PRIORITY CLAIM

This application claims the priority benefit of French Application for Patent No. 2003147, filed on Mar. 30, 2020, the content of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law.

TECHNICAL FIELD

The present disclosure relates to charge-coupled devices (CCDs) and, more particularly, aims at a two-phase charge-coupled device, that is, a device where a charge transfer is performed in two phases, also called two-phase transfer device. The present disclosure particularly aims at a light sensor, such as an image sensor, comprising such a device.

BACKGROUND

Charge-coupled devices comprising an array of insulated electrodes vertically penetrating into a semiconductor substrate are known. The properly-biased electrodes of the array define a plurality of potential wells in the substrate. When the substrate receives light, electron-hole pairs are photogenerated in the substrate and accumulate in potential wells defined between the electrodes. The charges accumulated in the potential wells are then transferred from one potential well to an adjacent potential well by modifying, in two phases, electrode biasings.

FIGS.1and2respectively correspond to the FIGS. 6A and 7 of U.S. Pat. No. 8,937,341 (incorporated by reference) and schematically show an example of an image sensor comprising a charge-coupled device provided with an array of insulated electrodes vertically penetrating into a semiconductor substrate.FIG.1is a top view of a portion of the image sensor andFIG.2is a perspective cross-section view schematically showing a portion of the sensor ofFIG.1.

A P-type doped silicon substrate1has an N-type doped layer3arranged thereon. Layer3and possibly the upper portion of substrate1form the sensor photoconversion area. A thin P-type doped layer5covers the surface of layer3.

The photoconversion area is divided into a plurality of parallel lines7, separated by rows9of insulated electrodes11extending vertically through layers5,3, and1. Each electrode11is formed of a T-shaped groove, comprising a longitudinal bar13and a transverse bar15, filled with doped polysilicon and insulated from the substrate by an oxide layer17. Longitudinal bars13are aligned and oriented parallel to the charge transfer direction. Transverse bars15are orthogonal to longitudinal bars13and are arranged upstream of longitudinal bars13with respect to charge transfer direction19. The electrodes11of two adjacent rows9are opposite two by two and form electrode columns21. The opposite ends of two transverse bars15of two successive electrodes of a same column21are separated from each other by a space e1.

A pixel is defined by the region comprised between two successive electrodes11G11, G12of a first row9, and the two corresponding electrodes G11, G12of a second row9adjacent to the first row.

All the electrodes of a same column21are interconnected and are set to a same potential. Electrodes11, properly biased, define in each line7a succession of potential wells where electric charges may be stored.

During an image acquisition step, the upper surface of the sensor is illuminated and electrons, resulting from the creation, by absorption of a photon, of an electron-hole pair in the photoconversion area, are stored in the potential wells. Substrate1is connected, in operation, to a reference potential, the ground, enabling the holes resulting from the photoconversion to flow.

After the acquisition step, a transfer step is provided, during which the charges stored in the potential wells are transferred, in the direction of arrows19. The charge transfer is ensured by successive switchings, between high and low states, in two phases, of control signals Φ11, Φ12applied to respective electrodes G11, G12.

FIG.3is a copy of FIG. 9 of the above-mentioned U.S. Pat. No. 8,937,341 and schematically illustrates the storage, during an image acquisition step, of photogenerated electrons in potential wells formed by application of control signals Φ11, Φ12to electrodes G11, G12of the sensor described in relation withFIGS.1and2.FIG.3further illustrates a transfer in two phases of the electrons, from one potential well to an adjacent potential well, by switching between high and low states of control signals Φ11and Φ12.

At a time t0corresponding to the end of an image acquisition step, control signal Φ11is in the low state, for example, at a potential in the order of 0 V, control signal Φ12is in the high state, for example, at a potential in the order of 5 V, and electrons, represented by the hatched areas of the drawing, are stored in potential wells.

In a column21, when two opposite electrodes G11or two opposite electrodes G12are set to a same potential, the two opposite electrodes, and more particularly the longitudinal bars13of these opposite electrodes, define together a potential well where photogenerated electrons may be stored. The higher the potential applied to the two electrodes, the deeper the potential well defined by the two electrodes.

Further, when two opposite electrodes G11or G12are set to the same potential, a potential barrier is created in space e1between the opposite ends of transverse bars15of the two electrodes.

When the two opposite electrodes G11or G12are at a same high potential, the potential barrier in the corresponding space e1is low and conducts electrons, in transfer direction19, from a shallower potential well to an adjacent deeper potential well of the same line7. More particularly, when the opposite electrodes G11and the opposite electrodes G12respectively are in the low state (G11) and in the high state (G12), the potential in space e1between electrodes G12is at an intermediate level between the potential well defined by the low level applied to electrodes G11and the potential well defined by the high level applied to electrodes G12and thus conducts electrons, in transfer direction19, in a same line7, from the shallower potential well between electrodes G11to the adjacent deeper potential well between electrodes G12.

When the two opposite electrodes G11or G12are at a same low potential, the potential barrier in the corresponding space e1is high and opposes the electron flow in transfer direction19.

