Microchannel plate, method for producing same, and corresponding image intensifier tube and night vision system

Abstract

The invention relates to a microchannel plate (18) comprising a body (39) provided with a plurality of tubular microchannels (25), the axes of revolution of which are parallel to one another, and also comprising a functionalisation layer (33) present on an inner surface of every microchannel (25). According to the invention, the material constituting the body (39) is pure silica.

Description

[0001] MICROCHANNEL DISC, PRODUCTION METHOD, IMAGE INTENSIFIER TUBE AND ASSOCIATED NIGHT VISION SYSTEM

[0002] FIELD OF INVENTION

[0003] The invention relates to the field of microchannel wafers, particularly for the amplification and detection of low-intensity signals. More specifically, the invention concerns a microchannel wafer and its manufacturing process, said wafer being intended for use as an electron multiplier for various applications, notably in the fields of night vision and scientific research.

[0004] The invention also relates to an image intensifier tube and a night vision system implementing such a microchannel wafer.

[0005] In the field of night vision, microchannel wafers, used as electron multipliers, make it possible to amplify the brightness of an observed scene, such wafers being implemented within an image intensifier tube.

[0006] In the field of scientific research, the unique properties of microchannel wafers offer significant advantages for a variety of applications requiring the detection and / or amplification of signals from diverse particles, such as ions, neutrons, alpha particles, or high-energy photons (UV and X-ray photons). For example, microchannel wafers are used in mass spectrometry, multispectral and hyperspectral imaging, electron spectroscopy and microscopy, X-ray photoelectron spectroscopy, and nuclear instrumentation.

[0007] This invention therefore presents a multitude of potential applications, particularly when there are problems with the use of potentially toxic materials.

[0008] For example, the invention finds a particularly advantageous application when it is sought to limit the use of lead in electronic and electrical equipment in order to contribute to the protection of human health, the recovery, and the environmentally sound disposal of this waste. PRINCIPAL OF THE TECHNOLOGY

[0009] As schematically illustrated in Figures 1 and 2 of the prior art, relating to the use of a microchannel wafer for a night vision application, an image intensifier tube 130 comprises at least three distinct elements: a photocathode 16, an electron multiplier 180, and a phosphorescent screen 20.

[0010] The photocathode 16 is in the form of a semi-transparent photosensitive layer receiving photons from the incident electromagnetic radiation, i.e. the photons transmitted by the lens 12. To do this, an entrance window 15 transmits the photons from the lens 12 onto the photocathode 16.

[0011] The photocathode 16 is generally made of a thin layer of metal or semiconductor bonded to a layer of glass or a light-transparent material. The material of the photocathode 16 is chosen according to its sensitivity to incident photons from the observed scene. The interaction of photons from the incident electromagnetic radiation of the observed scene with the photocathode 16 produces, through the photoelectric effect, the emission of electrons, called primary electrons 28 or photoelectrons.

[0012] The photocathode 16 is characterized by its quantum efficiency, also known as "quantum efficiency" and denoted QE in the English-language literature. This quantum efficiency, generally expressed as a percentage (μ%), represents the ratio of the number of photoelectrons, or primary electrons 28, emitted to the number of photons incident on the surface of the photocathode 16.

[0013] The primary electrons 28 are then subjected to a first electric field within a first acceleration zone 17, directing the primary electrons towards the electron multiplier 180. This first electric field is created by applying a voltage between the photocathode 16 and the electron multiplier 180, typically a voltage of the order of 50 to 500 volts to ensure a straight path for the electrons. This first electric field is conventionally configured to attract negative electrons generated by the photocathode 16 onto the electron multiplier 180. The electron multiplier 180, also called an electron amplifier, typically comprises a microchannel plate 25 covered by electrodes. This microchannel plate is also known by the acronym GMC or MCP for "microchannel plate" in the English-language literature.It is made in a body of resistive or dielectric material with a thickness typically between 0.15 and 1 millimeter.

[0014] As illustrated more precisely in Figure 3 of the prior art, the microchannels 25 pass completely through the body of the electron multiplier wafer 180 and are parallel to each other. They have a diameter between 3 and 12 micrometers. Their inner wall is conventionally treated by chemical reduction when the wafer is made of lead glass in order to achieve hopping conduction, known as "hopping-conduction" in the English-language literature, or with a functionalization layer when another material is used to constitute the electron multiplier wafer 180.

[0015] These microchannels 25 have an axis of revolution "2 inclined at an angle "3 of a few degrees, typically between 4 and 12 degrees, relative to the normal of the surface of the electron multiplier 180, so as to induce multiple collisions of the primary electrons 28 in the microchannels 25.

[0016] In addition to the first electric field applied between the photocathode 16 and the electron multiplier 180, a second electric field is applied between the two faces of the electron multiplier 180 by means of electrodes placed on either side of the microchannel wafer 25. This second electric field induces an electric current in each microchannel 25 of the electron multiplier 180. More precisely, the current is generated in a conduction layer located on the inner surface of the microchannels 25. This inner surface of the microchannels 25 is sized to maintain a constant number of electrons inside the microchannels 25.

[0017] With this second electric field, during the multiple collisions between the photoelectrons and the material constituting the microchannels 25, a greater number of secondary electrons are generated by cascade effect. These secondary electrons are accelerated within the microchannels 25 to cause further collisions with the surface of the microchannels 25, generating additional secondary electrons 29. These secondary electrons are then directed by this second electric field from the entrance of the microchannels 25 to the exit of the microchannels 25.

[0018] In more detail, the impact of a primary electron 28 with the surface of the microchannels 25 pulls electrons from the surface material of the microchannel 25, releasing secondary electrons 29 which are accelerated under the effect of the voltage between the input surface and the output surface of the electron multiplier 180.

[0019] These secondary electrons, once generated, leave a positive charge in the microchannel material 25 if this electron loss is not compensated. The conduction current within each microchannel 25 thus has the advantage of compensating, within the material and along the entire length of the microchannel 25, the electron loss due to the emission of secondary electrons 29.

