Gas Cluster Ion Research Articles

Combining advanced photoelectron spectroscopy approaches to analyse deeply buried GaP(As)/Si(1 0 0) interfaces: Interfacial chemical states and complete band energy diagrams

The epitaxial growth of the polar GaP(100) on the nonpolar Si(100) substrate suffers from inevitable defects at the antiphase domain boundaries (APDs), resulting from mono-atomic steps on the Si(100) surface. Stabilization of Si(100) substrate surfaces with As is a promising technological step enabling the preparation of Si substrates with double atomic steps and reduced density of the APDs. In this paper, 4–50-nm-thick GaP epitaxial films were grown on As-terminated Si(100) substrates with different types of doping, miscuts, and As-surface termination by metalorganic vapor phase epitaxy (MOVPE). The GaP(As)/Si(100) heterostructures were investigated by X-ray photoelectron spectroscopy (XPS) combined with gas cluster ion beam (GCIB) sputtering and by hard X-ray photoelectron spectroscopy (HAXPES). We found residuals of As atoms in the GaP lattice (∼0.2–0.3 at.%) and a localization of As atoms at the GaP(As)/Si(100) interface (∼1 at.%). Deconvolution of core level peaks revealed interface core level shifts. In As core levels, chemical shifts between 0.5 and 0.8 eV were measured and identified by angle-resolved XPS measurements. Similar valence band offset (VBO) values of 0.6 eV were obtained, regardless of the doping type of Si substrate, Si substrate miscut or type of As-terminated Si substrate surface. The band alignment diagram of the GaP(As)/Si(1 0 0) heterostructure was deduced.

How innovations in methodology offer new prospects for volume electron microscopy

Detailed knowledge of biological structure has been key in understanding biology at several levels of organisation, from organs to cells and proteins. Volume electron microscopy (volume EM) provides high resolution 3D structural information about tissues on the nanometre scale. However, the throughput rate of conventional electron microscopes has limited the volume size and number of samples that can be imaged. Recent improvements in methodology are currently driving a revolution in volume EM, making possible the structural imaging of whole organs and small organisms. In turn, these recent developments in image acquisition have created or stressed bottlenecks in other parts of the pipeline, like sample preparation, image analysis and data management. While the progress in image analysis is stunning due to the advent of automatic segmentation and server‐based annotation tools, several challenges remain. Here we discuss recent trends in volume EM, emerging methods for increasing throughput and implications for sample preparation, image analysis and data management.

Gas Cluster Ion Beams as a Versatile Soft-Landing Tool for the Controlled Construction of Thin (Bio)Films

Surface biofunctionalization with proteins is the key to many biomedical applications. In this study, a solvent-free method for the controlled construction of protein thin films is reported. Using large argon gas cluster ion beams, proteins are sputtered from a target (a pool of pure proteins), and collected on a chosen substrate, being nearly any solid material. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) revealed the presence of intact protein molecules on the collectors. Furthermore, lowering the energy per atom in the cluster projectiles down to 1 eV/atom allowed more than 60% of bradykinin molecules to be transferred intact. This protein deposition method offers a precise control of the film thickness as the transferred protein quantity is proportional to the argon clusters ion dose reached for the transfer. This major feature enables building protein films from (sub)mono- to multilayers, without upper limitation of the thickness. A procedure was developed to measure the film thickness in situ the ToF-SIMS instrument. The versatility and potential of this soft-landing alternative for further applications is demonstrated on the one hand by building a protein thin film at the surface of paper, a substrate hardly compatible with solution-based adsorption methods. On the other hand, the possibility to achieve alternated multilayer buildup is demonstrated with the construction of a bilayer composed of bradykinin and Irganox, with the two layers well separated. These results lay the first stone toward original and complex multilayers that could previously not be considered with solution-based adsorption methods, and this regardless of the substrate nature.

