Micro-electro-mechanical system and method of manufacturing the same

Abstract

The invention relates to a method of manufacturing a microelectromechanical system (10), comprising the steps of: forming (30) an electromechanical element (11) on a substrate (13); preparing (31) a packaging wrapper (18) to form a sealed chamber (12) integrating said electromechanical element (11), said sealed chamber (12) having a volume less than 10 mm 3 ; physically vapor depositing (32) a getter film (15) on said substrate (13) or on a wall (17) of said packaging wrapper (18) so that said getter film (15) has a specific absorption surface area less than 8 m 2 / g; and sealing (33) said packaging wrapper (18) on said substrate (13) by means of a heat sealing cycle having a temperature capable of activating said getter film (15).

Description

Technical Field

[0001] This invention relates to a method for manufacturing a microelectromechanical system, also known as MEMS in the literature.

[0002] More specifically, the present invention aims to manufacture vacuum-encapsulated containers typically having a diameter of less than 10 mm. 3 MEMS in small, sealed packages.

[0003] This invention can be applied to various types of microelectromechanical systems that need to be packaged in a small volume under vacuum, such as microbolometers for uncooled infrared imagers. Background Technology

[0004] Some microsystems must operate in a vacuum to achieve optimal performance. This is especially true for MEMS used in uncooled infrared imagers: microbolometers.

[0005] A microbolometer is formed by an absorber sensitive to incident infrared radiation in the spectral range of interest, typically ranging from 8 to 14 micrometers. The absorber is coupled to a thermistor, which measures the heat generated by the absorber. The thermistor has a resistance that changes as its temperature changes. Therefore, the thermistor converts the temperature change of the absorber into a resistance change. Furthermore, the thermistor is coupled to a readout circuit that measures this resistance change. By collecting the resistance changes of the microbolometer pixel array, an infrared image is generated using the readout circuit.

[0006] In order to maximize the temperature rise of the absorber, heat loss must be minimized, which requires thermal insulation between the readout circuit and the detection function ensured by the thermistor.

[0007] For this purpose, the microbolometer takes the form of a structure called a "plate," suspended above the readout circuit, which ensures both absorption and detection functions. The plate is coupled to the readout circuit via a suspension arm. This suspended design addresses the optical, mechanical, and most importantly, thermal requirements.

[0008] In fact, by placing the suspension structure in a housing under low-pressure rarefied atmosphere, the volume of the empty space between the board and the substrate of the integrated readout circuit forms an excellent thermal insulator.

[0009] The three main heat loss phenomena of the suspension structure are solid conduction through the arm of the plate coupled to the readout circuit, conduction through the gas surrounding the plate, and radiation. At the temperature used in the microbolometer, heat transfer through radiation is negligible. Solid conduction is determined by the geometry of the suspension structure. Heat transfer through the gas is minimized by reducing the gas pressure between the radiant plate and the readout circuit until it falls below the solid conduction level.

[0010] Therefore, thermal insulation is necessary for the optimal operation of the microbolometer by placing it under a vacuum. Below approximately 10 -2 A pressure of millibars is necessary to achieve maximum sensitivity.

[0011] To achieve such a vacuum level, the microbolometer pixel array is encapsulated in a chamber under vacuum. This encapsulation typically requires the use of hermetically sealed metal sealing technology to maintain the desired vacuum level throughout the entire lifespan of the component.

[0012] However, forming a metal weld implies a heating system, which leads to the desorption of gas molecules trapped in or on the chamber walls. Furthermore, when the chamber is sealed, the gas inside can no longer be expelled via an external pumping system. Therefore, a specific device for absorbing this gas must be present within the chamber. This device is called a "getter."

