Patent Publication Number: US-2013241346-A1

Title: Device for converting mechanical energy into electrical energy

Description:
The invention relates to devices for converting mechanical energy into electrical power, and in particular to standalone power-supply devices that generate electrical power from a vibrational movement. 
     In certain environments, it may be difficult to connect an electrical circuit to supply cables, for example in hostile mediums or on moving mechanisms. To overcome this problem, micromechanical devices converting vibrational energy into electrical power are known. These devices form microsystems that are generally adhesively bonded to vibrating supports, such as machines or vehicles. In a known technique, a resonant system is used to amplify the mechanical vibration of a support and to convert the amplified movement into electricity. The electrical circuit may thus be supplied with power without the need for cables coming from the outside. 
     One of the known principles for converting mechanical vibrational energy into electrical power is based on an electrostatic system. The electrostatic system uses a variable capacitor in order to convert the mechanical vibrational energy into electrical power. 
     Among these electrostatic systems, a first family comprises capacitors the plates of which are biased by sources of electrical power. The main problem encountered with this first family of electrostatic systems relates to the need to provide a source of electrical power that is available before energy conversion begins. On the one hand, such a source of electrical power complicates the electrical control structure of the electrostatic system. On the other hand, such a power source consumes some of the energy generated, thereby decreasing the overall efficiency of the energy conversion structure. 
     Because of these drawbacks, a second family of electrostatic systems has been developed. Such electrostatic systems are based on the use of electrets. An electret is a dielectric material having an almost permanent electrical polarization state. In contrast to a conventional capacitor the polarization of which is temporary (the charge stored finishes by disappearing by itself), an electret may keep its polarization for a very long time (for about several tens of years). In order to produce an electrostatic mechanical/electrical converter based on electrets, it is enough to place two electrodes facing each other and to create a relative movement between an electret and at least one of these two electrodes. The movement of the electret induces a variation of charge when the electrodes are located in a closed electrical circuit. Therefore an electrical current flows through a closed electrical circuit formed between the terminals of the electrodes when the system is subjected to vibrations. 
       FIG. 1  is a schematic diagram of an example of a mechanical/electrical converter CO based on the use of an electret. As illustrated, the converter CO comprises an electrode EL and a counter electrode CE formed from metal plates connected by an electrical impedance IE. An electret ET forming a plate is fixed to the electrode EL. The electrode EL and the electret ET are both securely fastened to a support SU. The counter electrode CE is mounted so as to be able to move in its plane via a spring RE relative to the support SU. 
     By virtue of the vibrations of the medium, the counter electrode moves and the influence of the electret on this electrode varies. Because of the law of conservation of charge, the sum of the charges on the electrode and the counter electrode is equal to the charge on the electret, which is constant. Therefore, charge is redistributed between the electrode and the counter electrode. The voltages/currents that result therefrom thus allow the electrical impedance to be supplied with power. 
     A whole wafer electret with an area greater than one centimeter squared may store a relatively large charge (a few mC/m 2 ) with a good stability (greater than 10 years). The stability is defined by the length of time the electret keeps its charge. 
     However, it has been shown that such a converter exhibits only a small variation in capacitance when it is subjected to small vibrational movements. Thus, the electrical power generated remains relatively small. 
     In order to increase the variation in the capacitance between two facing electrets under the effect of vibrations, the document “Electrostatic micro power generation from low-frequency vibration such as human motion” provides a fabrication process for forming electrets and electret absences in alternation in a direction parallel to a sliding direction. This document proposes to form electrets in succession with a relatively small pitch in order to increase the variation in capacitance during the vibrational movements. In this process, a planar layer of insulating silicon oxide is formed on a silicon substrate. An aluminum layer is deposited on the silicon oxide. The pattern of the electrets to be formed is then defined by etching the aluminum layer. Charge is then implanted locally in the silicon oxide layer in order to form the electrets. The residual aluminum is thus used as a mask to prevent charging of the zones that it covers. 
     The silicon substrate is mounted so as to be able to slide over a first glass sheet via a spring. A second glass sheet supports an alternation of electrodes with opposite polarities. The electrical impedance is connected between the electrodes of each polarity. The glass sheets are fastened to each other. The silicon support and its electrets are placed between the two glass sheets, facing the electrodes. The electrodes of a given polarity are distributed with a pitch identical to the distribution pitch of the electrets. 
     However, it has been observed that such electrets exhibit an unsatisfactory stability, in particular for relatively small distribution pitches (&lt;300 μm). Such a lack of stability limits the usefulness of such converters, since reduction in the pitch between the electrets improves the conversion efficiency when the structure is subjected to small-amplitude vibrations. 
     The document “HARVESTING ENERGY FROM VIBRATIONS BY A MICROMACHINED ELECTRET GENERATOR” written by Messrs. Sterken, Fiorini, Altena, Van Hoof and Puers and published on the occasion of the 14th International Conference on Solid-State Sensors held in Lyon from 10 to 14 Jun. 2007, describes a structure intended to benefit from an electret having a high stability. This structure is also structured so as to generate large variations in capacitance during the movement, thereby in theory resulting in an improved conversion efficiency. Specifically, the structure comprises a silicon wafer fastened above a glass support. The glass support supports a first electrode comprising features distributed with a pitch. A movable mass is housed in the silicon wafer and slides horizontally above the glass support. The movable mass supports a second electrode comprising features distributed with the same pitch. The second electrode is placed opposite the first electrode. The electret, formed from a large continuous layer, polarizes the second electrode through the movable mass of silicon. 
     In practice, a constant parasitic capacitor is added in series between the electret and the movable mass, thereby greatly limiting the conversion efficiency of the structure. 
     Moreover, all of the electret-comprising mechanical/electrical converters developed up to now have remained confined to laboratory prototypes and have never been produced on an industrial scale. 
     The invention aims to solve one or more of these drawbacks. The invention thus relates to a device for converting mechanical vibrational energy into electrical power, comprising:
         first and second collecting electrodes intended to be connected to the terminals of an electrical load;   an electret placed facing at least the first electrode, the electret being mounted so as to be able to move at least relative to the first electrode in at least one degree of freedom in a plane, so that a relative movement between the electret and the first electrode induces a potential difference across the first and second electrodes. In addition:   the electret comprises a continuous layer containing a series of protrusions extending in a direction perpendicular to said plane, the protrusions being distributed in said degree of freedom with a pitch smaller than the travel between the first electrode and the electret; and   the first electrode has faces facing the electret, these faces being distributed in said degree of freedom with a pitch identical to the pitch of the protrusions of the electret.       

