Pixel with an improved quantum efficiency

The present disclosure relates to a pixel comprising: a photodiode comprising a portion of a substrate of a semiconductor material, extending vertically from a first face of the substrate to a second face of the substrate configured to receive light; a layer of a first material covering each of the lateral surfaces of the portion; a layer of a second material covering the portion on the side of the first face, first and second material having refractive indexes lower than that of the semiconductor material; and a diffractive structure disposed on a face of the photodiode on the side of the second face.

BACKGROUND

Technical Field

The present disclosure relates generally to integrated electronic circuits, and more particularly to a pixel of an integrated image sensor.

Description of the Related Art

A pixel comprises a photosensitive area, or photoconversion area, configured to convert light in electrical charges. The photosensitive area of known pixels, typically a photodiode, is made of semiconductor material.

For a given semiconductor material, for example silicon, and given operating wavelengths, for example the wavelengths in the near infrared, the quantum efficiency of known pixels may be low, the quantum efficiency of a pixel being equal to the ratio of the number of electrical charges collected by the pixel with the number of incident photons of the pixel.

BRIEF SUMMARY

Improving the quantum efficiency of a pixel may be achieved with a pixel having a silicon photodiode being configured to operate with wavelengths in the near infrared.

For example, the quantum efficiency of a pixel having a silicon photodiode configured to operate as a single photon avalanche diode for wavelengths in the near infrared may be improved

One embodiment provides a pixel, comprising:a photodiode comprising a portion of a substrate made of a semiconductor material, extending vertically from a first face of the substrate to a second face of the substrate located opposite the first face and configured to receive light at an operating wavelength of the pixel;a layer of a first material with a refractive index lower than a refractive index of the semiconductor material covering each of the lateral surfaces of said portion;a layer of a second material with a refractive index lower than the refractive index of the semiconductor material covering said portion on the side of the first face; anda diffractive structure disposed on a face of the photodiode on the side of the second face of the substrate.

According to an embodiment, each of the first and second materials is configured so that light at an operating wavelength of the pixel reaching an interface between the photodiode and said material with an angle of incidence greater than 30° is fully reflected.

According to an embodiment, the semiconductor material is silicon, the first material being silicon oxide and/or the second material being silicon oxide.

According to an embodiment:the layer of the first material has a first surface in contact with said portion and a second surface located opposite the first surface covered with a metallic layer; and/orthe layer of the second material has a first surface in contact with said portion and a second surface located opposite the first surface covered with a metallic layer.

According to an embodiment, the diffractive structure is configured to diffract light at an operating wavelength of the pixel, which reaches the photodiode on the side of the second face of the substrate, mainly in two directions orthogonal to each other when projected onto a plane parallel to the second face.

According to an embodiment, the diffractive structure comprises trenches penetrating into said portion from the second face of the substrate, the trenches being filled with one or several third dielectric materials having a refractive index different from that of the semiconductor material.

According to an embodiment, the trenches of the diffractive structure, which are parallel to each other, are arranged at a pitch equal to twice an operating wavelength inside the pixel, at more or less 20%.

According to an embodiment, in a plane parallel to the second face, said photodiode has a square or rectangular shape.

According to an embodiment, in a plane parallel to the second face, the trenches comprise first trenches extending longitudinally in a first direction orthogonal to first and second opposite edges of the photodiode, and second trenches extending longitudinally in a second direction orthogonal to third and fourth opposite edges of the photodiode.

According to an embodiment, the first trenches do not contact the second trenches.

According to an embodiment, in a plane parallel to the second face:a part of the first trenches forms a first diffraction grating extending from the first edge towards the center of the photodiode;another part of the first trenches forms a second diffraction grating extending from the second edge of the photodiode towards the center of the photodiode;a part of the second trenches forms a third diffraction grating extending from the third edge of the photodiode towards the center of the photodiode; andanother part of the second trenches forms a fourth diffraction grating extending from the fourth edge of the photodiode towards the center of the photodiode.

