WO2024260569A1 - Printing-assisted hybrid bonding technique - Google Patents

Abstract

A first and a second substrate (2, 16) are bonded to each other by first depositing a polymer layer (4, 22) on at least one of them. Particle bumps (12) are printed into openings (8, 24) of the polymer layer (4, 22) by means of applying an ink that comprises particles of metal or a metal salt. The two substrates (2, 16) are joined with the polymer layer(s) (4, 22) between them, and they are bonded. During bonding, the polymer layers (4, 22) as well as the particle bumps (12) are deformed. The former form a polymer bond, and the latter form metal connections (17). The combination of deformable polymer layers (4, 22) and deformable particle bumps (12) reduces the requirements for machining accuracy, yet it still delivers a reliable bonding between the substrates.

Description

Printing-assisted hybrid bonding technique

Technical Field

The invention relates to a method for bonding a first and a second substrate by forming a polymer bond as well metal bonds between them. It also relates to a bonded first and second substrate producible, in particular produced, by said method.

Background Art

Bonding two substrates by means of one or more polymer layers between them while also forming metal connections is called “hybrid bonding” and has e.g. be known from US 11244920 or from Park et al., Adv. Mater. Technol. 2023, 2202134 (DOI: 10.1002/admt.202202134). It allows to not only bond substrates mechanically, but e.g. to also provide electrical interconnections between them.

Hybrid bonding involves forming structured polymer layers or other inorganic layers on the substrate and adding a solid metal over the structured polymer layers. The metal is then partially removed to remain only in recesses of the polymer layers, whereupon the two structures are bonded to each other. This procedure requires highly accurate machining and very clean surfaces, and it is therefore challenging to handle.

Disclosure of the Invention

The problem to be solved is to provide a method of the type above that is easy to carry out.

This problem is solved by the method of claim 1.

Accordingly, the method for bonding a first and a second substrate comprises at least the following steps

A) Forming, on the first substrate, a first polymer layer having a plurality of first openings.

B) Selectively printing an ink comprising particles of a metal and/or a metal precursor onto at least one of the substrates, thereby forming particle bumps at the locations of said first openings. In this context, “selectively printing” is to be understood such that the ink is applied to only select areas on at least one of the substrates. Advantageously, these areas are disjoint areas. Printing may take place on the first and/or the second substrate. The result of the printing process are particle or metal precursor bumps at the locations of the first openings.

The term “at (the) locations” is to be understood such that, during printing and/or at least once the two substrates are joined (see next step), the bumps are located at the first openings. In other words, all or at least some of the particle bumps may be directly printed on the first substrate at the location of the first openings, and/or all or some of the particle bumps may be printed on the second substrate at such positions that, once the substrates are joined, they are located at the first openings.

The term “metal precursor” refers to a solid containing metal atoms that can, in the later bonding step, be converted to a metal.

C) Joining the first and second substrate with the first polymer layer between them.

D) Bonding the first and the second substrate with the first polymer layer between them. As a result of this bonding, the following steps take place:

- The first polymer layer is deformed. By means of the first polymer layer (and, optionally, one or more further polymer layers), a first bond between the substrates is created.

- The particle bumps are deformed. By means of the particle bumps, metal connections are formed in the first openings and between the substrates. These metal connections provide a second bond between the substrates.

The present technique relies on using particle bumps, i.e. localized conglomerates of particles, that are inherently more deformable and softer than the prior art solid (bulk) metal structures located in the openings of the first polymer. Hence, the particle bumps as well as the polymer layer(s) are easily deformed when the substrates are bonded, and the process is more forgiving towards surface contaminations or an unevenness of the surfaces.

The deformation of the polymer layer(s) in the bonding process accommodates for minor surface unevenness and/or surface contaminations. The deformation of the particle bumps reshapes the bumps to connect to the opposing surfaces. Both deformations cooperate to provide a good bonding between the substrates even if the cleanliness and machining accuracy of the structures prior to bonding is limited.

The particles are advantageously nanoparticles, i.e. particles have diameters smaller than 500 nm, in particular smaller than 100, in particular smaller than 10 nm. This is to be understood such that the median diameters of the printed particles are smaller than 500 nm, in particular smaller than 100 nm, in particular smaller than 10 nm. This not only simplifies printing and increases spatial resolution, but small particles tend to have a lower melting point than the respective bulk material, thereby allowing to at least partially melt and/or sinter the particles without damaging the polymer layer(s).

Advantageously, at least some of the particle bumps are printed into the first openings of the first polymer layer. This allows to laterally confine the bumps, during printing, by means of the first openings.

In this case, advantageously, at least some of the particle bumps printed into the first openings project from the first openings after printing. In other words, they are “higher” (in a direction perpendicular to the first surface) than the first polymer layer. This expedites subsequent bonding by providing better contacting between the bumps and their counterparts as the substrates are joined.

At least some of the particle bumps may also be printed onto the second substrate. In this case, when the first and second substrates are joined, these particle bumps are located at the first openings.

In this case, advantageously, when the first and the second substrates are joined, the bumps printed onto the second substrate can reach into the first openings, thereby centering the two substrates against each other.

Each first opening may be surrounded by a wall structure formed by the first polymer layer, and the wall structure is in turn surrounded by a moat where the first polymer layer is absent. This design further improves lateral confinement of the bump or metal during printing and/or bonding. As described in more detail below, it also reduces capacitive coupling and undesired diffusion processes.

The wall structure may form a continuous wall enclosing the first opening, thereby making sure that all bump material is laterally confined securely.

Alternatively, though, one or more passages may be formed in the wall structure. Each passage extends between the first opening and the moat and has a width, along the direction of the circumference of the first opening, that is no more than 1/10 of the length of the circumference of the first opening, i.e. the passage or passages is/are narrow. In this case, and as described in more detail below, the wall structure still laterally confines the ink during printing, but it allows for bump material to be pushed into the moat during bonding, i.e. excess bump material can be drained into the moat.

In a first embodiment, the method further comprises the step of forming, on the second substrate, a second polymer layer. When joining the first and second substrate, at least some of the first openings and of the bumps are aligned with second openings in the second polymer layer, i.e. they come to lie at the same locations such that they can form the metal connections through pairs of adjacent first and second openings.

In this first embodiment, at least at some of the pairs of adjacent first and second openings, the bumps can be printed in both the first and second openings, thereby providing bumps on both sides, with the bumps contacting each other when the substrates are joined.

The second openings may be formed prior to joining the first and second substrate, or they may be formed while joining and/or bonding the substrates.

In a second embodiment, only a single polymer layer is used. In other words, when joining the first and second substrate, the first polymer layer is the only polymer layer between the first and second substrate. Using only one polymer layer simplifies the manufacturing process.

The particle bumps may be printed on both the first and the second substrate. When joining the first and the second substrate, at least some of the bumps on the first substrate come into contact with at least some of the bumps on the second substrate, thereby providing more reliable bonding.

Alternatively, though, in a simpler process, the method may be characterized by printing particle bumps on only one of the substrates.

The first and second substrates may each comprise metal pads, advantageously forming terminals for electrical leads in the substrate. When joining the first and second substrates, the particle bumps align with metal pads on the first and the second substrate. This allows to bond the particles and the metal pads, thereby forming electrical contacts between the first and second substrate.

