Abstract:
The invention concerns a method for connecting first contact studs of a structure bearing electrodes for measuring or for stimulating a physiological activity with second studs of at least a downstream circuit, each second stud being traversed by an opening perforating the downstream circuit. The method includes the following steps: a) placing the downstream circuit on the structure, so that the opening of a second stud is located opposite a first stud; and b) depositing in the opening of the second stud a conductive material providing the connection between the first second stud opposite.

Description:
FIELD OF THE INVENTION 
   The present invention relates to structures supporting electrodes intended to measure the electric activity of an organ or to stimulate it, and in particular to the connection of these structures to a circuit. 
   BACKGROUND OF THE INVENTION 
   In  FIG. 1 , a structure  1  with electrodes is intended to be connected to a downstream circuit  2 . Structure  1  comprises an insulating flexible support film  3 . On film  3  are deposited electrodes  4  connected by conductive tracks  5  to connection pads  6 . The electrodes, the pads, and the tracks connecting them are formed by deposition and etching of a conductive layer on film  3 . A thin insulating layer, not shown, covers the structure, except for the electrodes and the pads. Structure  1  is a flexible structure of small thickness (from a few micrometers to a few tens of micrometers), the electrodes of which are intended to be placed to contact an organ such as a nerve or the skin to measure the electric activity of the organ or to stimulate it. 
   Structure  1  must be connected to circuit  2 . Circuit  2  comprises pads  7 , the arrangement of which corresponds to that of pads  6 . Each pad  7  is connected to a conductive track  8  for the conduction of the signal provided to or by electrodes  4 . To connect a pad  6  to a pad  7 , a hole  10  thoroughly crossing the structure is made in each of pads  6 . 
   In  FIG. 2 , downstream circuit  2  is partially shown and a single contact pad  7  is visible. Structure  1  is here shown with support film  3 , a single pad  6 , and an upper insulating layer  11 . The thickness of circuit  2 , on the order of from one to two millimeters, is significant as compared to that of structure  1 , which is at most a few tens of micrometers. Structure  1  is placed on circuit  2  so that each of holes  10  is above a pad  7 . The surface of each pad  7  is greater than that of a hole  10 . A welding drop  12  is then deposited, by a conventional bonding technique well known in microelectronics, which fills hole  10  and overflows on pad  6 . Welding drop  12  ensures the electric contact between pads  6  and  7 . 
   This way of doing has several disadvantages. 
   The electrode-supporting structure must be drilled into in the middle of each pad  6 . This operation is delicate, due to the fragility of the structure, and this assumes that pads  6  are large enough, which limits their number. The drilling of pads  6  is generally performed by means of a laser beam, which results in having the metal layer forming the pad partially split, so that the pad no longer exhibits a uniform surface. The drilling of pads  0 . 6  may also be carried out by etching, which requires an additional mask, which brings about a cost problem, and greatly increases the pad drilling time. 
   Further, the impedance exhibited by this type of contact is relatively high. Indeed, the surface of pad  6  enabling flowing of an electric signal between pads  6  and  7  is a substantially ring-shaped surface  13  delimited by the base of welding drop  12  and the circumference of hole  10 . The surface of pad  7  enabling flowing of the electric signal is a substantially circular surface  14  equal to the surface area of hole  10 . Surfaces  13  and  14  have been shown in bold lines in  FIG. 2 . To avoid too much decreasing the impedance of the contact between pads  6  and  7 , surfaces areas  13  and  14  must be substantially equal. As a result, at most, the surface area enabling flowing of an electric current between pads  6  and  7  is equal to half the surface area of pad  6 , which makes the impedance of the contact between pads  6  and  7  relatively high. 
   Further, welding  12  forms a brittle dome of relatively great height, typically on the order of 50 micrometers. This may hinder the circuit encapsulation. 
