Defect reactions and impurity control in silicon

Abstract

This Ph.D. thesis has covered the scientific issues of defect reactions involving dopants and impurities in Si, and the applications of this knowledge to reactive ion etching (RIE) and Fe gettering processes. The reactions among self-interstitials (Sii), vacancies (V), impurities (C, 0), and dopants (B, P) in Si produce undesirable defects which affect device operation and control transport processes such as dopant diffusion. Electron beam irradiation has been used to generate Sii and V to initiate the defect reactions. Deep level transient spectroscopy (DLTS) has been used to identify specific defects and to measure defect concentrations. The experimental results can be summarized in terms of a complex hierarchy diagram of defect reactions. We describe the defect reactions as a three-step process: (i) the displacement reaction to generate Sii and V, (ii) the Watkins replacement reaction to generate C and B interstitials (Ci, Bi), and (iii) the diffusion limited pairing and clustering processes to generate defect pairs and clusters. On the basis of reaction kinetics, we have simulated the reaction processes. The interstitial migration enthalpy (Him) and capture radius (r,) are two parameters used in the model to describe the long range migration and the near neighbor capture of mobile interstitials. The calculated defect reaction rates are in good agreement with the experimental data. We conclude that the diffusion limited pairing reactions are the predominant processes in the defect reactions. The reaction kinetics are determined by Him, r,, and the background dopant and impurity concentrations. The model supports the defect assignments by DLTS. The model can be improved by including pair breakup processes and large interstitial clusters.

(cont.) RIE causes substrate surface contamination, substrate damage, and induces defect reactions at depths extending to um range. We have applied the defect reaction model to RIE and developed a model describing interstitial injection for the defect reaction region to evaluate the defect depth profile. The reaction kinetics is formulated as a series of coupled 1-D interstitial diffusion-reaction partial differential equations (PDEs) with a moving boundary. The model predicts the profiles which are consistent with that measured in the photoluminescence (PL) experiments. We conclude that the depth profile is determined by the interstitial diffusion coefficient, the etch rate, the etch time, the interstitial defect reaction rate, and the background dopant and impurity concentrations in the Si substrate. The um range depth profile can be explained as: (i) fast diffuser Sii injection to ,um depth range; (ii) the generation of Bi and Ci by the Watkins replacement reactions, and (iii) the formation of Bi- and Ci-related defects through diffusion limited pairing reactions. The injection of Bi or Ci is extremely shallow during a typical RIE process. Fe is incorporated into Si as a highly mobile and soluble interstitial species (Fei) during device processing or in the starting materials. Fei and Group III impurities (B, Al, Ga, In) form Fe-acceptor (FeiAs) pairs in Si. Both Fei and the FeiAs pairs introduce deep levels in the bandgap which act as recombination centers. The long range Coulomb interaction between Fei and As is the driving force for the FeiAs pair formation. The short range near neighbor interactions determine the specific FeiAs pair energetics and structures. We have studied the FeiAs pairs within the framework of an ionic model. ...