What Are Silicides?

Silicides are binary chemical compounds formed between silicon and a more electropositive element, most commonly a transition metal. More than 180 distinct silicide phases are known, spanning the periodic table from titanium and tungsten to cobalt, nickel, and platinum. In electronics engineering, the term most often refers to transition metal silicides: intermetallic compounds whose low electrical resistivity, thermal stability, and chemical compatibility with silicon substrates make them indispensable in CMOS fabrication and related semiconductor processes.

The study of silicides lies at the intersection of solid-state chemistry, thin-film physics, and semiconductor process engineering. Their properties vary widely by stoichiometry: a single metal-silicon system can produce several phases (for example, Ni₂Si, NiSi, and NiSi₂ in the nickel system), each with distinct resistivity, melting point, and stability window. Understanding which phase forms under a given thermal treatment is central to deploying silicides reliably in manufacturing.

Material Classes and Electrical Properties

Transition metal silicides are classified broadly by their resistivity. The lowest-resistivity group includes TiSi₂ (C54 phase, approximately 13–20 μΩ·cm), CoSi₂ (approximately 15–20 μΩ·cm), NiSi (approximately 14–20 μΩ·cm), and WSi₂ (approximately 70 μΩ·cm). These values are one to two orders of magnitude lower than doped polycrystalline silicon, which is why silicides replaced poly-Si alone for gate and source/drain contacts at sub-micron nodes.

A separate class of semiconducting silicides, including iron disilicide (FeSi₂) and chromium disilicide (CrSi₂), has optical band gaps in the 0.6–1.5 eV range and has attracted research interest for photovoltaic and thermoelectric applications. The Fraunhofer Institute review of transition metal silicides for thermoelectric applications surveys the structural and transport properties of higher manganese silicide and other candidates for energy conversion.

Synthesis and Thin-Film Processing

Silicides in integrated circuits are formed almost exclusively by thin-film solid-state reaction. A metal is sputter-deposited onto a silicon wafer, then heated in a rapid thermal processing system. The resulting solid-state diffusion and reaction produce a silicide film at the metal-silicon interface. The dominant diffusing species, the sequence of phases that form, and the kinetics of each transition are specific to each metal.

Bulk silicides for structural and refractory applications are synthesized by powder metallurgy, arc melting, or chemical vapor deposition. Molybdenum disilicide (MoSi₂) and tungsten disilicide (WSi₂) are used as heating elements in high-temperature furnaces and as protective coatings on refractory metal components because they oxidize to a self-healing SiO₂ layer at temperatures above 1000°C. The Chemical Reviews thermodynamics of solid transition-metal silicides provides a comprehensive treatment of formation enthalpies and phase stability across the major silicide systems.

Integration in Semiconductor Devices

In CMOS manufacturing, silicides are introduced through the self-aligned silicide (salicide) process, which selectively forms the silicide on exposed silicon and polysilicon while leaving oxide spacers and field oxide unreacted. Titanium disilicide (TiSi₂) was the industry standard through the 0.25 μm node; cobalt disilicide (CoSi₂) replaced it at 0.18–0.13 μm because of superior scaling behavior on narrow lines; nickel monosilicide (NiSi) has dominated from 90 nm onward because of its lower formation temperature, reduced silicon consumption, and compatibility with strained SiGe source/drain regions.

Tungsten silicide (WSi₂) is deposited by chemical vapor deposition directly on polysilicon gates, forming a polycide stack, and remains in use for gate electrodes in memory and analog processes where the salicide process is not applied.

Applications

Silicides have applications in a wide range of fields, including:

• Ohmic contacts and local interconnects in CMOS and bipolar integrated circuits
• High-temperature furnace heating elements (MoSi₂, WSi₂)
• Thermoelectric energy conversion devices (FeSi₂, higher manganese silicide)
• Protective oxidation-resistant coatings for aerospace alloys
• Gate electrodes in DRAM and flash memory word lines