Optical filter for multispectral sensor and method for manufacturing such a filter

Abstract

Optical filter for multispectral sensor and method for manufacturing such a filter. This description relates to an optical filter (101) intended to be positioned opposite an image sensor (103) comprising a plurality of pixels (105), the filter comprising, for each pixel, at least one resonant cavity (109) having a transparent region (111) having a first refractive index and laterally delimited by a peripheral reflective vertical wall (113), and at least one resonant element (115) formed in said region. Figure for the abstract: Fig. 1

Description

Title of the invention: Optical filter for multispectral sensor and method for manufacturing such a filter. Technical field

[0001] This description relates generally to multispectral sensors adapted for acquiring images of a scene in different wavelength ranges. More particularly, this description relates to an optical filter for a multispectral sensor, a multispectral sensor comprising such a filter, and a method for manufacturing an optical filter for a multispectral sensor. Previous technique

[0002] Multispectral sensors comprising a filter wheel positioned opposite an image sensor adapted to acquire successive images of a scene through different filters of the wheel have been proposed. Furthermore, other, more compact multispectral sensors have been proposed, comprising a single optical filter positioned opposite an image sensor, the filter being adapted to transmit incident radiation predominantly in a first wavelength range to certain pixels of the sensor and incident radiation predominantly in at least a second wavelength range, different from the first wavelength range, to other pixels of the sensor.

[0003] Existing multispectral sensors, however, have various drawbacks. Summary of the invention

[0004] An object of an embodiment is to overcome all or part of the drawbacks of known optical filters for multispectral sensors, known multispectral sensors incorporating such filters and known manufacturing processes of optical filters for multispectral sensors.

[0005] For this purpose, an embodiment provides an optical filter intended to be arranged opposite an image sensor comprising a plurality of pixels, the filter comprising, for each pixel, at least one resonant cavity having a transparent region having a first refractive index and laterally delimited by a peripheral reflective vertical wall, and at least one resonant element formed in said region.

[0006] According to one embodiment, said at least one resonant element located in one of said at least one resonant cavity has a different lateral dimension from that of said at least one resonant element located in another resonant cavity.

[0007] According to one embodiment, at least one of said resonant cavity has a different width from that of another resonant cavity.

[0008] According to one embodiment, each resonant element comprises a pad having a second refractive index greater than the first index.

[0009] According to one embodiment, each resonant element further comprises a transparent layer having a third refractive index greater than the first index and extending laterally into the resonant cavity.

[0010] According to one embodiment, the third index is substantially equal to the second index.

[0011] According to one embodiment, each resonant element comprises a portion of the transparent region located inside a through-opening formed in a transparent layer having a fourth refractive index greater than the first index and extending laterally into the resonant cavity.

[0012] According to one embodiment, the filter comprises, for each pixel, a single resonant cavity.

[0013] According to one embodiment, the filter comprises, for each pixel, a plurality of resonant cavities.

[0014] According to one embodiment, the peripheral reflective vertical wall is made of a metal.

[0015] According to one embodiment, the peripheral reflective vertical wall comprises a stack of electrically insulating layers of materials having different refractive indices.

[0016] According to one embodiment, the filter further comprises, for each cavity, a microlens located directly above said cavity.

[0017] One embodiment provides a multispectral image sensor comprising an image sensor having a plurality of pixels formed in and on a semiconductor substrate and an optical filter as described.

[0018] One embodiment provides a method for manufacturing an optical filter intended to be positioned opposite an image sensor comprising a plurality of pixels, the method comprising the following successive steps: a) to form, in a transparent layer, at least one resonant element for each pixel; and b) divide the transparent layer into several transparent regions, each comprising at least one of the resonant elements; and c) to line the sides of each transparent region with a peripheral reflective wall, in which the transparent region and the peripheral reflective wall form, for each pixel, a resonant cavity. Brief description of the drawings

[0019] 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 accompanying figures, among which:

[0020] [Fig.1] is a schematic and partial cross-sectional view of an example of a multispectral image sensor comprising an optical filter according to one embodiment;

[0021] [Fig.2] is a schematic and partial cross-sectional view of an example of a multispectral image sensor comprising an optical filter according to one embodiment;

[0022] [Fig.3] is a schematic and partial cross-sectional view of an example of a multispectral image sensor comprising an optical filter according to one embodiment;

