Abstract:
A color back-side illuminated image sensor including, on the side of the thin semiconductor layer opposite to the illuminated surface, periodic thickness unevennesses forming an optic network having characteristics which make it capable of reflecting a given wavelength chosen within the range of the wavelengths of an illuminating incident beam.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of French patent application number 09/56775, filed on Sep. 30, 2009, entitled “BACK-SIDE IMAGE SENSOR,” which is hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention generally relates to the monolithic construction of imagers. More specifically, the present invention relates to the structure of image sensors of back-side illuminated sensor type, presently called back-side sensors. 
     Discussion of the Related Art 
     An image sensor is formed of an array of cells capable of performing a photoelectric transformation. The cells are arranged at the nodes of an array of lines and columns. Each cell comprises a photodiode associated with a transfer transistor and possibly other transistors. 
     An example of a back-side sensor will be described in relation with  FIGS. 1A to 1E , which are partial simplified cross-section views illustrating successive steps of the construction of such a sensor. 
       FIG. 1A  shows a wafer of silicon-on-insulator type (SOI) comprising a thin single-crystal silicon layer  1  separated by an insulating layer  3  from a solid substrate  5 . Thin layer  1  is lightly P-type doped. The method comprises an initial step of etching trenches  7  intended to delimit the areas in which the elementary cells, especially comprising a photodiode and its transfer transistor, will be formed. At this stage of the process, insulating layers  7  filled with an insulating material are generally also formed at the limits between the areas where the elementary cell are formed, to provide a so-called STI isolation (shallow trench isolation) between these cells. 
       FIGS. 1A to 1E  only illustrate four insulating trenches  7  delimiting three areas where individual cells are to be formed, each cell being comprised of a photodiode associated with its transfer transistor. 
     As illustrated in  FIG. 1B , trenches  7  are generally surrounded with a P-type region  9  more heavily doped than thin layer  1 . Then, insulated gates  11  of the MOS transistors are formed on thin layer  1 . N-type doped drain and source regions  13  and  15  are formed by implantation/diffusion on either side of gates  11 . The transfer transistors are asymmetrical. In each cell, source  15  is smaller than drain  13  which forms the cathode of the photodiode. Drain  13  takes up the most part of each cell. 
     The surface of drain region  13  may be covered with a P-type inversion region  17  of small thickness, more heavily-doped than thin layer  1 . Thin layer  1  is covered with a succession of interconnect levels providing the necessary connections. These interconnect levels comprise metallizations and vias (not referenced) formed between successive insulating layers  21 ,  23 ,  25 . Only three insulating layers are shown. They may, however, be in a greater number, for example, between four and nine. 
     Then, as shown in  FIG. 1C , a wafer of a semiconductor material or handle wafer  30  is attached to the structure on an insulating layer  28  coating the last interconnect level. 
     As illustrated in  FIG. 1D , substrate  5  is then removed, generally by chem.-mech. polishing, to expose the back side of thin layer  1 . 
     As illustrated in  FIG. 1E , the back side is then successively covered with a heavily-doped P-type layer  32 , and with color filters  34  and lenses  36  associated with each cell. 
     In operation, an incident light beam of the visible spectrum is focused by lenses  36  in the various cells through filters  34 . Within the incident beam the photons of a wavelength range selected by filter  34  are converted by photoelectric effect into carriers (electrons) and are stored in cathode/drain region  13 , whereby they are transferred to read and analysis circuits, not shown, by the transfer transistors. 
     A problem encountered in the manufacturing of such sensors is the selection of the thickness of thin layer  1  which forms the area where the photons are transformed into electric charges. On the one hand, to avoid interference effects between neighboring cells, this layer should be as thin as possible: typically equivalent to the size of the pixel, that is, from 1 to 2 μm for a pixel having a 1.4-μm side. Such thicknesses guarantee a conversion quantum efficiency close to 100% for short wavelengths, that is, in blue and green, but result in an efficiency loss ranging from 30 to 60% for wavelengths in the red wavelength range. On the other hand, this layer should be sufficiently thick for most of the incident photons to be converted: it should be more than 5-μm thick to guarantee the absorption of 90% of the photons corresponding to the red portion of the visible spectrum. 
     Such constraints lead to use thin layers  1  having a thickness from 2 to 3 μm. 
     To overcome this loss, it has been provided to place a reflective metal layer on the side of the transistor gates so that the photons which have not been converted transit for a second time through the photoelectric conversion layer. 
     However, the insertion of an additional metal element in the structure has the disadvantage of causing stray capacitive couplings. 
     Further, in microelectronics, all metals are normally processed and deposited to be non-reflective. Introducing a reflective metal layer may require significant modifications of the manufacturing conditions. 
     SUMMARY OF THE INVENTION 
     An object of an embodiment of the present invention is to provide a back-side sensor having an improved quantum efficiency. 
     Another object of an embodiment of the present invention is to provide such a sensor without increasing the thickness of the thin layer in which the components are formed. 
     Another object of an embodiment of the present invention is to provide such a sensor without modifying the structure of the transfer transistor. 
     Another object of an embodiment of the present invention is to provide such a sensor without requiring the introduction of new materials in the manufacturing processes, nor of additional specific processings of currently-used materials. 
