Simulation of sputter-induced roughness for depth profiling of thin film structures

Sputtering induced surface roughening is the dominant factor that degrades depth resolution in sputter profiling of polycrystalline film samples. Due to the dependence of the sputtering yield on the crystallographic orientation, ion beam incidence angle and composition, the local sputtering rate differs from grain to grain. A simple computer program based on a model of Marton and Fine can simulate such a roughness development within one layer, an improved version can even be applied for interfaces. A further extension of the program using a model of Hauffe includes effects like shadowing and enhanced peak erosion leading to surface smoothing.     

Simulation of sputter-induced roughness for depth profiling of thin film structures

Sputtering induced surface roughening is the dominant factor that degrades depth resolution in sputter profiling of polycrystalline film samples. Due to the dependence of the sputtering yield on the crystallographic orientation, ion beam incidence angle and composition, the local sputtering rate differs from grain to grain. A simple computer program based on a model of Marton and Fine can simulate such a roughness development within one layer, an improved version can even be applied for interfaces. A further extension of the program using a model of Hauffe includes effects like shadowing and enhanced peak erosion leading to surface smoothing.     

Secondary ion mass spectrometry depth profiling of ultrathick films using an argon gas cluster source: Crater shape implications on the analysis area as a function of depth

Argon cluster ions have enabled molecular depth profiling to unprecedented depths, with minimal loss of chemical information or changes in sputter rate. However, depth profiling of ultrathick films (>100 μm) using a commercial ion source oriented at 45° to the surface causes the crater bottom to shrink in size because of a combination of the crater wall angle, sputter rate differences along the trailing-edge crater wall, and undercutting on the leading-edge. The shrinking of the crater bottom has 2 immediate effects on dual-beam depth profiling: first is that the centering of the analysis beam inside the sputter crater will no longer ensure the best quality depth profile because the location of the flat crater bottom progressively shifts toward the leading-edge and second, the shifting of the crater bottom enforces a maximum thickness of the film that could be depth profiled. Experiments demonstrate that a time-of-flight secondary ion mass spectrometry instrument equipped with a 20 keV argon cluster source is limited to depth profiling a 180 μm-thick film when a 500 μm sputter raster is used and a 100 μm square crater bottom is to be left for analysis. In addition, depth profiling of a multilayer film revealed that the depth resolution degrades on trailing-edge side of the crater bottom presumably because of the redeposition of the sputtered flux from the crater wall onto the crater bottom.     

Development of B‐doped Si multiple delta‐layer reference materials for SIMS profiling

B‐doped Si multiple delta‐layers (MDL) were developed as certified reference materials (CRM) for secondary ion mass spectrometry (SIMS) depth profiling analysis. Two CRMs with different delta‐layer spacing were grown by ion beam sputter deposition (IBSD). The nominal spacing of the MDL for shallow junction analysis is 10 nm and that for high energy SIMS is 50 nm. The total thickness of the film was certified by high resolution transmission electron microscopy (HR‐TEM). The B‐doped Si MDLs can be used to evaluate SIMS depth resolution and to calibrate the depth scale. A consistency check of the calibration of stylus profilometers for measurement of sputter depth is another possible application. The crater depths measured by a stylus profilometer showed a good linear relationship with the thickness measured from SIMS profiling using the calibrated film thickness for depth scale calibration. The sputtering rate of the amorphous Si thin film grown by sputter deposition was found to be the same as that of the crystalline Si substrate, which means that the sputtering rate measured with these CRMs can be applied to a real analysis of crystalline Si. Copyright © 2005 John Wiley & Sons, Ltd.     

