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The thermal stability of Ag and Ag/Au on different substrates has been investigated by transmission electron microscopy, scanning electron microscopy, Auger electron spectroscopy, and sheet resistance measurements. Ti thin films (~ 3 nm and ~ 10 nm) were first deposited in ultrahigh vacuum onto (001)Si substrate. The samples were then annealed at 600℃ to form TiSi2. Ag film (~ 50 nm) was deposited on TiSi2/Si. Island structure was formed upon annealing at 400℃. The morphological stability of the Ag-substrate interface is rather poor since it was prone to form islands upon annealing at a low temperature. To overcome the island formation problem, the utilization of a thin interposing Au layer (~ 3 nm) has been explored between Ag and the substrate. Au layer did stabilize the Ag thin films until the annealing etmperature was increased over 400℃. It is though that the surface energy of Ag is too high and lowering the surface energy may enhance the thermal stability. From thermodynamic consideration, forming solid solution can lower the surface energy. It was found that there were Ag or Ag-Au spikings between TiSi2 and Si layer after annealing at 400℃ fro 30 min.
The thermal stability of Ag/Ti/Si was investigated to compare with that of Ag/Si and Ag/Au/Si systems. The results were different from those of the Ag-Au system since Ag and Ti do not mix completely as that of Ag and Au.
Owing to the formation of Ag-Au spikings at the Ag-Au/Si interface, a diffusion barrier is needed to buffer the interdiffusion of Ag and Ag-Au with Si. Ag film (~ 50 nm) was deposited onto TiN/Si. Au and Ti (~ 3 nm) layers deposited between Ag and TiN have been investigated. TiN films were deposited by physical vapor deposition (PVD) with reactive dc mangnetron sputtering or chemical vapor deposition (CVD) using TDMAT as deposition source, as well as CVD process with plasma treatment. In Ag/Au/TiN system, PVD process had better thermal stability at a temperature as high as 450℃. But in Ag/Ti/TiN and Ag/TiN systems, the thermal stability in the sample on CVD-TiN with and without plasma treatment were superior to that on PVD-TiN. Ag layer was smooth in Ag/Ti/TiN and Ag/TiN systems after annealing at 350 and 100℃, respectively, in CVD-TiN system. The sheet resistance of PVD systems was lower than that of CVD systems because of more impurities were involved in CVD systems.
Ag, Ag/Au or Ag/Ti films deposited on BPSG/Si were also investigated. The Ag/Au film has found to stabilize the Ag films up to 300℃. Polycrystalline islands were formed on Ag/BPSG and Tg/Ti/BPSG systems after annealing at a temperature lower than 100 and 300℃, respectively.
The failure mechanisms of Ag, Ag/Au, or Ag/Ti films on TiSi2, TiN and BPSG were discussed. The stress due to difference of thermal expansion led the island structure formation.
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