Effect of Boron and BF[sub 2][sup +] Implant on Polysilicon Resistors

Abstract

Polycrystalline silicon is often implanted with boron or BF to fabricatep-type resistors in integrated circuits. This paper demonstratesthe differences between boron and BF-doped polysilicon resistorsexperimentally and describes the influence of fluorine on boron redistributionnear the interface of polysilicon and its underlying layer. Resultsshow the resistance of BF implanted layers to be much less than boron-implantedlayers at low dose levels irrespective of the underlying layer onwhich polysilicon is deposited. As the dose level increases, thedifference between sheet resistances reduce and after a transitiondose level, BF-implanted resistors start showing a higher resistancethan boron-doped samples. This transition level for poly-on-Si3N4samples occurs at a lower dose level than poly-on-SiO2 samples. Also,the change in resistance, R is always much higher in poly-on-Si3N4than poly-on-SiO2 samples for any given dose level. At high dopinglevels, BF and boron layers are comparable when the polysilicon layeris deposited on SiO2 surface. Secondary ion mass spectrometry analysisshows a peak in the fluorine concentration near the interface ofpolysilicon and SiO2 as well as polysilicon and Si3N4 in BF implant,which enhances the accumulation of boron near the interface significantly.The change in grain structure of polysilicon on Si3N4 surface coupledwith enhancement of boron accumulation near the interface in presenceof fluorine doubles the sheet resistance of BF-doped polysiliconon Si3N4 surface. The temperature coefficient of the resistors fora given sheet resistance is independent of the underlying layer andimplanted species. Amorphous silicon resistors fabricated with boronand BF show a trend very similar to polysilicon. ?2002 The ElectrochemicalSociety. All rights reserved.