Boron-interstitial Clusters Research Articles

Evolution of boron-interstitial clusters in crystalline Si studied by transmission electron microscopy

The thermal evolution of large boron-interstitials clusters (BICs) in crystalline Si has been studied by transmission electron microscopy (TEM). After ion implantation (20keV and 1×1014Si∕cm2) and annealing (815°C and 5min), large clusters (6–8nm) have been observed in correspondence of a narrow, highly doped Si:B layer (2×1020B∕cm3). Under prolonged annealing, such clusters dissolve, progressively shrinking their mean size below the TEM detection limit. The time evolution of such a BIC shrinking is fully compatible with the slow path dissolution kinetics recently published. These data suggest the identification of the slow dissolving BICs with the large observed clusters.

Physical insight into the phenomenon of B clustering in Si at room temperature

The interaction between the crystal defects and the implanted dopants in Si is at the origin of the diffusion and the electrical de-activation that are detrimental for the realization of the ultra-shallow junctions required by the semiconductor technology roadmap. In this contest, the B clustering phenomenon under non-equilibrium conditions is of particular interest for the scientific community.It has been shown that ion channeling technique is an extremely powerful tool to reveal the room temperature formation of small boron-interstitial-clusters (BICs) through the detection of subnanometric B off-lattice displacement.In this paper, we review our recent results on BIC formation in B-doped Si samples on a large range of B concentration (1019–1021at/cm3). At room temperature B atoms undergo an off-lattice displacement, with a characteristic channeling mark, if a supersaturation of Si self-interstitials (Is) is maintained by ion irradiation. The B displacement rate is limited by the fluence of excess Is per B atom and can be reproduced using a simple model of B–Is interaction with the formation of small B clusters. In the early stage of Is injection the formation of B2I clusters with the structure of split (100) has been detected by performing channeling angular scans along the <100> and <110> crystal axes.

Defects evolution and dopant activation anomalies in ion implanted silicon

The interactions between the defects and the implanted dopants are at the origin of the diffusion and activation anomalies that are among the major obstacles to the realisation of ultra-shallow junctions satisfying the ITRS requirements.In this paper, we present some recent results on the evolution of extended defects in technology relevant conditions for the fabrication of p+/n ultra-shallow junctions and elucidate for each of them the role of the defects in the diffusion and activation anomalies exhibited by the implanted dopants. The presented studies range from the formation of large Boron-Interstitial Clusters in high-fluence B+-implanted silicon, to the deactivation/reactivation of preamorphised ultra-shallow junctions and, finally, to the impact of co-implanted F on the thermal stability of preamorphised junctions.

Diffusion and activation of dopants in silicon and advanced silicon-based materials**Invited paper.

A quantitative description of the transient diffusion and activation of boron during post-implantation annealing steps is one of the most challenging tasks in the simulation of silicon doping processes. In industrially relevant situations, simulations needs to address diffusion at extrinsic concentrations, the agglomeration of self-interstitials, and the formation of boron-interstitial clusters. This paper describes the experimental work performed or used to calibrate model parameters as independently as possible. The combined model is then applied to ultra-shallow junction formation by annealing boron implanted into crystalline or preamorphized silicon. In comparison to bulk silicon, much less is known about diffusion of dopants in SiGe and germanium which are considered as technological options for future technology nodes. Therefore, dedicated experiments were performed to clarify open points in the diffusion behaviour of dopants in these materials.

Damage and recovery in boron doped silicon on insulator layers after high energy Si+ implantation

The effects of 2MeV Si+ implantation on silicon-on-insulator layers uniformly doped with B at concentrations 1.0 and 1.8×1020cm−3, and the kinetics of damage recovery were investigated by carrier density, mobility measurements, and transmission electron microscopy (TEM) observations. High energy implantation reduces the hole density by about 98%; the mobility is also reduced at an extent which increases with B concentration. Isochronal and isothermal annealings show that recovery of the hole density takes place in three stages: the first stage (α) is accompanied by a mobility decrease and is followed by the second stage (β) where mobility increases attaining values close to the ones of the reference undamaged samples. Mobility keeps nearly constant in the third recovery stage (γ), which takes place above 800°C. As a characterizing feature the mobility values for each B concentration only depend on the hole density, irrespective of the thermal history of the samples. Experiments and TEM observations allowed us to distinguish defect recovery from SiB3 precipitation, which can take place at temperatures higher than 700°C. Recovery stages are discussed, and it is concluded that dissolution of B rich clusters in stage (α) modifies the concentration, or the charge state, of the defects responsible of the second (β) stage. These defects are identified as boron interstitial clusters in consideration of their mobility behavior and of the activation energy Eβ for their recovery process, which results to be 3±0.2eV.

