Abstract:
The photodiode comprises an upper pn junction (D 1 ) formed between an upper layer and an intermediate layer supported by one portion of a semiconductor substrate. A lower junction is formed between the intermediate layer and the substrate portion. The forward bias voltage of the upper junction (D 1 ) is lower than the forward bias voltage of the lower junction (D 2 ). The charges are permitted to be stored in the photodiode until the said upper junction is forward-biased so as to favor (A 1 ) the recombination of the carriers coming from the intermediate layer with the carriers of the upper layer.

Description:
FIELD OF THE INVENTION 
   The invention relates to microelectronics, especially to integrated circuits which include photodiodes and more particularly to the blooming control of such a photodiode. 
   BACKGROUND OF THE INVENTION 
   Image sensors based on semiconductor components work on the principle of converting photons into electron/hole pairs in silicon. More specifically, the charges created in the photosensitive regions are stored in the photodiode and are then read by an electronic system. This electronic system, which operates the photodiode, includes, especially when the photodiode is a fully depleted photodiode, a transfer transistor permitting transfer of the charges stored in the photodiode. 
   If no precautions are taken, under blooming conditions, the electrons that can no longer be stored in the photodiode can then diffuse into the semiconductor substrate where they disturb all the photodiodes of the image sensor. The visual result of this is an increasingly large white halo around the image. 
   To remedy this problem, one solution consists in defining the photodiode saturation level by the voltage applied to the gate of the transfer transistor. More specifically, this voltage is chosen so that the charges will be drained away towards the supply (via controlled conduction of a resetting transistor) before they diffuse into the substrate and degrade the signal received by the neighbouring photodiodes. 
   However, this voltage must be adjusted by a specific supply and must take into account the dispersions associated with the technology. Furthermore, these dispersions result in a reduction in the storage dynamic range of the photodiode. 
   SUMMARY OF THE INVENTION 
   The invention aims to remedy this drawback and provides a radically different solution for controlling the blooming of a photodiode. 
   The invention therefore provides an integrated circuit comprising at least one cell which includes a photodiode, this photodiode having a lower pn junction, formed between one portion of a semiconductor substrate and an intermediate layer located on the said substrate portion, and an upper pn junction formed between the intermediate layer and an upper layer more highly doped than the intermediate layer. 
   Moreover, the forward bias voltage of the upper junction is lower than the forward bias voltage of the lower junction. 
   Thus, the photodiode will store charges until the surface diode (upper junction) is turned on and the carriers (for example electrons) “not storable” in the photodiode will then be recombined with the carriers (for example holes) of the surface layer (upper layer) instead of diffusing into the substrate. 
   Thus, according to the invention, the maximum storage level is no longer defined by the channel voltage of an MOS transistor (the transfer transistor) but by the forward bias voltage of a junction. 
   The storage dynamic range of the photodiode is consequently increased. The problems associated with technology dispersions are also solved. 
   Moreover, the blooming control means comprise in this case the upper junction and the upper layer. They therefore form an integral part of the photodiode, this also being a general characteristic of the integrated circuit according to the invention. 
   The doping of the upper layer must be greater than the doping of the intermediate layer so as to contain more carriers (holes, for example) than the carriers (electrons) of the intermediate layer, so as to allow recombination during blooming. 
   Moreover, even if the forward bias voltage of the upper junction is lower than the forward bias voltage of the lower junction of the photodiode, there nevertheless remains in general a current in the substrate which also results from the lower junction being forward biased. 
   A person skilled in the art will know how to adjust, according to the applications and the maximum desired storage level of the photodiode, on the one hand, the doping of the upper layer with respect to the doping of the intermediate layer and, on the other hand, the ratio of the forward bias current of the upper junction to the forward bias current of the lower junction. 
   This being so, it is preferable to choose the doping of the upper layer to be at least five times higher, for example ten times higher, than the doping of the intermediate layer. Moreover, it is preferable to choose respective forward bias voltages of the two junctions so as to obtain a forward bias current of the upper junction at least ten times higher than the forward bias current of the lower junction. 
   According to one way of implementing the invention, the upper layer is formed from indium-doped silicon, while the intermediate layer is formed from phosphorus-doped silicon and the said substrate portion is formed from boron-doped silicon. 
   Having said this, other materials could be envisaged. 
   Thus, it is possible to replace the phosphorus with arsenic or antimony. In general, it will be possible to choose any material which decreases the gap between the valence band and the conduction band of silicon. 
   The subject of the invention is also an image sensor which includes at least one integrated circuit as defined above. 
   The subject of the invention is also a blooming control method for a photodiode produced in a semiconductor substrate. 
   According to a general feature of this method according to the invention, the photodiode includes an upper pn junction formed between an upper layer and an intermediate layer supported by a portion of the substrate and the method permits storage of the charges in the photodiode until the said upper junction is forward-biased so as to favour the recombination of the carriers coming from the intermediate layer with the carriers of the upper layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages and features of the invention will become apparent on examining the detailed description of entirely non-limiting ways of implementation and of embodiments, together with the appended drawings in which: 
       FIG. 1  illustrates schematically an image sensor according to the invention, formed from several cells equipped with photodiodes, according to the invention; 
       FIG. 2  illustrates schematically a semiconductor substrate of a photodiode, according to the invention; 
       FIG. 3   a  illustrates schematically an example of the doping of the various layers forming a photodiode, according to the invention; 
       FIG. 3   b  illustrates schematically an example of self-biasing of a fully depleted photodiode according to the invention; 
       FIG. 4  is another schematic representation of a photodiode according to the invention, illustrating one way of implementing the method according to the invention; and 
       FIG. 5  shows two current-voltage curves relating to the two junctions of a photodiode according to the invention. 
