Optical filtering structure in the visible and/or infrared domain

Abstract

An optical filtering structure comprising a stack of layers forming a first filter letting pass a first spectral band, and a second filter adjacent to the first filter and which lets pass a second spectral band comprising: a plurality of dielectric layers common to the two filters and of different refractive indices, n first metal layers common to the two filters, m second metal layers arranged only in the second filter, and wherein at least one of said dielectric layers comprises, in the first filter, a thickness different to that in the second filter, and/or wherein at least one dielectric layer is arranged only in the second filter, n being an integer greater than or equal to 0, and m being an integer greater than or equal to 1.

Claims

1. An optical filtering structure comprising a stack of layers forming at least one first filter letting pass the wavelengths of a first spectral band and cutting off the wavelengths of a second spectral band, and a second filter adjacent to the first filter and letting pass the wavelengths of the second spectral band and cutting off the wavelengths of the first spectral band, wherein the stack of layers comprises: a plurality of dielectric layers common to the first filter and to the second filter and forming an alternation of layers of different refractive indices; n first metal layers common to the first filter and to the second filter, wherein each first metal layer comprises at least one of copper, aluminium, and silver; m second metal layers arranged only in the second filter, wherein each second metal layer comprises at least one of copper, aluminium, and silver, wherein at least one of said dielectric layers has a same thickness in the first and second filters; and wherein at least one of said dielectric layers comprises, in the first filter, a thickness different to that in the second filter, and/or wherein at least one dielectric layer is arranged only in the first or the second filter; n being an integer greater than or equal to 0, and m being an integer greater than or equal to 1.

2. The optical filtering structure according to claim 1, wherein the wavelengths of the first spectral band are less than around 600 nm and the wavelengths of the second spectral band are greater than around 600 nm.

3. The optical filtering structure according to claim 1, wherein the dielectric layers comprise at least one of SiN, SiO.sub.2, TiO.sub.2, Ta.sub.2O.sub.5, HfO.sub.2, ZrO.sub.2, ZnS, ZnSe, MgF.sub.2, SiOCH, and Na.sub.3AlF.sub.6.

4. The optical filtering structure according to claim 1, wherein the stack of layers comprises one or more layers of SiO.sub.2 each arranged between two layers of SiN.

5. The optical filtering structure according to claim 1, wherein each metal layer is arranged between two dielectric layers comprising a same dielectric material.

6. The optical filtering structure according to claim 5, wherein each metal layer comprises copper and is arranged between two layers of SiN.

7. The optical filtering structure according to claim 1, wherein, in the first filter, at least one dielectric layer comprises several portions of different thicknesses such that the first filter comprises several parts having different spectral responses.

8. The optical filtering structure according to claim 7, wherein the parts of the first filter having the different spectral responses form a filtering array of three colours red, green and blue.

9. The optical filtering structure according to claim 7, wherein the wavelengths of the first spectral band correspond to the colours green and blue, and the wavelengths of the second spectral band correspond to the colour red.

10. The optical filtering structure according to claim 1, further comprising an array of coloured filters arranged on the first filter.

11. The optical filtering structure according to claim 10, wherein the array of coloured filters arranged on the first filter form a filtering array of three colours red, green and blue.

12. The optical filtering structure according to claim 10, wherein the wavelengths of the first spectral band correspond to the colours green and blue, and the wavelengths of the second spectral band correspond to the colour red.

13. The optical filtering structure according to claim 1, wherein the first spectral band corresponds to the visible domain and the second spectral band corresponds to at least one part of the infrared domain.

14. A photodetector and/or photoemitter device comprising at least: one array of pixels able to carry out a photodetection and/or a photoemission in the range of wavelengths of the visible domain and/or the infrared domain; one filtering structure according to claim 1 arranged opposite the array of pixels such that the first filter is arranged opposite a first set of pixels of the array and that the second filter is arranged opposite a second set of pixels of the array.

15. The optical filtering structure of claim 1, wherein n being an integer greater than or equal to 1, and m being an integer greater than or equal to 2.

