Functional properties and applications of plasma polymerized hexamethyldisiloxane (ppHMDSO) thin films

Abstract

Plasma Polymerization is a novel technique for the preparation of polymer-like thin film coatings at low temperatures onto almost any type of substrates: plastic, metal, semiconductors, wood, textile fibers or membranes, to cite just a few. The films can be grown directly from liquid monomers that are introduced in the vapor phase into a vacuum chamber equipped with one or more electrodes that generate the plasma after a high voltage in continuous current mode (DC), low-frequency (AC) or high frequency (RF) is applied. The plasma state is a high-energy gas state in which the density of electrons, ions, excited species and radical fragments is abundant. The introduction of an organic monomer vapor into the plasma triggers the formation of molecular fragments capable of initiating multiple reactions: in the gas phase, recombination of radicals, oligomerization of high-weight molecules and aggregation into nanoscopic dust can occur, whereas adsorption and reaction onto any solid surface will result in the growth of highly adherent thin films. The structural, chemical and functional properties of these coatings are determined by the composition of the precursor gas mixture and the type of monomer, and also by several technological parameters that can be fine-tuned, such as the pressure, plasma power, frequency of change of electrode polarization, substrate location, flux of gas, etc... By controlling these technological parameters it is possible to modulate the value of the magnitudes that govern the physico-chemical mechanisms which are responsible for film growth: residence time of molecules, available energy per molecule, degree of monomer fragmentation, density and energy of ion bombardment on the substrates, and gas transport in the reactor, among others.

Plasma polymerization allows to grow films from virtually any kind of organic molecule which can be evaporated at low temperatures (<80 °C) and introduced in the reactor at sufficient flow rates (> lsccm), even when that molecule would not be the characteristic repeating unit of any conventional polymer synthesized by other physical or chemical means. The technique is also applicable to other types

of monomers (non-carbon based), such as organosilicon or organometallic molecules.

The use of organosilicon monomers allows to obtain films with a wide spectrum of properties, from those frequently attributed to an elastomeric polymer such as silicone (polydimethylsiloxane, PDMS) to those associated to a hard inorganic material, such as glass (amorphous silicon dioxide, silica). Regardless of their apparently opposed nature, these two materials share an extremely similar chemical backbone based on silicon-oxygen chemical bonds. During the investigations conducted in our study, different organosilicon monomers have been employed for plasma polymerization: hexamethyldisiloxane (HMDSO), hexamethyldisilazane (HMDSN) and tetraethoxysilane (TEOS), but the only results presented within the scope of this Thesis are those obtained for ppHMDSO films. This is due to the fact that HMDSO is the only monomer allowing the growth of polymer-like films, inorganic-like films, intermediate stoichiometry films and even graded films with properties varying with depth in a single plasma process.

With respect to power sources for plasma generation, most published works choose high frequency electrical power sources, such as radiofrequency (RF) or microwaves (MW), although plasma polymerization can also be carried on with direct-current high-voltage sources (HV-DC), from 500 V to 3000 V. In our investigations, these three types of sources have been employed, as can be found in our related publications, but again only results with the DC plasma source will be presented due to their simple design and use in industrial applications. A main objective of this Thesis is to establish its limitations, such as the limited film thickness attainable or the excessive heating of substrates, depending on the reactor configuration and the operating parameters.

As a consequence, the scope of this Thesis covers two main objectives: first, the study of DC plasma polymerization of hexamethyldisiloxane (DC ppHMDSO) with and without addition of carrier gases in the precursor mixture, in order to obtain polymer-like or inorganic silica-like coatings with specific mechanical, optical and corrosion protective functional properties, for further application to solving some practical problems of industrial interest; secondly, the study of modifications

produced by a different non-additive post-treatments in a polymer-like ppHMDSO film, in order to obtain a film with graded properties varying with depth.

La polimerització assistida per plasma és una tècnica novedosa per a la obtenció de recobriments polimèrics a baixa temperatura sobre qualsevol tipus de substrat: plàstic, metall, semiconductors, fusta, fibra tèxtil o membranes, per citar-ne només alguns. Els recobriments són obtinguts directament a partir de monòmers líquids que s'introdueixen controladament en fase vapor dins una cambra de buit equipada amb un o més elèctrodes que, en aplicació d'una tensió elèctrica constant (DC), alterna (AC) o d'alta freqüència (RF, MW), generen el plasma. Les propietats estructurals, químiques i funcionals d'aquests recobriments venen determinades per la composició de la mescla gasosa precursora i la naturalesa del monòmer, i per diferents paràmetres tecnològics controlables, com ara la pressió, la potència acoplada al plasma, la freqüència d'oscil•lació de la polarització, la posició dels substrats, el flux circulant de gas, etc.

L'ús de monòmers orgànics de base silici permet obtenir propietats amb característiques molt amples, des de les més pròpies d'un polímer elastomèric com la silicona (polidimetilsiloxà, PDMS) a les d'un material inorgànic dur, com el vidre (òxid de silici amorf, Si02. Aquests dos materials comparteixen una base química extraordinàriament similar fonamentada en un esquelet d'enllaços Silici-Oxigen. Durant el treball de desenvolupament d'aquest estudi s'han emprat diferents monòmers de base silici: l'hexametildisiloxà (HMDSO), l'hexametildisilazana (HMDSN) i el tetraetoxisilà (TEOS), però només es presentaran els resultats obtinguts en els dipòsits de polimerització plasma del primer, HMDSO, degut a ser l'únic amb capacitat de generar recobriments de caire polimèric, inorgànic, intermedi o fins i tot amb propietats variables amb la profunditat en un únic recobriment.

L’objectiu principal d’aquesta tesi és l’estudi de la polimerització assistida per plasma en corrent contínua de l’hexametildisiloxà (DC ppHMDSO) amb i sense addició de gasos portadors, per a l’obtenció de recobriments polimèrics o inorgànics de base silici amb especials propietats funcionals mecàniques, òptiques i protectores contra la corrosió, i l’aplicació pràctica d’aquests recobriments a la solució d’alguns problemes d’interès industrial. El segon objectiu és l’estudi de les modificacions de les capes de naturalesa polimèrica ja dipositades mitjançant un segon plasma o post-tractament sense contingut de monòmer per tal de modificar la superfície del recobriment i aconseguir un gradient en profunditat de les propietats del material.