Properties of EUVL masks as a function of capping layer and absorber stack structures

We have fabricated extreme ultraviolet lithography (EUVL) blank masks consisting of a TaN absorber, Ru capping layer, and Mo/Si multilayers using ion-beam sputter deposition and investigated their dependence on capping layer and absorber stack structure. At EUV wavelengths, the reflectivities of the multilayers, including their dependency on the thickness of the capping and absorber layers, are in good agreement with simulation results obtained using Maxwell equations and the refractive indexes of each layer. Ru, one of the most promising capping materials on Mo/Si multilayers due to its resistance to oxidation and selectivity to etching, also shows better EUV reflectivity than Si as a capping layer if we choose a thickness that produces a constructive interference. To meet the reflectivity requirements (<= 0.5 %) in the SEMI EUVL mask standard specifications, a TaN absorber at least 70 nm thick should be applied. However, aerial image results simulated by using EM-Suite show that 40 nm is sufficient for the TaN absorber to display the maximum image contrast. In addition, horizontal-vertical (HV) biasing effects due to mask shadowing become negligible if the TaN is reduced to about 40 nm. As a result, we suggest using a thin TaN absorber 40 nm thick since it is able to minimize mask shadowing effects without a loss of image contrast.