ITTO20110779A1 - ENCAPSULATING STRUCTURE FOR MICROELETTROMECHANICAL SYSTEMS AND MANUFACTURING METHOD - Google Patents

Description

DESCRIPTION

of the patent for industrial invention entitled:

"ENCAPSULATING STRUCTURE FOR MICROELECTROMECHANICAL SYSTEMS AND MANUFACTURING METHOD"

The present invention relates to an encapsulating structure for microelectromechanical systems (MEMS) and to a related manufacturing method.

Microsystem technology has allowed, and allows, an ever more efficient miniaturization of electronic components and sensors, in a plurality of technical fields. The miniaturization, in addition to guaranteeing considerable savings in terms of occupied space, also guarantees economic savings. Miniaturized systems are used, for example, in microelectronics, in the automation industry, in the automotive industry, in medical technologies, and in many other fields. The miniaturization also allows an increase in the integration density of the miniaturized systems, allowing the housing of a plurality of systems having different functions in the same chip.

In the field of sensors, in particular, the integration of sensors of different types in the same plate is not always possible. Depending on the requirements, different sensors may require different environmental conditions (e.g. ambient pressure) during use. Resonant systems (such as gyroscopes) have a quality factor Q ("quality value") strongly dependent on the pressure inside the package in which these resonant systems are housed. In particular, the Q factor is higher the lower the pressure of the environment in which they operate is. Conversely, other types of sensors, such as accelerometers, do not need a high Q factor, and can therefore operate at higher ambient pressures (for example, ambient pressure). In order to allow different types of sensors to operate at the desired pressures, hermetically sealed housings have been provided ("provided") containing the sensor and in which the internal atmosphere is suitably regulated in order to include a specific gas and / or a specific environmental pressure.

In order to ensure, for each sensor, the optimal operating conditions in terms of ambient pressure and / or gas present in the housing, sensors of different types (for example gyroscopes and accelerometers), are currently manufactured independently of each other. on the other hand on respective platelets. Separate manufacturing steps guarantee the realization of sensors encapsulated in appropriate respective housings, each of which has its own internal environmental pressure.

However, this leads to obtaining different packages for different sensors. In complex systems, which require, for their correct functioning, the use of a plurality of sensors of different types (for example both gyroscopes and accelerometers, and / or other sensors), the integration of a plurality of packages on the same support (for example an integrated circuit) is a limit for increasing the integration density. Each package, in fact, has an overall volume which is greater than the volume occupied by the sensor contained therein, and has its own external connection elements for the transfer of the power supply and control / detection signals.

The object of the present invention is to provide an encapsulating structure for microelectromechanical systems and a manufacturing method thereof capable of overcoming the drawbacks of the prior art, and in particular capable of increasing the integration density of MEMS sensors suitable for operating at ambient pressures. different from each other.

According to the present invention, an encapsulating structure for microelectromechanical systems and a related manufacturing method are provided as defined in the attached claims.

For a better understanding of the present invention, preferred embodiments are now described, purely by way of non-limiting example and with reference to the attached drawings, in which:

Figure 1 shows, in sectional view along the section line I-I of Figure 2, an encapsulating structure, according to an embodiment of the present invention;

Figure 2 shows the encapsulating structure of Figure 1 in a top view;

Figure 3 shows, in sectional view, an encapsulating structure according to another embodiment of the present invention;

Figure 4 shows, in sectional view, an encapsulating structure according to a further embodiment of the present invention; And

Figures 5-8 show wafers during subsequent manufacturing steps to obtain a plurality of encapsulating structures of the type shown in Figure 1.

Figure 1 shows, in sectional view along the section line I-I of Figure 2, an encapsulating structure 1, or package, for MEMS components or devices (in particular sensors) comprising at least a first and a second cavity 2, 4, of buried type, separated from each other and hermetically isolated from each other. The first cavity 2 houses a first sensor 6, for example a gyroscope, while the second cavity 4 houses a second sensor 8, for example an accelerometer.

More in detail, the package 1 comprises a substrate 10, for example of semiconductor material, lying on an XY plane. On a face 10a of the substrate 10, substantially parallel to the XY plane, the first sensor 6 and the second sensor 8 are made, in a per se known manner. The manufacturing steps of the first and second sensors 6, 8 are not subject of the present invention and are therefore not further described. The package 1 further comprises a cap 12, lying on a respective plane substantially parallel to the XY plane. In correspondence with a face 12a of the cap 12 recesses are formed which form, when the cap 12 is coupled to the substrate 10 so that the face 12a of the cap 12 is in contact with the face 10a of the substrate 10 (as shown in the figure 1), the first and second cavities 2, 4. The first and second cavities 2, 4 are, in this way, cavities inside the package 1, or cavities buried in the package 1.

The cap 12 is of semiconductor (for example, silicon) or insulating material (for example, silicon oxide).