According to an embodiment, in a plane parallel to the second face:a part of the second trenches forms a first diffraction grating extending from the first edge towards the center of the photodiode;another part of the second trenches forms a second diffraction grating extending from the second edge of the photodiode towards the center of the photodiode;a part of the first trenches forms a third diffraction grating extending from the third edge of the photodiode towards the center of the photodiode; andanother part of the first trenches forms a fourth diffraction grating extending from the fourth edge of the photodiode towards the center of the photodiode.

According to an embodiment, the first, second, third and fourth gratings do not intersect.

According to an embodiment, at least one of the first, second, third and fourth gratings comprises at least one periodicity defect among: an omitted trench, a modification of the width of a trench with respect to the other trenches of the network, a local modification of the pitch at which the trenches of the network are arranged.

According to an embodiment, the photodiode is configured to be used as a single photon avalanche photodiode, an operating wavelength of the pixel preferably belonging to a range from 700 nm to 2000 nm and being, for example, equal to 940 nm.

DETAILED DESCRIPTION

For the sake of clarity, only the operations and elements that are useful for an understanding of the embodiments described herein have been illustrated and described in detail. In particular, the implementation of the described pixel in an image sensor comprising a matrix of identical pixels has not been described, the implementation of such image sensor being in the abilities of those skilled in the art.

In the following disclosure, a pixel configured to operate with one or several wavelengths in the near infrared, that is to say in the range from 750 nm to 1400 nm, for example a pixel configured to operate with a wavelength equal to substantially 940 nm, is considered as an example. The photodiode of the pixel is further considered to be made of silicon, which has a poor light absorption for these wavelengths. It is further considered, as an example, that the pixel is configured so that its photodiode operates in a single photon avalanche mode, or, say in other words, that the pixel comprises a single photon avalanche diode or SPAD.

The inventors here propose to increase the quantum efficiency of a photodiode of a pixel by providing a structure for dispersing, out of the normal incident angle, the light which reaches the photodiode, and by providing, at the boundary of the volume of the photodiode, surfaces configured to reflect at least part of the dispersed light. As a result, the light pathlength in the photodiode is increased, which leads to an increase of the quantum efficiency.

FIG.1is a schematic and cross-sectional view of an embodiment of such a pixel1. In the example ofFIG.1, the pixel1(in the middle ofFIG.1) is part of a matrix of pixels1of an image sensor, and two adjacent pixels1(in the left and in the right ofFIG.1) are also represented.

Each pixel1comprises a photodiode PD. The photodiode PD comprises a portion100of a substrate102made of a semiconductor material, for example silicon. Said in other words, the photodiode PD occupies a volume corresponding to that of the portion100. The portion100extends vertically from a face104of the substrate102to a face106of the substrate102, the face106being located opposite the face104. The face106is configured for receiving light at an operating wavelength of the pixel1. Said in other words, the pixel1is configured so that its photodiode PD receives light from the side of the face106of the substrate102. Said in further other words, the pixel1is configured so that it receives light from the side of the face106of the substrate102.

As an example, when the pixel1has one or several operating wavelengths in the near infrared, for example when the pixel1has an operating wavelength substantially equal to 940 nm, and when the substrate102is made of silicon, the height of the substrate102, measured between faces104and106, is in the range from 2 μm to 10 μm, typically substantially equal to 4.5 μm.

As an example, in a plane parallel to the face106of the substrate102, the photodiode PD has a square or rectangular shape. As an example, when the pixel1has one or several operating wavelengths in the near infrared, for example when the pixel1has an operating wavelength substantially equal to 940 nm, and when the substrate102is made of silicon, in a plane parallel to the face106of substrate102, the photodiode PD has the shape of a square having sides whose length is in the range from 2 μm to 20 μm, for example in the range from 5 μm to 10 μm (or in the range of 5 and 10 μm, inclusive of both).

