Methods of forming a semiconductor device include providing a substrate having an area including a source and a drain region of a transistor. A nickel (Ni) metal film is formed on the substrate area including the source and the drain region. A first heat-treatment process is performed including heating the substrate including the metal film from a first temperature to a second temperature at a first ramping rate and holding the substrate including the metal film at the second temperature for a first period of time. A second heat-treatment process is then performed including heating the substrate including the metal film from a third temperature to a fourth temperature at a second ramping rate and holding the substrate at the fourth temperature for a second period of time. The fourth temperature is different from the second temperature and the second period of time is different from the first period of time. The sequentially performed first and second heat-treatment processes convert the Ni metal layer on the source and drain regions into a NiSi layer on the source and drain regions and a NiSi2 layer between the NiSi layer and the source and drain regions.