Infoscience

Thesis

Nouvelles technologies de microstructuration de type UV-LIGA et fabrication de composants micromécaniques

This study presents two techniques of structuration of type UV-LIGA (Lithographie, Galvanisierung und Abformung) based on the use of two thick photoresists which are a dry film resist called Riston® from DuPont and the new SU-8 negative-working resist from IBM. The relating processes to the structuration of these photoresists and some applications are presented too. The minimal resolution of the Riston® films has been found to be 30 µm for lines and trenches, and 40 µm for isolated holes in a 50 µm-thick film. A technique for the structuration of multilevel superimposed cavities has been set up. The verticality of the sidewalls (85.5°deg. to 87.5°deg) has been measured on film thicknesses between 25 and 100 µm. The SU-8 process has been studied and is presented. Some optical and mechanical properties have been analysed. We have determined a coefficient of light attenuation of 5.18 [db/mm] at 365 nm, a coefficient of thermal expansion of 52 [ppm/°C] and a biaxial modulus of 5.18 GPa. The verticality of the walls depends on the exposure energy and has been found to be higher than 89°deg. Layers from 750 nm to 2 mm can be exposed in one time, which seems to be the state-of-the art for a UV-photoresist to our knowledge. A specific process for the electroforming in multi-level superimposed cavities has been developed. With an adequate dosage of a leveling agent (coumarin) in the electrolytic Watt bath of nickel, a change in the growth rate of the high and the low level of the mold has been demonstrated. This rate is harmonised to obtain a flat deposition on the top surface of the formed structure. The UV-LIGA techniques developed in this thesis have found applications in the field of the-micromechanics. A flexible gearwheel, whose teeth are bound to the center via three integrated spring, and a multilevel gear system are the main realizations obtained by electroforming. These two structures have been used in a mechanism with a special watch mechanism and in a microreducer based on an electrostatic rotary micromotor. We have experimented with success the plastic injection of a gear system having 1.4 mm in height and 3 mm in diameter. Its mold has been electroformed on a master in SU-8. Theses microparts have demonstrated their functionality and have been mounted in commercialy-available-low-cost watches. All the process step of the LIGA technique have been established. The main advantages of this fabrication method are a better control of the tolerances and an improvement of the rugosity (against the wire electrodischarge machining technology). This technique is also cheaper and easier to start than X-LIGA.

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