METHOD FOR PRODUCING AT LEAST ONE OPTICAL WINDOW, OPTICAL WINDOW AND ASSOCIATED INFRARED DETECTOR

Abstract

This method for fabricating at least one optical window for an infrared detector (10a) comprises the following steps: – using a substrate consisting of a silicon or germanium layer (25), at least one face (16, 17) of the substrate being coated with an oxide film (13), itself coated with a silicon or germanium film (14); – reactive ion etching of the silicon or germanium film (14) with a stop on the oxide film (13) so as to form the patterns of at least one periodic diffraction grating (21); and – etching of the oxide film (13) so as to remove the oxide film (13) present at the patterns of at least one periodic diffraction grating (21). Figure for the abstract: Fig 11

Description

Title of the invention: METHOD FOR PRODUCING AT LEAST ONE OPTICAL WINDOW, ASSOCIATED OPTICAL WINDOW AND INFRARED DETECTOR Field of invention

[0001] The invention relates to the field of infrared detection, and in particular detection using micro-bolometers. The invention relates to a method for producing at least one optical window, i.e. a mechanical part intended to be hermetically sealed on a base substrate to form an infrared detector. The invention also relates to the associated optical window and infrared detector.

[0002] An infrared sensor is traditionally composed of a matrix of sensitive elementary detection detectors. Thus, the elementary infrared detectors form the pixels of an infrared image.

[0003] More particularly, the invention aims to improve the spectral properties of the boxes containing the sensitive detection elements. State of the art

[0004] An elementary detector of an infrared sensor is conventionally in the form of a micro-bolometer mounted in suspension on a base substrate, and encapsulated under vacuum in a hermetic housing or volume. The hermetic housing is generally constituted by an optical window and side walls fixed between the base substrate and the optical window. Intrinsically, the untreated optical window has a transmittance wider than the wavelength range of interest. Given the atmospheric absorption conditions in the infrared spectrum and the black body law for objects around 30°C, a detector formed by an imager based on microbolometers and associated with a window, is for example designed to have the maximum sensitivity on the 8-14 micrometer spectral band, while being insensitive to the flux outside this spectral band.

[0005] To improve the performance of the elementary detector, this optical window is structured with respect to the micro-bolometer(s). Typically, this structuring is intended to attenuate wavelengths between 2 and 8 micrometers, and to form an anti-reflection filter for wavelengths of interest between 8 and 14 micrometers.

[0006] In addition, the performance of the elementary detector is also linked to the level of vacuum prevailing within the hermetic housing. To ensure thermal insulation between the micro-bolometer and the lower substrate, necessary to achieve the performance expected, the vacuum level in the hermetic enclosure must typically be less than 0.1 Pa, or 103 mbar.

[0007] To obtain an optical window with a limited number of defects and achievable at low cost using standard large-scale collective manufacturing techniques, document FR 2 985 576 proposes producing an optical window 110 whose external face 17 is covered with an interference multilayer filter 20, and whose internal face 16 is provided with a periodic diffraction grating 210, as illustrated in [Fig.l] of the prior art.

[0008] It is therefore possible to use cooperation between the interference multilayer filter and the periodic diffraction grating to filter wavelengths between 2 and 8 micrometers.

[0009] For example, the multi-layer interference filter 20 may be designed to attenuate the overall optical transmission of the optical window 110 over a first wavelength range between 2 and 8 micrometers, for example over the wavelength range between 2 and 5 micrometers. Unlike filters which are made to filter wavelengths over the entire wavelength range between 2 and 8 micrometers, the reduced width of this range makes it possible in particular to provide an interference filter having a limited number of layers, and consequently a limited number and size of defects compared to an interference filter designed to attenuate the transmission of the optical window 110 over the complete interval from 2 to 8 micrometers.

[0010] Furthermore, the periodic diffraction grating 210 may be designed to attenuate the overall optical transmission of the optical window 110 over a second wavelength range between 2 and 8 micrometers, for example over the wavelength range between 5 and 8 micrometers.

