Point Defects and Dopant Diffusion in Silicon.
Selective incorporation of elements from group III and V of the periodic table, known as dopant atoms, into silicon substrates is the fundamental step by which all silicon-based intergrated circuits are made. During the actual fabrication sequence, the silicon substrate undergoes many high temperature steps in which the dopant concentration profiles redistribute due to diffusion. It is highly desireable to be able to model the changes in profile shapes using process simulation programs so that processing parameters from the fabrication sequence can be related to the resulting electrical operation of the integrated circuits. Since the technologies of integrated circuit fabrication are expected to continuously change as they have in the past, only diffusion models with a solid physical basis will remain relevant to future fabrication processes. This work examines, both theoretically and experimentally, the basic mechanisms underlying dopant diffusion in order to develop models of dopant diffusion appropriate for advanced process modeling programs. Since dopant atoms are substitutional in nature, they become mobile only by interacting with the native point defects of the silicon lattice. Dopant diffusion by the vacancy, interstitial, and interstitialcy mechanisms are examined from an energetics viewpoint. Both the charge states of the defects and the chemical identity of the dopant atoms are variables in the analysis. Relationships between dopant and point defect concentrations are examined under equilibrium and non-equilibrium conditions, and the appropriate equations governing diffusion presented. It is demonstrated that exposure of the silicon surface to NH(,3) ambient at elevated temperatures caused vacancy defects to be injected into the silicon bulk, while exposure of SiO(,2) layers to NH(,3) results in injection of interstitial type defects. These thermal nitridation processes are used to study the diffusion of the common dopants, P, As, Sb, and B. By selectively injecting vacancies or interstitials into the bulk and monitoring the changes in diffusion rates through spreading resistance or SIMS profile analysis, the types of mechanisms by which the dopants diffuse can be ascertained. It is determined that P and B diffuse almost exclusively by an interstital type mechanism under both intrinsic and extrinsic doping conditions, contrary to the current models used in process simulation programs. In contrast, Sb and As are found to diffuse predominantly by the vacancy mechanism as previously assumed.
- Pub Date:
- Physics: Electricity and Magnetism;
- Doped Crystals;
- Integrated Circuits;
- Point Defects;
- Crystal Lattices;
- Equilibrium Equations;
- Temperature Effects;
- Electronics and Electrical Engineering