XPS for non-destructive depth profiling and 3D imaging of surface nanostructures

Depth profiling of nanostructures is of high importance both technologically and fundamentally. Therefore, many different methods have been developed for determination of the depth distribution of atoms, for example ion beam (e.g. O₂⁺, Ar⁺) sputtering, low-damage C₆₀ cluster ion sputtering for depth profiling of organic materials, water droplet cluster ion beam depth profiling, ion-probing techniques (Rutherford backscattering spectroscopy (RBS), secondary-ion mass spectroscopy (SIMS) and glow-discharge optical emission spectroscopy (GDOES)), X-ray microanalysis using the electron probe variation technique combined with Monte Carlo calculations, angle-resolved XPS (ARXPS), and X-ray photoelectron spectroscopy (XPS) peak-shape analysis. Each of the depth profiling techniques has its own advantages and disadvantages. However, in many cases, non-destructive techniques are preferred; these include ARXPS and XPS peak-shape analysis. The former together with parallel factor analysis is suitable for giving an overall understanding of chemistry and morphology with depth. It works very well for flat surfaces but it fails for rough or nanostructured surfaces because of the shadowing effect. In the latter method shadowing effects can be avoided because only a single spectrum is used in the analysis and this may be taken at near normal emission angle. It is a rather robust means of determining atom depth distributions on the nanoscale both for large-area XPS analysis and for imaging. We critically discuss some of the techniques mentioned above and show that both ARXPS imaging and, particularly, XPS peak-shape analysis for 3D imaging of nanostructures are very promising techniques and open a gateway for visualizing nanostructures.     

High resolution depth profile analysis by elastic recoil detection with heavy ions

Elastic recoil detection (ERD) with energetic heavy ions (e.g. 60–120 MeV¹²⁷I) is a suitable method to measure depth profiles of light and medium heavy elements in thin films. The advantages of this method are reliable and quantitative results and elementally and isotopically resolved depth profiles. A relative energy resolution of 0.07% has been measured in real ERD-experiments using the Q3D magnetic spectrograph at the Munich tandem accelerator and a large solid angle of detection of 5 msr. The good energy resolution allows atomic depth resolution near to the surface which has been obtained at flat and smooth carbon samples. A large solid angle of detection is necessary to measure a depth profile with the desired accuracy before the sample is significantly altered by the ion beam. As an example carbon profiles of thin carbon layers, prepared by a laser plasma ablation deposition process, have been investigated revealing the high depth resolution and its power to resolve elemental profiles at gradiated interfaces.     

Laser ablation isotope ratio mass spectrometry for enhanced sensitivity and spatial resolution in stable isotope analysis

Stable isotope analysis permits the tracking of physical, chemical, and biological reactions and source materials at a wide variety of spatial scales. We present a laser ablation isotope ratio mass spectrometry (LA‐IRMS) method that enables δ¹³C measurement of solid samples at 50 µm spatial resolution. The method does not require sample pre‐treatment to physically separate spatial zones. We use laser ablation of solid samples followed by quantitative combustion of the ablated particulates to convert sample carbon into CO₂. Cryofocusing of the resulting CO₂ coupled with modulation in the carrier flow rate permits coherent peak introduction into an isotope ratio mass spectrometer, with only 65 ng carbon required per measurement. We conclusively demonstrate that the measured CO₂ is produced by combustion of laser‐ablated aerosols from the sample surface. We measured δ¹³C for a series of solid compounds using laser ablation and traditional solid sample analysis techniques. Both techniques produced consistent isotopic results but the laser ablation method required over two orders of magnitude less sample. We demonstrated that LA‐IRMS sensitivity coupled with its 50 µm spatial resolution could be used to measure δ¹³C values along a length of hair, making multiple sample measurements over distances corresponding to a single day's growth. This method will be highly valuable in cases where the δ¹³C analysis of small samples over prescribed spatial distances is required. Suitable applications include forensic analysis of hair samples, investigations of tightly woven microbial systems, and cases of surface analysis where there is a sharp delineation between different components of a sample. Copyright © 2011 John Wiley & Sons, Ltd.     

