Method for manufacturing semiconductor device

A method for manufacturing a semiconductor device comprising:

• forming a second layer over a first layer;
• forming a first mask over the second layer;
• forming a second mask over the first mask;
• forming a first resist mask over the second mask;
• forming a first opening portion in the first resist mask;
• reducing the first opening portion in size in a plan view;
• forming a second opening portion in a position of the second mask that overlaps with the first opening portion;
• removing the first resist mask;
• forming a second resist mask over the second mask provided with the second opening portion;
• forming a third opening portion in the second resist mask;
• reducing the third opening portion in size in the plan view;
• forming a fourth opening portion in a position of the second mask that overlaps with the third opening portion and does not overlap with the second opening portion;
• removing the second resist mask;
• forming a fifth opening portion in each position of the first mask that overlaps with the second opening portion or overlaps with the fourth opening portion;
• removing the second mask;
• forming a sixth opening portion in a position of the second layer that overlaps with the fifth opening portion;
• removing the first mask;
• and
• forming an oxide semiconductor in contact with a side surface of the second layer and a top surface of the first layer inside the sixth opening portion.