Inkjet printing and high aspect ratio structuring for polymer-based micro and nano systems

Inkjet printing of functional materials, photolithography and micromolding are 3 different techniques largely used in the field of micro-electromechanical systems (MEMS) with increasing interest for the fabrication of new devices and the development of new applications. This Ph.D. thesis provides new insights into each of these fields and, by combining them, aims to enable the access to new application fields. Complementary to photolithography, inkjet printing is increasingly considered a cost-effective and flexible microfabrication method to structure functional materials. The ease of mass fabrication and the inherent flexibility of inkjet technology make it a suitable method for the manufacturing of microsystems and components. The results presented in this dissertation summarize recent achievements in this relatively new technology for the development of miniaturized devices. First the piezo-acoustic inkjet printing technology is described and a theory to explain the drop generation process is developed and evaluated. The experimental work was focused on the drop-on-demand inkjet printing of viscous polymers, particularly SU-8, a negative tone photo-resist epoxy. It validates the theoretical predictions and shows the shortcomings of the existing theories. The camera-assisted high accuracy alignment (< 3 µm misalignment) technique developed in the frame of this work enables the precise positioning of microdrops on pre-structured substrates and accurate filling of high aspect ratio micromolds. Additionally, polymeric microlenses were also Inkjet-printed and characterized. The highly reproducible optical properties of such microlenses, i.e. the focal length in the range of 40-150 µm and the numerical aperture in the range of 0.25-0.75, can be adjusted for different applications in micro-optical systems. An innovative technique, based on micromolding, for the microfabrication of high aspect ratio ceramic structures using polymeric precursors is also presented. Polyureasilazane, a precursor for silicon-carbide-based ceramics was investigated. Pipettes and inkjet printers were used to fill high-aspect ratio SU-8 molds. The micromolded structures were released using a sacrificial layer. After a pyrolysis step which enables the polymer-to-ceramic transformation in a simple tube furnace, the final structures showed that this process is very reliable as the mold shapes were perfectly replicated and a smooth surface was obtained. This technique is an interesting candidate for the fabrication of ceramic devices for harsh environments and high temperatures. Incorporation of luminescent nanocrystals (NCs) in SU-8 is another contribution of this thesis. For this purpose, the selection of an appropriate solvent for both NCs and the epoxy-resist formulation is fundamental in directing the nanocomposite resist preparation. The NC-modified SU-8 was successfully patterned and high aspect ratio structures with micrometer resolution were fabricated, showing that the overall UV-structuring capability of the modified resist was not affected by the NC incorporation. The obtained microstructures were shown to retain the NC emission properties. Finally the possibility to combine inkjet printing, micromolding and photopolymerization for the fabrication of Hybrid AFM probes by a single-plane surface-micromachining technique was demonstrated. Hybrid AFM probes, with a tip made of a hybrid organic/inorganic polymer and a cantilever and a holder made of SU-8 were micro-structured, characterized and tested for AFM imaging. The results were compared with a commercial silicon probe and a SU-8 AFM cantilever. This technology can be used to convey new properties to scanning microscopy probes for new sensing applications or to improve the quality of the existing devices. Functional materials can be inkjet-printed to form the tips with a minimum material waste. This thesis is intended to contribute to the advancement of micromachining technologies in the area of micro-devices with enhanced properties. In particular, it shall contribute to the cost effective fabrication of hybrid components made of different functionalized materials.

Development of Implantable Electrodes Based on Polymer Derived Ceramics

Jonas Grossenbacher

The invention of heart pacemakers in the 50's aims to help patients to overcome heart strokes, which represent the first cause of death worldwide. In Europe and in the USA, more than 3000 pacemakers are implanted every day. Despite the constant evolution of the pacemaker system, the lifetime of the implants is limited to several years. State of the art pacemaker electrodes suffer from pacing signal inhibition related to fibrotic tissue growth. This thesis describes the development of novel electrically conductive ceramic based materials, which promise to be more bio-inert than the currently used metals electrodes. Several Polymer Derived Ceramic (PDC) materials (Ceraset, SMP10 and SU8) have been investigated. In order to induce electrical conductance our project partner at EMPA doped the ceramics by an increase in carbon contents. However, for SU8 this step was not necessary. Photolithography and moulding were the two main microfabrication methods used for shaping the PDC materials. Moulding showed to be the most suitable method for shaping Ceraset and SMP10 materials. Thanks to the optimization work of the mould coatings, the main advantage of moulding consists in the possibility to release the PDC from the mould prior the sintering at high temperature. The release can be further facilitated when using elastomeric mould. SU8 was shaped by means of photolithography on top of a water soluble Dextran release layer and released before sintering. The materials were characterized at each fabrication step. The material composition, surface analysis and hardness of the densified ceramic samples have been measured. Furthermore an electrochemical characterization was performed in PBS and 1M KCl solution, by means of cyclic voltammetry and electrochemical impedance spectrometry. The measurements of the new materials show comparable values and behaviours to standard Pt metal electrodes. As the electrodes are meant to be implanted, it was necessary to investigate their cytotoxicity and in vitro biocompatibility for non-conductive and conductive compositions. Low cytotoxicity levels comparable to the state of the art positive reference implant materials have been assessed. To elucidate the fibrosis inhibition, long-term implantation tests (1 week to 6 months) have been carried out without the presence of electric signals. The fibrosis level turned out to be low for the new materials and comparable the reference materials. The pacing functionality of the new electrodes has been experimentally assessed as follows: (a) In vitro pacing of isolated cardiomyocytes by applying an electric potential of 20-40 V between two electrodes. The cell contraction was quantified by correlating the pacing signal with the hemodynamic parameters by means of video recording and image processing; (b) In vivo and ex vivo tests on a rat model were conducted by implanting SU8 micro-electrodes into peripheral muscles and in an isolated and perfused heart, respectively. The analysis confirmed that both muscle tissues were successfully actuated by using typical signal parameters. In conclusion, this thesis presents the successful development, optimisation and fabrication of pacemaker electrodes with new materials, which possess properties that are comparable to state of the art platinum electrode. This work provides a selection of materials for further characterization of long term implanted electrodes, studying the influence of an electric pacing signal on the fibrosis.

