Simulation of interface broadening of sputter profiled isotopic silicon layers

Abstract

The experimental SIMS profiles of Si³⁰/Si²⁸ multilayers depth profiled using 2–10 KeV Ne⁺, Ar⁺ and Xe⁺ (K. Wittmaack and D. B. Poker, Nucl. Instr. Meth. B47 (1990) 224) have been simulated using a diffusion approximation to ion mixing. Both the leading and trailing edges of peaks in the depth profiles could be fitted for all ion energies by mixing efficiencies of 25, 40 and 65 Å⁵ eV^?1 respectively. These values are larger than the estimates of 5, 11 and 15 Å⁵ eV^?1 from ballistic mixing. The additional contributions to the mixing, 20, 29 and 50 Å eV^?1, scale with cascade energy density, suggesting that stimulated motion in thermal spikes is important. A simple spike model in which cascade defects are trapped at fixed sinks agrees with the experimental mixing efficiencies.     

A SIMS study of ion beam induced migration of nitrogen in thermally oxidized silicon

The potential use of thin silicon nitroxide films as gate dielectrics in VLSI MOS devices has motivated much recent work. The present study shows that positive ion bombardment, as encountered in sputter depth profiling or ion implantation, can induce considerable movement of nitrogen in thin thermal oxide films on silicon. Low energy N⁺₂ implants are performed in-situ in a SIMS apparatus and are subsequently depth profiled. The effect of implant dose and oxide thickness are examined and comparisons are made to films prepared by rapid thermal nitridation and LPCVD. Profiles obtained under O⁺₂, O^-, and Cs⁺ bombardment are also compared. SIMS depth profiles of implanted 200 Å oxides using positive ion bombardment show a depletion of nitrogen near the surface, a shoulder in the nitrogen concentration near the Si-SiO₂ interface, and a peak in this concentration at the interface. Negative ion bombardment did not induce a shoulder-peak structure at the interface. The implications of these results are discussed.     

A SIMS depth profiling study of the hydration layer formed at polycrystalline tin oxide surfaces by atmospheric exposure

Dynamic SIMS has been used to examine the hydrated surface layer formed on polycrystalline tin oxide films by exposure to atmospheric humidity. A strongly hydrated layer is found to occur within approximately the top 6 Å of the surface with a hydrogen-rich region extending to a depth of about 20 Å into the bulk. The Sn⁺ sputtering yield for the hydrated layer is greater than that of the bulk-oxide material. It is shown that the profile of the positive-ion 121 amu peak, which is attributed primarily to ¹²⁰SnH⁺, appears to yield the least ambiguous measure of the extent of hydration. High-temperature annealing of the films is found to affect the bulk-sputtering properties possibly by altering the structure of the film and/or the form of the hydrogen incorporated in the bulk of the film.     

Application of mechanical scanning to ion implantation

Abstract

Atomic depth profiles from Be-implanted Si have been examined as a function of implant fluence and annealing, and the results have been correlated with theoretically calculated implantation induced damage profiles. The Be atomic depth profiles were obtained by secondary ion mass spectrometry (SIMS) techniques from samples implanted at 300 keV to fluences ranging from 2 × 10¹² to 10¹⁵ cm^?2. Subsequent to annealing at 600°C for 30 min, the Be SIMS profiles exhibited anomalous redistribution effects. The Be profiles obtained from the annealed samples had the same general features as the depth distribution of implant energy deposited into damage, based on Brice's¹ calculations. The correlation of the SIMS atomic profiles and the theoretical damage profiles indicated that Be “decorates” the implantation induced damage regions while redistributing during the annealing process.     

Use of cluster secondary ions for minimization of matrix effects in the SIMS depth profiling of La/B4C multilayer nanostructures

The elemental composition of La/B₄C multilayer metal structures is studied using SIMS on a TOF.SIMS-5 experimental setup. Analysis conditions that make it possible to considerably enhance the depth resolution are found. They include using low-energy O₂⁺ and Cs⁺ ions for sputtering and cluster secondary ions for registering matrix elements. The roughness evolution in the etching crater region is studied in a layer-by-layer analysis. It is shown that, at an incidence angle of 45° for sputtering ions, the rms roughness increases slightly (from an initial value of 0.5–0.7 nm) in the etching crater of the (La/B₄C)₇₀/Si structure at a depth of 0.5 μm. The profiles of elements in multilayer structures grown using two different types of magnetron systems with stationary and high-frequency discharges are compared. The main contaminations in the structures are determined.     

