Semiconductor device and method for manufacturing semiconductor device

A semiconductor device comprising:

• a first oxide comprising a channel formation region;
• a first conductor over and in contact with a top surface of the first oxide, the first conductor functioning as one of a source electrode and a drain electrode;
• a second conductor over and in contact with the top surface of the first oxide, the second conductor functioning as the other of the source electrode and the drain electrode;
• a first insulator over the first conductor and the second conductor;
• a second oxide in contact with the top surface of the first oxide;
• a second insulator over the second oxide, the second insulator functioning as a gate insulator;
• and
• a third conductor over the second insulator, the third conductor functioning as a gate electrode,
• wherein the first insulator comprises aluminum,
• wherein the first oxide comprises indium, an element M, and zinc,
• wherein the element M is gallium, yttrium, or tin,
• wherein the first oxide comprises a first region overlapping with the third conductor, and
• wherein the first region comprises a region in which an atomic ratio of aluminum to the element M is less than 0.1.

A fabrication method of a semiconductor device comprising a first oxide, a second oxide, a third oxide, a first conductor, a second conductor, a third conductor, a first insulator, a second insulator, a third insulator, and a fourth insulator, comprising:

• forming the third insulator;
• forming the third oxide over the third insulator;
• forming the first oxide over the third oxide;
• forming a conductive film over the first oxide;
• forming the first insulator over the third insulator and the conductive film;
• forming the fourth insulator over the first insulator;
• forming an opening from which the first oxide is exposed in the fourth insulator, the first insulator, and the conductive film, so that the first conductor and the second conductor are formed;
• removing part of the exposed first oxide;
• forming the second oxide over the first oxide and the fourth insulator;
• forming the second insulator over the second oxide;
• forming the third conductor over the second insulator;
• and
• removing part of the second oxide, the second insulator, and the third conductor to expose part of the fourth insulator,
• wherein the first insulator comprises aluminum,
• wherein the first oxide comprises indium, an element M, and zinc, and
• wherein the element M is gallium, yttrium, or tin.