[0020] Typically, the primary electrons 28 are multiplied by a factor between 10 3 and 10 6 in the electron multiplier 180. The conversion rate of primary electrons 28 into secondary electrons 29 corresponds to the electrical gain of the electron multiplier 180.

[0021] At the exit of the microchannels 25, these secondary electrons 29 are then moved linearly towards the phosphorescent screen 20, within a second acceleration zone 19, under the effect of a third electric field generated between the output of the electron multiplier 180 and the phosphorescent screen 20, typically an electric field generated by a voltage between 4 and 10 kV.

[0022] The phosphorescent screen 20 converts secondary electrons 29 into photons. It takes the form of a phosphorescent layer or a layer of a luminescent material deposited on a substrate, typically glass. The measured conversion rate of secondary electrons 29 into photons corresponds to the efficiency of the phosphorescent screen 20.

[0023] To generate the three electric fields, electronic components 22 are conventionally arranged around an internal vacuum chamber 24. In order to limit collisions between electrons and gas molecules, particularly air, the two acceleration zones 17 and 19 are conventionally placed under vacuum at a pressure in the ultra-high vacuum range, on the order of 10⁻¹⁰⁰⁴. 7 at 10' 10 mbar. To do this, the photocathode 16, the electron multiplier 180, and the phosphorescent screen 20 are encapsulated in the internal vacuum chamber 24.

[0024] The strict operating conditions of the image intensifier tube 130 are such that defects in any of its components can cause premature wear, leading to reduced image intensity. This intensity is measured by the light gain, defined as the product of four quantities: the number of incident photons entering the image intensifier tube 130, the quantum efficiency of the photocathode 16, the electrical gain of the electron multiplier 180, and the efficiency of the phosphor screen 20.

[0025] Classically, it is known that the main phenomena affecting the light gain of the image intensifier tube 130 are the degradation of the quantum efficiency of the photocathode 16 and the reduction of the electrical gain of the electron multiplier 180; the phosphorescent screen 20 exhibiting good stability during the use of the image intensifier tube 130.

[0026] It has been observed that the degradation of the quantum yield of the photocathode 16 is due to a generation of positive ions by the electron multiplier 180 within the image intensifier tube 130. More specifically, as illustrated in Figure 4, these positive ions 32, generally in the form of hydrons or cations of hydrogen, sodium or potassium, are generated following the interaction of the primary electrons 28 with the internal surface of the microchannels 25 of the electron multiplier 180.

[0027] Given the initial electric field designed to transmit negatively charged electrons from the photocathode 16 to the electron multiplier 180, these positive ions 32 are attracted to the photocathode 16 and travel up the image intensifier tube 130, causing, through collisions, the destruction of chemical bonds between the atoms on the surface of the photocathode 16. These multiple collisions of positive ions 32 degrade, over time, the quantum efficiency of the photocathode 16. The degradation of quantum efficiency is particularly evident in image intensifier tubes 130 using electron multipliers 180 made of lead glass 30. Indeed, to create a wafer of microchannels 25, it is known to fabricate initial fibers with a core soluble in an acid, and a sheath surrounding the core, said sheath being insoluble in the same acid.These first fibers, called primary fibers, are obtained by drawing up to a first desired diameter and then assembled to form a hexagonal preform composed of a large number of primary fibers.

[0028] Secondary fibers, derived from the first base material, are then preferentially obtained by stretching the hexagonal preform to a second desired diameter and then assembled again to create a second longline base material obtained after melting and cooling the secondary fibers together.

[0029] During these fiber melting and cooling stages, it is preferable to operate at low temperatures, between 400°C and 600°C, in order to avoid these production stages being too long or energy-intensive.

[0030] The second long, basic material is then cut transversely along a certain cutting plane.

[0031] Generally, the cutting plane forms an angle between 4° and 12° with a cross-sectional plane perpendicular to the length of the second base material. This cutting step allows for the production of multiple base wafers. These base wafers are then soaked in acid to dissolve the core, exposing the microchannels. Thus, after the core is dissolved, the base wafers form microchannel wafers whose core is formed by the fusion of the sheaths of each fiber involved in the fabrication process. These microchannel wafers will then be functionalized so that they can be operational and enable electron multiplication.

[0032] The selectivity of the core and cladding materials based on their solubility in the acid is essential for the fabrication of the microchannels 25. Indeed, the cladding must be insoluble in this acid so that quenching can dissolve only the core. Lead glass 30, which is a lead silicate, is an example of a particularly effective cladding material for forming these microchannel wafers 25, given its insolubility in a variety of acids. Furthermore, this lead glass 30 has the characteristic of being readily chemically reduced by dihydrogen, generating metallic islands of lead 31 on the internal surface of the microchannels 25, ranging in size from tens to tens of nanometers, as illustrated in Figure 4.

[0033] This chemical reduction capacity makes lead glass 30 a material of choice for the production of microchannel wafers 25, since this single reduction step allows both the production of the microchannels 25, and the functionalization of the internal surface of these microchannels 25, thus enabling them to multiply electrons by secondary emission without a further functionalization step.

[0034] Finally, lead-30 glass has a sufficiently low glass transition temperature, around 460°C, to be easily used in forming microchannel wafers, particularly during the melting and cooling stages of the fibers used to create the base wafers. Pure silica, on the other hand, has a much higher glass transition temperature, around 1500°K.

[0035] The main disadvantage of this type of electron multiplier 180 is that it allows hydrogen atoms to be trapped in the lead glass following the reduction of this glass by dihydrogen, these hydrogen atoms subsequently generating hydrogen cations 32 when the primary electrons 28 interact with the internal surface of the microchannels 25 in order to produce the secondary electrons 29, as illustrated in Figure 4.

[0036] On the other hand, sodium or potassium cations can also be emitted, as these are also likely to accumulate on the surface of the microchannels 25 during reduction. These hydrogen cations 32, or sodium or potassium cations, are thus responsible for the degradation of the photocathode 16, and therefore for reducing the lifespan of the image intensifier tube 130.

