Real Time Measurement of Epilayer Strain Using a Simplified Wafer Curvature Technique

We describe a technique for measuring thin film stress using wafer curvature that is robust, compact, easy to setup, and sufficiently sensitive to serve as a routine diagnostic of semiconductor epilayer strain in real time during MBE or CVD growth. We demonstrate, using growth of SiGe alloys on Si, that the critical thickness for misfit dislocation can clearly be resolved, and that the subsequent strain relaxation kinetics during growth or post-growth annealing are readily obtained

Prepyramid-to-pyramid transition of SiGe islands on Si(001)

The morphology of the first three-dimensional islands appearing during strained growth of SiGe alloys on Si(001) was investigated by scanning tunneling microscopy. High resolution images of individual islands and a statistical analysis of island shapes were used to reconstruct the evolution of the island shape as a function of size. As they grow, islands undergo a transition from completely unfacetted rough mounds (prepyramids) to partially {105} facetted islands and then they gradually evolve to {105} facetted pyramids. The results are in good agreement with the predictions of a recently proposed theoretical model

Reciprocal space analysis of the initial stages of strain relaxation in SiGe epilayers

Metastable SiGe films were grown by MBE on Si (001) substrates and annealed to promote varying degrees of partial relaxation. X-ray diffraction reciprocal-space analysis was then used to monitor the structural evolution of the displacement fields of the dislocation array with increasing misfit density. The diffuse-x-ray-scattering patterns of the dislocated heterolayers were compared with lineal- misfit densities determined by defect etching, leading to the develop a geometric model which provides a framework for understanding the early-stage evolution of the displacement fields of the dislocation array, and which also explicitly links diffuse x-ray intensity to misfit density. At low misfit density, the diffuse intensity arises from two-dimensional displacement fields associated with single-nonoverlapping dislocations. As misfit density increases, the displacement fields of individual dislocations increasingly overlap producing three-dimensional displacements. The evolving diffuse intensity reflects the transition from 2-D to 3-D displacement fields. Finally, it is demonstrated that the diffuse x-ray intensity of the strained epilayer can be used to accurately measure lineal misfit- dislocation densities from 400 to 20,000 lines/cm

Final report on LDRD Project: In situ determination of composition and strain during MBE

Molecular Beam Epitaxy (MBE) of semiconductor heterostructures for advanced electronic and opto-electronic devices requires precise control of the surface composition and strain. The development of advanced in situ diagnostics for real-time monitoring and process control of strain and composition would enhance the yield, reliability and process flexibility of material grown by MBE and benefit leading-edge programs in microelectronics and photonics. The authors have developed a real-time laser-based technique to measure the evolution of stress in epitaxial films during growth by monitoring the change in the wafer curvature. Research has focused on the evolution of stress during the epitaxial growth of Si{sub x}Ge{sub 1{minus}x} alloys on Si(001) substrates. Initial studies have observed the onset and kinetics of strain relaxation during the growth of heteroepitaxial layers. The technique has also been used to measure the segregation of Ge to the surface during alloy growth with monolayer sensitivity, an order of magnitude better resolution than post-growth characterization. In addition, creation of a 2-dimensional array of parallel beams allows rapid surface profiling of the film stress that can be used to monitor process uniformity

Wetting layer thickness and early evolution of epitaxially strained thin films

We propose a physical model which explains the existence of finite thickness wetting layers in epitaxially strained films. The finite wetting layer is shown to be stable due to the variation of the non-linear elastic free energy with film thickness. We show that anisotropic surface tension gives rise to a metastable enlarged wetting layer. The perturbation amplitude needed to destabilize this wetting layer decreases with increasing lattice mismatch. We observe the development of faceted islands in unstable films.Comment: 4 pages, 3 eps figure

Real time measurement of epilayer strain using a simplified wafer curvature technique

We describe a technique for measuring thin film stress using wafer curvature that is robust, compact, easy to setup, and sufficiently sensitive to serve as a routine diagnostic of semiconductor epilayer strain in real time during MBE or CVD growth. We demonstrate, using growth of SiGe alloys on Si, that the critical thickness for misfit dislocation can clearly be resolved, and that the subsequent strain relaxation kinetics during growth or post-growth annealing are readily obtained

Modeling metallic island coalescence stress via adhesive contact between surfaces

