INFRARED RADIATION DETECTOR AND ASSOCIATED MANUFACTURING METHOD

Abstract

The invention relates to an infrared radiation detector (10a) comprising: – a cryostat (11) equipped with a cold finger (12) and a window (13) transparent to the infrared radiation to be detected; – a cold plate (14) mechanically fixed and in heat exchange with the cold finger (12); – a detection block (15) in heat exchange directly or indirectly with the cold plate (14); and – a cold screen or diaphragm (16), mechanically fixed and in heat exchange with the cold plate (14), and suitable for limiting stray radiation; the cryostat (11) incorporating at least one zeolite material (17a-17d) having a pore opening of between 5 and 8 Ångströms, configured to maintain a vacuum of less than 10-2 mbar in the cryostat (11) continuously throughout the lifetime of the infrared radiation detector (10a), even when the cold finger (12) is not cooled. Figure for the summary: Fig 2

Description

Title of the invention: INFRARED RADIATION DETECTOR AND ASSOCIATED PRODUCTION METHOD Field of invention

[0001] The invention relates to the field of infrared radiation detectors, more particularly those operating at low temperature, i.e. typically at temperatures between 50 and 210 Kelvin (K). The invention also relates to a method for producing such an infrared radiation detector.

[0002] Such a low-temperature infrared detector, operating according to quantum physics, is associated with a cryostatic enclosure, more conventionally called a "cryostat". The detector also integrates a plurality of unitary or elementary detectors, called photosites, associated with a reading circuit, the assembly, commonly called a "detection block", being mechanically fixed on a cold plane in heat exchange with a cold finger of the cryostat, so as to bring said detector to the desired temperature. The cryostat as well as all the elements internal to this enclosure, such as the detection block, the cold finger or the cold plane, form the infrared radiation detector, also called a "D ewar vessel".

[0003] To limit consumption and achieve cryogenic temperatures, below 210 Kelvin, it is necessary to minimize thermal losses between the so-called "cold" parts, grouping the detection block, the cold plane and the cold finger, and the so-called "hot" parts forming the external wall of the cryostat.

[0004] The heat exchange between these two parts, "hot" and "cold" is carried out according to the following three heat transfer mechanisms: radiation, solid conduction, and gaseous conduction. The loss by gaseous conduction is made negligible compared to the other two types of loss by establishing a vacuum between the "hot" and "cold" parts, that is to say by establishing a pressure lower than 103 mbar in the cryostat during operation.

[0005] The invention relates more particularly to the problem of maintaining a vacuum in the cryostat of an infrared radiation detector, making it possible to limit loss by gas conduction. State of the art

[0006] As illustrated in [Fig.l] of the state of the art, a Dewar vessel 100 is conventionally mounted on a base 20 on which rests a cold machine 21. The cold, generated by this cold machine 21, propagates in a cold finger 12 extending in a cryostat 11 until reaching a cold plane 14 on which is fixed a block of detection 15. A cold screen 16 extends around this detection block 15 to limit parasitic radiation.

[0007] To limit the consumption of the cold machine 21, the heat exchanges between the “hot” and “cold” parts of the Dewar vessel 100 are limited by maintaining the cryostat 11 under a vacuum of less than 103 mbar during operation. To do this, the cold finger 12 is encapsulated under vacuum in a hermetic housing consisting of an external envelope 23 and a window 13 transparent to the infrared radiation to be detected.

[0008] Conventionally, the vacuum inside the Dewar vessel is obtained and maintained thanks to the consecutive action of two devices: a queusot 22 and a pinned getter 190. The queusot 22 makes it possible to create a vacuum inside the cryostat 11 by connecting it to a pumping circuit. It is then hermetically sealed by queusoting. The pinned getter 190, for its part, maintains the vacuum by trapping the surrounding gas once activated.

[0009] However, this technique has drawbacks, particularly in terms of assembly complexity for electrically connecting the pinned getter 190 through the external casing 23 of the cryostat 11, risks for the hermeticity at the level of the power supply circuit of the pinned getter 190, and constraints for the miniaturization of the cryostat 11.

