Abstract:
A method manufactures semiconductor chips each comprising a component implanted in the semiconductor. The method includes collectively implanting components onto a front face of a semiconductor wafer and fixing a plate of a transparent material onto the front face of the wafer. Fixing the plate of transparent material is preceded by a step of depositing, on the front face of the wafer, at least one layer of polymer material forming an optical filter. Application is particularly to the manufacturing of imagers.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the manufacturing of a semiconductor wafer covered with a plate of glass and particularly to the manufacturing of a wafer comprising CMOS imagers.  
         [0003]     2. Description of the Related Art  
         [0004]     Imagers produced according to the CMOS (“Complementary Metal Oxide Semiconductor”) technology are the subject of an increasing number of applications due to their low cost price in comparison with CCD (“Charge Coupled Device”) imagers. Such CMOS imagers were initially used to produce low resolution image sensors of mediocre quality (for example web cameras). Today, after major investment in research and development, CMOS imagers can compete with CCD imagers. One embodiment of the present invention is in line with an effort to develop and improve this imager technology aiming to reduce the cost prices with at least equal image quality.  
         [0005]      FIG. 1  represents an example of a module for capturing images and/or video using a CMOS imager, intended for example to be mounted into a portable device such as a mobile telephone, a camera or a video camera. The module  1  comprises a frame  2 , an optical set  3 , lenses  4  fitted into the block  3 , an optical filter  5  and a base  6 . A semiconductor chip  100  integrating a CMOS imager  10 , is disposed on the base  6  and receives the light passing through the lenses  4  and the filter  5 . The filter  5  is generally an infrared filter (consumer applications) but can also be a specific filter for industrial applications needing to capture images in a certain frequency range or to reject certain frequency ranges.  
         [0006]     The CMOS imager  10  comprises photosites each forming one pixel (not visible in  FIG. 1 ). Each pixel comprises a photodiode and a control and interconnection circuit of the photodiode. The pixels are arranged as an array and a mosaic of red, green and blue filters is distributed over the pixel array, generally according to the Bayer architecture.  
         [0007]      FIG. 2  is a schematic cross-section of the chip  100  and of the imager  10  in a region corresponding to three pixels PIX 1 , PIX 2 , PIX 3 . A semiconductor substrate  15  into which the imager  10  is implanted, and a glass chip  20  fixed onto the imager through a layer of glue  19  can be distinguished.  
         [0008]     Going from bottom to top, the imager  10  comprises layers  10 - 1 ,  10 - 2 ,  10 - 3 ,  10 - 4 ,  10 - 5  and microlenses L 0  (L 0 - 1 , L 0 - 2 , L 0 - 3 ). The layer  10 - 1  represents the active part of the imager and comprises photodiodes and their associated control and interconnection circuits (not detailed). The layer  10 - 2  is a dielectric material that entirely covers the substrate  15 . The layer  10 - 3  is a passivating layer deposited on the imager at the end of the manufacturing process. The layer  10 - 4  is formed by colored resins and comprises red, green or blue areas R, G, B forming the above-mentioned primary color filters, with one color filter per pixel. The layer  10 - 5  is an intermediate layer of resin forming a base for the microlenses L 0  and providing good flatness (“planarization” layer). The microlenses L 0  are arranged in a “MLA” (Microlens Array) with one microlens per pixel, and are covered with the layer of glue  19  and with the glass chip  20 .  
         [0009]     Steps of a classic method of manufacturing the imager chip  100  are represented in  FIGS. 3A  to  3 E. The method first of all comprises the collective manufacturing of a plurality of imagers  10  on a silicon wafer  15 ′, as represented in  FIG. 3A . In the step shown in  FIG. 3B , a glass plate  20 ′ is fixed onto the front face of the wafer  15 ′ through a layer of glue  19 ′ covering the entire wafer, the glue comprising for example epoxy, urethane, silicon, etc. In the step in  FIG. 3C , the wafer  15 ′ is turned over and is put on a base (not represented) to treat its rear face. This treatment on the rear face comprises at least one step of thinning the wafer, a step of etching and a step of sawing the wafer to obtain a plurality of imager chips like the chip  100 .  
