Secondary Ion Mass Spectrometry Experiments Research Articles

Sputtering simulations of organic overlayers on metal substrates by monoatomic and clusters projectiles

This paper reviews our recent work on computer simulations of monoatomic and cluster bombardment of metal and organic surfaces. The investigated surfaces are irradiated with keV monoatomic (C, Ar, Ga) and polyatomic (C 60) projectiles that are recognized as valuable sources for desorption of high mass particles in secondary ion and neutral mass spectrometry (SIMS/SNMS) experiments. The analysis of the results reveals that the C 60 ion beam enables to perform chemical imaging with higher sensitivity, better depth resolution, and lower contamination than monoatomic projectiles with similar kinetic energy. For monoatomic projectiles, the development of a linear collision cascade is the predominant mechanism responsible for the ejection of particles. In contrast, strongly nonlinear, collective processes occur during C 60 bombardment. These nonlinear processes lead to the enhanced removal of material and the formation of a macroscopic crater. This paper presents theoretical insight into possible mechanisms responsible for the observed behavior with the emphasis on the phenomena important to the SIMS/SNMS community.

Magnetization of ultrathin (Ga,Mn)As layers

Kerr rotation and superconducting quantum interference device magnetometry measurements were performed on ultrathin $({\mathrm{Ga}}_{0.95}{\mathrm{Mn}}_{0.05})\mathrm{As}$ layers. The thinner layers (below 250 \AA{}) exhibit magnetic properties different than those of thicker ones, associated with different microstructure, and some degree of inhomogeneity. The temperature dependence of the field-cooled magnetization of the layers is recorded after successive low temperature annealings. While the Curie temperature of the thicker layer (250 \AA{}) is nearly unchanged, the critical temperature of the thinner layers is enhanced by more than 23 K after two annealings. Secondary ion mass spectrometry experiments on similar layers show that Mn is displaced upon annealing. The results are discussed considering a possible segregation of substitutional and interstitial Mn atoms at the surface of the (Ga,Mn)As layers.

Enhancement of sputtering yields due to C60 versus Ga bombardment of Ag[111] as explored by molecular dynamics simulations.

The mechanism of enhanced desorption initiated by 15-keV C60 cluster ion bombardment of a Ag single crystal surface is examined using molecular dynamics computer simulations. The size of the model microcrystallite of 165,000 atoms and the sophistication of the interaction potential function yields data that should be directly comparable with experiment. The C60 model was chosen since this source is now being used in secondary ion mass spectrometry experiments in many laboratories. The results show that a crater is formed on the Ag surface that is approximately 10 nm in diameter, a result very similar to that found for Au3 bombardment of Au. The yield of Ag atoms is approximately 16 times larger than for corresponding atomic bombardment with 15-keV Ga atoms, and the yield of Ag3 is enhanced by a factor of 35. The essential mechanistic reasons for these differences is that the C60 kinetic energy is deposited closer to the surface, with the deeply penetrating energy propagation occurring via a nondestructive pressure wave. The numbers predicted by the model are testable by experiment, and the approach is extendable to include the study of organic overlayers on metals, a situation of growing importance to the SIMS community.

Predictive Monte Carlo ion implantation simulator from sub-keV to above 10 MeV

In this paper is reported a general and accurate binary-collision-approximation- (BCA-)based Monte Carlo ion implantation model for implants into crystalline silicon. The combination of an improved semiempirical electronic stopping power model and Ziegler-Biersack-Littmark universal potential enables us to simulate a wide variety of implant species with only two different electronic stopping parameters for different implant species. With the model parameters fixed for a given implant species, excellent agreement is found with experimental secondary ion mass spectroscopy data for the energy range from sub-keV to above 10 MeV, and for different implant directions including random equivalent orientation, 〈100〉, 〈111〉, and 〈110〉 channeling directions. When compared with other BCA-based Monte Carlo simulators, it is demonstrated that more accurate results can be obtained for ultralow energy and very high energy implants. Furthermore, it is shown that, while the existing ion implantation simulators with the electronic stopping power based on the effective charge theory fail to predict the long tails of the deeply channeled implant species (such as Al), our model can predict these long tails successfully. Finally, an efficient damage model is also presented, which requires only one additional free parameter to accurately account for the damage accumulation and dechanneling effect. For high dose implants, substantial speed improvement over MARLOWE-based Monte Carlo simulators is observed.

