Molecular depth profiling of organic and biological materials

Atomic depth profiling using secondary ion mass spectrometry, SIMS, is common in the field micro-electronics; however, the generation of molecular information as a function of sample depth is difficult due to the accumulation of damage both on and beneath the sample surface. The introduction of polyatomic ion beams such as SF₅ and C₆₀ have raised the possibility of overcoming this problem as they deposit the majority of their energy in the upper surface of the sample resulting in increased sputter yields but with a complimentary reduction in sub-surface damage accumulation. In this paper we report the depth profile analysis of the bio-polymer polycaprolactone, PCL, using the polyatomic ions and and the monoatomic Au⁺. Results are compared to recent analysis of a similar sample using . depth profiling of cellulose is also demonstrated, an experiment that has been reported as unsuccessful when attempted with implications for biological analysis are discussed.     

Cesium redeposition artifacts during low energy ToF-SIMS depth profiling

H-terminated Si samples were preloaded with Cs by performing ToF-SIMS depth profiles (250 eV Cs⁺, 15 keV Ga⁺) until the steady state was reached both with and without a bias of +40 V applied to the ion extraction electrode. Xe⁺ depth profiles (350 eV Xe⁺, 15 keV Ga⁺) were obtained inside and around the Cs craters with and without applying the 40 V bias. The results indicate that the maximum of the Cs⁺ signal of the Xe⁺ depth profiles shifts to the surface if no bias is applied, either during the Cs⁺ sputtering or during the Xe⁺ sputtering (i.e., the profiles are broadest with both biases (Cs⁺ and Xe⁺) on and narrowest and closest to the surface if both biases are off). This effect can be explained by the electric field, caused by the bias, deflecting the sputtered low energy Cs⁺ ions back to the surface.     

Quantitative depth profiling of an alternating Pt/Co multilayer and a Pt-Co alloy multilayer by SIMS using a Buckminsterfullerene (C60) source

A Buckminsterfullerene ion beam has been applied to the depth profiling of an alternating pure Pt and pure Co multilayer. Quantitative depth profiling was performed by secondary ion mass spectrometry (SIMS) with C₆₀ ions using Pt-Co alloy films with different compositions. Relative sensitivity factors (RSFs) derived from a Pt₃₉Co₆₁ alloy film were used to convert an original depth profile to a composition depth profile. A severe interface artifact observed in the depth profile of a Pt/Co multilayer was quantitatively correlated with a gradual variation of matrix composition through the Pt/Co and Co/Pt interfaces by comparison with the depth profiling of an alloy multilayer film. Moreover, the interface artifact could be compensated by conversion of the profile to a composition profile using the same RSFs. The depth resolutions of a Pt/Co multilayer derived from the composition depth profile were much larger than the apparent interface widths measured from the original depth profile due to the nonlinear relationship between the Co and Pt ion intensities and their compositions.     

Caesium/xenon dual beam depth profiling: Velocity of the sputtered atom and ionization probability

In this work, a caesium/xenon co-sputtering gun was used to perform depth profiles of a RhSi layer with varying caesium beam concentration. The positive ion yields were monitored with respect to the varying work function of the solid and the intensities of the ions were plotted with respect to the caesium surface concentration. As expected by the tunneling model, all the M⁺ signals decrease exponentially with the increasing caesium beam concentration. Moreover, the heaviest ion yields decrease faster than the lighter ion ones. This phenomenon can be explained by the different velocities of the departing atoms, which has an important impact on the ionization processes. We then studied the variations of the MCs⁺ yields with respect to the caesium surface concentration and with respect to the nature of the departing atom. Finally, we applied models based on the tunneling model in order to fit our results.     

Compositional depth profiling of ultrathin oxynitride/Si interface using XPS

The atomic-scale compositional depth profiling of oxynitride/Si interface by applying the maximum entropy concept to the angle-resolved Si 2p photoelectorn spectra was performed after eliminating the effect of photoelectron diffraction on the Si 2p photoelectron spectra arising from Si substrate for oxynitride films containing the maximum nitrogen concentration of 3 and 6 at.% at the SiO₂/Si(1 0 0) interface. Although the distribution of intermediate oxidation states of Si depends on the amount of nitrogen atoms incorporated at the interface, total amount of intermediate oxidation states of Si does not depend on the nitrogen atoms incorporated at the interface.     

