Direct chemical in-depth profile analysis and thickness quantification of nanometer multilayers using pulsed-rf-GD-TOFMS

Nanometer depth resolution is investigated using an innovative pulsed-radiofrequency glow discharge time-of-flight mass spectrometer (pulsed-rf-GD-TOFMS). A series of ultra-thin (in nanometers approximately) Al/Nb bilayers, deposited on Si wafers by dc-magnetron sputtering, is analyzed. An Al layer is first deposited on the Si substrate with controlled and different values of the layer thickness, t _Al. Samples with t _Al = 50, 20, 5, 2, and 1 nm have been prepared. Then, a Nb layer is deposited on top of the Al one, with a thickness t _Nb = 50 nm that is kept constant along the whole series. Qualitative depth profiles of those layered sandwich-type samples are determined using our pulsed-rf-GD-TOFMS set-up, which demonstrated to be able to detect and measure ultra-thin layers (even of 1 nm). Moreover, Gaussian fitting of the internal Al layer depth profile is used here to obtain a calibration curve, allowing thickness estimation of such nanometer layers. In addition, the useful yield (estimation of the number of detected ions per sputtered atom) of the employed pulsed-rf-GD-TOFMS system is evaluated for Al at the selected operating conditions, which are optimized for the in-depth profile analysis with high depth resolution.     

Cathodoluminescence Depth Profiling in SiO2:Ge Layers

 For investigation of the luminescent center profile cathodoluminescence measurements are used under variation of the primary electron energy E ₀ = 2…30 keV. Applying a constant incident power regime (E ₀·I ₀ = const), the depth profiles of luminescent centers are deduced from the range of the electron energy transfer profiles dE/dx. Thermally grown SiO₂ layers of thickness d = 500 nm have been implanted by Ge⁺-ions of energy 350 keV and doses (0.5–5)10¹⁶ ions/cm². Thus Ge profiles with a concentration maximum of (0.4 – 4) at% at the depth of d_m≅240 nm are expected. Afterwards the layers have been partially annealed up to T _a = 1100 °C for one hour in dry nitrogen. After thermal annealing, not only the typical violet luminescence (λ = 400 nm) of the Ge centers is strongly increased but also the luminescent center profiles are shifted from about 250 nm to 170 nm depth towards the surface. This process should be described by Ge diffusion processes, precipitation and finally Ge nanocluster formation. Additionally, a Ge surface layer is piled-up extending to a depth of roughly 25 nm.     

Surface Characterisation of Water-Atomised Al–Zn–Mg–Cu Alloy Powders by SIMS and AES

 The present paper focuses on the characterisation of surface composition and alloying element in-depth distribution of water-atomised Al–Zn–Mg–Cu alloy powders by secondary ion mass-spectrometry and Auger electron spectroscopy. A pronounced segregation of Mg and some impurities (Fe, Ca, S) concurrently with some Zn depletion are observed on the powder surface. The oxide film formed on the powder surface mainly consists of Al and Mg oxides. The film is non-uniform in thickness: rather coarse surface oxide islands coexist with surface areas covered by a thin (<1.8 nm) oxide layer. The extent of surface oxidation is strongly affected by solidification conditions: The average thickness of the surface oxides increases with increasing particle size or with decreasing cooling rate. All alloying elements are homogeneously distributed in the bulk of individual particles. No significant differences in chemical composition between different particles of a given powder are observed. Received November 26, 1999. Revision September 25, 2001.     