[0037] Prior art presents several solutions to prevent the generation of positive ions when the electron multiplier 180 is in operation, or to avoid the use of leaded glass in order to comply with the limitations on the use of hazardous materials according to the European RoHS directive, an acronym for "Restriction of Hazardous Substances" in the Anglo-Saxon literature.

[0038] This European RoHS directive aims to limit the use of ten hazardous substances, including lead, in electrical and electronic equipment, as specified in the following version of the RoHS directive: "Directive (EU) 2017 / 2102 of the European Parliament and of the Council of 15 November 2017 amending Directive 2011 / 65 / EU on the restriction of the use of certain hazardous substances in electrical and electronic equipment".

[0039] Only two prior art solutions are known for eliminating the need for leaded glass: the fabrication of a borosilicate glass microchannel wafer, and the fabrication of a wafer in which the microchannels are created by laser machining. The implementation of these two solutions is detectable in the final product.

[0040] The first of these prior art solutions therefore consists of making a wafer of microchannels out of borosilicate glass, which does not contain lead.

[0041] Although this first solution complies with the RoHS directive, it requires a more complex manufacturing process, since it is no longer possible to fabricate the microchannels and functionalize their internal surface in a single step, as borosilicate glass is free of any metallic particles. However, borosilicate glass allows for operation at relatively low temperatures during the fiber melting and cooling stages, with a glass transition temperature of approximately 530°C.

[0042] However, although the generation of hydrogen cations is avoided thanks to the absence of lead, borosilicate glass generally contains sodium and potassium, generating sodium or potassium cations during its use which degrade the photocathode 16, and leading to a reduced lifespan of the image intensifier tube 130. The use of borosilicate glass is therefore not a satisfactory solution, despite the absence of lead, an appropriate glass transition temperature and the absence of generated hydrogen cations, since subsequent steps of functionalizing the microchannels are still necessary, in addition to not resolving the problem of the degradation of the photocathode 16 by cations.

[0043] On the other hand, given that this borosilicate glass has the same physical, chemical and thermal properties as lead glass 30, the implementation of this first solution is limiting without bringing any particular advantages compared to lead glass 30 in terms of chemical, physical or thermal properties, except for the absence of lead.

[0044] The second solution considered in the prior art is to use silica glass as the constituent material of the microchannel wafers 25, the microchannels 25 being produced by laser machining, as described in document US2022 / 267200, or by photolithography, as described in the publication BEETZET AL: "Silicon-micromachined microchannel plates", nuclear instruments & methods in physics research. Section A, vol. 442, no. 1-3, 11 March 2000.

[0045] Laser machining of microchannels 25 can be performed in two ways. The first method involves directly drilling the wafer to create the microchannels 25, a process known as "laser drilling" in the English-language literature. The second method, known as "laser-induced local modification drilling" in the English-language literature, involves locally treating the surface of the wafer with a laser to modify the material, and then chemically etching these modified areas to release them and form the microchannels 25.

[0046] Although this approach avoids the presence of cations since pure silica does not contain them, the fabrication of microchannels by laser machining is subject to technical limitations. The fabrication of microchannels with a diameter less than 10 micrometers is not feasible with current technology, thus limiting potential applications. Furthermore, the use of laser machining would impose strict and costly manufacturing conditions, resulting in reduced productivity. Finally, this second solution still requires subsequent functionalization of the internal surfaces of the fabricated microchannels to enable electron multiplication.

[0047] In the field of microchannel wafers for night vision applications, it is therefore necessary to reliably address the generation of positive ions within the image intensifier tube 130. More specifically, the aim is to provide a solution enabling the production of an electron multiplier 180 made from a microchannel wafer of material with improved physical, chemical and thermal properties, avoiding any generation of hydrogen, sodium or potassium cations, and which must be free of any material considered dangerous and presenting health risks for operators or technicians; or even environmental risks.

[0048] In other areas involving the manufacture and use of microchannel wafers, there is also a desire to limit the use of hazardous materials and substances that present health or environmental risks.

[0049] DESCRIPTION OF THE INVENTION

[0050] The invention proposes to address this technical problem by forming a microchannel wafer comprising a body provided with a plurality of tubular microchannels whose axes of revolution are parallel to each other, and a functionalization layer present on an internal surface of each microchannel, the constituent material of the body being pure silica.

[0051] The invention stems from the observation that, contrary to the technical prejudice of those skilled in the art, it is possible to produce microchannel wafers with a pure silica body intended for use as electron multipliers in image intensifier tubes.

[0052] Since the glass transition temperature of pure silica, which is 1200°C, is much higher than that of lead glass or borosilicate glass, which is 460°C and about 530°C respectively, pure silica was not usually chosen to form microchannel wafers, given that this temperature implies a complex manufacturing process, with higher energy consumption as well as longer fiber melting and cooling times.

[0053] However, the production of microchannel wafers in pure silica makes it possible to avoid the use of glass containing hazardous materials according to the European RoHS directive, such as lead or bismuth, or glass which may emit cations which may damage the photocathode during the operation of an image intensifier tube, while presenting improved physical, chemical and thermal properties compared to leaded glass or borosilicate glass.

[0054] The implementation of the invention also makes it possible to avoid complex processes of machining microchannels by laser machining.

[0055] A pure silica microchannel wafer thus increases the lifespan of an image intensifier tube compared to microchannel wafers used in the prior art.

[0056] Thus, even though a functionalization step is necessary for the realization of electron multipliers from microchannel wafers with a pure silica body according to the invention, and despite the high glass transition temperature of pure silica, the advantages brought by the invention justify this choice of material in view of the solutions proposed by the prior art.

[0057] Thus, according to a first aspect, the invention relates to a microchannel wafer comprising a body provided with a plurality of tubular microchannels whose axes of revolution are parallel to each other; and a functionalization layer present on an internal surface of each microchannel; the constituent material of the body being pure silica.

