited to
10 Hz and the output stability of the XRL follows that of the pumping laser. To overcome this situation a dedicated high
repetition rate XRL pumping laser will be introduced here. This concept is based on a fully diode pumped solid state laser using thin Yb:YAG disks as active material. In this paper we report about the first phase of the project aimed at a high average power XRL user station based on the GRIP scheme.
Output characteristics of an X-ray laser based on the GRIP geometry are analysed by both the theoretical and experimental methods. Detailed analysis of the last experiments on GRIP X-ray lasers with a single profiled pulse is given as well as the consequences of this pump variant for the injector-amplifier scheme being developed. Especially dynamics of the gain coefficient and the spontanous emission flux are important for the injector-amplifier scheme. Discussion on medium dynamics and kinetics is supported by numerical simulations. Additionally, some preliminary results on seeding a
Ni-like soft X-ray with high harmonic from neon at 13.9 nm are presented.
The present trends in the development work on X-ray lasers are shown and discussed on a background of a brief history of the collisionally pumped X-ray lasers. The presentation is focused on two variants of the transient inversion pump method succesfully applied in the experiments - slab target geometry and single profiled laser pulse. Recently, another scheme referred to as GRIP (GRazing Incidence Pumping) has been proposed and demonstrated. This pump geometry opens the new real possibility to construct a repetitive X-ray laser. Some
aspects of the pump scheme implementation are discussed in detail. Finally, a specific injector-amplifier system giving a new perspective on the future of X-ray lasers is dicussed briefly as well.
Recent development in the field of X-ray lasers is shown and discussed starting from transient inversion scheme in a double-pulse arrangement. Different variants of this scheme are discussed in detail from the point of view of reduction in the pump energy. The discussion is concentrated on the kinetic aspect of the plasma created
and heated by a profiled pulse. Recently, a scheme referred to as GRIP (GRazing Incidence Pumping) has been proposed and demonstrated. This pump geometry opens a new real possibility to construct a repetitive X-ray laser. Some aspects of the pump scheme implementation are analysed. Finally, a specific injector-amplifier system giving a new perspective on the future of X-ray lasers is dicussed briefly as well.
Technological reasons stimulated enormous interest in the spectral range between 10 nm and 15 nm. One of the most important, apart from the potential to be applied in the microlithography, was the existence of the high-efficiency, spectrally highly selective (narrow-band) reflective multi-layer (ML) optics in this spectral range. Applying these optics to plasma based XUV (extreme ultra violett) sources the debris from the plasma is a serious problem. For transmissive multi-layer optics we have additionally the low figures of merit. For example, the best beam splitters have an efficiency of about 30% (energy in both parts of the splitted beam). This type of element is crucial for efficient single-shot interferometry being the main application using table-top soft x-ray lasers.
We applied capillary optical elements, to our knowledge for the first time, to XUV radiation at 13.9 nm. These optical elements help overcome the limits discussed above or at least remarkably reduce the existing difficulties. A capillary beam splitter and a focussing capillary were applied to an incoherent XUV radiation source. For the beam splitter we measured a throughput of about 80%. With the focussing capillary we obtained a spot size of 27 μm (FWHM) with a gain (intensity in the focal spot compared to the intensity behind a pinhole of the focal spot size) of 600. Advantages and disadvantages of these optics in the discussed spectral range are analyzed.
The semiconductor industry has pushed linewidths on integrated-circuit chips down to 100 nm. To pattern ever finer lines by use of photolithography, the industry is now preparing the transition to extreme ultraviolet lithography (EUVL) at 13 nm by 2007. As EUVL matures, the requirements for the accuracy of reflectivity and wavelength measurements are becoming tighter. A high absolute accuracy and worldwide traceability of reflectance measurements are mandatory for worldwide system development. A direct comparison of EUV reflectance measurements at the Advanced Light Source (ALS) Center for X-Ray Optics (CXRO) and Physikalisch-Technische Bundesanstalt (PTB) yield perfect agreement within the mutual relative uncertainties of 0.14% for reflectance and 0.014% for wavelength.
