Polarizing filter and polarimetric image sensor with such a filter

Description

Domaine technique

[0001] This description relates generally to image sensors, and more specifically to so-called polarimetric image sensors, adapted to record information relating to the polarization of captured light. Technique antérieure

[0002] Measuring the polarization information of light during image acquisition can be useful for many applications. In particular, it enables the implementation of image processing. d'amélioration Images are adapted according to the application. For example, it allows for the attenuation or, conversely, the enhancement of reflections on an image of any surface that causes specular reflection, such as glass, water, or the surface of an eye. It also makes it possible to detect manufactured objects in a natural environment, as these objects generally exhibit a polarization signature. Other applications that can benefit from polarization information measurement include industrial control applications, biomedical applications (for example, applications for detecting cancer cells, which polarize light due to their fibrous nature), and contrast enhancement applications for image capture in diffusive environments (fog, underwater imaging, etc.).), or even distance mapping or depth image acquisition applications, in which polarization can provide information on the surface orientation of manufactured objects, and thus help 3D reconstruction in addition to another modality such as active illumination by structured light or by time-of-flight measurement.

[0003] To measure polarization information, it has already been proposed d'acquérir successively, using the same sensor, several images d'une The same scene is captured by placing a polarizer opposite the sensor for each acquisition, and changing the polarizer between successive acquisitions. This results in relatively bulky acquisition systems, with a mechanism in front of the sensor, such as a motorized rotating wheel or plate on which the different polarizers are mounted, allowing the polarizer to be changed between acquisitions. Another limitation is the need to successively acquire several images of the scene to record different polarization states. This can be particularly problematic when the scene changes over time.

[0004] To overcome these limitations, it has been proposed to place an array of polarizing filters opposite the image sensor. However, a limitation remains that the polarizing filters block part of the light signal received by the system. d'acquisition. Thus, the overall sensitivity or overall quantum efficiency of the acquisition system is relatively low.

[0005] Publication US 2021 / 389184 A1 constitutes a prior art document.

[0006] It would be desirable to at least partially overcome some of the limitations of known polarimetric image acquisition solutions. Summary of the invention

[0007] For this purpose, one embodiment provides an image sensor as defined in claim 9; and a polarization router comprising a two-dimensional metasurface disposed on the side of the polarizer filter opposite the photodetectors, the metasurface comprising a two-dimensional array of dots.

[0008] According to one embodiment, said plurality of pixels comprises at least first and second pixels adapted to measure radiation according to distinct first and second polarizations respectively, the polarization structure of the first pixel being adapted to transmit mainly radiation according to the first polarization and the polarization structure of the second pixel being adapted to transmit mainly radiation according to the second polarization.

[0009] According to one embodiment, a first part of the two-dimensional metasurface located directly above the first and second pixels is adapted to transmit primarily: radiations according to the first polarization towards the polarization structure of the first pixel; and radiations according to the second polarization towards the polarization structure of the second pixel.

[0010] According to one embodiment, the first and second polarizations are linear polarizations along first and second directions forming angles of 0° and 90° respectively with respect to a reference direction.

[0011] According to one embodiment, said plurality of pixels further comprises third and fourth pixels adapted to measure radiation according to third and fourth distinct polarizations respectively, different from the first and second polarizations, the polarization structure of the third pixel being adapted to transmit mainly radiation according to the third polarization and the polarization structure of the fourth pixel being adapted to transmit mainly radiation according to the fourth polarization.

[0012] According to one embodiment, a second part of the two-dimensional metasurface, different from the first part and located directly above the third and fourth pixels, is adapted to transmit primarily: radiation along the third polarization towards the polarization structure of the third pixel; and radiation along the fourth polarization towards the polarization structure of the fourth pixel.

[0013] According to one embodiment, the third and fourth polarizations are linear polarizations along third and fourth directions forming angles of 45° and 135° respectively with respect to the reference direction.

[0014] Claim 1 defines the polarizing filter of the invention.

[0015] According to one embodiment, the pads of the two-dimensional metasurface are made of amorphous silicon.

[0016] According to one embodiment, the pads of the two-dimensional metasurface are laterally surrounded by silicon oxide.

[0017] According to one embodiment, the plots of the two-dimensional metasurface have sub-wavelength lateral dimensions.

[0018] According to one embodiment, the sensor further comprises a plurality of first microlenses extending opposite a pair of adjacent pixels of the sensor.

[0019] According to one embodiment, the first microlenses each have an elongated shape.

[0020] According to one embodiment, the first microlenses are: A) arranged on the side of a face of the two-dimensional metasurface opposite to the photodetectors; or B) interposed between the photodetectors and the two-dimensional metasurface.

[0021] According to one embodiment, the sensor further comprises a plurality of second microlenses distinct from the first microlenses and arranged on the side of a face of the two-dimensional metasurface opposite the photodetectors, each second microlens extending opposite a pair of adjacent pixels of the sensor.

[0022] According to one embodiment, each second microlens has an elongated shape.

[0023] According to one embodiment, each first microlens further extends opposite the first part or the second part of the metasurface.

[0024] Furthermore, one embodiment provides for a polarimetric image sensor formed in and on a semiconductor substrate, the sensor comprising: a plurality of pixels each comprising a photodetector formed in the semiconductor substrate; a polarization router comprising a two-dimensional metasurface disposed on the side of an illumination face of the photodetectors, the metasurface comprising a two-dimensional array of dots; and a plurality of first microlenses extending opposite a pair of adjacent pixels of the sensor.

[0025] According to one embodiment, the first microlenses are arranged on the side of the polarization router opposite to the photodetectors.

[0026] According to one embodiment, the first microlenses are interposed between the photodetectors and the two-dimensional metasurface.

[0027] According to one embodiment, said plurality of pixels comprises, opposite one of the first microlenses, first and second pixels adapted to measure radiation according to respectively distinct first and second polarizations, a first part of the two-dimensional metasurface located directly above the first and second pixels being adapted to transmit predominantly: radiation according to the first polarization towards the first pixel; and radiation according to the second polarization towards the second pixel.

[0028] According to one embodiment, the rows of plots in the first part of the two-dimensional metasurface are identical to each other.

