Multispectral filter for electromagnetic radiation and method of manufacturing said filter

Abstract

Multispectral filter for electromagnetic radiation and method for manufacturing said filter. The invention relates to a multispectral filter (100) for electromagnetic radiation, said filter comprising at least two color filters (Fi), each color filter comprising: a metallic lattice having metallic patterns (102i) repeated according to a given period (P), each metallic pattern being spaced from an adjacent metallic pattern by a given non-zero gap (Wi); a continuous reflective layer (104i); a pattern (103i) of Fabry-Pérot cavity dielectric material between the metallic lattice and the continuous reflective layer; the thickness (Ti) of the dielectric material patterns of the two color filters being different. Figure to be published with the abstract: Figure 6

Description

Title of the invention: Multispectral filter for electromagnetic radiation and method of manufacturing said filter TECHNICAL FIELD OF THE INVENTION

[0001] The technical field of the invention is that of spectral filtering, in particular for imaging applications and the production of colored filters for CMOS type image sensors, liquid crystal display devices or light-emitting diodes. The invention can also be implemented in light-emitting devices.

[0002] The present invention relates to a multispectral filter for electromagnetic radiation. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] A spectral filter or a color filter makes it possible to filter light by wavelength, so as to provide information on the intensity of the light in certain wavelengths. Several color filters can be combined to form, for example, red-green-blue filters (RGB in English terminology) which provide information on the intensity of these three colors.

[0004] In particular, metal / dielectric color filters are known, made from a Fabry-Pérot cavity. These filters comprise one or more dielectric (or possibly semiconductor) cavities formed between two thin metal films having a metal mirror function so as to form a Fabry-Pérot cavity. Generally, the metal / dielectric stacks are different depending on the position on the optoelectronic component (an image sensor for example). The transmission wavelength of the filter is set by adjusting the thickness of the cavity. Thus, in operation, a portion of the incident light corresponding to the wavelength of the filter is transmitted through it in the form of a colored beam, while the remainder of the incident light is reflected.Generally speaking, the thickness of the dielectric layer sets the transmitted central wavelength, while the thickness of the metal layers allows the transmission spectral width to be adjusted. Furthermore, the use of several Fabry-Pérot cavities allows the spectral profile of the filter transmission to be modified. A filter of this type is made using conventional semiconductor manufacturing techniques. Thus, to obtain a red-green-blue filter, it is necessary to form at least one dielectric cavity whose thickness must have three different values. For certain applications, it may be interesting to adjust not only the central transmission wavelength (via the thickness of the dielectric layer) but also the bandwidth of . transmission.

[0005] The bandwidth of the transmitted spectra can in particular be adjusted by varying the thickness of the metal [cf. IL Gomes de Souza, VF Rodriguez-Esquerre and DF Rêgo, "Wide-angle filters based on nanoresonators for the visible spectrum", Appl. Opter. 57, 6755-6759 (2018)]. [Fig.l] shows the spectral response of a Fabry-Pérot cavity filter 1 comprising a dielectric layer 2 with a thickness of 140 nm between two metallic reflective layers 3 and 4 of the same thickness when the two thicknesses of the reflective metallic layers 3 and 4 vary between 10 and 60 nm. It is clear from the figure that by increasing the thickness of the metal of the two layers 3 and 4, the transmission bandwidth decreases (from approximately 100 nm to approximately 10 nm). However, a simultaneous decrease in maximum transmission from 0.9 to about 0.2 is observed.There is therefore a negative counterpart on the value of the maximum transmission when we seek to reduce the bandwidth of the filter.

[0006] The bandwidth adjustment can also be achieved by varying a single metal thickness, for example the thickness of the reflective layer 4, as illustrated in [Fig.2] (the thickness of the reflective layer 3 remaining constant and equal to 30 nm). However, from the simulation results, it is again observed that by increasing the thickness of a single metal layer, the maximum transmission decreases. It therefore appears to be difficult to obtain a narrow transmission bandwidth while maintaining a higher transmission coefficient with Fabry-Pérot cavity filters.

[0007] A known solution for finding a good compromise between obtaining a narrow bandwidth and maximum transmission that is not too reduced consists of using GMR filters (Guided Mode Resonance in English terminology). These filters can be implemented in the visible or infrared. An example of such a filter can be found in the publication “Metal-dielectric structures with guided mode resonance and applications to filtering and infrared imaging” (Optics / Photonic. Ecole Polytechnique X, 2013 -Sakat et al.). [Fig. 3] illustrates the structure of a GMR filter 10 in the infrared. The filter 10 comprises a metal grating 11 (“grating layer” in English terminology) comprising metal patterns spaced apart by a distance w (thus forming slots between each pattern) placed on a dielectric layer 12 which acts as a waveguide.The metal patterns are repeated with a period d. The coupling between the waveguide mode and the resonances of the metal patterns gives a resonance wavelength. The wavelength depends essentially on the thickness of the dielectric layer 12. By adjusting the various parameters, including the period p, the gap w and the thickness of the metal patterns tm, it is possible to reduce the transmission bandwidth while maintaining a . maximum acceptable transmission.

[0008] More recently, filter structures comprising a dielectric layer sandwiched between two metal gratings have been described, operating in both the visible and infrared ranges. An example of such a filter 20 is shown in [Fig.4]. The filter 20 comprises a first metal grating 21 comprising metal patterns arranged periodically according to a period d and spaced apart by a gap a, placed on a dielectric layer 22 which acts as a waveguide, itself above a second metal grating 23 similar to the first grating 21. Again, the various parameters, namely the thickness td of the dielectric layer, the period d, the gap a and the thickness of the metal patterns tm, will make it possible to adjust the transmission wavelength, the bandwidth and the maximum transmission.

