Light sensor

Abstract

Light Sensor This description relates to an image sensor (10) comprising a first layer (14) of photoelectric material and a diffraction grating (24) located between said first layer (14) and the face of the sensor configured to receive light rays. Figure for the abbreviation: Fig. 1

Description

Description Title of the invention: Light sensor technical field This description generally relates to optoelectronic devices, preferably light sensors. Previous technique Light sensors are optoelectronic devices capable of generating charges upon receiving light rays. Light sensors preferably include a layer or region made of a photoelectric material, that is, a material that absorbs photons and generates electrical charges. Light reaching the layer of a photoelectric material is not entirely absorbed by the layer. Some of the light is reflected off the layer, and some passes through it. A portion of the light received by the light sensor does not generate a charge. Light sensors thus lose efficiency. Summary of the invention One embodiment overcomes all or part of the disadvantages of known light sensors. One embodiment provides an image sensor comprising a first layer of photoelectric material and a diffraction grating located between said first layer and the face of the sensor configured to receive light rays. According to one embodiment, the diffraction grating has a periodic shape. According to one embodiment, the diffraction grating comprises a plurality of first blocks. According to one embodiment, the first blocks are identical to each other. According to one embodiment, the diffraction grating comprises second blocks having at least one dimension different from that of the first blocks. According to one embodiment, the diffraction grating and the layer are separated by a first electrode. According to one embodiment, the first electrode is a second conductive layer completely covering the first layer. According to one embodiment, the sensor includes second electrodes in contact with the first layer on the side opposite the diffraction grating. According to one embodiment, the sensor comprises a substrate on which the first layer rests. According to one embodiment, each second electrode comprises a stud conductor located in the substrate. According to one embodiment, each second electrode comprises a third conductive layer resting on one of the conductive pads and partially resting on the substrate. Brief description of the drawings These features and advantages, as well as others, will be described in detail in the following description of particular embodiments, given by way of non-limiting example, in relation to the attached figures, among which: [Fig.1] represents one embodiment of a light sensor; Figure 2 represents another embodiment of a light sensor; and Figure 3 schematically illustrates the operation of the embodiments of Figures 1 and 2. Description of the implementation methods The same elements have been designated by the same reference numerals in the different figures. In particular, structural and / or functional elements common to the different embodiments may have the same reference numerals and may have identical structural, dimensional and material properties. For the sake of clarity, only the steps and elements useful for understanding the implementation methods described have been represented and are detailed. Unless otherwise specified, when referring to two connected elements, this means directly connected without any intermediate elements other than conductors, and when referring to two coupled elements, this means that these two elements can be connected or linked through one or more other elements. In the description that follows, when referring to absolute positional qualifiers, such as the terms “front”, “back”, “top”, “bottom”, “left”, “right”, etc., or relative positional qualifiers, such as the terms “above”, “below”, “superior”, “inferior”, etc., or to orientational qualifiers, such as the terms “horizontal”, “vertical”, etc., it refers, unless otherwise specified, to the orientation of the figures. Unless otherwise specified, the expressions 'approximately', 'roughly', 'significantly', and 'on the order of' mean within 10%, preferably within 5%. Figure 1 shows an embodiment of a light sensor 10. The sensor 10 is configured to receive light rays 12 through a front or top face. The sensor 10 is configured to generate electrical charges upon receiving light rays having a wavelength λ equal to an operating wavelength of the sensor 10, i.e., equal to a value λ0 or within a range of wavelengths between λ01 and λ02. The values ​​λ0, λ01 and 102 are for example between 250 nm and 1500 nm, which corresponds to radiation ranging from ultraviolet (UV) to near infrared (NIR) via visible. The sensor 10 includes a layer 14 made of a photoelectric material. The photoelectric material is configured to generate charges when light rays, having a wavelength equal to the operating wavelength, are in contact with the photoelectric material. The longer the light is in contact with the photoelectric material, the greater the number of charges generated. The layer is preferably a planar layer, that is, the layer 14 preferably comprises a lower and an upper face parallel to each other. Layer 14 can be, for example, a quantum film, a quantum well, a quantum dot layer, a type III-V material, silicon, InGaS, or another photoelectric material. The choice of material depends, for example, on the operating wavelength of the sensor. Preferably, the top and bottom surfaces of layer 14 are parallel. A type III-V material is defined as an alloy of at least one material from group III (Boron, Aluminum, Gallium, Indium, Titanium) and at least one material from group V (Nitrogen, Phosphorus, Arsenic, Antimony, Bismuth, Moscovium). Layer 14 rests on a support 16. The support 16 is, for example, a semiconductor substrate, such as a semiconductor-on-insulator (SOI) substrate. Alternatively, the support 16 is a stack of insulating layers comprising conductive tracks and conductive vias. The sensor 10 comprises at least one lower electrode 17, preferably a plurality of lower electrodes 17. A single electrode 17 is shown in [Fig. 1]. The lower electrode 17 corresponds, for example, to a pixel of the light sensor. The electrode 17 comprises a conductive region 18, for example, made of metal, such as copper, located in the support 16. The region 18 is preferably flush with the upper surface of the support 16. The electrode 17 comprises, for example, a conductive layer 20, for example, made of metal, such as titanium nitride. The conductive layer 20 preferably covers the region 18 entirely and, for example, at least partially, the layer 16. The layer 20 is preferably separated from the layers 20 of the neighboring electrodes, for example, by a portion of the layer 14 or by a portion of insulating material. Layer 20 is thus in contact, on one side, with region 18, and on the other side with layer 14. Layer 20 electrically connects layer 14 to region 18. Thus, the charges generated