FR2971622A1 - Complementary metal oxide semiconductor image sensor cell for microelectronic device, has reading node that is in contact with doped region and connected to reading circuit, where node is arranged at distance away from edges of region - Google Patents

Abstract

The sensor cell has a photosensitive area (110) for capture and conversion of photons, where the area includes a doped region (112) and another doped region (111) forming a junction with the former doped region. A reading node (130) is in contact with the former doped region and connected to a reading circuit, where the reading node is arranged at minimum distance away from edges of the former doped region, of about one-tenth of a critical dimension (Dc2) of the former doped region. An independent claim is also included for a microelectronic device.

Description

IMAGE SENSOR PIXEL HAVING A READING NODE HAVING AN IMPROVED ARRANGEMENT

TECHNICAL FIELD The present invention relates to the field of imaging microelectronic devices or image sensors, in particular those made in CMOS technology. It relates to an image sensor pixel having an improved arrangement and is particularly applicable to the implementation of sensors comprising pixels of large size, for example greater than 10 μm. It brings improvements in particular in terms of signal-to-noise ratio compared to conventional imaging pixels. STATE OF THE PRIOR ART In a CMOS-type image sensor, the pixels are generally formed of a detector element 20 such as a photodiode and of a read circuit, for example in the form of a charge converter. voltage associated with transistors. An exemplary equivalent electrical diagram of a 3-transistor CMOS imager pixel commonly referred to as "3T pixel" is illustrated, for example, in FIG. 1A. The charge-voltage conversion is obtained at the terminals of a capacitance Cpix formed by the depletion capacitance of the PN junction of a photodiode 10, the capacitances of a reset transistor T1 and a transistor T2 reading and interconnection capacities. The voltage is read through a transistor T2 called "reading". The photodiode is merged with the read node and preloaded with the reset transistor at the beginning of each integration. The photodiode may be a photodiode of the type commonly known as "pinned" as described in "32 32 SOI CMOS Image Sensor with Pinned Photodiode on Handle Wafer", Cho et al., OPTICAL REVIEW Vol. 14, No. 3 (2007) 125-130, February 2007. There are also 4-transistor pixels commonly referred to as "4T pixels", in which the integration function is dissociated from that of conversion (Figure 1B). This configuration makes it possible to overcome the reset noise without having to resort to a memory external to the sensor. Compared with a 3T configuration, the 4T configuration also makes it possible to maximize the charge-voltage conversion factor of the pixel and to reduce the influence of the temporal noise of the reading transistor by amplifying the signal present on the capacity of the reading node.

A transfer gate electrode of a transistor T4 controls the charge transfer from the photodiode 10 to the read node. An example of an image sensor pixel structure 3T according to the prior art is given in FIGS. 2A and 2B. The pixel is formed of a photosensitive zone 11 which may be a photodiode in which electrons are to be generated. These electrons are recovered by a reading node 13 disposed on the periphery of the pixel sensitive zone 11, the node 13 being a conductive element connected to the diode and to a reading circuit, this circuit being located outside the sensitive zone 11 and near the latter.

In order not to mask the illuminated area of the sensitive area and facilitate the connection with the read circuit, the reading node is placed at the edge of the sensitive area. Most of the time, it is sought to produce smaller and smaller pixels in order to implement image sensors having a high pixel integration density. However, for some applications, particularly in the field of medical imaging, it may be advantageous to make large pixels, in particular having a sensitive area greater than 10 microns. The collection and, in the case of the 4T pixel, the transfer of charges from the photodiode to the reading node may be insufficient, especially when the pixels are large. Incomplete charge transfer can, when it concerns several pixels, create a phenomenon of remanence between images generated by a set of pixels, non-uniformity of response between the pixels of a sensor, and temporal noise. There is the problem of implementing a new improved image sensor pixel structure with respect to the disadvantages mentioned above.