[0009] Even if the structure of [Fig.3] offers satisfactory performance in terms of compromise between maximum transmission and bandwidth, it remains poorly suited to implementation for multispectral filters using pixelated sensors. Indeed, such an application would require producing in the same structure, several different color filters (typically red, blue and green) with, for each, two different metal networks. Such a structure 30 is illustrated in [Fig.5]. It comprises 3 filters (Pixel i, i varying from 1 to 3) each comprising respectively: • a first 3 IPi metal network comprising metal patterns arranged periodically according to a period Pi and spaced apart by a gap Wi, • a 32Pi dielectric layer which acts as a waveguide, • a second 33Pi metal network.

[0010] It will be noted that each dielectric layer 32Pi here has a different thickness to allow the corresponding wavelength to pass through.

[0011] Thus, two metal gratings (lower and upper) must be manufactured using standard lithography techniques. While the lower 3 IPi grating can be obtained in a standard manner, the upper 33Pi grating is much more complex to produce on dielectric layers of different thicknesses, particularly due to the necessary alignment of the two upper and lower metal gratings, and the need to carefully control the periods and the different gaps for the gratings of each pixel. Such manufacturing requires numerous technologically complex steps.

[0012] Furthermore, for the guided mode resonance filter to work efficiently, a higher repetition of the grating period is required. In addition, to achieve transmission of different wavelengths, a different grating period must be used for each pixel. The difficulty in mastering the technological realization of the gratings can therefore lead to performance problems of the image sensors. pixelated. Finally, with a predefined pixel size for each pixel, the number of metallic patterns is different for the different pixels, again leading to production difficulties. Summary of the invention

[0013] The invention offers a solution to the problems mentioned above, by proposing a multispectral filter for electromagnetic radiation with a good compromise between the maximum transmission for each wavelength and the width of the passband, while overcoming the manufacturing problems mentioned above.

[0014] To do this, the invention relates to a multispectral filter for electromagnetic radiation, said filter comprising at least two color filters, each color filter comprising: • a metal network comprising metal patterns repeated over a given period, each metal pattern being spaced from an adjacent metal pattern by a given non-zero distance; • a continuous reflective layer; • a Fabry-Pérot cavity dielectric material pattern between the metal lattice and the continuous reflective layer; the thickness of the dielectric material patterns of the two color filters being different.

[0015] Thanks to the invention, the Fabry Pérot cavity filter is produced with a conventional continuous metal reflector while the second reflector is a discontinuous metal grating of the "metal grating layer" type. Such an architecture offers both the adjustment flexibility of double grating filters and simplicity of manufacture insofar as only one discontinuous layer needs to be produced. Furthermore, it is perfectly possible to produce a filter in accordance with the invention making it possible to filter different colors (red, blue, green for example) with a repetition period of the patterns of the metal grating which remains constant, whatever the color, and by adjusting for example the gap between the patterns from one color to another.As we will see later, the filter according to the invention makes it possible to obtain more satisfactory performances in terms of compromise between maximum transmission and narrowness of the bandwidth (in particular compared to conventional filters in which the thickness of the metal is increased). It will be noted that the filter according to the invention acts as a Fabry-Pérot cavity resonator formed by the tri-layer "continuous metal layer - dielectric layer - metal network" and that the filtered wavelength is essentially determined by the thickness of the dielectric layer. The presence of at least two different thicknesses therefore ensures operation on at least two wavelengths and therefore the production of a pixelated filter, each pixel corresponding to a . wavelength.

[0016] In addition to the characteristics which have just been mentioned in the preceding paragraph, the filter according to the invention may have one or more additional characteristics among the following, considered individually or according to all technically possible combinations: • the repetition period of the metallic patterns is identical for the two color filters; the filter according to the invention can keep this period identical for any number of color filters greater than or equal to 2; • the thickness of the metal network of the two color filters is identical; the filter according to the invention can keep this identical thickness for any number of color filters greater than or equal to 2; • the thickness of the reflective layer of the two color filters is identical; the filter according to the invention can keep this identical thickness for any number of color filters greater than or equal to 2; • each of the two filters is opposite a photoelectric transducer. “Photoelectric transducers” are devices that can operate either as a collector of light from the filters or as an emitter of light towards the filters. In the case of collectors, the transducers may be, for example, CMOS photodiodes. In the case of emitters, the transducers may be, for example, LED diodes, QLED diodes or LASER diodes, the emitters in this case having a broader emission spectrum than that of the corresponding Fabry-Pérot cavities; • the space between each metal pattern is filled with the dielectric material forming the Fabry-Pérot cavity dielectric material patterns of the color filters; • the repetition period of the metallic patterns of each metallic network is chosen to be strictly less than the wavelength of the corresponding color filter; • it will be noted that the metal networks are preferentially located on the lower part of the filter while the continuous reflective layers are located on the upper part of the filter for reasons of ease of manufacture; we will see however that it is also possible to manufacture a multispectral filter according to the invention with metal networks located on the upper part of the filter and continuous reflective layers located on the lower part of the filter.

