Gate structures for semiconductor devices

A semiconductor device, comprising:

• a substrate;
• a first gate structure, disposed on the substrate, comprising:
• a first gate dielectric layer,
• a barrier layer disposed on the first gate dielectric layer,
• a first glue layer comprising a first aluminum-free layer disposed on the barrier layer;
• and
• a first metal layer comprising a first lower portion and a first upper portion, wherein the first lower portion is in contact with the first glue layer and the first upper portion is in contact with the barrier layer;
• and
• a second gate structure, disposed on the substrate, comprising:
• a second gate dielectric layer;
• a second glue layer comprising a second aluminum-free layer disposed on and in contact with the second gate dielectric layer;
• and
• a second metal layer comprising a second lower portion and a second upper portion, wherein the second lower portion is in contact with the second glue layer and the second upper portion is in contact with the second gate dielectric layer.

A method, comprising:

• forming a gate opening on a substrate;
• depositing a first metal layer in the gate opening;
• depositing a barrier layer on the first metal layer;
• depositing a second metal layer on the barrier layer;
• converting a portion of the second metal layer to a metal oxide layer;
• removing the metal oxide layer to expose an unconverted portion of the second metal layer;
• and
• depositing a third metal layer on the unconverted portion of the second metal layer.