A metal oxide containing one or more selected from hafnium, aluminum, gallium, yttrium, zirconium, tungsten, titanium, tantalum, nickel, germanium, neodymium, lanthanum, magnesium, and the like can be used as the insulator 544. Alternatively, silicon nitride oxide, silicon nitride, or the like can be used as the insulator 544.
It is particularly preferable to use an insulator containing an oxide of one or both of aluminum and hafnium, such as aluminum oxide, hafnium oxide, or an oxide containing aluminum and hafnium (hafnium aluminate), as the insulator 544. In particular, hafnium aluminate has higher heat resistance than a hafnium oxide film. Therefore, hafnium aluminate is preferable because it is less likely to be crystallized by heat treatment in a later step. Note that the insulator 544 is not an essential component when the conductor 542a and the conductor 542b are oxidation-resistant materials or do not significantly lose their conductivity even after absorbing oxygen. Design is appropriately set in consideration of required transistor characteristics.
When the insulator 544 is included, diffusion of impurities such as water and hydrogen contained in the insulator 580 to the oxide 530b through the oxide 530c and the insulator 550 can be inhibited.
The insulator 550 functions as a first gate insulating film. The insulator 550 is preferably positioned in contact with an inner side (the top surface and the side surface) of the oxide 530c. Like the insulator 524, the insulator 550 is preferably formed using an insulator that contains excess oxygen and releases oxygen by heating.