[0023] [Fig.4] is a schematic and partial cross-sectional view of an example of a multispectral image sensor comprising an optical filter according to one embodiment;

[0024] [Fig.5A] and [Fig.5B] are schematic and partial top and cross-sectional views along plane AA of [Fig.5A], respectively, of an example of a multispectral image sensor comprising an optical filter according to one embodiment;

[0025] [Fig.6] is a schematic and partial cross-sectional view of an example of a multispectral image sensor comprising an optical filter according to one embodiment;

[0026] [Fig.7A], [Fig.7B] and [Fig.7C] are cross-sectional views illustrating, schematically and partially, successive stages of an example of a manufacturing process for the optical filter of [Fig.1] according to an embodiment;

[0027] [Fig.8] is a cross-sectional view illustrating, schematically and partially, a step in a manufacturing process for a variant of the optical filter of [Fig.1];

[0028] Figures [Fig. 9A] and [Fig. 9B] are cross-sectional views illustrating, schematically and partially, successive stages of an example of a manufacturing process for an optical filter according to one embodiment; and

[0029] [Fig.1OA] and [Fig.1OB] are cross-sectional views illustrating, schematically and partially, successive stages of an example of a manufacturing process for the optical filter of figures 5A and 5B according to one embodiment. Description of the implementation methods

[0030] The same elements have been designated by the same reference numerals in the different figures. In particular, the 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.

[0031] For the sake of clarity, only the steps and elements necessary for understanding the described embodiments have been shown and detailed. In particular, the implementation of the photodiodes and the pixel control circuits has not been detailed, as the implementation of such pixels is within the capabilities of a person skilled in the art, based on the information provided in this description.

[0032] Unless otherwise specified, when referring to two elements connected together, this means directly connected without intermediate elements other than conductors, and when referring to two elements coupled together, this means that these two elements can be connected or linked through one or more other elements.

[0033] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "superior", "inferior", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures.

[0034] Unless otherwise specified, the expressions "approximately", "about", "substantially", and "in the order of" mean within 10%, preferably within 5%.

[0035] Fig. 1 is a schematic and partial cross-sectional view of an example of a multispectral image sensor 100 comprising an optical filter 101 according to one embodiment.

[0036] In the example shown, the optical filter 101 is positioned opposite an image sensor 103, for example a CMOS (Complementary Metal-Oxide-Semiconductor) sensor. The image sensor 103 comprises a plurality of pixels 105 formed in and on a substrate 107. The substrate 107 is, for example, a wafer or a piece of wafer made of a semiconductor material, for example, silicon. The pixels 105 can have any shape when viewed from above. For example, each pixel 105 has a polygonal perimeter when viewed from above, for example, a rectangular or square, or a circular one. The pixels 105 are, for example, arranged in a matrix of rows and columns. Although not detailed in [Fig.1], control and read circuits for pixels 105 can be formed in and on substrate 107.Moreover, although only two pixels 105 have been illustrated in [Fig.1], the image sensor 103 can of course contain a much larger number of pixels 105, for example several thousand or several million pixels 105.

[0037] According to one embodiment, the optical filter 101 intended to be positioned opposite the image sensor 103 comprises, for each pixel 105 of the image sensor 103, at least one resonant cavity 109 having a transparent region 111, laterally delimited by a peripheral reflective vertical wall 113, and at least one resonant element 115, formed in the transparent region 111. The region 111 is made of a material transparent at the operating wavelengths of the pixels 105 and having a refractive index ni. In the example illustrated in [Fig. 1], the optical filter 101 includes, for each pixel 105, a single resonant cavity 109 comprising a single resonant element 115.

[0038] The resonant cavities 109 may have, in top view, any shape, for example identical to that of the underlying pixels 105. By way of example, each resonant cavity 109 may have, in top view, a polygonal perimeter, for example rectangular or square, or a circular one. The resonant cavities 109 may, for example, have lateral dimensions less than or equal to those of the pixels 105, the perimeter of each resonant cavity 109 being, in top view, located respectively inside or directly above the perimeter of the underlying pixel 105.

[0039] The reflective peripheral wall 113 has, for example, in top view, an annular shape, for example with a polygonal cross-section, for example rectangular or square, or with a circular cross-section, surrounding or laterally bordering the transparent region 111 of each resonant cavity 109, the sides of the transparent region 111 being covered by the wall 113. By way of example, the reflective peripheral wall 113 has a height, or thickness, of the order of 1 pm.

