CH703289A2 - Encapsulating device for quartz tuning-fork resonator in e.g. electronic watch, has sealing unit comprising alloy formed by nickel with material having less melting point to permit interdiffusion of material with nickel at liquid state - Google Patents

Abstract

The device (3) has a sealing unit (6) for hermetically sealing a vacuum or controlled atmosphere cavity (10) forming a parallelepiped hollow main part (2) of a ceramic case (7) with a rectangular cover (4). The sealing unit comprises alloy formed by nickel with a material e.g. indium or tin, whose melting point is less than 250 degree Celsius, so as to permit interdiffusion of the material with the nickel at liquid state. Independent claims are also included for the following: (1) an electronic component comprising an encapsulating device (2) a method for fabricating an electronic component.

Description

Description

Field of the invention

The invention relates to an encapsulation system for an electromicromechanical system (also known by the abbreviation MEMS from the English term "Micro Electro Mechanical System") and, in particular, for a MEMS resonator type to quartz.

Background of the invention

The electronic components having a MEMS are generally formed by a hermetically sealed housing in which the MEMS is mounted. Such a housing generally comprises a hollow main portion closed by a cover.

The MEMS may for example be a piezoelectric resonator, such as a quartz resonator intended to be connected to an oscillator circuit. Most small crystal resonators, which are used for example in electronic or electromechanical watches, are tuning fork type resonators.

Such quartz resonators are usually enclosed under vacuum in housings, in the case of the generation of low frequency signals provided by the oscillator circuit, or under an inert gas atmosphere. In addition, a portion of the cover may be transparent at a specific wavelength of a light beam to allow optical adjustment of the quartz resonator.

Generally, such resonators are mounted in housings for example ceramic which are relatively flat. These housings comprise a hollow main part formed parallelepipedic inside which is mounted the resonator, and a rectangular cover fixed on the main part.

To ensure the seal between the lid and the main part, currently, using a metal alloy gasket based on gold and tin which is reported between the two parts and the whole is heated to to permanently seal the case.

These alloys based on gold and tin have the disadvantages of using intrinsically expensive materials and having a relatively low melting point, that is to say around 278 ° C. This last characteristic limits the possible methods during or after the connection of the housing. It is understood that no heat treatment greater than 280 ° C achieved later connection is possible under risk of loosening the housing.

Summary of the invention

The object of the present invention is to overcome all or part of the disadvantages mentioned above by proposing a new type of sealing means.

For this purpose, the invention relates to an encapsulation device for an electromicromechanical system comprising a housing comprising a main part forming a cavity which is hermetically closed by a cover by means of sealing means characterized in that the sealing means comprises an alloy formed of nickel with a material whose melting point is less than 250 ° C to allow its interdiffusion in the liquid state with nickel.

From the current housings whose metallizations already include nickel, it is therefore understood that only the low melting point material is necessary to be reported to achieve hermeticity of the housing. In addition, the nickel base is substantially less expensive than gold and also provides at least one intermetallic whose melting point is at a higher temperature than the current sealing means.

According to other advantageous features of the invention:

said material is indium or tin;

the housing is formed of ceramic.

The invention also relates to an electronic component comprising an encapsulation device according to one of the preceding variants in the cavity of which is mounted at least one electro-micromechanical system.

According to other advantageous features of the invention:

the cavity is under vacuum or in a controlled atmosphere;

the electromicromechanical system is a quartz tuning fork resonator;

the component comprises an oscillator circuit electrically connected to the electromicromechanical system;

the oscillator circuit is mounted on the main part.

[0014] Finally, the invention relates to a method for manufacturing an electronic component formed by an encapsulation device receiving at least one electromicromechanical system comprising the following steps: a) forming said at least one electromechanical system; micromechanical;

2 b) forming a main portion forming a cavity and a cover having metallizations comprising at least one layer of nickel; characterized in that the method further comprises the steps of: c) forming an intermediate layer of pure material having a melting point of less than 250 ° C; d) fixing said at least one electromicromechanical system in the cavity; e) assembling the cover against the main portion to cover the entire cavity and bringing into contact the metallizations and the intermediate layer; f) heating the metallizations and the intermediate layer to form solid-liquid interdiffusion intermetallic capable of sealing said at least one electromicromechanical system in the cavity.

