Optical device

Abstract

Title: Optical device The invention relates to an optoelectronic device (1) comprising in a stack: • A reflection polarizing filter (10), • A phase shift element (20) configured to add a phase shift of π / 4 in polarization, • An active region (30), • A reflector (40), such that the rays of light radiation reflected by the reflector (40) and passing through the phase shift element (20) have a new polarization phase shifted by π / 2 with respect to their initial polarization, said rays then being reflected again by the polarizing filter (10) in the direction of the active region (30). Figure for the abstract: Fig.1

Description

Title of the invention: Optical device Technical field

[0001] The present invention relates, in general, to an optical device, and more particularly, to an optical device for an infrared sensor pixel. STATE OF THE ART

[0002] Some spatially resolved infrared sensors have a pixel architecture. These pixels, typically silicon-based photodiodes, can be formed by processes derived from complementary metal-oxide-semiconductor (CMOS) transistor technologies.

[0003] Each pixel can be configured to detect a particular polarization component of infrared radiation. A polarizing filter is then added to each pixel upstream of the active region of the pixel.

[0004] A polarization-sensitive infrared imager thus typically comprises a matrix of pixels each comprising, in stack: - Possibly an optical element of the lens type, - A polarizing filter, - An active region of the photodiode type absorbing the light transmitted by the polarizing filter, - An underlying level of contacts or interconnections, possibly serving as a reflector for light not absorbed by the active region.

[0005] Although functional, such an imager remains poorly performing in the infrared due to low quantum efficiency of the pixels. The absorption of infrared light in the active region remains limited. This low quantum efficiency of the pixels limits the possibility of miniaturizing them. The resolution of the imager is limited. In addition, light not absorbed by the active region can potentially propagate into another pixel, thus creating a source of optical crosstalk also detrimental to the resolution of the imager.

[0006] To improve the quantum efficiency of pixels, it is possible to form a diffractive structure upstream of the active region, in order to increase the path length of the light propagating in the active region. The document “2021 -Photonics West - Pixels with add-on structures to enhance quantum efficiency in the near infrared. Félix Bardonnet et. al.” discloses such a diffractive structure for a pixel sensitive in the near infrared. The gain provided by this solution nevertheless depends on the thickness of the photodiode and the width of the pixels. A compromise between resolution and efficiency remains necessary.

[0007] There is therefore a need to improve the quantum efficiency of a pixel, particularly in the infrared, independently of the size of the pixel.

[0008] An objective of the present invention is to meet this need and to at least partially overcome the drawbacks mentioned above.

[0009] An objective of the present invention is to provide an optoelectronic device of the photosensitive pixel type which has improved quantum efficiency.

[0010] Other objects, features and advantages of the present invention will become apparent from a consideration of the following description and accompanying drawings. It is understood that other advantages may be incorporated. SUMMARY

[0011] To achieve this objective, according to one embodiment, an optoelectronic device is provided configured to capture light radiation of wavelength X, comprising in a stack along a first direction z: - A reflection polarizing filter, configured to transmit only rays of light radiation having a first polarization, - A phase shift element configured to add a phase shift of ir / 4 in polarization to the rays of light radiation passing through this phase shift element, - An active region configured to absorb at least part of the rays of light radiation, - A reflector configured to reflect at least part of the rays of light radiation.

[0012] Thus, the rays of light radiation reflected by the reflector and passing through the phase shift element have a new polarization phase shifted by ir / 2 with respect to the first polarization. The rays having the new polarization are then again reflected by the polarizing filter in the direction of the active region. The polarizing filter transmits only rays having the first polarization. The rays having a polarization other than the first polarization are reflected by the polarizing filter.

[0013] The path of the light rays in the active region is advantageously increased. The quantum efficiency is improved. The light rays can make up to two round trips between the polarizing filter and the reflector. A cavity, which depends on the polarization of the light rays, is thus advantageously formed. This cavity is thus also called a "polarizing optical cavity".

[0014] Structurally, the optical cavity is located between the polarizing filter and the reflector, and comprises the phase shift element and the active region. This cavity makes it possible to introduce a phase shift of ir / 4 in polarization at each path of the light radiation within said cavity.