     According to one variant, the first and second electrodes are housed on the same support facing the electret, the second electrode having faces distributed in said degree of freedom with a pitch identical to the pitch of the protrusions of the electret, the faces of the first and second electrodes being alternated. 
     According to another variant, the first and second electrodes are housed on respective supports placed on either side of the electret. 
     According to another variant, the electret is mounted so as to be able to slide relative to the first electrode in a direction contained in said plane, the protrusions being distributed in said plane in this sliding direction, the faces of the first electrode being distributed in this sliding direction. 
     According to yet another variant, the faces of the first electrode are separated by grooves having a width greater than the width of the faces. 
     According to one variant, the pitch of the protrusions is smaller than 200 μm, and preferably smaller than 100 μm. 
     According to another variant, the electret is mounted so as to be able to pivot relative to the first electrode about an axis normal to said plane, the protrusions being angularly distributed about this axis, the faces of the first electrode being angularly distributed about this axis. 
     According to another variant, the protrusions of the electret are separated by grooves having a depth between 10 μm and 500 μm. 
     According to another variant, the electret is separated from the first electrode by a distance smaller than 10 μm, and preferably smaller than 5 μm. 
     According to another variant, the pitch of the protrusions of the electret is at least 20 times larger than said distance. 
     According to one variant, the electret is housed on a support containing a relief pattern, the electret being formed from a dielectric layer of continuous thickness. 
     According to another variant, the electret is covered with a continuous protective layer. 
     According to another variant, the electret is formed from a layer of silicon oxide housed on a silicon substrate. 
     According to yet another variant, the electret is connected to the first electrode via a spring compressed by a relative movement in said degree of freedom between the first electrode and the electret. 
     The invention also relates to a process for fabricating a device for converting mechanical energy into electrical power, comprising steps of:
         forming a continuous layer of dielectric containing a series of protrusions extending in a direction and distributed with a pitch;   charging the continuous layer of dielectric formed so as to form an electret; and   assembling the electret facing first and second collecting electrodes, the electret being mounted so as to be able to move relative to the first electrode in a degree of freedom in a plane perpendicular to said direction with a travel in this degree of freedom larger than the pitch of the protrusions, so that a relative movement between the electret and the first electrode induces a potential difference across the first and second electrodes, the first electrode having faces facing the electret, which faces are distributed in said degree of freedom with a pitch identical to the pitch of the protrusions of the electret.       

     According to one variant, the formation of the continuous layer of dielectric comprises:
         etching a face of a support made of silicon in order to form protrusions with said pitch in a direction of a relative sliding motion between the electret and the first electrode; and   forming a continuous layer of dielectric on the etched face of the support.       

     According to yet another variant, the continuous layer of dielectric is formed by oxidizing the etched face of the silicon oxide support. 
    