[0040] In the example shown, each resonant element 115 is a pad 117 made of a material with a refractive index n2 strictly greater than the refractive index ni of the material of the transparent region 111. Each pad 117 is, for example, coated on all its faces with the material of the transparent region 111. The pads 117 may have, in top view, a cross-section of any shape. By way of example, each pad 117 has, in top view, a perimeter of polygonal shape, for example rectangular or square, oval, circular, etc.

[0041] The pads 117, for example, have the same height or thickness, within manufacturing variations. In the example shown, the pads 117 are centered laterally and vertically with respect to the resonant cavities 109. This example is not limiting, however; the pads 117 could, as an alternative, be off-center with respect to the resonant cavities 109. The pads 117 form, for example, a grid with a substantially constant pitch over the entire filter 101, the grid pitch corresponding to a center-to-center distance between two adjacent pads 117.

[0042] At least one of the resonant cavities 109 may have, as in the example illustrated in [Fig. 1], a width L different from that of another resonant cavity 109. The width L of a resonant cavity 109 corresponds to the maximum lateral dimension of that cavity. By way of example, the width L corresponds to the side, or the diameter, of the resonant cavity 109, in the case where the cavity has, in top view, a square or circular perimeter, or a circular perimeter.

[0043] Furthermore, at least one of the studs 117 may have, as in the example illustrated in [Fig. 1], a lateral dimension D different from that of another stud 117. The The lateral dimension D of a plot 117 corresponds, for example, to the side, respectively to the diameter, of the plot 117, in the case where the plot has, in top view, a perimeter of square shape, respectively circular shape.

[0044] By way of example, each resonant cavity 109 located opposite one of the pixels 105 has a width L different from the widths L of the resonant cavities 109 located opposite the pixels 105 adjacent to the pixel in question. In other words, the resonant cavities 109 located opposite two adjacent pixels 105 have, in this example, different widths L. Alternatively, adjacent resonant cavities 109 may have the same width L different from that of another resonant cavity 109.

[0045] Alternatively or complementaryly, the pad 117 of each resonant cavity 109 located opposite one of the pixels 105 may, for example, have a lateral dimension D different from the lateral dimensions D of the pads 117 of the resonant cavities 109 located opposite the pixels 105 adjacent to the pixel in question. In other words, the pads 117 of the resonant cavities 109 located opposite two adjacent pixels 105 have, in this example, different lateral dimensions D. As a variant, the pads 117 of adjacent resonant cavities 109 may have the same lateral dimension D different from that of the pad 117 of another resonant cavity 109.

[0046] By way of example, the optical filter 101 may comprise several groups of resonant cavities 109, the resonant cavities 109 of the same group having the same cavity width L and comprising pads 117 having the same lateral dimension D, within manufacturing variations. The width L of the resonant cavities 109 belonging to the same group is different from the widths L of the resonant cavities 109 belonging to the other groups of cavities. Alternatively, the lateral dimension D of the pads 117 of the resonant cavities 109 belonging to the same group is different from the lateral dimensions D of the pads 117 belonging to the other groups of cavities 109. The resonant cavities 109 belonging to the same group are, for example, arranged in a regular pattern.

[0047] Each resonant cavity 109 of the optical filter 101 is predominantly resonant for a range of wavelengths of incident radiation intended to be transmitted to a photosensitive area of ​​the underlying pixel 105. The range of wavelengths transmitted by each resonant cavity 109 is, among other things, a function of the width L of the resonant cavity 109 in question (the larger the width L of the resonant cavity 109, the longer the wavelength of the radiation predominantly transmitted to the underlying pixel 105). Thus, providing resonant cavities 109 of different widths L allows the filter 101 to transmit the incident radiation in different wavelength ranges.

[0048] The range of wavelengths transmitted by each resonant cavity 109 is, moreover, a function of the lateral dimension D of the pad 117 (the larger the lateral dimension D of the pad 117, the longer the wavelength of the radiation predominantly transmitted towards the underlying pixel 105). Thus, providing resonant cavities 109 whose pads 117 have different lateral dimensions D allows the filter 101 to transmit the incident radiation in different wavelength ranges.

