The internal structure of Mo/Si multilayers is investigated during and after thermal annealing. Multilayer period
compaction is shown to result from diffusion induced MoSi2 interlayer growth, reducing optical contrast and changing
the reflected wavelength. We focus on early-stage interface growth observed at relatively low temperatures (100 °C - 300
°C), determining diffusion constants from parabolic interface growth laws. Diffusion constants obey Arrhenius-type
behavior, enabling temperature scaling laws. Using the methods developed, we compare results on Mo/Si based
multilayers designed for enhanced thermal stability and discuss their relevant diffusion behavior. Arrhenius-type
behavior can be observed in all multilayers studied here, and demonstrates reduction of diffusion rates over several
orders of magnitude. The method described here is of general interest for any multilayer application that is subjected to
enhanced thermal loads and demonstrates the enormous technology gain that this type of optics has experienced the last
decade.
We investigated the use of separation, or substrate recovery, layers (SRLs), to enable the reuse of optical substrates after the deposition of multilayer reflective coatings, in particular Mo/Si multilayers as used for EUV lithography. An organic material (polyimide), known from other work to reduce the roughness of the substrate, was applied to the optical substrate. It appeared to be possible to remove the multilayer coating, including the SRL, without any damage or roughening of the substrate surface. The SRL was spin-coated at 1500 to 6000 rpm on different substrate types (Si, quartz, Zerodur) with diameters up to 100 mm. For this range of parameters, the multilayer centroid wavelength value remained unchanged, and its reflectivity loss on applying the SRL was limited typically to 0.7%. The latter was shown to be caused by a minor increase of the SRL surface roughness in the high-spatial-frequency domain. The roughness, characterized with an atomic force microscope, remained constant at 0.2 nm during all stages of the substrate recovery process, independent of the initial substrate roughness.
A new deposition technique that builds on the thermal particle characteristics typical for e-beam deposition is described.
This technique applies magnetron sputtering in a special scheme where these characteristics of the e-beam deposition
method are achieved. The method was used for interface engineering of Mo/Si multilayers, with different barrier layer
materials being tested. Composition of the barrier layers formed was studied using XPS. Results are shown on the
general example of a Mo/B4C/Si/B4C system. The ultra-thin reflectance enhancement B4C barriers can be deposited with
low added stress, resulting in a multilayer stress as low as about -150 MPa. The best interface engineered multilayers
reflect 70.5% at 13.3 nm and 70.15% at 13.5 nm. These results were achieved with 50 period multilayers terminated with
a standard Si layer.
We have investigated the use of separation, or substrate recovery layers (SRL) enabling the re-usage of optics
substrates after deposition of multilayer reflective coatings, in particular Mo/Si multilayers as used for Extreme UV
lithography. An organic material, a polyimide, was applied, from other work known to reduce the roughness of the
substrate 1, 2. It appeared to be possible to remove the multilayer coating, including the SRL, without any damage or
roughening of the substrate surface. The SRL was spin-coated at 1500 - 6000 rpm on different substrate types (Si,
quartz, Zerodur) with diameters up to 100 mm. For this range of parameters, the multilayer centroid wavelength value
remained unchanged, while its reflectivity loss, upon applying the SRL, was limited to typically 0.7%. The latter is
demonstrated to be caused by a minor increase of the SRL surface roughness in the high spatial frequency domain. The
AFM characterized roughness remained constant at 0.2 nm during all stages of the substrate recovery process,
independent of the initial substrate roughness.
E. Louis, A. Yakshin, E. Zoethout, R. van de Kruijs, I. Nedelcu, S. van der Westen, T. Tsarfati, F. Bijkerk, H. Enkisch, S. Müllender, B. Wolschrijn, B. Mertens
Reported is a summary of the development of EUV Mo/Si multilayer coating technology. Though the results are developed for application in Extreme Ultraviolet Lithography, they are of a broader relevance including optics for astronomy. The coating process used consists of electron beam evaporation in combination with low energy ion beam smoothening. The radiation hardness of these coatings is discussed and methods to reduce the multilayer induced substrate stress. The reflectance of the coatings, which are covered with a special protective capping layer, is typically around 65%, while the non correctable figure error added by the full multilayer stack is controlled to better than 15 picometer.
Reported is a summary of the coating of three elements of the illuminator and three of the projection optics of the EUVL Process Development Tool. The coating process used is e-beam evaporation in combination with low energy ion beam smoothening. The reflectance of the coatings, which are covered with a special protective capping layer, is typically around 65% and the non correctable figure error that is added by the full multilayer stack is controlled to better than 15 picometer.
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