Excess Interstitials Research Articles

Defect Evolution from Low Energy, Amorphizing, Germanium Implants on Silicon

ABSTRACTPlan-view transmission electron microscopy (PTEM) was used to characterize defect evolution upon annealing of low-to-medium energy, 5-30 keV, germanium implants into silicon. The implant dose was 1 × 1015 ions/cm2, sufficient for surface amorphization. Annealing of the samples was done at 750 °C in nitrogen ambient by both rapid thermal annealing (RTA) and conventional furnace, and the time was varied from 10 seconds to 360 minutes. Results indicate that as the energy drops from 30 keV to 5 keV, an alternate path of excess interstitials evolution may exist. For higher implant energies, the interstitials evolve from clusters to {311}'s to loops as has been previously reported. However, as the energy drops to 5 keV, the interstitials evolve from clusters to small, unstable dislocation loops which dissolve and disappear within a narrow time window, with no {311}'s forming. These results imply there is an alternate evolutionary pathway for {311} dissolution during transient enhanced diffusion (TED) for these ultra-low energy implants.

Atomistic Modeling of Amorphization in Silicon

ABSTRACTWe discuss atomistic simulations of ion implantation and annealing of Si over a wide range of ion dose and substrate temperatures. The DADOS Monte Carlo model has been extended to include the formation of amorphous regions, and this allows simulations of dopant diffusion at high doses. As the dose of ions increases, a continuous amorphous layer may be formed. In that case, most of the excess interstitials generated by the implantation may be swept to the surface as the amorphous layer regrows, instead of diffusing through the crystalline region. This process reduces the amount of transient enhanced diffusion during annealing. This model also reproduces the dynamic annealing during high temperature implants.

Boron-interstitial silicon clusters and their effects on transient enhanced diffusion of boron in silicon

The nature of ion-implantation induced clusters of boron and silicon-self interstitials (BICs), and their effects on transient enhanced diffusion (TED) of B in Si have been investigated in samples predoped with B at different concentrations. Excess Si interstitials have been introduced by Si+ implantation at 60 keV with doses of 1 and 5×1014 cm−2. The B diffusivity and the amount of B trapped in the clusters have been evaluated from the best fits of simulation-prediction profiles to experimental B profiles, after annealing at 740 and 800 °C for different times. Our results show that the BICs in the beginning act as a sink for interstitials, strongly reducing the TED in the early phases of the annealing. However, being more stable than the Si-interstitial clusters and the {113} defects, they dissolve slowly and can, therefore, sustain a moderate Si-interstitial supersaturation for longer annealing times, even when the Si-interstitial defects are completely dissolved. The data show that the amount of B in the BICs is higher than that of the interstitials; we estimate an average ratio between the B and interstitial concentrations to be about 1.5.

Influence of transient enhanced diffusion of the intrinsic base dopant profile on SiGe HBT DC and HF characteristics

The influence of transient enhanced boron out-diffusion from the intrinsic base, caused by excess silicon interstitials created during the extrinsic base implantation, has been investigated for a non-selective SiGe HBT process. Devices with different designs of the extrinsic base region were fabricated, where some designs allowed part of the epitaxial base to be implanted with a high boron dose, hereby increasing the number of silicon interstitials close to the intrinsic device. These devices showed a marked degradation of DC characteristics and HF performance. 2D-device simulations were used to investigate the sensitivity in DC and HF parameters to vertical base profile changes. Good agreement was obtained between measured and simulated DC and HF characteristics.