The cap 12 and the substrate 10 are coupled to each other by means of coupling structures 18 (having the shape of a wall or pillar) which are substantially portions of the cap 12 formed following the excavation to make the recesses. The shape and thickness of the coupling structures 18 are therefore defined by the shape and depth of the recesses. Figure 1 shows recesses having ideally vertical internal walls (parallel to the Z axis). It is evident that this representation is ideal and has the sole purpose of illustrating an embodiment of the present invention. Depending on the etching process used to form the recesses, the recesses may have inclined side walls, for example along a preferred etching direction.

The coupling between the substrate 10 and the cap 12 is carried out by known techniques, for example by gluing ("bonding") of the "glass frit" type, or by metal bonding, thermal compression bonding, eutectic bonding, anodic bonding , bonding with adhesives or glues, or still other types of bonding. The bonding layer is schematically shown in Figure 1 and indicated with the reference number 7.

The first and second cavities 2, 4 have, in top view, a shape chosen according to need, for example as shown in Figure 2 in a broken line, a quadrangular shape with rounded corners. Other shapes include circular, or generically polygonal. Still other forms are possible.

The first and second cavities 2, 4 are hermetically isolated from each other and from the environment external to the package 1. All the aforementioned methods for carrying out the bonding between the substrate 10 and the cap 12 ensure good insulation, a good seal over time, and minimal losses.

Following the coupling process of the substrate 10 and the cap 12, the first cavity 2 and the second cavity 4 possess a respective internal pressure which substantially corresponds to the ambient pressure present at the moment of coupling. For example, the first cavity 2 has an internal pressure P1, while the second cavity 4 has an internal pressure P2. The pressures PI and P2 can be the same or different from each other.

In particular, if the first sensor 6 is a gyroscope and the second sensor 8 is an accelerometer, the pressure P1 preferably has a lower value than the pressure P2. It is in fact known that the quality factor Q of a gyroscope is a function of the ambient pressure at which the gyroscope itself works. In this case, the pressure PI is preferably between 5 -IO <3> Pa and 5 -IO <4> Pa, while the pressure P2 can be one or more orders of magnitude higher (for example, between 1-10 <5> Pa and 5 -IO <6> Pa).

In order to obtain a pressure P1 lower than the pressure P2, the first cavity 2 further comprises an absorber element for gas or gaseous molecules, better known as "getter" 14. The getter 14 is usually made of material deposited in the form of a layer, and has the function of absorbing specific gas molecules. Materials used as a getter layer are known, and include for example metals such as aluminum (Al), barium (Ba), zirconium (Zr), titanium (Ti), vanadium (V), iron (Fe), or related mixtures or alloys such as zirconium-aluminum, zirconium-vanadium-iron, zirconium-nickel, zirconium-cobalt (in particular, an alloy of Zr / Co / O).

The getter 14 is, according to an embodiment, of the "non-evaporable" (NEG) type, made in the form of a layer at the internal surface 2 'of the first cavity 2 of the cap 12. The getter 14 is preferably formed at all the area available on the internal surface 2 'of the first cavity 2, in such a way as to maximize the surface area of the getter 14 which is exposed inside the first cavity 2 (active area during the "absorption" or "capture" phase "of gases). As is known, during the formation phase of the getter 14, the material of which the getter 14 is formed reacts with the surrounding air, causing the formation of a passivating layer (typically of oxide or oxide / nitride) which completely covers the area surface of the getter 14, making it inactive.

The activation of the getter 14 occurs after the hermetic sealing of the first cavity 2 by local activation in temperature. This activation step can be performed by locally externally heating the region of the package 1 corresponding to the area in which the getter 14 is arranged (for example by magnetic induction, or by heating by means of a generic heat source), and has the function of removing the layer passivating agent formed on the surface of the getter 14 during its formation phase. In this way, the getter 14 is activated and operates in a known way reacting with residual gases inside the first cavity 2 (with the exception of the noble gases) allowing a reduction of the environmental pressure inside the first cavity 2. In this way, after the activation of the getter 14, the first cavity 2 has an internal pressure P1 of a lower value than the value of the pressure P2 internal to the second cavity 4, which is devoid of the getter layer.

According to a further embodiment, shown in Figure 3, the second cavity 4 also comprises its own getter layer 17, extending internally to the second cavity 4. By suitably modulating the extension of the getter layer 14, 17 in both cavities 2 and 4 it is possible to modulate the pressure values PI and P2 independently of each other and / or independently of the ambient pressure external to package 1.

