Universidad de Valladolid

Universidad de Valladolid

Semiconductor doping

Dopant atoms, such as boron or phosphorous, are selectively introduced into the Si substrate during some of the process steps in the fabrication of Si-based electronic devices (e.g., formation of source/drain regions in CMOS fabrication, formation of emitters in solar cells,…) to modify its electrical features. Dopants are generally introduced by ion implantation, since it is a well established technique and it provides a precise control of the distribution and concentration of the dopants in the substrate. However, during ion implantation, lattice is damaged resulting in a highly non-equilibrium distribution of point defects (self-interstitials and vacancies). Generally, after implantation dopants atoms do not reside in lattice sites, thus, they are not electrically active. Subsequent thermal processing is needed to heal lattice damage and electrically activate dopants. Interactions of dopants with excess Si interstitials and vacancies (generated during the implantation, or released by extended defects during annealing) result in mobile dopant species and dopant-defect agglomerates. This has severe adverse consequences on the Si based devices as dopant diffusivity is enhanced and dopant activation is reduced compared to equilibrium values.

These processes are highly transient and their dynamics needs to be captured by models in order to provide comprehension of the physical mechanisms and define the optimum processes. Our group has developed a model for B clustering and diffusion (implemented in our kinetic Monte Carlo simulator) whose validity for Si processing evaluation and optimization has been confirmed in a very wide range of implant parameters and annealing conditions. Thus, our contributions in this field include: (i) the ripening and dissolution of Si interstitial defects and their influence on transient enhanced diffusion of dopants; (ii) the formation and dissolution of B-interstitial complexes (BICs) that affect boron electrical activation and diffusion; (iii) the influence of end-of-range interstitial defects resulting from preamorphizing implants on B activation and diffusion; (iv) explanation of the "apparently anomalous" uphill diffusion of B; (v) doping of advanced nanodevices with three-dimensional architectures; (vi) BICs modeling at very high B concentrations (as those required in ultra-shallow junctions and advanced nanodevices); (vii) the influence of BICs on hole mobility degradation that occurs at very high B concentrations.

Sheet resistance and junction depth
Sheet resistance and junction depth values obtained from simulations of B implanted in preamorphized Si an annealed at different temperatures. Solid lines represent simulation results and symbols correspond to experimental data taken from reference J.-Y. Jin et al., J. Vac. Sci. Technol. B 20, 422 (2002) (M. Aboy et al., J. Appl. Phys. 97, 103520 (2005) - Atomistic analysis of the evolution of boron activation during annealing in crystalline and preamorphized silicon).

Uphill diffusion
Evolution of the simulated B profile during annealing at 800 °C after recrystallization of a preamorphized layer. The initial B profile after recrystallization at 650 °C for 5 s is taken from SIMS measurements in reference W. Lerch et al., Proc.-Electrochem. Soc. 2004-01, 90 (2004). The B profile shows a progressive displacement towards the high B concentration region ("uphill diffusion") in the region of intermediate B concentrations for increased times 10 and 120 s. Longer anneal time 900 s results in "downhill" tail diffusion (M. Aboy et al., Appl. Phys. Lett. 88, 191917 (2006) - Physical insight into boron activation and redistribution during annealing after low-temperature solid phase epitaxial regrowth).