Depth profiling using C60 SIMS—Deposition and topography development during bombardment of silicon

A C₆₀⁺ primary ion source has been coupled to an ion microscope secondary ion mass spectrometry (SIMS) instrument to examine sputtering of silicon with an emphasis on possible application of C₆₀⁺ depth profiling for high depth resolution SIMS analysis of silicon semiconductor materials. Unexpectedly, C₆₀⁺ SIMS depth profiling of silicon was found to be complicated by the deposition of an amorphous carbon layer which buries the silicon substrate. Sputtering of the silicon was observed only at the highest accessible beam energies (14.5 keV impact) or by using oxygen backfilling. C₆₀⁺ SIMS depth profiling of As delta-doped test samples at 14.5 keV demonstrated a substantial (factor of 5) degradation in depth resolution compared to Cs⁺ SIMS depth profiling. This degradation is thought to result from the formation of an unusual platelet-like grain structure on the SIMS crater bottoms. Other unusual topographical features were also observed on silicon substrates after high primary ion dose C₆₀⁺ bombardment.     

Optical phase shift analysis of n + 32S

A theory of optical phase shifts (OPS) developed for analyzing low-energy resonant cross sections is applied to analyze the total cross section for low-energy neutrons on n + ³²S. Phase shifts of a model for the optical potential are calculated at complex energies and compared with the OPS in order to determine the depth of its real and absorptive potentials.     

Low energy SIMS study on multi-built-up layer of cobalt(II) stearate

The surface of multi-built-up layer of cobalt(II) stearate on copper substrate has been studied by means of low energy SIMS. The sampling surfaces were impacted with 700 eV, 8 × 10^?7 Å/cm², Ar⁺ ions. In each group (odd-numbered accumulated and even-numbered accumulated samples), the ⁵⁸Co⁺ intensities at the initial bombardment responded to the number of the accumulation, respectively. The ⁵⁸Co⁺ ion intensities from even-numbered accumulated samples were observed more than the expected values from a proportionality between the accumulation number and ⁵⁸Co⁺ intensities.     

Corrosion behavior of low energy,high temperature nitrogen ion-implanted AISI 304 stainless steel

This work presents the results of a low-energy nitrogen ion implantation of AISI 304 type stainless steel (SS) at a moderate temperature of about 500°C. The nitrogen ions are extracted from a Kauffman-type ion source at an energy of 30 keV, and ion current density of 100 μA cm⁻². Nitrogen ion concentration of 6 × 10¹⁷, 8 × 10¹⁷ and 10¹⁸ ions cm⁻², were selected for our study. The X-ray diffraction results show the formation of CrN polycrystalline phase after nitrogen bombardment and a change of crystallinity due to the change in nitrogen ion concentration. The secondary ion mass spectrometry (SIMS) results show the formation of CrN phases too. Corrosion test has shown that corrosion resistance is enhanced by increasing nitrogen ion concentration.      

Improved quantitative analysis of Cu(In,Ga)Se2 thin films using MCs+-SIMS depth profiling

