Differential Charging Effects Research Articles

Differential charging effects from impurities in pyrolytic graphite

This work reports differential charging effects in samples of pyrolytic graphite. Despite being considered a nuisance in XPS analyses, differential charging effects can be exploited for the identification and quantification of chemical species in heterogeneous materials. In this case, charging effects were useful to identify impurities at the surface of pyrolytic graphite. For these impurities, the analysis of the O 1s, Si 2p, and Al 2p core levels allowed distinguishing the contributions of SiO2 and Al2O3 from those of silicone/alumino-silicate oxides. Though both groups of compounds are insulators, a splitting of the above peaks was induced by the use of the flood gun device of the spectrometer when the samples were mounted on a conductive metallic sample holder. The shape and relative concentration of the chemical species ascribed to the split peaks served as a basis for the decomposition of the corresponding O 1s, Si 2p, and Al 2p peaks registered over a ceramic sample holder. The latter allowed for inducing a well controlled differential charging of the samples that led to the identification of true chemical shifts, i.e. those ascribed to variations on the chemical environment of the elements. A qualitative model for describing the observed charging effects was postulated. The model was based on an analogy between electrical circuits and the geometrical configuration of the impurity particles intercalated within the graphite particles.

Carbon doping controlled thermoluminescent defect centers in nanoporous alumina for ion beam dosimetry

The flexibility of amorphous anodized alumina (AAO) in developing radiation dosimeter for hadron therapy is reported by controlled carbon ion implantation, followed by thermoluminescence (TL) measurements. The efficacy of amorphous AAO in controlling TL sensitivity is found to be governed by an increase in F+ defect centers as a function of carbon concentration, as revealed from the close resemblance of the trend in photoluminescence intensity. Moreover, its nanoporous structure is demonstrated to be advantageous for defect engineering due to the increase in the surface-to-volume ratio. Detailed X-ray photoelectron spectroscopy analysis suggests the formation of F+ centers by substituting Al3+ ions with C2+ in the vicinity of oxygen vacancies, where depth-dependent study showed the evolution of conducting channels owing to sp2 hybridized C–C bonding, leading to a differential charging effect. This work provides a direction to tune nanoporous AAO in its amorphous form for future ion beam dosimetry.

MODIFICATION OF SURFACE OF DOUBLE WALL CARBON NANO TUBES BY FULLERENE C60

The composite samples based on double-wall carbon nano tubes and fullerenes followed by laser treatment were prepared. XPS confirmed existence of essential contact between two components of the composite. The differential charging effect disappears after laser processing, which induces photopolymerization of fullerene clusters. The TEM showed close-packed continuous coating of cross-linked C60 clusters formed on the surface of nano tubes after laser irradiation.

Initial Transient Phenomena Impact on Plasma CVD of Ultrathin Silicon Nitride and Silicon Carbon Nitride Dielectrics for Nano Devices Cu-Low k Interconnects

