Density–depth profiles of an As‐implanted Si wafer studied by repeated planar sputter etching and total reflection x‐ray fluorescence analysis

The implantation of a high dose of high‐energy ions into an Si wafer causes amorphization of the original monocrystalline structure within a near‐surface layer. The in‐depth distribution of both Si atoms of the wafer and As ions implanted at a dose of 1 × 10¹⁷ ions cm^?2 and an energy of 100 keV is studied. A novel method combining a repeated planar and broad sputter etching with differential weighing, surface analysis by total reflection x‐ray fluorescence and Tolansky interferometry is used for this investigation. Different depth profiles are recorded on the nanometre scale for the concentration defined as the mole ratio of As and Si, for the mass density of the implanted layer and for the number density of As and Si. The results generally correspond with measurements of Rutherford backscattering spectrometry and only deviate when the assumptions made for the mass density do not fit well. An appropriate approach to this quantity involves the number density of implanted ions but, furthermore, considers a variation of the number density of Si atoms during implantation, especially for a high dose and high‐energy implantation. The variation can be taken into account by a factor γ, where γ > 1 indicates compression and γ < 1 indicates extension of the original crystalline structure. For the above mentioned implantation, γ is measured separately for each sublayer to obtain accurate depth profiles. Copyright © 2003 John Wiley & Sons, Ltd.     

Piezoresistive properties of polycrystalline and crystalline silicon films

The piezoresistive properties of thin polycrystalline and crystalline boron-doped silicon films on thermally oxidized silicon substrates are reported, based on their calculated and measured gauge factors. A simple theoretical model for calculating the longitudinal and transverse gauge factors, including grain size, crystallite orientation and doping dependence, is described. Boron doping concentrations in the range 5 × 10¹⁸ cm⁻³ to 1 × 10²⁰ cm⁻³ have been investigated. Predictions of gauge factors using our model give good agreement with experimental results. Maximum gauge factors of K₁ ≅ 37 and K_t ≅ −9 for polycrystalline silicon with grain sizes of about 120 nm were obtained at doping concentrations of about 1 × 10¹⁹ cm⁻³. In the case of completely crystalline silicon films, the advantages of SOI technology are combined with excellent piezoresistive properties comparable to those of usual piezoresistors in monocrystalline silicon devices.     

Electrochemical etching process to tune the diameter of arrayed deep pores by controlling carrier collection at a semiconductor–electrolyte interface

An approach to control the diameter of high-aspect-ratio pores formed into a silicon wafer by an electrochemical etching process is reported. Hole (h⁺) was involved in the etching reaction and the collection of the h⁺ was the key factor. Artificial micro-cavities were fabricated on the silicon surface prior to the etching. The depth of the space charge region (SCR), Schottky barrier on the silicon-electrolyte interface, was adjusted regarding the depth of the micro-cavities by applied overpotential and specific resistance of the silicon wafer. The collection of h⁺ at the tip of the cavity site was widely controlled by the adjusted SCR. Consequently the electrochemically etched domain at the cavity site was actively tuned, and then high-aspect-ratio pore with the controlled diameter was formed. The diameter was tuned by the SCR depth which was controlled by the overpotential and the specific resistance. The diameter tuning mechanism worked under the mask-free condition.     

Study of thermal recrystallisation in Si implanted by 0.4-MeV heavy ions

A Si crystal layer on SiO₂/Si was implanted using 0.4-MeV Kr⁺, Ag⁺, and Au⁺ at ion fluences of 0.5 × 10¹⁵ to 5.0 × 10¹⁵ cm⁻². Subsequent annealing was performed at temperatures of 450° and 800° for 1 hour. The structural modification in a Si crystal influences ion beam channelling phenomena; therefore, implanted and annealed samples were investigated by Rutherford backscattering spectrometry under channelling (RBS-C) conditions using an incident beam of 2-MeV He⁺ from a 3-MV Tandetron in random or in aligned directions. The depth profiles of the implanted atoms and the dislocated Si atom depth profiles in the Si layer were extracted directly from the RBS measurement. The damage accumulation and changes in the crystallographic structure before and after annealing were studied by X-ray diffraction (XRD) analysis. Lattice parameters in modified silicon layers determined by XRD were discussed in connection to RBS-C findings showing the crystalline structure modification depending on ion implantation and annealing parameters.     