[0029] According to one embodiment, the first part of the two-dimensional metasurface is adapted to focus the incident radiation mainly along a direction parallel to the rows of studs.

[0030] According to one embodiment, said plurality of pixels further comprises third and fourth pixels adapted to measure radiation according respectively to distinct third and fourth polarizations, different from the first and second polarizations, a second part of the two-dimensional metasurface located directly above the third and fourth pixels being adapted to transmit predominantly: radiation according to the third polarization towards the third pixel; and radiation according to the fourth polarization towards the fourth pixel.

[0031] According to one embodiment, the sensor further comprises a polarizing filter interposed between said plurality of pixels and the two-dimensional metasurface, the filter comprising, for each pixel, a polarization structure.

[0032] According to one embodiment, the polarization structure of the first pixel is adapted to transmit mainly radiation according to the first polarization and the polarization structure of the second pixel is adapted to transmit mainly radiation according to the second polarization.

[0033] Furthermore, one embodiment provides a polarizing filter intended to be positioned opposite an image sensor comprising a plurality of pixels, the filter comprising, for each pixel, a polarizing structure comprising a plurality of parallel metal bars, each bar being coated with an absorbing stack comprising: a tungsten layer; a silicon layer, coating the tungsten layer; and a dielectric layer, coating the silicon layer.

[0034] According to one embodiment, the metal bars are made of a material other than tungsten, preferably aluminium.

[0035] According to one embodiment, the tungsten layer has a thickness greater than 40 nm, preferably greater than 60 nm.

[0036] According to one embodiment, the metal bars are made of tungsten.

[0037] According to one embodiment, the metal bars and the tungsten layer have a cumulative thickness greater than 40 nm, preferably greater than 60 nm.

[0038] According to one embodiment, the silicon layer has a thickness of between 20 and 100 nm, preferably between 30 and 50 nm, for example equal to about 39 nm.

[0039] According to one embodiment, the dielectric layer is made of silicon oxide.

[0040] According to one embodiment, the dielectric layer consists of a stack of several layers of dielectric materials with refractive indices lower than that of silicon.

[0041] Furthermore, one embodiment provides for a polarimetric image sensor formed in and on a semiconductor substrate, the sensor comprising: a plurality of pixels each comprising a photodetector formed in the semiconductor substrate; and a polarizing filter as described, the filter being disposed on the side of an illumination face of the photodetectors.

[0042] According to one embodiment, the first, second, third and fourth polarizations are linear polarizations along first, second, third and fourth directions forming angles of 0°, 90°, 45° and 135° respectively with respect to a reference direction.

[0043] According to one embodiment, the sensor further comprises a polarization router including a two-dimensional metasurface disposed on the side of the polarizer filter opposite the photodetectors, the metasurface comprising a two-dimensional array of dots.

[0044] According to one embodiment, the two-dimensional metasurface comprises: a first part located directly above the first and second pixels adapted to transmit mainly: radiation according to the first polarization towards the first pixel; and radiation according to the second polarization towards the second pixel, and a second part located directly above the third and fourth pixels adapted to transmit mainly: radiation according to the third polarization towards the third pixel; and radiation according to the fourth polarization towards the fourth pixel. Brève description des dessins

[0045] 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: there figure 1A and the figure 1B are respectively an exploded perspective view and a cross-sectional view of an example of a polarimetric image sensor according to one embodiment; the figure 2 is a schematic and partial top view of an example implementation of the sensor's polarizer filter figures 1A et 1B ; there figure 3 is a schematic and partial top view of an example implementation of the sensor polarization router figures 1A et 1B ; there figure 4A and the figure 4B are respectively an exploded perspective view and a cross-sectional view of another example of a polarimetric image sensor according to one embodiment; the figure 5 is a schematic and partial top view of an example implementation of the sensor polarization router figures 4A et 4B ; there figure 6A and the figure 6B are respectively an exploded perspective view and a cross-sectional view of another example of a polarimetric image sensor according to one embodiment; the figure 7 is a schematic and partial cross-sectional view of the sensor's polarizing filter figures 6A And 6B ; there figure 8 is a cross-sectional view of another example of a polarimetric image sensor according to one embodiment; and the figure 9 a cross-sectional view of another example of a polarimetric image sensor according to one embodiment. Description des modes de réalisation

[0046] 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.

[0047] For the sake of clarity, only the steps and elements necessary for understanding the described embodiments have been shown and detailed. In particular, the photodetection elements and the electronic circuits for controlling the described image sensors have not been detailed, as the described embodiments are compatible with common implementations of these elements. Furthermore, the applications of the described image sensors have not been detailed, as the described embodiments are compatible with all or most known applications of polarimetric image acquisition systems.

[0048] 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.

[0049] 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., reference is made, unless otherwise specified, to the orientation of the figures.

[0050] Unless otherwise specified, the expressions "approximately", "roughly", "about", and "in the order of" mean to within 10%, preferably to within 5%, or, when referring to angular values, to within 10°, preferably to within 5°.

[0051] There figure 1A and the figure 1B are respectively an exploded perspective view and a cross-sectional view of an example of a polarimetric image sensor 100 according to one embodiment.

[0052] The sensor 100 is formed in and on a substrate 101. The substrate 101 is for example made of a single-crystal semiconductor material, for example single-crystal silicon.

[0053] The sensor 100 comprises a plurality of pixels P formed in and on the semiconductor substrate 101. In top view, the pixels P are for example arranged in a matrix according to rows and columns.

[0054] The sensor 100 further includes, on the side of a first face of the substrate 101, called the front face, corresponding to the lower face of the substrate 101 in the orientation of the figures 1A et 1B , a 103 stack of insulating and conductive (e.g. metallic) layers, called an interconnect stack, in which interconnection elements (e.g., conductive interconnection tracks and vias) of the sensor's P pixels are formed.

[0055] In the example of figures 1A et 1B The sensor 100 is a backside illumination sensor, also called a BSI sensor (from the English "Back Side Illumination"), meaning that the light rays from the scene to be imaged illuminate the substrate 101 from its back side, that is, its side opposite the interconnect stack 103, namely its upper face in the orientation of the figures 1A et 1B .