As mentioned, the particles may be of metal, in particular of at least one of gold, palladium, indium, copper, aluminum and silver. In this case, the metal connections can be formed directly from the particles.

In another embodiment, the particles may be of a metal precursor. Advantageously, the metal precursor comprises a metal in an oxidized state. In particular, it comprises at least one of a metal salt, a metal oxide, and a metal hydroxide. In this case, the method comprises the step of converting the metal precursor to a metal. In particular, it comprises the step of reducing the metal salt, metal oxide, or metal hydroxide for forming said metal connections as described in more detail below.

The invention also relates to a bonded first and second substrate that is producible, in particular produced, by the method described herein. Brief Description of the Drawings

The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:

Fig. 1 shows a first substrate with a first polymer layer,

Fig. 2 shows the substrate of Fig. 1 with particle bumps printed thereon,

Fig. 3 shows the step of joining the first substrate with a second substrate,

Fig. 4 shows the step of bonding the two substrates,

Fig. 5 shows a single opening at the side of the first substrate,

Fig. 6 shows the opening with a particle bump printed therein,

Fig. 7 shows the situation of Fig. 6 while joining the two substrates,

Fig. 8 shows the situation of Fig. 7 while bonding,

Fig. 9 shows another embodiment while joining (A) and while bonding (B) the two substrates,

Fig. 10 shows another embodiment while joining (with A showing the alignment process and B showing the aligned situation) and while bonding (C),

Fig. 11 shows a wall-and-moat embodiment after printing the particle bumps (A) and while joining the substrates (B),

Fig. 12 shows the embodiment of Fig. 11 after printing when viewed perpendicularly to the first substrate,

Fig. 13 shows another wall-and-moat embodiment in a view corresponding to Fig. 12,

Fig. 14 shows another wall-and-moat embodiment prior to joining the substrates,

Fig. 15 shows another wall-and-moat embodiment, in a view perpendicular to the substrate, prior to bonding (A) and after bonding (B),

Fig. 16 shows another wall-and-moat embodiment prior to joining the substrates.

Fig. 17 shows another embodiment during plasma activation (A), during bonding (B), and after bonding (C),

Fig. 18 shows another embodiment after printing (A), during ligand exchange (B), and after ligand exchange (C), Fig. 19 shows a first substrate with a first polymer layer having auxiliary openings,

Fig. 20 shows the substrate of Fig. 19 with the particle bumps printed thereon,

Fig. 21 shows the step of joining the first substrate of Fig. 20 with the second substrate,

Fig. 22 shows the step of bonding the two substrates of Fig. 21,

Fig. 23 shows a sectional view (along a sectional plane parallel to the substrates) of the first polymer layer of Fig. 19, and

Fig. 24 illustrates an embodiment where one of the polymer layers is only partially crosslinked upon joining the substrates.

Modes for Carrying Out the Invention

Definitions

In the present context, the term “horizontal” describes any direction parallel to the surfaces of the substrates.

Similarly, the term “vertical” describes the direction perpendicular to the surfaces of the substrates.

Introduction, first embodiment

Figs. 1 - 8 illustrate some general concepts of the present technique and show a first embodiment thereof. Figs. 1 - 4 schematically illustrate the bonding of two complete substrates with several metal connections between them while Figs. 5 - 8 represent sectional drawings of the formation of a single connection.

As mentioned, the technique generally relates to a method for bonding a first and second substrate. It includes the step of forming, on the first substrate 2, a first polymer layer 4, as shown in Figs. 1 and 5.

Polymer layer 4 is arranged on a surface 6 of first substrate 2, and affixed thereto, e.g. by means of spin coating or lamination. It is structured to have a plurality of first openings 8. Possible processes of structuring polymer layer 4 are described in the section “The polymer layer(s)” below.

As can be seen in Fig. 5, first substrate 2 comprises a plurality of metal pads 10 arranged on its surface 6, with each first opening 8 having one of the metal pads 10 at its base. The metal pads 10 are advantageously connected to metal leads 11 connected e.g. to circuitry or terminals in/on the substrate. One such lead 11 is shown, by way of example, in Fig. 6.

In a next step, as depicted in Figs. 2 and 6, particle bumps 12 are formed in the first openings 6 by selectively printing an ink comprising particles of a metal and/or metal salt. Suitable techniques and materials are described in the sections “Bonding” and “Printing techniques and inks” below.

The particle bumps 12 come to rest on the metal pads 10 located at the base of each first opening 8.

As can be seen from Fig. 6, the particle bumps 12 project from the first openings 8 over the outer surface 14 of first polymer layer 4 by an amount h.

The amount h (measured in a direction perpendicular to the substrate) is selected to be large enough to provide secure bonding in the following steps, taking into account that the particles will be compressed during bonding and that the polymer layer(s) may expand when it is heated up during bonding. On the other hand, h should not be too large to avoid an excessive amount of particle material.

The amount h is advantageously be sufficient to compensate for the thermal expansion of the polymer layer during bonding. Considering that a typical polymer, e.g. a polymer with CTE 60, thermal expansion is 0.6% per 100 K, and assuming that the temperature increase during bonding is e.g. 300 K, h should be at least 1.8 % of the thickness of the polymer layer.

In addition or alternatively thereto, another consideration also provides for a minimum projection over the opening: During the bonding process, the particle bump is turned into the more compact and denser metal connection 17. In particular, if we assume that the particles of the particle bump are arranged in a close - packing of equal spheres, they take up a spherical volume of 74%. Assuming that, during bonding, the spheres are merely deformed into a solid metal connection, and said metal connection should fill at least 90% of the volume of the opening 8, the volume of the particle bump should be at least 20% larger than the volume of opening 8.

As to an upper limit of the projection and/or the volume of the particle bump, this depends various factors. Excess bump material must be able to be drained from opening 8 during bump material (for means to do so, see embodiments below). Without the option for such draining, without the option for the material to flow into the opening of the second polymer layer, and unless material can be removed from the bump before final bonding, advantageously the volume of the bump is no more that 100%, in particular no more than 50%, larger than the volume of the opening 8. Hence, in conclusion, at least one of the following conditions advantageously applies:

- The amount h is at least 1.8%, in particular at least 5%, of the thickness of polymer layer 8.

- The volume of the particle bump is at least 20% larger than the volume of the opening it is placed in, and it is no more than 100%, in particular no more than 50%, larger than the volume of the opening 8.

In a next step, as depicted in Figs. 3 and 7, first substrate 2 is joined with a second substrate 16, with the first polymer layer 4 between them.

As can be seen from Fig. 7, second substrate 16 also has, on its surface 18, a plurality of metal pads 20, and the two substrates 2, 16 are joined such that at least one of the metal pads 20 is positioned at the location of each of the particle bumps 12.

Again, the metal pads 20 are advantageously connected to electrical leads 21 in second substrate 16. One such lead 21 is shown, by way of example, in Fig. 7.

Next, and as shown in Figs. 4 and 8, the two substrates 2 are bonded, advantageously while compressing them (i.e. pushing them vertically against each other) and heating them, as described in the section “Bonding” below.

