Interstitial Si Atoms Research Articles

Thermal optimization of CZ bulk growth and wafer annealing for crystalline dislocation-free silicon

In this article, we consider a number of questions related to the thermal optimization of 200mm diameter Si Czochralski (CZ) single-crystal growth process using the new “Redmet-90M” puller and the rapid thermal annealing (RTA) of Si wafers cut from single crystals. The article includes results of the CZ numerical modeling of heat transfer, which takes into account the special thermal shield assemblies (with and without water cooling). The influences of various crucible rotation rates on the distribution of the azimuthal velocity of the melt and on the liquid–solid interface were also investigated. Differences in axial temperature profiles of the growing crystal were analyzed depending on whether the thermal shield was used with or without its water-cooling system and depending on the various crucible rotation rates. A numerical diffusion–recombination model of intrinsic point defects (vacancies and interstitial Si atoms), which takes into account the vacancy cluster formation, was applied for RTA optimization. Vacancy concentration, density and sizes of vacancy clusters were calculated along the thickness of a wafer. An experimental study on the defect structure in the annealed Si wafers was carried out by the light microscopy and the transmission electron microscopy. The type, density and morphology of defects in the thickness of a wafer were also analyzed. The experimental distribution of total defect density in the thickness of a wafer was compared with the calculated vacancy cluster density.

Calculation of fast pipe diffusion along a dislocation stacking fault ribbon

The diffusion of an interstitial Si atom along an edge dislocation in Al is studied by using a quantum-mechanics/molecular-mechanics coupling method. It is found that the diffusing Si atom follows approximately a sinusoidal trajectory. The diffusion energy barrier along the partial dislocation core is in excellent agreement with the experimental value. We predict that the stacking fault ribbon could provide a fast pathway for diffusion which increases the diffusivity by 6--7 orders of magnitude comparing to the bulk value at 700 K. At the same temperature, the diffusivity along the stacking fault is found to be three to four orders of magnitude greater than that along the partial dislocation core.

Modeling of the effect of the buried Si–SiO2 interface on transient enhanced boron diffusion in silicon on insulator

The effect of the buried Si–SiO2 interface on the transient enhanced diffusion (TED) of boron in silicon on insulator (SOI) structures has been investigated. To this purpose, boron marker layers were grown by chemical vapor deposition on Si and SOI substrates and implanted under nonamorphizing conditions with 40 keV Si+ ions. The experimental results clearly confirm that the Si–SiO2 interface is an efficient trap for the Si interstitial atoms diffusing out of the defect region. Based on these experiments, existing models for the simulation of B TED in silicon have been modified to include an additional buried recombination site for silicon interstitials. The simulation results provide an upper limit of ∼5 nm for the recombination length of interstitials at the Si–SiO2 interface.

Model of High-Temperature Diffusion of Interstitial Silicon Atoms in Silicon

Problem statement: Correct description of the anomalous phenomena determined by self-point defects in implanted silicon desires knowledge of their properties. Interstitial Si atoms themselves display anomalies in their behavior and firstly in existence of two very different values of the diffusion coefficient. Approach: We analyzed experimental results and proposed the model of diffusion of interstitial Si atoms in silicon in two shapes. Results: At low saturation Si atoms diffuse as isolated atoms with a low diffusion coefficient (~10-12 cm2 sec-1 at 900°C). At high supersaturation interstitial atoms diffused as Si-Si pairs, which had lower activation energy of migration and higher diffusion coefficients (~10-7 cm2 sec-1). Conclusion: The high diffusivity pairs were formed when two Si atoms hit in the same interstice. The atoms were not bound to one another by covalent bond. In a pair atoms were retained by a potential relief of the crystal.

STM study of Si(1 1 3) 3 × 2-Ga surface

The Ga-adsorbed structure on Si(1 1 3) surface at low coverage has been studied by scanning tunneling microscopy (STM). The bright protrusion corresponding to the position of the dimer without the interstitial Si atom of the clean surface disappeared in the filled-state STM image after Ga adsorption, although the protrusion due to the Si adatom still remained. On the basis of the adatom-dimer-interstitial (ADI) model, this result indicates that the Ga atom is adsorbed interstitially at the center of another pentamer that does not have the interstitial Si atom. An ab initio calculation was performed and STM images were simulated.

