Behavior and effects of fluorine in annealed n+ polycrystalline silicon layers on silicon wafers

Abstract

A comprehensive study is made of the behavior and effects of fluorine in n+-polysilicon layers. The polysilicon is deposited in a conventional low pressure chemical vapor deposition furnace on (100) silicon wafers, implanted with 1×1016 cm−2 F+ and 1×1016 cm−2 As+ and annealed at 850, 950, 1015, and 1065 °C. Sheet resistance, transmission electron microscopy (TEM), and secondary ion mass spectroscopy are used to obtain quantitative data for the breakup of the interfacial oxide, the epitaxial regrowth of the polysilicon layer, and the fluorine and arsenic distributions. The fluorine significantly increases both the initial oxide breakup (∼8×) and the initial polysilicon regrowth. It also produces inclusions in the layer which can affect the subsequent polysilicon regrowth and the arsenic distributions. Three regrowth stages and two regrowth mechanisms are distinguished and interpreted, and a value of ∼6×10−11 cm2 s−1 is deduced for the effective diffusivity of fluorine in polysilicon at 950 °C. The amounts of regrowth determined by TEM are compared with the corresponding changes in sheet resistance. The thermal budgets required to produce polysilicon layer regrowths of 1% and 50%, important for the performance of polysilicon emitter bipolar transistors, are given. All the thermal budgets are lower when fluorine is present.