Patent Publication Number: US-2016240864-A1

Title: Method for forming a microbattery

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of French patent application number 1251467, filed on Feb. 17, 2012, which is hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relate to a method for forming a microbattery, and to a microbattery capable of being formed by said method. 
     2. Discussion of the Related Art 
     Term “microbattery” generally designates an assembly comprising, on a support substrate, a stack of thin layers forming an active battery element, and contact pads connected to electrodes of the active element. The assembly further comprises a protective coating only leaving access to the contact pads of the microbattery. The total thickness of a microbattery typically approximately ranges from a few tens to a few hundreds of μm, for a surface area ranging from a few mm2 to a few cm 2 , which enables to house the battery in very small spaces and further enables to form flexible batteries. 
     Existing methods for forming microbatteries have the disadvantage that certain manufacturing steps comprise relatively meticulous and lengthy manipulations, necessitating the use of expensive specific equipment. In particular, the bonding of the protective coating of a microbattery is a delicate operation which requires specific equipment and significantly limits microbattery production rates. 
     Further, a disadvantage of existing microbatteries is that the means for bonding the protective coating to the support substrate have a relatively low resistance to heat. There thus is a risk of separation of the protective coating when the temperature exceeds a threshold, which limits the maximum temperature at which the battery can be used. 
     SUMMARY 
     An embodiment provides a method for forming a microbattery overcoming at least some of the disadvantages of existing methods. 
     Another embodiment provides a method for forming a microbattery enabling to bond a protective coating on microbatteries more easily and faster than in existing methods. 
     Another embodiment provides a microbattery at least partly overcoming some of the disadvantages of existing microbatteries. 
     Another embodiment provides a microbattery in which the protective coating bonding means have a higher resistance to heat than in existing microbatteries. 
     Thus, an embodiment provides a method for forming a microbattery comprising, on a surface of a first substrate, one active battery element and two contact pads, this method comprising the steps of: a) forming, on a surface of a second substrate, two contact pads with a spacing compatible with the spacing of the pads of the first substrate; and b) arranging the first substrate on the second substrate so that said surfaces face each other and that the pads of the first substrate at least partially superpose to those of the second substrate, where a portion of the pads of the second substrate is not covered by the first substrate. 
     According to an embodiment, at step a), a plurality of pairs of contact pads are formed on the second substrate and, at step b), a plurality of substrates, each supporting one active element and two contact pads, are arranged on the second substrate, a subsequent step of dicing of the second substrate being provided to separate the micro-batteries from one another. 
     According to an embodiment, the method comprises a step of bonding the first substrate to the second substrate by bonding means resistant to temperatures greater than 100° C. 
     According to an embodiment, the bonding means comprise an electrically-conductive glue connecting the pads of the first substrate to the pads of the second substrate. 
     According to an embodiment, the bonding means comprise solder paste connecting the pads of the first substrate to the pads of the second substrate. 
     According to an embodiment, the bonding means comprise a non-conductive glue connecting at least some regions of the first and second substrates. 
     According to an embodiment, the bonding means comprise a molded resin layer coating the surface of the first substrate opposite to the active element. 
     According to an embodiment, contact bumps are bonded to the portions of the pads of the second substrate which are not covered by the first substrate. 
     Another embodiment provides a microbattery comprising: on a surface of a first substrate, one active battery element and two contact pads; and, superposed to the first substrate on the active element side, a second substrate comprising two contact pads with a spacing compatible with the spacing of the contact pads of the first substrate, the pads of the first substrate at least partially facing the pads of the second substrate and being connected to these pads, where a portion of the pads of the second substrates is not covered by the first substrate. 
     According to an embodiment, the first and second substrates are connected by bonding means resistant to temperatures higher than 100° C. 