[0010] Added to this is the constraint of thermally protecting the infrared component of the detection block 15. Indeed, the temperature of the infrared component must not conventionally exceed 100°C while the activation temperature of the pinned getter 190 is conventionally between 500°C and 800°C. Sometimes, during activation, it is necessary to protect the pinned getter 190 by an internal thermal screen, and / or to apply a flow of cooling air to the external casing 23 so as to limit the thermal conduction of the getter.

[0011] A solution is therefore sought for maintaining the cryostat 11 under vacuum, without using a pinned getter 190.

[0012] To this end, US 4,474,036 describes a vacuum Dewar vessel containing a molecular gas trap sealed at the cold end, made of porous molecular sorption materials, such as a zeolite. This gas trap replaces the broached getter and is designed to cryosorb the gaseous species generated by degassing, thus maintaining the vacuum during operation of the detector. However, the vacuum in the cryostat is only guaranteed when the cold machine is in operation, and this type of gas trap may be ineffective if the cold machine is stopped for too long because the vacuum level could be greatly degraded during this period. Upon re-cold, a functional vacuum level may be impossible to re-establish or may require a overconsumption of the cold machine and / or a slowdown in the refrigeration time.

[0013] Document WO 90 / 04763 proposes to isolate the "hot" and "cold" parts of the Dewar vessel by a ballast gas inert at atmospheric pressure. This ballast gas is associated with a zeolitic material used to absorb the internal gases by cryosorption, making it possible to achieve a vacuum of pressure lower than 9.4.10 1 mbar when the detector is cooled. As with document US 4,474,036, this method does not maintain a high vacuum outside of the cooling periods and therefore has the same defects. In addition, it is more complex because it requires control of the ballast gas pressure.

[0014] Document US 5,111,049 proposes a method of activating the getter by radiofrequency induction rather than by Joule effect, in order to simplify the process of encapsulating the detection block. However, this activation method remains expensive and complex, and does not solve the problems associated with the high-temperature activation of traditional getters.

[0015] Document EP 0 397 251 describes a method for placing a Dewar vessel under vacuum based on a removable getter in the form of pellets placed in the queusot tube. Although this method avoids high-temperature activation in the Dewar vessel, it is difficult to carry out and does not allow for collective operation on an assembly line, thus limiting its effectiveness for mass production.

[0016] Document US 9,200,359 describes a hermetic vacuum enclosure such as a Dewar vessel, part of the internal walls of which is covered with a film with getter properties.

[0017] This film is composed of two layers, each dedicated to the sorption of at least one type of gas. Although this design eliminates the defects associated with the traditional getter pin, it still requires activation at a relatively high temperature, which can be problematic for some sensitive components of the detection block.

[0018] The teachings of these documents show that zeolites have been used mainly for the cryosorption of gases during periods of cooling of the detector, or to absorb water vapor outside these periods. However, none of these methods provides a solution for maintaining a high vacuum continuously and efficiently throughout the lifetime of the infrared radiation detector, without requiring inert ballast gas or high temperature activation, and without the addition of additional components which could limit the miniaturization of the cryostat. Brief description of the invention

[0019] The invention arises from an observation that; in a cryostat of a Dewar vessel, certain zeolites can exhibit gas sorption properties much higher than those estimated for the pressure and temperature conditions of the cryostat, so that it is possible to use a zeolitic material to maintain a vacuum of less than 102 mbar in the cryostat continuously throughout the lifetime of the infrared radiation detector, even when the cold finger is not cooled.

[0020] Indeed, as a microporous material, the zeolite sorbs gaseous molecules by physisorption, a sorption mechanism based on physical equilibria and therefore reversible as a function of temperature and dependent on the partial pressure of the gas to be sorbed, that is to say that the quantity of molecules adsorbed by physisorption by a zeolite is minimal under conditions of high temperatures and low pressure. The quantity of gas sorbed by a zeolite in contact with the cooled part is therefore maximized when the detector is put into cold.

[0021] Using a zeolite with a pore opening between 5 and 8 Angstroms, it has been observed that the amount of gas sorbed at room temperature by the zeolite is sufficient to maintain a high vacuum inside the Dewar vessel.

