Patent ID: 12202725

DETAILED DESCRIPTION OF THE DISCLOSURE

The following examples describe a method for bonding wafers in a wafer bonding process. A wafer refers here to a slice of solid material(s) that can be used for the fabrication of microelectromechanical system (MEMS) elements. A wafer may be formed of uniform material or include layers of different materials. A wafer may also undergo microfabrication processes that shape it so that a surface of the resulting wafer is not uniformly planar but includes carefully controlled and dimensioned recesses and/or regions of various materials and shapes.

A wafer may thus include, for example, a uniform slice of glass, crystalline silicon wafer, or a structural wafer like a crystalline silicon wafer with insulator and polysilicon layers on it, a silicon-on-insulator (SOI) wafer, a cavity SOI wafer (C-SOI) or a device wafer layer of C-SOI wafer. As an example of a structural wafer, a silicon-on-insulator (SOI) wafer typically comprises a handle wafer layer, a buried oxide (BOX) layer, and a device wafer layer. The handle wafer layer is usually the thickest part, some hundreds of microns thick, while the device wafer layer is typically tens of microns thick. The BOX layer is typically from fraction of a micron to a few microns thick. The BOX layer may be deposited either on the handle wafer layer, or the device wafer layer, and the two pieces of silicon may be bonded to each other so that the BOX layer is in between them and isolates the device wafer layer electrically from the handle wafer layer. Structures with electromechanical properties are typically manufactured into the device wafer layer of a SOI wafer by etching trenches into and/or through the device wafer layer. Sacrificial etching of the BOX layer can be used to mechanically release selected MEMS structures also from the BOX layer. The thickness of the device wafer layer may be in the order of a few tens of μm, but SOI wafers are available at different device wafer layer thicknesses ranging from a few μm to 100 μm or more.

In order to enable wafer bonding, a wafer includes bonding surface parts that are designed to get into permanent contact with bonding surface parts of another wafer in the wafer bonding process. Advantageously, but not necessarily, the bonding surface parts are locally planar and align to a contact plane that is common to all bonding surface parts of the wafer. However, both wafers to be bonded do not necessarily need to be microprocessed before bonding. One wafer may provide a planar surface that is bonded to a recessed surface of another wafer.

These basic concepts are illustrated with examples inFIG.1andFIGS.2ato2c.FIG.1illustrates an exemplary wafer100formed of a crystalline semiconductor material. Wafers typically have one or more flats102or notches cut into one or more sides of the wafer to indicate crystallographic planes of the wafer. If the wafer has undergone a microfabrication process, several microstructures104have been formed onto it. The microfabrication process may include one or more subprocesses, like doping, ion plantation, etching, thin-film deposition, and/or photolithographic patterning, for example. Through the wafer bonding process, each of these microstructures104gets into a permanent contact with a corresponding microstructure in another wafer and the microstructures thereby form a MEMS element. Finally, the individual MEMS elements can be separated into dies by wafer dicing and then further forwarded for device packaging.

Microstructures104in at least one of the wafers to be bonded often include recesses. These are needed, for example, in a final MEMS element to provide an open space in which the mechanically moving parts of the system can move. To enable controlled mechanical operation of the system, such open spaces preferably become hermetically closed when opposing microstructures are permanently attached to each other in the wafer bonding process.

FIGS.2ato2dillustrate some examples of microstructures ofFIG.1. As explained above, the wafer100ofFIG.1includes a plurality of microstructures, some exemplary forms of which are shown withFIGS.2ato2d. In order to minimize the cost per die, the number of dies that can be made from a single wafer is typically maximized.

FIG.2aillustrates a simple form of a microstructure104that has a non-patterned, planar surface. Such a microstructure in one wafer may be bonded to a microstructure in another wafer, and the latter may comprise one or more recesses and bonding surface parts. In the wafer bonding process, bonding surface parts in opposing microstructures become permanently fixed to each other such that the recesses form hermetically closed cavities. The microstructure of2amay be, for example, a glass layer element that is to be bonded to a microstructure that includes mirror and spring elements of an optical device. The microstructure of2amay also be, for example, a semiconductor plate designed to extend over a gap formed for a pressure sensor microstructure in the opposing wafer. These are examples only, other types of microstructures with the same form are included in the scope.