[0017] The invention also relates to a method for manufacturing a multispectral filter. for electromagnetic radiation according to a first embodiment of the invention comprising the following steps: • Deposition of a first layer of resin on a substrate; • Structuring the first layer of resin so as to obtain at least two series of trenches in the first layer of resin, each trench of a series being spaced from an adjacent trench of the series by a given non-zero gap, the trenches being repeated according to a given period; • Deposition of a layer of metal in the trenches and on the remaining resin; • Removal of remaining resin and metal on the remaining resin from way of maintaining two metal networks each comprising metal patterns repeated according to the given period of one of the series of trenches, each metal pattern being spaced from an adjacent metal pattern by the given non-zero gap of one of the series of trenches; • Deposition of a planar layer of dielectric material intended to form at least two patterns of Fabry-Pérot cavity dielectric material, said layer having the thickness of the maximum Fabry-Pérot cavity dielectric pattern; • Deposition of a layer of resin on the layer of dielectric material and removal of said layer of resin with stopping on the dielectric layer at the level of the zone above one of the two metal networks; • Etching of the dielectric layer at the resin removal zone in order to obtain a first dielectric Fabry-Pérot cavity pattern; • Removing the remaining resin so as to leave visible a second Fabry-Pérot cavity dielectric pattern having the maximum dielectric pattern thickness, the thickness of the first dielectric pattern being less than the thickness of the first dielectric pattern; • Deposition of a continuous metal layer on the first and second dielectric patterns.

[0018] According to this first manufacturing method, the metal networks are preferentially located on the lower part of the filter while the continuous reflective layers are located on the upper part of the filter.

[0019] The invention also relates to a method for manufacturing a multispectral filter for electromagnetic radiation according to a second embodiment of the invention comprising the following steps: • Deposition on a substrate of a layer of structuring material; • Structuring of the layer of structuring material so as to obtain at at least two patterns of structuring material of different heights, at least one of the patterns having a maximum reference height relative to tively to the substrate, the height being measured perpendicular to the plane of the substrate; • Deposition of a continuous metal layer on the at least two patterns of structuring material; • Deposition of a layer made in the dielectric material intended to form at least two dielectric patterns of Fabry-Pérot cavities, said layer of dielectric material covering all of the patterns of structuring material by presenting an upper surface of which each point is located at a height greater than the maximum reference height; • Planarization by removal of the dielectric material with selective stopping at the top of the highest structuring material pattern covered with the continuous metal layer; • Deposition of a layer made from the same dielectric material so as to finalize the formation of at least two Fabry-Pérot cavity dielectric patterns; • Deposition of a second metallic layer on the at least two dielectric cavity patterns of the Fabry-Pérot cavity; • Structuring the second metal layer so as to form at least two metal networks each comprising metal patterns repeated according to a given period, each metal pattern being spaced from an adjacent metal pattern by a given non-zero gap.

[0020] According to this second manufacturing method, the multispectral filter according to the invention comprises metal networks located on the upper part of the filter and continuous reflective layers located on the lower part of the filter.

[0021] Advantageously, the structuring material is a resin, said structuring step being carried out by a grayscale lithography step on the layer of structuring material.

[0022] The invention and its various applications will be better understood by reading the following description and examining the accompanying figures. BRIEF DESCRIPTION OF THE FIGURES

[0023] The figures are presented for information purposes only and in no way limit the invention. • [Fig.l] illustrates the effect of the thickness of the two reflective layers of a state-of-the-art Fabry-Pérot cavity filter on the transmission and bandwidth of the filter; • [Fig.2] illustrates the effect of the thickness of one of the two reflective layers of a state-of-the-art Fabry-Pérot cavity filter on the transmission and bandwidth of the filter; • [Fig.3] illustrates a state-of-the-art single metal grating GMR guided mode resonance filter; • [Fig.4] illustrates a state-of-the-art dual metal grating GMR guided mode resonance filter; • [Fig.5] illustrates a multispectral filter comprising a plurality of filters according to [Fig.4]; • [Fig.6] illustrates a multispectral filter according to a first embodiment of the invention; • [Fig.7] shows a comparison of the transmission spectra for three different color filter structures including a color filter used in a filter according to the invention; • [Fig.8] and [Fig.9] show the spectral responses of a multispectral filter according to the state of the art and of a multispectral filter according to the invention; • [Fig. 10] shows a transmission map as a function of the spacing between the patterns of a metal grating of a color filter of a multispectral filter according to the invention; • [Fig.l 1] shows another example of spectral response of a multispectral filter according to the invention with a period different from that of figures 8 and 9; • [Fig. 12] illustrates the different stages of a method of manufacturing a multispectral filter according to the first embodiment of the invention; • [Fig. 13] illustrates a multispectral filter according to a second embodiment of the invention • [Fig. 14] shows the superposition of the spectral response of two color filters used respectively in the filter of [Fig.6] and in the filter of [Fig.13]; • [Fig. 15] illustrates the different stages of a method of manufacturing a multispectral filter according to the second embodiment of the invention. DETAILED DESCRIPTION

[0024] The figures are presented for information purposes only and in no way limit the invention.

[0025] It should be recalled first that, generally and well known to those skilled in the art, a metal / dielectric type color filter made from a Fabry-Pérot cavity is obtained by dimensioning the thickness of the dielectric layer formed between the two metal layers. If the filtering of several colors is sought on the same component, it is then necessary to be able to obtain a dielectric thickness of variable thickness on this same component.

[0026] This dimensioning is for example carried out using an electronic calculation program tromagnetic such as the Abeles matrix transfer formalism or a diffraction calculation for pixels whose size is close to the wavelength such as the Fourier Expansion Modal Method formalism or rigorous coupled wave analysis (RCWA).

