Boron Doping Of Silicon Research Articles

Sub-2 nm boron doping in silicon using novel ultra-thin SiO2 film produced by sol–gel dip coating as a capping layer

Advancements in semiconductor electronics and sensor technologies demand a more robust doping approach and process to fabricate ultra-shallow doping with abrupt depth, particularly on complex structured surfaces. Molecular monolayer doping (MLD) is a potential approach, but is limited by the difficulty in depositing a high-quality SiO2 capping film without damaging the dopant molecular layer. In this work, a relative humidity condition during deposition of the film in the sol–gel dip-coating process is found to produce continuous, dense and stoichiometric ultra-thin SiO2 films of quality equivalent to that produced by complicated thermal oxidation/etching and sophisticated chemical vapor deposition and atomic layer deposition methods. Applying these ultra-thin SiO2 films as capping layers in the molecular MLD method, B doping in Si with sub-2 nm effective depth and sub-5 nm abrupt depth is achieved. Remarkably, around 82%–86% of doped B atoms in Si are found to be electrically active as estimated from sheet resistance measurements. The established sol–gel dip coating conditions to deposit ultra-thin SiO2 films are generic and can be extended to produce high-quality ultra-thin films of other metal oxide materials required for advanced technological applications.

Lattice contraction due to boron doping in silicon

In modern electronic devices, strain is used to increase carrier mobility. It is thus mandatory to know precisely the effect of doping on the lattice parameter of silicon. However, there are many experimental biases which prevent one from measuring this effect with high accuracy. For this reason, we have designed and fabricated a step-like structure consisting of five 50 nm-thick Si layers of increasing substitutional boron concentrations. Then, we have used Dark Field Electron Holography, a Transmission Electron Microscopy based technique, to measure the strain in these pseudomorphic and defect-free layers. Using Finite Element Modelling, we show that the observed out-of-plane strains can be reproduced by assuming that the expansion coefficient of boron is about − 6.5 × 10–24 cm3. This value is slightly larger (in absolute value) than those previously reported. It is otherwise about 20% larger than predicted from the size mismatch between B and Si atoms.

Use of B2O3 films grown by plasma-assisted atomic layer deposition for shallow boron doping in silicon

Plasma-assisted atomic layer deposition (PALD) was carried for growing thin boron oxide films onto silicon aiming at the formation of dopant sources for shallow boron doping of silicon by rapid thermal annealing (RTA). A remote capacitively coupled plasma source powered by GaN microwave oscillators was used for generating oxygen plasma in the PALD process with tris(dimethylamido)borane as boron containing precursor. ALD type growth was obtained; growth per cycle was highest with 0.13 nm at room temperature and decreased with higher temperature. The as-deposited films were highly unstable in ambient air and could be protected by capping with in-situ PALD grown antimony oxide films. After 16 weeks of storage in air, degradation of the film stack was observed in an electron microscope. The instability of the boron oxide, caused by moisture uptake, suggests the application of this film for testing moisture barrier properties of capping materials particularly for those grown by ALD. Boron doping of silicon was demonstrated using the uncapped PALD B2O3 films for RTA processes without exposing them to air. The boron concentration in the silicon could be varied depending on the source layer thickness for very thin films, which favors the application of ALD for semiconductor doping processes.

Application of atomic layer deposited dopant sources for ultra‐shallow doping of silicon

