Comparison of different gettering techniques for Cu–p+ versus polysilicon and oxygen precipitates

We performed measurements of gettering efficiencies for Cu in silicon wafers with competing gettering sites. Epitaxial wafers (p/p+) boron-doped with a polysilicon back side allowed us to compare p+ gettering with polysilicon gettering. We further measured metal distributions in p+/p- epitaxial test wafers, with the p- substrate wafers pretreated for oxygen precipitation to compare p+ gettering with oxygen precipitate gettering. Our test started with a reproducible spin-on contamination in the 10¹² atoms/cm² range, followed by thermal treatment in order to redistribute the metallic impurity. Wafers were then analyzed by a novel wet chemical layer-by-layer etching technique in combination with inductively coupled plasma mass spectrometry. This led to “stratigraphical” concentration profiles of the impurity, with typical detection limits of 5–10×10¹² atoms/cm³. Twenty-five percent of the total Cu contamination in the p/p+/poly wafer was found in the p+ layer, whilst 75% was gettered by the polysilicon. Obviously, polysilicon exhibits a stronger gettering than p+ silicon, but due to the large distance from the front surface, polysilicon was less effective in reducing impurities from the front side of a wafer compared with p+ gettering. An epitaxial layer p+ on top of p- substrates with oxygen precipitates gettered 50% of the total Cu; while the other 50% of the Cu was measured in the p- substrate wafer with oxygen precipitates. Without oxygen precipitates, 100% of the spiked Cu contamination was detected inside the p+ layer. Gettering by oxygen precipitates thus occurs in the same temperature range as that where p+ silicon begins to getter Cu. Received: 3 September 2001 / Accepted: 17 October 2001 / Published online: 27 March 2002     

MeV-boron implanted layer, oxygen precipitates and poly-silicon back side combined in one silicon wafer: at what defect will Cu and Ni be gettered?

We have measured the gettering efficiencies for Cu and Ni of various silicon wafers, such as MeV-boron-implanted p- polished wafers treated with two different implantation doses of 3×10¹³ atoms/cm² B and 1×10¹⁵ atoms/cm² B, respectively. A third kind of wafer was covered with a poly-silicon back side and thermally pretreated before the gettering test to form oxygen precipitates in the bulk. The gettering test started with a reproducible spin-on spiking on the front side of the wafers in the range around 10¹² atoms/cm², followed by a thermal treatment to redistribute the metallic impurities in the wafer. Then the gettering efficiencies were measured by a novel wet chemical layer-by-layer etching technique in combination with inductively coupled plasma mass spectrometry. This led to “stratigraphical concentration profiles” of the metallic impurities in the wafer with typical detection limits of (5–10)×10¹² atoms/cm³. The concentration profiles were compared with concentration profiles found after testing the gettering efficiency of p/p+ epitaxial wafers. Almost 100% of the total intentional Cu spiking was recovered in the boron buried layer for both implantation doses. On the front surface and in the region between the front surface and the buried layer a Cu concentration ∼20 times higher than on/in p/p+ epitaxial wafers/layers was measured for the implanted specimen. The lower implantation dose led to higher Cu-concentration levels on the front surface compared to the higher implantation dose. The wafer containing a MeV-boron-implanted layer as well as oxygen precipitates and a poly-silicon back side exhibited a Cu distribution of 30/∼0/70%, respectively. Thus, the gettering by poly-silicon exceeded both the gettering effects by the buried layer and by the oxygen precipitates. Ni gettering in MeV-boron-implanted wafers exhibited other characteristics. The gettering efficiency of the buried layer was 65%, while the remaining Ni contamination was equally distributed between the front-side region and the wafer back side. A wafer containing a buried layer obtained by a 1×10¹⁵ atoms/cm³ B dose and oxygen precipitates exhibited 17% of the total Ni contamination in the boron layer, while ∼80% of the total Ni contamination was gettered by oxygen precipitates. In the case of buried layer/oxygen precipitates/poly-silicon back side the distribution was found to be 13/37/45%, thus exhibiting equal gettering strengths for oxygen precipitates and the poly-silicon back side for Ni contamination. The results were discussed in terms of segregation and relaxation-induced gettering mechanisms including different reaction rates. Received: 30 May 2001 / Accepted: 16 June 2001 / Published online: 30 August 2001     