Defect engineering via ion implantation to control B diffusion in Si

The processes which are currently studied in the fabrication of B-doped ultra shallow junctions (USJ) usually involve a preamorphization step to reduce B channelling effect during implantation and to improve B electrical activation. At this stage a high amount of Si interstitial atoms (Is), which dramatically increases the B diffusivity, is introduced. The introduction of voids in Si is a promising tool to control B transient enhanced diffusion (TED), because of their ability to capture Is. In this work the efficiency of a cavity band to reduce B TED is checked in silicon interstitial supersaturation conditions, obtained by high dose Si implantation. He is implanted either at 10 keV or at 50 keV with a fluence of 5 × 10 16 cm −2. Conventional techniques to introduce and activate the B (conventional ion implantation and rapid thermal annealing (RTA)) are applied in order to have a better control of the technological process to focus on the benefit of the cavity layer. The samples were characterized by cross section transmission electron microscopy (XTEM), secondary ion mass spectroscopy (SIMS) and Hall Effect (HE). The latter shows that good activation of the B is achieved only after 1000 °C RTA, though a 900 °C RTA is sufficient for implantation-damage recovery, as it is confirmed by XTEM observations. B SIMS profiles show that the band of cavities plays its best effect in reducing B TED when it is located near the surface.

The effect of alloying elements on the vacancy defect evolution in electron-irradiated austenitic Fe–Ni alloys studied by positron annihilation

The vacancy defect evolution under electron irradiation in austenitic Fe–34.2 wt% Ni alloys containing oversized (aluminum) and undersized (silicon) alloying elements was investigated by positron annihilation spectroscopy at temperatures between 300 and 573 K. It is found that the accumulation of vacancy defects is considerably suppressed in the silicon-doped alloy. This effect is observed at all the irradiation temperatures. The obtained results provide evidence that the silicon-doped alloy forms stable low-mobility clusters involving several Si and interstitial atoms, which are centers of the enhanced recombination of migrating vacancies. The clusters of Si-interstitial atoms also modify the annealing of vacancy defects in the Fe–Ni–Si alloy. The interaction between small vacancy agglomerates and solute Al atoms is observed in the Fe–Ni–Al alloy under irradiation at 300–423 K.

Strain Loss in Epitaxial Si:C Films Induced by Phosphorus Diffusion

The influence of P diffusion on the thermal stability of epitaxial Si:C films has been studied. P diffusion is shown to greatly contribute to the strain loss in epitaxial Si:C films. It is proposed that the strain loss is mainly caused by the interaction of Si interstitial atoms injected by P diffusion with substitutional C atoms, driving them to non-substitutional lattice sites.

Strengths and Limitations of the Vacancy Engineering Approach for the Control of Dopant Diffusion and Activation in Silicon

ABSTRACTThe fabrication of highly doped and ultra-shallow junctions in silicon is a very challenging problem for the materials scientist. The activation levels which are targeted are well beyond the solubility limit of current dopants in Si and, ideally, they should not diffuse during the activation annealing. In practice, the situation is even worse and when boron is implanted into silicon excess Si interstitial atoms are generated which enhance boron diffusion and favor the formation of Boron-Silicon Interstitials Clusters (BICs). An elegant approach to overcome these difficulties is to enrich the Si layers where boron will be implanted with vacancies before or during the activation annealing. Spectacular results have been recently brought to the community showing both a significant control over dopant diffusion and an increased activation of boron in such layers. In general, the enrichment of the Si layers with vacancies is obtained by Si+ implantation at high energy. We have recently developed an alternative approach in which the vacancies are injected from populations of empty voids undergoing Ostwald ripening during annealing. While different, the effects are also spectacular. The goal of this work is to establish a fair evaluation of these different approaches under technologically relevant conditions. The application domains of both techniques are discussed and future directions for their development/improvement are indicated.

Modeling of diffusion mass transfer of implanted dopants in silicon. I. Formulation of the model

A generalized model of diffusion mass transfer of implanted dopant atoms (donors and acceptors) in crystalline silicon has been developed. It takes into account all possible charge states of diffusing species (vacancies, interstitial Si atoms, and “dopant atom-vacancy” and “dopant atom-interstitial silicon atom” pairs) and the difference between the diffusion coefficients of differently charged pairs. Expressions for the source/sink terms are derived which describe all bimolecular reactions between the species (generation and annihilation) in the course of diffusion with account for their charges.

Oxidation of Si(001) with a hyperthermal O-atom beam at room temperature: Suboxide distribution and residual order structure

Synchrotron radiation photoelectron spectroscopy (SR-PES) and crystal truncation rod (CTR) scattering profiles were used to investigate an ultrathin SiO2 overlayer on a Si(001) surface formed by a 5eV O-atom beam at room temperature. The SR-PES spectra indicated that the suboxides in the O-atom-beam oxidized film were concentrated on the SiO2 surface rather than at the Si∕SiO2 interface. The CTR scattering data of the O-atom-beam oxidation film had a lower intensity near (11L) (0.3<L<0.8), suggesting a lower content of the SiO2 ordered structure in the oxide film. An inverse diffusion of the interstitial Si atoms in the oxidation kinetics can explain the data.

