Optical filter for multispectral sensor

Abstract

Optical filter for multispectral sensor The present description relates to an optical filter (101) intended to be arranged opposite an image sensor (103) comprising a plurality of pixels (105), the filter comprising, for each pixel, a resonant cavity (109) comprising a first transparent layer (111), interposed between second and third mirror layers (113, 115), and a diffraction grating (117) formed in the first layer, in which at least one of the cavities has a thickness (T) different from another cavity. Figure for abstract: Fig. 1

Description

Title of the invention: Optical filter for multispectral sensor Technical field

[0001] The present description relates generally to multispectral sensors, adapted to acquire images of a scene in different wavelength ranges. The present description relates more particularly to an optical filter for a multispectral sensor, a multispectral sensor comprising such a filter and a method of manufacturing an optical filter for a multispectral sensor. Prior art

[0002] Multispectral sensors comprising a filter wheel placed opposite a sensor adapted to acquire an image for each filter of the wheel have been proposed. Other more compact multispectral sensors comprising a single optical filter arranged 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 predominantly in at least a second wavelength range, different from the first wavelength range, to other pixels of the sensor, have also been proposed.

[0003] These multispectral sensors, however, suffer from various drawbacks. In particular, it would be desirable to have compact multispectral sensors with both a wide spectral band and high resolution. Summary of the invention

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

[0005] For this, one embodiment provides an optical filter intended to be arranged opposite an image sensor comprising a plurality of pixels, the filter comprising, for each pixel, a resonant cavity comprising a first transparent layer, interposed between second and third mirror layers, and a diffraction grating formed in the first layer, in which at least one of the cavities has a thickness different from another cavity.

[0006] According to one embodiment, the filter comprises several groups of adjacent resonant cavities of the same thickness different from those of the resonant cavities forming part of the groups of resonant cavities adjacent to said group.

[0007] According to one embodiment, each group comprises exactly four adjacent resonant cavities of the same thickness.

[0008] According to one embodiment, the filter comprises several sets of groups of resonant cavities of the same thickness, non-adjacent, arranged in a regular pattern, the resonant cavities of each set having a thickness different from that of the resonant cavities of the other sets.

[0009] According to one embodiment, each resonant cavity has a thickness different from that of the resonant cavities adjacent to said cavity.

[0010] According to one embodiment, the diffraction grating of each resonant cavity has a filling factor different from those of the diffraction gratings of the resonant cavities adjacent to said cavity.

[0011] According to one embodiment, the diffraction grating of each resonant cavity comprises a plurality of regions made of a material having a refractive index greater than that of the first transparent layer.

[0012] According to one embodiment, each region has a plot shape.

[0013] According to one embodiment, each region has a band shape.

[0014] According to one embodiment, the first transparent layer comprises: - a first part made of a first material, extending vertically from the second mirror layer to the regions; and - a second part made of a second material, different from the first material, the second part extending laterally between the regions and extending vertically from the first part to the third mirror layer.

[0015] 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.

[0016] One embodiment provides a method for manufacturing an optical filter intended to be arranged opposite an image sensor comprising a plurality of pixels, the method comprising the following successive steps: a) depositing a first transparent layer covering a second mirror layer; b) forming, in the first transparent layer, a diffraction grating; and c) depositing a third mirror layer coating the first transparent layer, wherein the first transparent layer and the second and third mirror layers form, for each pixel, a resonant cavity, at least one of the cavities having a thickness different from another cavity.

[0017] According to one embodiment, the diffraction grating is formed from a fourth layer having a refractive index greater than that of the first transparent layer.

[0018] According to one embodiment, the first transparent layer is formed by successive deposits: - a first part made of a first material, extending vertically from the second mirror layer up to the diffraction grating; and - a second part made of a second material, different from the first material, the second part extending vertically from the first part to the third mirror layer.