Molecular depth profiling of trehalose using a C60 cluster ion beam

Molecular depth profiling of organic overlayers was performed using a mass selected fullerene ion beam in conjunction with time-of-flight (TOF-SIMS) mass spectrometry. The characteristics of depth profiles acquired on a 300-nm trehalose film on Si were studied as a function of the impact kinetic energy and charge state of the C₆₀ projectile ions. We find that the achieved depth resolution depends only weakly upon energy.     

Molecular depth profiling of multi-layer systems with cluster ion sources

Cluster bombardment of molecular films has created new opportunities for SIMS research. To more quantitatively examine the interaction of cluster beams with organic materials, we have developed a reproducible platform consisting of a well-defined sugar film (trehalose) doped with peptides. Molecular depth profiles have been acquired with these systems using C₆₀⁺ bombardment. In this study, we utilize this platform to determine the feasibility of examining buried interfaces for multi-layer systems. Using C₆₀⁺ at 20 keV, several systems have been tested including Al/trehalose/Si, Al/trehalose/Al/Si, Ag/trehalose/Si and ice/trehalose/Si. The results show that there can be interactions between the layers during the bombardment process that prevent a simple interpretation of the depth profile. We find so far that the best results are obtained when the mass of the overlayer atoms is less than or nearly equal to the mass of the atoms in buried molecules. In general, these observations suggest that C₆₀⁺ bombardment can be successfully applied to interface characterization of multi-layer systems if the systems are carefully chosen.     

Molecular depth profiling and imaging using cluster ion beams with femtosecond laser postionization

The emergence of cluster ion sources as viable SIMS probes has opened new possibilities for detection of neutral molecules by laser postionization. Previous studies have shown that with atomic bombardment multiphoton ionization using high-power femtosecond pulses leads to photofragmentation. The large amount of photofragmentation can be mostly attributed to high amounts of internal energy imparted to the sputtered molecules during the desorption process. Several pieces of preliminary data suggest that molecules subjected to cluster beam bombardment are desorbed with lower internal energies than those subjected to atomic beam bombardment. Lower energy molecules may then be less likely to photodissociate creating less photofragments in the laser postionization spectra. Here we present data taken from coronene films prepared by physical vapor deposition comparing a 40 keV ion source with a 20 keV Au⁺ ion source, which supports this hypothesis. Furthermore, the depth profiling capabilities of cluster beams may be combined with laser postionization to obtain molecular depth profiles by monitoring the neutral flux. In addition, imaging and depth profiling may be combined with atomic force microscopy (AFM) to provide three-dimensional molecular images.     

Quantitative ToF-SIMS studies of protein drug release from biodegradable polymer drug delivery membranes

Biodegradable polymers are of interest in developing strategies to control protein drug delivery. The protein that was used in this study is Keratinocyte Growth Factor (KGF) which is a protein involved in the re-epithelialization process. The protein is stabilized in the biodegradable polymer matrix during formulation and over the course of polymer degradation with the use of an ionic surfactant Aerosol-OT (AOT) which will encapsulate the protein in an aqueous environment. The release kinetics of the protein from the surface of these materials requires precise timing which is a crucial factor in the efficacy of this drug delivery system.Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used in the same capacity to identify the molecular ion peak of the surfactant and polymer and use this to determine surface concentration. In the polymer matrix, the surfactant molecular ion peak was observed in the positive and negative mode at m/z 467 and 421, respectively. These peaks were determined to be [AOT + Na⁺] and [AOT − Na⁺]. These methods are used to identify the surfactant and protein from the polymer matrix and are used to measure the rate of surface accumulation. The second step was to compare this accumulation rate with the release rate of the protein into an aqueous solution during the degradation of the biodegradable film. This rate is compared to that from fluorescence spectroscopy measurements using the protein autofluorescence from that released into aqueous solution [C.M. Mahoney, J. Yu, A. Fahey, J.A.J. Gardella, SIMS depth profiling of polymer blends with protein based drugs, Appl. Surf. Sci. 252 (2006), 6609-6614.].     