[0011] After forming the optical window 110, it is conventionally transferred onto a base substrate or onto a ceramic housing, on which micro-bolometers are mounted in suspension, so as to form an elementary detector. During this transfer, the optical window 110 is fixed onto the base substrate by means of a hermetic weld conventionally made between two layers of silicon by means of a weld bead made of gold-tin and heated to a temperature between 270 and 320°C.

[0012] With this technique, several optical windows 110 can be formed simultaneously on the same massive silicon or germanium wafer, the term “wafer” is better known by the Anglo-Saxon expression “wafer”.

[0013] However, a dispersion of the optical properties of the collectively formed optical windows is observed. More precisely, the production of the periodic diffraction grating 21 0 requires the implementation of photolithography and reactive ion etching, also known by the English acronym RIE for “Reactive Ion Etching”.

[0014] Conventionally, this RIE etching is carried out in an etching chamber using an appropriate reagent.

[0015] When collectively producing several periodic diffraction gratings 21 0 in connection with several optical windows 110, the etching depth of the solid substrate 12 0 is determined as a function of the duration of the RIE etching, according to a method called “time etching”. Indeed, the speed of an RIE etching is a parameter known to a person skilled in the art. This speed depends on the nature of the substrate 12 0 and the nature of the reagent used for the RIE etching.

[0016] The desired etching depth is obtained by the product of the etching speed and the duration of application of the reagent. However, the disparity of the reagent within the etching chamber, i.e. the variation in the quantity of reagent per unit area in the etching chamber, is the cause of non-uniformities in the etching depth, as illustrated schematically in [Fig. 2] of the prior art. It follows that certain periodic diffraction gratings 21 0 are etched with a depth P1 much greater than other periodic diffraction gratings 21 0 etched with a depth P2. This phenomenon is explained in particular by edge-center effects. In practice, differences in etching depth P1-P2 of the order of 15% have been observed on 12 0 substrates of 200 millimeters in diameter.This problem of uniformity on the engraving depth obtained during the formation of a diffraction grating is implicitly a limitation of the solution described in document FR 2 985 576.

[0017] When the optical windows 110 are separated, as illustrated in [Fig. 3], this non-uniformity of the periodic diffraction gratings 210 is so impactful that it results in a disparity in the transmission qualities of the optical windows 110. Indeed, it has been evaluated that the differences in engraving depth P1-P2 lead to differences in the transmittances within a batch of optical windows 110 of the order of 17.5%.

[0018] It follows that the performance of an infrared detector, mainly the sensitivity to afterglow phenomena linked to variable solar glare, varies from one detector to another due to the differences in transmittance of the optical windows.

[0019] The invention therefore aims to produce optical windows having uniform transmittance performances and which can also be produced on a large scale. Presentation of the invention

[0020] For this purpose, the invention proposes using a specific substrate to form an optical window comprising at least one oxide film, for example silicon oxide, intended to constitute a barrier layer for reactive ion etching of the patterns of the periodic diffraction gratings. This oxide film is then partially etched.

[0021] According to the invention, the substrate consists of a layer of silicon or germanium, topped by an oxide film, itself topped by a film of silicon or germanium. Thus, at least one of the two faces of the substrate has a film of silicon or germanium fixed on an insulator film. Considering a silicon substrate, this type of substrate is known by the acronym "SOI", for silicon on insulator or "Silicon-on-Insulator" in the English literature.

[0022] Furthermore, according to a variant, the two faces of the substrate intended to form at least one optical window are provided with a silicon or germanium film fixed on an oxide film.

[0023] For the purposes of the invention, the concepts of “film” and “layer” are distinguished due to the differences in respective thicknesses. For example, for an SOI substrate, the silicon layer may have a thickness of 300 nanometers, while the oxide film and the silicon film have thicknesses of the order of 10 to 100 nanometers.