[0015] Functionally, light radiation propagating from the polarizing filter towards the active region can thus undergo a first phase shift of ji / 4, a first reflection by the reflector, a second phase shift of ji / 4, a reflection by the polarizing filter, a third phase shift of ji / 4, a second reflection by the reflector, a fourth phase shift of ji / 4, a transmission by the polarizing filter. The combination of the polarizing filter, the phase shift element and the reflector advantageously makes it possible to lengthen the optical path in the active region, i.e. in the photosensitive part of the device according to the invention.

[0016] According to one aspect, the invention also relates to a system comprising a plurality of these devices organized in a matrix, forming an infrared imager. BRIEF DESCRIPTION OF THE FIGURES

[0017] The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of embodiments thereof which are illustrated by the following accompanying drawings in which:

[0018] [Fig.l] [Fig.l] schematically illustrates in cross-section an optoelectronic device, according to an embodiment of the present invention.

[0019] [Fig.2] [Fig.2] schematically illustrates in cross-section the operating principle of an optoelectronic device according to an embodiment of the present invention.

[0020] [Fig.3A][Fig.3B] Figures 3A, 3B schematically illustrate in top view different embodiments of a polarizing filter of an optoelectronic device according to the present invention.

[0021] [Fig.4A][Fig.4B][Fig.4C] Figures 4A, 4B, 4C schematically illustrate in top view different embodiments of a phase shift element of an optoelectronic device according to the present invention.

[0022] [Fig.5] [Fig.5] illustrates simulated curves of the absorption and reflection of an optoelectronic device according to the present invention, as a function of the thickness of the phase shifting element.

[0023] [Fig.6] [Fig.6] schematically illustrates in cross-section a system comprising two adjacent optoelectronic devices, according to an embodiment of the present invention.

[0024] [Fig.7A] [Fig.7A] schematically illustrates in top view a polarization sorting element of a system, according to an embodiment of the present invention.

[0025] [Fig.7B] [Fig.7B] schematically illustrates in top view a plurality of adjacent polarizing filters of a system according to an embodiment of the present invention.

[0026] [Fig.7C] [Fig.7C] schematically illustrates in top view a plurality of adjacent phase shifting elements of a system according to an embodiment of the present invention.

[0027] [Fig.8A] [Fig.8A] schematically illustrates in top view a sorting element of polarizations of a system, according to another embodiment of the present invention.

[0028] [Fig.8B] [Fig.8B] schematically illustrates in top view a plurality of adjacent polarizing filters of a system according to another embodiment of the present invention.

[0029] [Fig.8C] [Fig.8C] schematically illustrates in top view a plurality of adjacent phase shifting elements of a system according to another embodiment of the present invention.

[0030] [Fig.9A][Fig.9B] Figures 9A, 9B schematically illustrate in top view different embodiments of a reflector of an optoelectronic device according to the present invention.

[0031] The drawings are given as examples and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily on the scale of practical applications. In particular, on the schematic diagrams, the thicknesses of the different layers and portions, and the dimensions of the patterns are not necessarily representative of reality. DETAILED DESCRIPTION

[0032] Before commencing a detailed review of embodiments of the invention, optional features which may possibly be used in combination or alternatively are set out below:

[0033] According to one example, the active region is a silicon-based photodiode. Such a photodiode is advantageously produced by standard CMOS technology methods.

[0034] According to one example, the polarizing filter comprises a network of lines parallel to each other and perpendicular to the first polarization. The lines are orthogonal to the direction of the electric field. The lines may be based on aluminum or silicon. They are typically embedded in a SiO2-based matrix.

[0035] According to one example, the reflector comprises a portion of a contact level or interconnections. This allows the reflector to be used for both optical reflection and electrical conduction. For example, it comprises metal tracks forming an electrical contact with the active region of the device. These metal tracks can be arranged in the form of a grid so as to adjust the reflection properties of the reflector.

[0036] According to one example, the phase shifting element comprises an array of parallel lines between them and forming an angle of 45° with the first polarization. This makes it possible to obtain an efficient phase shift element, not requiring a large thickness to obtain an effective phase shift. The lines can be silicon-based. They are typically embedded in a SiO2-based matrix.

