V. Privitera

Depth profiles of vacancy- and interstitial-type defects in MeV implanted Si

We demonstrate that the depth distribution of defects in MeV implanted n-type and p-type crystalline Si is severely affected by the impurity content of the material. Silicon samples with different concentrations of dopants (P or B) and intrinsic contaminants (i.e., C and O) were implanted with 1 or 2 MeV He ions to fluences in the range 2.5×108–1×1013/cm2. Using deep-level transient spectroscopy and spreading resistance measurements, we have identified the defects and determined their concentration and depth distribution. It is found that less than 4% of the defects generated by the beam escape recombination and are stored in electrically active, room temperature stable defect clusters, such as divacancies and carbon–oxygen pairs. When the concentration of these defects is much smaller than the doping level, their profile mirrors the initial defect distribution, as calculated by transport of ions in matter (TRIM), a Monte Carlo code. In particular, the profile presents a maximum at the same depth predicte...

A phase-field approach to the simulation of the excimer laser annealing process in Si

We present a phase-field methodology applied to the simulation of dopant redistribution in Si during an excimer laser annealing process. The kinetic model derived in the framework of the Ginsburg–Landau thermodynamic formalism is made up of three coupled equations that rule the concurrent evolution of the thermal, phase, and impurity fields. The model was solved numerically by considering, as the initial conditions, the generic material modification due to an ion implant process, i.e., the implanted impurity profile in a SiO2/a–Si/c–Si stack. The model is parametrized for the cases of As and B doping, considering the thermal properties of the materials in the stack and the impurity-dependent diffusivity in the solid, liquid, and interfacial regions (the latter is characterized by a finite dimension). Simulated profiles are compared with the experimental results that have been obtained by secondary ion mass spectrometry and spreading resistance profiling. These comparisons demonstrate the reliability of th...

Electrical behavior of ultra-low energy implanted boron in silicon

In this paper an extensive characterization of the electrical activation of ultra-low energy implanted boron in silicon is reported. The Spreading Resistance Profiling technique has been used, in a suitable configuration, for measuring doped layers shallower than 100 nm, in order to extract the carrier concentration profiles. The dependence on the implant energy, dose, and annealing temperature allowed us to gain more insight into the mechanisms responsible for the electrical activation at implant energies below 1 keV. By measuring the electrical activation as a function of time for several annealing temperatures, the thermal activation energy for the electrical activation of the dopant was achieved. It slightly depends on the implant dose and it is in the range of 2–3 eV. In particular, for an implant dose of 1×1014/cm2 it is 2.0 eV, close therefore to the 1.7 eV activation energy found [Napolitani et al., Appl. Phys. Lett. 75, 1869 (1999)] for the enhanced diffusion of ultra-low energy implanted boron. ...

Microscopical aspects of boron diffusion in ultralow energy implanted silicon

The transient enhanced diffusion of ultralow energy implanted B is reported in this letter. The mechanism giving rise to an enhancement of the diffusion during postimplantation anneal is investigated in detail by monitoring the diffusion of B as a function of temperature in the range 600–750 °C, for implant energies of 500 eV and 1 keV. The contribution of several classes of defect clusters to the anomalous diffusion phenomenon has been detected and interpreted. Both an ultrafast diffusion, occurring during the ramp-up of the thermal process, and a transient enhancement of the diffusion with characteristic decay times shorter by orders of magnitude than the known transient enhanced diffusion lifetimes, have been evidenced. The activation energy for the enhanced diffusion has been measured and found to be 1.7 eV.

Ultra-shallow junction formation by excimer laser annealing and low energy (<1 keV) B implantation: A two-dimensional analysis

Formation of shallow junctions has been investigated by using excimer laser annealing in combination with two implantation schemes: BF2-ions at 20 keV and B-ions at low energies (<1 keV). The latter approach was shown to produce best results, with ultra-shallow profiles extending to a depth as low as 35 nm. The lateral distribution of the implanted B following laser annealing has been studied with two-dimensional measurements using selective etching and cross-section transmission electron microscopy (TEM) on samples where the implanted dopant was confined within an oxide mask. The results show that there is substantial lateral diffusion of B under the oxide mask when melting occurs in this region while, if melting under the oxide mask is prevented, the implanted B close to the oxide mask edge was not activated by laser annealing. The results have been explained by numerical heat-flow calculations and it is concluded that the melting of the Si under the masked region and, therefore, the lateral diffusion, can be controlled by the oxide mask thickness.

