Process for bonding and electrically connecting microsystems integrated in several distinct substrates

A process for bonding two distinct substrates that integrate microsystems, including the steps of forming micro-integrated devices in at least one of two substrates using micro-electronic processing techniques and bonding the substrates. Bonding is performed by forming on a first substrate bonding regions of deformable material and pressing the substrates one against another so as to deform the bonding regions and to cause them to react chemically with the second substrate. The bonding regions are preferably formed by a thick layer of a material chosen from among aluminum, copper and nickel, covered by a thin layer of a material chosen from between palladium and platinum. Spacing regions ensure exact spacing between the two wafers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for bonding and electrically connecting microsystems integrated in several distinct substrates.

2. Description of the Related Art

As is known, numerous technological approaches allow manufacturing integrated circuits wherein the electronic circuitry coexists with a sensor element or an actuator (micro-electromechanical device). The traditional approaches envisage the production of the sensors/actuators and circuitry in a same silicon substrate (surface and epitaxial sensors). The most recent approaches envisage, instead, several substrates and the electronic circuit, the micro-electromechanical device or parts thereof are formed in distinct wafers that are subsequently bonded together and finally diced.

Bonding of the wafers is obtained by causing one or more metals to react with one another, with the silicon of one of the substrates or with metal alloys. To this aim, one or more metals are deposited in sequence on the surface of one or both of the wafers. Then the surfaces to be bonded are brought into intimate contact through a piston that applies a pre-determined pressure, as shown inFIG. 1, which illustrates a substrate1, a first wafer2, a layer of bonding material3, a second wafer4, and a piston5which presses the second wafer4against the first wafer2.

Under the pressure of the piston4, the bonding material reacts only where the surfaces are in a mechanical contact, and the areas that are not in contact are not bonded.

With this solution, bonding between the wafers depends to a large extent upon the mechanical force of the piston; in particular, criticalities are linked, on the one hand, to the uniformity of pressure applied by the piston and, on the other, to the possible presence of foreign bodies.

In particular, for example in the presence of non-planar areas, the pressure applied by the piston may be non-uniform over the entire surface or over the entire area where bonding is to be obtained. In this case, the presence of areas of the two wafers that are not in contact prevents bonding of these areas.

In addition, the presence of particles, acting as spacers, also entails absence of contact, which prevents bonding, as shown, by way of example, inFIG. 1, wherein a particle7prevents bonding in an area of the surfaces of the wafers2,4.

On the other hand, application of excessive pressure in an attempt to achieve uniform contact in the areas to be bonded may be counterproductive. In fact the deformation of the substrate thus induced causes stresses in the material that persist over time, weakening the bonding joints and/or subsequently causing undesired deformations, in particular in case of suspended structures. For example, a mobile part (such as a rotor of a micro-actuator), once it is released, tends to relieve the accumulated stresses. In this case, the mobile part may get deformed and undergo an undesired spatial displacement, such as might impair proper operation of the structure or, in any case, reduce efficiency thereof.

BRIEF SUMMARY OF THE INVENTION

An embodiment of the present invention provides a manufacturing process allowing a good bonding quality to be achieved between wafers of semiconductor material.

According to the present invention there are provided a process for bonding distinct substrates, as well as a device obtained thereby.

According to an embodiment of the invention, a process for bonding two distinct substrates is provided, comprising the steps of forming micro-integrated devices in at least one of two substrates, using micro-electronic processing techniques and bonding the substrates by forming, on a first of the substrates, bonding structures of deformable material and by pressing the substrates against each other so as to deform the bonding structures and cause the bonding structures to react chemically with a second substrate.

The bonding structures may comprise a stack of layers including a soft layer and a bonding layer. They may also include a diffusion barrier layer between the soft layer and the bonding layer.

The method may also include forming spacing regions on the first substrate, the spacing regions having a thickness less than the thickness of the bonding structures.

Another embodiment of the invention provides an integrated device comprising first and second substrates, distinct from each other, and bonding structures arranged between said first and second substrates, including portions of deformed material and portions of material derived from the reaction between said structures and said second substrate.