In each pixel1, a layer108of a material with a refractive index lower than that of the semiconductor material of the substrate102covers the lateral surfaces of the portion100of the substrate102. More precisely, the layer108entirely covers the lateral surfaces of the portion100of the substrate102. The lateral surfaces of the portion100are, for example, those which are transverse or substantially orthogonal to the faces104and106of substrate102. Preferably, the material of the layer108is a dielectric material.

As the refractive index of the material of layer108is lower than that of the semiconductor material of the substrate102, at least part of the light which propagates into the photodiode PD and reaches the interface between the portion100and the layer108is totally reflected. For example, when the substrate102is made of silicon and the layer108is made of silicon oxide, light having wavelengths in the near infrared, for example a wavelength substantially equal to 940 nm, is totally reflected when its incident angle on the interface is greater than 30°, the incident angle of the light being, for example, the angle between the normal to the interface and the direction of the incident light on the interface.

According to an embodiment, as illustrated inFIG.1, a surface of layer108being located opposite a surface of layer108in contact with the portion100is covered, for example entirely covered, by a metal or metallic layer109. As a result, all the light which propagates into the photodiode PD and reaches the interface between layers108and109is reflected by the metallic layer109.

As an example, layer108and, possibly, layer109are formed by etching a vertical trench in the substrate102, at the boundaries of the portion100, by depositing layer108on the wall of the trench, and, possibly, by depositing layer109on the layer108. As it can be seen inFIG.1, a conductive material111, for example doped silicon or polysilicon, can be then deposited to fill the trench. The use of a conductive material111insulated from the substrate102by the layer108allows to apply a bias voltage on the material111, and thus to implement a capacitive deep trench insulation (CDTI).

In each pixel1, a layer110of material with a refractive index lower than the refractive index of the semiconductor material of the substrate102covers the portion100on the side of the face104of the substrate102. More precisely, the layer110entirely covers the portion100on the side of the face104of the substrate102. For example, the layer110covers the entire face104of the substrate102. Preferably, the material of the layer110is a dielectric material. Preferably, layer108and110are made of the same material.

Similarly to layer108, layer110allows to reflect at least part of the light propagating into the photodiode PD when the light reaches the interface between the portion100and the layer110.

According to an embodiment, not illustrated, a surface of the layer110being located opposite a surface of layer110in contact with the substrate102is covered with a metallic layer.

In the example ofFIG.1, an interconnection structure IT rests on the face104of the substrate102, or, said in other words, rests on the substrate102on the side of the face104. Layer110is, for example, a layer of this interconnection structure IT. The interconnection structure IT comprises portions of conductive, for example metal, conductive, or metallic, layers112. The portions of layers112are electrically insulated from each other by insulating layers of the structure IT, represented here by a single layer114comprising the layer110. Electrically conductive vias116of the structure IT are transverse to the metal layers112, such as vertically passing through the insulating layer114to couple adjacent metal layers. The vias116electrically connect portions of layers112to each other and/or to integrated components made in and/or on the substrate102on the side of face104and/or to connection or contact pads118disposed on the side of a face of the interconnection structure which is located opposite face104.

In each pixel1, a layer120of material with a refractive index lower than the refractive index of the semiconductor material of the substrate102may cover the portion100on the side of the face106of the substrate102. For example, the layer120entirely covers the portion100on the side of the face106, and, for example, covers the entire face106of the substrate102. Preferably, the material of the layer120is a dielectric material. Preferably, layers108and120are made of the same material.

Similarly to layer108, layer120allows to reflect at least part of the light propagating into the photodiode PD when the light reaches the interface between the portion100and the layer120.

In the example ofFIG.1, one or more layers122rest on the face106of the substrate102, for example on a face of the layer120which is located opposite a face of layer120in contact with the substrate102. The layers122, for example, correspond to an anti-reflective structure and/or to filters for filtering certain wavelengths of the light reaching the pixel1.

