Process for producing a microchannel plate, associated image intensifier tube and night vision system

Abstract

The invention relates to a process for producing a microchannel plate comprising the following steps:  functionalizing the internal surface of the microchannels (25) by depositing metal nanoparticles (40) with a view to allowing hopping conduction in the microchannels (25); and  fastening the metal nanoparticles (40) to the surface of the microchannels (25) by depositing a layer of dielectric on the functionalized surface of the microchannels (25). The functionalizing step comprises the following steps:  applying electrostatic charges of a first polarity (41) to the metal nanoparticles (40) within an aqueous solution;  applying electrostatic charges of opposite polarity (43) to the electrostatic charges applied to the metal nanoparticles (40), to the internal wall of the microchannels (25); and  dipping the microchannels (25) into the solution so as to deposit the metal nanoparticles (40) on the internal surface of the microchannels (25), before the step of fastening the metal nanoparticles (40).

Description

[0001] METHOD FOR PRODUCING A MICROCHANNEL PLATE, ASSOCIATED IMAGE INTENSIFIER TUBE AND NIGHT VISION SYSTEM

[0002] FIELD OF THE INVENTION

[0003] The invention relates to the field of microchannel plates, in particular for the amplification and detection of low intensity signals. The invention relates more particularly to a method for producing a microchannel plate which can be used as an electron multiplier for various applications, in particular in the field of night vision and scientific research.

[0004] In the field of night vision, microchannel plates, used as electron multipliers, make it possible to amplify the brightness of an observed scene, such plates being implemented within an image intensifier tube.

[0005] In the field of scientific research, the particular properties of microchannel plates provide certain advantages for a variety of applications requiring detection and / or amplification of the signal of various particles, such as ions, neutrons, a-particles or even high-energy photons (UV photons and X-ray photons). For example, microchannel plates are used in mass spectrometry, multispectral or hyperspectral imaging, electron spectroscopy and microscopy, X-ray photoelectron spectrometry or for nuclear instrumentation.

[0006] This invention therefore presents a multitude of potential applications, particularly when there are problems with the use of potentially toxic materials.

[0007] 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 ecological elimination of this waste.

[0008] STATE OF THE ART

[0009] As schematically illustrated in Figures 1 and 2 of the prior art, relating to the use of a microchannel plate 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. The photocathode 16 is in the form of a semi-transparent photosensitive layer receiving the photons of the incident electromagnetic radiation, i.e. the photons transmitted by the objective 12. To do this, an input window 15 transmits the photons from the objective 12 onto the photocathode 16.

[0010] The photocathode 16 is generally made in the form of a thin layer of metal or semiconductor applied to a layer of glass or material transparent to light. The material of the photocathode 16 is chosen according to its sensitivity to the incident photons from the observed scene. The interaction of the photons of the incident electromagnetic radiation from the observed scene with the photocathode 16 produces, by photoelectric effect, the emission of electrons, called primary electrons 28 or photoelectrons.

[0011] The photocathode 16 is characterized by its quantum efficiency, also known as "quantum efficiency" in English literature. This quantum efficiency, generally expressed as a percentage, refers to the ratio of the number of photoelectrons, or primary electrons 28, emitted compared to the number of photons incident on the surface of the photocathode 16.

[0012] The primary electrons 28 are then subjected to a first electric field within a first acceleration zone 17, making it possible to direct the primary electrons towards the electron multiplier 180. This first electric field is produced 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.

[0013] 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 literature. It is made from a plate of resistive or dielectric material having a thickness typically between 0.2 and 1 millimeter.

[0014] As illustrated more precisely in Figure 3 of the prior art, the microchannels 25 pass right through the wafer of the electron multiplier 180 and are parallel to each other. They have a diameter of between 3 and 12 micrometers. Their inner wall is conventionally treated in order to achieve transmission of an electric current by hopping, known as "hopping-conduction" in the English literature. 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.

[0015] 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 the electrodes placed on either side of the microchannel plate 25. This second electric field makes it possible to induce an electric current in each microchannel 25 of the electron multiplier 180. More precisely, the current is generated in a conduction layer located in the internal surface of the microchannels 25. This internal surface of the microchannels 25 is sized to keep the number of electrons inside the microchannels 25 constant.

