Abstract:
A capacitive microsensor formed on a wafer, including a conductive detection area arranged on a first surface or front surface of the wafer; a conductive via crossing the wafer and emerging on said area; and a structure to ensure contact with said via on the second surface or rear surface of the wafer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a static capacitive microsensor especially enabling determination of a distance variation between the microsensor and a charged surface having a relative motion with respect to the microsensor. 
   The present invention especially applies to a fingerprint detector. 
   2. Discussion of the Related Art 
     FIG. 1  shows a conventional capacitive microsensor for detection of fingerprints. This microsensor includes a set of detection sites  1  arranged in one or several rows. The microsensor is formed on an insulating wafer  2 , for example made of glass or silica or a flexible material. On wafer  2  are formed conductive regions  3 , end areas  4  of which are placed in front of the detection sites. Conductive regions  3  are coated with an insulating layer  5  above which is formed a conductive layer  6  including windows in front of the detection sites. Conductive layer  4  is coated with an insulating layer  7  coated with a conductive layer  8  also including windows at the level of each detection site. The substrate is crossed by conductive vias. First vias  11  contact regions  3 , a second via  12  contacts layer  6 , and a third via  13  contacts layer  8 . Each of the vias ends with a connection pad, respectively  14 ,  15 ,  16 . It should be understood that there exists one detection area  4  for each window  1 , while conductive layer  6  and conductive layer  8  are common to the entire structure. For practical reasons, the lower surface of substrate  2  is coated with an insulating layer  18 , this insulating layer being possibly coated with a conductive shielding layer (not shown) open at the pad locations. 
   Regions  3  are connected to a detection circuit by pads  14 . Layer  6  is connected to ground by pad  15 . Layer  8  is connected to a high-frequency generator by pad  16 . Thus, when an isolating surface, for example, a finger, is moved with respect to microsensor row  1 , this surface is charged by the high-frequency signal and the capacitance between the surface portion passing above a detection area and this detection area varies according to the distance between the surface portion passing above the detection area (hollow or relief of a fingerprint) and this area. The successive relieves of a fingerprint may thus be detected at the location of each detection area. Various means are known in the art to perform the measurements. Some of these means are described in European patent application 1058531 of Idex AS Company, which is incorporated herein by reference. 
   SUMMARY OF THE INVENTION 
   The present invention aims at improving the sensitivity of this type of capacitive micro sensor. 
   Another object of the invention is to provide a capacitive microsensor that can be integrated with other electronic components. 
   To achieve these and other objects, the present invention provides a capacitive microsensor formed on a wafer, including a conductive detection area arranged on a first surface or front surface of the wafer; a conductive via crossing the wafer and emerging on said area; and a means of contact recovery with said via on the second surface or rear surface of the wafer, wherein the substrate is made of silicon, the walls of the via and the surfaces of the substrate are oxidized. 
   The present invention also provides a fingerprint detector including a set of microsensors such as mentioned above. 
   The foregoing objects, features and advantages of the present invention, as well as others 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 
       FIG. 1 , previously described, is a partial perspective view of an example of a microsensor according to prior art; and 
       FIGS. 2 to 4  show various embodiments of a microsensor according to the present invention. 
   

   DETAILED DESCRIPTION 
   As is conventional in the representation of microcomponents, the various drawings are not drawn to scale. Further, those portions common to the structures of the various drawings have been designated with the same references. 
   The present invention is based on an analysis of the operation of the structure of  FIG. 1 . The microsensor sensitivity appears to depend on the distance between the surface to be sensed and the top of detection area  4 . This distance is set by the thicknesses of insulating layers  5  and  7 . Excessively reducing these thicknesses makes the structure too fragile or impossible to form. Thus, the present inventor has analyzed in further detail the structure and especially the function of the grounded shielding layer  6 . The main function of layer  6  is to avoid any interference between the high frequencies on upper electrode  8  and detection electrode  4 , and especially conductor  3  connected to this detection electrode. 
