This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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Cu (copper) reacts with Si (silicon) rapidly at temperatures ≥ 200 °C.  For the systems of Cu/TiN/TiSi₂/n⁺Si - p Si diode junctions, the TiN/TiSi₂ bilayer acts as a diffusion barrier between Cu and n⁺Si layers. In this system, no deterioration of n⁺-p diode junctions occurs when it is annealed up to 475 °C for 30 min in an N₂ ambient. For the sintering temperature at 500 °C, the leakage current increased abruptly and SIMS technique revealed a large amount of Cu atoms diffusing into the junctions of n⁺-p diodes [2]. The TEM image in Figure 1725 shows that a pyramidal-shaped Cu₃Si crystallite in size of 0.25 µm precipitated in the n⁺Si substrate. The phenomenon is related to Cu penetrating through the TiN/TiSi₂ bilayer and reacting with the n⁺Si substrate to form Cu₃Si. [1]  The Cu penetration can be induced by lattice diffusion or by diffusion through grain boundaries and defects without reaction with TiN and TiSi₂, and then Cu reacts with Si to form Cu₃Si crystallite in the n⁺Si.  A gap between TiSi₂ and n⁺Si may be generated during the growth of Cu₃Si precipitates, and then the electrical contact resistance is increased. SiO₂ can also be found underneath the Cu₃Si precipitates. The formation of the SiO₂ layer is due to the rapidly catalytic behavior of Cu₃Si at room temperature. Because of Cu depletion, voids can form at the interface between Cu and TiN. On the other hand, in the annealed TiN/TiSi₂ system (even though annealed in N₂), oxygen contents can easily accumulate near the interface between TiN and TiSi₂. [1]

Figure 1725. TEM image of the systems of Cu/TiN/TiSi₂/n⁺Si - p Si diode junctions after sintering at 500 °C/30 min in N₂. [2]

[1] A. Cross, M. 0. Aboelfotoh, and K. N. Tu, J. Appl. Phys. 67,332s (1990).
[2] Tzong-Sheng Chang, Wen-Chun Wang, Lih-Ping Wang, Jenn-Chang Hwang, and Fon-Shan Huang, Thermal stability study of TiN/TiSi₂ diffusion barrier between Cu and n⁺Si, J. Appl. Phys. 75 (12), (1994) 7847.