Angeliki Tserepi

Mechanisms of Oxygen Plasma Nanotexturing of Organic Polymer Surfaces: From Stable Super Hydrophilic to Super Hydrophobic Surfaces

Plasma processing is used to fabricate super hydrophilic or super hydrophobic polymeric surfaces by means of O2 plasma etching of two organic polymers, namely, poly(methyl methacrylate) (PMMA) and poly(ether ether ketone) (PEEK); a C4F8 plasma deposition follows O2 plasma etching, if surface hydrophobization is desired. We demonstrate high aspect ratio pillars with height ranging from 16 nm to several micrometers depending on the processing time, and contact angle (CA) close to 0 degrees after O2-plasma treatment or CA of 153 degrees (with CA hysteresis lower than 5 degrees) after fluorocarbon deposition. Super hydrophobic surfaces are robust and stable in time; in addition, aging of super hydrophilic surfaces is significantly retarded because of the beneficial effect of the nanotextured topography. The mechanisms responsible for the plasma-induced PMMA and PEEK surface nanotexturing are unveiled through intelligent experiments involving intentional modification of the reactor wall material and X-ray photoelectron spectroscopy, which is also used to study the surface chemical modification in the plasma. We prove that control of plasma nanotexture can be achieved by carefully choosing the reactor wall material.

Nanotexturing of poly(dimethylsiloxane) in plasmas for creating robust super-hydrophobic surfaces

A rapid, easy-to-implement, and potentially large-scale production method for fabricating high-aspect-ratio columnar-like nanostructures on poly(dimethylsiloxane) (PDMS) is demonstrated. Plasma treatment of PDMS under appropriate conditions in SF6 gas, followed by plasma-induced fluorocarbon film deposition, results in PDMS surfaces of fully controlled wetting properties, geometrical characteristics leading to robust superhydrophobic surfaces, and transparency. Potential applications to microfluidic devices are outlined.

From Superamphiphobic to Amphiphilic Polymeric Surfaces with Ordered Hierarchical Roughness Fabricated with Colloidal Lithography and Plasma Nanotexturing

Ordered, hierarchical (triple-scale), superhydrophobic, oleophobic, superoleophobic, and amphiphilic surfaces on poly(methyl methacrylate) PMMA polymer substrates are fabricated using polystyrene (PS) microparticle colloidal lithography, followed by oxygen plasma etching-nanotexturing (for amphiphilic surfaces) and optional subsequent fluorocarbon plasma deposition (for amphiphobic surfaces). The PS colloidal microparticles were assembled by spin-coating. After etching/nanotexturing, the PMMA plates are amphiphilic and exhibit hierarchical (triple-scale) roughness with microscale ordered columns, and dual-scale (hundred nano/ten nano meter) nanoscale texture on the particles (top of the column) and on the etched PMMA surface. The spacing, diameter, height, and reentrant profile of the microcolumns are controlled with the etching process. Following the design requirements for superamphiphobic surfaces, we demonstrate enhancement of both hydrophobicity and oleophobicity as a result of hierarchical (triple-scale) and re-entrant topography. After fluorocarbon film deposition, we demonstrate superhydrophobic surfaces (contact angle for water 168°, compared to 110° for a flat surface), as well as superoleophobic surfaces (153° for diiodomethane, compared to 80° for a flat surface).

Nanotextured super-hydrophobic transparent poly(methyl methacrylate) surfaces using high-density plasma processing

We present an environmentally friendly, rapid, no-rinse and mass-production amenable plasma process for the fabrication of super-hydrophobic (SH) poly(methyl methacrylate) (PMMA) surfaces using only a one load/unload step in a low-pressure, high-density plasma reactor. First, oxygen plasma is applied to nanotexture the PMMA surface via etching processes leading to high aspect ratio (HAR) topography, with dual-roughness characteristics for short process durations, as evidenced by AFM analysis. The duration of the process may range from 1 min to several min depending on the roughness amplitude and on the degree of transparency desired. The significance of the ion-bombardment is revealed and discussed. After this first step, the gas chemistry is changed to a fluorocarbon one which leads to a few nanometres-thick Teflon-like film deposition, thus altering the PMMA surface chemistry within a few seconds. Following this process, a very large area (depending on the reactor scale) of the PMMA may become SH in less than 1.5 min (total process duration) with a transparency as desired (from fully transparent to milky and antireflective). The contact angles (CA) measured are approximately 152° with 5° hysteresis. For short process durations, the dual-roughness character of PMMA surfaces favours the SH formation, despite the low roughness factor. Furthermore, the dry and low-temperature character of the process ensures the intactness of PMMAs shape and bulk mechanical properties.

Plasma micro-nanotextured, scratch, water and hexadecane resistant, superhydrophobic, and superamphiphobic polymeric surfaces with perfluorinated monolayers.

Superhydrophobic and superamphiphobic toward superoleophobic polymeric surfaces of polymethyl methacrylate (PMMA), polyether ether ketone (PEEK), and polydimethyl siloxane (PDMS) are fabricated in a two-step process: (1) plasma texturing (i.e., ion-enhanced plasma etching with simultaneous roughening), with varying plasma chemistry depending on the polymer, and subsequently (2) grafting of self-assembled perfluorododecyltrichlorosilane monolayers (SAMs). Depending on the absence or not of an etch mask (i.e., colloidal microparticle self-assembly on it), random or ordered hierarchical micro-nanotexturing can be obtained. We demonstrate that stable organic monolayers can be grafted onto all these textured polymeric surfaces. After the monolayer deposition, the initially hydrophilic polymeric surfaces become superamphiphobic with static contact angles for water and oils>153°, for hexadecane>142°, and hysteresis<10° for all surfaces. This approach thus provides a simple and generic method to obtain superamphiphobicity on polymers toward superoleophobicity. Hydrolytic and hexadecane immersion tests prove that superamphiphobicity is stable for more than 14 days. We also perform nanoscratch and post nanoscratch tests to prove the scratch resistance of both the texture and the SAM and demonstrate lower coefficient of friction of the SAM compared to the uncoated surface. Scanning electron microscope observation after the nanoscratch tests confirms the scratch resistance of the surfaces.