[0037] According to one example, the phase shift element is in the form of a metasurface comprising a network of ellipses whose major axes are directed in a direction forming an angle of 45° with the first polarization. This makes it possible to obtain a phase shift element having better transmission. The ellipses can be silicon-based. They are typically embedded in a SiO2-based matrix.

[0038] According to one example, the phase shift element is in contact with the active region. The height of the stack is thus reduced. This makes it possible to increase the compactness of the device.

[0039] According to one example, the device further comprises at least one reflective wall laterally bordering the active region. This makes it possible to improve the collection of rays propagating in a direction not parallel to the first direction z. This makes it possible to reduce optical crosstalk between two adjacent devices.

[0040] According to one example, the wavelength X of the light radiation belongs to the infrared range. The wavelength X is for example between 920 nm and 960 nm.

[0041] According to one example, the phase shift element has a thickness along the first direction z of the order of 0.8 μm. This makes it possible to optimize the function of the phase shift element for rays of wavelengths X in the infrared range having different directions of propagation.

[0042] According to one example, the system comprises at least a first device and a second device adjacent in a y direction normal to the first z direction, the first device comprising a polarizing filter configured to transmit only rays of the light radiation having a first polarization and the second device comprising a polarizing filter configured to transmit only rays of the light radiation having a second polarization. The system may further comprise, on the first and second devices, a polarization sorting element configured to direct the rays having the first polarization towards the first device and to direct the rays having the second polarization towards the second device. This makes it possible to produce polarization-sensitive sensors, typically to form an imager making it possible to obtain several polarized images from the same light radiation.

[0043] Unless incompatibility exists, it is understood that all of the above optional features may be combined to form an embodiment that is not necessarily illustrated or described. Such an embodiment is not obviously not excluded from the invention. The characteristics and advantages of the device according to the invention may apply, mutatis mutandis, to the characteristics and advantages of the system according to the invention, and vice versa.

[0044] It is specified that, in the context of the present invention, the terms "on", "overcomes", "covers", "underlying", "facing" and their equivalents do not necessarily mean "in contact with". Thus, for example, the deposition or formation of a first layer on a second layer does not necessarily mean that the two layers are directly in contact with each other, but means that the first layer at least partially covers the second layer by being either directly in contact with it, or by being separated from it by at least one other layer or at least one other element.

[0045] A layer may also be composed of several sub-layers of the same material or of different materials.

[0046] A substrate, a stack, a layer, an element “based” on a material A, a substrate, a stack, a layer means an element comprising this material A only or this material A and possibly other materials, for example alloying elements and / or doping elements.

[0047] A preferably orthonormal reference frame, comprising the axes x, y, z is shown in the attached figures. When a single reference frame is shown on the same sheet of figures, this reference frame applies to all the figures in this sheet.

[0048] In the present patent application, the thickness of a layer is taken along a direction normal to the main extension plane of the layer. Thus, a layer typically has a thickness along z. The relative terms "on", "overcomes", "under", "underlying", "intercalated" refer to positions taken along the z direction.

[0049] The terms "vertical", "vertically" refer to a direction along z. The terms "horizontal", "horizontally", "lateral", "laterally" refer to a direction in the xy plane. Unless explicitly stated, thickness, height and depth are measured along z.

[0050] An element located "perpendicular" or "straight" to another element means that these two elements are both located on the same line perpendicular to a plane in which a lower or upper face of a substrate mainly extends, that is to say on the same line oriented vertically in the figures.

[0051] In the context of the present invention, the phase shifting element is configured to phase shift the polarization of the light by ji / 4. This function corresponds to that of a quarter-wave plate. The phase shifting element can therefore also be referred to as a "quarter-wave plate" in the following.

[0052] [Fig.l] illustrates an embodiment of the optoelectronic device 1. This optoelectronic device 1 typically comprises in stacking along z: - An optional optical element 60, typically a lens or micro lens, intended to increase the numerical aperture of the device, - A polarizing filter 10 in reflection, configured to transmit polarized light, typically to transmit a polarization state, and reflect the polarization state orthogonal to the transmitted state, - One or more interposition elements 61, for example in the form of SiO2-based layers, configured to optimize the propagation of polarized light, - A phase shift element 20 configured to add a phase shift of ir / 4 in polarization to the light passing through this phase shift element 20, - An active region 30 configured to absorb at least part of the polarized light, - A reflector 40 configured to reflect at least part of the polarized light.