With the development of EUV lithography there is an increasing need for high-accuracy at-wavelength metrology. In particular, there is an urgent need for metrology at optical components like mirrors or masks close to the production line. Sources for metrology have to fit different demands on EUV power and spectral shape than sources for steppers systems. We present the results of the radiometric characterization of a laser produced plasma (LPP)-source, newly developed at Max-Born-Institute Berlin for use in an EUV reflectometer. It is operated with a high-power pointing-stabilized laser beam (energy per pulse up to 700 mJ, 10 ns pulse duration, < ± 25 μrad pointing stability) at 532 nm which is focussed on a rotating Au target cylinder. The incident angle of the laser beam is set to 63°, the detecting angle 55° to the target normal. The source has been characterized regarding spectral photon flux, source size and source point stability. Two independently calibrated instruments, an imaging spectrometer and a double multilayer tool for in-band power measurements were used to obtain highly reliable quantitative values for the EUV emission of the Au-LPP source. Both instruments were calibrated by Physikalisch-Technische Bundesanstalt in its radiometry laboratory at the electron storage ring BESSY II. We obtained a source size of 30 μm by 50 μm (2s horizontal by vertical) and a stability of better than 2s=5 μm horizontally and 2s=9 μm vertically. A spectral photon flux of 1*10e14 /(s sr 0.1 nm) at 13.4 nm at a laser pulse energy of 630 mJ is obtained. The shot-to-shot stability of the source is about 5% (1s) for laser pulse energies above 200 mJ. For pulse energies between 200 mJ and 700 mJ, there is a linear relation between laser pulse energy and EUV output. The spectrum shows a flat continuos emission in the EUV spectral range, which is important for wavelength scanning reflectometry. High stability in total flux and spectral shape of the plasma emission as well as low debris was only obtained using a new target position for each shot. There is also a trade off between source size and EUV power. For a slightly defocused laser, an increase in EUV power up to a factor of two is obtained, while the source size also increases by about a factor of two. It is shown that an Au-LPP source provides spectrally flat reproducible emission with sufficient power at low debris conditions for the operation of a laboratory based EUV reflectometer.
The development of EUV lithography is critically based on the availability of suitable metrology equipment. To meet the industries requirements the Physikalisch-Technische Bundesanstalt (PTB) recently has installed a new EUV reflectometer at the electron storage ring BESSY II. The new reflectometer is designed for at-wavelength metrology of full-size EUVL optics. Samples with a maximum weight of 50 kg and a diameter of up to 550 mm can be investigated. Besides wavelength and angle scans also the measurement of bi-directional scattering is possible within the full sample surface. Convex and concave shaped surfaces are allowed. Not only a single mirror of the projection optics but also up to five masks can be mounted simultaneously. For future lithography production tools the requirements for the optics and masks are very stringent. The homogeneity of the multilayer reflectivity across the surface and the wavelength matching of the peak reflection become even stronger than today. To meet the increasing demands not only regarding the sample size but also regarding the accuracy of the measurements the operation of the beamline was further optimized. Diffuse scattered light limits the uncertainty in the peak reflectance. A total relative uncertainty of 0.14% is achieved with a reproducibility of 0.07%. The uncertainty in the center wavelength is mainly given by the uncertainty for the reference wavelength of the Kr 3d5/2-5p resonance. The reduction of all other sources of uncertainty results in a total uncertainty of 0.014% in the center wavelength with a reproducibility of 0.008%. We present a detailed description of the EUV reflectometer and discuss the optimized beamline conditions with the different sources of uncertainties. The results are illustrated by recent measurements.
The quality assurance for production of optical components for EUV lithography strongly requires at-wavelength metrology. Presently, at-wavelength characterizations of mirrors and masks are done using the synchrotron radiation of electron storage rings, e.g. BESSY II. For the production process of EUV optics, however, the immediate access to metrology tools is necessary and availability of laboratory devices is mandatory. Within the last years a stand alone laboratory EUV reflectometer for large samples has been developed It consists of a laser produced plasma (LLP) radiation source, a monochromator and a large goniometer systme. The manipulation system of the reflectometer can handle samples with diameters of up to 500 mm, thicknesses of up to 200 mm and weights of up to 30 kg. The wavelength can be varied from 10 nm to 16 nm. The spot size on the sample surface is about 2mm. The angle of incidence can be varied from 3° to 60°. In this paper, we describe the laboratory reflectometer in detail and discuss the achieved performance. First measurements of 4 inch mirrors are presented and discussed in comparison to the results obtained at the PTB soft x-ray radiometry beamline at BESSY II.