[0027] These calculation programs make it possible to determine the optimal parameters of the metal-dielectric stacks for each pixel. The calculation takes into account in particular the thicknesses of the metal and dielectric layers as well as their indices, the spectrum and the angular distribution of the incident light. For example, in the case of Fabry-Pérot filters, the central wavelength of the filter is determined approximately by the following formula: _ 2ma!^ WHERE • hcavity is the thickness of the cavity, i.e. approximately the thickness of the dielectric layer • ncavity is the effective index of the cavity, • q> is the phase shifts on reflection on the metal mirror (determined by the nature of the materials involved and the wavelength considered), • 0in is the angle of incidence of the incident light on the filter (counted from the perpendicular to the filter surface).

[0028] Once the angle of attack is known, the index and the phase shift are known, all that remains is to determine an approximate thickness so that the cavity is centered on a particular wavelength. Once the filtering function is calculated for each filter and each wavelength, the thicknesses h of the dielectrics are then adjusted according to the desired performance (search for a good signal-to-noise ratio, maximum transmission, etc.).

[0029] Another more empirical method consists of calculating, for several thicknesses h, the response of the stack and choosing h such that the resonance peak of the filter (Xres) is positioned in accordance with the specifications.

[0030] [Fig.6] illustrates a multispectral filter 100 according to the invention.

[0031] The multispectral filter 100 comprises a plurality of color filters Fi (i being a integer greater than or equal to 2) made on a substrate 101 which may be for example a Si substrate, a Silicon on Insulator SOI substrate (for "Silicon On Insulator" according to English terminology) or a glass or sapphire substrate. Here, 4 color filters Fl to F4 (including two identical filters Fl and F4) are shown for illustration purposes only.

[0032] Each color filter Fi aims to pass a transmission spectrum with a given central length, a given maximum transmission and a given bandwidth and comprises: • A discontinuous reflective metal layer 102i formed by a substantially planar metal network comprising a plurality of metal patterns 103i, each metal pattern 103i being repeated with a period P measured along the axis Ox, being spaced from the adjacent pattern(s) by a non-zero gap Wi measured along the axis Ox and having a thickness EG measured along the axis Oy (Oxy being the plane of the figure and the axis Oy being in the direction perpendicular to the plane of the substrate 101); • A dielectric layer 103i of thickness Ti forming a Fabry-Pérot cavity dielectric pattern, said layer being in contact on its lower surface with the discontinuous metal layer 102i; • A continuous reflective metal layer 104i of thickness EC measured along the Oy axis in contact with the upper surface of the dielectric layer 103i.

[0033] The metallic patterns of the networks 102i are here represented in the form of parallelepiped blades extending along the axis Oz perpendicular to the Oxy, it being understood that these patterns can have other shapes such as shapes of cylindrical pillars (square, circular, elliptical, etc.) spaced from each other by a distance Wi and repeated according to a period P.

[0034] Preferably, the period P is constant for all the metal gratings 102i. The same applies to the thickness EG which is also preferably constant for all the metal gratings 102i and to the thickness EC which is also preferably constant for all the continuous reflective layers 104i. The repetition period P is advantageously chosen to be strictly less than the wavelengths of the color filters forming the filter according to the invention.

[0035] The material used for the reflective metal layers 102i and 104i is preferably identical and may be, for example, Ag or Au for applications in the visible range.

[0036] The material used for the dielectric patterns 103i can be for example SiO2, SiN or even a resin intended to be used in lithography.

[0037] Advantageously, each space between the metal patterns of the networks 102i is filled with the dielectric material used for the dielectric patterns 103i.

[0038] The set of metal layers 104i can be seen as a continuous metal layer of constant thickness EC with staircase steps covering the upper surfaces of the dielectric patterns 103i as well as, optionally, the sides of the latter.

[0039] The set of metallic gratings 102i can be seen as a planar metallic grating with a constant repetition period P of metallic patterns whose spacing Wi between adjacent patterns is likely to vary from one color filter to another.

[0040] The filter 100 according to the invention therefore corresponds to a state-of-the-art Fabry-Pérot type transmission filter modified in that a discontinuous metal grating with a pattern repetition period strictly less than the transmission wavelength replaces one of the continuous reflectors of the filter. The other reflector remains a continuous thin metal film as in a known Fabry-Pérot cavity filter.

[0041] Advantageously, the grating period is the same for all the color spectral filters Fi unlike the prior art where the grating period is different for the different color filters.

[0042] The transmission wavelength in the structure of each filter Fi can be obtained by optimizing the thickness Ti of the dielectric cavity 103i according to the methods mentioned above and to a lesser extent the gap Wi between each pattern of a metal grating 102i. The transmission bandwidth corresponding to a given wavelength can be controlled by adjusting the gap Wi between each pattern of a metal grating 102i and the thickness EG of the metal grating 102i.

[0043] The thickness EG of the metal networks 102i is preferably greater than the thickness EC of the continuous metal reflective layers 104i.

[0044] The thickness EG of the metal gratings 102i is preferably greater than the penetration depth of the metal, i.e. the (minimum) thickness of the metal at which electromagnetic radiation can be transmitted with minimal energy loss (the metal is more or less transparent), so that the metal grating acts as a reflector. The thickness EG must therefore be greater than this value (generally the penetration depth of metals is approximately 5 to 10 nm)

[0045] As we will see in the following, the filter 100 according to the invention makes it possible to obtain resonant filters at several wavelengths with a satisfactory maximum transmission for each wavelength and a sufficiently narrow and substantially constant bandwidth for all the wavelengths.

