Abstract:
An optical unit including a glass substrate supporting a diaphragm intended to receive a lens with a circular base, the surface of the diaphragm having one or several protruding elements for guiding the lens in horizontal positioning, and its manufacturing method.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of optical sensors associated with semiconductor devices and, more specifically, to the manufacturing of a semiconductor package with an optical sensor and especially of the optical unit thereof. 
   Such semiconductor packages with an optical sensor are used, for example, to form miniature cameras or shooting devices, for example in portable phones. 
   2. Discussion of the Related Art 
   Optical units for devices with an optical sensor of semiconductor packages are essentially formed of a hemispherical lens arranged on a diaphragm placed at a focal distance from a microlens array formed on an integrated circuit chip forming the optical sensor. 
   A first known family of optical units is formed of a rotatable plastic cap intended to hang up the lens and the diaphragm, and to be arranged on the integrated circuit chip. The cap rotation is especially used to adjust the focal distance, and thus as a focusing device. This optical unit family is generally intended for lenses made of plastic material having positioning and manufacturing tolerances (on the order of more or less 80 micrometers) incompatible with the small focal distance (a few millimeters) of this type of device. 
   A second known family of optical units consists of using a glass lens (generally hemispherical) arranged on a diaphragm formed on a glass spacer, itself resting on the integrated circuit chip supporting the microsensors (phototransistors, microdiaphragms, and microlenses). The present invention more specifically applies to this family of optical units. 
     FIG. 1  shows a conventional example of a semiconductor package with an optical sensor such as described in French patent application 03/12305 (03/GR2/190) which is incorporated herein by reference. 
   Such a package  10  forming a shooting element comprises an integrated circuit chip  1  forming an optical sensor, on which hemispherical microlenses  2  have been shown. In practice, these microlenses are integrated in the silicon of chip  1  above the phototransistors. 
   Package  10  also comprises a lens  3  arranged at a distance from chip  1  and a diaphragm  4  against the lower surface (opposite to the sensor) of lens  3 . A glass spacer  6 , relatively thick (on the order of a few millimeters) with respect to the thickness of chip  1 , is interposed between chip  1  and diaphragm  4  and defines the focal distance of the optical device. The above-described assembly is formed by batches in whole plates before being cut for individualization. The assembly then rests on a substrate  5 . The electric information provided by integrated circuit chip  1  is transferred, for example, from the upper surface of this chip supporting microlenses  2  by means of wires  52  of contact transfer to pads  53  at the upper substrate surface, pads  53  being electrically connected to pads  54  at the rear (free) surface of substrate  5  via vias  51  across its thickness. Pads  54  then generally receive conductive balls or bosses  55  for assembly of the semiconductor package on a printed circuit, as a BGA (ball grid array) package. 
   In practice, spacer  6  is obtained by means of a glass plate against the respective surfaces of which are arranged on the one hand a diaphragm grid  4  and a lens plate  3  and, on the other hand, integrated circuit chips  1  supported by substrates  5 , a spacer  7  of relatively small thickness with respect to the thickness of spacer  6  being provided to avoid for chip  1  to touch spacer  6  and to be damaged on assembly. 
   In the example of  FIG. 1 , it is assumed that optics  3  are formed of a hemispherical glass lens. As an alternative, the definitive curvature of the focusing lens may be adapted by plunging the hemispherical lens into a cavity containing an optical resin which polymerizes by a replication method, to add a surface layer (not shown) giving it the desired curvature. 
   Once the devices, formed of chip  1  supported by substrate  5 , of spacer  6 , and of optics  4 , have been individualized, a casing  8  generally made of plastic matter (resin) is arranged around the assembly to protect the system and to complete package  10 . 
     FIGS. 2A to 2D  illustrate the conventional assembly of hemispherical lens  3  on diaphragm  4 , supported by glass spacer  6 . This assembly is generally performed after cutting of integrated circuits  1  and of spacers  6  supporting diaphragms  4 . 
   A first step ( FIG. 2A ) consists of depositing a drop g of glue on diaphragm  4  supported by spacer  6 . 
   The glue spreads on the diaphragm ( FIG. 2B ) and fills opening  41  thereof. A layer g′ of glue is obtained on the structure. 
   A hemispherical glass lens  3  is then added ( FIG. 2C ) on glue layer g′ and the assembly is submitted to an ultraviolet processing to have the glue polymerize. The glue is selected to be thixotropic, that is, with no capillary effect, to avoid its upward migration on the lens, which would modify its optical properties. 
   Finally ( FIG. 2D ), integrated circuit  1  is arranged on the rear surface of glass spacer  6 , conductive bosses  54  are formed at the rear surface and the entire unit is topped with plastic matter to form casing  8 . 
   A problem of the assembly thus formed is that, when lens  3 ′ is laid on non-solidified glue layer g′, it floats thereon, which considerably increases horizontal positioning tolerances. This phenomenon is particularly notable when the lenses are desired to be assembled on whole wafers. A disadvantage is that this generates resolution problems. 