   German patent application DE 19530353 A1 generally describes a method for connecting contact areas of a flexible film to a printed circuit. In this document, the printed circuit has pads drilled with metallized holes, the metallization extending somewhat over the rear surface of the integrated circuit which does not support the pads. The portions of the film and of the circuit to be connected are opposite and a filler metal is deposited between them. Heat is provided to the rear circuit surface, while a relatively high pressure is applied on the assembly formed by the film and the circuit. The provided heat melts the filler metal, which forms a relatively thick gluing layer between the film and the circuit. The excess filler metal is absorbed by the metallized hole. 
   The abstract of Japanese patent JP 09312453 describes a technique similar to that described in application DE 19530353. 
   The techniques described in the two above documents require application of a high temperature and pressure. When such techniques are desired to be applied to connect a structure support electrodes intended to measure the electric activity of an organ or to stimulate it, several problems arise. Indeed, electrode structures are very thin and very brittle. The application of a pressure, even mild, crushes the structure and may damage it. Further, the application of a high temperature, necessary to melt the filler metal, may destroy the structure. It will be seen hereafter that the overthickness created by the welding layer may also be a disadvantage. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to form a connection between an electrode structure intended to measure or stimulate an activity of physiological origin and a circuit, without for a high pressure to be exerted between the parts to be connected. 
   Another object of the present invention is to form a connection between an electrode structure intended to measure or to stimulate an activity of physiological origin and a circuit, without for a high temperature to be applied to one of the parts to be connected. 
   Another object of the present invention is to form an easy connection between an electrode structure intended to measure or to stimulate an activity of physiological origin and a circuit. 
   Another object of the present invention is to form a connection enabling optimization of the contact impedance between an electrode structure intended to measure or to stimulate an activity of physiological origin and a circuit. 
   Another object of the present invention is to form a substantially planar connection between an electrode structure intended to measure or to stimulate an activity of physiological origin and a circuit. 
   Another object of the present invention is to form a connection between an electrode structure intended to measure or to stimulate an activity of physiological origin and a circuit enabling optimal use of the surface area of the structure and/or of the circuit. 
   To achieve these and other objects, the present invention provides a method for connecting first pads of an electrode-supporting structure capable of measuring or of stimulating an activity of physiological origin to second pads of at least one downstream circuit, each second pad being run through by an opening perforating the downstream circuit, comprising the steps of: 
   a) placing the downstream circuit on said structure, so that the opening of the second pad is placed opposite to a first pad; and 
   b) depositing in the opening of the second pad a conductive material ensuring the connection between the second pad and the first opposite pad. 
   According to an embodiment of the present invention, the opening made in the second pad has a surface area substantially equal to the surface area of the first opposite pad. 
   According to an embodiment of the present invention, the portion of said structure comprising the first pads is cut to form blade terminals, and the connection of the second pads to the first pads is such that the first pads of two adjacent blade terminals at least are connected to second pads of different downstream circuits, arranged substantially one above another. 
   According to an embodiment of the present invention, the opening perforating the downstream circuit is a metallized hole. 
   According to an embodiment of the present invention, the connection of the first pads to the second pads is made by means of a conductor glue, of a conductor paste, or of a welding. 
   According to an embodiment of the present invention, the second pad has a thickness on the order of from 20 to 50 micrometers and the first pad has a thickness equal to at most a few micrometers. 
   The present invention also provides a structure supporting electrodes capable of measuring or of stimulating an activity of physiological origin exhibiting first pads likely to be connected to second pads of downstream circuits by a method according to the present invention. The portion of said structure comprising the first pads is cut to form blade terminals each supporting first pads. 
   The present invention also provides a structure supporting electrodes capable of measuring or of stimulating an activity of physiological origin exhibiting first pads that can be connected to second pads of at least one downstream circuit by a method according to the present invention, the first pads being connected to the electrodes by conductive tracks. Said conductive tracks are arranged on at least two superposed levels separated by insulating layers, and at least one conductive track runs under a first pad. 
