US-12621612-B2 - Method for manufacturing a low-noise electroacoustic transducer

Abstract

A method for manufacturing an electroacoustic transducer includes a frame; an element movable relative to the frame, the movable element including a membrane and a membrane rigidifying structure; a first transmission arm, the movable element being coupled to one end of the first transmission arm; in which method the membrane of the movable element is moved away from the frame by using a sacrificial layer of greater thickness at least at the periphery of the membrane.

Inventors

• Samer DAGHER

Assignees

• Commissariat à l'Energie Atomique et aux Energies Alternatives

Dates

Publication Date

20260505

Application Date

20230124

Priority Date

20220128

Claims (18)

1. 1 . A method for manufacturing an electroacoustic transducer comprising: a frame; an element movable relative to the frame, the movable element comprising a membrane and a membrane rigidifying structure; a first transmission arm, the movable element being coupled to one end of the first transmission arm; the method comprising: forming a first sacrificial layer on a substrate, the first sacrificial layer comprising a first portion and a second portion adjacent to the first portion, the first portion having a greater thickness than the second portion; forming a first structural layer on the first sacrificial layer; forming a second sacrificial layer on the first structural layer; forming a second structural layer on the first structural layer and on the second sacrificial layer; etching the second structural layer so as to expose the second sacrificial layer and to delimit the rigidifying structure of the movable element; etching the first structural layer up to the first sacrificial layer so as to delimit the membrane of the movable element, a peripheral portion of the membrane being arranged facing the first portion of the first sacrificial layer; etching the second sacrificial layer so as to expose a first face of the membrane; etching the substrate so as to delimit the first transmission arm, the first sacrificial layer acting as a stop layer to etching of the substrate; and etching the first sacrificial layer so as to expose a second, opposite face of the membrane.
2. 2 . The method according to claim 1 , comprising, prior to forming the first sacrificial layer, etching a cavity in the substrate, the first portion of the first sacrificial layer completely filling the cavity, and the second portion of the first sacrificial layer being arranged outside the cavity.
3. 3 . The method according to claim 2 , wherein the cavity has a depth greater than or equal to 1 μm.
4. 4 . The method according to claim 3 , wherein the depth is between 2 μm and 10 μm.
5. 5 . The method according to claim 2 , wherein the cavity has an annular shape.
6. 6 . The method according to claim 5 , wherein the cavity extends over more than 80% of a periphery of the movable element.
7. 7 . The method according to claim 2 , wherein the cavity and the membrane are of the same shape.
8. 8 . The method according to claim 7 , wherein the shape is rectangular.
9. 9 . The method according to claim 2 , wherein the cavity has dimensions greater than or equal to dimensions of the membrane.
10. 10 . The method according to claim 1 , wherein the first structural layer and the second structural layer are simultaneously etched so as to delimit the membrane and the rigidifying structure of the movable element.
11. 11 . The method according to claim 1 , further comprising, after etching the second sacrificial layer and before etching the substrate: arranging a cap on the second structural layer, thereby forming an assembly; and turning over the assembly.
12. 12 . The method according to claim 1 , wherein the substrate is of silicon, the first sacrificial layer is of silicon oxide, and the first structural layer is of silicon.
13. 13 . The method according to claim 1 , wherein the second sacrificial layer is of silicon oxide.
14. 14 . The method according to claim 1 , wherein the first structural layer is formed by epitaxy on the first sacrificial layer.
15. 15 . The method according to claim 1 , wherein forming the first structural layer comprises: providing a transfer substrate comprising the first structural layer; bonding the first structural layer of the transfer substrate to the first sacrificial layer; thinning the transfer substrate until the first structural layer is reached.
16. 16 . The method according to claim 15 , wherein the transfer substrate is a multilayer silicon-on-insulator type structure.
17. 17 . An electroacoustic transducer comprising: a frame; an element movable relative to the frame, the movable element comprising a membrane and a membrane rigidifying structure; a first transmission arm, the movable element being coupled to one end of the first transmission arm; the membrane being formed by a first part of a first structural layer, the rigidifying structure being formed by a first part of a second structural layer arranged on the first structural layer, and the frame comprising a substrate, a second part of the first structural layer and a second part of the second structural layer, in which electroacoustic transducer a distance between the substrate and a peripheral portion of the membrane is greater than the distance between the substrate and the second part of the first structural layer.