[0056] Each pixel P of the sensor 100 comprises a photosensitive region 105 formed in the substrate 101. Each photosensitive region 105 includes, for example, a photodetection element 107, such as a photodiode or photodetector. In the example shown, the photosensitive regions 105 of the pixels P are separated laterally from each other by insulating walls 109. The insulating walls 109 are, for example, made of a dielectric material, such as silicon oxide. Alternatively (not detailed in the figures), the insulating walls 109 comprise outer lateral walls made of a dielectric material, such as silicon oxide, and a central wall made of an electrically conductive material, such as doped polycrystalline silicon or a metal. In this example, the insulating walls 109 extend vertically through the entire thickness of the substrate 101.The thickness of the substrate 101 is, for example, between 1 and 20 µm, for example between 3 and 10 µm. As an alternative, the insulation walls 109 can be omitted.

[0057] Each pixel P is surmounted by a polarization structure 111 arranged opposite the photosensitive region 105 of the pixel, on the side of the illumination face of the photodetector 107, that is to say on the side of the upper face of the substrate 101 in the orientation of the figures 1A et 1B . The set of polarization structures 111 located directly above the pixels P of the sensor 100 forms, for example, a polarization filter or polarizer filter FP.

[0058] Each polarization structure 111 is adapted to transmit mostly light radiation according to a predefined polarization.

[0059] In the example of figures 1A et 1B The sensor comprises several pixels P whose respective polarization structures 111 have different polarization orientations and are thus adapted to transmit predominantly light rays with different polarizations. This allows the measurement, by means of distinct pixels P, of intensities of received light radiation with different polarizations. In other words, the sensor includes at least first and second pixels P designed to measure intensities of received light radiation with first and second polarizations respectively, for example, orthogonal linear first and second polarizations.As an example, the 111 polarization structure of the first pixel has a radiation transmission coefficient along the first polarization greater than its radiation transmission coefficient along the second polarization, and the 111 polarization structure of the second pixel has a radiation transmission coefficient along the second polarization greater than its radiation transmission coefficient along the first polarization.

[0060] Polarizing structures are, for example, metallic structures with openings that primarily transmit radiation according to a predefined polarization, while absorbing or reflecting radiation according to other polarizations. Examples of such metallic structures are made of aluminum or copper. Alternatively, other metals can be used, such as silver, gold, tungsten, or titanium.

[0061] For example, a filling material 115, such as a dielectric material like silicon dioxide, silicon nitride, alumina (Al₂O₃), tantalum oxide, or hafnium oxide, fills the openings formed in the metallic structures. In this example, the material 115 further coats the polarization structures 111, forming a planarization layer 115. Alternatively, the openings in the polarization structures 111 may be left empty or filled with air.

[0062] In practice, the choice of patterns and the dimensioning of the polarization structures 111 can be carried out using known electromagnetic simulation tools.

[0063] For example, the P pixels are distributed into macropixels M, each comprising at least two adjacent P pixels, for example, four adjacent P pixels. Within each macropixel M, the P pixels have different polarization structures. Thus, within each macropixel M, the P pixels measure intensities of received light radiation with different polarizations.

[0064] The sensor 100 further includes a two-dimensional (2D) metasurface MS located opposite the P pixels. More precisely, the MS metasurface is positioned on the side of the polarizing filter FP opposite the photodetectors 107. The MS metasurface comprises a two-dimensional array of pads 117 of a first material, for example, amorphous silicon, laterally surrounded by a second material, for example, material 115, such as silicon oxide. More generally, the first material has a higher refractive index than the second material. The pads 117 of the MS metasurface have sub-wavelength lateral dimensions; that is, the largest lateral dimension of each pad 117 is smaller than the principal wavelength intended to be measured by the underlying P pixel, i.e., the wavelength at which the quantum efficiency of the P pixel is maximal.For example, for P pixels intended to measure visible or near-infrared radiation, for example radiation with a wavelength of less than 1 µm, the largest dimension of each 117 plot is between 10 and 500 nm, for example between 30 and 300 nm.

[0065] The metasurface MS, for example, corresponds to a polarization router or sorter adapted to implement an optical function of polarization routing or sorting to the various underlying polarization structures 111 of the polarizing filter FP. In practice, the metasurface MS comprises, opposite each pixel P, a plurality of pads 117 of varying lateral dimensions. The dimensions and arrangement of the pads 117 are defined according to the optical function to be performed. For example, to perform the polarization routing or polarized light routing function, pads 117 can be provided with asymmetrical shapes when viewed from above, for example rectangular or elliptical, it being understood that the pads 117 can have any shape when viewed from above.The pads 117 can have vertical flanks, i.e., orthogonal to the upper face of the substrate 101, oblique flanks, or stepped flanks, including at least one step. Furthermore, each pad 117 can be made of a single material or a stack of layers of different materials. The pattern of the MS metasurface can be defined using an electromagnetic simulation tool, for example, by using inverse design methods, such as those described in the article entitled "Phase-to-pattern inverse design paradigm for fast realization of functional metasurfaces via transfer learning" by Zhu, R., Qiu, T., Wang, J. et al. Nat. Commun. 12, 2974 (2021), or in the article entitled "Matrix Fourier optics enables a compact full-Stokes polarization camera" by Rubin et al. (SCIENCE - Volume 365 - Issue 6448 - July 5, 2019).

[0066] The 117 pads of the MS metasurface preferably all have the same height, for example, of the same order of magnitude as the principal wavelength intended to be measured by each pixel P, for example, between 20 nm and 2 µm, preferably between 50 nm and 750 nm, for radiation with wavelengths less than 1 µm. Providing pads 117 of constant height across the entire sensor surface advantageously simplifies the fabrication of the MS metasurface.

[0067] In the example illustrated in figures 1A et 1B The polarization sorter of sensor 100 comprises a single MS metasurface. As an alternative, the polarization sorter of sensor 100 could comprise several metasurfaces, for example analogous to the MS metasurface.

[0068] There figure 2 is a schematic and partial top view of an example implementation of the FP polarizer filter of the 100 sensor. figures 1A et 1B .

[0069] There figure 2 This illustrates more specifically the polarization structures of the pixels P of a single macropixel M. In this example, each macropixel M comprises four adjacent pixels P(1), P(2), P(3), and P(4) adapted to measure intensities of received light radiation according to four different polarization orientations PS1, PS2, PS3, and PS4, for example, linear polarizations along four directions forming angles of 0°, 90°, 45°, and 135° with respect to a reference direction. In this example, the four pixels P(1), P(2), P(3), and P(4), symbolized by dashed squares in figure 2 are arranged in a matrix with two rows and two columns.