Preferably, as it is represented inFIG.1, each pixel1comprises a micro-lens124configured to focalize the light received by the pixel1toward the photodiode PD of the pixel1. The micro-lens124rests on the face106of the substrate102, and is disposed in front of, or located opposite, the face of the portion100on the side of the face106of substrate102. In the example ofFIG.1, the micro-lens124rests on and in contact with layer122.

Layer108, the possible layer109, the possible layer110, the possible metallic layer which covers layer110, and the possible layer120allow to confine the light inside the photodiode PD of the pixel1, or, said in other words, inside the portion100of the substrate102.

Each pixel1further comprises a diffractive structure125. The diffractive structure125is disposed on the face of the photodiode PD on the side of the face106of the substrate102.

According to an embodiment, the diffractive structure125of a given pixel1is configured to diffract the incident light of the pixel1, before or when the light reaches the photodiode PD of the pixel1. Preferably, the diffractive structure125is configured to diffract the light mainly in two directions orthogonal to each other when these directions are projected onto a plane parallel to the face106of the substrate102. Light is said to be diffracted mainly in two direction when, for example, at least 30% of the light reaching the diffractive structure125is diffracted in a first of these two directions and at least 30% of the light reaching the diffraction structure125is diffracted in a second of these two directions. Said in other words, at the output of the diffractive structure, all the diffraction orders superior to the zero order are suppressed in directions other than the two orthogonal directions in which the diffractive structure mainly diffracts the light.

According to an embodiment, the diffractive structure125comprises trenches126, which penetrate into the portion100of the substrate102from the face106of the substrate102. The trenches126are filled with one or several dielectric materials having a refractive index different from that of the semiconductor material of the substrate102.

As an example, when the substrate102is made of silicon and the pixel1has operating wavelengths in the near infrared, for example an operative wavelength substantially equal to 940 nm, the trenches126penetrate the substrate102over a depth in the range from 100 to 500 nm, for example in the range from 200 to 400 nm. Further, when the substrate102is made of silicon and the pixel1has operating wavelengths in the near infrared, for example an operative wavelength substantially equal to 940 nm, the width of the trenches126is, for example, in the range from 100 to 300 nm.

As an example, when the substrate102is made of silicon and when the pixel1has operating wavelengths, for example, in the near infrared, the trenches126are filled with one or several dielectric materials among silicon nitride, aluminum oxide, tantalum oxide and lanthanum oxide.

According to an embodiment, the trenches126, which are substantially parallel to one another, are arranged at a pitch equal to twice the operating wavelength inside the pixel1, at more or less 20%. The wavelength inside the pixel is the effective wavelength inside the material of the photodiode PD and is equal to the operative wavelength of the pixel, taken in air or vacuum, divided by the refractive index n of the material of the photodiode PD. For example, when the operating wavelength (in air of vacuum) of the pixel1is in the near infrared, for example equal to 940 nm, and when the photodiode PD is in silicon having a refractive index n equal to 3.6, the pitch between parallel trenches126is in the range from 420 to 630 nm (in the range of 420 and 630 nm, inclusively).

More particularly, according to an embodiment, and as it will be described in more detail withFIGS.2to6, the trenches126of the diffractive structure125of each pixel1comprises first trenches1261and second trenches1262. The trenches1261,1262are disposed so that, in a plane parallel to face106of substrate102, the trenches1261extend longitudinally in a first direction, and the trenches1262extend longitudinally in a second direction orthogonal to the first direction. Preferably, the first direction is orthogonal to two opposite edges, or sides or lateral surfaces, of the photodiode PD, the second direction being orthogonal to two other opposite edges of the photodiode PD. According to this embodiment, the trenches1261allow to diffract the light mostly in the second direction, and the second trenches1262allow to diffract the light mostly in the first direction.

Preferably, the trenches126does not cross each other, or, said in other words, none of the trench126is in contact with another trench126. In particular, the trenches1261preferably do not contact the trenches1262, which simplifies the manufacturing of these trenches.

Preferably, the trenches126do not contact the layer108.