Quantitative Analysis of Thin Aluminium-Oxynitride Films by EPMA

 Thin films of aluminium oxynitride with diverse composition were prepared by dc-magnetron sputtering of aluminium, utilising sputtering power as well as argon, oxygen and nitrogen gas flows to vary the composition. Since film properties depend mainly on the content of incorporated oxygen and nitrogen, a method for quantitative analysis of the main constituents based on electron probe micro analysis with energy dispersive detection was developed. The excellent precision of the quantitative results for aluminium as well as oxygen and nitrogen are shown. Furthermore, a film layer analysis program was applied for the quantification of several films deposited under the same deposition parameters on silicon wafers, from 520 nm down to 40 nm thickness, showing that electron probe micro analysis with energy dispersive detection is a reliable method for quantitative compositional analysis of thin aluminium oxynitride films down to approximately 20 nm thickness. Since this method of analysis provides only bulk information, expected inhomogeneities of the depth distribution of the film components were checked by secondary ion mass spectrometry depth profiles of two thin films and correlated to the EPMA results. The thickness of the films was determined by ellipsometry. Received September 1, 1998     

XPS Depth Profile Analysis of a Thin Non-Conducting Titanate Superlattice

 New BaTiO₃-SrTiO₃ (BTO-STO)-superlattices which may be interesting for future electronic applications have been investigated by X-ray photoelectron spectroscopy (XPS) depth profiling. At first XPS measuring conditions were optimized for that non-conducting and thin layer systems (21 nm double layer thickness) considering the practical instrumental limitations. Second a simulation of the sputtering process for the concrete experimental conditions were done by a dynamic T-DYN code. By comparison of experimental and simulated depth profiles the maximum sample roughness could be estimated to be in the range of 2 nm.     

Potential of Total Reflection and Grazing Incidence XRF for Contamination and Process Control in Semiconductor Fabrication

 The actual detection limits of total reflection X-ray fluorescence (TXRF) are determined and compared to those of destructive physical analytical methods like secondary ion mass spectrometry (SIMS) and chemical methods like vapour phase decomposition in combination with inductively coupled plasma-mass spectrometry (VPD-ICP-MS). The elements Ca, Ti, Cr, Fe, Cu were analyzed on a Si wafer with 10 nm thermal oxide using TXRF and VPD-ICP-MS. The deviation of the TXRF and the VPD-ICP-MS results is less than 30%. The thickness, composition and density of a Co/Ti two-layer stack were determined using angle dependent total reflection and grazing incidence X-ray fluorescence (A-TXRF). The obtained data were compared with X-ray reflectometry (XRR) and energy filtered transmission electron microscopy (EFTEM). The agreement between TEM and A-TXRF is excellent for the determination of the thickness of the metal layers. From these results we conclude, that A-TXRF permits the accurate determination of composition, thickness and density of thin metallic layers. The results are discussed regarding detection efficiency, acquisition time, accuracy and reproducibility.     

Comparative Analysis of a Solar Control Coating on Glass by AES, EPMA, SNMS and SIMS

 A solar control coating was analysed by different methods of surface analysis with respect to the layer sequence and the composition and thickness of each sublayer. The methods used for depth profiling were Auger electron spectroscopy, electron probe microanalysis, secondary neutral mass spectroscopy and secondary ion mass spectroscopy based on MCs⁺. The structure of the coating was unknown at first. All methods found a system of two metallic Ag layers, embedded between dielectric SnO_X layers. Additionally, thin Ni-Cr layers of 1–2 nm were detected on top of the Ag layers. Thus the detected layer sequence is SnO_X/Ni-Cr/Ag/SnO_X/Ni-Cr/Ag/SnO_X/glass. The Ni:Cr ratio in the nm-thin layers could be quantified by every method, the Cr fraction corresponding to less than one monolayer. We compare the capabilities and limitations of each method in routinely investigating this solar control coating. Importance was attached to an effective investigation. Nevertheless, by combining all methods, measuring artefacts could be uncovered and a comprehensive characterisation of the system was obtained.     

