Andrea Leto

High-resolution stress assessments of interconnect/dielectric electronic patterns using optically active point defects of silica glass as a stress sensor

A piezospectroscopic (PS) cathodoluminescence (CL) study has been carried out on a Cu-Ta∕SiOx (carbon-doped) model chip prepared on a Si substrate. The PS approach was applied to CL spectra arising from optically active point defects in dielectric silica. The red CL emission arising from nonbridging oxygen hole centers (NBOHC) in the carbon-doped SiOx dielectric layer was calibrated and used as a stress sensor. This approach enabled us to locate the trace of the residual stress tensor, as locally developed during manufacturing process in the dielectric interlayers between Cu-Ta interconnects. A minimally invasive electron beam allowed probing local residual stress fields with an improved spatial resolution as compared to more conventional photostimulated PS techniques applied to the Si substrate. In addition, a two-dimensional deconvolution procedure was attempted to retrieve the “true” residual stress distribution piled up between adjacent Cu-Ta lines, according to a theoretical model for embedded struct...

Electron probe response function and piezo-spectroscopic behaviour of semiconductor materials in presence of highly graded stress fields

The response function of the electron probe and the stress dependence of cathodoluminescence spectra emitted by selected semiconductor materials have been evaluated by scanning across sharp bi-material interfaces and along highly graded residual stress fields generated at the tip of an equilibrium crack, respectively. These microscopic procedures can be made fully quantitative provided that the crack opening displacement of the investigated crack is preliminarily measured in a scanning electron microscope for an in situ estimate of the crack-tip stress intensity factor. Taking advantage of the high scanning flexibility of the electron probe, capable of nanometric lateral displacements, spectral shifts typical of the K-dominated zone along the axis of crack propagation were recorded as a function of distance from the crack-tip. A plot of equi-biaxial stress versus spectral band shift was then obtained whose slope represents the piezo-spectroscopic coefficient of the selected material band. A theoretical analysis was attempted to analyse the in-plane interaction between sample and electron probe and to put forward suitable conditions for a reliable assessment of highly graded stress fields. Conditions were worked out into generalized plots as a function of spectroscopic and mechanical parameters for GaN, 3C–SiC and GaAs, as paradigm semiconductor materials.

Spatially resolved residual stress assessments of GaN film on sapphire substrate by cathodoluminescence piezospectroscopy

Two cathodoluminescence piezospectroscopic (CL/PS) approaches for measuring the residual stress distribution in thin films are critically examined and compared using an intrinsic GaN film sample (2.5μm in thickness) grown on a (0001)-oriented sapphire substrate. The first approach invokes an analytical model to fit experimental stress distributions as retrieved in both film and substrate at the edge of an artificially created cross section of the sample. Such an edge-stress distribution takes into account both the thermal expansion mismatch between the film and substrate and the mechanistics of film growth process. In the second approach, we directly and nondestructively measure the bulk residual stress field from the sample top surface on the film side using an increase in electron beam voltage (maintaining a constant beam power) as a means for screening the film subsurface. In this latter case, the combined effects of self-absorption and misfit dislocations on the GaN spectrum severely affect the CL/PS ...

In situ measurements of local temperature and contact stress magnitude during wear of ceramic-on-ceramic hip joints.

Fluorescence microprobe spectroscopy was applied to in situ assessments of contact stress and local temperature at the contact point of dry-sliding couples during wear tests of two commercially available ceramic-on-ceramic femoral heads. The investigated ceramic hip implants consisted of either monolithic Al2O3 or Al2O3/ZrO2 composite. A specially designed pin-on-ball tribometer was employed, which enabled directly testing the femoral head components as received from the maker without further manipulation. The strong fluorescence emission from Cr(3+) impurities contained in Al2O3 served as a responsive sensor for both temperature and stress. Analytical corrections for the averaging effects arising from the finite size of the laser probe were made according to a probe response formalism in which geometrical conditions of the sliding couple were incorporated as boundary conditions. The sample-probe interaction at the contact point was then experimentally calibrated by obtaining probe response functions for the two materials investigated. Based on such theoretical and experimental procedures, deconvolutive computational routines could be set up and the true variations of local temperature and stress at the contact point of the bearing surfaces retrieved from the observed time-dependent broadening and shift of a selected spectral band, respectively. The main result of the in situ investigation was that the monolithic sliding couple showed both significantly lower temperature and lower magnitude of compressive stress at the contact point as compared to the composite one, although the composite couple wore at a significantly lower specific wear rate than the monolithic one.

Kinetics and the role of off-stoichiometry in the environmentally driven phase transformation of commercially available zirconia femoral heads.

The low-temperature polymorphic transformation behavior of two types of commercially available femoral head, both made of 3 mol.% Y(2)O(3)-stabilized tetragonal ZrO(2) polycrystals (3Y-TZP), was examined by in vitro experiments. Both materials contained a small amount (0.25 wt.%) of Al(2)O(3), but they differed slightly in their SiO(2) impurity content, in the morphology and crystallinity of the dispersed Al(2)O(3) phase, and in grain size. In vitro experiments were conducted in a water-vapor environment at temperatures in the range 90-134°C and for periods of time up to 500 h. Despite the materials having the same nominal composition, quite different behaviors were found in the hydrothermal environment for the two types of femoral head investigated. A phenomenological description of the kinetics of monoclinic nuclei formation/growth led to the experimental determination of activation energy values for the environmentally driven polymorphic transformation. From the material physics viewpoint, cathodoluminescence spectroscopy enabled us to rationalize the role of surface stoichiometry on the mechanisms leading to polymorphic transformation. Spectroscopic experiments unveiled some new relevant aspects of surface off-stoichiometry, which lie behind the different phase transformation kinetics experienced by the investigated femoral heads.