[0016] 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 is generated by cascade effect.

[0017] These secondary electrons are accelerated within the microchannels 25 in order to cause more collisions with the surface of the microchannels 25 to generate other secondary electrons 29. These secondary electrons are finally 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 tears 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 material of the microchannel 25, if this loss of electrons is not compensated. The conduction current inside each microchannel 25 thus has the advantage of compensating, inside the material and all along the microchannel 25, the loss of electrons due to the emission of secondary electrons 29.

[0020] Typically, the primary electrons 28 are multiplied by a factor of between 10 3 and 10 6in the electron multiplier 180. The conversion rate of the primary electrons 28 into secondary electrons 29 corresponds to the electrical gain of the electron multiplier 180. At the outlet 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 outlet of the electron multiplier 180 and the phosphorescent screen 20, typically an electric field generated by a voltage of between 4 and 10 kV.

[0021] The phosphorescent screen 20 makes it possible to transform the secondary electrons 29 into photons. It is in the form of a phosphorescent layer or a layer of a luminophore material deposited on a substrate, conventionally made of glass. The measured conversion rate of the secondary electrons 29 into photons corresponds to the efficiency of the phosphorescent screen 20.

[0022] To generate the three electric fields, electronic components 22 are conventionally arranged around an internal vacuum enclosure 24.

[0023] 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, of the order of 10' 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 enclosure 24.

[0024] The strict operating conditions of the image intensifier tube 130 are such that defects in one of its components can cause premature wear, leading to reduced intensification of the radiation of the observed image. This intensification is measured by the light gain, defined as the product of four quantities: the number of incident photons at the input of 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 phosphorescent screen 20.

[0025] Conventionally, it is known that the main phenomena affecting the luminous 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 having good stability during use of the image intensifier tube 130.

[0026] It has been observed that the degradation of the quantum efficiency of the photocathode 16 is due to a generation of positive ions by the electron multiplier 180 within the image intensifier tube 130. More particularly, as illustrated in FIG. 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. Given the first electric field intended to transmit the negatively charged electrons from the photocathode 16 to the electron multiplier 180, these positive ions 32 are attracted towards the photocathode 16 and rise up the image intensifier tube 130, causing by collision a destruction of the chemical bonds between the atoms of the surface of the photocathode 16.These multiple collisions of positive ions 32 degrade, over time, the quantum efficiency of the photocathode 16.

[0027] The degradation of the quantum efficiency is particularly present in the image intensifier tubes 130 using electron multipliers 180 made with lead glass 30. Indeed, to produce a microchannel plate 25, it is known to produce first fibers with a core soluble in an acid, and a sheath surrounding the core insoluble in the same acid. These first fibers, called primary fibers, are obtained by drawing to a first desired diameter 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 obtained by stretching the hexagonal preform to a second desired diameter and then assembled again in order to produce a second elongated base material obtained after melting and cooling the secondary fibers together.

[0029] The second elongated base material is then cut transversely along a certain cutting plane. Typically, the cutting plane forms an angle between 4° and 12° with a sectional plane perpendicular to the length of the second base material. This cutting step makes it possible to produce multiple base wafers. These base wafers are then dipped in acid to dissolve the core in this acid, revealing the microchannels 25.

[0030] The selectivity of the core and cladding materials according to their solubility in the acid is essential for the realization of the microchannels 25. Indeed, it is necessary that the cladding is insoluble in this acid, so that the quenching can only dissolve the core.

[0031] Lead glass 30, which is in the form of a lead silicate, is an example of a material constituting the sheath that is particularly effective for forming these microchannel plates 25, given its insolubility in a plurality of acids. On the other hand, this lead glass 30 has the particularity of being easily chemically reduced by means of dihydrogen, generating on the internal surface of the microchannels 25 metal islands of lead 31, of a size between about ten and a few tens of nanometers, as illustrated in FIG. 4. These metal islands of lead 31, located under a layer of a few nanometers of glass having a function of emitting secondary electrons 29, allow the regeneration of the “holes” created during the emission of secondary electrons 29.

