A wafer-to-wafer bonding structure includes a first wafer including a first conductive pad in a first insulating layer and a first barrier layer surrounding a lower surface and side surfaces of the first conductive pad, a second wafer including a second conductive pad in a second insulating layer and a second barrier layer surrounding a lower surface and side surfaces of the second conductive pad, the second insulating layer being bonded to the first insulating layer, and at least a portion of an upper surface of the second conductive pad being partially or entirely bonded to at least a portion of an upper surface of the first conductive pad, and a third barrier layer between portions of the first and second wafers where the first and second conductive pads are not bonded to each other.