Roman Klein

A plane-grating monochromator beamline for the PTB undulators at BESSY II.

At the BESSY II storage ring, the Physikalisch-Technische Bundesanstalt (PTB) will operate insertion devices dedicated to radiometric use. One branch of the appendant beamline system will be equipped with a grazing-incidence monochromator, described here. The monochromator concept is based on a plane grating operated in parallel light; therefore exact focusing is maintained for all photon energies irrespective of the angular setting at the grating. The monochromator has been optimized for small higher-order transmittance and high power throughput, as required by radiometric applications in the wide photon energy range from 20 eV to 1900 eV.

Beam diagnostics at the BESSY I electron storage ring with Compton backscattered laser photons: measurement of the electron energy and related quantities

The electron storage ring BESSY I in Berlin-Wilmersdorf is used by the Physikalisch-Technische Bundesanstals (PTB) as a primary radiation source standard in the vacuum-UV and soft X-ray spectral range. This requires the determination of the storage ring parameters relevant to the calculation of the spectral photon flux with small relative uncertainties. Within this framework, a new set-up for measuring the electron energy is described: CO2-laser light is scattered in a head-on collision with the electron beam in the electron storage ring and the backscattered photons are measured with an energy-calibrated high-purity germanium (HPGe) detector. The exact energy and shape of the high energy edge of the measured Compton spectrum can then be used to determine the electron energy and also the electron beam energy spread and related storage ring parameters. Moreover, at BESSY I, we have the unique possibility of measuring the electron energy by the alternative method of resonant spin depolarization. Comparison of the results obtained by these two techniques shows agreement within the combined relative uncertainties of 2×10−4. An overview of the current experimental set-up at the storage ring BESSY I and first experimental results are given.

High-accuracy EUV metrology of PTB using synchrotron radiation

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.

The electron storage ring BESSY II as a primary source standard from the visible to the the X-ray range

The Physikalisch-Technische Bundesanstalt (PTB) has used the BESSY I 800 MeV electron storage ring as a primary source standard since 1984. The calculable spectral photon flux ΦE of a bending magnet with relative uncertainties of 4 × 10−4 to 4 × 10−3 for the photon energy 1 eV to 15 keV was used in a broad spectral range from the visible to the soft X-ray range for the calibration of radiation sources and energy-dispersive X-ray detectors. In 1998, BESSY II was brought into operation. Owing to its higher electron energy, of 1.7 GeV, the useful spectral range could be extended significantly into the X-ray region up to a photon energy of about 50 keV, while special storage-ring operation at 900 MeV results in a spectrum up to about 7 keV. The PTB has established BESSY II as a European primary source standard from the visible to the X-ray range by setting up equipment for the measurement of all storage-ring parameters and geometrical quantities involved in the calculation of ΦE according to Schwinger. At BESSY II, ΦE from a bending magnet can be calculated with a relative standard uncertainty of 3 × 10−4 for photon energies below 3 keV, rising to 2 × 10−3 at 50 keV.

Characterization of detectors for extreme UV radiation

Accurate measurements of the radiant power and other quantities are a prerequisite for the development and optimization of suitable radiation sources for extreme ultraviolet (EUV) lithography. Photodiodes are established as easy-to-operate detectors also in the EUV range. The calibrations at the Physikalisch-Technische Bundesanstalt are based on the comparison of the detector to be calibrated with the cryogenic electrical substitution radiometer as a primary detector standard using monochromatized synchrotron radiation at the soft x-ray radiometry beamline in the spectral range from 1 nm up to 25 nm. The spectral responsivity is measured with a relative uncertainty of 0.3% or better. For the dissemination of these high-accuracy calibrations, we investigated the stability and linearity of silicon n-on-p junction photodiodes under intense EUV irradiation in ultra-high vacuum. The maximum current in linear operation (1% relative saturation) depends on the size of the photon beam and ranges from about 3 mA for a 6 mm photon beam diameter to 0.2 mA for a 0.25 mm diameter spot. Diodes with diamond-like carbon or a TiSiN top layer proved to be stable up to a radiant exposure of about 100 kJ cm−2. Furthermore, examples of the calibration of spectrally and spatially resolving radiometric tools for EUV-source characterization are presented.