In this bonding process, first polymer layer 4 is slightly deformed and creates a first bond between the substrates 2, 16, and the particle bumps 12 are deformed as well to form metal contacts 17 providing a second bond between the substrates.

Two polymer layers, symmetric setup

Fig. 9 shows an embodiment with two polymer layers 4, 22 and with particle bumps 12 of symmetric design.

First polymer layer 4 is mounted to first substrate 2 and it is structured to have first openings 8 as in the previous embodiment. In addition, a second polymer layer 22 is mounted to second substrate 16 and provided with second openings 24. The first and second openings 8, 24 are aligned with the metal pads 12, 20 on the respective substrates 2, 16.

Prior to joining the substrates 2, 16, the particle bumps 12 are printed into the first as well as into the second openings 8, 24, advantageously using two separate printing steps. In the embodiment of Fig. 9, the particle bumps 10 on both substrates project over the openings in the respective polymer layer 4, 12, again by an amount h is described in reference with Fig. 6 above.

The first and second openings 8, 24 are positioned such that, when joining the two substrates 2, 16, as shown in section A of Fig. 9, they are at the same location, i.e. the particle bumps 12 on both substrates 2, 16 align with each other.

During bonding, using the techniques described below, the substrates 2, 16 are compressed and the particle bumps 12 and polymer layers 4, 22 are deformed, thereby forming the two types of bonds as mentioned above.

Two polymer layers, asymmetric setup

Fig. 10 shows a design similar to the one of Fig. 9.

Again, the first and second polymer layers 4, 22 with their first and second openings 8, 24 are arranged on the first and second substrate 2, 16, respectively.

In this embodiment, however, the heights of the particle bumps 12 (i.e. their extension perpendicular to the respective surface 6, 18 of their substrate 2, 16) on the first and second substrate 2, 16 differ. For a given pair of adjacent first and second openings 8, 24, the particle bumps project over one of the openings 8, 24 but not over the other opening 24, 8. In the embodiment of Fig. 10, for example, particle bump 10 on first substrate 2 does not project over first polymer layer 4 and therefore it does not project over first opening 8, but particle bump 10 on second substrate 16 does project over second polymer layer 22 and therefore over second opening 24.

This asymmetric design encourages an automatic alignment of the two substrates 2, 16 as they are joined and/or can promote a locking-mechanism between the two substrate as they are heated up during bonding, to prevent any temperature-induced movements. This is illustrated in section A of Fig. 10, where there is a slight misalignment of the two openings 8, 24. When the tapered forward end of the projecting particle bump 10 enters the opposite opening, this generates a lateral force that automatically aligns the two substrates 2, 16 with each other, as shown in section B of Fig. 10.

In the embodiment shown, there is a small, non -projecting bump 10 located at the female sign of the device (in the shown embodiment at first opening 8). It must be noted, however, that this smaller bump can be completely dispensed with if the projecting particle bump is sufficiently high to reach all the way through the opposite opening and contact the metal pad (here: metal pad 10) at its bottom. Once the two substrates 2, 16 are properly aligned (as shown in section B of Fig. 10), bonding can take place to create a metal contact 17 (as shown in section C of Fig. 10).

Hence, in more general terms, at least at some pairs of adjacent first and second openings 8, 24, the particle bumps may project over one of the pair of openings but not over the other. For example, at a given contact structure, the particle bump 12 may project over the second opening 24 while the particle bump 12 at the first opening 8 (if any bump is present there) does not project over the first opening 8, as shown, or vice versa.

Wall-and-moat structures

Figs. 11 and 12 illustrate another aspect of the present technique, which relies on forming a wall structure and a “moat” around the first and/or second opening(s).

In this present context, the term “moat” designates an opening (i.e. an area where the polymer layer is absent) looped around the first or second opening in the first or second substrate, with a wall structure arranged between the moat and the first/second opening. The moat may e.g. be annular (i.e. circular), polygonal, or it may be any other closed-loop shape.

Figs. 11 and 12 show an embodiment similar to the one of Figs. 1 - 8 with only one polymer layer, i.e. the first polymer layer 4, arranged on first substrate 2, and with second substrate 16 not having a polymer layer. However, the “wall-and-moat” design can also be used with two polymer layers, in particular where each polymer layer has a wall-and-moat structure surrounding the first and second openings, respectively.

In the shown embodiment, each of some or of all of the first openings 8 is surrounded by a wall structure 26, which may be annular as shown. Wall structure 26 is in turn surrounded by a moat 28 where the first polymer layer 4 is absent.

The wall structure 26 as shown forms a continuous wall that completely separates (along horizontal directions) first opening 8 from moat 28.

The particle bumps 12 are printed into the first openings 8 as shown in section A of Fig. 11. Any ink overflowing the first openings 8 is typically confined at the outer edge of wall structure 26 or overflows into the surrounding moat 28 to be trapped there.

When the first and second substrate are joined, as shown in section B of Fig. 11, and subsequently compressed for bonding, the wall structures 26 will yield easily to make room for any residual ink material at their top edge, thereby allowing to form a good bond between the two substrates 2, 16.

The wall-and-moat structure also reduces stress in the final product, i.e. it reduces stress that might arise from differences in the thermal expansion coefficients between the polymer layer(s) and the bonded bumps. Further, the moats 28 reduce the capacitive electrical coupling between the resulting metal connection 17 and structures arranged laterally (i.e. offset horizontally) beside it. For example, in the embodiment of Fig. 12, the two moats 28 reduce the capacitive coupling of the two metal connections formed by the two neighboring bumps 12.

The moat also acts as a barrier for metal diffusion. Without the moat, metal from the metal particles may diffuse laterally through the polymer layer, but the moat acts as a diffusion stop.

In Figs. 11, 12, the two moats 28 of neighboring contact structures are separate. Alternatively, however, and as illustrated in Fig. 13, moats 28 around neighboring first openings 8 may intersect, i.e. communicate with each other, which provides for a more compact design.

However, and in more general terms, in an advantageous embodiments, any two first openings 8 are separated by at least two wall structures 26 and by at least one moat 28. Similarly, if a wall-and-moat design is used in second substrate 16, any two second openings 24 are separated by at least two wall structures 26 and by at least one moat 28.

Fig. 14 shows an example with wall-and-moat structures in two polymer layers 4, 16. In other words, we have a first polymer layer 4 on first substrate 2 and a second polymer layer 22 of second substrate 16, as e.g. in the embodiment of Fig. 9, but both polymer layers have wall-and-moat structures.

The first openings 8 in first polymer layer 4 are surrounded by wall structures 26 and moats 28, e.g. having the design of Figs. 11 and 12. Similarly, the second openings 24 in second polymer layer 22 are surrounded by wall structures 26 and moats 28, e.g. again having the design of Figs. 11 and 12.

In the embodiments describes so far, wall structure 26 forms a continuous wall surrounding the first/second opening 8, 24. Alternatively, and as illustrated in Fig. 15, one or more passages 30 may be formed in wall structure 26, with each passage 30 extending between the central opening 8, 24 and the surrounding moat 28.