[0019] According to one embodiment, the third mirror layer comprises at least two parts made of different materials. Brief description of the drawings

[0020] These characteristics and advantages, as well as others, will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures among which:

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

[0022] [Fig.2A], [Fig.2B], [Fig.2C], [Fig.2D], [Fig.2E] and [Fig.2F] are sectional views illustrating, schematically and partially, successive steps of an example of a method of manufacturing the device of [Fig.1] according to one embodiment; and

[0023] [Fig.3A] and [Fig.3B] are sectional views illustrating, schematically and partially, successive steps of a variant of the manufacturing process of the device of [Fig.1]. Description of the embodiments

[0024] The same elements have been designated by the same references in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same references and may have identical structural, dimensional and material properties.

[0025] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been shown and are detailed. In particular, the production of the photodiodes and the pixel control circuits has not been detailed, the production of such pixels being within the scope of the person skilled in the art from the indications of the present description.

[0026] Unless otherwise specified, when referring to two elements connected to each other, this means directly connected without intermediate elements other than conductors, and when referring to two elements connected (in English "coupled") to each other, this means that these two elements can be connected or be connected by means of one or more other elements.

[0027] In the following description, when referring 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", "upper", "lower", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures.

[0028] Unless otherwise specified, the expressions “about”, “approximately”, “substantially”, and “of the order of” mean to within 10%, preferably to within 5%.

[0029] [Fig.l] is a schematic and partial sectional view of an example of a multispectral image sensor 100 comprising an optical filter 101 according to one embodiment.

[0030] In the example shown, the optical filter 101 is arranged opposite an image sensor 103, for example a CMOS (Complementary Metal-Oxide-Semiconductor) sensor. The sensor 103 comprises a plurality of pixels 105 formed in and on a substrate 107, for example a wafer or a piece of wafer made of a semiconductor material, for example silicon. The pixels 105 are for example arranged in a matrix according to rows and columns. Each pixel 105 of the sensor 103 has, for example, a rectangular or square shape in top view. Although this has not been detailed in [Fig. 1], control and reading circuits for the pixels 105 of the sensor 103 may also be formed in and on the substrate 107. Furthermore, although only six pixels 105 have been illustrated in [Fig.l], the image sensor 103 can of course comprise a much larger number of pixels 105, for example several thousand or several million pixels 105. .

[0031] According to one embodiment, the optical filter 101 intended to be arranged opposite the image sensor 103 comprises, for each pixel 105 of the sensor 103, a resonant cavity 109 comprising a first transparent layer 111, for example an electrically insulating layer, interposed between second and third mirror layers 113 and 115. The layer 111 is transparent in the operating wavelength range of the optical filter 101. Furthermore, at least one of the cavities 109 of the optical filter 101 has a thickness T different from another cavity 109. Each resonant cavity 109 has, for example, in top view, a rectangular or square shape, for example identical to the shape of the underlying pixel 105. Furthermore, each resonant cavity 109 has, for example, lateral dimensions substantially equal to those of the underlying pixel 105.

[0032] For example, the resonant cavities 109 of the optical filter 101 are Fabry-Perot cavities.

[0033] The optical filter 101 may, as in the example illustrated in [Fig.l], comprise several groups of adjacent resonant cavities 109, the resonant cavities 109 of the same group having the same thickness T, apart from manufacturing dispersions, and being located respectively opposite adjacent pixels 105 of the sensor. of images 103. For example, the filter 101 may comprise groups of four adjacent resonant cavities 109 of the same thickness T located respectively opposite four adjacent pixels 105, the group of adjacent resonant cavities 109 having, for example, in top view, a substantially square-shaped perimeter. The thickness T of the resonant cavities 109 forming part of the same group is different from the thicknesses T of the resonant cavities 109 forming part of the groups of cavities 109 adjacent to the group considered. The resonant cavities 109 of the same group form, for example, a single resonant cavity of thickness T.