Modeling the suppression of boron diffusion in Si∕SiGe due to carbon incorporation

We used the process simulator FLOOPS-ISE to implement a consistent model describing the diffusion behaviors of boron and carbon in silicon and silicon germanium. In particular, our model successfully accounts for boron and carbon behaviors in a wide range of sample structures and experimental conditions over the complete temperature range of 750–1070°C in inert and oxidizing ambients, and in the presence of implant damage. The structures studied include cases where the boron and carbon profiles are separated as well as cases where profiles overlap, cases with carbon in silicon and in SiGe, and our own recent experiments where boron diffusion within a SiGeC region has been characterized. We model carbon diffusion by the kickout and Frank-Turnbull mechanisms, and interstitial capture by substitutional carbon, and demonstrate that a model must incorporate all three effects to satisfactorily explain published data. We also include standard models for boron-interstitial clusters and {311} defects.

Vacancy-engineering implants for high boron activation in silicon on insulator

The formation of boron interstitial clusters is a key limiting factor for the fabrication of highly conductive ultrashallow doped regions in future silicon-based device technology. Optimized vacancy engineering strongly reduces boron clustering, enabling low-temperature electrical activation to levels rivalling what can be achieved with conventional preamorphization and solid-phase epitaxial regrowth. An optimized 160keV silicon implant in a 55∕145nm silicon-on-insulator structure enables stable activation of a 500eV boron implant to a concentration ∼5×1020cm−3.

Boron pocket and channel deactivation in nMOS transistors with SPER junctions

In this paper, we demonstrate the consequences of extension junction formation by low-temperature solid-phase-epitaxial-regrowth in nMOS transistors. Atomistic simulations, experimental device results, sheet resistance, and scanning spreading resistance microscopy data indicate that the high concentration of silicon interstitials associated with the end-of-range defect band promote the local formation of boron-interstitial clusters, and thus deactivate boron in the pocket and channel. These inactive clusters will dissolve after the high concentration silicon interstitial region of the end-of-range defect band has been annihilated. This nMOS requirement is in direct opposition to the pMOS case where avoidance of defect band dissolution is desired, to prevent deactivation of the high concentration boron extension profile.

Theoretical Study of Boron Clustering in Silicon

Ion implantation is the method of choice to introduce dopants such as boron into silicon. Thermal anneals are used to heal the implant damage as well as to activate the dopant electrically. The implant-anneal cycle causes transient enhanced diffusion (TED) of boron and clustering of boron atoms at concentrations far below the solubility limit. The formation of these small immobile boron-interstitial clusters (BICs) causes the deactivation of boron. In this work, we use density-functional theory calculations to study the boron clustering process in Si. We determine the minimum-energy structures of these clusters at different sizes embedded in bulk Si and calculate the energy and charge state of each cluster within density-functional theory. Special care is taken with regard to structural minimization, charge state effects and energy corrections. In contrast to previous work, we argue that substitutional-boron clusters as defined previously are meaningless due to the high repulsive energy of nearest-neighbor boron atoms. We compare the larger clusters to the phase of precipitates at higher boron concentrations, Si1.8B5.2, and suggest the boron icosahedron as logical final BIC before the formation of more macroscopic precipitates in the absence of kinetic constraints. We also describe in detail the implementation of our ab-initio results into a continuum model, which we have used in previous work to simulate diffusion and deactivation of boron.

Atomistic study of the dissolution of small boron interstitial clusters in c-Si

The dissolution of boron-interstitial clusters (BICs) in crystalline silicon, often formed after ion implantation, is investigated by temperature accelerated dynamics of their formation using the Si–B Stillinger-Weber potential. We find that the dominant breakup event for small BICs is the emission of either Si mono- or di-interstitials, though the dominant reaction for the reactivation of boron is via emission of a boron interstitial defect B1I1. Most reactions are well approximated by the assumption that they are diffusion limited. Finally, the rate limiting step for breakup of clusters with three B atoms involves the B3I2 cluster, which also dissolves via emission of B1I1.

