Elastocapillary self-folding of micro-machined structures

Abstract

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 interconnected 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 75 µm. The yield reduced to (18 ± 2) % when the length increased above 100 µm. 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 5 µm, 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) × 10 6 A/cm 2. 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 angle v piques. Pour ceux-ci, la fabrication et l'assemblage sont très simples : plongez les ru...