Auger electron spectroscopy depth profiling studies on stationary and rotated samples of a new model metal/semiconductor multilayer structure

Multilayer structures for application in microelectronics are becoming increasingly complex. A sputter deposited multilayer structure composed of chromium, nickel and silicon layers with a total thickness of 310 nm on a smooth silicon substrate was characterized by transmission electron microscopy (TEM) and by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) depth profiling. AES depth profiles of the Ni/Cr/Si multilayers were obtained with Ar⁺ ion bombardment at various angles of incidence using stationary and rotated samples. In some cases a strong influence of semiconductor structure on the experimentally obtained metal-metal and metal-semiconductor interface widths was observed. Owing to ion beam induced Si(LVV) Auger electrons in the crater wall of the Ni/Cr/Si sample, a distortional influence on depth resolution during simultaneous AES analysis and ion sputtering was found. Silicide formation during sputtering at the silicon-metal interfaces was confirmed by XPS. The measured compositional depth profiles are explained with respect to the influence of polycrystalline metallic and amorphous semiconductor structures; the effects of ion beam induced topography, atomic mixing and silicide formation are discussed.     

Depth resolution during C60+ profiling of multilayer molecular films

Time-of-flight secondary ion mass spectrometry is utilized to characterize the response of Langmuir-Blodgett (LB) multilayers under the bombardment by buckminsterfullerene primary ions. The LB multilayers are formed by barium arachidate and barium dimyristoyl phosphatidate on a Si substrate. The unique sputtering properties of the C60 ion beam result in successful molecular depth profiling of both the single component and multilayers of alternating chemical composition. At cryogenic (liquid nitrogen) temperatures, the high mass signals of both molecules remain stable under sputtering, while at room temperature, they gradually decrease with primary ion dose. The low temperature also leads to a higher average sputter yield of molecules. Depth resolution varies from 20 to 50 nm and can be reduced further by lowering the primary ion energy or by using glancing angles of incidence of the primary ion beam.     

Carbon-13 labeled polymers: an alternative tracer for depth profiling of polymer films and multilayers using secondary ion mass spectrometry

13C labeling is introduced as a tracer for depth profiling of polymer films and multilayers using secondary ion mass spectrometry (SIMS). Deuterium substitution has traditionally been used in depth profiling of polymers but can affect the phase behavior of the polymer constituents with reported changes in both bulk-phase behavior and surface and interfacial interactions. SIMS can provide contrast by examining various functional groups, chemical moieties, or isotopic labels. 13C-Labeled PS (13C-PS) and unlabeled PS (12C-PS) and PMMA were synthesized using atom-transfer radical polymerization and assembled in several model thin-film systems. Depth profiles were recorded using a Cameca IMS-6f magnetic sector mass spectrometer using both 6.0-keV impact energy Cs+ and 5.5-keV impact energy O2+ primary ion bombardment with detection of negative and positive secondary ions, respectively. Although complete separation of 12C1H from 13C is achieved using both primary ion species, 6.0-keV Cs+ clearly shows improved detection sensitivity and signal-to-noise ratio for detection of 12C, 12C1H, and 13C secondary ions. The use of Cs+ primary ion bombardment results in somewhat anomalous, nonmonotonic changes in the 12C, 12C1H, and 13C secondary ion yields through the PS/PMMA interface; however, it is shown that this behavior is not due to sample charging. Through normalization of the 13C secondary ion yield to the total C (12C + 13C) ion yield, the observed effects through the PS/PMMA interface can be greatly minimized, thereby significantly improving analysis of polymer films and multilayers using SIMS. Mass spectra of 13C-PS and 12C-PS were also analyzed using a PHI TRIFT I time-of-flight mass spectrometer, with 15-keV Ga+ primary ion bombardment and detection of positive secondary ions. The (12)C7(1)H7 ion fragment and its 13C-enriched analogues have significant secondary ion yields with negligible mass interferences, providing an early indication of the potential for future use of this technique for cluster probe depth profiling of high molecular weight 13C-labeled fragments.     

