Methods of forming field effect transistors having t-shaped gate electrodes using carbon-based etching masks

A method of forming a field effect transistor, comprising the steps of:

• forming a first electrically insulating layer comprising mostly carbon on a surface of a semiconductor substrate having an electrically insulating device isolation region therein;
• patterning the first electrically insulating layer to define an opening therein by etching the first electrically insulating layer in an ambient containing oxygen to thereby expose the surface of the semiconductor substrate and expose the device isolation region;
• forming a trench in the semiconductor substrate by etching the surface of the semiconductor substrate using the patterned first electrically insulating layer as an etching mask;
• and
• forming a gate electrode of the field effect transistor in the trench;
• wherein said step of etching the first electrically insulating layer comprises thickening the device isolation region in the ambient containing oxygen.