Marcus Trost

Influence of the substrate finish and thin film roughness on the optical performance of Mo/Si multilayers

Scattering resulting from interface imperfections critically affects the image contrast and optical throughput of multilayer coatings for 13.5 nm. To investigate the scattering mechanisms, at-wavelength scattering measurements in combination with atomic force microscopy are analyzed for an in-depth characterization of the roughness properties. The different impacts of substrate finish and intrinsic thin film roughness on the scattering distribution are separated and analyzed in detail. Furthermore, a novel approach to characterize the roughness of large extreme ultraviolet substrates is presented, based on light scattering measurements at 442 nm.

Angle-resolved scattering and reflectance of extreme-ultraviolet multilayer coatings: measurement and analysis

Roughness-induced light scattering critically affects the performance of optical components, in particular at short wavelengths. We present a stand-alone instrument for angle-resolved scattering and reflectance measurements at 13.5 nm in the extreme-ultraviolet (EUV) spectral range. The achieved dynamic range allows even the scattering of high-quality EUV mirrors on extremely smooth substrates to be investigated. For Mo/Si multilayers, total scatter losses of several percent have been observed, depending on the substrate qualities as well as on roughening and smoothing effects during coating. Different approximate models for estimating the impact of roughness on scatter losses are discussed and compared with experimental results.

Comprehensive nanostructure and defect analysis using a simple 3D light-scatter sensor

Light scattering measurement and analysis is a powerful tool for the characterization of optical and nonoptical surfaces. A new 3D scatter measurement system based on a detector matrix is presented. A compact light-scatter sensor is used to characterize the scattering and nanostructures of surfaces and to identify the origins of anisotropic scattering features. The results from the scatter sensor are directly compared with white light interferometry to analyze surface defects as well as surface roughness and the corresponding scattering distributions. The scattering of surface defects is modeled based on the Kirchhoff integral equation and the approach of Beckmann for rough surfaces.

Evaluation of subsurface damage by light scattering techniques

Subsurface damage (SSD) in optical components is almost unavoidably caused by mechanical forces involved during grinding and polishing and can be a limiting factor, in particular for applications that require high laser powers or an extreme material strength. In this paper, we report on the characterization of SSD in ground and polished optical surfaces, using different light scattering measurement techniques in the visible and extreme ultraviolet spectral ranges. The materials investigated include fused silica, borosilicate glass, and calcium fluoride. The scattering results are directly linked to classical destructive SSD characterization techniques, based on white light interferometry, optical microscopy, and atomic force microscopy of the substrate topography and cross sections obtained after etching in hydrofluoric acid and fracturing.

Structured Mo/Si multilayers for IR-suppression in laser-produced EUV light sources

Laser produced plasma sources are considered attractive for high-volume extreme-ultraviolet (EUV) lithography because of their high power at the target wavelength 13.5 nm. However, besides the required EUV light, a large amount of infrared (IR) light from the CO2 drive laser is scattered and reflected from the plasma as well as from the EUV mirrors in the optical system. Since these mirrors typically consist of molybdenum and silicon, the reflectance at IR wavelengths is even higher than in the EUV, which leads to high energy loads in the optical system. One option to reduce this is to structure the EUV multilayer, in particular the collector mirror, with an IR grating that has a high IR-suppression in the zeroth order. In this paper, the characterization of such an optical element is reported, including the IR-diffraction efficiency, the EUV performance (reflectance and scattering), and the relevant surface roughness. The measurement results are directly linked to the individual manufacturing steps.

Roughness and optical losses of rugate coatings

Light-scattering measurements on rugate coatings made out of mixtures of Si(x)Ta(y)O(z) and Si(x)Hf(y)O(z) were performed. Through successive optimization steps for the substrate roughness and deposition parameters, the overall scattering loss could be reduced by 96% to 3.5 ppm. In order to analyze the relevant scattering mechanisms in such coatings, different theoretical models for scattering from bulk and surface imperfections are compared to measured data. The best accordance between simulated and measured data could be achieved for the theory based on bulk imperfections, while the classical roughness based theory, which is used for conventional multilayer systems, gives reasonable good results.

Roughness characterization of large EUV mirror optics by laser light scattering

Optical components for extreme ultraviolet (EUV) lithography at a wavelength of 13.5 nm face tremendous requirements on the surface finish, because large amounts of the EUV light are lost as a result of roughness-induced scattering. In this paper we present a novel approach for the roughness characterization of large EUV mirror optics based on light scattering measurements at a wavelength of 442 nm. The high sensitivity to roughness and the robustness of this method are exemplified for a 660 mm diameter collector mirror substrate. Area covering images of the high-spatial frequency roughness are retrieved which enable a detailed prediction of the EUV reflectance prior to coating. The results are compared to EUV reflectance measurements after coating.

Sub-aperture EUV collector with dual-wavelength spectral purity filter

The combination of a 10.6 μm main pulse CO2 laser and a 1064 nm pre-pulse Nd:YAG laser in EUV source concepts for HVM would require collector mirrors with an integrated spectral purity filter that suppresses both laser wavelengths. This paper discusses a new approach of a dual-wavelength spectral purity filter to suppress 10.6 μm and 1064 nm IR radiation at the same time. The dual-wavelength spectral purity filter combines two binary phase gratings that are optimized for 10.6 μm and 1064 nm, respectively. The dual phase grating structure has been realized on spherical sub-aperture EUV collector mirrors having an outer diameter of 150 mm. IR suppression factors of 260 at 10.6 μm and 620 at 1064 nm have been measured on the sub-aperture EUV collector while its EUV reflectance exceeded 64 % at 13.5 nm.

Sophisticated light scattering techniques from the VUV to the IR regions

Light scattering based characterization techniques are well suited to meet the challenging requirements for fast and sensitive finish assessment of optical surfaces. Further advantages are the high flexibility and robustness which enable the inspection of large geometries and freeform optics that are sometimes too complex for characterization techniques like atomic force microscopy or white light interferometry. In this paper, we report on the development of instruments for total and angle resolved light scattering measurements at wavelengths ranging from the vacuum ultraviolet to the infrared spectral regions. Extremely high sensitivities equivalent to surface roughness levels of below 0.1 nm and dynamic ranges of up to 15 orders of magnitude have been achieved. In addition to laboratory-based equipment, compact and table-top tools are discussed which enable the advantages of light scattering metrology to be used for characterization tasks close to or even in manufacturing processes. Instructive examples of applications are presented ranging from the characterization of diamond-turned and polished substrates to interference coatings, diffraction gratings, and IR window materials.

Efficient specification and characterization of surface roughness for extreme ultraviolet optics

EUV mirrors are cutting-edge optical surfaces. Meeting the roughness specifications over the entire range of relevant spatial frequencies is a challenging process. Recent developments that might help to increase the efficiency of EUV mirror production will be discussed including relaxed roughness specifications using the new Generalized Harvey Shack theory as well as a new approach for roughness measurements during and after manufacturing based on light scattering measurements and analysis. The method provides area covering images of the distribution of high-spatial frequency roughness (HSFR) over entire mirrors. Results will be presented for 660 mm diameter EUV collector mirror substrates.