On the potential performance of micro-fabricated subminiature microphones based on electron surface tunneling

Abstract

The principle of electron tunneling across a vacuum potential energy barrier, as utilized in surface tunneling microscopy (STM), is a well known, highly sensitive, method for the detection of surface features and displacements in the subnanometer regime. In this paper, the basic properties of the surface tunneling principle and its possible utilization in acoustic sensors are discussed, with particular focus on the expected noise properties of a microphone based on tunneling in comparison to more conventional devices based on capacitive detection. It is found that the noise performance is largely limited by thermal noise due to the acoustical radiation impedance of the device in combination with electronic noise in the circuit used to establish and control the tunneling current. Finally, possible methods for sensor fabrication are discussed with special attention aimed at resolving the common problem of vibration sensitivity associated with STM. This problem stems from the relative large inertial mass and low flexural rigidity of the tunneling tip suspension system in most detector designs. Alternative approaches suitable for acoustic sensors in which the tunneling tip is fixed are described and it is shown that low vibration sensitivities comparable to capacitive devices can be achieved.