Frank Scholz

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.

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.

Status of EUV reflectometry at PTB

The development of EUV lithography is critically based on the availability of suitable metrological equipment. To meet the industrys requirements, the Physikalisch-Technische Bundesanstalt (PTB) operates an EUV reflectometry facility at the electron storage ring BESSY II. It is designed for at-wavelength metrology of full-sized EUVL optics with a maximum weight of 50 kg and a diameter of up to 550 mm. A micro-reflectometry station was installed for reflectometry with high spatial resolution for, e.g., structured masks. A photon beam size of 10 μm FWHM has presently been achieved. To meet the increasing demands of metrology for future lithography production tools, the measurement uncertainty was permanently reduced. For peak reflectance, a total uncertainty of 0.10 % is achieved with a reproducibility of 0.05 %. The uncertainty of 2 pm in the center wavelength is given mainly by the uncertainty for the reference wavelength of the Kr 3d5/2-5p resonance. A long-term reproducibility of 0.8 pm has been demonstrated over a period of about 4 years. We have recently demonstrated repeatability below 0.06 pm. This good repeatability is important for the determination of the coating-thickness gradient in alpha-tool optics. We present a long-term series of measurements at a set of EUV mirrors and discuss our recent results in improving wavelength reproducibility.

Characterization of large off-axis EUV mirrors with high accuracy reflectometry at PTB

CZ SMT AG produced large off-axis EUV mirrors as they are used e.g. in ASMLs alpha demo tools, the predecessor for Extreme Ultraviolet Lithography (EUVL) production tools by ASML. The coating development and a large part of the actual coatings were done by the FOM-Institute. The Physikalisch-Technische Bundesanstalt (PTB) operates an EUV reflectometry facility at the electron storage ring BESSY II for at-wavelength metrology of full-size EUVL optics with a weight of up to 50 kg and a diameter of 550 mm. Critical issues for EUVL mirrors are the high reflectivity close to the theoretical limit, the matching of the period to the operating wavelength of the stepper (13.5 nm) and the imaging properties of the EUV optics. The full multilayer stack needs to be controlled laterally to such extend that the initial sub-nanometre surface figure of the substrate is preserved. The so-called added figure error should not exceed 100 pm in order to ensure faultless imaging at 13.5 nm wavelength. Here, we discuss representative results obtained at large off-axis EUV mirrors. We especially discuss the challenges of measurements at higher local angles of incidence according to the optical design and the accuracy needed in sample alignment for measurement of the coating profiles. PTB has shown excellent reproducibility for measurements of the near normal incidence reflectance of flat homogeneous mirrors over several years. For large off-axis EUV mirrors, measurements have to be done at angles significantly off normal, which dramatically increases the influence of angular alignment errors of the sample on the measured peak wavelength. Furthermore, according to the optical design, these optics have gradients of the coating thickness which require exact knowledge of the measurement position in the mirror coordinates. Extensive studies were done to estimate and validate the uncertainties connected to the sample alignment. Our results clearly show that it is possible to meet and verify the tight specifications for the lateral coating profiles of EUV multilayer mirrors. The non-correctable added figure error is significantly better than required and the overall reflectance of the coatings with a special protective capping layer is 65%.

Polarization dependence of multilayer reflectance in the EUV spectral range

The Physikalisch-Technische Bundesanstalt (PTB) with its laboratory at the electron storage ring BESSY II supports the national and European industry by carrying out high-accuracy at-wavelength measurements in the EUV spectral region, particularly to support the development of EUV lithography, which holds the key to the next generation of computer technology. PTB operates an EUV reflectometry facility, designed for at-wavelength metrology of full-size EUVL optics with a maximum weight of 50 kg and a diameter of up to 550 mm and a micro-reflectometry station for reflectometry with sub 10 μm spatial resolution. An absolute uncertainty of 0.10 % is achieved for peak reflectance, with a reproducibility of 0.05 %. For the center wavelength an uncertainty of 2 pm is achieved with a long-term reproducibility of 1.1 pm and a short-term repeatability below 0.06 pm. Measurements at PTB use linearly polarized radiation, whereas EUV optics are operated with unpolarized sources and the status of polarization changes throughout the optical system. Therefore, to transfer these high-accuracy measurements to the EUV optical components under working conditions, it is essential to study the polarization dependence in detail. The degree of linear polarization in the EUV reflectometer is 97%. Representative polarization dependencies obtained on Mo/Si multilayer coatings over a wide range of angles of incidence reveal that the accuracy of calculations with the IMD-code is presently limited by the optical data available.

Characterization of a laser-produced plasma source for a laboratory EUV reflectometer

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.

Accumulation of 38?mg of bismuth in a cylindrical collector from a 2.5?mA ion beam

An experiment for the direct measurement of the atomic mass unit is under way by accumulating ions from an ion beam up to a weighable mass. In the first experiment, a mass of more than 38 mg of bismuth was accumulated, and the atomic mass unit was reproduced with a relative deviation of 9 × 10−4 from the CODATA value. Details of the experimental setup and accumulation process are presented.

Using an intense bismuth ion beam for the accumulation of a weighable mass of atoms

At the Physikalisch-Technische Bundesanstalt, an experiment for the direct measurement of the atomic mass unit is underway by accumulating ions from an ion beam up to a weighable mass. Until now, an intense bismuth ion beam of /spl ap/5 mA can be extracted out of a cold or hot reflex discharge ion source (CHORDIS). Different arrangements of the beamline ensuring a high transmission will be discussed. Finally, suitable geometries of an ion collector are shown based on SIMION-3D simulations.

Optimum Extraction From a CHORDIS by Pressure Variation

The influence of the working gas pressure on the extraction behavior of ion beams out of a cold or hot reflex discharge ion source was investigated using xenon ions in the energy range from 17 to 30 keV. Optimal extraction conditions were achieved by transmission measurements through a 90 doubly focusing dipole magnet. The measured currents obtained at the optimized transmission reproduce the Child-Langmuir equation. The results are compared with IGUN simulations of the extraction system.

The determination of the atomic mass constant with ion accumulation: status and perspectives

An experiment is described for determining the atomic mass constant by accumulating ions from an ion beam up to a weighable mass. It establishes a link between the international prototype of the kilogram, the realization of the associated SI unit and an atomic mass. The items necessary for such an experiment are a high-current ion source, an ion optical system with high transmission, a suitable ion collector and a vacuum balance. With the most recent measurement, a mass of more than 320 mg of bismuth was accumulated and its atomic mass was determined with a relative standard uncertainty of 9.4 × 10−5. Special emphasis is placed on determining the mass loss of the accumulated ions caused through sputter effects in the ion collector.Although work on this experiment at the PTB has now been stopped, we conclude with a number of suggestions that could lead to much smaller relative uncertainty in a future experiment.