Formation of cuprous oxide layers in Cu(II) solutions containing gluconic acid

In accordance with thermodynamic analysis, cuprous oxide layers are formed spontaneously in the Cu|Cu(II), gluconic acid system at pH > 3.7 under open-circuit conditions. A current peak of Cu₂O reduction is observed on cathodic voltammograms at ca −0.7 V, its height being dependent on the exposure time. The analysis of the charge transferred in this region yields the rate of Cu₂O formation equal to 1.25 × 10⁻¹⁰ mol cm⁻² s⁻¹. The light perturbation of Cu electrode under open-circuit conditions results in the generation of a negative photopotential, which is indicative of n-type conductivity. The threshold wavelength is equal to ∼590 nm and is consistent with a band gap of ∼2.1 eV. Anodic photocurrents, which are observed near the open-circuit potential, decrease with cathodic polarization and change their sign at ∼0.05 V. Analysis of impedance data was performed, invoking the equivalent circuit that accounts for the two-step charge transfer. In the presence of Cu₂O, some retardation of Cu(II) reduction was found to occur with a slight increase in the admittance of the double layer. The suggestion has been made that oxide layers formed in Cu(II) gluconate solutions cannot be compact and uniformly distributed over the entire electrode surface. Relevant investigations of surface morphology support this conclusion.     

Application of Sputter-Assisted EPMA to Depth Profile Analysis

 A common problem in depth profile measurement is the calibration of the depth scale. The new technique of sputter assisted electron probe microanalysis offers the possibility of calculating the composition as well as the depth scale solely from the acquired X-ray intensity data without further information, e.g. sputter rates. To achieve a depth resolution that is smaller than the depth of information of the electron probe, i.e. 0.1–1 μm, special deconvolution algorithms must be applied to the acquired data. To assess the capabilities of this new technique it was applied to a Ti/Al/Ti multilayer on Si under different measurement conditions. Quantitative depth profiles were obtained by application of a deconvolution algorithm based on maximum entropy analysis. By comparison of these profiles with AES depth profiles and AFM roughness measurements, it was shown that the limiting factor to the achievable depth resolution is the occurrence of surface roughening induced by the sputtering process rather than the relatively large depth of information of the electron probe. We conclude that for certain applications sputter-assisted EPMA can be regarded as a valid depth profiling technique with a depth resolution in the nm range.     

Synthesis of metallic copper nanoparticles coated with polypyrrole

This paper describes a method for polypyrrole (PPy) coating of metallic Cu nanoparticles in aqueous solution in atmosphere. Colloid solution of Cu nanoparticles was prepared by reducing Cu ions with the use of hydrazine in an aqueous solution dissolving citric acid and cetyltrimethylammonium bromide as stabilizers. The PPy coating was performed by polymerizing pyrrole with the use of hydrogen peroxide as an initiator in an aqueous colloid solution of the Cu nanoparticles. Ultraviolet–visible extinction measurements, transmission electron microscopy observation, and X-ray diffraction measurements revealed that the metallic Cu nanoparticles with a size of 27.6 ± 11.1 nm were coated with PPy. The obtained PPy-coated Cu particles were chemically stable even in atmosphere.     

Photoelectrical properties of Langmuir–Blodgett films of poly(methyl phenylsilane)s

Langmuir–Blodgett (LB) films were prepared from poly(methylphenylsilane) bearing electron acceptor π-conjugated substituents. The small limiting area (0.078 nm²) per one repeating unit of polysilane (PSi) in monomolecular film and the large thickness of the film (6 nm) suggest that the polymer chains are not fully spread on water surface. The electrical and photoelectrical properties of Al/LB film/Au sandwich cells containing various numbers of the polysilane layers were studied. Holes were transported from the Al electrode through the LB film to the Au electrode when the light was absorbed by the polysilane. The highest photovoltaic effect occurred in the first monolayer of polysilane at the Al contact. The cell resistivity and the photovoltage were decreased by parallel conductance of defects in the films consisting of small numbers of PSi layers.     

