1.7 Oxidation and Doping of Silicon

• The purity of silicon is destroyed by introducing controlled amounts of electrically active impurities - called dopants
• Dopants determine current flow in semiconductor, and the introduction of electron rich dopants (donors) and electron-deficient dopants (acceptors) determine the formation of p-n junctions, which are the heart of transistors
• After the wafers have been cut from the ingot and etch polished, one of the first step in IC processing is growth of an oxide layer on the silicon
• Oxide layers can be grown in selected areas of the wafer by depositing and patterning nitride layers which act as a barrier, masking portion of the silicon from the penetration of the oxidizing species
• Dopants are introduced by ion implantation - energetic As ions penetrate the exposed portion of Si but are blocked from the silicon in areas covered by SiO2.
• Oxide and nitride layers are deposited on silicon during later stages of circuit fabrication as insulating layers between metal lines or as an insulating protective cover