The chalcopyrite semiconductor, Cu(InGa)Se₂ (CIGS), is popular as an absorber material for incorporation in high-efficiency photovoltaic devices because it has an appropriate band gap and a high absorption coefficient. To improve the efficiency of solar cells, many research groups have studied the quantitative characterization of the CIGS absorber layers. In this study, a compositional analysis of a CIGS thin film was performed by depth profiling in secondary ion mass spectrometry (SIMS) with MCs⁺ (where M denotes an element from the CIGS sample) cluster ion detection, and the relative sensitivity factor of the cluster ion was calculated. The emission of MCs⁺ ions from CIGS absorber elements, such as Cu, In, Ga, and Se, under Cs⁺ ion bombardment was investigated using time-of-flight SIMS (TOF-SIMS) and magnetic sector SIMS. The detection of MCs⁺ ions suppressed the matrix effects of varying concentrations of constituent elements of the CIGS thin films. The atomic concentrations of the CIGS absorber layers from the MCs⁺-SIMS exhibited more accurate quantification compared to those of elemental SIMS and agreed with those of inductively coupled plasma atomic emission spectrometry. Both TOF-SIMS and magnetic sector SIMS depth profiles showed a similar MCs⁺ distribution for the CIGS thin films.     

Inelastic electron scattering in nickel

Using a 200 keV electron spectrometer, with an energy resolution of ～0.25 eV and a momentum resolution of ～0.2 A^-1, we have measured the energy loss spectra for transmission of electrons through thin (～600 Å) Ni films. These results address the general question of the validity of momentum transfer estimates in electron loss scattering.Using low-energy electron backscattering, we have observed the dipole forbidden M₁ transition at 112 eV. For high-energy scattering, we have observed this transition only at high momentum transfer (q? 2 A^-1). These results indicates sizable contributions from high momentum transfer collisions in the low-energy experiments.     

Model multilayer structures for three-dimensional cell imaging

The prospects for SIMS three-dimensional analysis of biological materials were explored using model multilayer structures. The samples were analyzed in a ToF-SIMS spectrometer equipped with a 20 keV buckminsterfullerene (C₆₀⁺) ion source. Molecular depth information was acquired using a C₆₀⁺ ion beam to etch through the multilayer structures at specified time intervals. Subsequent to each individual erosion cycle, static SIMS spectra were recorded using a pulsed C₆₀⁺ ion probe. Molecular intensities in sequential mass spectra were monitored as a function of primary ion fluence. The resulting depth information was used to characterize C₆₀⁺ bombardment of biological materials. Specifically, molecular depth profile studies involving dehydrated dipalmitoyl-phosphatidylcholine (DPPC) organic films indicate that cell membrane lipid materials do not experience significant chemical damage when bombarded with C₆₀⁺ ion fluences greater than 10¹⁵ ions/cm². Moreover, depth profile analyses of DPPC-sucrose frozen multilayer structures suggest that biomolecule information can be uncovered after the C₆₀⁺ sputter removal of a 20 nm overlayer with no appreciable loss of underlying molecular signal. The experimental results support the potential for three-dimensional molecular mapping of biological materials using cluster SIMS.     

Microfabrication techniques used in Japan for VLSI and other devices

Lead films of thickness 100 Å, 250 Å. and 350 Å were vacuum deposited on AI and laser treated in air using single pulses (7 ns FWHM) from a Nd: glass laser operating in TEM₀₀ mode, at peak energy densities of 1.5-5.0 J/cm². Rutherford back-scattering of 1.8 MeV He⁺ ions was employed to determine the depth profiles of Pb in Al. Up to about 1.4 J/cm², we observe only evaporation loss of Pb and formation of Pb-rich cells on the surface. At higher energies, liquid phase diffusion of Pb is observed up to 4 J/cm², beyond which convection effects are seen. A quantitative analysis of data for 350 Å film at 3.0 J/cm² shows evidence of nonequilibrium segregation effects.     

Effect of MeV electron irradiation on the free volume of polyimide

The free volume of the microvoids in the polyimide samples, irradiated with 6 MeV electrons, was measured by the positron annihilation technique. The free volume initially decreased the virgin value from ～13.70 to ～10.98 Å³ and then increased to ～18.11 Å³ with increasing the electron fluence, over the range of 5?×?10¹⁴ – 5?×?10¹⁵ e/cm². The evolution of gaseous species from the polyimide during electron irradiation was confirmed by the residual gas analysis technique. The polyimide samples irradiated with 6 MeV electrons in AgNO₃ solution were studied with the Rutherford back scattering technique. The diffusion of silver in these polyimide samples was observed for fluences >2?×?10¹⁵ e/cm², at which microvoids of size ≥3 Å are produced. Silver atoms did not diffuse in the polyimide samples, which were first irradiated with electrons and then immersed in AgNO₃ solution. These results indicate that during electron irradiation, the microvoids with size ≥3 Å were retained in the surface region through which silver atoms of size ～2.88 Å could diffuse into the polyimide. The average depth of diffusion of silver atoms in the polyimide was ～2.5 μm.     