[0013] It is a thin metallic film deposited in the chamber of a MEMS. It is initially passivated by its natural oxide, but when heated, the diffusion of this surface layer into the volume makes its surface reactive. It then traps gaseous substances present in its environment, thereby reducing the chamber pressure. The getter is thus referred to as "activated." Therefore, the vacuum level achieved in the chamber is controlled by the amount of gas molecules absorbed by the getter film. This amount depends on:

[0014] ■Inherent properties of getters: the chemical properties of the film, its microstructure, and the surface of the deposit; and

[0015] ■ Getter activation conditions: These are determined by the sealing cycle, as the heat generated by the metal sealing method is typically used to activate the getter film in order to reduce production costs.

[0016] For operation, the active getter must come into contact only with the gas to be absorbed. Therefore, getter films are deposited continuously or discontinuously on unused areas of the MEMS substrate or on the walls of the encapsulation package.

[0017] The ongoing research into miniaturization of MEMS has led to a reduction in chamber volume. The ratio of wall surface area to chamber volume increases as the volume decreases, and the partial pressure of the gas desorbed into the chamber also increases. Therefore, increasingly effective getters are necessary.

[0018] To develop functional getters, i.e., those capable of absorbing all molecules desorbed into the MEMS chamber during sealing, there are a variety of different solutions that can be implemented for small-volume chambers.

[0019] The solution described in document US 7,998,319 involves using the membrane by means of cathode sputtering deposition of a porous getter membrane. The porosity of the getter membrane allows for improved getter efficiency by creating microchannels within which molecules can be trapped by the getter membrane.

[0020] Another known getter film deposition method is physical vapor deposition. However, for small chamber volumes, the literature US 7,998,319 shows that this deposition method is unsuitable, mainly because the deposited film becomes dense and therefore does not have the large effective surface area and high porosity characteristics required for proper operation of the getter.

[0021] Other solutions propose increasing the effective surface area of ​​the getter film by depositing a textured surface. These solutions are used, for example, in documents US 7,789,949, US 5,701,008, US 2014 / 0175590, US 6,923,625, US 9,637,377, US 9,511,998, US2009 / 0261464, EP 2 897 162, or US 7,871,660.

[0022] This porosity can also be achieved by depositing a film on a sub-layer, as described in documents US 2016 / 0040282 or US 9,005,353. In these solutions, cathodic sputtering is the preferred deposition method for developing them because the getter film must have a large effective surface area, as it can produce porous films.

[0023] In addition, in order to minimize gas desorption during sealing and thus limit the efficiency constraints of getter materials, US 9,240,362 proposes to perform degassing annealing under vacuum at a temperature below the activation temperature of the getter before sealing the chamber.

[0024] However, this method prolongs the cycle time and is only applicable to limited encapsulation methods. In fact, if the getter is exposed to ambient air again between its pre-annealing and sealing cycles, its porous surface traps the gas again: therefore, degassing is ineffective. However, if degassing annealing is performed in a sealing furnace, and the sealing cycle immediately follows, the sealing beads in the chamber must also undergo pre-degassing annealing. This can now reduce weld quality, especially for cryogenic sealing techniques, typically close to 250°C. Therefore, pre-degassing in a sealing furnace can only be implemented using techniques that typically seal at temperatures above 350°C, for which the amount of gas desorbed into the chamber during sealing is significant.

[0025] For example, close to 1mm3 For very small chambers, existing solutions cannot achieve the typical size of 5 × 10⁻⁶. -2 A very high vacuum level of millibars.

[0026] Therefore, the technical problem that this invention aims to solve is to determine how to achieve a very high vacuum level while reducing the volume of the MEMS chamber using the simplest feasible manufacturing method. Summary of the Invention

[0027] This invention stems from the observation that argon is the gas limiting the vacuum level achievable by existing technological solutions. To achieve this observation, the gas desorbed into the MEMS chamber, particularly a microbolometer, was analyzed after the chamber was sealed. This analysis revealed that the desorbed gas primarily consists of hydrogen, nitrogen, carbon oxides, and carbon and hydrogen compounds such as methane.

[0028] Unexpectedly, the analysis also revealed a large amount of argon.

[0029] Currently, among these gases, the only one that cannot be captured by the getter is argon, and this gas is heavily used in microfabrication methods within cleanrooms. Therefore, after sealing the chamber, each desorbed argon atom irreparably increases the pressure within the chamber.

