Thomas Foltyn

Multi component EUV multilayer mirrors

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.

Highly reflective coatings for micromechanical mirror arrays operating in the DUV and VUV spectral range

High reflecting low-stress optical coatings for the next-generation of micro mechanical mirrors have been developed. The optimized metal systems are applicable from VUV and DUV down to the UV and VIS spectral region and can be integrated in the technology of MOEMS, such as spatial light modulators (SLM) and micro scanning mirrors. This optimized metal designs enable to reconcile high optical performances with adequate mechanical properties and convenient CMOS compatibility. Currently, micro-mirror arrays with enhanced highly reflective coatings for DUV (λ = 193 nm) and VUV (λ = 157 nm) exist as prototypes.

Highly reflective optical coatings for high-power applications of micro scanning mirrors in the UV-VIS-NIR spectral region

This paper addresses different highly reflective optical coatings on micro scanning mirrors (MSM) for applications in the NIR-VIS-UV- spectral region to enable new applications at high optical power density like laser marking and material treatment. In the common case of MSM with an unprotected Al coating, the absorption limits the maximal power density because of induced heating. In contrast to macroscopic optics HR-micro mirror coatings have to guarantee additional demands like low-stress and CMOS compatibility. Hence, to enable novel high power applications of MSM in the NIR-VIS-UV spectral region highly reflective low-stress coatings have been developed according to a triple strategy: (a) broadband metallic reflectors, (b) dielectric multilayers and (c) enhanced hybrid coatings. For Au and Ag based NIR-coatings an excellent mirror planarity and a reflectance around 99 % (@ 1064 nm) have been achieved, whereas dielectric coatings reached 99.7 % for a (LH)4 design and thinner low-stress hybrid NIR-coatings reached up to 99.8 % enabling an improved mirror planarity and excellent laser damage threshold. For the VIS and UV spectral region enhanced hybrid HR-coatings have been favored, because they enable high reflectance of up to 99.7 % @ 633 nm or 98.8 % @ 308 nm in combination with low stress, high mirror planarity and CMOS compatibility.

Micromechanical scanning mirrors with highly reflective NIR coatings for high power applications

This paper addresses different highly reflective optical coatings on micro scanning mirrors (MSM) for applications in the NIR-spectral region to enable new applications like laser marking and material treatment at high optical power density. In the case of MSM with an unprotected Al coating, the absorption limits the maximal power density because of induced heating. The damage threshold for unprotected Al coatings was investigated. In addition highly reflective enhanced metallic and dielectric multilayer coatings for the NIR have been developed and characterized. These coatings resolve the problems of unprotected aluminum coatings related to NIR absorption and the resulting limitation of applicable laser power density. The coatings ensure a high reflectance even in corrosive environments. Enhanced metallic broadband reflectors reach a reflectivity of 98.7% at 1064 nm whereas narrow-band dielectric multilayer coatings reach a reflectivity of 99.7% at 1064 nm.

Highly reflective thin film coatings for high power applications of micro scanning mirrors in the NIR-VIS-UV spectral region

This paper addresses different highly reflective optical coatings on micro scanning mirrors (MSM) for applications in the NIR-VIS-UV-spectral region to enable new applications at high optical power density like laser marking and material treatment. In the common case of MSM with an unprotected Al coating, the absorption limits the maximal power density because of induced heating. In contrast to macroscopic optics HR-micro mirror coatings have to guarantee additional demands like low-stress and CMOS compatibility. Hence, to enable novel high power applications of MSM in the NIR-VIS-UV spectral region highly reflective low-stress coatings have been developed according to a triple strategy: (a) broadband metallic reflectors, (b) dielectric multilayers and (c) enhanced hybrid coatings. For Au and Ag based NIR-coatings an excellent mirror planarity and a reflectance around 99 % (@ 1064 nm) have been achieved, whereas dielectric coatings reached 99.7 % for a (LH)4 design and thinner low-stress hybrid NIR-coatings reached up to 99.8% enabling an improved mirror planarity and excellent laser damage threshold. For the VIS and UV spectral region enhanced hybrid HR-coatings have been favored, because they enable high reflectance of up to 99.7 % @ 633 nm or 98.8 % @ 308 nm in combination with low stress, high mirror planarity and CMOS compatibility.

