A semiconductor device includes a semiconductor substrate, an underlying insulation layer, a conductive via and a sidewall insulation layer. The underlying insulation layer is over the semiconductor substrate. The conductive via is through the semiconductor substrate and the underlying insulation layer. The sidewall insulation layer is between the semiconductor substrate and the conductive via. The sidewall insulation layer includes a protrusion stopping at an interface between the semiconductor substrate and the underlying insulation layer, and protruding outwardly into the semiconductor substrate.