[0070] Polarization structures 111 are, for example, metallic grids, each comprising a plurality of regularly spaced parallel metallic bars, transmitting radiation mainly according to a linear polarization perpendicular to the metallic bars, and absorbing radiation according to other polarizations.

[0071] Alternatively, the number of pixels P per macropixel M may be different from four. Furthermore, the described embodiments are not limited to linear polarization structures. Alternatively, each macropixel M may comprise one or more linear polarization structures and / or one or more circular polarization structures.

[0072] As an example, the polarization structures 111 of the pixels P in the same position in the different macropixels M of the sensor are adapted to transmit predominantly the same polarization orientation.

[0073] As an example, the polarization structures 111 of the same polarization orientation in the different macropixels M of the sensor are all identical, except for manufacturing dispersions.

[0074] As an alternative, the polarization structures 111 of the same polarization orientation in the different macropixels M of the sensor have patterns adapted according to the position of the macropixel M on the sensor, to take into account the principal direction of incidence of the light rays received by the macropixel.

[0075] Although this was not illustrated in figures 1A et 1B Each pixel P of the sensor 100 can include a color filter positioned above the polarization structure and adapted to transmit light predominantly within a specific wavelength range. The color filters are, for example, positioned above the planarization layer 115, for instance, in contact, on their underside, with the upper side of the layer 115. Different pixels P can include different color filters. For example, the sensor includes pixels P with a color filter adapted to transmit predominantly red light, pixels P with a color filter adapted to transmit predominantly green light, and pixels P with a color filter adapted to transmit predominantly blue light. For example, the pixels P of the same macropixel M include identical color filters, and the pixels P of neighboring macropixels M include different color filters.For example, coloured filters are made of coloured resin.

[0076] As an example, the polarization structures 111 of the same polarization orientation in the different macropixels M of the sensor exhibit patterns adapted according to the color of the pixel, that is to say the range of wavelengths transmitted predominantly by the color filter of the pixel.

[0077] As a non-limiting example, for linear polarizers of the type illustrated in figure 2 Made of aluminum with a silicon oxide filling, and designed to operate at visible and / or infrared wavelengths, the dimensions of the metal bars can be as follows: for blue light with a wavelength of around 450 nm (and / or for light with a longer wavelength, for example infrared light), the metal bars can have a height of between 50 and 100 nm, a width of around 60 nm and a repetition period of around 180 nm; for green light with a wavelength of around 530 nm (and / or for light with a longer wavelength, for example infrared light), the metal bars can have a height of between 50 and 100 nm, a width of around 70 nm and a repetition period of around 210 nm; for red light with a wavelength of around 610 nm (and / or for light with a longer wavelength, for example infrared light), the metal bars can have a height between 50 and 100 nm, a width of around 80 nm and a repetition period of around 240 nm;and for infrared light with a wavelength of approximately 940 nm or higher, the metal bars can have a height between 50 and 100 nm, a width of approximately 90 nm and a repetition period of approximately 280 nm.

[0078] In practice, to simplify manufacturing processes, it is preferable for the height of the metal bars in the polarization structures 111 to be the same in all P pixels of the sensor. This allows for compromises to be made between the manufacturing complexity of the polarizers 111 and their polarization filtering performance.

[0079] There figure 3 is a schematic and partial top view of an example implementation of the sensor polarization router figures 1A et 1B .

[0080] There figure 3 illustrates in particular an example of the shape and arrangement of the plots 117 of the metasurface MS directly above a macropixel M. In this example, the macropixel M includes, as previously explained in relation to the figure 2 , the four adjacent pixels P(1), P(2), P(3) and P(4) adapted to measure intensities of light radiation received according respectively to the four different polarization orientations PS1, PS2, PS3 and PS4.

[0081] In the example shown, the metasurface MS comprises a first part MS(1), located directly above pixels P(1) and P(2), and a second part MS(2), located directly above pixels P(3) and P(4). In this example, the first part MS(1) of the metasurface MS has a pattern adapted to implement a routing function for light rays received according to the two polarization states PS1 and PS2 towards the polarization structures 111 of the two pixels P(1) and P(2) of the macropixel M, respectively. Similarly, the second part MS(2) of the metasurface MS has a pattern adapted to implement a routing function for light rays received according to the two polarization states PS3 and PS4 towards the polarization structures 111 of the two pixels P(3) and P(4) of the macropixel M, respectively. The components of the incident flux polarized according to states PS1, PS2, PS3, and PS4 are thus deflected towards pixels P(1), P(2), P(3), and P(4) of the macropixel M, respectively.A photon arriving above pixel P(1) or pixel P(2) will then be sorted into PS1 or PS2, and a photon arriving above pixel P(3) or pixel P(4) will be sorted into PS3 or PS4.

[0082] Compared to a polarimetric sensor based on polarizing filters, this advantageously improves the quantum efficiency of the sensor since the entire flux collected opposite each macropixel M is transmitted to the four pixels P(1), P(2), P(3) and P(4) of the macropixel.

[0083] In the example shown, each part MS(1), MS(2) of the metasurface MS is adapted to focus the incident light along two orthogonal axes Ox and Oy, the Ox and Oy axes corresponding respectively to the horizontal and vertical axes, in the orientation of the figure 3 .

[0084] The 117 plots, for example, have a constant pitch, meaning a constant center-to-center distance, across the entire MS metasurface. For instance, the pitch of the 117 plot matrix of the MS metasurface is between 250 and 300 nm. In the example shown, parts MS(1) and MS(2) of the MS metasurface each have a roughly rectangular perimeter. This corresponds, for example, to a case where the sensor's P pixels each have, in top view, a roughly square perimeter. The rectangle formed by each part MS(1), MS(2) of the MS metasurface then has lateral dimensions roughly equal to those of a rectangle formed by a pair of adjacent P pixels. For example, each part MS(1), MS(2) of the MS metasurface has a length between 4 and 8 µm and a width approximately half its length, for example, between 2 and 4 µm.This example is not limiting, however, the parts MS(1) and MS(2) of the MS metasurface may for example have lateral dimensions smaller than those mentioned above, for example less than 1 µm in a case where the pixels P have a pitch of the order of 0.5 µm or 1 µm.

