2.3.2 Polycrystalline Silicon

Polycrystalline silicon (polysilicon) has found many applications in integrated circuits. Depending on the doping concentration it is used as conductor (gate in metal oxide semiconductor field effect transistors  (MOSFET) [20]) or as a resistor (high value resistor in memory cells [3]). Polysilicon is deposited on the wafer by low pressure chemical vapor deposition (LPCVD) making use of the decomposition of silane (SiH$_4$). It consists of small crystallites called grains separated by thin regions called grain boundaries. The grain size increases with an increase in the deposition temperature [48] and it depends on the in-situ (dopants are added during the deposition process) doping concentration. The average grain size ranges from 0.3 $\mu$m to 1.0 $\mu$m while the grain boundaries (interface between two grains with different crystal orientation) are only 0.5 nm to 1 nm wide.