Be diffusion behavior in InGaAs, InGaAsP and InGaAs/InGaAsP GSMBE structures

Results on Be diffusion during post-growth rapid thermal annealing at 750°C and 850°C in InGaAs, InGaAsP homostructures and InGaAs/InGaAsP heterostructures, grown by gas source molecular beam epitaxy, are reported. Profiles calculated with the kick-out model can be fitted to experimental secondary ion mass spectrometry profiles assuming Be to diffuse as neutral interstitial species.

Plasma deposition of Ultrathin polymer films on carbon nanotubes

Ultrathin films of pyrrole were deposited on the surfaces of carbon nanotubes using a plasma polymerization treatment. High-resolution electron transmission microscopy images revealed that an extremely thin film of the polymer layer (2∼7 nm) was uniformly deposited on the outer and inner surfaces of the nanotubes. The nanotubes of all sizes exhibited equally uniform ultrathin films, indicating well-dispersed nanotubes in the fluidized bed reactor during the plasma treatment. In particular, the inner wall of the nanotube was also coated with a uniform ultrathin film of only ∼1–3 nm. Time-of-flight secondary ion mass spectroscopy experiments confirmed the highly branched and cross-linked polymer thin films on the carbon nanotubes. The plasma deposition mechanism is discussed in this letter.

SPM and TOF-SIMS investigation of the physical and chemical modification induced by tip writing of self-assembled monolayers

The nanoelectrochemical modification of alkyl self-assembled monolayers (SAMs) obtained on hydrogenated silicon surfaces via radical-initiated reactions of 1-octadecene has been investigated. Scanning Probe Microscopy (SPM) showed that the modification of the organic layer occurs by applying either positive or negative biases to the tip at a threshold of about ±5 V. When the bias absolute value was ≤6 V, the height of the monolayer was only faintly modified, whereas a consistent increase in tip/sample friction force was observed, in agreement with the formation of hydrophilic moieties at the organic surface. In addition to the increase of friction, bias absolute values larger than 6 V led to a significant raise of the surface height, the application of negative biases resulting in stronger effects. This suggests the occurrence of a concomitant growth of silicon oxide underneath the organic layer. Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) experiments, including chemical imaging and analysis of the retrospective spectra, were performed by writing patterns of some microns in size on the SAM. These experiments allowed to investigate the features of the chemical modification as a function of the applied bias. Positive spectra from the modified regions display the presence of CxHyO and CxHyN type peaks that increase with the tip bias, whereas an intensity reduction of the SiCxHy signals with respect to the unmodified regions was observed.

Secondary ion mass spectrometry using cluster primary ion beams

We have a developed a capability for conducting cluster secondary ion mass spectrometry (SIMS) experiments on commercially available SIMS instrumentation. This paper reviews our recent work on cluster ion source development, elemental depth profiling with cluster primary ion beams and the use of cluster ion beams for organic surface characterization. An area of particular interest is the observation that beam-induced damage for some organic materials is substantially reduced under cluster bombardment. This unique feature of cluster SIMS is utilized for molecular depth profiling of selected polymer films and for studying the spatial distribution of high explosive particles by SIMS imaging. We also describe recent studies that may provide additional insight into possible mechanisms for the molecular secondary ion yield enhancement observed for organic thin films under cluster bombardment.

Influence of increasing zinc contents in brass in the early stages of corrosion investigated by in-situ TM-AFM and SIMS.