FIG.2billustrates another type of a microstructure that still includes a planar surface but has been microfabricated to include regions of different materials. These regions may exist in the microstructure, for example, to form bonding surface parts that through their material characteristics enable or even induce a chemical, electrical, adhesive or other mechanism necessary to implement the wafer bonding process and form a desired permanent contact between bonding surface parts in opposing microstructures. The microstructure104ofFIG.2bmay include, for example, a silicon substrate200onto which regions of glass have been microfabricated to form bonding surface parts202. By such bonding surface parts the microstructure can be sealed to silicon or metal surfaces in an opposing microstructure in an anodic bonding process. However, this is an example only, other type of materials, microfabricated forms and bonding methods with similar surface forms may be applied in the scope.

FIG.2cillustrates a microfabricated microstructure104that includes a recess204and mobile structure parts206that extend into the recess. The mobile structure parts206are thus surrounded by a void space in which they can move in order to implement the designed function of the MEMS element the microstructure is part of. The microstructure ofFIG.2cmay be, for example, a part of a microelectromechanical accelerometer device and the mobile structure parts206may include comb structures for capacitive detection. However, this is an example only, other type of materials, microfabricated forms and bonding methods with similar surface forms may be applied in the scope.

FIG.2dillustrates a further example where bonding surface parts202of the microstructure104are not planar but include some inwards and/or outwards extending formations that are intended to bond with matching formations in bonding surface parts of the opposite microstructure.FIG.2dshows an example where the formation is a protuberance208.

As mentioned above, a wafer bonding process refers here to any process where a permanent contact is formed between the bonding surface parts of the first wafer and the bonding surface parts of the second wafer. This permanent contact encapsulates said microstructures in bonded wafers into MEMS elements where sensitive internal structures are protected from environmental influences such as temperature, moisture, high pressure and oxidizing species, for example. Examples of wafer bonding processes include direct bonding, surface activated bonding, plasma activated bonding, anodic bonding, eutectic bonding, glass frit bonding, adhesive bonding, thermocompression bonding, reactive bonding and transient liquid phase diffusion bonding. However, the invention is applicable in any wafer bonding process where mechanisms and reactions that form the permanent bonding between the bonding surface parts of the microstructures may be adversely affected by in-plane offset of wafers to be bonded.

Returning back toFIG.1, embodiments of the present disclosure disclose an exemplary arrangement where one of the wafers includes a prebonding element106that enables fastening of a first wafer to a second wafer for the duration of the wafer bonding process in a way that maintains accurate alignment of microstructures in opposing wafer surfaces before and during the wafer bonding process, but avoids the adverse effects of the conventional fixed prebonding methods on the wafer bonding process. It should be noted that the disclosed solution is a simple example, in implementations, each wafer to be bonded may include one or more prebonding elements and prebonding elements may be arranged to one, both or all wafers to be bonded. When the individual MEMS elements are separated into dies by wafer dicing, the one or more prebonding elements are also released from the wafer stack.

FIG.3aillustrates a simple example of a prebonding element300. The example ofFIG.3ashows the prebonding element in a top view to a surface of the first wafer100. As shown inFIG.3a, the first wafer100is considered to extend in two orthogonal in-plane directions IP1and IP2. As discussed withFIGS.2bto2dand also shown inFIG.3a, the first wafer may include microfabricated recesses or protuberances, but the in-plane directions IP1and IP2are easily conceived through the planar surface of the initial wafer disc from which the first wafer has been formed of.FIG.3ashows also an out-of-plane direction OP that is orthogonal to the first in-plane direction IP1and to the second in-plane direction IP2. In this disclosure, the in-plane directions IP1and IP2are also called as horizontal directions, and the out-of-plane direction OP as the vertical direction. The prebonding element300in this example is formed of the first wafer100. In other words, the prebonding element300is microfabricated into the first wafer100by removing material from the first wafer100, for example by etching, laser ablation, or some similar process. Alternatively, the prebonding element300may be formed as a layered element on the first wafer100. For example, layers of polysilicon may be applied for the purpose.

