Reduced boron diffusion under point defect injection in fluorine implanted silicon

Abstract

This thesis reports the results of experiments aimed at understanding the behaviour of fluorine under various device processing conditions and hence aims to identify the mechanisms responsible for the reduced boron diffusion.

Point defect injection studies are performed to study how the injection of interstitials and vacancies from the surface influences the fluorine SIMS peaks and the diffusion of boron marker layers which are placed to coincide with major fluorine peaks of the fluorine profile.  SIMS analysis of a sample implanted with 185keV, 2.3 x 10¹⁵ cm^-2 F⁺ and annealed at 1000°C shows fluorine peaks at depths of 0.3R_p and R_p and a shoulder between 0.5-0.7R_p.  The shallow fluorine peak at a depth of 0.3R_p­ is smaller under interstitial injection than inert anneal and it decreases in size with anneal time.  The presence of this shallow peak correlates with the suppression of boron diffusion in a boron marker located at the same depth.

In fluorine implanted samples, less boron diffusion is surprisingly observed under interstitial injection than inert anneal for boron marker layers located in the interstitial-rich region (at R_p) of the fluorine damage profile.

A systematic study is made of the effect of device processing on the V-F clusters.  SIMS analysis shows that the V-F clusters are stable for anneals of 1.5 hours at 820°C or 45s at 1050°C.  The clusters are stable in the presence of a low dose dopant co-implant (1x10¹³ cm^-2), but are eliminated by a high dose dopant co-implant (2x10¹⁵ cm^-2).  Investigation of the effect of decreasing fluorine implant energy on the V-F clusters shows that a low thermal budget anneal gives a better retention of the V-F clusters and allows a reduction of the fluorine implant energy for application in junction depths down to ~20 nm.

Finally, this thesis reports the implementation of a fluorine implant in production silicon bipolar technology at STMicroelectronics, Sicily, Italy.  A novel approach was used and it was demonstrated that fluorine dramatically suppresses boron diffusion in the base and leads to a world record f_T of 110GHz in an appropriately optimised device.

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