[0032] Thus, these metallic lead islands 31 allow the passage of electronic current, by an electronic conduction known as jump conduction: the electrons “jump” from one metallic lead island to another metallic lead island under the effect of the electric field generated between the two metallic electrodes of the faces of the electron multiplier 180. The lead glass 30, which allows this jump conduction when it is subjected to a chemical reduction step of its microchannels 25, is characterized by high resistivity values, of the order of 10 6 at 10 9 ohm.cm, by a significant insensitivity to the applied electric field, the value of the resistance or resistivity of the lead glass being relatively constant over a wide range of electric field values, and by a negative variation in resistance as the operating temperature of the electron multiplier 180 decreases.

[0033] 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 FIG. 4. On the other hand, sodium or potassium cations can also be emitted since these are also likely to accumulate on the surface of the microchannels 25 during the reduction.

[0034] The 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 lead 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 English literature.

[0035] 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”. A first solution of the prior art illustrated in Figure 5, consists of coating the internal surface of the microchannels 25 of the electron multiplier 180, made of lead glass 30, with a first passivation layer 33 topped by an emissive layer 34.

[0036] The passivation layer 33 is typically nanolaminated on the surface of the lead glass 30 and it makes it possible to avoid the production and emission of hydrogen, sodium or potassium cations 32 during the interaction with the primary electrons 28. Indeed, the primary electrons 28 are blocked at the surface of the passivation layer 33, as illustrated in FIG. 5. Furthermore, the passivation layer 33 also allows the transfer of electrons coming from the lead glass to the emissive layer 34, in order to provide sufficient electrons to the emissive layer 34 for the generation of secondary electrons 29.

[0037] The electrons coming from the lead metal islands 31 are thus transferred to the emissive layer 34 through the passivation layer 33. The emissive layer 34, conventionally made of alumina or aluminum oxide, generates secondary electrons 29 upon contact with the primary electrons 28, thanks to the electrons received from the lead metal islands 31 through the passivation layer 33. In this first solution, the passivation layer 33 and the emissive layer 34 are made by deposition of atomic thin layers, also known by the acronym ALD for "atomic layer deposition" in the English literature.

[0038] This first solution effectively avoids the generation of positive ions, but has several drawbacks. Firstly, the deposition of the first passivation layer 31 by ALD and of the second emissive layer 32, also by ALD, requires an excessively long production time, typically several days.

[0039] On the other hand, the use of lead 30 glass and the reduction step of this glass represent a health hazard for operators and technicians, as well as environmental issues. Thus, this solution does not comply with the European RoHS directive and presents risks for human health and the environment, with regard to the life cycle of a microchannel wafer made with lead 30 glass.

[0040] To avoid using lead glass 30 to form the wafer of the electron multiplier 180, a second solution of the state of the art proposes to directly form the secondary electrons 29 in a conductive layer 36, as illustrated in FIG. 6. This second solution is notably described in document US 8921799. In this second solution, the sheath can thus be made of lead-free glass 35, ceramic, or any other material suitable for resisting dissolution of the core and having a high resistivity to electric current, of the order of at least 10 8 at 10 9 ohm.cm.

[0041] Since the sheath does not incorporate metal molecules allowing the rapid formation of metal islands, said sheath therefore only serves as a mechanical support. The internal surface of the microchannels 25 in this type of electron multiplier 180 is thus coated with a first conductive layer 36 providing the secondary electrons 29, produced by ALD, and a second emissive layer 34 of secondary electrons 29 on the first conductive layer 36, also produced by ALD. As mentioned for the first solution of the prior art, the production time of such an electron multiplier 180 is excessively long.

[0042] In the field of microchannel plates for a night vision application, it therefore appears necessary to reliably remedy the generation of positive ions within the image intensifier tube 130. More particularly, it is sought to provide a solution while taking into account the production time of the electron multiplier 180, which must be reasonable, and the base material of the electron multiplier 180, which must be free of any material considered to be dangerous and presenting health risks for operators or technicians; or even environmental risks.

[0043] In other areas involving the manufacture and use of microchannel wafers, it is also sought to limit the use of hazardous materials and substances that present health or environmental risks.

