Abstract:
A method for making an optical structure and a multilayer optical structure provided with optical guide elements for transmitting at least a light wave, comprises: providing a substrate with a first and second layer; etching out in the second electrically conducting layer a groove having two opposite ends and partly enclosing a first zone of the layer; filling the groove with electrically insulating material; etching out a cavity in the second layer then in the first non-electrically conducting layer partly enclosing a second zone of the second layer, adjacent to or extending the first zone wherein emerges the groove; the cavity comprises a clearance in the second layer; such that the closed part of the second layer corresponding to the zones is electrically isolated from the rest of the layer and the cleared part beneath the second zone constitutes a beam.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to the field of optical wave transmission in optical guiding structures. 
   DESCRIPTION OF THE RELATED ART 
   To transport an optical wave, optical guides are used that generally consist of optical fibers or microfibers and/or of integrated components that include optical microguides. In general, optical fibers comprise an optical wave transmission core surrounded by a tubular cladding, the refractive index of the constituent material or materials of the core being higher than the refractive index of the constituent material of the cladding. In general, components with integrated microguides comprise an optical wave transmission core formed between two layers, the refractive index of the constituent material of the core being higher than the refractive index of the constituent material or materials of these layers. 
   Various optical guiding structures have in particular been disclosed in the patents FR-A-90/03902 and FR-A-95/00201. 
   Patent FR-A-90/03902 discloses integrated optical switches in which a flexible beam carrying longitudinally an optical microguide is capable of being deformed with respect to a fixed body so as to selectively bring the end of the core of the microguide of the beam into coincidence with the end of the fixed microguides. 
   Patent FR-A-95/00201 discloses integrated optical switches which comprise a movable platform placed between two parts of a body and connected to the latter by arms. The platform carries integrated optical microguides that are placed so as to switch, when the platform is moved transversely, a light wave arriving via an optical microguide from one part of the body selectively to two optical microguides of its other part. 
   The above documents also propose members for actuating the flexible beam and the movable platform. The beam and the platform are both made of an electrically insulating material and have arms covered with a metal layer, and the bodies have parts covered with a metal layer. These layers are placed a certain distance apart so as to constitute the electrodes for a capacitive or inductive effect and are connected to supply lines, for example via tracks and/or wire bridges. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is in particular to improve and simplify the construction of actuating members in optical structures having movable or deformable parts, in particular those for optical switching. 
   The subject of the invention is firstly a process for producing an optical structure provided with at least one optical guiding means for the purpose of transmitting at least one optical or light wave. 
   According to the invention, this process consists in providing a substrate with an electrically nonconducting first layer and with an electrically conducting second layer; in producing a wall of electrical insulation in the second layer at least up to the first layer, having two opposed ends and partly surrounding a first region of this layer; and in excavating a cavity in the second layer and then in said first layer, partly surrounding a second region of the second layer, adjacent or extending said first region and into which said wall emerges, this cavity including a recess in the second layer beneath at least part of the second region; in such a way that the closed part of the second layer corresponding to said regions is electrically isolated from the rest of this layer and in such a way that the exposed part underneath said second region constitutes a beam. 
   According to a preferred variant of the invention, the process consists in providing a substrate with an electrically nonconducting first layer and with an electrically conducting second layer; in excavating a groove in the second layer at least down to the first layer, having two opposed ends and partly surrounding a first region of this layer; in at least partly filling the groove with an electrically insulating material; and in excavating a cavity in the second layer and then in said first layer, partly surrounding a second region of the second layer, adjacent or extending said first region and into which said groove emerges, said cavity including a recess in the second layer beneath at least part of the second region; in such a way that the closed part of the second layer corresponding to said regions is electrically isolated from the rest of this layer and in such a way that the exposed part beneath said second region constitutes a beam. 
   According to the invention, the process preferably consists in producing a recess such that said beam is cantilevered. 
   According to the invention, the process preferably consists in producing at least one optical guiding means that passes over said filled groove and between said ends of this filled groove. 
   According to the invention, the process preferably consists, between the operation of filling said groove and the operation of producing said cavity, in producing at least one optical microguide on the second layer, which passes over said filled groove and passes between said ends of this groove. 
   According to the invention, the process preferably consists, between the operation of filling said groove and the operation of producing said cavity, in producing at least one surface groove in said second layer, which passes over said filled groove and passes between said ends of this groove and, after the operation of producing said cavity and said recess, in fastening an optical microfiber to this groove. 
   According to the invention, the process preferably consists in producing at least one surface groove which passes over the cavity to be produced. 
