Thin film transistor substrate, liquid crystal display device including same, and method for producing thin film transistor substrate

A method for producing a thin film transistor substrate comprising:

• a first patterning step of forming a conductive film on a base substrate and patterning the conductive film with a first photomask to form a gate electrode;
• a gate insulating film forming step of forming a gate insulating film to cover the gate electrode;
• a second patterning step of forming a first semiconductor film containing a first oxide semiconductor on the gate insulating film and patterning the first semiconductor film with a second photomask to form a first semiconductor layer;
• a third patterning step of forming a second semiconductor film containing a second oxide semiconductor to cover the first semiconductor layer and patterning the second semiconductor film with a third photomask to form a second semiconductor layer to cover the first semiconductor layer;
• and
• a fourth patterning step of forming a conductive film to cover the first semiconductor layer and the second semiconductor layer and patterning the conductive film by dry etching with a fourth photomask to form a source electrode and a drain electrode.