Shallow junctions by out-diffusion from BF₂ implanted polycrystalline silicon

We describe reverse bias diode leakage and physical analysis (secondary ion mass spectroscopy, Rutherford backscattering and transmission electron microscopy) data from shallow p⁺/n junctions made by implanting various doses of BF₂ into polycrystalline Si and out-diffusing at various temperature/time conditions, into the underlying (100) Si substrate. The polycrystalline Si is cobalt disilicided to provide the first level of metallization. Subsequent metallization consists of electroless plated Co followed by sputtered Al. The minimum process specifications giving good junction quality (reverse bias diode leakage current density ≤10 nA/cm ² at 10 V) are 5×10¹⁵ cm^-2 BF₂ outdiffused at 900 °C, 30 min with a junction depth 1200 Å below the polycrystalline Si. At these conditions, most of the BF ₂ (>90%) uniformly redistributes in the polycrystalline Si. There is a slight increase in B concentration towards the polycrystalline Si/Si interface which is characterized by a porous, <50-Å thin silicon oxide interface. The polycrystalline Si/Si interface is also characterized by a peaked fluorine concentration. The lower boron dose diffusing into the Si substrate may be used to explain the observation that these p⁺/n diodes are ∼10 times leakier than similar n⁺/p diodes made by outdiffusing As or P implanted from the polycrystalline Si diffusion source.