Abstract:
A microelectromechanical vibration sensor includes: a first chamber; a second chamber; a semiconductor membrane between the first chamber and the second chamber; a reference electrode, capacitively coupled to the membrane; and a package structure, which encapsulates and insulates acoustically from the outside world the first chamber, the second chamber, and the membrane.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a microelectromechanical vibration sensor. 
         [0003]    2. Description of the Related Art 
         [0004]    As is known, one way to detect vibrations in a body is to use a microelectromechanical accelerometer rigidly connected to the body itself A microelectromechanical accelerometer presents the advantage of having small dimensions, together with a very high sensitivity and very low consumption levels. It is thus easy to incorporate a microelectromechanical accelerometer even in small-sized portable devices and thus extend significantly the range of available functions. In particular, the signals supplied by the sensors may be processed for extracting information on the nature of the events detected. For example, some portable communication and/or processing devices (smartphones, tablets, portable computers) are provided with touch screens. Touch-detection systems normally enable only locating the touch events and, possibly, tracking the movement on the screen. Use of an accelerometer may enable discrimination of how a touch event has been generated (by the fingertip, a nail, a knuckle, a hard tip, etc.). Further, the majority of current portable communication and/or processing devices are already provided with accelerometers for functions different from detection of vibrations (for example, microelectromechanical accelerometers are commonly used for determining the orientation of the device or for recognizing free-fall conditions). 
         [0005]    Microelectromechanical accelerometers generally comprise a mobile mass elastically constrained to a supporting structure. The mobile mass is further capacitively coupled to the supporting structure by a system of mobile and fixed electrodes. 
         [0006]    However, the structure of the microelectromechanical accelerometers commonly used is complex, and production thereof is costly. In addition, the bandwidth of microelectromechanical accelerometers sometimes is not sufficient to enable classification of the events (such as touch events on a screen). 
       BRIEF SUMMARY 
       [0007]    One or more embodiments of the present invention are directed to a microelectromechanical vibration sensor and a method of forming same. 
         [0008]    One embodiment is directed to a microelectromechanical vibration sensor comprising a first chamber, a second chamber, and a semiconductor membrane between the first chamber and the second chamber. The sensor further includes a reference electrode capacitively coupled to the membrane. The sensor further includes a package structure that encapsulates and acoustically isolates the first chamber, the second chamber and the membrane from environments outside of the package structure. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]    For a better understanding of the invention, an embodiment thereof will now be described, purely by way of non-limiting example and with reference to the attached drawings, wherein: 
           [0010]      FIG. 1  is a partially sectioned side view of an electronic device incorporating a microelectromechanical vibration sensor according to an embodiment of the present invention; 
           [0011]      FIG. 2  is a cross-section through the microelectromechanical vibration sensor of  FIG. 1 ; 
           [0012]      FIG. 3  is a cross-section at an enlarged scale through a component of the microelectromechanical vibration sensor of  FIG. 1 ; 
           [0013]      FIG. 4  is an exploded perspective view of the microelectromechanical vibration sensor of  FIG. 1 ; 
           [0014]      FIG. 5  is a simplified block diagram of the microelectromechanical vibration sensor of  FIG. 1 ; 
           [0015]      FIG. 6  is a simplified block diagram of the electronic device of  FIG. 1 ; 
           [0016]      FIG. 7  is a cross-section through a component of a microelectromechanical vibration sensor according to a different embodiment of the present invention; 
           [0017]      FIG. 8  is a cross-section through a microelectromechanical sensor according to a further embodiment of the present invention; and 
           [0018]      FIG. 9  is a cross-section through a microelectromechanical sensor according to a further different embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The ensuing treatment will make reference, for convenience, to a specific example of application, i.e., use of a vibration sensor in a portable communication/processing device provided with touch-screen, for detecting and classifying touch events. It is understood, however, that the example is non-limiting and what is described extends to any possible use of a vibration sensor. 
         [0020]    By “touch event” is meant here and in what follows a contact of a body with the touch-screen, said contact producing vibrations that may be detected by the vibration sensor described. The body may, for example, be a fingertip, a nail, a knuckle, the tip of a stylus or of a pen, whether dielectric or conductive. 
         [0021]    In  FIG. 1 , a portable communication/processing device is designated by the reference number  1 . In the embodiment of  FIG. 1 , the device  1  is a smartphone. Purely by way of example, the device  1  could alternatively be a tablet, a portable computer, a wearable device, such as a smart watch, or a filming device such as a video camera or a photographic camera. 
         [0022]    The device  1  comprises a package  2 , housed in which is a processing unit  3 , and is provided with a touch-screen  4  arranged for closing the package  2 . Further, a vibration sensor  5  is fixed to the touch-screen  4  and is coupled in communication with the processing unit  3 . In one embodiment, one face of the vibration sensor  5  is directly joined to an internal face of the touch-screen  4 , for example by an adhesive layer, here not illustrated. In this way, the touch-screen  4  and the vibration sensor  5  are rigidly connected together. Consequently, vibrations of the touch-screen  4 , for example following upon a touch event, cause corresponding oscillatory movements of the vibration sensor  5 . 
