Abstract:
A micro-electromechanical package includes a casing and a microelectronic circuit. At least one portion of the casing includes a piezoelectric material arranged such that, in use, dynamically changing mechanical strain in the at least one portion produces an electrical charge usable as a power source by the microelectronic circuit.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Application No. EP 06111849.3 filed on Mar. 28, 2006, entitled “Energy Scavenger,” the entire contents of which are hereby incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to the area of energy scavenging and particularly to a piezoelectric energy scavenger. 
       BACKGROUND 
       [0003]    Energy scavengers, also sometimes known as energy harvesters are increasingly used in electronic systems to reduce the strain on main power sources or, in some cases, to act as main sources of power. 
         [0004]    An energy scavenger is a device which converts ambient energy, which would otherwise be wasted, into energy which can be used for a specific purpose. Energy scavengers may be used in a wide variety of applications ranging from indirectly powering emergency telecommunication equipment to powering microelectronic sensors. 
         [0005]    A system for indirectly powering telecommunication devices could, for example, comprise a solar panel which would help charge a battery connected to a stationary emergency wireless telephone box located on the side of a remote motorway. Unfortunately, solar power is not always ideal for powering microelectronic circuits in that, often, these circuits are found in relatively dark places. 
         [0006]    However, many micro electronic devices, such as acceleration sensors or passive sensors are constantly being exposed to kinetic forces. Consequently, kinetic energy scavengers have been developed to harness a part of the residual ambient energy caused by the acceleration, vibration or dynamic compression of a device. 
         [0007]    Kinetic energy scavengers may harness vibratory energy using piezoelectric devices, thereby converting strain in a material into electrical impulses. These devices can be used in a wide variety of applications. 
         [0008]    Most energy scavengers comprise multiple interconnected parts which are assembled in a package in order to protect the highly sensitive parts from environmental conditions. This has numerous drawbacks. The first of which is that the multiple interconnected parts must be assembled and placed in the package itself. Thus, known energy scavengers are expensive and difficult to manufacture. What is needed is an energy scavenger which comprises few parts and is thereby cheaper and easier to manufacture. 
         [0009]    One fundamental challenge with energy scavengers for kinetic energy is the need for large seismic masses or large diaphragms to reach certain levels of efficiency. A second fundamental challenge for piezoelectric energy scavengers is that the efficiency is directly related to the amount of piezoelectric material that is exposed to resulting high strains. Thus, the efficiency of the scavenger is directly related to the volume and/or area of the device. 
         [0010]    There is a need to for an energy scavenger which maximizes the piezoelectric material exposed to high strain while minimizing the complexity of the microelectronic system itself. 
       SUMMARY 
       [0011]    A micro-electromechanical package according to the present invention includes a casing and a microelectronic circuit. At least one portion of the casing includes a piezoelectric material arranged such that, in use, dynamically changing mechanical strain in the at least one portion produces an electrical charge usable as a power source by the microelectronic circuit. 
         [0012]    The at least one portion of the casing can comprise at least one outer wall of the casing. The at least one outer wall of the casing may comprise an inertial mass formed as part of the casing. 
         [0013]    The at least one portion of the casing may comprise a cantilever arm, and the cantilever arm may comprise an inertial mass. 
         [0014]    Electrode layers can be embedded in the piezoelectric material such that, in use, they collect the electrical charges produced by the piezoelectric material. 
         [0015]    The casing may be made of Aluminum Nitride. 
         [0016]    The microelectronic circuit may further comprise at least one of: a power management circuit; a power storage circuit; and a micro-electromechanical sensor circuit. 
         [0017]    The present invention further provides a method of manufacturing the micro-electromechanical package of the types described above. The method involves forming the MEMS package using a ceramic multi-layering technique or a polymer molding technique. 
         [0018]    The above and still further features and advantages of the present invention will become apparent upon consideration of the following definitions, descriptions and descriptive figures of specific embodiments thereof, wherein like reference numerals in the various figures are utilized to designate like components. While these descriptions go into specific details of the invention, it should be understood that variations may and do exist and would be apparent to those skilled in the art based on the descriptions herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Exemplary embodiments of an energy scavenger device according to the present invention, and methods for manufacturing the same, will now be described with reference to the accompanying drawings in which: 
           [0020]      FIG. 1  is a cross-sectional view of a micro-electromechanical package according to a first embodiment of the present invention; 
           [0021]      FIG. 2  is a cross-sectional view of a micro-electromechanical package according to a second embodiment of the present invention; 
           [0022]      FIG. 3  is a cross-sectional view of a micro-electromechanical package according to a third embodiment of the present invention; 
           [0023]      FIG. 4  is a cross-sectional view of a micro-electromechanical package according to a fourth embodiment of the present invention; 
           [0024]      FIG. 5  is a cross-sectional view of a micro-electromechanical package according to a fifth embodiment of the present invention; and 
           [0025]      FIG. 6  is a cross-sectional view of a micro-electromechanical package according to a sixth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    With reference to  FIG. 1 , an energy scavenger device in the form of a micro-electromechanical package  1  according to an exemplary embodiment of the present invention includes a casing comprising piezoelectric material  2  and a diaphragm  3  including electrodes  5  interleaved with layers of the piezoelectric material  2 . The electrodes  5  collect the electrical charge produced in the piezoelectric material  2  layers of the diaphragm  3 . 