The initial transient (ITS) phenomena in plasma enhanced Chemical Vapor Deposition (PECVD) process was identified in early 1980’s (1). This ITS phenomenon has impact on the bonding structure and composition of plasma deposited silicon nitride (1) and amorphous silicon (2) films at the initial film-substrate interface. When the deposited film is in micron thickness, the ITS impact is generally smaller. The shrinking of semiconductor devices and the corresponding push to increase device density is continuing in the quest for improved performance of integrated circuits. Nano scale thickness low temperature plasma CVD dielectric films such as silicon nitride and silicon carbon nitride used as cap materials (3) for Cu interconnects that are expected to reach to sub-10 nm dimension next few years. The compositional variation in PECVD films at the interface by ITS which had a relatively small impact in past semiconductor technologies is becoming a significant factor in film’s properties as we scale film thickness down to sub-10nm. In this paper, we will present in detail the impact of the initial transient plasma in CVD process on the ultrathin nano thick cap and-low k dielectric films that are being used for advanced device interconnect fabrication, We will show that we can modify the current standard PECVD process to alleviate the impact of ITS phenomena. Some ITS impact on nano thick film are: a) PECVD silicon nitride / silicon carbon nitride dielectric cap films are normally deposited with a mixture of silane/ammonia or other carbosilane/ammonia precursors. The initial breakdown of silane is strongly affected by the plasma conditions and the dissociation thermodynamic of the precursors. Deposition rates can be different on Cu versus dielectric surfaces due differences in charging effects (see Figure 1). Additionally, the silane molecule is relatively unstable and more reactive as compared to ammonia, and typically dissociate rapidly first to form more conformal SiHx deposition radicals before the plasma reaches a steady state. Subsequently, PECVD with SiH4/NH3 precursors will normally produce a Si-rich silicon nitride composition at the interface. Silicon carbon nitride films can be deposited from more stable carbosilane precursors. In this case, the thickness of the Si-rich interface zone can be reduced, and a C-rich film surface may be deposited under specific deposition condition. This modulation may be exploited to control electrical (k, breakdown/leakage) and adhesion properties of the film for Cu low k interconnect structures. For a very thin dielectric cap deposited on a patterned structure of Cu and ultra-low k dielectric, the film thickness deposited on Cu versus the dielectric surface will be slightly different due to the differential local plasma charging effect during initial transient deposition period as shown in fig 1. This deposition rate difference will impact the film’s barrier and electrical properties, gap fill performance and subsequent in Cu-low k fabrication. In this case, plasma ITS charging is the principle mechanism that cause differences in film’s thickness. b) For typical PECVD SiN deposition process, the initial dissociation of SiH4 and/or NH3 will create reactive species that subsequently etch the deposited ultra-low k pSiCOH dielectric surface. These reactive species can produce substantial damage to pSiCOH, increase dielectric constant and subsequently poor reliability in final electronic devices. This damage can be reduced significantly by using an initially low rf plasma power and then increasing to a higher rf plasma power for the remainder of the deposition (4). ITS negative impact to devices and mitigation method will be discussed. c) For ultrathin cap films for Cu interconnects, such as SiN, variations in film composition at the interface will cause significant variation in electrical properties and conformality. To mitigate the ITS effect, a cyclic, multi-step deposition process utilizing plasma surface treatment was developed. A multilayers film with better conformality and consistent film’s properties was achieved. Using this novel cyclic process, high quality dielectrics SiN and SiNO at sub-10 nm thickness can be deposited consistently and reproducibly for Cu-low k interconnect fabrication as showed in Figure 2. In summary, the ITS phenomena on plasma CVD, its impact in nano thick film fabrication and methods to mitigate the ITS’s negative impact will be presented in the paper.

XPS characterization of MWCNT and C60-based composites

ABSTRACTX-ray photoelectron spectroscopy was used for characterization of MWNT-C60 composites with assistance of scanning and transmission electron microscopy. The composites were received by immerison of nanotubes into fullerene solution in CS2 with consequent processing with green laser irradiation at aproximately 10 W/cm2. It was shown that C1s spectra of composites are different from simple addition of MWNT and C60 features. It found that preliminary nanotube purification by oxidative treatment influences strongly the nature of fullerene-nanotube bonding. Hydrophilic surface of oxidized nanotube material has lower affinity to C60 molecules and provides for concentration of C60 molecules into clusters, which show differential charging effect in XPS spectra. The differential charging effect disappears after laser processing, which induces photopolymerization of fullerene clusters. The as-synthesized nanotubes form composites with more evenly distributed fullerene.