   According to the invention, the process preferably consists in producing a groove that has end parts close together. 
   According to the invention, this process preferably consists in producing a groove that defines a T-shaped first region having a central branch directed toward said second region and two opposed side branches. 
   According to the invention, the process preferably consists in producing an electrical connection pad in said first region. 
   According to another variant of the invention, the process consists in producing said wall of insulation by locally doping said second layer. 
   The subject of the present invention is also an optical structure provided with at least one optical guiding means for the purpose of transmitting at least one optical or light wave. 
   According to the invention, this structure comprises in succession, on a substrate, an electrically nonconducting first layer, an electrically conducting second layer, and optical guiding means. 
   According to a variant of the invention, the second layer comprises a closed part, the periphery of which is bounded by a groove and a cavity which are excavated at least up to the first layer so as to electrically isolate this part from the rest of this layer, said groove being at least partly filled with an electrically insulating material and said cavity extending into said first layer, leaving a recess beneath said second layer so that the second layer constitutes a beam above this recess. 
   According to the invention, the structure preferably includes at least one optical guiding means extending along said second layer and passing over said filled groove and between the ends of this groove. 
   According to another variant of the invention, the second layer includes a closed part, the periphery of which is bounded by a wall of electrical insulation and a cavity which are excavated at least as far as the first layer so as to electrically isolate this part from the rest of this layer, said cavity extending into said first layer leaving a recess under said second layer so that the second layer constitutes a beam above this recess. 
   The structure includes at least one optical guiding means extending along said second layer and passing above said wall of insulation between the ends of said wall of insulation. 
   According to the invention, said groove preferably defines a T-shaped or C-shaped first region having a central branch directed toward said second region and two opposed side branches. 
   According to the invention, said recess preferably extends so as to constitute a cantilevered beam. 
   According to the invention, said optical guiding means preferably includes an integrated optical microguide. 
   According to the invention, said optical guiding means preferably includes a surface groove made in said second layer and at least one optical microfiber installed along this groove. 
   According to the invention, said optical guiding means preferably extends as far as the edge of said cavity. 
   According to the invention, the structure preferably includes an electrical connection pad formed in said first region. 
   According to the invention, the structure preferably includes an actuating member, said beam comprising at least part of this actuating member. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood on examining the optical structures and their manufacturing processes, these being described by way of nonlimiting examples and illustrated by the drawing in which: 
       FIG. 1  shows a section through a first base structure according to the present invention; 
       FIG. 2  shows a top view of the base structure in  FIG. 1  after a first fabrication operation has been carried out; 
       FIG. 3  shows a cross section on III—III of the structure in  FIG. 2 ; 
       FIG. 4  shows a top view of the base structure after a second fabrication operation; 
       FIG. 5  shows a cross section on IV—IV of the structure in  FIG. 4 ; 
       FIG. 6  shows a top view of the base structure after a third fabrication operation; 
       FIG. 7  shows a cross section on VII—VII of the structure in  FIG. 6 ; 
       FIG. 8  shows a top view of the base structure after a fourth fabrication operation; 
       FIG. 9  shows a section on IX—IX of the structure in  FIG. 8 ; 
       FIG. 10  shows a top view of the base structure after a fifth fabrication step, which constitutes a first final optical structure according to the present invention; 
       FIG. 11  shows a cross section on XI—XI of the final optical structure in  FIG. 10 ; 
       FIG. 12  shows a longitudinal section of a second base structure after a first fabrication step, which corresponds to the first base structure in  FIGS. 2 and 3 ; 
       FIG. 13  shows a cross section of the second base structure after a second fabrication step; 
       FIG. 14  shows a top view of the second base structure after a second fabrication step, which constitutes a second final optical structure according to the present invention; 
       FIG. 15  shows a cross section on XV—XV of the final optical structure in  FIG. 14 ; and 
       FIG. 16  shows a cross section on XVI—XVI of the final optical structure in  FIG. 14 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A description will firstly be given, with reference to  FIGS. 1 to 11 , of the various operations leading to an optical structure  100  shown in its final form in  FIGS. 10 and 11 . 
     FIG. 1  shows, on the one hand, a multilayer base structure  1  comprising a substrate  2 , made of silicon for example, which is covered with a first layer  3  made of an insulating material such as silicon dioxide (SiO 2 ), which is itself covered with a second layer  4  made of an electrically conducting material, for example silicon. 
   As shown in  FIGS. 2 and 3 , the conducting, layer  4  is excavated so as to produce a groove  5  extending depthwise as far as the conducting layer  3 . This groove  5  defines and partly surrounds a first region  6  of the conducting layer  4 . 