   In certain cases, diaphragms  4  are formed of a metal grid arranged on the glass wafer forming spacers  6 . In such a case, an additional problem of horizontal shift of the metal gates with respect to the centers of the lenses appears. Further, the etch tolerances of openings  41  in these grids are greater (on the order of more or less 5 micrometers) with respect to the tolerances in the lens diameter (more or less 3 micrometers for a diameter on the order of 3 millimeters). 
   The lens floating on the glue mat however remains the most critical since it adds a much greater tolerance, on the order of more or less 100 micrometers. 
   SUMMARY OF THE INVENTION 
   The present invention aims at overcoming the disadvantages of conventional assemblies of hemispherical lenses on diaphragms of miniaturized optical units. 
   The present invention especially aims at providing an assembly method which optimizes the centering of the lenses with respect to the centers of the diaphragm openings and to the sensor centers. 
   The present invention also aims at providing a solution which improves the number of steps that can be performed by batches on whole wafers. 
   To achieve these and other objects, the present invention provides a method for manufacturing an optical unit of a semiconductor package with an optical sensor, comprising: 
   depositing, on a front surface of a glass wafer having a thickness selected to form a spacer between the optical sensor and a focusing lens with a circular base, an opaque metal layer; 
   etching this layer according to a pattern for defining openings of diaphragms between the lens and the spacer, and one or several guiding elements at the periphery of a circle in which the base of the lens to be placed on the diaphragm inscribes; and 
   cutting the wafer to individualize optical units on which are placed said lenses positioned due to said guiding elements. 
   According to an embodiment of the present invention, a front surface of a wafer containing the optical sensors is placed, before cutting, on a rear surface of the glass wafer. 
   According to an embodiment of the present invention, electric connection conductive bosses are formed, before cutting, at the rear surface of the wafer supporting the optical sensors. 
   According to an embodiment of the present invention, a definitive hold casing is placed around the unit. 
   According to an embodiment of the present invention, the temporary hold of the lens on the diaphragm is performed by molecular bonding. 
   The present invention also provides an optical unit comprising a glass substrate supporting a diaphragm intended to receive a lens with a circular base, the surface of the diaphragm comprising one or several protruding elements for guiding the lens in a horizontal positioning. 
   According to an embodiment of the present invention, at least three punctual elements are distributed at the periphery of a circle in which the circular base of the lens inscribes. 
   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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 ,  2 A,  2 B,  2 C, and  2 D, previously described, are intended to show the state of the art and the problem to solve; 
       FIGS. 3A ,  3 B, and  3 C very schematically illustrate in cross-section views an embodiment of the assembly method according to the present invention, on an individual optical unit; 
       FIGS. 4 and 5  are top views of embodiments of diaphragms provided with positioning elements according to the present invention; and 
       FIGS. 6A ,  6 B, and  6 C illustrate characteristic steps of the method for manufacturing semiconductor packages with an optical sensor according to the present invention. 
   

   DETAILED DESCRIPTION 
   For clarity, the same elements have been referred to with the same reference numerals in the different drawings. Further, as usual in the representation of integrated circuits, the various drawings are not to scale. Further, only those elements and steps that are necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the manufacturing of the integrated circuit chip forming the actual microlens sensor has not been described in detail, the present invention being compatible with any conventional sensor. Further, those steps which are derived from methods currently used in semiconductor manufacturing technology have not been described in detail because they are readily accessible to those skilled in the art. 
   A feature of the present invention is to provide, in the layer forming the diaphragm or in a layer arranged thereon, a guide for positioning the hemispherical lenses. 
   Preferably, the lenses deposited on the diaphragms are temporarily maintained by molecular bonding before they are definitively maintained by the casing of the unit. 
     FIGS. 3A ,  3 B, and  3 C very schematically illustrate the structure of an individual unit according to the present invention during assembly thereof. 
   The assembly of a hemispherical lens  3  ( FIG. 3B ) on a diaphragm  4  supported by a glass spacer  6  according to the present invention is performed on a structure such as shown in  FIG. 3A  in which one or several elements  9  for horizontally guiding the lens protrude with respect to the surface of diaphragm  4 . 
   Once the lenses have been positioned, a casing  8  is conventionally arranged and the semiconductor package with an optical sensor is obtained. 
   It could have been devised to improve the lens positioning by temporarily placing them in a positioning grid. This solution may be problematic since this would, on the one hand, pose a problem of temporary grid hold and, on the other hand, generate a risk of scratching or of separating the hemispheres forming the lenses when this grid would be removed. 