A statistical approach to delta layer depth profiling

We present a simple statistical model describing the removal and relocation of material during a sputter depth profiling experiment. All input parameters are determined from low‐fluence molecular dynamics simulations, making the model de facto parameter free. The model can be used to extrapolate data from the molecular dynamics simulations to projectile fluences relevant to sputter depth profiling experiments. As a result, the erosion of the surface is calculated in terms of fluence‐dependent filling factors of different sample layers. Using input data determined for the 20‐keV C₆₀ cluster bombardment of silicon, it is found that a steady‐state erosion profile is reached after removal of approximately 20 monolayer equivalents of material. Plotting the contribution of particles from a specific layer to the instantaneous sputtered flux, one can directly determine the delta layer response function predicted from such a model. It is shown that this function can be parameterized by the semiempirical Dowsett response function, and the resulting fitting parameters are compared with published depth profile data. The model is then used to study the role of different processes influencing the observed depth resolution. We find that the statistical nature of the sputtering process suffices to explain many features of experimentally measured delta layer depth profiles. Copyright © 2012 John Wiley & Sons, Ltd.     

A new approach to express ToF SIMS depth profiling

We propose a new approach to express SIMS depth profiling on a TOF.SIMS‐5 time‐of‐flight mass spectrometer. The approach is based on the instrument capability to independently perform raster scans of sputter and probe ion beams. The probed area can be much smaller than the diameter of a sputter ion beam, like in the AES depth profiling method. This circumstance alleviates limitations on the sputter beam–raster size relation, which are critical in other types of SIMS, and enables analysis on a curved‐bottomed sputter crater. By considerably reducing the raster size, it is possible to increase the depth profiling speed by an order of magnitude without radically degrading the depth resolution. A technique is proposed for successive improvement of depth resolution through profile recovery with account for the developing curvature of the sputtered crater bottom in the probed area. Experimental study of the crater bottom form resulted in implementing a method to include contribution of the instrumental artifacts in a nonstationary depth resolution function within the Hofmann's mixing–roughness–information depth model. The real‐structure experiment has shown that the analysis technique combining reduction of a raster size with a successive nonstationary recovery ensures high speed of profiling at ~100 µm/h while maintaining the depth resolution of about 30 nm at a 5 µm depth. Copyright © 2015 John Wiley & Sons, Ltd.     

Depth resolution in sputter profiling revisited

Based on a brief review of the well‐established framework of definitions, measurement and evaluation principles of the depth resolution in sputter profiling for interfaces, delta layers, single layers and multilayers, an extension to additional definitions is presented, which include the full‐width‐at‐half‐maximum of layer profiles and non‐Gaussian depth resolution functions as defined by the Mixing‐Roughness‐Information depth (MRI) model. Improved evaluation methods for adequate analysis of sputter depth profiles as well as improved definitions of depth resolution are introduced in order to meet new developments in ToF‐SIMS and GDOES, and in cluster ion sputtering of so‐called delta layers in organic matrices. In conclusion, the full‐width‐at‐half‐maximum definition and measurement of depth resolution, Δz(FWHM), is found to be more appropriate than the traditional Δz(16–84%) in order to characterize depth profiles of single layers and multilayers, because it is also valid for non‐Gaussian depth resolution functions. Copyright © 2016 John Wiley & Sons, Ltd.     

Use of C60 cluster projectiles for sputter depth profiling of polycrystalline metals

We have investigated the merits of fullerene cluster ions as projectiles in time‐of‐flight secondary neutral mass spectrometry (ToF‐SNMS) sputter depth profiling of an Ni:Cr multilayer sample similar to the corresponding NIST depth profiling standard. It is shown that sputter erosion under bombardment with C₆₀⁺ ions of kinetic energies between 10 and 20 keV provides good depth resolution corresponding to interface widths of several nanometres. This depth resolution is maintained during the complete removal of the multilayer stack with a total thickness of 500 nm. This finding is in contrast to the case where atomic Ga⁺ projectile ions of comparable kinetic energy are used, demonstrating the unique features of cluster projectiles in sputter depth profiling. Copyright © 2004 John Wiley & Sons, Ltd.     