         [0010]     The step of thinning, represented in  FIG. 3D , is generally performed by backlapping, and comprises the grinding and the polishing of the rear face. The thickness of the wafer, initially in the order of a few hundred micrometers is in the order of a few tens or one hundred micrometers at the end of the process.  
         [0011]     The step of etching, represented in  FIG. 3E , enables grooves  25  to be obtained on the rear face of the wafer. These grooves can be provided for various reasons, for example to facilitate the subsequent cutting of the wafer or to produce contacts on the rear face. The grooves  25  are generally produced by chemical etching or plasma etching.  
         [0012]     A micro-module for capturing images as represented in  FIG. 1  has the disadvantage of having a relatively complex structure and of requiring considerable assembly time, which increases its cost price.  
         [0013]     In particular, the filter  5  is an additional component of a high cost price and requires a dedicated manufacturing line, then steps of storing, handling and assembling.  
       BRIEF SUMMARY OF THE INVENTION  
       [0014]     One embodiment is a filter that is less costly to produce and simpler to assemble than the classic filter previously described.  
         [0015]     One embodiment is based on the simple but no less inventive principle of adding on the surface of the finished wafer as represented in  FIG. 3A , prior to or at the time of fixing the glass plate  20 ′ ( FIG. 3B ), layers of polymer material having different optical indices so as to produce a filtering function, for example of infrared filtering.  
         [0016]     Thus, the filter  5  in the micro-module represented in  FIG. 1  is no longer necessary.  
         [0017]     More particularly, one embodiment provides a method for manufacturing semiconductor chips each comprising at least one component implanted in the semiconductor, the method comprising a step of collectively implanting components on a front face of a semiconductor wafer, a step of fixing a plate of a transparent material onto the front face of the wafer, wherein the step of fixing the plate of transparent material is preceded by a step of depositing, on the front face of the wafer, at least one layer of a polymer material forming an optical filter.  
         [0018]     According to one embodiment, the step of fixing the plate of transparent material is preceded by a step of depositing, on the front face of the wafer, several layers of polymer material(s) forming together a multi-layer optical filter.  
         [0019]     According to one embodiment, at least the last layer of the filter is a layer of glue fixing the plate of transparent material.  
         [0020]     According to one embodiment, any layer forming the filter is a layer of glue.  
         [0021]     According to one embodiment, the method comprises a step of covering the optical filter with a layer of glue fixing the plate of transparent material and not involved in the optical filtering function.  
         [0022]     According to one embodiment, the polymer material of each layer of the filter is chosen from the group comprising light-sensitive resins, planarizing resins and glues.  
         [0023]     According to one embodiment, each layer of the filter is polymerized prior to depositing the next layer.  
         [0024]     According to one embodiment, at least one layer of the filter is not polymerized prior to depositing the next layer, which is miscible with the non-polymerized layer, so as to form a resulting layer with a progressively varying optical index that is then polymerized.  
         [0025]     According to one embodiment, at least one layer of the optical filter is deposited by spin coating.  
         [0026]     According to one embodiment, the method comprises a step of cutting the wafer into individual chips.  
         [0027]     According to one embodiment, the step of implanting components onto a face of the wafer comprises the implantation of imagers intended to capture images through the plate of transparent material and the optical filter.  
         [0028]     According to one embodiment, the plate of transparent material is made of glass.  
         [0029]     One embodiment relates to an imager comprising a plate of transparent material fitted onto the semiconductor chip, through which the imager receives images to be captured, and comprising, between the semiconductor chip and the plate of transparent material, at least one layer of polymer material forming an optical filter.  
         [0030]     According to one embodiment, the imager comprises between the semiconductor chip and the plate of transparent material several layers of polymer material(s) forming together a multi-layer optical filter.  