[0044] STATEMENT OF THE INVENTION

[0045] The invention proposes to address this technical problem by forming a microchannel plate made with a first deposit of conductive nanoparticles, according to a chemical process of depositing electrostatic charges on the internal surface of the microchannels, followed by a second deposit of a dielectric material emitting secondary electrons, for example by ALD. These two layers thus protect the internal surface of the microchannels from electronic collisions generating positive ions, while ensuring sufficient generation of secondary electrons and conduction of these secondary electrons from the inlet of the microchannels to the outlet of the microchannels.The invention arises from the observation that, contrary to the technical prejudice of those skilled in the art, it is possible to deposit several layers on the surface of the microchannels, to ensure both conduction, the generation of secondary electrons in sufficient quantity and the passivation of the internal surface of the microchannels, in other words the non-generation of positive ions, without resorting to the ALD process alone, thus reducing the production time.

[0046] Thus, instead of conventionally reducing a lead-doped glass or using the ALD process alone to produce a microchannel plate, the inventors have found a method allowing the production of a microchannel plate in a reasonable time, without generating positive ions, while making it possible to produce functional microchannel plates in a plurality of materials free from hazardous materials such as lead or bismuth, and more specifically free from hazardous materials according to the European RoHS directive.

[0047] According to a first aspect, the invention therefore relates to a method for producing a microchannel plate, said method comprising the following steps:

[0048] ■ functionalization of the internal surface of the microchannels by deposition of metallic nanoparticles to enable hopping conduction in the microchannels; and

[0049] ■ fixing of metallic nanoparticles on the surface of the microchannels by the deposition of a layer of dielectric material on the functionalized surface of the microchannels.

[0050] According to the invention, the functionalization step comprises the following steps:

[0051] ■ application of electrostatic charges of a first polarity on the metallic nanoparticles within an aqueous solution;

[0052] ■ application of electrostatic charges, of opposite polarity to those of the electrostatic charges applied to the metallic nanoparticles, on the internal wall of the microchannels; and

[0053] ■ dipping the microchannels into the solution so as to deposit the metal nanoparticles on the internal surface of the microchannels, before the step of fixing the metal nanoparticles.

[0054] In addition to the functionalization steps specific to the invention, the method may also comprise the following steps for forming the microchannels before functionalizing them:

[0055] ■ forming first fibers, comprising a core soluble in an acid, and a sheath insoluble in the same acid, said sheath surrounding the core; ■ drawing the first fibers to a first desired diameter;

[0056] ■ production of a first lateral stack of the first fibers within a first preform;

[0057] ■ melting and cooling of the first stack within the first preform, in order to produce a first base material having an elongated shape extending along a length;

[0058] ■ drawing second fibers from the first base material to a second desired diameter;

[0059] ■ production of a second lateral stack of the second fibers within a second preform;

[0060] ■ melting and cooling the second stack within the second preform, in order to produce a second base material having an elongated shape extending along a length;

[0061] ■ transverse cutting of the second base material according to a certain cutting plane, said cutting plane forming an angle of between 4° and 12° with a sectional plane perpendicular to said length of said second base material, in order to produce multiple base pancakes; and

[0062] ■ dipping base wafers into an acid to dissolve the core in this acid in order to create microchannels.

[0063] Preferably, the sheath is made of lead-free glass. To achieve sheath shrinkage with lead-free glass, the core is preferably soluble in hydrochloric acid, while the sheath is insoluble in hydrochloric acid. For the purposes of the invention, a sheath that is "insoluble" in hydrochloric acid means that the sheath is less soluble in hydrochloric acid by at least a factor of 100 compared to the solubility of the core in hydrochloric acid.

[0064] Preferably, the sheath is made of glass, silica-based glass, silicate or ceramic, or any perforable dielectric material having a resistivity to electric current of between 10 8 and 10 11 ohm.cm, or even a resistivity greater than 10 10 ohm.cm or 10 11 ohm.cm; as well as a capacity to be insoluble in an acid allowing the core to be dissolved.

[0065] The core is generally made of boro-borate glass, or glass containing boron, alkali cations, and / or alkaline earth cations. In the chemical process of depositing electrostatic charges on the internal surface of the microchannels, the step of applying electrostatic charges of a first polarity to the metal nanoparticles can be carried out by applying citrate molecules to the surface of the metal nanoparticles within an aqueous solution, by means of a mixture of an aqueous solution of sodium citrate and an aqueous solution of an inorganic metal compound.