The structures may comprise a stack of layers including a soft layer and a bonding layer. A diffusion barrier layer may be formed between the soft layer and the bonding layer.

DETAILED DESCRIPTION OF THE INVENTION

The invention is based upon the use of a material having characteristics allowing good-quality bonding of two substrates (namely, two wafers of semiconductor material in which electronic devices and/or micro-electromechanical structures are integrated), even in presence of non-planar areas and/or undesired particles acting as spacers, as generally happens in the case of substrates that have undergone previous fabrication processes.

According to one aspect of the invention, on one of two substrates a layer (possibly a composite layer) is formed having characteristics of high deformability (soft material) and capacity for reaction with the other substrate (bonding material).

In what follows, the term “soft material” or “deformable material” refers to a material which at standard bonding pressures and at a low temperature (of less than 450-500° C., usable in the final fabrication steps) undergoes deformation without causing stresses on the substrates (for example, a material that has a modulus of elasticity of less than one tenth that of silicon).

According to another aspect of the invention, as shown inFIG. 2, on the first substrate (first wafer10) there is deposited and possibly defined a stack of layers comprising at least one soft layer11, of a material having good plastic characteristics and low cost (such as aluminum, copper or nickel), and at least one bonding layer12, which reacts with the material present on the surface of the second substrate (second wafer13), forming a eutectic or a silicide. A suitable material is, for example, palladium or platinum.

According to yet another aspect of the invention, between the soft layer11and the bonding layer12a diffusion barrier layer14may extend, with the dual function of enabling good adhesion between the soft layer11and the bonding layer12and of constituting a barrier against the diffusion of the various materials of the soft layer11and bonding layer12. A suitable material is, for instance, chromium or titanium.

Typically, the material of the bonding layer12has a high cost, such as to require minimization of its use, adopting low thicknesses.

In order to carry out bonding, the first wafer10and second wafer13are brought into mechanical contact with one another. A temperature cycle (for example, at 400° C.) and mechanical pressure (through a piston similar to the piston5ofFIG. 1) is carried out so as to cause the bonding layer12and the second wafer13to react and bond. In this step, the soft layer11undergoes deformation and adapts to the existing geometry, compensating for any non-planar regions and/or for the presence of foreign bodies. In practice, the soft layer11, which has a low cost and hence can be deposited with a large thickness, behaves like a cushion and enables a more even distribution of the pressure exerted by the piston, in such a way as to obtain uniform mechanical contact over the entire area to be bonded and in such a way that any foreign bodies are completely surrounded and embedded in said layer.

Hereinafter there will be described an example of a sequence of steps of a process for bonding and electrically connecting two wafers, one of which houses electrical circuits and the other houses a micro-electromechanical device.

With reference toFIG. 3, first electrical components18are formed, in a known way, in a body17of semiconductor material. On top of the body17various insulating layers—illustrated, for simplicity, as a single insulating layer19—and various conductive layers, including polycrystalline-silicon regions and various metal levels (not shown in detail) are formed and defined. On top of the insulating layer19a top metal layer, for example of aluminum, is formed and defined, thereby forming a contact region20, connected to the electrical components18, as schematically shown inFIG. 3. Then a protection layer21, preferably of silicon dioxide, is deposited and opened, so as to form an opening above the contact region20.

Next (FIG. 4), a first and a second conductive layers23,24are deposited in succession. For example, the first conductive layer23may be of tantalum/aluminum, and the second conductive layer24may be of aluminum. Then (FIG. 5), a spacing layer25, for example of silicon dioxide, is deposited.

Next (FIG. 6), the spacing layer25is defined so as to form spacing regions25′, and (FIG. 7) first the second conductive layer24and then the first conductive layer23are defined. Thus, connection lines26are formed by the overlaid first and second conductive layers23,24, and stator electrodes27formed by the first conductive layer23alone.