     The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1C  illustrate steps of an example of a method for manufacturing microbatteries; 
         FIG. 2  is a perspective view of a microbattery obtained by the method of  FIGS. 1A to 1C ; 
         FIGS. 3A and 3B  illustrate steps of an embodiment of a method for manufacturing microbatteries; 
         FIG. 4  is a perspective view illustrating an embodiment of a microbattery capable of being obtained by the method described in relation with  FIGS. 2A and 2B ; 
         FIG. 5  is a perspective view illustrating another embodiment of a microbattery; and 
         FIGS. 6 to 10  are top views schematically illustrating examples of the shape of microbattery contact pads. 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, the various drawings are not to scale. Further, only those elements which are useful to the understanding embodiments have been shown and will be described. In particular, in the embodiments described hereafter, the structure and the method for manufacturing the active battery element used have not been detailed, embodiments being compatible with all types of active battery elements known by those skilled in the art. 
       FIGS. 1A to 1C  illustrate steps of an example of a method for manufacturing a microbattery. 
       FIG. 1A  is a top view illustrating, in its right-hand portion, the forming, on a surface of a support substrate  101 , of a plurality of cells  102  each corresponding to a microbattery, and each comprising one active battery element  103  and two metal contact pads  105   a  and  105   b  electrically connected to a positive electrode and to a negative electrode of active element  103 . To simplify the diagrams, the connections between contact pads  105   a  and  105   b  and battery element  103  have not been shown. Support substrate  101 , for example, is a substrate made of mica, glass, or any other adapted material. The thickness of substrate  101  is preferably smaller than a few hundreds of μm, for example, smaller than 200 μm. In a specific example, substrate  101  has a substantially rectangular shape of approximately 5.5 by 2.5 cm, and four cells  102  each having a substantially rectangular shape of approximately 2.5 by 1.3 cm are formed on the upper surface of substrate  101 . More generally, other substrate and cell shapes and/or dimensions may be provided. In the shown example, battery element  103  occupies an approximately central region of cell  102 , and contact pads  105   a  and  105   b  are located at two corners of the cell. 
     As illustrated in the left-hand portion of  FIG. 1A , in addition to support substrate  101  on which cells  102  are formed, it is provided to use a second substrate  107  which will be used, in subsequent steps ( FIGS. 1B and 1C ), to form a protective coating for microbatteries. Substrate  107  for example is a substrate of same nature and of same dimensions as substrate  101 , but on which no contact pad and no active battery element has been formed. 
       FIG. 1B  is a top view illustrating, in its right-hand portion, a step during which elementary cells  102  are separated from one another by dicing of support substrate  101  into elementary pieces. 
     Simultaneously, as illustrated in the left-hand portion of  FIG. 1B , substrate  107  is also diced into elementary pieces intended to be used as a protective coating for cells  102 . Each elementary piece of substrate  107  has approximately the same shape and the same dimensions as the portion of substrate  101  of a cell  102  which is not coated with contact pads  105   a  and  105   b.  As an illustration, in the specific example mentioned hereabove, each substrate piece  107  has the shape of a rectangle of approximately 2.5 by 1.3 cm with two cut-out corners. 
       FIG. 1C  is a perspective view illustrating a step subsequent to the dicing step described in relation with  FIG. 1B , during which, on each elementary cell  102 , a substrate piece  107  is arranged so that substrate  107  covers active element  103  and substrate  101  (on the active element side of the cell), thus only leaving access to contact pads  105   a  and  105   b.  To bond substrate  107  to substrate  101 , the internal surface of substrate piece  107  is coated with glue, and then adjusted against cell  102 . 
       FIG. 2  is a perspective view of a microbattery  200  obtained by the method described in relation with  FIGS. 1A to 1C . Active element  103  (not visible in  FIG. 2 ) is protected from outside elements such as humidity, air, dust, etc. by support substrate  101 , coating  107 , and the glue for bonding coating  107  to substrate  101 . Contact pads  105   a  and  105   b  allows electric contacts on the electrodes of active element  103 . 
