Boron-enhanced Diffusion Research Articles

SIMULATION OF BORON DIFFUSION IN THE NEAR-SURFACE REGION OF SILICON SUBSTRATE

The mechanism of boron-enhanced diffusion from a thin boron layer deposited on the surface in the case of silicon crystal doping is proposed and investigated. It was supposed that lattice contraction occurs in the vicinity of the surface due to the difference between the atomic radii of boron and silicon. This lattice contraction provides a stress-mediated diffusion of silicon self-interstitials from the near-surface region to the bulk of a semiconductor. Due to the stress-mediated diffusion, the near-surface region is depleted of silicon self-interstitials, and simultaneous oversaturation of this species occurs in the bulk. In this way, a strong nonuniform distribution of silicon self-interstitials in the vicinity of the surface is formed without regard to the large migration length of this species. The oversaturation of the bulk of a semiconductor with nonequilibrium self-interstitials allows one to explain the boron-enhanced diffusion of impurity atoms. The strong nonuniform distribution of these point defects also results in a specific form of boron concentration profile in the vicinity of the surface. Good agreement of the calculated boron profile with the experimental data for the entire doped region was obtained within the limit of the proposed model.

Application of high energy ion beam for the control of boron diffusion

For the purpose of optimizing the process of co-implantation of MeV Si ions to reduce boron transient enhanced diffusion and boron-enhanced diffusion in Si, multiple MeV implantations and annealing at different temperatures have been performed. A slight improvement on the suppression of B diffusion is observed by adding a low temperature annealing step after the MeV implantation. No differences in B diffusion are observed when the Si doses are increased from 1×1015 to 1×1016cm−2. This dose independent behavior is speculated to be a quasi-steady state of vacancy cluster evaporation.

An investigation on the modeling of boron-enhanced diffusion of ultralow energy implanted boron in silicon

We have simulated the boron-enhanced diffusion (BED) of boron implanted in silicon. We have used published data for B implantation with an energy of 500 eV in crystalline silicon. The simulation well reproduces the experimental profiles when sinks for interstitials are considered. Sources and sinks of self-interstitials are discussed as function of temperature.

Ultra-Shallow Junctions for the 65nm Node Based on Defect and Stress Engineering

AbstractThe co-implantation of germanium, carbon, and boron with the optimum implant energies and doses makes it possible to create p+/n junctions with the sheet resistance of less than 600 Ohm/square and the slope of less than 3 nm/decade. The narrow process window is based on careful engineering of the amorphization, point defects, and stresses and includes standard 1050°C spike annealing. The germanium pre-amorphization suppresses the ion channeling for the subsequent boron implant. The tensile stress induced by the substitutional carbon atoms and the compressive stress induced by the substitutional germanium atoms slow down boron diffusion and help to make the junctions shallower. The stress gradient in the transition region from the strained carbon and germanium doped layers to the relaxed silicon underneath creates an uphill boron flux that makes the junction slope steeper.The optimum amount of carbon is placed in between the implanted boron and the implant damage, which is located below the amorphized layer. During the annealing, the carbon atoms capture silicon interstitials that are coming from the implant damage and form carbon-interstitial clusters. The analysis demonstrates that it is possible to capture over 95% of the interstitials this way before they have a chance to reach boron-doped layer. This completely suppresses the transient-enhanced boron diffusion (TED) and drastically reduces the amount of boron that is deactivated in boron-interstitial clusters (BICs). In fact, the point defect engineering with an optimized carbon profile allows to remove all non-equilibrium silicon interstitials that are generated by the following three sources: the implant damage below the amorphized layer, the rapid temperature ramp down, and the interstitials generated by boron at high concentrations (due to the effect known as boron-enhanced diffusion (BED)).The latter effect leads to significant increase of the apparent boron activation level beyond the well-characterized solid-state solubility level. We explain this effect as a reduction in formation of BICs due to the lack of interstitial supersaturation. In carbon-free silicon, high concentration boron is always accompanied by the non-equilibrium interstitials, coming from either the implant damage or the BICs even if boron is introduced into silicon by pre-deposition instead of the implantation. Extensive experiments and theoretical analysis based on simulation of the interaction of Ge, C, I, and B atoms, as well as the stress effects, point to the optimized process flow that improves the shape and parameters of the p+/n USJs.

Boron-enhanced diffusion in excimer laser annealed Si

The effect of excimer laser annealing (ELA) and rapid thermal annealing (RTA) on B redistribution in B-implanted Si has been studied by secondary ion mass spectrometry (SIMS) and spreading resistance probe (SRP). B has been implanted with an energy of 1 keV and a dose of 10 16 cm −2 forming a distribution with a width of 20–30 nm and a peak concentration of ∼5 × 10 21 cm −3. It has been found that ELA with 10 pulses of the energy density of 850 mJ/cm 2 results in a uniform B distribution over the ELA-molten region with an abrupt profile edge. SRP measurements demonstrate good activation of the implanted B after ELA, with the concentration of the activated fraction (∼10 21 cm −3) exceeding the solid solubility level. RTA (30 s at 1100 °C) of the as-implanted and ELA-treated samples leads to a diffusion of B with diffusivities exceeding the equilibrium one and the enhancement is similar for both of the samples. It is also found that RTA decreases the activated B in the ELA-treated sample to the solid solubility limit (2 × 10 20 cm −3). The similarity of the B diffusivity for the as-implanted and ELA-treated samples suggests that the enhancement of the B diffusivity is due to the so-called boron-enhanced diffusion (BED). Possible mechanisms of BED are discussed.

