Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B81—MICROSTRUCTURAL TECHNOLOGY
  • B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
  • B81B7/00—Microstructural systems ; Auxiliary parts of microstructural devices or systems
  • B81B7/0032—Packages or encapsulation
  • B81B7/0035—Packages or encapsulation for maintaining a controlled atmosphere inside of the chamber containing the MEMS
  • B81B7/0038—Packages or encapsulation for maintaining a controlled atmosphere inside of the chamber containing the MEMS using materials for controlling the level of pressure, contaminants or moisture inside of the package, e.g. getters
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10W—GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
  • H10W76/00—Containers; Fillings or auxiliary members therefor; Seals
  • H10W76/40—Fillings or auxiliary members in containers, e.g. centering rings
  • H10W76/42—Fillings
  • H10W76/48—Fillings including materials for absorbing or reacting with moisture or other undesired substances
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B81—MICROSTRUCTURAL TECHNOLOGY
  • B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
  • B81C1/00—Manufacture or treatment of devices or systems in or on a substrate
  • B81C1/00015—Manufacture or treatment of devices or systems in or on a substrate for manufacturing microsystems
  • B81C1/00261—Processes for packaging MEMS devices
  • B81C1/00277—Processes for packaging MEMS devices for maintaining a controlled atmosphere inside of the cavity containing the MEMS
  • B81C1/00285—Processes for packaging MEMS devices for maintaining a controlled atmosphere inside of the cavity containing the MEMS using materials for controlling the level of pressure, contaminants or moisture inside of the package, e.g. getters
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B81—MICROSTRUCTURAL TECHNOLOGY
  • B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
  • B81C2203/00—Forming microstructural systems
  • B81C2203/01—Packaging MEMS
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B81—MICROSTRUCTURAL TECHNOLOGY
  • B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
  • B81C2203/00—Forming microstructural systems
  • B81C2203/01—Packaging MEMS
  • B81C2203/0145—Hermetically sealing an opening in the lid

Definitions

• the invention relates to a method of manufacturing a microelectromechanical system, also called MEMS for "microelectromechanical System” in the English literature.
• the invention relates to the manufacture of a MEMS encapsulated under vacuum in a hermetic case of small volume, typically of volume less than 10 mm 3 .
• the invention can be applied to several types of microelectromechanical systems requiring vacuum encapsulation with low volume, for example microbolometers of uncooled infrared imagers.
• microsystems must operate under vacuum to achieve optimum performance. This is particularly the case with MEMS which are the basis of uncooled infrared imagers: microbolometers.
• a microbolometer consists of an absorber sensitive to infrared radiation incident in a spectral range of interest, typically between 8 and 14 micrometers.
• the absorber is connected to a thermistor, whose function is to measure the heating of the absorber.
• the thermistor has a resistance that varies as it changes in temperature. Thus, the thermistor converts the change in temperature of the absorber into a change in resistance.
• the thermistor is connected to a read circuit which measures this variation in resistance. By collecting the resistance variations of a micro-bolometric pixel array, an infrared image is generated using the read circuit.
• the microbolometer is in the form of a structure suspended above the reading circuit, called a "board", which performs the functions of absorption and detection.
• the board is connected to the reading circuit by suspension arms. This suspended design meets optical, mechanical and above all thermal requirements.
• the volume of empty space between the board and a substrate incorporating the reading circuit constitutes an excellent thermal insulator.
• the three main phenomena of thermal loss of a suspended structure are formed by solid conduction through the arms connecting the board to the reading circuit, by conduction through the gas surrounding the plate and by radiation. At temperatures in which a microbolometer is used, radiant heat transfer is negligible. Solid conduction is determined by the geometry of the suspended structure. The heat transfer by the gas is minimized by lowering the gas pressure between the bolometric board and the reading circuit, to be less than the solid conduction.
• Thermal insulation essential for the optimal functioning of a microbolometer, is therefore achieved by placing the microbolometer under vacuum. A pressure of less than approximately 10 2 mbar is necessary to obtain maximum sensitivity.