Chiral recognition in electron scattering by S- and R-2-butanol

Experiments are described involving the low-energy scattering of electrons from the two optical enantiomers S- and R-2-butanol. Using a synchrotron radiation photoionisation source on the ASTRID storage ring, scattering spectra are reported between a few meV and 140 meV at an electron energy resolution of 1.6 meV. These data show electron dichroism at 35 meV, for scattering into the backward hemisphere, with a cross-section of 202.1 ± 8.0 Ų (4σ) and 182.8 ± 9.0 Ų (4σ) for S- and R-, respectively, with the racemic mixture lying at the average value. No dichroic effect could be detected in the integral cross-section within the accuracy of the experiment. A brief qualitative discussion is given of a possible electron dissociative attachment mechanism for the creation of homochirality in space, including a new feature involving feedback through enhancement of electron polarisation through scattering from non-racemic mixtures of enantiomers.     

X-ray photoelectron spectroscopic studies of PbO surfaces bombarded with He+, Ne+, Ar+, Xe+ and Kr+

The surface composition of PbO has been studied with X-ray photoelectron spectroscopy after bombardment with several inert gas ions of 400 eV. The results show reduction of PbO to metallic Pb with the degree of damage following the order He⁺ > Ne⁺ >Ar⁺. Both Kr⁺ and Xe⁺ did not reduce the oxide. The depth of damage varied from ≈9 Å for He⁺ to ≈1 Å for Ar⁺ bombardment. The results were compared to a collisional and a thermal model of the sputtering process.     

Changes in gold concentration at the surface of a AuCu alloy sputtered at low temperature

A series of depth profiles in Au_0.56Cu_0.44 produced by bombarding at ? 120°C with an argon ion beam of 2 keV energy and current densities of 4, 6, 12 and 24 μA/cm², respectively, are presented. Gold is enriched in the top layer but the concentration rapidly decreases to a minimum at an average depth of about 2 Å, then increases slowly and saturates at about 30 Å. The change in the composition at this point is called a dip. The magnitude of the dip depends on the ion current density, but the depth is nearly independent of it. Segregation and diffusion effects in establishing the surface composition are stressed.     

Improvedp/n junctions in Ge-doped GaAs grown by molecular beam epitaxy

The effects of single-pulse ruby laser irradiation have been investigated in Si samples with disorder layers located at a depth of 2000 Å from the crystal surface and extending up to 8000 Å. This disorder was obtained by implantation with 350 keV N⁺ to a fluence of 2×10¹⁶/cm². Channeling, diffraction and transmission electron microscopy were used to characterize the structure of the irradiated layers. After 1.5 J/cm² irradiation the damaged layer reorders partially, while for about 2.0J/cm² the surface single crystal becomes polycrystalline. At a higher energy density all the material undergoes the transition to single crystal. Calculations based on the liquid model accounts in part for the experimental results.     

Correlation between Si diffusion and Si-induced disordering in AlGaAs/GaAs superlattices

Compositional disordering of AlGaAs superlattices induced by Si ion implantation and subsequent annealing has been studied by secondary ion mass spectrometry (SIMS). Distinct correlation is found between the induced disordering and the rapid Si diffusion which occurs above the critical concentration of about 3×10¹⁸ cm⁻³. The annealing-condition dependence of the disordering suggests that AlGa intermixing is induced by the vacancy flow enhanced by the Si_IIISi_V pair movement which causes the rapid Si diffusion. SIMS depth profiles of the heat treated superlattices co-doped with Si and Be do not show any appreciable Si diffusion and induced disordering. This is well-explained by the formation of SiBe pairs which prevents that of Si_IIISi_V pairs.