Electronic tube using low-temperature solders and degassed at room temperature

Abstract

The invention relates to a method for degassing an electronic tube, comprising the steps of: - pumping (1) a sealed element of the electronic tube to be placed under ultra-vacuum, by means of a pumping structure connected to the tube by a pumping tip, until a pressure falling below a threshold is obtained in the sealed element; and - irradiating (2) the electronic tube with ionizing rays during the pumping step (1) at ambient temperature.

Description

DESCRIPTION Title of the invention: Electronic tube using low temperature solders and degassed at room temperature.

[0001] The invention relates to a method for degassing an electronic tube and an electronic tube obtained by such a method.

[0002] The invention concerns electronic tubes, such as traveling wave tubes with the acronym TOP, klystrons, gyrotrons, magnetrons, or even X-ray tubes. It applies whenever an ultra-high vacuum (<10 -7 at 10' 9 Torr) is necessary for the tube to operate properly in the presence of an intense (>1 mA) or powerful (>10 watts) electron beam, or both.

[0003] The only known solution today to obtain a satisfactory vacuum is to bake the electron tubes at a temperature between 450 and 550°C for several hours. The electron tube is brought to these temperatures while it is still mounted on its pumping frame by its evacuation pipe called tail. The temperature causes the surfaces to degas, and this gas is extracted by the pumping frame. After this long baking period (typically 24 hours), the electron tube is brought back to room temperature and the copper tail is cut by crushing, which causes it to be sealed by a cold welding phenomenon.

[0004] At bake temperatures, low-melting metals evaporate into the vacuum and pollute the inside of the tube. This pollution is unacceptable because it poisons the thermionic cathode and degrades the high-voltage insulation of the ceramics present in the electronic tube. This pollution is present with all metals that have a high saturation vapor pressure at these temperatures. These solders are typically alloys of copper, gold, silver, and nickel, but are not limited to these alloys. Silver is said to be the last compatible metal for bake. Its saturation vapor pressure is 4 E -9 Torr at 550°C.

[0005] The invention aims to eliminate high-temperature brazing in the design of an electronic tube. They require brazing temperatures ranging from approximately 800 to 1000°C, which places significant limitations on the materials that can be used and causes significant thermomechanical constraints during brazing. These constraints require having thermal expansion coefficients very close between the brazed materials, as well as very mechanically resistant parts, allowing them to withstand residual mechanical stresses after brazing. The design is complicated, the suitable materials are very limited and expensive (for example Mo-Cu and W-Cu composites for brazing with ceramics). Brazing processes are expensive, because to avoid oxidation it is often necessary to braze in a vacuum or in controlled hydrogen atmospheres. Furthermore, gold mining has a very high environmental cost and the invention aims to do without it.

[0006] Brazing constraints even prohibit the use of certain ceramics such as aluminum nitride AIN because we do not know of sufficiently robust metallizations on their surface that can withstand high-temperature brazing, especially if this brazing must ensure the tube's watertightness. However, AIN allows for much better cooling of tube components thanks to its very high thermal conductivity compared to alumina (by a factor of about 10), which is the main ceramic used today.

[0007] Baking tubes below 450°C is not possible. The main reason is that the last water desorption peak is at this temperature. Water is one of the main molecules bound to the inner walls of the tube, which must be removed during baking. Otherwise, the interception of the electron beam on these walls, during nominal operation of the tube, causes a strong degassing of these molecules and prevents the tube from functioning properly. Poisoning of the cathode, due to ion formation and ionic erosion impacting the focusing of the electron beam are problems that arise in the event of insufficient degassing.

[0008] The problem of surface degassing stems from the binding energy of molecules with the surfaces. One aim of the invention is to eliminate the risk of these molecules being released during nominal operation of the tube. The main problem then is the poisoning of the cathode. Molecules very weakly or very strongly bound to the surfaces are not a problem because the former are evacuated during pumping and the latter cannot be released. The problem comes from the intermediate binding energies. Today, the excitation of these bonds by a high temperature is the only process used to release these molecules.

[0009] Once a very good vacuum is achieved, the electron beam from the tube is sometimes used to degas the residual molecules. In such a process, illustrated in [Fig. 1], the beam is scanned over all surfaces accessible to it (e.g. by deflecting it with a magnet external to the tube and varying the bias voltages of the electrodes).

[0010] This electron beam degassing process is expensive because the focusing magnets must be mounted on the tube and equipped with high-voltage cables, while it is still connected to the pumping frame. Then the electron tube must be operated and an operator comes to scan the beam. These operations are time-consuming, require a lot of equipment and keep the operator busy for a long time.

