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A study is made of fluorine implantation into n+ polycrystalline silicon (polysilicon), with the aim of producing completely epitaxially regrown polysilicon emitters at lower temperatures. Polysilicon-on-silicon sheet resistance measurements are made to obtain an indication of the amount of polysilicon regrowth, and cross-section transmission electron microscope examinations to directly observe the regrowth. Samples given a fluorine implant, followed by a 1000 °C/10 min preanneal, show 50% epitaxial regrowth of the polysilicon after an emitter drive-in of only 850 °C/30 min. In contrast, in the absence of fluorine, negligible regrowth occurs even after an emitter drive-in of 850 °C/480 min.

A comprehensive study is made of the behavior and effects of fluorine in n ϩ -polysilicon layers. The polysilicon is deposited in a conventional low pressure chemical vapor deposition furnace on ͑100͒ silicon wafers, implanted with 1ϫ10 16 cm Ϫ2 F ϩ and 1ϫ10 16 cm Ϫ2 As ϩ and annealed at 850, 950, 1015, and 1065°C. Sheet resistance, transmission electron microscopy ͑TEM͒, 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 ͑ϳ8ϫ͒ 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 ϳ6ϫ10 Ϫ11 cm 2 s Ϫ1 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.

The role of fluorine (F) in the break-up of the native oxide and the regrowth of As doped poly-Si layers on unpatterned Si wafers and on patterned regions of Si device wafers at temperatures of 900°C to 1000°C are investigated by TEM and by the fabrication of npn poly-Si emitter bipolar devices. Results for unpatterned wafers with F show i) a 950°C dopant drive-in anneal causes oxide break-up and regrowth after a time suitable for the fabrication of devices, ii) a pre-anneal, before the As implant, further enhances the break-up and regrowth and iii) there is an optimum F dose of 5×1015/cm2. Based on these results poly-Si emitter bipolar devices were fabricated using F=5×1015/cm2, a pre-anneal and a 900°C As drive-in anneal. The results establish quantitatively the relationship between the interface structures and the specific emitter resistance, i.e. with no F there is no break-up or regrowth and the emitter resistance is high (114Ωμm2) while with F there is break-up and regrowth and the emitter resistance is low (17Ωμm2).

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