Combining In-situ Mechanical, Ellipsometric and Spectrometric Tools during Growth of 3D Porous Anodic Silica

Abstract

Capacitors store energy in the form of an electric field. At small scales, the use of 3D structures increasing the effective surface of the capacitor is a potential route for the development of high capacitance components [1]. In addition, there is an interest for low temperature processes to avoid thermal stress damages in more complex structures and to reduce the energy consumption of the fabrication process [2], which is profitable from the economical and environmental point of view. In this context, electrochemical processes are a potential route for the fabrication of 3D capacitors because anodic porous silicon formation can be used to shape the 3D structure and a dielectric layer can be grown by anodic oxidation of silicon. However, the electrical properties of silicon oxide obtained by anodic oxidation are inferior to those of thermal oxide [2]. To produce anodic silica layers of higher quality, it is first necessary to better understand and control the growth of anodic silica. In this work, we report on the electrochemical formation of anodic silicon oxide, the in-situ characterization of the growing film by different techniques, and the comprehension of the growth mechanism. The growth of anodic silica films is intimately linked to electrochemical oscillations related to the periodic variation of the electrical or morphological properties of the film [3]. To understand the link between the growth, the silica properties and the oscillations, we monitored the silica film during its electrochemical growth by combined in-situ Optical Stress Sensing (MOSS) and Spectroscopic Ellipsometry (SE), as well as by in-situ Inductive Coupled Plasma Spectrometry (ICP-OES). The high temporal resolutions (0.3 sec for the MOSS and 2 sec for the SE) allow the very precise monitoring of the change of internal stress and morphology of the silica film during the anodic polarization of silicon. ICP provides in-situ information about the film dissolution rate in hydrofluoric acid solutions. It appears that, depending on the growth regime and the associated oscillatory behavior, some of the thin film properties are either constant or variable with time. From the mechanical point of view, the internal stress is about -325 MPa in the constant regime and varies between -100 MPa and -300 MPa during the growth in the variable regime. We determined the relation between the two growth regimes in order to advance the understanding and control of 3D porous anodic silica formation