[0046] The filter 100 according to the invention also optionally comprises a dielectric overlayer 105 deposited above the continuous metal layers 104i.

[0047] The filter 100 according to the invention may also comprise a plurality of photoelectric transducers (at least two) not shown (here 4 photoelectric transducers would be necessary) opposite each color filter Fi. The transducers may be under the substrate 101, formed inside the latter or above the latter. The photoelectric transducers may operate either as a collector of light coming from the filters or as an emitter of light towards the filters. If they are collectors, the transducers may be for example CMOS type photodiodes. If they are emitters, the transducers may be for example LED diodes, QLED type diodes or LASER diodes, the emitters presenting in this case a broader emission spectrum than that of the corresponding Fabry-Pérot cavities.

[0048] In order to illustrate the performance of the filter according to the invention, [Fig.7] shows a comparison of the transmission spectra for three different color filter structures: • SI spectrum with a color filter having continuous thin metal layers of a Fabry-Pérot filter according to the state of the art, • Spectrum S2 with a color filter having a continuous layer and a metal network as used in the present invention, • Spectrum S3 with a filter having a thick metal layer and a thinner metal layer as shown in [Fig.2].

[0049] The comparison is made for a constant dielectric cavity thickness of 140 nm and the following filter structures: • (a) For the SI spectrum: Fabry-Pérot filter with thin metal layers 30 nm thick, • (b) For the S2 spectrum: Color filter structure according to the invention where the The lower metal layer is a metal lattice. The lattice period used for this illustration is 290 nm and the gap between the metal patterns is 150 nm. The thickness of the metal lattice is 60 nm, • (c) For S3 spectra: Fabry-Pérot filter with a continuous upper metal layer thickness of 30 nm and a continuous lower metal layer thickness of 60 nm.

[0050] In the presented simulation, the angle of incidence is a normal angle while a transverse magnetic polarization is used.

[0051] Silver Ag is used here as the metal for the reflective layers and the cavity, substrate and covering material are made of SiO2 with a constant refractive index of 1.46 for illustration purposes. The choice of materials and dimensions is obviously not limited to that presented here.

[0052] As can be seen in [Fig.7], the use of a greater thickness of continuous metal layer (spectrum S3) makes it possible to make the transmission bandwidth narrower compared to that of the SI spectrum of a standard Fabry-Pérot filter: however, the increase in thickness results in a decrease in the maximum transmission. The use of a thick metal grating structure according to the invention makes it possible to increase the transmission coefficient while having a narrow bandwidth (~ 25 nm).

[0053] [Fig.8] shows the spectral response of a multispectral filter such as that of [Fig.6] with the following dimensions for each color filter Fi: Fl F2 F3 F4 P(nm) 290 290 290 290 Ti (nm) 110 140 160 180 Wi (nm) 50 150 120 90

[0054] The thickness of the continuous reflective layer is equal to 30 nm for each of the color filters and the thickness of the patterns of the metallic network for each color filter is 60 nm.

[0055] It can be seen in [Fig.8] and the table above that it is possible, thanks to the filter according to the invention, to obtain a quasi-constant and narrow bandwidth (the width at half-maximum LMH is approximately equal to 25 nm whatever the color) for all the wavelengths each corresponding to a given color while keeping an identical period P of repetition of the patterns of the metallic networks and by adjusting only the value of the gap Wi between the patterns for each wavelength (the value of the wavelength being essentially fixed by the thickness Ti of the dielectric patterns of the Fabry-Pérot cavity).

[0056] [Fig.9] shows the spectral response of [Fig.8] on which is superimposed (in dotted lines) the spectral response of a multispectral filter according to the state of the art with Fabry-Pérot cavities with continuous reflective layers with a thickness of 30nm.

[0057] The table below gives the widths at half height for each of the color filters respectively of the filter according to the invention and of the filter according to the state of the art. Fl F2 F3 F4 X (nm) 485 555 600 655 LMH (nm) filter according to the invention ~25 ~25 ~25 ~25 LMH (nm) filter according to the state of the art ~60 ~40 ~35 ~30

[0058] It can therefore be seen that despite a higher maximum transmission, the filter according to the state of the art does not make it possible to obtain a substantially identical bandwidth for each color filter.

[0059] Furthermore, from the point of view of the manufacturing process, it is clear that the filter according to the invention offers more parameters to be varied to adjust both the wavelength, the maximum transmission and the bandwidth while the filter of the state of the art requires the wavelength to be precisely fixed as a function of the thickness of the dielectric pattern without it being possible to correct or compensate for certain manufacturing-related limitations. Conversely, with the filter according to the invention, the fact of being able to vary the Wi gap makes it possible to influence the transmission and the bandwidth. Similarly, adjusting the period makes it possible to influence the transmission and the bandwidth. Of course, it is also possible to vary the thickness of the patterns of the metal network and / or the thickness of the metal layer in order to obtain the desired filter properties.

[0060] [Fig. 10] shows a transmission map as a function of the spacing W between the patterns of a metal grating of a color filter of a multispectral filter according to the invention, for a grating period of 290 nm and a dielectric cavity thickness of 180 nm. It can be seen that fine adjustment of the wavelength can also be achieved by appropriately choosing the spacing W.