[0022] Pore opening in porous materials, such as zeolites, is a key concept in materials science and catalysis. It refers to the size of the pore openings in the structure of the material, which is crucial in determining the resulting adsorption and catalysis properties. More specifically, in the case of zeolites, pore opening is the size of the pore entrance or the diameter of the orifice through which molecules can enter or exit the porous cavity of the material.

[0023] This dimension is crucial because it determines which molecules can be adsorbed, trapped or converted by the material.

[0024] It is known to measure pore opening by several methods: by transmission electron microscopy (TEM), by gas adsorption and / or by X-ray diffraction.

[0025] Thus, the invention provides an infrared radiation detector with a cryostat incorporating a zeolitic material configured to maintain a vacuum of less than 102 mbar in the cryostat continuously throughout the lifetime of the infrared radiation detector, even when the cold finger is not cooled.

[0026] According to a first aspect, the invention relates to an infrared radiation detector comprising: - a cryostat equipped with a cold finger, capable of ensuring heat exchange with a cold source, and a window transparent to the infrared radiation to be detected; - a cold plane fixed mechanically and in thermal exchange with the cold finger; - a detection block comprising at least one detection circuit sensitive to the range of infrared wavelengths to be detected and in thermal exchange directly or indirectly with the cold plane; and - a cold screen or diaphragm, mechanically fixed and in thermal exchange with the cold plane, and capable of limiting parasitic radiation.

[0027] According to the invention, the cryostat incorporates at least one zeolitic material having a pore opening of between 5 and 8 Angstroms, configured to maintain a vacuum of less than 102 mbar in the cryostat continuously throughout the lifetime of the infrared radiation detector, even when the cold finger is not cooled.

[0028] Zeolite is presented as a powder of microcrystals. The latter, introduced into the Dewar vessel, could become a source of particulate pollution. However, it is possible to shape zeolites in order to avoid these constraints. For example, it is possible to synthesize zeolites directly on a metal surface, with the aim of forming a thin layer of zeolitic material, integral with the surface on which it was synthesized.

[0029] Thus, according to one embodiment, the zeolite material is deposited in a thin layer on the cold finger, on an external envelope of the cryostat and / or on the cold screen or diaphragm. This allows efficient integration of the zeolite in the most extensive areas of the cryostat, thus maximizing its gas adsorption properties.

[0030] Furthermore, it is possible to compress the zeolite powder (and optionally add a binder thereto) to form pellets, which have higher mechanical strength and increased compactness compared to zeolite powder. The zeolite material can therefore be deposited in the form of pellets around the cold finger and / or on an external casing of the cryostat. This embodiment makes it possible to offer greater flexibility in the placement of the zeolite material and ease of integration into the cryostat.

[0031] In addition, the zeolite powder can also be encapsulated in a jacket retaining the zeolite microcrystals, but permeable to gas. Thus, the Dewar vessel is protected from the particulate pollution that the powdered zeolite is likely to generate.

[0032] There is a very wide variety of zeolites. According to the invention, the zeolite has a pore opening of between 5 and 8 Angstroms. For example, the zeolite material is made at least partially with faujasite X, silicalite-1 and purely silicic zeolite Beta, because their pore size is suitable for the physisorption of non-cryosorbable gases encountered in an infrared detector cryostat, such as carbon oxides, hydrocarbons and argon.

[0033] Faujasite X (FAU) is characterized by a caged crystal structure. It has a large pore opening of about 7.4 Angstroms, as well as a supercage in the center of its crystal lattice with a diameter of about 11.24 Angstroms. It is mainly composed of silica and alumina, with a ratio of metal atoms constituting its aluminosilicon Si / Al framework typically between 1 and 3, lower than that of faujasite Y. This aluminosilicon composition gives faujasite X a hydrophilic character. It implies that part of the porosity of the zeolite is occupied by a cationic phase, the nature and composition of which can be modified.

[0034] Silicalite-1 (MFI) shares the same zeolite structure as ZSM-5 (MFI), but is composed almost entirely of silica, without alumina. This gives it a hydrophobic character. It has a pore system consisting of interconnected straight and sinusoidal channels with dimensions of about 5.5 Angstroms.

[0035] Purely silicic Beta zeolite is a form of Beta zeolite where aluminum is almost absent, composed almost exclusively of silica. It has a three-dimensional structure of channel pores with pore openings of about 6.6 Angstroms.