According to a further embodiment of the present invention, shown in Figure 4, the second cavity 4 has a height h2 greater than the respective height hi of the first cavity 2. The heights hi and h2 are the maximum distances, measured along the axis Z orthogonal to the plane. XY, existing between, respectively, the face 10a of the substrate and the face 2 'in the first cavity 2 (hi) and the face 4' in the second cavity 4 (h2). In other words, the height hi is the maximum depth reached in the first cavity 2 inside the cap 12, measured along the axis Z starting from the face 10a of the substrate 10; while the height h2 is the maximum depth reached in the second cavity 4 inside the cap 12, measured along the axis Z starting from the face 10a of the substrate 10.

Forming first and second cavities 2, 4 of different heights hi and h2 has the advantage of guaranteeing greater flexibility, since sensors of different types may require grooves of different heights.

According to a manufacturing embodiment of the present invention, both the substrate 10 and the cap 12 are formed starting from respective wafers of semiconductor material.

With reference to Figures 4-7, a method for manufacturing a plurality of packages 1 is described.

Initially, figure 5, a substrate wafer 20 is provided ("provide"). The substrate wafer 20 is processed, according to known microfabrication steps, in order to form a plurality of first and second sensors 6, 8 in correspondence with a face 20a of the substrate wafer 20.

Then, figure 6, a cap slice 21 is arranged, which is worked in order to form a plurality of recesses 24 which form, in successive steps, the first and second cavities 2, 4. The recesses 24 are formed by one or more etching phases using one or more suitable masks designed to define the desired shape for the recesses 24. The etching phases may include a wet etching ("wet etching") or a dry etching ("dry etching") , of anisotropic or isotropic type. In the case in which it is desired to form some recesses 24 of greater depth than other recesses 24 (for example in order to obtain, at the end of the process phases, the package of figure 4), it is possible to carry out a first etching step common to all the recesses 24, and a second selective etching step for only the recesses 24 whose depth it is desired to increase. This second attack step can be carried out by using a suitable mask able to protect, during the second attack, the recesses 24 which it is not desired to attack.

A step of selective deposition of the getters 14 is then carried out inside the recesses 24 only corresponding to the first sensors 6, whose operation takes place at low environmental pressures.

Following or prior to the step of forming the getters 14 it is possible to form, in correspondence with the regions 23 of the cap wafer 21 which will contact the substrate wafer 20, the bonding layer 7 (for example an adhesive or metallic layer, or of another type a depending on the bonding technique used), for the subsequent bonding step between the cap slice 21 and the substrate slice 20.

Alternatively, in a way not shown, the bonding layer 7 can be formed on the substrate wafer 20, or on both the substrate wafers 20 and cap 21.

Then, the substrate wafers 20 and cap 21 are aligned, by means of per se known alignment techniques, so that the recesses 24 are aligned with the respective sensors 6, 8 formed on the substrate wafer 20.

A bonding step is then proceeded, according to one of the previously mentioned techniques, forming a plurality of first and second buried cavities 2, 4. In this phase, the first and second cavities 2, 4 have the same internal pressure (P1 = P2), which corresponds to the environmental pressure present in the environment in which the previous alignment and bonding phases were carried out.

The activation step of the getters 14 can take place immediately after the bonding step or at a later time, indifferently.

Figure 8 then proceeds with cutting the assembly thus obtained to obtain a plurality of packages 1 of the type shown and described above with reference to Figures 1-4. Each package 1 obtained following the cutting step therefore comprises a single die, or chip, housing both cavities 2, 4.

Known techniques for carrying out the assembly of generic slices with each other and the subsequent cutting are known, and generally known under the name of wafer-level-packaging, or WLP.

It is evident that, according to further embodiments, other sensors of different types and / or generic electronic circuits can be formed on the same substrate wafer 20, in addition to the first and second sensors 6, 8. In particular, the substrate wafer 20 comprises electrical connections for supplying the sensors housed therein.

From an examination of the characteristics of the invention made according to the present invention, the advantages that it allows to be obtained are evident.

In particular, the realization on the same plate ("die") of multi-axis structures (for example, accelerometers, gyroscopes, etc.), operating at different internal atmospheres, allows to optimize the occupation of the area without decreasing the performance of these structures. multi-axis.

Finally, it is clear that modifications and variations can be made to what is described and illustrated herein without thereby departing from the scope of protection of the present invention, as defined in the attached claims.

For example, the first sensor 6 can be a sensor of any type whose operating characteristics (sensitivity and / or quality factor Q and / or generic performance characteristics) vary as the ambient pressure in which the sensor operates varies. In particular, the first sensor 6 is of such a type that, in use, it preferably operates at a pressure PI lower than the ambient pressure at which the package housing it is, or, alternatively, at a pressure PI lower than a pressure P2 acceptable for the operation of other sensors present on the same plate. Sensors of this type include, for example, gyroscopes, rotational accelerometers, and generic vibrational sensors (or sensors equipped with moving parts) whose performance varies with the variation of the Q factor.