[0053] The polarizing filter 10 in reflection may be in the form of a layer comprising a network of lines 11, 12 embedded in a SiO2-based matrix. These lines 11, 12 may be based on silicon or aluminum. The polarizing filter 10 typically has a thickness ei0. Such a polarizing filter 10 is easily integrated and produced by CMOS technology processes. Different embodiments of the polarizing filter 10 in reflection are described and illustrated below.

[0054] The phase shift element 20 may be in the form of a layer comprising a network of lines or patterns 21, 22 embedded in a SiO2-based matrix. These lines or patterns 21, 22 may be silicon-based. The phase shift element 20 typically has a thickness e20. Such a phase shift element 20 is easily integrated and produced by CMOS technology methods. The phase shift element 20 may be directly formed on the active region 30, for example by etching. There is not necessarily an interposition element 61 between the phase shift element 20 and the active region 30. Different embodiments of the phase shift element 20 are described and illustrated below.

[0055] The active region 30 is preferably silicon-based. It is preferably bordered by reflective walls 31, for example in the form of deep aluminum-based trenches.

[0056] The reflector 40 is typically formed by a network of metal lines in electrical contact with the active region 30. It thus provides an optical reflection function and an electrical conduction function. It can typically correspond to a part of a metal level of CMOS technology.

[0057] [Fig.2] illustrates an operating principle of the optoelectronic device 1. The Optoelectronic device 1 functions partly as an optical cavity between the polarizing filter 10 and the reflector 40, for certain polarized rays of the incident light. In [Fig.2], four successive phases (A1), (RI), (A2), (R2), corresponding to back and forth movements of the light within the optical cavity of the device, are detailed. During the first phase (A1), typically unpolarized incident rays Ri propagate from top to bottom, towards the polarizing filter 10. These incident rays Ri are filtered by the polarizing filter 10. Only the rays Rt having the PI polarization are transmitted by the polarizing filter 10. The PI polarization corresponds for example to a Transverse Electric (TE) polarization. The PI polarized rays Rt then propagate towards the phase shift element 20.After passing through the phase shift element 20, the Rt rays become Rtl rays having a Pl'd polarization phase shifted by ir / 4 relative to the PL polarization. In the case of a TE type PI polarization, the Pl'd polarization typically corresponds to a right circular polarization. A portion of the Pl'd polarized Rtl rays is absorbed by the active region 30. The Pl'd polarized Rtl rays not absorbed by the active region 30 are then reflected by the reflector 40. During the second phase (RI), the rays propagate from bottom to top. The Pl'd polarized Rtl rays reflected by the reflector 40 become Rtl' rays having a Pl'g polarization. In the case of a TE type PI polarization, the Pl'g polarization typically corresponds to a left circular polarization. A part of the Pl'g polarized Rtl' rays is absorbed by the active region 30.The Pl'g polarized Rtl' rays not absorbed by the active region 30 propagate towards the phase shift element 20. After passing through the phase shift element 20, the Rtl' rays become Rt2 rays having a PI” polarization phase shifted by ir / 4 relative to the Pl'g polarization. In the case of a TE type PI polarization, the PI” polarization typically corresponds to a Transverse Magnetic (TM) polarization. The PI” polarized Rt2 rays are then advantageously reflected by the polarizing filter 10. During the third phase (A2), the rays propagate from top to bottom. The PI” polarized Rt2 rays reflected by the polarizing filter 10 become Rt2' rays having the PI” polarization. The PI” polarized Rt2' rays propagate again towards the phase shift element 20.After passing through the phase shifting element 20, the Rt2' rays become Rt3 rays having the Pl'g polarization phase shifted by ir / 4 with respect to the PI polarization. A part of the Pl'g polarized Rt3 rays is again absorbed by the active region 30. The Pl'g polarized Rt3 rays not absorbed by the active region 30 are then reflected by the reflector 40. During the fourth phase (R2), the rays propagate from the bottom to the top. The Pl'g polarized Rt3 rays reflected by the reflector 40 become Rt3' rays having the Pl'd polarization. A part of the Pl'd polarized Rt3' rays is again absorbed by the region. active 30. The Pl'd polarized Rt3' rays not absorbed by the active region 30 propagate towards the phase shift element 20. After passing through the phase shift element 20, the Rt3' rays become Rt4 rays having the PI polarization phase shifted by ir / 4 relative to the Pl'd polarization. The PI polarized Rt4 rays are then transmitted by the polarizing filter 10, and become PL polarized Rtb rays