[0066] Furthermore, the step of applying electrostatic charges, of opposite polarity to those of the electrostatic charges applied to the metal nanoparticles, on the internal wall of the microchannels, is preferably carried out using a silanization process.

[0067] The step of fixing the metal nanoparticles on the surface of the microchannels by the deposition of a layer of dielectric material can be carried out by an ALD deposition process, or in a variant by a sol-gel deposition process.

[0068] Furthermore, after the step of fixing the metallic nanoparticles, a thermal annealing step can be implemented in order to eliminate any organic residues.

[0069] As regards the materials used to obtain efficient conduction and efficient generation of secondary electrons, the metal nanoparticles are preferably made of a non-oxidizable material, for example gold, silver or platinum. The dielectric material layer, ensuring the emissive function of secondary electrons, is preferably made of aluminum oxide AI2O3 / SAI2O3 or magnesium oxide MgO.

[0070] According to a second aspect, the invention also relates to an image intensifier tube comprising:

[0071] ■ an input window configured to receive and transmit photons;

[0072] ■ a photocathode fixed on an internal face of said input window, capable of converting the photons transmitted by the input window into primary electrons;

[0073] ■ an electron multiplier, incorporating a microchannel plate, capable of multiplying primary electrons into secondary electrons; and

[0074] ■ a phosphorescent screen transforming secondary electrons into photons.

[0075] According to the invention, the microchannel plate is produced by implementing a method according to the first aspect of the invention. According to a third aspect, the invention relates to a night vision system comprising an objective, an image intensifier tube according to the second aspect of the invention, and an eyepiece and / or a sensor.

[0076] BRIEF DESCRIPTION OF THE FIGURES

[0077] The invention will be better understood from reading the following description, given solely by way of example, and drawn up in relation to the appended drawings, in which identical references designate identical or similar elements, and in which:

[0078] Figure 1 illustrates a schematic sectional view of a night vision system comprising a state-of-the-art image intensifier tube;

[0079] Figure 2 illustrates a perspective view of the image intensifier tube of Figure 1;

[0080] Figure 3 illustrates a schematic sectional view of the electron multiplier of the night vision device of Figure 1;

[0081] Figure 4 illustrates a schematic sectional view of a microchannel of a microchannel plate according to a prior art embodiment, generating secondary electrons and positive ions;

[0082] Figure 5 illustrates a schematic sectional view of a microchannel of a microchannel plate according to another embodiment of the state of the art, generating secondary electrons;

[0083] Figure 6 illustrates a schematic sectional view of a microchannel of a microchannel plate according to another embodiment of the state of the art, generating secondary electrons;

[0084] Figure 7 illustrates a schematic sectional view of a microchannel of a microchannel plate according to an embodiment of the invention, generating secondary electrons;

[0085] Figure 8a illustrates a schematic sectional view of the step of applying electrostatic charges to metal nanoparticles, according to a method of producing the microchannel plate of Figure 7;

[0086] Figure 8b illustrates a schematic sectional view of the step of applying electrostatic charges to the internal surface of the microchannel, according to a method of producing the microchannel plate of Figure 7;

[0087] Figure 8c illustrates a schematic sectional view of the step of contacting metal nanoparticles of Figure 8a on the inner surface of the microchannel of Figure 8b; Figure 8d illustrates a schematic sectional view of a microchannel of the microchannel plate of Figure 8c after interaction of the metal nanoparticles on the inner surface of the microchannel;

[0088] Figure 8e illustrates an overview of an apparatus allowing the contacting of the charged metallic nanoparticles with the charged microchannel plate;

[0089] Figure 8f illustrates a schematic sectional view of a microchannel of the microchannel plate of Figure 8d after application of a secondary electron emissive layer to the inner surface of the microchannel;

[0090] Figure 9 illustrates a schematic sectional view of a night vision system comprising an image intensifier tube according to another aspect of the invention.

[0091] DETAILED DESCRIPTION OF THE INVENTION

[0092] Figure 7 illustrates the internal surface of a microchannel 25, said microchannel 25 forming part of a microchannel plate 18 made of lead-free glass, according to a method in accordance with the invention.