The quantum efficiency of the pixel1is increased compared to a similar pixel which is devoid of the diffractive structure125and/or of the layers108and110. For example, there is an improvement of the quantum efficiency by a factor greater than 2 between the pixel1and a similar pixel which is devoid of the diffractive structure.

A first pixel similar to pixel1but which is devoid of the structure125and of all the layers108,109,110,120configured to confine light inside the photodiode PD, a second pixel similar to pixel1but which is devoid of the structure125, and a third pixel similar to pixel1but which is devoid of all the layers108,109,110,120configured to confine light inside the photodiode PD, are here considered. The quantum efficiency increase between pixel1and the first pixel is more than the sum of the quantum efficiency increase between the second and first pixels and of the quantum efficiency increase between the third and first pixels. Said in other words, the effect of the diffractive structure125and the effect of the layers108and110cooperate to improve the quantum efficiency of the pixel1.

Example embodiments of the diffractive structure125of the pixel1ofFIG.1will be now described in relation with theFIGS.2to6. Each of theFIGS.2to6is a schematic view of the diffractive structure125, taken in a plane parallel to the face106of substrate102(FIG.1). More precisely, the diffractive structure125of each of theFIGS.2to6is represented as if the structure125is seen from the below inFIG.1.

In these example embodiments, the photodiode PD of each pixel1has a square shape in a plane parallel to the face106of substrate102. Further, the diffractive structure125of each pixel1is formed by the trenches126comprising first trenches1261and second trench1262as described above, in relation withFIG.1. In each of theFIGS.2to6, the layer108, which in practice surrounds all the illuminated faces of the photodiode PD, or of the corresponding portion100of the substrate102, is represented.

In theFIGS.2to6, the trenches1261,1262are disposed so that, in a plane parallel to face106of substrate102, trenches1261extend longitudinally in a first direction (vertically inFIGS.2to6) orthogonal or transverse to two opposite edges200and202of the photodiode PD, and trenches1262extend longitudinally in a second direction (horizontally inFIGS.2to6) orthogonal or transverse to two other opposite edges204and206of the photodiode PD. The trenches1261are transverse to the trenches1262.

InFIGS.2,3and6, a part of the trenches1261forms a first diffraction grating G1extending from the edge200towards the center O of the photodiode PD, and another part of the trenches1261forms a second diffraction grating G2extending from the edge202towards the center O of the photodiode PD, the gratings G1et G2being delimited by dotted lines inFIGS.2,3and6. Similarly, a part of the trenches1262forms a third diffraction grating G3extending from the edge204of the photodiode PD towards the center O of the photodiode PD, and another part of the trenches1262forms a fourth diffraction grating G4extending from the edge206towards the center O of the photodiode PD, the gratings G3et G4being delimited by dotted lines inFIGS.2,3and6.

InFIGS.4and5, a part of the trenches1262forms a first diffraction grating G1extending from the edge200towards the center O of the photodiode PD, and another part of the trenches1262forms a second diffraction grating G2extending from the edge202towards the center O of the photodiode PD, the gratings G1et G2being delimited by dotted lines inFIGS.4and5. Similarly, a part of the trenches1261forms a third diffraction grating G3extending from the edge204of the photodiode PD towards the center O of the photodiode PD, and another part of the trenches1261forms a fourth diffraction grating G4extending from the edge206towards the center O of the photodiode PD, the gratings G3et G4being delimited by dotted lines inFIGS.4and5.

According to an embodiment, the gratings G1, G2, G3and G4do not intersect.

According to an embodiment, in a plane parallel to the face106of the substrate102(FIG.1), each of the gratings G1, G2, G3and G4occupies an area having a triangular shape. A base of this triangular shape is parallel to the edge respectively200,202,204and206from which the grating extends, and is disposed on the side of this edge, for example on this edge. The summit of the triangular shape, which is located opposite the base, is disposed near the center O of the photodiode PD, for example on the center O.