Applications of nanostructured Pt-Au hybrid film for the simultaneous determination of catecholamines in the presence of ascorbic acid

Nanostructured platinum-gold (Pt–Au) hybrid film modified glassy carbon electrode (GCE) was fabricated by electro-deposition method in the presence of 2 × 10⁻⁴ mol l⁻¹ l-cysteine. To examine the surface morphological analysis, the (Pt–Au) hybrid film were electrochemically deposited on transparent semiconductor indium tin oxide (ITO) electrodes for scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) studies. From the SEM analysis, it was observed that the deposited nanoplatinum (250–400 nm) was formed as a cauliflower-shaped structure with the gold nanoparticles (30–90 nm). The concentration variation of additive l-cysteine results in the formation of cauliflower-shaped platinum nanoparticles. Further, the Pt–Au hybrid film modified GCE could be used for the detection of catecholamine neurotransmitters epinephrine (EP), norepinephrine (NEP) individually and in the presence of ascorbic acid (AA) in pH 7 phosphate-buffered solutions (PBS). Furthermore, the proposed Pt–Au hybrid film could be applied for the detection of epinephrine in injection solution and ascorbic acid from commercially available vitamin C tablets.     

Tailored modification of quartz surfaces by covalent immobilization of small molecules (γ-aminopropyltriethoxysilane), monodisperse macromolecules (dendrimers), and poly(styrene/acrolein/divinylbenzene) microspheres with narrow diameter distribution

Quartz plates were modified by consecutive immobilization of γ-aminopropyltriethoxysilane (APTS), phosphorus containing dendrimers with aldehyde groups (generation 5 – G5), Starburst PAMAM dendrimers generation 4 (G4-PAMAM), and poly(styrene/acrolein/divinylbenzene) microspheres [P(SAD)]. In this way surfaces with heterogeneity on molecular, macromolecular, and microscopic levels, and which were equipped with functional amino or aldehyde groups were obtained. Surface layers were characterized by X-ray photoelectron spectroscopy (XPS) and by contact-angle measurements. Analysis of XPS spectra revealed that the thickness of the layer of G5 on the SiO₂-APTS substrate was 3.7 nm, i.e., the thickness was typical for macromolecular dimensions. The average thickness of the layer of PAMAM dendrimers on SiO₂-APTS-G5 was found to be 0.35 and 0.29 nm, depending on whether calculations were based on attenuation of the intensity of the Si2p or the P2p signal respectively. This thickness was unreasonably low for a monolayer of PAMAM dendrimers and indicated that the surface of the SiO₂-APTS-G5 substrate was incompletely covered with these macromolecules. The XPS method was also used for the determination of the degree of coverage of the surface of a SiO₂-APTS-G5-PAMAM plate with P(SAD) microspheres. The degree of coverage was found to be 0.60 and approaches the maximum theoretically possible value (0.62) for microspheres attached chaotically and irreversibly to the surface in an arrangement one microsphere thick. Subsequent coverage of the SiO₂-APTS-G5-PAMAM-P(SAD) substrate with PAMAM dendrimers resulted in the formation of a PAMAM adlayer 3.2 nm thick, close to the molecular dimensions of these dendrimers. Contact-angle measurements revealed considerable differences in the hydrophobicity of the surfaces of the quartz plates, depending on their modification. Hydrophobicity increased in the order SiO₂ < SiO₂-APTS-G5-PAMAM < SiO₂-APTS ≤ SiO₂-APTS-G5 < SiO₂-APTS-G5-PAMAM-P(SAD). Received: 17 March 1998 Accepted: 14 September 1998     

Applying Digital Image Processing to SEM-EDX and BSE Images to Determine and Quantify Porosity and Salts with Depth in Porous Media

