Fabrication and characterization of nanometer thin films for low-voltage DEAs

Nanometer-thin films are the essential components of a low-voltage dielectric elastomer actuator (DEA). Comprising of two electrodes sandwiching a dielectric elastomeric material DEAs have evoked versatile materials research. Before choosing the materials used to manufacture low-voltage DEAs one should carefully consider the targeted application. This project aims at finding new techniques to realize nanometer-thin films to obtain low-voltage DEAs with possible future application as artificial muscle to treat urinary incontinence. Therefore, the materials used should be highly biocompatible. Two promising materials are gold and polydimethylsiloxane (PDMS) both show high biocompatibility due to their inherent chemical inertness. Additionally, their physical properties exhibit most of the desired qualities such as for example high electric conductivity for gold and high elasticity for the PDMS.
To deposit metal electrodes with nanometer scale thickness, the most frequently used techniques are radio frequency (RF) magnetron sputtering, thermal- or electron beam evaporation, chemical vapor deposition (CVD) methods and electrochemical deposition. Deposition from the liquid/dissolved state by applying a potential between the conducting substrate and a counter electrode, as done in electrochemical techniques, is not applicable for DEA production since the metal has to be deposited on the dielectric elastomer. Regarding the listed deposition methods from the gaseous phase only the physical vapor deposition (PVD) was considered with focus on thermal evaporation and RF magnetron sputtering. The electromechanical properties of simple one layer DEAs with either sputtered or evaporated gold electrodes were investigated taking advantage of the bending of asymmetric planar DEA structures on a flexible substrate. The bending of these cantilever-like structures is induced by applying a voltage. It was found that the actuation at the same voltage was up to 39 % higher for the RF magnetron sputtered actuator compared to the thermally evaporated one. This finding will have a big impact on the stiffness of future multi-stack actuators (cp. Section 2.1).
Considering the fabrication of elastomeric nanometer-thin films two methods were established and proven to lead to obtain the targeted nanometer scale in film thickness. Both methods, electro-spray deposition (ESD) and organic molecular beam deposition (OMBD), have advantages and disadvantages regarding the applicable materials, deposition rates, costs and up scaling. In the following sections each method and the corresponding findings will be discussed in more detail.
The in house built electro-spray deposition system, which can be coupled to a spectroscopic ellipsometer (SE) to acquire inter alia real time data of film growth, was evaluated as a possible method for the creation of nanometer-thin elastomeric PDMS films. Therefore, the appropriate deposition mode, solvent and pre-polymer had to be identified. Since the aim was to fabricate multi-stack actuators it had to be considered that the conducting substrate needed for direct current (dc) experiments could not be assured throughout the whole manufacturing process. Therefore, it was decided on using the alternating current mode (ac). This mode, according to literature, prevents surface charge accumulation on non-conducting substrate due to neutralization by incoming opposite charged species. As a solvent ethyl acetate was chosen since it dissolves PDMS pre-polymer chains and it is not poisonous to humans. Considering the pre-polymer the commonly used two components PDMS Elastosil 745 A/B was first applied. After deposition and subsequent heat treatment for curing the Elastosil was still a viscous liquid. This finding was attributed to the reduction/oxidation of the Pt catalyst by the applied electric field of ± 5 kV (18 Hz). At this point it was decided to use vinyl-terminated PDMS which has been approved to work with UV curing in OMBD. Having determined the fundamentals to obtain a stable electro-spray and a curing process to manufacture PDMS films, investigations on deposition parameters towards optimization of the resulting films were conducted. First of all the influence of the deposition rate on the resulting film morphology was studied applying in-situ SE, atomic force microscopy (AFM) and interferometry. The results revealed that the surface roughness of the deposited films increases with increasing deposition rate but smoothed to values in the same range by UV irradiation for all deposition rates. The obtained surface roughnesses vary between 0.20 and 0.28 nm determined by atomic force microscopy on areas of 25 μm2 and between 2 and 20 nm on an area of 0.72 mm2 as obtained by interferometry for deposition rates between 0.02 to 5.54 nm/s. With thicknesses in the scale of a few hundreds of nanometer to micrometer these films qualify for use in DEA manufacturing (cp. Section 2.2).