    
     
       Other features and advantages of the invention will become clear from the completely non-limiting description given thereof below by way of indication, with reference the appended drawings, in which: 
         FIG. 1  schematically illustrates an example of an electret-comprising mechanical/electrical converter; 
         FIG. 2  is a cross-sectional view of an electret-comprising electrical/mechanical conversion structure according to a first embodiment of the invention; 
         FIG. 3  is a schematic top view of the configuration of the electrodes in this first embodiment; 
         FIG. 4  is a cross-sectional view of an electret-comprising electrical/mechanical conversion structure according to a second embodiment of the invention; 
         FIG. 5  is a cross-sectional view of an electret-comprising electrical/mechanical conversion structure according to a third embodiment of the invention; 
         FIG. 6  is a bottom view of an example of a combination of electret patterns allowing exploitation of vibrational excitation along separate axes; 
         FIGS. 7   a  to  7   e  illustrate various steps in a first variant process for fabricating an electret for producing a conversion structure according to the invention; 
         FIGS. 8   a  to  8   g  illustrate various steps in a second variant process for fabricating an electret for producing a conversion structure according to the invention; 
         FIG. 9  is a cross-sectional side view of an electret-comprising electrical/mechanical conversion structure according to a fourth embodiment of the invention; 
         FIGS. 10 and 11  are respectively top and bottom views of a pair of electrodes and an electret of the structure in  FIG. 9 ; 
         FIG. 12  is a cross-sectional side view of an electret-comprising electrical/mechanical conversion structure according to a fifth embodiment of the invention; and 
         FIGS. 13 and 14  are respectively top and bottom views of a pair of electrodes and an electret of the structure in  FIG. 12 . 
     
    
    
