Abstract:
A MEMS microphone ( 100 ) is disclosed comprising: a substrate ( 101 ), a membrane ( 102 ) attached to the substrate ( 101 ), and an electrode ( 104 ) attached to the substrate ( 101 ), wherein the membrane ( 102 ) and the electrode ( 104 ) have the same resonance frequency.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to MEMS microphones. 
       BACKGROUND OF THE INVENTION 
       [0002]    Prior art microphones are prone to the influence by sound which is transferred via solid connection, which is not induced by air pressure but is induced by movements of the body of the microphone or the body the microphone (body noise) is fixed to. This movement generates a noise signal in the microphone not relating to sound or voice, which is to be recorded by the microphone, therefore leading to a degrading of the performance of the microphone. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0003]    One object of the invention is to provide an alternative microphone having a low level of susceptibility to body noise. 
         [0004]    In order to achieve the object defined above a Micro-Electro-Mechanical System (MEMS) microphone according to the independent claims is provided. Enhancements are described in the dependent claims. 
         [0005]    According to an exemplary aspect of the invention, a Micro-Electro-Mechanical System microphone is provided, wherein the MEMS microphone comprises a membrane attached to a substrate and an electrode attached to the substrate, wherein the membrane and the electrode have the same resonant frequency. 
         [0006]    In this application the term “attached to a substrate” may particularly denote the fact that the part, e.g. the membrane or the electrode, may be attached to or even may be formed as a part of the substrate, i.e. are fixedly connected to the substrate. In particular, the term may also include that the substrate and the membrane or electrode may be fixed to each other by a connection element or that the membrane and/or electrode forms an integrally part of the substrate. Furthermore, the term may also include an arrangement wherein the membrane is fixed or attached directly to the electrode, which in turn is fixed or attached to the substrate. In this arrangement the electrode may comprise a stiff outer region, e.g. an annular outer region in case of a circular membrane and electrode, having elastic strips or elastic bands implemented so that an oscillation of the membrane and electrode is possible. These elastic strips may be arranged in radial direction of the electrode. 
         [0007]    Furthermore, in this application the term “MEMS” or “Micro-Electro-Mechanical System” may particularly denote the technology of the very small, and merges at the nano-scale into nanoelectromechanical systems (NEMS) and nanotechnology. MEMS may also be referred to as micro-machines, or  Micro Systems Technology  ( MST ). MEMS may be separate and distinct from the hypothetical vision of molecular nanotechnology or molecular electronics. MEMS generally ranges in size from a micrometer to a millimeter. Thus, a MEMS microphone may denote a microphone having a size in the above-described range, i.e. in the range from micrometer to some millimeters. 
         [0008]    By providing a MEMS microphone, whose electrode and membrane have the same resonant frequency, a new type of microphone is provided, i.e. a small microphone, which is still not object to body noise, i.e. is not be influenced by movements of a body the microphone is fixed to (e.g. by fixing or mounting the electrode to said body). The so-called body noise is particularly induced by body movements and not by variations in air pressure. By ensuring that the membrane and the electrode have the same resonant frequency it is possible to ensure that the membrane and the electrode are moving parallel or synchronous when the MEMS microphone is moved. In other words, it is possible to avoid a relative movement between the membrane and the electrode, which relative movement otherwise would cause an output signal (e.g. to an amplifier and a downstream speaker) and thus decrease the performance of the microphone. 
         [0009]    It should be noted that due to the small size of MEMS microphones compared to standard microphones the resonant frequencies of the MEMS components are much higher, e.g. in the range of about 100 kHz. Thus, emphasis should be put on the matching of the resonance frequencies for a proper performance of the microphone while the resonance quality is less important than for normal microphones. 
         [0010]    A gist of an exemplary aspect of the invention may be seen in providing a MEMS microphone having a reduced susceptibility to movements performed by a body the MEMS microphone is fixed to, by ensuring that a membrane and an electrode of the MEMS microphone have the same or almost the same resonant frequency. Thus, it is ensured that the membrane and the electrode move synchronous when the microphone is moved so that no relative movement between the electrode and membrane is induced, which relative movement may lead to an additional unintended signal. 
         [0011]    Next, further exemplary embodiments of the MEMS microphone are described. 
         [0012]    According to an exemplary embodiment of the MEMS microphone, the electrode is formed as a back-electrode. Furthermore, the MEMS microphone may be a capacity type MEMS microphone. In particular, the MEMS microphone may be a microphone that is based on a detection mechanisms of the group of detection mechanisms consisting of: optical detection mechanism, electret mechanism, electromechanic mechanism, and electrodynamic mechanism. 