Positive and negative cluster ions of amino acids formed by electrospray droplet impact/secondary ion mass spectrometry (EDI/SIMS)

Cluster ions of amino acids (AAs) have been measured by several ionization methods such as electrospray, sonic spray, thermal desorption, and gas cluster ion beam (Arn+) bombardment. Prior research has primarily focused on positively charged cluster ions, whereas the formation of negatively charged cluster ions remains largely unexplored. In this study, mass spectra of both positive and negative cluster ions for 20 amino acids were measured by electrospray droplet impact/secondary ion mass spectrometry (EDI/SIMS). Because EDI is capable of generating abundant negative ions as well as positive ions, it is best suited for the comparative study on the dependence of polarity for AA cluster ions. For AAs with apolar side chains such as Ala, Val, Leu, and Ile, while large cluster sizes of positive cluster ions [n(AA) + H]+ (n = 11–21) were observed, negative cluster ions [n(AA) − H]− were observed with only n up to 3. The AAs with polar side chains (Ser, Thr, Asn, Gln, Tyr and Cys) gave similar n up to 7–12 for both [n(AA) + H]+ and [n(AA) − H]−. For basic AAs (Lys and Arg), the maximum n values observed for [n(AA) + H]+ are larger than those for [n(AA) − H]− and vice versa for acidic AA (Asp and Glu). A sharp intensity drop with n→(n+1) was observed for [n(AA) + H]+ with n of 12, 8, 8 and 11 for Pro, Ser, Thr, and Tyr, respectively. The magic number 8 for positive and 6 for negative cluster ions of Ser and Thr are in agreement with previous work. No magic numbers were observed for [n(AA) + H]+ and [n(AA) − H]− for acidic amino acids (Asp and Glu). The ligands (n−1)AAs in the negative clusters [AA − H]−⋅⋅⋅⋅(n−1)AA are bound more loosely than those in the positive clusters [AA + H]+ ⋅⋅⋅⋅(n−1)AA. The stability of amino acid cluster ions is critically affected by the polarities and structures of amino acids.

Young’s Modulus of Fatty Acid Adsorption Films on Si Surface Measured Using Ar Gas Cluster Ion Beams

Young’s Modulus of Fatty Acid Adsorption Films on Si Surface Measured Using Ar Gas Cluster Ion Beams

Gas cluster etching for the universal preparation of polymer composites for nano chemical and mechanical analysis with AFM

Atomic force microscopy (AFM) based techniques that probe nanomechanical and nanochemical features are valuable tools for studying composite surfaces at high-resolution. However, the application of AFM, particularly at resolutions less than 50 nm per pixel, is limited to sample surfaces that are smooth relative to the tip dimensions and damage-free. As a result, high-resolution AFM is limited in technologically relevant polymer composite systems where current sample preparation techniques can fail to produce the necessary pristine surfaces. Here, we apply gas cluster ion beam (GCIB) etching to prepare composite materials for nanomechanical and nanochemical analysis. We demonstrate that the GCIB can effectively remove material and produce damage free surfaces for composites comprised of PMMA or PS confined in the 7 nm diameter pores of an organosilicate matrix. AFM-IR measurements on the etched region indicate a high degree of hydrogen bonding between the carbonyl groups of the confined PMMA and the hydroxyl groups on the pore walls, while maps of loss tangent can identify the 7 nm polymer domains within the organosilicate matrix. This work demonstrates that GCIB is an effective tool for nanocomposite surface preparation and could enable a new range of materials to be studied with AFM at high resolution.

In-depth analysis of iodine in artificial biofilm model layers by variable excitation energy XPS and argon gas cluster ion sputtering XPS.