[0049] By way of example, modifying the width L of a resonant cavity 109 induces a shift in the wavelength range of the radiation transmitted to the underlying pixel 105 that is greater than the shift obtained by a similar modification of the lateral dimension D of the cavity's pad 117. In other words, modifying the lateral dimension D of the pad 117 of a resonant cavity 109 allows, in this example, a finer adjustment of the wavelength range transmitted to the underlying pixel 105 than is possible by modifying the width L of the resonant cavity 109 under consideration.

[0050] Thus, providing resonant cavities 109 of different widths L allows, for example, the optical filter 101 to cover a wider spectral band than could be covered by a filter consisting only of pads 117 of different lateral dimensions D. For example, when the image sensor 100 is adapted to capture visible light and near-infrared radiation, the filter 101 can cover a spectral band extending over several hundred nanometers in a case where the extreme widths L of the resonant cavities 109 are separated by several tens of nanometers. Furthermore, providing pads 117 of different lateral dimensions D allows, for example, the optical filter 101 to exhibit a higher spectral resolution than would be achieved by a filter consisting only of resonant cavities 109 of different widths L.

[0051] The multispectral sensor 100 incorporating the optical filter 101 advantageously has a greater compactness than multispectral sensors comprising a filter wheel, and also has a wider spectral band and / or a higher spectral resolution than existing multispectral sensors comprising a single filter adapted to transmit incident radiation predominantly in a first range of wavelengths to certain pixels of the sensor and incident radiation predominantly in at least a second range of wavelengths, different from the first range of wavelengths, to other pixels of the sensor.

[0052] One advantage of the optical filter 101 lies in the fact that the presence of the peripheral reflective wall 113 makes it possible to promote the transmission of radiation towards the pixels 105 of the image sensor 103, in particular when the radiation reaches the optical filter 101 at an oblique angle (i.e. not orthogonal to the surface). upper part of the optical filter 101, in the orientation of [Fig.1]).

[0053] Optionally, the multispectral sensor 100 includes microlenses 119. Each microlens 119 is, for example, positioned opposite a single resonant cavity 109 of the optical filter 101. The microlenses 119 cover the face of the optical filter 101 opposite the pixels 105 (the upper face of the optical filter 101, in the orientation of [Fig. 1]). The microlenses 119 facilitate the transmission of incident radiation towards the pixels 105 of the image sensor 103.

[0054] Figure 2 is a schematic, partial cross-sectional view of an example of a multispectral image sensor 200 comprising an optical filter 201 according to one embodiment. The sensor 200 of Figure 2 includes elements common to the sensor 100 of Figure 1. These common elements will not be detailed again below.

[0055] The sensor 200 of [Fig. 2] differs from the sensor 100 of [Fig. 1] in that each resonant element 115 of the optical filter 201 of the sensor 200 comprises, in addition to the pad 117, a layer 203 extending laterally inside the corresponding resonant cavity 109. The layer 203 extends, for example, as illustrated in [Fig. 2], over the entire width L of the resonant cavity 109, the layer 203 then being laterally delimited, or bordered, by the peripheral reflective wall 113.

[0056] In the example shown, layer 203 is made of the same material with index n2 as plot 117. Layer 203 is transparent to the wavelength range of the incident radiation intended to be transmitted to the photosensitive area of ​​the underlying pixel 105.

[0057] Figure 3 is a schematic, partial cross-sectional view of an example of a multispectral image sensor 300 comprising an optical filter 301 according to one embodiment. The sensor 300 of Figure 3 includes elements common to the sensor 200 of Figure 2. These common elements will not be detailed again below.

[0058] The sensor 300 of [Fig.3] differs from the sensor 200 of [Fig.2] in that the transparent layer 203 of the optical filter 301 of the sensor 300 is made of a different material than that of the pad 117. The transparent layer 203 of the optical filter 301 of the sensor 300 is for example made of a material having a refractive index n3 strictly greater than the refractive index ni of the material of the transparent region 111 and different from the refractive index n2 of the material of the pad 117.

[0059] Although Figures 2 and 3 illustrate examples in which each plot 117 covers part of a face of layer 203 (the upper face of layer 203, in the orientation of Figures 2 and 3), each plot 117 may, alternatively, be disjoint from the underlying layer 203. In this case, each plot 117 is, for example, separated from layer 203 by a portion of the transparent region 111.