[0030] Based on this observation, all argon capture sources capable of desorbing argon within the substrate or chamber wall after chamber sealing have been identified. It has been observed that getter deposition via cathode sputtering results in the introduction of argon into the getter film during deposition. Furthermore, the highly desirable porosity of the getter film also leads to the capture of argon molecules present in the air.

[0031] The invention also stems from a second observation that all gas molecules, except argon, can be absorbed and degassed into a small-volume chamber having a specific surface area as small as that obtained by non-porous deposition, as can be easily obtained by evaporation on a planar and textureless substrate.

[0032] To address this technical problem, the present invention provides a MEMS fabrication method in which a getter film is deposited via physical vapor deposition to limit the introduction of argon gas during the deposition of the getter film. Furthermore, this deposition technique enables the acquisition of a very slightly porous getter film, limiting the capture of argon gas present in the air.

[0033] For this purpose, according to a first aspect, the present invention relates to a method for manufacturing a microelectromechanical system, comprising the following steps:

[0034] ■ Forming electromechanical components on a substrate;

[0035] ■Preparing a packaging assembly to form a sealed chamber integrating the electromechanical components, the sealed chamber having a diameter of less than 10 mm. 3 Volume;

[0036] ■ A getter film is physically vapor-deposited on the substrate or on the wall of the package, such that the getter film has a thickness of less than 8 μm. 2 / g specific absorption surface area; depositing the getter film on the substrate in the absence of any pre-cleaning involving rare gases; and

[0037] ■The package is sealed onto the substrate by means of a heat-sealing cycle with a temperature that can activate the getter film.

[0038] In the context of this invention, "specific absorption surface area" is measured using a method known as BET (Brunauer, Emmett, and Teller), for example, as described in... In the scientific publication "Texture des matériaux divisés-Aire spécifique des matériaux pulvérulents ou nanoporeux" by ROUQUEROL, Jean ROUQUEROL, Isabelle BEURROIES, Philip LLEWELLYN, and Renaud DENOYEL, Techniques de l'Ingénieur, ref. p1050, 2017, this method can measure the specific surface area of ​​a solid by absorbing a certain amount of gas to determine its porosity.

[0039] According to the present invention, the term "specific absorption surface area" describes the absorption and adsorption capacity of a solid, that is, the ability of a solid to allow gaseous substances to permeate and the ability of a solid to fix gaseous substances on its surface.

[0040] Furthermore, due to the presence of the getter, less than 8m 2 The specific absorption surface area of ​​ / g makes the desorption of argon into the chamber negligible. Therefore, the specific absorption surface area of ​​the getter membrane can be estimated by analyzing the argon desorption from the getter membrane.

[0041] To achieve this, residual gas analysis can be used, as described in the scientific publication of P.-L. Charvet, P. Nicolas, D. Bloch, B. Savornin, “MEMS Packaging Reliability Assessment: Residual Gas Analysis,” Microelectronics Reliability 53 (2013) 1622-1627. This type of analysis involves placing the chamber to be analyzed within a shell under ultra-high vacuum. By puncturing the chamber present within the shell, the volume and properties of all gases extracted from the shell due to the puncture can be measured.

[0042] By heating the MEMS at 300°C for 30 minutes, a large amount of argon gas can be forced to desorb from the substrate and the walls of the chamber.

[0043] Therefore, for both MEMS with and without an integrated getter membrane, the amount of gas desorbed after the heating step can be measured. If the difference in the argon proportion measured between the two MEMS is less than 10%, the argon desorbed by the getter membrane is considered negligible compared to the argon desorbed by the substrate and chamber walls, and it can be concluded that the getter membrane has a thickness of less than 8 μm. 2 Specific absorption surface area per g.

[0044] Physical vapor deposition of the getter membrane results in a getter membrane with a thickness of less than 8 μm. 2 With a specific absorption surface area of ​​ / g, a chamber with a small amount of argon gas can be obtained. Since other gases can be captured by the getter film, this invention can improve the vacuum level achievable in a small-volume chamber.