[0061] The selection of the period P of repetition of the patterns of the metal grating can be arbitrary provided that this period is of a dimension smaller than the wavelength (in other words, the period P must be strictly smaller than the smallest wavelength to be filtered). The spacing Wi between the patterns can be optimized separately once the thickness Ti of the cavity is determined. [Fig.l 1] shows another example of spectral response of a multispectral filter according to the invention with a period P of 260 nm and cavity thicknesses Ti predetermined to filter 3 color channels (here specifically blue, green and red). The spacing Wi between the patterns of each metal grating corresponding to a color has been optimized separately to obtain the highest possible transmission while having a narrow bandwidth (still of the order of 25 nm) for each of the wavelengths.

[0062] The dimensions of the multispectral filter according to the invention simulated in [Fig. 11] are given in the table below for each color filter Fi: Fl F2 F3 P(nm) 260 260 260 Ti (nm) 90 140 180 Wi (nm) 70 130 70 X (nm) 450 555 650 LMH (nm) ~25 ~25 ~25

[0063] It will be noted in passing that the deviations WI and W3 are identical for different wavelengths of the respective color filters F1 and F2.

[0064] [Fig. 12] shows the different steps of a method 200 for manufacturing a multispectral filter according to the invention.

[0065] The first step 201 of the method 200 consists of starting from a substrate 300 which can for example be an Si substrate, a Silicon on Insulator SOI substrate (for “Silicon On Insulator” according to the English terminology) or a glass or sapphire substrate.

[0066] Step 201 is followed by a step 202 consisting of depositing a layer of resin 301 on the substrate 300.

[0067] The method 200 then comprises a step 203 of structuring the resin layer 301 aimed at creating spaces 302i by lithography by removing material in the resin layer 301: the spaces 302i are intended to receive the metal patterns of the metal network of each color filter. According to the invention, it is appropriate to have at least two series of spaces corresponding to at least two color filters. Here, for the sake of illustration only, two series of spaces 3021 and 3022 are shown, each corresponding to a color filter. This step thus makes it possible to fix not only the repetition period P in each metal network (corresponding to the width of the resin pattern of each series to which the width of a space in the series is added) but also the gap Wi (corresponding to the width of a pattern) between each pattern of a metal network.

[0068] The method 200 then comprises a step 204 of depositing metal both in the spaces 302i but also above the remaining resin zones of the layer 301. This deposition makes it possible to form the metal patterns 303i in the spaces 302i for each of the metal networks corresponding to a color filter. The metal can be deposited in a conformal manner by chemical vapor deposition (CVD) or physical vapor deposition (PVD). The thickness of the deposited metal is for example of the order of 60 nm.

[0069] The metal deposition step is followed by a step 205 of removing the metal remaining on the resin and the remaining resin in order to retain only the metal patterns 303i.

[0070] The method 200 according to the invention then comprises a step 206 of depositing a layer of dielectric material 304 intended to form the dielectric patterns of each of the color filters of the filter according to the invention. This layer 304 is optionally planarized by an etch-back type etching step and / or a CMP (Chemical Mechanical Polishing) step. The material of the layer 304 is preferably but not limited to a transparent material in the visible range such as an organic material of the polymer or inorganic type (oxide, silicon nitride, alumina, etc.).The deposit is for example a deposit made by a technique of physical vapor deposition PVD (Physical Vapor Deposition) or chemical vapor deposition CVD (Chemical Vapor Deposition in English) or low pressure chemical vapor deposition LPCVD (Low Pressure Chemical Vapor Deposition in English) or plasma-assisted chemical vapor deposition PECVD (Plasma . Enhanced Chemical Vapor Deposition in English). It should be noted that the invention is not limited to the visible range and that other materials transparent to other wavelengths, in the infrared for example (for example using silicon), can be used. It will be noted that the dielectric material fills the spaces between the metal patterns 303i. The thickness of the dielectric layer 304 (if applicable after planarization) is chosen to correspond to the height of the thickest dielectric cavity (here designated by the thickness T2) for all the color filters.

[0071] Step 206 is followed by a step 207 of depositing a second layer of resin 305 on the dielectric layer 304.

[0072] According to step 208, the resin layer 305 is structured, for example by lithography, so as to leave visible the zone P of the dielectric layer 304 corresponding to the dielectric pattern of the first color filter to be formed.

[0073] Step 208 is followed by a step 209 during which the dielectric layer in the zone P is etched to a given thickness T1 so as to obtain the desired thickness of the dielectric pattern 3061 of the Fabry-Pérot cavity. This etching can be a dry or wet etching.

[0074] Steps 207 to 209 may be repeated in the event that more than two color filters are to be obtained.

[0075] Step 210 then consists of removing the remaining resin 305 so as to release on the surface the dielectric pattern 3062 of Fabry-Pérot cavity of thickness T2.

[0076] Step 211 then consists of depositing a reflective metal layer 307, for example with a thickness of between 20 and 50 nm, covering the dielectric patterns 3061 and 3062 so as to form the continuous reflective layer of the color filters F1 and F2 of the filter according to the invention. The two color filters F1 and F2 are thus formed respectively by: • The networks of metallic patterns 3031 and 3032 of period P and respective gap W1 and W2; • Cavity dielectric patterns 3061 and 3062; • The continuous metallic reflective layer 307.

[0077] This continuous planar reflective layer 307 is deposited conformally by chemical vapor deposition (CVD) or physical vapor deposition (PVD).

[0078] At the end of the manufacturing process according to this first embodiment of the invention, a multispectral filter is obtained in accordance with that of [Fig.6] in which the metal networks are located on the lower part of the filter while the continuous reflective layers are located on the upper part of the filter.

[0079] We will see in the following that it is also possible to have a multi-filter spectral according to a second embodiment of the invention.