Gettering efficiencies for Cu and Ni as a function of size and density of oxygen precipitates in p/p- silicon epitaxial wafers

We have measured the gettering efficiencies for Cu and Ni in p/p-Si epitaxial wafers. The wafers were pretreated to obtain oxygen precipitates of different sizes and densities in the bulk. Gettering tests started with a reproducible spin-on spiking in the range of 10¹² atoms/cm², followed by thermal treatment to drive-in and redistribute the impurities in the wafer. Subsequently, the wafers were analyzed by a novel stratigraphical layer-by-layer etching technique in combination with inductively coupled plasma mass spectrometry. Gettering efficiencies for Ni did not depend on oxygen precipitate sizes and densities as long as ΔO_i was larger than 0.2×10¹⁷ atoms/cm³ and the bulk micro defect densities were detectable by preferential etching (10⁷ cm^-3). In these cases, gettering efficiencies were 96–99% for Ni, while wafers not containing any measurable BMDs exhibited no detectable gettering. Cu exhibited a more complex behavior because the total Cu contamination was found to be divided into two species, one mobile and the other immobile species. A dependence on BMD size and BMD density of the Cu distributions in the wafers was also detected. Gettering effects were increased with increasing BMD densities and sizes. For BMD densities <10⁹ cm^-3, Cu was not efficiently gettered by oxygen precipitates. Even for BMD densities >10¹⁰ cm^-3, gettering effects due to oxygen precipitates were one order of magnitude lower than in heavily boron-doped silicon. Received: 19 January 2001 / Accepted: 31 January 2001 / Published online: 20 June 2001     

Modeling of Cu gettering in p- and n-type silicon and in poly-silicon

Based on experimental findings we set up calculations of numerical modeling of gettering efficiencies for Cu in various silicon wafers. Gettering efficiencies for Cu were measured by applying a reproducible spin-on contamination in the 10¹² atoms/cm² range, followed by a thermal treatment to redistribute the metallic impurity. Subsequently, the wafers were analyzed by a novel wet chemical layer-by-layer etching technique in combination with inductively coupled plasma mass spectrometry. We investigated p/p+ and n/n+ epitaxial wafers with different doping levels and different substrate-doping species. We have also investigated gettering efficiencies of phosphorus-diffused p- and n-type wafers. Heavilyboron doped silicon exhibited a gettering efficiency of ∼100%, while gettering by n+ silicon occurred for doping levels >3×10¹⁹ atoms/cm³ only. In another set of experiments we investigated the dependence of the gettering efficiency of p-type wafers with poly-silicon back sides for different cooling rates and Cu spiking levels. A strong dependence on both parameters was found. Cu gettering in p/p+ epitaxial wafers was modeled by calculating the increased solubility of Cu in p+ silicon compared to non-doped silicon taking into account the Fermi-level effect, which stabilizes donors in p+ silicon, and the pairing reaction between Cu and B. Calculated gettering efficiencies were in very good agreement with experimental results. Gettering in n+ silicon was similarly modeled in terms of pairing reactions and the Fermi-level effect. But, for n-type silicon, many experimental uncertainties existed; thus, we applied our expressions to solubility data of Hall and Racette to obtain the unknown parameters. The empirical calculations were in good agreement even with results on n/n+ wafers. For phosphorus-diffused wafers we had to consider an excess vacancy concentration of 1.2–5.5 times the equilibrium concentration to explain the experimental findings by the model. Gettering by poly-silicon back sides was simulated by solving the time-dependent diffusion equation with boundary conditions that take into account different surface reaction rates of silicon point defects. Using this advanced model, the experimentally measured gettering efficiencies were reproduced within the uncertainty of the measurement. Received: 3 September 2001 / Accepted: 4 September 2001 / Published online: 20 December 2001     

Ni reactions with surfaces: dependence of gettering efficiencies for Ni on crystal-growth conditions, back-side-gettering techniques, oxygen precipitates and thermal treatments