Molecular depth profiling of multilayer polymer films using time-of-flight secondary ion mass spectrometry

The low penetration depth and high sputter rates obtained using polyatomic primary ions have facilitated their use for the molecular depth profiling of some spin-cast polymer films by secondary ion mass spectrometry (SIMS). In this study, dual-beam time-of-flight (TOF) SIMS (sputter ion, 5 keV SF(5)(+); analysis ion, 10 keV Ar(+)) was used to depth profile spin-cast multilayers of poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate) (PHEMA), and trifluoroacetic anhydride-derivatized poly(2-hydroxyethyl methacrylate) (TFAA-PHEMA) on silicon substrates. Characteristic positive and negative secondary ions were monitored as a function of depth using SF(5)(+) primary ion doses necessary to sputter through the polymer layer and uncover the silicon substrate (>5 x10(14) ions/cm(2)). The sputter rates of the polymers in the multilayers were typically less than for corresponding single-layer films, and the order of the polymers in the multilayer affected the sputter rates of the polymers. Multilayer samples with PHEMA as the outermost layer resulted in lowered sputter rates for the underlying polymer layer due to increased ion-induced damage accumulation rates in PHEMA. Additionally, the presence of a PMMA or PHEMA overlayer significantly decreased the sputter rate of TFAA-PHEMA underlayers due to ion-induced damage accumulation in the overlayer. Typical interface widths between adjacent polymer layers were 10-15 nm for bilayer films and increased with depth to approximately 35 nm for the trilayer films. The increase in interface width and observations using optical microscopy showed the formation of sputter-induced surface roughness during the depth profiles of the trilayer polymer films. This study shows that polyatomic primary ions can be used for the molecular depth profiling of some multilayer polymer films and presents new opportunities for the analysis of thin organic films using TOF-SIMS.     

TOF-SIMS analysis using C60. Effect of impact energy on yield and damage

C60 has been shown to give increased sputter yields and, hence, secondary ions when used as a primary particle in SIMS analysis. In addition, for many samples, there is also a reduction in damage accumulation following continued bombardment with the ion beam. In this paper, we report a study of the impact energy (up to 120 keV) of C60 on the secondary ion yield from a number of samples with consideration of any variation in yield response over mass ranges up to m/z 2000. Although increased impact energy is expected to produce a corresponding increase in sputter yield/rate, it is important to investigate any increase in sample damage with increasing energy and, hence, efficiency of the ion beams. On our test samples including a metal, along with organic samples, there is a general increase in secondary ion yield of high-mass species with increasing impact energy. A corresponding reduction in the formation of low-mass fragments is also observed. Depth profiling of organic samples demonstrates that when using C60, there does not appear to be any increase in damage evident in the mass spectra as the impact energy is increased.     

Molecular depth profiling of buried lipid bilayers using C(60)-secondary ion mass spectrometry

An organic delta layer system made of alternating Langmuir-Blodgett multilayers of barium arachidate (AA) and barium dimyristoyl phosphatidate (DMPA) was constructed to elucidate the factors that control depth resolution in molecular depth profile experiments. More specifically, one or several bilayers of DMPA (4.4 nm) were embedded in relatively thick (51 to 105 nm) multilayer stacks of AA, resulting in a well-defined delta layer model system closely resembling a biological membrane. 3-D imaging time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profile analysis was performed on this system using a focused buckminsterfullerene (C(60)) cluster ion beam. The delta layer depth response function measured in these experiments exhibits similar features as those determined in inorganic depth profiling, namely an asymmetric shape with quasi-exponential leading and trailing edges and a central Gaussian peak. The effects of sample temperature, primary ion kinetic energy, and incident angle on the depth resolution were investigated. While the information depth of the acquired SIMS spectra was found to be temperature independent, the depth resolution was found to be significantly improved at low temperature. Ion induced mixing is proposed to be largely responsible for the broadening, rather than topography, as determined by atomic force microscopy (AFM); therefore, depth resolution can be optimized using lower kinetic energy, glancing angle, and liquid nitrogen temperature.     

