Multi-Scale Deconvolution of Mass Spectrometry Signals

High detection sensitivity in bulk analysis or depth profiling by secondary ion mass spectrometry (SIMS) can only be achieved, for positively charged ions, if the nearsurface regions of the sputter eroded sample are fully oxidized. Using oxygen primary ions, a stationary oxidation state is established after some time of bombardment during which period the sputtering yield decreases and the ionization probability increases. The physical and chemical processes occurring during the transient period are reviewed with emphasis on the results for impurity analysis in silicon, i.e. the matrix material that has been studied most thoroughly in the past. The transient decrease in sputtering yield gives rise to a depth scale offset and an associated apparent shift of impurity profiles towards the surface. The effect is largest at normal beam incidence, ca. 1 nm /keV (O + 2 ), in which case silicon is fully oxidized. The transition depth, i.e. the depth sputtered before achieving a stable ion yield is about twice as large and increases as the impact angle is turned away from normal. The shift and the depth scale offset can be measured safely using thin (delta) layers of isotopically pure tracers, for example 30 Si in 28 Si. Boron delta layers can serve as secondary standards because this impurity behaves almost the same as the host silicon atoms. The profile shift observed with other common dopants may contain contributions due to unidirectional relocation, often driven by segregation away from the surface. At O + 2 energies below about 0.7 keV the transition depths fall below the thickness of typical native oxides, so that the transient changes in sputtering yield and ionization probability can disappear. The transient phenomena may be described by a simple sputtering-oxidation model that connects the depth scale offset to other observable parameters like the initial and final sputtering yield, the transition fluence and the oxide thickness.

Depth profiling analysis of In x Ga1x As heterolayers grown by MBE on Fe doped InP(100) substrates was performed in the SAJW-02 secondary ion mass spectrometry analyser equipped with 4.5 keV O+2 ion source and a specially designed sample manipulator enabling depth profiling in the standard as well as in so-called Zalar rotation operation modes. The fairly high energy of the primary ion beam required for sputtering in secondary ion mass spectrometry measurements causes changes in surface topography, usually of different origin. Depth resolution parameters and roughness formation monitored by scanning electron microscopy were analysed for a set of samples with composition x changing in the range 0.33 to 0.60. The results were compared with the same data for a layer of z = 0.53 (best lattice-matched to InP) grown on the top of a three monolayer thick AlAs film deposited previously on the InP substrate. Improvement in the depth profile resolution was revealed for the structure with an AlAs layer indicating sharper interface transition. Moreover, sample rotation applied for this structure improves further the depth profaling resolution. Thus, we showed for the first time that a very thin AlAs layer grown by MBE between the InP substrate and the m053 Ga047 As improves considerably the heterointerface properties and that Zalar rotation applied for depth profiling of the investigated material system diminishes further the negative effects of ion etching on depth resolution in secondary ion mass spectrometry analysis.

Challenges of biological sample preparation for SIMS imaging of elements and molecules at subcellular resolution

Applications of semiconductor sputter profiling in the near future will demand nm or even sub-nm depth resolution, i.e. the ability to distinguish between adjacent features in a concentration profile which are separated by 1 nm or less (not just the ability to achieve a 16-84% width of 1 nm on some edge transient). Recent developments in SIMS indicate that sputter profiling can meet this challenge—provided that instruments with high current microfocus beams less than 1 keV in energy become widely available, and data processing methods are developed to cope with the results. In this paper we discuss the nature of SIMS sputter profile data in detail and explore the consequences of noise, finite sample interval and the destruction of information by processes such as atomic mixing. The use of empirically determined response functions is discussed. We use simple arguments to show that convolution is only a valid model for atomic mixing in the low concentration limit, and contrast the use of Fourier transform and maximum entropy methods for deconvolution in these circumstances. A depth resolution of 1 nm is demonstrated without deconvolution, showing that the technique has real potential for depth resolution on the scale of 1 atomic plane.