[0085] Alternatively, each part MS(1), MS(2) of the metasurface MS can have a perimeter of any shape, for example, a square. This corresponds, for example, to a case where the pixels P of the sensor each have, in top view, a perimeter of approximately rectangular shape, the square formed by each part MS(1), MS(2) of the metasurface then having, for example, lateral dimensions approximately equal to those of a square formed by a pair of pixels P adjacent along their longer side. One advantage of ensuring that each part MS(1), MS(2) of the metasurface MS has a square shape is that it facilitates spatial sampling of the image acquired by the pixels P of the sensor 100.

[0086] The pattern of the portion of the MS metasurface located directly above the macropixel M can be repeated identically (within manufacturing dispersions) with respect to all other macropixels M of the sensor.

[0087] As an alternative, the pattern of the portion of the MS metasurface can vary from one macropixel M to another, depending on the position of the macropixel on the sensor, to take into account in particular the principal direction of incidence of the rays arriving on the MS metasurface from the scene to be imaged.

[0088] There figure 4A and the figure 4B are respectively an exploded perspective view and a cross-sectional view of another example of a 400 polarimetric image sensor according to one embodiment.

[0089] The 400 polarimetric image sensor of figures 4A et 4B includes common elements with the 100 polarimetric image sensor figures 1A et 1B These common elements will not be detailed again below.

[0090] The 400 sensor of figures 4A et 4B differs from the 100 sensor of figures 1A et 1B in that the sensor 400 lacks the polarizing filter FP and comprises, on the side of the polarization router opposite the photodetectors 107, a plurality of microlenses 401. In the example shown, each microlens 401 has an elongated shape extending opposite a pair of adjacent pixels P of the sensor 400. Each microlens 401 presents, for example, in top view, an oval or rectangular cross-section with rounded corners. This corresponds, for example, to a case where the pixels P of the sensor each have, in top view, a substantially square perimeter, each microlens 401 then having, for example, lateral dimensions equal to those of a rectangle formed by a pair of adjacent pixels P. In the example shown, the microlenses 401 are refractive microlenses.

[0091] Alternatively, each microlens 401 can have any shape, for example, a circular or rounded square perimeter, when viewed from above. This corresponds, for example, to a case where the sensor's P pixels each have, when viewed from above, a substantially rectangular perimeter, the circle or square formed by the perimeter of each microlens 401 then having, for example, respectively a diameter or a side approximately equal to the side of a square formed by a pair of adjacent P pixels along their longer side.

[0092] As an example, each 401 microlens is made of flowable resin.

[0093] There figure 5 is a schematic and partial top view of an example implementation of the 400 sensor polarization router figures 4A et 4B .

[0094] There figure 5 illustrates in particular an example of the shape and arrangement of the 117 plots of the MS metasurface of the 400 sensor directly above a macropixel M. In this example, the macropixel M comprises, as previously explained in relation to the figure 2 , the four adjacent pixels P(1), P(2), P(3) and P(4) adapted to measure intensities of light radiation received according respectively to the four different polarization orientations PS1, PS2, PS3 and PS4.

[0095] In the example shown, the MS metasurface includes, as previously explained in relation to the figure 3 The first part, MS(1), is located directly above pixels P(1) and P(2), and the second part, MS(2), is located directly above pixels P(3) and P(4). In this example, the first part, MS(1), of the metasurface MS has a pattern adapted to implement a routing function for light rays received in the two polarization states PS1 and PS2, directed respectively to the two pixels P(1) and P(2) of the macropixel M. Similarly, the second part, MS(2), of the metasurface MS has a pattern adapted to implement a routing function for light rays received in the two polarization states PS3 and PS4, directed respectively to the two pixels P(3) and P(4) of the macropixel M. The components of the incident flux polarized according to states PS1, PS2, PS3, and PS4 are thus deflected towards pixels P(1), P(2), P(3), and P(4) of the macropixel M, respectively.A photon arriving above pixel P(1) or pixel P(2) will then be sorted into PS1 or PS2, and a photon arriving above pixel P(3) or pixel P(4) will be sorted into PS3 or PS4.

[0096] In the example shown, the lines of plots 117 in part MS(1) of the MS metasurface are identical to each other. Similarly, the lines of plots 117 in part MS(2) of the MS metasurface are identical to each other.

[0097] In the example shown, each microlens 401 extends over a portion MS(1) or MS(2) of the metasurface MS and is adapted to focus the incident light along the orthogonal axes Ox and Oy (the horizontal axis and the vertical axis, in the orientation of the figure 5 ) on the underlying MS(1) or MS(2) portion of the MS metasurface. For example, each microlens 401 exhibits astigmatism and, more specifically, has a focal length along the Oy axis strictly shorter than its focal length along the Ox axis. This corresponds, for example, to a case where the sensor's P pixels each have, in top view, a substantially square perimeter. Alternatively, each microlens 401 does not exhibit astigmatism. This corresponds, for example, to a case where the sensor's P pixels each have, in top view, a substantially rectangular perimeter, with each microlens 401 having a substantially circular perimeter. For example, each microlens 401 has a focal length along the Oy axis on the order of the distance separating the microlens from the upper face of the substrate 101 (the face of the substrate 101 opposite the interconnect stack 103).Furthermore, in this example, each part MS(1), MS(2) of the MS metasurface is adapted to focus the incident light primarily, or even exclusively, along an axis parallel to the rows of studs 117, here the Ox axis. Thus, in this example, focusing is achieved jointly by the microlenses 401 and by the MS metasurface.

[0098] This simplifies the design and implementation of the MS metasurface of the 400 sensor compared to the MS metasurface of the 100 sensor. For example, this allows the 400 sensor to have a thinner MS metasurface than the 100 sensor, and the shape of the MS metasurface of the 400 sensor can also be adapted to generate smaller phase jumps and phases in the vicinity of the corners of each MS(1) and MS(2) part compared to the case of the MS metasurface of the 100 sensor.