[0060] Figure 4 is a schematic and partial cross-sectional view of an example of a sensor multispectral image 400 comprising an optical filter 401 according to one embodiment. The sensor 400 of [Fig. 4] includes elements common to the sensor 100 of [Fig. 1]. These common elements will not be detailed again below.

[0061] The sensor 400 of [Fig. 4] differs from the sensor 100 of [Fig. 1] in that each resonant element 115 of the optical filter 401 of the sensor 400 comprises a portion of the transparent region 111 located within a through-hole 403 formed in a layer 405 extending laterally within the corresponding resonant cavity 109. The openings 403 may have, in top view, a cross-section of any shape, for example, one of the shapes previously described for the studs 117.

[0062] The layer 405 extends, for example, as illustrated in [Fig. 4], over the entire width L of the resonant cavity 109, the layer 405 being then laterally delimited, or bordered, by the peripheral reflective wall 113. The layer 405 is transparent to the wavelength range of the incident radiation intended to be transmitted to the photosensitive area of ​​the underlying pixel 105. By way of example, the layer 405 is made of the same material with refractive index n2 as the pads 117 of the filters 101, 201, and 301.

[0063] What has been previously described in relation to [Fig. 1] in the case where the resonant elements 115 are pads 117 of lateral dimension D is transposable by the person skilled in the art to the case where each resonant element 115 includes a part of transparent region 111 located inside an aperture 403 of lateral dimension D formed in the layer 405. In particular, a modification of the lateral dimension D of an aperture 403 of the optical filter 401 induces a shift in the range of wavelengths of the radiation transmitted to the underlying pixel 105 (the larger the lateral dimension D of an aperture 403, the shorter the wavelength of the radiation predominantly transmitted to the underlying pixel 105).

[0064] The optical filter 401 of the sensor 400 thus presents advantages identical or analogous to those of the optical filters 101, 201 and 301 of the sensors 100, 200 and 300, respectively.

[0065] Figures 5A and 5B are schematic and partial top and cross-sectional views, respectively, along plane AA of Figure 5A, of an example of a multispectral image sensor 500 comprising an optical filter 501 according to one embodiment. The sensor 500 of Figures 5A and 5B includes elements common to the sensor 100 of Figure 1. These common elements will not be described in detail again below.

[0066] The sensor 500 of Figures 5A and 5B differs from the sensor 100 of [Fig. 1] in that the optical filter 501 of the sensor 500 comprises, for each pixel 105 of the image sensor 103, several resonant cavities 109 (nine resonant cavities 109 in the example shown in [Fig. 5A]). In the example shown, each resonant cavity 109 has a single resonant element 115. As an example, the pixels 105 of the 500 sensor have larger lateral dimensions than the pixels 105 of the 100 sensor.

[0067] In the example shown, the resonant elements 115 are pads 117, each having, in top view, a substantially square perimeter. In the illustrated example, the pads 117 of the resonant cavities 109 located opposite the same pixel 105 have substantially identical lateral dimensions D, within manufacturing variations. The pads 117 of the resonant cavities 109 located opposite one of the pixels 105, for example, have lateral dimensions D (equal, in this example, to the side of the square formed, in top view, by each pad 117) different from those of the pads 117 of the resonant cavities 109 located opposite another pixel 105.

[0068] By way of example, resonant cavities 109 located opposite the same pixel 105 have identical widths L, within manufacturing variations. To simplify the drawing, the resonant cavities 109 have the same width L in the example illustrated in Figures 5A and 5B. This example is not limiting, however, as resonant cavities 109 located opposite one pixel 105 may have different widths L than resonant cavities 109 located opposite another pixel 105.

[0069] Providing several resonant cavities 109 per pixel 105 advantageously allows the optical filter 501 to promote the transmission of incident radiation towards the pixels 105 of the sensor 103, particularly when the radiation reaches the optical filter 101 at an oblique angle of incidence. This also advantageously allows the optical filter 501 to exhibit greater angular acceptance, i.e., greater invariance of the spectral response of the filter 501 with respect to the angle of incidence, than a nanostructured filter lacking reflective walls 113.

[0070] Figure 6 is a schematic, partial cross-sectional view of an example of a multispectral image sensor 600 comprising an optical filter 601 according to one embodiment. The sensor 600 of Figure 6 includes elements common to the sensor 500 of Figures 5A and 5B. These common elements will not be detailed again below.

