A device includes a Backside Illumination (BSI) image sensor chip, which includes an image sensor disposed on a front side of a first semiconductor substrate, and a first interconnect structure including a plurality of metal layers on the front side of the first semiconductor substrate. A device chip is bonded to the image sensor chip. The device chip includes an active device on a front side of a second semiconductor substrate, and a second interconnect structure including a plurality of metal layers on the front side of the second semiconductor substrate. A first via penetrates through the BSI image sensor chip to connect to a first metal pad in the second interconnect structure. A second via penetrates through a dielectric layer in the first interconnect structure to connect to a second metal pad in the first interconnect structure, wherein the first via and the second via are electrically connected.