MALDI Mass Spectrometry Imaging of Neuronal Cell Cultures

Mass spectrometry imaging (MSI) provides the ability to detect and identify a broad range of analytes and their spatial distributions from a variety of sample types, including tissue sections. Here we describe an approach for probing neuropeptides from sparse cell cultures using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MSI—at single cell spatial resolution—in both MS and tandem MS modes. Cultures of Aplysia californica neurons are grown on an array of glass beads embedded in a stretchable layer of Parafilm M. As the membrane is stretched, the beads/neurons are separated physically and the separated beads/neurons analyzed via MALDI TOF MS. Compared with direct MS imaging of samples, the stretching procedure enhances analyte extraction and incorporation into the MALDI matrix, with negligible analyte spread between separated beads. MALDI tandem MSI using the stretched imaging approach yields localization maps of both parent and fragment ions from Aplysia pedal peptide, thereby confirming peptide identification. This methodology represents a flexible platform for MSI investigation of a variety of cell cultures, including functioning neuronal networks.     

Depth profiling of light elements in PAMBE‐grown GaN and helium‐implanted titanium with heavy ion time‐of‐flight elastic recoil detection

The heavy ion time‐of‐flight elastic recoil detection analysis (HI‐ERDA) technique was used to investigate the possibility of measuring near‐surface elemental depth profiles of light and mid‐Z elements in thin films of plasma‐assisted molecular beam epitaxy (PAMBE)‐grown GaN and helium‐implanted titanium. The great advantage of HI‐ERDA is the ability to measure mass‐separated elemental depth profiles simultaneously. However for some materials it is not certain whether HI‐ERDA can be used successfully because significant sputtering or other beam‐induced damage may occur. The damage to the surfaces by a 77 MeV iodine beam was assessed using RBS, AFM and profilometry. The results show that for thin PAMBE‐grown polycrystalline GaN films and for titanium that has been heavily implanted with helium a significant modification of the near‐surface region is caused by the probing heavy ion beam. For the PAMBE‐grown GaN films the most significant loss trend is observed for nitrogen. Surprisingly this was not accompanied by a change in surface topology. In contrast, an almost complete removal of the heavily helium‐implanted surface layer was measured for the titanium specimens. The investigation shows that reference measurements with additional techniques such as RBS, AFM and profilometry have to be performed to ascertain sample integrity before HI‐ERDA data can be used. Copyright © 2004 John Wiley & Sons, Ltd.     

Toward accurate characterization of nitrogen depth profiles in ultrathin oxynitride films

Good accuracy in depth profile analyses of nitrogen in ultrathin oxynitride films is desirable for process development and routine process monitoring. Low energy SIMS is one of the techniques that has found success in the accurate characterization of thin oxynitride films. This work investigated the artifacts in a typical depth profile analysis of nitrogen with the current SIMS technique and the ways to improve the accuracy by selecting optimal analytical conditions. It was demonstrated that surface roughness developed rapidly in a SiO₂/Si stack when it was bombarded with an O₂⁺ beam at 250 eV and angle of incidence from 70 to 79° . The roughness caused distortion in the measured depth profiles of nitrogen and the major component elements. However, the above roughness and the distortion in the depth profiles can be eliminated by using a 250 eV O₂⁺ beam at an angle of incidence above 80° . Depth profile analyses with a 250 eV 83° O₂⁺ beam exhibited minimal surface roughening and insignificant variation in the secondary ion yield of SiN^? from SiO₂ bulk to the SiO₂/Si interface, facilitating an accurate analysis of nitrogen distribution in a SiO₂/Si stack. In addition, depth profiles of the major component elements such as ¹⁸O^? and ²⁸Si^? delivered clear information on the location of the SiO₂/Si interface. Using the new approach, we compared nitrogen distribution in thin SiNO films with the decoupled‐plasma nitridation (DPN) at various powers. Copyright © 2008 John Wiley & Sons, Ltd.     

Two photon excitation of the vacuum ultraviolet fluorescence of carbon monoxide

The vacuum ultraviolet fourth positive band spectrum (A¹Πχ¹σ⁺) of CO was excited by two photon absorption of ultraviolet radiation which was produced by frequency doubling the output of a nitrogen laser pumped tunable dye laser. The (0–0) through (9–0) bands were observed. Position monitoring of the ultraviolet beam provided feedback control for continuous adjustment of the orientation of the angle tuned frequency doubling crystal. In this way the orientation for maximum efficiency of the doubling crystal and the point of focus of the ultraviolet beam in the sample cell was maintained during the wavelength scan of a band. A solar-blind photomultiplier was used to detect the vacuum ultraviolet fluorescence. O, P, Q, R, and S rotational branches were observed, and the new band-head positions agree with those predicted from the previously observed single-photon spectra.     

Nanoscale imaging of hydrogen and sodium in alteration layers of corroded glass using ToF-SIMS: Is an auxiliary sputtering ion beam necessary?