[0036] It should be noted that the zeolitic material can be a mixture of zeolites, with or without binder.

[0037] These possible shapings of the zeolite also make it possible to locate the gas traps strategically in the cryostat. Thus, the zeolite pellet can be deposited on the cooled part of the cold finger because their mechanical, thermal and optical properties are compatible. The thermal mass of zeolite necessary for maintaining the vacuum is negligible and therefore does not disrupt the cooling time of the detector. The thin film of zeolite absorbs infrared radiation in the range 0.8 to 16 micrometers and can therefore be deposited on the walls of the cold screen as an absorbent coating, thus combining two functions of “gas absorber” and “optical absorber”.

[0038] In addition to the sorption properties of the zeolite material, the cryostat can also additionally integrate a getter film deposited in a thin layer on an external envelope of the cryostat.

[0039] The getter sorbs by chemisorption, a type of sorption that is irreversible depending on temperature and pressure, but which requires activation (or reactivation if multiple activations are necessary) by heat treatment. A getter placed on the hot part is therefore easier to activate.

[0040] In addition, the getter also acts as a suppressor of degassing of the surfaces on which it is deposited. As such, it is also advantageous to deposit it on the hot part which is most likely to degas since degassing is activated by temperature.

[0041] Preferably, the getter film is made of transition metal alloys selected from yttrium (Y), scandium (Sc), titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), and tantalum (Ta). These metals make it possible to obtain efficient sorption of the gases present in the cryostat of a Dewar vessel. They can be deposited by physical vapor deposition with a low thickness, typically less than 3 micrometers.

[0042] The getter film can also be doped with graphite (C), aluminum (Al), cobalt (Co), iron (Fe), nickel (Ni) and / or one or more elements from the lanthanide group, such as cerium (Ce), which can modify and potentially improve the sorption properties of the getter.

[0043] In addition, the getter film can be topped with an overlayer of gold (Au), palladium (Pd) or platinum (Pt), which can protect the getter against corrosion and improve its performance.

[0044] According to a second aspect, the invention relates to a method for producing an infrared radiation detector according to the first aspect of the invention, comprising the steps of depositing the zeolite material and, where appropriate, a getter film, followed by a final degassing cycle before the cryostat is quenched, the zeolite material and the getter film being activated during the final degassing step preceding the cryostat is quenched. This method ensures that the zeolite material and possibly the getter film are sufficiently activated to maintain the vacuum from the start of the detector's lifetime. Brief description of the figures

[0045] The manner in which the invention can be implemented and the advantages which result therefrom will emerge more clearly from the following example of implementation, given for informational and non-limiting purposes, with the support of the appended figures.

[0046] [Fig.l] is a schematic sectional view of an infrared detector according to the state of the art;

[0047] [Fig.2] is a schematic sectional view of an infrared detector according to a first embodiment of the invention;

[0048] [Fig. 3] is a schematic sectional view of an infrared detector according to a second embodiment of the invention; and

[0049] [Fig.4] is a schematic sectional view of an infrared detector according to a third embodiment of the invention. Detailed description of the invention

[0050] [Fig.2] illustrates a quantum infrared detector or Dewar vessel 10a comprising a cryostat 11 whose cold finger 12 is connected to a cold source 21, typically based on liquid helium, liquid argon or liquid nitrogen. This cryostat 11 is closed in the upper zone by a window 1 3 transparent to the infrared radiation to be detected.

[0051] At the upper end of the cold finger 12 of the cryostat 11, a cold plane 14 is mechanically fixed, moreover in heat exchange with the cold finger 12. An infrared detection block 15 is mounted on the cold plane 14 in heat exchange with the cold plane 14. This detection block 15 is typically made up of a plurality of elementary detectors, associated with an operating circuit, suitable for converting the signals emanating from these detectors into usable electrical signals. The resulting information is conveyed to the outside of the cryostat 11 by connections, in particular wired connections, not shown so as not to unnecessarily load the figure.

[0052] Also reported on this cold plane 1 4 is a cold screen or diaphragm 1 6, also comprising an opening located opposite the transparent window 1 3, arranged at the upper end of the cryostat.