Cryogenic XPS study of fast-frozen sulfide minerals: Flotation-related adsorption of n-butyl xanthate and beyond

Cryogenic XPS of wet particulate samples separated via centrifugation and fast-frozen allows quasi in situ examination of solid surfaces, adsorbates, and reaction products, largely preventing the loss both of volatiles and hydrated species at mineral/water interfaces. Here, the cryo-XPS has been applied to characterize the surfaces and interfacial layers of natural pyrite (FeS2), chalcopyrite (CuFeS2), and galena (PbS) in solutions of a common flotation collector, potassium n-butyl xanthate (KBX), in conjunction with zeta-potential measurement. It was found, in particular, that dibutyl dixanthogen was the major adsorbate at pyrite in 0.1mM KBX and 10mM KBX solutions; dixanthogen and cuprous xanthate in the next stage were formed on chalcopyrite, and predominant chemisorbed butyl xanthate was present at galena, including in 10mM KBX solution. The results may suggest that the production of dixanthogens at the interface has been underestimated while the quantities of surface metal xanthates could be over evaluated in previous studies. Pronounced differential charging effects were observed in the XPS experiment for the samples moderately hydrophobized by the xanthate treatment; we proposed that the effect was due to electrically isolated mineral particles with hydrophobic and ice-repellent surfaces, which retained, however, some frozen water islets.

Development of plasma etching processes to pattern sub-15 nm features with PS-b-PMMA block copolymer masks: Application to advanced CMOS technology

The best strategies to transfer nanoholes formed from the self-assembly of Polystyren/Polymethylmethacrylate (PS/PMMA) based block copolymers into a silicon substrate are investigated. The authors show that specific issues are associated with the plasma etching of materials through the PS masks obtained from self-assembly. Indeed, due to the nanometric size of sub-15 nm contact holes and to their inherently high aspect ratio (>5), plasma etching processes typically used to etch SiO2 and silicon in the microelectronic industry must be revisited. In particular, processes where the etching anisotropy relies on the formation of passivation layer on the feature's sidewalls are not adapted to nanometric dimensions because these layers tend to fill the holes leading to etch stop issues. At the same time, the ion bombarding energy must be increased as compared to a typical process to overcome differential charging effects in high aspect-ratio nanoholes. However, by developing appropriate processes—such as synchronized pulsed plasmas—the authors show that it is possible to etch 70 nm deep holes into silicon by using block copolymers and a hard mask strategy. Another interesting observation resulting from these experiments is that for sub-15 nm holes, a critical dimension (CD)-dispersion of few nm leads to strong aspect ratio dependent etch rates. In addition, a careful analysis of the dispersion of the holes' CD after each plasma steps shows that the CD control is far from satisfying advanced CMOS technology requirements. A critical issue comes from the uncompleted PMMA removal from the PS/PMMA matrix during our self-assembly process: variable amount of PMMA remains in the PS holes, leading to microloading effects during the etching steps, which in turn generates CD-control loss. This problem perhaps can be solved by combining UV exposure to acetic acid treatment to provide PS masks free of PMMA residues before plasma etching.

Investigation of HfO2 high-k dielectrics electronic structure on SiO2/Si substrate by x-ray photoelectron spectroscopy

In this Letter, x-ray photoelectron spectroscopy (XPS) was used to investigate electronic structures of HfO2 with various thicknesses on SiO2/Si substrate prepared by atomic layer deposition with post deposition annealing of 600 °C, which is compatible with a gate last process in the fabrication of complementary metal oxide semiconductor. The hafnium silicate formed between HfO2 and SiO2 interface plays a key role in generating an internal electric field. This can be attributed to the presence of oxygen vacancies (Vo) at the interface. The XPS binding energy shifts of O 1s, Si 2p, and Hf 4f spectra with increasing HfO2 thicknesses can be explained by the presence of the internal field and differential charging effects. Electrical measurements of capacitor structures support the XPS result.