We have performed measurements on the gettering efficiencies for Ni in different silicon wafers. Gettering efficiencies were measured of wafers grown by different crystal-growth techniques, such as Czochralski-grown (CZ) and floating zone (FZ), as well as wafers containing crystal-originated particles (COPs) of different size and density. Lightly boron doped CZ wafers covered with an epitaxial layer were also evaluated. In another set of experiments, we compared different back-side-gettering techniques, like poly-silicon, stacking faults and He-implanted back sides and the dependence of back-side gettering on cooling rate and contamination level. Internal surfaces of oxygen precipitates were also investigated. The gettering test started with a reproducible spin-on contamination in the range around 10¹² atoms/cm² and was followed by a thermal treatment to redistribute the Ni impurity in the wafer. Subsequently, wafers were analyzed for their surface and bulk contamination by a novel layer-by-layer etching, stratigraphical technique in combination with inductively coupled plasma mass spectrometry. No detectable gettering effect of COPs was found. FZ wafers differed remarkably in their gettering behavior from CZ wafers, obviously due to differences in aggregated self-point defects. Most remarkably, the deposition process of an epitaxial layer changed the gettering behavior of p/p- wafers. Comparing the gettering efficiencies of different back sides, an extraordinarily high gettering efficiency of He-implanted voids can be anticipated, which was higher than the gettering efficiency of poly-silicon and stacking faults. High cooling rates at the end of the drive-in cycle and low contamination levels lowered the gettering efficiencies of back-side-gettering techniques, suggesting a diffusion-limited gettering process. Based on the dependence of the gettering efficiencies on different drive-in cycles, a surface reaction as a mechanistic initiation of the drive-in must be assumed. Oxygen precipitates exhibited a high gettering effect for Ni contamination. All experimental results are interpreted by available active surfaces in the gettering phases. Received: 30 May 2001 / Accepted: 16 June 2001 / Published online: 30 August 2001     

Integrity of ultrathin gate oxides with different oxide thickness, substrate wafers and metallic contaminations

The integrity of ultrathin gate oxides was investigated as a function of polished and epitaxial wafer surfaces with various gettering sites. After intentional contamination of wafers with 1×10¹¹ atoms/cm² and 5×10¹² atoms/cm² Cu and Ni by a spin-on technique of high reproducibility, we performed 0.18-μm low-thermal-budget CMOS process runs. Thermal oxides were grown with various gate oxides in the range of 5–17 nm. After a MOS-capacitor fabrication we applied a ramped current-density test to study the gate-oxide integrity. Generally, thinner gate oxides exhibited a much more robust behavior than thicker oxides. The gate-oxide integrity was strongly influenced by different gettering sites. Although a higher Ni contamination led to a higher number of gate-oxide failures, Cu contamination exhibited a higher impact on the gate-oxide integrity than Ni. Received: 12 September 2000 / Accepted: 21 September 2000 / Published online: 22 November 2000     

Level lifetimes in 3p and 3d configurations of Fe XII

Transitions between the five fine-structure levels in the 3s ²3p ³ ground configuration of Fe XII (P-like) are of interest in astrophysics and terrestrial plasma diagnostics. The decay rates give rise to level lifetimes in the millisecond range, which have been measured recently at a heavy-ion storage ring. While most of the 3s ²3p ²3d levels are short-lived, two of these levels have no E1 decay channels and may also have millisecond lifetimes. We present HFR and MCDF calculations of the E1, M1, E2 and M2 transition rates between the 3s ²3p ³, 3s3p⁴ and 3s ²3p ²3d levels and compare the lifetime results to most recent experimental data as well as to other predictions. Received 2 October 2001 / Received in final form 22 January 2002 Published online 28 June 2002     

Effects of phosphorus diffusion gettering on minority carrier lifetimes of single-crystalline,multi-crystalline and UMG silicon wafer

Phosphorus diffusion gettering, which can effectively reduce the transition-metal impurities in the bulk of Si wafer and enhance the minority carrier lifetime (MCLT), is a well-known process to improve the performances of solar cells. Especially, the appropriate gettering process is further required for manufacturing solar cells using an upgraded metallurgical-grade silicon (UMG Si) wafer. In this work, an improvement in the MCLT of the UMG Si wafer including the single-crystalline and multi-crystalline Si wafer after phosphorus diffusion gettering was confirmed by using the quasi-steady state photo-conductivity (QSSPC) measurement and the microwave photo-conductance decay (μW-PCD) method. The experimental results were compared with the MCLT variations calculated through the simulation of the Fe distributions in the Si wafers. It was also observed that the efficiency of the UMG Si solar cell increased by 0.53% due to the two-step gettering process.     