in layer 14 during the absorption of light rays are transmitted through electrode 17, for example to a data processing circuit. The sensor 10 includes an upper electrode 22. Electrode 22 is preferably a conductive layer, for example, made of a metal. Electrode 22 covers, for example, the entire layer 14. Electrode 22 is, for example, common to all pixels of the sensor 10. Electrode 22 is at least partially transparent. Electrode 22 is preferably transparent to wavelengths λ equal to an operating wavelength. By transparent, it is understood that electrode 22 is configured to allow at least 75% of the radiation to pass through, having a wavelength λ equal to an operating wavelength. The more transparent the upper electrode 22 is, i.e., the higher the proportion of radiation that is allowed to pass through, the greater the efficiency of the sensor will be. The sensor 10 further includes a diffraction grating 24. The diffraction grating 24 is, for example, located on the electrode 22. In other words, the grating 24 is, for example, separated from the layer 14 by the electrode 22. The grating 24 is located on the front side relative to the layer 14. In other words, the grating 24 is located between the layer 14 and the front side. The grating 24 is therefore located between the layer 14 and the side configured to receive the light rays 12. The rays 12 become, after passing through the diffraction grating, guided rays 27. The diffraction grating thus allows the coupling of the light in the layer, or the guide, 14. The network 24 is covered by a layer 28, preferably an insulating protective layer. The material of layer 28 is preferably transparent to wavelengths λ equal to an operating wavelength. The array 24 comprises blocks 26, preferably at least three blocks 26. The blocks 26 are separated from each other by portions of layer 28. The blocks 26 rest, for example, on electrode 22. The blocks 26 may, for example, have a rectangular parallelepiped shape. The blocks 20 may, for example, have another shape, chosen to improve detection efficiency. Circular shapes, for example, improve the symmetry of the sensor with respect to polarization. Elliptical shapes favor one polarization over another. The shape of the blocks 26 therefore depends on the intended application. The sensor 10 is configured so that at least a part, preferably the majority, of the diffracted light 27 having a wavelength substantially equal to an operating wavelength has, in layer 14, a direction substantially parallel to the upper or lower face of layer 14. In other words, the sensor 10 is configured so that at least a part, preferably the majority, of the light 27 having a wavelength equal to an operating wavelength moves, after diffraction by the diffraction grating, in a plane entirely contained by layer 14. Thus, the duration for which the radiation is in contact with layer 14 is increased. Otherwise That said, the duration for which charges are generated is increased. The light sensor is thus more sensitive. The materials of blocks 26 and layers 14 and 28, more specifically their optical indices, the dimensions of the grating and the dimensions of layer 14 are chosen so as to optimize, for example for a given wavelength, the quantity of diffracted light rays lying in a plane entirely within layer 14. An example of the determination of these criteria is described in more detail in relation to [Fig.3]. In the example in [Fig. 1], the blocks 26 are identical. In other words, the blocks 26 in this example all have the same dimensions. Furthermore, the distance between neighboring blocks 26 is approximately the same. Preferably, the period of the grating 24, that is, the sum of the width of a block 26 and the distance between two neighboring blocks, is approximately equal to the operating wavelength in the material of layer 14. Preferably, the blocks 26 are made of a material that does not absorb radiation at the operating wavelength. Alternatively, the blocks 26 adjacent to a pixel all have the same dimensions, and the distance between neighboring blocks 26 adjacent to that pixel is approximately the same. Furthermore, the dimensions of the lattice 24—that is, the dimensions of the blocks 26 and the distance between two neighboring blocks 26 adjacent to another pixel—can be different. Thus, a pixel can be configured to be more sensitive to one wavelength, and a neighboring pixel can be configured to be sensitive to another wavelength. Similarly, a pixel can be configured to be more sensitive at a given angle of incidence, and a neighboring pixel can be configured to be more sensitive at a different angle of incidence. This allows the device to be optimized according to the angle of incidence of the source, as pixels do not receive radiation from the same source at the same angle of incidence. Therefore, the grating period will also be adjusted according to the target angle of incidence for each pixel. Figure [Fig. 2] represents another embodiment of a light sensor 40. Sensor 40 includes the elements of sensor 10, which will not be described again. Sensor 40 differs from sensor 10 in the shape of its diffraction grating 24. The grating 24 in [Fig. 2] comprises a superposition of two diffraction gratings. The two gratings are, for example, adapted to operate at the same wavelength but at different angles of incidence. The grating 24 therefore comprises blocks 24a forming one of the two diffraction gratings and blocks 24b forming the other diffraction grating. The 24a blocks all have the same dimensions and the distance between each 24a block and The nearest neighbor block 24a is substantially the same. Preferably, the period of the lattice formed by the blocks 24a, that is to say the sum of the width of a block 24a and the distance between each block 24a and the nearest neighbor block 24a, is substantially equal to the operating wavelength in the material of layer 14. Preferably, the blocks 24a are made of a material that does not absorb radiation at said operating wavelength. Similarly, the 24b blocks all have the same dimensions, and the distance between each 24b block and its nearest neighbor is substantially the same. Preferably, the period of the grating formed by the 24b blocks—that is, the sum of the width of a 24b block and the distance between each 24b block and its nearest neighbor—is substantially equal to the operating wavelength. Preferably, the 24a blocks are made of a material that does not absorb radiation at the other operating wavelength in the material of layer 14. Preferably, the 24a and 24b blocks are made of the same material, said material not absorbing radiation at the operating wavelengths of both diffraction gratings. The 24 network, that is to say the two sets of blocks 24a and 24b, is preferably periodic, each period comprising one block 24a and one block 24b. For example, at least one of the dimensions of blocks 24a is different from the equivalent dimension of blocks 24b. Figure 3 schematically illustrates the operation of the embodiments of Figure 1. Figure 3 also illustrates the determination