Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F39/00—Integrated devices, or assemblies of multiple devices, comprising at least one element covered by group H10F30/00, e.g. radiation detectors comprising photodiode arrays
  • H10F39/10—Integrated devices
  • H10F39/12—Image sensors
  • H10F39/18—Complementary metal-oxide-semiconductor [CMOS] image sensors; Photodiode array image sensors
  • H10F39/186—Complementary metal-oxide-semiconductor [CMOS] image sensors; Photodiode array image sensors having arrangements for blooming suppression

Definitions

• the invention relates to microelectronics, in particular integrated circuits comprising photodiodes, and more particularly the control of over-illumination (“blooming” in English) of such a photodiode.
• Image sensors based on semiconductor components take advantage of the principle of converting photons into electron / hole pairs in silicon. More precisely, the charges created in the photosensitive zones are stored in the photodiode and are then read using an electronic system.
• This electronic system which controls the photodiode, comprises, in particular when the photodiode is a fully depleted photodiode (“fully depleted” in English), a transfer transistor authorizing the transfer of the charges stored in the photodiode.
• the electrons which can no longer be stored in the photodiode can then diffuse in the semiconductor substrate to disturb all the photodiodes of the image sensor, this which visually leads to the generation of an increasingly important white halo on the image.
• one solution consists in defining the saturation level of the photodiode by the voltage applied to the gate of the transfer transistor. More precisely, this voltage is chosen so that the charges will be evacuated towards the supply (via a conduction controlled by a reset transistor) before they diffuse in the substrate and come to degrade the signal received by neighboring photodiodes.
• this voltage must be adjusted by a specific power supply and must take into account the dispersions linked to the technology. In addition, these dispersions lead to a reduction in the storage dynamics on the photodiode.
• the invention aims to remedy this drawback and proposes a radically different solution for controlling the over-illumination of a photodiode.
• the invention therefore provides an integrated circuit comprising at least one cell comprising a photodiode, this photodiode comprising a lower PN junction formed between a part of a semiconductor substrate and an intermediate layer situated on said substrate part, and an upper PN junction formed between the intermediate layer and an upper layer more heavily doped than the intermediate layer.
• the direct conduction voltage of the upper junction is lower than the direct conduction voltage of the lower junction.
• the photodiode will store charges until the surface diode (upper junction) goes into conduction and the carriers (electrons for example) “not storable” in the photodiode will then be recombined with the carriers (holes for example) of the surface layer (upper layer) instead of diffusing into the substrate.
• the maximum level of storage is no longer defined by the voltage of the channel of an MOS transistor (the transfer transistor), but by the voltage of direct conduction of a junction.
• the storage dynamic on the photodiode is therefore increased. And we have solved the problems related to the dispersions of technology.
• the over-lighting control means here comprise the upper junction and the upper layer. They therefore form an integral part of the photodiode, which is also 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 layer intermediate, and this in order to allow recombination during over-illumination.
• a doping of the upper layer is preferably chosen at least five times greater than the doping of the intermediate layer, and for example at least ten times greater.
• the respective voltages of the direct conductions of the two junctions will preferably be chosen so as to obtain a direct conduction current for the upper junction at least ten times greater than the direct conduction current for the lower junction.
• the upper layer is formed of silicon doped with indium, while the intermediate layer is formed of silicon doped with phosphorus and that said portion of substrate is formed of silicon doped with boron.
• the intermediate layer is formed of silicon doped with phosphorus and that said portion of substrate is formed of silicon doped with boron.
• other materials are possible. One can thus replace phosphorus by arsenic or antimony. And, in general, we can choose any material which reduces the difference between the valence band and the silicon conduction band.
• the invention also relates to an image sensor comprising at least one integrated circuit as defined above.
• the invention also proposes a method for controlling the over-lighting of a photodiode produced in a semiconductor substrate.
• the photodiode comprises an upper junction PN formed between an upper layer and an intermediate layer supported by a portion of the substrate and, storage of the charges in the photodiode is authorized until direct conduction of said upper junction so as to promote the recombination of the carriers coming from the intermediate layer with the carriers of the upper layer.
• FIG. 1 schematically illustrates an image sensor according to the invention, formed of several cells equipped with photodiodes, according to the invention
• FIG. 2 schematically illustrates a semiconductor structure of a photodiode, according to the invention
