METHOD FOR PRODUCING A MICROCANAL SHEET, IMAGE INTENSIFIER TUBE AND ASSOCIATED NIGHT VISION SYSTEM

Abstract

This process for producing a wafer of microchannels includes the following steps: – functionalization of the internal surface of the microchannels (25) by deposition of metallic nanoparticles (40) to allow jump conduction in the microchannels (25); and – fixation of the metallic nanoparticles (40) on the surface of the microchannels (25) by deposition of a layer of dielectric material on the functionalized surface of the microchannels (25).The functionalization step comprises the following steps: – application of electrostatic charges of one polarity (41) to the metallic nanoparticles (40) within an aqueous solution; – application of electrostatic charges of opposite polarity (43) to those applied to the metallic nanoparticles (40) on the inner wall of the microchannels (25); and – immersion of the microchannels (25) in the solution so as to deposit the metallic nanoparticles (40) on the inner surface of the microchannels (25), before the fixation step of the metallic nanoparticles (40). Figure for the abstract: Fig 8c.

Description

Title of the invention: METHOD FOR PRODUCING A MICROCANAL SHEET, IMAGE INTENSIFIER TUBE AND ASSOCIATED NIGHT VISION SYSTEM Scope of the invention

[0001] The invention relates to the field of microchannel wafers, particularly for the amplification and detection of low-intensity signals. More specifically, the invention relates to a method for producing a microchannel wafer that can be used as an electron multiplier for various applications, particularly in the fields of night vision and scientific research.

[0002] 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.

[0003] In the field of scientific research, the particular properties of microchannel wafers offer distinct advantages for a variety of applications requiring the detection and / or amplification of signals from various 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.

[0004] This invention therefore presents a multitude of potential applications, particularly when there are problems with the use of potentially toxic materials.

[0005] 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, to the recovery and to the ecological disposal of this waste. State of the art

[0006] 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 13 0 comprises at least three distinct elements: a photocathode 16, an electron multiplier 18 0, and a phosphorescent screen 20.

[0007] The photocathode 16 is in the form of a semi-transparent photosensitive layer receiving photons from the incident electromagnetic radiation, that is- that is to say the photons transmitted by the lens 12. To do this, an input window 15 transmits the photons from the lens 12 onto the photocathode 16.

[0008] The photocathode 16 is generally made in the form of a thin layer of metal or semiconductor deposited on 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, by the photoelectric effect, the emission of electrons, called primary electrons 28 or photoelectrons.

[0009] The photocathode 16 is characterized by its quantum efficiency, also known as "quantum efficiency" in the English-language literature. This quantum efficiency, generally expressed as a percentage, refers to 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.

[0010] The primary electrons 28 are then subjected to a first electric field within a first acceleration zone 17, enabling the primary electrons to be directed 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 on 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.

[0011] The electron multiplier 180, also called an electron amplifier, conventionally 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 of a resistive or dielectric material plate typically with a thickness between 0.2 and 1 millimeter.

[0012] As illustrated more precisely in Figure 3 of the prior art, the microchannels 25 pass completely 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 to achieve the transmission of an electric current by hopping, known as "hopping conduction" in the English-language literature.

[0013] These microchannels 25 have an axis of revolution a2 inclined at an angle a3 of a few degrees, typically between 4 and 12 degrees, with respect to the normal to the surface of the electron multiplier 18 0, so as to induce multiple collisions of the primary electrons 28 in the microchannels 25.

[0014] 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 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 maintain a constant number of electrons inside the microchannels 25.

[0015] With this second electric field, during the multiple collisions between the photoelectrons and the material constituting the microchannels 25, a larger number of secondary electrons is generated by cascade effect.

[0016] 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.

[0017] 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.

[0018] These secondary electrons, once generated, leave a positive charge in the microchannel material 25 if this electron loss is not compensated. The conduction current inside 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.

[0019] Typically, the primary electrons 28 are multiplied by a factor between 103 and 106 in the electron multiplier 18 0. The conversion rate of the primary electrons 28 into secondary electrons 29 corresponds to the electrical gain of the electron multiplier 18 0.

[0020] At the exit of the microchannels 25, these secondary electrons 29 are then moved linearly in the direction of 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.