Here, we present a study on agarose thin-film samples that represent a model system for the exopolysaccharide matrix of biofilms. Povidone-iodide (PVP-I) was selected as an antibacterial agent to evaluate our x-ray photoelectron spectroscopy (XPS)-based methodology to trace specific marker elements, here iodine, commonly found in organic matrices of antibiotics. The in-depth distribution of iodine was determined by XPS analyses with variable excitation energies and in combination with argon gas cluster ion beam sputter cycles. On mixed agarose/PVP-I nanometer-thin films, both methods were found to solve the analytical task and deliver independently comparable results. In the mixed agarose/PVP-I thin film, we found the outermost surface layer depleted in iodine, whereas the iodine is homogeneously distributed in the depth region between this outermost surface layer and the interface between the thin film and the substrate. Depletion of iodine from the uppermost surface in the thin-film samples is assumed to be caused by ultrahigh vacuum exposure resulting in a loss of molecular iodine (I2) as reported earlier for other iodine-doped polymers.

A Microscopic View of Macromolecule Transfer in the Vacuum Using Gas and Bismuth Clusters

Transferring large nonvolatile molecules in the vacuum is key to both their characterization by mass spectrometry (ion mobility, etc.) and their use as nanofabrication building blocks in soft-landing-type experiments. Recently, our group performed the successful transfer and redeposition of intact and bioactive lysozyme (14 kDa) with large gas cluster ion beams, in the absence of a solvent or matrix, demonstrating that such beams could serve as tools for macromolecule manipulation. A number of fundamental questions arose in relation with this experimental proof-of-concept, concerning for instance the maximum molecular size for successful transfer, the effect of the cluster projectile incidence angle, the dependence of molecular dissociation on the projectile energy and size, the internal energy uptake ultimately leading to metastable decay reactions, or the influence of the surface interactions on desorption. To address these questions, a series of molecular dynamics simulations were conducted involving cluster impacts on an adsorbed globular polystyrene (PS) macromolecule of 60 kDa, with a mass and shape comparable to those of a protein such as bovine serum albumin. Conditions of intact desorption of this PS molecule by Ar clusters are identified, in terms of energy per atom and incidence angle, and the results are compared to 30 keV Bi5 impacts, commonly used for molecular analysis and 2D imaging in our time-of-flight secondary ion mass spectrometry experiments. When Ar cluster impact conditions are appropriate for intact desorption, the take-off angle of the macromolecule is larger than the projectile incidence. Bombardment by a massive methane projectile (6.6 × 104 methane molecules) does not induce qualitatively different results, and in general, the energy per atom or energy per unit mass remains the deciding factor concerning the survival or dissociation of the bombarded molecule. In contrast with large gas clusters, Bi5 projectiles largely fail at desorbing the intact macromolecules, because of the detrimental effect of the collision cascade and the insufficient momentum transferred by the crater expansion for molecular lift off. Additionally, 10 keV Ar5000 projectiles with 45–75° incidence with respect to the surface normal directly transfer momentum to the macromolecule via their backscattered Ar atoms and small clusters, inducing molecular desorption with center-of-mass velocities of the order of 1 km/s. The implications for future experiments are discussed.

Microscopic Structure and Binding Mechanism of the Corrosion-Protective Film of Oleylpropanediamine on Copper in Hot Water

The structure of the film formed by oleylpropanediamine (OLDA) on the copper (Cu) metal surface in water at 150 °C was investigated by combining quantitative NMR and surface characterization methods. We succeeded in quantifying the amount of film formation by precisely determining all mass balances in the systems examined. 2D IR microscopic mapping showed that the film thickness is uneven in the horizontal direction with a length scale of ∼100 μm and hundreds of OLDA layers. This film thickness was also confirmed by AFM. The analysis of the C–H stretching vibrational frequency disclosed that the alkyl chains are highly ordered in the layers close to the Cu surface and are conformationally disordered in the layers distant from the Cu surface in the thicker portion of the film. Combining XPS measurements using argon gas cluster ion beam etching with the ICP-AES analysis, we revealed that the key to multiple layering is the formation of a coordination complex of the unprotonated amino groups of OLDA with Cu that presumably results in polymer chain-like network structures. Contact angle measurements at different OLDA concentrations and treatment times showed that the water repellency of the film originated from the thick layering of OLDA molecules with disordered hydrophobic chains.

