Process for fabricating a silicon hairspring

A process for fabricating a hairspring comprising the following steps:

• a) providing an SOI wafer successively comprising a silicon device layer, a silicon oxide bonding layer and a silicon handle layer;
• b) growing a first silicon oxide layer on a surface of the wafer;
• c) carrying out photolithography on the device layer to form a resist mask;
• d) etching the first silicon oxide layer through the resist mask formed previously;
• e) carrying out deep reactive-ion etching in order to form the silicon hairspring;
• f) growing a second silicon oxide layer on the surface of the silicon, the second silicon oxide layer serving as protection for the hairspring formed;
• g) etching the handle layer to expose the bonding layer and then release the hairspring, the hairspring being held on the wafer by at least one attachment;
• h) determining a stiffness of the hairspring formed and calculating an excess material of the coil in order to obtain a hairspring of final stiffness;
• i) oxidizing the hairspring formed in order to transform said thickness of silicon-based material to be removed into silicon oxide and to thus form an oxidized hairspring;
• j) removing the oxide from the oxidized hairspring to form a silicon-based hairspring having overall dimensions necessary for obtaining the final stiffness;
• k) re-oxidizing the silicon-based hairspring to form the hairspring having the final stiffness and to adjust thermal performances of said hairspring.