[0021] The phosphorescent screen 20 allows the secondary electrons 29 to be transformed into photons. It is in the form of a phosphorescent layer or a layer of a luminescent material deposited on a substrate, conventionally glass. The rate of conversion of secondary electrons 29 into measured 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, on the order of 10⁷ to 10¹⁰ mbar. To achieve 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 one of its components can cause premature wear, leading to reduced intensification of the observed image radiation. 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 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 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.

[0027] Given the first electric field aimed at transmitting the negatively charged electrons from the photocathode 16 to the electron multiplier 180, these positive ions 32 are attracted towards the photocathode 16 and travel 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.

[0028] The degradation of quantum efficiency is particularly present in image intensifier tubes 130 using electron multipliers 180 made with lead glass 30. Indeed, to make a wafer of microchannels 25, it is known to make first fibers with a core soluble in an acid, and a sheath surrounding the core, which is insoluble in the same acid. These first fibers, called primary fibers, are obtained by drawing them to a first desired diameter and then assembled to form a hexagonal preform composed of a large number of primary fibers.

[0029] Secondary fibers, from the first base material, are then obtained by stretching the hexagonal preform to a second desired diameter and then assembled again to produce a second long-stretched base material obtained after melting and cooling the secondary fibers together.

[0030] The second elongated base material is then cut transversely along a specific cutting plane. 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 in the acid, revealing the microchannels 25.

[0031] The selectivity of the core and sheath materials according to their solubility in the acid is essential for the realization of the microchannels 25. Indeed, it is necessary that the sheath be insoluble in this acid, so that quenching can only dissolve the core.

[0032] Lead glass 30, which is in the form of a lead silicate, is an example of a particularly effective sheathing material for forming these microchannel wafers 25, given its insolubility in a plurality of acids. Furthermore, this lead glass 30 has the particularity of being easily chemically reduced by means of dihydrogen, generating metallic islands of lead 31 on the internal surface of the microchannels 25, with a size ranging from tens to a few tens of nanometers, as illustrated in [Fig. 4].

[0033] These metallic 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.

[0034] Thus, these lead metallic islands 31 allow the passage of electronic current, by an electronic conduction known as jump conduction: electrons "jump" from one lead metallic island to another lead metallic island under the effect of the electric field generated between the two metallic electrodes on the faces of the electron multiplier 180. The lead glass 30, which allows this jump conduction when subjected to a chemical reduction step of its microchannels 25, is characterized by high resistivity values, on the order of 10⁶ to 10⁹ ohm.cm, by 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 variation negative resistance when the operating temperature of electron multiplier 180 decreases.

[0035] The main drawback of this type of electron multiplier 180 is that it allows hydrogen atoms to be trapped in the lead glass following its reduction by dihydrogen. These hydrogen atoms subsequently generate hydrogen cations 32 when the primary electrons 28 interact with the internal surface of the microchannels 25 to produce the secondary electrons 29, as illustrated in [Fig. 4]. Furthermore, sodium or potassium cations may also be emitted since these are also likely to accumulate on the surface of the microchannels 25 during the reduction.

[0036] 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 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.

[0037] 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".

[0038] A first prior art solution illustrated in [Fig.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 surmounted by an emissive layer 34.

[0039] The passivation layer 33 is typically nanolaminated onto the surface of the leaded glass 30 and prevents the production and emission of hydrogen, sodium, or potassium cations 32 during 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 from the leaded glass to the emissive layer 34, in order to provide sufficient electrons to the emissive layer 34 for the generation of secondary electrons 29.

[0040] Electrons from the lead metallic 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 in contact with the primary electrons 28, thanks to the electrons received from the lead metallic islands 31 through the passivation layer 33. In this first In the solution, the passivation layer 33 and the emissive layer 34 are made by deposition of atomic thin films, also known by the acronym ALD for "atomic layer deposition" in the Anglo-Saxon literature.

[0041] This first solution does indeed prevent the generation of positive ions, but has several drawbacks. Firstly, the deposition of the first passivation layer 31 by ALD and the second emissive layer 32, also by ALD, requires an excessively long production time, typically several days.

[0042] Furthermore, the use of lead 30 glass and the glass reduction step pose a health hazard to operators and technicians, as well as environmental problems. Therefore, this solution does not comply with the European RoHS directive and presents risks to human health and the environment, considering the life cycle of a microchannel wafer made with lead 30 glass.

