Disclosed is a flash memory device. The flash memory device includes a plurality of trench lines in an isolation region of a semiconductor device, a common source region along a word line (WL) direction under a surface portion of the semiconductor substrate, a plurality of gate lines along a vertical direction of the trench line, a drain region on an opposite side of the gate line to the common source region, a drain contact over the drain region, and a uniform by-product layer on the common source region.