Article pubs.acs.org/Langmuir
Control of Surface Energy of Silicon Oxynitride Films Kaishi Wang,† Martin Günthner,§ Günter Motz,§ Brian D. Flinn,† and Rajendra K. Bordia*,† †
Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195-2120, United States Ceramic Materials Engineering (CME), University of Bayreuth, D-95440 Bayreuth, Germany
§
ABSTRACT: Microstructure and chemical composition determine the wetting property of solid surfaces. To achieve hydrophobicity or hydrophilicity, recent efforts have mostly focused on designed patterns and sophisticated surface modification. Here we show the fabrication of a dense amorphous silicon oxynitride (SiON) film by simple annealing of perhydropolysilazane (PHPS), which experiences significant and abrupt transition in surface energy as a function of temperature. The polar component of surface energy, derived from contact angle measurements, exhibits an increase of 20−40 times in an annealing temperature window of ∼100 °C, which leads to a 5-fold increase of its total surface energy. On the basis of the chemical analyses, we propose a compositional gradient in the film. Due to this gradient, the hydrophilic SiON film, for instance, can be used as the bond coat material in a double-layer environmental barrier coating system with outstanding oxidation resistant properties.
1. INTRODUCTION Ceramic films are widely used in the components of electronic devices, solar cells, light-emitting diodes, and wear resistant parts. SiOx, as an example, is a good dielectric material that can be further modified into low-k dielectrics, for use with Cu wiring, to replace the conventional Al/SiO2 technology in microelectronics industry.1,2 In addition, it also can be used as environmental barrier or protective coatings in various applications due to its impermeability to gas species, such as moisture, O2, N2, Ar, Kr, and Xe.3 Requirements for such coatings include low weight, low atomic diffusion or permeability, and, sometimes, high transparency.4−7 Moreover, in certain applications, e.g., self-cleaning coatings8 and multilayer-structured MEMS,9,10 hydrophobicity or hydrophilicity is needed to realize their designed functionality.11 The wetting property of solid surfaces is mainly governed by microstructure and chemical composition. Recent research in the field has mostly focused on patterned surfaces12−15 and composite materials with one phase that carries the hydrophobic/ hydrophilic property.16−19 Typical methods of making such films are self-assembled monolayer (SAM),9,10,15 lithography/ imprinting techniques,13,14 plasma-enhanced chemical vapor deposition (PECVD),20−23 physical vapor deposition (PVD),24 reactive sputtering,25 and sol−gel processing.8,26,27 The vapor phase processes have a variety of obvious limitations, including high capital cost, environmental concerns, potentially high defect density,20 and the need for line-of-sight geometries. Also, sol−gel derived coatings suffer from the high shrinkage associated with the ceramic conversion or low temperature tolerance. Polymeric precursors can be converted to ceramics28,29 in various forms, including foams,30 fibers,31,32 and tapes.33 When using them for coatings, shrinkage during pyrolysis is a concern © 2013 American Chemical Society
as it usually leads to cracking if the coating thickness exceeds a critical thickness. However, the shrinkage of polymer-derived coatings made from high-yield preceramic polymers is significantly lower than that for sol−gel derived coatings. The shrinkage can be further lowered by adding inert and reactive particles. Such approaches have demonstrated the ability to make crack free coatings of practical thicknesses.28 Perhydropolysilazane (PHPS) is a versatile polymer. It is a precursor to both SiO2 and Si3N4, depending on annealing atmosphere and conditions. To make the process suitable for roll-to-roll mass production, research has been conducted on converting PHPS into amorphous ceramics at or near room temperatures by exposing the as-deposited film to either reactive environments, like ammonia gas34−36 or hydrogen peroxide solution,37 or vacuum ultraviolet (VUV) light sources, resulting in SiOx in oxygen5,38 or Si3N4 in nitrogen.39 In addition, thermal annealing of PHPS has also been extensively investigated,40−42 revealing that the polymer can be fully converted to an inorganic at intermediate temperatures (600 °C) with a high ceramic yield.41 In this paper, a dense silicon oxynitride (SiON) film on either metal (dip coating) or silicon (spin coating) substrate is made by simple annealing of the PHPS polymer. We report on the abrupt surface energy transition from hydrophobic to hydrophilic within a small temperature window during annealing. The outstanding performance of the SiON film is then illustrated in an application of a multilayered environmental barrier coating system. Received: October 30, 2012 Revised: January 17, 2013 Published: January 29, 2013 2889
dx.doi.org/10.1021/la304307y | Langmuir 2013, 29, 2889−2896
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In order to obtain the constants a and b, which corresponded to the square roots of dispersive and polar surface energy components of the solid phase, the contact angle θ with different known liquids needs to be measured. Three different liquids were used in the measurements, and their surface energies (both polar and dispersive components),45 in units of mN m−1, are the following: deionized water (P 50.3, D 21.5), ethylene glycol (P 15.2, D 32.8), and formamide (P 23.5, D 34.4). Contact angle data were collected on PHPS films annealed at RT, 200, 250, 300, 350, 400, 600, and 800 °C. A larger number of films were investigated between 200 and 400 °C because this was the temperature range of significant changes in film’s surface chemistry. 2.3. Chemical and Microscopic Analysis. Chemical analysis on the PHPS films was performed using Fourier transform infrared spectroscopy (FTIR) (powder) and Raman spectroscopy (785 nm laser, Renishaw inVia Raman Microscope, Renishaw Inc., Hoffman Estates, IL). Pellet samples for FTIR were made from powders (particle size