[0024] According to a first aspect, the invention relates to a method for producing at least one optical window for an infrared detector, comprising the following steps: - use of a substrate consisting of a layer of silicon or germanium, at least one face of the substrate being coated with an oxide film, itself coated with a fixed film of silicon or germanium; - reactive ion etching of the silicon or germanium film with stopping of the etching on the oxide film so as to form the patterns of at least one periodic diffraction grating; and - etching the oxide film so as to remove the oxide film present at the level of the patterns of the at least one periodic diffraction grating.

[0025] Unlike the state of the art which provides for using a solid silicon or germanium substrate to form a periodic diffraction grating, the invention thus proposes to implement an oxide film intended to constitute a stop layer for etching the patterns of the periodic diffraction grating. It follows that the invention makes it possible to guarantee, due to the definition of a fixed reference frame, the regularity of the depth of the patterns of the periodic gratings thus formed.

[0026] The invention is particularly counter-intuitive because it is known that an oxide layer or film degrades the transmittance performance. However, the use of an oxide film, to stop the reactive ion etching of a silicon or germanium film in which the patterns of the periodic diffraction grating are formed, requires retaining at least part of the oxide film to ensure the mechanical attachment of the silicon or germanium film to the silicon or germanium layer that forms the substrate. Thus, the presence of this oxide film outside the patterns of the periodic diffraction grating degrades the transmittance performance of the optical windows.

[0027] This degradation in the transmittance performance of the optical windows is counterbalanced by obtaining uniform transmittance performance.

[0028] Preferably, the etching of the oxide film is isotropic and partial in nature. In this embodiment, against all expectations, the windows thus obtained have a transmittance greater than or identical to the transmittance of a window of the state of the art, in addition to benefiting from improved uniformity over the etching depth obtained.

[0029] Indeed, isotropic etching has the properties of etching a portion of the oxide film present outside the patterns, thus limiting the degradation of the transmittance performance of the optical windows. However, it is only necessary to partially etch the oxide film by isotropic etching to guarantee the mechanical strength of the patterns made in the silicon or germanium film. For example, under real conditions, an etching time of less than 5 minutes makes it possible to obtain isotropic and partial etching of the oxide film.

[0030] According to one embodiment, a plurality of periodic diffraction gratings of several optical windows are produced simultaneously, the method ultimately comprising a step of separating the optical windows.

[0031] Furthermore, to limit the degradation of performance due to the presence of the oxide film outside the patterns of the periodic diffraction grating, it is preferable to use a very thin oxide film, i.e. with a thickness of between 10 and 100 nanometers.

[0032] On the contrary, the patterns defining the periodic diffraction grating must be sufficiently thick to guarantee the optical properties of said grating. Thus, the silicon or germanium film preferably has a thickness of between 1 and 3 micrometers.

[0033] Alternatively, to increase the thickness of the patterns of the periodic diffraction grating with a substrate in which the silicon or germanium film is not thick enough, it is possible to carry out epitaxy on the silicon or germanium film.

[0034] Thus, according to one embodiment, the method comprises a step of epitaxy of the silicon or germanium film, carried out before the production of the patterns of the periodic diffraction grating.

[0035] With regard to stopping the reactive ion etching on the oxide film, it is preferable to use an oxide film with high selectivity compared to the reactive ion etching used to produce the patterns of the periodic diffraction grating.

[0036] Typically, reactive ion etching conventionally uses reactive gases which are fluorinated or chlorinated derivatives, such as sulfur hexafluoride in the presence of dioxygen. These reactive gases are known to efficiently etch silicon or germanium, but on the other hand prove less effective in attacking the oxide film.

[0037] Thus, preferably, the reactive gases used during the reactive ion etching are selected so that the etching process of the oxide film has a selectivity of at least 5 with respect to the silicon or germanium film.

[0038] For the purposes of the invention, selectivity means the ratio between the etching speeds of the same process on two types of target layers to be etched. For example, the etching speed in nanometers per minute is determined by bringing an oxide layer into contact with a sulfur hexafluoride SF6 plasma, then the etching speed in nanometers per minute is determined by bringing a silicon or germanium layer into contact with the same sulfur hexafluoride plasma.

