An image sensor device includes a substrate having a front surface and a back surface, and a deep trench disposed at the front surface of the substrate. The deep trench has sidewalls, a bottom and an opening. A dielectric layer is disposed along the sidewalls and the bottom of the deep trench. An epitaxial layer is disposed on the front surface of the substrate. The deep trench and the epitaxial layer collectively define an air chamber. The deep trench has a chamfered portion at an interface between the epitaxial layer and the front surface of the substrate. The chamfered portion is free of dielectric layer.