D. C. Jacobson

On the mechanism of the hydrogen-induced exfoliation of silicon

We have investigated the fundamental mechanism underlying the hydrogen-induced exfoliation of silicon, using a combination of spectroscopic and microscopic techniques. We have studied the evolution of the internal defect structure as a function of implanted hydrogen concentration and annealing temperature and found that the mechanism consists of a number of essential components in which hydrogen plays a key role. Specifically, we show that the chemical action of hydrogen leads to the formation of (100) and (111) internal surfaces above 400u2009°C via agglomeration of the initial defect structure. In addition, molecular hydrogen is evolved between 200 and 400u2009°C and subsequently traps in the microvoids bounded by the internal surfaces, resulting in the build-up of internal pressure. This, in turn, leads to the observed “blistering” of unconstrained silicon samples, or complete layer transfer for silicon wafers joined to a supporting (handle) wafer which acts as a mechanical “stiffener.”

Carbon diffusion in silicon

Carbon diffusion in silicon has been investigated by using a superlattice structure of carbon spikes (10 nm-wide, carbon concentration >1019u2009cm−3, spikes spaced 100 nm apart) grown epitaxially by Si molecular beam epitaxy. Samples were annealed in the range between 680 and 850u2009°C. The diffusive behavior of carbon was monitored by secondary ion mass spectrometry. Carbon diffusion profiles observed at temperatures above 800u2009°C show highly nonregular behavior. The diffusion results are interpreted in terms of the kick-out mechanism.

Interactions of ion‐implantation‐induced interstitials with boron at high concentrations in silicon

Ion implantation of Si (60 keV, 1×1014/cm2) has been used to introduce excess interstitials into silicon predoped with high background concentrations of B, which were varied between 1×1018 and 1×1019/cm3. Following post‐implantation annealing at 740u2009°C for 15 min to allow agglomeration of the available interstitials into elongated {311} defects, the density of the agglomerated interstitials was determined by plan‐view transmission electron microscopy observation of the defects. We report a significant reduction in the fraction of excess interstitials trapped in {311} defects as a function of boron concentration, up to nearly complete disappearance of the {311} defects at boron concentrations of 1×1019/cm3. The reduction of the excess interstitial concentration is interpreted in terms of boron‐interstitial clustering, and implications for transient‐enhanced diffusion of B at high concentrations are discussed.

Reduction of transient diffusion from 1–5 keV Si+ ion implantation due to surface annihilation of interstitials

The reduction of transient enhanced diffusion (TED) with reduced implantation energy has been investigated and quantified. A fixed dose of 1×1014u2002cm−2u2002Si+ was implanted at energies ranging from 0.5 to 20 keV into boron doping superlattices and enhanced diffusion of the buried boron marker layers was measured for anneals at 810, 950, and 1050u2009°C. A linearly decreasing dependence of diffusivity enhancement on decreasing Si+ ion range is observed at all temperatures, extrapolating to ∼1 for 0 keV. This is consistent with our expectation that at zero implantation energy there would be no excess interstitials from the implantation and hence no TED. Monte Carlo modeling and continuum simulations are used to fit the experimental data. The results are consistent with a surface recombination length for interstitials of <10 nm. The data presented here demonstrate that in the range of annealing temperatures of interest for p-n junction formation, TED is reduced at smaller ion implantation energies and that this is d...

MECHANISM FOR THE REDUCTION OF INTERSTITIAL SUPERSATURATIONS IN MEV-IMPLANTED SILICON

We demonstrate that the excess vacancies induced by a 1 MeV Si implant reduce the excess interstitials generated by a 40 keV Si implant during thermal annealing when these two implants are superimposed in silicon. It is shown that this previously observed reduction is dominated by vacancy annihilation and not by gettering to deeper interstitial-type extended defects. Interstitial supersaturations were measured using B doping superlattices (DSL) grown on a silicon-on-insulator (SOI) substrate. Implanting MeV and keV Si ions into the B DSL/SOI structure eliminated the B transient enhanced diffusion normally associated with the keV implant. The buried SiO2 layer in the SOI substrate isolates the deep interstitials-type extended defects of the MeV implant, thereby eliminating the possibility that these defects getter the interstitial excess induced by the keV Si implant.