[0039] To obtain this selectivity, the oxide film is preferably made of silicon dioxide SiO2. This embodiment makes it possible to obtain particularly effective selectivity for reactive ion etching.

[0040] Furthermore, the isotropic etching of such a film made of silicon dioxide can be easily carried out using hydrofluoric acid. This process is very selective and it makes it possible to guarantee the physical integrity of the upper face of a silicon substrate, while efficiently etching the silicon dioxide SiO2.

[0041] To form the complete optical window, it is possible to produce an interference multilayer filter on one face of the substrate and the periodic diffraction grating on the other face. The cooperation of the interference multilayer filter and the periodic diffraction grating thus makes it possible to attenuate the incident radiation in a wavelength range between 2 and 8 micrometers.

[0042] To do this, the interference multilayer filter and the periodic diffraction grating can be shaped to each filter the incident radiation between 2 and 8 micrometers.

[0043] Alternatively, the multilayer interference filter and the periodic diffraction grating can be shaped to perform two separate filterings of the incident radiation: a first filtering between 2 and x micrometers, and a second filtering between y and 8 micrometers.

[0044] Considering x and y equal to 5, the interference multilayer filter attenuates the transmission of the optical window over a first range of wavelengths between 2 and 5 micrometers, and the periodic diffraction grating attenuates the transmission of the optical window over a second wavelength range between 5 and 8 micrometers.

[0045] According to another example, considering x equal to 6 and y equal to 4, the interference multilayer filter attenuates the transmission of the optical window over a first range of wavelengths between 2 and 6 micrometers, and the periodic diffraction grating attenuates the transmission of the optical window over a second range of wavelengths between 4 and 8 micrometers.

[0046] To obtain this cooperation of the interference multilayer filter with the periodic diffraction grating over the entire wavelength range between 2 and 8 micrometers, it is necessary that x be between 2 and 8 micrometers, and that y be less than or equal to x.

[0047] Furthermore, it is also possible to replace the multi-layer interference filter with a second periodic diffraction grating.

[0048] In this embodiment, in which the substrate consists of a layer of silicon or germanium, each face of the substrate being provided with a film of silicon or germanium fixed on an oxide film, the method comprises the following steps: - reactive ion etching of the silicon or germanium film with stopping on the oxide film so as to form the patterns of a first periodic diffraction grating on a first face of the substrate; - reactive ion etching of the silicon or germanium film with stopping on the oxide film so as to form the patterns of a second periodic diffraction grating on a second face of the substrate; - etching the oxide film so as to remove the oxide film present at the patterns of the first periodic diffraction grating of the first face of the substrate; and - etching the oxide film so as to remove the oxide film present at the patterns of the second periodic diffraction grating of the second face of the substrate.

[0049] Preferably, the two etchings of the oxide film are of an isotropic and partial nature.

[0050] The two reactive ion etchings and the two isotropic etchings can be carried out successively or simultaneously.

[0051] The periodic diffraction grating may have a hexagonal or square mesh. Preferably, at least one periodic diffraction grating has an Archimedean or Penrose grating. Indeed, a hexagonal periodic diffraction grating induces an average attenuation of 80% over the 5-8 micrometer range, while a grating having an Archimedean tiling allows an average attenuation greater than 90%, or even 95%, over this same range. An advantageous high-order symmetry can be obtained if the grating comprises a “quasicrystal” type mesh. For example, at least one periodic diffraction grating can be made up of a two-dimensional pattern network derived from Penrose tilings. These tilings have periodically repeated patterns, whose symmetry is of order 5 or 10. Such gratings can be produced, for example, according to the method described in the publication: “Construction of plane tilings by the multi-grid method”, Denis Gratias, LEM-CNRS / ONERA, 2002.