Use of the differential charging effect in XPS to determine the nature of surface compounds resulting from the interaction of a Pt/(BaCO3 + CeO2) model catalyst with SO x

Changes in the chemical composition of the surface of a Pt/(BaCO3 + CeO2) model NOx storage-reduction catalyst upon its interaction with SOx (SO2 (260 Pa) + O2 (2600 Pa) + H2O (525 Pa)) followed by regeneration in a mixture of CO (2100 Pa) with H2O (525 Pa) were studied by X-ray photoelectron spectroscopy (XPS). Model catalyst samples were prepared as a thin film (about several hundreds of angstrom units in thickness) on the surface of tantalum foil coated with a layer of aluminum oxide (∼100 A). It was found that the Pt/BaCO3 and Pt/CeO2 catalyst constituents acquired different surface charges (differential charging) in the course of photoelectron emission; because of this, it was possible to determine the nature of surface compounds formed as a result of the interaction of the catalyst with a reaction atmosphere. It was found that barium carbonate was converted into barium sulfate as a result of reaction with SOx on the surface of BaCO3 at 150°C. As the treatment temperature in SOx was increased to 300°C, the formation of sulfate on the surface of CeO2 was observed. The sulfatization of CeO2 was accompanied by the reduction of Ce(IV) to Ce(III). The regeneration reaction of the catalyst treated in SOx at 300°C resulted in the consecutive decomposition of cerium(III) sulfate at ≤500°C and then barium sulfate at 600–700°C. Upon the decomposition of BaSO4, a portion of sulfur was converted into a sulfide state, probably, because of the formation of BaS.

X‐ray photoelectron spectroscopy study of MgH2 thin films grown by reactive sputtering

AbstractIn this work the indepth compositional variations of MgH2 thin films grown directly by reactive sputtering in H2 gas were studied over time. XPS was performed using a Thermo Scientific Theta Probe with monochromatic Al Kα radiation (hν = 1486.6 eV). Measurements at different sample depths were obtained by Ar+ ion sputtering, and high‐resolution Mg 2p and O 1s spectra were used to quantify the data. Indications of differential charging effects were observed in the O 1s spectra. At the unsputtered surface a layer of Mg(OH)2 was found to grow with air exposure time. Underneath this layer, both MgH2 and MgO were present in near equal amounts, and deeper in the film there was an increasing amount of metallic Mg. Apart from the growth of Mg(OH)2 the film appeared to be stable in air over the time period of the experiment. Copyright © 2010 John Wiley & Sons, Ltd.

Use of the differential charging effect in XPS to determine the nature of surface compounds resulting from the interaction of a Pt/BaCO3/Al2O3 model catalyst with NO x

Use of the differential charging effect in XPS to determine the nature of surface compounds resulting from the interaction of a Pt/BaCO3/Al2O3 model catalyst with NO x

New method to calibrate binding energy using Au nanocolloids in X-ray photoelectron analysis of diamondlike carbon films with different electrical resistivities

A new method to calibrate the binding energy ( E B) using Au nanocolloids as a calibrant in XPS analysis of diamondlike carbon (DLC) is proposed by considering the DLC films with different electrical resistivities. A few microliters of a dilute aqueous solution containing Au nanocolloids were dropped onto a small local surface area of the DLC film, which became a stain before XPS measurements by gradually drying in vacuo. The observed peak E B of the C 1s spectrum at another native surface (an area without Au nanocolloids) of the DLC film was calibrated by setting that of the Au 4f 7/2 spectrum of the Au nanocolloids to 84.0 (83.98 ± 0.02) eV. The adequacy of this method was investigated by considering the correlation among the full width at half maximums (FWHMs) of the Au 4f 7/2 spectra of the Au nanocolloids on the DLC surfaces and that of a Au plate as a reference. Consequently, the FWHM of the Au 4f 7/2 spectrum of the Au nanocolloids on the DLC surface is a candidate to investigate the differential charging effect of the DLC surface, and the calibration method is reliable if the FWHM agrees with that of the Au plate.