16. A method for making an optical filtering structure comprising the implementation of steps of depositing, photolithography and etching of dielectric and metal materials, forming a stack of layers forming at least one first filter letting pass the wavelengths of a first spectral band and cutting off the wavelengths of a second spectral band, and a second filter adjacent to the first filter and letting pass the wavelengths of the second spectral band and cutting off the wavelengths of the first spectral band, and wherein the stack of layers comprises: a plurality of dielectric layers common to the first filter and to the second filter and forming an alternation of layers of different refractive indices; n first metal layers common to the first filter and to the second filter wherein each first metal layer comprises at least one of copper, aluminium, and silver; m second metal layers arranged only in the second filter, wherein each second metal layer comprises at least one of copper, aluminium, and silver, wherein at least one of said dielectric layers has a same thickness in the first and second filters; and wherein at least one of said dielectric layers comprises, in the first filter, a thickness different to that in the second filter, and/or wherein at least one dielectric layer is arranged only in the first or the second filter; n being an integer greater than or equal to 0, and m being an integer greater than or equal to 1.

17. The method of claim 16, wherein n being an integer greater than or equal to 1, and m being an integer greater than or equal to 2.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The present invention will be better understood on reading the description of embodiment examples given by way of indication and in no way limiting and by referring to the appended drawings, among which:

(2) FIG. 1 represents a photodetector device of the prior art functioning in the visible and infrared domains;

(3) FIG. 2 represents partially and schematically a photodetector device comprising a filtering structure according to a first embodiment;

(4) FIG. 3 represents the spectral responses of the filters of the filtering structure according to the first embodiment;

(5) FIG. 4 represents partially and schematically a photodetector device comprising a filtering structure according to a second embodiment;

(6) FIG. 5 represents the spectral responses of the filters of the filtering structure according to the second embodiment;

(7) FIGS. 6 and 7 represent the spectral responses of the filters of the filtering structure according to different variants of the first embodiment;

(8) FIG. 8 represents the spectral responses of the filters of the filtering structure according to a variant embodiment.

(9) Identical, similar or equivalent parts of the different figures described hereafter bear the same numerical references so as to make it easier to go from one figure to the next.

(10) In order to make the figures more legible, the different parts (particularly the layers) shown in the figures are not necessarily according to a uniform scale.

(11) The different possibilities (variants and embodiments) should be understood as not being mutually exclusive and may be combined together.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(12) Reference is made to FIG. 2 which represents partially and schematically a photodetector device 100 able to carry out a light detection in the visible domain and in the infrared domain, according to a first embodiment.

(13) The device 100 comprises a sensor 102, for example of CCD or CMOS type, comprising an array of pixels. A first set of pixels of the sensor 102 is intended to carry out a photodetection in the visible domain, and a second set of pixels of the sensor 102 is intended to carry out a photodetection in the infrared domain. In the example of FIG. 2, four pixels 104 to 110 are shown. The three pixels 104, 106 and 108 are intended to carry out a photodetection in the visible domain (the pixel 104 for blue, the pixel 106 for green and the pixel 108 for red), whereas the pixel 110 is intended to carry out a photodetection in the infrared domain.

(14) The two sets of pixels may optionally be intermingled, for example in the form of a Bayer array integrating a RGB and IR filtering. In such an array, for each group of 4 pixels arranged one adjacent to the other in a square, 3 pixels are each intended to detect one of the colours red, green and blue, and the fourth pixel is intended to detect the infrared.

(15) The sensor 102 is covered by a filtering structure 111 intended to filter the light received by the photodetector device 100. The pixels of the first set which are intended to carry out a photodetection in the visible domain (corresponding to the pixels 104, 106 and 108 in the example of FIG. 2) are covered by a first visible pass and infrared cut off filter 112, in other words the spectral response of which lets pass the wavelengths of the visible domain and cuts off the wavelengths of the infrared domain. The pixels of the second set intended to carry out a photodetection in the infrared domain (corresponding to the pixel 110 in the example of FIG. 2) are covered by a second infrared pass and visible cut off filter 114, in other words the spectral response of which lets pass the wavelengths of the infrared domain and cuts off the wavelengths of the visible domain. These two filters 112, 114 are obtained from a stack of layers made in a common manner for the two filters 112, 114. In these two filters 112, 114, the spectral transmission and rejection is obtained on account of the alternating layout of dielectric layers of different refractive indices (here layers of SiN of refractive index equal to around 2, and layers of SiO.sub.2 of refractive index equal to around 1.45), as well as the presence of an additional metal layer in the second filter 114 compared to the first filter 112. Dielectric layers having a high refractive index may for example be composed of SiN (for example in the stoichiometric form Si.sub.3N.sub.4 or non-stoichiometric form Si.sub.XN.sub.Y) and/or TiO.sub.2 and/or Ta.sub.2O.sub.5 and/or HfO.sub.2 and/or ZrO.sub.2 and/or ZnS and/or ZnSe. Dielectric layers having a low refractive index may for example be composed of SiO.sub.2 and/or MgF.sub.2 and/or SiOCH and/or Na.sub.3AlF.sub.6. The materials of the alternating dielectric layers (alternation of high refractive index/low refractive index) having different refractive indices may be chosen such that they have a difference of refractive index greater than around 0.3, and preferably greater than around 0.5.