[0043] To avoid using leaded glass 30 to form the wafer of the electron multiplier 180, a second prior art solution proposes to directly form the secondary electrons 29 in a conductive layer 36, as illustrated in [Fig. 6]. This second solution is described in particular in US patent 8921799.

[0044] In this second solution, the sheath can thus be made of lead-free glass 35, ceramic, or any other material suitable for resisting the dissolution of the core and having a high resistivity to electric current, on the order of at least 108 to 109 ohm.cm.

[0045] Since the sheath does not incorporate metallic molecules that would allow for the rapid formation of metallic islands, said sheath serves only as mechanical support. The internal surface of the microchannels 25 in this type of electron multiplier 180 is thus coated by a first conductive layer 36 supplying 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 prior art solution, the fabrication time for such an electron multiplier 180 is excessively long.

[0046] In the field of microchannel wafers 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 manufacturing 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 dangerous and presenting health risks for operators or technicians; or even environmental risks.

[0047] In other fields 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. Description of the invention

[0048] The invention proposes to address this technical problem by forming a microchannel wafer 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 electron 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.

[0049] The invention stems from the observation that, contrary to the technical prejudice of a person skilled in the art, it is possible to carry out a deposit of 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 time of realization.

[0050] Thus, instead of classically reducing lead-doped glass or using the ALD process alone to produce a microchannel wafer, the inventors have found a method for producing a microchannel wafer within a reasonable time, without generating positive ions, while also allowing functional microchannel wafers to be produced 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.

[0051] According to a first aspect, the invention therefore relates to a method for producing a wafer of microchannels, said method comprising the following steps: - functionalization of the internal surface of the microchannels by deposition of metallic nanoparticles to allow jump conduction in the microchannels; and - fixation of the metallic nanoparticles on the surface of the microchannels by deposition of a layer of dielectric material on the functionalized surface of the microchannels.

[0052] According to the invention, the functionalization step comprises the following steps: - application of electrostatic charges of a first polarity on the metallic nanoparticles within an aqueous solution; - application of electrostatic charges, of opposite polarity to those of the electrostatic charges applied to the metallic nanoparticles, on the inner wall of the microchannels; and - dipping the microchannels in the solution so as to deposit the metallic nanoparticles on the internal surface of the microchannels, before the step of fixing the metallic nanoparticles.

[0053] In addition to the functionalization steps specific to the invention, the process may also include the following steps for forming the microchannels before functionalizing them: - formation of 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 up to a first desired diameter; - creation of a first lateral stacking 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 basic material having an elongated shape extending along a length; - drawing of second fibers from the first base material up to a second desired diameter; - creation of a second lateral stacking of the second fibers within a second preform; - melting and cooling of 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 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, in order to produce multiple base discs; and - soaking the base wafers in an acid to dissolve the core in this acid in order to create microchannels.

[0054] Preferably, the sheath is made of lead-free glass. To achieve sheath removal 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.

[0055] Preferably, the sheath is made of glass, silica-based glass, silicate or ceramic, or any perforable dielectric material having an electric current resistivity between 10⁸ and 10¹¹ ohm.cm, or even a resistivity greater than 1010 ohm.cm or 1011 ohm.cm; as well as the ability to be insoluble in an acid capable of dissolving the core.

[0056] The core is generally made of boro-borate glass, or of glass containing boron, alkali cations, and / or alkaline earth cations.

[0057] In the chemical process of depositing electrostatic charges on the internal surface of microchannels, the step of applying electrostatic charges of a first polarity on the metallic nanoparticles can be carried out by applying citrate molecules to the surface of the metallic nanoparticles within an aqueous solution, by means of a mixture of an aqueous solution of sodium citrate and an aqueous solution of a metallic inorganic compound.

[0058] In addition, the step of applying electrostatic charges, of opposite polarity to those of the electrostatic charges applied on the metallic nanoparticles, to the internal wall of the microchannels, is preferably carried out according to a silanization process.

[0059] The step of fixing the metallic nanoparticles on the surface of the microchannels by depositing a layer of dielectric material can be carried out by an ALD deposition process, or in a variant by a sol-gel deposition process.

[0060] Furthermore, after the step of fixing the metallic nanoparticles, a thermal annealing step can be implemented in order to eliminate any organic residues.