   In the example shown, the groove  5  is, seen from above, in the form of a C whose ends are extended by two parallel longitudinal end parts  5   a  and  5   b  that are close together, in such a way that the region  6  is, seen from above, in the form of a T with thick branches, its two opposed side branches being bounded by the C and its central branch being bounded by the two longitudinal end parts  5   a  and  5   b.    
   Next, the groove  5  is filled with an electrically insulating material  7 , for example with silica or silicon dioxide. A wall of electrical insulation formed by the groove  5  filled with material  7  is thus obtained. 
   Next, as may be seen in  FIGS. 4 and 5 , a longitudinal groove  8 , for example of V-shape cross-section is excavated in the surface of the conducting layer  4 , which groove crosses over the filled groove  5  and passes axially through the region  6 , passing between its longitudinal end parts  5   a  and  5   b.    
   Next, as may be seen in  FIGS. 6 and 7 , a cavity  9  is excavated in the conducting layer  4  as far as the insulating layer  3 , which cavity  9  partly surrounds a second region  10  of this layer  4  and into which the filled cavity  5  runs. 
   In the example shown, the second region  10  longitudinally extends the central branch of the first region  6  and has a transverse end surface  11 , on the opposite side from the region  6 , which lies a short distance from a transverse wall  12  of the cavity  9  so as to define a space  13 . In addition, the surface groove  8  runs longitudinally to the second region  12  and this groove  8  is interrupted by the space  13 . 
   As a result, the first region  6  and this second region  10  are adjacent and peripherally completely surrounded by the groove  5  and the cavity  9 , so that the closed part  14  of the conducting layer  4  thus defined is electrically isolated from the rest of this layer surrounding the groove  5  and the cavity  9 . 
   Next, as shown in  FIGS. 8 and 9 , the insulating layer  3  is excavated around and beneath the region  10  of the isolated part  14  of the conducting layer  4 , so that the deepened cavity  9  has a recess  15  beneath the second region  10  of the isolated part  14  of the layer  4 . 
   As a result, the region  10  of the isolated part  14  of the conducting layer  4  constitutes a cantilevered beam  16  that lies freely in the cavity  9  and is firmly set in by its region  6  surrounded by the filled groove  5  and attached from below to the insulating layer  3 . 
   Next, as may be seen in  FIGS. 10 and 11 , a longitudinal optical microfiber  17  is fastened in the surface part of the groove  8  in such a way that one end of this microfiber  17  coincides with the end  11  of the beam  16  and a longitudinal optical fiber  18  is fastened in the other part of the groove  8  in such a way that one end of this microfiber  18  coincides with the wall  12  of the cavity  9 . 
   Thus, the optical microfibers  17  and  18  are optically coupled via their ends and any bending of the beam  16 , obtained by an actuating member that will be described later, allows this coupling to be modified. 
   Next, an electrical connection pad  19  is produced, for example in the region  6  of the isolated part  14  of the conducting layer  4  and an electrical connection pad  20  is produced at a point in the rest of this conducting layer  4 , so that the abovementioned actuating member can be electrically controlled, as will be explained later. 
   A description will now be given, with reference to  FIGS. 12 to 16 , of the various steps in the fabrication of a second optical structure  200 , shown in its final form in  FIGS. 16 and 17 . 
     FIG. 12  in particular shows that the process starts with a base structure  21  identical to the base structure  1  of the previous example, which comprises a substrate  22  on which a first layer  23  of an insulating material is deposited, a second layer  24  of an electrically conducting material being deposited on this first layer  23 . 
   As described with reference to  FIGS. 2 and 3 , a groove  25 , of the same shape as the groove  5  in the previous example, is excavated in the conducting layer  25 , this groove  25  being filled with an insulating material  26  and partly surrounding and defining a first region  27 . 
   Next, as shown in  FIG. 13 , an integrated longitudinal microguide  28  is produced in the conducting layer  24  by depositing a layer  29  and by producing a longitudinal optical transmission core  30 , the surface layer  29  being made of an electrically insulating material, for example undoped silica, and the transmission core  29  being made of doped silica or of silicon nitride. 
   Like the surface groove  8  of the previous example, the transmission core  30  extends longitudinally, passing over the filled groove  25  and between the end parts  25   a  and  25   b  of this groove. 
   Next, as shown in  FIGS. 14 to 16 , a cavity  31  is excavated in the nonconducting surface layer  29 , then in the conducting layer  24  and then in the nonconducting layer  23 , said cavity  31  having a recess  32  and the filled cavity  25  running into this cavity  31 . 
