Patent Publication Number: US-7709916-B2

Title: Optical semiconductor device having photosensitive diodes and process for fabricating such a device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based upon and claims priority from prior French Patent Application No. 05 00408, filed on Jan. 14, 2005, the entire disclosure of which is herein incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of optical semiconductor devices, and more particularly to an optical semiconductor device having a structure of photosensitive diodes. 
   2. Description of the Related Art 
   U.S. Pat. No. 6,455,836 discloses a photosensitive semiconductor device that comprises, on top of a base layer, an intermediate layer made of a photosensitive material, projecting lower electrodes in this layer and an upper layer that includes an upper electrode. A lateral well passes through the intermediate layer and the upper layer. The base layer includes connection means that are connected selectively to the lower electrodes and comprise connection pads placed in the bottom of the aforementioned well. A local layer made of an electrically-conductive non-transparent material covers some of the diodes in order to form an optical barrier and extends along the sidewalls and in the bottom of the well so as to electrically connect the upper electrode to the electrical connection pads. Such an arrangement is not suitable for implementation on standard fabrication machines, especially planarization machines, and requires a tricky final operation to produce the local layer. 
   Accordingly, there exists a need for overcoming the disadvantages of the prior art as discussed above. 
   SUMMARY OF THE INVENTION 
   An embodiment of the present invention provides an optical semiconductor device comprising photosensitive diodes of simplified structure and simplified fabrication. 
   The first subject of the present invention is a semiconductor device comprising, in a zone, a structure of photosensitive diodes comprising a matrix of lower electrodes, an intermediate layer made of a photosensitive material formed on the matrix of lower electrodes, and at least one upper electrode formed on the intermediate layer. 
   According to the invention, the device comprises, beneath the intermediate layer, electrical connection means that include at least one electrical contact pad and at least one electrical connection pad. 
   According to the invention, the device comprises at least one electrical connection via passing through at least the intermediate layer and connecting the upper electrode to the electrical contact pad. 
   According to the invention, the device has at least one well formed outside the zone and passing through at least the intermediate layer, this well exposing the electrical connection pad. 
   According to the invention, the device preferably comprises a lower dielectric layer in which the matrix of lower electrodes and the at least one electrical contact pad are formed and, beneath this lower layer, a base layer in which the at least one electrical connection pad is formed, the at least one well passing through this lower layer in order to expose this electrical connection pad. 
   According to the invention, the device preferably includes an upper layer comprising a first layer constituting the second electrode and a second layer made of an electrically conductive material, the at least one via and the at least one well passing through this upper layer. 
   According to the invention, the device preferably includes a non-transparent local layer that partly covers the structure of photosensitive diodes. 
   According to the invention, the local layer is preferably made of an electrically conductive material, the at least one via being connected to this local layer. 
   According to the invention, the device preferably includes at least one transparent outer protective layer. 
   Another subject of the present invention is a process for fabricating an optical semiconductor device comprising a multiplicity of photosensitive diodes comprising lower electrodes and at least one upper electrode that are separated by an intermediate layer made of a photosensitive material. 
   According to the invention, this process consists of: producing a base layer that includes electrical connection means; forming a dielectric lower layer above the base layer; forming, in the lower layer, a matrix of lower electrodes that are selectively connected to the electrical connection means; producing an intermediate layer made of a photosensitive material on the lower layer; forming an upper layer made of an electrically conductive transparent material; forming at least one via made of an electrically conductive material through the intermediate and upper layers so as to electrically connect the upper layer and at least one of the lower electrodes; and producing at least one well through the lower, intermediate and upper layers so as to expose at least one electrical connection pad of the electrical connection means, the at least one electrical connection pad being connected to the at least one via. 
   According to the invention, the at least one via is preferably produced above the corresponding lower electrode. 
   According to the invention, the process preferably consists in forming an upper layer comprising a first layer made of a first electrically conductive material and a second layer made of a second electrically conductive material. 
   According to the invention, the process preferably consists in forming a local layer made of a non-transparent material above the upper layer and extending partly above the multiplicity of photosensitive diodes. 
   According to the invention, the process preferably consists in forming a protective outer layer made of a transparent material. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from studying the optical semiconductor devices having photosensitive diodes that are described by way of non-limiting examples and illustrated schematically by the drawing in which: 
       FIG. 1  shows a sectional view of an optical semiconductor device according to the invention; 
       FIG. 2  shows a top view of the device of  FIG. 1 ; 
       FIG. 3  shows a first step in the fabrication of the device of  FIG. 1 , in a sectional view; 
       FIG. 4  shows a top view of  FIG. 3 ; 
       FIG. 5  shows a second step on the fabrication of the device of  FIG. 1 , in a sectional view; 
       FIG. 6  shows a third step in the fabrication of the device of  FIG. 1 , in a sectional view; 
       FIG. 7  shows a fourth step in the fabrication of the device of  FIG. 1 , in a sectional view; 
       FIG. 8  shows a sectional view of an alternative embodiment of the device of  FIG. 1 ; and 
       FIG. 9  shows a sectional view of another alternative embodiment of the device of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   Referring to  FIGS. 1 and 2 , these show a semiconductor device  1  in the final state. 
   This device  1  comprises a dielectric base layer  2  that includes electrical connection means  3  flush with its surface and, on this base layer, a dielectric lower layer  4 . 