[0053] In the embodiment of [Fig.2], the Dewar vessel 10a incorporates different deposits of zeolitic material making it possible to maintain a vacuum of less than 10 2 mbar in the cryostat 11. The zeolitic material 17a is deposited on the cold screen 16 participating in the adsorption of the gases. It is also deposited 17b on the cold finger 12, exploiting the heat exchange with the cold source to improve the efficiency of the adsorption of the gases when the detector is in operation.

[0054] The zeolitic material is also deposited 17c on the external casing 23 of the cryostat 11, allowing adsorption of gases over a large internal surface, and in the form of pellets 17d around the cold finger 12, providing a modular and easily integrated method for maintaining the vacuum.

[0055] [Fig. 3] shows a Dewar 10b which combines the zeolite material with a getter film for improved performance. The zeolite material is deposited 17a-17b on the cold shield 16 and on the cold finger 12, as in [Fig. 2]. In addition, the zeolite material is deposited 17e in the form of pellets on the outer shell 23 of the cryostat of the cryostat 11. The getter film 19 is also deposited in a thin layer on the outer shell 23 of the cryostat 11.

[0056] [Fig.4] illustrates a Dewar vessel 10c in which the zeolite material is encapsulated and a getter film is used. The zeolite material 17f is deposited in a gas-porous envelope but filtering out any particles, protecting the detector from particulate pollution while allowing the adsorption of gases. The getter film 19 is deposited in a thin layer on the external envelope 23 of the cryostat 11.

[0057] Each configuration described in Figures 2 to 4 offers unique advantages for maintaining the vacuum in the cryostat, thereby contributing to the long-term performance and reliability of the infrared radiation detector. Of course, these examples are not limiting and illustrate multiple possible uses of the material zeolitic. In practice, it is possible to use only a small quantity of the zeolitic material by covering only a part of the volumes or surfaces mentioned.

[0058] The development of the vacuum in the Dewar vessel 10a-10c is obtained by connecting the Dewar vessel 10a-10c to a pumping system via the pipe 22.

[0059] A heat treatment called "degassing" also allows the effective degassing of the species that have physisorbed onto the zeolite after its deposition in the 10a-10c Dewar vessel and the assembly of the rest of the hybridized detector. Indeed, the quantity of species adsorbed onto a zeolite decreases with increasing temperature and decreasing pressure. The degassing heat treatment therefore progressively "empties" the zeolite of the species physisorbed onto its surface, until a new state of equilibrium is reached between the quantity of species adsorbed onto the zeolite and the pressure.

[0060] In parallel, when present, the getter film will also be activated during this degassing heat treatment. The progressive dissolution of its native oxide during the treatment will release active sites on the surface, i.e. dangling metal bonds, which will be capable of chemisorbing gaseous molecules.

[0061] Given the low activation temperature, only a partial dissolution of the native oxide is achieved, but sufficient nonetheless to release active sites. At the end of the heat treatment, both the getter and the zeolite can therefore be considered as being "active", i.e. capable of sorbing gaseous species: the getter because it has active sites, and the zeolite because it has reached a state of equilibrium in which the quantity of physisorbed species on its surface is very low, and in fact minimal compared to the temperature and pressure conditions that the zeolite will subsequently experience.

[0062] Once the Dewar 10a-10c vessel has returned to room temperature, the queusotage then takes place, which hermetically isolates the Dewar 10a-10c vessel from the pumping system. From this moment and until the end of the detector's lifetime, typically at least twenty years, the pressure in the Dewar 10a-10c vessel deteriorates according to three pressure build-up mechanisms: leakage, degassing, permeation.

[0063] Four main types of gaseous molecules are generated by these mechanisms: carbon oxides, nitrogen, hydrocarbons and argon. Two types of molecules, carbon oxides and nitrogen, can be chemisorbed by the active sites of the activated getter, as they are generated. The getter cannot, however, chemisorb hydrocarbons and argon. The generation of these molecules is preferentially countered by the zeolite, which is also capable of countering the generation of nitrogen and carbon oxides. A portion of the hydrocarbon and argon molecules is trapped by physisorption by the zeolite, the other remains in phase gaseous, according to the thermodynamic equilibrium of physisorption. Although the sorption by the zeolite is only partial, it is still sufficient to maintain a high vacuum in the Dewar flask 10a-10c.

