(Invited) Silicon Oxide Films: Plasma Assisted Formation of Nanostructures from Glass to Organic Polymers

Abstract

Thin silicon oxide films produced by means of plasma enhanced deposition techniques cover a large area of applications with a long industrial tradition. It is long-recognized that the chemical composition, physical properties and particularly the functionalisation of SiOx films can be adjusted in a wide range by choosing the appropriate plasma conditions for the deposition process. For PECVD, the proper choice of the silicon-organic molecule as thin film precursor is also substantial. Next to the production of amorphous SiO2films by PVD for optical coatings, also SiOx films by PECVD are known that mimic the functionality of a glass surface. Prominent examples include films for corrosion protection, as passivation and insulation layer, and for permeation barrier coatings. Here, dense, pin-hole free films are required that are usually characterized by a stoichiometry close to x=2, with low carbon content (below 5%). With increasing polymeric character and depending on the degree of cross-linking the films become attractive for applications such as adhesion promoting interfaces in multilayer systems, for the integration of catalysts into surfaces or as selective absorber in sensor applications. For these purposes, films with micro- and nano-porous morphologies are preferred. It is generally supposed that films with superior qualities (e.g. spatial morphological homogeneity) are produced by using low pressure methods due their technological maturity in comparison to atmospheric pressure thin film deposition techniques which have found increasing interest over the last decades. In this contribution, several examples of deposition processes for SiOx at atmospheric pressure are presented. The nanostructure of the films is emphasized in particular. An atmospheric pressure dielectric barrier discharge (apDBD) using Ar, O2and OMCTS (D4, octamethylcyclotetrasiloxane) is presented for the deposition of SiOx release films. Deposition using a microwave plasma jet with TTMS (Tetrakis(trimethylsilyloxy)silane) is shown as an example for the deposition of nanostructured dendritic films. A similar experiment with HMDSO (hexamethyldisiloxane) and with an RF plasma jet demonstrates advantages of self-organisation mechanisms of plasma sources under atmospheric pressure for the chemical homogeneity of the film. Compared to other non-thermal plasmas, the geometry of the plasma jet is advantageous for local selective deposition, particularly on 3D shaped substrates. The jet geometry allows better flexibility in terms of distance between active plasma and substrate position and exposure to high electrical fields, as the surfaces to be treated are not necessarily placed between electrodes. Furthermore, the low pressure deposition of amorphous SiO2films using PVD with additional kinetic ions is reported. These films serve as low refractive index material for multiple thin film stacks in applications for high-precision thin film optics, such as laser mirrors or optical filters. The refractive index of the deposited film depends crucially on the plasma conditions. The chemical composition of films is systematically characterized using different surface analysis techniques (XPS, FTIR). The stoichiometry and carbon content are evaluated and robustly classified regarding the chemical structure (polymer-like, crosspolymer-like, inorganic) and allocated to an overview structure diagram proposed as a general classification model for SiOxCy films. The correlation of the chemical properties is thoroughly discussed in relation to the morphologic characterisation (based on SEM and AFM). This research has been supported by the projects R&D center for low-cost plasma and nanotechnology surface modifications CZ.1.05/2.1.00/03.0086, by the project No 7AMB12DE005 funded by the Czech Ministry of Education, by the DAAD project No 54437076 and by the German Research Foundation under grant LO 623/3-1 and TRR 24 ‘Complex Plasmas’.