X-ray detector calibration in the PTB radiometry laboratory at the electron storage ring BESSY II

Abstract The Physikalisch-Technische Bundesanstalt (PTB) currently operates a laboratory for radiometry from the VUV to the hard X-ray spectral range at the 1.7 GeV electron storage ring BESSY II. Detector calibration, performed by the PTB at BESSY I up to photon energies of about 1.5 keV, can now be extended up to 10.5 keV based on an electrical substitution radiometer as a primary detector standard and on the use of monochromatized bending magnet and undulator radiation of high spectral purity. This energetic range will be further extended up to 50 keV by using radiation from a 7 T wavelength shifter. Moreover, BESSY II is a primary source standard offering calculable bending magnet radiation. The PTB uses these primary standards for the absolute calibration of detectors with relative uncertainties below 1%. Up until now, photodiodes, energy-dispersive semiconductor detectors and also superconducting tunnel junctions have been characterized for photon energies up to 10.5 keV.

Molecular contamination mitigation in EUVL by environmental control

Abstract EUVL tools operate under vacuum conditions to avoid absorption losses. Under these conditions, the MoSi multilayer mirrors are contaminated, resulting in reduced reflection and thus throughput. We report on experiments on MoSi mirrors exposed to EUV radiation from a synchrotron. To mimic the effects of EUV radiation we also exposed samples using an electron gun. The oxidation rate was found to be ∼0.016 nm/h per mW/mm 2 of EUV radiation under conditions expected for a high throughput EUVL system. This oxidation can to a large extent be suppressed by using smart gas blend strategies during exposure, e.g. using ethanol. A carbon growth rate of 0.25 nm/h was found for a hydrocarbon pressure of 10 −9 mbar Fomblin. We demonstrate that carbonisation can be suppressed by admitting oxygen during electron gun exposure.

New PTB beamlines for high-accuracy EUV reflectometry at BESSY II

High-accuracy characterization of optical components has been one of the main services of the PTB radiometry laboratory at BESSY I. Now, after the shut down of BESSY I with the end of 1999, PTB is operating two new beamlines suitable for EUV reflectometry at their new laboratory at BESSY II. As at BESSY I, synchrotron radiation from a bending magnet is used for reflectometry but additionally a beamline at an undulator covering the same spectral range from 50 eV to 1800 eV can be used for special applications where, e.g., high radiant power or very high spectral purity is needed. In this paper, the characteristics of the beamlines are presented. We present the results of the beamline characterization on photon flux, spectral resolution, spectral purity and beam stability with special respect to the EUV photon energy range. During the phase of simultaneous operation of BESSY I and II in 1999, a direct comparison was done for reflectance measurements at high equality Mo/Si EUV mirrors. The results showed perfect agreement: (68.98 +/- 0.17)% at BESSY I and (69.10 +/- 0.24)% at BESSY II. The wavelength scale was calibrated using the absorption resonances of Ar, Kr, and Xe whose energies are known with a relative uncertainty of about 10-4. The measured peak positions agreed within this uncertainty.

Current capabilities at the Metrology Light Source

The Physikalisch-Technische Bundesanstalt (PTB) has set up the 630?MeV electron storage ring Metrology Light Source (MLS) in close cooperation with the Helmholtz-Zentrum Berlin (HZB). This electron storage ring has been in regular user operation since April 2008. It is dedicated to synchrotron-radiation-based metrology and technological developments in the far-IR/THz, IR, UV, VUV and EUV spectral ranges, with the use as primary source standard as the key activity. In a permanent process of improvement, the storage ring itself was optimized regarding its regular performance (beam current and lifetime) as well as for special operations (e.g. variable electron energies and electron bunch lengths). The measurement capabilities at the seven different beamline ports were set up sequentially, first in the UV/VUV and IR spectral ranges. This first phase of instrumentation set-up will be finished in 2011 by completing the beamlines for EUV metrology, for the calibration of radiation sources and for the application of undulator radiation.

Irradiation stability of silicon photodiodes for extreme-ultraviolet radiation

Photodiodes are used as easy-to-operate detectors in the extreme-ultraviolet spectral range. At the Physikalisch-Technische Bundesanstalt photodiodes are calibrated with an uncertainty of spectral responsivity of 0.3% or less. Stable photodiodes are a prerequisite for the dissemination of these high-accuracy calibrations to customers. Silicon photodiodes with different top layers were exposed to intense extreme-ultraviolet irradiation. Diodes coated with diamondlike carbon or TiSiN proved to be stable within a few percent up to a radiant exposure of 100 kJ/cm2. The changes in responsivity could be explained as being due to carbon contamination and to changes in the internal charge collection efficiency. In ultrahigh vacuum, no indication of oxidation was found.