Micro-electromechanical system and method for producing same

Abstract

A method of manufacturing a microelectromechanical system includes forming of an electromechanical element on a substrate. The method further includes preparation of an encapsulation package to form a sealed cavity integrating the electromechanical element, with the sealed cavity having a volume smaller than 10 mm.sup.3. The method includes physical vapor deposition of a getter film on the substrate or on a wall of the encapsulation package so that the getter film has a specific absorption surface area smaller than 8 m.sup.2/g, and sealing of the encapsulation package on the substrate by means of a thermal sealing cycle having a temperature enabling to activate said getter film.

Claims

1. A method of manufacturing a microelectromechanical system, comprising the following steps: forming of an electromechanical element on a substrate; preparation of an encapsulation package to form a sealed cavity integrating said electromechanical element, said sealed cavity having a volume smaller than 10 mm.sup.3; physical vapor deposition of a getter film on said substrate or on a wall of the encapsulation package so that the getter film has a specific absorption surface area smaller than 8 m.sup.2/g; the getter film being deposited on the substrate in the absence of any previous cleaning of said substrate involving a noble gas; and sealing of the encapsulation package on the substrate by means of a thermal sealing cycle having a temperature enabling to activate the getter film.

2. The microelectromechanical system manufacturing method according to claim 1, wherein the physical vapor deposition of the getter film is performed at a pressure lower than 10.sup.−7 mbar.

3. The microelectromechanical system manufacturing method according to claim 1, wherein the sealing is performed at a temperature in the range from 250° C. to 350° C.

4. The microelectromechanical system manufacturing method according to claim 1, wherein the getter film comprises at least one of the following elements: Baryum, Lanthanum, Scandium, Titanium, Zirconium, Niobium, Yttrium, Vanadium, Hafnium, Tantalum, Iron, Cobalt, Nickel, Palladium, Platinum, and Aluminum, alone or in a mixture.

5. The microelectromechanical system manufacturing method according to claim 4, wherein the getter film is made of a Titanium-Yttrium alloy.

6. A microelectromechanical system comprising: a substrate supporting an electromechanical element; an encapsulation package attached to said substrate to form a sealed cavity integrating said electromechanical element; and a getter film deposited in the sealed cavity on said substrate or on a wall of said encapsulation package; wherein the sealed cavity has a volume smaller than 10 mm.sup.3; and the getter film has a specific absorption surface area smaller than 8 m.sup.2/g; the getter film being deposited on the substrate in the absence of any previous cleaning of said substrate involving a noble gas.

7. The micro-electromechanical system according to claim 6, wherein the getter film comprises at least one of the following elements: Baryum, Lanthanum, Scandium, Titanium, Zirconium, Niobium, Yttrium, Vanadium, Hafnium, Tantalum, Iron, Cobalt, Nickel, Palladium, Platinum, and Aluminum, alone or in a mixture.

8. The microelectromechanical system according to claim 7, wherein the getter film is made of a Titanium-Yttrium alloy.

9. The microelectromechanical system according to claim 6, wherein the sealed cavity has a vacuum level smaller than 5.10.sup.−2 mbar.

10. The microelectromechanical system according to claim 6, wherein the sealed cavity has a volume smaller than 2 mm.sup.3.

11. The microelectromechanical system according to claim 6, wherein the electromechanical element corresponds to at least one microbolometer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The way to implement the present invention, as well as the resulting advantages, will better appear from the description of the following non-limiting embodiments, given as an indication, based on the accompanying drawing:

(2) FIG. 1 is a flowchart of the steps of the microelectromechanical system manufacturing method according to an embodiment of the invention.

DETAILED DESCRIPTION

(3) In the rest of the description, the invention will be described in reference to an optoelectronic component 10, although the invention may also apply to other MEMS devices without changing the invention.

(4) FIG. 1 illustrates the steps of forming of an optoelectronic component 10 encapsulated in an enclosure 12 under a predetermined pressure, for example, under a pressure lower than 5.10.sup.−2 mbar. Enclosure 12 is formed by sealing of lateral walls 17 of a sealed package 18 on a substrate 13 by means of a metal seal 20.

(5) To achieve this, a first step 30 comprises forming electromechanical element 11 on substrate 13. For example, substrate 13 may integrate a readout circuit and electromechanical element 11 may correspond to an uncooled microbolometer mounted in suspension above substrate 13 by means of pads and of support arms. To achieve this, one or a plurality of sacrificial layers are used and are structured to form the pads and the different layers of the microbolometer membrane. Further, under this membrane, microbolometer 11 may comprise a reflector 16.