18. 18 . The electroacoustic transducer according to claim 17 , wherein the substrate comprises a cavity arranged facing the peripheral portion of the membrane.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to French Patent Application No. 2200754, filed Jan. 28, 2022, the entire content of which is incorporated herein by reference in its entirety. FIELD The technical field of the invention is that of microelectromechanical system (MEMS) or nanoelectromechanical system (NEMS) type devices. The invention more particularly relates to a method for manufacturing an electroacoustic transducer comprising a device for transmitting movement and force between two zones that are sealingly insulated from each other. Such an electroacoustic transducer can be employed as a microphone or loudspeaker. BACKGROUND Microelectromechanical or nanoelectromechanical microphones represent a rapidly expanding market, particularly by virtue of the development of nomadic devices, such as tablets, smartphones and other connected objects, wherein they are gradually replacing electret microphones. Microphones measure a rapid change in atmospheric pressure, also known as acoustic pressure. They therefore have at least one part in contact with the outside. Most MEMS or NEMS microphones manufactured today are capacitive detection microphones. Patent application FR3059659 describes an example of a capacitive detection microphone comprising a movable element, capacitive detection means and a device for transmitting movement between the movable element and the capacitive detection means. The movable element is capable of collecting the pressure variation. It can be formed by a rigid piston comprising a membrane, also called a thin film, and a membrane rigidifying structure. The membrane forms a separation between a cavity open to the external environment and a back volume of the microphone, also called a reference volume because a reference pressure prevails therein. Thus, one face of the membrane is subjected to the reference pressure, and the opposite face of the membrane is subjected to the atmospheric pressure (whose variation is desired to be detected). The movable element is connected to the movement transmission device in a first zone of the microphone. The capacitive detection means allow the displacement of the piston, and thus the variation in pressure, to be measured. They are arranged in a second zone that is sealingly insulated from the first zone. They comprise a movable electrode and at least one fixed electrode arranged facing the movable electrode. The electrodes form the plates of a capacitor whose capacitance varies as a function of the displacement of the piston. The second zone is a controlled atmosphere (typically under vacuum) chamber to reduce viscous friction phenomena and associated noise. The transmission device comprises at least one first transmission arm extending in the first zone and at least one second transmission arm extending in the second zone. The piston is coupled to a first end of the first transmission arm, while the movable electrode of the capacitive detection means is coupled to one end of the second transmission arm. The first and second transmission arms are connected at their second end via a pivot hinge. This pivot hinge allows rotation of the transmission arms relative to the microphone frame and simultaneously ensures sealing of the first and second zones. Such a capacitive detection microphone may suffer from squeeze-film damping, which is caused by air being squeezed between the piston and the frame upon operating the microphone. Squeeze-film damping generates mechanical noise and leads to a decrease in microphone performance. SUMMARY More generally, there is a need to manufacture an electroacoustic transducer in which the damping phenomenon is reduced, the electroacoustic transducer comprising: a frame;an element movable relative to the frame, the movable element comprising a membrane and a membrane rigidifying structure;a first transmission arm, the movable element being coupled to one end of the first transmission arm. According to a first aspect of the invention, this need tends to be satisfied by providing a manufacturing method comprising the following steps of: forming a first sacrificial layer on a substrate, the first sacrificial layer comprising a first portion and a second portion adjacent to the first portion, the first portion having a greater thickness than the second portion;forming a first structural layer on the first sacrificial layer;forming a second sacrificial layer on the first structural layer;forming a second structural layer on the first structural layer and on the second sacrificial layer;etching the second structural layer so as to expose the second sacrificial layer and to delimit the rigidifying structure of the movable element;etching the first structural layer up to the first sacrificial layer so as to delimit the membrane of the movable element, a peripheral portion of the membrane being arranged facing the first portion of the first sacrificial layer;etching the second sacrificial layer so