In the embodiment ofFIG.2, the gratings G1and G2, respectively G3and G4, are, for example, identical to each other, but disposed according to different orientations, the gratings G1, G2, G3, and G4being, for example, identical to each other, but disposed according to different orientations.

In the embodiment ofFIG.2, the grating G1is not symmetrical with respect to a plane210orthogonal to the edge200from which the grating G1extends, the plane210passing through the middle of the edge200, or, said in other words, dividing the edge200in two parts having the same length. Similarly, in the embodiment ofFIG.2, the grating G2is not symmetrical with respect to the plane210.

In the embodiment ofFIG.2, the grating G3is not symmetrical with respect to a plane212orthogonal to the edge204from which the grating G2extends, the plane212passing through the middle of the edge204. Similarly, in the embodiment ofFIG.2, the grating G4is not symmetrical with respect to the plane212.

The embodiment ofFIG.3differs from the one ofFIG.2in that each of the gratings G1and G2is symmetrical with respect to the plane210, and each of the gratings G3and G4is symmetrical with respect to the plane212.

The dissymmetry between the gratings of the diffractive structure125of the pixel1ofFIG.2allows, for example, that the trenches density near the center O of the photodiode PD is higher than in the pixel1ofFIG.3, in which the gratings are symmetrical. This results from the fact that, inFIG.2, the spacing between a trench belonging to given grating and a trench belonging to an adjacent grating is normal to a longitudinal side of one of these trenches, and extends between this longitudinal side and an extremity of the other of these trenches, whereas, inFIG.3, the spacing between these two trenches is taken along a diagonal passing by a corner of an extremity of one of these trenches and by a corner of an extremity of the other of these trenches.

In the embodiment ofFIG.4, the gratings G1and G2, respectively G3and G4, are, for example, identical to each other, but disposed according to different orientations, the gratings G1, G2, G3, and G4being, for example, identical to each other, but disposed according to different orientations.

In the embodiment ofFIG.4, the grating G1is not symmetrical with respect to the plane210. Similarly, in the embodiment ofFIG.4, the grating G2is not symmetrical with respect to the plane210.

In the embodiment ofFIG.4, the grating G3is not symmetrical with respect to the plane212. Similarly, in the embodiment ofFIG.4, the grating G4is not symmetrical with respect to the plane212.

The embodiment ofFIG.5differs from the one ofFIG.4in that each of the gratings G1and G2is symmetrical with respect to the plane210, and each of the gratings G3and G4is symmetrical with respect to the plane212. Further, the gratings G1and G2are disposed symmetrically with respect to the plane212, the gratings G3and G4being disposed symmetrically with respect to the plane210.

As already explain in relation with the pixels1ofFIGS.2and3, the trenches density near the center O of the photodiode PD may be higher in the pixel1ofFIG.4than in the pixel1ofFIG.5.

FIG.6illustrates an alternative embodiment of the diffractive structure125illustrated byFIG.2.

In this alternative embodiment, at least one of the gratings G1, G2, G3and G4comprises a periodicity defect or variation, that is to say a defect in the periodicity of the grating.

In the example ofFIG.6, each of the gratings G1, G2, G3and G4comprises the same periodicity defect.

In the example ofFIG.6, the periodicity defect corresponds to two trenches126which have been omitted in the grating (at respective locations700and701for each grating inFIG.6). In the example ofFIG.6, in each grating G1, G2, G3and G4, the two periodicity defects are symmetrical to each other, with respect to the plane of symmetry of the grating.

Other type of periodicity defect can be provided in at least one of the grating G1, G2, G3and G4, possibly in a symmetrical manner in each grating, possibly in combination with at least one missing trench126. These other types of periodicity defects, which can be used in combination in a given grating G1, G2, G3or G4, are, for example, a modification of the width of a trench and a local modification of the pitch between the trenches of the grating.

