A lithography mask includes a substrate that contains a low thermal expansion material (LTEM). The lithography mask also includes a reflective structure disposed over the substrate. The reflective structure includes a first layer and a second layer disposed over the first layer. At least the second layer is porous. The mask is formed by forming a multilayer reflective structure over the LTEM substrate, including forming a plurality of repeating film pairs, where each film pair includes a first layer and a porous second layer. A capping layer is formed over the multilayer reflective structure. An absorber layer is formed over the capping layer.