[0080] This filter 400 according to a second embodiment of the invention is shown schematically in [Fig. 13]. Its structure is identical to that of the filter according to the first embodiment of the invention shown in [Fig.6]. The only difference between the filter 400 and the filter 100 consists in reversing the position of the continuous metallic reflective layers 104i and the metallic gratings 102i; thus, in [Fig.13], the metallic gratings 102i are located on the upper part of the Fabry-Pérot cavities whereas they were in the lower part in the filter of [Fig.6]. For the sake of clarity, the reference numbers of the structural elements and dimensions are common to both filters 100 and 400.

[0081] [Fig. 14] shows the superposition of the spectral response of two color filters, one used in filter 100 of [Fig. 6] (solid line) and the other used in filter 400 of [Fig. 13] (dotted line). The dimensions chosen are the same for both color filters with a dielectric pattern thickness T of 180 nm, a repetition period P of the metal patterns of the grating of 260 nm and a spacing W between the metal patterns of the grating of 70 nm. It can be seen that the two spectral responses are superimposed: it is therefore possible to use the structure of filter 100 and filter 400 indifferently without modification of the spectral response.

[0082] [Fig. 15] shows the different steps of a method 500 for manufacturing a multispectral filter according to the second embodiment of the invention.

[0083] The first step 501 of the method 500 consists of starting from a substrate 600 which can for example be an Si substrate, a Silicon on Insulator SOI substrate (for “Silicon On Insulator” according to the English terminology) or a glass or sapphire substrate.

[0084] Step 501 is followed by a step 502 consisting of depositing a layer of resin 301 on the substrate 300.

[0085] According to step 503, the resin layer 601 is then structured. The structuring of the resin layer 601 is carried out by a lithography step. This lithography may preferably be a grayscale lithography according to the English electronic or optical terminology. Other lithography techniques such as two-photon lithography or nano-imprint lithography may also be used for producing the resin structure 601.

[0086] The structured resin layer 601 comprises a plurality of patterns 601Ai (here 4 patterns 601A1, 601A2, 601A3 and 601A4).

[0087] According to the invention, it is appropriate to have at least two resin patterns 601Ai of different heights. Among all of these patterns 601Ai, one of them or several, here the patterns 601 A1 and 601 A4, has a maximum height Hmax, called reference, the height being measured perpendicular to the plane of the substrate 600. More generally, the height of the pattern 601Ai will be noted hresist i. Thus, in [Fig. 15], Hmax is equal to hresist-l hre^Hl 4.

[0088] Step 503 is followed by a step 504 of depositing a reflective metal layer 602, for example with a thickness of between 20 and 50 nm, covering the patterns 601Ai of the structured sub-layer 401. This reflective layer 602 forms the first reflective layer of the future Fabry-Pérot cavity type color filters. The reflective layer 602 must at least continuously cover each upper surface of the patterns 601Ai, it being understood that it could also be deposited on the sides of the patterns 601Ai. This reflective layer 602 is for example deposited conformally by chemical vapor deposition (CVD) or physical vapor deposition (PVD). The conformal deposition makes it possible to have a constant thickness of the reflective layer 602, at least on the top of the patterns.

[0089] The next step, not shown, consists of depositing a layer made of the dielectric material intended to form the dielectric patterns of the Fabry-Pérot cavities. The layer of dielectric material covers all of the patterns 601Ai covered with the reflective layer 602. The layer has an upper surface (not necessarily flat) each point of which is located at a height, relative to the substrate 600, greater than the maximum reference height Hmax. The material of the layer is preferably, but not limited to, a transparent material in the visible range such as an organic material of the polymer or inorganic type (oxide, silicon nitride, alumina, etc.).The deposit is preferably a conformal deposit, for example carried out using a physical vapor deposition technique (PVD) or chemical vapor deposition (CVD) or low pressure chemical vapor deposition (LPCVD) or plasma-enhanced chemical vapor deposition (PECVD).

[0090] The method 500 then comprises a step 505 of planarization of the layer made in the dielectric material intended to form the dielectric patterns of the Fabry-Pérot cavities so as to form a surface-planarized layer 603 by removing the dielectric material from the previously deposited dielectric layer. The planarization is carried out with a stop on the reflective layer 602 located at its highest level (i.e. at the level of the reference height of the patterns 601A1 and 601A4).

[0091] The method 500 according to the invention then comprises a step 506 of conformal deposition of a layer 604 of the same dielectric material intended to form the dielectric patterns of the Fabry-Pérot cavities. This step is for example carried out by a PVD, CVD, LPCVD or PECVD deposition technique. The height of the layer 604 is noted hc, the height being measured perpendicular to the plane of the substrate 600. After In this step, a plurality of Fabry-Pérot cavity dielectric patterns 605Ai are obtained (here four patterns 605A1, 605A2, 605A3 and 605A4).

[0092] The height hdiel_i of the pattern 605Ai can be determined according to the methods men mentioned above. According to the method according to the invention, the height hdiel_i is fixed by the following formula: hdiel_i = Hmax - hresist i+ hc.

[0093] Thus, for patterns 605A1 and 605A4, hdiel_l and hdiel_4 are here directly equal to the height hc of layer 604.

[0094] More generally, for each 605Ai pattern, the height is technologically determined by the difference in height between that of the highest 601 Al resin pattern Hmax and the height of the 601Ai resin pattern to which the height of the second dielectric layer 604 is added.

