A semiconductor device includes a first chip and a second chip stacked on the first chip. The first chip includes a first substrate, a first upper pad on an upper surface of the first substrate, a first upper insulating layer surrounding a lower portion of the first upper pad and a sacrificial layer surrounding an upper portion of the first upper pad. The second chip includes a second substrate, a second upper pad on an upper surface of the second substrate and a second upper insulating layer surrounding the second upper pad, wherein a thickness of the second upper pad is less than a thickness of the first upper pad.