Method of gate work function adjustment and metal gate transistor

A method of gate work function adjustment, comprising:

• providing a substrate comprising a metal gate thereon, wherein a source predetermined region and a drain predetermined region is defined at opposite sites of the metal gate, and the metal gate is divided into a source side adjacent to the source predetermined region, and a drain side adjacent to the drain predetermined region;
• forming a mask layer covering the source predetermined region and the drain predetermined region;
• performing an implantation process to implant nitrogen into the metal gate so as to make a first nitrogen concentration of the source side higher than a second nitrogen concentration of the drain side;
• removing the mask layer;
• and
• forming a source doping region within the source predetermined region, and a drain doping region within the drain predetermined region.

A metal gate transistor, comprising:

• a substrate;
• a metal gate disposed on the substrate;
• and
• a source doping region and a drain doping region disposed in the substrate at opposite sites of the metal gate, wherein the metal gate is divided into a source side adjacent to the source doping region, and a drain side adjacent to the drain doping region, and wherein the source side has a first nitrogen concentration, and the drain side has a second nitrogen concentration lower than the first nitrogen concentration.