WO2015091673A1

WO2015091673A1 - Bonded assemblies with pre-deposited polymer balls on demarcated areas and methods of forming such bonded assemblies

Info

Publication number: WO2015091673A1
Application number: PCT/EP2014/078268
Authority: WO
Inventors: Keith Redford; Helge Kristiansen; Mark SUGDEN; Bente Gilbu Tilset; Piotr Warszynski; Changqing Liu; David Whalley; David HUTT
Original assignee: Conpart As
Priority date: 2013-12-17
Filing date: 2014-12-17
Publication date: 2015-06-25
Also published as: GB2523983A; GB201322318D0

Abstract

A method of forming a bonded assembly is described. A liquid-based dispersion of fine polymer balls (< 50 μm diameter) is applied to a first substrate (16) having a plurality of demarcated areas (14) of width less than five ball diameters. The balls (12) are urged into registration through an electrical, chemical or physical stimulus, and the liquid from the dispersion is removed to leave the polymer balls in the demarcated areas. The first substrate is aligned with respect to a second substrate (18) and pressed together to partially compress the polymer balls. The geometry of the assembly is fixed by bonding the assembly. In preferred embodiments the balls (12) have a diameter of between 2-6 μm and they are urged into place using electrophoresis and surface tension. The polymer balls may be coated in Au and In and placed as individual balls on the substrate.

Description

Bonded Assemblies with Pre-deposited Polymer Balls on Demarcated Areas and Methods of Forming Such Bonded Assemblies

Technical Field

The present invention relates to bonded assemblies with pre-deposited polymer balls on demarcated areas, as well as to methods of forming such bonded assemblies. Introduction

When assembling electronic components and substrates there is often a need to add electrical, optical or mechanical functionality (or a combination of these) in highly localised areas. This includes providing electrical connections, optical paths or controlled spacing between the different members of the assembly.

There are numerous examples of the use of balls of different type and materials in electronic assemblies. They generally fall into three categories.

Relatively large balls, having diameters typically much greater than 100pm and made of solder or a solderable metal. The balls are mechanically assembled onto components or wafers to provide electrical interconnect between two components or between a component and a substrate.

Small particles, which are typically between 3-10μηη, that are pre-dispersed in an adhesive which is applied to the substrate to provide electrical connections. In most cases the particles are randomly dispersed in the adhesive, whereas in some cases the particles have some kind of ordered distribution, to avoid that two particles coincidentally are in contact. In all these arrangements, an arbitrary number of particles (typically a small fraction of the total) are contributing to the electrical contact.

· Air spraying of small dry particles onto substrates to provide accurate gap/ stand-off height. This procedure gives a random positioning as well as non-uniform distribution of the particles. It has been previously used by the LCD industry, but now to a large extent has been replaced by photo-spacer technology due to the lack of control of particle position. With the reduction in size and increased operating speed of electronic components there has been a move away from conventional wire bonding technology towards the use of so-called "flip chip" technology. To provide the electrical connections the interconnection pads of the chips are usually "bumped", typically while still in wafer format. After singulating the individual chips, each chip is then "flipped" with the active side (and the bumps) towards the substrate unto which it is to be mounted. The substrate may be in the form of a circuit board, or in more specialised applications, a wafer or other semiconductor/insulator base having circuits provided on it.