         [0013]    According to another exemplary embodiment, the MEMS microphone further comprises an elastic suspension, which is arranged between the electrode and the substrate. In particular, the elastic suspension may comprise a material having a lower Young modulus than the Young modulus of the electrode and/or the substrate. For example, the elastic suspension may be formed or may consist of the material. The provision of an elastic suspension, i.e. an elastic fixation, between the electrode and the substrate or frame may be a suitable way to ensure that the fixation of the electrode to the microphone substrate or microphone frame is realized in a way that the stiffness of the elastic suspension together with the mass of the electrode possibly results into the same fundamental resonance frequency as the resonance frequency of the microphone membrane itself. Thus, it may be possible to ensure that no relative movement between the membrane and the electrode is induced when body movements are applied to the microphone while sound pressure may not induce a significant movement of the electrode due to the relative high mass of the electrode compared to the membrane. 
         [0014]    According to yet another exemplary embodiment of the MEMS microphone, the elastic suspension is adapted in such a way that the membrane and the electrode have the same resonant frequency. In particular, the elastic suspension may have a spring constant, which is adapted in such a way that the resonant frequency of the membrane and the electrode are the same. The spring constant of the suspension depends on the shape of the suspension and the material of the suspension. Thus, the spring constant may be set by choosing an appropriate form (dimension) and/or an appropriate material for the suspension. 
         [0015]    According to another exemplary embodiment of the MEMS microphone, the suspension and/or the electrode and/or the substrate comprises the same material. In particular, the suspension and/or the electrode and/or the substrate may be formed of or may consist of the same material. In this respect the term “attached to” just relates to functional entities and does neither mean the parts of the microphone consist of different materials nor that separate parts of a microphone are connected to one another. By contrast, these functional entities can be formed of one “block” of material, e.g. by etching. 
         [0016]    According to another exemplary embodiment of the MEMS microphone, the membrane and the electrode comprise the same material. In particular, the membrane and the electrode may be formed of or may consist of the same material. 
         [0017]    According to another exemplary embodiment of the MEMS microphone, the membrane and/or the electrode comprises silicone. In particular, the membrane and/or the electrode may be formed or may be consist of silicone. 
         [0018]    Summarizing, according to an exemplary aspect of the invention, a MEMS microphone of a capacity type is provided having a membrane and a fixed back-electrode. The MEMS microphone may be adapted for body noise compensation, which is induced by body movement and transmitted to the microphone via different movements of the membrane and the back electrode. The compensation is achieved by ensuring that a resonant frequency of the membrane and the back-electrode have the same value so that the body and thus of the back-electrode move in parallel and not relative to each other. 
         [0019]    The aspects and exemplary embodiments defined above and further aspects of the invention are apparent from the example of embodiment to be described hereinafter and are explained with reference to this example of embodiment. It should be noted that features described in connection with one exemplary embodiment or exemplary aspect may be combined with other exemplary embodiments and other exemplary aspects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The invention will be described in more detail hereinafter with reference to an example of embodiment but to which the invention is not limited. 
           [0021]      FIG. 1  schematically illustrates a MEMS microphone according to an exemplary embodiment of the invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0022]      FIG. 1  schematically shows a MEMS microphone  100  comprising a microphone frame or substrate  101 , which a membrane  102  is fixed to, and an elastic suspension  103 , which is fixed to the microphone frame  101  as well as to a back-electrode  104 . The membrane  102  and/or the back-electrode  104  may comprise or may consist of silicone and may have a circular shape. However, any other shape is applicable as well, e.g. rectangular with or without rounded corners, elliptical, etc. The back-electrode  104  comprises holes so that air can pass it, either for exposing the membrane  102  to sound or ventilating the small back volume between membrane  102  and back-electrode  104 . In particular, the elastic suspension  103  comprises or consists of a material having a lower Young modulus than the Young modulus of the electrode  104  and/or the substrate  101 . The provision of an elastic suspension  103 , i.e. an elastic fixation, between the electrode  104  and the substrate or frame  101  may be a suitable way to ensure that the stiffness of the elastic suspension  103  together with the mass of the electrode  104  results in the same fundamental resonance frequency as the resonance frequency of the microphone membrane  102  itself. 
         [0023]    However, the elasticity of the suspension  103  is not just defined by the properties of the material the suspension consists of but also by its shape. The shape of the suspension  103  and the material, which the suspension  103  consists of, define the spring constant of the suspension  103 . Said spring constant and the mass of the electrode  104  define a mass spring system with characteristic oscillation properties. 
         [0024]    Finally, it should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word “comprise” and its conjugations do not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. In a device claim enumerating several means, several of these means may be embodied by one and the same item. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.