     FIG. 4  illustrates in a top view of a diaphragm  4  a first embodiment of guiding means according to the present invention. It is a continuous change of circular section (circle  91 ) with an inner diameter slightly greater than the diameter of the hemispherical surface of lens  3 . “Slightly greater” means a diameter enabling engagement of this lens without allowing the lens play to be greater than the desired positioning tolerances (a few micrometers). This play should be compatible with manufacturing tolerances of the glass hemispherical lens (for example, more or less three micrometers for a lens with a three-millimeter diameter). Guiding element  9  is thus formed of the periphery of circle  91  in which the lens inscribes. To illustrate the plane change in diaphragm  4 , different hatchings have been used for the two different planes around opening  41 . 
   The height of protruding element  9  with respect to the diaphragm plane ranges, for example, between 50 and 100 micrometers. 
     FIG. 5  illustrates, in a top view of a diaphragm  4 ′, a second preferred embodiment of guiding means according to the present invention. It is formed of local pads  9 ′ protruding from the surface of diaphragm  4 . The number of pads  9 ′ is at minimum 3. These pads are positioned around an imaginary circle  91  within which a lens  3  must be positioned. 
   According to a preferred embodiment, the hold of lens  3  on the surface of diaphragm  4  is temporarily ensured by molecular bonding. This molecular bonding is made possible by the fact that the surface of lens  3  is polished for optical reasons and the upper surface of diaphragm  4  exhibits a surface state of optical type, obtained by chemical etch, by using usual semiconductor manufacturing techniques. 
   The present invention then takes advantage from the fact that, to preserve the optical operation of the device, the opposite surfaces of the diaphragm and of the lens need be as planar as possible, to obtain a blind flange. Such a planeness precisely enables obtaining a molecular bonding. 
   For the seal to be full under the entire lens surface, the metal layer of the diaphragm will be preferentially directly formed on the glass wafer, in which islands intended to subsequently form openings  41  in the opaque layer will have been previously formed. 
   The obtained molecular bonding is sufficient to enable handling of the units until their positioning in a mold of formation of casing  8  which ensures the definitive fixation. 
   In the embodiment of  FIG. 5 , a small amount of glue may be used instead of the molecular bonding. It will however be preferred to avoid a temporary gluing step in the manufacturing process. 
   In the embodiment of  FIG. 4 , the use of glue will be avoided, since said glue would risk to migrate upwards by capillary effect along the entire lens surface. 
     FIGS. 6A ,  6 B, and  6 C illustrate in very simplified fashion an embodiment of the optical sensor semiconductor package manufacturing method according to the present invention. 
   It is started ( FIG. 6A ) from a glass plate  11  on which a metal layer  12  is deposited full plate (in one or several steps). The thickness of layer  12  is selected to be greater than the final desired thickness for guiding or pre-positioning elements  9  (or  9 ′). 
   Layer  12  is then etched according to patterns of definition of openings  41  of the diaphragms and of guiding elements  9  or  9 ′ ( FIG. 6B ). This etching uses usual photographic etch techniques (by means of a deposition of photosensitive layers). Several successive etch steps may be necessary according to the used techniques. 
   The plate thus formed is placed ( FIG. 6C ) on an integrated circuit wafer  13  containing the actual sensors. This plate is conventionally formed and in particular comprises the phototransistor network. Conductive bosses  54  of the finished package have also been shown in  FIG. 6C . Indeed, the present invention is compatible with the forming by whole wafers of these bosses  54 . At the end of the steps shown in  FIG. 6C , the optical units are individualized by conventional cutting means, for example, by sawing, by water, or by laser. This then provides the structures shown in  FIG. 3A  on which are then arranged the hemispherical lenses ( FIG. 3B ), and which are molded in a casing  8  ( FIG. 3C ). 
   In the case where the temporary hold of lens  3  is performed by molecular bonding, it may be necessary to perform an additional etch step at the beginning of the method to provide for openings  41  of diaphragms  4  to be formed by glass or resin pads or islands and thus ensure the full junction between the lens and the diaphragm. 
   According to another embodiment, a small amount of thixotropic glue is used to hold the lens and fill the space of the diaphragm opening. As indicated previously, this embodiment is however preferentially dedicated to the case where the temporary hold elements are punctual to avoid trapping the glue, which would cause an upward migration thereof on the edges of lens  3 . 
   An advantage of the present invention is that it solves the problems of horizontal hemispherical lens positioning on optical units of small dimension. 
   Another advantage of the present invention is that it optimizes the number of manufacturing steps that may be performed by whole wafers. In particular, and although this is not compulsory, the processing by whole wafers may go as far as the forming of conductive bosses at the rear surface. 
   Another advantage of the present invention is that it is compatible with the other conventional optical sensor semiconductor device manufacturing steps and especially with the forming of the actual phototransistor sensor. 
   Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the dimensions to be given to temporary hold  9  and  9 ′ and the play of circle  91  around which they inscribe for the lens positioning are within the abilities of those skilled in the art according to the actual lens manufacturing tolerances. Further, although the present invention has been described in relation with hemispherical lenses, it applies to lenses of different curvature, provided that they have a circular base. 
   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.