The passages are narrow enough to discourage, while printing, the ink from seeping through, confining it to the central opening due to its surface tension and the wetting behavior of the ink. In particular, and as illustrated in section A of Fig. 15, the width w of each opening 30 is advantageously no more than 1/10 of the circumference c of the opening 8, 24.

Section A of Fig. 15 shows the situation after printing and before joining and compressing the substrates 2, 16. As can be seen, the particle bump 12 is confined by wall structure 26.

However, when compressing the substrates 2, 16, if there is excess particle bump material within opening 8, 24, the pressure within opening 8, 24 pushes the excess material into or through the passages 30 as shown in section B of Fig. 15.

This design makes it easier to fill the opening(s) 8, 24 with material enough particles for forming the metal connection 17 while it makes it less likely that excess material enters the interface between the first and second polymer layers 4, 22, or between the first polymer layer 4 and the second substrate 16, where it would be detrimental for a tight bonding. This is particularly advantageous for embodiments where at least some of the particle bumps 12 printed into the first openings 8 project from the first openings 8 after printing.

It must be noted that the design of Fig. 15 can be used in combination with a single polymer layer (as e.g. shown in Fig. 11) as well as in combination with a first and second polymer layers 2, 16 (as e.g. shown in Fig. 14). In the latter case, the passages 30 may be provided, for a given pair of adjacent first and second openings, at either the first opening 8 only, at the second opening 24 only, or at both openings 8, 24.

Fig. 16 illustrates further aspects of a wall-and-moat design.

Here, there are wall structures 26 and moats 28 in first polymer layer 4 at the first openings 8. However, there are no wall structures and moats formed in second polymer layer 22 at the second openings 24.

The design of Fig. 16 can also be used without a wall-and-moat structure in that the first openings 8 can be formed with a smaller diameter, in particular by at least 25% and/or 1 pm, than the second openings 24. Ink is printed into the first openings 8 to form particle bumps that extend over the first openings. Further, there are no particle bumps printed into the second openings 24 or, if particle bumps are printed there, they do not project over the second openings 24. This allows to provide additional volume in the second openings 24 to receive excess bump material during bonding. Bonding

Advantageously, bonding the substrates involves compressing the first and second substrate along the vertical direction with the polymer layer(s) between them. This expedites the deformation of the particle bumps and the polymer layers.

Advantageously, heat is applied to the joined structure while bonding for sintering and/or fusing the particles in the particle bumps 12 and/or for improving the polymer-based bond.

Advantageously, the maximum temperature of the bonding process is only reached after compression of the two substrates because strong heating can lead to mutual a horizontal displacement of the two substrates 2, 16, and/or it may affect the mechanical properties of the particle bumps.

Hence, advantageously, the method comprises the step of heating the substrates while compressing them, wherein compression starts before the maximum temperature is reached. In particular, after the start of the compression, temperature is increased by at least 100°C, in particular for metals with a low melting or sintering temperature. For metals with a higher melting or sintering temperatures, the temperature may be increased by at least 150°C or even more. Sintering/melting may take place while the substrates are compressed, or sintering/melting may, at least in part, take place after compression ends. In other words, initial bonding may take place under compression, whereupon the assembly of the two substrates is moved e.g. to an oven for sintering/hardening. This is particularly advantageous when using thermos - setting polymers.

As described in the section “Printing techniques and inks” below, each particle used in the ink for printing may be enclosed (coated) by a ligand of a material different from the particle. Such ligands can e.g. be used for passivating the particles, tuning the shapes of particles, and/or expediting bump formation, see, e.g. Kanelidis et al. in Beilstein J. Nanotechnol. 2017, 8, 2625 - 2639, DOI: 10.3762/ bjnano.8.263.

Such surface ligands may, however, impede the formation of the metal connections 17 during bonding. Hence, they may be removed or modified prior to or during bonding.

For example, the method may include the step of plasma-activation prior to the formation of the conductive structures 17 as illustrated in section A of Fig. 17. In particular, oxygen plasma activation can be used by subjecting the particle bumps 12 to ionized oxygen, thereby decomposing the surface ligands partially or fully. Such plasma activation is not suggested though with metal precursors or particles that are prone to oxidation. Further methods for pre-treatment of particles are discussed below.

Plasma-activation can also be used to activate surfaces of the polymer layer(s). For example, water-plasma or 02 plasma at presence of water can be used to create dangling OH bonds at the polymer surface, which expedite the formation of covalent bonds during bonding, as shown e.g. Meng et al. in Materials 2022, 15(7), 2529; https://doi.org/10.3390/mal5072529. The process of surface activation may be executed before or after printing the metal bumps. In particular, if the metal is prone to oxidation, it can be advantageous to execute polymer surface activation prior to bump formation.

Hence, advantageously, the present method comprises the step of plasma-activating the particle bumps and/or polymer layer(s), in particular prior to compressing the first and second substrates 2, 16.

In another embodiment, instead of using plasma activation for removing the surface ligand, such removal can be achieved by treating the particle bumps with any other ligand-removing fluid, e.g. with a gaseous oxidant and/or with a solvent that removes and/or decomposes the ligands.

The final ligands, in particular organic ligands, will desorb from the particles at sufficiently high sintering temperature.

Hence, advantageously, the method comprises the step of removing the ligand by heating the particle bumps. Such heating may be executed prior or subsequent to the bonding process. Removal of the ligands during heating will make metal particles touch each other which will induce cross-diffusion and partial fusion which again will lead to increased mechanical strength of the structure. If executed prior to bonding, such sintering will therefore oppose the compressibility of the bump during bonding. On the other hand, by retaining surfactants and by sintering only during the bonding process, surfactants may not be properly extracted from the bonding site and reduce the quality of the bonded metal bump.

For example, an as-printed film of gold nanoparticles stabilized with Dodecanethiol ligands was shown to obtain a more than lOOtimes lower E-Mod- ulus than a reference created by PVD deposition, while in a sintered state it reaches 50% of the reference value, as shown by Reiser et al. in https://doi.org/10.1002/adfm.201910491. In this example a high sintering temperature of 400°C was used. However, desorption of the same ligand on gold nanoparticles may already be induced at tempafter feratures as low as 120°C (Sook Young Moon et al. in DOI: 10.1007/s 11671-010-9565-6). Higher temperature annealing not only promotes desorption of ligands but also diffusion in the fused particle and simultaneous crystal growth. Hence, if sintering is executed before bonding, low -temperature sintering may be executed to remove a large portion of the ligands. In the case of above- mentioned particles this may be achieved by sintering at temperatures in the range of 120-150°C. In this case the E-modulus of the obtained structure will be lower than the above-mentioned factor 0.5 compared to PVD -reference. Therefore pre -bond sintering can be used to improve bonding results when considering both ligand pollution and compressibility during bonding. The exact procedure will strongly depend on the used ink material though.

In yet another embodiment, an optimized sintering process may be achieved by ligand exchange procedures, by which the ligand used during printing is exchanged against another ligand once the bump has formed on the substrate.

