Elastocapillary self-folding of micro-machined structures: capillary origami

Standard lithographic techniques have proven to be inadequate at machining true 3D micro-structures - structures with similar dimensions in all directions or with large height to width ratio. New fabrication paradigms are necessary. Combining the assets of mask-based techniques with self-assembly, self-folding is a proven and still promising approach to bring micro-structures to the third dimension. Pre-tethered parts are assembled using an external stimuli such as magnetic forces, pre-stressed layers, thermal shrinking... The work presented in this thesis deals with the self-folding of micro-machined three-dimensional silicon nitride structures using the surface tension of water, method termed elastocapillary folding or capillary origami. A technique for the controllable capillary folding of 3D micro-structures by means of through-wafer liquid application was developed. For the first time hydro-mechanical, repeatable, actuation of capillary folded structures via the addition or retraction of water on demand has been demonstrated. Silicon nitride objects made of thick flaps inter-connected by flexible hinges were machined with a central through-wafer tube and connected to a dedicated pumping system to enable assembly. When remaining wetted, structures can be assembled and reopened up to several dozens of times and still reach the same final folding angle. Objects were actuated up to 60 times without signs of wear. Extracted curves from the self-folding experiments are in agreement with a two-dimensional elastocapillary folding model. When structures are allowed to dry in between foldings, an increase in the bending stiffness of the hinges is observed, by a factor 50% after first folding and subsequent drying. This stiffening causes a decrease of the finally achieved angle. Residue from the fabrication process found on the structures after folding is suspected to be the cause of the stiffening. Using the same type of structures, Platinum electrodes running from the substrate to the plates via the bendable hinges were introduced. The fabrication yield was as high as (77 2)% for hinges with a length less than 75μm. The yield reduced to (18 2)% when the length increased above 100μm. Most of the failures in conductivity were due to evaporation of metal during the plasma cleaning step at the end of the fabrication. The bi-layer hinges survived the capillary folding process, even for extremely small bending radii of 5μm, nor does the bending have any impact on the conductivity. Once assembled, the conductive hinges can withstand a current density of (1:6 0:4) 106 A/cm2. Stress in the different layers caused apparent deformation of the hinges. This introduction of conductive electrodes to elastocapillary self-folded silicon based micro-objects extends the range of possible applications by allowing electronic functionality on the folded parts. Elastocapillary folding of silicon nitride objects with accurate folding anglev between flaps of (70:6 0:1)° was shown and the feasibility of such accurate micro-assembly with a final folding angle of 90° was demonstrated. The folding angle is defined by stop-programmable hinges that are fabricated starting from silicon molds thanks to accurate three-dimensional corner lithography. This nano-patterning method exploits the conformal deposition and the subsequent timed isotropic etching of a thin film in a 3D shaped silicon template. The technique leaves a residue of the thin film in sharp concave corners which can be used as an inversion mask in subsequent steps. Hinges designed to stop folding at 70:6° were fabricated batch-wise by machining V-grooves obtained by KOH etching in (110) silicon wafers ; 90° stop-programmable hinges were obtained starting from silicon molds obtained by dry etching on (100) wafers. The technique we present here is applicable to any folding angles. It opens a new route towards creating structures with increased complexity, which will ultimately lead to a novel method for device fabrication. The assembly of side by side ribbons under the action of surface forces was demonstrated. The beams - which are anchored on both sides - rotate and bend to form a highly symmetric three dimensional structures with a central joined part and two opposite opened parts. Wafer-scale assembly of silicon nitride beams was demonstrated and control experiments with macro-sized mylar ribbons were carried out. A model that connects ribbons dimensions, pulling tension and surface forces was developed and compared to the experiments at both scales. The found opening length is conformed to the macro-experiments and is found to be linear with the width of the beams, the separation between them and more than proportional to the applied tension. The opening is independent of the ribbons length. For micro-ribbons, the induced stress by deformation cause the opening length to increase with the width and the separation and to decrease with the length. The theoretical model does not represent the trends well, probably because it does not capture the 3D aspects of the assembly.