     The invention makes it possible to exploit electrets having very high stabilities and allowing large variations in capacitance to be generated with small movements. The amount of electrical power that can be generated using a conversion device of a given size may be substantially increased. A large variation in capacitance per unit of relative movement of the electret can be obtained because of the permitted fineness of the electret structure. Moreover, this structural fineness means a large range of vibrational amplitudes can be exploited. Whereas conventional technical best practice would suggest it makes sense to form a discontinuous electret, this electret only being formed on the protrusions (the presence of electret in the grooves in theory decreasing the variation in capacitance during the movement of a movable mass), the inventors have in fact demonstrated that a continuous electret with protrusions is particularly advantageous. 
     The embodiments illustrated with reference to  FIGS. 2 to 6  relate to devices for converting vibrational energy in which an electret is mounted so as to be able to slide relative to a facing electrode. The electret slides in a plane, and comprises protrusions extending perpendicularly to this plane. 
       FIG. 2  is a cross-sectional view of a first embodiment of a structure  10  for converting mechanical vibrational energy into electrical power. The structure  10  comprises a support  50  intended to be securely fastened to the system generating the vibrational energy. A silicon-based structure is fastened plumb with the support  50  using a resin  54 . The silicon-based structure comprises a fixed frame  56  and a movable support  51 . The movable support  51  is connected to the fixed frame  56  via a spring  55 . The movable support  51  is mounted so as to be able to slide relative to the support  50  in the x direction. The support  51  compresses the spring  55  during its movements along this x axis. The spring  55  may be produced by processing of the silicon-based structure. 
     The support  50  is made of a dielectric, for example glass. The support  50  comprises a first electrode  20  and a second electrode  30  on its upper surface. The electrodes  20  and  30  are formed of metal strips extending in the y direction. The metal strips forming the electrode  20  comprise faces  21  oriented upward. The metal strips forming the electrode  30  comprise faces  31  oriented upward. The metal strips of the electrode  20  are isolated from the metal strips of the electrode  30 . The faces  21  are distributed in the x direction with a pitch P. The faces  31  are also distributed in the x direction with a pitch P. The faces  21  are separated from each other by the faces  31 . The faces  21  and  31  therefore alternate in the x direction. 
       FIG. 3  is a top view of the configuration of the electrodes  20  and  30  on the support  50 . The electrode  20  and the electrode  30  are connected to respective terminals of an electrical load  60 . The electrical load  60  may be an electronic circuit, for example including a recharging circuit including a capacitor for storing energy and a functional circuit powered by this capacitor. The metal strips forming the electrode  20  are all connected to a first terminal of the electrical load  60 . The metal strips forming the electrode  30  are connected to a second terminal of the electrical load  60 . In this embodiment, the electrode  20 , the electrode  30  and the electrical load  60  are fixed to the same support  50 , thereby making their fabrication easier. 
     An electret  40  is housed on the lower face of the movable support  51 . The electret  40  comprises a continuous layer of dielectric material storing charge. The dielectric layer of the electret  40  closely follows the relief pattern in the movable support  51  in order to form a series of protrusions  42  separated by grooves  41 . The electret  40  may especially comprise a layer of SiO 2  or a layer of a polymer such as parylene. The electret  40  is advantageously formed from a uniform material layer. The protrusions  42  extend in the z direction. The protrusions  42  are distributed in the x direction with a pitch P identical to the pitch of the metal strips of the electrodes  20  and  30 . The protrusions  42  and the grooves  41  extend in the y direction. The electret  40  is placed facing faces  21  and  31  of the first and second electrodes  20  and  30 , respectively. The movable support  51  has a travel in the x direction larger than the distribution pitch P of the protrusions  42 . The assembly formed by the movable support  51 , the electret  40  and the spring  55  has a resonant frequency centered on a frequency range for vibrations for which an optimal conversion gain is sought. 
     When a vibration pushes the movable support  51  and the electret  40  in the x direction (i.e. generates a relative movement between the support  50  and the support  51 ), transfers of electrical charge are induced between the electrodes  20  and  30 . Due to these charge transfers, a potential difference appears across the terminals of the electrical load  60  and an electrical current flows through this electrical load  60 . 
     When the relative movement of the electret  40  is larger than the pitch P several electrical alternations are generated during the travel. With an open electrical circuit, the polarity of the potential difference changes when the electret slides a distance equal to half the pitch P. The amount of electrical power recovered when the electret travels its entire travel is thus maximized. Moreover, efficient electrical power conversion is obtained even when the amplitude of the sliding motion of the electret  40  varies greatly over time, several alternations being generated even with a limited sliding motion. The frequency of the potential difference generated across the terminals of the load  60  may be higher than the resonant frequency of the resonant system or of the vibration frequency of the source of vibrations. This performance is obtained while benefiting from a stable electret  40  because a continuous dielectric layer is used. 
     The invention proves to be particularly advantageous when the gap or distance G between the electret  40  and the electrode  20  is relatively small, for example when this gap G is smaller than 10 μm, even smaller than 5 μm. Specifically, the inventors have observed that edge effects may be particularly appreciable at such dimensions, further increasing the conversion gain. 
     In order to limit the impact of such edge effects, the pitch P of the protrusions  42  is advantageously at least 20 times larger than this gap G. If LS denotes the width of the protrusions  42  and LR the width of the grooves  41 , it proves to be advantageous for the following relationships to be respected: 
         LR&gt; 10 ·G    
         LS&gt; 10 ·G    
       FIG. 4  is a cross-sectional view of a second embodiment of a structure  10  for converting mechanical vibrational energy into electrical power. The structure  10  comprises a support  52  intended to be securely fastened to the system generating the vibrational energy. The support  52  is made of a semiconductor, for example from a silicon wafer. 
     A semiconductor-based structure (silicon wafer) is fixed plumb with the support  52  using a resin  54 . The silicon-based structure comprises a fixed frame  56  and a movable support  53 . The movable support  53  is connected to the fixed frame  56  via a spring  55 . The movable support  53  is mounted so as to be able to slide relative to the support  52  in the x direction. The support  53  compresses the spring  55  during its movements along this X axis. The spring  55  may be produced by processing of a silicon wafer in which the fixed frame  56  and the movable support  53  are formed. 
     The support  53  contains, in the z direction, a relief pattern formed by alternating protrusions and grooves. The protrusions and the grooves in the support  53  extend in the y direction. The protrusions and grooves in the support  53  are distributed in the x direction with a pitch P. The support  52  also contains, in the z direction, a relief pattern formed by alternating grooves  22  and protrusions. The protrusions and the grooves in the support  52  extend in the y direction. The protrusions and the grooves in the support  52  are distributed in the x direction with a pitch P. The relief patterns in the supports  52  and  53  are placed facing each other. 
     The electret  40  comprises a continuous layer covering the protrusions and the grooves in the support  53 . The electret  40  may especially comprise a layer of SiO 2  or a layer of a polymer such as parylene. The electret  40  closely follows the relief pattern in the support  53  and thus exhibits an alternation of protrusions  42  and grooves  41  distributed in the x direction with the pitch P. The pitch P is smaller than the travel of the support  53  and of the electret  40  in the x direction. The protrusions  42  and the grooves  41  extend in the y direction. The electret  40  is advantageously made from a dielectric layer of continuous thickness formed on the support  53  containing the relief pattern. 
     The support  52  forms the first electrode  20  by having faces  21  at the ends of its protrusions and by being sufficiently conductive to conduct electric charge to and from these faces  21 . The support  53  forms the second electrode  30  by being sufficiently conductive to conduct the charge to and from its protrusions. The electrodes  20  and  30  are thus formed in supports placed on either side of the electret  40 . The electrical load  60  is connected between the support  52  and the support  53 . 
     When the protrusions  42  lie opposite the faces  21 , the capacitance of the capacitor formed is maximized and corresponds to the sum of the capacitances C 1  between protrusions  42  and faces  21  and of the capacitances C 2  between the grooves  41  and the grooves  22 . The capacitance Cmax of the capacitor formed is given by the following relationship: 
     