On the origin of nanoripples on silicon by gas cluster-ion irradiation

The first stage of nanoripples formation on silicon surface under inclined gas cluster ion beam irradiation is investigated. It was shown that ordered ripples appear as the result of emerging and self-organization of single standing protrusions spontaneously developed on the surface. The shapes of protrusions and ripples depending on the ion fluence and the angle of incidence are discussed.

Surface preparation of metal films by gas cluster ion beams using organic acid vapor for Cu–Cu bonding

Surface-activated bonding (SAB) of Cu by gas cluster ion beam (GCIB) irradiation with acetic acid vapor was studied. GCIB irradiation realizes surface smoothing and surface reaction enhancement without severe damage. Therefore, it is promising for SAB. In this study, acetic acid vapor was introduced during Ar-GCIB irradiation to assist the removal of surface oxides on the Cu surface. XPS results showed that Cu(OH)2 was effectively removed by reaction with adsorbed acetic acid, and there was no residue by acetic acid adsorption. In addition, surface roughness decreased by Ar-GCIB irradiation with acetic acid because of the preferential removal of protrusion. Preliminary bonding experiments showed an increase of Cu–Cu bond strength by Ar-GCIB irradiation with acetic acid vapor.

Ion distribution in iron oxide, zinc and manganese ferrite nanoparticles studied by XPS combined with argon gas cluster ion beam sputtering

In this work, the influence of X-ray irradiation and two types of surface sputtering methods is thoroughly studied. The first one, monoatomic sputtering, resulted in the reduction of iron oxide to the metallic iron in a short time of sputtering. The second studied technique is argon cluster sputtering, which uses lower energies per atom, hence the modification of the structure of the samples is greatly reduced. Based on the XPS characterization of Fe3O4 single crystal the fitting procedure for the Fe 2p spectra for nanoparticles is established. The preliminary results showed that X-rays combined with cluster sputtering resulted in alteration of the morphology and solidification of the surface of the samples. In the response to the morphology modification, the Spot-by-Spot Cluster Procedure is proposed. It allows to thoroughly characterize the layer-by-layer structure of the nanoparticles, without changing it. The Procedure was used to determine the structure of zinc, manganese and zinc-manganese ferrite nanoparticles. Finally, the results show, that depending on the used dopant, its incorporation and the state of iron are changed.

Time of Flight – Secondary Ion Mass Spectroscopy Profiling of Self-Assembled Monolayer Patterns Based on Vapor Deposition Technique

It is crucial to develop novel metrology techniques in a semiconductor fabrication process for characterizing a thin film with a thickness of a few nanometers, as well as the material profile of the film accurately. Uniform trichlorosilane (1H,1H,2H,2H-perfluorodecyltrichlorosilane) derived self-assembled monolayer film patterns were fabricated by several conventional semiconductor fabrication techniques incorporated, including photolithography, vapor deposition, and lift-off technique. The patterned trichlorosilane self-assembled monolayer patterns were thoroughly characterized by Time-of-Flight Secondary Ion Mass Spectrometry, and precise two-dimensional self-assembled monolayer patterns were reconstructed through corresponding ions and ion clusters feature peaks. Additionally, three-dimensional self-assembled monolayer patterns were reconstructed layer by layer through gas cluster ion beam etching and two-dimensional mapping analysis in an alternate way, which verifies that vapor-based trichlorosilane self-assembled monolayer patterns were precisely fabricated and Time-of-Flight Secondary Ion Mass Spectrometry is highly surface-sensitive to obtain accurate information about the self-assembled monolayer pattern in a point-by-point manner. Time-of-Flight Secondary Ion Mass Spectrometry technique could be used as a metrology technique in the semiconductor process with high-quality self-assembled monolayer micro and nanostructures.

