Silicon (Si) stands as the primary base material for the majority of semiconductor chips due to several essential properties that make it exceptionally well-suited for integrated circuit (IC) fabrication:
Ideal Band Gap: Silicon possesses an ideal band gap of 1.1 electron volts (eV) for most circuit applications. This particular value is advantageous because it ensures that intrinsic conduction, the flow of electrons without any external influence, doesn’t occur in silicon until temperatures exceed 200°C. This means silicon remains non-conductive at typical operating temperatures for electronic devices. Moreover, the 1.1 eV band gap allows for the creation of field-effect transistors (FETs) with very low threshold voltages, a crucial factor in modern IC design.
Silicon Dioxide Insulator: Silicon dioxide (SiO2), often referred to as oxide, is a highly stable and intrinsic oxide with excellent insulating properties. It can be readily manufactured and is widely used as an insulator between interconnects in ICs. Additionally, SiO2 serves as a dielectric material in capacitors and field-effect transistors, contributing to the miniaturization and efficient operation of ICs.
Thermal Conductivity: Silicon exhibits good thermal conductivity, which is a vital characteristic for miniaturized structures. In modern semiconductor devices, where components are densely packed on a chip, efficient heat dissipation is crucial to prevent overheating, maintain performance, and avoid intrinsic conduction or damage. Silicon’s ability to conduct heat efficiently helps in managing these power losses.
Large Monocrystal Growth: Silicon can be easily grown as large monocrystals, which are then sliced into wafers for IC fabrication. These monocrystals have an ordered atomic structure in all directions without breaks or irregularities, minimizing the risk of unwanted current paths caused by lattice defects. The growth process involves melting silicon at just above its melting point and using a small silicon seed crystal to initiate the growth. Doping agents can also be added during this process to tailor the silicon’s electrical properties.
Silicon Material Fabrication Method
The two primary methods for growing silicon monocrystals are the Czochralski process and zone melting. In the Czochralski process, a silicon monocrystal is pulled from a silicon melt while being rotated about the axis of pulling. This process essentially grows the crystal from the bottom up. In contrast, the zone melting method involves melting a thin strip of a polycrystalline silicon bar using an annular heater. As the heater and melting zone move along the bar, atoms align in the thin strip, effectively producing a monocrystal in place.
Both crystal growth methods must be conducted under a vacuum or in an inert gas atmosphere to prevent contamination. Once grown, the silicon monocrystal is shaped into round bars from which wafers, typically 1 mm thick, are cut using a process known as internal diameter sawing (IDS).
Why is silicon the preferred material for the vast majority of chips in the semiconductor industry?
Silicon is preferred because it possesses several useful properties, including an ideal band gap for most circuit applications, excellent thermal conductivity, the ability to grow large monocrystals, and the availability of stable silicon dioxide (SiO2) for insulation and dielectric purposes.
What are the two common technologies for crystal growth in silicon manufacturing?
The two common crystal growth technologies are the Czochralski process, where a silicon monocrystal is drawn from a silicon melt while rotating, and the zone melting method, where a thin strip of a polycrystalline bar is melted and then solidified to create a monocrystal in place.
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