Ligands are generally coated as a sufficiently thick layer on the particle surface to keep individual particles apart from each other when in solution. Certain ligands are of similar chemical composition, for example with a functional group that docks to the particle (e.g. thiol or amine group) and a differing number of methylene units that make the particles dispersible in nonpolar solvents. The longer the chain of methylene units the more stable the particles are in solution. For the same reason, a printed bump of particles will have softer mechanical properties the longer the chain of methylene units is, which acts like a lubricant. Also, the removal of surfactants with longer methylene chains will require higher temperature sintering as compared to shorter chains, as described in Kanelidis et al. above. For example, a gold particle functionalized with oleylamine (C18H35NH2) shows higher sintering temperature than the same particle functionalized with octylamine (C8H27NH2). A possible way of performing a ligand exchange on deposited particles is shown by Fafarman et al. in ACS Nano 2014 8 (3), 2746-2754, DOI: 10.1021/nn406461p. In case of amine or thiol-terminated ligands, exchange may be executed by presenting to the particles a solution of high concentration of the new ligand which will lead to replacement of the old ligand over time. The use of a polar solvent thereby prevents the particles from getting redispersed during the ligand exchange process. In this way, the ligand concentration in the printed structure can be reduced prior to bonding but without the particles making touch each other and start diffusing, particularly not at elevated temperature.

The process of replacing long ligands with shorter ones is illustrated in Fig. 18, where first the particle bump 12 is printed (section A). Then, the particle bump 12 is exposed to a fluid, in particular liquid, ligand exchange agent 32 (section B), which dissolves and/or decomposes the original ligand and replaces it with a different coating material. This may lead to a more compact bump 12 (section C), which then can be subjected to the bonding process.

Hence, advantageously, the method comprises the step of replacing the ligand with a coating different from the ligand after forming the particle bumps 12. As mentioned, the particles in the particle -bumps may be metals or metal salts.

If the particles are metal, the bonding process should be suitable to fuse the metal particles. Such fusing is advantageously carried out under a temperature where the metal atoms readily diffuse between particles such that they combine into larger assemblies. For example, gold nanoparticles stabilized with branched thiols have a melting point around 140°C (see S. Hamacher et al. in Informatics, Electronics and Microsystems: TechConnect Briefs 2017, pp. 141 - 144 or Kanelidis et al. above).

Advantageous materials for metal particles are noble metals, such as gold or palladium. Other suitable metals e.g. include indium and copper.

If the particles are metal precursors, the metal precursors should first be turned into a metal.

In particular, the metal precursor may comprise a metal in an oxidized state that can be reduced to form the pure metal. As mentioned above, advantageous examples of this type of metal precursor include metal salts, metal oxides and/or metal hydroxides.

The step of turning the metal precursor into a metal may e.g. comprise a reduction step, e.g. a step including polyol reduction as described by Teichert et al. in Dalton Trans, 2018, 47, 14085 - 14093 (DOI: 10.1039/c8dt03034k).

A suitable ink composition is e.g. described in US2015299489A1 or W02018118460A1.

In one example, the two substrates are bonded as follows:

1. The used polymer layer is Perminex (Kayaku Advanced Materials) and is spin-coated onto the first substrate 2, followed by a soft bake and subsequent lithography step and development that creates the first openings 8.

3. The polymer surface is activated in a O2 plasma at the presence of water or in a dedicated water plasma as for example sold by the company Samco.

4. Immediately following surface activation an ink consisting of ~5nm diameter Au nanoparticles, stabilized with dodecane thiol, and dispersed in a nonpolar solvent, e.g. dodecane, is printed by one of the printing techniques described below into the first openings. Printing is executed such that the solvent of a given droplet is essentially evaporated before arrival of the next droplet, such that the dry nanoparticle material is stacked on top of each other instead of spreading out as a liquid film. A printhead and method to accelerate this process is shown in W02021008817.

5. The same process as illustrated above is repeated for the second substrate 16.

6. A commercial wafer bonding machine, e.g. an Ayumi AD-300 wafer bonder, is employed to load the first and second substrates directly after printing the bumps and is used to align them such that the first openings 8 and the second openings are aligned to each other, without making contact between the wafers.

7. A vacuum atmosphere is created inside the bonding tool.

8. Temperature of the two wafers is linearly ramped to 150°C and once the temperature is reached the wafers are brought into contact and pressure of approximately 0.58 MPa is applied for 1 minute.

9. The bonded wafers are removed from the machine and are introduced to an oven and where the sintering and polymer hard-bake process is commenced at 200°C for another 60minutes.

For higher accuracy bonding, wafer contact may be initiated prior to temperature ramping, such that initial bonding of the activated surfaces occurs already before temperature ramping.

In case the polymer is a non-photosensitive material, for example the thermosetting polymer BCB, then the polymer is preferably first applied by spincoating or other suitable techniques, followed by a soft-cure at elevated temperatures, in a particular example for 40 min at 210C. Subsequently, a lithographically formed etch mask can be arranged on the surface of the polymer, either a soft mask consisting of another polymer, or a hard mask consisting of a metal or another inorganic material with high etch resistance to an oxygen plasma or similar etch chemistry that is mainly targeted at etching carbon-containing polymers. In a subsequent step, the substrate containing polymer and mask are being etched, preferably in a dry etching process that results in at least partly anisotropic etch profile, i.e. with vertical side-walls in the etched polymer. Such etching may be executed in a reactive ion etching system using a composition of oxygen and a fluor containing etch gas like SF6 or CF4. Once the polymer is etched, the etch mask is removed by appropriate commercial stripping chemistry. The polymer layer(s)

As mentioned, there needs to be at least one polymer layer (the “first polymer layer”) between the substrate while they are bonded, and in some embodiments, at least two polymer layers (“first” and “second” polymer layers) are provided on the substrates.

In the present context, the term “polymer layer” refers to a layer that comprises a polymer or that comprises or a resin to be at least partially polymerized during bonding. In addition to said polymer or resin, the polymer layer may comprise further constituents, such additives for reducing the coefficient of thermal expansion (CTE) and/or for increasing thermal conductivity. Advantageously, the polymer layer comprises, in polymerized form, at least 10 weight-%, in particular at least 50 weight-% of a polymer.

The at least one polymer layer needs to be structured. To do so, the polymer layer(s) may be a photoresist, such as SU-8, that can be directly structured using lithography. In a specific example, the polymer layer(s) may be PermiNex (R) by Kayaku, which is a photoresist that has been optimized to have good bonding properties.

Hence, advantageously, the polymer layer(s) is/are a photoresist.

In another example, if the polymer layer(s) is not a photoresist, it can be structured using conventional photolithographic masking and etching techniques.

Advantageously, the polymer layer(s) is/are applied to their respective substrate by means of spin-coating or lamination.

In another embodiment, the polymer layer(s) may be printed into the substrate(s) by printing a resin and subsequent polymerization, see e.g. F. lervo- lino et al., ACS Omega 2021, 6, 24, 15892-15902, https ://pubs .acs .org/doi/ 10.1021/acsomega. IcO 1488.