Surface preparation of metal films by gas cluster ion beams using organic acid vapor for Cu-Cu bonding

Abstract Surface activated bonding of Cu by gas cluster ion beam (GCIB) irradiation with acetic acid vapor was studied. GCIB irradiation realizes surface smoothing and surface reaction enhancement without severe damage. Therefore, it is promising for surface-activated bonding (SAB). In this study, acetic acid vapor was introduced during Ar-GCIB irradiation to assist removal of surface oxides on Cu surface. XPS results showed that Cu(OH)2 was effectively removed by reaction with adsorbed acetic acid, and there was no residue by acetic acid adsorption. In addition, surface roughness decreased by Ar-GCIB irradiation with acetic acid because of the preferential removal of protrusion. Preliminary bonding experiments showed increase of Cu-Cu bond strength by Ar-GCIB irradiation with acetic acid vapor.

X-ray Photoelectron Spectroscopy Equipped with Gas Cluster Ion Beams for Evaluation of the Sputtering Behavior of Various Nanomaterials

The selection of ion-beam species and parameters for depth profiling requires experience to obtain accurate depth information and an efficient sputter rate on diverse samples. Sputter damage that causes changes in the chemical state and composition should be avoided. Monoatomic ion-beam sources are commonly utilized for the rapid sputtering of metals and inorganics, but sputter damage to highly oxidized metal oxides and organics is well-known. Cluster ion beams have become popular in recent years for soft materials because of their capability for low damage to organics, but sputter yields of metal and inorganic materials are very low. Currently, the material properties and structures of semiconductor devices have become more complicated because of rapid technical development. Thus, it is necessary to explore more sputtering methods for samples with hybrid architecture that contain metals/inorganics/organics together. Recently, gas cluster ion beams (GCIBs) with small cluster sizes were considered to be one of the methods with the advantages of both monatomic ion beams and conventional large-cluster GCIBs (>Ar2000+). A smaller-cluster GCIB results in higher energy per atom with a sputtering behavior similar to that of the monatomic ion beam and leads to enhancement of the sputter yield in metal and inorganic layers. However, the discussion of the cluster size selection for X-ray photoelectron spectroscopy (XPS) chemical state analysis during depth profiling is rather limited. In this work, a GCIB cluster measurement kit is developed and installed in an XPS system. The actual distribution of the cluster size of each GCIB setting can be measured before depth profiling. Depth profiling using GCIBs with a series of cluster sizes is performed to examine the sputter yield and evaluate the sputter effect on various organic, inorganic, and hybrid nanomaterials. The results also show the different sputtering behaviors of GCIBs with different cluster sizes (Ar500–2000+).

Optical constants of diamond-like carbon films formed by gas cluster ion beam assisted deposition

We investigated the refractive index (n) and extinction coefficient (k) of diamond-like carbon (DLC) films formed by gas cluster ion beam (GCIB)-assisted deposition. DLC films are classified into four types based on n and k values (at λ = 2.25 eV). We formed the nonclassified DLC film which has high n (2.4–3.0) and k (0.75–1.0) values. The Raman peak intensity ratio (I D/I G) of this film was 0.601, which corresponds to the value of a-C:H or ta-C:H. The film density was 2.71 g cm−3. The nonclassified DLC film has intermediate properties between a-C:H and ta-C:H with respect to the density and k.

Effects of Cs+ and Arn+ ion bombardment on the damage of graphite crystals

Intercalation mechanisms and diffusion or segregation phenomena in graphitic materials play a crucial role in different applied science fields. The investigation of such phenomena is usually accomplished through depth profiling experiments. Ar-GCIBs (Argon- Gas Cluster Ion Beams) are commonly adopted for in-depth concentration profiling of organic or soft materials; on the other hand, cesium ions are in general more suitable for the sputtering of inorganics. During such experiments, the beam-target interaction could alter chemistry and structure of the material. In this work, we define the optimal conditions in terms of both sputtering ion source and energy to preserve the crystal features. HOPG was used as a model system to compare morphological, physical, and chemical effects induced by different Arn+ clusters, and ultra-low energy Cs+ beam during ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometry) depth profiling experiments. We demonstrated, through in-situ AFM (Atomic Force Microscopy) analysis, that the monoatomic Cs+ beam alters to a lower extent the HOPG structure. On the contrary, Ar-GCIBs strongly modify the graphite surface basal plane and underlying layers. However, HOPG crystals treated with the cesium monoatomic source undergo a chemistry modification leading to the formation of graphite oxide (GOx) together with the presence of hydrogen, and cesium adducts.