   The present invention also provides an assembly formed of a structure supporting electrodes capable of measuring or of stimulating an activity of physiological origin and a downstream circuit, wherein the electrode-supporting structure and the downstream circuit are connected by an above method. 
   In an embodiment of the present invention, the portion of the electrode-supporting structure that comprises pads and at least the portion of the downstream circuit that is connected to said structure are covered with a biocompatible sheath. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, in which: 
       FIG. 1 , previously described, shows an electrode structure and a downstream circuit; 
       FIG. 2  shows a known type of connection between an electrode structure and a downstream circuit; 
       FIG. 3A  shows the connection of an electrode structure and of a downstream circuit according to a first embodiment of the present invention; 
       FIG. 3B  shows the connection of an electrode structure and of a downstream circuit according to a second embodiment of the present invention; 
       FIGS. 4A and 4B  respectively show a novel electrode structure and its mode of connection to a downstream circuit according to a third embodiment of the present invention; 
       FIG. 5  shows a novel electrode structure illustrating an advantage of the connection according to the present invention; and 
       FIGS. 6 and 7  show examples of application of a connection according to the present invention. 
     In the drawings, same reference numerals represent same elements. The scales have not been respected, especially regarding thicknesses. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3A  shows a first embodiment of the present invention. The end of an electrode structure  1  of the above-mentioned type comprises an insulating support film  3 , a pad  16 , and an upper insulating layer  11 . A circuit  2  comprising a pad  17  to be connected to pad  16  is here placed above structure  1 . Circuit  2  comprises an opening  15  which completely crosses it, substantially located at the center of pad  17 . In the example shown, opening  15  has a size substantially equal to that of pad  16 . Opening  15  is filled with a material  18  ensuring the electric connection of pads  16  and  17 . Material  18  is preferably a conductive paste or glue, but it may also be a welding. For example, a drop of conductive glue is deposited in opening  15  and set by means of ultraviolet rays. Material  18  fills opening  15  and has a substantially planar surface which very slightly extends beyond the surface of circuit  2 . 
   According to the present invention, circuit  2  is placed above structure  1 , rather than the opposite as in  FIG. 2 . This has many advantages. 
   First, in the present invention, the drilling is performed in circuit  2  and not in structure  1 . Now, circuit  2  is generally much thicker than structure  1  (typically a hundred times as thick) and it is much easier to regularly drill into circuit  2  than structure  1 . Further, the conductive layer forming pads  17  often is much thicker than the conductive layer forming pads  16 , and it is accordingly much less brittle (typically, the thickness of a pad  17  is on the order of from 20 to 50 micrometers while the thickness of a pad  16  is of a few micrometers at most, or even under one micrometer). The holes of circuit  2  may be formed by means of various techniques, like by means of conventional mechanical drilling techniques, and they are more regular. The holes of circuit  2  may also be formed before deposition of the conductive layer forming pads  17 . This enables, for example, using a laser beam drilling without risking to damage pads  17 . 
   Further, the fact that the thickness of pad  17  is generally much greater than that of pad  16  plays a role for the connection impedance. Indeed, the surface area of cylindrical crown  19  (in bold lines in  FIG. 3A ) corresponding, in opening  15 , to the thickness of pad  17 , is significant and non-negligibly takes part in the flowing of the electric current. As an example, it is assumed that pads  16  and  17  have a same radius R and that thickness e of pad  17  is also equal to R. If an opening  15  that represents 75% of the surface area of pad  17  is selected, the surface area of pad  16  taking part in the current conduction is 0.75.πR 2 . Cylindrical surface  19  represents, as for itself, approximately 1.7.πRe, and thus 1.7.πR 2 , that is, twice as much. Since pads  16  and  17  generally have a small radius (typically some ten micrometers), thickness e is generally greater than the pad radius and, as concerns pad  17 , it is the thickness which mainly takes part in the current flowing. As a result, connection material  18  needs not widely extend over pad  17  and pad  17  needs not have a large surface area, which optimizes the used surface area. Further, if need be, material  18  may significantly overflow over the conductive layer which, in circuit  2 , leads to pad  17 . Also, pad  17  may, if desired, be simply formed by a portion of a conductive track of circuit  2 , pierced by an opening  15 . 