Surface topography effects on glow discharge depth profiling analysis

Glow discharge optical emission spectroscopy (GD‐OES) has been shown to be of immense value in elemental depth profiling of thin or thick films on conductive or non‐conductive substrates. For aluminium, GD‐OES has been employed to examine locations of markers and tracers in anodic films, thereby assisting understanding of transport phenomena. In order to investigate the influence of surface topography on depth profiling analysis, anodic aluminium oxide films of various thicknesses, with incorporated electrolyte species, were produced on superpure aluminium substrates of controlled roughnesses. The distributions of incorporated species in the films were subsequently probed. Surface topography modifications and consequent depth resolution degradation were examined during depth profiling analysis performed by GD‐OES. The results reveal that the sputtering process leads to the roughening or smoothing of the surface topography of the specimen for a ratio of the film thickness to the amplitude of the substrate texture less, or greater, than 1 respectively. As a consequence of the surface topography dependence of the ion bombardment, analysis of thin films over rough surfaces suffers from depth resolution limitations due to sputtering‐induced topography changes, thereby limiting quantification of the resultant spectra. Copyright © 2010 John Wiley & Sons, Ltd.     

Molecular depth profiling with cluster ion beams

Peptide-doped trehalose thin films have been characterized by bombardment with energetic cluster ion beams of C60+ and Aux+ (x = 1, 2, 3). The aim of these studies is to acquire information about the molecular sputtering process of the peptide and trehalose by measurement of secondary ion mass spectra during erosion. This system is important since uniform thin films of approximately 300 nm thickness can be reproducibly prepared on a Si substrate, allowing detailed characterization of the resulting depth profile with different projectiles. The basic form of the molecular ion intensity as a function of ion dose is described by a simple analytical model. The model includes parameters such as the molecular sputtering yield, the damage cross section of the trehalose or the peptide, and the thickness of a surface layer altered by the projectile. The results show that favorable conditions for successful molecular depth profiling are achieved when the total sputtering yield is high and the altered layer thickness is low. Successful molecular depth profiles are achieved with all of the cluster projectiles, although the degree of chemical damage accumulation was slightly lower with C60. With C60 bombardment, the altered layer thickness of about 20 nm and the damage cross section of about 5 nm2 are physically consistent with predictions of molecular dynamics calculations available for similar chemical systems. In general, the model presented should provide guidance in optimizing experimental parameters for maximizing the information content of molecular depth profiling experiments with complex molecular thin film substrates.     

Molecular sputter depth profiling using carbon cluster beams

Sputter depth profiling of organic films while maintaining the molecular integrity of the sample has long been deemed impossible because of the accumulation of ion bombardment-induced chemical damage. Only recently, it was found that this problem can be greatly reduced if cluster ion beams are used for sputter erosion. For organic samples, carbon cluster ions appear to be particularly well suited for such a task. Analysis of available data reveals that a projectile appears to be more effective as the number of carbon atoms in the cluster is increased, leaving fullerene ions as the most promising candidates to date. Using a commercially available, highly focused C₆₀^q+ cluster ion beam, we demonstrate the versatility of the technique for depth profiling various organic films deposited on a silicon substrate and elucidate the dependence of the results on properties such as projectile ion impact energy and angle, and sample temperature. Moreover, examples are shown where the technique is applied to organic multilayer structures in order to investigate the depth resolution across film-film interfaces. These model experiments allow collection of valuable information on how cluster impact molecular depth profiling works and how to understand and optimize the depth resolution achieved using this technique.     

Three-dimensional depth profiling of molecular structures

Molecular time of flight secondary ion mass spectrometry (ToF-SIMS) imaging and cluster ion beam erosion are combined to perform a three-dimensional chemical analysis of molecular films. The resulting dataset allows a number of artifacts inherent in sputter depth profiling to be assessed. These artifacts arise from lateral inhomogeneities of either the erosion rate or the sample itself. Using a test structure based on a trehalose film deposited on Si, we demonstrate that the “local” depth resolution may approach values which are close to the physical limit introduced by the information depth of the (static) ToF-SIMS method itself.     