Boron-enhanced diffusion of boron from ultralow-energy ion implantation

We have investigated the diffusion enhancement mechanism of boron-enhanced diffusion (BED), wherein boron diffusivity is enhanced four to five times over the equilibrium diffusivity at 1050 °C in the proximity of a silicon layer containing a high boron concentration. It is demonstrated that BED is driven by excess interstitials injected from the high boron concentration layer during annealing. For evaporated layers, BED is observed above a threshold boron concentration between 1% and 10%, though it appears to be closer to 1% for B-implanted layers. For sub-keV B implants above the threshold, BED dominates over the contribution from transient-enhanced diffusion to junction depth. For 0.5 keV B, this threshold implantation dose lies between 3×1014 and 1×1015 cm−2. It is proposed that the excess interstitials responsible for BED are produced during the formation of a silicon boride phase in the high B concentration layers.

Boron-enhanced diffusion of boron: Physical mechanisms

Silicon layers containing B in excess of a few atomic percent create a supersaturation of Si self-interstitials in the underlying Si, resulting in enhanced diffusion of B in the substrate [boron-enhanced diffusion (BED)]. The temperature and time dependence of BED is investigated here. Evaporated boron as well as ultralow energy 0.5 keV B-implanted layers were annealed at temperatures from 1100 to 800 °C for times ranging from 3 to 3000 s. Isochronal 10 s anneals reveal that the BED effect increases with increasing temperature up to 1050 °C and then decreases. In contrast, simulations based on interstitial generation via the kick-out mechanism predict a decreasing dependence leading to the conclusion that the kick-out mechanism is not the dominant source of excess interstitials responsible for BED. The diffusivity enhancements from the combined effects of BED and transient-enhanced diffusion, measured in 2×1015 cm−2, 0.5 keV B-implanted samples, show a similar temperature dependence as seen for evaporated B, except that the maximum enhancement occurs at 1000 °C. The temperature-dependent behavior of BED supports the hypothesis that the source of excess interstitials is the formation of a silicon boride phase in the high-boron-concentration silicon layer.

Temperature- and Time-Dependence of Boron-Enhanced Diffusion From Evaporated- and Ultra-Low Energy Ion-Implanted Layers

ABSTRACTSilicon layers containing B in excess of a few atomic percent create a supersaturation of Si self-interstitials in the underlying Si, resulting in enhanced diffusion of B in the substrate (boron-enhanced diffusion, BED). The temperature- and time-dependence of BED is investigated here. Evaporated-boron as well as ultra-low energy 0.5-keV B-implanted layers were annealed at temperatures from 1100 to 800°C for times ranging from 3 to 3000s. Isochronal 10s anneals reveal that the BED effect increases with increasing temperature up to 1050°C and then decreases. In contrast, a simulation of interstitial generation via the kick-out mechanism predicts a decreasing dependence on increasing temperature leading to the conclusion that the kick-out mechanism is not the dominant source of excess interstitials responsible for BED. The diffusivity enhancements from the combined effects of BED and TED (transient-enhanced diffusion), measured in 2×105cm−2, 0.5-keV B-implanted samples, show a similar temperature dependence as seen for evaporated B, except that the maximum enhancement occurs at 1000°C. The temperature-dependent behavior of BED supports the hypothesis that the source of excess interstitials is the formation of a silicon boride phase in the high B concentration silicon layer.

Damage, defects and diffusion from ultra-low energy (0–5 keV) ion implantation of silicon

Continued use of ion implantation for doping of silicon integrated circuits will soon require implantation energies below 5 keV to form electrical junctions less than 50 nm deep. At such low energies, dopant diffusion and formation of extended defects is modified by both the proximity of the surface and by the large volume concentrations of point defects and dopant atoms that arise from reduced range straggling. This brief review summarizes our recent experiments which measured defect formation and evolution, as well as enhanced diffusion, in silicon implanted with Si + and B + ions at energies as low as 0.5 keV. The results have demonstrated that {311}-type extended defects are generated from Si + implants even within 3 nm of the surface. However, when these defects eventually dissolve, the surface acts as a perfect sink to efficiently annihilate the released interstitials. As a result, the amount of TED from Si + implantation measured by epitaxially-grown B markers decreases approximately linearly with decreasing ion energy. For sub-keV B + implants typical doses currently used for source-drain doping lead to a boron diffusion enhancement of 3–4× despite the proximity of the surface. Enhanced diffusion is also observed from molecular beam-deposited silicon layers containing a high boron concentration. This newly emerged diffusion enhancement mechanism, boron-enhanced-diffusion (BED), is associated with the formation of a fine-grain polycrystalline silicon boride phase in the implanted layer during activation annealing. These investigations of ultra-low energy (ULE) implantation have thus reinforced and validated our understanding of the role of implantation damage in enhancing dopant diffusion in silicon, while simultaneously revealing some important new materials issues which will impact semiconductor processing in coming device generations.

BF +2 implantation in predamaged Si with Ge + or Si + at doses lower than amorphization

The B diffusion dependence on the preimplantation energy and dose of Ge + or Si + was studied. Boron-enhanced diffusion is reduced by moderate lattice disorder below amorphization. The residual damage was examined by cross-sectional transmission electron microscopy, which indicated that amorphization causes the formation of extended defects. A gate-controlled diode was designed and fabricated for the evaluation of preimplanted layer. The dependence of the leakage current on the preimplantation dose indicates that no serious defects are caused by Ge + implantation at 40 keV and 5 × 10 13 cm −2 or Si + at 40 keV and 2 × 10 14 cm −2.

Radiation-enhanced diffusion of boron in germanium during ion implantation

An effect of radiation-enhanced diffusion during boron ion implantation into 200 to 500 deg C germanium substrates was found. The boron-enhanced diffusion coefficient is independent of the temperature over the range 200 to 500 deg C during ion implantation. It depends upon the dose rate of the incident ions and its value corresponds to the thermal diffusion of boron in germanium at 800 deg C. (auth)