[0058] The rays thus typically make two round trips in the optical cavity before possibly exiting the device. The path of the light rays is thus advantageously doubled in this device provided with a quarter-wave plate type phase shift element, compared to a device without such a phase shift element. The absorption of polarized rays in the active region 30 can be done four times, during each of the round-trip phases (Al), (RI), (A2), (R2).The absorption of polarized rays by the device is thus improved.

[0059] [Fig.3A] illustrates an embodiment of the polarizing filter 10. The polarizing filter 10 is here formed by a network of lines 11 based on aluminum in a silicon oxide matrix. In the case of a polarizing filter with a metal grid, the orientation of PI is perpendicular to that of the grid. The lines 11 are oriented along the y axis. The pitch of the network is measured along x. and it is indeed the x polarization which is 90% and the y polarization is 1%. Such a polarizing filter transmits light rays having a polarization parallel to the x axis and rejects light rays having a polarization perpendicular to the x axis. The polarizing filter 10 comprising metal lines 11 is advantageously insensitive to the angle of incidence of the light rays.For light rays in the infrared range, a polarizing filter 10 with a thickness ei0 of the order of 250 nm will preferably be chosen, with a grating pitch p < 250 nm along x, and a filling factor f < 30% (the filling factor f being the ratio of the width of a line along y to the grating pitch). This makes it possible to obtain a transmission of approximately 90% for rays polarized along x, while maintaining a transmission of less than 1% for rays polarized along y.

[0060] [Fig.3B] illustrates another embodiment of the polarizing filter 10. The polarizing filter 10 is here formed by a network of lines 12 based on silicon in a silicon oxide matrix. The lines 11 are oriented along the y axis. The pitch of the network p, along x, can here be larger, typically of the order of 500 nm. This makes it possible to relax the constraint in spatial resolution necessary for the manufacture of such a network. The cost of the polarizing filter 10 can thus be reduced. For light rays in the infrared range, typically 920 nm < X < 960 nm, a polarizing filter 10 will preferably be chosen having a thickness ei0 of the order of 550 nm, with a network pitch p of the order of 505 nm along x, and a filling factor f of the order of 42%. Alternatively, a polarizing filter 10 can be chosen with a thickness ei0 of the order of 120 nm, with a network pitch p of the order of 525 nm along x, and a filling factor f of the order of 42%.

[0061] [Fig.4A] illustrates an embodiment of the phase shift element 20. The phase shift element 20 is here formed by a network of lines 21 based on silicon in a silicon oxide matrix. The lines 21 are typically oriented at 45° with respect to the x axis. For light rays in the infrared range, a phase shift element 20 will preferably be chosen having a thickness e20 of the order of 300 nm, with a network pitch p < 300 nm, typically 50 nm < p < 250 nm, and a filling factor f of the order of 50%. This makes it possible to obtain an effective quarter-wave plate for TE and TM light propagation modes, with a transmission greater than 90%, close to 100%.

[0062] Figures 4B, 4C illustrate another embodiment of the phase shift element 20. The phase shift element 20 is here formed by a network of silicon-based ellipses 22 in a silicon oxide matrix. The ellipses 22 typically have a major axis (O) oriented at 45° relative to the x axis. For light rays in the infrared range, a phase shift element 20 will preferably be chosen having a thickness e2o of between 700 nm and 800 nm, with a network pitch p < 300 nm, and a filling factor f of the order of 25%. The ellipses 22 typically have a dimension Ix of 225 nm along the major axis (O), and a dimension ly of 75 nm along the minor axis.