[0034] As a variant, each resonant cavity 109 located opposite one of the pixels 105 of the sensor 103 has a thickness T different from the thicknesses T of the resonant cavities 109 located opposite the pixels 105 of the sensor 103 adjacent to the pixel 105 considered, that is to say a thickness T different from those of the resonant cavities 109 adjacent to the cavity considered. In other words, the resonant cavities 109 located opposite two adjacent pixels 105 have, in this variant, different thicknesses T.

[0035] By way of example, the pixels 105 of the sensor 103 have maximum lateral dimensions of less than 4 μm, for example between 1 and 2 μm, in the case where the resonant cavities 109 are distributed into groups of adjacent cavities of the same thickness T, and between 4 and 10 μm, for example of the order of 5 μm, in the case where the resonant cavities 109 located opposite two adjacent pixels 105 have different thicknesses T.

[0036] The optical filter 101 may comprise several assemblies, for example between two and ten assemblies, each comprising several non-adjacent cavities or groups of resonant cavities 109 of the same thickness T, apart from manufacturing dispersions. The thickness T of the resonant cavities 109 forming part of the same assembly is different from the thicknesses T of the resonant cavities 109 forming part of the other assemblies. The resonant cavities 109 or the groups of resonant cavities 109 forming part of the same assembly are for example arranged in a regular pattern.

[0037] Each cavity 109 of the filter 101 is predominantly resonant for a wavelength range of incident radiation intended to be transmitted to a photosensitive zone of the underlying pixel 105. The wavelength range transmitted by each resonant cavity 109 is, among other things, a function of the thickness T of the cavity 109 considered (the greater the thickness T of the resonant cavity 109, the greater the wavelength of the radiation predominantly transmitted to the underlying pixel 105). Thus, providing resonant cavities 109 or groups of resonant cavities 109 of different thicknesses T allows the filter 101 to transmit the incident radiation in different wavelength ranges and to cover a wide spectral band extending for example over several hundred of nanometers in a case where the extreme thicknesses T are separated by several tens of nanometers.

[0038] According to one embodiment, each resonant cavity 109 of the optical filter 101 further comprises a diffraction grating 117 formed in the first layer 111.

[0039] In the example shown, the diffraction grating 117 of each resonant cavity 109 comprises a plurality of regions 119 made of a material different from that of the transparent layer 111, the regions 119 being separated from each other by parts of the transparent layer 111. The regions 119 are furthermore separated from the mirror layers 113 and 115 by other parts of the transparent layer 111.

[0040] Each region 119 has, for example, a stud shape extending vertically in the thickness of the layer 111. The studs may in this case have, in top view, a section of any shape, for example circular, rectangular or square. As a variant, the regions 119 may be strips parallel to each other, extending for example laterally between two opposite sides of the optical filter 101 along a direction orthogonal to the section plane of [Fig. 1].

[0041] In the example shown, the regions 119 have the same height, or thickness, that is to say the same dimension in a direction orthogonal to the mirror layers 113 and 115.

[0042] The regions 119 form, for example, a grating having a substantially constant pitch over the entire filter 101, the pitch of the grating corresponding to a center-to-center distance between two neighboring pads or to a distance between two center lines of two neighboring bands. For example, the pitch of the grating formed by the regions 119 is of the order of a hundred or several hundred nanometers, for example equal to approximately 150 nm. In the example shown, the diffraction grating 117 of each resonant cavity 109 located opposite one of the pixels 105 of the sensor 103 has a fill factor different from the fill factors of the diffraction gratings 117 of the resonant cavities 109 located opposite the pixels 105 of the sensor 103 adjacent to the pixel in question. In other words, the diffraction gratings 117 of the resonant cavities 109 located opposite two adjacent pixels 105 have different filling factors.In particular, in the example illustrated in [Fig.l] where the filter 101 comprises groups of adjacent resonant cavities 109 of the same thickness T, the diffraction gratings 117 of the resonant cavities 109 forming part of the same group have different filling factors.

