Substrate recovery layers for EUVL optics: Effects on multilayer reflectivity and surface roughness

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.