Thus, at time t0, electron bunches corresponding to points of the acquired image are stored in the N layer, in the potential well defined between the electrodes G12of each pixel.

At a time t0+T, T being the clock period driving the charge transfer, the control signals Φ11and Φ12applied to respective electrodes G11and G12are switched. This results in the shifting, in transfer direction19, of the potential wells and of the potential barriers. This causes the synchronous shifting of the electron bunches, in transfer direction19(towards the right in the shown example). The position of transverse bars15relative to longitudinal bars13(on the left-hand side in the shown example) determines charge transfer direction19(towards the right in the shown example).

At a time t0+2T (not shown), in each line7, the electrons stored in a pixel have been shifted to an adjacent pixel located on its right-hand side.

There is a need to improve at least certain aspects of known charge-coupled devices comprising an array of insulated vertical electrodes, and particularly of known light sensors comprising such charge-coupled devices, such as for example the sensor of U.S. Pat. No. 8,937,341.

SUMMARY

An embodiment overcomes all or part of the disadvantages of known charge-coupled devices.

An embodiment overcomes all or part of the disadvantages of light sensors, and in particular of image sensors, comprising known charge-coupled devices.

An embodiment provides a charge-coupled device comprising: an array of insulated electrodes vertically penetrating into a semiconductor substrate, the array comprising rows of alternated longitudinal and transverse electrodes, each end of a longitudinal electrode of a row being opposite and separated from a portion of an adjacent transverse electrode of said row; and electric insulation walls parallel to one another and to the longitudinal electrodes, the electric insulation walls penetrating vertically into the substrate deeper than the longitudinal electrodes, and at least two adjacent rows of electrodes being arranged between each two successive electric insulation walls.

According to an embodiment, the electrodes of the array form columns of longitudinal electrodes alternated with columns of transverse electrodes, said columns being orthogonal to the rows, preferably in each column of transverse electrodes, the transverse electrodes of the column are aligned and, in each column of longitudinal electrodes, the longitudinal electrodes of the column are parallel to one another.

According to an embodiment, between each two successive electric insulation walls, the electrodes of said at least two rows belonging to a same column are electrically coupled together.

According to an embodiment, two successive electric insulation walls laterally delimit pixels of a pixel row, preferably the device comprising a plurality of pixel rows.

According to an embodiment, two transverse electrode columns separated from each other by at least another column of transverse electrodes laterally delimit pixels of a same pixel column, preferably the device comprising a plurality of pixel columns.

According to an embodiment, the transverse electrodes laterally delimiting pixels penetrate into the substrate down to a same depth as said electric insulation walls.

According to an embodiment, the semiconductor substrate is doped with a first conductivity type and comprises, between each two successive electric insulation walls, a doped well of a second conductivity type penetrating less deeply into the semiconductor substrate than the longitudinal electrodes.

According to an embodiment, between each two successive electric insulation walls: the well extends widthwise at least from the longitudinal electrodes of said at least two rows which is the closest to a first one of the two successive electric insulation walls all the way to at least the longitudinal electrodes of that of said at least two rows which is closest to a second one of the two successive electric insulation walls; and the doped well of the second conductivity type extends lengthwise at least along the entire length of said at least two rows.

According to an embodiment, the semiconductor substrate comprises a doped layer of the first conductivity type covering each doped well of the second conductivity type, preferably said layer being more heavily doped with the first conductivity type than the semiconductor substrate, preferably said doped layer of the first conductivity type extending, between each two successive electric insulation walls, from one to the other of said two successive electric insulation walls and from one end to another of each of said two successive electric insulation walls.

According to an embodiment, said electric insulation walls are insulated conductive walls.

An embodiment provides a light sensor comprising the described device, and an interconnection structure resting on a front surface of the semiconductor substrate from which the electrodes and the electric insulation walls penetrate into the semiconductor substrate, the sensor being configured to receive light on the front surface side of the semiconductor substrate or on a back side of the semiconductor substrate opposite to the front surface.

An embodiment provides a method of two-phase charge transfer in the described device, wherein: during a charge transfer in a given direction, each longitudinal electrode of each row is electrically coupled to the adjacent transverse electrode in said row which is arranged upstream of said longitudinal electrode with respect to the transfer direction; and in a row of electrodes, a first shift signal is applied to one longitudinal electrode out of two, and a second shift signal is applied to the other longitudinal electrodes.

According to an embodiment: during a first one of the two phases of the transfer, the first signal is in a high state and the second signal is in a low state; and during the second one of the two transfer phases, the first signal is switched to the low state and the second signal is switched to the high state.

According to an embodiment: during a first one of the two phases of the transfer, the first and second signals are in the low state during a portion of the first phase, and the first signal is set to the high state during the other portion of the first phase; and during the second one of the two transfer phases, the first and second signals are in the low state during a portion of the second phase, and the second signal is set to the high state during the other portion of the second phase.

According to an embodiment, the signal received by each transverse electrode is phase leading over the signal received by the longitudinal electrode to which the transverse electrode is electrically coupled.