In-situ tapping mode atomic force microscopy (TM-AFM), a powerful, high-resolution imaging technique for determining the structure of surfaces and ex-situ secondary ion mass spectroscopy (SIMS), a multielement, high-depth-resolution method, were used to examine the influence of increasing zinc contents in brass in the early stages of corrosion. Four different samples (pure Cu, pure Zn, Cu/Zn=90/10 wt% and Cu/Zn=70/30 wt%) were studied in order to determine their chemical behaviour under various atmospheric conditions. The in-situ TM-AFM investigations were carried out in synthetic air with 60% relative humidity (RH) and 80% RH with 250 ppb SO(2). The samples for the ex-situ SIMS experiments were weathered over a period of 60 h in 80% RH and 250 ppb SO(2). The in-situ TM-AFM investigations have shown that an increasing Zn content in brass increases the corrosion rate.

Interfacial Particle Bonding Via an Ultrathin Polymer Film on Al2O3 Nanoparticles by Plasma Polymerization

To study interfacial particle-to-particle bonding mechanisms, an ultrathin film of pyrrole was deposited on alumina nanoparticles using a plasma polymerization treatment. High resolution transmission electron microscopy experiments showed that an extremely thin film of the pyrrole layer (2 nm) was uniformly deposited on the surfaces of the nanoparticles. In particular, the particles of all sizes (10–150 nm) exhibited equally uniform ultrathin films indicating well-dispersed nanoparticles in the fluidized bed during the plasma treatment. Time-of-flight secondary ion mass spectroscopy experiments confirmed the nano-surface deposition of the pyrrole films on the nanoparticles. The pyrrole-coated nanoparticles were consolidated at a temperature range (approximately 250 °C) much lower than the conventional sintering temperature. The density of consolidated bulk alumina has reached about 95% of the theoretical density of alumina with only a few percent of polymer in the matrix. After low-temperature consolidation, the micro-hardness test was performed on the bulk samples to study the strength that was related to particle-particle adhesion. The underlying adhesion mechanism for bonding of the nanoparticles is discussed.

Dynamical screening effects in surface ionization of mono(di)atomic fragmentation: A finite size approach

An atomic particle ejected with kinetic energy K from a sample is not perfectly followed by the electron screen neutralizing it in the sample. We study this phenomenon on a limited-size sample with six atoms and electrons. We use a complete determinant basis model, already presented in a previous paper, more accurate than the usual Hartree–Fock one-determinant method. We calculate the R + ionization probability of Li, Na and K at various energies. A study of R +( K 2) is also performed, our result R +( K 2)> R +( K) agrees with the general observation in secondary ion mass spectroscopy experiments on metals.

Improved binary collision approximation ion implant simulators

An efficient binary collision approximation (BCA) ion implant code with good prediction capabilities for semiconductor materials (Si, GaAs, SiC) with only one fitting parameter for low implantation doses is presented. It includes specific interatomic potentials and recent improvements in physical models for inelastic stopping. A periodic ab initio full bond electron density for the target is used. Damage accumulation is supported using a modified Kinchin–Pease model [G. H. Kinchin and R. S. Pease, Rep. Prog. Phys. 18, 1 (1955)]. Also, some of the BCA integration algorithms and target selection procedure have been refined. An algorithm commonly used for statistical noise reduction has been modified to also improve the noise reduction in the lateral and shallow zones. The agreement with experiments is good, even under channeling conditions and for different target materials. A comparison with experimental secondary ion mass spectroscopy results for several projectiles and targets is presented.

Structural studies of erbium-implanted LiNbO 3 single crystals

Some LiNbO 3 Y-cut single crystals ((1 0 0) in hexagonal notations) have been implanted with erbium ions and annealed at various temperatures (300, 500 and 1100 °C) for different durations (1, 20 and 100 h). X-ray absorption spectroscopy (XAS) measurements have been then performed at Nb K edge and erbium L III edge in order to provide some information about the structure around the niobium ion of the matrix and the implanted dopant, respectively. A wide angle X-ray spectroscopy (WAXS) study has also been achieved to have some knowledge about the global structure of the Er-implanted layer after annealing. Finally secondary ion mass spectrometry (SIMS) experiments have been carried out to estimate the erbium concentration in the implanted zone after the annealing treatment. The data analysis enables us to conclude that the two annealing temperatures, 500 and 1100 °C, appears like steps as well in the crystallization state of the implanted matrix, as in the local surrounding of the incorporated trivalent rare earth, as in the nature of the solid solutions which appear after the post-implantation annealing treatment. This is probably due to the fact that the Er diffusion strongly increases between 500 and 1100 °C. Indeed, the diffusion coefficient value of the trivalent rare earth is weak for an annealing temperature of 500 °C, while it is high for an annealing temperature of 1100 °C, leading to processes which are governed by kinetic laws in the first case and by thermodynamic laws in the second case.