CD metrology requirements have increased dramatically within the last years. For the coming technology generations, it is not clear which CD measurement method will be standard for mask manufacturing. An interesting approach is to use the diffracted signal of periodic mask patterns for determination of CD. For wafer CD measurement, CD scatterometry tools using visible or UV wavelengths are already commercially available. For this experiment, diffracted EUV light was used. Dense lines of pitches 1:1, 2:1 and 5 :1 and nominal CDs of 150 nm, 200 nm, 300 nm, 400 nm and 500nm have been illuminated with EUV light of ?= 13.35 nm at the BESSY II storage ring in Berlin. The reflected signal has been collected with a movable detector in a range of -1 ° to 200 relative to the specular reflection. With the angular position of the peak, the pitch can be calculated. The CD, however, is related to the intensity of the peaks. Several effects as mask topography and measurement uncertainties are discussed. The results are compared to CD-SEM measurements of the same patterns.
Extensive investigations on the lifetime of EUVL optics using synchrotron radiation [1, 2, 3] have been performed at the radiometry laboratory  of the Physikalisch-Technische Bundesanstalt (PTB) at the BESSY II electron storage ring in the past. Nevertheless, synchrotron radiation shows a very different time structure as compared to the radiation of EUVL sources to be used in lithography tools.
To assess the question, whether the different time structure of the radiation has an impact on the contamination behavior of EUVL optics, an irradiation experiment was performed using synchrotron radiation of different time structure available at the BESSY II electron storage ring: Keeping all other parameters constant, radiation from the normal operation mode of BESSY II, which resembles quasi-cw- illumination, and the special single bunch operation mode, which gives pulsed synchrotron radiation with 1.25 MHz repetition rate were used to irradiate samples in a defined residual gas environment. The reflectance of the samples were measured before and after the illumination to determine the loss in reflectance due to irradiation.
Although the time structure of the single bunch mode still differs considerably from those of potential EUVL sources, trends in the contamination behavior could possibly be observed.
Since 1986, the Physikalisch Technische Bundesanstalt (PTB), Germany's national metrology institute, has been working on the 'at-wavelength' characterization of VUV and EUV optical components with synchrotron radiation. Today, PTB operates a laboratory at the electron storage ring BESSY II. Here, at several beamlines, high-accuracy at-wavelength characterization of EUVL components is routinely carried out. Reflectometry is performed on a bending magnet beamline at which a relative uncertainty of 0.25 percent is achieved for the spectral reflectance of a mirror in the EUV spectral region. For the investigation of very large optical components, a new reflectometer will be set up at the soft x-ray radiometry beamline in March 2002. The reflectometer allows the characterization of full size EUVL optical components. Raster scans across the full sample surface can be performed in. Convex or concave profiles are allowed. An additional detector movement out of the scattering plane allows the measurement of bi-directional scattering. Similar measurements can be performed by mounting a cooled CCD camera at different fixed positions on the vacuum chamber. The motor step size for all translational and rotational movements will be 1 micrometers and 0.001 degrees, respectively. We present a detailed description of the new reflectometer for large EUV optics and discuss the uncertainties to be achieved for reflectance measurements.