[0058] In this way, the microchannel wafers are free of any hazardous materials as defined by the European RoHS directive, such as lead or bismuth, or of glass that could emit cations potentially damaging the photocathode during the operation of an image intensifier tube. The lifespan of devices using a microchannel wafer whose body is made entirely of silica is thus significantly extended.

[0059] According to a first alternative, in order to produce an electron multiplier from a wafer of microchannels according to the invention, the functionalization layer comprises a first doped silica sublayer comprising a dopant selected from the group comprising the following materials: transition materials, alkaline earth materials, rare earths; and a second secondary electron emitting sublayer; the first doped silica sublayer being located between the internal surface of the microchannel and the second secondary electron emitting sublayer.

[0060] According to a second alternative, in order to produce an electron multiplier from a wafer of microchannels according to the invention, the functionalization layer comprises metallic nanoparticles and a fixation sublayer; the fixation sublayer being intended to encapsulate and fix the metallic nanoparticles on the inner wall of each microchannel, and being intended to emit secondary electrons.

[0061] According to a second aspect, the invention relates to a method for producing a microchannel wafer according to the first aspect, comprising the following steps:

[0062] ■ formation of first fibers, comprising a core soluble in an acid and a sheath insoluble in the same acid, said sheath surrounding the core;

[0063] ■ drawing the first fibers up to a first desired diameter;

[0064] ■ realization of a first lateral stacking of the first fibers within a first preform;

[0065] ■ drawing of second fibers from the first base material up to a second desired diameter;

[0066] ■ realization of a second lateral stacking of the second fibers within a second preform;

[0067] ■ transverse cutting of the second base material along a certain cutting plane, said cutting plane forming an angle between 4° and 12° with a section plane perpendicular to said length of said second base material, so as to obtain fiber slabs; and

[0068] ■ dipping the fiber wafers in acid to dissolve the core in this acid in order to create microchannels. The second aspect of the invention is characterized in that the sheath is made of pure silica glass, and in that the core is made of doped silica glass, the doped silica glass corresponding to silica glass doped with phosphorus pentoxide, and / or fluorine, and / or germanium oxide and / or boron oxide.

[0069] In this way, a very strong chemical selectivity is obtained between the core and the sheath with respect to the dissolving acid during the quenching of the fiber wafers aimed at completely dissolving the core to form the microchannels.

[0070] This high selectivity is ideally a factor of 100 with respect to an acid corresponding to a mixture of hydrofluoric acid and acetic acid.

[0071] Preferably, a melting and cooling step of the first and second stack is also implemented within the first and second preform, in order to produce a first and second base material having an elongated shape extending over a predetermined length.

[0072] Preferably, the first fiber formation stage includes the following steps:

[0073] ■ production or functionalization of a pure silica glass tube forming the sheath of the first fibers;

[0074] ■ fabrication of a doped silica glass cylinder intended to be placed within the silica tube, forming the core of the first fibers; and

[0075] ■ Formation of the first fibers can be carried out according to one of the following processes: MCVD, MOCVD or OVD.

[0076] Thus, it is possible to produce the first fibers in a simplified manner according to a plurality of possible configurations, these processes being adaptable as needed to influence the nature and thickness of the deposited layers. The terms MCVD, MOCVD, and OVD are acronyms corresponding respectively to the following expressions in the English-language literature: "modified chemical vapor deposition," "metalorganic chemical vapor deposition," and "outside vapor deposition." Preferably, the first fiber formation step is carried out by MCVD / MOCVD by creating a doped silica glass cylinder within a pure silica glass tube, said doped silica glass cylinder being produced by MCVD / MOCVD deposition of successive layers of doped silica glass onto the inner surface of the pure silica glass tube.

[0077] Preferably, in order to maintain the cylindrical shape and mechanical strength of the core, a core support made of pure silica glass is made within the doped silica glass cylinder, by MCVD / MOCVD deposition of successive layers of pure silica glass.

[0078] Preferably, the first fiber formation stage is carried out by OVD according to the following successive steps:

[0079] ■ production of a substrate for OVD deposition, in the form of a cylinder in doped silica glass or in pure silica glass, forming at least in part the core of the first fibers;

[0080] ■ creation of a first layer on the substrate by OVD deposition, the first layer being made of doped silica glass or pure silica glass, the constituent material of this first layer being different from the constituent material of the substrate;

[0081] ■ creation of a second layer by OVD deposition around the first layer, the second layer being made of pure silica, until the pure silica glass tube is made, forming at least part of the sheath of the first fibers.

[0082] Thus, it is also possible to produce a pure silica glass support for the core using OVD, again in order to maintain the cylindrical shape and mechanical resistance of the core.

[0083] According to one embodiment, the substrate is made of pure silica glass and the first layer is made of doped silica glass, so that a core support is made of the substrate, so that the core of the first fibers is made of the first layer, and so that the sheath of the first fibers is made of the second layer.

[0084] In another embodiment, the substrate is made of doped silica glass and the first layer is made of pure silica glass, such that the core of the first fibers consists of the substrate, and the cladding of the first fibers consists of the first and second layers. Preferably, the first fiber formation step includes a step for creating an intermediate cladding between the cladding and the core. This intermediate cladding is made of an additional doped glass. This additional doped glass corresponds to pure silica glass or an aluminosilicate glass doped with metallic nanoparticles on the one hand, and with transition materials and / or alkaline earth materials and / or rare earths on the other. This intermediate cladding forms the functionalization layer after the core dissolution step.

[0085] In this way, the intermediate sheath prevents even partial dissolution of the core formed by the sheaths of the first fibers. This intermediate sheath also allows for at least partial functionalization of the microchannel wafers after the core dissolution step.