[0052] Alternatively, at least one periodic diffraction grating may consist of a grating of so-called Archimedean patterns, repeated periodically so as to form larger patterns, the symmetry of which is of order 12. In particular, a periodic diffraction grating of patterns derived from an Archimedean tiling makes it possible to obtain a transition of small width between the attenuated range and the transmitted range, namely a width of approximately 2 micrometers for a cutoff around 8 micrometers. Conversely, the cutoff of a square mesh grating will extend over a larger spectral band, typically a width of approximately 2.5 micrometers for a cutoff around 8 micrometers. Preferably, at least one periodic diffraction grating comprises pads having a period, a diameter and a height of between 1 and 2.5 micrometers.

[0053] The invention thus makes it possible to produce an optical window having improved transmittance performance and which can be produced in mass.

[0054] According to a second aspect, the invention relates to an optical window comprising: - a substrate consisting of a layer of silicon or germanium; - a periodic diffraction grating arranged on a first face of the substrate and having silicon or germanium patterns; and - a multi-layer interference filter or a periodic diffraction grating arranged on a second face of the substrate; the periodic diffraction grating(s) being fixed to the substrate by means of an oxide film), said oxide film being absent directly above the openings forming the patterns of the periodic diffraction grating(s).

[0055] According to a third aspect, the invention relates to an infrared detector comprising: - a base substrate on which at least one micro-bolometer is mounted in suspension; - side walls extending substantially perpendicularly relative to the base substrate; and - an optical window, of the type previously mentioned, fixed on an upper end of the side walls. Brief description of the figures

[0056] The invention will be better understood on reading the following description, given solely by way of example, and carried out in relation to the appended drawings, in which identical references designate identical or similar elements, and in which:

[0057] [Fig. 1] schematically illustrates a state-of-the-art optical window;

[0058] [Fig.2] schematically illustrates a first stage of realization of several state-of-the-art optical windows;

[0059] [Fig.3] schematically illustrates a second step in the production of several optical windows of the state of the art;

[0060] [Fig.4] schematically illustrates a first step in producing an optical window according to a first embodiment of the invention;

[0061] [Fig.5] schematically illustrates a second step of producing an optical window according to the first embodiment of the invention;

[0062] [Fig.6] schematically illustrates a third step of producing an optical window according to the first embodiment of the invention;

[0063] [Fig.7] schematically illustrates a fourth step of producing an optical window according to the first embodiment of the invention;

[0064] [Fig.8] schematically illustrates a first step in producing an optical window according to a second embodiment of the invention;

[0065] [Fig.9] schematically illustrates a second step of producing an optical window according to the second embodiment of the invention;

[0066] [Fig. 10] schematically illustrates a third step of producing an optical window according to the second embodiment of the invention;

[0067] [Fig. 11] schematically illustrates an infrared detector incorporating an optical window according to the first embodiment of [Fig.7]; and

[0068] [Fig. 12] schematically illustrates an infrared detector integrating an optical window according to the second embodiment of [Fig. 10]. Detailed description of the invention

[0069] Figures 4 to 7 and 8 to 10 illustrate two distinct embodiments of an optical window 24a, 24b.

[0070] For these two embodiments, the optical window 24a, 24b contributes to the production of an infrared detector 10a, 10b. Depending on the type of infrared detector 10a, 10b, the optical window 24a, 24b may be intended to cover one or more elementary detectors, for example micro-bolometers 19. Indeed, it is possible to form the optical window 24a, 24b to cover a single micro-bolometer 19 in a chip-to-substrate implantation, also known as “chip to wafer”, or to use the optical window 24a, 24b as a substrate cover in a substrate-to-substrate implantation, also known as "wafer to wafer".

[0071] Furthermore, it is also possible to integrate the elementary detector 19 into a vacuum-sealed housing, for example a ceramic housing, by fixing the optical window 24a, 24b above the elementary detector 19. The fixing of the elementary detector 19 and the optical window 24a, 24b in a housing can be carried out by soldering or sealing.

[0072] Whatever the type of implantation envisaged, the optical window 24a, 24b has at least one face 16, 17 structured with a periodic diffraction grating 21.