[0093] This microchannel 25 comprises on its surface metallic nanoparticles 40, in this case metallic gold nanoparticles, and a layer of dielectric material 34. Figure 7 also represents the generation of secondary electrons 29 following the interaction of the surface of the microchannel 25 with a primary electron 28, when the microchannel plate 18 is used as an electron multiplier.

[0094] Several production steps are carried out in order to obtain a wafer provided with a plurality of microchannels 25.

[0095] First, several first fibers, comprising a core soluble in an acid, and a sheath insoluble in the same acid, are formed, stretched, and assembled laterally within a first mold. Within this first mold, the first fibers are fused in order to produce a first block, which has an elongated shape.

[0096] Second, second fibers are made from this first block, and similarly to the first fibers, are stretched and assembled laterally within a second mold. In this second mold, a second elongate block is made by merging the stack of second fibers. Finally, this second block is cut at a certain cutting angle relative to the elongation axis of the second block, with an angle between 4° and 12°, resulting in the production of several base wafers. These base wafers are finally soaked in an acid allowing the core to be dissolved without dissolving the cladding, resulting in the production of the microchannel wafers 18. The acid used may be hydrochloric acid, nitric acid or hydrofluoric acid, these three acids may also be used successively, alternately, or independently of each other.

[0097] The sheath is made of glass, silica-based glass, silicate or ceramic, or any perforable dielectric material having a resistivity to electric current of between 10 8 and 10 11 ohm. cm. The core, for its part, is generally made of boro-borate glass, or glass containing boron, alkali cations, and / or alkaline earth cations.

[0098] Advantageously, this choice of distinct materials for the sheath and the core allows for dissolution selectivity when the base wafers are soaked in a specific acid, resulting in dissolution of the core without degrading the material constituting the sheath.

[0099] The microchannel plate 18 produced has a thickness L of between 170 and 190 micrometers, with microchannels 25 having a diameter D of approximately 3 micrometers. The pitch of the microchannel plate 18, namely the spacing of the microchannels 25 understood as the spacing between the axes of revolution of two microchannels 25 in a direction parallel to the two faces of the microchannel plate 18, is between 4 and 5 micrometers. Generally, the ratio L / D, also called the form factor, is equal to 45.

[0100] Still according to Figure 7, the internal surface of a microchannel 25 of a microchannel plate 18 is shown in longitudinal section, said microchannel 25 being made, as mentioned previously, of a material having a resistivity to electric current of between 10 8 and 10 11ohm.cm. On the internal surface of the microchannel 25, a first deposition of metal nanoparticles 40 and a second deposition of a layer of dielectric material 34 are carried out on the surface of the microchannels. These successive depositions are carried out after a plurality of successive steps corresponding to the method according to the invention, illustrated in FIGS. 8a to 8f.

[0101] With reference to Figure 8a, the first step in producing a microchannel plate 18 according to the invention corresponds to an application of negative electrostatic charges 41 to the surface of the metal nanoparticles 40. These metal nanoparticles 40 are in the form of gold metal nanoparticles, in a HauCh 42 chlorauric acid solution. This chlorauric acid solution 42 is then added to a sodium citrate solution for mixing, in order to obtain a solution comprising charged gold metal nanoparticles 40. These charged metal nanoparticles 40 thus have on their surface negative charges 41 present in the form of negatively charged citrate molecules, obtained by means of the sodium citrate solution.

[0102] According to a particular embodiment, obtaining functionalized gold metal nanoparticles 40 is obtained as follows: a solution of chloroauric acid 42 having a molar concentration of 0.25 mM is brought to a boil, then an appropriate volume of sodium citrate concentrated to 1% is then added to the chloroauric acid solution 42. After intensive mixing of the solutions for 20 minutes, the suspension of gold metal nanoparticles 40 is cooled to room temperature and reconcentrated to 25 mM by tangential filtration. This method for preparing gold metal nanoparticles 40, known in the state of the art as the Turkevich citrate reduction method, makes it possible to obtain stabilized spherical gold-citrate metal nanoparticles 40.Ideally, the colloidal stability of the suspension of 40 gold metal nanoparticles is characterized by UV-Vis spectroscopy and by DLS, acronym for "Dynamic Light Scattering", in the English literature.