     Thus, an embodiment of the present invention provides a color back-side illuminated image sensor comprising, on the side of the thin semiconductor layer opposite to the illuminated surface, periodic thickness unevennesses forming an optic network having characteristics which make it capable of reflecting a given wavelength of the visible spectrum selected within the range of the wavelengths of the incident illuminating beam. 
     According to an embodiment of the present invention, the unevennesses correspond to recesses in the silicon, which are filled with silicon oxide. 
     According to an embodiment of the present invention, the unevennesses are pads separated by trenches. 
     According to an embodiment of the present invention, optic networks are only provided for the sensor cells associated with filters transmitting red light. 
     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. 1A to 1E  partially and schematically illustrate in cross-section view successive steps of the manufacturing of an image sensor; 
         FIG. 2A  is a partial simplified cross-section view of an image sensor according to an embodiment of the present invention at an intermediary stage of its manufacturing; and 
         FIG. 2B  is a partial simplified cross-section view of an image sensor according to an embodiment of the present invention at a final manufacturing stage. 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale. 
       FIG. 2A  shows a single-crystal substrate, of the same type as that of  FIG. 1A , for example, formed of a thin semiconductor layer  1  of a structure of silicon-on-insulator type separated by a thin insulator  3 , presently silicon oxide, from a solid substrate  5 . The method comprises an initial step of delimitation of portions of thin layer  1  in which elementary cells of the sensor are to be formed. This delimitation is performed by the etching of trenches  7 . At this stage, recesses  40  are simultaneously etched into thin layer  1  to form, at the surface of thin layer  1 , an unevenness area  41 . According to an embodiment, recesses  40  are etched so that unevennesses  41  are square pads rather than rectilinear trenches. The location of area  41  and the unevenness distribution will be described subsequently. 
     The method carries on with steps similar to those described in relation with  FIGS. 1B to 1E , especially comprising the filling of trenches  7  and  40 , the forming of insulated gates  11 , the forming of different implanted regions  9 ,  13 ,  15 , and  17 , the forming of interconnect levels  21 ,  23 , and  25 , the arranging of a handle wafer on the metallization levels, a thinning-down capable of exposing the back side of thin layer  1  by removal of substrate  5  and of at least a portion of insulating layer  3 , and the forming of filters  34  and of lenses  36 . The result of these different steps is illustrated in  FIG. 2B . 
     As shown in  FIG. 2B , unevenness area  41  is arranged to be located in an elementary cell having a red filter  34 , outside of the transfer transistor forming area, at the level of the portion of the surface of thin layer  1  located right above cathode/drain region  13 . 
     Unevennesses  41  form a periodic pattern formed of pads surrounded with trenches. These pads may have a square base, but will preferably have a circular or hexagonal basis to avoid privileging a specific type of light biasing. Period Λ of the pattern, height d of the pads, as well as the ratio between the width of the pads and the width of the interval separating them are selected so that the periodic unevennesses altogether form a two-dimensional optic network capable of reflecting wavelengths corresponding to the color red of the visible spectrum. 
     Thus, an incident illuminating beam crossing a red filter  34  is reflected by unevenness area  41  and crosses back cathode region  13 . This amounts to virtually doubling the thickness of cathode region  13 . The probability of occurrence of a photoelectric effect is thus doubled. 
     An advantage of such a structure is that the reflection effect is better with a two-dimensional network than with a reflective layer. 
     Another advantage is that this result is obtained without introducing any new material with respect to known structures. The manufacturing method is thus simple. In particular, unevennesses  41  may be formed without adding additional steps, by etching thin layer  1  to form the pads at the same time as STI-type insulation areas are etched in other portions of thin layer  1 . Although the pads and their insulation areas have been shown with different widths, these widths may be identical. 
     According to an embodiment, thin layer  41  is a single-crystal silicon layer having a thickness from 3 to 5 μm and the insulating layer filling the interval between pads is a silicon oxide layer. It should be noted that the optical indexes of silicon and of silicon oxide are clearly differentiated, as they are respectively close to 4 and to 1.45, which is favorable to the obtaining of an efficient optic network. 
     For a photodiode associated with a red filter, unevennesses  41  will, for example, have the following characteristics:
         period Λ: on the order of from 350 to 400 nm;   height d of the pads: on the order of from 50 to 300 nm;   ratio between the pad width and the distance between pads close to 1.       

     Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, the present invention has been described as applied to a specific type of back-side sensor based on photodiodes. This sensor is likely to have various alterations. Further, all back-side sensors are confronted to the same problem and the solution provided herein also applies to other back-side sensors. For example, the present invention also applies to sensors based on phototransistors or on photogates. 
     Further, it should be noted that unevennesses  41  have only been described in relation with an elementary cell associated with a red filter. This is a preferred embodiment of the present invention. Indeed, presently-used filters are always imperfect, and by avoiding reflection of red in green and blue cells, the conversion of the parasitic red light is limited in these cells. A specific array of pads with a specific step may, however, be provided for each color to reflect the desired color of the visible spectrum chosen within the range of the wavelengths of an illuminating incident beam. 
     Generally, although the present invention has been described in the context of a silicon process, it applies to any integrated circuit manufacturing process. 
     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.