XPS and QCM Studies of Hydrocarbon and Fluorocarbon Polymer Films Bombarded by 1−20 keV C60 Ions

Poly(methyl methacrylate) or PMMA, Teflon AF1600, and poly(3-hexylthiophene) films are studied after bombardment with different fluences of C60 ions with energies of 1−20 keV by a quartz crystal microbalance (QCM) and X-ray photoelectron spectroscopy (XPS). C 1s XP spectra show little or no change in the film chemistry upon ion bombardment up to 1013−1014 ions/cm2. This result supports prior observations from secondary ion yields that at least some organic films have no apparent static limit in secondary ion mass spectrometry (SIMS) using C60 ion projectiles until the film is sputtered away. Changes in C 1s XP spectra observed at the highest ion energies and fluences >1015 ions/cm2 are explained predominantly by differential charging effects. Some changes in film composition are also observed, but their extent varies with the polymer studied. Measurements for the total sputter yield of films acquired using a QCM show that each C60 cluster leads to an efficient emission of ∼105 amu per C60 ion at 20 keV im...

Characterization of Self-Assembled Organic Films Using Differential Charging in X-ray Photoelectron Spectroscopy

Differential charging is often regarded as a problem in X-ray photoelectron spectroscopic studies, especially for insulating or partially conducting samples. Application of a positive bias can reduce the effect of differential charging by attracting stray electrons from the system, thereby compensating for the electron loss. On the other hand, differential charging effect can be enhanced by the application of a negative bias to the sample during spectrum acquisition. The successful use of the differential charging technique to distinguish between multi- and monolayer organophosphonate films on oxide-covered silicon has been reported. A detailed description of this technique is now presented which shows how differential charging can be used as an important tool for the characterization of self-assembled films deposited on various surfaces. The dependence of this technique on the conductivity of the substrate has been investigated by studying the spectral behavior of the deposited films of phosphonic acid on conducting, semiconducting, and insulating samples (stainless steel, silicon, and glass). Application of either positive or negative dc electrical bias affects the carbon core-level (C1s) line shape and intensity, which is dependent on the atom's physical location above the surface.

Monolayer vs. multilayer self-assembled alkylphosphonate films: X-ray photoelectron spectroscopy studies

Combining functional organic self-assembled monolayers (SAMs) with conventional semiconductor materials is a key step in the development of integrated electronics-based devices. T-BAG (Tethering by Aggregation and Growth) has been shown to be a simple and reliable method to grow SAMs of alkylphosphonates on oxide surfaces. However, distinguishing SAMs from ultra-thin multilayers is a challenge for most conventional surface characterization techniques. Self-assembled films of octadecylphosphonic acid (ODPA) were deposited on oxide-covered silicon coupons, converted to the corresponding phosphonates, and characterized by high resolution angularly resolved X-ray photoelectron spectroscopy (XPS). It was our goal to distinguish among different bonding configurations for phosphorous in the phosphonate head groups, namely, mono-dentate, bi-dentate or tri-dentate interactions with the oxide surface, as well as to assess quantitatively the near-surface layer composition. We also present an innovative method that allows us to distinguish between monolayer and multilayer films of ODPA on silicon oxide surfaces. This method is based on differential surface charging effects in XPS. It was found that variation in the ODPA film thickness causes differential responses of various spectral characteristics to an electrical bias applied to the sample during XPS measurements. Both positive and negative applied biases were found to affect the carbon core-level (C1s) line-shape and intensity in the case of the multilayer ODPA film, whereas line-shapes and intensities of all XPS lines measured for the monolayer film were unaffected by the application of a dc bias in the ±30 V range.

Precise determination of band offsets and chemical states in SiN∕Si studied by photoemission spectroscopy and x-ray absorption spectroscopy

We have investigated chemical states and band offsets in SiN∕Si by photoemission spectroscopy and x-ray absorption spectroscopy. N1s photoemission spectra in SiN for three kinds of layer-thickness films are fitted by a single component, suggesting that a nitrogen atom is surrounded by three silicon and nine nitrogen atoms for the first and the second nearest neighbor, respectively. Valence-band offsets between SiN and the Si substrates are determined to be 1.6 eV using valence-band spectra by subtracting the contribution from Si substrates. Band gap of SiN is estimated to be 5.6–5.7 eV from valence-band, N1s core level, and NK-edge-absorption spectra. Furthermore, time-dependent measurements of N1s photoemission spectra reveal that the x-ray irradiation time is a significant factor to determine the precise valence-band offsets excluding the differential charging effects.