[0043] By filling factor of the diffraction grating 117, we mean a ratio between, on the one hand, surfaces or volumes occupied by the regions 119 and, on the other hand, surfaces or volumes occupied by the parts of the transparent layer 111 extending laterally between the regions 119.

[0044] In the example shown, the diffraction grating 117 of each resonant cavity 119 of the optical filter 101 has, inside the cavity considered, a substantially constant filling factor. In other words, the regions 119 of the diffraction grating 117 are, inside the same resonant cavity 109, uniformly spaced from each other and have substantially identical lateral dimensions.

[0045] For example, the gratings 117 are resonant waveguide diffraction gratings (RWG), or resonant filters with guided mode.

[0046] The wavelength range transmitted by each resonant cavity 109 is a function, in addition to the thickness T of the cavity, of the filling factor of the diffraction grating 117 (the higher the filling factor of the grating 117, the higher the wavelength of the radiation mainly transmitted to the underlying pixel 105). Each resonant cavity 109 has an optical index depending on the material of the transparent layer 111, this material being for example identical for all the cavities 109 of the optical filter 101, and of the filling factor of the grating 117 of the cavity in question. Thus, providing resonant cavities 109 whose diffraction gratings 117 have different filling factors makes it possible to obtain different optical indices inside these cavities, and therefore to transmit the incident radiation in different wavelength ranges.

[0047] The presence of resonant cavities 109 of different thicknesses T allows the optical filter 101 to access a wider spectral band than that which would be obtained by means of a filter comprising only diffraction gratings 117 having different filling factors. Furthermore, the presence of the diffraction gratings 117 having different filling factors allows the optical filter 101 to have a spectral resolution higher than that which would be obtained by means of a filter comprising only resonant cavities 109 of different thicknesses T, for example due to limits inherent in the methods for producing cavities of variable thicknesses.

[0048] Furthermore, the fact of providing, as in the example illustrated in [Fig.l], groups of adjacent resonant cavities 109 of the same thickness T but whose diffraction gratings 117 have, within the same group, different filling factors allows the optical filter 101 to benefit from a spatial resolution greater than that which would have, for example, the optical filter 101 without the diffraction gratings 117, for example due to limits inherent in the methods implemented. Thus, the fact of combining, in the optical filter 101, resonant cavities 109 of different thicknesses T and, within the cavities, diffraction gratings 117 having different filling factors makes it possible to access a wider spectral band or a resolution greater than that of an optical filter having only one or the other of these characteristics.

[0049] The multispectral sensor 100 integrating the filter 101 advantageously has a compactness greater than that of the multispectral sensors comprising a filter wheel placed opposite a sensor adapted to acquire an image for each filter of the wheel while also having a wider spectral band and / or a higher resolution than the existing multispectral sensors comprising a filter adapted to transmit the incident radiation in at least two different wavelength ranges.

[0050] In the example shown, the sensor 100 further comprises a planarization layer 121 coating the mirror layer 115. The layer 121 makes it possible to compensate for the differences in thickness T between the cavities 109 of the filter 101 in order to obtain a face (the upper face, in the orientation of [Fig. 1]) that is substantially planar. By way of example, the layer 121 is made of an organic material, for example a resin.

[0051] In the example illustrated in [Fig.l], the sensor 100 further comprises a layer of microlenses 123 coating the planarization layer 121. Each microlens 123 is arranged opposite one of the pixels 105 of the sensor 103 and makes it possible, for example, to focus incident radiation onto a photosensitive area of ​​the underlying pixel 105. The microlenses 123 are, for example, made of the same material as the layer 121, for example a resin.

[0052] Alternatively, the planarization layer 121 may be omitted. The microlenses 123 then coat the mirror layer 115, and at least one of the microlenses 123 is located at a height different from that at which another microlens 123 is located due to the difference in thickness T between the cavities 109 of the filter 101.