(6) In parallel with this first step 30, a second step 31 comprises preparing an encapsulation package 18 intended to form a sealed cavity 12 around electromechanical element 11. To achieve this, lateral walls 17 are structured substantially vertically in line with a substrate intended to form the top of encapsulation package 18. An optical window 14 may also be structured in this upper substrate to filter the electromagnetic radiation captured by microbolometer 11.

(7) After this second step 31, a third step 32 comprises depositing a getter film 15 on substrate 13 or on a wall of encapsulation package 18. As illustrated in FIG. 1, getter film 15 may be deposited next to optical window 14 on the upper wall of encapsulation package 18. As a variant, getter film 15 may be deposited next to reflector 16 or on lateral walls 17.

(8) In all cases getter film 15 is intended to be arranged inside of enclosure 12 to capture the gases desorbed into said enclosure, and to maintain a vacuum level smaller than 5.10.sup.−2 mbar therein.

(9) According to the invention, this getter film 15 is deposited by physical vapor deposition to obtain a specific absorption surface area smaller than 8 m.sup.2/g. Preferably, the deposition surface of this getter film 15 is not cleaned by a method involving argon before the deposition of getter film 15 to avoid the incorporation of argon into the wall of encapsulation package 18 or on substrate 13.

(10) Preferably, getter film 15 comprises at least one of the following elements Baryum, Lanthanum, Scandium, Titanium, Zirconium, Niobium, Yttrium, Vanadium, Hafnium, Tantalum, Iron, Cobalt, Nickel, Palladium, Platinum, and Aluminum, alone or in a mixture. For example, getter film 15 may be made of a Titanium-Yttrium alloy.

(11) The physical vapor deposition of getter film 15 consists in heating a crucible integrating the getter material to obtain its evaporation. This evaporation is performed under vacuum, preferably at a pressure lower than 10.sup.−7 mbar. The evaporation of the getter material is controlled by an electric current or an electron beam so that the evaporated particles of the getter material agglomerate on the target surface, that is, on a surface of substrate 13 or of a wall of encapsulation package 18. To achieve this, the evaporation by Joule effect consists in heating the crucible with an electric current while the electron beam evaporation consists in applying an electron beam directed onto the crucible.

(12) When getter film 15 is deposited on substrate 13 or on a wall of encapsulation package 18, said package may be sealed to substrate 13, during a step 33. To achieve this, a metallic weld bead 20 is deposited between substrate 13 and the lower end of the lateral walls 17 of encapsulation package 18. This weld bead 20 is then heated to obtain a sealed surface between the lower end of the lateral walls 17 of encapsulation package 18 and substrate 13.

(13) The heating temperature of this weld bead 20 is preferably in the range from 250 to 350° C., to allow an activation of getter film 15 during the temperature rise of weld bead 20. A very simple thermal sealing cycle may be implemented: a first temperature rise phase, a second phase of temperature stabilization at the heating temperature for a predetermined duration, and a third phase of progressive temperature decrease. During this sealing cycle, the stabilization time as well as the temperature rise and fall times may be adjusted according to the getter material and to the material of weld bead 20 to obtain an efficient activation of getter film 15 and a sealed welding of enclosure 12. Further, it is also possible to use more complex sealing cycles with degassing stages.

(14) The activation of the getter film is obtained by means of a migration of the passivation layer formed at the surface of getter film 15 after the contact between the getter film and oxygen. This passivation layer may correspond to a nitride layer if a specific anneal method has been used, as described in document U.S. Pat. No. 9,051,173, or to a thin gold, palladium, or nickel layer, as described in documents U.S. Pat. Nos. 6,923,625 and 9,240,362.

(15) These different micro-manufacturing steps enable to obtain an enclosure 12 with a very low argon level, since these steps limit the desorption of argon into cavity 12. Further, the getter film 15 deposited by evaporation enables to absorb all the gas molecules degassed into cavity 12. Thereby, it is possible to obtain a vacuum level smaller than 5.10.sup.−2 mbar in a sealed cavity 12 having a volume smaller than 2 mm.sup.3, typically a cavity 12 with a volume of approximately 1 mm.sup.3.

(16) The invention thus enables to obtain a microelectromechanical system 10 with a sealed cavity 12 of very small volume and with a high vacuum level.