     A disadvantage of the method described in relation with  FIGS. 1A to 1C  is that the steps of dicing of coating substrate  107 , of coating of substrate pieces  101  and  107  with glue, and of arranging of substrate pieces  107  on substrate pieces  101  are relatively delicate and imply the use of expensive equipment. Further, these steps are relatively lengthy, which decreases the microbattery production rate. 
     Another disadvantage is that the glues available to bond protective coating  107  have a relatively low resistance to heat. As an example, beyond from 60 to 70° C., the glue softens, to such an extent that coating  107  risks separating from the rest of the microbattery, thus partially or totally exposing active element  103 . This limits the temperature at which microbatteries can be used, and further forbids, on assembly of microbatteries in electronic systems, exceeding temperatures on the order of from 60 to 70° C., which is particularly constraining. 
       FIGS. 3A and 3B  are top views illustrating steps of an embodiment of a method for manufacturing microbatteries. 
     The method described in relation with  FIGS. 3A and 3B  uses elementary cells  102  of the type described in relation with  FIG. 1B , that is, comprising, on a surface of a support substrate  101 , an active battery element  103  and two contact pads  105   a  and  105   b  respectively connected to a positive electrode and to a negative electrode of active element  103 . Cells  102  may be manufactured with the method described in relation with  FIGS. 1A and 1B , or by any other method capable of forming cells of this type. The method described hereafter more specifically relates to the bonding of a protective coating on cells  102 . 
       FIG. 3A  illustrates a step of forming, on a substrate  307 , or wafer, intended to be used as a protective coating for cells  102 , a plurality of pairs of contact pads, each comprising a pad  305   a  and a pad  305   b  spaced apart by a distance compatible with the spacing of contact pads  105   a  and  105   b  of a cell  102 . Contact pads pairs  305   a,    305   b  are sufficiently spaced apart from one another to enable cells  102  to be laid flat on substrate  307 , so that their contact pads  105   a  and  105   b  respectively face contact pads  305   a  and  305   b  of substrate  307  and that cells  102  do not overlap. In this example, substrate  307  has, in top view, much greater dimensions than a cell  102 , that is, substrate  307  can receive a large number of cells  102 , for example at least  10  cells  102 . 
     In one embodiment, substrate  307  has substantially the same shape and the same dimensions as substrates conventionally used to manufacture semiconductor chips. In other words, in one embodiment, substrate  307  is compatible with equipment already existing in semiconductor chip manufacturing processes, and can be processed and manipulated by using such equipment. As an example, substrate  307  is a glass or mica substrate of generally circular shape, with a diameter of approximately 8 inches, that is, approximately 20 cm, with a thickness smaller than a few hundred μm, for example, smaller than 200 μm. Of course, substrates made of other materials and/or having other shapes and other dimensions may be used. 
     As an example, contact pads  305   a,    305   b  are made of copper or of a stack of thin metal layers having different compositions, for example, a titanium-nickel-copper-gold stack currently designated as UBM in the art, and have a total thickness on the order of a few μm, for example, ranging between 1 and 5 μm. Pads  305   a,    305   b  are for example formed by using already existing equipment for forming metal contact pads on semiconductor wafers. 
       FIG. 3B  illustrates a step during which cells  102  are arranged on substrate  307  so that the surface of support substrate  101  of the cells supporting active element  103  and contact pads  105   a  and  105   b  faces the surface of substrate  307  supporting contact pads  305   a  and  305   b.  Cells  102  are arranged so that their contact pads  105   a  and  105   b  at least partially superpose to contact pads  305   a  and  305   b  of substrate  307 , and so that a portion of pads  305   a  and  305   b  is not covered with substrate  101 . Pads  305   a,    305   b  of substrate  307  preferably have a larger surface area than pads  105   a,    105   b  of the cells. In the shown example, each pad  305   a,    305   b  comprises a triangular portion having a shape substantially corresponding to the shape of a pad  105   a,    105   b  of a cell  102  and to which the corresponding pad  105   a,    105   b  superposes, and a square or rectangular portion adjacent to the triangular portion, not covered with substrate  101  of cell  102 . Of course, the contact pads of substrate  307  and of cells  102  may have other shapes than those described hereabove. 