Advantageously, it is no longer necessary to report an alloy to form the hermeticity of the encapsulation device but a pure material. In addition, compared to current sealing means, it has emerged that the intermetallics formed from nickel have slower growth kinetics, which advantageously makes it possible to better control their formation.

According to other advantageous features of the invention:

said material whose melting point is less than 250 ° C. is indium or tin;

during step c), the intermediate layer is formed on at least one of the metallizations or in a part whose projection surface is of a shape corresponding to at least one of the metallizations;

step c) involves an electroplating or screen printing process;

the metallizations formed during step b) are produced by screen printing and electroplating;

step f) is carried out under vacuum or in a controlled atmosphere;

said at least one electromicromechanical system is a quartz tuning fork resonator.

Brief description of the drawings

Other features and advantages will become apparent from the description which is given below, for information only and in no way limitative, with reference to the accompanying drawings, in which:

- fig. 1 is a top view of the electronic component according to the invention;

- fig. 2 is a view according to section A-A of FIG. 1 of the electronic component according to the invention;

- fig. 3 is an enlarged view located on the interface between the cover and the hollow body of the housing;

- fig. 4 is a block diagram of the manufacturing method according to the invention.

Detailed Description of the Preferred Embodiments

In the following description, all parts of the electronic component that are well known to those skilled in this technical field, will not be explained in detail.

The electronic component 1 is shown in a simplified manner in FIGS. 1 and 2. It mainly comprises an encapsulation device 3 for receiving a MEMS 5 hermetically. The encapsulation device 3 comprises a housing 7 formed by a hollow main part 2 and a cover 4 intended to close the hollow part 2 by means of sealing means 6.

In the example illustrated in FIGS. 1 and 2, the MEMS 5 shown is a quartz tuning fork resonator, however other types of MEMS requiring encapsulation under vacuum or controlled atmosphere are also applicable.

The hollow portion 2 is generally of parallelepipedal shape and has a bearing surface 8 in the inner cavity 10 for fixing the MEMS 5 cantilevered. The free ends of the walls surrounding the cavity 10 are intended to receive the lid 4 of substantially rectangular shape by means of the sealing means 6 in order to hermetically seal the MEMS 5 in the encapsulation device 3.

For example, the housing 7, that is to say the hollow portion 2 and the cover 4, may be 5 mm long, 3.2 mm wide and 1.08 mm high. . In addition, the housing 7 is preferably made of ceramic according to a usual technique.

The sealing means 6 are formed by a succession of layers intended to adhere to the ceramic and to form the layer allowing the hermeticity. Advantageously according to the invention, the sealing means 6 comprise a nickel-based alloy associated with a material whose melting point is low, that is to say much lower than that of nickel.

3 as for example of the order of 250 ° C maximum. Preferably, the material used may be indium or tin.

These alloys Ni-ln or Ni-Sn, which may comprise several intermetallic, are obtained by a weld involving a solid-liquid interdiffusion, that is to say that the melting point difference between indium or tin relative to that of nickel melts one of these first and spread it in the solid nickel layer to form intermetallic.

These welds can thus be made at "low" temperature, that is to say below 250 ° C while accepting subsequent heat treatments at much higher temperatures induced by the melting points of the resulting intermetallic, that is to say between 400 ° C and 800 ° C.

Advantageously according to the invention, the ceramic packages 7 currently marketed include metallizations 9, 1 1 which already include at least one layer of nickel. Typically, the metallizations 9, 1 1 comprise several layers formed with molybdenum and nickel, or tungsten and nickel, the nickel of each metallization 9, 1 1 being protected against oxidation with a flash of gold. Usually, the layers of molybdenum and nickel, or tungsten and nickel have thicknesses of 10 μm and 5 μm, respectively, while that of gold is approximately 0.75 μm.

Therefore, it is understood that, to form the sealing means 6 in a nickel-indium or nickel-tin intermetallic, a single layer 12 of pure indium or pure tin is necessary to achieve the solid-state interdiffusion welding. liquid according to the invention.

Therefore, the MEMS 5 can, with the aid of the sealing means 6, be sealed under vacuum or in a controlled atmosphere in the cavity 10 of the encapsulation device 3 with less expensive materials and at least obtaining an intermetallic whose melting point is at a higher temperature than the current sealing means.