PRESENTATION OF THE INVENTION The present invention relates to an image sensor cell comprising a photosensitive zone, capture of (s) photon (s) and conversion of (s) photon (s) into charges, comprising at least a first doped zone and at least one second doped zone making a junction with said first doped zone, at least one reading node connected to a read circuit, the device being characterized in that said reading node is arranged at a minimum distance Amin of edges of said first doped zone of at least 1/10 of the critical dimension Dc2 of said first doped zone. Thus, the average path that loads must travel to the read node is decreased. This improves the collection and transfer of charges in a pixel, in particular if the pixel is large and comprises, for example, a first doped zone of critical dimension greater than 10 μm. By critical dimension is meant throughout the description, the smallest dimension of a zone or a layer or a stack of layers except its thickness.

According to one implementation possibility, the pixel may comprise several reading nodes connected to the reading circuit. This also improves the collection and transfer of charges in a pixel, particularly if this pixel is large. The pixel may be implemented so that at least one reading node is located in the center of said first doped area.

The pixel may also be implemented so that at least one reading node is located closer to the center of said first doped zone or to another reading node than the edges of said first doped zone.

This reduces the average path that loads are likely to travel to the reading node. The reading node (s) may comprise a given doped zone in contact with said first doped zone. According to one possible implementation, said given doped zone may be located at a distance d from the second doped zone, with d lying between 0.2 μm and 1 μm.

The given doped zone may thus be provided sufficiently far from the second doped zone to avoid breakdown and sufficiently close to the second doped zone to allow charge transfer.

The cell may further comprise a plurality of lenses disposed opposite said first doped zone about an axis passing through a reading node and orthogonal to said first doped zone. The cell may further comprise a gate electrode disposed around at least one read node, and connected to said read circuit. The invention also relates to a microelectronic device comprising a matrix of cells as defined above. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood on reading the description of exemplary embodiments given, purely by way of indication and in no way limiting, with reference to the appended drawings in which: FIGS. 1A and 1B respectively illustrate a example of a prior art 3T image sensor device, and an example of a prior art image sensor device 4T, FIGS. 2A and 2B, illustrate an exemplary pixel structure of a next imaging device FIGS. 3 and 4 illustrate an example of an imaging device according to the invention in which a reading node connected to a reading circuit is arranged in the center or in a central region of the pixel, FIGS. 5C illustrate an example of a diode of the type commonly called "pinned", implemented within an image sensor pixel according to the invention; - FIG. 6 illustrates an example of a pixe structure 1 of the image sensor according to the invention in which a reading node connected to a read circuit is arranged at the center of the pixel between micro-lenses, - FIG. 7 illustrates an example of the mode of operation of this pixel, and in particular a potential profile during an integration of photogenerated charges in this pixel, - Figures 8A-8B and 9, illustrate another example of an image sensor pixel according to the invention, in which a reading node is disposed at the center of the pixel and surrounded by a charge transfer control gate electrode to a read circuit; - FIGS. 10A-10B illustrate an exemplary mode of operation of this other pixel, - FIGS. 11A-11B illustrate another example of an image sensor pixel according to the invention, in which a reading node is arranged at the center of the pixel and surrounded by a charge transfer control gate electrode to a read circuit, figure 12 illustrates another example of an image sensor pixel according to the invention, provided with a plurality of reading nodes, each reading node being closer to another reading node or to the center of the pixel than to the edges of the latter; FIGS. 13A-13B illustrate another example of a pixel according to the invention. Identical, similar or equivalent parts of the different figures bear the same numerical references in order to facilitate the passage from one figure to another. The different parts shown in the figures are not necessarily in a uniform scale, to make the figures more readable. DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS An example of an elementary cell also called a "pixel" of an image sensor device according to the invention will now be described in connection with FIGS. 3, 4, 5A-5C, 6 and 7. This pixel is formed of a photo-sensitive zone 110 made in a support layer 100. The support layer 100 may have been formed for example by epitaxy and may be doped, for example by P-type doping. photosensitive zone 110 comprises at least two superimposed doped zones forming a diode junction, for example a PN type junction. In this example, the photosensitive zone comprises a first doped zone 112 of a first type, for example N type, surmounted by a second doped zone 111 doping a second type, for example P + type . The diode thus formed may be a type of diode commonly called "pinned", whose doping profile, and the threshold voltage profile may be such as those illustrated for example respectively in Figures 5C and 5B. In FIG. 5C, the curve referenced C2 is representative of a concentration profile of the P-doped support layer 100, and in particular of the concentration of doping species in this layer as a function of its thickness, while the referenced curve C1 gives an example of the implantation profile of the first doped zone 112, the curve referenced Co being, for its part, representative of an exemplary implantation profile of the second doped zone 111 disposed above the first zone. In FIG. 5B, curves C3 and C4 illustrate a potential variation of the photodiode as a function of the depth in the photosensitive zone, depending on whether charges have been created (curve C3) or not (curve C4) in this zone. The second P + doped area 111 at the surface of the photodiode may have a thickness or depth e1 measured in a direction parallel to the vector k of an orthogonal reference [O; 1; j; k]) for example between 100 nanometers and 200 nanometers , while the first N-doped zone 112 situated under the P + doped zone may have a thickness or depth e2 for example between 700 nanometers and 1000 nanometers (FIG. 5A). The pixel may be delimited by isolation trenches 180 and may have a width or critical dimension Dc which may be of the order of several micrometers or for example greater than 10 micrometers. By critical dimension of an area is meant throughout the present description the smallest dimension of this area (measured in a plane parallel to the plane [0; i; j] of an orthogonal reference [0; i; j; k] defined in Figures 3, 4 and 6) except its thickness 10 (the thickness being measured in a direction parallel to the vector k). The first doped zone 112 may, for its part, have a critical dimension Dc2 which may for example be at least 10 μm between 10 μm and 20 μm. while the second doped zone 111 has a critical dimension Dc1 greater than that Dc2 of the first zone 111, for example between 12} and 25} gym.