[0063] [Fig.5] illustrates a modeling of the absorption (curve C1) in the active region 30 as well as the reflection (curve C2) of the light above the device, as a function of the thickness e20 of the phase shift element 20. The phase shift element 20 corresponds here to the network of ellipses 22 oriented at 45°, based on silicon. The curves C1, C2 are averaged over 5 wavelengths between 920 nm and 960 nm. Absorption is a criterion to be maximized and reflection is a criterion to be minimized. The optimal thickness e20, corresponding to a quarter-wave plate, is of the order of 0.8 pm.

[0064] [Fig. 6] illustrates in cross-section a system 2 comprising two devices 1a, 1b adjacent along y. The first device 1a comprises a polarizing filter 10a configured to transmit only R' rays of the light radiation having a first polarization PL. The second device 1b comprises a polarizing filter 10b configured to transmit only R” rays of the light radiation having a second polarization P2. The system 2 further comprises, on the first and second devices 1a, 1b, a polarization sorting element 51, 52 configured to direct the R' rays towards the first device 1a and to direct R” rays towards the second device 1b. In the first device 1a, the polarizing filter 10a transmits the R' rays to the phase shift element 20a and, as described previously for the device, the R' rays make several round trips between the phase shift element 20a and the reflector 40 passing through the active region 30a. The R' rays are thus more efficiently absorbed by the first device 1a. In the same way, in the second device 1b, the polarizing filter 10b transmits the R” rays to the phase shift element 20b and the R” rays make several round trips between the phase shift element 20b and the reflector 40 passing through the active region 30b. The R” rays are thus more efficiently absorbed by the second device 1b. The active regions 30a, 30b are preferably separated by reflective walls 31, typically made of aluminum, to avoid or limit a phenomenon of optical crosstalk between devices, and to maximize absorption in the active regions 30a, 30b.The system 2 thus comprises devices, typically pixels, sensitive to different polarizations of the incident light. Such a system 2 makes it possible, for example, to produce an infrared imager sensitive to polarization, capable of producing different polarized images from the same light source.

[0065] [Fig.7A] illustrates in top view an embodiment of a polarization sorting element 51 covering an elementary matrix of four pixels 1a, 1b, 1c, 1d. Each pixel 1a, 1b, 1c, 1d is sensitive to a polarization, for example respectively 0°, 45°, 90°, 135°. The polarization sorting element 51 is here formed by an array of silicon-based patterns in a silicon oxide matrix. The patterns are defined and oriented according to the polarization to be transmitted and according to the position of the pixel configured to receive the rays polarized according to this polarization.

[0066] [Fig.7B] illustrates in top view polarizing filters 10a, 10b, 10c, 10d of the corresponding pixels 1a, 1b, 1c, 1d. Each polarizing filter 10a, 10b, 10c, 10d is configured to transmit rays having a given polarization, for example respectively 0°, 45°, 90°, 135°. The polarizing filters 10a, 10b, 10c, 10d are here formed by networks of silicon-based lines in a silicon oxide matrix. The orientation of these lines is defined as a function of the polarization of the rays that it is desired to transmit to the underlying active region.

[0067] [Fig.7C] illustrates in top view phase shift elements 20a, 20b, 20c, 20d of the corresponding pixels 1a, 1b, 1c, 1d. Each phase shift element 20a, 20b, 20c, 20d is configured to phase shift the polarization of the transmitted rays by ir / 4. The phase shift elements 20a, 20b, 20c, 20d are here formed by arrays of silicon-based ellipses in a silicon oxide matrix. The orientation of the major axis of these ellipses is defined as a function of the orientation of the lines of the polarizing filters 10a, 10b, 10c, 10d, respecting a shift of 45° with respect to the orientation of the lines of the corresponding polarizing filters 10a, 10b, 10c, 10d. The major axes of the ellipses of the phase shift elements 20a, 20b, 20c, 20d are for example respectively oriented at 45°, 90°, 135°, 180°.

[0068] A system 2 comprising one or more elementary matrices of pixels 1a, 1b, 1c, 1d sensitive to polarization forms for example a polarization-sensitive imager.