Suppression of boron interstitial clusters in SOI using vacancy engineering

As CMOS devices scale into the 45 nm process window, the requirements for the individual devices become even more stringent, with levels of activation well above solid solubility with minimal dopant diffusion. Boron interstitial clusters (BICs) are known to hinder the activation of typical boron implants reducing the level of activation even below solid solubility. This paper reports on an optimised vacancy engineering technique to reduce the interstitial population, which would normally occur after ion implantation. Hence, the BIC formation is suppressed creating a highly active layer, which remains active over a 700–1000 °C temperature window. Using this technique, it has been estimated that at 700 °C the level of activation may be around 5 × 10 20 cm −3 rivaling techniques such pre-amorphisation combined with solid phase epitaxy re-growth.

Optimizing boron junctions through point defect and stress engineering using carbon and germanium co-implants

We report the fabrication of p+∕n junctions using Ge+, C+, and B+ co-implantation and a spike anneal. The best junction exhibits a depth of 26nm, vertical abruptness of 3nm∕decade, and sheet resistance of 520Ohm∕square. The junction location is defined by where the boron concentration drops to 1018cm−3. These junctions are close to the International Technology Roadmap specifications for the 65nm technology node and are achieved by careful engineering of amorphization, stresses, and point defects. Advanced simulation of boron diffusion is used to understand and optimize the process window. The simulations show that the optimum process completely suppresses the transient-enhanced diffusion of boron and the formation of boron-interstitial clusters. This increases the boron solubility to 20% above the equilibrium solid-state solubility.

Electronic structure of boron-interstitial clusters in silicon

Hybrid functional calculations within density functional theory are carried out toinvestigate the electronic structure of boron-interstitial clusters (BICs). A one-parameterhybrid functional is chosen is to give accurate results for the whole electronic structure(including the gap) and the elastic properties of crystalline silicon. It is shown that thisapproach provides dependable defect level positions in the gap. Investigation of theboron+vacancy and boron+self-interstitial centres gives a consistent description of the experimentally observed G10 andG28 centres. The electronic structure of BICs, which may affect the activation rate ofboron implantation, are reported. The one-electron level positions of isolatedBnIm defects are given.

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.

Hydrogen plasma enhancement of boron activation in shallow junctions

The ability to activate large concentrations of boron at lower temperatures is a persistent contingency in the continual drive for device scaling in Si microelectronics. We report on our experimental observations offering evidence for enhancement of electrical activation of implanted boron dopant in the presence of atomic hydrogen in silicon. This increased electrical activity of boron at lower anneal temperature is attributed to the creation of vacancies in the boron-implanted region, lattice-relaxation caused by the presence of atomic hydrogen, and the effect of atomic hydrogen on boron-interstitial cluster formation.

Impact of the end of range damage from low energy Ge preamorphizing implants on the thermal stability of shallow boron profiles

A fundamental understanding of the effect of scaling amorphous layers on the thermal stability of active concentrations is required for the formation of ultrashallow junctions. A study on the influence of boron on the evolution of the end of range defects for samples containing shallow amorphous layers formed by low energy germanium implants is conducted. Czochralski grown (100) silicon wafers are preamorphized with 1×1015cm−2, 10keV Ge+ and subsequently implanted with 1×1015cm−2, 1keV B+ such that high boron levels are attained in the end of range region. A sequence of anneals are performed at 750°C, under nitrogen ambient for times ranging from 1s to 6h and the end of range defect evolution is imaged via plan-view transmission electron microscopy (TEM). Defect analyses are conducted utilizing quantitative TEM which indicates substantial differences in the defect evolution for samples with boron in the end of range. The extended defects observed are very unstable and undergo a fast dissolution. In contrast, stable defects are observed in the experimental control in which the evolution follows an Ostwald ripening behavior. Secondary ion mass spectroscopy analyses confirm the ephemeral nature of the defects observed and also demonstrates drastic reductions in interstitial supersaturation. In addition, uphill-type diffusion is observed to occur for a short time frame, which emphasizes a transient interstitial supersaturation. Correlation of this data with sheet resistance and active dose measurements conducted on a Hall measurement system strongly indicates the formation of boron interstitial clusters. The high boron concentrations and supersaturation levels attained at the anneal temperature enables the cluster formation. An estimate of the boron concentrations trapped in the clusters is determined from the active dose obtained from the Hall measurements and indicates concentrations much higher than those available in the end of range. This suggests an interstitial migration from the end of range to regions of higher boron levels. Since the end of range is in the vicinity of the highly doped layer it is not isolated from the strain effects induced by the high initial activation levels. Hence it is proposed that the tensile strain stimulates the interstitial migration from the end of range to the boron-doped layer. Consequently, the end of range defects dissolve as the interstitial supersaturation falls below levels required to sustain their evolution.