   

   DETAILED DESCRIPTION 
   In  FIG. 1 , the reference IMS denotes in general an image sensor formed from a matrix of cells (or pixels) PX i , each including one photodiode PD and means for controlling this photodiode. 
   In the example described here, the photodiode is a fully depleted photodiode and the control means are means consisting of four transistors TT, RST, FT and ST. Having said this, the invention is not limited to a fully depleted diode and applies to any photodiode, especially one whose associated control means comprise only three transistors. 
   The structure and the operation of the four-transistor control means of a fully depleted photodiode are conventional and well known to those skilled in the art. The main characteristics thereof will be recalled here with reference to  FIG. 1 . 
   These control means comprise a transfer transistor TT connected on one side to the photodiode PD and on the other side to the gate of a follower transistor FT. The gate of the follower transistor FT is also connected to the supply voltage Vdd via a resetting transistor, labelled RST. 
   Finally, a transistor ST for selecting the cell PX i  connects the follower transistor FT to a capacitive output register. 
   With regard to the operation of the cell, the first phase is a phase of integrating or of storing the charges in the photodiode, during which phase the transfer transistor TT is off. During most of the integration phase, the resetting transistor is on. Then, just before the charges are transferred from the photodiode to the output, the transistor RST is turned off and then the selection transistor ST is turned on, thereby making it possible to determine the level of charge in the photodiode before transfer. 
   During transfer, the transfer transistor TT is turned on and then, while the resetting transistor is still off, the transistor TT is turned off again, marking the end of transfer. The selection transistor ST is then again turned on so as to be able to determine the level of charge after transfer. 
   After this second measurement, the resetting transistor RST is again turned on, while a new integration phase has already started. 
   The amount of light received by the photodiode is then determined by the difference between the two charge levels measured before and after transfer, respectively. 
   Although in the prior art the blooming control means include a specific supply for regulating the low level of the voltage applied to the gate of the transfer transistor TT, the control means according to the invention form an integral part of the photodiode PD, as explained in detail below. 
   The image sensor IMS, and more particularly each cell or pixel PX i , is made in complementary metal oxide semiconductor (CMOS) technology. 
     FIG. 2  illustrates in greater detail the semiconductor structure of the photodiode PD of a cell PX i . 
   In this  FIG. 2 , the reference SB denotes a silicon semiconductor substrate, in this case p-doped. This substrate SB may, for example, be a p-doped well and lie within a p + -doped semiconductor wafer. The substrate SB is earthed. 
   The transfer transistor TT is a MOS transistor whose n − -doped source  2  forms, for the photodiode PD, an intermediate layer which extends above the portion  1  of the substrate SB. 
   Produced above this intermediate layer  2 , for example by implantation, is a p+-doped upper layer  3  that extends as far as an isolating region IS, for example a region of a local oxidation of silicon (LOCOS) type, using the name well known to those skilled in the art. Moreover, the portion  1  of the substrate SB comes into contact with the p+-doped upper layer  3 . 
   The photodiode PD is therefore in this case formed from these three layers, which define two pn junctions (diodes), namely an upper junction formed from the layers  2  and  3  and a lower junction formed from the layer  2  and the subjacent portion of the substrate  1 . 
   An example of the doping profile of this photodiode PD is illustrated in  FIG. 3   a . More specifically, the p + -doped upper layer  3  is more highly doped than the intermediate layer  2 , which is itself more highly doped than the subjacent portion of the substrate  1 . 
   By way of indication, the doping of the upper layer  3  may be chosen between 10 18  and 10 19  at./cm 3 . 
   The doping of the intermediate layer  2  may itself be chosen between 10 17  and 10 18  at./cm 3 , while the doping of the subjacent substrate portion  1  may be chosen to be less than 10 16  at./cm 3 . 
   With such doping of the intermediate layer  2 , the photodiode is then of the fully depleted type, that is to say it self-biases at a defined voltage when the concentration of the electrons in the layer  2  is zero at the end of transfer. In the example described, the photodiode self-biases at 1.5 volts ( FIG. 3   b ). 
   As illustrated in  FIG. 4 , and as indicated above, the photodiode PD is composed of two junctions (diodes) D 1  and D 2 . These two diodes (D 1  and D 2  are chosen so that the forward bias voltage V 1  of the diode D 1  ( FIG. 5 ) is lower than the forward bias voltage V 2  of the diode D 2 . Consequently, the photodiode PD will store charges during its illumination until the upper junction D 1  is forward biased. 
   The electrons “not storable” in the n-doped intermediate layer  2  will then be preferably transferred (arrow A 1 ,  FIG. 4 ) into the upper layer  3  to be recombined with the holes of this upper layer  3 , and they will not diffuse into the substrate and disturb the neighbouring photodiodes. 
   Thus, depending on the characteristics CD 1 , CD 2  ( FIG. 5 ) of the junctions, there may also exist, simultaneously with the forward bias current I 1  of the junction D 1 , a small forward bias current I 2  of the junction D 2 . 
   However, in the example described here, a ratio of the current I 1  to the current I 2  of ten ( 10 ) generally allows good recombination of the electrons with the holes of the upper layer without the electrons that diffuse into the substrate (arrow A 2 ,  FIG. 4 ) being detrimental to the neighboring photodiodes. 
   As an indication, if a forward bias voltage V 2  for the diode D 2  is taken as 0.6 volts, a forward bias voltage V 1  of 0.4 volts may for example be chosen for the junction D 1 . 
   Physically, to obtain this difference in forward bias voltage, it may be possible to choose a p + implantation obtained with indium, while the p-doping of the substrate portion will be obtained with boron. The region  2  may itself be n-doped with phosphorus.