KrF excimer laser dry and steam cleaning of silicon surfaces with metallic particulate contaminants

KrF excimer laser-assisted dry and steam cleaning of single-crystal silicon wafers contaminated with three different types of metallic particles was studied. The laser fluence used was 0.3 J/cm². In the dry process, for samples cleaned with 100 laser pulses the cleaning efficiency was 91, 71 and 59% for Au, Cu and W particles, respectively, whilst in steam cleaning the efficiency is about 100% after 5 laser pulses, independently of the type of contaminant. The effects of laser irradiation on the Si surface are investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Laser processing at 0.3 J/cm² does not deteriorate the Si-wafer surface, either in dry or steam cleaning. However, the measured XPS intensity coming from the metallic component is greater on the cleaned surfaces than in the initial condition. Quantification of the XPS results, assuming a stratified overlayer model for the detected species and accounting for the presence of the metallic particles on the surface, showed that the obtained results can be explained by the formation of a fractional metallic monolayer on the cleaned surfaces, due to partial vaporisation of small particles initially present on the sample surface. This contamination of the substrate could be considered excessive for some applications and it shows that the process requires careful optimisation for the required efficiency to be achieved without degradation of the substrate. Received: 14 January 2001 / Accepted: 19 February 2001 / Published online: 20 June 2001     

A rutherford scattering study of the diffusion of heavy metal impurities in silicon to ion-damaged surface layers

Rutherford backscattering of 1.75 and 2 MeV ⁴He⁺ ions has been utilized to study the high temperature gettering of Fe, Co, Ni, Cu and Au from silicon by ion-damaged surface layers. In a typical experiment a metal film was evaporated onto one side of a silicon wafer (125 microm thick) which had received ion implantation damage (10¹⁶/cm² Si⁺ ions at 100 keV) on the opposite side; the wafer was then annealed at 900°C, usually for 30 min. The results of such experiments show that the metals studied may be divided into two classes, those which are gettered slowly - Fe, Co, and Au, and those gettered rapidly - Cu and Ni. Fe, Co, and Au were found at levels of 1×10¹³?1×10¹⁴/cm² in the damaged layer, whereas Cu and Ni appeared at levels of 6×10¹⁴ to 5 ×10¹⁶cm². The gettered level of Au, one of the “slow” group, was increased ten-fold by an equal increase in the anneal time to 300 min. The gettered Cu and Au exhibited double peaks in the scattered ion spectra, corresponding to metal concentrated at the most heavily damaged region (end of range for Si implant) and also at the outer surface, with a separation of ~ 1300 Å. A simple model is proposed to explain the slow and fast gettering, based on published interstitial diffusivities and solubilities of the five elements studied.Rutherford scattering has proven to be well suited for the quantitative identification of low levels of impurities on Si surfaces and for impurities gettered at damaged layers close to the surface.     

Stopping powers of C,N and O ions in Cr,Fe, Co,Ni, Cu and Zn in the 1 MeV region

The stopping powers of C, N, and O ions in Cr, Fe, Co, Ni, Cu and Zn in the energy range 500 keV to 2 MeV have been measured relative to that for He ions in the same material. The measurements were made utilizing energy spectra of particles backscattered from thick substrates of the target material into which a heavy material (Bi) had been previously implanted to a shallow depth. Assuming that dE/dx for He is known, the dose of implanted Bi can be determined from a He backscattering spectrum. Then this dose is used to calculate the unknown stopping power from a backscattering spectrum of the ion to be studied.

The data are analysed in terms of a function dE/dx=aE ^p +b The experimental results give values of p ranging from 0.3 to 0.5. Our absolute dE/dx values (normalized to known dE/dx values for He ions) are somewhat higher than those tabulated by Northcliffe and Schilling⁽³⁾ and considerably higher than predicted by the LSS⁽⁶⁾ theory.     