[0095] Step 507 of the method 500 then consists of depositing a second metal layer 606, with a thickness preferably greater than that of the first metal layer 602, for example of the order of 60 nm, covering the Fabry-Pérot cavity patterns 605Ai. This planar and continuous metal layer 602 is intended to form the upper network of metal patterns of the Fabry-Pérot cavity type color filters. The metal layer 606 is deposited conformally by CVD or PVD deposition.

[0096] The method 500 then comprises a step 508 of structuring a resin layer aimed at creating spaces 607Ai by lithography by removing material in the resin layer.

[0097] The spaces 607Ai are intended to receive the metal patterns of the metal network of each color filter. According to the invention, it is appropriate to have at least two series of spaces corresponding to at least two color filters. Here, for the sake of illustration only, four series of spaces 607A1 to 607A4 are shown, each corresponding to a color filter. This step thus makes it possible to fix not only the repetition period P in each metal network (corresponding to the width of the resin pattern of each series to which the width of a space in the series is added) but also the gap Wi (corresponding to the width of a space in the series) between each pattern of a metal network.

[0098] Step 509 then consists of etching the metal of the metal layer 606 not protected by the resin to create the metal patterns 308i in for each of the metal networks corresponding to a color filter.

[0099] Step 510 consists of optionally depositing a planarized dielectric overlayer 609 above and between the continuous metal pattern networks.

Claims

Claims

1. Multispectral filter (100, 400) for electromagnetic radiation, said filter comprising at least two color filters (Fi), each color filter comprising: • a metal grating comprising metal patterns (102i) repeated according to a given period (P), each metal pattern being spaced from an adjacent metal pattern by a given non-zero gap (Wi); • a continuous reflective layer (104i); • a pattern (103i) of Fabry-Pérot cavity dielectric material between the metal grating and the continuous reflective layer; the thickness (Ti) of the dielectric material patterns of the two color filters being different.

2. Multispectral filter according to claim 1 characterized in that the repetition period of the metallic patterns is identical for the two color filters.

3. Multispectral filter according to one of the preceding claims, characterized in that the thickness of the metal network of the two color filters is identical.

4. Multispectral filter according to one of the preceding claims, characterized in that the thickness of the reflective layer of the two color filters is identical.

5. Multispectral filter according to one of the preceding claims, characterized in that each of the two filters faces a photoelectric transducer.

6. Multispectral filter according to one of the preceding claims characterized in that the space between each metallic pattern is filled with the dielectric material forming the Fabry-Pérot cavity dielectric material patterns of the color filters.

7. Multispectral filter according to one of the preceding claims, characterized in that the repetition period of the metallic patterns of each metallic network is chosen to be strictly less than the wavelength of the corresponding color filter.

8. A method (200) of manufacturing a multispectral filter for radiation electromagnetic according to one of the preceding claims, characterized in that it comprises the following steps: - Deposition (202) of a first layer of resin on a substrate; - Structuring (203) the first layer of resin so as to obtain at least two series of trenches in the first layer of resin, each trench of a series being spaced from an adjacent trench of the series by a given non-zero gap, the trenches being repeated according to a given period; - Deposition (204) of a layer of metal in the trenches and on the remaining resin; - Removing (205) the remaining resin and the metal on the remaining resin so as to retain two metal networks each comprising metal patterns repeated according to the given period of one of the series of trenches, each metal pattern being spaced from an adjacent metal pattern by the given non-zero gap of one of the series of trenches; - Deposition (206) of a planar layer of dielectric material intended to form at least two patterns of Fabry-Pérot cavity dielectric material, said layer having the thickness of the maximum Fabry-Pérot cavity dielectric pattern; - Deposition (207, 208) of a layer of resin on the layer of dielectric material and removal of said layer of resin with stopping on the dielectric layer at the level of the zone above one of the two metal networks; - Etching (209) of the dielectric layer at the resin removal zone so as to obtain a first dielectric Fabry-Pérot cavity pattern; - Removing (210) the remaining resin so as to leave visible a second Fabry-Pérot cavity dielectric pattern having the maximum dielectric pattern thickness, the thickness of the first dielectric pattern being less than the thickness of the first dielectric pattern; - Deposition (211) of a continuous metal layer on the first and second dielectric patterns.

9. Method (500) of manufacturing a multispectral filter for radiation electromagnetic according to one of claims 1 to 7 characterized in that it comprises the following steps: • Deposition (502) on a substrate of a layer of structuring material; • Structuring (503) of the layer of structuring material so as to obtain at least two patterns of structuring material of different heights, at least one of the patterns having a maximum reference height relative to the substrate, the height being measured perpendicular to the plane of the substrate; • Deposition (504) of a continuous metal layer on the at least two patterns of structuring material; • Deposition of a layer made in the dielectric material intended to form at least two dielectric patterns of Fabry-Pérot cavities, said layer of dielectric material covering all of the patterns of structuring material by presenting an upper surface of which each point is located at a height greater than the maximum reference height; • Planarization (505) by removal of the dielectric material with selective stopping at the top of the highest structuring material pattern covered with the continuous metal layer; • Deposition (506) of a layer made from the same dielectric material so as to finalize the formation of at least two Fabry-Pérot cavity dielectric patterns; • Deposition (507) of a second metallic layer on the at least two dielectric cavity patterns of the Fabry-Pérot cavity; • Structuring (508, 509) of the second metal layer so as to form at least two metal networks each comprising metal patterns repeated according to a given period, each metal pattern being spaced from an adjacent metal pattern by a given non-zero gap.

10. Manufacturing method (500) according to the preceding claim, characterized in that the structuring material is a resin, said structuring step being carried out by a grayscale lithography step on the layer of structuring material.

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