Interfacial microstructures of rf-sputtered TiNi shape memory alloy thin films on (100) silicon

Interfacial microstructures of TiNi thin films rf sputtered on to Si(100) and post-annealed at 400-700°C for 30 mins have been investigated using analytical and high-resolution transmission electron microscopy. For annealing temperatures below 600°C, a very thin amorphous (Si, O)-rich layer is observed at the interface. Ni atoms are the primary diffusing species and NiSi₂ forms triangularly and epitaxially towards the Si substrate. TiNi films initially crystallize after 30 min at 500°C. Si and Ti atoms begin to migrate in specimens annealed at 600°C for 30 min. At this temperature, a near-Ti₄Ni₄Si₇ phase in triangular NiSi₂ and a near-TiNiSi phase in the TiNi film are simultaneously nucleated and grown at the interface. For the specimens annealed at 700°C for 30 min, two layers of Ti₄Ni₄Si₇ and TiNiSi form at the interface with the sequence TiNi/TiNiSi/Ti₄Ni₄Si₇/Si. Triangular NiSi₂ islands are now embedded in the Ti₄Ni₄Si₇ layer. A mechanism of interfacial microstructure evolution is proposed to explain the temperature effect on the interfacial reaction layers between the TiNi film and the Si(100).