of the various characteristics of sensor 10. Fig. 3 represents a waveguide 50, whose operation is considered identical to the operation of layer 14, and a diffraction grating 52, whose operation is considered identical to the operation of diffraction grating 24. The waveguide 50 has a thickness Hg. The thickness of the waveguide corresponds to the dimension along a vertical axis Z of layer 14, that is, the dimension of layer 14 between the face in contact with the support 16 and the face in contact with layer 22. The width of the waveguide, corresponding to a dimension along a horizontal axis X of layer 14, that is, a dimension in a plane parallel to the upper or lower face of layer 14, is, for example, theoretically infinite. The X-axis is orthogonal to the Z-axis. The width preferably corresponds to the horizontal dimension in the plane of Figures 1 to 3. The length of the waveguide, that is, the dimension of the waveguide along an axis Y orthogonal to the X and Z axes, is, for example, considered infinite. The diffraction grating in this example comprises 54 identical blocks. Each 54 block has a thickness Hr. The thickness of a block corresponds to the The dimension of the block along a vertical axis Z. Each block 54 is, in this example, separated from neighboring blocks by the same distance. The diffraction grating, and more precisely the location of the blocks 54, is therefore periodic, with a period T. The width of each block 54 has a value L1. The distance between two neighboring blocks 54, along the X-axis, has a value L2. The length of the blocks 54, that is, the dimension along the Y-axis, is considered infinite. The period T of the diffraction grating is equal to the sum of the values ​​L1 and L2, that is, the width of a block 54 and the distance between blocks 54. The light rays reaching the diffraction grating, i.e. the incident rays, are represented by an arrow 56. The incident rays reach the diffraction grating at an angle © with respect to the Z axis. The incident rays have a wavelength λ, the wavelength λ being an operating wavelength of the sensor 10 or 40, i.e. a wavelength equal to a value AO or in a range of wavelengths between A01 and A02. The diffraction grating 52 and the face of the waveguide 50 closest to the incident rays are coated with a material 58, having a refractive index n58. The face of the waveguide 50 opposite the face closest to the incident rays is coated with a material 59, having a refractive index n59. The material of the blocks 54 of the grating 52 has a refractive index n54. The material of the waveguide 50 has a refractive index n50. The guided rays, that is to say the rays obtained by the coupling of the incident rays 56 diffracted by the diffraction grating 52, are represented by arrows 60. The guided rays 60 form an angle œ with the Z axis. In the embodiments shown in Figures 1 and 2, the dimensions and materials of the waveguide and diffraction grating are chosen to increase the number of diffracted rays traveling in a plane contained within the waveguide, i.e., a plane parallel to the upper and lower faces of the waveguide. In other words, the dimensions and materials of the waveguide and diffraction grating are chosen so that the angle @ is equal to 90° or -90°. The characteristics of the diffraction grating and waveguide are determined by calculating the effective refractive index (neff) of the diffracted rays. The effective refractive index (neff) of the diffracted rays can be calculated using a first method, corresponding in this example to the following coupling equation: [Math.1] nave f =n58*sin (0) -m*4*sin (o) in which n58 is the optical index of material 58, © is the angle of incidence of the rays 56 with respect to the Z axis, m is an integer, T is the period of the diffraction grating, A is the wavelength of the incident rays, @ is the desired value of the angle rays guided with respect to the Z axis. The effective refractive index (neff) can also be calculated by determining the mode of the guided diffracted ray, for example, by a second method, such as calculating the eigenmodes of a multilayer structure. The effective refractive index (neff) is thus calculated from the thickness Hr of the 54 blocks, the thickness Hg of the waveguide, the optical indices n58 of the 58 material, n50 of the waveguide, and n52 of the diffraction grating, and the filling factor f of the diffraction grating. The filling factor is defined as the ratio of the width L of a block to the period T of the grating, i.e., to the sum of the width L1 of a 54 block and the distance L2 between two adjacent blocks. For example, consider a case in which the incident radiation received is predominantly radiation traveling along the Z-axis, that is, rays with an angle θ of 0°. Let's say, for instance, incident rays have a wavelength λ of 940 nm. In other words, the light sensor 10 or 40 in Figures 1 and 2 is configured to generate charges when a light ray with a wavelength approximately equal to 940 nm comes into contact with the material of layer 14. Let's arbitrarily choose a value m of 1. The effective refractive index neff is thus equal to: [Math.2] Nef at = # On the other hand, we choose, for example, the thicknesses Hr and Hg of the blocks 54 and the waveguide 50 to be equal to 60 nm. We also consider that the material 58 is air and therefore that the index n58 is equal to 1, and that the blocks 54 and the waveguide 50 are made of a quantum film type material and therefore have an optical index equal to 2.43. We further choose a filling factor f equal to 50%. The value of the effective refractive index (neff) is determined. In the example considered, the effective refractive index (neff) has a value of 1.63. Next, the period T required to obtain guided rays traveling in a plane contained within the waveguide is calculated; this period T is equal to 401 nm in this example. The period T is therefore approximately equal to the wavelength, in the quantum film (387 nm), of radiation having a wavelength of 940 nm in air. Various optimization algorithms can be used to determine the characteristics of the light sensor based on criteria chosen by the manufacturer. Examples of optimization algorithms include particle swarm optimization, genetic optimization, and Monte Carlo optimization. One advantage of the described embodiments is that it is possible to increase the absorption of light radiation and thus improve the sensitivity of the light sensors. Another advantage of the described embodiments is that, at the same absorption rate, it is possible to reduce the thickness of the absorbing layer, and therefore reduce the thickness of the sensor. Various embodiments and variations have been described. A person skilled in the art will understand that some features of these various embodiments and variations could be combined, and other variations will become apparent to a person skilled in the art. Finally, the practical implementation of the described methods and variants is within the reach of the person in the trade, based on the functional indications given above.