The formation mechanism of grown-in defects in CZ silicon crystals based on thermal gradients measured by thermocouples near growth interfaces

The thermal distributions near the growth interface of 150 mm CZ crystals were measured by three thermocouples installed at the center, middle (half radius) and edge (10 mm from surface) of the crystals. The results show that larger growth rates produced smaller thermal gradients. This contradicts the widely used heat flux balance equation. Using this fact, it is confirmed in CZ crystals that the type of point defects created is determined by the value of the thermal gradient ( G) near the interface during growth, as already reported for FZ crystals. Although depending on the growth systems the effective lengths of the thermal gradient for defect generation are varied, we defined the effective length as 10 mm from the interface in this experiment. If G is roughly smaller than 20°C cm −1, vacancy rich CZ crystals are produced. If G is larger than 25°C cm −1, the species of point defects changes dramatically from vacancies to interstitials. The experimental results after detaching FZ and CZ crystals from the melt show that growth interfaces are filled with vacancies. We propose that large G produces shrunk lattice spacing and in order to relax such lattice excess interstitials are necessary. Such interstitials recombine with vacancies which were generated at the growth interface, next occupy interstitial sites and residuals aggregate themselves to make stacking faults and dislocation loops during cooling. The shape of the growth interface is also determined by the distributions of G across the interface. That is, the small G and the large G in the center induce concave and convex interfaces to the melt, respectively.

First-principles calculations of interstitial boron in silicon

We perform first-principles total-energy calculations to identify the stable and metastable configurations of interstitial B in Si. We calculate formation energies and ionization levels for several equilibrium ionic configurations in different possible charge states. In all charge states the ground state consists of a B atom close to a substitutional site and a Si self-interstitial nearby. The binding energy of the self-interstitial to the substitutional B is, however, rather weak, of the order of 0.2--0.3 eV. The ground state has negative-$U$ properties in accordance with experiments. We find several charge-state-dependent metastable configurations of interstitial B energetically close to the ground state. We discuss on the basis of formation energies the role of excess Si interstitials in the activation of B diffusion and the charge-assisted transport mechanism in the activation of B diffusion.

Implantation damage effect on boron annealing behavior using low-energy polyatomic ion implantation

We have studied ion-implantation damage effects on boron clustering and transient enhanced diffusion (TED) by using polyatomic boron (Bn−, n=1–3) ion implantation with the same atomic boron dose and energy. This Bn− series implantation can produce different amounts of damage with the same boron as-implanted profile and same amount of excess interstitials, hence a net effect of implantation damage can be extracted. Secondary ion mass spectrometry measurements indicate that for 1 keV boron atomic energy implantation and 10 s 1050 °C rapid thermal annealing, B1− implantation has less TED and less boron–interstitial clustering than B2− and B3− implantation. A boron trapping peak at the SiO2/Si interface is also speculated since the amount of boron trapped is correlated to the size of implanted ions.

Modelisation of extended defects to simulate the transient enhanced diffusion of boron

Process conditions to create ultra-shallow junctions for silicon devices are known to result in a transient enhanced diffusion (TED) of boron. The diffusion of boron is due to its coupling with silicon self-interstitials. An anomalous behavior of these atoms is responsible for the boron TED. More precisely, it has been experimentally evidenced that a great amount of excess Si interstitials is created after implantation and that a part of this supersaturation precipitates into extended defects during annealing. A modelisation of this phenomenon is presented, aimed at enlarging the predictness of the process simulations, such as in the simulator IMPACT-4, and at increasing the understanding of boron TED. The continuous description of the nucleation, ‘pure growth’ and Ostwald ripening of one kind of extended defects, dislocation loops, is exposed. The comparison of the calculated sizes and densities of dislocation loops with their experimental values validates the modelisation. It was demonstrated that the concomitant evolution of silicon free interstitials results in the right amount of boron diffusion. TED is correctly simulated throughout the annealing thanks to this enhancement of the physical basis of IMPACT-4.

