A structure includes a substrate, a transistor, a contact, an oxygen-free etch stop layer, an oxygen-containing etch stop layer, a dielectric layer, and a via. The transistor is on the substrate. The contact is on a source/drain region of the transistor. The oxygen-free etch stop layer spans the contact. The oxygen-containing etch stop layer extends along a top surface of the oxygen-free etch stop layer. The dielectric layer is over the oxygen-containing etch stop layer. The via passes through the dielectric layer, the oxygen-containing etch stop layer, and the oxygen-free etch stop layer and lands on the contact. The memory stack lands on the via.