         [0027]    The micro-electromechanical package  1  may also comprise a variety of electronic circuits  7 . These may include, for example, power management and power storage circuits as well as micro-electromechanical sensors. The micro-electromechanical package  1  of the energy scavenger device may also comprise solder terminals  8  for connecting the package to a substrate (not shown) and a lid  6  for shielding the electronic circuits  7 . 
         [0028]    With reference to the embodiment shown in  FIG. 1 , an external force F acting upon the diaphragm will create mechanical strain in the interleaved layers of piezoelectric material  2  included in the diaphragm  3 . Consequently, the piezoelectric layers generate charges which are collected by the electrodes  5  and sent to the electronic circuits  7  or, alternatively, to the substrate (not shown) via the solder terminals  8 . 
         [0029]    Thus, the energy scavenger device found in the embodiment of  FIG. 1  collects the energy related to the external force F applied directly to the micro-electromechanical package  1  itself. A possible application for which this particular embodiment of the energy scavenger device would be well suited is, for example, that of a sensor integrated into the rubber of a tire. Each time the appropriate surface of the tire hits and leaves contact with the road surface the diaphragm will either be compressed or decompressed and the piezoelectric material will be strained. 
         [0030]    With reference to the embodiment shown in  FIG. 2 , a first external force F is applied to a first diaphragm  3  and a second external force F′ is applied to a second diaphragm  3 ′. Both diaphragms  3 ;  3 ′ function in the same way as the diaphragm of the embodiment shown in  FIG. 1 . The metal bond  9  is used to hermetically join the two parts of the package. One reliable method of making such a bond, for example, would be using a metal-to-metal bonding technique. 
         [0031]    With reference now to the embodiment shown in  FIG. 3 , the micro-electromechanical package  1  according to another exemplary embodiment of the present invention comprises a casing made of piezoelectric material  2  and a diaphragm  3  comprised of electrodes  5  interleaved with layers of the piezoelectric material  2 . Again, the electrodes  5  collect the electrical charge produced in the piezoelectric material layers of the diaphragm  3 . 
         [0032]    This particular embodiment further comprises an inertial mass  4  located inside the package and formed at the center of the diaphragm  3 . Thus, when the micro-electromechanical package  1  is moved or vibrated, the inertial mass  4  is used to amplify the strain in the piezoelectric material  2  in the diaphragm  3 . In this case, the diaphragm acts as the elastic spring element of a mass-spring system. A possible application for which this particular embodiment of the energy scavenger device would be well suited is, for example, that of sensors integrated onto the wheel of a car. When the car is driving the wheel will vibrate and the casing mounted to the wheel will feel the same vibrations. The mass-spring system will vibrate and the piezoelectric material will be strained. 
         [0033]    With reference now to the embodiment shown in  FIG. 4 , the micro-electromechanical package  1  according to another exemplary embodiment of the present invention can comprise an inertial mass  4  located outside of the package and formed at the center of the diaphragm  3 . This particular embodiment of the energy scavenger device may provide advantages related to manufacturing costs and application integration. 
         [0034]    With reference now to the embodiment shown in  FIG. 5 , the micro-electromechanical package  1  according to another exemplary embodiment of the present invention may comprise a cantilever arm  11  integrally formed with one wall of the micro-electromechanical package  1 . The cantilever arm  11  also includes an inertial mass  4  formed at the free end of the cantilever arm  11 . This particular embodiment of the energy scavenger device will provide a significant advantage when no displacement or movement of any outer wall of the casing is possible (e.g., when the component is potted or encased in hard epoxy). 
         [0035]    With reference now to the embodiment shown in  FIG. 6 , the micro-electromechanical package  1  according to another exemplary embodiment of the present invention further comprises a weight  10  embedded in the inertial mass  4 . The embedded weight  10  may be made of a metal. Each of the previous embodiments which comprise an inertial mass  4  may also comprise a weight  10  embedded in the inertial mass  4 . As mentioned above, the efficiency of a vibratory energy scavenger depends much on the seismic mass. In light of the fact that most casing materials (ceramics and plastics) are light weight materials, the efficiency of an energy scavenger device will be significantly improved by inserting a high density metal during the casing manufacturing process. 
         [0036]    A method of manufacturing the micro-electromechanical package ( 1 ) as described above in relation to  FIGS. 1-6  includes forming a micro-electromechanical system (i.e., MEMS) package using a ceramic multi-layering technique or a polymer molding technique. 
         [0037]    While the invention has been described in detail with reference to specific embodiments thereof, it will be apparent to one of ordinary skill in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. Accordingly, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.