Method for manufacturing sputtering target, method for forming oxide film, and transistor

A method for manufacturing a semiconductor device, comprising the steps of:

• forming a gate electrode;
• forming a first oxide film by using a first sputtering target comprising In, M1 and Zn, so as to overlap the gate electrode;
• forming a second oxide film by using a second sputtering target comprising In, M2 and Zn over the first oxide film;
• and
• forming a source electrode and a drain electrode electrically connected to the first oxide film and the second oxide film,
• wherein M1 represents an element selected from the group consisting of Al, Ti, Ga, Y, Zr, La, Cs, Nd and Hf,
• wherein M2 represents an element selected from the group consisting of Al, Ti, Ga, Y, Zr, La, Cs, Nd and Hf,
• wherein a first atomic ratio of M2 to In in the second oxide film is larger than a second atomic ratio of M1 to In in the first oxide film, and
• wherein an atomic ratio of metal elements of the second sputtering target satisfies M2>In and Zn >M2.