AbstractThe advanced silicon semiconductor technology requires doping methods for production of ultra‐shallow junctions with sufficiently low sheet resistance. Furthermore, advanced 3‐dimensional topologies may require controlled local doping that cannot be achieved by ionimplantation. Here, the application of the atomic layer deposition (ALD) method for pre‐deposition of dopant sources is presented. Antimony oxide and boron oxide were investigated for such application. Ozone‐based ALD was carried out on silicon wafers by using triethylantimony or tris‐(dimethylamido)borane. Very homogeneous Sb2O5 deposition could be achieved on flat silicon wafers and in trench structures. The thermal stability of antimony oxide layers was investigated by rapid thermal annealing experiments. The layers were not stable above 750 °C. Therefore, this material failed to act as dopant source so far.In contrast, ultra‐shallow boron doping of silicon from ALD grown boron oxide films was successful. However, pure B2O3 films were highly unstable after exposure to ambient air. The boron oxide films could be protected by thin Sb2O5 or Al2O3 films that were in‐situ grown by ALD. Low temperature ALD of Al2O3 at 50 °C from trimethylaluminium (TMA) and ozone was investigated in detail with respect of its protective effect on boron oxide. Interestingly, it was observed that already one ALD cycle of TMA and O3 resulted in significant increase in stability of the boron oxide in air.(© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Modeling the Microstructure Curvature of Boron-Doped Silicon in Bulk Micromachined Accelerometer.

Microstructure curvature, or buckling, is observed in the micromachining of silicon sensors because of the doping of impurities for realizing certain electrical and mechanical processes. This behavior can be a key source of error in inertial sensors. Therefore, identifying the factors that influence the buckling value is important in designing MEMS devices. In this study, the curvature in the proof mass of an accelerometer is modeled as a multilayered solid model. Modeling is performed according to the characteristics of the solid diffusion mechanism in the bulk-dissolved wafer process (BDWP) based on the self-stopped etch technique. Moreover, the proposed multilayered solid model is established as an equivalent composite structure formed by a group of thin layers that are glued together. Each layer has a different Young’s modulus value and each undergoes different volume shrinkage strain owing to boron doping in silicon. Observations of five groups of proof mass blocks of accelerometers suggest that the theoretical model is effective in determining the buckling value of a fabricated structure.

Comparison of beryllium oxide and pyrolytic graphite crucibles for boron doped silicon epitaxy

This article reports on the comparison of beryllium oxide and pyrolytic graphite as crucible liners in a high-temperature effusion cell used for boron doping in silicon grown by molecular beam epitaxy. Secondary ion mass spectroscopy analysis indicates decomposition of the beryllium oxide liner, leading to significant incorporation of beryllium and oxygen in the grown films. The resulting films are of poor crystal quality with rough surfaces and broad x-ray diffraction peaks. Alternatively, the use of pyrolytic graphite crucible liners results in higher quality films.

Imaging of the boron doping in silicon using low energy SEM

Scanning electron imaging of plan views of boron-doped patterns in silicon is examined, together with the mechanism of formation of the electronic contrast in this kind of structures. Main to-date published results are critically reviewed and new data are presented concerning the secondary, backscattered and total-emission electron contrasts, including their qualitative and quantitative behaviour, particularly in the low energy range achieved with the help of the cathode lens (the scanning low energy electron microscopy mode, SLEEM). Surface analysis of the structure by means of Auger electron spectrometer has been performed, too, both before and after ion beam bombardment. The scanning electron microscope micrographs, acquired after the oxide mask removal in HF, are examined in a variety of detection modes, aiming at identification of the signal component primarily bearing the contrast. The energy dependence of the contrasts is presented as well as its change owing to alteration in the vacuum conditions. The most important findings include an extremely high contrast obtained in the SLEEM mode and even more enhanced under medium vacuum conditions at which the carbonaceous layer of surface contamination plays its role. The observed phenomena are partly explained in the frame of the “flat band” model of a passivated surface. The increased contrast in the SLEEM mode is understood as connected with the above-surface electric field of the cathode lens, generating space charge layers inside the semiconductor. In addition, charge carriers, injected via the primary electron beam, are considered as influencing the contrast vs. energy dependence.