Transition from small interstitial clusters to extended {311} defects in ion-implanted Si

We have investigated the transition from small interstitial clusters to {311} defects in ion-implanted Si. Czochralski Si wafers were implanted with 1.2 MeV Si ions to fluences in the range 1012–5×1013/cm2 and annealed at temperatures of 600–750 °C for times as long as 15 h. Photoluminescence and transmission electron microscopy analyses allowed us to analyze the transition of small interstitial clusters, formed by the agglomeration of the excess interstitials introduced by the beam, into {311} defects. It is found that {311} defects form only at fluences ⩾1013/cm2 and at temperatures above 600 °C. When {311} are observed in transmission electron microscopy, the luminescence spectrum is dominated by a sharp signal at 1376 nm which has been correlated with optical transitions occurring at or close to these defects. At lower temperatures or at lower fluence, no extended defects are observed in transmission electron microscopy and the luminescence spectrum present two broad signatures arising from carrier recombination at interstitial clusters. These data strongly indicate that a severe structural transformation occurs in the evolution from small interstitial clusters to extended {311} defects.

Quantitative Depth Profiles of Vacancy Cluster Defects Produced by MeV Ion Implantation in Si: Species and dose Dependence

AbstractMonte Carlo simulation codes such as TRIM or MARLOWE show a net displacement of interstitials with respect to vacancies from the Frenkel pairs produced in implant cascades. As a result, defect profiles after recombination have an excess of vacancies (Vex) in the shallow region of these implants, with excess interstitials near the projected range Rp. Typically, accurate estimates of the Vex distribution are computer intensive as the complete history of each damage cascade must be recorded. In this work we introduce a method for fast estimation of the Vex profile based on the derivative of the recoil distribution obtained from binary collision simulations using the Kinchen-Pease approximation for damage production. Also, Au labeling has been used to quantitatively analyze and compare the experimental depth profiles of Vex produced by implants of B, Si, and Ge into Si for a range of doses. Specific conditions like matching of the projected range of the implanted ions and matching of the number of Frenkel pairs created have been compared. This detailed study and comparison with simulation provides the first systematic, quantitative measurements of the Vex concentrations over such a range of species and doses. In analogy to the “+ number” for excess interstitials from ion implantation, we determine the range of “minus (-) numbers”, that is the number of excess vacancies per implanted ion for the different species. For Si it is constant in the dose range studied at -0.05, while for B it is -0.001 to -0.004 and for Ge, -0.1 to -0.2. The general shapes of the Vex profiles are similar to those obtained from simulation. Evidence is presented that one significant difference, namely an experimentally observed vacancy depletion near the surface, may be due to the diffusion and annihilation of vacancies at the surface. In addition, an increased nucleation probability of vacancy defects close to Rp may account for the growth of excess vacancies from close to Rp towards the surface with increasing implant dose.

Phosphorus / Silicon Interstitial Annealing After Ion Implantation

AbstractThe release of interstitials from extended defects after ion implantation acts as a driving force behind transient enhanced diffusion (TED). Implantation of Si+ ions into regions of phosphorus-doped silicon provides experimental insight into the interaction of silicon interstitials and dopant atoms during primary damage annealing. The presence of phosphorus influences the morphology of secondary defects during initial nucleation. Transmission electron microscopy (TEM) is used to differentiate between defect types and quantify the interstitials trapped in extended defects. This analysis reveals that phosphorus results in a reduction of interstitials trapped in observable extended defects. The interstitial flux released from the implanted region is also affected by the phosphorus doping. This phenomenon is closely studied using secondary ion mass spectrometry (SIMS) to monitor diffusion enhancements of dopant layers. Shifts in diffused dopant profiles are correlated with the different morphologies of the extended defects and the decay of the silicon interstitial supersaturation. This correlation is used to understand the interaction of excess silicon interstitials with phosphorus atoms.