The hydrogen (H)/sodium (Na) interface is of great interest in glass corrosion research. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is one of the few techniques that can provide nanoscale H and Na imaging simultaneously. However, the optimized condition for ToF-SIMS imaging of H in glass is still unclear. In H depth profiling using ToF-SIMS, H background control is a key, in which an analysis ion beam and a sputtering ion beam work together in an interlaced mode to minimize it. Therefore, it is of great interest to determine if an auxiliary sputtering ion beam is also necessary to control H background in ToF-SIMS imaging of H. In this study, H imaging with and without auxiliary sputtering beams (Cs⁺, O₂⁺, and Ar_n⁺) was compared on a corroded international simple glass (ISG). It was surprising that the H/Na interface could be directly imaged using positive ion imaging without any auxiliary sputtering ion beam under a vacuum of 2 to 3 × 10⁻⁸ mbar. The H⁺ background was about 5% atomic percent on the pristine ISG glass, which was significantly lower than the H concentration in the alteration layer (~15%). Moreover, positive ion imaging could show distributions of other interesting species simultaneously, providing more comprehensive information of the glass corrosion. If an auxiliary O₂⁺ sputtering ion beam was used, the H⁺ background could be reduced but still higher than that in the depth profiling. Besides, this condition could cause significant loss of signal intensities due to strong surface charging.     

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.     

共聚焦拉曼测量中激光焦点在球形液滴中的穿透深度

针对在共聚焦拉曼测量中激光在球形液滴中的聚焦问题建立理论模型, 推导出激光焦点在球形液滴中的穿透深度公式, 将激光焦点在球形液滴中的实际穿透深度和拉曼平台的垂直移动距离关联起来. 研究表明, 平台移动距离和激光焦点移动距离是非线性的, 激光焦点分别聚焦于液滴上表面和球心处时, 激光焦点的移动距离等于平台移动距离, 而当激光焦点在液滴上表面和液滴球心之间时, 激光焦点的移动距离大于平台移动距离. 并利用此结论初步获得MgSO₄球形液滴胶态结构的空间分布信息. 发现MgSO₄液滴在低湿度下形成的胶态结构是具有一定厚度的壳状结构, 且其厚度与液滴所处环境的相对湿度有关.     

Novel approach to enhance sensitivity in surface‐assisted laser desorption/ionization mass spectrometry imaging using deposited organic‐inorganic hybrid matrices

Sample pretreatment is key to obtaining good data in matrix‐assisted laser desorption/ionization mass spectrometry imaging (MALDI‐MSI). Although sublimation is one of the best methods for obtaining homogenously fine organic matrix crystals, its sensitivity can be low due to the lack of a solvent extraction effect. We investigated the effect of incorporating a thin film of metal formed by zirconium (Zr) sputtering into the sublimation process for MALDI matrix deposition for improving the detection sensitivity in mouse liver tissue sections treated with olanzapine. The matrix‐enhanced surface‐assisted laser desorption/ionization (ME‐SALDI) method, where a matrix was formed by sputtering Zr to form a thin nanoparticle layer before depositing MALDI organic matrix comprising α‐cyano‐4‐hydroxycinnamic acid (CHCA) by sublimation, resulted in a significant improvement in sensitivity, with the ion intensity of olanzapine being about 1800 times that observed using the MALDI method, comprising CHCA sublimation alone. When Zr sputtering was performed after CHCA deposition, however, no such enhancement in sensitivity was observed. The enhanced sensitivity due to Zr sputtering was also observed when the CHCA solution was applied by spraying, being about twice as high as that observed by CHCA spraying alone. In addition, the detection sensitivity of these various pretreatment methods was similar for endogenous glutathione. Given that sample preparation using the ME‐SALDI‐MSI method, which combines Zr sputtering with the sublimation method for depositing an organic matrix, does not involve a solvent, delocalization problems such as migration of analytes observed after matrix spraying and washing with aqueous solutions as sample pretreatment are not expected. Therefore, ME‐Zr‐SALDI‐MSI is a novel sample pretreatment method that can improve the sensitivity of analytes while maintaining high spatial resolution in MALDI‐MSI.     

Microstructure fabrication with a CO2 laser system: characterization and fabrication of cavities produced by raster scanning of the laser beam

In this paper we describe the use of a CO(2) laser for production of cavities and microstructures in poly(methyl methacrylate) (PMMA) by moving the laser beam over the PMMA surface in a raster pattern. The topography of the cavities thus produced is studied using stylus and optical profilometry and scanning electron microscopy (SEM). The microstructures display artifacts from the laser ablation process and we describe how the laser ablation parameters can be optimized in order to minimize these artifacts. Using this technique it is possible to generate structures with a depth from 50 microm and a minimum width of approximately 200 microm up to depth and widths of several mm, governed by the beam size and the laser settings.     