[0071] The sensor 600 of [Fig. 6] differs from the sensor 500 of Figures 5A and 5B in that each resonant cavity 109 of the optical filter 601 of the sensor 600 comprises several resonant elements 115. The resonant elements 115 of the filter 601 are, in the example shown, pads 117 having, for example, in top view, a substantially square shape. By way of example, each resonant cavity 109 of the optical filter 601 has nine pads 117.

[0072] Fig. 7A, Fig. 7B and Fig. 7C are cross-sectional views illustrating, in a schematic and partial, of the successive steps of an example of a manufacturing process of optical filter 101 of [Fig.1] according to an embodiment.

[0073] Fig. 7A illustrates more precisely a structure obtained at the end of the realization of the image sensor 103, in particular after the formation of the pixels 105 in and on the substrate 107, and after the successive depositions of two layers 701 and 703. In the example shown, layer 703 covers a face of layer 701 opposite to the substrate 107 (the upper face of layer 701, in the orientation of Fig. 7A).

[0074] By way of example, layer 701 is made of silicon dioxide (SiO2) and layer 703 is made of silicon (Si), silicon nitride (SiN) or titanium dioxide (TiO2).

[0075] Fig. 7B illustrates more precisely a structure obtained after a structuring step, for example by photolithography and then engraving, of layer 703 so as to form the plots 117.

[0076] In the example shown, the portions of layer 703 extending laterally between the pads 117 are completely removed during the etching step. Alternatively, portions of layer 703 extending laterally between the pads 117 and having a thickness less than that of the pads 117 can be retained after etching, thus forming, for example, the layers 203 of the optical filter 201 previously described in relation to [Fig. 2].

[0077] Fig. 7C illustrates more precisely a structure obtained at the end of a step of formation of the transparent regions 111 surrounding the pads 117.

[0078] By way of example, a transparent layer 705 of the same material as layer 701 is first deposited on the upper face of the structure, the transparent layer 705 covering, for example, the studs 117 and parts of the upper face of layer 701 not covered by the studs 117. The layers 701 and 705 are then structured, for example by photolithography and then etching, so as to form the transparent regions 111. In other words, the transparent layers 701 and 705, which here form a single layer, are split into several transparent regions 111, each comprising at least one of the resonant elements 115.

[0079] In the example shown, the transparent regions 111 are separated laterally by trenches 707.

[0080] Although not detailed, subsequent steps including the deposition of a reflective layer of a metal, for example silver (Ag) or aluminium (Al), or of a metallic alloy coating the transparent regions 111 and filling the trenches 707, and then the planarization of the reflective layer, for example by mechano-chemical polishing, are for example implemented from the structure illustrated in [Fig.7C] so as to form the peripheral reflective walls 113. In this case, the walls 113 are made of the material of the reflective layer.

[0081] An optional step of manufacturing the microlenses 119 may also be provided afterwards.

[0082] Figure 8 is a cross-sectional view illustrating, schematically and partially, a step in a manufacturing process for a variant of the optical filter 101 of Figure 1. The step illustrated in Figure 8 follows, for example, steps identical or similar to those previously described in relation to Figures 7A to 7C.

[0083] In the example shown, the sides and bottom of the trenches 707 are coated with a reflective layer 801, for example, of a metal or a metal alloy. In the example illustrated in [Fig. 8], the reflective layer 801 does not fill the trenches 707.

[0084] By way of example, the reflective layer 801 is first deposited on the upper face side of the structure of [Fig. 7C], for example by a conformal deposition technique. Parts of the reflective layer 801 coating the face of the transparent regions 111 opposite the substrate 107 (the upper face of the regions 111, in the orientation of [Fig. 8]) are then removed, for example by chemical polishing.

[0085] Although not detailed, subsequent steps including the deposition of a filling layer, for example of an electrically insulating material, coating the reflective layer 801 and filling the trenches 707, and then the planarization of the filling layer, for example by mechano-chemical polishing, are for example implemented from the structure illustrated in [Fig.8] so as to form the peripheral reflective walls 113. The peripheral reflective walls 113 in this case include parts of the filling layer flush with the upper face of the transparent regions 111, and whose bottom and sides are coated with the reflective layer 801.

[0086] An optional step of manufacturing the microlenses 119 may also be provided for subsequently.