The prebonding element300includes a first element point302and a second element point304and an elastic element that extends between the first element point302and the second element point304. The first element point302is at a region through which the prebonding element300is fixed to the first wafer100. The elastic element is configured to deflect between the first element point302and the second element point304and to thereby enable movement of the second element point304in the out-of-plane direction of the first wafer. This ability of the prebonding element300to deflect in the out-of-plane direction and deflect only minimally or practically none at all in the in-plane direction is utilised to minimise in-plane offset of wafers to be bonded. This is implemented by forming an adherent contact between a region around the second element point on the prebonding element and the second wafer.

In some cases, the adherent contact between the region around the second element point on the prebonding element and the second wafer can be formed without any intermediate material layers. For example, when the interface between the prebonding element and the second wafer is a silicon-glass interface the adherent contact can be made with a silicon-to-glass laser pre-bonding arrangement that allows accurate creation of bond spots to the silicon-glass interface.

Alternatively, an intermediate prebonding layer may be applied to form the adherent contact. In the following, this alternative is illustrated with an example where a patch layer of prebonding material306that covers a region around the second element point304is deposited on the prebonding element300.

It should be noted, however, that the patch layer of prebonding material306can alternatively be created on the second wafer, opposite the prebonding element300and the second element point304. The patch layer of prebonding material may be a layer deposited on the second wafer so that the second element point coincides with the patch layer of prebonding material in the out-of-plane direction. In other words, we may consider a reference plane that is defined by two intersecting lines that run in the two in-plane directions IP1and IP2. The second element point then coincides with the patch layer of prebonding material in the out-of-plane direction if the projection of the second element point on the reference plane is within the projection of the patch layer of prebonding material on the reference plane.

The term patch implies here that the horizontal dimensions of the deposited material forming the patch layer covers only a very limited region around the second element point. The term layer implies here that the vertical dimension of the deposited material forming the patch layer is considerably (at least ten times) smaller than the horizontal dimensions. The vertical and horizontal dimensions of the material deposited for the patch layer of prebonding material are optimised to enable an adequate adherent contact between the prebonded wafers until creation of the permanent contact in the wafer bonding process, but to avoid adding any unnecessary fixed vertical offset between the wafers.

In the wafer bonding process, the first wafer100needs to be aligned with the second wafer so that bonding surface parts of the first wafer100are aligned with bonding surface parts of an opposing second wafer. In other words, in a projection in the out-of-plane direction OP, bonding surface parts of a first wafer are adjusted to maximally overlap bonding surface parts of the second wafer. As a result of the alignment, the patch layer of prebonding material306on the prebonding element300gets also in contact with the second wafer, or the prebonding element300layer on the second wafer gets to be aligned with the prebonding element300in or on the first wafer. The patch layer of prebonding material306is then activated to maintain the extremely important accurate horizontal alignment of the wafers. Activation in this context refers to an operation that temporarily or permanently transforms the patch layer of prebonding material306so that it forms an adherent contact between the prebonding element300and the second wafer. This transformation may be activated, e.g. by heating the layer of a prebonding metal material so that it melts between the prebonding element300and the second wafer, and after cooling attaches the prebonding element300and the second wafer to each other. The patch layer of prebonding material may include, for example, Titanium, and it may be activated with laser. Other materials and activation methods, like room temperature bonding, well known to a person skilled in the art may be applied in the field. After activation of the prebonding material, the attached wafers are ready to be processed in the wafer bonding process where the actual permanent contact between the bonding surface parts of the first wafer and the bonding surface parts of the second wafer is created.