Next (FIG. 8), a sacrificial layer28, for example of polyimide, is deposited and opened where the bonding regions are to be formed. As shown inFIG. 9, a soft layer30(for example of aluminum) and a bonding layer31(for example of palladium) are deposited and defined, thus forming bonding regions32that extend in part on top of the sacrificial layer28. In particular, the thickness of the soft layer30(which determines, to a first approximation, the thickness of the bonding regions32) is greater than the thickness of the spacing layer25. The bonding regions32are thus deeper than the spacing regions25′. Subsequently, the sacrificial layer28is removed, and finally (FIG. 10) the wafer33thus obtained is bonded to a second wafer34in which micromechanical structures (not shown) have been formed.

In this step, the wafers33,34are pressed against one another at a low temperature (for instance, at about 400° C.). Consequently, the aluminum of the soft layer30, which melts at 600° C., softens and spreads out, thus enabling the second wafer34to abut against the spacing regions25′, which thus ensure proper spacing between the wafers33,34, while the bonding layer31reacts with the second wafer34to form a silicide or a eutectic, ensuring bonding of the wafers. Then bonding joints35are formed, that buckle with respect to the bonding regions32. The spacing regions25′ may moreover be shaped in such a way as to surround the bonding joints35and isolate them from the outside environment.

By making the bonding regions32of an appropriate depth, equal to at least the sum of the depth of the spacing regions25′ and the possible depressions in the second wafer34, it is possible to ensure bonding even in the non-planar areas of the wafers33,34, as shown inFIGS. 11 and 12, wherein the second wafer34has a central depression which would prevent bonding thereof to the first wafer33. As shown inFIG. 12, the central bonding region32is deformed less than the lateral regions, but ensures bonding even so.

Finally, the final fabrication steps are performed, which include, if so envisaged, thinning-out of the first wafer33and/or second wafer34, freeing of the suspended structures, dicing, packaging, etc.

In certain applications, it may be necessary to have two or more bonding regions32arranged in parallel, so as to obtain a section with adequate contact. In fact, to ensure a sufficient deformability of the bonding regions32, the portion of soft material cannot have an excessive width, i.e., a width greater than a certain value, which can be determined experimentally. In this case, it is possible to arrange a plurality of bonding joints35in parallel. For example, as shown inFIG. 13, two bonding joints35are connected to a same connection line26on the first wafer33and to a same conductive region36in the second wafer34. The conductive region36is electrically insulated from the remainder of the second wafer34by insulating regions37.

FIG. 13also shows two spacing regions25″ which do not surround bonding joints35and are formed at regions removed from the second wafer34. In this case, the spacing regions25″ have a function of mechanical support to the second wafer, wherein a linear electrostatic motor is formed, in order to prevent collapse of the suspended regions. Here, the spacing regions25″ are arranged in the proximity of a “spring”40which connects a fixed region41of the second wafer34(which houses the conductive region36) to a mobile region (“rotor”)42provided with mobile electrodes43. The spacing regions25″ face a removed portion that surrounded the spring40. Possibly further spacing regions25″ having the function of temporary mechanical suspension may be provided also at the suspended regions and must be removed after the bonding step.

Finally, it is clear that numerous modifications and variations may be made to the process and device described herein, without thereby departing from the scope of the present invention.

In particular, the invention may be applied to integrated devices of any type formed in at least two substrates.

The material of the bonding regions may vary. The diffusion barrier material may be present or not, according to the materials used and to the requirements. The soft layer may be modified in terms of hardness, for example by adding copper to the aluminum. Alternatively, the soft layer may be made entirely of copper, possibly coated with a thin layer of platinum, which forms the bonding layer. The soft material may be nickel protected by a very thin layer of palladium, which is exhausted during bonding and enables the formation of a nickel silicide; in this case, then, the nickel layer works both as a soft material, which undergoes deformation and enables adaptation of the bonding joints to the existing geometry, and as a bonding material, which ensures mechanical connection between the two wafers.

Obviously, the same process can be used to bond three or more wafers together.