   This lower layer  4  includes, in a zone  5 , a matrix  6  of lower electrodes  7  that are flush with its surface and are selectively connected to the electrical connection means  3  by electrical connections vias  8 . The lower electrodes  7  may in particular be made of chromium or n-doped silicon. 
   The device  1  includes, on the lower layer  4 , an intermediate layer  9  made of a photosensitive material which may in particular be amorphous or polycrystalline silicon. 
   The device  1  includes, on the intermediate layer  9 , a transparent upper layer  10  formed by a first layer  10   a  of a first electrically conductive material, constituting an upper electrode, and a second layer  10   b  made of a second electrically conductive material. The first layer  10   a  may in particular be p-doped silicon and the second layer  10   b  may in particular be ITO (indium tin oxide). 
   The device  1  includes, on the intermediate layer  10 , an insulating transparent outer protective layer  11 . 
   The device  1  includes, on the outer layer  11 , a non-transparent local layer  12  that, in the example, extends in the form of a ring covering the lower electrodes of two or three outer rows of the matrix  6 . This local layer  12  may in particular be made of a metal of the Al or Ti/TiN type. 
   The device  1  has holes  13  that extend through the aforementioned layers  9 ,  10  and  11 , between the local layer  12  and the lower electrodes  7   a  of the outer row of the matrix  6  of lower electrodes. These holes are filled with an electrically conductive material, for example copper, constituting vias  14  for electrically connecting the layer  10  by lateral contact to the lower electrodes  7   a  that constitute electrical connection pads. The local layer  12  extends or passes over the vias  14  and so is connected to these vias and to the branch  3   a  through these vias. 
   Advantageously, the local layer  12  and the vias  14  can be made with the same material. They can be realized in a single step of the process. 
   It follows from the foregoing that, with the exception of the peripheral zone corresponding to the vias  14  associated with the lower electrodes  7   a , the device  1  has a structure  6   a  of photosensitive diodes formed between the remaining lower electrodes  7   b  and the upper electrode  10   a . The photosensitive diodes located facing the central aperture in the local annular layer  12  are sensitive to the external light, while the photosensitive diodes located beneath this non-transparent local annular layer  12  are not sensitive thereto. 
   Outside the zone  5 , and therefore outside the local annular layer  12 , the device  1  has a multiplicity of wells  15  distributed around its periphery, which pass through the layers  4 ,  9 ,  10  and  11 . These wells expose electrical connection pads  16  that are selectively connected to the electrical connection means  3 . 
   In particular, a well  15   a  exposes an electrical connection pad  16   a  that is exclusively connected to the electrodes  7   a , and therefore to the upper electrode  10 , by the intermediary of the vias  14  and of a branch  3   a.    
   The device  1  can be connected to any other electrical or electronic means by the intermediary of electrical connection wires  17 , the ends of which are respectively introduced into the wells  15  and bonded to the corresponding electrical connection pads. 
   One method of fabricating the semiconductor device  1  described above, employing known techniques, especially etching, deposition and planarization, will now be described with reference to  FIGS. 3 to 8 . 
   As  FIGS. 3 and 4  show, after the connection means  3  have been produced in the base layer  2  and its surface has been planarized, the process continues with the deposition of the upper layer  4 . The vias  8  are produced, and then the matrix  6  of lower electrodes  7 . 
   As  FIG. 5  shows, after the surface of the layer  4 , including the vias  8  and the lower electrodes  7 , have been planarized, the process continues with the deposition of the intermediate layer, of the layer  10 , by successively depositing its first layer  10   a  and its second layer  10   b , and of the outer layer  11 , optionally with intermediate planarization operations being carried out. 
   As  FIG. 6  shows, holes  13  are produced through the layers  9 ,  10  and  11 , down to just above the peripheral lower electrodes  7   a.    
   As  FIG. 7  shows, the vias  14  are then produced, followed by the local layer  12 . 
   Finally, the wells  15  are produced through the layers  4 ,  9 ,  10  and  11  down to the connection pads  16  so as to finally obtain the device as shown in  FIGS. 1 and 2 . 
   As shown in  FIGS. 2 and 4 , preferably several devices  1  located beside one another are produced on a common base layer  2 , by carrying out the aforementioned operations jointly. Each individual device  1  is then produced, for example by sawing. 
   Referring to  FIG. 8 , it may be seen that, according to an alternative embodiment, it would optionally be possible to deposit an additional transparent outer protective layer  11   a  on the front of the outer layer  10  and of the local layer  12 . 
   Referring to  FIG. 9 , this shows that, according to another alternative embodiment, it would be possible to form the vias  14  through the layers  9  and  10 , to form the local layer  12  on the upper layer  10  and then optionally to form the outer protective layer  11  on the front of this upper layer  10  and of this local layer  12 . 
   The present invention is not limited to the examples described above. 
   In particular, the local layer  12  does not go right around the matrix  6  but, for example, it could be limited to one side of this matrix. The number of vias  14  could be reduced, and these vias could be provided only around part of the periphery of the matrix  6 . Many other alternative embodiments are possible without departing from the framework defined by the appended claims. 
   While there has been illustrated and described what is presently considered to be embodiments of the present invention, it will be understood by those of ordinary skill in the art that various other modifications may be made, and equivalents may be substituted, without departing from the true scope of the present invention. 
   Additionally, many modifications may be made to adapt a particular situation to the teachings of the present invention without departing from the central inventive concept described herein. Furthermore, an embodiment of the present invention may not include all of the features described above. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the invention include all embodiments falling within the scope of the appended claims.