       
         
           
             
               
                 
                   
                     C 
                     max 
                   
                   = 
                     
                    
                   
                     
                       nC 
                       1 
                     
                     + 
                     
                       
                         ( 
                         
                           n 
                           - 
                           1 
                         
                         ) 
                       
                        
                       
                         C 
                         2 
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     
                       n 
                        
                       
                         
                           
                             ɛ 
                             0 
                           
                           * 
                           LS 
                           * 
                           LO 
                         
                         
                           G 
                           + 
                           
                             EP 
                             ɛ 
                           
                         
                       
                     
                     + 
                     
                       
                         ( 
                         
                           n 
                           - 
                           1 
                         
                         ) 
                       
                        
                       
                         
                           
                             ɛ 
                             0 
                           
                           * 
                           LS 
                           * 
                           LO 
                         
                         
                           
                             2 
                              
                             
                                 
                             
                              
                             DE 
                           
                           + 
                           G 
                           + 
                           
                             EP 
                             ɛ 
                           
                         
                       
                     
                   
                 
               
             
           
         
       
     
     where n is the number of protrusions, EP is the thickness of the electret, c is the permittivity of the electret, LO is the length of the protrusions  42  and of the faces  21 , DE is the depth of the grooves  41  and of the grooves  22 , and LS is the width of the protrusions  42  and the faces  21 . 
     When the protrusions  42  lie opposite the grooves  22 , the capacitance of the capacitor formed is minimized and corresponds to the sum of the capacitances C 3  between protrusions  42  and grooves  22  and of the capacitances C 4  between the grooves  41  and the faces  21 . The capacitance Cmin of the capacitor formed is given by the following relationship: 
     
       
         
           
             
               
                 
                   
                     C 
                     min 
                   
                   = 
                     
                    
                   
                     
                       nC 
                       3 
                     
                     + 
                     
                       
                         ( 
                         
                           n 
                           - 
                           1 
                         
                         ) 
                       
                        
                       
                         C 
                         4 
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     
                       ( 
                       
                         
                           2 
                            
                           n 
                         
                         - 
                         1 
                       
                       ) 
                     
                      
                     
                       
                         
                           ɛ 
                           0 
                         
                         * 
                         LS 
                         * 
                         LO 
                       
                       
                         DE 
                         + 
                         G 
                         + 
                         
                           EP 
                           ɛ 
                         
                       
                     
                   
                 
               
             
           
         
       
     
     For a high value of n and for DE&gt;&gt;G, the ratio between Cmax and C min may then be expressed as follows: 
     
       
         
           
             
