Noise,resolution and entropy in sputter profiling

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.

It has become more important to measure accurate depth profiles in developing more advanced devices. To this aim, Depth profiling in secondary ion mass spectrometry (SIMS) has been extensively used as an informative technique in the semiconductor and electronic devices fields due to its high sensitivity, quantification accuracy and depth resolution (Fujiyama et al, 2011; Seki et al, 2011). However, the depth resolution in SIMS analysis is still limited to provide reliable and precise information in very thin structures such as delta layers, abrupt interfaces, etc. By optimization of the experimental conditions, the depth resolution can be enhanced. In particular, lowering the primary energy seems to be a good solution, but this increases the measurement time and leads to other limitations, owing to the wrong focalization of primary ion beam, such as roughness in the crater bottom, not flat crater, etc. Therefore, the depth resolution remains so far to its perfect limit. It is only by numerical processing like deconvolution that the depth resolution can be improved beyond its experimental limits.

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.

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. >

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

The effect of cascade mixing on profile broadening during secondary-ion mass spectrometry (SIMS) analysis has been thoroughly investigated for ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ structures of five different compositions ($x=0.1,$ 0.3, 0.5, 0.73, or 1) and layers with varying thicknesses (from one monolayer to 1000 \AA{}). The SIMS analyses were performed using primary sputtering ions of ${}^{20}{\mathrm{Ne}}^{+},$ ${}^{40}{\mathrm{Ar}}^{+},$ ${}^{84}{\mathrm{Kr}}^{+},$ and ${}^{136}{\mathrm{Xe}}^{+}$ with an impact energy (E) ranging from 1.8 to 13.2 keV and an angle of incidence, with respect to the surface normal (\ensuremath{\theta}), from 62\ifmmode^\circ\else\textdegree\fi{} to 35\ifmmode^\circ\else\textdegree\fi{}. Within the experimental accuracy, the decay length of the trailing edge was found to be proportional to ${E}^{1/2}\mathrm{cos}\ensuremath{\theta}$ where the proportionality constant displays a relatively weak dependence on primary ion mass. However, the leading edge is strongly affected by the extension of the collision cascade as demonstrated by a comparison of the results for the different ions at a given energy. As long as the cascade is fully developed before reaching an interface no dependence on the sample depth is obtained for the profile broadening. Furthermore, the decay length for the trailing edges is extracted in the dilute limit and no effect of the marker thickness or the x value is revealed. A numerical treatment of the profile broadening within a diffusional model, where the diffusion coefficient is assumed to be proportional to the energy deposited in elastic collisions, gives a surprisingly good agreement with the experimental data.

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.

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.

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.

Reactive ion beams such as O2+and Cs+are used in Secondary Ion Mass Spectrometry (SIMS) to analyze solids for trace impurities. Primary beam properties such as energy, dose, and incidence angle can be systematically varied to optimize depth resolution versus sensitivity tradeoffs for a given SIMS depth profiling application. However, it is generally observed that the sputtering process causes surface roughening, typically represented by nanometer-sized features such as cones, pits, pyramids, and ripples. A roughened surface will degrade the depth resolution of the SIMS data. The purpose of this study is to examine the relationship of the roughness of the surface to the primary ion beam energy, dose, and incidence angle. AFM offers the ability to quantitatively probe this surface roughness. For the initial investigations, the sample chosen was <100> silicon, and the ion beam was O2+.Work to date by other researchers typically employed Scanning Tunneling Microscopy (STM) to probe the surface topography.

Beam-induced broadening effects in sputter depth profiling

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.

Factors controlling the depth resolution in radio frequency glow discharge optical emission spectroscopy (rf‐GDOES) depth profiling analysis of thin films have been investigated using anodic alumina films of varying thicknesses with delta function marker layers of ∼2 nm width located at various depths from the oxide surface. In rf‐GDOES depth profiling analysis, where a relatively large area of ∼4 mm in diameter is analysed, it was found that the depth resolution is determined mainly by the large‐scale variation of sputtering rate across the crater. Despite the relatively large area analysed, the uniformity of sputtering rate across the crater was constant to within 1% under the optimum Ar pressure except for near‐edge regions of ∼0.4 mm width, where the sputtering rate was ∼6% higher than in the central crater region. Owing to this relatively minor non‐uniformity of sputtering rate, the depth resolution degrades approximately linearly with depth. A depth resolution of ∼1.3 nm was realized to depths of ∼25 nm, where the degradation of depth resolution due to the non‐uniformity in sputtering rate is insignificant. The depth resolution is close to the ultimate depth resolution in sputter depth profiling, claimed to be 0.7– 1.0 nm, and arises from the minimal atomic mixing associated with film sputtering with Ar + ions of very low energy, ∼50 eV, in rf‐GDOES analysis. Copyright © 2001 John Wiley & Sons, Ltd.

Optimization of depth resolution parameters in AES sputter profiling of GaAs/AlAs multilayer structures

Sputter depth profiles of an Rh/C microscopically modulated thin film (double layer thickness 7 nm) were obtained by low energy SIMS and SNMS. The depth resolution was obtained using the linear superposition of error functions at the interfaces. In optimal cases the depth resolution was shown to be of the order of 1–2 nm for both techniques.