The first and (if used) second openings in the polymer layer(s) must have diameters sufficient for printing into the particle bumps. Advantageous values depend on the used printing technique. When using electrohydrodynamic inkjet printing, printing spot sizes of below 100 nm are obtainable. Hence first openings in the polymer should be no smaller than 100 nm. Advantageously, the diameter (minimum diameter for non-circular openings) is optimized to equip bumps providing sufficiently low electrical resistance in a given circuit, particularly if the metal is a noble metal. For example, if a bump needs to provide a better resistance than 10 mil, then, if the height of the bump after bonding is 2 pm, and if the bump material is gold with post-sintering resistivity of 5- 10'⁸ Qm, the bump diameter should accordingly have an optimal diameter of approximately 3.6 pm. Creating larger bumps will consume more than the required metal. Hence, advantageously, the diameter should not be larger than 10 pm, in particular not be larger than 5 pm.

Reducing stress in the polymer layer and improving thermal conductivity

Figs. 19 - 23 show an embodiment of the present technique where the polymer layer(s) is/are provided with auxiliary openings 38 to reduce stress.

In more detail, first polymer layer 4 and/or second polymer layer 22 may be provided with auxiliary openings 38, in addition to the first/second openings 8, 24, in order to reduce stress in the device. Such stress can be caused by different expansions of the substrates 2, 16 and the polymer layer(s) 4, 22, e.g. with temperature or aging. By adding auxiliary openings 38 to at least one polymer layer, in particular all polymer layers, such stress can be reduced. In addition, dielectric coupling between the substrates 2, 16 can be reduced as well.

Advantageously, and as shown, the auxiliary openings 38 form a repetitive pattern arranged at least in the area(s) between the first/second openings 8, 24.

Fig. 23 illustrates that the auxiliary openings 38 advantageously form a hexagonal pattern that, with its high symmetry, is particularly well suited to reduce stress within the structure.

Hence, in an advantageous embodiment, the first polymer layer 4 and/or the second polymer layer 22 comprise(s) a plurality of auxiliary openings 38, in addition to the first and/or second openings 8, 24, wherein no particle bumps are arranged in the auxiliary openings 38.

Advantageously, the auxiliary openings 38 form a repetitive pattern between the first and/or second openings 8, 24, in particular a hexagonal pattern.

As shown, the auxiliary openings may form closed cavities once the two substrates 2, 16 have been bonded to each other. Such closed cavities are advantageous because no humidity or other undesired substances can enter the auxiliary openings 38.

The auxiliary openings 38 may be manufactured in the same process steps as the first and/or second openings 8, 24.

Another benefit of using auxiliary openings is the fact that they make it easier to bond two flat substrates at non-vacuum conditions since the auxiliary openings can absorb gas that may otherwise get trapped between the substrates and thereby impede bonding. Advantageously, at least some of the auxiliary openings 38 can be additionally filled, at least in part, with printed auxiliary bumps 42 prior to bonding. One such auxiliary bump 42 is shown, by way of example, in Figs. 20 - 22. Just like the particle bumps 12, the auxiliary bumps 42 comprise particles of a metal or metal precursor. In this way, the thermal conductivity between the joined substrates can be strongly improved. The metal or metal precursor selected for being placed in the auxiliary openings 38 can be different from the metal or metal precursor used for forming the primary particle bumps 12. For example, for forming primary particle bumps 12, the noble metal gold may be employed while in the formation of the auxiliary bumps 42 inside auxiliary openings 38, copper may be employed. Since the impact on thermal conductivity by partial metal oxidation is less severe than that on electrical conductivity, in this example it is anticipated that the noble metal gold will show good long-term performance both in its thermal as well as electrical conductivity while copper, which more easily degrades by oxidation, will mainly degrade in electrical conductivity but to a lesser degree in thermal conductivity. Hence, for the purpose of improving thermal conductivity even a partly oxidized metal can be beneficial. Other advantageous metals for filling auxiliary openings 38 are e.g. silver or aluminum.

In contrast to the primary particle bumps 12 that are advantageously connected, via the metal pads 10, to electrical leads 11, 21 in the two substrates 2, 16 (such that they can be used as electrical connectors between the substrates), the auxiliary bumps 42 need not be connected to such electrical leads because they are not being used as electrical connectors between the substrates.

In the embodiment of the previous paragraph, therefore, the method is characterized in that the first polymer layer 4 and/or, where applicable, the second polymer layer 22, comprise(s) a plurality of auxiliary openings 38, in addition to the first and/or second openings 8, 24, wherein auxiliary bumps 42 are printed in at least some of the auxiliary openings 38, and wherein

(a) the auxiliary bumps 42 are of a material different from the particle bumps 12 and/or

(b) the particle bumps 12 are connected to electrical leads (e.g. the leads 11, 21) on the first as well as on the second substrate 2, 16 while the auxiliary bumps 42 not connected to electrical leads at at least one of the first and the second substrate 2, 16.

In general, the concept of the auxiliary openings 38 is advantageously combined with using partially crosslinked polymers as described below. Printing techniques and inks

As mentioned, the particle bumps 12 are applied by means of a printing process. In this process, the particles are suspended in an ink, and the ink is applied to the openings.

Advantageously, an inkjet printing process is used because it is well suited for generating small structures. Other techniques that can be used include laser- induced forward transfer (LIFT) or Aerosol printing.

In a particularly advantageous embodiment, an electrohydrodynamic printing process is used, e.g. using a device such as shown in WO 2021/052580 or WO 2022/174907, as this technique obtains substantially higher printing resolution than other inkjet or related techniques.

Suitable materials for the particles have been discussed above, in particular in the section “Bonding”. Each particle may, as mentioned, be coated by a ligand, see e.g. the reference Kanelidis et al. cited above.

Alternatively, particles without ligands may be used. Such particles may e.g. be stabilized by electrical charges, and they may be manufactured using laser ablation.

In the ink, the particles are suspended in a liquid. This liquid should be volatile for being evaporated while forming the particle bump. Depending on the used surfactants, either polar or nonpolar solvents are used. For example, in case of a nanoparticle ink that uses an alkylamine to stabilize the particle dispersion, the solvent may be chosen from different alkanes, e.g. decane, dodecane, etc. A benefit of this solvent class is that with increasing length of the methylene chain, the boiling temperature increases.

At least some of the particle bumps 12 are advantageously formed with a convex apex 40 as shown, for example, in Fig. 6. Such a convex apex 40 can be created using inkjet printing when the particle bump projects over the opening 8, 24. If the profile of the apex has to be specifically engineered, it is possible to lay down a higher droplet count at the center of the apex than at its edge region. In this case, however, the resolution of printing needs to be at least twice as high at the width of the bump. Its convex shape reduces the risk of enclosing gas bubbles during the bonding process. In addition, if the bump projects over its opening and reaches into the adjacent opening on the opposite substrate, as described in reference to Fig. 10 above, the apex supports proper alignment of the two substrates. Substrates

The present technique is suitable to bond a large number of different substrates. For example, each of the substrates 2, 16 may be a semiconductor substrate or a dielectric substrate. Examples of dielectric substrates include glass substrates or polymer substrates. At least one of the substrates may also be a metallic substrate, e.g. used as a heat sink.

In a particularly advantageous embodiment, at least one of the substrates is a semiconductor substrate, and the present technique is used to electrically and mechanically connect the substrate to another substrate, such as e.g. an interposer layer or package substrate.