New horizons in sputter depth profiling inorganics with giant gas cluster sources: Niobium oxide thin films

X‐ray photoelectron spectroscopy is used to study a wide variety of material systems as a function of depth (“depth profiling”). Historically, Ar⁺ has been the primary ion of choice, but even at low kinetic energies, Ar⁺ ion beams can damage materials by creating, for example, nonstoichiometric oxides. Here, we show that the depth profiles of inorganic oxides can be greatly improved using Ar giant gas cluster beams. For NbO_x thin films, we demonstrate that using Ar_x⁺ (x = 1000‐2500) gas cluster beams with kinetic energies per projectile atom from 5 to 20 eV, there is significantly less preferential oxygen sputtering than 500 eV Ar⁺ sputtering leading to improvements in the measured steady state O/Nb ratio. However, there is significant sputter‐induced sample roughness. Depending on the experimental conditions, the surface roughness is up to 20× that of the initial NbO_x surface. In general, higher kinetic energies per rojectile atom (E/n) lead to higher sputter yields (Y/n) and less sputter‐induced roughness and consequently better quality depth profiles. We demonstrate that the best‐quality depth profiles are obtained by increasing the sample temperature; the chemical damage and the crater rms roughness is reduced. The best experimental conditions for depth profiling were found to be using a 20 keV Ar₂₅₀₀⁺ primary ion beam at a sample temperature of 44°C. At this temperature, there is no, or very little, reduction of the niobium oxide layer and the crater rms roughness is close to that of the original surface.     

Status monitoring of ion sputter relevant parameters of an XPS depth profiling instrument

An X-ray photoelectron spectroscopy (XPS) instrument is utilized for sputter depth profiling of thin films. Relevant instrumental parameters are the ion gun sputter rate, the contamination level of the sputter ion gun, and the purity of the sputter ion gun gas supply as well as the vacuum quality of the instrument at the sample position. A long-term recording of these instrumental parameters ensures the reliability of the measured depth profile data. The ion gun sputter rate was estimated using the standard ISO conform depth profiling of a SiO₂ reference layer of known thickness. Two new procedures are developed to determine the other relevant parameters. Gases that are emitted by the ion sputter gun get implanted into a Si target. An analysis of the implanted gases allows judging on the contamination level of the sputter gun and the purity of the sputter gun gas supply. The vacuum condition of an XPS microprobe at the sample position is monitored by the recontamination of a sputtered Ti surface by adsorbed residual gas particles.     

An analytical depth resolution function for the MRI model

The mixing roughness information depth model is frequently used for the quantification of sputter depth profiles. In general, the solution of the convolution integral for any kind of in‐depth distributions is achieved by numerical methods. For a thin delta layer, an analytical depth resolution function is presented, which enables a simple and user‐friendly quantification of measured delta layer profiles in AES, XPS and SIMS. Copyright © 2013 John Wiley & Sons, Ltd.     

Backscattering effect in quantitative AES sputter depth profiling of multilayers

AES sputter depth profiles of multilayers with constituents of very different backscattering factors show characteristic distortions in the shape of the intensity–depth profiles. These distortions are quantified by introducing an extension of the local effective backscattering factor concept developed in an earlier paper in the mixing‐roughness‐information depth (MRI) model for profile quantification. The extension is based on a linear superposition of two newly defined parameters, the effective backscattering factors for each interface that are diminished with distance from the respective interface by another characteristic parameter, the mean effective backscattering decay length. As shown for a Ni/C multilayer structure of six alternating layers of Ni (38 nm) and C (25 nm) on a Si substrate, AES intensity depth profiles calculated with the presented modification of the MRI model, yield an excellent agreement with the measured profile after some adjustment of the initial mean effective backscattering decay lengths and, sometimes, after a slight change of the backscattering factors given by the Ichimura–Shimizu relations. The backscattering effect is studied as a function of the single layer thickness. A critical layer thickness can be determined, below which the backscattering influence becomes negligible for typical AES depth profiling results. Copyright © 2007 John Wiley & Sons, Ltd.     