[0087] Fig. 9A and Fig. 9B are cross-sectional views illustrating, schematically and partially, successive stages of an example of a manufacturing process for an optical filter according to an embodiment.

[0088] Figure 9A illustrates more precisely a structure obtained by successive deposition of two layers 901 and 903 on the upper face of the structure shown in Figure 7C. In the example shown, layer 901 covers the sides and upper face of the transparent regions 111, and also covers parts of the upper faces of the pixels 105 not covered by the transparent regions 111. In the example shown in Figure 9A, layer 903 covers layer 901 and fills the trenches 707. As an example, layers 901 and 903 are obtained by conformal deposition.

[0089] Layer 901 is, for example, made of a material having a refractive index superior to that of the material of layer 903. As an example, the respective materials of layers 901 and 903 are chosen from the following pairs: silicon and silicon dioxide, silicon and silicon nitride, and titanium dioxide and silicon dioxide. As an alternative, layer 901 is, for example, made of silicon nitride and layer 903 is omitted, thus creating an air gap inside trenches 707.

[0090] Fig. 9B illustrates more precisely a structure obtained after a planarization step of the stack formed by layers 901 and 903. As an example, structuring steps by photolithography and then etching, followed by a mechano-chemical polishing step on the upper face side of the structure illustrated in Fig. 9A, are implemented to planarize the stack formed by layers 901 and 903.

[0091] In the example shown, the parts of layers 901 and 903 located above the transparent regions 111 are eliminated, and the parts of layers 901 and 903 remaining between the transparent regions 111 are flush with the upper face of the transparent regions 111. In this example, the peripheral reflective walls 113 include parts of layer 903 flush with the upper face of the transparent regions 111, and whose bottom and sides are coated with layer 901.

[0092] The peripheral reflective walls 113 form, for example, in this case, Bragg mirrors. Although two layers 901 and 903 have been illustrated in figures 9A and 9B, a stack comprising more than two electrically insulating layers of different refractive indices may, as an alternative, be provided.

[0093] Figure 9B further illustrates a subsequent step in the formation of masks 905 covering the portions of layers 901 and 903 located between the transparent regions 111. For example, a metallic layer, for instance tungsten, is first deposited on the upper face of the structure, the metallic layer covering the upper face of the transparent regions 111 and the portions of layers 901 and 903 located between the transparent regions 111. The metallic layer is then, for example, structured, for example by photolithography and then etching, so as to form the masks 905. Alternatively, the masks 905 can be formed before the deposition of layers 901 and 903. In this case, the masks 905 cover, for example, the bottom of the trenches 707.

[0094] The masks 905 make it possible to avoid diaphotic phenomena between the pixels 105 of the image sensor 103 as well as parasitic couplings and resonances.

[0095] An optional step of manufacturing the microlenses 119 may also be provided for subsequently.

[0096] Fig. 1OA and Fig. 1OB are cross-sectional views illustrating, schematically and partially, successive stages of an example of a filter manufacturing process optics 501 of figures 5A and 5B according to one embodiment.

[0097] Fig. 1OA illustrates more precisely a structure obtained at the end of a structuring step, for example by photolithography and then etching, of the layer 703 of the structure previously described in relation to Fig. 7A so as to form the pads 117 of the optical filter 501.

[0098] In the example shown, the portions of layer 703 extending laterally between the pads 117 are completely removed during the etching step. Alternatively, portions of layer 703 extending laterally between the pads 117 and having a thickness less than that of the pads 117 may be retained after etching.

[0099] Fig. 1OB illustrates more precisely a structure obtained at the end of a step of formation of the transparent regions 111 surrounding the plots 117.

[0100] By way of example, a transparent layer of the same material as that of layer 701 is first deposited on the upper face side of the structure, the transparent layer covering for example the studs 117 and parts of the upper face of layer 701 not covered by the studs 117. The transparent layers are then structured for example, for example by photolithography and then etching, so as to form the transparent regions 111.

[0101] In the example shown, the transparent regions 111 are separated laterally by trenches 707.

[0102] Further steps in the formation of the peripheral reflective walls 113, for example analogous to those previously described in relation to [Fig.7C], are implemented for example.

[0103] An optional step of manufacturing the microlenses 119 may also be provided for subsequently.