Bioinertness and fracture toughness evaluation of the monoclinic zirconia surface film of Oxinium™ femoral head by Raman and cathodoluminescence spectroscopy.

Raman and cathodoluminescence spectroscopic methods were employed for clarifying important stoichiometric and mechanical properties so far missing in the specification of the physical origin of the structural behavior of Oxinium™ femoral head components. Spectroscopy proved helpful in rationalizing the actual physical and chemical reasons behind the mechanical integrity of the ceramic-film/metal-substrate interface, which is responsible for both the good adherence and the surface durability reported in prosthetic applications of Oxinium™ components. Raman spectroscopy coupled with the crack opening displacement (COD) method was used to evaluate the intrinsic fracture toughness of the surface oxide film. In addition, cathodoluminescence spectroscopy provided new evidences on both the oxygen vacancy gradient developed during the metal-oxidation manufacturing process and the bioinertness of Oxinium™ femoral components.

Stress stabilization of a new ferroelectric phase incorporated into SrTaO2N thin films

Microstructural analyses of highly stressed SrTaO2N thin films deposited on SrTiO3 substrates by cathodoluminescence spectroscopy revealed coexistence of ferroelectric and relaxor-ferroelectric-like phases in the films. These two phases are, respectively, associated with “trans-type” and “cis-type” anion orders, as supported by the relative difference of the band gap energies calculated by first principles calculations based on the density functional theory. The formation of the new ferroelectric phase is considered to occur upon stabilization by the high compressive residual stress stored into the film structure, with the length/size of the “trans-type” region strongly depending upon the local stress state in the film.

Cathodoluminescence evaluation of oxygen vacancy population in nanostructured titania thin films for photocatalytic applications.

Room-temperature results of cathodoluminescence (CL) spectroscopy investigations are presented for nanostructured titanium dioxide (anatase) thin films (500 nm thick) deposited via RF magnetron sputtering on high-purity silica substrates. The collected CL bands of the anatase thin films, as deposited and after different annealing cycles, showed a broad morphology consisting of three Voigtian bands located at 500, 550, and 610 nm that were partially overlapping. The overall CL emission increased with increasing temperature and time of the annealing cycle as a consequence of the increased crystallinity of the thin film. A clear trend was found for the oxygen-vacancy-related band (located at 610 nm), whose relative intensity decreased, as compared with the as-grown sample, after annealing in air; the higher the annealing temperature, the lower the relative intensity. We evaluated the photoactivity of the nanostructured thin film samples by measuring their photocatalytic activity in aqueous solution toward the degradation of phenol. A relationship between the decrease in oxygen vacancy concentration as a consequence of the annealing and the increase in the photoactivity was highlighted.

Innovative tribometer for in situ spectroscopic analyses of wear mechanisms and phase transformation in ceramic femoral heads.

The literature on tribological assessments of artificial hip joints usually focuses on correlations between joint composition, size, and specific wear rates, but conspicuously ignores the physical aspects behind the occurrence of degradation mechanisms of friction and wear. Surface degradation in artificial joints occurs because of increases in temperature and local exacerbation of contact stresses inside the moving contact as a consequence of physical and chemical modifications of the sliding surfaces. This article reports about the development of a new pin-on-ball spectroscopy-assisted tribometer device that enables investigating also physical rather than merely engineering aspects of wear processes using in situ Raman and fluorescence techniques. This innovative tribometer is designed to bring about, in addition to conventional tribological parameters, also information of temperature, stress and phase transformations in the femoral heads as received from the manufacturer. Raman and fluorescence spectra at the point of sliding contact are recorded durilng reciprocating hard-on-hard dry-sliding tests. Preliminary results were collected on two different commercially available ceramic-on-ceramic hip joint bearing couples, made of monolithic alumina and alumina-zirconia composites. Although the composite couple showed direct evidence of tetragonal-to-monoclinic phase transformation, which enhanced the coefficient of friction, the specific wear rate was significantly lower than that of the monolithic one (i.e., by a factor 2.63 and 4.48 on the pin and head side, respectively). In situ collected data compared to ex situ analyses elucidated the surface degradation processes and clarified the origin for the higher wear resistance of the composite as compared to the monolithic couple.

Cathodoluminescence and Raman spectroscopic analyses of Nd- or Yb-doped Y2O3 transparent ceramics.

In this article, we discuss a study of the influence of Nd(3+) and Yb(3+) dopants on the spectroscopic behavior of Y2O3 ceramic polycrystals, by using Raman and cathodoluminescence (CL) spectroscopy. Doping with Yb at 1 at % results in a blue shift of the Raman band but has no pronounced influence on the intrinsic emission of Y2O3, while doping with Nd shows a red shift of the Raman band and markedly enhances the oxygen vacancy related 380 nm CL band. Lattice distortion induced by the alien ion incorporation and variations of effective absorption coefficients by adding different dopants were assessed using Raman spectroscopy. Moreover, both CL and Raman spectroscopies were applied to examine the homogeneity and distribution of the dopants throughout the ceramic microstructure. Visualization of sample homogeneity was made available by hyperspectral imaging of the local intensity of CL bands, while spectral deconvolution was performed to retrieve local structural variations at grain boundaries. We also confirmed that the combination of Raman and CL spectroscopies leads to a reliable and useful methodology for the examination of dopants in yttria ceramic materials.