 Digital image processing is applied to X-ray element maps and back-scattered electron (BSE) images obtained with scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX), to quantify and identify salts and porosity with depth in porous media. Until now, only salt chemical composition with depth has been characterised and salt mineral composition inferred, so that no salt quantification was possible. The method we present can be used stand-alone or integrated with other techniques to fill certain gaps in our knowledge. It can be used to quantify texture and porosity of clay minerals, with implications in petroleum research. Also, salts may be identified and quantified with depth in porous media; this is important when selecting among salt extraction procedures in order to conserve cultural heritage. Finally, the method can help in interpreting salt weathering processes and links between stone decay forms at different scales, with consequences for geomorphology. To demonstrate the method, salt and pore percentages with depth are determined in two limestone types from monuments in the Aunis province, SW France: i) Crazannes sparite, showing alveolisation and black crust, and ii) La Pallice micrite, suffering from micro-delamination. In most of the micrites no salts are identified. Porosity data show that micro-fissures are confined between the surface and 9 mm depth in LPAM5, the only micrite with loading salts with depth (NaCl = 1.9% at the surface versus 1.4% at 18 mm; and Na₂SO₄ · H₂O and MgCl₂ ∼ 0.02% in the first 3 mm). In the sparites, those samples with no crust display an alveolisation zone with lower porosity and higher diversity and percentage of salts. In CRZM6, the stone crust-alveolisation front lies at a depth of 9 mm. In the crust, porosity is lower than in the alveolisation zone (12%). Salt concentration, by contrast, is higher in the crust (2.91% versus 0.14% in the alveolisation zone). Received September 1, 2000 Revision November 2, 2001     

Determination of the Mg-Concentration Profile in Near-Surface Layers of Materials Using the 24Mg(α,p)27Al Nuclear Reaction

 The ²⁴Mg(α, p)²⁷ Al nuclear reaction was applied for the determination of the magnesium distribution in near-surface layers of materials. The cross sections of this reaction were determined in the energy region between 4.5 and 5.5 MeV in steps of 5 to 10 KeV (θ_lab : 158°) using thin magnesium films. The investigated projectile energy region included five main resonances allowing the determination of magnesium. The uncertainty of the cross-section determination was of the order of 7%. The applicability of the technique was tested using Mg-implanted AISI 321 steel samples. Depth resolution of 100 nm and detection limits of the order of 0.1 ppm were achieved for the determination of magnesium in steel samples using the 4805 keV resonance of the ²⁴Mg(α, p)²⁷ Al nuclear reaction. The shape and height of the magnesium depth-profile in the Mg-implanted steel samples were compared with corresponding values obtained by X-ray photoelectron spectroscopy. Received July 15, 1999. Revision March 30, 2000.     

Theoretical study of group 11 metal–silonyl complexes: M–SiO and M–(SiO)2 (M = Cu, Ag, or Au)

 The spectroscopic properties of M–SiO and M–(SiO)₂ (1–1 and 1–2 complexes with M = Cu, Ag, or Au) have been theoretically studied. It has been shown that both M–SiO and M–(SiO)₂ compounds in their ground state are bent with a metal–Si bonded structure. The calculated M(ns) spin density agrees well with the electron spin resonance experimental data. From a topological analysis, it has been shown that a rather large charge transfer occurs from the metal towards the SiO moiety, and that the M–Si bond energy correlates with the electron density located at the M–Si bond path (bond critical point). Received: 6 July 2000 / Accepted: 11 October 2000 / Published online: 19 January 2001     

Analysis of photocurrent spectra at polybithienyl film coated on Pt

Photoelectrochemical measurements have been performed at a polybithienyl (PBT) film (doping level of 1 × 10¹⁸/cm³) deposited on a platinum electrode. The cathodic photocurrents and negative slope of the Mott-Schottky plot indicate that the PBT film has the features of a p-type semiconductor. The cathodic photocurrents are interpreted in terms of the Gaertner-Butler model on the basis of the theory of the semiconductor|solution interface. The (i _ph hν)^2/n vs. hν plots taken from the photocurrent spectra show two linearities for n=1 in the wavelength range from 460 nm to 490 nm and for n=4 in the wavelength range λ > 490 nm. The band gaps of the PBT film were determined to be 2.05 ± 0.05 eV for n=1 and 1.55 ± 0.05 eV for n=4. The flat-band potential is 0.33 V (vs SCE). From the slope of the Mott-Schottky plot at the modulation frequency of 3 kHz, the dielectric constant ɛ of the film and the thickness of the depletion layer W ₀ of the PBT film were determined to be 7.4 and 0.29 μm, respectively. Received: 6 January 1999 / Accepted: 6 June 1999     