Chemical maps of patterned samples by microline‐imaging laser‐induced plasma spectrometry

The potential of a microline‐imaging laser‐induced plasma spectrometry (LIPS) system for surface and depth analysis of heterogeneous solid samples in air at atmospheric pressure has been demonstrated. A pulsed Nd : YAG laser beam operating at 532 nm, with a homogeneous energy distribution (flat top laser), was used to generate a microline plasma on the sample surface. Subsequent light from the microline plasma was resolved spectrally and spatially and detected with an imaging spectrograph and an intensified charged‐coupled device detector. A patterned metal sample was chosen as the most appropriate for this study. Three‐dimensional chemical maps of Ni and Cu from the edge connectors of a printed circuit board have been obtained. With this experimental configuration, the lateral resolution (limited by crater width) was 42 µm and the spatial resolution along the spectrometer slit was 17.4 µm. The results illustrate the capability of microline imaging for fast mapping of large‐area samples and for depth profiling purposes. Copyright © 2003 John Wiley & Sons, Ltd.     

Depth profiling of Irganox‐3114 nanoscale delta layers in a matrix of Irganox‐1010 using conventional Cs+ and O2+ ion beams

Depth profiling of an organic reference sample consisting of Irganox 3114 layers of 3 nm thickness at depths of 51.5, 104.5, 207.6 and 310.7 nm inside a 412 nm thick Irganox 1010 matrix evaporated on a Si substrate has been studied using the conventional Cs⁺ and O₂⁺ as sputter ion beams and Bi⁺ as the primary ion for analysis in a dual beam time‐of‐flight secondary ion mass spectrometer. The work is an extension of the Versailles Project on Advanced Materials and Standards project on depth profiling of organic multilayer materials. Cs⁺ ions were used at energies of 500 eV, 1.0 keV and 2.0 keV and the O₂⁺ ions were used at energies of 500 eV and 1.0 keV. All four Irganox 3114 layers were identified clearly in the depth profile using low mass secondary ions. The depth profile data were fitted to the empirical expression of Dowsett function and these fits are reported along with the full width at half maxima to represent the useful resolution for all the four delta layers detected. The data show that, of the conditions used in these experiments, an energy of 500 eV for both Cs⁺ beam and O₂⁺ beam provides the most useful depth profiles. The sputter yield volume per ion calculated from the slope of depth versus ion dose matches well with earlier reported data. Copyright © 2013 John Wiley & Sons, Ltd.     

Azobenzene Liquid‐Crystalline Polymer for Optical Switching of Grating Waveguide Couplers with a Flat Surface

Grating waveguide couplers with a flat surface were fabricated in an azobenzene liquid‐crystalline polymer film by holographic lithography using Ar⁺ laser beams at 488 nm. When a probe beam at 633 nm was incident to one grating of a grating waveguide coupler, the beam propagated in the waveguide and an output beam came out from the other grating with the throughput coupling efficiency of ≈5%. Upon irradiation of the film between two gratings with UV light to cause trans–cis photoisomerization of the azobenzene moiety, the intensity of the output beam was repeatedly switched. It was found that the alternating irradiation at 366 and 436 nm induced reversible changes in the intensity of the guided probe beam.

     

Micro and surface analysis with fast heavy ions

The desorption of atomic and molecular species from surfaces bombarded by fast heavy ions (Z ? 20; E ? 0.5 MeV/amu) is attractive for surface and microscopic characterization. Only a low-intensity probe beam is needed, the escape depth of desorbed species is shallow (ca. 10 Å), and desorbed ions are efficiently detected with a time-of-flight mass spectrometer. Thus, particle-induced desorption mass spectrometry (PDMS) maintains sample integrity and charging effects are avoided. PDMS is useful for surface analysis of glasses and plastics by using californium-252 fission fragments for bombardment. Inorganic and organic surface constituents can be detected simultaneously; mass resolution is good. For lithium in glass, the detection limit is about 1 pg (ca. 100 μg g^?1. The PDMS technique can be combined with sequential ion etching for depth profiling. The feasibility of PDMS for microscopic analysis with a collimated 84-MeV Kr⁷⁺ beam (target diameter ca. 11 μm) is discussed.     

