A MEMS structure includes a substrate, an inter-dielectric layer on a front side of the substrate, a MEMS component on the inter-dielectric layer, and a chamber disposed within the inter-dielectric layer and through the substrate. The chamber has an opening at a backside of the substrate. An etch stop layer is disposed within the inter-dielectric layer. The chamber has a ceiling opposite to the opening and a sidewall joining the ceiling. The sidewall includes a portion of the etch stop layer.