[0061] With regard to the materials used to achieve efficient conduction and efficient generation of secondary electrons, the metallic nanoparticles are preferably made of a non-oxidizable material, for example gold, silver, or platinum. The dielectric layer, which performs the secondary electron emission function, is preferably made of aluminum oxide Al₂O₃ / sAl₂O₃ or magnesium oxide MgO.

[0062] According to a second aspect, the invention also relates to an image intensifier tube comprising: - an input window configured to receive and transmit photons; - a photocathode fixed on an inner face of said entrance window, capable of converting the photons transmitted by the entrance window into primary electrons; - an electron multiplier, incorporating a wafer of microchannels, capable of multiplying primary electrons into secondary electrons; and - a phosphorescent screen transforming secondary electrons into photons.

[0063] According to the invention, the microchannel wafer is produced by implementing a process according to the first aspect of the invention.

[0064] According to a third aspect, the invention relates to a night vision system comprising a lens, an image intensifier tube according to the second aspect of the invention, and an eyepiece and / or a sensor. Brief description of the figures

[0065] 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 elements, and in which:

[0066] Fig. 1 illustrates a schematic cross-sectional view of a night vision system comprising an image intensifier tube of the prior art;

[0067] [Fig.2] illustrates a perspective view of the image intensifier tube of [Fig.1];

[0068] [Fig.3] illustrates a schematic cross-sectional view of the electron multiplier of the night vision device of [Fig.1];

[0069] Fig. 4 illustrates a schematic cross-sectional view of a microchannel of a wafer of microchannels according to a prior art embodiment, generating secondary electrons and positive ions;

[0070] Figure 5 illustrates a schematic cross-sectional view of a microchannel of a wafer of microchannels according to another prior art embodiment, generating secondary electrons;

[0071] Figure 6 illustrates a schematic cross-sectional view of a microchannel of a wafer of microchannels according to another prior art embodiment, generating secondary electrons;

[0072] Fig. 7 illustrates a schematic cross-sectional view of a microchannel of a wafer of microchannels according to an embodiment of the invention, generating secondary electrons;

[0073] Fig. 8a illustrates a schematic cross-sectional view of the step of applying electrostatic charges to metallic nanoparticles, according to a method of producing the microchannel wafer of Fig. 7;

[0074] Fig. 8b illustrates a schematic cross-sectional view of the step of applying electrostatic charges to the internal surface of the microchannel, according to a method of making the microchannel wafer of Fig. 7;

[0075] Fig. 8c illustrates a schematic cross-sectional view of the step of bringing metallic nanoparticles from Fig. 8a into contact with the internal surface of the microchannel from Fig. 8b;

[0076] Fig. 8d illustrates a schematic cross-sectional view of a microchannel of the microchannel wafer of Fig. 8c after interaction of metallic nanoparticles on the internal surface of the microchannel;

[0077] Fig. 8e illustrates an overview of an apparatus enabling the contact of charged metallic nanoparticles with the charged microchannel wafer;

[0078] Fig. 8f illustrates a schematic cross-sectional view of a microchannel of the microchannel wafer of Fig. 8d after application of a secondary electron emissive layer on the internal surface of the microchannel;

[0079] Figure 9 illustrates a schematic cross-sectional view of a night vision system comprising an image intensifier tube according to another aspect of the invention. Detailed description of the invention

[0080] Fig. 7 illustrates the internal surface of a microchannel 25, said microchannel 25 being part of a wafer of microchannels 18 made of lead-free glass, according to a process according to the invention.

[0081] This microchannel 25 comprises on its surface metallic nanoparticles 40, in this case metallic gold nanoparticles, and a layer of dielectric material 34. [Fig.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 wafer 18 is used as an electron multiplier.

[0082] Several manufacturing steps are carried out in order to obtain a wafer provided with a plurality of microchannels 25.

[0083] 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 to form a first block, which has an elongated shape.

[0084] Secondly, second fibers are produced 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 elongated block is produced by fusing the stack of the second fibers.

[0085] Finally, this second block is cut at a specific 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 then soaked in an acid that dissolves the core without dissolving the sheath, resulting in the production of the microchannel wafers 18. The acid used can be hydrochloric acid, nitric acid, or hydrofluoric acid; these three acids can can also be used successively, alternately, or independently of each other.