         [0023]    As shown in  FIG. 2 , in one embodiment the vibration sensor  5  comprises a package structure  7 , housed in which are a membrane microelectromechanical transducer  8  of a capacitive type and a read and control circuit  10 , which are provided in distinct chips and are connected together by wire bonding  11 . 
         [0024]    The package structure  7 , for example an integrated-circuit package of a plastic or ceramic type, delimits a cavity  9  and seals it acoustically from the outside world. In particular, the package structure  7  is closed and is made in such a way that the incident acoustic waves are dampened and are not transmitted to the microelectromechanical transducer  8  inside the cavity  9 . In one embodiment, a vacuum may be formed in the cavity  9 . Alternatively, the cavity  9  may be filled with a gas (for example, air) or with a solid filling material (for example, a resin). 
         [0025]    The microelectromechanical transducer  8  is shown in greater detail in  FIGS. 3 and 4  and comprises a substrate  12 , an anchorage layer  14 , a membrane  15  of semiconductor material, a rigid plate  16 , and a reference electrode  17 . 
         [0026]    In the substrate  12  a through cavity is formed, which defines a first chamber  18  delimited on one side by a wall of the package structure  7  ( FIG. 2 ) and on the other by the membrane  15  ( FIGS. 3 and 4 ). 
         [0027]    The membrane  15  is fixed to the substrate  12  through anchorages  14   a  of the anchorage layer  14  and is spread out to cover the first chamber  18 . In one embodiment, the membrane  15  has a generally quadrangular shape and has the four vertices fixed to respective anchorages  14   a.  Further, the membrane  15  is elastically deformable and is doped to be electrically conductive. The mechanical properties of the membrane  15  are basically determined by the type of material (for example, epitaxial silicon), by the mass, and by the relation between the size and the thickness of the membrane  15  itself. The mechanical properties in turn determine the frequency response of the microelectromechanical transducer  8  and thus the detectable bandwidth. 
         [0028]    The plate  16 , which is made, for example, of silicon carbide or silicon nitride, is substantially undeformable and is fixed to the substrate  12  through an outer frame  14   a  of the anchorage layer  14 . The plate  16  is located on the opposite side of the membrane  15  with respect to the first chamber  18  and delimits, with the membrane  15  itself, a second chamber  19 . The second chamber  19  may be in fluid communication with the first chamber  18  and with the cavity  9  (when this is not filled with a solid filling material) or else may be fluidically decoupled from one of the two or from both. 
         [0029]    In one embodiment, the plate  16  carries the reference electrode  17  on one face, for example an outer face. In one embodiment, the plate  16  and the reference electrode  17  have openings, thus placing the second chamber  19  in fluid communication with the cavity  9 . 
         [0030]    The membrane  15  and the reference electrode  17  define the plates of a variable capacitor  20 , the capacitance of which is determined by the state of deformation of the membrane  15 . Consequently, reading of the capacitance of the variable capacitor  20  provides information on the accelerations perpendicular to the membrane  15  that modify the state of the membrane  15  itself 
         [0031]    Through an opening  21  in the plate  16 , a membrane electrode  22  contacts a coplanar pad  23  electrically connected to the membrane  15 . 
         [0032]    The vibration sensor  5  described presents the advantage of using a microelectromechanical transducer that is simple to manufacture and has a wider detection bandwidth as compared to alternative transducers, in particular as compared to conventional microelectromechanical accelerometers. The passband of the capacitive membrane microelectromechanical transducer  8  may in fact extend up to some tens of kilohertz and may be easily controlled during the manufacturing step by acting on the mass and dimensions of the membrane. For example, the capacitive microelectromechanical transducer may make it possible to achieve an output data rate higher than 30 kHz, as against 4-5 kHz that may be reached with the microelectromechanical accelerometers normally used. 
         [0033]    The package structure  7  provides acoustic insulation of the membrane  15  and makes it possible to eliminate interferences in detection of the mechanical vibrations. The membrane  15  is in fact extremely sensitive to stresses and responds also to acoustic waves. The insulation afforded by the package structure  7  makes it possible, instead, to eliminate the source of disturbance and to abate the contribution of noise on the signals generated by the microelectromechanical transducer  8 , which represent in practice only the oscillations of the membrane  15  due to the accelerations. 
         [0034]    In one embodiment, the vibration sensor  5  may comprise a microelectromechanical microphone, the input port of which has been sealed for obtaining acoustic insulation of the membrane from the surrounding environment. 
         [0035]    With reference to  FIG. 5 , the read and control circuit  10  may comprise a bias stage  25 , a reference stage  26 , a phase-generator stage  27 , an amplifier stage  28 , and an oversampling converter, for example a sigma-delta converter  29 . The phase-generator stage  27  supplies clock signals to the sigma-delta converter  29 , which produces a bitstream with high output rate on the basis of transduction signals coming from the microelectromechanical transducer  8  and amplified by the amplifier stage  28 . 
