Electroacoustic Transducer Sacrificial Layer Etching
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Solution Overview
Problem
The manufacturing of electroacoustic transducers, such as microphones, faces challenges in preventing air leaks between different volumes and reducing mechanical noise due to squeeze-film damping, which affects the performance and accuracy of sound pressure detection.
Innovation Solution
A manufacturing process involving a stack of substrates and sacrificial layers is used to form a movable element with a membrane and stiffening structure, where a second sacrificial layer serves as a protective layer to prevent air leaks and reduce overlap distance between the substrate and stiffening structure, thereby minimizing damping phenomena.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If drilling is used to shape the piston and define transmission arms, then manufacturing is simplified, but air leaks occur between the cavity and rear volume
Solution Approach 1:
A sacrificial layer is introduced as an intermediary element between the piston and the substrate. This layer is deposited conformally on the piston structure and then selectively removed through etch access holes to define the transmission arms. The sacrificial layer acts as a temporary mediator that enables the formation of movable transmission arms without creating air leaks, as it maintains the sealing integrity during the manufacturing process and is subsequently removed in a controlled manner.
Solution Approach 2:
The sacrificial layer is deposited conformally on the piston structure before the transmission arms are defined. This preliminary action creates a protective coating that prevents air leaks during the subsequent etching process. The sacrificial layer is then selectively removed through etch access holes to define the transmission arms, allowing the movable elements to be released without compromising the sealing integrity of the piston.
2Ease of manufacture
If the overlap distance between substrate and stiffening structure is large, then manufacturing is easier, but squeeze-film damping increases causing mechanical noise
Solution Approach 1:
The overlap distance between the substrate and the stiffening structure is precisely controlled and reduced to minimize squeeze-film damping effects. By changing this geometric parameter to a smaller value, the mechanical noise caused by viscous friction is significantly reduced while maintaining manufacturing feasibility through the conformal deposition process and controlled etching steps.
3Reliability
If the pivot joint seals between zones, then hermetic sealing is achieved, but rotation capability is constrained
Solution Approach 1:
The pivot joint utilizes a flexible membrane or thin film structure that allows rotational movement while maintaining hermetic sealing between the first zone (atmospheric pressure) and the second zone (controlled atmosphere). The flexible nature of this sealing element enables it to accommodate the rotation of the transmission arms without compromising the seal integrity, thus resolving the contradiction between sealing and rotation capability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The process effectively prevents air leaks and reduces mechanical noise, enhancing the performance and accuracy of electroacoustic transducers by minimizing the overlap distance between the substrate and stiffening structure, leading to improved sound pressure detection capabilities.
Implementation Method 1
a second sacrificial layer arranged so as to be encapsulated by the stiffening structure... The second portion of the sacrificial layer, encapsulated within the membrane's stiffening structure, acts as a protective layer or shield against etching of the stiffening structure
Implementation Method 2
etch the second structural layer so as to expose the first portion of the second sacrificial layer and to delimit the stiffening structure of the moving element... etch the first structural layer up to the first sacrificial layer so as to delimit the membrane of the moving element
Implementation Method 3
Capacitive sensing devices measure piston displacement and thus the pressure variation. They are located in a second zone, hermetically sealed and isolated from the first zone. Each device comprises a moving electrode and at least one fixed electrode positioned opposite the moving electrode. The electrodes form the plates of a capacitor whose capacitance varies with piston displacement.
Implementation Method 4
The transmission device comprises at least one first transmission arm extending into the first zone and at least one second transmission arm extending into the second zone. The piston is coupled to one end of the first transmission arm, while the moving electrode of the capacitive sensing means is coupled to one end of the second transmission arm.
Implementation Method 5
The first and second transmission arms are connected at their second ends by means of a pivot joint. This pivot joint allows rotation of the transmission arms relative to the microphone frame and simultaneously ensures a seal between the first and second zones.
Implementation Method 6
The diaphragm forms a separation between a cavity open to the external environment and a rear volume of the microphone, also called the reference volume because it contains a reference pressure. Thus, one face of the diaphragm is subjected to the reference pressure, and the opposite face is subjected to atmospheric pressure (whose variation we wish to detect).
Implementation Method 7
The second zone is a chamber under controlled atmosphere (typically vacuum) to minimize viscous friction and associated noise.
Data Source
Figure 1~2B
Figure 2C~2E
Figure 2F~2H
AI summary
The invention relates to a method for manufacturing an electroacoustic transducer (1) comprising: - a frame; - a movable element (13) relative to the frame, the movable element (13) comprising a membrane (131) and a stiffening structure (132) for the membrane; - a first transmission arm, the movable element (13) being coupled to one end of the first transmission arm; method in which a shield (24b) is used to protect the stiffening structure (132) during a step (S7) of etching a substrate (21), the etching of the substrate allowing the first transmission arm to be delimited and the movable element (13) to be lightened.