Since SIMOX (separation by implanted oxygen) material is formed by a high-fluence process and subsequent high-temperature anneal, any contamination in the starting material and any residual contamination from the implant process may be critical for the further device performance and yield. The residual contamination level in SIMOX substrates was evaluated with respect to a variety of wet and dry cleaning techniques. The authors used total X-ray fluorescence analysis (TXRF) and SIMS (secondary ion mass spectrometry) analysis to determine the residual contamination from transition metals and aluminum, respectively Utilizing the TXRF sensitivity, incoming bulk wafers were examined for heavy metal contamination. Unacceptable residual levels of zinc ( approximately 4*10/sup 12/ cm/sup -2/) were found on the wafers. Wafers were also examined by TXRF and SIMS after the ion implantation process. An aluminum peak areal density on the order of 1*10/sup 12/ to 1*10/sup 13/ atoms/cm/sup 2/ was identified by SIMS analysis on the ion implanted wafers. Effective reduction in aluminum contamination after traditional wet cleans for the as-implanted wafers is shown. >

The complementary metal-oxide-semiconductor (CMOS) industry continues to push the boundaries of what is possible. Along with this, secondary ion mass spectrometry (SIMS) depth profiling continues to support CMOS R&D and high volume manufacturing (due to its unparalleled sensitivity and detection limits over predefined volumes and within acceptable time periods). New developments in SIMS are also being realized to support the move of the CMOS industry from planar structures to three-dimensional (3D) structures. This article presents a perspective of existing SIMS research areas for beyond one-dimensional structural analysis along with potential future SIMS implementation and data processing scenarios. The topics covered include: (1) existing SIMS depth profiling approaches for 3D structures, (2) in-fab SIMS deployment, (3) data analysis of in-fab SIMS derived depth profiles from 3D structures via pattern recognition, and (4) hybrid characterization approaches. With SIMS depth profiling of optimized (for SIMS) 3D structures already demonstrated, the remaining topics may serve to extend these and other capabilities and open new application areas for SIMS.

3D chemical microscopy is one of the emerging applications of secondary ion mass spectrometry (SIMS) in biology. Tissues, cells, extracellular matrices, and polymer films can be imaged at present with a lateral resolution of 50 nm and depth resolution of 1 nm using the latest generation of CAMECA magnetic sector NanoSIMS 50 or with a lower lateral resolution (above 100 nm) using IMS 4f Cameca SIMS equipped with cold stage. Dynamic mode SIMS analysis is performed in ultrahigh vacuum and thus requires specific and careful preparation of biological samples aimed at preserving and minimizing destruction of the original structural and chemical properties of the samples. Here we describe a methodology based on the ultrafast plunge-freezing of biological tissues, preparation of the sample for SIMS analyses and transfer to the SIMS cold stage without interruption of the cold chain during the mounting procedure and subsequent SIMS analyses. Using this strategy, SIMS chemical microscopy can be performed on biological tissue in which unwanted molecular and/or structural reorganization, loss of constituents and chemical modifications are minimized and in which structures are therefore optimally preserved.

Liquid crystal (LC) concentrations in stratified holographic polymer-dispersed liquid crystal (PDLC) films have been quantified from depth profiles obtained by time-of-flight secondary ion mass spectrometry (SIMS). The volatile nature of the LCs was hindered during SIMS analysis by capping the PDLC samples with poly(vinyl alcohol) and cooling to cryogenic temperatures. It remains difficult to gain quantitative results from SIMS analysis due to matrix effects yielding complex SIMS fingerprint spectra. With the multivariate statistical method discriminant function analysis the LC contents in the stratified holographic PDLC films were quantified. The concentration of the LC-rich layers was determined to be 32.9 ± 3.4 wt % LC, and for the polymer-rich layers it was 28.8 ± 2.7 wt % LC. The low concentration difference was supported by imaging analysis and modeling results.