(16) The metal layer(s) of the filtering structure 111 may be composed of copper (which makes it possible particularly to carry out a good filtering for the spectral band of the colour red and the infrared), and/or aluminium and/or silver and/or gold.

(17) During the conception of the filters 112, 114, the thicknesses of the different layers of the filters are evaluated initially in an approximate manner via a calculation algorithm of multilayer filters optimising the stacks of filters considered individually, compared to the desired spectral templates. These values are then used as input data in a code that implements an overall numerical optimization technique to adjust more finely the thicknesses of the different layers of the filters 112, 114 considered simultaneously. Equality constraints are fixed between certain thicknesses of layers of the filters, to impose a constant thickness between the two filters. The filters 112, 114 are thus designed by trying to obtain a maximum of common layers of same thickness from one filter to the next.

(18) Each filter may firstly be designed individually using multilayer optical calculation software, such as for example the Optilayer software, which uses the Abls matrix theory to calculate the optical response of a stack of thin layers, and a needle type optimization method (as described in the document of A. V. Tikhonravov, M. K. Trubetskov, and G. W. DeBell, Application of the needle optimization technique to the design of optical coatings, Applied Optics, 1996, 35, Vol. 28, pp. 5493-5508) to provide a stack of spectral responses approaching as best as possible the desired template. The filters are then considered simultaneously from the individual solutions provided by the preceding calculation software.

(19) The optimisation making it possible to consider the layers of filters as common between them may use techniques of optimisation under constraint of simulated annealing type.

(20) The difference between the spectral responses of the two filters 112, 114 stems from the variation in the thickness of at least one of the dielectric layers between the two filters (here a layer of SiO.sub.2, the thickness of which in the second filter 114 is greater than its thickness in the first filter 112), as well as the presence of an additional metal layer in the second infrared pass and visible cut off filter 114 compared to the first infrared cut off and visible pass filter 112.

(21) The realization of the different layers of the stack of the filtering structure 111 according to the first embodiment of FIG. 2 will now be described.

(22) The filters 112 and 114 are made by depositing firstly, for example by PECVD, a first layer of SiN 116, the thickness of which is equal to around 85 nm. A first metal layer 118, for example composed of copper and the thickness of which is equal to around 38 nm, is then deposited on the first layer of SiN 116. This first layer of copper 118 is photolithographied and etched in order to only conserve this first layer of copper 118 at the second filter 114, in other words opposite the pixels intended to carry out a photodetection in the infrared domain (in other words the pixel 110 in the example of FIG. 2). The resin used to carry out this photolithography is then eliminated.

(23) The deposit is then carried out of a second layer of SiN 120, the thickness of which is for example equal to around 15 nm. On account of the presence of the first layer of copper 118 only in the second filter 114, the part of the second layer of SiN 120 in the first filter 112 is not arranged on a same plane as that in the second filter 114.

(24) It is then desired to make, on the second layer of SiN 120, a first layer of SiO.sub.2 122 comprising, at the first filter 112, a thickness equal to around 84 nm, and at the second filter 114, a thickness equal to around 191 nm. To do this, a first deposit is firstly carried out, on the second layer of SiN 120, of SiO.sub.2 of thickness equal to around 107 nm (191 nm84 nm=107 nm). This deposit of SiO.sub.2 is photo-lithographied and etched in order to only conserve the SiO.sub.2 at the second filter 114, in other words opposite the pixels intended to carry out a photo-detection in the infrared domain (pixel 110 in the example of FIG. 2). The resin used to carry out this photolithography is then eliminated. The first layer of SiO.sub.2 122 is then finished by making a second deposit of SiO.sub.2 of thickness equal to around 84 nm. This second deposit of SiO.sub.2 covers, at the first filter 112, the second layer of SiN 120, and at the filter 114, the first deposit of SiO.sub.2.

(25) The filters 112 and 114 of the filtering structure 111 are then finished by the deposits of the following layers which are common and which comprise a same thickness in the two filters 112, 114: a third layer of SiN 124 of thickness equal to around 27 nm; a second metal layer 126, here composed of copper and of thickness equal to around 23 nm; a fourth layer of SiN 128 of thickness equal to around 62 nm; a second layer of SiO.sub.2 130 of thickness equal to around 15 nm; a fifth layer of SiN 132 of thickness equal to around 62 nm.