Synchrotron Radiation Photoelectron Emission Study of SiO2 Film Formed by Hyperthermal O-Atom Beam at Room Temperature

An ultrathin SiO2 overlayer on a Si(001) surface formed by a 5 eV O-atom beam at room temperature was analyzed by synchrotron radiation photoelectron spectroscopy (SR-PES). SR-PES spectra clearly indicated that the SiO2 layer formed by a hyperthermal O-atom beam at room temperature contains a small amount of suboxides compared with that formed by high-temperature oxidation in O2 atmosphere. Quantitative analysis suggests that the thickness of the structural transformation layer was less than a monolayer and the amount of suboxides was independent of the film thickness. The translational energy dependence of SR-PES spectra suggests that the reaction probability with a Si-atom increases with the translational energy of the O atoms in the range between 1 to 5 eV. The role of inverse diffusion of interstitial Si atoms in the kinetics of hyperthermal O-atom-beam oxidation is suggested.

Displacement of Boron from the Silicon Crystal Nodes by Interstitial Si Atoms During Implantation and Annealing

The process of boron displacement from the nodes into interstitial positions of interstitial Si atoms in silicon (Watkins effect) on the conditions of implantation and annealing has been investigated with the help of X-ray diffraction and electrical methods. It was revealed that the efficiency of the Watkins substitution is determined by the ion current density (level of ionization). With increasing of the ionization level in the implanted layer during implantation or annealing (additional low-energy electron irradiation) the replacement process may be suppressed

X-ray multiple diffraction on the shallow junction of B in Si(0 0 1)

We apply X-ray multiple diffraction (XRMD) as a high resolution probe for analyzing the amorphous-crystalline interface (Si interstitial rich region) for different implantation energies and thermal treatment conditions during the formation of a shallow junction of B implanted in Si(0 0 1) crystals. Renninger scans (RS) (ϕ-scans) of the three sets of samples were measured using the (0 0 2) primary reflection, forbidden by the Si space group. Si(0 0 2) RS of as-implanted samples showed an extra peak coming from the Si interstitial atoms in the implanted region. This hybrid peak provides a very sensitive probe for analyzing the occurrence of interstitial Si atoms close to the amorphous-crystalline interface. We report its behavior as a function of the thermal treatment. The separation between the vacancy rich and the interstitial rich regions explains these results on the basis of the Si interstitial annihilation for shallow implantation. Samples with deep implantation are outside the detection range of the technique, in agreement with results for etched samples.

Current Understanding and Modeling of B Diffusion and Activation Anomalies in Preamorphized Ultra-Shallow Junctions

ABSTRACTThe formation of ultra-shallow junctions (USJs) for future integrated circuit technologies requires preamorphization and high dose boron doping to achieve high activation levels and abrupt profiles. To achieve the challenging targets set out in the semiconductor roadmap, it is crucial to reach a much better understanding of the basic physical processes taking place during USJ processing. In this paper we review current understanding of dopant-defect interactions during thermal processing of device structures – interactions which are at the heart of the dopant diffusion and activation anomalies seen in USJs. First, we recall the formation and thermal evolution of End of Range (EOR) defects upon annealing of preamorphized implants (PAI). It is shown that various types of extended defect can be formed: clusters, {113} defects and dislocation loops. During annealing, these defects exchange Si interstitial atoms and evolve following an Ostwald ripening mechanism. We review progress in developing models based on these concepts, which can accurately predict EOR defect evolution and interstitial transport between the defect layer and the surface. Based on this physically based defect modelling approach, combined with fully coupled multi-stream modelling of dopant diffusion, one can perform highly predictive simulations of boron diffusion and de/re-activation in Ge-PAI boron USJs. Agreement between simulations and experimental data is found over a wide range of experimental conditions, clearly indicating that the driving mechanism that degrades boron junction depth and activation is the dissolution of the interstitial defect band. Finally, we briefly outline some promising methods, such as co-implants and/or vacancy engineering, for further down-scaling of source-drain resistance and junction depth.