EPFL2015

A new versatile hybrid MEMS technology for high sensitivity, fluid proof sensing applications.

Matthias Neuenschwander

The field of micro electromechanical systems (MEMS) evolved from the microelectronic industry and the technologies developed to fabricate integrated circuits. As a result, MEMS are commonly fabricated on silicon wafers. The development of MEMS has been driven by three main merits: miniaturisation, microelectronics integration, and parallel fabrication with high precision. Many operational properties scale well with smaller sizes. In addition, integrated electronic circuitry allows embedding MEMS with computing or networking capabilities, while parallel manufacturing enables the fabrication of many identical devices on a single wafer, reducing the unit cost. The main MEMS application is transducers which transform signals from one form of energy to another, and can be used for perception (sensors) or to produce actions (actuators). Silicon and other microelectronic materials like metals have enabled very high-performance MEMS transducers because these materials can be used for a range of very effective actuating and sensing principles.More recently, polymers have started to be used in MEMS because of their unique electrical, physical and chemical properties which include biocompatibility, viscoelasticity and mechanical shock tolerance. They are also much softer than silicon or metals, and they can be processed using many techniques that allow unique low-cost, batch-style fabrication and packaging. However, polymers have relatively low glass-transition and melting temperatures. As a result, the advantages of polymers cannot easily be combined with high-performance actuating and sensing elements as these are based on silicon technology and require high-temperature fabrication steps. Here, a hybrid-MEMS fabrication process is used to address this issue. The developed devices are based on a trilayer structure, where a thick polymer core is sandwiched between two hard thin films, while electronic layers are embedded within in a fluid-compatible way.The objective of this thesis is to turn hybrid-MEMS into a benchmark MEMS technology. This involves many different aspects. First, sensing and actuation elements are integrated into trilayer devices, and their performance is analysed. Then, some more unique possibilities offered by hybrid-MEMS are exploited. A way to fabricate electronics on multiple layers is developed, which allows different electronic features to be integrated in parallel. In addition, three-dimensional bulk features fabricated at several levels are demonstrated. This is possible because the hybrid-MEMS process is based on the bonding of multiple wafers.All these new fabrication features are demonstrated in atomic force microscopy (AFM) applications. Self-actuated trilayer AFM cantilevers with sharp silicon tips are used to boost imaging speeds in the off-resonance tapping (ORT) mode, thanks to an order of magnitude faster surface probing rates. Furthermore, fully integrated ORT imaging is demonstrated with self-actuated piezoresistive cantilevers. Next, trilayer cantilevers with shielded conductive tips are introduced for electrical AFM modes. Finally, ongoing research projects are touched upon, including approaches to fabricate hybrid-MEMS with single crystal silicon piezoresistors for improved sensitivity, and an analysis of reproducibility of fabricated trilayer cantilevers.

EPFL2022

Improved and new properties of resist materials by doping with nanoparticles and nanocrystals

Jürgen Brugger, Vahid Fakhfouri

Increasing interest in specific applications in the field of micro- and nano-electromechanical systems (MEMS and NEMS) has raised the request for new functional polymeric materials. Epoxy based photoresists are being broadly used for the fabrication of micromechanical devices and for the definition of patterns with high aspect ratio in the micrometer scale. SU-8, the well known thick photo resist with superior lithographic properties over a wide range of thicknesses, have made a tremendous impact on the development in this field by simple fabrication of permanent plastics elements with vertical sidewalls and a high aspect ratio. For new low cost applications organic materials are required which can be patterned and exhibit properties comparable to inorganic materials, such as high hardness, magnetism, conductivity or luminescence. For the manufacture of polymer waveguide systems materials with comparable lithographic processing but with sufficient difference in the refractive index are required. Hybrid materials consisting of inorganic nanoparticles in polymer matrices represent a novel class of materials for nano patterning with unique properties, which arise from the synergism between the properties of both components. The resulting novel materials can be processed and patterned using established techniques whereas the incorporated nanoparticles and nanocrystals convey their inherent functionalities and properties to the final material or composites. The suitability of silica epoxy photosensitive material as cladding material for waveguiding application and luminescent nanocrystal doped epoxy resist as model composite for the investigations of different core/ shell surface modifying methods and for light sensing and emitting application will be demonstrated.