Semiconductor device and method of manufacturing the same

A method of forming a semiconductor device, comprising:

• forming a gate insulating film on a memory cell transistor forming region, a select gate transistor forming region neighboring the memory cell transistor forming region, and a contact region neighboring the select transistor region respectively defined on a semiconductor substrate;
• removing the gate insulating film on the contact region;
• forming a first polycrystalline silicon layer having a first thickness on the contact region, the gate insulating film of the memory cell transistor forming region and the select gate transistor forming region;
• forming a second polycrystalline silicon layer having a second thickness via an inter-gate insulating film on the first polycrystalline silicon layer formed on the memory cell transistor forming region, the select gate transistor forming region and the contact region;
• removing the first and the second polycrystalline silicon layers and the inter-gate insulating film situated at boundaries between the memory cell transistor forming region and the select gate transistor forming region, and between the select gate transistor forming region and the contact region;
• removing the second polycrystalline silicon layer and the inter-gate insulating film over the contact region to expose an upper surface of the first polycrystalline silicon layer;
• and
• forming a metal plug on the exposed upper surface of the first polycrystalline silicon layer.