               
                 C 
                 max 
               
               
                 C 
                 min 
               
             
             ≅ 
             
               
                 3 
                 4 
               
               + 
               
                 DS 
                 
                   2 
                    
                   
                     ( 
                     
                       G 
                       + 
                       
                         EP 
                         ɛ 
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
     The proposed structure thus allows the variations in capacitance per unit sliding movement of the electret  40  to be optimized and the conversion gain of the converter  10  to be increased. 
     In order to promote optimal capacitance variation between the electret  40  and electrode  20  during their relative movement, the grooves  41  separating the protrusions  42  of the electret  40  advantageously have a depth (relief in the z direction) of between 10 and 500 μm. 
     The inventors have furthermore observed that using a continuous layer to form the electret  40  allows edge effects to be limited for small protrusions  42 , for example when their pitch P is smaller than 200 μm, and in particular when their pitch P is smaller than 100 μm. 
     Advantageously, the grooves in the support  53  are wider than the protrusions in the same support  53 . Likewise, the grooves in the support  52  are wider than the protrusions in the same support  52 . 
       FIG. 5  is a cross-sectional view of a third embodiment of a structure  10  for converting mechanical vibrational energy into electrical power. The structure  10  comprises a support  50  intended to be securely fastened to the system generating the vibrational energy. The support  50  is made of an insulator, for example from a glass sheet. 
     A semiconductor-based structure (silicon wafer) is fixed plumb with the support  50  using a resin  54 . The silicon-based structure comprises a fixed frame  56  and a movable support  53 . The movable support  53  is connected to the fixed frame  56  via a spring  55 . The movable support  53  is mounted so as to be able to slide relative to the support  50  along this X axis. The support  53  compresses the spring  55  during its movements along this X axis. The spring  55  may be produced by processing of a silicon wafer in which the fixed frame  56  and the movable support  53  are formed. 
     The support  53  contains, in the z direction, a relief pattern formed by alternating protrusions and grooves. The protrusions and the grooves in the support  53  extend in the y direction. The protrusions and grooves in the support  53  are distributed in the x direction with a pitch P. 
     The support  50  comprises a substantially flat upper face on which a first electrode  20  is housed. The electrode  20  is advantageously housed in relief on the support  50  in order to increase the variation in capacitance during the sliding movement of the electret  40 . The electrode  20  is formed from metal strips extending in the y direction. The metal strips forming the electrode  20  comprise faces  21  pointed upward. The faces  21  are distributed in the x direction with a pitch P. 
     The electret  40  comprises a continuous layer covering the protrusions and the grooves of the support  53 . The electret  40  may especially comprise a layer of SiO 2  or a layer of a polymer such as parylene. The electret  40  closely follows the relief pattern in the support  53  and thus exhibits an alternation of protrusions  42  and grooves  41  distributed in the x direction with a pitch P. The pitch P is smaller than the travel of the support  53  and of the electret  40  in the x direction. The protrusions  42  and the grooves  41  extend in the y direction. The electret  40  is advantageously made from a dielectric layer of continuous thickness formed on the support  53  containing the relief pattern. The electret  40  is opposite the electrode  20 . 
     The support  53  forms of the second electrode  30  by being sufficiently conductive to conduct the charge to and from its protrusions. The electrodes  20  and  30  are thus formed on supports placed on either side of the electret  40 . The electrical load  60  is connected between the electrode  20  and the electrode  30 . 
     The electrodes  20  and  30  of the supports  51 ,  52  and  53  of the embodiments described above may also be formed by a conductive layer that closely follows the relief patterns formed therein. The electrode  30  housed on a support  51  or  53  may for example be formed from a conductive metal layer placed between the silicon of the support and the electret  40 . 
       FIG. 6  is a bottom view of a support  53  comprising two groups of electrets  40 . The electrets of a first group contain protrusions  42  distributed in the y direction. The electrets of a second group contain protrusions  42  distributed in the x direction. The electrets  40  are plumb with electrodes  20  and  30  containing corresponding distributions. The support  53  is mounted so as to be able to slide in the x and y directions relative to the electrode  20 . Thus, the converter  10  is capable of making optimal use of vibrations having various orientations or having orientations that vary over time. 
     Various electrets having different respective distribution pitches may also be provided. Furthermore, various electrets having different phase shifts relative to the electrodes  20  placed opposite may be provided. 
       FIGS. 7   a  to  7   e  schematically illustrate a first variant of a process for fabricating an electret  40  on a support  57  made of silicon. For the sake of simplicity, certain optional steps of this process, such as the production of a spring connecting the support or the electret assembly formed in a conversion device, are not described. 
     In  FIG. 7   a , a silicon wafer  57  having two substantially flat sides is provided. As illustrated in  FIG. 7   b , a resist is then deposited. Using a photolithography process known per se, a pattern  58  of hardened resist is formed on one side of the silicon wafer. 
     As illustrated in  FIG. 7   c , a relief pattern is formed in the silicon wafer  57  by an etching step, using the pattern  58 . Etching processes known per se in the art may be used. Wet etching processes (such as KOH etching) or dry etching processes (such as DRIE etching) may be employed. In the context of the invention, DRIE etching is advantageously used, thereby allowing protrusions to be produced with very straight sidewalls, even for groove depths exceeding 100 μm. After the resist has been removed, the etching may be followed by a heat treatment. The relief pattern formed thus contains protrusions and grooves in alternation, housed in the silicon wafer  57 . 