[0045] Therefore, this invention challenges the technical biases discussed above, which state that only by forming a porous getter or by structuring the getter surface can a density of 10 be achieved in a small-volume chamber of a MEMS. -2 A vacuum level of millibars. In fact, physical vapor deposition of the getter film enables processes such as those less than 2 mm in diameter. 3 Achieving 5×10⁵ cells in a very small volume chamber -2 The vacuum level of millibars is achieved because this deposition technique limits the introduction of argon gas during deposition and enables the production of a very slightly porous getter film, limiting the capture of argon atoms present in the air during the inevitable exposure of the getter to ambient air.

[0046] Therefore, the present invention is particularly advantageous for MEMS with integrated microbolometers, for which a vacuum level is necessary to maintain its performance over time.

[0047] Furthermore, some manufacturing techniques involve cleaning the substrate with argon plasma before depositing the getter film. It has been observed that this technique results in argon gas being absorbed into the substrate prior to sealing. To limit argon desorption during sealing, the getter film is deposited on the substrate without prior cleaning involving rare gases.

[0048] Preferably, the physical vapor deposition of the getter film is carried out at a temperature below 10°C. -7 The pressure is increased by heating the crucible containing the evaporating charge under millibar pressure via the Joule effect or by an electron beam to limit the amount of argon introduced into the getter layer.

[0049] Preferably, the sealing is performed at a temperature in the range of 250 to 350°C, typically 300°C. By using a sealing temperature close to 300°C, the activation of the seal and getter can be achieved using a trial-and-error method.

[0050] Preferably, 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, either alone or in mixtures.

[0051] For example, high-performance results have been achieved using getters made of titanium-yttrium alloys. In fact, for a 1mm seal at 300°C... 3 The chamber has been obtained with 3×10 -2 A vacuum of millibars.

[0052] According to a second aspect, the present invention relates to a microelectromechanical system (MEMS) comprising:

[0053] ■The substrate supports electromechanical components;

[0054] ■ Encapsulation and packaging, which is attached to the substrate to form a sealed chamber for integrating the electromechanical components; and

[0055] ■ Getter film, which is deposited on the substrate in the sealed chamber or on the wall of the package;

[0056] The sealed chamber has a diameter of less than 10 mm. 3 Volume;

[0057] The getter membrane has a thickness of less than 8m. 2 / g specific absorption surface area; the getter film is deposited on the substrate in the absence of any pre-cleaning involving rare gases on the substrate.

[0058] Preferably, a depth of less than 8m is obtained through the aforementioned physical vapor deposition. 2 Specific absorption surface area of ​​the getter membrane per g.

[0059] For example, this specific absorption surface area can be obtained by evaporation along an axis that is perpendicular or slightly inclined relative to the substrate (i.e., with an angle of less than 40°). Then, during the evaporation of the getter material, the substrate itself rotates about the evaporation axis to form a getter film having the said specific absorption surface area.

[0060] According to another example, this specific absorption surface area can be obtained by cathode sputtering. The carrier gas pressure is then determined based on the properties of the getter film to be sputtered and the plasma power. Then, during the evaporation of the getter material, the substrate itself rotates about the evaporation axis to form a getter film having the stated specific absorption surface area.

[0061] As a variant, it can be used to obtain less than 8m 2 / g of any other deposition of specific absorption surface area.

[0062] Observations show that argon degassing due to exposure to ambient air and argon trapping in the pores of the getter is related to the specific absorption surface area. Therefore, measuring the specific absorption surface area allows us to determine whether argon trapping will occur based on the specific absorption surface area characteristics of a particular getter. Attached Figure Description

[0063] The manner in which the invention is practiced and the advantages thereof will become more apparent from the following description of non-limiting embodiments given by way of indication, based on the accompanying drawings:

[0064] Figure 1 This is a flowchart of the steps of a method for manufacturing a microelectromechanical system according to an embodiment of the present invention. Detailed Implementation

[0065] In the remainder of the specification, the invention will be described with reference to optoelectronic component 10, but the invention can also be applied to other MEMS devices without changing the invention.