Positioning of cationic silver nanoparticle by using AFM lithography and electrostatic interaction

One-dimensional metal lines of silver nanoparticles with a nano-sized width were generated onto silicon surface by using a nano-level lithography technique, field induced oxidation (FIO) by AFM, on self-assembled monolayer-modified Si wafers. This FIO technique provided SiO₂ lines a width of less than 100 nm. Short-time immersion of partially anodized silicon surface which is covered by a cationic silanol surfactant ((CH₃O)₃SiCH₂CH₂CH₂N(CH₃)₃⁺Cl⁻)-monolayer into quaternary ammonium (HSCH₂CH₂N(CH₃)₃⁺Br⁻)-covered silver nanoparticles readily and reproducibly gave nano-metal lines of silver onto silicon wafers. Hydrophilicity of the whole wafer surface was indispensable for homogeneously wetting the anodized SiO₂ area with a nanodimensional width.     

Reactivity and magnetism of Fe/InAs(100) interfaces

Interface reaction and magnetism of epitaxially-grown Fe on InAs(100) are studied by core-level photoemission (As 3d and In 4d) and Fe 2p X-ray magnetic circular dichroism using synchrotron radiation. The reactivity of Fe/InAs(100) is relatively low compared to that of other interfaces involving deposition of 3d metals on III-V semiconductors. As a consequence, we observe a magnetic signal at Fe L_{2, 3} edges for the lowest thicknesses studied (1 ML). The atomic magnetic moment reaches a value close to that of the bulk α-Fe (2.2 μ _B) for Fe coverages exceeding 5 ML. A ferromagnetic compound with approximate stoichiometry of FeAs is formed at the interface. The orbital magnetism represents between 12 and 20% of the total momentum, due to 3d density of states depletion and to crystal-field modification of the electronic levels. These properties make the Fe/InAs(100) interface very promising for spin-tunneling devices. Received 4 April 2002 / Received in final form 13 May 2002 Published online 31 July 2002     

K +-meson production inpBe interactions atT p=2.9 GeV

The production ofK ⁺ andπ ⁺ mesons and protons inpBe collisions atT _p=2.9 GeV has been studied at the ITEP proton synchrotron. Ejectiles with a momentum ofp=545 MeV/c were observed under an emission angle ?=17°. The detectors which have been developed for the identification of kaons out of a six orders of magnitude more intense background of pions and protons are described. A cross-section ratio d²σ _K ⁺/dΩdp: d² σ _p /dΩdp: d²σ _p /dΩdp of (1±0.34):(85±1):(31±1) has been measured. Normalization with existing pion data yields an invariant differential cross sectionE·d³σ _K ⁺/d³ p=(3.1±1.2) mbGeV^?2c³sr^?1 and a total cross section of σ_tot(pBe)=(3.7±1.5) mb. These cross sections are compared with existing data and theoretical predictions. TheA dependence ofK ⁺ production in the few-GeV range is analyzed.     

n‐type silicon solar cells with surface‐passivated screen‐printed aluminium‐alloyed rear emitter

Aluminium‐doped p‐type (Al‐p⁺) silicon emitters fabricated by means of a simple screen‐printing process are effectively passivated by plasma‐enhanced chemical‐vapour deposited amorphous silicon (a‐Si). We measure an emitter saturation current density of only 246 fA/cm², which is the lowest value achieved so far for a simple screen‐printed Al‐p⁺ emitter on silicon. In order to demonstrate the applicability of this easy‐to‐fabricate p⁺ emitter to high‐efficiency silicon solar cells, we implement our passivated p⁺ emitter into an n⁺np⁺ solar cell structure. An independently confirmed conversion efficiency of 19.7% is achieved using n‐type phosphorus‐doped Czochralski‐grown silicon as bulk material, clearly demonstrating the high‐efficiency potential of the newly developed a‐Si passivated Al‐p⁺ emitter. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Intensity and detuning dependence of fine structure changing collisions in a 85 Rb magneto-optical trap