[0086] The sheath is made of glass, silica-based glass, silicate or ceramic, or any perforable dielectric material having an electric current resistivity between 108 and 1011 ohm.cm. The core, on the other hand, is generally made of boro-borate glass, or of glass containing boron, alkali cations, and / or alkaline earth cations.

[0087] Advantageously, this choice of distinct materials for the sheath and the core allows for selectivity of dissolution when the base wafers are soaked in a determined acid, resulting in dissolution of the core without degrading the constituent material of the sheath.

[0088] The microchannel wafer 18 produced has a thickness L between 170 and 190 micrometers, with microchannels 25 having a diameter D of approximately 3 micrometers. The pitch of the microchannel wafer 18, namely the spacing of the microchannels 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.

[0089] According to [Fig. 7], the internal surface of a microchannel 25 of a microchannel wafer 18 is shown in longitudinal section, said microchannel 25 being made, as previously mentioned, of a material having an electrical resistivity between 10⁸ and 10¹¹ ohm.cm. On the internal surface of the microchannel 25, a first deposit of metallic nanoparticles 40 and a second deposit of a layer of dielectric material 34 are made on the surface of the microchannels. These successive deposits are made after a plurality of successive steps corresponding to the process according to the invention, illustrated in Figures 8a to 8f.

[0090] With reference to [Fig. 8a], the first step in producing a microchannel wafer 18 according to the invention involves applying negative electrostatic charges 41 to the surface of the metallic nanoparticles 40. These metallic nanoparticles 40 are in the form of gold metallic nanoparticles in a solution of HauCl4 chloroauric acid 42. This chloroauric acid solution 42 is then added to a sodium citrate mixing solution to obtain a solution comprising charged gold metallic nanoparticles 40. These charged metallic nanoparticles 40 thus present negative charges 41 on their surface in the form of negatively charged citrate molecules, obtained using the sodium citrate solution.

[0091] According to a particular embodiment, functionalized gold metal nanoparticles 40 are obtained as follows: a chlorauric acid solution 42 with a molar concentration of 0.25 mM is brought to a boil, and then an appropriate volume of 1% concentrated sodium citrate is added to the chlorauric 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 flow filtration. This process for preparing gold metal nanoparticles 40, known in the prior art as the Turkevich citrate reduction process, yields stabilized spherical gold-citrate metal nanoparticles 40.Ideally, the colloidal stability of the suspension of 40 gold metallic nanoparticles is characterized by UV-Vis spectroscopy and by DLS, an acronym for "Dynamic Light Scattering" in the Anglo-Saxon literature.

[0092] The solution of gold metallic nanoparticles 40 loaded with citrate molecules on their surface is then reconcentrated up to 200 times by tangential flow filtration. According to one embodiment, polysorbate 80 can be added to the chloroauric acid-sodium citrate mixture to improve colloidal stability.

[0093] As shown in [Fig. 8b], the second step in producing a lead-free glass microchannel wafer 35 18 according to the invention consists of 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 one embodiment, the microchannel wafer 18 can be made with leaded glass, but not reduced lead, thus preventing exposure to lead particles and protecting the environment and operators from the harmful effects of lead particles.

[0094] 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 5% concentrated solution of a silane is prepared, in this case the utility silane is EDMPS (N-(-aminoethyl)3-aminopropylmethyldimethoxisilane (95%)), which is diluted in ultrapure water. The microchannel wafer 18 is then soaked in this solution and subsequently subjected to sonication for one hour at a temperature of 50°C. The microchannel wafer 18 is then rinsed two to 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 airflow. Advantageously, a thermal annealing phase can be considered to eliminate certain organic residues.

[0095] According to an unrepresented variant, the silanization of the microchannel wafers 18 does not require any heat treatment, in order to avoid the aggregation of the metallic nanoparticles 40.

[0096] With reference to [Fig.8c], the negatively charged metallic nanoparticles 40 are deposited on the positively charged surface of the microchannel 25. Since the respective polarizations of the metallic nanoparticles 40 and the surface of the channel are reversed, the negative charges 41 on the metallic nanoparticles 40 will be attracted by the positive charges 43 on the surface of the microchannel 25, these positive charges 43 acting as a ligand, leading to the bonding of the gold metallic nanoparticles 40 to the wall of the internal surface of the microchannel 25, as shown in [Fig.8d].

