María Aboy

Atomistic modeling of amorphization and recrystallization in silicon

We propose an atomistic model to describe the evolution of the damage generated by irradiation in Si, going from isolated point defects to the formation of continuous amorphous layers. The elementary units used to reproduce the defective zones are Si interstitials, vacancies and the bond defect, which is a local distortion of the Si lattice without any excess or deficit of atoms. More complex defect structures can be formed as these elementary units cluster. The amorphous pockets are treated as agglomerates of bond defects characterized by their local coordination. The model is able to reproduce the abrupt regime in the crystal-amorphous transition in Si and the epitaxial recrystallization upon annealing as observed in the experiments. The model extends the atomistic kinetic Monte Carlo simulation technique to high implant doses, adequately describing the amorphization and regrowth in Si.

Atomistic analysis of the evolution of boron activation during annealing in crystalline and preamorphized silicon

We use kinetic nonlattice Monte Carlo atomistic simulations to investigate the physical mechanisms for boron cluster formation and dissolution in complementary metal-oxide semiconductor (MOS) processing, and the role of Si interstitials in the different processes. For this purpose, B implants in crystalline Si as well as B implants in preamorphized Si are analyzed. For subamorphizing B implants, a high concentration of Si interstitials overlaps with the B profile and this causes a very quick B deactivation for both low- and high-dose B implants. For B implants in preamorphized silicon, B is activated during the regrowth of the amorphous layer if the B concentration is lower than 1020cm−3 and remains active upon annealing. However, if B concentrations higher than 1020cm−3 are present, as occurs in the formation of extensions in p-channel MOS transistors, B atoms are not completely activated during the regrowth. Moreover, the injection of Si interstitials from the end-of-range defects leads to additional B ...

Atomistic modeling of deactivation and reactivation mechanisms in high-concentration boron profiles

We use kinetic nonlattice Monte Carlo atomistic simulations to investigate the physical mechanisms for boron cluster formation and dissolution at very high B concentrations, and the role of Si interstitials in these processes. For this purpose, high-dose, low-energy B implants and theoretical structures with fully active box shaped B profiles were analyzed. Along with the theoretical B profile, different Si interstitial profiles were included. These structures could be simplifications of the situation resulting from the regrowth of preamorphized or laser annealed B implants. While for B concentrations lower than 1020 cm−3, B clusters are not formed unless a high Si interstitial concentration overlaps the B profile, our simulation results show that for higher B concentrations, B clusters can be formed even in the presence of only the equilibrium Si interstitial concentration. The existence of a residual concentration of Si interstitials along with the B boxes makes the deactivation faster and more severe.

Atomistic analysis of defect evolution and transient enhanced diffusion in silicon

Kinetic Monte Carlo simulations are used to analyze the ripening and dissolution of small Si interstitial clusters and {113} defects, and its influence on transient enhanced diffusion of dopants in silicon. The evolution of Si interstitial defects is studied in terms of the probabilities of emitted Si interstitials being recaptured by other defects or in turn being annihilated at the surface. These two probabilities are related to the average distance among defects and their distance to the surface, respectively. During the initial stages of the defect ripening, when the defect concentration is high enough and the distance among them is small, Si interstitials are mostly exchanged among defects with a minimal loss of them to the surface. Only when defects grow to large sizes and their concentration decreases, the loss of Si interstitials through diffusion to the surface prevails, causing their dissolution. The presence of large and stable defects near the surface is also possible when the implant energy i...

Physical insight into boron activation and redistribution during annealing after low-temperature solid phase epitaxial regrowth

Kinetic Monte Carlo simulations of B diffusion and activation in preamorphized Si during annealing after solid phase epitaxial regrowth have been used to provide insight into the mechanisms that drive these phenomena. Simulations show that the presence of an initially high active B concentration along with a Si interstitial supersaturation set by end of range defects leads to simultaneous B deactivation and uphill diffusion through the capture of mobile interstitial B in the high concentration region during subsequent anneal treatments. Once the Si interstitial supersaturation decays close to equilibrium values, B clusters dissolve and emitted B diffuses downhill, following the B concentration gradient. The active B concentration at the minimum state of activation becomes higher as the annealing temperature increases as a consequence of a faster increase of the B cluster dissolution rate compared with the formation rate.