   Further, in the present invention, connection material  18  fills the hole defined by opening  15 , which generally is a deep hole, conversely to hole  10  of  FIG. 2 . As a result, a sufficient quantity of conductive material is systematically used and forms strong connections, without forming a significant dome as in  FIG. 2 . It should further be noted that it is possible to scrape material  18  from the surface of circuit  2  and obtain a practically planar surface of circuit  2 . 
   As compared to the connections described in documents DE 19530353 and JP 09312453, the connection of  FIG. 3A  has required no application of an excessive pressure or heat. Indeed, structure  1  and circuit  2  are simply placed against each other during the connection and maintained in place with no excessive pressure during the connection. If need be, structure  1  may be glued to circuit  2  by means of a thin insulating layer of insulating glue which does not cover pads  16 . 
     FIG. 3B  shows a second embodiment of the present invention. In  FIG. 3B , a conductive track  20  of circuit  2  ends on opening  15  of pad  17 . The hole defined by opening  15  is metallized. A conductive layer  22  covers the walls of opening  15 . Layer  22  may, as shown, slightly overflow over the surface of circuit  2 , but this is not necessary. Pad  17  is thus defined by conductive layer  22 , connected to track  20 . In this embodiment, the surface area of pad  17  taking part in the electric current conduction is very large. Connection material  18  needs not fill the entire opening  15  to ensure a good mechanical strength and a good conduction of the connection. The surface of circuit  2  remains planar. This embodiment enables particularly significant decrease in the impedance of the formed connection and pad  17  takes up no more space than opposite pad  16 . This is a significant advantage, especially when many pads  16  are arranged on a reduced surface of structure  1 . 
     FIGS. 4A and 4B  illustrate a third embodiment of the present invention, which enables a great versatility, and forming pads  17  of large size. 
     FIG. 4A  shows a novel structure of electrodes  1 ′. In  FIG. 4A , the end of structure  1 ′ comprises eight pads  16 - i , with i ranging from 1 to 8. The end of the structure is cut by three longitudinal openings  24  separating pads  16 - i  in groups of two. Openings  24  thus divide the end of structure  1 ′ into four longitudinal blade terminals A, B, C, D, arranged in this order and comprising two pads each. 
     FIG. 4B  illustrates the way of connecting structure  1 ′. Structure  1 ′ is connected to two downstream circuits  2   a  and  2   b . Non-adjacent blade terminals A and C are connected to circuit  2   a  in the way described in relation with  FIG. 3A  or  3 B. Similarly, non-adjacent blade terminals B and D are connected to circuit  2   b  in the way described in relation with  FIG. 3A  or  3 B. 
   Circuits  2   a  and  2   b  are arranged one above the other. The insulating support film of structure  1  may by itself ensure the electric insulation between circuits  2   a  and  2   b , or an additional insulator such as an insulating sheet will separate circuits  2   a  and  2   b.    
   By so operating, a stacking of circuits  2   a  and  2   b  is formed. Pads  17  of each of circuits  2   a  or  2   b  may have a surface area which is double that of pads  16 - i  and extend over the width of two blade terminals A, B, C, D. Thus, for example, pad  17  connected to pad  16 - 1  of blade terminal A may take up a surface area corresponding to pad  16 - 1  and to pad  16 - 3  of blade terminal B. It will easily exhibit an opening  15  of same surface area as pad  16 - 1 . 