     As an example, cells  102  may be arranged on substrate  307  by using equipment already existing in semiconductor chip device manufacturing processes to arrange discrete chips on a substrate wafer. 
     It should already be noted that an advantage of the method described in relation with  FIGS. 3A and 3B  is that it can be implemented by using equipment already existing in conventional semiconductor chip manufacturing processes. This makes it possible to use technologies already known for semiconductor chip device manufacturing, but which were up to now not available in microbattery manufacturing processes. In particular, this enables to use, to bond cells  102  to substrate  307 , bonding means, and in particular glues, resistant to higher temperatures than glues used in methods of the type described in relation with  FIGS. 1A to 1B . It may, for example, be provided to use an electrically-conductive glue resistant to temperatures higher than from 100 to 130° C., and preferably higher than 150° C., to connect contact pads  105   a,    105   b  to contact pads  305   a,    305   b,  and a non-conductive glue, for example, based on epoxy, resistant to temperatures higher than from 100 to 130° C., and preferably higher than 150° C., to connect substrates  101  and  307 . The non-conductive glue is for example deposited at the periphery of substrate  101 , after cell  102  has been arranged on substrate  307 , and penetrates by capillarity between substrate  101  and substrate  307 . As a variation, the glue may be deposited on substrate  307  and/or on substrate  101  before cell  102  is arranged on substrate  307 , or injected between substrate  101  and substrate  307  after cell  102  has been arranged on substrate  307 . Other encapsulation and sealing techniques may be used, it being understood that it will be preferably be chosen to use bonding means at least resistant to a temperature from 100 to 130° C., and preferably higher than 150° C. 
     As a variation, rather than conductive glue, a low-temperature soldering, for example, by means of a solder paste deposited at from approximately 100 to 130° C., may be provided to connect pads  105   a,    105   b  to pads  305   a,    305   b.  It should be noted that a high-temperature soldering should not be used since this would damage the active microbattery element, since usual active battery elements generally do not resist temperatures higher than from approximately 150 to 160° C. 
     After cells  102  have been arranged on and bonded to substrate  307 , a dicing step, not shown, is provided to separate the microbatteries from one another. The dicing step is, for example, carried out by using equipment already existing in semiconductor chip manufacturing processes to dice a semiconductor wafer into discrete chips. 
     An advantage of the method described in relation with  FIGS. 3A and 3B  is that the steps of arranging and bonding of cells  102  on substrate  307  and of dicing of substrate  307  may be carried out by using existing equipment of conventional semiconductor chip device manufacturing processes. This enables, on the one hand, to decrease equipment costs, and on the other hand to significantly increase microbattery production rates. This further enables to provide microbattery inspection and testing steps, such as are already provided in semiconductor chip manufacturing. 
       FIG. 4  is a perspective view of a microbattery  400  capable of being obtained by the manufacturing method described in relation with  FIGS. 3A and 3B . Microbattery  400  comprises, on one surface of a support substrate  101 , one active battery element  103  and two contact pads  105   a,    105   b  (not visible in the drawing) and, superposed to substrate  101  (on the active element and contact pad side), a coating substrate  307  comprising two contact pads  305   a  and  305   b  respectively partially facing pads  105   a  and  105   b,  where a portion of pads  305   a,    305   b  is not covered with substrate  101 . Support substrate  101 , coating substrate  307 , and the bonding means connecting substrate  101  to substrate  307  form elements of a package protecting active element  103  from outside elements such as humidity, air, dust, etc. Conductive pads  305   a  and  305   b  allow electric contacts on the electrodes of active element  103 . 