[0011] An alternative solution, as illustrated in [Fig. 2], is to equip the tube with a small ion pump that serves only to capture the molecules released when the electron beam is first emitted into the electron tube. This solution is also expensive. In addition to the cost of the ion pump, it must be removed from the tube at a later date, which also requires splitting the tube dressing operation into two steps. One solution is to replace this ion pump with a getter (gas-absorbing material) placed in the tube and which remains active throughout the tube's operating time (but with a lower pumping efficiency than a pump).

[0012] The tube cladding is the name given to all the parts mounted on the tube after it has been pumped: mainly the focusing magnets, the high-voltage wires, the base on which the tube is fixed, the waveguides, various glues and potting, RF absorbers and the cover.

[0013] These solutions all require high-temperature processes that present enormous constraints in terms of choice of materials, their geometries, ceramic metallization processes and require finding compatible materials in terms of thermal expansion coefficients, particularly for brazing. Such solutions also present an environmental problem linked to the use of gold in many of these brazings. In the case of multi-step assemblies it is necessary to find brazings with decreasing melting temperatures. Baking at 450°C ultimately requires brazing or extremely high temperature processes when the number of steps becomes significant (>2), commonly above 900°C.

[0014] One aim of the invention is to overcome the problems mentioned above.

[0015] According to one aspect of the invention, there is provided an electronic tube comprising a ceramic substrate, a sealed cover delimiting a volume under ultra-high vacuum, and a solder connecting the substrate and the cover among: Sn 48% / In 52%; Sn 42% / Bi 58%; In 80% / Pb 15% / Ag 5%; In 100%; In 70% / Pb 30%; Sn 62% / Pb 36% / Ag 2%; Sn 63% / Pb 37%; Sn 96.2% / Ag 2.5% / Cu 0.8% / Sb 0.5%; Sn 96.52% / Ag 3% / Cu 0.5%; Sn 95.3 to 95.7% / Ag 3.8 to 4% / Cu 0.5 to 0.7%; Sn 96.5% / Ag 3.5%; In 19% / Pb 81%; Pb 92.5% / In 5% / Ag 2.5%; Pb 97.5% / Ag 1.5% / Sn 1%.

[0016] In one embodiment, the ceramic substrate comprises a metallized portion facing the solder and the cap comprises metal or a metallized ceramic facing the solder.

[0017] According to another aspect of the invention, there is also proposed a method for producing an ultra-high vacuum electronic tube, comprising a ceramic substrate (8), a sealed cover (9) delimiting a volume (10) under ultra-high vacuum, comprising the steps of: producing a low-temperature solder (11) connecting the substrate (8) and the cover (9) among: Sn 48% / In 52%; Sn 42% I Bi 58%; In 80% / Pb 15% / Ag 5%; In 100%; In 70% / Pb 30%; Sn 62% / Pb 36% / Ag 2%; Sn 63% / Pb 37%; Sn 96.2% / Ag 2.5% / Cu 0.8% / Sb 0.5%; Sn 96.52% / Ag 3% / Cu 0.5%; Sn 95.3 to 95.7% / Ag 3.8 to 4% / Cu 0.5 to 0.7%; Sn 96.5% / Ag 3.5%; In 19% / Pb 81%; Pb 92.5% / In 5% / Ag 2.5%; And Pb 97.5% / Ag 1.5% / Sn 1%; pumping of a sealed element of the electronic tube to be placed under ultra-high vacuum, by a pumping frame connected to the tube by a pumping pipe, until a pressure in the sealed element is obtained below a threshold; and irradiation at room temperature by ionizing rays of the electronic tube during the pumping step.

[0018] According to one embodiment, the method further comprises a step of pumping the electronic tube in operation, in the absence of irradiation.

[0019] In one embodiment, the threshold is 5.10' 7 Torr.

[0020] Ultra-high vacuum means a pressure in the sealed part of the electron tube of less than 10' 7Torr. The process according to the invention even allows a pressure of 10' to be reached 8 Torr.

[0021] According to one embodiment, the irradiation step uses an irradiation device movable in translation relative to the electron tube.

[0022] In one embodiment, the irradiation step uses X-rays as ionizing rays, emitted by an X-ray tube movable in translation relative to the electron tube.

[0023] According to one embodiment, the irradiation step uses X-rays emitted by an X-ray tube with an electrical voltage of between 50 keV and 300 keV.

[0024] In one embodiment, the irradiation step lasts so as to deposit a dose of at least 10 19 X-ray photons on every square centimeter of the surfaces of the electron tube to be degassed.

