A backside illuminated (BSI) image sensor device includes a device layer, a doped isolation region and a doped radiation sensing region. The device layer has a front side and a backside, in which the device layer has a thickness greater than or equal to 4 μm. The doped isolation region having a first dopant of a first conductivity is through the device layer to define a plurality of pixel regions of the device layer, in which the doped isolation region includes a first upper region adjacent to the front side and a first lower region between the first upper region and the backside, and the first upper region has a width less than a width of the first lower region. The doped radiation sensing region having a second dopant of a second conductivity opposite to the first conductivity is in one of the pixel regions of the device layer.