Carbon buffer layers for smoothing substrates of EUV and x-ray multilayer mirrors

Smoothing of surfaces by thin film deposition is facilitated by methods which release hyperthermal particles on the substrate. One of these techniques is pulsed laser deposition (PLD), with high kinetic particle energies of up to several 100 eV. The concrete energy distribution of the particles can be widely influenced by the laser power density. We investigated the deposition of carbon layers by PLD on numerous substrates with rms-roughnesses between 0.15 and 0.75 nm using different laser power densities and film thicknesses. It turns out that a better smoothing can be obtained with higher laser power densities, whereby diamond-like carbon films are created. With typical thicknesses of dC = 100 nm, the rms-roughness is reduced from 0.75 nm to 0.55 nm and from 0.32 nm to 0.18 nm. Accordingly by applying smoothing carbon buffer layers, the EUV reflectance of Mo/Si multilayers on rough substrates is increased from typically 60% to > 65% on substrates with initial roughnesses of 0.75 nm.

Design and development of an optical system for EUV-microscopy

In the paper we describe the development of a reflective optical system for EUV-microscopy containing an ellipsoidal formed collector optics and a Schwarzschild objective (magnification M=21, numerical aperture NA=0.2) for EUV radiation of a wavelength λ=13.5nm. In order to collect the maximum intensity of an EUV gas discharge plasma source, the grazing incidence collector has been inside-coated with molybdenum by Pulsed Laser Deposition (PLD). This method enables the deposition of uniform and highly reflective molybdenum layers, which have been protected against oxydation by using thin carbon top layers. The two mirrors of the Schwarzschild objective consist of highly reflective Mo/Si- multilayers produced by Magnetron Sputter Deposition (MSD). In order to obtain the best optical performance, laterally graded multilayers with rotational symmetry have been deposited by using a new mask-deposition technique. Thus the multilayer thickness corresponds at each point of the curved mirrors to wavelength and incidence angle of the EUV beam. Ray tracing simulations were performed for the two optical elements, collector optics and Schwarzschild objective. The results of these calculations are shown and compared with the results obtained by the EUV-microscope.

Deposition of multilayer mirrors with arbitrary period thickness distributions

In this paper we will describe a new approach for the deposition of multilayers with arbitrary period thickness distributions. The standard technique of magnetron sputter deposition has been extended to a design where a special mask with lateral varying particle transmission is placed in front of the substrate to be coated. Planar and curved substrates have been used to deposit multilayers with prescribed period thickness gradients. The realized laterally graded multilayer interference mirrors have been investigated by using X-ray and EUV-reflectometry. For the example of Mo/Si multilayers as normal incidence reflectors for EUV light at 13.5 nm it has been demonstrated that high-reflection multilayer mirrors can be deposited using the new deposition technique. Typical EUV reflectance of Mo/Si multilayer with carbon barrier layers are in the order of 70%. In addition, non-uniform masks have been used and several 1- and 2- dimensional period thickness gradients have been coated. The nominal thicknesses were compared with the actual values. Furthermore, steep gradients of the period thickness with thickness changes of typically 5% along a length of 5mm have been prepared and characterized.

Smoothing of substrate roughness by carbon-based layers prepared by pulsed laser deposition (PLD)

In order to obtain high reflectance of EUV and X-ray multilayer mirrors, highly polished substrate surfaces with rms roughness σrms = 0,1-0.2 nm are necessary. However, the simultaneous achievement of low micro-roughness and precise surface figure is very challenging and often not accomplished. Therefore deposition techniques capable to deposit layers with smoothing properties are very desirable. One potential method that enables the formation of such layers is the pulsed laser deposition (PLD). This technique generates particles with high kinetic energies of up to several 100 eV. We investigated the deposition of carbon based smoothing layers by PLD on numerous substrates with roughness between σrms = 0.15 and 0.75 nm using different laser power densities and film thicknesses. Besides pure carbon layers we also used metal/carbon (metal = Ni, W, Pt) multilayers with respect to their capabilities to smooth surface roughness. As a general trend it turns out that a better smoothing can be obtained with higher laser power densities, whereby diamond-like carbon films are created. Furthermore, the intrinsic stress of the smoothing layers has been investigated. Due to the high kinetic energy of the impinging particles during the film growth, the layers show compressive stress. The degree of the stress depends on the concrete metal that is combined with carbon in the multilayer stack. Up to now the lowest compressive stress is obtained with Ni/C multilayers.