Semiconductor device and method for manufacturing the same

Abstract

To reduce oxygen vacancies in an oxide semiconductor film and the vicinity of the oxide semiconductor film and to improve electric characteristics of a transistor including the oxide semiconductor film. A semiconductor device includes a gate electrode whose Gibbs free energy for oxidation is higher than that of a gate insulating film. In a region where the gate electrode is in contact with the gate insulating film, oxygen moves from the gate electrode to the gate insulating film, which is caused because the gate electrode has higher Gibbs free energy for oxidation than the gate insulating film. The oxygen passes through the gate insulating film and is supplied to the oxide semiconductor film in contact with the gate insulating film, whereby oxygen vacancies in the oxide semiconductor film and the vicinity of the oxide semiconductor film can be reduced.

What is claimed is: 
     
         1 . A semiconductor device comprising:
 an oxide semiconductor film;   a gate electrode overlapping with the oxide semiconductor film; and   a gate insulating film provided between the oxide semiconductor film and the gate electrode,   wherein the gate electrode comprises a first region and a second region,   wherein the first region is closer to the gate insulating film than the second region, and   wherein an oxygen concentration of the first region is lower than an oxygen concentration of the second region.   
     
     
         2 . The semiconductor device according to  claim 1 , wherein the gate electrode includes one or more elements selected from silver, copper, ruthenium, iridium, platinum, and gold. 
     
     
         3 . The semiconductor device according to  claim 1 , wherein the gate electrode includes ruthenium. 
     
     
         4 . The semiconductor device according to  claim 1 , wherein the gate electrode includes a substance having higher Gibbs free energy for oxidation than a substance of the gate insulating film. 
     
     
         5 . The semiconductor device according to  claim 1 , wherein the gate insulating film has an oxygen-transmitting property. 
     
     
         6 . The semiconductor device according to  claim 1 , further comprising:
 an insulating film in contact with a side surface of the gate electrode, wherein oxygen does not pass through the insulating film.   
     
     
         7 . The semiconductor device according to  claim 1 ,
 wherein a thickness of the gate electrode is grater than or equal to 30 nm,   wherein the first region is at a distance of 3 nm from an interface with the gate insulating film, and   wherein the second region is at a distance of 15 nm from the interface with the gate insulating film.   
     
     
         8 . The semiconductor device according to  claim 1 , wherein the gate electrode further comprises a conductive film with the first region and the second region between the gate insulating film and the conductive film.