[0069] It is also possible to implement the invention in a system 2 not sensitive to polarization.

[0070] [Fig.8A] illustrates in a top view an embodiment of a polarization sorting element 52 covering an elementary matrix of four pixels 1e, 1f, 1g, 1h. The pixels 1e, 1h are here sensitive to the same first polarization, typically 0°. The pixels 1e, 1g are here sensitive to the same second polarization complementary to the first polarization, typically 90°. The polarization sorting element 52 is here formed by a network of silicon-based patterns in a silicon oxide matrix. The patterns are defined and oriented according to the polarization to be transmitted and according to the position of the pixels configured to receive the rays polarized according to this polarization. The polarization sorting element 52 typically has a central symmetry.

[0071] [Fig.8B] illustrates in top view polarizing filters 10e, 10f, 10g, 10h of the corresponding pixels 1e, 1f, 1g, 1h. Each polarizing filter 10e, 10f, 10g, 10h is configured to transmit rays having a given polarization, for example respectively 0°, 90°, 0°, 90°. The polarizing filters 10e, 10f, 10g, 10h are here formed by networks of silicon-based lines in a silicon oxide matrix. The orientation of these lines is defined according to the polarization of the rays that it is desired to transmit to the underlying active region.

[0072] [Fig.8C] illustrates in top view phase shift elements 20e, 20f, 20g, 20h of the corresponding pixels 1e, 1f, 1g, 1h. Each phase shift element 20e, 20f, 20g, 20h is configured to phase shift the polarization of the transmitted rays by 1r / 4. The phase shift elements 20e, 20f, 20g, 20h are here formed by arrays of silicon-based ellipses in a silicon oxide matrix. The orientation of the major axis of these ellipses is defined as a function of the orientation of the lines of the polarizing filters 10e, 10f, 10g, 10h, respecting an offset of 45° with respect to the orientation of the lines of the corresponding polarizing filters 10e, 10f, 10g, 10h. The major axes of the ellipses of the phase shift elements 20e, 20f, 20g, 20h are for example all oriented at 45° in this embodiment.

[0073] By coupling the pixels 1e, 1f and the pixels 1g, 1h in pairs, it is possible to obtain a polarization-insensitive imager, having improved light detection efficiency (via the polarizing optical cavity integrated in each pixel). The invention can therefore advantageously be implemented in polarization-insensitive systems.

[0074] Figures 9A and 9B illustrate in top view respectively two embodiments of a reflector under an elementary matrix of four pixels separated by walls 31. In these two embodiments, the reflector comprises an array of metal pads 41a, 41b, for example based on copper, in silicon 4. These metal pads 41a, 41b advantageously serve to electrically contact the active regions located above. Only the size of the metal pads and the metal filling factor (ratio of the metal surface to the silicon surface) differ between the two embodiments. Table 1 presents different results (quantum efficiency QE and contrast C) from simulations for systems comprising different device configurations. The first row “direct illumination” of Table 1 corresponds to a configuration comprising an active region with an underlying reflector, without any other element surmounting the active region. The second row “sorter + reflector” of Table 1 corresponds to a configuration comprising an active region surmounted by a polarization sorting element, and an underlying reflector. The third row “sorter + phase shifter” of Table 1 corresponds to a configuration comprising an active region surmounted by a polarization sorting element and a phase shifter, without an underlying reflector.The fourth row "sorter + phase shifter + reflector" of Table 1 corresponds to a configuration comprising an active region surmounted by a polarization sorting element and a phase shifting element, and comprising an underlying reflector. The fifth row "sorter + diffraction + reflector" of Table 1 corresponds to a configuration comprising an active region surmounted by a polarization sorting element and a known diffractive structure, and comprising an underlying reflector. Two columns "reflector A" and "reflector B" correspond respectively to the reflector configurations illustrated in Figures 9A and 9B.

[0075] [Tables 1] Reflector A Reflector B direct illumination QE: 22.0% QE: 26.4% sorter + reflector QE: 22.4% - C: 10 QE: 26.0%-C: 8.1 sorter + phase shifter QE: 32.0% - C: 1.6 sorter + phase shifter + reflector QE: 33.0% - C: 8.2 QE: 56% - C: 14 sorter + diffraction + reflector - QE: 48.0% - C: 9.2

[0076] Best pixel performance is achieved for maximum quantum efficiency QE and maximum contrast C.