Boron-interstitial clusters in crystalline silicon: stoichiometry and strain

In this work, we have investigated the strain induced in a Si crystal by Boron-interstitial clusters (BICs). BICs were formed by Si implantation and subsequent annealing of two Si samples, grown by molecular beam epitaxy (MBE), containing thin buried layers doped with different B concentrations (10 19 and 10 20 at./cm 3). By B chemical profile diffusion analysis the doses of Si self-interstitials (Is) and Boron atoms trapped in the BICs were extracted. The B/I stoichiometric ratio is about 1 for the low B concentration and about 3.5 for the high B concentration sample. High-resolution X-ray diffraction (HRXRD) analyses provided a measure of the strain profile. While in the low B concentration sample, no appreciable strain was detected after BICs formation, at the higher B concentration, we found that the tensile strain present in the as-grown B doped layer changes to a strong compressive strain as a consequence of BIC formation. For this kind of clusters, the mean volume expansion with respect to the Si matrix is 29 ± 6 Å 3 for each B atom trapped in the BICs.

Impact of dopant profiles on the end of range defects for low energy germanium preamorphized silicon

As the industry continues to aggressively scale CMOS technology, the shift to lower energy ion implantation becomes essential. The consequent shallower amorphous layers result in dopant profiles that are in closer proximity to the end of range (EOR) damage and therefore a better understanding of the interaction between the dopant atoms and the EOR is required. A study is conducted on the influence of dopant profiles on the behavior of the EOR defects. Czochralski-grown silicon wafers are preamorphized with 1 × 10 15 cm −2, 10 keV Ge + ions and subsequently implanted with 1 × 10 15 cm −2, 1 keV B + ions. A sequence of rapid thermal and furnace anneals are performed at 750 °C under a nitrogen ambient for periods of 1 s up to 6 h. Plan view transmission electron microscopy (PTEM) reveals a significant difference in the defect evolution for samples with and without boron, suggesting that the boron influences the evolution of the EOR defects. The extended defects observed for samples which contain boron appear as dot-like defects which are unstable and dissolve after very short anneal times. The defect evolution however, in samples without boron follows an Oswald ripening behavior and form {3 1 1}-type defects and dislocation loops. Hall effect measurements denote a high initial activation and subsequent deactivation of the dopant atoms which is characteristic of the formation of boron interstitial clusters. Diffusion analyses via secondary ion mass spectroscopy (SIMS) support this theory.

Electrical activation in silicon-on-insulator after low energy boron implantation

We have investigated the electrical activation of implanted boron in silicon-on-insulator (SOI) material using Hall effect, four-point probe, and secondary ion mass spectrometry. Boron was implanted at energies ranging from 1keVto6.5keV with a dose of 3×1014cm−2 into bonded SOI wafers with surface silicon thickness ranging from 300Åto1600Å. In one sample set, furnace anneals at 750°C were performed in a nitrogen ambient for times ranging from 5minto48h. A second sample consisted of isochronal furnace anneals performed from 450°Cto1050°C for 30min. Significantly less activation of boron is observed in SOI at temperatures below 750°C, regardless of the implant energy and surface silicon thickness. Between 750°C and 900°C, the active dose of boron in SOI is similar to that of bulk Si. As the implant energy increases, the fractional activation in thin SOI increases, due to a reduction in boron interstitial clusters (BIC) in the surface Si layer. It is concluded that an increase in the BIC population is the likely source of the low activation observed in SOI. This may be due to an increase in the interstitial supersaturation within the surface Si layer, due to the interface acting as a reflective boundary for interstitials.

Boron ripening during solid-phase epitaxy of amorphous silicon

By means of large-scale molecular dynamics simulations we provide a thorough atomic-scale picture of boron incorporation in crystalline silicon upon solid-phase epitaxy. The present results show that boron can either be incorporated as a substitutional dopant or form clusters with a low content of silicon self-interstitials. A full characterization of the formation process of boron-interstitial clusters and their stoichiometry is presented. The present results are consistent with available experimental information and also provide a deep physical insight into B-doped silicon solid-phase epitaxy.