[0025]

[0026] The invention will be better understood by studying a few embodiments described as non-limiting examples and illustrated by the appended drawings in which:

[0027] [Fig.1] schematically illustrates a process for degassing an electronic tube, according to the state of the art;

[0028] [Fig.2] schematically illustrates another method of degassing an electronic tube, according to the state of the art;

[0029] [Fig.3] and [Fig.4] schematically illustrate a method of degassing an electron tube, according to one aspect of the invention; and

[0030] [Fig.5] schematically illustrates an electronic tube, according to one aspect of the invention.

[0031] Throughout the figures, elements with identical references are similar.

[0032] [Fig.3] schematically illustrates the method according to one aspect of the invention.

[0033] The method for degassing an electronic tube comprises the steps of: pumping 1 of a sealed element of the electronic tube to be placed under ultra-high vacuum, by a pumping frame connected to the tube by a pumping pipe, until a pressure in the sealed element is obtained which is lower than a threshold; and irradiation 2 at room temperature by ionizing rays of the electronic tube during the pumping step 1.

[0034] The method of degassing an electronic tube may also comprise a step 3 of pumping the electronic tube in operation, in the absence of irradiation 2.

[0035] Ultra-high vacuum is considered to correspond to a threshold of 5.10' 7 Torr.

[0036] The irradiation steps 2, 3 may use a mobile irradiation device 5 in translation relative to the electron tube, as illustrated in [Fig.4],

[0037] The sealed element of the electronic tube to be placed under ultra-high vacuum is connected to a pumping frame 6 by a pumping pipe 7.

[0038] The irradiation step can use X-rays as ionizing rays, emitted by an X-ray tube moving in translation relative to the electron tube.

[0039] For example, the irradiation step may use X-rays emitted by an X-ray tube with an electrical voltage between 50 kV and 300 keV.

[0040] The irradiation step lasts so as to deposit a dose of at least 10 19 X-ray photons on every square centimeter of the surfaces of the electron tube to be degassed.

[0041] [Fig.4] schematically illustrates an implementation of the method of [Fig.3],

[0042] The present invention consists in degassing the electron tube at room temperature by means of a beam of ionizing rays, such as X-rays. The electron tube is mounted on a pumping frame 6 by means of a tail 7. During the pumping operation, the electron tube is illuminated by ionizing rays sufficiently energetic (in the range of 50 to 300 keV, depending on the thickness to be crossed) to completely cross the vacuum envelope.

[0043] Ionizing rays pass through the surfaces to be degassed. They release the particles trapped on the surface by photoelectric effect, as explained in the documents "Cern Accelerator School, vacuum for particle accelerators 2017, 6-16 June 2017, Glumslôv Sweden, Dr. Oleg B. Malyshev", "Journal of Vacuum Science & Technology A 17, 635 (1999); doi: 10.1116 / 1.581630" and "Journal of Vacuum Science & Technology A 25, 791 (2007); doi: 10.1116 / 1.2746876)".

[0044] Surfaces emit photoelectrons, which in turn release the trapped molecules. This parasitic effect in particle accelerators is known as "Photon Stimulated Desorption" or "Photon Induced Desorption."

[0045] The invention consists in implementing this effect to degas the electron tube at room temperature. The source 5 of ionizing radiation may be a conventional tube, such as, in the case of X-rays, those used for example for the sterilization of food.

[0046] Alternatively, irradiation steps with, for example, gamma rays are also possible, such as using Cobalt 60 sources also used for food sterilization.

[0047] The present invention has several advantages, as follows: - The elimination of high-temperature soldering expands the choice of materials and solders that can be used in electronic tubes. - This makes it possible to seal the aluminum nitride. Thermal expansion stresses are greatly reduced. - Some technologies developed in the field of solid state electronics can be used for vacuum electronics. - In particular, the manufacture of electronic cards under vacuum, i.e. equipped with a sealed cover to form an ultra-high vacuum, becomes possible.

[0048] [Fig.5] schematically illustrates an electron tube, according to one aspect of the invention.

[0049] The electronic tube comprises a ceramic substrate 8, a sealed cover 9 delimiting a volume 10 under ultra-high vacuum, and a solder 11 connecting the substrate 8 and the cover 9.

[0050] The invention allows the use of low temperature solder such as tin and silver alloys (in particular Sn 96.5% / Ag 3.5%, whose melting point is 221°C), and tin, silver, copper (for example Sn 96.2% / Ag 2.5% / Cu 0.8% / Sb 0.5% and Sn 96.52% / Ag 3% / Cu 0.5%, whose melting points are around 217°C).

