MEMS Microphone Assembly with Nested Cavity Back Volume
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Solution Overview
Problem
Conventional MEMS microphones face challenges with decreasing back volumes due to size reduction, leading to increased acoustic impedance, which deteriorates audio performance in terms of sensitivity, frequency response, and signal-to-noise ratio.
Innovation Solution
The design incorporates a MEMS microphone assembly with an increased effective back volume by creating a gap between the MEMS diaphragm and the ASIC, connected via pressure ventilation openings, which distributes air compression across a larger volume, reducing acoustic impedance. This is achieved through a two-die structure with a gap height significantly larger than conventional designs, allowing for improved airflow and reduced squeeze impedance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the MEMS microphone size is reduced to meet space constraints, then the device becomes more compact, but the back volume decreases leading to increased acoustic impedance and deteriorated audio performance
Solution Approach 1:
The patent implements a nested cavity structure where a first cavity and a second cavity are positioned adjacent to each other, with the diaphragm forming part of both cavities. The first cavity serves as the back volume while the second cavity provides additional acoustic compliance volume, creating a nested spatial arrangement that maximizes effective back volume within a compact footprint.
Solution Approach 2:
The patent extends the acoustic volume in a spatial dimension by creating two adjacent cavities (first and second cavities) rather than relying on a single back volume. This dimensional expansion allows the effective back volume to be larger than the physical envelope of the microphone itself, resolving the contradiction between compact size and sufficient back volume.
2Length of moving object
If the gap between the diaphragm and ASIC is reduced to improve integration, then the device becomes more compact, but the acoustic compliance decreases leading to higher acoustic impedance
Solution Approach 1:
The patent creates a nested cavity arrangement where the second cavity is positioned adjacent to and connected with the first cavity. This nesting strategy allows the second cavity to provide additional acoustic compliance volume without increasing the overall device footprint, maintaining compact gap dimensions while improving acoustic compliance.
Solution Approach 2:
The patent resolves the gap height limitation by extending the acoustic volume in a different spatial dimension through the adjacent second cavity. Instead of increasing gap height vertically, the solution adds volume horizontally through the connected second cavity, maintaining compact integration while improving acoustic compliance.
3Reliability
If pressure ventilation openings are added to connect the cavities, then the acoustic compliance increases, but the device complexity increases
Solution Approach 1:
The diaphragm serves multiple functions simultaneously: it acts as the acoustic membrane separating the acoustic inlet from the back volume, forms part of both the first and second cavities, and provides the interface between the two cavities. This multi-functionality reduces the need for separate dedicated structures, thereby reducing overall device complexity while achieving improved acoustic compliance.
Solution Approach 2:
The patent merges the functions of the back volume and the acoustic compliance volume into an integrated dual-cavity structure. The first cavity and second cavity are combined through the diaphragm interface, creating a unified acoustic system that improves compliance without requiring separate independent structures, thus managing device complexity.
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 increased back volume results in reduced acoustic impedance, enhancing the microphone's sensitivity and signal-to-noise ratio, while maintaining a compact size, suitable for use in portable devices with limited space.
Implementation Method 1
the back volume that is typically defined by the gap between the MEMS diaphragm and the ASIC is connected via the pressure ventilation openings to the volume of the first cavity defined by the enclosure, which typically serves for packaging purposes. This has the effect that a compression of the air within the gap due to a moving diaphragm, for example, is distributed across a significantly larger amount of air, hence increasing its acoustic compliance.
Data Source
AI summary
A micro-electro-mechanical system, MEMS, microphone assembly comprises an enclosure defining a first cavity, and a MEMS microphone arranged inside the first cavity. The microphone comprises a first die with bonding structures and a MEMS diaphragm, and a second die having an application specific integrated circuit, ASIC. The second die is bonded to the bonding structures such that a gap is formed between a first side of the diaphragm and the second die, with the gap defining a second cavity. The first side of the diaphragm is interfacing with the second cavity and a second side of the diaphragm is interfacing with the environment via an acoustic inlet port of the enclosure. The bonding structures are arranged such that pressure ventilation openings are formed that connect the first cavity and the second cavity.