[0086] According to another embodiment, the first fiber formation stage includes a stage of creating an intermediate sheath present between the sheath and the core, this intermediate sheath comprising two distinct intermediate sublayers:

[0087] ■ The first intermediate sublayer is made of a primary doped glass, this primary doped glass corresponding to a pure silica glass or an aluminosilicate glass doped with metallic nanoparticles; and

[0088] ■ the second intermediate sublayer being made in a secondary doped glass, this secondary doped glass corresponding to a pure silica glass or an aluminosilicate glass doped with transition materials and / or alkaline-earth materials and / or rare earths; this intermediate sheath forming the functionalization layer after the core dissolution step.

[0089] Preferably, the core dissolution step is carried out by soaking in hydrofluoric acid or in a mixture of hydrofluoric acid and acetic acid.

[0090] The first intermediate sublayer and / or the second intermediate sublayer can be produced by direct nanoparticle deposition, also known as "Direct nanoparticle deposition" in the English-language literature. According to a third aspect, the invention relates to an image intensifier tube comprising:

[0091] ■ an input window configured to receive and transmit photons;

[0092] ■ a photocathode fixed on an inner face of said entrance window, capable of converting photons transmitted through the entrance window into primary electrons;

[0093] ■ an electron multiplier, incorporating a wafer of microchannels according to the first aspect of the invention and implemented according to the second aspect of the invention, capable of multiplying primary electrons into secondary electrons; and

[0094] ■ a phosphorescent screen transforming secondary electrons into photons.

[0095] According to a fourth aspect, the invention relates to a night vision system comprising:

[0096] ■ an objective;

[0097] ■ an image intensifier tube according to the third aspect of the invention; and

[0098] ■ an eyepiece and / or a sensor.

[0099] BRIEF DESCRIPTION OF THE FIGURES

[0100] The invention will be better understood upon reading the following description, given solely by way of example, and carried out in conjunction with the accompanying drawings, in which identical reference numerals designate identical or analogous features, and in which:

[0101] Figure 1 illustrates a schematic cross-sectional view of a prior art night vision device;

[0102] Figure 2 illustrates a perspective view of the night vision device of Figure 1;

[0103] Figure 3 illustrates a schematic cross-sectional view of the electron multiplier of the night vision device in Figure 1;

[0104] Figure 4 illustrates a schematic cross-sectional view of a microchannel of a prior art microchannel wafer during interaction with a primary electron;

[0105] Figure 5 illustrates a schematic cross-sectional view of the body of a microchannel wafer according to the invention;

[0106] Figure 6 illustrates a schematic cross-sectional view of a first fiber, according to one embodiment of the invention; Figure 7 illustrates a schematic cross-sectional view of a first fiber, according to another embodiment of the invention;

[0107] Figure 8 illustrates a schematic cross-sectional view of a first fiber, according to another embodiment of the invention;

[0108] Figure 9 illustrates a schematic cross-sectional view of a first fiber, according to another embodiment of the invention;

[0109] Figure 10 illustrates a schematic cross-sectional view of a first fiber, according to another embodiment of the invention;

[0110] Figure 11 illustrates a schematic longitudinal section view of a first fiber, including a core support, before core dissolution;

[0111] Figure 12 illustrates a view similar to Figure 11, after the core dissolution step; Figure 13 illustrates a schematic cross-sectional view of a microchannel of a microchannel wafer according to the invention, provided with a functionalization layer according to one embodiment;

[0112] Figure 14 illustrates a schematic cross-sectional view of a microchannel of a microchannel wafer according to the invention, provided with a functionalization layer according to another embodiment;

[0113] Figure 15 illustrates a schematic cross-sectional view of a microchannel wafer according to the invention, where a microchannel is provided with a functionalization layer according to one embodiment;

[0114] Figure 16 illustrates a schematic cross-sectional view of a microchannel wafer according to the invention, where a microchannel is provided with a functionalization layer according to another embodiment;

[0115] Figure 17 illustrates a schematic cross-sectional view of a night vision system comprising an image intensifier tube according to another aspect of the invention.

[0116] DETAILED DESCRIPTION OF THE INVENTION

[0117] Figure 5 illustrates the body 39 of a microchannel wafer 18 according to the invention, provided with a plurality of microchannels 25. The body 39 of this microchannel wafer 18 is made of pure silica, following the dissolution of the core 41 of each fiber which was used for the formation of the microchannel wafer 18. In general, the microchannel wafers 18 made according to the invention have a thickness L between 170 and 190 micrometers, with microchannels 25 of a diameter D of about 3 micrometers.

[0118] The pitch of the microchannel wafer 18, namely the microchannel spacing 25 understood as the spacing between the axes of revolution of two microchannels 25 along a direction parallel to the two faces of the microchannel wafer 18, is between 4 and 5 micrometers. Generally, the L / D ratio, also called the aspect ratio, is equal to 45.

[0119] Several manufacturing steps are carried out in order to obtain a wafer with a plurality of microchannels 25.

[0120] First, several first fibers 40, comprising a core 41 soluble in an acid, and a sheath 42 insoluble in the same acid, are formed, stretched, and assembled laterally within a first mold. Within this first mold, the first fibers 40 are preferentially fused to form a first block, which has an elongated shape.

[0121] Secondly, second fibers are produced from this first block, and similarly to the first 40 fibers, are stretched and assembled laterally within a second mold. In this second mold, a second elongated block is preferentially produced by fusing the stack of second fibers.

[0122] Finally, this second block is cut at a specific angle relative to its elongation axis, with an angle between 4° and 12°, resulting in several base wafers. These base wafers are then soaked in an acid that dissolves the core 41 without dissolving the sheath 42, leading to the formation of the microchannel wafers 18.

[0123] The first fibers 40 used to fabricate these base wafers comprise, according to a first embodiment illustrated in Figure 6, a core 41 made of doped silica glass, and a sheath 42 of pure silica. The doped silica glass here corresponds to silica glass doped with phosphorus pentoxide, and / or fluorine, and / or germanium oxide, and / or boron oxide. The doped silica glass exhibits a much higher solubility than pure silica glass in the acid used to dissolve the cores 41 of the microchannels 25, the useful acid being in this case a mixture of hydrofluoric acid and acetic acid.