In the example ofFIG.3a, the prebonding element includes a cantilever beam that has been formed of the first wafer100by removing material from a region that surrounds the cantilever beam all the way through the first wafer. If the wafer is formed of a layer of a SOI wafer, like a device wafer layer, the cantilever beam is also mechanically released from the underlying layer, like a BOX layer. One end of the cantilever beam is thus firmly attached and supported by the first wafer and the other end of the cantilever beam is unsupported and free to move. Accordingly, when placed under a load in the unsupported end, the cantilever beam can deform and thus deflect in the out-of-plane direction OP. The first element point302is in a region through which the prebonding element300is fixed to the first wafer100, which in this example means the position where the cantilever beam structure begins. The second element point304is in a region at or close to the unsupported end of the cantilever beam so that the cantilever beam extends between them to implement the function of the elastic element. The second element point304is covered by the patch layer of prebonding material306. The patch layer of prebonding material can then be activated so that it becomes attached both to the unsupported end of the prebonding element300and to a region in the second wafer.

Under a force acting on the second element point304, the deflection of the cantilever beam depends on the length of the beam, the modulus of elasticity of the material of the first wafer and the area moment of inertia of the beam's cross section. In this configuration, the material of the cantilever beam is the material of the first wafer, and the thickness (the vertical dimension) of the cantilever beam equals the thickness of the wafer, but the length of the cantilever beam, the constitution of the cantilever beam (e.g. recessed or not uniform) and the form factor of the cross section of the cantilever beam can be adjusted so that a desired level of rigidity in the in-plane direction and in the out-of-plane direction is achieved.

The in-plane rigidity of the prebonding element, alone or in combination with other prebonding elements, effectively reduces the risk of in-plane misalignment in transferring the wafers to a chamber where the wafer bonding process is implemented, or in wafer bonding process stages within the chamber. Furthermore, the microscale size of the prebonding element300enables creating the desired attachment into one position or more carefully selected positions so that the pressing force of the clamps can be reduced, or the conventional clamps are not needed at all. The risk that the prebonded wafers locally bend closer to each other at locations under the clamps and further from each other at locations between them is significantly reduced, which improves the accuracy of the alignment in the wafer bonding process. On the other hand, the out-of-plane elasticity of the prebonding element enables moving the first wafer and the second wafer temporarily away from each other (e.g. by means of spacers) and thus maintain an open space for fluid flows in the wafer processing stage.

FIG.3billustrates another example of an alternative structure for the prebonding element300. The prebonding element300includes now four first element points302, a second element point304, and four elastic elements310. The first element points302are in regions through which the prebonding element300is fixed to the first wafer100, which in this example means in regions where elastic elements connect to the surrounding wafer100. Two of the first element points can be connected with a line that is parallel to the first in-plane direction IP1and two of the first element points can be connected with a line that is parallel to the second in-plane direction IP2. The elastic elements310connect to a pad312and the second element point is in the middle of the pad312. Each of the elastic elements310is configured to elastically deflect between the respective first element point302and the second element point304and thus enable movement of the second element point304in the out-of-plane direction of the first wafer.

Again, as discussed above, the adherent contact can be formed without any intermediate layers or a patch layer of prebonding material may be used for the purpose. A patch layer of prebonding material306is deposited on the first wafer or on the second wafer so that after activation, the patch layer of prebonding material306covers a region around the second element point304, and attaches the first wafer and the second wafer to each other.

The structures described withFIGS.3aand3bare examples that describe the elements and function of the prebonding element in simple terms.FIGS.4aand4billustrate a further example embodiment, which applies the form of the prebonding element300ofFIG.3aand introduces some additional features that may be applied separately or jointly to enhance the advantageous effects of the basic concept.

FIG.4ashows a top view a prebonding element400that may be included in a wafer100ofFIG.1. The prebonding element includes a plurality of cantilever beams described in more detail withFIG.3a. The prebonding element400includes cantilever beams402that extend in parallel in the first in-plane direction IP1. InFIG.4a, one cantilever beam402that extends in the first in-plane direction IP1has been denoted with a reference number. All shown cantilever beams extending in the same orientation can be considered to form a first set of cantilever beams. As already discussed, material properties and the thickness of the wafer typically depend on the functionality of the resulting MEMS element so in this respect there are many parameters that limit the design of the prebonding element. With the added number of cantilever beams it is possible to create a controlled stiffness for the adherent but elastic contact that is provided by the prebonding element.