Deconvolution of very low primary energy SIMS depth profiles

In this paper, the deconvolution of SIMS profiles analysed at very low primary energy (0.5 keV/O₂⁺) is addressed. The depth resolution function (DRF) of the SIMS analysis in presence of roughness is established and a deconvolution procedure is implemented without or in presence of roughness on samples containing delta-doped layers of boron in silicon. It is shown that the deconvolution procedure can lead to a great improvement of the full width at half maximum (FWHM) of the measured peaks in the case where no roughness in detected in the crater bottom. In the case where it is present, the conditions required to use a deconvolution procedure are discussed, and the deconvolution is implemented using precise and restrictive assumptions.     

On the temperature dependence of Na migration in thin SiO2 films during ToF-SIMS O2 depth profiling

The detection of Na in insulating samples by means of time of flight-secondary ion mass spectrometry (ToF-SIMS) depth profiling has always been a challenge. In particular the use of O₂⁺ as sputter species causes a severe artifact in the Na depth distribution due to Na migration under the influence of an internal electrical filed. In this paper we address the influence of the sample temperature on this artifact. It is shown that the transport of Na is a dynamic process in concordance with the proceeding sputter front. Low temperatures mitigated the migration process by reducing the Na mobility in the target. In the course of this work two sample types have been investigated: (i) A Na doped PMMA layer, deposited on a thin SiO₂ film. Here, the incorporation behavior of Na into SiO₂ during depth profiling is demonstrated. (ii) Na implanted into a thin SiO₂ film. By this sample type the migration behavior could be examined when defects, originating from the implantation process, are present in the SiO₂ target. In addition, we propose an approach for the evaluation of an implanted Na profile, which is unaffected by the migration process.     

Thin films of polymer blends deposited by matrix-assisted pulsed laser evaporation: Effects of blending ratios

In this work, we show successful use of matrix-assisted pulsed laser evaporation (MAPLE) for obtaining thin films of PEG:PLGA blends, in the view of their use for controlled drug delivery. In particular, we investigate the influence of the blending ratios on the characteristics of the films. We show that the roughness of the polymeric films is affected by the ratio of each polymer within the blend. In addition, we perform Fourier transformed infrared spectroscopy (FTIR) measurements and we find that the intensities ratios of the infrared absorption bands of the two polymers are consistent with the blending ratios. Finally, we assess the optical constants of the polymeric films by spectroscopic ellipsometry (SE). We point out that the blending ratios exert an influence on the optical characteristics of the films and we validate the SE results by atomic force microscopy and UV-vis spectrophotometry. In all, we stress that the ratios in which the two polymers are blended have significant impact on the morphology, chemical structure and optical characteristics of the polymeric films deposited by MAPLE.     

SIMS as a subnanometer probe: A new tool for chemical profile analysis of grafted molecules

The complexity of modern engineered surfaces requires the development of very powerful methods to analyze and characterize them. We demonstrate that it is possible to obtain chemical information about the skeleton of organic molecules constituting SAMs grafted on a silicon surface by using a new type of SIMS method. A profile can be achieved by the investigation of the temporal variation of secondary ion intensities that correspond to the fractional parts of the molecule constituting the SAMs. The equivalent ablation rate is less than 0.5 nm/min.     

Temperature-controlled depth profiling in polymeric materials using cluster secondary ion mass spectrometry (SIMS)

Secondary ion mass spectrometry (SIMS) employing an SF₅⁺ polyatomic primary ion source was used to depth profile through poly(methylmethacrylate) (PMMA), poly(lactic acid) (PLA) and polystyrene (PS) thin films at a series of temperatures from −125 °C to 150 °C. It was found that for PMMA, reduced temperature analysis produced depth profiles with increased secondary ion stability and reduced interfacial widths as compared to analysis at ambient temperature. Atomic force microscopy (AFM) images indicated that this improvement in interfacial width may be related to a decrease in sputter-induced topography. Depth profiling at higher temperatures was typically correlated with increased sputter rates. However, the improvements in interfacial widths and overall secondary ion stability were not as prevalent as was observed at low temperature. For PLA, improvements in signal intensities were observed at low temperatures, yet there was no significant change in secondary ion stability, interface widths or sputter rates. High temperatures yielded a significant decrease in secondary ion stability of the resulting profiles. PS films showed rapid degradation of characteristic secondary ion signals under all temperatures examined.     