       FIG. 5  is a perspective view of an alternative embodiment of a microbattery  500 . Microbattery  500  differs from microbattery  400  of  FIG. 4  in that it comprises, in addition to the elements already described in relation with  FIG. 4 , two contact bumps  506   a  and  506   b  respectively bonded to the portions of contact pads  305   a  and  305   b  which are not coated with substrate  101 . Further, microbattery  500  comprises a resin layer  508  coating the surface of substrate  101  opposite to active element  103 , as well as the portion of the surface of substrate  307  located on the side of active element  103  and not coated with substrate  101 . Resin layer  508  only leaves access to the upper portion of contact bumps  506   a,    506   b.  Bumps  506   a  and  506   b  ease the contact with the active battery element electrodes. Layer  508  strengthens the bonding of substrate  101  to substrate  307 , thus improving the protection of the active element. 
     As an example, to form microbattery  500 , it may first be provided, after the forming of contact pads  305   a,    305   b  on substrate  307  ( FIG. 3A ) and before the step of arranging substrates  101  on substrate  307 , to bond contact pads  506   a,    506   b  to the portion of contact pads  305   a,    305   b  which is not intended to be covered with substrates  101 . This enables bonding bumps  506   a,    506   b  at relatively high temperatures without risking damage to active battery elements  103 . 
     Then, after the step of arranging and bonding cells  102  on substrate  307  ( FIG. 3B ) and before the step of dicing substrate  307 , it may be provided to deposit a resin layer  508  having a thickness smaller than the bump height, over the entire surface of substrate  307 . A mold is, for example, positioned above substrate  307  to avoid for resin to deposit on the upper portion of bumps  506   a,    506   b,  and resin  508  is then injected between the mold and substrate  307 . The thickness of resin  508  coating substrates  101  may be on the order of a few tens of μm, for example, between 20 and 60 μm. 
     A step of dicing resin layer  508  and substrate  307  is then provided, to separate microbatteries  500  from one another. 
       FIGS. 6 to 10  are top views schematically illustrating, as an example, various shapes that may be taken by the contact pads formed, at the step described in relation with  FIG. 3A , on substrate  307  intended to be used as a protective coating for cells  102 . In the drawings, the left-hand portion of the pad corresponds to the portion of the pad on which, at the step described in relation with  FIG. 3B , a corresponding pad  105   a,    105   b  of a cell  102  superposes, and the right-hand portion of the pad corresponds to the pad portion which is not covered with substrate  101  of a cell  102 . In these examples, a rectilinear track portion connects the left-hand portion of the contact pad to its right-hand portion. 
       FIG. 6  shows a pad  605  having approximately circular left-hand and right-hand portions. 
       FIG. 7  shows a pad  705  having a substantially square or rectangular left-hand portion and having an approximately circular right-hand portion. 
       FIG. 8  shows a pad  805  having a substantially triangular left-hand portion and having an approximately circular right-hand portion. 
       FIG. 9  shows a pad  905  having a substantially circular left-hand portion and having an approximately square or rectangular right-hand portion. 
       FIG. 10  shows a pad  1005  having a substantially circular left-hand portion and having an approximately triangular right-hand portion. 
     Contact pads  105   a,    105   b  of cells  102  may be adapted to the corresponding shapes of the left-hand portions of pads  605 ,  705 ,  805 ,  905 , and  1005 . Of course, the contact pads of substrate  307  and of cells  102  may have other shapes than those described hereabove. 
     Specific embodiments have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. In particular, microbatteries each comprising a single active battery element  103  have been described hereabove. Embodimenets are not limited to this specific case and it may be provided to form microbatteries each comprising several interconnected active elements, where these elements may be juxtaposed or superposed, and covered with a same protective coating. 
     Further, embodiments are not limited to the substrate and cell shapes, dimensions, and materials mentioned as an example in the present description. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.