A method includes bonding a front side surface of a first wafer to a second wafer; performing a multi-trimming process on the first and second wafers from a back side surface of the first wafer, the multi-trimming process comprising: performing a first trimming step from the back side surface of the first wafer to cut through a periphery of the first wafer; performing a second trimming step on the second wafer to partially cut a periphery of the second wafer to form a first step-like structure; and performing a third trimming step on the second wafer to partially cut the periphery of the second wafer to form a second step-like structure connecting down from the first step-like structure; after performing the multi-trimming process, forming a coating material at least over the periphery of the second wafer.