     As illustrated in  FIG. 7   d , a dielectric film  43  is formed on the relief pattern in the silicon wafer  57 . In this case, the film  43  has a uniform thickness and closely follows the relief pattern in the wafer  57 . The film  43  thus exhibits an alternation of protrusions  42  and grooves  41 . The film  43  is for example made of a polymer such as parylene. This material promotes the stability of the electret to be formed since it is hydrophobic and thus limits charge loss due to moisture. This material furthermore has a good capacity for storing electrical charge permanently. The film  43  may for example be between 10 nm and 9 μm in thickness. 
     As illustrated in  FIG. 7   e , charge is then implanted in the film  43  in order to form a continuous electret  40 . The implantation of charge in order to form the electret  40  may be carried out in any appropriate way. The charge may especially be implanted using what is called a corona discharge technique. A corona discharge is an electrical discharge that appears when the electric field on a conductor exceeds a certain value, under conditions that prevent an electric arc from striking. The medium surrounding the electrical conductor is then ionized and a plasma is created. The ions generated transfer their charge to the surrounding molecules with the lowest energy. The charge will advantageously be implanted using a triode corona discharge process in which a metal grid is used to control the surface potential and homogenize the charge in the electret. The charging step will possibly be followed by a heat treatment. 
       FIGS. 8   a  to  8   g  schematically illustrate a second variant of a process for manufacturing an electret  40  on a support  57  made of silicon. 
     In  FIG. 8   a , a silicon wafer  57  having two substantially flat sides is provided. As illustrated in  FIG. 8   b , a resist is then deposited. Using a photolithography process known per se, a pattern  58  of hardened resist is formed on one side of the silicon wafer  57 . 
     As illustrated in  FIG. 8   c , a relief pattern is formed in the silicon wafer  57  by an etching step, using the pattern  58 . Etching processes known per se in the art may be used. The relief pattern formed thus contains protrusions and grooves in alternation, housed in the silicon wafer  57 . The resist is then removed. 
     As illustrated in  FIG. 8   d , a dielectric layer  44  is formed on the relief pattern in the silicon wafer  57 . The layer  44  is an SiO 2  layer for example created by thermal oxidation of the side of the wafer  57  containing the relief pattern. The layer  44  thus exhibits an alternation of protrusions  42  and grooves  41 . The layer  44  may for example be formed with a thickness of between 50 nm and 5 μm. 
     As illustrated in  FIG. 8   e , a layer  45  of a stabilizing material is advantageously formed on the layer  44 . The layer  45  is for example made of silicon nitride Si 3 N 4 . Such a layer  45  allows the stability of the electret formed to be improved by trapping the charge. The layer  45  may be produced by low-pressure chemical vapor deposition (LPCVD). The layer  45  may for example be between 50 and 500 nm in thickness. Deposition of the layer  45  may be followed by a heat treatment step, typically at a temperature above 400° C. for several hours. 
     As illustrated in  FIG. 8   f , a protective layer  46  may be deposited. The aim of the protective layer  46  is to prevent contact between moisture and the electret formed, in order to prevent loss of the charge stored in the dielectric. The protective layer  46  may typically be made of parylene or HMDS, which have good hydrophobic properties. The layer  46  may for example be between 10 nm and 10 μm in thickness. 
     As illustrated in  FIG. 8   g , charge is then implanted in the film  44  in order to form a continuous electret  40 . The charge used to form the electret  40  may be implanted in any appropriate way. The charge may for example be implanted using a corona discharge technique. 
     Other processes for forming a continuous electret may of course be envisioned. It is especially possible to deposit a dielectric on a support by sputtering. 
     The embodiments illustrated with reference to  FIGS. 9 to 14  relate to vibrational energy conversion devices in which an electret is mounted so as to be able to pivot relative to a facing electrode. The electret pivots in a plane, and contains protrusions extending perpendicularly to this plane. 
       FIG. 9  is a cross-sectional view of a fourth embodiment of a structure for converting mechanical vibrational energy into electrical power. The structure  10  comprises a support  50  intended to be securely fastened to the system generating the vibrational energy. A silicon-based structure is fastened plumb with the support  50 . The silicon-based structure comprises a fixed frame  56  and a movable support  51 . The movable support  51  is connected to the fixed frame  56  via a torsion spring  55  and a rigid beam  70 . The movable support  51  is mounted so as to be able to pivot about a vertical axis  59  (z direction) relative to the fixed frame  56 . An eccentric mass  511  is fixed to the movable support  51 . The mass  511  is eccentric relative to the axis  59 . Because the assembly formed by the mass  511  and the movable support  51  is unbalanced relative to the axis  59 , a relative movement between the movable mass  51  and the support  50  generates a rotation of the movable mass  51  relative to the support  50 . The support  51  thus compresses the spring  55  when it is subjected to a vibration, due to the presence of the eccentric mass  511 . 
       FIG. 10  is a top view of the support  50  supporting the electrodes.  FIG. 11  is a bottom view of the support  51  supporting an electret  40 . 
     The support  50  is made of a dielectric, for example of glass. The support  50  comprises a first electrode  20  and a second electrode  30  on its upper side. The electrodes  20  and  30  are formed from angular segments distributed about a geometric centre. The angular segments forming the electrode  20  comprise faces  21  oriented upward. The angular segments forming the electrode  30  also comprise faces that are oriented upward. The angular segments of the electrode  20  are isolated from the angular segments of the electrode  30 . The faces  21  of the electrode  20  are distributed about the geometric centre with an angular pitch β. The faces of the electrode  30  are distributed about the geometric centre with an angular pitch β. The respective faces of the electrodes  20  and  30  are alternated about the geometric centre. The electrode  20  and the electrode  30  are connected to respective terminals of an electrical load  60 . The angular segments forming the electrode  20  are all connected to a first terminal of the electrical load  60 . The angular segments forming the electrode  30  are connected to a second terminal of the electrical load  60 . In this embodiment, the electrode  20 , the electrode  30  and the electrical load  60  are fixed to the same support  50 , thereby making their fabrication easier. 