[0077] Performance increases as the metal fill factor increases. (reflector B has a higher metal fill factor than reflector A).

[0078] The absence of a reflector impairs the contrast (expressed as the ratio between the transmitted polarization component and the rejected polarization component), despite the presence of the polarization sorting element.

[0079] It appears that the best performances are obtained for the configuration comprising the polarization sorting element, the phase shifting element, and the reflector in combination.

[0080] Such a configuration is furthermore more efficient than a known configuration comprising a diffractive structure. It therefore appears that the configuration proposed by the present invention can advantageously be implemented both in polarization-sensitive systems and in polarization-insensitive systems.

[0081] Other system configurations including an optoelectronic device as described above are possible. These variants are not necessarily illustrated but can be easily deduced by combining the features of the described embodiments.

[0082] The invention is not limited to the embodiments previously described.

Claims

Claims

1. Optoelectronic device (1) configured to capture light radiation (Ri) of wavelength X, comprising in a stack in a first direction z: • A polarizing filter (10) in reflection, configured to transmit only rays (Rt, Rtb) of the light radiation having a first polarization (PI), • A phase shift element (20) configured to add a phase shift of ir / 4 in polarization to the rays (Rtl, Rt2, Rt3, Rt4) of the light radiation passing through this phase shift element (20), • An active region (30) configured to absorb at least in part the rays (Rtl, Rtl', Rt3, Rt3') of the light radiation, • A reflector (40) configured to reflect at least in part the rays (Rtl, Rtl', Rt3, Rt3') of the light radiation, so that the rays (Rtl',Rt2) of the light radiation reflected by the reflector (40) and passing through the phase shift element (20) have a new polarization (PI”) phase shifted by ir / 2 with respect to the first polarization (PI), said rays (Rt2, Rt2') having the new polarization (PI”) then being reflected again by the polarizing filter (10) in the direction of the active region (30).,

2. Device (1) according to the preceding claim in which the active region (30) is a silicon-based photodiode.

3. Device (1) according to any one of the preceding claims in which the polarizing filter (10) comprises a network of lines (11, 12) parallel to each other and perpendicular to the first polarization (PD-

4. A device (1) according to any preceding claim wherein the reflector (40) comprises part of a contact or interconnection level.

5. Device (1) according to any one of the preceding claims in which the phase shift element (20) comprises a network of lines (21) parallel to each other and forming an angle of 45° with the first polarization (PI).

6. Device (1) according to any one of claims 1 to 4 in which the phase shift element (20) is in the form of a meta-surface comprising a network of ellipses (22) whose major axes (0) are directed in a direction forming an angle of 45° with the first polarization (PI).

7. Device (1) according to any one of the preceding claims in which the phase shifting element (20) is in contact with the active region (30).

8. Device (1) according to any one of the preceding claims further comprising at least one reflective wall (31) laterally bordering the active region (30).

9. Device (1) according to any one of the preceding claims in which the wavelength X of the light radiation belongs to the infrared range.

10. Device (1) according to any one of the preceding claims in which the phase shift element (20) has a thickness e2o in the first direction z of the order of 0.8 pm.

11. System (2) comprising a plurality of devices (1a, 1b) according to any one of the preceding claims organized in a matrix, forming an infrared imager.

12. System (2) according to the preceding claim comprising at least a first device (1a) and a second device (1b) adjacent in a direction (y) normal to the first direction (z), the first device (1a) comprising a polarizing filter (10a, 10e) configured to transmit only rays (R') of the light radiation having a first polarization (PI) and the second device (1b) comprising a polarizing filter (10b, 10f) configured to transmit only rays (R”) of the light radiation having a second polarization (P2), the system (2) further comprising, on the first and second devices (1a, 1b), a polarization sorting element (51, 52) configured to direct the rays (R') having the first polarization (PI) towards the first device (1a) and to direct the rays (R”) having the second polarization (P2) towards the second device (1b).

Images