Microstructure evolutions upon Ni(Pt) silicidation and the different responses to the metal etch. (November 2020) - Qin, Wentao
Abstract: Ni(Pt) films were silicided at nominally 400 °C. Electrostatic discharging (ESD) failure of devices occurred, which prompted root cause investigation. The silicide stack on the wafer was Ni(Pt)/Ni2 Si(/Si). Flakes of SiO2 emerged on the wafer after the metal etch. With a higher silicidation temperature of 570 °C, the extent of the silicidation was greater. The stack on the wafer became Ni-oxide/Si-oxide/(NiPtx )Si/NiSi(/Si). The flaking issue was resolved and the desired etch selectivity achieved. It is the decomposition of Ni2 Si that led to the bi-layer structure of NiSi, and the minimization of free energy that drove the out-diffusion of Ni and Si from the NiSi to form the surface oxides. Ni-oxide formed on Si-oxide because Si-oxide is thermodynamically more stable than Ni-oxide. The two layers of oxides and the density of bond energy of (NiPtx )Si being higher than that of Ni2 Si made the stack more resistant to the metal etch and resulted in the desired etch selectivity. No reliability failure was observed after the flaking issue was addressed. Highlights: Ni(Pt) silicide Schottky diodes failed ESD test because of missing silicide. The silicidation temperature was too low. Ni(Pt) was not entirely silicided, and the Ni-rich Ni2 Si formed. Ni(Pt)/Ni2 Si was not resistant to metal etch. The Ni(Pt) was removed, and Ni2 Si oxidized, leading to missing silicide. The silicidation temperature was raised for complete Ni(Pt) silicidation. (NiPtx )Si \ Ni-oxide \ Si-oxide formed. Since Si-oxide is more stable than Ni-oxide, thermodynamics dictated Si-oxide covered Ni-oxide. The stack was more resistant to the metal etch, which had prevented the oxidation of the (NiPtx )Si.