   Rather than modifying the structure of shielding electrode  6 , the present invention provides modifying the connection mode of detection area  4  to avoid the need for the shielding electrode. 
     FIG. 2  illustrates a first embodiment of the present invention. Substrate  2  is a doped, and thus conductive, silicon substrate  19 , and is grounded. The via intended to contact detection area  4  is arranged directly under this area. Before forming the conductive via layer in the opening arranged under detection area  4 , the entire structure is oxidized to insulate the silicon  19 . The via is designated with reference  21 . If the contact pad has to be displaced on the lower surface, a conductive layer  22 , extending to reach a connection pad  23 , is provided on the lower surface side. The electrode receiving a high frequency, or biasing metallization, designated with reference  31 , is etched in the same conductive layer as detection area  4 . An insulating layer  32  covers at least the detection area  4 . Due to the large thickness of wafer  19 , for example, on the order of from 0.3 to 0.5 mm, as compared to the thickness of an insulating layer, the coupling between upper electrode  31  and connection  22  is negligible. Shielding electrode  6  can thus be omitted. Thus, there is a single insulating layer thickness,  20 , between the substrate and upper electrode  31 , this layer having the function of insulating area  4  from the surface, for example, a finger, to be analyzed. The substrate grounding avoids any parasitic coupling between upper electrode  31  and the other conductive layers of the structure. 
     FIG. 3  shows a second embodiment of the present invention. In this embodiment, biasing metallization  31  is eliminated. The biasing of the surface to be analyzed is performed by a contact with a portion of the surface to be analyzed external to the sensor area. For example, if the fingerprint of one finger is desired to be analyzed, a high-frequency voltage will be applied to another finger. A conductive shielding layer  35 , which is preferably grounded, has also been shown in  FIG. 3 , on the rear surface side. 
     FIG. 4  shows a third embodiment of the present invention formed from a silicon substrate  19  of a first conductivity type, for example, P. This substrate is first pierced with openings located at the locations of the vias to be formed. A doping of opposite type is formed by diffusion in the vicinity of the openings, doped area  41  extending beyond detection areas  4 , to partially extend under a biasing metallization  31 . As in the embodiment of  FIG. 2 , high-frequency biasing metallization  31  and detection areas  4  are formed from a same conductive layer. (As in  FIG. 2 , the via of contact with metallization  31  is not shown). The upper and lower substrate surfaces and the via walls are coated with a thin insulating layer  20 , for example by thermal oxidation to insulate the various conductive layers of the substrate. Insulating layer  20  is opened, for example, in a ring, in front of a portion of the lateral extension of N-type region  41 , so that metallization  31  is in contact with this region. Metallization  31  is, as previously, connected to a high-frequency generator, but the voltage which is applied thereto also includes a positive D.C. bias voltage to reversibly bias NP junction  41 - 19 . Substrate  19  being grounded, an integrated protection against possible overvoltages is thus obtained. On the other hand, the parasitic coupling between the conductor of via  21  and the substrate is avoided due to the positive biasing of the N region. This biasing may, for example, be on the order of from 2 to 3 volts. 
   Those skilled in the art will be able to implement the various embodiments of the present invention by using the various materials currently used in semiconductor manufacturing. The biasing and detection conductive layers will, for example, be doped polysilicon layers, or layers made of a metal such as aluminum or another known metal for forming metallizations. The insulating layers on the upper surface side will, for example, be silicon oxide and/or nitride layers or layers made of another conventional insulator. The insulating layers on the lower surface side will, for example, be silicon oxide layers or polymer layers. The layer thicknesses will, for example, be on the order of from 0.1 to 0.2 μm for insulating layers and on the order of from 0.1 to 2 μm for conductive layers. 
   The present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. Various means other than vias may for example be used to ensure a contact with the conductive biasing layer. The features illustrated in the various drawings may be combined, for example, shielding layer  35  shown in  FIG. 3  on the lower surface side may be used in the various embodiments. Additionally, various active or passive electronic components can be implemented in or above the silicon substrate  19 . Also, the substrate could be of the SOI type. 
   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.