With the development of EUV-lithography, high-accuracy at-wavelength metrology has increasingly gained in importance. Characterization of detectors and sources using synchrotron radiation has been performed by the Physikalisch--Technische Bundesanstalt (PTB) for almost 20 years. At their new laboratory at BESSY II, PTB now has set up instrumentation which is suitable for high-accuracy EUV detector calibration. It uses synchrotron radiation from a bending magnet for detector characterization at a plane grating monochromator beamline. The detector calibration at PTB uses a cryogenic electrical substitution radiometer as the primary detector standard. For the measurement of radiant power of about 1 (mu) W, the systematic uncertainty contributions from the electrical substitution principle of about 0.03 percent relative dominate the measurement uncertainty of the radiometer. Careful adjustment of the temperature control circuit reduced the statistical noise of the measured power to about 0.2 nW. This allows the radiant power to be measured down to 0.1(mu) W with an uncertainty of 0.3 percent or better. This uncertainty is lower than the results achieved elsewhere by more than one order of magnitude. In this paper, the current status of EUV detector calibration at PTB is presented. The high performance of the radiometer, together with the improved stability and spectral purity of the beamline, is illustrated by typical results. In the EUV spectral range, photodiodes can be calibrated with a relative uncertainty of about 0.3 percent. This low uncertainty permits systematic studies of the homogeneity and stability of detectors with unprecedented sensitivity for even minor changes. The responsivity of individual photodiodes has been observed over a period of up to six years. We present a first investigation of the long-term stability of AXUV photodiodes which are widely assumed to be stable in the EUV spectral range. The results are of sufficient accuracy to show that even diodes which are rarely used and carefully stored, degrade. After a period of three years, the degradation becomes ever stronger.
One major goal in x-ray tomography is to increase the resolution in space and time. For the methods with high temporal resolution we will present pink beam imaging and tomography. Experiments were realised at the ESRF undulator beamline ID22 with hard x-rays in the range from 11 keV to 20 keV. For the tomographic scans the exposure time per image was reduced by one to two orders of magnitude to less than 50 ms per image. The obtained image quality was comparable to that done with monochromatic beam. Further time reducing for a tomographic scan is possible with an improved acquiring and control system. The goal in the future is to realise tomographic scans within a minute with micrometer resolution. In order to achieve in the hard x-ray range sub-micrometer resolution we will show first results of x-ray magnified tomography. Different lens systems are available for this purpose. We obtained with aluminium parabolic compound refractive lenses a resolution of 1 micrometers and expect to overcome this limit hand in hand with the improvement of lens technology.
Degradation of EUV optics during irradiation is a crucial topic as regards lifetime and performance in EUV lithography. To simulate irradiation conditions for future lithography tools, PTB (the German national metrology institute) operates two dedicated beamlines at the electron storage ring BESSY II. Both, undispersed undulator radiation from an EUV optimized undulator as well as focused and filtered bending magnet radiation can be used. Both beamlines provide EUV radiation with power densities of several mW / mm2. A dedicated irradiation chamber with sample load lock and differential pumping allows components such as substrates, multilayer mirrors or filters to be exposed to EUV radiation under different vacuum conditions. At the same laboratory, high-accuracy EUV reflectometry can be performed for proximate assessment of the resulting performance.
The development of EUV lithography, has made high-accuracy at-wavelength metrology necessary. Radiometry using synchrotron radiation has been performed by the German national metrology institute, the Physikalisch-Technische Bundesanstalt (PTB), for almost 20 years. Recently, PTB has set up four new beamlines for EUV metrology at the electron storage ring BESSY II. At a bending magnet, a monochromator for soft X-ray radiometry is routinely used for reflectometry and detector characterisation. A reflectometer designed for mirrors up to 550 mm in diameter and 50 kg in mass will be operational in January 2002. Detector characterisation is based on a primary detector standard, a cryogenic electrical substitution radiometer. Measuring tools for EUV source characterisation are calibrated on this basis. Detector testing at irradiation levels comparable to the anticipated conditions in EUV tools is feasible at a plane grating monochromator, installed at an undulator optimised for EUV radiation. A test beamline for EUV optics alignment and system metrology has been installed, using undispersed undulator radiation. Bending magnet radiation is available at a station for irradiation testing. A focusing mirror collects a radiant power of about 10 mW within the multilayer bandwidth and a 1 mm² focal spot.