The bumps are made of metals and typically formed by either a mechanical placement of a solder ball, or by depositing metals using a chemical or electrochemical plating process, or by thermo-compression bonding a ball formed by melting the end of a fine wire of for example gold, on the bond pads of the component. The direct placement processes, where one ball is placed on each contact point, can be performed in several ways; one is to place the balls on a surface of a tool (typically by vacuum suction) and then the tool is used to transfer the solder balls across to the component during a printing operation. Alternatively, the solder balls can be "printed" through a metal stencil. Both of these processes, often called "gang ball placement", require accurately designed mechanical structures adapted for each design of wafer or chip. Alternatively, individual solder balls can be jetted using highly sophisticated equipment (e.g., US-A-8,393,526). These fabrication techniques function well at the 150μηη or larger level but become much more difficult when producing finer connections, for example, of the order of 50pm or less, because of the difficulties in mechanical handling of the small balls. All of these processes also require a relatively accurate mechanical alignment procedure. During the interconnect process, the solder will melt, and form a solder joint between the opposite contacts to be electrically joined. Bumps can also be made "in situ" by chemical or electro-chemical plating of metal or combination of metals. Typical metal combinations are solder and nickel followed by a thin layer of gold. Electro-plating of gold is also frequently performed for manufacturing of bumps for driver ICs for LCD applications. Developments using micron-sized metal balls or non-spherical particles in an adhesive matrix have allowed other forms of electrical connections to be made. These are commercially attractive because the finer scale of the metal particles, e.g., metal balls can be made small with a diameter of less than 10μηη or so, offers the potential to achieve more contacts per unit area of the components. However, there are a number of shortcomings with respect to these particles. To give the needed electrical conduction, individual particles have to make reliable mechanical contacts across the contact gap separating the contacts to be joined. All metals tend to have a very limited recoverable deformation. For harder metals like nickel and copper, other parts of the contact structure (bump or pad) need to deform plastically to accommodate lack of planarity and particle uniformity. High local mechanical stresses can easily form cracks in brittle semiconductor structures or substrates causing severe yield and / or reliability problems. For softer particles like silver and gold, the particles themselves will undergo a plastic deformation;

however, they will not be able to adapt to even minute changes in contact gap caused by thermal expansion, stress relaxation or humidity absorption in the adhesive. To reduce problems with oxidation, noble metals are preferred, but at the expense of high cost. Also the high density of the conductive particles can easily cause sedimentation and as a consequence, the particle distribution in the adhesive will be non-uniform.

Metallised glass particles have also been made available for conductive adhesives. However, these particles are not inherently compliant and have similar problems as the metal particles in terms of mechanical performance. The lower density and reduced amount of noble metals, however, gives an advantage in terms of cost and reduced sedimentation.

For a number of years, fine (e.g., less than 50pm diameter) polymer balls having a metal coating have been available and used to a large extent in display

interconnect (anisotropic conductive adhesives (ACAs) and conductive spacers). They have also been used infrequently in other types of electronics. Until now, these balls have needed a size classification for use in fine pitch applications, a process that becomes expensive as the particle size is reduced. Recently, such balls have been produced without need for size classification e.g., from Conpart AS, a company based in Norway. The resilience of the polymer cores allows them to be compressed to a significant degree (typically 50% or more) during the bonding process without being harmed, enabling the chip or other component to be pressed against the substrate, and thereby accommodate shortcomings in planarity as the adhesive cures. The compliance of the polymer ball also helps to adapt to changes in contact gap caused by thermal expansion, stress relaxation or humidity absorption in the adhesive within the finished component assembly, in contrast to the non-compliant solid metal ball alternative. Moreover, utilising such metallised balls that have a polymer core also reduces the amount of noble metal needed to form an electrical connection, enabling production costs to be reduced.

With advances in the production of the metal-coated polymer balls, narrow size distributions of balls having specific ball diameters (i.e., mono-size balls) can be achieved to a high level of accuracy. The micron-sized balls may have a diameter, say, of the order of 10μηη or less and a size distribution (CV) of less than 5%.

In a paper entitled, "Metal-coated mono-sized polymer core particles for fine pitch flip-chip interconnects", by Sugden et al., which was presented at IEEE 62nd Electronic Components and Technology Conference on 29 May 2012, there is a description of how bump connections can be formed on a surface of a wafer using these metal-coated, mono-sized, polymer core balls. The balls were 9.8μηι in diameter and were deposited using an electric field to move the particles onto contact pads that were around 75μιη across and had a pitch of about 170μηη. Using the technique described in the paper, many tens or hundreds of balls are deposited randomly on each pad.

It would be desirable to provide a method of forming a bonded assembly with pre- deposited micron-sized polymer balls on demarcated areas, where balls are placed individually or in small clusters, thus offering the possibility of finer connections and higher component densities. It would also be desirable to provide a bonded assembly comprising micron-sized balls that have been accurately placed at predetermined positions.

Summary of the Invention According to a first aspect, the present disclosure can be seen to describe a bonded assembly where two or more substrates (for example, one of the substrates may be an electronic component and the other may be a supporting substrate) are bonded together with an adhesive, wherein the adhesive provides a mechanical fixation of the assembly (eg., by filling the whole gap or by providing the adhesive bond at the edges (periphery) of the substrates only) that comprises pre-deposited polymer balls that have been placed individually or in small clusters at predetermined locations, and where the balls contribute certain functionalities such as electrical, optical or mechanical or combinations of these.

