Low temperature reactions at Si/metal interfaces; What is going on at the interfaces?

Si-metal contacts play one of the most important roles in Si-device or integrated circuit technology represented by IC, LSI and VLSI. The role of the contacts is to interconnect individual devices (transistors, diodes and so on) in a Si chip and connect them as a whole to the outer circuit. These contacts are numerous in the chip and can easily total more than one million. Therefore, the establishment of criteria for providing stable and reproducible electrical (ohmic or Schottky barrier) contacts has been a key problem in Si-LSI technology. Si is a typical covalent semiconductor with a large bond energy (≈eV/ bond) and consequently its melting point is high (≈1440 °C). However,crucial to the Si-metal contact problem is that when Si is in contact with metal it readily reacts with it at a temperature as low as ≈100 °C. This interfacial reaction induces large scale transport of materials across the Si/metal interface to give rise to several interesting phenomena. Examples of this are thick (≈1000 Å) SiO2 growth for a short time (≈10 min) due to Si-Au reaction and uniform silicide layer formation at Si/Pd, Pt, Ni interfaces. Interesting point is that since the Si-Si covalent bonding is very strong without the presence of such effect of metal to weaken the Si bond adjacent to the metal, the above contact or interfacial reactions can rarely occur. As possible mechanism of this bond-weakening, several models have been proposed. One of them is by the present author postulating electronic screening of Coulomb interaction responsible for the covalent bonding due to mobile electrons in the metal films. In the present article, this “screening model” is critically discussed and compared with other models on the basis of experiments done by several laboratories on microscopic or atomic-scale observation of initial stages of the Si-Au and Si-Pd reactions by both electron and ion scattering spectroscopies. In addition new usage of the channeling effect of MeV He+ ions is demonstrated to be a powerful tool for interface and surface studies.