Two-dimensional delineation of ultrashallow junctions obtained by ion implantation and excimer laser annealing

Junctions shallower than 100 nm, obtained by ion implantation and excimer laser annealing, have been characterized in two dimensions by transmission electron microscopy ~TEM! on chemically treated samples. The chemical treatment selectively removes silicon as a function of the B concentration, making thinner the regions where B is present in the cross section of the sample, with respect to the n-type substrate. Both secondary ion mass spectrometry and spreading resistance profiling measurements have been performed, in order to quantify the contour line obtained by TEM in terms of B concentration. The results achieved by the two-dimensional technique show interesting features, related to the particular redistribution of B occurring when silicon is melted by excimer laser annealing irradiation. In particular, a rectangular shape of the doped region obtained by laser annealing could be evidenced, caused by the fast diffusion in the melted material, completely different from the typical half-moon-shaped, thermally annealed, two-dimensional B profile. The feasibility of ultrashallow junctions by laser annealing, with depths below 100 nm and high electrical activation, is demonstrated. However, a huge lateral diffusion in the melted silicon is also to be taken into account when considering excimer laser treatments as an alternative to standard rapid thermal annealing.

High-resolution scanning capacitance microscopy of silicon devices by surface beveling

High-resolution scanning capacitance measurements were carried out magnifying the sample dimensions by a double beveling method. A magnification of ten times has been reached, but in principle even higher magnifications can be obtained. For depth magnifications the reverse junction carrier spilling has to be considered. The measurements indicate that the amount of the spilling effect is in agreement with the models developed to date. The method was successfully applied directly to silicon devices and it demonstrates that accuracy well below tip dimensions can be easily reached. Junction depths as well as channel lengths can be determined with a high resolution.

ROOM-TEMPERATURE MIGRATION AND INTERACTION OF ION BEAM GENERATED DEFECTS IN CRYSTALLINE SILICON

We have investigated the room‐temperature migration properties of ion generated defects in crystalline Si. The defects were injected into the bulk of a pure epitaxial Si layer by a low energy (40 keV) Si implant and monitored using a preexisting defect distribution, produced by a high energy He implant, as a marker. The depth of this defective marker layer was changed by varying the He implant energy in the range 1–3 MeV. Spreading resistance measurements show that the injected defects produce a partial annihilation of the pre‐existing damage. The magnitude of the annihilation process is strongly dependent on the depth of the defective marker, being very large when this marker is confined within 5 μm from the surface and negligible when it lies beyond a depth of ∼10 μm. From these results, detailed information on the nature of ion‐generated defects which are injected and on their migration properties is obtained. It is found that the observed phenomena are due to the annihilation of divacancies and phosphorus‐vacancy defect complexes, generated by the He implant, by Si self‐interstitials injected by the shallow Si implant.

Redistribution and electrical activation of ultralow energy implanted boron in silicon following laser annealing

The electrical activation of B in Si following excimer laser annealing has been investigated with transmission electron microscopy (TEM) and spreading resistance profiling. Ultrashallow profiles, extending to a depth of 35 nm, have formed in Si following laser annealing. The lateral distribution of the implanted B following laser annealing has been studied with two-dimensional measurements using selective etching and cross-sectional TEM on samples where the implanted dopant was confined within an oxide mask. The results show that there is substantial lateral diffusion of B under the oxide mask when melting occurs in this region. However it is shown in this article that the melting of the Si under the masked region can be controlled by the oxide thickness. Dopant diffusion into the bulk was observed after a combination of laser and rapid thermal annealing (RTA). The TEM results suggest that there is considerable lattice strain at the liquid–crystal interface after regrowth of the layer, which was subsequen...

N-type doping of Ge by As implantation and excimer laser annealing

The diffusion and activation of arsenic implanted into germanium at 40 keV with maximum concentrations below and above the solid solubility (8 × 1019 cm−3) have been studied, both experimentally and theoretically, after excimer laser annealing (λ = 308 nm) in the melting regime with different laser energy densities and single or multiple pulses. Arsenic is observed to diffuse similarly for different fluences with no out-diffusion and no formation of pile-up at the maximum melt depth. The diffusion profiles have been satisfactorily simulated by assuming two diffusivity states of As in the molten Ge and a non-equilibrium segregation at the maximum melt depth. The electrical activation is partial and decreases with increasing the chemical concentration with a saturation of the active concentration at 1 × 1020 cm−3, which represents a new record for the As-doped Ge system.