Molecular depth profiling by wedged crater beveling

Time-of-flight secondary ion mass spectrometry and atomic force microscopy are employed to characterize a wedge-shaped crater eroded by a 40-keV C(60)(+) cluster ion beam on an organic film of Irganox 1010 doped with Irganox 3114 delta layers. From an examination of the resulting surface, the information about depth resolution, topography, and erosion rate can be obtained as a function of crater depth for every depth in a single experiment. It is shown that when measurements are performed at liquid nitrogen temperature, a constant erosion rate and reduced bombardment induced surface roughness is observed. At room temperature, however, the erosion rate drops by ～(1)/(3) during the removal of the 400 nm Irganox film and the roughness gradually increased to from 1 nm to ～4 nm. From SIMS lateral images of the beveled crater and AFM topography results, depth resolution was further improved by employing glancing angles of incidence and lower primary ion beam energy. Sub-10 nm depth resolution was observed under the optimized conditions on a routine basis. In general, we show that the wedge-crater beveling is an important tool for elucidating the factors that are important for molecular depth profiling experiments.     

Sputter deposition of multilayered structures for use in sputter depth profile calibration

The development of standards and methods for calibration and comparison of depth scales, sputter removal rates and sputter depth profiling by surface analytical techniques (e.g. AES, SIMS, RBS and X-ray absorption) requires well controlled deposition of multilayered structures. The plasma beam-sputter deposition technique was used to produce combinations of multilayered structures, consisting of metal and oxide layers on Si(100) substrates. Ni, Cr, Ag, Ta, Au, Cr₂O₃ and Ta₂O₅ films were combined and produced in three structural designs of standard reference materials (SRM's), to be used for sputter depth profiling calibration namely, single thin film types, periodically modulated multilayered thin film structures and multilayered ‘marker’ structures. Tetrode sputtering equipment (SPUTRON II¹ of Balzers) was found to be an appropriate apparatus for the deposition of the chosen materials on the small production scale. Four in situ interchangeable targets were used to make highly reproducible layers, having the required quality and especially minimal layer and inter-layer contamination. Characterization of some of the multilayered structures developed showed, that periodically modulated system Ni/Cr/Ni … with well defined repetitive profiles and interface depth resolution is primarily suited for use in sputter depth profiling calibration. This multilayered structure is issued by NBS, Washington as Standard Reference Material No 2135.     

A New Limitation of the Depth Resolution in TOF-SIMS Elemental Profiling: the Influence of a Probing Ion Beam

New data concerning the influence of a probing beam of bismuth ions on the depth resolution in elemental depth profiling by secondary ion mass spectrometry (SIMS) have been obtained on a TOF.SIMS-5 system using the principle of two separate ion beams. It is established that the existing criterion of nondestructive character of the probing beam, on which this principle is based, is insufficient. Additional processes must be taken into account so as to formulate a more adequate criterion. A regime of depth profiling is proposed that allows the depth resolution to be improved at low energies of sputtering ions.     

Evaluation of the depth resolutions of Auger electron spectroscopic, X-ray photoelectron spectroscopic and time-of-flight secondary-ion mass spectrometric sputter depth profiling techniques

Concentration-depth profiles of sputter-deposited Si/Al multilayered specimens were determined by model fitting to measured data obtained by depth profiling, using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary-ion mass spectrometry (TOF-SIMS). The model used for calculation of the concentration-depth profile accounts for the broadening (“smearing”) upon experimental depth profiling owing to the effects of atomic mixing, preferential sputtering, surface roughness and information depth of either the Auger electrons (for AES depth profiling) or the photoelectrons (for XPS depth profiling) or the secondary ions (for SIMS depth profiling). The depth resolution for each technique was derived directly from the values determined for the fitting parameters in the model.     

Applications of Secondary Ion Mass Spectrometry (SIMS) in Materials Science

Secondary Ion Mass Spectrometry (SIMS) is a mature surface analysis technique with a vast range of applications in Materials Science. In this review article the SIMS process is described, the fundamental SIMS equations are derived and the main terminology is explained. The issue of quantification is addressed. The various modes of SIMS analysis including static SIMS, imaging SIMS, depth profiling SIMS and three-dimensional (3D) SIMS are discussed as are specialized analysis strategies such as the imaging of shallow bevels and cross-sections and reverse side analysis. SIMS is shown to be a useful sample preparation tool based on ion beam milling (with SIMS and Scanning Electron Microscopy (SEM) analysis providing end-point detection). The case studies shown illustrate the application of SIMS to several important materials including semiconductors, superconductors, glass, stainless steel, micrometeoroids, solid oxide fuel cell components, museum artifacts, aerospace alloys and biomaterials. Strategies for introducing SIMS into undergraduate education and thus increasing awareness are described. Finally some informed guesses are made as to the future directions of SIMS.     