• the array of microbolometric pixels is vacuum encapsulated in a cavity.
• This encapsulation conventionally requires the use of a hermetic metal sealing technique to maintain the desired level of vacuum throughout the life of the component.
• getter activation conditions they are set by the sealing cycle, since thermal heating of the metal sealing process is conventionally used to activate the getter film, in order to reduce the production cost.
• the active getter must simply be in contact with the gas to be absorbed.
• the getter film is therefore deposited indiscriminately on the unused areas of the MEMS substrate, or on the walls of the encapsulation box, continuously or discontinuously.
• This porosity can also be obtained by a film deposited on an underlayer, as described in documents US 2016/0040282 or US 9,005,353.
• the preferred deposition method for producing them is cathode sputtering, since it allows porous films to be obtained.
• this method lengthens the cycle time and it is applicable only to limited cases of encapsulation process. Indeed, if the getter is again exposed to ambient air between its pre-annealing and the sealing cycle, its porous surface again traps gas: degassing is therefore not effective. On the other hand, if the degassing annealing is carried out in the sealing furnace, and the annealing is immediately followed by the sealing cycle, then the sealing beads of the cavity also necessarily undergo the pre-degassing annealing. However, this can degrade the quality of the weld, in particular for low temperature sealing techniques, typically close to 250 ° C.
• Pre-degassing in the sealing furnace can therefore only be carried out with high temperature sealing techniques, typically greater than 350 ° C., for which the quantity of gas desorbed in the cavity during sealing is large.
• high temperature sealing techniques typically greater than 350 ° C.
• These solutions of the state of the art do not make it possible to achieve a very high vacuum level, typically 5.10 2 mbar, for cavities of very low volumes, for example close to 1 mm 3 .
• the technical problem which the invention proposes to solve is therefore to determine how to obtain a very high vacuum level by reducing the volume of a cavity of a MEMS while using the simplest possible manufacturing process.
• the invention arises from a first observation that argon is the gas which limits the vacuum level that can be reached by the solutions of the state of the art.
• gases desorbed in a MEMS cavity in particular that of a microbolometer, were analyzed after the sealing of the cavity. This analysis showed that the desorbed gases consist mainly of hydrogen, nitrogen, carbon oxides, and carbon and hydrogen compounds, such as methane.
• each desorbed argon atom irreparably increases the pressure in the cavity.
• the invention proposes to use a method of manufacturing a MEMS in which the getter film is deposited by a physical vapor deposition so as to limit the incorporation of argon during the deposition of the film. getter.
• this deposition technique makes it possible to obtain a very weakly porous getter film that limits the capture of argon present in the air.
• the invention relates to a method of manufacturing an electromechanical microsystem comprising the following steps:
• the “specific absorption surface” is measured by the BET method, for Brunauer, Emmett and Teller, as described in the scientific publication by devise ROUQUEROL, Jean ROUQUEROL, Isabelle BEURROIES, Philip LLEWELLYN, and Renaud DENOYEL, "Texture of divided materials - Specific area of powdery or nanoporous materials", Techniques de l'Ingur, reference P1050, 2017. This method makes it possible to measure the specific surface area of absorption of a quantity of gas to determine the porosity of a solid.
• the expression “specific absorption surface” describes both the absorption capacity and the adsorption capacity of the solid, that is to say the capacity of a solid to allow penetration. of a gaseous species and the ability of a solid to fix a gaseous species on its surface.
• this specific absorption surface area of less than 8 m 2 / g makes the desorption of argon in the cavity due to the presence of the getter negligible.
• the getter film has a specific absorption surface area of less than 8 m 2 / g.
• the physical vapor deposition of the getter film such that the getter film has a specific absorption surface area of less than 8 m 2 / g, thus makes it possible to obtain a cavity with a small amount of argon.
• the invention makes it possible to improve the vacuum level that can be reached in a low volume cavity. The invention thus calls into question the technical prejudice exposed above according to which the vacuum level of 10 2 mbar can be reached in a small volume cavity of a MEMS only by producing a porous getter or by structuring the surface of the getter. .