PbZr0.52Ti0.48O3 and SrBi2Nb2O9 ferroelectric oxides integrated with YBa2Cu3O7 superconductor in multilayers epitaxially grown by pulsed laser deposition

Multilayers of PbZr 0.52 Ti 0.48 O 3 (PZT) or SrBi2Nb 2 O 9 (SBN) ferroelectrics (F) and YBa 2 Cu 3 O 7 (YBaCuO) superconductor (S) have been grown by pulsed laser deposition. F/S or S/F bilayers as well as S/F/S trilayers deposited on SrTiO 3 and MgO were epitaxially grown, as evidenced by x-ray diffraction (XRD) in θ-2θ and (p-scans modes, reflection high energy electron diffraction (RHEED) or electron channeling patterns (ECP). Superconducting YBaCuO films deposited on PZT exhibit a critical temperature, T c , of about 86 K slightly below the value routinely obtained in the same deposition conditions on bare (100)SrTiO 3 substrate (typically 88-89 K). By contrast, the T c of YBaCuO films on SBN, either on (100)SrTiO 3 or on (100)MgO is close to 88-89 K suggesting that SBN can be a good candidate as ferroelectric buffer layer for the growth of YBaCuO, especially on MgO for which a graphoepitaxial mechanism tends to limit the YBaCuO growth quality. In the case of F/S layers, hysteresis loops of PZT on YBaCuO show a saturated polarization larger than 30 μC/cm 2 and a coercive field of about 70 kV/cm. Secondary ion mass spectrometry (SIMS) experiments have been performed in order to correlate interdiffusion mechanisms with both structural data and physical properties.

Static secondary ion mass spectrometry to monitor solid-phase peptide synthesis

Insights into the direct monitoring of supported peptide synthesis were realized through the design of time of flight static secondary ion mass spectrometry (TOF-S-SIMS) experiments. The mass spectrometric method was carried out at the resin bead level and was found reproducible (intra- and inter-day assays), sensitive (femtomol level) and non-destructive (only 0.01% of the peptides were destroyed by the primary ion beam bombardment). The nature of the peptide–resin linkage governed the recovery of ions characterizing the whole peptide sequence. A S-SIMS cleavable bond was thus required solely in that position to achieve the release of the growing structures from the insoluble support into the gas phase without any fragmentation. Results are presented with standard solid-phase resins allowing linkage through an amide or an ester bond. The latter was orthogonally broken upon the bombardment and thus constituted a convenient S-SIMS cleavable bond.

P-to-n type-conversion mechanisms for HgCdTe exposed to H2/CH4 plasmas

The role of hydrogen incorporation in H2/CH4 reactive ion etching (RIE) induced type-conversion of p-type HgCdTe is investigated. A model is proposed in which hydrogen is incorporated into the HgCdTe crystal lattice in at least three different forms. It is proposed that the junction formation mechanism is a mixture of RIE-induced damage and Hg interstitial formation to which hydrogen forms strong bonds, and hydrogen-induced neutralization of acceptors. Confirmation of the model is presented based on experimental secondary ion mass spectroscopy of RIE-induced junctions, transport measurements reported previously, and initial diode bake stability testing.