[0053] [Fig.2A], [Fig.2B], [Fig.2C], [Fig.2D], [Fig.2E] and [Fig.2F] are sectional views illustrating, schematically and partially, successive steps of an example of a manufacturing method of the device 100 of [Fig.1].

[0054] [Fig.2A] illustrates more precisely a structure obtained at the end of the production of the image sensor 103, in particular after the formation of the pixels 105 in and on the substrate 107.

[0055] [Fig.2B] illustrates more precisely a structure obtained at the end of successive steps of deposition of the mirror layer 113, of a transparent layer 201, for example an electrically insulating layer, and of a layer 203 made of a material different from that of the transparent layer 201. In the example shown, the mirror layer 113 covers a face of the pixel matrix 105 opposite the substrate 107 (the upper face of the pixel matrix 105, in the orientation of [Fig.2B]). The mirror layer 113 is coated with the transparent layer 201, itself coated with the layer 203.

[0056] For example, the mirror layer 113 is a thin metal layer, for example a layer of gold, silver or aluminum, or a layer made of a alloy of one or more of these metals. Alternatively, the mirror layer 113 is a Bragg mirror comprising a stack of dielectric layers having different refractive indices.

[0057] For example, the transparent layer 201 is made of an oxide, for example silicon dioxide (SiO2).

[0058] The layer 203 is for example electrically insulating and has a refractive index higher than that of the transparent layer 201. For example, the layer 203 is made of silicon nitride (SiN), amorphous silicon, titanium dioxide (TiO2), alumina (A12O3) or tantalum pentoxide (Ta2O5).

[0059] [Fig.2C] illustrates more precisely a structure obtained at the end of a structuring step, for example by photolithography then etching, of the layer 203 so as to form the regions 119.

[0060] [Fig.2D] illustrates more precisely a structure obtained at the end of successive steps of depositing a transparent layer 205 and a layer 207 made of a photosensitive resin. In the example shown, the transparent layer 205 fills, or completely fills, all the free spaces extending laterally between the regions 119 previously formed from the layer 203 and coats the faces of the regions 119 opposite the mirror layer 113 (the upper faces of the regions 119, in the orientation of [Fig.2D]). This thus forms the diffraction grating 117. In the example illustrated in [Fig.2D], the layer 205 is made of the same material as the layer 201, the layers 201 and 205 jointly forming the transparent layer 111 previously described in relation to [Fig.1]. Photosensitive resin layer 207 coats layer 205, i.e. layer 111 in this example.

[0061] A planarization step, for example by chemical and mechanical polishing (CMP), of the upper face of the layer 205 can be implemented prior to the deposition of the photosensitive resin layer 207 in order to improve its surface condition.

[0062] [Fig.2E] illustrates more precisely a structure obtained at the end of a step of exposing the photosensitive resin of the layer 207 through a gray scale optical mask 209, then a step of developing the resin. In the example shown, the mask 209 comprises more precisely three regions 209-1 (D1), 209-2 (D2) and 209-3 (D3) having increasing absorption rates, or increasing optical densities, at the wavelength of the radiation used for the exposure of the resin layer 207. Thus, the layer 207 is exposed to a greater depth, from its upper face, in a part of the layer 207 located substantially directly above the region 209-1 of the mask 209 than in another part of the layer 207 located substantially directly above the region 209-3 of the mask 209, yet another part of the layer 207 located substantially directly above the region 209-2 of the mask being exposed to an intermediate depth.

[0063] After development, the photosensitive resin layer 207 has, as in the example illustrated in [Fig.2E], a staircase-shaped upper face comprising steps each having, relative to the adjacent step(s), a height of, for example, between 25 and 50 nm.

[0064] As a variant, it is possible to provide, instead of carrying out photolithography through the grayscale optical mask 209 and then etching the resin thus exposed, to successively link together several photolithography and then etching steps in order to obtain the resonant cavities 109 of variable thicknesses T.