[0099] Alternatively, focusing can be achieved primarily or even solely by the 401 microlenses, with the MS metasurface then, for example, lacking a focusing function. This further simplifies the design and fabrication of the MS metasurface.

[0100] There figure 6A and the figure 6B are respectively an exploded perspective view and a cross-sectional view of another example of a 600 polarimetric image sensor according to one embodiment.

[0101] The 600 polarimetric image sensor of figures 6A And 6B includes common elements with the 400 polarimetric image sensor of the figures 4A et 4B These common elements will not be detailed again below.

[0102] The 600 sensor of figures 6A And 6B differs from the 400 sensor figures 4A et 4B in that the 600 sensor comprises, in addition to the microlenses 401 and the metasurface MS, a polarizing filter FP interposed between the substrate 101 and the metasurface MS. The FP polarizing filter of the 600 sensor is, for example, analogous to the FP polarizing filter of the 100 sensor previously described in relation to the figures 1A et 1B .

[0103] There figure 7 is a schematic and partial cross-sectional view of the FP polarizer filter of the 600 sensor. figures 6A And 6B . There figure 7 is more specifically a detailed view of a portion of the FP polarizing filter delimited by a dotted box 601 in figure 6B .

[0104] According to one embodiment, the FP filter comprises, for each pixel P, a polarization structure 111 comprising a plurality of parallel metal bars 701, each bar being coated with an anti-reflective or absorbing stack 703 comprising, in order from the top face of the bars 701: a tungsten layer 705; a silicon layer 707, for example amorphous silicon, coating the tungsten layer 705; and a dielectric layer 709, coating the silicon layer 707.

[0105] For example, 701 metal bars are made of tungsten or aluminum.

[0106] The 709 layer is, for example, a silicon oxide layer or a silicon nitride layer. Alternatively, the 709 layer can consist of a stack of several layers of dielectric materials with refractive indices lower than that of silicon, for example, one or more silicon oxide layers and one or more silicon nitride layers.

[0107] The thicknesses of the layers 705, 707 and 709 of the stack 703 are chosen so that the stack 703 has, for a central wavelength λ0 of the photodetector 107, an absorption coefficient greater than that of the material 115. The stack 703 is dimensioned so that, for the central wavelength λ0 of the photodetector 107 and for incident radiation substantially orthogonal to the mean plane of the stack 703, more than 50%, preferably more than 80%, even more preferably more than 95%, of the radiation entering the stack 703 is absorbed in the stack 703 in a single pass. In other words, more than 50%, preferably more than 80%, even more preferably more than 95%, of radiation entering through the top face of stack 703 is absorbed in stack 703 and is not reflected back to the region in material 115.For example, for the center wavelength λ0, approximately 90% of the radiation entering the 703 stack is absorbed in the 703 stack in a single pass. For example, more than 50%, preferably more than 80%, and even more preferably more than 90% of all radiation in the wavelength range between 920 nm and 960 nm is absorbed in a single pass in the 703 stack. Preferably, more than 50%, preferably more than 80%, and even more preferably more than 90% of all radiation in the FP filter's passband is absorbed in a single pass in the 703 stack.

[0108] To maximize absorption in the stack 703, the thickness of the tungsten layer 705 is preferably relatively high, for example, greater than 40 nm, and preferably greater than 60 nm in the case where the metal bars 701 are made of a material other than tungsten. Alternatively, in the case where the metal bars 701 are made of tungsten, the bars 701 and the layer 705 have, for example, a combined thickness greater than 40 nm, and preferably greater than 60 nm.

[0109] Numerical simulations allow adjustment of the required silicon thickness to maximize absorption, depending on the various parameters of the 703 stack and / or the central wavelength λ0. As an example, the thickness of the 705 silicon layer is between 20 and 100 nm, preferably between 30 and 50 nm, for example approximately 39 nm.

[0110] In the FP polarizer filter of the 600 sensor, the presence of the absorbing structure 703 advantageously reduces the detection of parasitic radiation in the near infrared, and thus improves image quality.

[0111] There figure 8 is a cross-sectional view of another example of a polarimetric image sensor 800 according to one embodiment.

[0112] The 800 polarimetric image sensor of the figure 8 includes common elements with the 400 polarimetric image sensor of the figures 4A et 4B These common elements will not be detailed again below.

[0113] The 800 sensor of the figure 8 differs from the 400 sensor figures 4A et 4B in that, in the 800 sensor, the 401 microlenses are interposed between the 107 photodetectors of the P pixels and the two-dimensional MS metasurface of the polarization router.

[0114] In this example, the microlenses 401 are located in the planarization layer 115. The microlenses 401 are made of a material having a higher optical index than the material of the planarization layer 115. In the case where the planarization layer 115 is made of silicon oxide, the microlenses 401 are formed for example, for example by shape transfer, in a layer of silicon nitride or amorphous silicon.

[0115] The provision for interposing the microlenses 401 between the metasurface MS and the photodetectors 107 allows, for example, the incident radiation to be distributed within a cone defined by a lens (not shown) placed in front of the sensor 100, more precisely defined by an aperture f / Dp of this lens, where f represents the focal length and Dp the diameter of the lens's entrance pupil. An advantage of interposing the microlenses 401 below the metasurface MS is that it allows the angles of incidence related to the cone of incident radiation distribution to be smaller than those that would be produced, under similar conditions, if the microlenses 401 were positioned above the metasurface MS.This advantageously facilitates the design and manufacture of the MS metasurface, the MS metasurface being, for example, designed and optimized to be used under a given angle of incidence, for example under normal incidence, and exhibiting an angular acceptance lower than that of a "classical" refractive optic such as a lens.

[0116] However, placing the microlenses 401 between the metasurface MS and the photodetectors 107 can lead to undesirable optical crosstalk, meaning that some of the radiation incident directly above a given macropixel M can reach at least one of the macropixels adjacent to that macropixel. This is particularly true when the metasurface MS has a weak or even nonexistent focusing power (i.e., a very long or even infinite focal length), as the incident radiation is then mostly, or entirely, deflected by the metasurface MS according to its polarization. The crosstalk phenomenon is more pronounced when the deflection caused by the metasurface MS is significant and the distance between the metasurface MS and the microlenses 401 is large.