In the example illustrated in FIGS. 1 and 2, the MEMS 5 is a conventional quartz tuning fork consisting of two parallel branches 14, 16 for vibrating in bending mode, the common base 13 of which is fixed on the bearing surface 8. The metallization layers of the MEMS 5 necessary for the Piezoelectric actuation as well as the connection pads to an integrated circuit 15 having, for example, an oscillator stage are not presented in detail because these elements are not critical for the application of the invention.

The method 21 for manufacturing the encapsulation device 3 will now be explained with reference to FIG. 4. The method 21 comprises a first step 23 intended to manufacture, independently and habitually, during phases 20, 22 and 24, respectively the MEMS 5, the cover 4 and the hollow portion 2.

Thus, if the MEMS 5 is a quartz diapason resonator, the phase 20 may consist of etching a wafer in a quartz monocrystal, then etching the body of the tuning fork in the thickness of this wafer to finally instrument the tuning fork, that is to say deposit the electrically conductive layers necessary for its operation.

The lid 4 and the hollow portion 2 are preferably formed in a similar manner using a ceramic. To do this, in the usual way, several ceramic sheets are carved, stacked and fixed on one another. Then, the lid 4 and the hollow portion 2 are partially metallized to a corresponding surface to allow their future cooperation.

As explained above, these metallizations 9, 1 1 may comprise for example a succession of layers of molybdenum, nickel and gold, or tungsten, nickel and gold. These deposits are made, for example, by screen printing for molybdenum or tungsten and, for example, by electroplating for nickel and gold.

Advantageously according to the invention, the second step 25 is intended to form a single layer 12 of pure material whose melting point is low, that is to say preferably less than 250 ° C, such as for example indium or tin. According to the embodiment chosen for the assembly step 29, this step 25 may consist of forming a separate piece or a thickening of material of the metallization 9, 1 1 of the cover 4 and / or of the hollow portion 2.

It is therefore clear that step 25 can also be performed according to various types of deposit. Preferably, an electroplating step is used either in a dedicated mold or directly on at least one of the metallizations 9, 1 1. However, a screen printing process is also possible.

Preferably, the layer 12 has a thickness of 10 microns to ensure a sufficient solder thickness and the total consumption of the layer while keeping a relatively short process time.

In a third step 27, the MEMS 5 is mounted in the cavity 10 of the hollow portion 2 and, in the fourth step 29, the housing 7 is assembled by putting the layers 9, 12 opposite each other. , 1 1 in order to put them in contact. The assembly step 29 may comprise several embodiments. Thus, as explained above, step 29 can consist only of placing the lid 4 against the hollow portion 2 or interposing therebetween an intermediate piece forming the layer 12 comprising the material which will diffuse into the nickel included in the metallizations 9, 1 1.

Finally, whatever the embodiment, the method 21 comprises a last step 31 of welding the sealing means 6 to permanently seal the encapsulation device 3. As explained above, according to the MEMS 5 to encapsulate, step 31 and, optionally, step 29 is (are) carried out under vacuum or controlled atmosphere.

4

Claims (10)