[0073] Preferably, the periodic diffraction grating 21 is circumscribed in a transmission zone intended to come opposite a micro-bolometer 19. Thus, for a substrate-on-substrate implantation, the optical window has several juxtaposed transmission zones, and each transmission zone is intended to come opposite a micro-bolometer 19 arranged in a matrix manner on a base substrate 18.

[0074] As described in document FR 2 985 576, the periodic diffraction grating 21 may be formed by pads or holes extending perpendicularly from a face 16, 17 of a substrate 11, 12. For example, the periodic diffraction grating 21 may have an Archimedean or Penrose grating. Indeed, a hexagonal periodic diffraction grating induces an average attenuation of 80% over the 5-8 micrometer range, whereas a grating having an Archimedean tiling allows an average attenuation greater than 90%, or even 95%, over this same range. An advantageous high-order symmetry may be obtained if the grating comprises a mesh of the “quasi-crystal” type. For example, the periodic diffraction grating 21 may correspond to a network of two-dimensional patterns derived from Penrose tilings. It follows that the periodic diffraction grating 21 can exhibit effective attenuation in the wavelength range between y and 8 micrometers.

[0075] According to the invention, this periodic diffraction grating 21 is obtained by reactive ion etching with stopping on an oxide film 13. To do this, the substrate 11, 12 used to form the periodic diffraction grating 21 has a particular structure.

[0076] According to a first embodiment, as illustrated in [Fig. 4], the substrate 11 used to produce the optical window 24a comprises a layer of silicon or germanium 25 whose upper face 16 is coated with an oxide film 13, itself coated with a film of silicon or germanium 14.

[0077] The silicon or germanium layer 25 may have a thickness of between 3 and 10 micrometers while the oxide film 13 and the silicon or germanium film 14 may have a thickness of between 0.1 and 3 micrometers. For example, the oxide film 13 may be made of silicon dioxide with a thickness of 10 to 100 nanometers, while the silicon or germanium film 14 may have a thickness of between 1 and 3 micrometers. The manufacturing process of the substrate 11 may correspond to the “Smart-Cut” technology of the company Soitec, described in document WO 2018 / 137937.

[0078] Preferably, the reactive gases used during the reactive ion etching are selected so that the etching process has a selectivity of at least five with respect to the silicon or germanium 14 film.

[0079] If the thickness of the silicon or germanium film 14 of the substrate 11 used does not correspond to the thickness sought to form an effective periodic diffraction grating 21, it is possible to implement an epitaxy step to obtain growth of the silicon or germanium film 14.

[0080] For example, it is possible to obtain an SOI 11 substrate with a 20 nanometer silicon dioxide 13 film and a silicon 14 film with a thickness of between 100 nanometers and 1.5 micrometers and to carry out an epitaxy step to obtain a 2 micrometer silicon 14 film.

[0081] [Fig.5] illustrates a second step of the first embodiment, consisting of producing an interference multilayer filter 20 on a lower face 17 of the substrate 11. For example, this interference multilayer filter 20 can be obtained by producing successive deposits of layers of zinc sulfide and germanium.

[0082] The upper face 16 of the substrate 11 is then structured, as illustrated in [Fig.6], so as to form the periodic diffraction grating 21 by means of reactive ion etching with stopping on the oxide film 13. To do this, a resin is previously deposited and structured by photolithography to define the shape of the periodic diffraction grating.

[0083] Reactive gases are then used during reactive ion etching to form the patterns of the periodic diffraction grating 21. After reactive ion etching, the resist is removed, for example by etching with an oxygen, ammonia, hydrogen fluoride or acetone plasma.

[0084] An isotropic etching is then implemented to remove the oxide film 13 present at the patterns of the periodic diffraction grating 21, as illustrated in [Fig. 7]. This isotropic etching therefore makes it possible to remove the oxide film 13 present in the bottom of the openings forming the patterns of the periodic diffraction grating 21. In addition, as illustrated in [Fig. 7], a portion of the oxide film 13 present under the patterns of the periodic diffraction grating 21 can also be etched by the isotropic etching.