Role of silicon interstitials in boron cluster dissolution

We present kinetic nonlattice Monte Carlo atomistic simulations to investigate the role of Si interstitials in B cluster dissolution. We show that the presence of Si interstitials from an oxidizing anneal stabilize B clusters and slow down B cluster dissolution, compared to anneal in inert ambient. We have also analyzed the influence of injected Si interstitials from end of range defects, due to preamorphizing implants, on B deactivation and reactivation processes. We have observed that the B cluster evolution can be clearly correlated to the evolution of Si interstitial defects at the end of range. The minimum level of activation occurs when the Si interstitial supersaturation is low because the end of range defects have dissolved or reach very stable configurations, such as dislocation loops.

Kinetics of large B clusters in crystalline and preamorphized silicon

We present an extended model for B clustering in crystalline or in preamorphized Si and with validity under conditions below and above the equilibrium solid solubility limit of B in Si. This model includes boron-interstitial clusters (BICs) with BnIm configurations—complexes with n B atoms and m Si interstitials—larger (n > 4), and eventually more stable, than those included in previous models. In crystalline Si, the formation and dissolution pathways into large BICs configurations require high B concentration and depend on the flux of Si interstitials. In the presence of high Si interstitial flux, large BICs with a relatively large number of interstitials (m ≥ n) are formed, dissolving under relatively low thermal budgets. On the contrary, for low Si interstitial flux large BICs with few interstitials (m ≪ n) can form, which are more stable than small BICs, and whose complete dissolution requires very intense thermal budgets. We have also investigated the kinetics of large BICs in preamorphized Si, both ...

Atomistic process modeling based on Kinetic Monte Carlo and Molecular Dynamics for optimization of advanced devices

Combined Molecular Dynamics and Kinetic Monte Carlo simulations are used in hierarchical models to gain physical understanding for process optimization in advanced devices. Thermal budget for the removal of defects in advanced millisecond anneals is evaluated. Alternatives to overcome the imperfect regrowth of narrow Si structures are proposed. The compromise between implant and anneal parameters for doping of FinFETs are presented, considering lateral diffusion and activation.

Kinetic Monte Carlo simulations of boron activation in implanted Si under laser thermal annealing

We investigate the correlation between dopant activation and damage evolution in boron-implanted silicon under excimer laser irradiation. The dopant activation efficiency in the solid phase was measured under a wide range of irradiation conditions and simulated using coupled phase-field and kinetic Monte Carlo models. With the inclusion of dopant atoms, the presented code extends the capabilities of a previous version, allowing its definitive validation by means of detailed comparisons with experimental data. The stochastic method predicts the post-implant kinetics of the defect-dopant system in the far-from-equilibrium conditions caused by laser irradiation. The simulations explain the dopant activation dynamics and demonstrate that the competitive dopant-defect kinetics during the first laser annealing treatment dominates the activation phenomenon, stabilizing the system against additional laser irradiation steps.

Atomistic modeling of impurity ion implantation in ultra-thin-body Si devices

Source/drain formation in ultra-thin body devices by conventional ion implantation is analyzed using atomistic simulation. Dopant retention is dramatically reduced by backscattering for low-energy and low-tilt angles, and by transmission for high angles. For the first time, molecular dynamics and kinetic Monte Carlo simulations, encompassing the entire Si body, are applied in order to predict damage during implant and subsequent recovery during anneal. These show that amorphization should be avoided as recrystallization in ultra-thin-body Si leads to twin boundary defects and poly-crystalline Si formation, despite the presence of a mono-crystalline Si seed. Rapid dissolution of end-of range defects in thin-body Si, caused by surface proximity, does not significantly reduce diffusion lengths. The conclusions of the atomistic modeling are verified by a novel characterization methodology and electrical analysis.