Semiconductor devices and methods for fabricating the same

A method for fabricating a semiconductor device, comprising:

• forming a first active region in a substrate;
• forming an impurity particle on the substrate;
• sequentially forming a first gate dielectric layer, a first lower gate electrode and a first upper gate electrode on the substrate;
• forming a first spacer on a first sidewall of the first upper gate electrode, wherein the first spacer exposes a first sidewall of the first lower gate electrode and a first sidewall of the first gate dielectric layer;
• forming a second spacer on a second sidewall of the first upper gate electrode and covering a second sidewall of the first lower gate electrode and a second sidewall of the first gate dielectric layer, wherein a first portion of the first gate dielectric layer and a first portion of the lower gate electrode which are formed on the impurity particle are higher than a second portion of the first gate dielectric layer and a second portion of the first lower gate electrode which are not formed on the impurity particle;
• etching the first sidewall of the first lower gate electrode and the first sidewall of the first gate dielectric layer exposed by the first spacer to form an undercut region, wherein the undercut region is defined by a bottom surface of the first upper gate electrode and the first sidewall of the first lower gate electrode and the first sidewall of the first gate dielectric layer;
• forming a third spacer on the second spacer, wherein the third spacer is spaced apart from the first lower gate electrode and the first gate dielectric layer by the second spacer;
• and
• forming a fourth spacer on the first spacer, wherein the fourth spacer extends into the undercut region so as to be in contact with the first sidewall of the first lower gate electrode and the first sidewall of the first gate dielectric layer.