Iron and Nickel Solubilities in Heavily Doped Silicon and their Energy Levels in the Silicon Band Gap at Elevated Temperatures

AbstractWe have directly measured the solubility of iron and nickel in high and low boron-doped silicon using Instrumental Neutron Activation Analysis. Boron doping levels were 1.5×1019 and 6.5× 1014 atoms/cm3. Iron and nickel impurity concentrations were measured after extended indiffusions at 800, 900, 1000 and 1100°C for iron and 600, 700 and 800°C for nickel. We have measured a significant enhancement of Fe and Ni concentrations in high boron-doped silicon as compared to low boron-doped silicon. Based on these measurements, we show the iron donor energy level shifts towards the valence band with increased temperature, e.g. at 900°C the donor level is 0.24eV above the valence band as opposed to 0.39eV at room temperature. These results demonstrate that the impurity energy level shift with temperature must be accounted for in any prediction of segregation gettering of metal impurities into heavily doped substrates and heavily implanted doping layers. Additionally, our results suggest that either Ni solubility is greatly enhanced and/or the Ni diffusivity is greatly decreased with high boron doping of silicon.

Boron doping of silicon by plasma source ion implantation

We have performed an extensive analysis of the structural and electrical characteristics of 150 and 200 mm Si wafers boron doped by low-energy (−0.5 to −5 kV) plasma source ion implantation (PSII) from a BF 3 plasma and subsequent rapid thermal annealing. The formation of shallow junctions (60 nm or less) required for 0.18 μm device geometries has been verified by secondary ion mass spectrometry measurements and electrical characterization. Rutherford backscattering spectrometry and high resolution electron microscopy analyses have shown a thin (≤5 nm) amorphous region in the as-implanted silicon; no evidence of dislocations or other extended defects has been observed. After annealing, the implanted silicon appears to be perfectly crystalline. Good doping uniformity across a wafer and good wafer-to-wafer repeatability have been observed in the PSII process. Excellent device characteristics (threshold voltage rolloffs, junction leakage, etc.) have been obtained from MOSFET structures fabricated using this doping technology. All results suggest that boron doping of silicon by PSII results in materials and device characteristics that are suitable for advanced device fabrication.

Boron doping of silicon using coalloying with aluminium

We present a new technique for boron (B) doping of silicon. In this letter we show that a B doping profile of more than 2 μm thickness with a maximum active doping concentration of 3×1019 cm−3 can be formed by a fast (<1 min) alloying process at a temperature of 850 °C. Thick-film aluminum is used to obtain an alloying and epitaxial regrowth process in accordance with the Al–Si phase diagram. The atomic concentration profile of the B- and Al-doped Si layer was determined by secondary ion mass spectrometry. The active doping concentration was measured with an automatic electrochemical capacitance/voltage profiler. By the addition of B to the Al paste, the epitaxially regrown layer is doped with B and Al to the solid solubility of these elements in Si at the particular regrowth temperature. The applicability of this fast low temperature B doping process in Si solar cell processing is discussed.

Damage Formed by Plasma Boron Doping in Silicon

Damage formed by DC plasma doping of boron to silicon is studied. Damage density is measured by 1.5 MeV He+ Rutherford backscattering spectrometry and photoacoustic displacement techniques. These damage density measurements show that lower-damage doping can be achieved in plasma doping compared with that of ion implantation. Impurity diffusion does not occur as rapidly in this doping as seen in B+ and BF2+ implantations. The diffusion coefficient of boron is as low as 4.9× 10-15 cm2/s at 950° C in this doping. This coefficient is 5 and 40 times lower than that of B+ and BF2+ implantations, respectively. Correlation of the diffusion coefficient with damage density is found. A shallow and high peak carrier concentration p+ layer can be formed by plasma boron doping.

Formation of shallow p+ layer in silicon by plasma doping

Boron doping of silicon is achieved by employing a diode parallel-plate-type dc plasma reactor, where diborane (B2H6) gas diluted with Ar (0.5 mol %) is used as doping gas. Total boron concentration changes from 1.17×1015 to 1.82×1015 atoms with varying applied dc bias voltage, Vdc, from 1600 to 3000 V. Shallow and low sheet resistance highly doped p(p+) layer is formed with annealing of the layer doped above total doping concentration of 1.20×1015 atoms. Peak carrier concentration of 2.20×1020/cm3 and a sheet resistance of 86 Ω/⧠ are attained in 140 nm depth p+ layer.