EFTEM and EELS Analysis of a Pt/NiO Interface

 Energy-filtering transmission electron microscopy (EFTEM) is more and more becoming an important nanoanalytical technique in both materials science and biology. The main advantage of the method lies in the possibility to obtain two-dimensional chemical information from large specimen areas as well as from features on a nanometer scale. Due to its excellent lateral resolution it is perfectly suited for the investigation of nanometer sized features (e.g. interfaces). In this paper we will show how EFTEM can be used to characterize the interface between a Pt layer and a NiO crystal as part of a coulometric titration cell. In addition to elemental distribution maps electron energy-loss spectra (EELS) across the interface (EELS linescans) have been acquired to obtain quantitative compositional profiles. By employing these methods the following interfacial layers could be identified, all of which containing Pt, Ni and O in different proportions: 13 nm Pt-rich, 32 nm Ni-rich and 29 nm Pt-rich. The origin of these is discussed in terms of displacement reactions.     

Enzyme biosensor based on the immobilization of HRP on SiO<Subscript>2</Subscript>/BSA/Au composite nanoparticles

In this work, an enzyme biosensor based on the immobilization of horseradish peroxidase (HRP) on SiO₂/BSA/Au/thionine/nafion-modified gold electrode was fabricated successfully. Firstly, nafion was dropped on the surface of the gold electrode to form a nafion film followed by chemisorption of thionine (Thi) as an electron mediator via the ion-exchange interaction between the Thi and nafion. Subsequently, the SiO₂/BSA/Au composite nanoparticles were assembled onto Thi film through the covalent bounding with the amino groups of Thi. Finally, HRP was immobilized on the SiO₂/BSA/Au composite nanoparticles due to the covalent conjugation to construct an enzyme biosensor. The surface topographies of the SiO₂/BSA/Au composite nanoparticles were investigated by using scanning electronic microscopy. The stepwise self-assemble procedure of the biosensor was further characterized by means of cyclic voltammetry and chronoamperometry. The enzyme biosensor showed high sensitivity, good stability and selectivity, a wide linear response to hydrogen peroxide (H₂O₂) in the range of 8.0 × 10^-6 ∼ 3.72 × 10^-3 mol/L, with a detection limit of 2.0 × 10^-6 mol/L. The Michaelies-Menten constant K_M^app K_M^{app} value was estimated to be 2.3 mM.     

Oxidation, Diffusion and Segregation in CuNi(Mn) Films Studied by AES

 The surface and in-depth compositions of sputter-deposited Cu_0.57Ni_0.42Mn_0.01 thin films were studied by Auger electron depth profiling after thermal treatment. The samples were thermally cycled to maximum temperatures of 300 °C to 550 °C in air, argon and forming gas (N₂, 5 vol. % H₂). Linear least-squares fit to standard spectra and factor analysis were applied to separate the overlapping Auger transitions of Cu and Ni. Under bombardment by 4 keV argon ions, CuNi(Mn) layers display bombardment-induced surface enrichment of Ni in the same extent as binary CuNi alloys. At sufficiently high oxygen partial pressures, a duplex oxide layer is formed and a thick surface copper oxide overgrows the initial nickel oxide. In reducing atmosphere selective oxidation of manganese takes place. A capping NiCr layer prevents CuNi(Mn) from being oxidized, but the film configuration is degraded with increasing annealing temperature due to formation of a surface chromium oxide and diffusion of Ni from the CuNi(Mn) layer into the NiCr/CuNi(Mn) interface.