Semiconductor device and forming method thereof

A method of forming a semiconductor device, comprising:

• forming a fin structure having a stack of alternating first semiconductor layers and second semiconductor layers over a substrate;
• forming a dummy gate structure across the fin structure;
• forming gate spacers on opposite sidewalls of the dummy gate structure, respectively;
• removing the dummy gate structure to form a gate trench between the gate spacers;
• etching the first semiconductor layers in the gate trench, such that the second semiconductor layers are suspended in the gate trench to serve as nanosheets;
• performing a plasma deposition process to form titanium nitride materials between each of the nanosheets, wherein the plasma deposition process is a cyclic process comprising one or more repetitions of a tetrakisdimethyl titanium (TDMAT) providing step and an N2 plasma providing step;
• and
• depositing a metal fill layer over the titanium nitride materials but not into a gap between adjacent two of the nanosheets.

A semiconductor device, comprising:

• a plurality of nanosheets extending in a first direction above a semiconductor substrate and arranged in a second direction substantially perpendicular to the first direction;
• and
• a gate structure extending in a third direction perpendicular to both the first and second directions, the gate structure surrounding each of the plurality of nanosheets, wherein the gate structure comprises:
• a first titanium nitride layer surrounding each of the plurality of nanosheets;
• a second titanium nitride layer surrounding the first titanium nitride layer, the second titanium nitride layer has an atomic ratio of titanium to nitrogen greater than that of the first titanium nitride layer;
• and
• a metal fill layer surrounding the second titanium nitride layer and absent between adjacent two of the plurality of nanosheets.