In another embodiment the substrates may be used to combine a MEMS substrate with a semiconductor substrate. In this case it is possible that at least one of the two substrates does not have a flat surface, but instead it may at its center contain a recess in which the functional MEMS structure is embedded. In this case, the bonding process bonds the two substrates and hermetically seals the MEMS structure inside.

Using partially crosslinked polymers

Fig. 24 illustrates an embodiment where second polymer layer is only partially crosslinked upon joining the substrates. First polymer layer is structured to form first openings 8 as in the previous embodiments, and the particle bumps 12 are printed at the location of these first openings 8. Advantageously, each first opening 8 is surrounded by a moat 28 as described above. Passages between the first openings 8 and the moats 28 may or may not be present.

Second polymer layer 22 has first regions 22A where, upon joining the two substrates 2, 16, it has not yet substantially cross -linked. In addition, it has second regions 22B where second polymer layer 22 is more cross-linked that at the first regions 22A.

The first regions 22A are arranged at the locations of the first openings 8 and the particle bumps 12.

Section A of Fig. 24 shows the situation immediately prior to joining the two substrates 2, 16. As can be seen, at this point, second polymer layer 22 is continuous and does not yet have second openings.

During joining and bonding of the two substrates, the particle bumps 12 are pushed into the first regions 22A. Thereby, they displace the still soft second polymer at the first regions 22A and contact the metal pads (not shown) of second substrate 16. Thereby, they form the second openings 24.

The still soft polymer may e.g. flow into the moats 28.

Advantageously, after this step, the still soft parts of second polymer layer 22 are hardened, in particular crosslinked.

As a result, as shown in Section B of Fig. 24, a very tight bond can be formed by the resulting polymer structure.

Advantageously, second polymer layer 22 is thinner than first polymer layer 4, in particular by at least a factor 2, to make displacing the polymer easier.

To implement this scheme, second polymer layer 22 may e.g. be a negative-tone photopolymer that hardens only after a so-called post-exposure bake (PEB) but that remains deformable after exposure but before PEB, wherein second regions 22B represents regions that are light-exposed and will therefore cross-link during PEB, while first regions 22 A represent regions of polymer that are not light -exposed and will therefore remain soft during PEB. They will only partially cross -link during a more extensive heat treatment, e.g. at 180°C, while PEB generally takes place at around 100°C.

PEB does not take place before joining the two substrates 2, 16.

The step of joining the two substrates 2, 16 is advantageously carried out without heating or only limited heating up to e.g. 50°C to make the exposed polymer in second regions 22B flow better before it fully cross-links and becomes hard. The polymer in first regions 22A does not cross-link upon heat treatment and in fact will flow better with increasing temperature. Only once the temperature increases far beyond the intended PEB temperature, e.g. 180°C instead of 100°C, it will start to partly solidify as well but will remain reasonably soft.

When using a polymer like those mentioned above, it is useful to execute joining of the substrates quickly after the light exposure to prevent unwanted cross-linking that may commence even at room temperature, just at very low rate.

Alternatively to a photo-sensitive polymer, it is also possible to use a non-photo-sensitive polymer, such as thermosetting polymers BCB or polyimide. In this case, it is not possible to create different regions in the second polymer layer 22 by light exposure, i.e. the second polymer layer 22 is not structured in first regions 22A and second regions 22B. Instead joining the two substrates simply takes place when the second polymer layer 22 is still soft enough for being displaced by the particle bumps for forming the second openings 24. For thermosetting polymers, the softness will reduce with extended treatment of the polymer at high temperatures, but the process is much slower and requires more heat than in the case of a photo-sensitive polymer. Hence, the softness of the polymer can be adjusted by adjusting its curing state and temperature during joining, with higher temperature making it flow better. After joining, the assembly is heated for hardening second polymer layer 22, which may take place in an oven.

In yet another embodiment, second polymer layer 22 may be of a different material in the first and second regions 22A, 22B.

In yet another embodiment, before adding second polymer layer 22, a metal particle bump may be printed onto the second substrate at the location of the desired bonding positions. Such bump should be printed with smaller thickness than the intended thickness of the second polymer layer 22, most advantageously with a thickness of less than 50% of the intended thickness of the second polymer layer 22, such that the bump can be subsequently enclosed by the second polymer layer 22, e.g. by means of spin-coating, spray-coating or most advantageously by lamination, the latter of which can achieve flattest polymer topography. Such particles may be processed by any means described here before the polymer is applied. When the second polymer layer 22 is displaced during bonding, the connection between two bumps can yield better results than the connection between the bump and a flat metal landing pad.

One such particle bump 44 is shown, by way of example, in dotted lines, in Fig. 24, Section A.

Hence, advantageously, the method comprises the steps of

- during joining the first and second substrates 2, 16, mechanically displacing and penetrating the previously continuous second polymer layer 22 by means of the particle bumps 12 in at least first regions 22A in order to form the second openings 24, and

- hardening displaced parts of the second polymer layer 22, in particular by means of heating.

This method may further comprise the step of

- prior to joining the first and second substrates, at least partially light-exposing the second polymer layer 22 in second regions 22B different from said first regions, in particular using photolithography, while not light -exposing the second polymer layer 22 in the first regions.

The method may also comprise the step of

- before applying the second polymer layer 22 to the second substrate, printing, on the second substrate 16 in at least some of the first regions 22A, particle bumps 44. In that case, advantageously, the method also comprises the step of covering the particle bumps 44 in the first regions 22A by means of the second polymer layer 22 prior to joining the substrates 2, 16. The coverage of the particle bumps 44 is then, at least in part, removed when joining and/or bonding the substrates 2, 16.

As mentioned above, the concept of using a soft polymer layer 22 is advantageously combined with providing auxiliary openings 38 as described above. In this case, while joining the two substrates, the soft polymer layer can be deformed to be pushed into the auxiliary openings 38, thereby generating a better bond between the two substrates.

In other words, the method advantageously comprises the step of

- during joining the first and second substrates 2, 16, mechanically displacing the second polymer layer 22 to reach into the auxiliary openings 38.

Notes

Hence, advantageously, a first and a second substrate 2, 16 are bonded to each other by first depositing a polymer layer 4, 22 on at least one of them. Particle bumps 12 are printed into openings 8, 24 of the polymer layer 4, 22 by means of applying an ink that comprises particles of metal or a metal salt. The two substrates 2, 16 are joined with the polymer layer(s) 4, 22 between them, and they are bonded. During bonding, the polymer layer(s) 4, 22 as well as the particle bumps 12 are deformed. The former form(s) a polymer bond, and the latter form metal connections.

The combination of at least one deformable polymer layer and deformable particle bumps reduces the requirements for machining accuracy, yet it still delivers a reliable bonding between the substrates.

While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

Claims

1. A method for bonding a first and a second substrate (2, 16) comprising the steps of forming, on the first substrate (2), a first polymer layer (4) having a plurality of first openings (8), selectively printing an ink comprising particles of at least one of a metal and a metal salt onto at least one of the substrates (2, 16), thereby forming particle bumps (12) at locations of said first openings (8), joining the first and second substrates (2, 16) with the first polymer layer (4) between them, bonding the first and the second substrates (2, 16) with the first polymer layer (4) between them, thereby

- deforming the first polymer layer (4) and providing, by means of the first polymer layer (4), a first bond between the substrates (2, 26) and

- deforming the particle bumps (12) and forming metal connections (17) in the first openings (8) and between the substrates (2, 16) to provide a second bond between the substrates (2, 16).