Similarly, the second sensor 8 can be different from an accelerometer, and can be of a different type or the same as the first sensor 6. For example, the second sensor 8 can be a rotational accelerometer, a resonant accelerometer, a magnetometer, a generic resonator, and / or a generic vibrational sensor, or others.

Claims (13)

CLAIMS 1. Encapsulating structure (1) for a first and a second microelectromechanical component (6, 8), comprising: - a cap ("cap") (12) including a first and a second recess (2, 4); - a substrate (10) housing the first and second microelectromechanical components (6, 8), said substrate and said cap being coupled together in such a way that the first recess defines, in said encapsulating structure (1), a first buried cavity (2) housing the first microelectromechanical component (6), and the second recess defines, in said encapsulating structure (1), a second buried cavity (4) housing the second microelectromechanical component (8), said first and second buried cavities (2, 4) being hermetically sealed with respect to each other, the first buried cavity also housing a first absorber element ("getter") (14) configured to generate a first ambient pressure (PI) internal to the first cavity less than a second ambient pressure (P2) internal to the second cavity. Encapsulating structure according to claim 1, wherein the first getter element (14) comprises at least one of aluminum, barium, zirconium, titanium, vanadium, iron, or related mixtures or alloys. Encapsulating structure according to claim 1 or 2, wherein the substrate (10) is of semiconductor material and the cap (12) is of semiconductor or insulating material. 4. Encapsulating structure according to any one of the preceding claims, in which the second buried cavity also houses a second absorber element ("getter") (17) configured to generate a second ambient pressure (P2) inside the second cavity different from the first ambient pressure (Pi) internal to the first cavity, the second getter element comprising at least one of aluminum, barium, zirconium, titanium, vanadium, iron, or related mixtures or alloys. 5. Encapsulating structure according to any one of the preceding claims, in which the first buried cavity (2) has a first maximum height (hi), the second buried cavity (4) has a second maximum height (h2), called first and second height maximum being measured along a direction (Z) substantially orthogonal to a storage plane (XY) of the substrate (10), in which the first maximum height is less than the second maximum height. 6. Encapsulating structure according to claim 5, wherein the first microelectromechanical component (6) is adapted to operate with alternating current signals, and the second microelectromechanical component (8) is adapted to operate with direct current signals. 7. Encapsulating structure according to any one of the preceding claims, in which the first microelectromechanical component (6) is a sensor whose performance during use varies as the ambient pressure value at which it operates varies. 8. Encapsulating structure according to any one of the preceding claims, in which the substrate (10) and the cap (12) are coupled by bonding selected from: bonding "glass frit", metal bonding, thermal compression bonding, eutectic bonding, anodic bonding, adhesive bonding, or with glue. 9. Assembly of wafers comprising: - a first wafer ("20") of semiconductor material, including a plurality of first and second microelectromechanical components (6, 8); - a second wafer ("21") of semiconductor or insulating material including a plurality of first and second recesses (24), said first wafer and said second wafer being coupled together in such a way that said plurality of first and second recesses defines a respective plurality of first and second buried cavities (2, 4), each first buried cavity housing a respective first microelectromechanical component of said plurality of first microelectromechanical components and each second buried cavity housing a respective second microelectromechanical component of said plurality of second microelectromechanical components, said first and second buried cavities (2, 4) being hermetically sealed with respect to each other, the first buried cavities also housing a respective absorber element ("getter") (14) configured to generate a first environmental pressure (PI) internal to the first cavities lower than a second environmental pressure (P2) internal to the second cavities. 10. Method of manufacturing an encapsulating structure (1) adapted to house a first and a second microelectromechanical component (6, 8), comprising: - arrange a cap ("cap") (12); - forming in said cap a first and a second recess (2, 4); - arranging a substrate (10) comprising the first and second microelectromechanical components (6, 8); - forming a getter element (14) in the first recess; - couple the substrate and the cap together in such a way that the first recess defines a first buried cavity (2) housing the first microelectromechanical component (6), and in such a way that the second recess defines a second buried cavity (4) housing the second microelectromechanical component (8); And - hermetically seal the first and second buried cavities (2, 4) with respect to each other. Method according to claim 10, further comprising activating the getter element (14) so as to generate a first ambient pressure (Pi) internal to the first cavity lower than a second ambient pressure (P2) internal to the second cavity. Method according to claim 10 or 11, wherein the step of forming in said cap a first and a second recess (2, 4) comprises: - carrying out a first etching so as to form first and second recesses having a first depth (hi); And - perform a second attack in correspondence of the second recesses only so as to form second recesses having a second depth (h2) greater than the first depth (hi). Method according to any one of claims 10-12, wherein the step of coupling the substrate and the cap together comprises carrying out a bonding step chosen from: bonding "glass frit", metal bonding, bonding a thermal compression, eutectic bonding, anodic bonding, adhesive bonding, or with glue.