         [0031]     According to one embodiment, the optical filter is covered with a layer of glue fixing the plate of transparent material and not involved in the optical filtering function.  
         [0032]     According to one embodiment, the last layer of polymer material of the optical filter is a layer of glue fixing the plate of transparent material.  
         [0033]     According to one embodiment, any layer of polymer material forming the multi-layer optical filter is a layer of glue.  
         [0034]     According to one embodiment, any layer of the optical filter is formed by a material belonging to the group comprising light-sensitive resins, planarizing resins and glues.  
         [0035]     According to one embodiment, the optical filter has a graduated optical index.  
         [0036]     According to one embodiment, the multi-layer optical filter has a progressively varying optical index.  
         [0037]     According to one embodiment, the plate of transparent material is made of glass.  
         [0038]     One embodiment also relates to a portable device comprising a photographic or video module equipped with an imager. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0039]     These and other, features and advantages will be explained in greater detail in the following description of the process, given in relation with, but not limited to the following figures, in which:  
         [0040]      FIG. 1  described above represents a module for capturing images for portable device,  
         [0041]      FIG. 2  described above is a schematic cross-section of an imager chip,  
         [0042]      FIGS. 3A  to  3 E described above show a method for manufacturing imagers on a semiconductor wafer,  
         [0043]      FIGS. 4A  to  4 D represent steps of producing a filter according to one embodiment, and  
         [0044]      FIG. 5  represents an imager chip according to one embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0045]     An example of implementation of the method will be described here within the scope of manufacturing imagers on a semiconductor wafer as shown in  FIGS. 3A  to  3 E described above.  
         [0046]     One embodiment provides for depositing several layers of a hardenable polymer material on the finished wafer  15 ′ as represented in  FIG. 3A  (i.e., the wafer  15 ′ equipped with imagers  10 ), prior to mounting the glass plate  20 ′.  
         [0047]     These layers, and at least the last layer deposited prior to mounting the glass plate  20 ′, are for example layers of glues (epoxy, urethane, silicone, etc.) having determined optical indices. If necessary, colored or non-colored additives are added to the formulas of standard glues so as to obtain determined optical indices. Generally speaking, it is currently easy to obtain any type of glue or other polymer material defined by precise specifications stipulating in particular the color and the optical index desired.  
         [0048]     The layers can be produced with the same polymer material but containing different additives, or with different polymer materials, or a combination of these two possibilities.  
         [0049]     In prior art, the layer of glue  19 ′ ( FIG. 3B ) which enables the glass plate  20 ′ to be fixed has substantially the same optical index as glass, so that its presence does not cause parasitic diopters to appear. On the contrary, the various layers  31 - 33  of polymer material (or of polymer materials) described below have distinct optical indices, so that the set of the layers form a multi-layer optical filter.  
         [0050]     These layers of polymer material are preferably deposited by spin coating. The polymer material can be liquid or gelatinous. It is then polymerized according to the required conditions (UV polymerization or thermal polymerization).  
         [0051]     Thus, in the step represented in  FIG. 4A , a small quantity of polymer material  30 , in the order of a few milliliters, is deposited on the wafer  15 ′, preferably in the center of the wafer. In the step represented in  FIG. 4B , the material  30  is spread over the entire surface of the glass plate  20 ′ by centrifugation. The speed of rotation of the wafer is initially quite low, for example 1,500 rpm. A strong acceleration, for example up to 20,000 rpm, is then applied to the wafer to favor the formation of a uniform layer  31  over the entire surface of the glass plate while eliminating the surplus polymer material. The speed of rotation is then stabilized at a lower value, for example in the order of 2,000 to 7,000 rpm, so as to define the thickness of the layer  31 . The rotation is further maintained for a certain time to enable the solvent to evaporate. The evaporation of the solvent substantially decreases the viscosity of the polymer material on which the thickness of the film depends. As the evaporation is fast, it helps to make sure that the spreading time of the polymer material is short. During the rotation at high speed, the largest part of the solvent contained in the material evaporates and finally produces a solid film.  