         [0036]    As shown in  FIG. 6 , in one embodiment the processing unit  3  comprises an interface module  30 , a transform module  31 , a classification engine  32 , and a memory module  33 . 
         [0037]    The interface module  30  is coupled to the vibration sensor  5  for receiving transduction signals S T , which are converted into signals in the frequency domain by the transform module  31 . 
         [0038]    The classification engine  32 , by carrying out spectral analysis of the transduction signals S T , recognizes and classifies the touch events using information present in the memory module  33 . In one embodiment, the classification engine  32  may be an inferential engine that operates on the basis of a set of rules and templates stored in the memory module  33 . For example, the classification engine  32  may discriminate touch events caused by tapping on the touch-screen  4  with a fingertip, a nail, a knuckle, the tip of a stylus, a resilient element (a rubber), etc. The templates may, for example, be in the form of power spectral distributions over significant bands that correspond to typical touch events, or else spectra of sets of parameters that define power spectral distributions (such as frequency, amplitude, and width of power spectral peaks). 
         [0039]    In one embodiment, to which  FIG. 7  refers, in a microelectromechanical transducer  108  of a membrane capacitive type, the plate  116  and the reference electrode  117  are continuous and without openings in the portion corresponding to the membrane  115 . In this case, the membrane  115  is arranged between a first chamber  118  in a substrate  112  of the microelectromechanical transducer  108  and a second chamber  119  delimited and sealed by the plate  116 . 
         [0040]    According to a further embodiment of the invention, illustrated in  FIG. 8 , a vibration sensor  205  comprises a package structure  207 , housed in which are a microelectromechanical membrane transducer  208  of a capacitive type and a read and control circuit  210 , which are provided in distinct chips and are connected together by wire bonding  211 . 
         [0041]    The microelectromechanical transducer  208  and the read and control circuit  210  may be substantially of a type already described previously. 
         [0042]    The package structure  207  in this case comprises a shell  207   a  that contains the microelectromechanical transducer  208  and the read and control circuit  210 , and is open on a side coupled to a closing body, for example an internal face of the touch-screen  4 . In this case, the closing body, i.e., the touch-screen  4 , is an integral part of the package structure  207 . 
         [0043]    A further embodiment of the invention is illustrated in  FIG. 9 . In this case, a vibration sensor  305  comprises a die, which is formed by a chip  301  and a chip  302  and incorporates a microelectromechanical transducer  308  and a read and control circuit  310 . 
         [0044]    The microelectromechanical transducer  308  comprises a semiconductor membrane  315  integrated in the chip  301  and a reference electrode  317 . 
         [0045]    The membrane  315  is spread out to cover one side of a first chamber  318 , defined by a through cavity in a substrate  312  of the chip  301 . Furthermore, the membrane  315  is elastically deformable and is doped to be electrically conductive. An auxiliary mass  315   a  is fixed to the membrane  315  in order to increase the sensitivity of the microelectromechanical transducer  308 . The auxiliary mass  315   a  may extend in the first chamber  318 , in a second chamber  319 , or partially in both. On the opposite side of the chamber  318  with respect to the membrane  315 , the chamber  318  is delimited by an internal face of the touch-screen  4 , to which the chip  301  is joined. Fixing of the chip  301  to the touch-screen  4  is obtained for insulating the chamber  318  acoustically from the external environment. 
         [0046]    The reference electrode  317 , which is substantially planar and rigid, is arranged on a face  302   a  of the chip  302  oriented in the direction of the chip  301  and is capacitively coupled to the membrane  315  for forming a variable capacitor  320 . The face  302   a  of the chip  302  also functions as supporting plate for the reference electrode  317 . More precisely, in one embodiment, the face  302   a  of the chip  302  is joined to the chip  301  by an adhesion layer  303  that has an opening in a region corresponding to the membrane  315  and to the reference electrode  317 . The membrane  315  and the reference electrode  317  are separated by a gap, which defines the second chamber  319  having a thickness substantially equal to the thickness of the adhesion layer  303 . Furthermore, the chip  302  and the adhesion layer  303  complete acoustic insulation of the membrane  315  from the surrounding environment. In practice, the substrate  312  of the chip  301 , a portion of the touch-screen  4 , the chip  302 , and the adhesion layer  303  define a package structure in which the membrane  315  is sealed and acoustically insulated from the outside world. 
         [0047]    In one embodiment, the read and control circuit  310  is integrated in the chip  302  and is coupled to the membrane  315  by a connection  304  through the adhesion layer  303  and is coupled to the capacitor  320 . 
         [0048]    Finally, it is evident that modifications and variations may be made to the microelectromechanical vibration sensor described, without thereby departing from the scope of the present invention. 
         [0049]    The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.