Argon cluster cleaning of Ga+ FIB-milled sections of organic and hybrid materials

Secondary ion mass spectrometry studies have been made of the removal of the degraded layer formed on polymeric materials when cleaning focused ion beam (FIB)-sectioned samples comprising both organic and inorganic materials with a 30-keV Ga⁺ FIB. The degraded layer requires a higher-than-expected Ar gas cluster ion beam (GCIB) dose for its removal, and it is shown that this arises from a significant reduction in the layer sputtering yield compared with that for the undamaged polymer. Stopping and Range of Ions in Matter calculations for many FIB angles of incidence on flat polymer surfaces show the depth of the damage and of the implantation of the Ga⁺ ions, and these are compared with the measured depth profiles for Ga⁺-implanted flat polymer surfaces at several angles of incidence using an Ar⁺ GCIB. The Stopping and Range of Ions in Matter depth and the measured dose give the sputtering yield volume for this damaged and Ga⁺-implanted layer. These, and literature yield values for Ga⁺ damaged layers, are combined on a plot showing how the changing sputtering yield is related to the implanted Ga density for several polymer materials. This plot contains data from both the model flat poly(styrene) surfaces and FIB-milled sections showing that these 2 surfaces have the same yield reduction. The results show that the damaged and Ga⁺-implanted layer's sputtering rate, after FIB sectioning, is 50 to 100 times lower than for undamaged polymers and that it is this reduction in sputtering rate, rather than any development of microtopography, that causes the high Ar⁺ GCIB dose required for cleaning these organic surfaces.     

Depth-resolved analysis by laser-induced breakdown spectrometry at reduced pressure

The 581 nm output from a dye laser in a fluence range between 2.86 and 11.47 J cm⁻² was used to ablate pure Zn and Fe foils. The average ablation rate (AAR, μm per shot) was calculated for different experimental variables (buffer gas, pressure, laser fluence and focal conditions). Deposition of previously ablated material in the ablation crater results in large variation of the observed AAR values. This effect was observed in air and argon buffer gases at atmospheric pressure. The situation is largely alleviated at reduced pressure due to free expansion of the ablated material. Under these circumstances the capability of laser-induced plasmas to resolve interfacial structures is improved. The effect on depth-resolved studies was checked with a commercial Zn-coated steel sample. Due to the Gaussian-like energy distribution of the incident laser beam, the material is ablated to produce a conical crater. This fact ensures that the Zn signal remains for a longer time because the ablated region spreads over the edge gradually. At low pressure the emission peaks are better defined and the background becomes flat. However, these conditions produce also the lowest net intensities and some peaks are not detected. An Ar atmosphere produces more intense spectral lines at both pressure levels. Best analytical results were obtained at reduced pressure, with a slight improvement in depth resolution in the presence of Ar. © 1998 John Wiley & Sons, Ltd.     

On the use of single-photon ionization for inorganic surface analysis

Summary A general surface analysis method has been developed based on non-selective photoionization of sputtered or desorbed neutral atoms and molecules above the surface, followed by time-of-flight mass spectrometry. The approach, currently utilizes two main types of ionizing radiation and a variety of desorption probes. For photoionization, coherent untuned sources are used; an intense focused pulsed UV laser beam is used for non-resonant multiphoton ionization to give elemental and limited chemical information, usually used for inorganic analysis; a coherent VUV source is used for single-photon ionization at 118 nm (10.5 eV) produced by frequency tripling of 355 nm light from a Nd:YAG laser. This paper focuses on single-photon ionization for inorganic systems. The desorption probes used are ion, electron, and laser beams as well as thermal desorption. For depth profiling, ion beams are specifically used. Any focused desorption probe beam can provide lateral spatial resolution.     

Studies of Sb and Bi cluster produced by laser desorption

Direct production of cations and anions of metal clusters of Sb and Bi by laser evaporation in a vacuum has been studied. Bulk sample substrates are irradiated by 1064, 532 and 355 nm beams at variable intensity, and the ions produced are accelerated and identified by time-of-flight mass spectrometry. At 1064 nm, the cation distributions show that Sb ₃ ⁺ and Bi ₃ ⁺ are the most abundant species, while the monomer and dimer cations are almost non-existent. The anion spectra indicate very low yields of Sb^? and Bi^? with dominant dimer anion species. These patterns persist with laser power variation within the stable operation domain. With lower incident laser wavelength, the mass distributions are modified, favouring the production of the light cluster ions. In no circumstances were Sb and Bi ions withn>5 observed. Many of the observed phenomena can be explained if one assumes that for these elements, clusters withn<6 are formed on the substrate surface. Cluster ions are produced via a prompt desorption process, and are subjected to photon induced reactions due to the same incident laser beam. However, more detailed investigation of the desorption properties will be necessary to confirm such a desorption mechanism.