Papers by Author: Giuseppe D'Arrigo

Abstract: The following paper explores the development the bulge test technique combined with the micro-Raman analysis and a refined load-deflection model for high quality 3C-SiC squared-membranes. By the minimization of the total elastic energy, starting from the isotropic relation between the stress tensor and the strain tensor, it is possible to calculate the relationship between the maximum deflection and the applied pressure, in both regime of small and large deflection. From the measured breaking pressure through the refined model it is possible to evaluate the breaking strain of the membrane. Furthermore, the relationship between the measured shift of Raman Transverse Optical (TO) phonon modes and the total residual strain (Δa/a) within the epitaxial 3C-SiC layer was found.

Abstract: In this work, Raman microscopy is used to study the stress distribution on 3C-SiC cantilevers. Also we compare the strain distribution observed on the microstructure, using the shift of the transverse optical (TO) mode in micro-Raman maps, with the values predicted using a recent analytic theory [1]. Along the width of the cantilever is observed a reduction of stress ascribed to the etching processes that removes a thin layer of the interface between the 3C-SiC film and the substrate close to the edge of the microstructure. It is possible to show that this variation can be ascribed to a non-linearity of the strain field along the 3C-SiC film thickness. Also, helped by Finite Element Modelling (FEM), we determined the stress tensor along the cantilever. This result shows that, for a complete stress description of the cantilevers, it is necessary taking into account the role of diagonal and off-diagonal stress tensorial components.

Abstract: Raman microscopy has been used to study the stress distribution on 3C-SiC/Si(100) micro-machined free standing structures. Linear scans along different structures reveal similar trends of the TO mode Raman Shift. We have found that, independently of the microstructure considered, the Raman frequency decreases close to the undercut. We compare our experimental measurements with FEM simulations finding that, close to the undercut, the stress tensor becomes non-diagonal, modifying the Raman shift to stress relation.

Abstract: SiC is a candidate material for micro- and nano-electromechanical systems (MEMS and NEMS). The fabrication of SiC MEMS-based sensors requires new processes able to realize microstructures on either bulk material or on the SiC surface. The hetero-epitaxial growth of 3C-SiC on silicon substrates allows one to overcome the traditional limitations of SiC micro-fabrication, but the high residual stress created during the film grow limits the development of the material for these applications. In order to evaluate the amount of residual stress released from the epi-film, different micro-machined structures were developed. Finite elements simulations of the micro-machined structures have also been carried out in order to evaluate, in detail, the stress field inside the structures and to test the analytical model used. With finite element modeling a exponential approximation of the stress relationship was studied, yielding a better fit with the experimental data. This study shows that this new approximation of the total residual stress function reduces the disagreement between experimental and simulated data.

Abstract: The fabrication of SiC MEMS-based sensors requires new processes able to realize microstructures on either bulk material or on the SiC surface. The hetero-epitaxial growth of 3C-SiC on silicon substrates allows one to overcome the traditional limitations of SiC micro-fabrication. In this work a comparison between single crystal and poly crystal 3C-SiC micro-machined structures will be presented. The free-standing structures realized (cantilevers and membrane) are also a suitable method for residual field stress investigation in 3C-SiC films. Measurement of the Raman shift indicates that the mono and poly-crystal 3C-SiC structures release the stress in different ways. Finite element analysis was performed to determine the stress field inside the films and provided a good fit to the experimental data. A comprehensive experimental and theoretical study of 3C-SiC MEMS structures has been performed and is presented.

Abstract: SiC is a candidate material for micro- and nano-electromechanical systems (MEMS and NEMS). In order to understand the impact that the growth rate has on the residual stress of CVD-grown 3C-SiC hetero-epitaxial films on Si substrates, growth experiments were performed and the resulting stress was evaluated. Film growth was performed using a two-step growth process with propane and silane as the C and Si precursors in hydrogen carrier gas. The film thickness was held constant at ~2.5 µm independent of the growth rate so as to allow for direct films comparison as a function of the growth rate. Supported by profilometry, Raman and XRD analysis, this study shows that the growth rate is a fundamental parameter for low-defect and low-stress hetero-epitaxial growth process of 3C-SiC on Si substrates. XRD (rocking curve analysis) and Raman spectroscopy show that the crystal quality of the films increases with decreasing growth rate. From curvature measurements, the average residual stress within the layer using the modified Stoney’s equation was calculated. The results show that the films are under compressive stress and the calculated residual stress also increases with growth rate, from -0.78 GPa to -1.11 GPa for 3C-SiC films grown at 2.45 and 4 µm/h, respectively.

Abstract: 3C-SiC devices are hampered by the defect density in heteroepitaxial films. Acting on the substrate, it is possible to achieve a better compliance between Si and 3C-SiC. We present here an approach to favorite defect geometrical reduction in both [ ] and [ ] directions by creating Inverted Silicon Pyramids (ISP). A study of 3C-SiC growth on ISP is reported showing benefits in the film quality and a reduction in the linear density of stacking faults. Growth on ISP leads also to a decrease in the 3C-SiC residual stress as well as in the bow of the Si/SiC system.

Abstract: The fabrication of SiC MEMS-based sensors requires new processes able to realize microstructures on bulk material or on the SiC surface. The hetero-epitaxial growth of 3C-SiC on silicon substrates allows one to overcome the traditional limitations of SiC micro-fabrication. This approach puts together the standard silicon bulk microfabrication methodologies with the robust mechanical properties of 3C-SiC. Using this approach we were able to fabricate SiC cantilevers for a new class of pressure sensor. In the present research, chemical vapour deposition (CVD) in the low pressure regime of 3C–SiC on silicon substrates was carried out using silane (SiH4), propane (C3 H8) and hydrogen (H2) as the silicon supply, carbon supply and gas carrier, respectively. The resulting bow in the MEMS structures was evaluated optically and the residual stress in the films calculated using the modified stoney equation and determined to be approximately 300 MPa.

Abstract: The aim of this work is to improve the heteroepitaxial growth process of 3C-SiC on Si substrates using Trichlorosilane (SiHCl3) as the silicon growth precursor. With this precursor it has been shown that it is possible to simultaneously increase the growth rate of the process and avoid the nucleation of silicon droplets in the gas phase. Growth experiments were conducted on three (3) Si substrate orientations in order to assess the impact of the Si substrate on the resulting 3C-SiC film. X-ray Diffraction (XRD), Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) analysis show the important role of the substrate orientation for the growth process. The different orientation of the substrate modifies the morphology of the 3C-SiC crystalline structure, mostly by changing the density of micro-twins and stacking faults inside the film.

Abstract: In this work a comparison between atmospheric pressure (AP) and low pressure (LP) carbonization as the first step in the growth process of 3C-SiC on Si substrates is presented. Three different Si substrate orientations have been studied and compared. Characterization analysis has been performed by Atomic Force Microscopy (AFM), X-ray Diffraction Spectroscopy (XRD) and Transmission Electron Microscopy (TEM). XRD and AFM analysis show a lower roughness and a better quality for LPCVD carbonized samples. Substrate orientation plays an important role both in the generation as well as in the effect of such defects in the subsequent growth process, leading to a rougher SiC surface for growth on (110) Si while micro-twin effects are limited for growth on (111) Si, resulting in an extremely flat film.