[0124] In practice, during the dissolution step, the parameters of the dissolution process are chosen so as to obtain a difference in solubility of a factor of 100 between the cladding 42 and the core 41 of the first fiber, so that only the core 41 is dissolved and the structure of the cladding 42 is preserved as much as possible.

[0125] Following this dissolution step, we obtain microchannel wafers 18 according to the invention, made up solely of pure silica.

[0126] Other forms of implementation are conceivable for the production of the first 40 fibers.

[0127] According to another embodiment illustrated in Figure 7, the first fibers 40 have a core 41 made of doped silica glass and a silica cladding 42, but also have a core support 43 made of pure silica glass, this core support 43 being arranged within the core 41. This core support 43 makes it possible to maintain the cylindrical shape and the mechanical strength of the doped silica glass core 41.

[0128] According to yet another embodiment, illustrated in figure 8, the first fibers 40 have a core 41 made of doped silica glass, a sheath 42 made of silica, and also an intermediate sheath 44 made of an additional doped glass of different composition, the doped glass of the intermediate sheath 44 being in the form of pure silica glass or an aluminosilicate glass doped with metallic nanoparticles on the one hand, and with transition materials and / or alkaline earth materials, and / or rare earths on the other hand.

[0129] According to yet another embodiment illustrated in Figure 9, the first fibers 40 comprise a core support 43 of pure silica, a core 41 of doped silica glass as shown above, an intermediate sheath 44 of auxiliary silica glass as shown above, and finally a sheath 42 of pure silica glass. This embodiment thus offers the combined advantages of the preceding embodiments. The main advantage of the intermediate sheath 44 is that it minimizes the dissolution of the body 39 formed by the sheath 42 of the first fibers 40. Indeed, as illustrated in Figures 11 and 12, during the dissolution step, it is common for the structure of the sheath 42 to be partially dissolved by the dissolving acid while the core 41 is completely dissolved.

[0130] In practice, the target dissolution selectivity between the core 41 and the cladding 42 is estimated to be around a factor of 100, although lower selectivities may be possible to produce microchannel wafers 18 according to the invention. However, even a high selectivity is insufficient to prevent all dissolution of the cladding 42: thus, if we consider a selectivity of 100 and a core dissolution rate VI, the dissolution rate V2 of the cladding 42 is equal to VI multiplied by 100. The cladding 42 will also dissolve over a distance D, this distance corresponding to V2 multiplied by the core dissolution time T.

[0131] According to Figure 12, a partial dissolution of the sheath 42 of the first fibers 40 constituting the body 39 of the microchannel wafers 18 is shown, these first fibers comprising a sheath 42, a core 41 and a core support 43 as illustrated in Figure 11.

[0132] As illustrated, the final surface of the sheath 47 is at a transverse dissolution distance 49 from the initial surface of the sheath 48, the transverse dissolution distance 49 corresponding to the gap between the initial surface 48 and the final surface 47 of the sheath 42 at a surface of the microchannel wafer 18.

[0133] The arrangement of an intermediate sheath 44 thus ensures that the structure of the sheath 42 is preserved while the core 41 is completely dissolved.

[0134] However, dissolving the sheath 42 of the first fibers 40 forming the body 39 of the microchannel wafer 18 offers certain advantages for specific applications. Indeed, the partial dissolution of the sheath 42 structures the longitudinal section of the microchannels 25 into a V-shape when the microchannel wafer 18 is viewed in cross-section; this V-shape, in certain applications, improves the signal-to-noise ratio of the microchannel wafer 18. In practice, this intermediate sheath 44 can be produced in two distinct variants.

[0135] According to a first variant, illustrated for example in figure 9, the intermediate sheath 44 is made of an additional doped glass, which corresponds to a pure silica glass or an aluminosilicate glass doped with metallic nanoparticles on the one hand, and with transition materials and / or alkaline-earth materials, and / or rare earths on the other hand, this intermediate sheath 44 forming the functionalization layer after the dissolution step of the core 41.

[0136] According to a second variant, illustrated in figure 10, the intermediate sheath 44 comprises two distinct intermediate sub-layers:

[0137] ■ a first intermediate sublayer 45 made of a primary doped glass, this primary doped glass corresponding to a pure silica glass or an aluminosilicate glass doped with metallic nanoparticles; and

[0138] ■ a second intermediate sublayer 46 made in a secondary doped glass, this secondary doped glass corresponding to a pure silica glass or an aluminosilicate glass doped with transition materials and / or alkaline-earth materials and / or rare earths.

[0139] The intermediate sheath 44 can also be made to impart color to the microchannel wafer 18. When the wafer is used for night vision applications, this color can provide a significant advantage in terms of absorbing all radiation in the visible spectrum. This intermediate sheath 44 can, for example, be made of black glass, or at least of glass that absorbs between 90% and 99% of visible radiation.

[0140] In practice, when the microchannel wafer 18 is used as an electron multiplier within an image intensifier tube 13, the electrons amplified by the multiplier must be converted into light to display the observed scene to the operator. This scene is amplified by the image intensifier tube 13 through the action of a phosphor screen 20 made of light-emitting particles, which convert electrons into photons and are deposited on the surface of a fiber block. A thin aluminum film is deposited on the light-emitting particles of the phosphor screen 20 to reflect any light emitted by the particles and direct it back into the image intensifier tube 13 to the photocathode 16. The interaction of these photons can then produce photoelectrons, which are a source of noise.Indeed, these photoelectrons do not correspond to photons from the observation of the scene, but they are amplified within the tube, thus generating a "parasitic image." This effect, known as light feedback, can significantly reduce the signal-to-noise ratio. The aluminum layer is very effective but not perfect. Some defects can be observed on the surface, resulting in the leakage of some photons within the image intensifier tube.13

[0141] The fact of making an electron multiplier for image intensifier tube 13 from a wafer of microchannels 18 in a material which strongly absorbs the light from the phosphorescent screen thus presents a particular advantage, since the electron multiplier will absorb this light before it reaches the photocathode.