         [0052]     A step of baking is then conducted. This step ensures the rapid elimination of the residual solvents and the polymerization of the layer  31 . A contraction (volume shrinkage) of the polymer material follows, that causes a loss of thickness in the order of a fraction of a percent to a few percent, depending on the material used.  
         [0053]     The wafer is for example heated by means of hot plates taken to a temperature in the order of one hundred to a few hundred degrees (generally speaking 100-300° C.). The duration of the heating cycle is adjusted to reach the desired solvent rate, and can be of a few minutes to more than one hour. The baking is followed by a step of cooling the wafer, for example by means of plates at ambient temperature.  
         [0054]     As an alternative, and depending on the material chosen, the step of polymerizing can also be conducted by exposing the wafer to an ultraviolet radiation. Ultraviolet polymerization is generally shorter than thermal polymerization and only lasts a few minutes at the most.  
         [0055]     In the step shown in  FIG. 4C , two other polymer layers  32 ,  33  have been deposited on the layer  31 , the last layer  33  also serving as a layer of glue for the step of sticking the glass plate  20 ′ represented in  FIG. 4D . A multi-layer optical filter with graduated optical index has thus been produced below the glass plate  20 ′.  
         [0056]     Those skilled in the art will note that it is possible for the polymer layers extending beneath the last layer (here the layer  33 ) not to be glues, and that they could for example be light-sensitive resins (for example the “CT” resins used to manufacture the microlenses of the imagers) or “planarizing” resins (resins used to form flat surfaces in microelectronics), color resins, etc.  
         [0057]     Furthermore, the classic layer of glue  19 ′ ( FIG. 2 ,  FIG. 3B ) may be deposited on the wafer prior to depositing the layers  31  to  33 , to flatten the surface of the wafer above the microlenses L 0 . The layer of glue  19 ′ can also be deposited on the layers  31  to  33  so as to stick the glass plate. In this case, it is possible for all the layers forming the filter not to be glues. Furthermore, a layer of glue  19 ′ or any flatness layer (“planarizing” layer) can be deposited beneath the layers  31  to  33  and a layer of glue  19 ′ be deposited on the layers  31  to  33 .  
         [0058]     In an alternative embodiment, the layer  31  is not polymerized prior to depositing the layer  32  and the layer  32  is also not polymerized prior to depositing the layer  33 . The polymer materials forming the layers are then provided miscible (it may be the same material with different additives, as indicated above) such that the final layer obtained has an optical index varying progressively. A final step of polymerization is applied to the set of the layers after they are deposited. If no layer of glue is added onto the multi-layer filter to fix the glass plate, this polymerization is performed after mounting the glass plate so that the multi-layer structure sticks to the glass plate.  
         [0059]     As represented in  FIG. 5 , an imager chip  101 , obtained by cutting the wafer, differs from the classic chip  100  represented in  FIG. 2  by the presence of the layers  31 ,  32 ,  33  forming a filter between the silicon  15  and the glass chip  20 .  
         [0060]     It will be understood by those skilled in the art that various other applications and alternative embodiments of the method are possible. In particular, the number of layers deposited to form the filter can vary depending on the filter desired (infrared, ultraviolet, etc.) and the polymer materials used, the example of the three layers  31  to  33  described above being in no way limitative.  
         [0061]     A filter can also comprise a single layer only, if it is possible to produce a polymer composition offering by itself all the optical properties sought.  
         [0062]     The method thus enables modules for capturing images embedded in portable devices such as mobile telephones, cameras and video cameras, etc., to be produced without the need to arrange a filter distinct from the imager chips in these modules.  
         [0063]     Moreover, although the method has been described above in relation with the manufacturing of a semiconductor wafer comprising a glass plate, a transparent material other than glass can be used, for example Plexiglas.  
         [0064]     The method can further be applied to the production of any type of optical filters on wafers designed to let an electromagnetic radiation through, in the visible, infrared, ultraviolet or even beyond, or to filter a large or narrow frequency band.