A read node 130 connected to a read circuit 150 and to the first N-doped zone 112 is disposed at the center or in a central region of the pixel, and in particular at the center or in a central region of the first N-doped zone 112. By arranging the node 130 closer to the center 114 of the N-doped zone than the edges 113 of the latter, the collection of the charges is thus facilitated, by reducing the path taken by collected charges towards the reading node 130. This reading node 130 may be formed of a more doped contact zone 132 than the first zone 112 and in contact therewith. The contact zone 132 may have been formed, for example, by N + doping in the first N-doped zone 112. The node 130 also comprises a conductive pad 131 formed on the contact zone 132. This conductive pad 131 may be metallic and connected to the reading circuit via at least one metal track 140 passing through a portion of the pixel and terminating at the periphery of the latter (FIGS. 3 and 4). The second doped zone 111 and the contact zone 132 are spaced a distance d, chosen sufficiently large to prevent a breakdown between the P + and N + zone and sufficiently low to allow a charge transfer from the photodiode to the node of reading 130. The distance d can be for example between 0.2 pm and 1 pm (Figure 6). The read circuit 150 associated with the pixel can be dedicated to it and located on the periphery of the latter. This read circuit 150 may comprise, for example, at least one charge / voltage converter for converting a quantity of charges originating from the reading node into an electrical voltage. The read circuit 150 may be for example a 3T circuit formed of 3 transistors, including a read transistor T20 having its gate connected to the read node 130 via the conductive track 140, a line control transistor T30, and a reset transistor T10 (FIG. 3).