[0066] Figure 1 This shows that under a predetermined pressure, for example, below 5 × 10⁻⁶ -2 The step of forming the optoelectronic component 10 encapsulated in the housing 12 under a pressure of millibars. The housing 12 is formed by sealing the sidewalls 17 of the sealed package 18 to the substrate 13 by means of a metal seal 20.

[0067] To achieve this, the first step 30 includes forming an electromechanical element 11 on the substrate 13. For example, the substrate 13 may integrate readout circuitry, and the electromechanical element 11 may correspond to an uncooled microbolometer suspended above the substrate 13 by means of a pad and a support arm. To achieve this, one or more sacrificial layers are used and configured to form a pad and different layers that form the microbolometer film. Furthermore, beneath this film, the microbolometer 11 may include a reflector 16.

[0068] In parallel with the first step 30, the second step 31 includes preparing an encapsulation package 18 intended to form a sealed chamber 12 around the electromechanical component 11. To achieve this, sidewalls 17 are configured to be substantially perpendicular to the substrate on top of the encapsulation package 18. An optical window 14 may also be constructed in the upper substrate to filter electromagnetic radiation captured by the microbolometer 11.

[0069] Following the second step 31, the third step 32 includes depositing a getter film 15 on the substrate 13 or on the walls of the package 18. Figure 1 As shown, the getter film 15 can be deposited on the upper wall of the encapsulation package 18 next to the optical window 14. As a variation, the getter film 15 can be deposited next to the reflector 16 or on the sidewall 17.

[0070] In all cases, the getter membrane 15 is intended to be disposed inside the housing 12 to capture and retain gas desorbed into the housing for less than 5 × 10⁻⁶ ppm. -2 The vacuum level of millibars.

[0071] According to the present invention, the getter film 15 is deposited by physical vapor deposition to obtain a film smaller than 8 μm. 2 / g specific absorption surface area. Preferably, the deposition surface of the getter film 15 is not cleaned by a method involving argon gas before deposition of the getter film 15 to avoid argon gas being introduced into the walls of the encapsulation package 18 or onto the substrate 13.

[0072] Preferably, the getter membrane 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, either alone or in mixtures. For example, the getter membrane 15 may be made of a titanium-yttrium alloy.

[0073] The physical vapor deposition of the getter film 15 involves heating a crucible containing the integrated getter material to achieve its evaporation. This evaporation occurs under vacuum, preferably below 10 °C. -7The process is carried out under millibar pressure. Evaporation of the getter material is controlled by an electric current or an electron beam, causing the evaporated particles of the getter material to accumulate on the target surface, i.e., the surface of the substrate 13 or the surface of the wall of the encapsulation package 18. To achieve this, evaporation by the Joule effect involves heating the crucible with an electric current, while electron beam evaporation involves applying an electron beam directed towards the crucible.

[0074] When the getter film 15 is deposited on the substrate 13 or the wall of the package 18, the package can be sealed to the substrate 13 during step 33. To achieve this, a metal solder bead 20 is deposited between the substrate 13 and the lower end of the sidewall 17 of the package 18. The solder bead 20 is then heated to obtain a sealing surface between the lower end of the sidewall 17 of the package 18 and the substrate 13.

[0075] The heating temperature of the weld bead 20 is preferably in the range of 250 to 350°C, so that the getter film 15 is activated during the temperature rise of the weld bead 20. A very simple heat-sealing cycle can be implemented: a first temperature rise phase, a second phase in which the temperature stabilizes for a predetermined duration at the heating temperature, and a third phase in which the temperature gradually decreases. During this sealing cycle, the stabilization time and the temperature rise and fall times can be adjusted according to the getter material and the material of the weld bead 20 to achieve effective activation of the getter film 15 and a sealed weld of the housing 12. In addition, a more complex sealing cycle with a degassing phase can also be used.