An investigation of hydrogen depth profiling using ToF‐SIMS

Hydrogen depth distributions in silicon, zinc oxide, and glass are of great interest in material research and industry. Time‐of‐flight SIMS has been used for hydrogen depth profiling for many years. However, some critical information, such as optimal instrumental settings and detection limits, is not easily available from previous publications. In this work, optimal instrumental settings and detection limits of hydrogen in silicon, zinc oxide, and common glass were investigated. The recommended experimental settings for hydrogen depth profiling using time‐of‐flight SIMS are: (i) keeping pressure in the analysis chamber as low as possible, (ii) using a cesium beam for sputtering and monitoring the H^– signal, (iii) employing monatomic ion analysis beams with the highest currents, and (iv) using interlace mode. In addition, monatomic secondary ions from a matrix are recommended as references to normalize the H^– signal. Detection limits of hydrogen are limited by the pressure of residual gases in the analysis chamber. The base pressure of the analysis chamber (with samples) is about 7 × 10^?10 mbar in this study, and the corresponding detection limits of hydrogen in silicon, zinc oxide, and common glass are 1.3 × 10¹⁸ atoms/cm³, 1.8 × 10¹⁸ atoms/cm³, and 5.6 × 10¹⁸ atoms/cm³, respectively. Copyright © 2011 John Wiley & Sons, Ltd.     

The mechanisms of removal of heavy metals from water by ionizing radiation

The removal of heavy metal ions from water using electron beam and gamma irradiation has been investigated for the cases of Pb²⁺ and Hg²⁺ ions. These metal ions are reduced by hydrated electrons and hydrogen atoms to lower or zero valence state and eventually precipitate out of solution. Ethanol is applied as a relatively non-toxic additive to scavenge ^·OH radicals, to enhance reduction and inhibit oxidation. Mercury can be completely (>99.9%) removed from aqueous solution of 1×10⁻³ mol L⁻¹ mercury (II) chloride by using a 3 kGy dose. However, a 40 kGy dose is required to remove 96% of lead ions from a 1×10⁻³ mol L⁻¹ of PbCl₂ solution. The effect of dissolved oxygen and carbonate were also investigated. E-beam irradiation of 1×10⁻³ mol L⁻¹ lead ions complexed with ethylenediamine tetraacetic acid (EDTA) in deoxygenated as well as air-saturated solutions in the absence of ethanol resulted in removal of about 97% of the lead.     

Determination of metallic impurities in a silicon wafer by local etching and electrothermal atomic absorption spectrometry.

An etching technique for the determination of the metallic impurities distribution in silicon wafers has been developed. An area of 10 mmphi and 10 microm depth was etched by 100 microL of an etching solution with a HF and HNO3 mixture. The acid matrix was evaporated on the wafer surface by IR lamp illumination and vacuum exhaust. Metallic impurities remaining on the wafer surface were redissolved into the collection solution, which was measured by electrothermal atomic absorption spectrometry (ET-AAS). The recovery invested by local etching/ET-AAS was within 95 - 112% for Fe, Cu and Ni. The detection limit (3sigma) for Fe, Cu and Ni in silicon was 1 x 10(13) atoms/cm3. To confirm the applicability, local etching was applied to evaluate the effects of metallic impurities in a gettering study and the electronic properties of semiconductor devices. It was found that local etching is a useful sample preparation technique for the analysis of metallic impurities in a specific area on a silicon wafer.     

Summary of ISO/TC 201 Standard: X ISO 17560:2002—Surface chemical analysis—Secondary ion mass spectrometry—Method for depth profiling of boron in silicon

This International Standard specifies a secondary ion mass spectrometric method using magnetic‐sector or quadrupole mass spectrometers for depth profiling of boron in silicon, and using stylus profilometry or optical interferometry for depth calibration. This method is applicable to single‐crystal, polycrystal or amorphous silicon specimens with boron atomic concentrations between 1 × 10¹⁶ and 1 × 10²⁰ atoms cm^?3, and to the crater depth of 50 nm or deeper. Optical interferometry is generally applicable to crater depths in the range 0.5–5 µm. Copyright © 2004 John Wiley & Sons, Ltd.     