Structural Analysis of Silicon Doped by Plasma Source Ion Implantation

ABSTRACTWe have used high resolution x-ray diffraction to analyze the structural changes that accompany boron doping of silicon by BF3 plasma source ion implantation (PSII). Triple crystal diffraction analysis of as-implanted PSII doped silicon showed little excess x-ray diffuse scattering, even when analyzed using the asymmetric (113) reflection for increased surface sensitivity. This result suggests that PSΠ is capable of providing high dose implantation with low damage. Annealing of the PSII-doped silicon showed the development of a compressive surface layer, indicated by enhanced x-ray scattering directed perpendicular to the surface. Virtually all of the scattering from the annealed samples was concentrated in the so-called “surface streak” which arises due to dynamical diffraction from the perfect crystal Si structure. Little if any diffuse scattering due to kinematic scattering from crystal defects was detected. These observations indicate that plasma source doping can be used to achieve both a shallow implant depth and an extremely uniform incorporation of boron into the silicon lattice.

Two-Step Doping Using Excimer Laser in Boron Doping of Silicon

A two-step doping method consisting of the deposition of dopant films and the incorporation of dopant atoms using excimer laser has been studied in boron doping of silicon. These processes are carried out in the same chamber successively. Sheet resistance can be varied widely by changing both the number and the energy density of the depositing laser pulses. In this method, dopants diffuse in the melted silicon from a fixed total dopant source with a simple Gaussian distribution when the number of melting pulses is small. The diffusion coefficient of B in liquid Si is determined experimentally.

A comparative study of heavy boron doping in silicon and Si 1− x Ge x layers grown by molecular beam epitaxy

Different aspects of boron doping have been studied for silicon and Si 1− x Ge x layers grown by molecular beam epitaxy. Boron segregation at high doping levels was measured in situ in both cases and it was found to be relatively suppressed in the alloy samples. Some possible mechanisms for this effect are discussed. Germanium segregation was also measured during the growth of the first monolayers in Si 1− x Ge x structures. The limits for boron incorporation and activation as dopant in such structures are discussed on the basis of electrical measurements.

Precise control of sheet resistance in boron doping of silicon by excimer laser irradiation

To control the sheet resistance in boron doping of silicon by the excimer laser irradiation, two-step doping consisting of the deposition of boron films and the incorporation of dopant atoms is proposed. These processes are carried out in the same chamber successively. Precise control of the sheet resistance can be realized by this method and a very low sheet resistance can also be obtained by only one pulse irradiation for the melting of silicon. >

Dislocations Induced by Heavy Boron Doping of Silicon

Diffusion-induced dislocations in silicon have long been of interest in semiconducting devices. Several investigations concerning the dislocations induced by doping of Si were carried out during the early sixties. However, there are still a number of unresolved questions concerning this problem. Because the boron doped chemical etch-stop technique is widely used in the fabrication of microsensors, new interest has arisen in the role of dislocations induced by boron diffusion and the behavior of these dislocations on the mechanical properties of microsensors. The doping of Si in this technique is heavy (>5x1019cm-3) and, consequently, the density of induced dislocations in the wafer is large. The dislocations will probably then play a critical role on the mechanical properties of thin Si wafers used in the etch-stop technique.

Boron doping of silicon by ArF excimer laser irradiation in B2H6

Boron doping of silicon using a pulsed ArF excimer laser (λ=193 nm, FWHM=17 ns) with B2H6 has been investigated. Doping atoms are supplied mainly from both thermal and photochemical decomposition of the adsorbed layers on the surface. UV laser irradiation realizes both high surface carrier concentration (1×1021 cm−3) and a very shallow junction (0.1 μm). Diodes fabricated by laser doping show good electrical characteristics (n factor=1.10, reverse current density=1×10−7 A/cm2 at −5 V). Thermal annealing improves the reverse current density (3×10−9 A/cm2 at −5 V). A numerical calculation of boron concentration profiles has been performed by solving the diffusion equation in the melted region, which is evaluated by the heat conduction equation. From this calculation the diffusion coefficient of boron in liquid phase silicon (3×10−4 cm2/s) is obtained.