We deposited copper films which have a different grain size on the silicon single crystal wafer with dc magnetron sputtering. The crystal grain size of the copper films was varied by changing the substrate temperature Ts
, which is one of the sputtering deposition conditions, between 64°C and 200°C. The obtained copper films were set on an optical microscope equipped with a vacuum heating device. The films surface were observed with the microscope throughout the thermal cycling between room temperature to 800°C. Next, we examined the temperature which hillocks begin to form. As a result, we confirmed the clear relationship between the temperature which the hillocks begin to form and the initial grain size of the copper film. In the case of low substrate temperature of Ts
= 64°C, the initial grain size of copper film was very small, and many hillocks were formed at low heating temperature of 300°C. On the other hand, in the case of high substrate temperature of Ts
= 200°C, the initial grain size of copper film was large, and hillocks were not formed below heating temperature of 800°C.