• the physical vapor deposition of a getter film makes it possible to obtain a vacuum level of 5.10 2 mbar in very small volume cavities, for example less than 2 mm 3 , because this deposition technique limits the incorporation of argon during deposition and makes it possible to obtain a very weakly porous getter film, limiting the trapping of the argon atoms present in the air during the inevitable exposure of the getter to ambient air.
• the invention is therefore particularly advantageous for MEMS incorporating microbolometers, for which the vacuum level is essential to maintain their performance over time.
• some manufacturing techniques involve cleaning the substrate with argon plasma before depositing the getter film on the substrate. This technique has been observed to cause the absorption of argon into the substrate prior to sealing. To limit argon desorption during sealing, the getter film is deposited on the substrate without prior cleaning of the substrate involving a noble gas.
• the physical vapor deposition of the getter film is carried out by heating a crucible containing the evaporation charges by the Joule effect or by means of an electron beam, under a pressure of less than 10 7 mbar so as to limit the quantity of argon incorporated into the getter layer.
• the sealing is carried out at a temperature between 250 and 350 ° C, typically 300 ° C.
• a sealing temperature close to 300 ° C it is possible to use known and proven methods to achieve sealing and getter activation.
• the getter comprises at least one of the following elements: Barium, Lanthanum, Scandium, Titanium, Zirconium, Niobium, Yttrium, Vanadium, Hafnium, Tantalum, Iron, Cobalt, Nickel, Palladium, Platinum and Aluminum, alone or in mixed.
• the getter made of a titanium-yttrium alloy.
• a vacuum of 3.10 2 mbar was obtained for a 1 mm 3 cavity sealed at 300 ° C.
• the invention relates to an electromechanical microsystem comprising:
• the specific absorption surface of the getter film of less than 8 m 2 / g is preferably obtained by the physical vapor deposition described above.
• this specific absorption surface can be obtained by evaporation along a normal or slightly inclined axis, that is to say with an angle of less than 40 °, relative to the substrate.
• the substrate is then rotated on itself around the evaporation tax during the evaporation of the getter material so as to form a getter film with the specific absorption surface described.
• this specific absorption surface can be obtained by cathodic sputtering.
• the carrier gas pressure is then determined as a function of the nature of the getter film to be sprayed and of the power of the plasma.
• the substrate can also be rotated on itself around the spray base during the evaporation of the getter material so as to form a getter film with the specific absorption surface described.
• any other deposit making it possible to obtain a specific absorption surface area of less than 8 m 2 / g can be used.
• Observations have shown that the outgassing of argon due to exposure to ambient air and to the entrapment of argon in the pores of the getter is related to the specific absorption surface.
• a measurement of the specific absorption surface makes it possible to know whether or not the phenomenon of argon trapping could occur as a function of the specific absorption surface characteristic of a specific getter.
• Figure 1 is a flowchart of the steps of the manufacturing process of an electromechanical microsystem according to one embodiment of the invention.
• FIG. 1 illustrates the steps for producing an optoelectronic component 10 encapsulated in an enclosure 12 under a predetermined pressure, for example under a pressure less than 5.10 2 mbar.
• the enclosure 12 is formed by the sealing of side walls 17 of a hermetic housing 18 to a substrate 13 by means of a metal seal.
• a first step 30 consists in producing the electromechanical element 11 on the substrate 13.
• the substrate 13 can integrate a read circuit and the electromechanical element 11 can correspond to a microbol or be uncooled mounted in suspension above the substrate 13 by means of studs and support arms.
• one or more sacrificial layers are used and are structured to form the pads and the different layers of the microbolometer membrane.
• the microbolometer 11 may include a reflector 16.
• a second step 31 consists in preparing an encapsulation box 18 intended to form a hermetic cavity 12 around the electromechanical element 11.
• the side walls 17 are structured substantially vertically. of a substrate intended to form the top of the encapsulation box 18.
• An optical window 14 can also be structured in this upper substrate so as to filter the electromagnetic radiation picked up by the microbolometer 11.