(26) The table below summarises the thicknesses of the dielectric and metal layers used to make the two filters 112 and 114 of the filtering structure 111. The order of the layers indicated in this table corresponds to the stacking order, from the bottom to the top of the table, of the layers in the filtering structure 111.

(27) TABLE-US-00001 1.sup.st filter 112 2.sup.nd filter 114 SiN layer 132 62 nm SiO.sub.2 layer 130 15 nm SiN layer 128 62 nm Cu layer 126 23 nm SiN layer 124 27 nm SiO.sub.2 layer 122 84 nm 191 nm SiN layer 120 15 nm Cu layer 118 0 38 nm SiN layer 116 85 nm

(28) The spectral responses (value of the coefficient of transmission as a function of the wavelength in nm) of the filters 112 and 114, respectively referenced 140 and 142, are shown in FIG. 3.

(29) The value 0 indicated for the copper layer 118 at the first filter 112 indicates that this layer is absent at the first filter 112 and that it is thus only present in the second filter 114. The second filter 114, which is infrared pass and visible cut off, thus comprises an additional metal layer (first layer of copper 118) compared to the first filter 112 which is visible pass and infrared cut off. The presence of a single layer of copper in the stack of layers forming the first filter 112 makes it possible to minimise the absorption of visible light by the metal, disadvantageous in particular in the blue and the green, whereas the presence of two layers of copper in the stack of the infrared pass filter 114 makes it possible to improve the rejection of this filter in the visible domain.

(30) The variation in thickness of the first layer of SiO.sub.2 122 between the two filters 112 and 114 makes it possible to improve the transmission and the rejection of certain spectral bands in a more specific manner for each of the two filters 112, 114. The other layers are common to these two filters 112, 114 and have similar thicknesses in the two filters 112, 114. The difference between the spectral responses of the two filters 112, 114 is due to: the presence of an additional metal layer (layer 118) in the second filter 114 compared to the first filter 112; the variation in thickness of a single dielectric layer (layer 122) between the two filters 112 and 114, all the other layers being common and similar in the two filters 112, 114.

(31) In this example, the maximum transmission of the two filters 112, 114, of the order of 50%, and the rejection outside of the resonance, are sufficient for numerous envisaged applications. The spectral responses of the filters 112, 114 could be optimised if a greater number of layers is envisaged to make these filters.

(32) In order to carry out a detection of the red, green and blue components of the visible spectrum, coloured filters 134 are made on the fifth layer of SiN 132, opposite the pixels of the sensor 102 intended to carry out a photo-detection in the visible domain, in other words on the first filter 112, opposite the pixels of the first set of pixels. These coloured filters 134 are for example made from coloured resins. In the example of FIG. 2, a coloured filter 134a of blue colour is arranged opposite the pixel 104, a coloured filter 134b of green colour is arranged opposite the pixel 106 and a coloured filter 134c of red colour is arranged opposite the pixel 108. The spreading and the control of the thickness of the resins on the layers of the filters 112 and 114 is possible from the moment that the relief (difference in total thickness between the filters 112 and 114) is less than the thickness of the resins to be deposited. In the example described previously, the difference in thickness between the two filters 112, 114 is only around 145 nm, which is less than the thickness of the coloured organic resins used to make the coloured filters 134 (comprised between around 800 to 1000 nm).

(33) To make the coloured filters 134, a first coloured resin (for example red) is firstly spread by spin-coating on the fifth layer of SiN 132, at the two filters 112 and 114. The spread resin is then hardened by annealing, then photo-lithographied and finally developed to define the patterns provided for the red coloured filters (in other words localised above the pixels intended to carry out a photodetection in the spectral band of the colour red, corresponding to the pixel 108 in FIG. 2). These operations are then repeated with resins of two other colours.

(34) In the example described previously, the device 100 is a photodetector device. In a variant, the device 100 could be a photoemitter device. In this case, compared to the device 100 described, the sensor 102 could be replaced by a substrate comprising light emitting means (emitting in the visible and infrared spectral bands) on which the filters 112 and 114 would be arranged, as well as optionally the coloured filters 134 if it is wished to carry out, in the visible domain, emissions in spectral bands restricted to precise colours. It would also be possible to have a photodetector and photoemitter device, the light emitting means being in this case combined with the photodetectors under the filters 112 and 114.

