G. Brandt

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.

Characterization of the PTB EUV reflectometry facility for large EUVL optical components

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.

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.

High-accuracy detector calibration for EUV metrology at PTB

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.

The Metrology Light Source operated as a primary source standard

The Metrology Light Source (MLS)?the dedicated electron storage ring of the Physikalisch-Technische Bundesanstalt (PTB), designed for metrology and technological applications in the spectral range from the far IR to the VUV?started user operation in April 2008. The MLS is used as a primary source standard from the NIR to the VUV spectral region and is therefore equipped with all the instrumentation necessary to measure with low uncertainty the storage ring parameters and the geometrical parameters needed for the calculation of the spectral photon flux according to the Schwinger theory. It can be operated at any electron beam energy between 105?MeV and 630?MeV and at electron beam currents varying from 1?pA (one stored electron) up to 200?mA, which allows conditions to be tailor-made for special applications.

High-Accuracy Calibration of the HXDS Flow Proportional Counter for AXAF at the PTB Laboratory at BESSY

The Smithsonian Astrophysical Observatory uses the HRMA X- ray Detection System (HXDS) to calibrate the High-Resolution Mirror Assembly of the Advanced X-ray Astrophysics Facility AXAF. Apart from two high-purity-germanium solid-state detectors (SSDs) with good energy resolution and very high efficiency at higher energies, the detection system comprises seven flow proportional counters (FPCs) and one microchannel-plate High-Speed Imager. For the lower energy range, the FPCs are more appropriate. They have been calibrated at the radiometry laboratory of the Physikalisch- Technische Bundesandstalt, using the electron storage ring BESSY. For the determination of the absolute quantum efficiency two methods have been applied. First, the detector response was measured in the lower energy range 0.1 keV to 1.7 keV at several discrete energies using monochromatized radiation. The absolute photon flux has been determined by Si n-on-p photodiodes, calibrated against a cryogenic electrical-substitution radiometer used as primary detector source standard BESSY, which can be calculated very accurately. Combining both measurements the determination of the detection efficiency over the entire desired spectral range was possible with a typical relative uncertainty around 1 percent to 2 percent in the central energy range.

Radiometric comparison of the primary source standard 'Metrology Light Source' to a primary detector standard

The spectral radiant intensity of synchrotron radiation from electron storage rings can be calculated from basic electrodynamic relations (Schwinger equation). A storage ring can, thus, be used as a radiometric primary source standard. The Metrology Light Source (MLS), the dedicated electron storage ring of the Physikalisch-Technische Bundesanstalt, can be operated as a primary radiation source standard from the near infrared up to the soft x-ray region and its operational parameters can be adjusted and accurately measured in a wide range. The MLS electron beam current can be varied from 1 pA (one stored electron) up to 200 mA and, thus, the radiant intensity, being directly proportional to the stored current, can be varied by more than 11 decades. The electron energy, which has a large effect on the spectral shape of the spectrum, can be varied from 105 MeV up to 630 MeV. For the radiometric comparison, the total power irradiated into a well-defined solid angle was directly measured by a cryogenic radiometer as a primary detector standard and compared with the power calculated from the storage ring parameters. These measurements were performed for various electron energies in the range from 200 MeV to 630 MeV, thus varying the shape of the spectrum. For each electron energy setting, the power level was varied within the dynamic range of the cryogenic radiometer by a variation of the electron beam current. Good agreement was found for all parameter settings, thus validating the performance of the cryogenic radiometer and the storage ring parameter measurements within the related uncertainty budget.

The Metrology Light Source — the New Dedicated Electron Storage Ring of PTB

The Physikalisch‐Technische Bundesanstalt (PTB) is currently constructing a low‐energy electron storage ring in the close vicinity of BESSY II where PTB operates a laboratory for synchrotron‐radiation‐based metrology, mainly in the X‐ray spectral region. The new storage ring, which will be called ‘Metrology Light Source’ (MLS), will mainly be dedicated to metrology and technological development in the UV, EUV and VUV spectral range and will thus fill the gap in the spectral range that has opened up since the shut‐down of BESSY I. Moreover, the MLS will deliver intense radiation in the IR and FIR/THz spectral range. The MLS can be operated with parameters optimized for special calibration tasks, which, at a multi‐user facility such as BESSY II is rarely possible. The electron energy can be tuned in the range from 200 MeV up to 600 MeV and the electron beam current can be adjusted from 1pA (single electron) up to 200 mA. All relevant storage ring parameters can be measured with high accuracy, thus making th...