Junction Depth Reduction of ion Implanted Boron in Silicon Through Fluorine ion Implantation

AbstractThe interaction between boron and excess silicon interstitials caused by ion implantation hinders the formation of ultra-shallow, low resistivity junctions. Previous studies have shown that fluorine reduces boron transient enhanced diffusion, however it is unclear whether this observed phenomenon is due to the fluorine interacting with the boron atoms or silicon self-interstitials. Amorphization of a n-type Czochralski wafer was achieved with a 70 keV Si+ implantation at a dose of 1×1015/cm2. The Si+ implant produced a 1500Å deep amorphous layer, which was then implanted with 1.12 keV 1×1015/cm2 B+. The samples were then implanted with a dose of 2×1015/cm2F+ at various energies ranging from 2 keV to 36 keV. Ellipsometry measurements showed no increase in the amorphous layer thickness from either the boron or fluorine implants. The experimental conditions allowed the chemical species effect to be studied independent of the implant damage caused by the fluorine implant. Post-implantation anneals were performed in a tube furnace at 750° C. Secondary ion mass spectrometry was used to monitor the dopant diffusion after annealing. Transmission electron microscopy (TEM) was used to study the end-of-range defect evolution. The addition of fluorine reduces the boron transient enhanced diffusion for all fluorine energies. It was observed that both the magnitude of the boron diffusivity and the concentration gradient of the boron profile vary as a function of fluorine energy.

Structural Defects in Ion Implanted 4H-SiC Epilayers

ABSTRACTTransmission electron microscopy (TEM) was used to investigate Al, Ar, C and Si ionimplanted 4H-SiC epilayers. After the implantation the samples were thermally annealed for 30 minutes at 1700°C. During the annealing process dislocation loops are formed and the generation of such dislocation loops upon annealing, is investigated with respect to dopant electrical activation, peak ion concentration and calculated interstitial/vacancy concentrations. It is concluded that the dislocation loops are generated as the result of a combination of residual damage and excess interstitials generated in a “plus one” (+1) process.

Effect of Nitrogen Implants on Boron Transient Enhanced Diffusion

AbstractThe effect of nitrogen implants on boron transient enhanced diffusion was studied for nitrogen-only, boron-only, and boron plus nitrogen implants. A boron buried layer was used as a detector for interstitial supersaturation in the samples. Boron dose ranged from 1×1014 to 1×1015 cm−2 and N2+ dose from 5×1013 and 5×1014 cm−2. The energies were chosen such that the location of the nitrogen and boron peaks matched. After the implants, RTA and low temperature furnace anneals were carried out. The diffusivity enhancements were extracted from the buried layer profiles by simulation. Nitrogen-only implants were found to cause significant enhanced diffusion on the buried boron layer. For lower doses, the enhancement of the nitrogen implant is about half as that of boron whereas the enhancements are equal at higher doses. Nitrogen coimplant with boron increases the transient enhanced diffusion of boron at low boron doses, which implies that nitrogen does not act as a strong sink for excess interstitials unlike carbon. At high boron doses, nitrogen co-implant does not significantly change boron diffusion. Sheet resistance measurements indicate that low nitrogen doses do not affect the activation of boron whereas high nitrogen doses either reduce the activation of boron or the mobility of the holes.

Transient enhanced diffusion after laser thermal processing of ion implanted silicon

The effect of laser thermal processing (LTP) on implantation-induced defect evolution and transient enhanced diffusion (TED) of boron was investigated. A 270-Å-thick amorphous layer formed by 10 keV Si+ implantation was melted and regrown using a 20 ns ultraviolet laser pulse. Transmission electron microscopy revealed that recrystallization of the amorphous layer following LTP results in a high concentration of stacking faults and microtwins in the regrown region. Also, the end-of-range loop evolution during subsequent 750 °C furnace annealing, is different in a LTP sample compared to a control sample. Secondary ion mass spectroscopy of a boron marker layer 6000 Å below the surface showed that LTP alone produced no enhanced diffusion. However, during subsequent furnace annealing, the boron layer in the LTP sample experienced just as much TED as in the control sample which was only implanted and furnace annealed. These results imply that laser melting and recrystallization of an implantation-induced amorphous layer does not measurably reduce the excess interstitials released from the end-of-range implant damage.