[0117] To overcome this drawback, the metasurface MS is, for example, designed and manufactured to exhibit a focusing function such that the radiation deflected according to its polarization—for example, the radiation deflected by the first part MS(1) of the metasurface MS according to PS1 and PS2 for pixels P(1) and P(2) of a macropixel M—reaches only the underlying microlens 401. Each part MS(1), MS(2) of the metasurface MS has, for example, a focal length f1 greater than the focal length f2 of the underlying microlens 401.

[0118] As an example, the focal length f1 of each part MS(1), MS(2) of the metasurface MS is given by the following formula: f 1 ≤ D × tan θ 2

[0119] In equation [Math 1] above, θ represents the angle of deflection of a polarization by the metasurface (polarization PS1 or PS2, for part MS(1) of the metasurface MS, or polarization PS3 or PS4, for part MS(2) of the metasurface MS), and D represents the maximum lateral dimension of the underlying microlens 401 (e.g. the major axis of microlens 401, in the case where microlens 401 has an ellipsoidal section).

[0120] The deflection angle θ corresponds for example to a deviation angle, by the metasurface MS, of a radiation arriving, under normal incidence, at the center of the part MS(1), respectively MS(2), of the metasurface MS as a function of the polarization PS1 or PS2, respectively PS3 or PS4, of the incident radiation.

[0121] Another solution for avoiding, or mitigating, the phenomenon of diaphoty is explained below in relation to the figure 9 .

[0122] There figure 9 is a cross-sectional view of another example of a 900 polarimetric image sensor according to one embodiment.

[0123] The 900 polarimetric image sensor of the figure 9 includes common elements with the 800 polarimetric image sensor of the figure 8 These common elements will not be detailed again below.

[0124] The 900 sensor of the figure 9 differs from the 800 sensor of the figure 8 in that the sensor 900 further comprises, on the side of the polarization router opposite the photodetectors 107, a plurality of microlenses 901 distinct from the microlenses 401. The microlenses 901 are, for example, analogous to the microlenses 401. In the example shown, each microlens 901 has an elongated shape extending opposite a pair of adjacent pixels P of the sensor 900, for example, pixels P(1) and P(2) or pixels P(3) and P(4) of one of the macropixels M. Each microlens 901 has, for example, in top view, an oval or rectangular cross-section with rounded corners. This example is not, however, limiting; the microlenses 401 can, alternatively, have any shape, for example, circular or square with rounded corners, as previously described. In the example shown, the 901 microlenses are refractive microlenses.

[0125] As an example, each 901 microlens is made of flowable resin.

[0126] In the example shown, the 901 microlenses have a focal length f3 greater than the focal length f2 of the 401 microlenses.

[0127] As an example, the focal length f3 of each 901 microlens is given by the following equation: f 3 ≤ D × d 1 2 tan θ × d 2

[0128] In equation [Math 2] above: θ represents the deflection angle of a polarization by the metasurface (polarization PS1 or PS2, for part MS(1) of the metasurface MS, or polarization PS3 or PS4, for part MS(2) of the metasurface MS); D represents the maximum lateral dimension of the underlying microlens 401 (for example, the major axis of microlens 401, in the case where microlens 401 has an ellipsoidal cross-section); d1 represents the distance between the lower face of microlens 901 and the lower face of the metasurface MS, in the orientation of the figure 9 ; and d2 represents the distance between the lower face of the MS metasurface and the lower face of the 401 microlens, in the orientation of the figure 9 .

[0129] Various embodiments and variations have been described. Those skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will become apparent to them. Based on the indications in this description, those skilled in the art can, in particular, foresee a polarizing filter in sensors 100 and 400 having a structure analogous to that of the polarizing filter in sensor 600, that is, comprising a plurality of parallel metal bars coated with an absorbing stack.

[0130] Furthermore, although it was not shown, the 800 and 900 sensors previously discussed in relation to the figures 8 et 9 may include a polarizing filter interposed between the photodetectors 107 and the microlenses 401, for example a polarizing filter identical or similar to the FP polarizing filter of the sensor 600 or the FP polarizing filter of the sensor 100.

[0131] Furthermore, the embodiments described are not limited to the examples of dimensions and materials mentioned in this description for the realization of metasurfaces.

[0132] Although examples of backside illumination (BSI) sensor embodiments have been described above, the embodiments described can be adapted to frontside illumination (FSI) sensors. In this case, the photosensitive region of each pixel is illuminated through the interconnect stack 103. The polarizing filter and / or the polarizing router are then formed on the front side (lower face in the orientation of the figure 1B ) of the substrate, before the formation of the interconnection stack 103.

[0133] Furthermore, in the case of a front-facing illumination sensor, the polarization structures 111 can be formed in one or more metallic levels of the interconnect stack 103. This avoids introducing an additional step for the fabrication of the polarization structures 111.

[0134] Furthermore, the described embodiments are not limited to the examples presented above of polarization structures 111 made of opaque metallic patterns laterally surrounded by a transparent dielectric material. As an alternative, the polarization structures 111 can be made of transparent or semi-transparent materials exhibiting a refractive index contrast, so as to improve the transmission of the polarizers. For example, for polarization structures intended to operate in the near-infrared, for example at a wavelength of approximately 940 nm, silicon patterns, for example amorphous silicon, surrounded by a dielectric material with a lower refractive index, for example silicon oxide, can be used.For pixels intended to measure visible radiation, patterns made of silicon nitride or titanium oxide can be used, surrounded by a dielectric material with a lower refractive index, for example silicon oxide.

[0135] It should also be noted that, in a rear-facing illumination sensor of the type described in relation to the figures 1A et 1B Depending on the thickness of the substrate 101 and the wavelength intended to be measured by the pixels, part of the incident light radiation can pass through the entire thickness of the substrate and be reflected off metallic tracks of the interconnect stack 103, before being absorbed in the photosensitive region 105 of the pixels.

[0136] Reflection on the metal tracks of the interconnect stack can lead to at least partial polarization of the light in a direction dependent on the orientation of said metal tracks. Preferably, for each pixel P of the sensor, the metal tracks of the interconnect stack 103 located opposite the pixel are oriented in a direction chosen according to the pixel's polarization, for example, so as to favor the polarization of the reflected light in the polarization orientation intended to be measured by the pixel. Thus, preferably, the metal tracks of the interconnect stack 103 located opposite pixels intended to measure different polarizations have different orientations.