DEPTH PROFILING OF VACANCY CLUSTERS IN MEV-IMPLANTED SI USING AU LABELING

A technique for profiling the clustered-vacancy region produced by high-energy ion implantation into silicon is described and tested. This technique takes advantage of the fact that metal impurities, such as Au, are trapped in the region of excess vacancies produced by MeV Si implants into silicon. In this work, the clustered-vacancy regions produced by 1-, 2-, and 8-MeV Si implants into silicon have been labeled with Au diffused in from the front surface at 750u2009°C. The trapped Au was profiled with Rutherford backscattering spectrometry. The dynamics of the clustered-vacancy region were monitored for isochronal annealing at 750–1000u2009°C, and for isothermal annealing at 950u2009°C, for 10–600 s. Cross-sectional transmission electron microscopy analysis revealed that after the drive-in anneal, the Au in the region of vacancy clusters is in the form of precipitates. The results demonstrate that the Au-labeling technique offers a convenient and potentially quantitative tool for depth profiling vacancies in clusters.

The interstitial fraction of diffusivity of common dopants in Si

The relative contributions of interstitials and vacancies to diffusion of a dopant A in silicon are specified by the interstitial fraction of diffusivity, fA. Accurate knowledge of fA is required for predictive simulations of Si processing during which the point defect population is perturbed, such as transient enhanced diffusion. While experimental determination of fA is traditionally based on an underdetermined system of equations, we show here that it is actually possible to derive expressions that give meaningful bounds on fA without any further assumptions but that of local equilibrium. By employing a pair of dopants under the same point-defect perturbance, and by utilizing perturbances very far from equilibrium, we obtain experimentally fSb⩽0.012 and fB⩾0.98 at temperatures of ∼800u2009°C, which are the strictest bounds reported to date. Our results are in agreement with a theoretical expectation that a substitutional dopant in Si should either be a pure vacancy, or a pure interstitial(cy) diffuser.

Interstitial defects in silicon from 1–5 keV Si+ ion implantation

Extended defects from 5-, 2-, and 1-keV Si+ ion implantation are investigated by transmission electron microscopy using implantation doses of 1 and 3×1014u2009cm−2 and annealing temperatures from 750 to 900u2009°C. Despite the proximity of the surface, {311}-type defects are observed even for 1 keV. Samples with a peak concentration of excess interstitials exceeding ∼1% of the atomic density also contain some {311} defects which are corrugated across their width. These so-called zig-zag {311} defects are more stable than the ordinary {311} defects, having a dissolution rate at 750u2009°C which is ten times smaller. Due to their enhanced stability, the zig-zag {311} defects grow to lengths that are many times longer than their distance from the surface. It is proposed that zig-zag {311} defects form during the early stages of annealing by coalescence the high volume density of {311} defects confined within a very narrow implanted layer. These findings indicate that defect formation and dissolution will continue to con...

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

Abstract 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.

Iron gettering mechanisms in silicon

Boron implantation into silicon offers a unique system for studying the gettering mechanisms of Fe. Using deep level transient spectroscopy to monitor the remaining Fe in the gettered region and secondary‐ion‐mass spectroscopy to measure the concentration of Fe redistributed to the B region, we show that the gettering mechanisms can be quantitatively described. A combination of Fermi‐level‐induced Fe+ charge‐state stabilization and Fe+–B− pairing acts to lower the free energy of Fe in p+ regions. This can lead to Fe partition coefficients as high as 106 at a p+/p interface at temperatures below ≊400u2009°C. The dynamic response of the system is diffusion limited during the cooling cycle. B gettering is more effective than gettering produced by Si implantation damage and more effective than trapping by a neutral impurity such as C. These mechanisms also make a large contribution to the effective gettering of Fe by p/p+ epitaxial silicon wafers. The Fermi‐level/pairing gettering mechanism is also expected to op...