It is easily understood that after the alignment and before the wafer bonding, forces upon the adherent contact provided by the prebonding element(s) do not appear only in the out-of-plane direction OP. It may be that dimensional and material properties of the cantilever beam(s) can be designed to provide the required elasticity in the out-of-plane direction OP but necessary stiffness to prevent misalignment in the in-plane directions IP1, IP2. However, this is not always the case. A cantilever beam does not easily compress or stretch in the direction of its length, but it tends to deflect under a force in the direction that is perpendicular to the direction of its length. For example, the cantilever beam402that extends in the first in-plane direction IP1may deflect also in the second in-plane direction IP2. However, in the configuration ofFIG.4a, such deflections are limited by adjacent cantilever beams that extend to the same direction. Accordingly, the multiplication of parallel cantilever beams provides an effective way to restrict in-plane deflections before and during the wafer bonding process.

In order to further reduce such in-plane deflections, the prebonding element may include a first set of cantilever beams that extend in the first in-plane direction IP1and a second set of cantilever beams that extend in the second in-plane direction IP2. InFIG.4a, one cantilever beam404that extends in the second in-plane direction IP2has been denoted with a reference number. All shown cantilever beams extending in the same orientation can be considered to form a second set of cantilever beams.

FIG.4aillustrates an example where layers of prebonding material410are advantageously deposited on the first wafer or on the second wafer so that after activation, the layers of prebonding material attach regions on cantilever beams in the first and the second set of cantilever beams to the second wafer. As discussed earlier, in some applications, the adherent attachment can be implemented through direct contact between the prebonding element and the second wafer, without the separate layer410of prebonding material. The first set of cantilever beams limits parasitic in-plane motions in the second in-plane direction IP2and the second set of cantilever beams limits parasitic in-plane motions in the first in-plane direction IP1. Through this combination of cantilever beams in two orthogonal in-plane directions, possible in-plane misalignment between wafers to be bonded are effectively suppressed by the prebonding element, but the controlled elasticity in the out-of-plane direction desired for the wafer process is maintained.

To further restrict the possible in-plane misalignments, the prebonding element400may include one or more longitudinal stopper elements406. A stopper element406may be a longitudinal structural element that is fixedly attached to an underlying layer and extends parallel to a cantilever beam but is separated by a non-zero distance from the cantilever beam. The underlying layer refers here to a layer from which the prebonding element is mechanically released. For example, if the first wafer is a device wafer layer of a structural wafer, and the cantilever beam has been separated from an underlying handle wafer layer by sacrificial etching in the BOX wafer layer, the stopper element406is fixed to the handle wafer layer. The stopper element406may be formed of the BOX layer wafer and the device layer wafer of the original structure wafer, or it may be of a different material, deposited on the handle wafer layer or remains of the BOX layer wafer after the sacrificial etching.

Advantageously, but not necessarily, for each cantilever beam there are two longitudinal stopper elements406,408, which extend parallel to the cantilever beam402on both sides of the cantilever beam402, as shown inFIG.4a. These pairwise arranged stopper elements that in this example run parallel to the first in-plane direction IP1reduce and by far eliminate in-plane deflections of the cantilever beam402in the positive second in-plane direction IP2and in the negative in-plane direction IP2.

FIG.4billustrates the prebonding element400in a side view along line A-A shown inFIG.4a. As shown, the wafer100may be of a uniform material, or formed of a device wafer layer of a structural wafer. Boundaries of wafer layers in a structural wafer are illustrated with dashed lines inFIG.4b. When the adherent contact is made directly, the protrusion of the patch layer of prebonding material can be omitted. However, the prebonding element could be intentionally formed to have such protrusion to ensure that it is the first and only point in touch with the second wafer at the time the adherent contact is activated.

FIG.4cillustrates the prebonding element400in another side view along line B-B shown inFIG.4a.FIG.4cshows also the optional stopper elements406,408.

FIG.5illustrates another example structure for the prebonding element500. The structure ofFIG.5provides a way to effectively limit and reduce parasitic in-plane deflections and thereby suppress possible in-plane misalignment between wafers to be bonded. For the prebonding element500an opening512is made to the first wafer and the prebonding element includes two or more first element points502in separate regions through which the prebonding element is fixed to the first wafer. The prebonding element includes two or more elastic elements,510each of which extends from one first element point to a common second element point504in the middle of the opening.