Summary of ISO/TC 201 Standard: XIX ISO 17331:2004—Surface chemical analysis—Chemical methods for the collection of elements from the surface of silicon‐wafer working reference materials and their determination by total‐reflection x‐ray fluorescence (TXRF) spectroscopy

This international standard specifies chemical methods for the collection of iron and/or nickel from the surface of silicon‐wafer working reference materials by the vapour‐phase decomposition method or the direct acid droplet decomposition method. The determination of the elements collected may be carried out by total‐reflection x‐ray fluorescence spectroscopy, as well as by graphite‐furnace atomic absorption spectroscopy or inductively coupled plasma mass spectroscopy. This international standard applies to iron and/or nickel atomic surface densities from 6 × 10⁹ to 5 × 10¹¹ atoms cm^?2. Copyright © 2005 John Wiley & Sons, Ltd.     

Semiconductor applications of nanoliter droplet methodology with total reflection x-ray fluorescence analysis

In this study, the nanoliter dried spot method was applied to semiconductor contamination analysis to enhance vapor phase decomposition processes with total reflection X-ray fluorescence detection. Nanoliter-sized droplets (10 and 50 nl) were deposited onto native silicon oxide wafer surfaces in a clean room environment from both single and multielemental standards containing various concentrations of iron in different matrices. Direct comparisons were made to droplets formed by conventional VPD with similar iron standards. Nanoliter dried spots could be reproducibly deposited and dried in air with typical drying times ranging from 20 s to 2 min depending on the nanoliter volume deposited, compared to VPD spots which have drying times ranging from tens of minutes to several hours. Both types of residues showed a linear relationship between Fe intensity and mass deposited. Variable angle experiments showed that both nanoliter and VPD deposits of single element standards were film-like in character, while residues formed from much more complex matrices and higher mass loadings were particulate in character. For the experimental conditions used in this study (30 kV, 100 mA), typical TXRF spectral Fe limits of detection were calculated to be on the order of picograms or ∼1×10¹⁰ atoms/cm² for a 0.8 cm² X-ray excitation beam area for both nanoliter dried spots and VPD spots prepared from single elemental standards. Calculated Fe detection limits for 200 mm diameter silicon wafers used in this study were in the ∼1×10⁸ atoms/cm² range. By using nanoliter sized droplets, the required sample volume is greatly reduced resulting in higher sample throughput than with conventional VPD methods.     

Determination of phosphorus death profiles in semiconductor silicon by chemical etching and filament vaporization inductively-coupled plasma atomic emission spectrometry

Silicon slices are dissolved by anodizin and removal of the silica film by hydrofluoric acid. The phosphorus in the etching solution is determined by the filament vaporization technique and the depth of silicon is measured by determining the silicon content in the etching solution by conventional inductively-coupled plasma atomic emission spectrometry. A lower limit of 10¹⁸ phosphorus atoms cm^?3 can be determined by sectioning intervals of 30–50 nm.     

Determination of boron in the thin surface layer of a silicon wafer by instrumental charged particle activation analysis

Instrumental charged particle activation analysis (CPAA) for determining boron in a thin surface layer of silicon was developed. The nuclear reaction and incident energy were selected in order to minimize any interference from surface or bulk impurities. Thin boron film was used as a standard sample and its boron content was determined by neutron induced prompt -ray analysis. As a result, we were able to determine¹¹B and¹⁰B at 10¹⁵ atoms/cm² with an accuracy of better than 3% by 4 MeV proton and 7 MeV -bombardment, respectively. Each boron isotope could be determined down to 10¹³ atoms/cm². Our CPAA was applied to determine boron in a boron implanted silicon wafer of a SIMS standard sample.     

Instrumental neutron activation analysis of silicon wafers using the silicon matrix as the comparator

For the instrumental neutron activation analysis of trace impurities in high purity silicon wafer, a modified single comparator method has been applied. The energy distribution of the neutrons at the irradiation position was measured using the two flux monitors, Au and Co, and elemental contents were calculated using the silicon matrix in the wafer as a comparator. This has advantage of reducing the cross contamination from an external monitor during sample preparation and irradiation, the uncertainties from the non-homogeneity of the neutron flux and the error on the weight of comparators. Determination limits for 49 elements were presented under the condition of 72 hours irradiation at a neutron flux of 3.7·10¹³ n·cm^-2·s^-1 and 4000 s measurement. The analytical results obtained by this method and the conventional single comparator method were compared and were found to agree well within 5%.     

