Cobalt germanide contacts: growth reaction, phase formation models, and electrical properties

State of the art of cobalt germanide contacts to semiconductor devices is reviewed in this article. First, evolution of contacts is covered from the dawn of the transistor to present day. The history of contact has three stages: (a) elemental metals as direct contacts to the semiconductor with focus on aluminum, (b) self-aligned silicide contacts, and, recently, (c) the paradigm shift that emphasizes the interface contact resistivity. The second section outlines the current role of germanium in the semiconductor industry and the reasons cobalt germanide is an ideal contact material to germanium and silicon germanium semiconductor devices. Fundamental physical properties of cobalt germanides are presented next. Models for phase formation sequence are, then, detailed. This is followed by a comprehensive survey of the experimental results of formation of cobalt germanides. Those results are discussed and reconciled. Factors affecting the resulting phases and their quality are identified and some optimum choices for the experimental parameters are pointed based on the survey. After that, electrical properties of the contact are discussed. The role of germanium crystal orientation in ohmic and Schottky properties of the contact is analyzed. Fermi level pinning (FLP) plays a role mainly on metal/(100) n-type Ge interfaces. The role of FLP is minimal on p-type Ge and other crystalline orientations. Schottky barrier heights (SBH's) for cobalt and cobalt germanide contacts reported in the literature are surveyed. Mechanisms of FLP and methods adopted by the industry to depin the fermi level at the interface are outlined. The electrical properties section is concluded with a subsection that focuses on the effect of the crystallinity of the contact material on its electrical behavior. Crystalline cobalt germanides are expected to have lower interface resistivities compared to those calculated based on the SBH survey. The role of heat during Co deposition to obtain epitaxial germanides is pointed. Finally, current challenges and future trends of cobalt germanide contacts are summarized.