The effect of stoichiometric disturbance on activation of MeV Si implantation in GaAs

MeV Si implantation into semi-insulating (SI) GaAs is a common technique used to create a deep doping layer. It is observed that the maximum of the carrier concentration is at a shallower depth than that of the implanted atom concentration. This unexpected phenomenon has previously been explained as depth measurement error. Based on our theoretical calculations and experiments, the phenomenon can be attributed to the effect of non-uniform stoichiometric disturbances (NSD) on the activation of implanted atoms. The effect of NSD in GaAs substrate is caused by MeV implantation. After implantation, an excess concentration of the heavier element As exists at shallower depth, while an excess concentration of the lighter element Ga is seen at greater depth. It is likely that Si atoms move more easily to Ga sites when they are heated in an As-rich environment. Excess lattice interstitials build up around the depth of the peak in the implanted Si+ concentration and prevent Si atoms from becoming activated there.

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.

Atomic transport properties and electrical activation of ultra-low energy implanted boron in crystalline silicon

The growing importance of ultra-low energy implantation in Si processing imposes extensive characterization and understanding of such a novel energy regime. In this paper we investigate the evolution of ultra-low energy B implants (0.25–1 keV) after post-implantation annealing, both in terms of atomic diffusion and electrical activation of the doping atoms. Transient enhanced diffusion (TED) of boron after annealing at 900°C is observed even for boron implanted at 250 eV, and also when the implant dose is below the amorphization threshold. At higher temperatures for long anneal times, the TED is overwhelmed by the equilibrium diffusion and it is not visible. However, provided the correct combination of temperatures and times is chosen, the TED can always be observed in samples implanted with a dose at least of 1×10 14 cm 2. We suggest a possible microscopic mechanism to justify the dependence of the enhanced diffusion of ultra-low energy implanted boron on the implant dose, energy and annealing temperature. An excess of interstitials occurs giving rise to the formation of interstitials-like complexes containing B, probably due to enhanced annihilation of vacancies at the surface. Such an excess of interstitials is able to promote enhanced diffusion of implanted boron, provided the implant dose is high enough to generate a significant total number of point defects. The electrical activation of the ultra-low energy implanted B is shown to be strictly connected to its diffusion. With increasing the dose, the motion of B is supported by the increased amount of ion beam generated interstitials. We have also observed, at the same time, that the electrical activation is favored. The electrical activation, which can be achieved by the ultra-low energy implants we have investigated, is between 10 and 40% after annealing at 1100°C, depending on the implanted dose. For the highest dose we have studied, i.e. 1×10 15 cm 2, the sheet resistance measured after annealing at 1100°C in a range between 250 eV and 1 keV, is below 1000 Ω. Ultra-low energy implantation is therefore extremely appealing for the future generations of semiconductor devices.

Bulk properties of very large diameter silicon single crystals

It has been experimentally found that it will be difficult to grow 300 mm or larger diameter crystals with similar quality as for 200 mm or smaller diameter crystals. This phenomenon can be understood within the frame of the Voronkov theory in which the value of the parameter V/ G ( V=pull rate, G=temperature gradient at the growth interface) determines which type of defect forms in the growing crystal. Due to fundamental technological constraints, the pull rate of silicon single crystals has to be reduced as the diameter increases. For crystal diameters beyond 300 mm, the reduction of pull rate is so large, that V/ G( r) ( r=radial position) can probably no longer be kept above the critical value C crit=1.34×10 −3 cm 2 K −1 min −1 over the entire crystal volume by the present growth technology. As a result, the defect behavior of the silicon bulk changes. The aggregation of defects is now dominated by excess Si interstitials instead of vacancies and, hence, L-pits (dislocation loops) are observed instead of microvoids. Unless new methods for the suppression of L-pits can be developed, this will seriously challenge the use of polished wafers in very large diameter device manufacturing lines, as L-pits can severely damage the device performance. A promising solution to the defect problem appears to be p+p− epi wafers.