[0076] Activation of the getter film is achieved by the migration of a passivation layer formed on the surface of the getter film 15 after the getter film comes into contact with oxygen. This passivation layer may correspond to a nitride layer, as described in US 9,051,173, or a thin gold, palladium, or nickel layer, as described in US 6,923,625 and US 9,240,362, if a specific annealing method has been used.

[0077] These different microfabrication steps enable the production of a shell 12 with very low argon levels, as these steps restrict the desorption of argon into the chamber 12. Furthermore, the getter film 15, deposited by evaporation, is able to absorb all gas molecules degassing into the chamber 12. Thus, a shell 12 with a diameter of less than 2 mm can be produced. 3 The sealed chamber 12 has a volume of approximately 1 mm. 3 The volume of chamber 12 is less than 5 × 10 -2 The vacuum level of millibars.

[0078] Therefore, the present invention enables the creation of a microelectromechanical system 10 with a sealed chamber 12 having a very small volume and a high vacuum level.

Claims

1. A method for manufacturing a microelectromechanical system (10), comprising the following steps: ■ An electromechanical component (11) is formed on the substrate (13); ■Preparing a packaging (18) to form a sealed chamber (12) integrating the electromechanical components (11), the sealed chamber (12) having a diameter of less than 10 mm. 3 Volume; ■ A getter film (15) is physically vapor-deposited on the substrate (13) or on the wall (17) of the package (18), such that the getter film (15) has a thickness of less than 8 μm. 2 / g specific absorption surface area; the getter film (15) is deposited on the substrate (13) in the absence of any pre-cleaning of the substrate (13) involving rare gases; as well as ■The encapsulation package (18) is sealed on the substrate (13) by means of a heat-sealing cycle with a temperature that can activate the getter film (15).

2. The method for manufacturing a microelectromechanical system according to claim 1, wherein, The getter film (15) undergoes physical vapor deposition at a temperature below 10°C. -7 It is carried out under millibar pressure.

3. The method for manufacturing a microelectromechanical system according to any one of claims 1 to 2, wherein, The sealing is performed at a temperature ranging from 250°C to 350°C.

4. The method for manufacturing a microelectromechanical system according to any one of claims 1 to 2, wherein, The getter membrane (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, either alone or in mixtures.

5. The method for manufacturing a microelectromechanical system according to claim 4, wherein, The getter membrane (15) is made of titanium-yttrium alloy.

6. The method for manufacturing a microelectromechanical system according to claim 1, wherein the microelectromechanical system (10) comprises: ■ Substrate (13), which supports electromechanical components (11); ■ Encapsulation package (18), which is attached to the substrate (13) to form a sealed chamber (12) for integrating the electromechanical components (11); and ■ Getter film (15), which is deposited on the substrate (13) in the sealed chamber (12) or on the wall (17) of the package (18); in ■The sealed chamber (12) has a diameter of less than 10 mm. 3 Volume; ■And the getter membrane (15) has a thickness of less than 8m 2 / g specific absorption surface area; the getter film (15) is deposited on the substrate (13) in the absence of any pre-cleaning of the substrate (13) involving rare gases.

7. The method for manufacturing a microelectromechanical system according to claim 6, wherein, The getter membrane (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, either alone or in mixtures.

8. The method for manufacturing a microelectromechanical system according to claim 7, wherein, The getter membrane (15) is made of titanium-yttrium alloy.

9. The method for manufacturing a microelectromechanical system according to any one of claims 6 to 8, wherein, The sealed chamber (12) has a size of less than 5 × 10⁻⁶. -2 The vacuum level of millibars.

10. A method for manufacturing a microelectromechanical system according to any one of claims 6 to 8, wherein, The sealed chamber (12) has a diameter of less than 2 mm. 3 The volume.

11. The method for manufacturing a microelectromechanical system according to any one of claims 6 to 8, wherein, The electromechanical component (11) corresponds to at least one microbolometer.

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