Transient Enhanced Diffusion Of Boron Research Articles

Channel size-dependent TED effect in MOSFETs with infrared rapid thermal anneal

Boron transient enhanced diffusion (TED) causes reverse short channel effect, and threshold voltage (Vt) increases abnormally, which seriously affects the device performance and yield. The Vt increment is different with different channel size. To investigate channel impurity distribution and the influence of channel size on TED, Vt adjustment ion-implantation, LDD ion-implantation and carbon co-implantation experiments were performed on 40 nm CMOS process, measuring Vt with different channel size and process, and using TCAD tools for process simulation. Experiments results show that Vt rises first with channel length decreasing and then drops sharply after reaching the maximum at 0.55 μm. The Vt -L curve slope decreases when channel width decreases. Vt decreases with channel width and it is decreasing faster and faster when channel length is constant. The interstitial Si atoms are injected by LDD ion-implantation and diffuse into channel, while boron pile up at the LDD-channel boundary. However, boron leak into the shallow trench isolation (STI) area at the corner formed by LDD and channel. Both pile-up and leakage control boron concentration distribution in channel. The Vt shift induced by boron TED is channel size-dependent, and high energy carbon co-implantation and infrared rapid thermal anneal can suppress TED effect effectively.

Segregation and Clustering Effects on Complex Boron Redistribution in Strongly Doped Polycrystalline-Silicon Layers

This work deals with the investigation of the complex phenomenon of boron (B) transient enhanced diffusion (TED) in strongly implanted silicon (Si) layers. It concerns the instantaneous influences of the strong B concentrations, the Si layers crystallization, the clustering and the B trapping/segregation during thermal post-implantation annealing. We have used Si thin layers obtained from disilane (Si2H6) by low pressure chemical vapor deposition (LPCVD) and then B implanted with a dose of 4×1015 atoms/cm2 at an energy of 15 keV. To avoid long redistributions, thermal annealing was carried out at relatively low-temperatures (700, 750 and 800 °C) for various short-times ranging between 1 and 30 minutes. To investigate the experimental secondary ion mass spectroscopy (SIMS) doping profiles, a redistribution model well adapted to the particular structure of Si-LPCVD layers and to the effects of strong-concentrations has been established. The good adjustment of the simulated profiles with the experimental SIMS profiles allowed a fundamental understanding about the instantaneous physical phenomena giving and disturbing the TED process in strongly doped Si-LPCVD layers. It was found that boron TED is strongly affected by the simultaneous complex kinetics of clustering, crystallization, trapping and segregation during annealing. The fast formation of small Si–B clusters enhances the B diffusivity whereas the evolution of the clusters and segregation reduce this enhancement.

Impact of carbon junction implant on leakage currents and defect distribution: Measurement and simulation

In this paper we present a detailed investigation on the influence of carbon co-implantation in the source/drain extension on the leakage current and defect density in PFET transistors. Carbon is used to reduce the transient enhanced boron diffusion, to decrease short channel effects and to control the overlap length of the transistor. The leakage currents are measured and separated in order to analyze the influence of the carbon on the different MOSFET regions. The carbon implantation dose is varied between 3.5×1014atm/cm2 and 4×1014atm/cm2. This small increase in implantation dose leads to an enhanced source/drain extension leakage which is caused by carbon induced defects. No effect of the carbon implantation on the source/drain junction leakage was found as the co-implant is located above the source/drain depletion region. In addition an increase of the gate induced drain leakage with carbon dose was observed.

Leakage current and defect characterization of p +n-source/drain diodes

A good control of the transient enhanced dopant diffusion is needed for MOSFET scaling down to the sub 50 nm regime. Carbon ion implant is known to significantly suppress the transient enhanced boron diffusion. However, carbon implantation is also reported to increase diode leakage current. This paper investigates the impact of ion implantation and annealing conditions during source/drain extension formation on leakage current behavior of boron/phosphorous diodes of PFET transistors. Analyzing the leakage current it is difficult to distinguish between the influence of the increased electric field due to the reduced diffusion and possible additional trap centers in the space charge region. This distinction can be made by electrical characterization, as shown in this paper. The leakage current mechanism is found to be trap assisted tunneling with phonon interaction. The corresponding trap energy within the band gap is 0.58 ± 0.10 eV. The carbon concentration in the space charge region measured by SIMS is below the detection limit. Also in electrical measurements, which are more sensitive, no significant influence of carbon related traps is observed. The leakage current is increased by the application of a Flash Anneal additionally to a Rapid Thermal Anneal for recrystallization of the silicon substrate.