SIMS Analysis of Thin EUV Photoresist Films.

This study reports on the application of secondary ion mass spectrometry (SIMS) for examining thin (20-50 nm) chemically amplified resist films on silicon. SIMS depth profiling was carried out using a gas cluster ion beam to ensure minimal sputter-induced damage to the organic constituents of interest. Specific attention concerned the distribution of the photo acid generator (PAG) molecule within these films, along with the photo-induced fragmentation occurring on extreme ultra-violet photo exposure. Positive secondary ion spectra were collected using a traditional time of flight (ToF)-SIMS and the latest generation IONTOF Hybrid SIMS instrumentation equipped with an OrbitrapTM mass analyzer. Tandem mass spectrometry (MS/MS) capability within the OrbitrapTM secondary ion column was utilized to verify that the C19H17S+ secondary ion did indeed have the molecular structure consistent with the PAG structure. The superior mass resolving power of the OrbitrapTM mass analyzer (∼20× of the ToF mass analyzer) along with improved mass accuracy (a few ppm) proved pivotal in the mass spectral and depth profile analysis of these films. This was not the case for the ToF-SIMS experiments, as the mass spectra, as well as the associated depth profiles, exhibited severe molecular (isobaric) interferences.

Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) Profiling of Self Assembled Monolayer (SAM) Patterns Based on Vapor Deposition Technique

It is crucial to develop novel metrology techniques in the semiconductor fabrication process to accurately measure a film’s thickness in a few nanometers, as well as the material profile of the film. Highly uniform trichlorosilane (1H,1H,2H,2H-perfluorodecyltrichlorosilane, FDTS) derived SAM film patterns were fabricated by several conventional semiconductor fabrication methods combined, including photolithography, SAM vapor deposition, and the lift-off technique. Substantial information can be collected for FDTS SAM film patterns when Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) techniques are incorporated to investigate this material. Precise two-dimensional (2D) FDTS SAM film patterns were reconstructed through mapping analysis of corresponding elements and chemical state peaks by AES and XPS. Additionally, three-dimensional (3D) FDTS SAM film patterns were also reconstructed layer by layer through gas cluster ion beam (GCIB) etching and XPS analysis. These characterization results demonstrate that FDTS SAM film patterns based on the vapor deposition method are highly uniform because the vacuum and precise gas-delivery system exclude ambient environmental interference efficiently and ensure reaction process repeatability. AES and XPS techniques could be used for metrology applications in the semiconductor process with high-quality SAM microstructures and nanostructures.

Sensitivity enhancement using chemically reactive gas cluster ion beams in secondary ion mass spectrometry (SIMS)

We report for the first time on significant molecular secondary ion yield increases by modifying the chemistry of a water cluster primary ion beam. This was demonstrated using 70‐keV ion beams of 0.15 eV/amu. For the neutral drug Bezafibrate, secondary ion yield enhancements ×5–10 were observed when replacing the Ar carrier gas in a water gas cluster ion beam (GCIB) source with a mixture containing 12% CO2 and 2% O2 in Ar. For the cationic drug Ranitidine, the ion yield enhancements using the CO2‐containing carrier gas were up to ×20–50 in positive mode and ×2–4 in negative mode. The extent of molecular fragmentation was very similar from both cluster beams. We conclude that additional chemically reactive species are present in the impact zone using the (H2O/CO2)n projectile, which promote the formation of secondary ions of both polarity through projectile impact‐induced chemical reactions. This methodology can be applied to further extend the capabilities of high‐resolution 3‐dimensional mass spectral imaging using reactive GCIB‐SIMS.