Vias are formed within a stack of alternating active and insulating layers by forming a first sub stack, a second sub stack over the first sub stack, a first buffer layer therebetween and a second buffer layer under the first sub stack. An upper layer of the first sub stack is exposed through a set of vias by first and second etching processes. The first etching process forms a first set of etch vias through the second sub stack and stops at or in the first buffer layer. The second etching process etches through the first buffer layer to the upper layer of the first sub stack. A third etching process etches through the first set of etch vias, through the first sub stack and stops at or in the second buffer layer. A fourth etching process and etches through the second buffer layer.