Repair of soft-x-ray optical elements by stripping and redeposition of Mo/Si reflective coatings

Experiments aimed at recovering soft-x-ray projection lithography mirrors that have defective reflective coatings by stripping and redeposition of Mo/Si multilayers are being reported. Two wet etchants were found that would remove the multilayers from composite glass substrates. One etchant is a solution of potassium ferricyanide and sodium hydroxide in water; the other is nitric acid with a minute quantity of hydrofluoric acid. Although each of these solutions offers preferential etch selection of the multilayers over the substrate glass, the selectivities are insufficient. The result is a surface roughening that lowers the reflectance from subsequently deposited multilayer coatings to an unacceptable level. With the addition of a carbon barrier layer between the substrate and the multilayer coating, either etchant can be used to strip the multilayers without damage to the underlying substrate surface.