Quantification of line-edge roughness of photoresists. II. Scaling and fractal analysis and the best roughness descriptors

A search for the best and most complete description of line-edge roughness (LER) is presented. The root mean square (rms) value of the edge (sigma value) does not provide a complete characterization of LER since it cannot give information about its spatial complexity. In order to get this missing information, we analyze the detected line edges as found from scanning electron microscope(SEM)image analysis [see Paper I: G. P. Patsis et al., J. Vac. Sci. Technol. B 21, 1008 (2003)] using scaling and fractal concepts. It is shown that the majority of analyzed experimental edges exhibit a self-affine character and thus the suggested parameters for the description of their roughness should be: (1) the sigma value, (2) the correlation length ξ, and (3) the roughness exponent α. The dependencies of ξ and α on various image recording and analysis parameters (magnification, resolution, threshold value, etc.) are thoroughly examined as well as their implications on the calculation of sigma when it is carried out by averaging over the sigmas of a number of segments of the edge. In particular, ξ is shown to be connected to the minimum segment size for which the average sigma becomes independent of the segment size, whereas α seems to be related to the relative contribution of high frequency fluctuations to LER.

Controlling roughness: from etching to nanotexturing and plasma-directed organization on organic and inorganic materials

We describe how plasma–wall interactions in etching plasmas lead to either random roughening/nanotexturing of polymeric and silicon surfaces, or formation of organized nanostructures on such surfaces. We conduct carefully designed experiments of plasma–wall interactions to understand the causes of both phenomena, and present Monte Carlo simulation results confirming the experiments. We discuss emerging applications in wetting and optical property control, protein immobilisation, microfluidics and lab-on-a-chip fabrication and modification, and cost-effective silicon mould fabrication. We conclude with an outlook on the plasma reactor future designs to take advantage of the observed phenomena for new micro- and nanomanufacturing processes, and new contributions to plasma nanoassembly.

Quantification of line-edge roughness of photoresists. I. A comparison between off-line and on-line analysis of top-down scanning electron microscopy images

An off-line image analysis algorithm and software is developed for the calculation of line-edge roughness (LER) of resist lines, and is successfully compared with the on-line LER measurements. The effect of several image-processing parameters affecting the fidelity of the off-line LER measurement is examined. The parameters studied include the scanning electron microscopy magnification, the image pixel size dimension, the Gaussian noise-smoothing filter parameters, and the line-edge determination algorithm. The issues of adequate statistics and appropriate sampling frequency are also investigated. The advantages of off-line LER quantification and recommendations for the on-line measurement are discussed. Having introduced a robust algorithm for edge-detection in Paper I, Paper II [V. Constantoudis et al., J. Vac. Sci. Technol. B 21, 1019 (2003)] of this series introduces the appropriate parameters to fully quantify LER.

Plasma Nanotextured PMMA Surfaces for Protein Arrays: Increased Protein Binding and Enhanced Detection Sensitivity

Poly(methyl methacrylate) (PMMA) substrates were nanotextured through treatment in oxygen plasma to create substrates with increased surface area for protein microarray applications. Conditions of plasma treatment were found for maximum uniform protein adsorption on these nanotextured PMMA surfaces. Similar results were obtained using both a high-density plasma (HDP) and a low-density reactive ion etcher (RIE), suggesting independence from the plasma reactor type. The protein binding was evaluated by studying the adsorption of two model proteins, namely, biotinylated bovine serum albumin (b-BSA) and rabbit gamma-globulins (RgG). The immobilization of these proteins onto the surfaces was quantitatively determined through reaction with fluorescently labeled binding molecules. It was found that the adsorption of both proteins was increased up to 6-fold with plasma treatment compared to untreated surfaces and up to 4-fold compared to epoxy-coated glass slides. The sensitivity of detection was improved by 2 orders of magnitude. Moreover, highly homogeneous protein spots were created on optimized plasma-nanotextured surfaces through deposition with an automated microarray spotter, revealing the potential of plasma-nanotextured surfaces as protein microarray substrates.

Two‐photon absorption laser‐induced fluorescence of H atoms: A probe for heterogeneous processes in hydrogen plasmas

Two‐photon absorption laser‐induced fluorescence has been used to obtain the spatial distribution of H atoms in the interelectrode space of a parallel plate rf reactor. Continuous and pulsed operation of the discharge allows the monitoring of H atoms, which are produced at the sheath boundary and are removed on the confining surfaces, at rates that depend on the conditions of the discharge and the nature of the surfaces. A variety of metallic and semiconducting surfaces have been loaded on the ground electrode resulting in widely varying H concentration decay rates and concentration gradients characteristic of the surfaces, which range from significantly absorbing to nearly reflecting. A simple analytical model that simulates spatial and temporal profiles determines from the experimental data surface loss coefficients γ in the range 0.1%–20% for H atoms. Finally, we demonstrate the sensitivity of the method in probing, in situ and in real‐time, surface modifications induced by energetic plasmas.