FIG. 7 illustrates a potential profile during an integration of photogenerated charges within the device of FIG. 6. On this profile, OV trays illustrate the isolation of the pixel, while "Vpinned diode" trays correspond to the maximum voltage of the photodiode, fixed by the respective doping of the zones 111, 112 and 100. The voltage Ref corresponds to the potential of precharging of the pixel, Ref-Signal corresponding to the potential of the reading node with 12 storage charges during the integration of a luminous flux. The pixel may also be provided with convergent microlenses 190 arranged opposite the photosensitive zone 110. The microlenses 190 may be arranged around an axis passing through the reading node 130 and which is orthogonal to the main plane of the doped zone. 132 (the main plane of the doped zone being a plane passing through the latter and parallel to the plane [0; i; j] defined in FIG. 4). According to one possible embodiment, microlenses 290 (shown in broken lines in FIG. 4) may be provided with a shape, for example square, to cover the entire photodiode and may also cover the node of reading. In Figures 8A-8B and 9, an alternative embodiment of the pixel is illustrated. For this variant, the pixel is associated with a reading circuit 250, with 4 transistors and which may be of the type shown in FIG. 1B. In this exemplary embodiment, the reading circuit 250 comprises an additional transistor provided with a gate electrode 251 surrounding the reading node 130. This gate electrode 251 makes it possible to control the transfer of the charges from the photodiode to the reading circuit. 150, depending on a control signal. The control signal is applied to this transfer gate 251 via a control line 240 connected to the transfer gate 251 via vertical connection elements 245, 246. The reading node 130 is also centered and further away from the edges of the first doped zone 112 than from its center, in order to reduce the path to be traveled for the collected charge carriers. In FIGS. 10A and 10B, a charge integration phase and then a reading of the charges transferred via the transfer gate 251 are illustrated. According to another exemplary embodiment, an image sensor pixel according to the invention may be provided with several reading nodes.

FIGS. 11A and 11B illustrate a pixel provided with several reading nodes, in particular 4 nodes 2302r 2302, 2303, 2304 connected to the same reading circuit 150, via conductive tracks 341, 342, 343, 344.

In order to limit the path of charge carriers to each of the reading nodes 2302r 2302, 2303r 2304 are located at a given distance Amin of at least 10% of the critical dimension of said first doped zone, for example at least 1 the edge 113 of the first zone 112 doped with the photosensitive zone when the latter has a critical dimension Dc2 of the order of 10 μm. In Fig. 11B, each read node 2302r 2302, 2303, 2304 is surrounded by a plurality of microlenses. According to an alternative embodiment of this pixel structure with several reading nodes, each reading node may be located closer to the center of said first doped zone 111 or to another reading node than the edges of said first doped zone. . Thus, a distance A 'less than the given distance Amin can be provided between each node so as to allow better collection of charges (FIG. 12).

Another example of a pixel is illustrated in FIGS. 13A-13B. In this example, the junction is formed of the first doped zone surmounted by a second doped zone in the form of several distinct regions 211a, 211b, 211c, 211d.

Claims (9)

REVENDICATIONS1. An image sensor cell comprising: - a photosensitive zone (110) for capturing photon (s) and for converting photon (s) into charges, comprising at least a first doped zone (112) and at least one second doped region (111) forming a junction with said first doped zone, - at least one reading node (130, 2301r 2302r 2303, 2304) connected to a read circuit (150, 250), the device being characterized in that said reading node is arranged at a minimum distance (Amin) from edges of said first doped area of at least 1/10 of the critical dimension (Dc2) of said first doped area. An image sensor cell according to claim 1, comprising one or more read nodes (2301r 2302, 2303, 2304) connected to the read circuit (150). An image sensor cell according to claim 1 or 2, comprising one or more read nodes (2301r 2302, 2303, 2304), at least one read node being located closer to the center of said first area (132). doped or other reading node as edges (133) of said first doped area (112). 4. An image sensor cell according to one of claims 1 to 3, comprising one or more reading nodes 16, at least one reading node (130) being disposed at the center or in a central region of said first doped zone ( 112). An image sensor cell according to one of claims 1 to 4, wherein the at least one reading node comprises a given doped area (132) in contact with said first doped area (112), said given doped area ( 132) being located at a distance d from the second doped zone, with d lying between 0.2 μm and 1 μm. An image sensor cell according to one of claims 1 to 5, further comprising a plurality of lenses (191) disposed opposite said first doped region (111) about an axis passing through a reading node. (130, 2301r 2302, 2303, 2304) and orthogonal to said first doped zone. An image sensor cell according to one of claims 1 to 6, further comprising a gate electrode (251) disposed around at least one read node (130), and connected to said read circuit. 8. image sensor cell according to one of claims 1 to 7, the first doped zone having a dimension greater than 10}. 9. Microelectronic device comprising a matrix of cells according to one of claims 1 to 8.