Direct evidence of the recombination of silicon interstitial atoms at the silicon surface

In this experiment, a Si wafer containing four lightly doped B marker layers epitaxially grown by CVD has been implanted with 100 keV Si + ions to a dose of 2 × 10 14 ions/cm 2 and annealed at 850 °C for several times in an RTA system in flowing N 2. TEM and SIMS analysis, in conjunction with a transient enhanced diffusion (TED) evaluation method based on the kick-out diffusion mechanism, have allowed us to accurately study the boron TED evolution in presence of extended defects. We show that the silicon surface plays a key role in the recombination of Si interstitial atoms by providing the first experimental evidence of the resulting Si ints supersaturation gradient between the defect region and the surface. Our results indicate an upper limit of about 200 nm for the surface recombination length of Si interstitials at 850 °C in a N 2 ambient.

Modeling of the Ostwald ripening of extrinsic defects and transient enhanced diffusion in silicon

We present an atomistic simulation of the Ostwald ripening of extrinsic defects (clusters, {1 1 3}s and dislocation loops) which occurs during annealing of ion implanted silicon. The model describes the capture and emission of Si interstitial atoms to and from extrinsic defects of sizes up to thousands of atoms and includes a loss term due to the flux of interstitials to the recombining surface. Key input parameters of the simulation are the variations of the formation energy and of the capture efficiency with the size of the different defects. This model shows that the kinetics of the well-known dissolution of {1 1 3} defects is only driven by the recombination efficiency at the surface and the distance from the defects to the sample surface. We have subsequently used this model to study defect evolution in low and ultra low energy (ULE) B implanted Si during annealing. Defect dissolution occurs earlier and at smaller sizes in the ULE regime. Consequently, TED is mostly characterized by its “pulse” component which occurs at the very beginning of the anneal.

Effect of the Ge preamorphization dose on boron diffusion and defect evolution in silicon

In this paper, we study the effect of the Ge + preamorphization dose on boron diffusion and on the thermal evolution of end of range (EOR) defects during annealing. Amorphizations were carried out by implanting Ge + at 150 keV to doses ranging from 1×10 15 to 8×10 15 ions/cm 2 . Boron was subsequently implanted at 3 keV with a dose of 1×10 14 ions/cm 2 . Rapid thermal annealing (RTA) was performed for various time/temperature combinations in nitrogen ambient. Secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM) were used to study boron diffusion and defect evolution, respectively. We have found that after a given annealing, both the defect size and boron diffusivity are independent on the Ge ion dose. Increasing this dose only results in an increase of the defect density. These results are discussed and definitely show that EOR defects are involved in a quasi-conservative Ostwald ripening process during annealing. The diffusive behavior of boron suggests that the coupling of boron atoms with the “mean field” of Si interstitial atoms in dynamical equilibrium with the defects is responsible for transient enhanced diffusion (TED).

Influence of the annealing ambient on the relative thermal stability of dislocation loops in silicon

In this work, we have studied the relative stability of perfect (PDLs) and faulted (FDLs) dislocation loops formed during annealing of preamorphised silicon. In particular, we have investigated the effect of the annealing ambient (N 2, O 2) on their thermal evolution. Transmission Electron Microscopy analysis shows that after short annealing times FDLs are the more stable defects if the ripening process is conservative or if the surface acts as a sink for Si interstitial atoms. On the contrary, when the surface injects Si atoms (oxidising conditions) or for long annealing times under conservative conditions, PDLs are stabilised. We have calculated the formation energies of these defects as a function of their sizes and we have found that a stability inversion occurs for sizes of about 80 nm. All the experimental results presented here are explained by making use of these calculations.

Effect of the Ge preamorphisation dose on the thermal evolution of End of Range defects

ABSTRACTIn this paper, we study the effect of the Ge+ preamorphisation dose on the thermal evolution of End of Range (EOR) defects upon annealing. Amorphisations were carried out by implanting Ge+ at 150 keV to doses ranging from 1×1015 ions/cm2 to 8×1015ions/cm2. Rapid Thermal Annealing (RTA) was performed for various time/temperature combinations in nitrogen ambient. Plan view transmission electron microscopy under specific imaging conditions was used to measure the size distributions and densities of the EOR defects. We found that for a fixed thermal budget, the increase in the Ge ion dose results in an increase in the defect density but has no effect on the defect size distribution. This invariance of the mean size of defects with respect to the initial supersaturation introduced in the matrix is an expected characteristic of a conservative Ostwald ripening mechanism. Moreover, the total number (Nb) of Si interstitial atoms bound to the EOR defects is a monotonically increasing function of the Ge ion dose. Furthermore, we found that Nb is directly proportional to the number of Si atoms in excess of the vacancies found below the a/c interface as calculated by MonteCarlo simulations. This is consistent with the “excess interstitial” model which explains the origin of the EOR defects.