Three-dimensional doping and diffusion in nano scaled devices as studied by atom probe tomography

Abstract

Nowadays, technological developments towards advanced nano scale devices such as FinFETs and TFETs require a fundamental understanding of three-dimensional doping incorporation, activation and diffusion, as these details directly impact decisive parameters such as gate overlap and doping conformality and thus the device performance. Whereas novel doping methods such as plasma doping are presently exploited to meet these goals, their application needs to be coupled with new metrology approaches such as atom probe tomography, which provides the 3D-dopant distribution with atomic resolution. In order to highlight the relevant processes in terms of dopant conformality, 3D-diffusion, dopant activation and dopant clustering, in this paper we report on 3D-doping and diffusion phenomena in silicon FinFET devices. Through the use of atom probe tomography we determine the dopant distribution in a fully completed device which has been doped using the concept of self-regulatory plasma doping (SRPD). We extract the dopant conformality and spatial extent of this doping process and demonstrate that after annealing the resulting 3D-doping profiles and gate overlap are dependent on the details of the plasma doping process. We also demonstrate that the concentration-dependent 3D-diffusion process leads to concentration gradients which are different for the vertical versus the lateral direction. Through a statistical analysis of the dopant atom distributions we can identify dopant clustering in high concentration regions and correlate this with details of the dopant activation and, eventually, the device performance.

Number series of atoms, interatomic bonds and interface bonds defining zinc-blende nanocrystals as function of size, shape and surface orientation: Analytic tools to interpret solid state spectroscopy data

“…In a similar fashion, N bnd /N NC can be useful for investigating dopant clustering in ULSI devices as function of Si nanovolume shape, size and interface orientation, using Atom Probe Tomography as a mass spectroscopic method with ultra-high spatial resolution. 33,34 The ratio N IF (d NC )/N NC (d NC ) presents a gauge for the impact of interface charge transfer 24,35 and interface dipoles [36][37][38] onto NCs. Both phenomena have a major influence on NC electronic and optical properties.…”

Improved physical models for advanced silicon device processing

“…Nevertheless, conformal doping is more difficult for n-type doping. This is due to the low adsorption efficiency of n-type dopants on Si surfaces that results in poor incorporation of As or P on the fin sidewalls [18,19,160,162,180]. Dopants located at the fin surface are then vulnerable to subsequent resist strip and etching processes (i.e.…”

“…dopant losses for As doped fins from 75% [180] to a recently improved value of around 25% after the strip process have been reported [162]). To avoid dopant loss after the strip process, recent studies considered the use of a rapid thermal annealing (RTA) after plasma doping in order to drive the dopants inside the fin (in-diffusion) and electrically activate them [18,19]. As an example, Fig.…”

“…affecting RSD) as well as significant gate overlap (thus degrading SCE) were observed due to the increased thermal budget [18]. More recently, Lee et al have considered the deposition of an oxide buffer layer prior to plasma doping instead of RTA, achieving about 76% of the As doped concentration retained in the Si fin after the strip process and even after ashing [162].…”

“…The compromise to remove damage and to control dopant activation and diffusion has driven sophisticated implantation and annealing schemes. Modern devices involve a mixture of implant species, cold and hot implants followed by processing steps at low temperatures (<650°C) combined with high temperature anneals with rapid temperature-time variations (flash and laser anneals) [18,19].…”