A sample of low-temperature epitaxial Si grown with five B delta-doped layers 5.4 nm apart has been profiled using secondary ion mass spectrometry under a variety of O/sub 2/ bombardment conditions. Energies from 400 eV to 15 keV were used with angles of incidence from 0/spl deg/ to 70/spl deg/. Analyses were performed using oxygen flooding of the sample surface during analysis, as well as without using oxygen flooding. The apparent spacing between the B delta layers was used to determine the magnitude and extent of increased sputtering rate at the beginning of an analysis. The width of the B peaks was used to evaluate depth resolution and to measure the effect of surface roughening of the profiles. It was found that sputtering with 500 eV O/sub 2/ at an angle of 50/spl deg/ while flooding with oxygen gave no measurable error in sputtering rate and resulted in no unexpected shift towards the surface of the B delta layers. These analysis conditions also resulted in depth resolution which was as good as that obtained using 400 ev O/sub 2/ bombardment at 0/spl deg/ incidence. This 0/spl deg/ method of analysis, however, resulted in a 1.1 nm shift of the topmost B delta layer towards the surface, and the 0/spl deg/ method had a sputter rate only 1/5 that of the 50/spl deg/ method of analysis.

Pyroxene standards for SIMS oxygen isotope analysis and their application to Merapi volcano, Sunda arc, Indonesia

Irradiation damage in fuel cladding material is mainly caused by the neutron flux that results from fission reactions occurring in the fuel. To avoid the constraints inherent in handling radioactive material, the irradiation effects on the corrosion resistance of zirconium alloys can be studied by irradiating the materials with ions. We performed an original experiment using ion irradiation to more specifically study the influence of irradiation damage in the oxide on the corrosion rate of M5®. It has been established that irradiation with a 1.3-MeV helium ion at a fluence of 1017 cm−2 results in significant modifications of oxide properties, oxygen diffusion flux, and oxidation kinetics, as evidenced by Raman spectroscopy, secondary ion mass spectrometry (SIMS) analyses, and measurements of mass gains. A newly identified Raman vibration band at 712 cm−1 was linked to the presence of irradiation defects and allowed the evolution of their concentrations to be followed. The oxygen diffusion flux was significantly reduced after irradiation partly due to a surface concentration decrease of oxygen. The defects remained present in the oxide after 100 days of annealing in pressurized water reactor (PWR) conditions and were thus very stable in PWR conditions, which probably means that these defects would be stable in the reactor. According to the kinetics and in agreement with the results obtained by SIMS analyses, the oxidation rate was significantly reduced after ion irradiation, and this effect remained beyond 100 days in agreement with the high stability of irradiation defects in PWR conditions. An original model described quite well the oxidation kinetic results.

Loss of depth resolution with depth in secondary ion mass spectrometry (SIMS) due to variations in ion dose density across the rastered area

SIMS (secondary ion mass spectrometry) is the most commonly used analytical technique for characterizing low energy ion implants. To obtain accurate near-surface SIMS profiles, instrumental depth resolution must be optimized, and transient ion yield changes during primary ion beam equilibration must be minimized. Best depth resolution is obtained by sputtering with a low energy primary ion beam incident on the sample at a large angle with respect to the surface normal. These analytical conditions also reduce the depth range over which transient ion yield changes occur. For oxygen primary ion bombardment, flooding the sample surface with oxygen gas during analysis can largely eliminate transient ion yield changes. Of the readily available SIMS instrument configurations, Quadrupole SIMS can deliver the best depth resolution. However, the low mass resolving power of Quadrupole SIMS results in important limitations due to its inability to resolve mass interferences from surface contamination. Magnetic Sector SIMS can eliminate the interference problem by operating in a high mass resolution mode. SurfaceSIMS, using oxygen flooding, provides high sensitivity and uniform near-surface ion yields required for accurate and reproducible depth profiling of shallow ion implants and surface contaminants related to ion implantation.

A review is presented on quantitative analysis of impurities in III-V compounds semiconductors, especially GaAs, by secondary ion mass spectrometry (SIMS).The accuracy of SIMS analysis was investigated in comparison with chemical analysis results in the round robin study which was supported by Japan Society for the Promotion of Science Commitee 145. This study shows that the accuracy of SIMS quantitative analysis is ±10-30%. Highly sensitive and highly accurate quantitative SIMS analysis using chemical preparation is reviewed. This method has the very low detection limits (0.5-5ppb) and very high accuracy (±5-10%). SIMS quantitative analysis of GaAs using LTE model is also described. The accuracy of LTE model was considered to be factor 2.

Variation of yield with thickness in SIMS and PDMS: Measurements of secondary ion emission from organized molecular films