[0104] 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 those skilled in the art. In particular, the pads 117 of the optical filters 501 and 601 previously described in connection with Figures 5A, 5B and 6 can be replaced by resonant elements 115 identical or analogous to those of the optical filter 401 previously described in connection with [Fig. 4], for example, resonant elements 115 comprising a portion of the transparent region 111 located inside a through-hole formed in a transparent layer with a refractive index higher than that of the region 111.

[0105] Furthermore, a person skilled in the art is able to adapt the process described above in relation to Figures 7A and 7C to produce filter 301 of [Fig. 3] by inserting, between layers 701 and 703, a layer having, for example, a a thickness less than that of layer 703, this layer serving, for example, as a stop layer during the etching step described in relation to [Fig. 7B]. A person skilled in the art is also capable of adapting the process previously described in relation to Figures 7A and 7C to produce the filter 401 of [Fig. 4], in particular by etching layer 703, in the step described in relation to [Fig. 7B], so as to form the openings 403.

[0106] The person skilled in the art is also able to produce the peripheral reflective walls 113 of the filters 101, 201, 301, 401, 501 and 601 according to one of the variants described in relation to figures 7C, 8, 9A and 9B from the indications of this description.

[0107] 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 is notably able to adapt the width L of each resonant cavity 109 and the lateral dimensions D of the resonant elements 115 according to the desired wavelength range, for example using optical simulation software.

Claims

Demands

1. Optical filter (101; 201; 301; 401; 501; 601) intended to be disposed opposite an image sensor (103) comprising a plurality of pixels (105), the filter comprising, for each pixel, at least one resonant cavity (109) having a transparent region (111) having a first refractive index and laterally delimited by a peripheral reflective vertical wall (113), and at least one resonant element (115) formed in said region, in which each resonant element comprises: A) a pad (117) having a second refractive index greater than the first index; or B) a portion of the transparent region (111) located inside a through opening (403) formed in a transparent layer (405) having a fourth refractive index greater than the first index and extending laterally into the resonant cavity (109).

2. Filter according to claim 1, wherein said at least one resonant element (115) located in one of said at least one resonant cavity (109) has a lateral dimension (D) different from that of said at least one resonant element located in another resonant cavity.

3. Filter according to claim 1 or 2, wherein at least one of said resonant cavity (109) has a width (L) different from that of another resonant cavity.

4. Filter according to any one of claims 1 to 3 in its option A), wherein each resonant element (115) further comprises a transparent layer (203) having a third refractive index greater than the first index and extending laterally into the resonant cavity (109).

5. Filter according to claim 4, wherein the third index is substantially equal to the second index.

6. Filter (101; 201; 301; 401) according to any one of claims 1 to 5, comprising, for each pixel (105), a single resonant cavity (109).

7. Filter (501; 601) according to any one of claims 1 to 5, comprising, for each pixel (105), a plurality of resonant cavities (109).

8. Filter according to any one of claims 1 to 7, wherein the peripheral reflective vertical wall (113) is made of a metal.

9. Filter according to any one of claims 1 to 7, wherein the peripheral reflective vertical wall (113) comprises a stack of electrically insulating layers of materials having different refractive indices.

10. Filter according to any one of claims 1 to 9, further comprising, for each cavity (109), a microlens (119) located directly above said cavity.

11. Multispectral image sensor (100; 200; 300; 400; 500; 600) comprising an image sensor (103) having a plurality of pixels (105) formed in and on a semiconductor substrate (107) and an optical filter (101; 201; 301; 401; 501; 601) according to any one of claims 1 to 10.

12. A method for manufacturing an optical filter (101; 201; 301; 401; 501; 601) intended to be positioned opposite an image sensor (103) comprising a plurality of pixels (105), the method comprising the following successive steps: a) forming, in a transparent layer (701, 705), at least one resonant element (115) for each pixel; and b) dividing the transparent layer into several transparent regions (111) each comprising at least one of the resonant elements; and c) coating the sides of each transparent region with a peripheral reflective wall (113), in which the transparent region and the peripheral reflective wall form, for each pixel, a resonant cavity (109), in which each resonant element comprises: A) a pad (117) having a second refractive index greater than the first index;or B) a portion of the transparent region (111) located inside a through-opening (403) formed in a transparent layer (405) having a fourth refractive index higher than the first index and extending laterally into the resonant cavity (109).;

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