(35) The following table gives the thicknesses of dielectric and metal layers used to make the two filters 112 and 114 according to a first variant of the filtering structure 111 of the first embodiment. As for the previous table, the order of the layers indicated in this table corresponds to the stacking order, from the bottom to the top of the table, of the layers in the filtering structure according to this first embodiment variant.

(36) TABLE-US-00002 1.sup.st filter 112 2.sup.nd filter 114 SiN layer 108 nm SiO.sub.2 layer 144 nm SiN layer 105 nm SiO.sub.2 layer 157 nm SiN layer 0 82 nm Cu layer 0 38 nm SiN layer 90 nm SiO.sub.2 layer 143 nm SiN layer 106 nm SiO.sub.2 layer 144 nm SiN layer 107 nm The 0 values indicated for the copper layer and one of the layers of SiN at the first filter 112 indicate that these layer are absent at the first filter 112 and that they are present only in the second filter 114.

(37) As previously, the filtering structure according to this first embodiment variant comprises dielectric layers of different refractive indices arranged in an alternating manner (alternation of high and low refractive indices). On the other hand, in this first embodiment variant, the second infrared pass and visible cut-off filter 111 comprises a single metal layer, here composed of copper, and the first visible pass and infrared cut off filter 112 does not comprise any metal layer and is only composed of an alternation of dielectric layers of different refractive indices. In addition, in this first embodiment variant, one of the dielectric layers is only present in the second filter 114.

(38) The spectral responses (value of the coefficient of transmission as a function of the wavelength in nm) of the filters 112 and 114 of the filtering structure according to this first embodiment variant, respectively referenced 150 and 152, are shown in FIG. 6.

(39) The following table gives the thicknesses of dielectric and metal layers used to make the two filters 112 and 114 according to a second variant of the filtering structure 111 of the first embodiment. As for the preceding table, the order of the layers indicated in this table correspond to the stacking order, from the bottom to the top of the table, of the layers of the filtering structure according to this second embodiment variant.

(40) TABLE-US-00003 1.sup.st filter 112 2.sup.nd filter 114 SiN layer 108 nm SiO2 layer 148 nm SiN layer 108 nm Cu layer 0 15 nm SiN layer 0 175 nm Cu layer 0 37 nm SiN layer 0 85 nm SiO2 layer 148 nm SiN layer 108 nm SiO2 layer 148 nm SiN layer 108 nm SiO2 layer 148 nm SiN layer 109 nm The 0 values indicated for the layers of copper and two of the layers of SiN at the first filter 112 indicate that these layer are absent at the first filter 112 and that they are thus only present in the second filter 114.

(41) As previously, the filtering structure according to this second embodiment variant comprises dielectric layers of different refractive indices arranged in an alternating manner with respect to each other. In this second embodiment variant, the second infrared pass and visible cut off filter 114 comprises two metal layers, here composed of copper, and the first visible pass and infrared cut off filter 112 do not comprise any metal layer and is only composed of an alternation of dielectric layers of different refractive indices. In addition, all the layers that are common to these two filters 112 and 114 have a constant thickness from one filter to the next. Two dielectric layers composed of SiN are present only in the second filter 114.

(42) The spectral responses (value of the coefficient of transmission as a function of the wavelength in nm) of the filters 112 and 114 of the filtering structure according to this second embodiment variant, respectively referenced 160 and 162, are shown in FIG. 7.

(43) In FIGS. 6 and 7, it can be seen that the spectral responses 150 and 160 of the visible pass and infrared cut off filters of the filtering structures show a rise in their coefficient of transmission in the infrared domain, above around 950 nm. Nevertheless, such a rise is not bothersome when such a filtering structure is coupled to a sensor and/or an emitter composed of silicon which is not very sensitive to these wavelengths.

(44) Reference will now be made to FIG. 4 which shows partially and schematically a photodetector device 200 able to carry out a light detection in the visible and infrared domains, according to a second embodiment.

(45) Like the device 100 described previously, the device 200 comprises the sensor 102 described previously. The sensor 102 is covered by a filtering structure 211 which comprises a first filter 212 of visible pass and infrared cut off type, and a second filter 214 of infrared pass and visible cut off type. As for the filters 112, 114 described previously, the filters 212 and 214 are made from a stack of layers, most of which are common to these two filters. In addition, as for the filters 112, 114, the transmission and the spectral rejection carried out by the filters 212 and 214 are obtained on account of the use of dielectric layers of different refractive indices (layers of SiN and layers of SiO.sub.2 in the example described here), as well as the presence of an additional metal layer in the second filter 214 compared to the first filter 212.