[0051] Solders can include the following: Sn 48% / In 52%; Sn 42% / Bi 58%; In 80% / Pb 15% / Ag 5%; In 100%; In 70% / Pb 30%; Sn 62% / Pb 36% / Ag 2%; Sn 63% / Pb 37%; Sn 96.2% / Ag 2.5% / Cu 0.8% / Sb 0.5%; Sn 96.52% / Ag 3% / Cu 0.5%; Sn 95.3 to 95.7% / Ag 3.8 to 4% / Cu 0.5 to 0.7%; Sn 96.5% / Ag 3.5%; In 19% / Pb 81%; Pb 92.5% / In 5% / Ag 2.5%; Pb 97.5% / Ag 1.5% / Sn 1%.

[0052] The ceramic substrate 8 comprises a metallized portion 12 facing the solder 11 and the cover 9 comprises metal or a metallized ceramic 13 facing the solder 11.

[0053] The parameters for choosing these brazes are a low melting temperature (between 200 and 300°C), and a low saturated vapor pressure during the brazing operation (<10 -8 Torr).

Claims

CLAIMS 1. Electronic tube comprising a ceramic substrate (8), a sealed cover (9) delimiting a volume (10) under ultra-high vacuum, and a solder (11) connecting the substrate (8) and the cover (9) among: Sn 48% / In 52%; Sn 42% / Bi 58%; In 80% / Pb 15% / Ag 5%; In 100%; In 70% / Pb 30%; Sn 62% / Pb 36% / Ag 2%; Sn 63% / Pb 37%; Sn 96.2% / Ag 2.5% / Cu 0.8% / Sb 0.5%; Sn 96.52% / Ag 3% / Cu 0.5%; Sn 95.3 to 95.7% / Ag 3.8 to 4% / Cu 0.5 to 0.7%; Sn 96.5% / Ag 3.5%; In 19% / Pb 81%; Pb 92.5% / In 5% / Ag 2.5%; And Pb 97.5% / Ag 1.5% / Sn 1%.

2. Electronic tube according to claim 1, in which the ceramic substrate (8) comprises a metallized portion (12) facing the solder (11) and the cover (9) comprises metal or a metallized ceramic (13) facing the solder (11).

3. Method for producing an ultra-high vacuum electronic tube, comprising a ceramic substrate (8), a sealed cover (9) delimiting a volume (10) under ultra-high vacuum, comprising the steps of: producing a low-temperature solder (11) connecting the substrate (8) and the cover (9) from among: Sn 48% / In 52%; Sn 42% / Bi 58%; In 80% / Pb 15% / Ag 5%; In 100%; In 70% / Pb 30%; Sn 62% I Pb 36% I Ag 2% ; Sn 63% / Pb 37%; Sn 96.2% / Ag 2.5% / Cu 0.8% / Sb 0.5%; Sn 96.52% / Ag 3% / Cu 0.5%; Sn 95.3 to 95.7% / Ag 3.8 to 4% / Cu 0.5 to 0.7%; Sn 96.5% / Ag 3.5%; In 19% / Pb 81%; Pb 92.5% / In 5% / Ag 2.5%; And Pb 97.5% / Ag 1.5% / Sn 1%; pumping (1) of a sealed element of the electronic tube to be placed under ultra-high vacuum, by a pumping frame connected to the tube by a pumping pipe, until a pressure in the sealed element is obtained below a threshold; and irradiation (2) at room temperature by ionizing rays of the electronic tube during the pumping step (1).

4. Method for degassing an electronic tube according to claim 3, further comprising a step of pumping (3) the electronic tube in operation, in the absence of irradiation (2).

5. Method for degassing an electronic tube according to claim 3 or 4, in which the threshold is 5.10' 7 Torr.

6. Method for degassing an electronic tube according to one of claims 3 to 5, in which the irradiation step (2) uses an irradiation device movable in translation relative to the electronic tube.

7. Method for degassing an electron tube according to one of the preceding claims 3 to 6, in which the irradiation step (2) uses X-rays as ionizing rays, emitted by an X-ray tube movable in translation relative to the electron tube.

8. Method according to claim 6, in which the irradiation step (2) uses X-rays emitted by an X-ray tube with an electrical voltage of between 50 kV and 300 keV.

9. Method according to claim 7, in which the irradiation step (2) lasts so as to deposit a dose of at least 10 19 X-ray photons on every square centimeter of the surfaces of the electron tube to be degassed.

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