[0097] The gold metallic 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 pm2. According to other embodiments, the density of the gold metallic nanoparticles deposited on the surface of the microchannels can be up to 1500 to 1600 nanoparticles per pm2.

[0098] The deposition phase of the loaded metallic nanoparticles 40 on the surface of the microchannels 25 is carried out by means of a recirculating impregnation installation, by means of a solution reservoir 44 containing the metallic gold nanoparticles 40, a support 45 to hold the microchannel wafer 18, a recirculation loop 46, and a peristaltic pump 47, as shown in [Fig.8e].

[0099] Figure 8f represents the final step in the fabrication of a microchannel wafer 18 according to the invention, in which a layer of dielectric material 34 is deposited on the surface of the functionalized microchannel 25, enabling the metallic nanoparticles 40 to be fixed to the surface of the microchannel 25. This layer is made of a dielectric material, and more specifically of aluminum oxide Al₂O₃ / sAl₂O₃ or magnesium oxide MgO, advantageously enabling the generation of secondary electrons when the microchannel wafer 18 is used as an electron multiplier. This layer of dielectric material 34 is produced by ALD deposition or by sol-gel deposition.

[0100] Figure 9 represents a night vision system 11 comprising an image intensifier tube 13, a lens 12, and an eyepiece 14, aligned on an optical axis 1a1. 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 18 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 lifespan of the image intensifier tube 13, and allowing the avoidance of the use of leaded glass.

[0101] Thus, the invention makes it possible to produce a functional microchannel wafer 18, avoiding the use of metals that present health and environmental risks, while considerably reducing the manufacturing time of said wafer. Furthermore, the invention advantageously allows for the fabrication of a microchannel wafer 18 in a plurality of resistive materials, such as ceramics. Finally, in applications using microchannel wafers 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 lifespan.

Claims

Demands

1. A method for producing a microchannel wafer (18), said method comprising the following steps: - functionalizing the internal surface of the microchannels (25) by depositing metallic nanoparticles (40) to allow jump conduction in the microchannels (25); and - fixing the metallic nanoparticles (40) to 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: - applying electrostatic charges of a first polarity (41) to the metallic nanoparticles (40) within an aqueous solution (42); - applying electrostatic charges of opposite polarity (43) to those of the electrostatic charges applied to the metallic nanoparticles (40), on the internal wall of the microchannels (25);and - dipping the microchannels (25) in the solution (42) so as to deposit the metallic nanoparticles (40) on the internal surface of the microchannels (25), before the step of fixing the metallic nanoparticles (40).;

2. A method for producing a microchannel wafer (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; - drawing the first fibers to a first desired diameter; - making a first lateral stack of the first fibers within a first preform; - melting and cooling 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; - making a second lateral stack of the second fibers within a second preform; - melting and cooling of 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 along a certain cutting plane, said cutting plane forming an angle between 4° and 12° with a plane in section perpendicular to said length of said second base material; and - quenching of the base wafers in acid in order to dissolve the core in this acid in order to produce microchannels (25).

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

4. Method of making a wafer of microchannels (18) according to claim 2 or 3, wherein the sheath is made of lead-free glass (35).

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

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

7. A method for producing a wafer of microchannels (18) according to any one of claims 1 to 5, wherein the step of applying electrostatic charges of opposite polarity (43) to those of electrostatic charges of a first polarity (41) applied to the metallic nanoparticles (40) on the inner wall of the microchannels (25), is carried out according to a silanization process.

8. Method for producing a microchannel wafer (18) according to any one of claims 1 to 7, wherein the metallic nanoparticles (40) are made of a non-oxidizable material.

9. Method for producing a microchannel wafer (18) according to claim 8, wherein the metallic nanoparticles (40) are made of gold, silver or platinum.

10. Method of producing a microchannel wafer (18) according to any one of claims 1 to 9, wherein the dielectric material layer (34) is made of aluminum oxide.

11. A method for producing a microchannel wafer (18) according to any one of claims 1 to 10, wherein the step of fixing the metallic nanoparticles (40) to the surface of the microchannels by depositing a layer of dielectric material (34) is carried out by an ALD deposition process.

12. 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 input window (15), capable of converting photons transmitted by the input window (15) into primary electrons (28); - an electron multiplier, incorporating a wafer of microchannels (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 wafer of microchannels (18) is made by implementing a method according to any one of claims 1 to 11.

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

Images