Accurate depth profiling of dry oxidized SiGeC thin films by extended Full Spectrum ToF-SIMS

The abundance of work on SiGe based devices demonstrates the importance of compositional characterization of such materials. However accurate SIMS depth profiling of SiGe, and especially of SiGe/silicon dioxide interfaces can be difficult due to matrix effects.Therefore, we highlight here the improvements brought by the extended Full Spectrum protocol, presented in previous works and allowing minimization of matrix effects. Previous studies on this protocol showed that it was extremely precise and reproducible for Ge and impurity quantification in non oxidized matrices. In this study we thus investigated its accuracy for simultaneous quantitative depth profiling of both matrix elements (Si, Ge, O) and impurities (C) in Si_0.82Ge_0.16C_0.02 layers annealed in oxidizing atmosphere, by comparing results with more classic protocols. The profiles provided by the extended Full Spectrum protocol were found to be more accurate than the others, especially around interfaces. This results in a better comprehension of the behaviour of SiGeC layers under oxidizing anneal and thus allows the fabrication of very well controlled three dimensional Ge nanowire structures for next generation devices.     

Effect of primary oxygen ion implantation on SIMS depth profiling in glasses

The influence of the primary oxygen ion implantation on SIMS in-depth profiles in halide and chalcogenide glasses was examined. Various behaviours of particular profiles were generally explained in terms of the chemical affinity of analysed reactants and modifications of the glass structure induced by primary ions.     

Chemical effects in C60 irradiation of polymers

The C₆₀ erosion behaviour of poly(methyl)methacrylate (PMMA), poly(α-methyl)styrene (PAMS) and polystyrene (PS) has been studied at various temperatures and compared with that under Ga⁺ irradiation. Strong variations of erosion yields are observed, indicating that chemical degradation mechanisms are operating. In particular, our results suggest that fast depolymerization mechanisms are important in leaving the surface of the sputter crater virtually undamaged. Since such mechanisms are connected with the chemical nature of the polymer, the possibility of performing molecular depth profiling of polymers with C₆₀ appears to depend strongly on the chemical nature of the system under study.     

Correction of optical distortions in dry depth profiling with confocal Raman microspectroscopy

We present a generalized approach to obtain improved Raman intensity profiles from in‐depth studies performed by confocal Raman microspectroscopy (CRM) with dry objectives. The approach is based on regularized deconvolution of the as‐measured confocal profile, through a kernel that simulates optical distortions due to diffraction, refraction and collection efficiency on the depth response. No specific shape or restrictions for the recovered profile are imposed. The strategy was tested by probing, under different instrumental conditions, a series of model planar interfaces, generated by the contact of polymeric films of well‐defined thickness with a substrate. Because of the aforementioned optical distortions, the as‐measured confocal response of the films appeared highly distorted and featureless. The signal computed after deconvolution recovers all the films features, matching very closely with the response expected. Copyright © 2011 John Wiley & Sons, Ltd.     

常态中子深度分析物理模型

带电离子在样品内输运过程中的能量岐离、探测立体角有效范围及探测系统噪声都将改变中子深度分析（NDP）的多道离子能谱分布。假定上述三种因素导致的离子探测谱能量展宽都可以采用高斯函数描述,基于一般条件下的NDP装置设计,建立了预测一定深度核素产生的离子测量谱的探测器响应函数物理模型。该模型的理论分析结果与实验测量能谱的峰位、半高宽均符合良好,能够作为中子深度分析测量谱反演分析的基础。     

Expanding the metrology of Coulombic efficiency using neutron depth profiling

The metrology of Coulombic efficiency (CE) is widely used for the qualification of rechargeable batteries particularly in assessing their state of health and the expected lifetime. The quantity of CE is conventionally defined as the ratio of the electric charge capacity in one round-trip cycle of discharge and charge of a battery. For better diagnosis of the health, failure and lifetime of rechargeable batteries, it is desirable to expand the traditional CE of the round-trip electric charge to including that of the round-trip CE of ionic charges as well as those of single trip CEs of ionic/electric charges for charging and electric/ionic charges for discharging. To enable such expansion, in operando ion metrology is needed. For the mainstream technology of lithium-containing batteries, neutron depth profiling is one of the techniques suited for Li ion metrology. In this report, we demonstrate the application of in situ neutron depth profiling to expanding the metrology of Coulombic efficiencies to include all four CEs. The methodology of expanded CE is exemplified using measurements and analyses on a model battery of LiFePO₄ vs. Li.