Compound refractive lenses (CRL) for hard x-rays are genuine imaging devices like glass lenses for visible light. They are ideally suited for both full field and scanning microscopy with hard x-rays in the range from 2 to 100keV. They are robust and can withstand the heat load of the white beam of an ESRF undulator source. In full field microscopy, resolutions down to 300nm have been achieved so far using aluminium lenses. Resolutions below 100nm are expected for beryllium lenses currently under development. For scanning microbeam techniques, a monochromatic microbeam of 550nm by 5.5micrometers with a 1.1 1010ph/s (gain 1120) has been achieved with aluminium lenses at a third generation undulator source. For beryllium as lens material, a flux up to two orders of magnitude higher is expected. At planned FEL beamlines, the source size and distance from the source are favorable to microbeams produced by compound refractive lenses, and a diffraction limited microbeam is expected both horizontally and vertically. For beryllium lenses the diffraction limit can be below 100nm. A typical FEL beam size of approximately 1mm at the experiments hutch ideally matches the aperture of compound refractive lenses. Estimates of the heat load on the CRL as well as expected photon fluxes and micro beams sizes are given.
We describe parabolic compound refractive lenses for hard x- rays that are genuine imaging devices similar to glass lenses for visible light. They open considerable possibilities in both full field and scanning x-ray microscopy, microanalysis, and coherent scattering. They can operate in a range from about 2 keV to 100 keV, are robust, and withstand the white beam of a third generation undulator source. Using aluminum lenses in full field microscopy a field of view of about 300 micrometer can be imaged with magnifications between 10 and 50 and a resolution of about 300 nm. With beryllium lenses an improvement of the resolution to below 100 nm is expected. For microbeam applications, the synchrotron source is imaged onto the sample in a strongly demagnifying setup. With focal distances between 0.3 m and 2 m, the source can be demagnified by a factor 20 to 200 producing a beam with lateral extensions in the micron and sub-micron range. For aluminum lenses, monochromatic microbeams with fluxes above 1010 ph/s and a gain above 1000 are routinely produced at third generation undulator sources. Compound refractive lenses will allow to produce microbeams at energies up to at least 100 keV, making for example, microfluorescence experiments at the K-edges of heavy elements possible. The modular setup of compound refractive lenses allows to adjust the focal length to ideally match the experimental requirements. Assembling and aligning the lens take about 15 minutes. No order sorting apertures are required and the straight optical path allows to remove the lens to align other components.
X-ray fluorescence element micro tomography allows to determine the element specific inner structure of a sample with resolutions in the micron range. It has a wide range of applications in many disciplines and is ideally suited for investigating element distributions inside of biological bulk samples at a cellular level with minimal sample preparation. The high intensity hard x-ray microbeam required for this scanning technique is produced using parabolic compound refractive lenses at a third generation undulator source. The sample is scanned through the microbeam in both translation and rotation and the fluorescence radiation created in the sample is recorded by an energy dispersive detector. From this data, the element distribution on a virtual section through the sample is recovered by tomographic techniques. The excitation of the fluorescence by monochromatic x-rays yields a high signal to background ratio and a low detection limit. As an example, we have investigated the distribution of physiologically relevant ions on a virtual section through a freeze dried root of the mahogany plant. Absorption of the fluorescence radiation inside the sample has to be taken into account in tomographic reconstruction and ultimately limits the size of the sample that can be investigated. A self-consistent reconstruction technique not requiring the explicit knowledge of the absorption inside the sample has been developed. Further developments of the technique are discussed.
Focusing of hard X-rays by refraction has been a long time been considered as unfeasible due to strong absorption and weak refraction of X-rays in matter. Recently it has been shown that compound refractive lenses can overcome the problem. It was demonstrated that the best candidates for lenses are low Z, high density materials. Linear and 2D lenses from aluminum, boron carbide, beryllium, pyrographite and Teflon were produced and tested. Focusing of 2 - 3 microns was achieved at an energy range from 9 to 30 keV. Compound refractive lenses have low sensitivity to heatload and are extremely well suited for focusing of undulator radiation. Two-plane focusing lenses have been optimized, built and installed in the white beam of the undulator on the machine diagnostic beamline of the ESRF to be used as an X-ray emittance diagnostic. The future potentials of the refractive lenses will be discussed as well.