   As in the previous example, the cavity  31  defines and partly surrounds a second region  33  adjacent the first region  27 , in such a way that the conducting layer  24  has an isolated part  34 , the periphery of which is surrounded by the groove  25  and the cavity  31 , the region  27  and the groove  25  being this time covered with the surface layer  29 . 
   As in the previous example, a longitudinal cantilevered beam  35  is thus produced in the cavity  31 , in the second region  33  and in the extension of the first region  27 , the transverse end surface  36  of this beam  35  being separated from a transverse wall  37  of the cavity  31  by a space  38 . 
   The optical microguide  28 , being cut by the space  38 , has a part  28   a , the transmission core of which extends along the beam  35  as far as its end  36 , and a part  28   b , the transmission core of which extends as far as the transverse wall  37  of the cavity  31 , these parts  28   a  and  28   b  being optically coupled via the space  38 . 
   Next, holes are excavated in the surface layer  29  as far as the conducting layer  24 , these being filled with electrically conducting material so as to constitute, on the one hand, an electrical connection pad  39  in the region  27  of the part  34  of the layer  24  and, on the other hand, an electrical connection pad  34  in the other part of this layer, these pads  39  and  40  being intended to establish a potential difference between these isolated parts of the conducting layer  24 . 
   Complementarily, the cavity  31  and the recess  32  are excavated so as furthermore to constitute, by the same operations, a member  41  for actuating the beam  35 . 
   This actuating member  41  comprises, in the cavity  31 , a transverse arm  42  integral with the beam  34 , at a point close to its end  36 , which arm lies parallel to the wall  37  of the cavity  31  and has, laterally, opposed branches  43  and  44 . 
   The actuating member  41  also includes opposed branches  44  and  46  which project into the cavity  31  from the transverse wall  37  and from an opposed transverse wall  47  of the cavity  31 , these branches  45  and  46  extending between, on the one hand, the branches  43 , and on the other hand, the branches  44 . 
   Like the beam  35 , the side arm  42  and the branches  43 ,  44 ,  45  and  46  are made of the material of the layers  24  and  29 . Thus, the side arm  42  and the branches  43  and  44  form part of the electrically isolated second region  33  and are electrically connected to the pad  39  via the beam  35 , whereas the branches  45  and  46  are electrically connected to the electrical connection pad  40 . 
   Preferably, the opposing faces of the branches  43  and  45 , on the one hand, and of the branches  44  and  46 , on the other hand, are subsequently covered with metal layers (not shown) in such a way that these branches constitute electrodes. 
   It follows from the foregoing that the existence of the groove  25  filled with insulation and the embodiment of the cavity  31  allow electrical isolation of the beam  35  and those parts of the actuating member  41  formed by the side arm  42  and the branches  43  and  44 , in such a way that it is simple to supply this actuating member  41  with electrical energy. 
   This is because all that is required is for the pads  39  and  40  to be electrically connected, in particular by electrical connection wires, to a control device (not shown) so as to activate the actuating member  41  in order to make the beam  35  bend as required and to thus modify the optical coupling between the two parts  28   a  and  28   b  of the optical microguide  28 . 
   Of course, the actuating member  41  as has just been described may be produced in the optical structure  100  described with reference to  FIGS. 1 to 11  during the operation of the cavity  9 . 
   In a specific embodiment of the optical structure  200 , the thickness of the conducting layer  24  could be about 60 microns and the width of the optical microguide core  28  could be about 8 microns. Under these conditions, the width and the length of the branches of the first region could be about 50 microns and the width of the groove could be about 5 microns, the cavity  31  being wide enough to allow the beam  35  to bend and for the operation of the actuating member  41  not to be electrically disturbed. Such dimensions are also applicable to the optical structure  100 . 
   According to a variant of the invention, the groove  5  filled with material  7  could be replaced with a doping of the layer  4  so as to create an integrated wall of electrical insulation. 
   In general, the operations for producing the grooves, the cavities and the holes, together with the operations for producing the optical microguides, may be carried out by photolithographic, etching, deposition and chemical-mechanical planarization processes known in microelectronics. 
   The present invention is not limited to the examples described above. The optical structures could have cavities and filled grooves suitable for forming beams of any desired shape, particularly platforms supported by electrically isolated arms that can move in translation. The optical structures could have a multiplicity of optical microfibers or of integrated microguides. The actuating members could have different arrangements. The shape of the grooves filled with insulation could have different shapes. Many variants are in fact possible without departing from the scope defined by the appended claims.