(46) Nevertheless, unlike the filtering structure 111 which uses coloured resins to carry out, within the visible spectrum, the filtering of the colours red, green and blue, the first filter 212 comprises layers that have variable thicknesses even within the first filter 212 in order to carry out the different filterings desired in the visible domain, for example a function of RGB filtering. Thus, the first filter 212 comprises a first part 212a arranged opposite the pixels intended to capture the spectral band corresponding to the colour blue (pixel 104 in the example of FIG. 4) and forming a filter letting pass only this spectral band. In the same way, parts 212b and 212c arranged opposite the pixels intended to capture respectively the spectral bands corresponding to the colours green and red (pixels 106 and 108 in the example of FIG. 4) form filters letting pass only these spectral bands.

(47) The table below gives the thicknesses of the dielectric and metal layers used to make the two filters 212, 214.

(48) TABLE-US-00004 1st filter 212 212a 212b 212c 2nd filter 214 SiN 236 66 nm SiO2 234 69 nm SiN 232 36 nm 55 nm 71 nm 156 nm Cu 230 34 nm SiN 228 27 nm 50 nm 69 nm 139 nm SiO2 226 68 nm SiN 224 86 nm SiO2 222 95 nm SiN 220 31 nm Cu 218 0 16 nm SiN 216 0 102 nm

(49) The spectral responses of the parts 212a, 212b, 212c of the first filter 212, respectively referenced 240a, 240b and 240c, and the spectral response of the second filter 214, bearing the reference 242, are shown in FIG. 5.

(50) The differences between the two filters 212 and 214 are: the presence of a single layer of Cu in the stack of layers of the first filter 212, and two layers of Cu in the stack of the second filter 214; the variation in thickness of two dielectric layers (composed of SiN in this example) between the two filters 212 and 214; the presence of an additional dielectric layer (here comprising SiN) in the second filter 214 compared to the first filter 212.

(51) In addition, within the first filter 212, the filtering of the different RGB spectral bands is obtained on account of the fact that the two dielectric layers of different thicknesses between the two filters 212 and 214 also have different thicknesses within the different parts 212a, 212b and 212c of the first filter 212.

(52) All the other layers of the stack of the filtering structure 211 are common and of constant thickness in the two filters 212 and 214.

(53) As for the filtering structure 111, the stack of layers of the filtering structure 211 makes it possible to make the second filter 214 which cuts off the wavelengths of the visible domain without using a black resin.

(54) As for the device 100, the device 200 could be a photoemitter device, or instead a photodetector and photoemitter device.

(55) In a variant, the devices 100 and 200 could be able to carry out a light detection and/or a light emission only in the visible domain, or in the infrared domain. In this case, the presence of an additional metal layer in the second filter could be taken advantage of so that the first filter is able to carry out a filtering of a spectral band corresponding to the colours blue and green, and that the second filter, which comprises the additional metal layer, is able to carry out a filtering of a spectral band corresponding to the colour red. The table below corresponds to an example embodiment of a filtering structure of a device capable of performing a light detection and/or a light emission only in the visible domain. In this structure, the first filter comprises a first part adapted to carry out a filtering of the spectral band corresponding to the blue color and a second part adapted to carry out a filtering of the spectral band corresponding to the green color, and the second filter, which comprises an additional metal layer, is able to perform a filtering of a spectral band corresponding to the color red.

(56) TABLE-US-00005 1st filter 1st part 2nd part 2nd filter SiN 66 nm SiO2 69 nm SiN 36 nm 55 nm 71 nm Cu 34 nm SiN 27 nm 50 nm 69 nm SiO2 68 nm SiN 0 130 nm Cu 0 16 nm SiN 86 nm SiO2 95 nm SiN 31 nm

(57) The additional layer of copper included in the second filter and not in the first filter further improves the selectivity, and especially the rejection, of the second filter.

(58) The spectral responses of the first part of the first filter, the second part of the first filter and the second filter, respectively referenced 250a, 250b and 250c, are shown in FIG. 8.

(59) In all embodiments and examples previously described, when the metal layer(s) comprise copper, the sharp drop in the value of the refractive index of the copper around wavelengths comprised between around 550 nm and 600 nm is then judiciously used.

(60) The stacks of layers described above may be formed on a substrate SiO.sub.2-based, and the upper layer of these stacks may be in contact with a superstrate made of air.