Effect of Channel Dopant Profile on Difference in Threshold Voltage Variability Between NFETs and PFETs

The mechanism behind the threshold voltage V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> variability difference between n- and p-type field-effect transistors (NFETs and PFETs, respectively) is investigated from the viewpoint of the channel dopant profile. First, the effect of the depth profile is investigated by comparing the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> variability among FETs with various depth channel profiles. It is clarified that the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> variability of NFETs is larger than that of PFETs with similar depth profiles. The effect of the lateral channel profile is also examined. As one of the causes of the modulation of the lateral channel profile, the effect of halo implantation is evaluated. It is confirmed that V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> variability is enhanced with halo implantation. However, the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> variability of NFETs is still larger than that of PFETs even without halo implantation. Without halo implantation, the reverse short-channel effect appears in NFETs but not in PFETs. From this result, we find that lateral channel profile nonuniformity appears in NFETs even without halo implantation because of the transient-enhanced diffusion (TED) of the channel dopant of boron in NFETs. Since arsenic or phosphorus, which shows no TED characteristic, is used as the channel dopant in PFETs, lateral channel profile nonuniformity does not appear in PFETs without halo implantation. Channel profile nonuniformity, which is caused by boron TED, is thought to be the origin of the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> variability difference between NFETs and PFETs.

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.

10.1007/s11453-008-2017-6

High-voltage (900 V) 4H-SiC Schottky diodes terminated with a guard p-n junction were fabricated and studied. The guard p-n junction was formed by room-temperature boron implantation with subsequent high-temperature annealing. Due to transient enhanced boron diffusion during annealing, the depth of the guard p-n junction was equal to about 1.7 μm, which is larger by approximately 1 μm than the projected range of 11 B ions in 4H-SiC. The maximum reverse voltage of fabricated 4H-SiC Schottky diodes is found to be limited by avalanche breakdown of the planar p-n junction; the value of the breakdown voltage (910 V) is close to theoretical estimate in the case of the impurity concentration N = 2.5 × 1015 cm−3 in the n-type layer, thickness of the n-type layer d = 12.5 μm, and depth of the p-n junction r j = 1.7 μm. The on-state diode resistance (3.7 mΩ cm2) is controlled by the resistance of the epitaxial n-type layer. The recovery charge of about 1.3 nC is equal to the charge of majority charge carriers that are swept out of an epitaxial n-type layer under the effect of a reverse voltage.

Effect of Si and He implantation in the formation of ultra shallow junctions in Si

He, Si and B implantation are successively combined in order to produce ultra shallow junctions (USJs). Si is implanted at 180 keV to create a ‘vacancy-rich’ layer. Such a layer is however followed by a deeper interstitial rich one. He implantation is performed at a dose high enough (5 × 10 16 cm − 2 ) to create a cavity layer in Si sample. Eventually, B is introduced by low energy ion implantation. Since cavities are known to be sinks for self-interstitials (Is), they might be able to decrease the transient enhanced diffusion (TED) of B during the dopant activation annealing in all processes investigated in this study. The samples are divided in two groups (named S1 and S2) to check the benefits of defect engineering of each implantation element. Hence, sample 1 is first implanted with He, followed by cavity formation annealing and second with B. Si implantation is performed on S2 after cavity formation, and followed by B implantation in the same condition as S1. All samples are characterized by secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM) and Hall effect measurements. Results show that, in all cases, boron TED is suppressed while working with a cavity layer created before activation annealing. This layer acts as an Is barrier. Finally, an USJ with a low junction depth ( X j = (13 ± 1) nm, determined at 10 18 cm − 3 boron level) is successfully realized.