[0142] On the other hand, since pure silica is a material transparent to light in the visible spectral range, at least partial doping of the microchannel wafer 18 so as to absorb visible radiation is a desirable characteristic for night vision applications.

[0143] All the methods of first fiber 40 realization described above can be carried out by the MCVD, MOCVD or OVD techniques, well known to those skilled in the art.

[0144] When the MCVD or MOCVD process is used, the production of the first 40 fibers is carried out as follows:

[0145] A silica glass tube, intended to form the sheath 42 of a first fiber 40, is used as a substrate for the deposition of silica dust within it. This deposition is achieved through a chemical reaction activated by an oxy-hydrogen torch, inducing the oxidation of chlorinated gases such as SiCl₂. This deposit is then liquefied and vitrified in a second pass to form a glass layer. These glass layers are created as many times as necessary to achieve the desired shape within the tube. Of course, the silica dust can be doped during the oxidation phase by reacting with other chlorinated gases such as GeCl₂ or POCh₂, in order to form, for example, glass layers doped with germanium dioxide (GeCl₂) or potassium pentoxide (P₂O₅). The nature, doping of the layers, their thickness, and the number of layers are obviously adaptable according to the requirements of those skilled in the art.

[0146] When the OVD process is used, the production of the first 40 fibers is carried out as follows:

[0147] A cylinder made of pure silica glass or doped silica glass is used as a substrate for depositing silica dust, doped or undoped, onto its surface. As with the MCVD process, the pure or doped silica dust is obtained by oxidizing chlorinated gases, such as SiCl₂, GeCl₂, or POCI₃. This pure or doped silica dust is then liquefied and vitrified to form as many glass layers as required on the substrate surface. As with the MCVD process, the nature, doping of the layers, their thickness, and the number of layers can be easily adapted to the needs of those skilled in the art.

[0148] When the microchannel wafers 18 made according to the invention are used for night vision applications or for any application requiring an electron multiplier, the surface of the microchannels 25 is functionalized with a functionalization layer 33, as illustrated in Figures 13 to 16, so as to allow the generation of secondary electrons.

[0149] Thus, according to an embodiment illustrated in Figures 13 and 15, the internal surface of the microchannels 25 has a functionalization layer 33 made up of two sublayers: a first sublayer 34 of doped silica comprising a dopant 35 selected from the group including the following materials: transition metals, alkaline earth metals, rare earths; and a second sublayer 36 emitting secondary electrons, which may be in the form of an alumina layer. In practice, the first sublayer 34 is arranged directly on the surface of the microchannels 25, and the second emitting secondary electron layer 36 is deposited on the first layer.

[0150] According to another embodiment illustrated in Figures 14 and 16, the functionalization layer 33 comprises metallic nanoparticles 37, deposited first on the surface of the microchannels 25, and a fixation sublayer 38, intended to fix and encapsulate the metallic nanoparticles 37 on the surface. In practice, the metallic nanoparticles 37 correspond to gold nanoparticles, and the fixation sublayer 38 corresponds to a dielectric layer of alumina, preferentially deposited by the ALD process, an acronym for "atomic layer deposition" in the English-language literature.

[0151] In this embodiment, the gold metallic nanoparticles 37 are present on the surface of the glass of the microchannel 25 in a homogeneous manner, with a density of between 400 and 500 nanoparticles per pm 2According to other embodiments, the density of the gold metallic nanoparticles deposited on the surface of the microchannels 25 can be up to 1500 to 1600 nanoparticles per pm 2

[0152] According to one variant, the functionalization layer 33 can also be produced, at least partially, directly during the first fiber formation step 40, when an intermediate sheath 44 is provided in the structure of the first fibers 40. After the dissolution step carried out on fiber wafers made from first fibers 40 comprising an intermediate sheath 44, all or part of this intermediate sheath 44 is possibly retained and can be used as a functionalization layer 33.

[0153] Figure 17 represents a night vision system 11 comprising an image intensifier tube 13, an objective 12 and an eyepiece 14, aligned on an optical axis al.

[0154] This image intensifier tube comprises at least three distinct elements: a photocathode 16 attached to an entrance window 15, an electron multiplier 18, and a phosphorescent screen 20. The electron multiplier 180 is made with a wafer of microchannels 18 according to the present invention, making it possible to avoid the generation of positive ions following the generation of secondary electrons within the microchannels 25, which degrade the structure of the photocathode 16 and the life of the image intensifier tube 13, and making it possible to avoid the use of hazardous materials according to the European RoHS directive.

[0155] Thus, the invention makes it possible to produce a functional microchannel wafer 18, avoiding the use of metals presenting health and environmental risks according to the RoHS directive, while facilitating the production of the first fibers 40 intended to make the microchannel wafer 18. The microchannel wafers 18 according to the invention, whose body 39 is made solely of silica, also make it possible to extend the life of the image intensifier tubes 13 in which they are used, the silica constituting the body 39 of the microchannel wafers 18 not generating cations that could potentially damage the photocathode 16.

Claims

DEMANDS 1. Microchannel wafer (18) comprising: ■ a body (39) provided with a plurality of tubular microchannels (25) whose axes of revolution are parallel to each other; and ■ a functionalization layer (33) present on an internal surface of each microchannel (25); the constituent material of the body (39) being pure silica; characterized in that the functionalization layer (33) comprises, in a first alternative: ■ a first doped silica sublayer (34) comprising a dopant (35) selected from the group comprising the following materials: transition materials, alkaline earth materials, rare earths; and ■ a second secondary electron emitting subshell (36); the first doped silica subshell (34) being located between the internal surface of the microchannel (25) and the second secondary electron emitting subshell (36); or in that the functionalization layer (33) comprises, in a second alternative: ■ metallic nanoparticles (37); and ■ a fixation sublayer (38); the fixation sublayer (38) being intended to encapsulate and fix the metallic nanoparticles (37) on the inner wall of each microchannel (25), and being intended to emit secondary electrons.