Step 31 consists mainly in pressing the lid 4 against the hollow portion 2 while liquefying by heating the layer 12 so that it diffuses into the nickel metallizations 9, 1 1 to form at least one intermetallic promoting the hermeticity of the encapsulant 3 even for temperatures between 400 and 800 ° C. In addition, compared to current sealing means, it has emerged that the intermetallics formed from nickel have slower growth kinetics, which advantageously makes it possible to better control their formation. As an optional feature, if the MEMS 5 is a quartz tuning fork resonator, it may be necessary to adjust or adjust it. This setting can be performed after step 27 or after step 31. In the latter case, that is to say when the lid 4 is already sealed under vacuum the hollow portion 2 of the housing 7, the cover 4 should comprise at least one transparent portion at a given wavelength of a beam of light, such as a laser beam, used to effect said adjustment. With the aid of the present method 21, the electronic component 1 formed is thus configured as an SMD-type component (abbreviation of the English term "Surface Mounting Device"). As such, it can be mounted and connected by soldering. for example on a printed circuit board. Of course, the present invention is not limited to the example shown but is susceptible to various variations and modifications that will occur to those skilled in the art. In particular, the electronic component 1 may comprise only the resonator element 5 or, alternatively, the method 21 could be adapted to perform a process of the "wafer level packaging" type, that is to say a series encapsulation from two plates against each other which are subsequently cut to form the electronic component 1. It may also be envisaged to mount the oscillator circuit in the same cavity 10 as the quartz resonator 5. This oscillator circuit may also include a real-time clock function (RTC) or other functions. It may be envisaged also to mount one or more MEMS 5 in each housing 7 or alternatively to use alternative materials for the housings 7 such as metal or glass, without departing from the scope of the invention. Similarly, the shape of the metallizations 9, 1 1 can not be limited to that of FIGS. 1 and 2. It is also possible that the phases 20, 22 and 24 are not completely independent depending on the technology of the MEMS used. It is thus conceivable that the phase 24 of forming the hollow portion 2 is performed before the formation phase 20 of the MEMS 5 in the case where the latter 5 is etched directly in this first 2. Finally, a material of the "getter" type can be arranged in the encapsulation device 3 to serve as a vacuum pump, that is to say perfect the vacuum in the device 3 already manufactured, when it is activated for example by means of a laser or during the thermal sealing / diffusion process, simply by means of temperature and time. claims 1. Encapsulation device (3) for an electromicromechanical system (5) comprising a housing (7) comprising a main part (2) forming a cavity (10) which is hermetically closed by a cover (4) with the aid of sealing means (6) characterized in that the sealing means (6) comprise an alloy formed of nickel with a material whose melting point is less than 250 ° C in order to allow its interdiffusion in the liquid state with nickel. 2. Device (3) according to claim 1, characterized in that said material is indium. 3. Device (3) according to claim 1, characterized in that said material is tin. 4. Device (3) according to one of the preceding claims, characterized in that the housing (7) is formed of ceramic. 5. Electronic component (1) comprising an encapsulation device (3) according to one of the preceding claims in the cavity (10) which is mounted at least one electromicromechanical system (5). 6. Component (1) according to the preceding claim, characterized in that the cavity (10) is under vacuum or controlled atmosphere. 7. Component (1) according to claim 5 or 6, characterized in that the electromicromechanical system (5) is a quartz tuning fork resonator. 8. Component (1) according to one of claims 5 to 7, characterized in that it comprises an oscillator circuit (15) electrically connected to the electromicromechanical system (5). 9. Component (1) according to the preceding claim, characterized in that the oscillator circuit (15) is mounted on the main part (2). 10. A method of manufacturing (21) an electronic component (1) formed by an encapsulation device (3) receiving at least one electromicromechanical system (5) comprising the following steps: a) forming (20) said less an electromicromechanical system (5); b) forming (22, 24) a main portion (2) forming a cavity (10) and a cover (4) having metallizations (9, 1 1) comprising at least one layer of nickel; characterized in that the method (21) further comprises the steps of: c) forming (25) an intermediate layer (12) of pure material having a melting point of less than 250 ° C; D) securing (27) said at least one electromicromechanical system (5) in the cavity (10); e) assembling (29) the cover (4) against the main portion (2) to cover the entire cavity (10) and contacting the metallizations (9, 1 1) and the intermediate layer (12); f) heating (31) the metallizations (9, 1 1) and the intermediate layer (12) to form solid-liquid interdiffusion intermetallics capable of sealing said at least one electromicromechanical system (5) in the cavity ( 10). 1 1. Method (21) according to the preceding claim, characterized in that said material whose melting point is less than 250 ° C is indium. 12. Method (21) according to claim 10, characterized in that said material whose melting point is less than 250 ° C is tin. 13. Method (21) according to one of claims 10 to 12, characterized in that, in step c), the intermediate layer (12) is formed on at least one of the metallizations (9, 1 1). 14. Method (21) according to one of claims 10 to 12, characterized in that, in step c), the intermediate layer (12) is formed in a part whose projection surface is of corresponding shape to at least one of the metallizations (9, 1 1). 15. Process (21) according to one of claims 10 to 14, characterized in that step c) comprises an electroplating process. 16. Method (21) according to one of claims 10 to 14, characterized in that step c) comprises a screen printing process. 17. Method (21) according to one of claims 10 to 16, characterized in that the metallizations (9, 1 1) formed in step b) are performed by screen printing and electroplating. 18. Process (21) according to one of claims 10 to 17, characterized in that step f) is carried out under vacuum or in a controlled atmosphere. 19. Method (21) according to one of claims 10 to 18, characterized in that said at least one electromicromechanical system (5) is a quartz tuning fork resonator. 6