• a third step 32 consists in depositing a getter film 15 on the substrate 13 or on a wall of the encapsulation box 18. As illustrated in FIG. 1, the getter film 15 can be deposited at side of the optical window 14 on the upper wall of the encapsulation box 18. As a variant, the getter film 15 can be deposited next to the reflector 16 or on the side walls 17.
• the getter film 15 is intended to be placed inside the enclosure 12 so as to capture the gases desorbed in said enclosure, and to maintain a vacuum level of less than 5.10 2 mbar in the latter.
• this getter film 15 is deposited by physical vapor deposition so as to obtain a specific absorption surface area of less than 8 m 2 / g.
• the deposition surface of this getter film 15 is not cleaned by a process involving argon before the deposition of the getter film 15 so as to avoid the incorporation of argon into the wall of the encapsulation box. 18 or on the substrate 13.
• the getter film 15 comprises at least one of the following elements Barium, Lanthanum, Scandium, Titanium, Zirconium, Niobium, Yttrium, Vanadium, Hafnium, Tantalum, Iron, Cobalt, Nickel, Palladium, Platinum and Aluminum, alone or mixed.
• the getter film 15 can be made of a titanium-yttrium alloy.
• the physical vapor deposition of the getter film 15 consists in heating a crucible incorporating the getter material so as to obtain its evaporation. This evaporation is carried out under vacuum, preferably at a pressure of less than 10 7 mbar.
• the evaporation of the getter material is controlled by an electric current or an electron beam so that the particles of the evaporated getter material agglomerate on the target surface, that is to say on a surface of the substrate 13 or d 'a wall of the encapsulation box 18.
• the evaporation by Joule effect consists in heating the crucible by an electric current while the evaporation by electron beam consists in applying a beam of electrons directed on the crucible.
• the getter film 15 When the getter film 15 is deposited on the substrate 13 or on a wall of the encapsulation box 18, said box can be sealed on the substrate 13, during a step 33.
• a metal weld bead 20 is used. deposited between the substrate 13 and the lower end of the side walls 17 of the encapsulation box 18. This weld bead 20 is then heated so as to obtain a hermetic interface between the lower end of the side walls 17 of the encapsulation box 18 and substrate 13.
• the heating temperature of this weld bead 20 is preferably between 250 and 350 ° C, so as to allow activation of the getter film 15 when the temperature of the weld bead 20 rises.
• a very simple thermal sealing cycle can be achieved. be implemented: a first phase of temperature rise, a second phase of stabilization of the temperature at the heating temperature for a predetermined time, and a third phase of gradual decrease in temperature.
• the stabilization time as well as the rise and fall times of the temperature can be adjusted depending on the getter material and the material of the weld bead 20 so as to obtain an efficient activation of the getter film 15 and hermetic welding of the enclosure 12.
• Activation of the getter film is obtained by means of migration of the passivation layer formed on the surface of the getter film 15 following contact between the getter film and oxygen.
• This passivation layer can correspond to a layer of nitride if a specific annealing process has been used, as described in document US Pat. No. 9,051,173, or to a thin metallic layer of gold, palladium or nickel, as described in the documents. documents US 6,923,625 and US 9,240,362.
• the invention thus makes it possible to obtain a microelectromechanical system 10 with a hermetic cavity 12 of very low volume and with a high vacuum level.

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• Engineering & Computer Science (AREA)
• Microelectronics & Electronic Packaging (AREA)
• Manufacturing & Machinery (AREA)
• Computer Hardware Design (AREA)
• Micromachines (AREA)

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• 2020
  • 2020-05-07 FR FR2004519A patent/FR3109936B1/fr active Active
• 2021
  • 2021-04-08 CN CN202180025464.6A patent/CN115397767B/zh active Active
  • 2021-04-08 EP EP21716744.4A patent/EP4146586A1/de active Pending
  • 2021-04-08 KR KR1020227033621A patent/KR20230006453A/ko active Pending
  • 2021-04-08 US US17/913,956 patent/US11685645B2/en active Active
  • 2021-04-08 WO PCT/EP2021/059150 patent/WO2021223951A1/fr not_active Ceased

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