   This connection mode is advantageous. For example, the pads of structure  1 ′ may be twice as small and thus twice as many as in prior art, or the width of structure  1 ′ may be twice as small. The obtained assembly forms a very versatile compact assembly, practical to arrange. “Circuits  2   a  and  2   b ” is used to designate either two separate downstream circuits, coupled or not, or merely two superposed elements of a multiple-stage three-dimensional connector, associated with a single downstream circuit. 
   Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily appear to those skilled in the art. In particular, the electrode-supporting structure has been described as having an elongated shape, with electrodes at one end and pads at the other end. However, the structure may have any shape, for example, a circular shape, and the electrodes and pads of the structure may be arranged in any part of the structure. Also, the pads may be in any number in the structure, for example, as many as several hundreds. 
   It should be noted that the connection method of the present invention applies for a diversity of thicknesses of the electrode-supporting structure and of circuit  2 . 
   It should also be noted that pads  17  may have any shape. 
   Also, in the connection mode illustrated in relation with  FIGS. 4A and 4B , the structure may be cut into a number of blade terminals different from four and the number of downstream circuits or of superposed elements of a connector of the down-stream circuit may be greater than two. Also, even though each of the blade terminals of  FIG. 4A  is shown with a single row of pads, the blade terminals may comprise several rows of pads, for example, two. Also, it is not necessary for all adjacent blade terminals to be connected to different downstream circuits. For example, blade terminals A and D of  FIG. 4B  may be connected to circuit  2   a  and blade terminals B and C to circuit  2   b , providing the same advantages as those mentioned in relation with  FIG. 4B . 
   It should also be noted that other advantages of the connection method according to the present invention will occur to those skilled in the art. For example, in the case where the structure comprises not one electrode layer, but several, the present invention has a significant advantage. 
     FIG. 5  shows a structure  1 ″ with two electrode layers. Support film  3  of structure  1 ″ is covered with a first conductive layer  30 . Layer  30  is etched to form, at one end of the structure, a pad  16   a  connected by a conductive track  31  to an electrode not shown. On layer  30  is an insulating layer  32 . On layer  32  is arranged a second conductive layer  34 . Layer  34  is etched to form a pad  16   b , connected by a conductive track  35  to an electrode not shown. Layer  34  is topped with an insulating layer  36 . Layers  32  and  36  are properly etched to expose pads  16   a  and  16   b . Pad  16   a  and track  31  are located at a level lower than pad  16   b  and track  35 . In prior art, as described in relation with  FIG. 2 , since pads  16   a  and  16   b  must be drilled into with perforating holes, layer  31  cannot be located under pad  16   b , unless track  31  follows a complicated path and a complex mask is used to etch it. The surface area required to form track  31  may then be relatively large. In the present invention, track  31  may run under pad  16   b  and be rectilinear. The structure surface is better used and the mask used for the etching of layer  31  is simpler. 
   Finally, the connection according to the present invention may have various applications. For example, as illustrated in  FIG. 6 , the electrode structure may be connected to another flexible structure, and not to a rigid downstream circuit. 
   In  FIG. 6 , an electrode structure  1  having its layer  38  supporting electrodes and connection pads shown in bold lines, is connected to a flexible film  40 , playing the role of a downstream circuit. Film  40  comprises an insulating base  41  and, at its upper surface, a conductive layer  42  in which are formed the pads to be connected to the pads of layer  38 . The connection between structure  1  and pad  40  is formed by means of the connection method according to the present invention, and the pads of structure  1  and of film  40  are not shown for simplicity. A biocompatible sheath  46  surrounds film  40 , or at least the portion of film  40  intended to contact one or several organs, and the portion of structure  1  comprising the pads. 