     The electret  40  is housed on the lower side of the movable support  51 . The electret  40  comprises a continuous dielectric layer storing charge. The dielectric layer of the electret  40  closely follows the relief pattern in the movable support  51  thus forming a series of protrusions  42  taking the shape of angular segments, separated by grooves  41  also taking the shape of angular segments. The protrusions  42  extend in the z direction relative to a plane in which the support  51  pivots. The protrusions  42  are distributed about the axis  59  with an angular pitch β identical to the angular pitch of the angular segments of the electrodes  20  and  30 . The electret  40  is placed facing the faces of the first and second electrodes  20  and  30 . The movable support  51  exhibits a pivotal travel about the axis  59 , which travel is larger than the angular pitch β of the distribution of the protrusions  42 . When a vibration drives the movable support  51  with a rotational component about the axis  59 , the movable support  51  pivots relative to the support  50 . Electrical charge is then induced to move back and forth between the electrodes  20  and  30 . Because of this movement of charge, a potential difference appears across the terminals of the electrical load  60  and an electrical current flows through this electrical load  60 . 
     When the relative movement of the electret  40  is larger than the angular pitch β between the protrusions  42 , a number of electrical alternations are generated during the travel. For an open electrical circuit, the polarity of the potential different changes when the electret slides a distance equal to half the angular pitch β between the protrusions  42 . The amount of electrical power recovered when the electret travels its entire travel is thus maximized. 
       FIG. 12  is a cross-sectional view of a fifth embodiment of a structure for converting mechanical vibrational energy into electrical power. The structure  10  comprises a support  50  intended to be securely fastened to the system generating the vibrational energy. A silicon-based structure is fastened plumb with the support  50 . The silicon-based structure comprises a fixed frame  56  and a movable support  51 . The movable support  51  is connected to the fixed frame  56  via a beam  70 . The beam  70  has an end embedded in the fixed frame  56  and another end embedded in the support  51 . The beam  70  has dimensions that allow it to flex about a vertical axis (z direction) when it is subjected to vibrations, under the effect of the inertia of the movable support  51 . The movable support  51  thus pivots about a vertical axis passing through the point  71  where the beam  70  joins the fixed frame  56 . 
       FIG. 13  is a top view of the electrode-supporting support  50 .  FIG. 14  is a bottom view of the support  51  supporting an electret  40 . 
     The support  50  is made of a dielectric. The support  50  comprises a first electrode  20  and a second electrode  30  on its upper side. The electrodes  20  and  30  are formed from angular segments of a ring having the junction point  71  as its geometric centre. The geometric centre is placed substantially on the axis about which the movable support  51  pivots. The angular segments forming the electrode  20  comprise faces  21  oriented upward. The angular segments forming the electrode  30  comprise faces  31  also oriented upward. The angular segments of the electrode  20  are isolated from the angular segments of the electrode  30 . The faces  21  of the electrode  20  are distributed with an angular pitch β over an arc of a circle having the junction point  71  as its geometric centre. The faces  31  of the electrode  30  are distributed with an angular pitch β over an arc of a circle having the junction point  71  as its geometric centre. The respective faces of the electrodes  20  and  30  are alternated about the geometric centre. The electrode  20  and the electrode  30  are connected to respective terminals of an electrical load  60 . The angular segments forming the electrode  20  are all connected to a first terminal of the electrical load  60 . The angular segments forming the electrode  30  are connected to a second terminal of the electrical load  60 . In this embodiment, the electrode  20 , the electrode  30  and the electrical load  60  are fixed to the same support  50 , thereby making their fabrication easier. 
     The electret  40  is housed on the lower side of the movable support  51 . The electret  40  comprises a continuous dielectric layer storing charge. The dielectric layer of the electret  40  closely follows the relief pattern in the movable support  51  thus forming a series of protrusions  42  taking the shape of angular segments of a ring, separated by grooves  41  also taking the shape of angular segments of a ring. The protrusions  42  extend in the z direction relative to a plane in which the support  51  pivots. The protrusions  42  are distributed over an arc of a circle, having the junction point  71  as its geometric center, with an angular pitch β identical to the angular pitch of the angular segments of the electrodes  20  and  30 . The electret  40  is placed facing the faces of the first and second electrodes  20  and  30 . The movable support  51  exhibits a pivotal travel about the junction point, which travel is larger than the angular pitch of the distribution of the protrusions  42 . When a vibration drives the movable support  51  with a rotational component about the junction point  71 , the movable support  51  pivots relative to the support  50 . Electrical charge is then induced to move back and forth between the electrodes  20  and  30 . Because of this movement of charge, a potential difference appears across the terminals of the electrical load  60  and an electrical current flows through this electrical load  60 . 
     When the relative movement of the electret  40  is larger than the angular pitch β between the protrusions  42 , a number of electrical alternations are generated during the travel. For an open electrical circuit, the polarity of the potential different changes when the electret slides a distance equal to half the angular pitch β between the protrusions  42 . The amount of electrical power recovered when the electret travels its entire travel is thus maximized.