[0085] The optical window 24a thus formed therefore has an upper face 16 provided with a periodic diffraction grating 21 whose patterns are fixed on the substrate 11 by means of an oxide film 13 having openings at the level of the patterns of the grating. periodic diffraction 21. Furthermore, the lower face 17 of the optical window 24a has an interference multilayer filter 20.

[0086] This optical window 24a can be produced individually or simultaneously with other similar optical windows 24a. To do this, the method also comprises a step of separating the collectively formed optical windows 24a.

[0087] In the second embodiment of the optical window 24b, two periodic diffraction gratings 21 are formed on each face 16, 17 of the substrate 12. To do this, as illustrated in [Fig.8], the substrate 12 used to produce the optical window 24b comprises a layer of silicon or germanium 25, the upper face 16 of which is coated with an oxide film 13, itself coated with a silicon or germanium film 14. Unlike the first embodiment, the lower face 17 is also coated with an oxide film 13, itself coated with a silicon or germanium film 14.

[0088] The silicon or germanium layer 25 as well as the oxide film 13 and the silicon or germanium film 14 may correspond to those described with reference to the first embodiment.

[0089] In this second embodiment, the two faces 16, 17 of the substrate 12 are structured to form two periodic diffraction gratings 21, as illustrated in [Fig. 9]. To do this, two reactive ion etchings with stopping on the corresponding oxide film 13 can be implemented, using the method described with reference to the first embodiment.

[0090] Then, an isotropic etching is implemented to remove the oxide film 13 at the level of the patterns of each periodic diffraction grating 21 of each face 16, 17 of the substrate 12, as illustrated in [Fig. 10].

[0091] The optical window 24b thus formed therefore comprises an upper face 16 having a periodic diffraction grating 21 whose patterns are fixed on the substrate 11 by means of an oxide film 13 having openings at the level of the patterns of the periodic diffraction grating 21. In addition, the lower face 17 of the optical window 24b also has a periodic diffraction grating 21 whose patterns are fixed on the substrate 11 by means of an oxide film 13 having openings at the level of the patterns of the periodic diffraction grating 21.

[0092] This optical window 24b can be produced individually or simultaneously with other similar optical windows 24b. To do this, the method also comprises a step of separating the collectively formed optical windows 24b.

[0093] After forming the optical window 24a or 24b, it can be transferred onto a base substrate 18, on which a micro-bolometer 19 is mounted in suspension, as illustrated in Figures 11 and 12. During this transfer, the optical window can be turned over, as illustrated in [Fig. 11] in which the upper face 16 of the substrate 11 is fixed in contact with an upper surface of walls 27 rising perpendicularly relative to the base substrate 18.

[0094] To fix the substrate 11, 12 to the walls 27, a hermetic weld is conventionally carried out by heating a weld bead 29 to a temperature between 270 and 320°C in an environment under controlled pressure so as to guarantee the required vacuum level in the cavity formed around the micro-bolometer 19.

[0095] Alternatively, several elementary detectors can be formed simultaneously on the same substrate with a step consisting of cutting the base substrate 18 and the optical window 24a, 24b to separate the different elementary detectors thus formed.

[0096] In conclusion, the invention makes it possible to obtain an optical window 24a, 24b having uniform transmittance performances and which can also be produced on a large scale.

[0097] The invention finds a particularly advantageous application for elementary detectors in the infrared range, such as micro-bolometers. The invention can also be applied to cooled detectors or photodiodes based on mercury-cadmium telluride, indium-galium-arsenide or germanium.

Claims

Claims

1. Method for producing at least one optical window (24a, 24b) for an infrared detector (10a, 10b), obtained by transferring the optical window (24a, 24b) onto a base substrate (18), comprising the following steps: - use of a substrate (11, 12) consisting of a layer of silicon or germanium (25), at least one face (16, 17) of the substrate (11, 12) being coated with an oxide film (13), itself coated with a film of silicon or germanium (14); - reactive ion etching of the film of silicon or germanium (14) with stopping on the oxide film (13) so as to form the patterns of at least one periodic diffraction grating (21); and - etching the oxide film (13) so as to remove the oxide film (13) present at the patterns of the at least one periodic diffraction grating (21).