In an experimental study, the multi-ionisation of metallic clusters (Na_n) has been analysed in collisions with light ions in low charge states (H⁺, He⁺, He²⁺, O³⁺) at collision velocities below 1 a.u. Cluster ions are produced in charge states up to 5+. The average charge of the nano-particles is found to increase linearly with the variation of projectile velocity and the square of the effective projectile charge, well in agreement with the electronic stopping power of the bulk material. A fraction of 50% to 30% of the total projectile energy loss (decreasing with velocity) is transferred into vibrational modes in good agreement with recent theoretical predictions. Received 8 November 2000 and Received in final form 26 January 2001     

Comparison of profile tailing in SIMS analyses of various impurities in silicon using nitrogen,oxygen, and neon ion beams at near-normal incidence

We have performed a systematic SIMS study into the effect of (i) the chemical nature and (ii) the energy of the primary ions on the decay length which characterizes the exponential fall-off of impurity sputter profiles. The samples consisted of low resistivity, p-type Si covered with thin metallic overlayers. Bombardment was carried out at 2° off normal. Aspect (i) was investigated for tracers of Cu and Ga using N ₂ ⁺ , O ₂ ⁺ , and Ne⁺ primary ions at an energy of 5 keV/atom. The effect of the beam energy, aspect (ii), was studied for eight different tracer species and N ₂ ⁺ primary ions at energies between 2 and 5 keV/atom. In the case of Ga, was found to be shorter with N ₂ ⁺ or O ₂ ⁺ primary ions (=7.0 and 7.5 nm, respectively) than with Ne⁺ (=12 nm). This effect is attributed to beam induced formation of Si₃N₄ or SiO₂ layers, whereby the effective width of the internal distribution of intermixed Ga impurities in the Si subsystem is reduced significantly. In contrast to Ga, the decay length for Cu is smallest under bombardment with Ne⁺ (=16 nm), quite large with N ₂ ⁺ (26 nm) and extremely large with O ₂ ⁺ (2.2 m). Segregation of Cu atoms at the Si₃N₄/Si and the SiO₂/Si interface, respectively, is responsible for this depressed impurity removal rate. Within experimental accuracy the observed variation of the decay length with N ₂ ⁺ energy E [keV/atom] can be written in the form =kE ^p, where k and p are element specific parameters which range from k=1.2 nm for Pb to 10 nm for Cu and from p=0.6 for Cu and Ag to 1.0 for Pb. The results are discussed with reference to conceivable shapes of the distribution of intermixed impurity atoms.On leave from NTT Applied Electronics Laboratories, 3-9-11, Midori-cho, Musashino-shi, Tokyo 180, Japan     

Theoretical study of the MgAr molecule and its ion Mg<Superscript>+</Superscript>Ar: potential energy curves and spectroscopic constants

The adiabatic potential energy curves of the low-lying electronic states of the MgAr molecule dissociating into Mg (3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p)+Ar have been investigated. The electronic structure of the Mg-Ar molecule is calculated using [Mg²⁺] and [Ar] core pseudopotentials complemented by the core polarization operators for both atoms, considering the molecule to be a two-electron system. The derived spectroscopic constants of the ground state and lower excited states are in good agreement with available experimental and theoretical work. In addition, for the purpose of checking the pseudopotential accuracy on a simpler related system, low lying potential energy curves of the single active electron Mg⁺Ar ion are also reported and the corresponding molecular constants are compared with those in the existing literature.     

Multielectron effects in photoemission from quasi-atomic copper in Cu-phthalocyanine

Photoelectron energy distribution curves for Cu in Cu-Phthalocyanine have been investigated around the Cu 3p threshold at hv = 75 eV. A sharp 3d-electron satellite with two peaks with initial energies of 5.5 and 8.5 eV below the center of the Cu 3d main peak was found which exhibits a strong resonant enhancement at threshold. These results are explained as a two-electron shake-up multiplet with a 3p⁶3d⁸4s² + e^? final state configuration. The results are compared with metallic Cu, and the different chemical, relaxation (screening) and configuration shifts are discussed.     

Splitting of the Cr 2p ions states in some ternary sulphides and selenides

X-ray photoelectron spectroscopy was used to investigate ACr₂S₄ (, Zn, Mn, Fe, Fe:Cu) and BCr₂Se₄ (, Cu, Hg, Hg:Cu) single crystals. Well defined splitting of the Cr 2p core level has been found. The local magnetic moments of the Cr ions are responsible for the observed effect. Received 2 November 1999