In a further study of the electro-spray deposition the focus was put on the film growth mechanism of the deposited droplets/islands. This investigation was based on quasi-dynamic observations of the deposited and subsequently cured PDMS islands. Techniques used to evaluate the film growth ranged from AFM images to select appropriate pre-polymer molecular weight, optical micrograph segmentation to spectroscopic ellipsometry. The most convenient pre-polymer molecular weight, from the four investigated in this study, turned out to be 6,000 g/mol. Furthermore, studies of the deposited and cured islands of this pre-polymer revealed an average height of 30 nm. During the early stages of deposition a 3D growth is observed which eventually, with increasing deposition time, turns into a 2D growth. With a flow rate of 267 nL/s and an average deposition time of 155 s a confluent layer with a thickness of about 91 nm, which still exhibits a rough surface, can be obtained. Prolonging the deposition time will smoothen the surface to a scale of a few nanometers (cp. Section 2.3).
OMBD deposition, possible after assembling a small ultra-high vacuum (UHV) chamber, was applied to get a proof of concept for thermal evaporation, deposition and UV curing of PDMS pre-polymer chains. In a later stage a more elaborate UHV chamber was assembled with e.g. a mounted SE to conduct sophisticated investigations. Based on the structure of a standard (DMS-V05) pre-polymer, approved for thermal evaporation, a new pre-polymer was synthesized. The new poly((chloropropyl)methylsiloxane-co-dimethylsiloxane) copolymer showed higher dielectric permittivity and higher break down strength in liquid state. Therefore, a comparison study of film growth with in-situ curing as well as their resulting films after post deposition cure was conducted. The results suggest the use of the new copolymer for low-voltage DEA application since it has enhanced dielectric and elastic properties. Due to the inherent higher polarity a different growth mode during the early deposition stages could be detected by real time SE. The resulting films showed an increased surface roughness by a factor of two but still in the subnanometer scale for an area of 5 μm × 5 μm as determined by AFM (cp. Section 2.4). These results show that a major step towards low-voltage DEA has been accomplished with this work.
Salient points
The investigation on the impact of compliant metal electrode preparation for DEA application revealed an increase of actuation of 39 % for RF magnetron sputtered as compared to thermally evaporated gold electrodes. Imaging the surface of the deposited electrodes using AFM revealed the origin of this discrepancy of actuation. The micro-structure of the thermally evaporated electrode shows circular cluster formation with heights of about 20 nm whereas the sputtered electrode has a smoother surface with randomly distributed cracks in the range of 100 to 200 nm in width (cp. Section 2.1). Without the technical support of Yves Pellmont for thermal evaporation of gold and the assistance on the AFM of Monika Schönenberger this investigation would not have been possible.
Sections 2.2 and 2.3 deals with the ac electro-spray deposition of vinyl terminated PDMS in a 5 vol. % ethyl acetate solution. It was shown that during deposition the surface roughness of the deposited film proportionally depends on the flow rate. During UV curing the films seem to recover and end up in a similar surface roughness for all flow rates. Furthermore, the growth in early stages is rather three dimensional whereas in more advanced deposition times a 2D growth was observed using spectroscopic ellipsometry. Deposited and cured single circular islands show a mean height of about 30 nm observed after 13 s of deposition while a confluent layer with a rather high surface roughness is obtained within an averaged deposition time of 155 s with a mean height of 91 nm. The observed surface roughness is decreasing with successive time evolution after 155 s. Based on Dr. Gabor Kovacs (Empa, Dübendorf) idea electro-spray deposition of PDMS was evaluated. Due to valuable support of Tino Töpper (SE data analysis), Bekim Osmani (AFM morphology determination) and Dr. Hans Deyhle (Segmentations of the optical micrographs) these investigations could be evaluated.
After the proof of concept with the assembled small UHV chamber the new sophisticated UHV chamber (assembled by Dr. Vanessa Leung) allowed for the investigation of polymeric materials in UHV conditions. A commercially available pre-polymer (DMS-V05) was compared to a newly synthesized pre-polymer (found and optimized by Dr. Frederikke Bahrt Madsen, DTU) similar in structures to the DMS-V05. Due to the incorporated copolymer an permittivity increase of 33 % at 100 Hz was measured as well as an enhanced dielectric break down strength of 25 %. During early stages of growth two varying mechanisms were observed using real time SE. The presence of the copolymer increased the dipole of the pre-polymer molecule which causes this initial island growth observed compared to a 2D growth of DMS-V05. Nano-indentation experiments after the same UV irradiation time showed Young’s moduli differing in a factor of about two. These enhancements of dielectric and mechanical properties lead to an increase of 4.6 for the according figure of merit (cp. Section 2.4). Again the AFM (Bekim Osmani) and SE (Tino Töpper) measurements and data analysis lead to the evaluation of the collected data from the manufactured samples. Additional support for the dielectric measurements was given by Liyun Yu (DTU).