• FIG. 3a schematically illustrates an example of doping of the different layers forming a photodiode, according to the invention
• FIG. 3b schematically illustrates an example of self-polarization of a photodiode, according to the invention, completely depleted
• FIG. 4 is another schematic representation of a photodiode, according to the invention, illustrating an embodiment of the method according to the invention.
• FIG. 5 represents two voltage-current curves concerning the two junctions of a photodiode according to the invention.
• the reference CIM generally designates an image sensor formed from a matrix of PX i5 cells (or pixels) each comprising a photodiode PD as well as means for controlling this photodiode.
• the photodiode is a totally depleted photodiode and the control means are means with four transistors TG, RST, SF and RS. That said, the invention is not limited to a completely depleted diode and applies to all photodiodes, especially those whose associated control means only have three transistors.
• control means with four transistors of a totally depleted photodiode are conventional and well known to those skilled in the art. We recall here, with reference to Figure 1, the main features.
• control means include a transfer transistor
• TG connected on the one hand to the photodiode PD and, on the other hand, to the gate of a follower transistor SF.
• the gate of the follower transistor SF is also connected to the supply voltage Vdd by means of a reset transistor, referenced RST.
• PX connects the follower transistor SF to a capacitive output register.
• the first phase is a phase of integration or storage of the charges in the photodiode during which the transfer transistor TG is blocked.
• the reset transistor is on.
• the selection transistor RS is made conductive, which makes it possible to determine the level of charge in the photodiode before the transfer.
• the transfer transistor TG is made conductive, then, while the reset transistor is still blocked, the transistor TG is again blocked, marking the end of the transfer.
• the selection transistor RS is then again made conductive so as to be able to determine the charge level after transfer.
• the reset transistor RST is again made conductive, 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.
• the means for controlling the over-illumination comprise a specific supply for adjusting the low level of the voltage applied to the gate of the transfer transistor TG
• the control means are, according to the invention, part integral of the PD photodiode, as detailed below.
• the CIM image sensor, and more particularly each cell or pixel PX ⁇ is produced in CMOS technology.
• FIG. 2 illustrates in more detail the semiconductor structure of the photodiode PD of a cell PXj.
• the reference SB denotes a silicon semiconductor substrate, doped here P.
• This substrate SB can for example be a P doped well and located within a P + doped semiconductor plate.
• the substrate SB is connected to ground.
• the transfer transistor TG is a MOS transistor whose source 2, N " doped, forms for the photodiode PD an intermediate layer which extends above part 1 of the substrate SB.
• this intermediate layer 2 is produced, for example by implantation, an upper layer 3, P + doped, which extends to an isolation region IS, for example a region of the LOCOS type, according to a name well known to those skilled in the art. Furthermore, part 1 of the substrate SB comes into contact with the upper layer 3, P + doped.
• the photodiode PD is therefore here formed of these three layers, which define two PN junctions (diodes), namely an upper junction formed by layers 2 and 3, and a lower junction formed by layer 2 and the underlying part of substrate 1.
• FIG. 3a An example of the doping profile of this photodiode PD is illustrated in FIG. 3a. More precisely, the upper layer 3, P + doped, is more strongly doped than the intermediate layer 2, itself more strongly doped than the underlying part of the substrate 1.
• a doping of the upper layer 3 will be chosen between 10 18 and 10 19 at./cm 3 .
• the doping of the intermediate layer 2 could be chosen between 10 17 and 10 18 at./cm 3 , while the doping of the underlying part 1 of the substrate could be chosen less than 10 16 at./ cm 3 .
• the photodiode With such doping of the intermediate layer 2, the photodiode is then of the totally depleted type, that is to say that it self-polarizes at a determined voltage when the concentration of electrons in layer 2 is zero at the end of transfer. In the example described, the photodiode self-polarizes at 1.5 volts ( Figure 3b).
• the photodiode PD consists of two junctions (diodes) D1 and D2. These two diodes Dl and D2 are chosen so that the voltage VI for direct conduction of the diode Dl (FIG. 5) is less than the voltage for direct conduction V2 of the diode D2. As a result, the photodiode PD will store charges during its illumination until the upper junction D1 goes into direct conduction.

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• 2001
  • 2001-12-12 FR FR0116047A patent/FR2833408B1/fr not_active Expired - Fee Related
• 2002
  • 2002-12-12 JP JP2003551836A patent/JP2005512341A/ja active Pending
  • 2002-12-12 US US10/498,731 patent/US7586172B2/en not_active Expired - Lifetime
  • 2002-12-12 EP EP02799810A patent/EP1454357A2/de not_active Withdrawn
  • 2002-12-12 WO PCT/FR2002/004302 patent/WO2003050874A2/fr not_active Ceased

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