The provision of at least one periodicity defect in at least one of the gratings G1, G2, G3and G4allows further improve the quantum efficiency or to reduce the crosstalk between two adjacent photodiodes PD (FIG.1) or to prioritize a specific mode of the light, that is to say a specific wavelength allowed to propagate in the photodiode PD of the pixel.

Depending on the targeted result, those skilled in the art are capable of choosing the number, the type and the location of the periodicity defect in each grating G1, G2, G3, G4, for example by using a simulation tool such that the tool designated by the commercial appellation “Lumerical”.

Those skilled in the art are capable of implement the alternative embodiment of theFIG.6in the diffractive structure125of each of theFIGS.3to5. The diffractive structure125of the types described in relation withFIGS.2to6allows a better quantum efficiency of the pixel compared to a similar pixel wherein the diffractive structure would be implemented (in a top view) by concentric circular trenches, by line-shaped trenches all parallel to each other, by grating patterns which are not line-shaped, by periodic trenches or grooves which intersect, for example, in a lattice pattern arrangement, or by a random structuration.

In the embodiments described in relation withFIGS.2,3and6, the trenches1261of each of the gratings G1and G2are orthogonal (in the direction of their lengths) to the edge200, respectively202, from which the grating extends, and the trenches1262of each of the gratings G3and G4are orthogonal (in the direction of their lengths) to the edge204, respectively206, from which the grating extends. This allows to obtain a quantum efficiency higher than in the case ofFIGS.4and5where the trenches1262of each of the gratings G1and G2are parallel (in the direction of their lengths) to the edge200, respectively202, from which the grating extends, and the trenches1261of each of the gratings G3and G4are parallel (in the direction of their lengths) to the edge204, respectively206, from which the grating extends.

The pixel1which has been described in relation withFIG.1, is said to be a back side illuminated pixel as it receives light on the side of the face106of the substrate102which is located opposite the face104of the substrate102on which the structure IT rests. Those skilled in the art are capable of adapting the above description to the case of a front side illuminated pixel, that is to say a pixel which receives light on the side of the face104of the substrate102, on which the structure IT rests.

Further, although in all the embodiments of the diffractive structure125which have been described above, the diffractive structure comprises trenches126penetrating into the substrate102, those skilled in the art are capable of adapting the above description to the case where the diffractive structure125is made of bars in one or several first dielectric materials, embedded in a layer made of one or several second dielectric materials having a refractive index different from those of the first dielectric materials, the bars being then, for example, arranged as described for the trenches1261,1262in relation with theFIGS.2to6.

Further, the present description is not limited to a substrate102made of silicon. For example, in other embodiments, the substrate102comprises or is made of germanium or silicon-germanium.

More generally, the present description is not limited to the case where the operating wavelength(s) of the pixel1are in the near infrared. Those skilled in the art are capable of adapting the above description to the case where the operating wavelength(s) of the pixel1belong to another wavelengths range, for example by adapting the dimensions and/or the pitch of the trenches126and/or by adapting the material of the substrate102, the layer108and/or the layer110.

Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these embodiments can be combined and other variants will readily occur to those skilled in the art. In particular, although it has been indicated that the photodiode PD is preferably configured to be used as a SPAD, this is not a necessary feature of the photodiode PD. Further, those skilled in the art are capable of providing a pixel1in different imaging applications, for example in adaptative optics, in a LiDAR (“Light Detection And Ranging) sensor, in a direct or indirect TOF (“Time Of Flight”) sensor, etc.

Finally, the practical implementation of the embodiments and variants described herein is within the capabilities of those skilled in the art based on the functional description provided hereinabove.

A pixel, (1) may be summarized as including a photodiode (PD) including a portion (100) of a substrate (102) made of a semiconductor material, extending vertically from a first face (104) of the substrate (102) to a second face (106) of the substrate (102) located opposite the first face (104) and configured to receive light at an operating wavelength of the pixel (1); a layer (108) of a first material with a refractive index lower than a refractive index of the semiconductor material covering each of the lateral surfaces of said portion (100); a layer (110) of a second material with a refractive index lower than the refractive index of the semiconductor material covering said portion (100) on the side of the first face (104); and a diffractive structure (125) disposed on a face of the photodiode (PD) on the side of the second face (106) of the substrate (102).