[0103] The solution of 40 gold metal nanoparticles loaded with citrate molecules on their surface is then reconcentrated up to 200 times by tangential filtration. Alternatively, polysorbate 80 can be added to the chlorauric acid-sodium citrate mixture to improve colloidal stability.

[0104] As shown in Figure 8b, the second step of producing a microchannel wafer 18 made of lead-free glass 35 according to the invention corresponds to charging the internal surface of the microchannels 25 of the wafer by applying positive electrostatic charges 43. The surface of the microchannels 25 is thus treated according to a silanization process, and more particularly according to a liquid phase silanization process. According to a variant, the microchannel wafer 18 can be produced with leaded glass, but not reduced, thus preventing the exposure of the lead particles and protecting the environment and the operators from the harmful effects of the lead particles.

[0105] According to a particular embodiment, the application of positive electrostatic charges 43 to the internal surface of the microchannels 25 by silanization is carried out as follows: a concentrated 5% solution of a silane is prepared, in this case the silane used is EDMPS (N-(-aminoethyl)3-aminopropylmethyldimethoxisilane (95%)), which is diluted in ultrapure water. The microchannel wafer 18 is then soaked in this solution, then subjected to sonication for one hour at a temperature of 50°C. The microchannel wafer 18 is then rinsed between two and three times with methanol, and optionally twice with ultrapure water. The treated microchannel wafer 18 is then dried for a few seconds with a nitrogen or air spray gun, or by a laminar air flow. Advantageously, a thermal annealing phase can be considered to eliminate certain organic residues.

[0106] According to a variant not shown, the silanization of the microchannel plates 18 does not require any heat treatment, in order to avoid the aggregation of the metal nanoparticles 40.

[0107] With reference to Figure 8c, the negatively charged metal nanoparticles 40 are deposited on the positively charged surface of the microchannel 25. The respective polarizations of the metal nanoparticles 40 and the surface of the channel being inverse, the negative charges 41 on the metal nanoparticles 40 will be attracted by the positive charges 43 on the surface of the microchannel 25, these positive charges 43 playing the role of ligand, leading to the attachment of the gold metal nanoparticles 40 to the wall of the internal surface of the microchannel 25, as shown in Figure 8d.

[0108] The metallic gold nanoparticles 40 are thus 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 μm. 2 . According to other embodiments, the density of the gold metal nanoparticles deposited on the surface of the microchannels can be up to 1500 to 1600 nanoparticles per μm 2 .

[0109] The phase of deposition of the charged metal nanoparticles 40 on the surface of the microchannels 25 is carried out by means of a recirculation impregnation installation, by means of a solution reserve 44 containing the gold metal nanoparticles 40, a support 45 for holding the microchannel plate 18, a recirculation loop 46, and a peristaltic pump 47, as shown in FIG. 8e.

[0110] Figure 8f represents the final step of producing a microchannel plate 18 according to the invention, during which a layer of dielectric material 34 is applied to the surface of the functionalized microchannel 25, making it possible to fix the metal nanoparticles 40 on the surface of the microchannel 25. This layer is made of a dielectric material, and more precisely of aluminum oxide AI2O3 / SAI2O3 or of magnesium oxide MgO, advantageously making it possible to generate the secondary electrons when the microchannel plate 18 is used as an electron multiplier. This layer of dielectric material 34 is produced by ALD deposition or by sol-gel deposition. Figure 9 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.This image intensifier tube comprises at least three distinct elements: a photocathode 16 attached to an input window 15, an electron multiplier 18, and a phosphorescent screen 20. The electron multiplier 180 is produced with a microchannel plate 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 lifetime of the image intensifier tube 13, and making it possible to avoid the use of lead glass.

[0111] Thus, the invention makes it possible to achieve a functional microchannel plate 18, avoiding the use of metals presenting health and environmental risks, while considerably reducing the production time of said plate. Furthermore, the invention advantageously makes it possible to produce a microchannel plate 18 in a plurality of resistive materials, such as ceramic. Finally, in applications involving the use of microchannel plates as electron multipliers within image intensifier tubes, the invention makes it possible to avoid the generation of positive ions within the image intensifier tube and to extend its service life.