Surface cavities produced by high‐dose nitrogen ion implantation into silicon

Nitrogen ion implantation (24 keV, 4.6 × 10¹⁷ cm^?2) into (100) a p‐type silicon wafer material and a subsequent electron beam annealing at 1100 °C for 15 s under high vacuum conditions leads to the formation of an uneven surface in the implanted region caused by nitrogen bubbles beneath the surface. Annealing at 1200 °C for 300 s results in surface cavities with a mean diameter of 350 nm and a surface coverage of 3–4% and an average depth of ～60 nm. Nuclear reaction analysis reveals that the nitrogen concentration in the as‐implanted state exceeds 57 at%, the value of stoichiometric Si₃N₄. Annealing at 1100 °C for 15 s slightly reduces the nitrogen peak concentration, whereas annealing at 1200 °C for 300 s induces a significant alteration to the shape of the nitrogen depth profile coupled with the lowering of the concentration close to the stoichiometry of Si₃N₄. The results present a new method of producing sub‐micrometre cavities embedded in a thin silicon nitride film on wafer silicon which may lead to novel micro‐electronic and biotechnology applications. Copyright © 2007 John Wiley & Sons, Ltd.     

Opto-structural characterization of proton (3 MeV) irradiated polycarbonate and polystyrene

Polycarbonate (Makrofol-N) and polystyrene thin films were irradiated with protons (3 MeV) under vacuum at room temperature with the fluence ranging from 1×10¹⁴ to 1×10¹⁵ protons cm⁻². The change in optical properties, degradation of the functional groups and crystallinity of the proton-irradiated polymers were investigated with UV–vis, Fourier-transform infrared (FTIR) and X-ray diffraction (XRD) techniques, respectively. The UV–vis analysis revealed that the optical band gap of irradiated Makrofol-N is reduced by 30% as compared to 27.5% in polystyrene at highest fluence of 1×10¹⁵ protons cm⁻², owing to higher electronic energy loss of protons in Makrofol-N. The calculations of the number of carbon atoms per conjugation length, N and number of carbon atoms per clusters, M embedded in the network of polymers further revealed that Makrofol-N is more modified as compared to polystyrene on proton irradiation. FTIR results reveal the reduction in absorption intensity of the main characteristic bands of both the polymers after irradiation. The proton-irradiated Makrofol-N shows a strong decrease of almost all of its characteristic absorption bands at about 1×10¹⁴ protons cm⁻². Beyond a critical dose an increase of almost all its characteristic bands are noticed, however, no such effect had been observed in polystyrene at this particular fluence. Appearance of new –OH groups was observed at the higher fluences in the FTIR spectra of both proton-irradiated polymers. XRD measurements show the decrease of the main peak intensity and the crystallite size, confirming the increase of amorphization in polymers under irradiation.     

Effect of post-deposition annealing temperature on RF-sputtered HfO2 thin film for advanced CMOS technology

Structural and electrical properties of HfO₂ gate-dielectric metal-oxide-semiconductor (MOS) capacitors deposited by sputtering are investigated. The HfO₂ high-k thin films have been deposited on p-type <100> silicon wafer using RF-Magnetron sputtering technique. The Ellipsometric, FTIR and AFM characterizations have been done. The thickness of the as deposited film is measured to be 35.38 nm. Post deposition annealing in N₂ ambient is carried out at 350, 550, 750 °C. The chemical bonding and surface morphology of the film is verified using FTIR and AFM respectively. The structural characterization confirmed that the thin film was free of physical defects and root mean square surface roughness decreased as the annealing temperature increased. The smooth surface HfO₂ thin films were used for Al/HfO₂/p-Si MOS structures fabrication. The fabricated Al/HfO₂/p-Si structure had been used for extracting electrical properties such as dielectric constant, EOT, interface trap density and leakage current density through capacitance voltage and current voltage measurements. The interface state density extracted from the G–V measurement using Hill Coleman method. Sample annealed at 750 °C showed the lowest interface trap density (3.48 × 10¹¹ eV⁻¹ cm⁻²), effective oxide charge (1.33 × 10¹² cm⁻²) and low leakage current density (3.39 × 10⁻⁹ A cm⁻²) at 1.5 V.     