Secondary ion mass spectrometry of dopant and impurity elements in wide bandgap semiconductors

Based on secondary ion mass spectrometry (SIMS) measurements, we have compiled state-of-the-art data concerning dopant elements and natural impurities in the wide bandgap semiconductor materials diamond, SiC, ZnSe, GaN and AlN. Samples were prepared by ion implantation of different elements into these materials and post-implantation thermal annealing. SIMS depth profiling techniques were used to determine atomic depth profiles of implanted elements and subsequent changes produced by annealing. Range statistics and SIMS relative sensitivity factors were established for major dopant and impurity elements in these wide bandgap materials. Results of these studies are presented in tabular form along with representative depth profile figures.     

Cluster secondary ion mass spectrometry and the temperature dependence of molecular depth profiles

The quality of molecular depth profiles created by erosion of organic materials by cluster ion beams exhibits a strong dependence upon temperature. To elucidate the fundamental nature of this dependence, we employ the Irganox 3114/1010 organic delta-layer reference material as a model system. This delta-layer system is interrogated using a 40 keV C(60)(+) primary ion beam. Parameters associated with the depth profile such as depth resolution, uniformity of sputtering yield, and topography are evaluated between 90 and 300 K using a unique wedge-crater beveling strategy that allows these parameters to be determined as a function of erosion depth from atomic force microscope (AFM) measurements. The results show that the erosion rate calibration performed using the known Δ-layer depth in connection with the fluence needed to reach the peak of the corresponding secondary ion mass spectrometry (SIMS) signal response is misleading. Moreover, we show that the degradation of depth resolution is linked to a decrease of the average erosion rate and the buildup of surface topography in a thermally activated manner. This underlying process starts to influence the depth profile above a threshold temperature between 210 and 250 K for the system studied here. Below that threshold, the process is inhibited and steady-state conditions are reached with constant erosion rate, depth resolution, and molecular secondary ion signals from both the matrix and the Δ-layers. In particular, the results indicate that further reduction of the temperature below 90 K does not lead to further improvement of the depth profile. Above the threshold, the process becomes stronger at higher temperature, leading to an immediate decrease of the molecular secondary ion signals. This signal decay is most pronounced for the highest m/z ions but is less for the smaller m/z ions, indicating a shift toward small fragments by accumulation of chemical damage. The erosion rate decay and surface roughness buildup, on the other hand, exhibit a rather sudden delayed onset after erosion of about 150 nm, indicating that a certain damage level must be reached in order to influence the erosion dynamics. Only after that onset does the depth resolution become compromised, indicating that the temperature reduction does not significantly influence parameters like ion-beam mixing or the altered-layer thickness. In general, the wedge-crater beveling protocol is shown to provide a powerful basis for increased understanding of the fundamental factors that affect the important parameters associated with molecular depth profiling.     

SIMS studies of low-K materials

We report progress in conducting quantitative SIMS analyses of low-K materials. Electron-beam (e-beam) pre-irradiation of SIMS measurement sites was used to study the e-beam-induced effects on SIMS depth profiling of a porous organosilicate low-K material. Pre-irradiation of the sample surface using the e-beam causes a reduction in the thickness of the low-K film. SIMS profiling was used to sputter to identifiable marker positions within the pre-irradiated film. Physical measurement of the thickness of the remaining film was used to show that the e-beam-induced reduction in thickness occurs uniformly throughout the pre-irradiated film. Exposure of the film to the e-beam prior to SIMS analysis also resulted in minor changes in the composition of the film. However, pre-irradiation of the film is not part of the normal SIMS measurement procedure. We conclude that when the e-beam irradiation is used only for charge compensation during SIMS depth profiling, the SIMS analysis of the low-K material will not be significantly affected.     

Using matrix peaks to map topography: increased mass resolution and enhanced sensitivity in chemical imaging

It is well known in secondary ion mass spectrometry (SIMS) that sample topography leads to decreased mass resolution. Specifically, the ion's time of flight is dependent on where it was generated. Here, using matrix-enhanced SIMS, it is demonstrated that, in addition to increasing the yield of intact pseudomolecular ions, the matrix allows the user to semiquantitatively record the topography of a sample. Through mapping the topography-related mass shifts of the matrix (which leads to decreased mass resolution), the analogous mass shifts of higher mass ions can be deconvoluted and higher resolution and greater sensitivity obtained. Furthermore, the semiquantitative topographical map obtained can be compared with any chemical images obtained, allowing the user to quickly ascertain whether local intensity maximums are due to topological features or represent genuine features of interest.     