Each of the first and second materials may be configured so that light at an operating wavelength of the pixel (1) reaching an interface between the photodiode (PD) and said material with an angle of incidence greater than 30° is fully reflected.

The semiconductor material may be silicon, the first material being silicon oxide and/or the second material being silicon oxide.

The layer (108) of the first material may have a first surface in contact with said portion (100) and a second surface located opposite the first surface covered with a metallic layer (109); and/or the layer (110) of the second material may have a first surface in contact with said portion (100) and a second surface located opposite the first surface covered with a metallic layer.

The diffractive structure (125) may be configured to diffract light at an operating wavelength of the pixel (1), which reaches the photodiode (PD) on the side of the second face (106) of the substrate, mainly in two directions orthogonal to each other when projected onto a plane parallel to the second face (106).

The diffractive structure (125) may include trenches (126;1261,1262) penetrating into said portion (100) from the second face (106) of the substrate (102), the trenches (126;1261,1262) being filled with one or several third dielectric materials having a refractive index different from that of the semiconductor material.

The trenches (126;1261,1262) of the diffractive structure (125) which are parallel to each other may be arranged at a pitch equal to twice an operating wavelength inside the pixel (1), at more or less 20%.

In a plane parallel to the second face (106), said photodiode (PD) may have a square or rectangular shape.

In a plane parallel to the second face, the trenches (126) may include first trenches (1261) extending longitudinally in a first direction orthogonal to first (200) and second (202) opposite edges of the photodiode (PD), and second trenches (1262) extending longitudinally in a second direction orthogonal to third (204) and fourth (206) opposite edges of the photodiode (PD).

The first trenches (1261) may not contact the second trenches (1262).

In a plane parallel to the second face (106) a part of the first trenches (1261) may form a first diffraction grating (G1) extending from the first edge (200) towards the center (O) of the photodiode (PD); another part of the first trenches (1261) may form a second diffraction grating (G2) extending from the second edge (202) of the photodiode (PD) towards the center (O) of the photodiode (PD); a part of the second trenches (1262) may form a third diffraction grating (G3) extending from the third edge (204) of the photodiode (PD) towards the center (O) of the photodiode (PD); and another part of the second trenches (1262) may form a fourth diffraction grating (G4) extending from the fourth edge (206) of the photodiode (PD) towards the center of the photodiode (PD).

At least one of the first, second, third and fourth gratings (G1, G2, G3, G4) may include at least one periodicity defect among an omitted trench, a modification of the width of a trench with respect to the other trenches of the network, and a local modification of the pitch at which the trenches of the network are arranged.

In a plane parallel to the second face (106) a part of the second trenches (1262) may form a first diffraction grating (G1) extending from the first edge (200) towards the center (O) of the photodiode; another part of the second trenches (1262) may form a second diffraction grating (G2) extending from the second edge (202) of the photodiode (PD) towards the center (O) of the photodiode (PD); a part of the first trenches (1261) may form a third diffraction grating (G3) extending from the third edge (204) of the photodiode (PD) towards the center (O) of the photodiode (PD); and another part of the first trenches (1261) may form a fourth diffraction grating (G4) extending from the fourth edge (206) of the photodiode (PD) towards the center (O) of the photodiode (PD).

At least one of the first, second, third and fourth gratings (G1, G2, G3, G4) may include at least one periodicity defect among an omitted trench, a modification of the width of a trench with respect to the other trenches of the network, and a local modification of the pitch at which the trenches of the network are arranged.

The photodiode (PD) may be configured to be used as a single photon avalanche photodiode, an operating wavelength of the pixel preferably belonging to a range from 700 nm to 2000 nm and being, for example, equal to 940 nm.