1. Field of the Invention
The present invention relates to a MEMS (microelectrical mechanical system) element having a microphone structure implemented in a layer structure. The microphone structure includes at least one sound pressure-sensitive diaphragm, an acoustically permeable counter element and a capacitor system for detecting diaphragm deflections. The diaphragm and the counter element are situated on top of each other, spaced a distance apart, in the layer structure of the element, and each is equipped with at least one electrode of the capacitor system.
2. Description of the Related Art
Under the impact of sound, the microphone diaphragm is deflected at a right angle to the planes of the layers of the layer structure. Thereby the distance between the microphone diaphragm and the stationary counter element changes. The microphone diaphragm and the counter element are each equipped with at least one capacitor electrode, so that the “out-of-plane” deflections of the microphone diaphragm are detectable as changes in capacitance of the capacitor system. However, the connection of the diaphragm to the layer structure of the element causes bending or warping of the diaphragm, which may have negative effects on the microphone performance. In particular such a warping may result in the relationship between the diaphragm deflection and the measuring signal being no longer linear at higher sound pressures.
A high signal-to-noise ratio (SNR) is always sought with high-performance MEMS microphones. One possibility for improving the SNR is to reduce the flow resistance of the counter element.
In addition, for high-performance MEMS microphones, a preferably large frequency bandwidth is frequently sought, i.e., a preferably flat response function, which also includes high frequencies, ideally into the ultrasonic range. The microphone structure is advantageously designed to have a preferably high resonant frequency co. The greater the response function of the upper cutoff frequency, the higher is the resonant frequency co of a capacitive MEMS microphone. Resonant frequency co depends decisively on the mass and stiffness of the microphone diaphragm. The thinner the diaphragm having a given diaphragm area and diaphragm stiffness, the higher is the resonant frequency. In a homogeneous diaphragm, however, the stiffness also depends on the thickness of the diaphragm. The lower stiffness of a thin microphone diaphragm in turn favors its warping, which has negative effects on the microphone performance, and results in nonlinearities in the microphone signal as well as a reduction of the resonant frequency. A high resonant frequency and good microphone performance are therefore not readily compatible.