2. Method for producing at least one optical window for an infrared detector according to claim 1, in which the etching of the oxide film (13) is of an isotropic and partial nature.

3. Method for producing at least one optical window for an infrared detector according to claim 1 or 2, in which several patterns of several periodic diffraction gratings (21) of several optical windows (24a, 24b) are produced simultaneously, the method comprising a step of separating the optical windows.

4. Method for producing at least one optical window for an infrared detector according to one of claims 1 to 3, in which the oxide film (13) has a thickness of between 10 and 100 nanometers.

5. Method for producing at least one optical window for an infrared detector according to one of claims 1 to 4, in which the silicon or germanium film (14) has a thickness of between 1 and 3 micrometers.

6. Method for producing at least one optical window for an infrared detector according to one of claims 1 to 5, in which the method comprises a step of epitaxy of the silicon or germanium film (14), carried out before producing the patterns of the periodic diffraction grating (21).

7. Method for producing at least one optical window for an infrared detector according to one of claims 1 to 6, in which the reactive gases used during the reactive ion etching are selected so that the etching method has a selectivity of at least five with respect to the silicon or germanium film (14).

8. Method for producing at least one optical window for an infrared detector according to one of claims 1 to 7, in which the oxide film (13) consists of silicon dioxide.

9. Method for producing at least one optical window for an infrared detector according to one of claims 1 to 8, in which each face (16, 17) of the substrate (12) is provided with a silicon or germanium film (14) fixed on an oxide film (13), the method comprising the following steps: - reactive ion etching of the silicon or germanium film (14) with stopping on the oxide film (13) so as to form the patterns of a first periodic diffraction grating (21) on a first face (16, 17) of the substrate (12); - reactive ion etching of the silicon or germanium film (14) with stopping on the oxide film (13) so as to form the patterns of a second periodic diffraction grating (21) on a second face (16, 17) of the substrate (12); - etching the oxide film (13) so as to remove the oxide film (13) present at the level of the patterns of the first periodic diffraction grating (21) of the first face (16, 17) of the substrate (12);and - etching the oxide film (13) so as to remove the oxide film (13) present at the level of the patterns of the second periodic diffraction grating (21) of the second face (16, 17) of the substrate (12).;

10. Method for producing at least one optical window for an infrared detector according to claim 9, in which the two etchings of the oxide film (13) are of an isotropic and partial nature.

11. Method for producing at least one optical window for an infrared detector according to claim 9 or 10, in which the two reactive ion etchings are carried out simultaneously so as to form the patterns of the first and second periodic diffraction gratings (21).

12. Method of producing at least one optical window for an infrared detector according to claim 10, in which the two etchings isotropic are carried out simultaneously so as to remove the oxide film (13) present at the level of the patterns of the first and second periodic diffraction gratings (21).

13. Optical window for infrared detector obtained by transferring the optical window (24a, 24b) onto a base substrate (18), comprising: - a substrate (11, 12) consisting of a layer of silicon or germanium; - a periodic diffraction grating (21) arranged on a first face (16, 17) of the substrate (11, 12) and having silicon or germanium patterns; and - an interference multilayer filter (20) or a periodic diffraction grating (21) arranged on a second face (16, 17) of the substrate (11, 12); characterized in that the periodic diffraction grating(s) (21) is (are) fixed to the substrate (11, 12) by means of an oxide film (13), said oxide film (13) being absent directly above the openings forming the patterns of the periodic diffraction grating(s) (21).

14. Infrared detector 10a, 10b) comprising: - a base substrate (18) on which at least one micro-bolometer (19) is mounted in suspension; - side walls (27) extending substantially perpendicularly relative to the base substrate (18); and - an optical window (24a, 24b), according to claim 13, fixed on an upper end of the side walls (27).

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