Kinetic Monte Carlo study on transient enhanced diffusion: posterior to amorphisation process

We report our theoretical investigations on the suppression of boron diffusion in the silicon substrate posterior to pre-amorphisation implant (PAI). We numerically investigated the defect-generating behaviour of silicon atoms and their subsequent effect on the transient enhanced diffusion (TED) of boron as a new species for PAI. Our kinetic Monte Carlo simulation revealed that Si–PAI produces more interstitials than the case of Ge–PAI, while Ge–PAI makes interstitial moves further up to the surface than the Si–PAI case during the annealing process, which results in the suppression of the boron TED.

Role of Boron TED and Series Resistance in SiGe/Si Heterojunction pMOSFETs

AbstractWe investigate boron transient enhanced diffusion (TED) and series resistance in SiGe/Si heterojunction channel pMOSFET. The stress gradient at the SiGe/Si interface near the gate edge in high Ge concentrations are found to determine boron TED as well as extension junction shape, which has a significant impact on the parasitic LDD and source/drain (S/D) series resistance. In addition, high Ge concentrations in the epitaxial SiGe layer on top of Si substrate result in a high sheet resistance during a 1000°C/5s rapid thermal processing (RTP), which is mainly due to alloy scattering and interface roughness scattering.

High-voltage (900 V) 4 H-SiC Schottky diodes with a boron-implanted guard p-n junction

High-voltage (900 V) 4H-SiC Schottky diodes terminated with a guard p-n junction were fabricated and studied. The guard p-n junction was formed by room-temperature boron implantation with subsequent high-temperature annealing. Due to transient enhanced boron diffusion during annealing, the depth of the guard p-n junction was equal to about 1.7 μm, which is larger by approximately 1 μm than the projected range of 11 B ions in 4H-SiC. The maximum reverse voltage of fabricated 4H-SiC Schottky diodes is found to be limited by avalanche breakdown of the planar p-n junction; the value of the breakdown voltage (910 V) is close to theoretical estimate in the case of the impurity concentration N = 2.5 × 1015 cm−3 in the n-type layer, thickness of the n-type layer d = 12.5 μm, and depth of the p-n junction rj = 1.7 μm. The on-state diode resistance (3.7 mΩ cm2) is controlled by the resistance of the epitaxial n-type layer. The recovery charge of about 1.3 nC is equal to the charge of majority charge carriers that are swept out of an epitaxial n-type layer under the effect of a reverse voltage.

Reduced boron diffusion under interstitial injection in fluorine implanted silicon

Point defect injection studies are performed to investigate how fluorine implantation influences the diffusion of boron marker layers in both the vacancy-rich and interstitial-rich regions of the fluorine damage profile. A 185keV, 2.3×1015cm−2 F+ implant is made into silicon samples containing multiple boron marker layers and rapid thermal annealing is performed at 1000°C for times of 15–120s. The boron and fluorine profiles are characterized by secondary ion mass spectroscopy and the defect structures by transmission electron microscopy (TEM). Fluorine implanted samples surprisingly show less boron diffusion under interstitial injection than those under inert anneal. This effect is particularly noticeable for boron marker layers located in the interstitial-rich region of the fluorine damage profile and for short anneal times (15s). TEM images show a band of dislocation loops around the range of the fluorine implant and the density of dislocation loops is lower under interstitial injection than under inert anneal. It is proposed that interstitial injection accelerates the evolution of interstitial defects into dislocation loops, thereby giving transient enhanced boron diffusion over a shorter period of time. The effect of the fluorine implant on boron diffusion is found to be the opposite for boron marker layers in the interstitial-rich and vacancy-rich regions of the fluorine damage profile. For marker layers in the interstitial-rich region of the fluorine damage profile, the boron diffusion coefficient decreases with anneal time, as is typically seen for transient enhanced diffusion. The boron diffusion under interstitial injection is enhanced by the fluorine implant at short anneal times but suppressed at longer anneal times. It is proposed that this behavior is due to trapping of interstitials at the dislocation loops introduced by the fluorine implant. For boron marker layers in the vacancy-rich region of the fluorine damage profile, suppression of boron diffusion is seen for short anneals and then increased diffusion after a critical time, which is longer for inert anneal than interstitial injection. This behavior is explained by the annealing of vacancy-fluorine clusters, which anneal quicker under interstitial injection because the injected interstitials annihilate vacancies in the clusters.

