Method for fabricating an encapsulation device for a micro-electromechanical system characterized in that the method includes the following steps: a) forming the micro-electromechanical system, a main portion forming a cavity and a cover (4, 44); b) depositing, on the main portion and the cover (4, 44) respectively, a first and a second metallization (9, 11, 49) respectively including at least a first metal (13, 14, 15, 16, 35, 55) protected by a first anti-oxidation material (18, 19, 57), and at least a second metal (13, 14, 15, 16, 35, 55) protected by a second anti-oxidation material (18, 19, 57); c) depositing a layer (12, 52, 72) of a material (12, 52) whose melting point is lower than 250°C on solely one of the first and second metallizations (9, 11, 49); d) partially diffusing the material (12, 52) whose melting point is lower than 250°C in solely one of the first and second anti-oxidation materials (18, 19, 57) in order to entirely transform said one of the first and second anti-oxidation materials (18, 19, 57) into an intermetallic compound formed of said one of the first and second anti-oxidation materials (18, 19, 57) and of said material whose melting point is lower than 250°C in order to protect one of the at least one first and at least one second metals (13, 14, 15, 16, 35, 55), and to leave a non-diffused portion (12', 52') of the material whose melting point is lower than 250°C; e) mounting the micro-electromechanical system in the cavity of the main portion; f) assembling the portion (12', 52') of the material whose melting point is lower than 250°C which was not diffused in step d) against the other of the first and second metallizations (9, 11, 49); g) totally diffusing the portion (12', 52') of material whose melting point is lower than 250°C which was not diffused in step d) in the other of the first and second metallizations (9,11, 49) in order to entirely transform the portion (12', 52') of material whose melting point is lower than 250°C which was not diffused in step d) into a second intermetallic compound formed by the other of the first and second anti-oxidation materials (18, 19, 57), the material (12, 52) whose melting point is lower than 250°C and the other of the at least one first and at least one second metals (13, 14, 15, 16, 35, 55), so as to hermetically close the micro-electromechanical system inside the encapsulation device. a) forming the micro-electromechanical system, a main portion forming a cavity and a cover (4, 44); b) depositing, on the main portion and the cover (4, 44) respectively, a first and a second metallization (9, 11, 49) respectively including at least a first metal (13, 14, 15, 16, 35, 55) protected by a first anti-oxidation material (18, 19, 57), and at least a second metal (13, 14, 15, 16, 35, 55) protected by a second anti-oxidation material (18, 19, 57); c) depositing a layer (12, 52, 72) of a material (12, 52) whose melting point is lower than 250°C on solely one of the first and second metallizations (9, 11, 49); d) partially diffusing the material (12, 52) whose melting point is lower than 250°C in solely one of the first and second anti-oxidation materials (18, 19, 57) in order to entirely transform said one of the first and second anti-oxidation materials (18, 19, 57) into an intermetallic compound formed of said one of the first and second anti-oxidation materials (18, 19, 57) and of said material whose melting point is lower than 250°C in order to protect one of the at least one first and at least one second metals (13, 14, 15, 16, 35, 55), and to leave a non-diffused portion (12', 52') of the material whose melting point is lower than 250°C; e) mounting the micro-electromechanical system in the cavity of the main portion; f) assembling the portion (12', 52') of the material whose melting point is lower than 250°C which was not diffused in step d) against the other of the first and second metallizations (9, 11, 49); g) totally diffusing the portion (12', 52') of material whose melting point is lower than 250°C which was not diffused in step d) in the other of the first and second metallizations (9,11, 49) in order to entirely transform the portion (12', 52') of material whose melting point is lower than 250°C which was not diffused in step d) into a second intermetallic compound formed by the other of the first and second anti-oxidation materials (18, 19, 57), the material (12, 52) whose melting point is lower than 250°C and the other of the at least one first and at least one second metals (13, 14, 15, 16, 35, 55), so as to hermetically close the micro-electromechanical system inside the encapsulation device.