Memory device and manufacturing method thereof

A method includes depositing a gate dielectric film over a substrate. A floating gate layer and a control gate layer are deposited over the gate dielectric film. The control gate layer is patterned to form a control gate over the floating gate layer. A top portion of the floating gate layer is patterned. A spacer structure is formed on a sidewall of the control gate and over a remaining portion of the floating gate layer. The remaining portion of the floating gate layer is patterned to form a bottom portion of a floating gate. A ratio of the bottom width of the bottom portion to the middle width of the bottom portion is in a range between about 103% and about 108%. The gate dielectric film is patterned form a gate dielectric layer.