2. A method for producing a microchannel wafer (18) according to claim 1, comprising the following steps: ■ formation of first fibers (40), comprising a core (41) soluble in an acid and a sheath (42) insoluble in the same acid, said sheath (42) surrounding the core (41); ■ drawing of the first fibers (40) up to a first desired diameter; ■ realization of a first lateral stacking of the first fibers (40) within a first preform; ■ drawing of second fibers from the first base material up to a second desired diameter; ■ realization of a second lateral stacking of the second fibers within a second preform; ■ transverse cutting of the second base material along a certain cutting plane, said cutting plane forming an angle between 4° and 12° with a section plane perpendicular to said length of said second base material, so as to obtain fiber slabs; and ■ dipping the fiber wafers in acid to dissolve the core (41) in this acid to create microchannels (25); characterized in that the sheath (42) is made of pure silica glass, and in that the core (41) is made of doped silica glass, the doped silica glass corresponding to a silica glass doped with phosphorus pentoxide, and / or fluorine, and / or germanium oxide and / or boron oxide.

3. A method for producing a microchannel wafer (18) according to claim 2, wherein the first fiber formation step (40) comprises the following steps: ■ fabrication or functionalization of a pure silica glass tube forming the sheath (42) of the first fibers (40); and ■ fabrication of a doped silica glass cylinder intended to be placed within the silica tube, forming the core (41) of the first fibers (40); the formation of the first fibers (40) being able to be carried out according to one of the following processes: MCVD, MOCVD or OVD.

4. Method of producing a microchannel wafer (18) according to claim 3, wherein the step of forming the first fibers (40) is carried out by MCVD / MOCVD by creating a doped silica glass cylinder within a pure silica glass tube, said doped silica glass cylinder being produced by MCVD / MOCVD deposition of successive layers of doped silica glass on the internal surface of the pure silica glass tube.

5. Method of producing a microchannel wafer (18) according to claim 4, wherein a core support (43) of pure silica glass is produced within the doped silica glass cylinder, by MCVD / MOCVD deposition of successive layers of pure silica glass.

6. Method for producing a microchannel wafer (18) according to claim 3, wherein the step of forming the first fibers (40) is carried out by OVD according to the following successive steps: ■ production of a substrate for OVD deposition, in the form of a cylinder in doped silica glass or in pure silica glass, forming at least in part the core (41) of the first fibers (40); ■ creation of a first layer on the substrate by OVD deposition, the first layer being made of doped silica glass or pure silica glass, the constituent material of this first layer being different from the constituent material of the substrate; ■ production of a second layer by OVD deposition around the first layer, the second layer being made of pure silica, until production of the pure silica glass tube forming at least in part the sheath (42) of the first fibers (40).

7. Method of producing a microchannel wafer (18) according to claim 6, wherein the substrate is made of pure silica glass and the first layer is made of doped silica glass, such that a core support (43) is made of the substrate, such that the core (41) of the first fibers (40) is made of the first layer, and such that the sheath (42) of the first fibers (40) is made of the second layer.

8. Method of producing a microchannel wafer (18) according to claim 6, wherein the substrate is made of doped silica glass and the first layer is made of pure silica glass, so that the core (41) of the first fibers (40) is made of the substrate, and so that the sheath (42) of the first fibers (40) is made of the first layer and the second layer.

9. A method for producing a microchannel wafer (18) according to any one of claims 3 to 8, wherein the step of forming the first fibers (40) comprises a step of producing an intermediate sheath (44) located between the sheath (42) and the core (41), this intermediate sheath (44) being made of an additionally doped glass, this additionally doped glass corresponding to a pure silica glass or an aluminosilicate glass doped with metallic nanoparticles on the one hand, and with transition materials and / or alkaline-earth materials, and / or rare earths on the other hand, this intermediate sheath (44) forming the functionalization layer after the dissolution step of the core (41).

10. Method for producing a microchannel wafer (18) according to any one of claims 3 to 8, wherein the step of forming the first fibers (40) includes a step of producing an intermediate sheath (44) present between the sheath (42) and the core (41), this intermediate sheath (44) comprising two distinct intermediate sublayers: ■ the first intermediate sublayer (45) is made of a primary doped glass, this primary doped glass corresponding to a pure silica glass or an aluminosilicate glass doped with metallic nanoparticles; and ■ the second intermediate sublayer (46) being made in a secondary doped glass, this secondary doped glass corresponding to a pure silica glass or an aluminosilicate glass doped with transition materials and / or alkaline-earth materials and / or rare earths; this intermediate sheath (44) forming the functionalization layer after the dissolution step of the core (41).

11. Method for producing a microchannel wafer (18) according to any one of claims 3 to 10, wherein the core dissolution step (41) is carried out by soaking in hydrofluoric acid or in a mixture of hydrofluoric acid and acetic acid.

12. Method for producing a microchannel wafer (18) according to claim 10, wherein at least one sublayer (45, 46) is produced by direct deposition of nanoparticles.

13. Method for producing a microchannel wafer (18) according to any one of claims 3 to 12, wherein a melting and cooling step of the first and second stack is implemented within the first and second preform, in order to produce a first and second base material having an elongated shape extending over a predetermined length.

14. Image intensifier tube (13) comprising: ■ an input window (15) configured to receive and transmit photons; ■ a photocathode (16) fixed on an inner face of said entrance window (15), capable of converting photons transmitted by the entrance window (15) into primary electrons (28); ■ an electron multiplier, incorporating a wafer of microchannels (18) according to claim 1 and made according to claims 2 to 13, capable of multiplying primary electrons (28) into secondary electrons (29); and ■ a phosphorescent screen (20) transforming secondary electrons (29) into photons.

15. Night vision system (11) comprising: ■ an objective (12); ■ an image intensifier tube (13) according to claim 14; and ■ an eyepiece (14) and / or a sensor.

Images