[0064] The quantity of physisorbed molecules can be adjusted by the quantity of zeolite introduced into the 10a-10c Dewar vessel. The internal pressure can rise to a maximum of 102 mbar at room temperature: the 10a-10c Dewar vessel therefore remains under high vacuum throughout the lifetime of the detector. The internal pressure is, however, at least lower than 103 mbar when the detector is put into operation, i.e. cooled to a cryogenic temperature.

[0065] Indeed, at these very low temperatures, the surface of the cold walls physisorbs gas molecules according to the well-known cryosorption effect, and the same applies to the zeolitic gas trap placed on the cold part which also has a very large developed surface: when cooled, the quantity of molecules physisorbed on the surface of the zeolite is increased.

[0066] The excess vacuum thus generated during cooling is sufficient to ensure optimal thermal insulation of the hot and cold parts. The lower the cooling temperature, the better the vacuum obtained. This optimal vacuum can thus be generated quickly from the vacuum existing during storage of the product at room temperature, and this throughout the lifetime of the detector, because the so-called “storage” vacuum is maintained by the zeolite and the getter.

Claims

Claims

1. Infrared radiation detector (10a-10c) comprising a cryostat (11) provided with a cold finger (12), capable of ensuring heat exchange with a cold source, and a window (13) transparent to the infrared radiation to be detected, said cryostat integrating: - a cold plane (14) mechanically fixed and in heat exchange with the cold finger (12); - a detection block (15) comprising at least one detection circuit sensitive to the range of infrared wavelengths to be detected and in heat exchange directly or indirectly with the cold plane (14); and - a cold screen or diaphragm (16), mechanically fixed and in heat exchange with the cold plane (14), and capable of limiting parasitic radiation;characterized in that the cryostat (11) incorporates at least one zeolitic material (17a-17f) having a pore opening of between 5 and 8 Angstroms, configured to maintain a vacuum of less than 10 2 mbar in the cryostat (11) continuously throughout the lifetime of the infrared radiation detector (10a-10c), even when the cold finger (12) is not cooled.;

2. An infrared radiation detector according to claim 1, wherein the zeolitic material (17a-17f) is deposited in a thin layer on the cold finger (12), on an external casing (23) of the cryostat (11) and / or on the cold screen or diaphragm (16).

3. An infrared radiation detector according to claim 1 or 2, wherein the zeolitic material (17a-17f) is deposited in the form of pellets around the cold finger (12) and / or on an outer casing (23) of the cryostat (11).

4. Infrared radiation detector according to one of claims 1 to 3, in which the zeolitic material (17a-17f) is encapsulated in a hermetic envelope (18) with zeolite microcrystals, but permeable to gas.

5. Infrared radiation detector according to one of claims 1 to 4, wherein the zeolitic material (17a-17f) is constituted at least partially with faujasite X, silicalite-1 and purely silicic Beta zeolite.

6. Infrared radiation detector according to one of claims 1 to 5, in which the cryostat (11) further incorporates a getter film (19) deposited in a thin layer on an external envelope (23) of the cryostat (H).

7. An infrared radiation detector according to claim 6, wherein the getter film (19) is made of transition metal alloys selected from yttrium (Y), scandium (Sc), titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), and tantalum (Ta).

8. An infrared radiation detector according to claim 6 or 7, wherein the getter film (19) is doped with graphite (C), aluminum (Al), cobalt (Co), iron (Fe), nickel (Ni) and / or one or more elements from the lanthanide group.

9. Infrared radiation detector according to one of claims 6 to 8, in which the getter film (19) is topped with an overlayer of gold (Au), palladium (Pd) or platinum (Pt).

10. Method for producing an infrared radiation detector according to one of the preceding claims, comprising the steps of depositing the zeolitic material (17a-17f) and, where appropriate, a getter film (19), followed by a final degassing cycle before the queusoting of the cryostat (11), the zeolitic material (17a-17f) being activated during the final degassing step preceding the queusoting of the cryostat (11).