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.
It is well known that molybdenum and silicon is the combination with the highest EUV reflectivity of two-component multilayers in the wavelength range lambda=12.5-20nm. Using the magnetron sputter deposition method multilayers with reflectivities of typically 69% can be prepared. A further increase to R=70% was demonstrated by the introduction of tiny barrier layers (C and/or B4C) between the Mo and Si layers, which reduce the interdiffusion of both chemical elements. However, still higher reflectivities are desirable for the use of the multilayers as reflectors for EUV lithography. From model considerations we have concluded that the replacement of the Mo absorber layer by a multi-component layer consisting of two or three layers could result in an EUV reflectivity increase of up to 0.5% compared to the pure Mo/Si system assuming sharp interfaces without any roughness. Particularly Ag and Ru are promising candidates as additional elements within the absorber. Therefore we have systematically changed the thicknesses of the individual layers under the boundary condition of a fixed period thickness of dp=(6.90±0.05)nm. Microstructure and optical parameters of the multilayers have been investigated by HRTEM, X-ray diffractometry and Cu-Ka reflectometry. The most promising multilayers have also been characterized by EUV reflectometry.
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.