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.

Effect of Fluorine Implantation Dose on Boron Transient Enhanced Diffusion and Boron Thermal Diffusion in Si&amp;lt;tex&amp;gt;$_1-x$&amp;lt;/tex&amp;gt;Ge&amp;lt;tex&amp;gt;$_x$&amp;lt;/tex&amp;gt;

This paper studies how boron transient enhanced diffusion (TED) and boron thermal diffusion in Si/sub 1-x/Ge/sub x/ are influenced by a high-energy fluorine implant at a dose in the range 5 /spl times/ 10/sup 14/ cm/sup -2/ to 1 /spl times/ 10/sup 16/ cm/sup -2/. Secondary ion mass spectroscopy (SIMS) profiles of boron marker layers are presented for different fluorine doses and compared with fluorine SIMS profiles and transmission electron microscopy (TEM) micrographs to establish the conditions under which boron diffusion is suppressed. The SIMS profiles show that boron thermal diffusion is reduced above a critical F/sup +/ dose of 7 - 9 /spl times/ 10/sup 14/ cm/sup -2/, whereas boron TED is suppressed at all doses. Fitting of the measured boron profiles gives suppressions of boron TED diffusion coefficients by factors of 6.8, 10.6, and 12.9 and of boron thermal diffusion coefficient by factors of 1.9, 2.5, and 3.5 for F/sup +/ implantation doses of 9 /spl times/ 10/sup 14/, 1.4 /spl times/ 10/sup 15/, and 2.3 /spl times/ 10/sup 15/ cm/sup -2/ respectively. The reduction of boron thermal diffusion above the critical fluorine dose correlates with the appearance of a shallow fluorine peak on the SIMS profile in the vicinity of the boron marker layer, which is attributed to vacancy-fluorine clusters. This reduction of boron thermal diffusion is explained by the effect of the clusters in suppressing the interstitial concentration in the Si/sub 1-x/Ge/sub x/ layer. The suppression of boron TED correlates with a deep fluorine peak around the range of the fluorine implant and TEM micrographs show that this peak is due to a band of dislocation loops. This suppression of boron TED is explained by the retention of interstitials in the dislocation loops, which suppresses their backflow to the surface. The fluorine SIMS profiles show that the fluorine concentration in the Si/sub 1-x/Ge/sub x/ layer increases with increasing germanium concentration and that the fluorine concentration in the Si/sub 1-x/Ge/sub x/ layer after anneal is much higher than after implant. This indicates that fluorine is transported into the Si/sub 1-x/Ge/sub x/ layer from the adjacent silicon, and is explained by the lower formation energy for vacancies in Ge than in Si. This accumulation of fluorine in the Si/sub 1-x/Ge/sub x/ layer during anneal is advantageous for devices like SiGe heterojunction bipolar transistors, where the boron must be kept within the Si/sub 1-x/Ge/sub x/ layer.

Interstitial charge states in boron-implanted silicon

It is becoming increasingly clear that simulation models of transient enhanced diffusion (TED) in silicon need to incorporate interstitial charging effects accurately in order to adequately reproduce experimental data near the surface and near the underlying junction. However, in the case of boron TED, the relevant charge states and ionization levels of both boron and silicon interstitial atoms are known only imperfectly. The present work attempts to describe this behavior more accurately via simulations of implanted profiles that employ a model whose kinetic parameters have been determined with considerable confidence by rigorous systems methods. The results suggest that B has two relevant charge states: (+) and (−). The corresponding states for Si are (++) and (0). The effective ionization levels for B and Si are 0.33±0.05 and 0.12±0.05eV above the valence band maximum, respectively.

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.