[0137] Furthermore, the described embodiments are not limited to the application examples described above for visible sensors. Other wavelength ranges can benefit from polarizing pixels. For example, the described embodiments can be adapted to infrared sensors designed to measure radiation with wavelengths between 1 and 2 µm, for example, based on InGaAs or germanium.

[0138] Furthermore, although above examples have been described in which PS1 and PS2 polarizations are orthogonal and PS3 and PS4 polarizations, rotated 45° with respect to PS1 and PS2 polarizations respectively, are orthogonal, a person skilled in the art is able to adapt the embodiments of this description to cases where PS1 and PS2 polarizations are not orthogonal and / or PS3 and PS4 polarizations are not orthogonal, PS3 and PS4 polarizations being able to be further rotated by an angle other than 45° with respect to PS1 and PS2 polarizations.

[0139] Finally, the practical implementation of the described embodiments and variants is within the reach of a person skilled in the art, based on the functional specifications given above. In particular, a person skilled in the art will be able, based on the specifications in this description, to design, dimension, and fabricate the metasurfaces and polarizing filters and / or microlenses of the sensor pixels, so that these structures cooperate to achieve the desired effect of improving the trade-off between sensitivity and the polarization extinction coefficient of the pixels.

Claims

1. Polarizing filter (FP) intended to be arranged in front of an image sensor (100; 400; 600; 800; 900) comprising a plurality of pixels (P), the filter comprising, for each pixel, a polarizing structure (111) comprising a plurality of parallel metal bars (701), each bar being coated with an absorbing stack (703) comprising: - a tungsten layer (705); the polarizing filter being characterized in that it further comprises: - a silicon layer (707), coating the tungsten layer; and - a dielectric layer (709), coating the silicon layer.

2. Filter according to claim 1, wherein the metal bars (701) are made of a material different from tungsten, preferably of aluminum.

3. Filter according to claim 2, wherein the tungsten layer (705) has a thickness greater than 40 nm, preferably greater than 60 nm.

4. Filter according to claim 1, wherein the metal bars (701) are made of tungsten.

5. Filter according to claim 4, wherein the metal bars (701) and the tungsten layer (705) have a cumulated thickness greater than 40 nm, preferably greater than 60 nm.

6. Filter according to any of claims 1 to 5, wherein the silicon layer (707) has a thickness in the range from 20 to 100 nm, preferably from 30 to 50 nm, for example equal to approximately 39 nm.

7. Filter according to any of claims 1 to 6, wherein the dielectric layer (709) is made of silicon oxide.

8. Filter according to any of claims 1 to 6, wherein the dielectric layer (709) is formed of a stack of a plurality of layers of dielectric materials having refraction indices lower than that of silicon.

9. Polarimetric image sensor (100; 400; 600) formed inside and on top of a semiconductor substrate (101), the sensor comprising: - a plurality of pixels (P), each comprising a photodetector (107) formed in the semiconductor substrate; and - a polarizing filter (FP) according to any one of claims 1 to 8, the filter being arranged on the side of an illumination surface of the photodetectors.

10. Sensor (100; 600; 800; 900) according to claim 9, wherein said plurality of pixels (P) comprises at least first and second pixels (P(1), P(2)) adapted to measuring radiations according to respectively first and second distinct polarizations, the polarization structure (111) of the first pixel (P(1)) being adapted to predominantly transmitting a radiation according to the first polarization and the polarization structure (111) of the second pixel (P(2)) being adapted to predominantly transmitting a radiation according to the second polarization.

11. Sensor (100; 600; 800; 900) according to claim 10, wherein said plurality of pixels (P) further comprises third and fourth pixels (P(3), P(4)) adapted to measuring radiations according to respectively third and fourth distinct polarizations, different from the first and second polarizations, the polarization structure (111) of the third pixel (P(3)) being adapted to predominantly transmitting a radiation according to the third polarization and the polarization structure (111) of the fourth pixel (P(4)) being adapted to predominantly transmitting a radiation according to the fourth polarization.

12. Sensor (100; 600; 800; 900) according to claim 11, wherein the first, second, third and fourth polarizations are linear polarizations along first, second, third and fourth directions respectively forming 0°, 90°, 45° and 135° angles with a reference direction.

13. Sensor (600; 800; 900) according to any of claims 9 to 12, further comprising a polarization router comprising a two-dimensional metasurface (MS) arranged on the side of the polarizing filter (FP) opposite to the photodetectors, the metasurface comprising a two-dimensional array of pads (117).

14. Sensor (600; 800; 900) according to claim 13, as dependent on claim 11 or 12, wherein the two-dimensional metasurface (MS) comprises: - a first portion (MS(1)) located vertically in line with the first and second pixels (P(1), P(2)) adapted to predominantly transmitting: a radiation according to the first polarization towards the first pixel (P(1)); and a radiation according to the second polarization towards the second pixel (P(2)), and - a second portion (MS(2)) located vertically in line with the third and fourth pixels (P(3), P(4)) adapted to predominantly transmitting: a radiation according to the third polarization towards the third pixel (P(3)); and a radiation according to the fourth polarization towards the fourth pixel (P(4)).

15. Sensor (600; 800; 900) according to any of claims 9 to 14, further comprising a plurality of first microlenses (401) extending in front of a pair of adjacent pixels (P(1), P(2)) of the sensor.

16. Sensor (600; 800; 900) according to claim 15, wherein the first microlenses (401) each have an elongated shape.

17. Sensor (600; 800; 900) according to claim 15 or 16, as dependent on claim 13 or 14, wherein the first microlenses (401) are: A) arranged on the side of a surface of the two-dimensional metasurface (MS) opposite to the photodetectors (107); or B) interposed between the photodetectors and the two-dimensional metasurface.

18. Sensor (900) according to claim 17, in its option B), further comprising a plurality of second microlenses (901) distinct from the first microlenses (401) and arranged on the side of a surface of the two-dimensional metasurface (MS) opposite to the photodetectors (107), each second microlens extending in front of a pair of adjacent pixels (P(1), P(2)) of the sensor.

19. Sensor (900) according to claim 18, wherein each second microlens (901) has an elongated shape.