SIMS studies of scale growth processes

Abstract

The penetration of ¹⁸O tracer into single layered and duplex NiO scales formed on Ni has been examined by sputter depth profiling and imaging modes of secondary ion mass spectrometry (SIMS). Preoxidation annealing in hydrogen and superficial contamination during substrate surface preparation were found to promote inward scale growth. The principles for selecting SIMS techniques for studying oxide scales and the influence of substrate condition on mechanisms for gaseous oxygen transport in NiO are discussed.

MST/950     

Characterization of coatings

The variety of physical and chemical properties of coatings is determined by their thickness, structure and chemical composition. A fundamental understanding of coating properties, as well as of their reproducibility, therefore requires a good knowledge of these parameters.During the last few years great progress has been made in the field of chemical analysis (including depth distribution) of thin films and coatings. This progress is mainly the result of the combination of recently developed surface analytical techniques such as Auger electron spectroscopy (AES), photoelectron spectroscopy (UPS, XPS), ion scattering spectroscopy (ISS) and secondary ion mass spectroscopy (SIMS), on one hand, and controlled simultaneous surface etching by sputtering on the other.With these surface analytical techniques the chemical composition of the uppermost monolayers is detected by energy or mass analysis of ion- (ISS, SIMS), electron- (AES) or photon- (UPS, XPS) induced emission of ions (ISS, SIMS) or electron (AES, UPS, XPS), respectively. By combining these techniques with sputtering, the depth distribution of elements and compounds can also be determined with a lateral resolution of some microns.In order to recognize the capabilities and limitations of these techniques for coating analysis, the fundamental emission processes as they appear in the various analytical techniques, as well as the details of the sputtering process, have to be taken into account. The main features such as detection limits, isotope sensitivity, detection of compounds etc. of these techniques will be compared for some typical examples.Other methods, such as high energy ion backscattering and the detection of sputtered particles in the gas phase, will also be considered.     

Sputter etch profiles of spheres,cylinders and slab-like silica targets

The intermediate and quasi-equilibrium contours generated by ion bombardment on silica targets under the condition of stationary target and ion beam direction are shown predictable by Frank's construction method. An expression based on ion reflection is reported which yields values for the apex angle of conical silica ion etch structures in agreement with those derived graphically from sputter yield data. The effect of the initial target shape on the etch topography is discussed.     

Depth profiling of fullerene-containing structures by time-of-flight secondary ion mass spectrometry

A new variant of depth profiling for thin-film fullerene-containing organic structures by the method of time-of-flight (TOF) secondary ion mass spectrometry (SIMS) on a TOF.SIMS-5 setup is described. The dependence of the yield of C₆₀ molecular ions on the energy of sputtering ions has been revealed and studied. At an energy of sputtering Cs⁺ ions below 1 keV, the intensity of C₆₀ molecular ions is sufficiently high to make possible both elemental and molecular depth profiling of multicomponent (multilayer) thin-film structures. Promising applications of TOF-SIMS depth profiling for obtaining more detailed information on the real molecular composition of functional organic materials are shown.     

砷化镓中掺硅元素的SIMS定量分析

通过对体均匀掺硅和离子注入硅样品的二次离子质谱（ＳＩＭＳ）深度剖面分析，采用均匀体标样法、剖面二次离子强度积分法和ＬＳＳ理论关系计算获得了比较一致的、且与手册值接近的ＧａＡｓ中硅元素相对灵敏度因子值。将相对灵敏度因子法应用于双能量、双剂量硅离子注入ＧａＡｓ样品及分子束外延多层ＧａＡｓ膜渗硅的ＳＩＭＳ定量分折中。从实验结果初步讨论了ＧａＡｓ中氧和碳元素的存在对２８Ｓｉ－、１０３ＳｉＡｓ－二次离子信号强度的影响，２８ＳｉＡｓ－和１０３Ｓｉ－二次离子信号强度的线性关系和可靠性。