A multipurpose apparatus for studying the interaction of low energy ions with solid state surface

A new concept of a multipurpose device for the investigation of a low energy in beam interactions with solid state surfaces is realized in an apparatus for surface analysis by means of low energy ion scattering (LEIS), direct recoil spectrometry (DRS), mass spectrometry of recoil ions (MRSI), and secondary-ion mass spectrometry (SIMS). This device provides broad alkali ion beams, homogeneous and much more intense than in standard commercial realizations available, as well as various high quality intense gas ion beams. A long tube with a magnetic quadrupole triplet is introduced. This construction disjoins the high vacuum from the ultrahigh vacuum (UHV) parts of the system and makes possible low cost differential pumping. The use of an UHV broad alkali ion beam in SIMS experiments should importantly decrease crater effect problems without use of rastering and electrical gating systems. In order to demonstrate the possibilities and the advantages of the proposed concept, we present the results of experimental analysis of primary ion beam characteristics, as well as LEIS and DRS experimental results obtained by use of gas ion beams scattered from silver polycrystalline surfaces. This design concept results in a simple and low cost device with operating characteristics comparable to those of high quality devices of similar type.

Copper gettering at half the projected ion range induced by low-energy channeling He implantation into silicon

He + ions were implanted at 40 keV into Si 〈100〉 channel direction at room temperature (RT) and at 350 °C. The Si samples were subsequently doped with Cu in order to study the gettering of Cu atoms at the defective layer. A subsequent annealing at 800 °C was performed in order to anneal the implantation damage and redistribute the Cu into the wafer. The samples were analyzed by Rutherford backscattering channeling and transmission electron microscopy techniques. The Cu distribution was measured by secondary ion mass spectrometry (SIMS). The SIMS experiments show that, while the 350 °C implant induces gettering at the He projected range (Rp) region, the same implant performed at RT has given as a result, gettering at both the Rp and Rp/2 depths. Hence, this work demonstrates that the Rp/2 effect can be induced by a light ion implanted at low energy into channeling direction.

Acceptor ion-implantation in SiC

Single and multiple energy Al, B and Ga ion-implantations were performed into n-type 6H–SiC epitaxial layers. Empirical formulae for the range statistics of the implant depth distributions in the energy range 50 keV–4 MeV were developed by analyzing the experimental secondary ion mass spectrometry (SIMS) implant depth profiles. The Al and Ga implants were electrically activated at an annealing temperature of 1500°C, but the B implants required an annealing temperature of 1650°C. In this study annealings were performed by using AlN or graphite encapsulant. The Al and Ga implants are thermally stable but the B implants showed redistribution during annealing.

SiO 2 thickness determination by x-ray photoelectron spectroscopy, Auger electron spectroscopy, secondary ion mass spectrometry, Rutherford backscattering, transmission electron microscopy, and ellipsometry

As the Rp of ion implants steadily decreases an ever-increasing percentage of the implant species lies in the oxide layer and is, therefore, not electrically active. For this reason, it is important to have analytical techniques capable of accurately measuring the thickness of ultrathin oxide layers. A round-robin study was performed on a series of SiO2 films ranging from 0.3 to 20 nm in order to evaluate the advantages and disadvantages of five commonly used analytical techniques. High-resolution cross-section transmission electron microscopy (TEM) offers the only true measurement of oxide thickness because no density assumptions are made. In this study, TEM is used as the standard for all the other techniques. X-ray photoelectron spectroscopy and Auger electron spectroscopy offer precise measurements for ultrathin (<3 nm) films, but are limited for thicker films (>15 nm) due to the exponential decay functions that describe the sampling depth in both techniques. Secondary ion mass spectrometry (SIMS) has historically been used for characterizing relatively thick films (>10 nm) but not for thinner films because of atomic mixing effects. Encapsulating oxides with amorphous silicon prior to performing a SIMS experiment can negate these effects. A comparison of SIMS on encapsulated and as received films is made. Rutherford backscattering is an excellent technique for determining oxide thickness over a wide thickness range by channeling the Si signal from the crystalline substrate and analyzing oxygen from the amorphous oxide. Ellipsometry, being both rapid and low cost, is one of the most widely used techniques capable of accurate measurements on thick films (>10 nm).