[0065] [Fig.2F] illustrates more precisely a structure obtained at the end of an etching step, for example a dry etching of the RIE type (from the English “Reactive Ion Etching”), of the layers 205 and 207 and a subsequent step of deposition of the mirror layer 115. The greater the thickness of the layer 207 coating the upper face of the layer 205, the smaller the etching depth of the layer 205, from the upper face of the layer 205. This makes it possible to transfer the relief of the upper face of the layer 207 onto the upper face of the layer 205 and to obtain resonant cavities 109 having different thicknesses T.

[0066] In the example shown, the mirror layer 115 covers the upper face of the layer 207. The mirror layer 115 has, for example, a structure and a composition identical or similar to those of the mirror layer 113. By way of example, the mirror layer 115 is made of a material that reflects in the entire operating wavelength range of the optical filter 101. As a variant, the mirror layer 115 comprises at least two parts made of different materials, each part of the mirror layer 115 covering one or more parts of the same thickness of the layer 207. This makes it possible, for example, to modify the composition of the mirror layer 115 to optimize the reflection of the radiation as a function of the thickness T and the filling factor of the diffraction grating 117 of the resonant cavity 109 in question.

[0067] [Fig.3A] and [Fig.3B] are sectional views illustrating, schematically and partially, successive steps of a variant of the manufacturing method of the device 100 of [Fig.1],

[0068] This variant of the method for manufacturing the device 100 comprises, for example, initial steps identical or similar to those previously described in relation to FIGS. 2A to 2C.

[0069] [Fig. 3A] illustrates more precisely a structure obtained at the end of a deposition step, on the side of the upper face of the structure of [Fig. 2C], of a transparent layer 301 made of a material different from that of the layer 201, for example an electrically insulating material. The layer 301 is for example made of a photosensitive resin. In the example shown, the transparent layer 301 fills, or fills to tationally, all the free spaces extending laterally between the regions 119 previously formed from the layer 203 and covers the face of the regions 119 opposite the mirror layer 113 (the upper face of the regions 119, in the orientation of [Fig.2D]). This thus forms a diffraction grating 117' analogous to the diffraction grating 117.

[0070] In the example illustrated in [Fig.3A], the layers 201 and 301 jointly form a layer 111' similar to the layer 111 previously described in relation to [Fig. 1]. In other words, the layer 111' comprises, in this example: - a first part 201 made of a first material, extending vertically from the first mirror layer 113 to the regions 119; and - a second part 301 made of a second material, different from the first material, the second part extending laterally between the regions 119 and extending vertically from the first part to the second mirror layer 115.

[0071] [Fig.3B] illustrates more precisely a structure obtained at the end of a photolithography step, through a grayscale mask, then etching of the layer 301 so as to obtain a stepped upper face similar to that obtained for the layer 205 at the end of the steps previously described in relation to [Fig.2F].

[0072] An advantage of the variant illustrated in Figures 3A and 3B is that it allows, compared to the method of Figures 2A to 2F, to omit the step of depositing the transparent layer 207.

[0073] [Fig.3B] further illustrates a subsequent step of depositing the mirror layer 115 on the side of the upper face of the layer 207.

[0074] From the structure illustrated in [Fig.2F] or in [Fig.3B], the planarization layer 121 is for example deposited on the side of the upper face of the mirror layer 115 then planarized, for example by CMP, so that the layer 121 has an upper face that is substantially flat and parallel to the mirror layer 113. The microlenses 123 can then be produced on the side of the upper face of the planarization layer 121.

[0075] Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these various embodiments and variants could be combined, and other variants will occur to those skilled in the art. In particular, the manufacture of the optical filter 101 whose mirror layer 115 comprises several parts made of different materials is within the ability of those skilled in the art from the indications of the present description.