An effect of measurement conditions on the depth resolution for low‐energy dual‐beam depth profiling using TOF‐SIMS

An effect of measurement conditions on the depth resolution was investigated for dual‐beam time of flight‐secondary ion mass spectrometry depth profiling of delta‐doped‐boron multi‐layers in silicon with a low‐energy sputter ion (200 eV – 2 keV O₂⁺) and with a high‐energy primary ion (30 keV Bi⁺). The depth resolution was evaluated by the intensity ratio of the first peak and the subsequent valley in B⁺ depth profile for each measurement condition. In the case of sputtering with the low energy of 250 eV, the depth resolution was found to be affected by the damage with the high‐energy primary ion (Bi⁺) and was found to be correlated to the ratio of current density of sputter ion to primary ion. From the depth profiles of implanted Bi⁺ primary ion remaining at the analysis area, it was proposed that the influence of high‐energy primary ion to the depth resolution can be explained with a damage accumulation model. Copyright © 2013 John Wiley & Sons, Ltd.     

Depth profiling organic/inorganic interfaces by argon gas cluster ion beams: sputter yield data for biomaterials,in‐vitro diagnostic and implant applications

Argon gas cluster ion beam sources are likely to become much more widely available on XPS and SIMS instruments in the next few years. Much attention has been devoted to their ability to depth profile organic materials with minimum damage. What has not been the focus of attention (possibly because it has been very difficult to measure) is the large ratio of sputter yield for organic materials compared with inorganic materials using these sources and the special opportunities this presents for studies of organic/inorganic interfaces. Traditional depth profiling by monatomic argon ions introduces significant damage into the organic overlayer, and because sputter rates in both organic and inorganic are similar for monatomic ions the interface is often ‘blurred’ due to knock‐on and other damage mechanisms. We have used a quartz crystal technique to measure the total sputter yield for argon cluster ions in a number of materials important in medical implants, biomaterials and diagnostic devices, including polymethyl methacrylate, collagen, hydroxyapatite, borosilicate glass, soda lime glass, silicon dioxide and the native oxides on titanium and stainless steel. These data fit a simple semi‐empirical equation very well, so that the total sputter yield can now be estimated for any of them for the entire range of cluster ion energy typical in XPS or SIMS. On the basis of our total sputter yield measurements, we discuss three useful ‘figures‐of‐merit’ for choosing the optimum cluster ion energy to use in depth profiling organic/inorganic samples. For highest selectivity in removing the organic but not the inorganic material the energy‐per‐atom in the cluster should be below 6 eV. A practical balance between selectivity and reasonably rapid depth profiling is achieved by choosing a cluster ion energy having between around 3 and 9 eV energy‐per‐atom. Copyright © 2013 John Wiley & Sons, Ltd.     

Observations on X‐ray enhanced sputter rates in argon cluster ion sputter depth profiling of polymers

Traditionally polymer depth profiling by X‐ray photoelectron spectroscopy (XPS) has been dominated by the damage introduced by the ion beam rather than the X‐rays. With the introduction of polyatomic and especially argon gas cluster ion‐beam (GCIB) sources for XPS instruments, this is no longer the case, and either source of damage may be important (or dominate) under particular conditions. Importantly, while ion‐beam damage is a near‐surface effect, X‐ray damage may extend micrometres into the bulk of the sample, so that the accumulation of X‐ray damage during long depth profiles may be very significant. We have observed craters of similar dimensions to the X‐ray spot well within the perimeter of sputter craters, indicating that X‐rays can assist GCIB sputtering very significantly. We have measured experimentally sputter craters in 13 different polymers. The results show that X‐ray exposure can introduce much more topography than might previously have been expected, through both thermal and direct X‐ray degradation. This can increase the depth of a crater by a remarkable factor, up to three in the case of poly‐L‐lactic acid and polychlorotrifluorothylene under reasonably normal XPS conditions. This may be a major source of the loss of depth resolution in sputter depth profiles of polymers. Copyright © 2012 John Wiley & Sons, Ltd.