X-ray photoelectron spectroscopy studies of laterite standard reference material

X-ray photoelectron spectroscopy is used to characterize two natural samples of laterite standard reference materials (certified geologically), a mechanical mixture of Fe 2O 3, SiO 2, and Al 2O 3 in a 4:2:1 proportion. Fe 2O 3, SiO 2, and Al 2O 3 samples were also analyzed for comparison purposes. Special attention was paid to peak shape, FWHM, comparison of the same XPS peaks of different samples, and peak shifts. The XPS results reveal the presence of some kind of iron aluminate prevailing on the surface of the laterite standard reference materials. It is also observed that the mechanical mixture exhibits a differential charging effect. Our report provides important information for the researchers doing XPS on these materials when they are used in catalytic reactions.

Electrical conductivity during XPS of heated PMMA: detection of core line and valence band tacticity effects

Electrical conductivity during the XPS of ∽100 nm thick films of poly(methyl methacrylate) (PMMA) on silicon was induced by heating to above 100°C. Rapid x-ray-induced depolymerization set in at about the same temperature. The electrical conductivity allowed acquisition of high-resolution core line and valence band spectra from s- and i-PMMA, free of differential charging effects. Within a time-scale of ∽25 min the depolymerization did not affect the appearance of the spectra. The presence of methoxy groups at the uppermost surface of the two heated samples was confirmed by angle-dependent XPS, with an indication of a higher surface concentration of such groups for the s-polymer. At ambient temperature the valence band spectra of the two stereoisomers were identical, but when heated to above 100°C a difference of ∽0.15 eV developed in the O 2s binding energies; this is interpreted as being due to preferential crystallization of the s-polymer. Copyright © 1999 John Wiley & Sons, Ltd.

XPS characterization of nanosized overoxidized polypyrrole films on graphite electrodes

X-Ray photoelectron spectroscopy (XPS) analysis was used to verify the structure of nanosized overoxidized polypyrrole (OPPy) films that were grown on glassy carbon. The films were electrochemically grown to ca. 1.2–1.6 nm thickness from acetonitrile with tetrabutylammonium perchlorate (TBAP) and from water with adenosine triphosphate (ATP) as dopant. The films have been previously characterized by electrochemical methods, but this represents the first study of the ultrathin films by XPS. XPS analysis verifies that the key structural features of nanosized OPPy films are the same as those of much thicker films previously studied by XPS on metal substrates, where differential charging effects are much less severe. Some differences between the microstructures of OPPy and OPPy/ATP films are highlighted by the results. In addition, XPS results confirm a two-domain model of the polymer films that has been proposed from previous studies of PPy and OPPy. The results show that XPS can be used to characterize nanosized films on graphite, after correction for differential charging. Graphite represents a substrate of choice for the fabrication of permselective ultrathin membranes for biosensors from materials such as OPPy.

Differential charging in XPS. Part III. A comparison of charging in thin polymer overlayers on conducting and non-conducting substrates

Previous work has used small-area and imaging XPS to show that differential charging in bulk insulators develops as a result of non-uniform x-ray flux across the surface, causing lateral differential charging. Charging in heterogeneous samples can be further affected by the local sample environment and sample mounting. The current work extends these studies through an analysis of differential charging effects in thin overlayers on conducting and insulating substrates. The charging observed in PnBMA overlayers on indium tin oxide, glass, Ag and Al is discussed as a function of substrate conductivity and photoelectron cross-sections. Substrate conductivity is the most significant factor in determining the magnitude of the overlayer charging observed when no charge compensation is utilized. Differential charging in the PnBMA overlayer was used to image a patterned substrate containing insulating and conducting areas. © 1997 John Wiley & Sons, Ltd.