Wafer-Scale Fabrication of Graphene-Based Condenser Microphones

Abstract

The evolution of microphone technology can be linked to the growing multimedia and communication market. The first is linked to the music and audiovisual industry, while the latter is linked to smartphones, smart mobility, and home automation. In all of these audio applications, the common trend is toward downsizing and performance improvement.
State-of-the-art silicon and nickel suspended membranes generally require a large diameter (in the millimeter range) to ensure high sensitivity. To this end, new materials have been studied. The properties of these materials should allow for miniaturization without losing performance.
As a matter of fact, over the past decade, researchers have been studying how to implement graphene in sensing applications. The properties of graphene have enabled the realization of a number of devices capable of outperforming their silicon counterparts while decreasing the sizes of the sensors.
The properties of these new materials should allow for greater flexibility and a thinner membrane that will ensure a greater mechanical response. This will allow for the creation of a smaller membrane. On the other hand, suspended graphene-based as membranes of a capacitive microphone generally featured a low-yield process in which the graphene is transferred from the grown substrate to the target device using polymers. The low-yield is related to a not fully automated transfer method. These restrictions limit the mass production of these devices, making large-scale production difficult.
Therefore, a possible method to obtain a transfer-free, wafer-scale, high-yield process for the realization of a graphene-based capacitive microphone is presented. The use of graphene will allow for miniaturization and improved performance.
The presented work will be structured as follows:
Chapter 1 - Introduction: The rationale for the thesis is provided, underlying the advantages of using graphene in the applications of capacitive microphones. The physics and history of capacitive microphones will be explored. A method to improve the sensitivity while scaling the device will be investigated. The research questions that started this thesis work are listed.
Chapter 2 - Literature Review: A study on state-of-the-art capacitive microphones will be introduced, focusing on the transfer and transfer-free methods for the realization of graphene-based capacitive microphones. The knowledge gained will provide an overview of the beneficial properties of graphene in condenser microphone devices.
Chapter 3 - Device Simulations & Concept Design: The device will be simulated with 2 different models. The used models will define the geometry of the final device. Subsequently, the design of the mask will be presented. An overview of the used materials and the reason behind their choice will be presented.
Chapter 4 - Fabrication Results: Some tests will be exposed and the process used for the realization of the final device will be presented, highlighting all the critical steps of the presented flowchart.
Chapter 5 - Measurement of the Processed Device: The results and the measured data will be discussed.
Chapter 6 - Final Conclusions & Future Perspective: The results of the presented work will be highlighted and suggestions for a future improvement of the device will be presented.