Marc Füldner

Silicon microphone based on surface and bulk micromachining

In this paper a silicon microphone which can be fabricated using standard semiconductor processes is presented. The acoustic-electrical transducer is based on the capacitance change of a movable 400 nm thin poly-silicon membrane with different diameters (800-1200 µm). A source follower was integrated to transform the impedance. The complete chip is 2×2×0.5 mm3 in size. The sensitivity achieved is in the range of 0.4 to 3.2 mV Pa-1.

Analytical analysis and finite element simulation of advanced membranes for silicon microphones

In this paper, advanced membrane designs are simulated in order to improve the sensitivity of micromachined silicon condenser microphones. Analytical analyzes and finite element simulations have been carried out to derive algebraic expressions for the mechanical compliance of corrugated membranes and membranes supported at spring elements. It is shown that the compliance of both types of membranes can be modeled with the help of an enhanced theory of circular membranes. For spring membranes, a numerically derived and design dependent constant takes into account the reduced suspension. The mechanical stress in corrugated membranes is calculated using a geometrical model and is confirmed by finite element simulations. A very good agreement between theory and experimental results is demonstrated for spring membranes of different shape and for membranes with varying number of corrugations. In a silicon microphone application, a high electro-acoustical sensitivity up to 8.2 mV/Pa/V is achieved with a membrane diameter of only 1 mm.

Design of a poly silicon MEMS microphone for high signal-to-noise ratio

This paper reports on the state of the art silicon micromachined microphone utilizing a dual poly silicon membrane system. MEMS chips from 1.4mm down to 1.0mm side length are applied for mobile communication. Design aspects related with key performance parameters such as sensitivity, signal to noise ration and distortion are discussed. Sensitivity of - 38BV/Pa is achieved for different microphone membrane diameters. A maximum signal to noise ration of 66dB(A) for the largest system could be achieved. The perfect fit of simulation versus measurements enables deeper analysis and balancing of noise contributors. Environmental noise suppression of 5dB by acoustical high pass design is demonstrated.

Silicon Microphones with Low Stress Membranes

A new silicon condenser microphone process for low stress diaphragm is presented. The diaphragm is formed either by a polysilicon layer or by a monocrystalline silicon layer of a silicon on insulator substrate (SOI). Acoustical sensitivities up to 8.2 mV/Pa with a 1×1 mm2 diaphragm for a bias voltage of only 1 V have been achieved. The A-weighted noise voltage was found to be 7 eV in the band of 100 Hz to 10 kHz.

Improved signal-to-noise ratio of silicon microphones by a high-impedance resistor

This paper presents the improvement of the signal-to-noise ratio of a silicon microphone by utilizing a high-impedance load resistor. A model is described which considers the acoustical and electrical parts of the microphone. Based on an equivalent circuit diagram, the model is able to simulate the sensitivity and the noise thus the signal-to-noise ratio. Describing the noise spectrum differentiated by parts, the thermal noise of the load resistor proves to be a main noise source as long as the resistance is relatively low.

CAE environment for the design of capacitive micromachined sound and ultrasound transducers

We present a newly developed computer aided engineering (CAE) tool for the design of capacitative micromachined sound and ultrasound transducers. The numerical scheme is based on the finite element (FE) method and takes all relevant nonlinearities (geometric nonlinearity of the mechanical structures, electrostatic force, moving body in an electrostatic field) and couplings between the different physical fields (electrostatic, mechanical and acoustical fields) into account. The applicability of the CAE tool is demonstrated by two practical examples: mechanical and acoustic crosstalk in capacitative micromachined ultrasound transducers (CMUTs) and total harmonic distortion of micromachined silicon microphones.