FIG.5shows an example where each elastic element includes a cantilever beam and has a first element point502in a region through which the prebonding element is fixed to the first wafer. The first element points502are separate from each other, but the elastic elements510connect to each other at a common second element point504. If two adjacent cantilever beams of elastic elements510are considered to align with sides of an angle, advantageously angles between all elastic elements510are the same. In other words, the prebonding element is formed within an opening of the first wafer, and the first element points502are evenly distributed to the circumference of the opening.

The example prebonding element500ofFIG.5includes eight elastic elements510, each of which includes a cantilever beam. One end of each cantilever beam is connected to a first element point502. The first element points502are in regions through which the prebonding element500is fixed to the first wafer100. In this example this again means that the first element points are in regions where elastic elements connect to the surrounding wafer100. The first element points502are evenly distributed to a circumference of a circular opening, which in this example means that an angle between two adjacent cantilever beams is 45 degrees. It should be noted that the number of elastic elements and first element points is exemplary only, depending on the configuration, some other number of elastic elements may be applied for the purpose. For example, a combination of sixteen elastic elements and sixteen first element points302, evenly distributed to a circumference of a circular opening is possible.

The prebonding element500includes also a common second element point504that is in the point where the elastic elements connect to each other in the middle of the opening. Each of the elastic elements510is configured to deflect between the respective first element point502and the second element point504and thus enable movement of the second element point504in the out-of-plane direction of the first wafer. A patch layer of prebonding material506is deposited on the first wafer or on the second wafer so that after activation, the patch layer of prebonding material506covers a region around the second element point504.

FIG.6illustrates stages of the method explained through the examples ofFIGS.1to5, so additional description to the terms and expressions may be referred from those Figures and their description. The method begins by forming (stage600) at least one prebonding element that includes a first element point, a second element point, and an elastic element. The first element point is in a region through which the prebonding element is fixed to the first wafer. The elastic element extends from the first element point to the second element point and the elastic element is configured to deflect between the first element point and the second element point to enable movement of the second element point in an out-of-plane direction of the first wafer.

A patch layer of prebonding material is created (stage602) on the first wafer or on the second wafer. The layer coincides in the out-of-plane direction with a region around the second element point. The first wafer is aligned (stage604) with the second wafer so that bonding surface parts of the first wafer are aligned with bonding surface parts of the second wafer. After this alignment, the patch layer of prebonding material can be activated (stage606) to form an adherent contact between the prebonding element and the second wafer.

When the first wafer and the second wafer have been attached to each other through the prebonding element, the set of wafers can be safely manoeuvred (stage608) in the wafer bonding process without compromising the critical alignment between them. The wafer bonding process can then be implemented in a normal manner so that a permanent contact is formed between the bonding surface parts of the first wafer and the bonding surface parts of the second wafer.

It should be noted that the method described withFIG.6refers only to parts of a process that are relevant in view of the invention. For a person skilled in the art it is clear that handling of wafers to be bonded before and during a wafer bonding process includes several stages that are well known and thus not described here in detail. For example, as mentioned earlier, wafers to be bonded may be initially separated by spacers to enable exchange of gases or other process-related fluids between them in early stages of the wafer bonding process. Furthermore, the prebonding element(s) may be used to fully replace clamps that press wafers against the jig, or as a complement to said clamps to ensure more even distribution of the pressuring forces.

The basic concept has been so far described with the elementary combination of two wafers. The invention is not, however, limited to wafer bonding of two wafers but can be applied in wafer bonding of two or more wafers.FIG.7illustrates an example of three wafers700,702,704that may be prebonded to each other before a wafer bonding process. The above described prebonding method can be applied to maintain the desired alignment between wafers700,702,704when they are wafer bonded to each other in a common wafer bonding process. The method can be applied also to first maintain the desired alignment between two wafers700,702when they are wafer bonded to each other in a first wafer bonding process, and then between a wafer bonded wafer700,702and the further wafer704in a second wafer bonding process.