Molecular Modification of Single Cobalt Sites Boosts the Catalytic Activity of CO2 Electroreduction into CO

Electroreduction of CO₂ into carbonaceous fuels or industrial chemicals using renewable energy sources is an ideal way to promote global carbon recycling. Thus, it is of great importance to develop highly selective, efficient, and stable catalysts. Herein, we prepared cobalt single atoms (Co SAs) coordinated with phthalocyanine (Co SAs-Pc). The anchoring of phthalocyanine with Co sites enabled electron transfer from Co sites to CO₂ effectively via the π-conjugated system, resulting in high catalytic performance of CO₂ electroreduction into CO. During the process of CO₂ electroreduction, the Faradaic efficiency (FE) of Co SAs-Pc for CO was as high as 94.8 %. Meanwhile, the partial current density of Co SAs-Pc for CO was −11.3 mA cm⁻² at −0.8 V versus the reversible hydrogen electrode (vs RHE), 18.83 and 2.86 times greater than those of Co SAs (−0.60 mA cm⁻²) and commercial Co phthalocyanine (−3.95 mA cm⁻²), respectively. In an H-cell system operating at −0.8 V vs RHE over 10 h, the current density and FE for CO of Co SAs-Pc dropped by 3.2 % and 2.5 %. A mechanistic study revealed that the promoted catalytic performance of Co SAs-Pc could be attributed to the accelerated reaction kinetics and facilitated CO₂ activation.     

Kinetics of the reactions of hydroxyl radicals (OH) and of chlorine atoms (Cl) with methylethylether over the temperature range 274–345 K

The rate constants for the gas-phase reactions between methylethylether and hydroxyl radicals (OH) and methylethylether and chlorine atoms (Cl) have been determined over the temperature range 274–345 K using a relative rate technique. In this range the rate constants vary little with temperature and average values of k_MEE+OH = (6.60_−2.62^+3.88) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ and k_MEE+Cl= (34.9 ± 6.7) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ were obtained. The atmospheric lifetimes of methylethylether have been estimated with respect to removal by OH radicals and Cl atoms to be ca. 2 days and ca. 30–40 days, respectively. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 231–236, 1997.     

Rate coefficients for the reactions of chlorine atoms with methanol and acetaldehyde

Rate coefficients have been measured for the reactions of Cl atoms with methanol (k₁) and acetaldehyde (k₂) using both absolute (laser photolysis with resonance fluorescence) and relative rate methods at 295 ± 2 K. The measured rate coefficients were (units of 10⁻¹¹ cm³ molecule⁻¹ s⁻¹): absolute method, k₁ = (5.1 ± 0.4), k₂ = (7.3 ± 0.7); relative method k₁ = (5.6 ± 0.6), k₂ = (8.4 ± 1.0). Based on a critical evaluation of the literature data, the following rate coefficients are recommended: k₁ = (5.4 ± 0.9) × 10⁻¹¹ and k₂ = (7.8 ± 1.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ (95% confidence limits). The results significantly improve the confidence in the database for reactions of Cl atoms with these oxygenated organics. Rate coefficients were also measured for the reactions of Cl₂ with CH₂OH, k₅ = (2.9 ± 0.6) × 10⁻¹¹ and CH₃CO, k₆ = (4.3 ± 1.5) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, by observing the regeneration of Cl atoms in the absence of O₂. Based on these results and those from a previous relative rate study, the rate coefficient for CH₃CO + O₂ at the high pressure limit is estimated to be (5.7 ± 1.9) × 10⁻¹² cm³ molecule⁻¹ s⁻¹. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 776–784, 1999     

One-Step Approach for Constructing High-Density Single-Atom Catalysts toward Overall Water Splitting at Industrial Current Densities

The construction of highly active, durable, and cost-effective catalysts is urgently needed for green hydrogen production. Herein, catalysts consisting of high-density Pt (24 atoms nm⁻²) and Ir (32 atoms nm⁻²) single atoms anchored on Co(OH)₂ were constructed by a facile one-step approach. Remarkably, Pt₁/Co(OH)₂ and Ir₁/Co(OH)₂ only required 4 and 178 mV at 10 mA cm⁻² for hydrogen evolution reaction and oxygen evolution reaction, respectively. Moreover, the assembled Pt₁/Co(OH)₂//Ir₁/Co(OH)₂ system showed mass activity of 4.9 A mg_{noble metal}⁻¹ at 2.0 V in an alkaline water electrolyzer, which is 316.1 times higher than that of Pt/C//IrO₂. Mechanistic studies revealed that reconstructed Ir−O₆ single atoms and remodeled Pt triple-atom sites enhanced the occupancy of Ir−O bonding orbitals and improved the occupation of Pt−H antibonding orbital, respectively, contributing to the formation of the O−O bond and the desorption of hydrogen. This one-step approach was also generalized to fabricate other 20 single-atom catalysts.