Fluorine-enhanced boron diffusion induced by fluorine postimplantation in silicon

In this article, the postimplanted fluorine effect on boron transient-enhanced diffusion (TED) and dose loss during a 750°C annealing is shown. F19 implants at 2keV, after B11 implant at an energy of 1keV,3×1014∕cm2, have been investigated in the dose range between 1×1013∕cm2 and 6×1014∕cm2 without a preamorphizing implant. Below a F-implant dose of 1×1014∕cm2, the reduction or non-enhancement of boron-TED effect is observed. In the case of a F-implant dose of 6×1014∕cm2, the enhanced boron TED (∼2.6×) in crystalline Si and the increased dose loss(∼2×) than that of a normal boron TED is shown, and this anomalous diffusivity enhancement persists for 120min at 750°C. The B+F6×1014∕cm2 consecutive implant damage is smaller than that of the BF25keV implant. In the case of the B+F6×1014∕cm2, a high content of fluorine is retained around the end-of-range (EOR) damage region within 120min. These results indicate that the fluorine retained around the EOR region may affect the enhanced boron TED in crystalline Si at 750°C. Boron-diffusion model, which describes both the native interstitial fluorine and the boron-fluorine chemical effect, explains that the suppression of boron TED with a low F-implant dose is due to the reduction of the interstitial supersaturation.

Molecular dynamics simulations of boron diffusion in SiGe

Transient enhanced diffusion (TED) of boron poses a limit to the formation of ultrashallow junctions. To suppress TED of boron, other species (e.g., Ge) can be introduced into the Si substrate. Experiments have shown that boron diffusivity decreased rapidly as Ge concentration increased to 40% Ge. There is no consensus on the cause for this decrease in diffusivity. Here, an ab initio derived energy database was used to fit classical potential models in order to simulate boron diffusion in SiGe on larger length and time-frames than are possible using ab initio models. In this paper, a set of Stillinger-Weber potential parameters for Ge-B and Si-Ge-B has been constructed, allowing a molecular dynamics study of boron diffusion in SiGe alloys to be carried out. Molecular dynamics simulations of boron B diffusion in Si compared to that in SiGe alloys suggest that different trapping mechanisms dominate: B in Si is trapped in substitutional positions, whereas B in SiGe alloys is trapped in interstitial positions. The number of boron interstitials increases as Ge concentration increases, reaches a maximum at 50% Ge, and then decreases as the amount of Ge increases to 100%. Concordantly, the number of Si∕Ge interstitials followed the opposite trend: Their numbers decreased, reached a minimum at 50% Ge and then increased again as the Ge concentration increased. This confirmed ab initio predictions and provided an exploration to the origin of retarded boron diffusion in SiGe alloys.

Boron Diffusion in Intrinsic, n-Type and p-Type 4H-SiC

The diffusion of boron in 4H-SiC has been studied by secondary ion mass spectrometry. Three kinds of epitaxial layers have been used, highly p-type, highly n-type and low doped n-type (intrinsic). A boron diffusion source has been introduced in the samples by ion implantation. Subsequent anneals have been carried out in Ar atmosphere in a RF-heated furnace between 1220degreesC and 2000degreesC for 5 min to 3 h. For the boron diffusion in the highly p-doped layer, 4x10(19) Al atoms/cm(3) an activation energy of 5.3 eV has been determined. A similar activation energy has been extracted in the highly n-doped layer, 1x10(19) N atoms/cm(3), although the absolute diffusivity values differ by four orders of magnitude. In the lightly n-doped layer, transient enhanced boron diffusion is observed at intrinsic conditions.

Carbon-mediated aggregation of self-interstitials in silicon: A large-scale molecular dynamics study

The carbon-mediated aggregation of silicon self-interstitials is investigated with large-scale parallel molecular dynamics. The presence of carbon in the silicon matrix is shown to lead to concentration-dependent self-interstitial cluster pinning, dramatically reducing cluster coalescence and thereby inhibiting the nucleation process. The extent of cluster pinning increases with cluster size for the range of cluster sizes observed in the simulation. The direct effect of carbon on single self-interstitials is shown to be of secondary importance, and the concentration of single self-interstitials as a function of time is essentially unchanged in the presence of carbon. A quasi-single-component mean-field interpretation of the atomistic simulation results is proposed and further confirms these conclusions. Based on these results, it is suggested that the experimentally observed effect of carbon on transient-enhanced diffusion of boron could be due to the direct interaction between carbon atoms and self-interstitial clusters.