Claims

CLAIMS 1. Method for producing a microchannel plate (18), said method comprising the following steps: ■ functionalization of the internal surface of the microchannels (25) by deposition of metallic nanoparticles (40) to allow hopping conduction in the microchannels (25); and ■ fixing the metal nanoparticles (40) on the surface of the microchannels (25) by depositing a layer of dielectric material (34) on the functionalized surface of the microchannels (25); characterized in that the functionalization step comprises the following steps: ■ application of electrostatic charges of a first polarity (41) on the metallic nanoparticles (40) within an aqueous solution (42); ■ application of electrostatic charges, of reverse polarity (43) to those of the electrostatic charges applied to the metal nanoparticles (40), on the internal wall of the microchannels (25); and ■ quenching the microchannels (25) in the solution (42) so as to deposit the metal nanoparticles (40) on the internal surface of the microchannels (25), before the step of fixing the metal nanoparticles (40).

2. Method for producing a microchannel plate (18) according to claim 1, wherein the method comprises the following steps: ■ formation of first fibers, comprising a core soluble in an acid, and a sheath insoluble in the same acid, said sheath surrounding the core; ■ stretching the first fibers to a first desired diameter; ■ production of a first lateral stack of the first fibers within a first preform; ■ melting and cooling of the first stack within the first preform, in order to produce a first base material having an elongated shape extending along a length; ■ drawing second fibers from the first base material to a second desired diameter; ■ production of a second lateral stack of the second fibers within a second preform; ■ melting and cooling the second stack within the second preform, in order to produce a second base material having an elongated shape extending along a length; ■ transverse cutting of the second base material according to a certain cutting plane, said cutting plane forming an angle of between 4° and 12° with a sectional plane perpendicular to said length of said second base material; and ■ dipping the base wafers into the acid in order to dissolve the core in this acid in order to create microchannels (25).

3. Method for producing a microchannel plate (18) according to claim 2, in which the core is soluble in hydrochloric acid, and in which the sheath is insoluble in hydrochloric acid.

4. Method for producing a microchannel plate (18) according to claim 2 or 3, in which the sheath is made of lead-free glass (35).

5. Method for producing a microchannel wafer (18) according to one of claims 1 to 4, in which the method comprises, after the step of fixing the metal nanoparticles (40), a thermal annealing step in order to eliminate any organic residues.

6. Method for producing a microchannel plate (18) according to one of claims 1 to 5, in which the step of applying electrostatic charges of a first polarity (41) to the metal nanoparticles (40) is carried out by applying citrate molecules to the surface of the metal nanoparticles (40) within the aqueous solution (42), by means of a mixture of an aqueous solution of sodium citrate and an aqueous solution of an inorganic metal compound.

7. Method for producing a microchannel plate (18) according to one of claims 1 to 5, in which the step of applying electrostatic charges of reverse polarity (43) to those of the electrostatic charges of a first polarity (41) applied to the metal nanoparticles (40) on the internal wall of the microchannels (25), is carried out according to a silanization method.

8. Method for producing a microchannel plate (18) according to one of claims 1 to 7, in which the metal nanoparticles (40) are made of a non-oxidizable material.

9. Method for producing a microchannel plate (18) according to claim 8, in which the metal nanoparticles (40) are made of gold, silver or platinum.

10. Method for producing a microchannel plate (18) according to one of claims 1 to 9, in which the layer of dielectric material (34) is made of aluminum oxide.

11. Method for producing a microchannel plate (18) according to one of claims 1 to 10, in which the step of fixing the metal nanoparticles (40) on the surface of the microchannels by the deposition of a layer of dielectric material (34) is carried out by an ALD deposition method.

12. Image intensifier tube (13) comprising: ■ an input window (15) configured to receive and transmit the photons; ■ a photocathode (16) fixed on an internal face of said input window (15), capable of converting photons transmitted by the input window (15) into primary electrons (28); ■ an electron multiplier, incorporating a microchannel plate (18), capable of multiplying the primary electrons (28) into secondary electrons (29); and ■ a phosphorescent screen (20) transforming the secondary electrons (29) into photons; characterized in that the microchannel plate (18) is produced by implementing a method according to one of claims 1 to 11.

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

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