2. The method of claim 1 wherein said particles have diameters smaller than 500 nm, in particular smaller than 100 nm.

3. The method of any of the preceding claims wherein at least some of the particle bumps (12) are printed into the first openings (8) of the first polymer layer (4).

4. The method of claim 3 wherein at least some of the particle bumps (12) printed into the first openings (8) project from the first openings (8) after printing.

5. The method of any of the preceding claims wherein at least some of the particle bumps (12) are printed onto the second substrate (16) such that, when the first and second substrates (2, 16) are joined, they are located at the first openings (8).

6. The method of claim 5 wherein, when the first and the second substrates (2, 16) are joined, the bumps (12) printed onto the second substrate (26) reach into the first openings (8).

7. The method of any of the preceding claims wherein each first opening (8) is surrounded by a wall structure (26) formed by the first polymer layer (4), and the wall structure (26) is surrounded by a moat (28) where the first polymer layer (4) is absent.

8. The method of claim 7 wherein the wall structure (26) forms a continuous wall enclosing the first opening (8).

9. The method of claim 7 wherein one or more passages (30) are formed in the wall structure, with each passage extending between the first opening (8) and the moat (28), and having a width (w) of no more than 1/10 of the circumference (c) of the first opening (8).

10. The method of any of the claims 7 to 9, wherein at least some of the particle bumps (12) printed into the first openings (8) project from the first openings (8) after printing.

11. The method of any of the preceding claims comprising the step of forming, on the second substrate (16), a second polymer layer (22) layer, wherein, after joining the first and second substrates (2, 16), at least some of the first openings (8, 24) and of the particle bumps (12) are aligned with second openings (24) in the second polymer layer to form the metal connections (17) through adjacent pairs of the first and second openings (8, 24).

12. The method of claim 11 comprising the step of printing, at least at some of the pairs of adjacent first and second openings (8, 24), particle bumps (12) into both the first and second openings (8, 24).

13. The method of any of the claims 11 or 12 wherein at least at some of the pairs of adjacent first and second openings (8, 24), the particle bumps (12) project over one opening but not over the other opening.

14. The method of any of the claims 11 to 13 wherein the first openings (8) have smaller diameters, in particular by at least 25%, than the second openings (24), and wherein any particle bump (12) printed into the second openings (24) does not project over the second openings

15. The method of any of the claims 11 to 14 and of any of the claims 7 - 10, wherein one of the following applies: there are wall structures (26) and moats (28) at the first openings (8) but no wall structures and moats (26, 28) are formed at the second openings (24) or there are wall structures (26) and moats (28) at the second openings (24) but no wall structures and moats (26, 28) are formed at the second openings (8).

16. The method of any of the claims 11 to 15 comprising the steps of

- during joining the first and second substrates (2, 16), mechanically displacing and penetrating the second polymer layer (22) by means of the particle bumps (12) in at least first regions (22A) in order to form the second openings (24), and

- hardening displaced parts of the second polymer layer (22).

17. The method of claim 16 further comprising the step of prior to joining the first and second substrates (2, 16), at least partially light-exposing the second polymer layer (22) in second regions (22B) different from said first regions (22A) while not light-exposing the second polymer layer (22) in the first regions (22 A).

18. The method of any of the claims 16 or 17 comprising the step of before applying the second polymer layer (22) to the second substrate (16), printing, on the second substrate (16) in at least some of the first regions (22A), particle bumps (44) and, in particular, covering the particle bumps (44) in the first regions (22A) by means of the second polymer layer (22) prior to joining the substrates (2, 16).

19. The method of any of the claims 1 to 10 wherein, when joining the first and second substrates (2, 16), the first polymer layer (4) is the only polymer layer between the first and second substrates (2, 16).

20. The method of any of the preceding claims comprising the step of printing particle bumps (12) on both the first and the second substrate (2, 16), wherein, when joining the first and the second substrates (2, 16), at least some of the bumps (12) on the first substrate (2) come into contact with at least some of the bumps (12) on the second substrate (16).

21. The method of any of the claims 1 to 19 wherein particle bumps (12) are printed on only one of the substrates (2, 16).

22. The method of any of the preceding claims wherein said particles are enclosed by a ligand of a material different from the particles.

23. The method of claim 22 wherein said method comprises the step of removing said ligand after forming said particle bumps (12).

24. The method of claim 23 wherein the ligand is removed by at least one of heating the particle bumps (12) and/or applying a ligand-removing fluid to the particle bumps (12).

25. The method of any of the claims 22 to 24 comprising the step of replacing the ligand with a coating different from the ligand after forming said particle bumps (12).

26. The method of any of the preceding claims wherein both the first and the second substrate (2, 16) comprise a plurality of metal pads (10, 20), wherein, when joining the first and second substrates (2, 16), each particle bump (12) aligns with a pair of metal pads (10, 20) on the first and the second substrates (2, 16).

27. The method of any of the preceding claims wherein the particles are of metal, in particular of at least one of gold, palladium, indium, copper, aluminum, and silver.

28. The method of any of the claims 1 to 26 wherein the particles are of a metal precursor, in particular comprising a metal in an oxidized state, and wherein the method comprises the step of converting the metal precursor to a metal, in particular reducing the metal, for forming said metal connections (17).

29. The method of any of the preceding claims wherein each of the substrates (2, 16) is at least one of semiconductor substrate, a dielectric substrate, and a metallic substrate.

30. The method of any of the preceding claims wherein the step of bonding the first and second substrates (2, 16) comprises compressing the first and second substrates (2, 16).

31. The method of claim 30 wherein the step of bonding the first and second substrates (2, 16) comprises a heating process, wherein compressing starts before a maximum temperature of the heating process is reached.

32. The method of any of the preceding claims comprising the step of plasma-activating the particle bumps (12) and/or polymer layer(s), in particular prior to bonding the first and second substrates (2, 16) (2, 16).

33. The method of any of the preceding claims, wherein the first polymer layer (4) and/or, where applicable, the second polymer layer (22) comprise^) a plurality of auxiliary openings (38), in addition to the first and/or second openings (8, 24), wherein

- no particle bumps (12) are arranged in the auxiliary openings (38) or

- auxiliary bumps (42) are printed in at least some of the auxiliary openings (38), and wherein

(a) the auxiliary bumps (42) are of a material different from the particle bumps (12) and/or

(b) the particle bumps (12) are connected to electrical leads (11, 21) on the first as well as on the second substrate (2, 16) while the auxiliary bumps not connected to electrical leads at at least one of the first and the second substrate (2, 16).

34. The method of claim 33 wherein the auxiliary openings (38) form a repetitive pattern between the first and/or second openings (8, 24), in particular a hexagonal pattern.

35. The method of any of the claims 33 or 34 comprising the step of during joining the first and second substrates (2, 16), mechanically displacing the second polymer layer (22) to reach into the auxiliary openings (38).

36. A bonded first and second substrate producible, in particular produced, by the method of any of the preceding claims.