Claims

Demands

1. Image sensor (10, 40) comprising a first layer (14) of photoelectric material and a diffraction grating (24) located between said first layer (14) and the sensor face configured for to receive light rays.

2. Sensor according to claim 1, wherein the diffraction grating has a periodic form.

3. Sensor according to claim 1 or 2, wherein the dif- fraction comprises a plurality of first blocks (26, 24a).

4. Sensor according to claim 3, wherein the first blocks (26) are identical to each other.

5. Sensor according to claim 3 or 4, wherein the dif- fraction includes second blocks (24b) having at least one different dimension than that of the first blocks (24a).

6. Sensor according to any one of claims 1 to 5, wherein the diffraction grating (24) and layer (14) are separated by a first electrode (22).

7. Sensor according to claim 6, wherein the first electrode (22) is a second conductive layer completely covering the first layer (14).

8. Sensor according to any one of claims 1 to 7, comprising the second electrodes (17) in contact with the first layer (14) on the opposite side of the diffraction grating (24).

9. A sensor according to any one of claims 1 to 8, comprising a substrate (59) on which the first layer (14) rests.

10. Sensor according to claims 8 and 9, wherein every second The electrode comprises a conductive pad (18) located in the substrate.

11. | Sensor according to claim 10, wherein every second electrode (17) comprises a third conductive layer (20) resting on one of the conductive pads (18) and rests partially on the substrate.