[0076] Finally, the practical implementation of the embodiments and variants described is within the reach of the person skilled in the art from the functional indications given above. In particular, the person skilled in the art is capable, from the present description, of determining the thicknesses T and the filling factors of the networks of diffraction 117 of each resonant cavity 109 as a function of the wavelength ranges to be transmitted to the pixels 105 of the underlying image sensor 103.

Claims

Claims

1. Optical filter (101) intended to be arranged opposite an image sensor (103) comprising a plurality of pixels (105), the filter comprising, for each pixel, a resonant cavity (109) comprising a first transparent layer (111; 111'), interposed between second and third mirror layers (113, 115), and a diffraction grating (117; 117') formed in the first layer, in which at least one of the cavities has a thickness (T) different from another cavity, in which the third mirror layer (115) is made of a material reflecting in the entire operating wavelength range of the optical filter and has a structure and composition identical to those of the second mirror layer (113).

2. Filter according to claim 1, comprising several groups of adjacent resonant cavities (109) of the same thickness (T) different from those of the resonant cavities forming part of the groups of resonant cavities adjacent to said group.

3. Filter according to claim 2, in which each group comprises exactly four adjacent resonant cavities (109) of the same thickness (T).

4. Filter according to claim 2 or 3, comprising several sets of groups of resonant cavities (109) of the same thickness (T) non-adjacent arranged in a regular pattern, the resonant cavities of each set having a thickness different from that of the resonant cavities of the other sets.

5. Filter according to claim 1, in which each resonant cavity (109) has a thickness (T) different from those of the resonant cavities (109) adjacent to said cavity.

6. Filter according to any one of claims 1 to 5, in which the diffraction grating (117; 117') of each resonant cavity (109) has a filling factor different from those of the diffraction gratings of the resonant cavities adjacent to said cavity.

7. A filter according to any one of claims 1 to 6, wherein the diffraction grating (117; 117') of each resonant cavity (109) comprises a plurality of regions (119) made of a material having a refractive index higher than that of the first transparent layer (111; 111').

8. Filter according to claim 7, wherein each region (119) has a plot shape.

9. A filter according to claim 7, wherein each region (119) has a band shape.

10. A filter according to any one of claims 7 to 9, wherein the first transparent layer (111') comprises: - a first portion (201) made of a first material, extending vertically from the second mirror layer (113) to the regions (119); and - a second portion (301) made of a second material, different from the first material, the second portion extending laterally between the regions (119) and extending vertically from the first portion to the third mirror layer (115).

11. A multispectral image sensor (100) comprising an image sensor (103) having a plurality of pixels (105) formed in and on a semiconductor substrate (107) and an optical filter (101) according to any one of claims 1 to 10.

12. Method for manufacturing an optical filter (101) intended to be arranged opposite an image sensor (103) comprising a plurality of pixels (105), the method comprising the following successive steps: a) depositing a first transparent layer (111; 111') coating a second mirror layer (113); b) forming, in the first transparent layer, a diffraction grating (117; 117'); and c) depositing a third mirror layer (115) coating the first transparent layer, wherein the first transparent layer and the second and third mirror layers form, for each pixel, a resonant cavity (109), at least one of the cavities having a thickness (T) different from another cavity, and wherein the third mirror layer (115) is made of a material reflecting throughout the operating wavelength range of the optical filter and has a structure and composition identical to those of the second mirror layer (113).

13. A method according to claim 12, wherein, in step b), the diffraction grating (117; 117') is formed from a fourth layer (203) having a refractive index higher than that of the first transparent layer (111; 111').

14. Method according to claim 12 or 13, in which the first transparent layer (111; 111') is formed by successive deposits: - a first part (201) made of a first material, extending vertically from the second mirror layer (113) to the diffraction grating (117; 117'); and - a second part (301) made of a second material, different from the first material, the second part extending vertically from the first part to the third mirror layer (115).

Images