Tensile stress generation associated with island coalescence is almost universally observed in thin films that grow via the Volmer-Weber mode. The commonly accepted mechanism for the origin of this tensile stress is a process driven by the reduction in surface energy at the expense of the strain energy associated with the deformation of coalescing islands during grain boundary formation. In the present work, we have performed molecular statics calculations using an embedded atom interatomic potential to obtain a functional form of the interfacial energy vs distance between two closely spaced free surfaces. The sum of interfacial energy plus strain energy provides a measure of the total system energy as a function of island separation. Depending on the initial separation between islands, we find that in cases where coalescence is thermodynamically favored, gap closure can occur either spontaneously or be kinetically limited due to an energetic barrier. Atomistic simulations of island coalescence using conjugate gradient energy minimization calculations agree well with the predicted stress as a function of island size from our model of spontaneous coalescence. Molecular dynamics simulations of island coalescence demonstrate that only modest barriers to coalescence can be overcome at room temperature. A comparison with thermally activated coalescence results at room temperature reveals that existing coalescence models significantly overestimate the magnitude of the stress resulting from island coalescence.Comment: 20 pages, 8 figures, 2 tables, submitted to PR

Detección de pacientes con riesgo de desarrollar diabetes en farmacias comunitarias de Pontevedra

OBJETIVO: Evaluar la utilidad del test de findrisk en la farmacia comunitaria para la detección de personas en riego de padecer diabetes tipo 2 en la provincia de Pontevedra. METODOLOGÍA: Estudio descriptivo transversal. Pilotaje de un proyecto con diseño similar que se realizará con ámbito estatal. Ámbito y población de estudio: mayores de 18 años no diagnosticados de diabetes y/o sin tratamiento hipoglucemiante, usuarios durante el mes de enero de 2013 de trece farmacias en Pontevedra, seleccionados al azar. Método: se administró el cuestionario Findrisk a los sujetos incluidos en el estudio por los alumnos en prácticas tuteladas de la Facultad de Farmacia de la Universidad de Santiago. Se clasificaron en función del riesgo bajo, ligero, moderado, alto y muy alto, y se realizó a todos ellos una intervención educativa. A los que tuvieron ≥12 puntos se les recomendó la visita al médico. RESULTADOS Y DISCUSIÓN: Cumplimentaron correctamente el cuestionario 381 sujetos. Hombres 142 (37,3%), mujeres 239 (62,7%). La media de puntuación del test findrisk fue menor en mujeres (7,8 ± 4,6) que en hombres (8,7 ± 4,5), pero la diferencia no fue estadísticamente significativa (t de Student= 1,7355, p= 0,0835). Se encontró también mayor porcentaje de hombres con riesgo medio a alto (27,5% vs 23,0%). CONCLUSIÓN: Se han encontrado cifras de riesgo de desarrollar diabetes a diez años superiores a las halladas en otros ámbitos. La aplicación del cuestionario findrisk no plantea problemas en su administración a los usuarios en la farmacia comunitaria. Todo ello la avala como un centro sanitario idóneo para realizar cribado de pacientes con diabetes no diagnosticada obteniendo un alto grado de eficiencia

Sobrecarga, psicopatologías y relación con el farmacéutico comunitario en cuidadores informales de enfermos de Alzheimer

Introducción: El cuidado diario de un familiar enfermo de Alzheimer (EA) supondrá un estrés emocional y físico importante para el cuidador. El resultado puede desembocar en un intenso sentimiento de sobrecarga y conducir a diversas psicopatologías como depresión, ansiedad, agresividad, astenia psicofísica, etc. El aumento en la incidencia de la enfermedad está provocando escasez de cuidadores y falta de apoyo social e institucional a las familias que se encargan de cuidar a un EA.Objetivos: Revisar las características del apoyo que las estructuras sociales y sanitarias prestan a los cuidadores informales (CI) de EA, conocer su perfil, la percepción que tienen de su situación y su estado de ánimo, detectar posibles psicopatologías que les afecten, cuantificar el nivel de sobrecarga, evaluar el apoyo social percibido, el grado de satisfacción familiar y la relación con su farmacia habitual.Métodos: Estudio descriptivo transversal en el que participan CI de EA no institucionalizados, reclutados en las asociaciones de familiares de EA de Galicia, mayores de 18 años y que den su consentimiento informado.Variable principal: puntuación del test de Zarit de sobrecarga del cuidador. Otras: socio-demográficas, ansiedad (STAI), depresión (Beck) y apoyo social (Duke-UNC) y familiar (APGAR) percibido.Discusión: Los resultados de la metodología implementada en este estudio pueden servir para desarrollar programas similares en la farmacia comunitaria, en la que los farmacéuticos comunitarios colaboren en la detección de posibles psicopatologías, y contribuir, mediante la prestación de servicios profesionales, a conseguir una mejora en la calidad de vida y estado de salud de los cuidadores