   The example of  FIG. 6  is particularly advantageous. Indeed, structure  1 , to be placed at the contact of an organ, is biocompatible and its selfcost is high. Further, the manufacturing of structure  1  is performed by deposition of layers on a mother wafer, and it is advantageous to simultaneously form the largest possible number thereof. Thus, it is advantageous to form relatively short structures  1  (typically, on the order of 2 centimeters). Now, in certain applications, the organ to be tested or stimulated is at a non-negligible depth of the body surface. For example, in surgery of the base of the skull (retrosigmoid approach), the acoustic nerve is located at a 5-centimeter depth and a short electrode structure does not reach it. In  FIG. 6 , the assembly formed by structure  1  and film  40  may be relatively long, for example, it may reach 20 cm, and biocompatible sheath  46  enables introduction of the assembly to the desired depth. Further, the assembly of  FIG. 6  is relatively inexpensive. Indeed, the materials covered by the biocompatible sheath need not be biocompatible and have a lesser cost. 
   In the example of  FIG. 6 , it is advantageous to keep the thickness of the assembly formed by structure  1  and film  40  as small as possible. With the connection method according to the present invention, structure  1  and film  40  may be arranged directly against each other, with no gluing layer in between, the pads of structure  1  and of film  40  being on opposite surfaces. This is an advantage with respect to previously-mentioned prior art documents DE 19530353 and JP 09312453. Indeed, in these two documents, the pads to be interconnected face one another, conversely to the present invention, and a welding layer forming a relatively thick gluing layer is present between the thin structure and the thick structure. In the present invention, if for example a structure from 1 to 3 microns and a 20-micron film  40  are used, the assembly of structure  1  and of film  40  has a 23-micron thickness. The addition of a bonding layer, as in documents DE 19530353 and JP 09312453, between structure  1  and film  40 , would considerably increase the thickness of the assembly formed by the structure and the film, which can make it inoperative in certain applications. Further, the presence of a rigid and brittle gluing layer may be a disadvantage (lack of flexibility, risk of connection breakage). Moreover, the fact that, in the present invention, the surface of the structure comprising the pads faces the surface of the downstream circuit devoid of pads leaves the surface of the downstream circuit comprising the pads free. This enables, for example, the downstream circuit to comprise many pads and many connection tracks without risking for these to form undesired contacts with the structure pads and/or tracks. 
     FIG. 7  illustrates another example of application of the connection method according to the present invention. In  FIG. 7 , an electrode structure  1 , having a layer  38  supporting electrodes and connection pads, is connected to one end of a rigid element  50  supporting, on its upper surface  52 , connection pads and metal tracks. The other end of rigid element  50  is connected to a flexible film of small thickness  56 , which supports connection pads on its upper surface  58 . The connections between rigid element  50  and, respectively, structure  1  and film  56 , are formed according to the method of the present invention. Rigid element  50  may be relatively short, for example, 5 mm. A biocompatible sheath  60  surrounds film  56 , rigid element  50  and the portion of structure  1  supporting the pads. As in  FIG. 6 , sheath  60  enables connecting structure  1  to a relatively long flexible film, the assembly being biocompatible and relatively inexpensive. Element  50  may have various functions. For example, the surgeon may seize it by means of pliers to more easily introduce the structure. Element  50  may also be used, after the placing of the structure, to attach the assembly to the operating theatre napkin. Element  50  needs not be made of a biocompatible material. It may have various thicknesses, for example, on the order of 50 micrometers. Further, the masks of manufacturing of element  50  need not be as accurate as those used in the manufacturing of structure  1  and they are accordingly less expensive. 
   It has already been signaled that no high pressure needs be exerted for the connection according to the present invention of an electrode structure to a downstream circuit. This is particularly advantageous in certain cases, for example, where the electrode structure exhibits protruding elements at the pads, for example, elements having a 20-micron thickness or more, made of a relatively soft insulating material, which would crush upon application of a high voltage. 
   Finally, it should be noted that the electrode structures described in relation with  FIGS. 4A and 5  may also be connected by any other method without departing from the scope of the present invention. Also, an electrode structure resulting from a combination of the structures of  FIGS. 4A and 5 , for example, a structure in which one or several blade terminals comprise pads connected to superposed tracks, is part of the present invention.