An interconnect structure and a method of forming an interconnect structure are disclosed. The interconnect structure includes a lower conductive feature in a lower low-k (LK) dielectric layer; a first etch stop layer (ESL) over the lower conductive feature, wherein the first ESL comprises a metal compound; an upper LK dielectric layer over the first ESL; and an upper conductive feature in the upper LK dielectric layer, wherein the upper conductive feature extends through the first ESL and connected to the lower conductive feature. The interconnect structure may further include a second ESL between the upper LK dielectric layer and the first ESL, or between the first ESL and the lower conductive feature, wherein the second ESL comprises a silicon compound.