According to a second aspect, there is provided a method of forming a bonded assembly comprising:

applying a liquid-based dispersion, containing polymer balls having a diameter of less than 50 μιτι, to a first substrate having a plurality of demarcated areas, the demarcated areas having a width dimension of less than five ball diameters;

urging the balls into registration with the demarcated areas through an electrical, chemical or physical stimulus;

removing the liquid from the liquid-based dispersion to leave the polymer balls in the demarcated areas;

aligning the first substrate with respect to a second substrate and pressing the substrates together to partially compress the polymer balls and form an assembly having a given geometry; and

fixing the geometry of the assembly by bonding the assembly, to form the bonded assembly.

The method may include providing an adhesive around the edge(s) of the first substrate and/or locally at the site of the particles and/or between the first and second substrates, e.g. the whole area, and curing the adhesive while pressing the substrates together to fix the geometry and thereby form the bonded assembly.

In another embodiment the step of fixing the geometry may include the application of heat to melt solder either on the polymer ball and/or on the substrates, to form the bonded assembly. ln another embodiment the polymer balls include a metal coating, preferably comprising Au and/or In, and the step of fixing the geometry comprises a thermo compression bonding step. Combinations of metal elements, such as gold and indium will, with intimate contact, co-diffuse to form a bond, providing a form of cold welding. Other metal element combinations are also possible, for example, comprising Au or In in a binary, tertiary or other metallic system, as well as other non-Au or non-ln metal element combinations that are able to diffuse into one another to form a bond, and are included herein. The polymer balls are placed on the demarcated areas as a single layer, and are placed either individually or in small clusters (e.g., three or seven balls, or as a thin line that is one, two or three balls wide). The demarcated areas represent predefined positions for the polymer balls. Thus the placement of such micron- sized polymer balls, which can be coated or uncoated, can be carefully controlled to achieve fine electrical interconnects or optical waveguides, accurate patterning, accurately placed spacers, etc. in a bonded assembly that can be produced for a lower cost and offer better reliability than the prior art counterparts.

The polymer balls have a diameter of less than 50pm, more preferably less than 30μιη. For example, they may have a diameter of 15μιη or less. The polymer balls may have a diameter greater than 0.5μηη, more preferably greater than 1 μιη. In one embodiment the diameter is 10μιη or less, more preferably the diameter of the polymer balls is about 5pm. The polymer balls are preferably of a substantially uniform size, i.e., they can be regarded as "mono-sized". The polymer balls may have a size distribution of less than 5% (where the size distribution is equal to 100 times the standard deviation of diameter divided by the average diameter), more preferably less than 4% and more preferably still less than 3%. An advantage of using "mono-sized" polymer balls that have a small size distribution is that the chances of having an off-size particle (e.g., balls with a diameter more than 50% larger than the mean particle size) is much reduced. Preferably the likelihood of such an off-size polymer ball being present is less than 1 in a million. This will significantly reduce the risk of electrical short-circuiting of nearby contacts or non-uniform gaps in an optical assembly, thereby increasing manufacturing yield. The polymer balls are bonded in a compressed state to take up any lack of planarity. Thus the gap between the substrates will be smaller than the ball diameter. Depending on the intended function, the polymer balls may be compressed by more than 5%. Preferably the compression is less than 60%, more preferably less than 50% (for example, 5-25% in applications such as spacers for LCDs where a uniform gap is required and possibly 20-50% for electrical contacts).

The polymer balls may be uncoated. The surface of the polymer material may be treated, for example, to provide a surface charge that can be used to urge the polymer balls into registration with the demarcated areas. The placement of such uncoated polymer balls on a substrate may be of use to facilitate an accurate spacing of two components at a prescribed separation. Controlling the placement of the polymer balls in this way so that they are present on the substrate as individual balls or small clusters of up to 7 balls, particularly where they are of a narrow size distribution, can result in more precise assemblies being fabricated.

Preferably the polymer balls have a coating of some form. The coating may be an insulating, semi-conducting or conducting oxide material, for example, silica, zinc oxide, silver oxide or titanium oxide. The oxide coating may be treated or adapted to provide the balls with a charge or to promote a chemical interaction with the demarcated areas, for example, to allow them to respond to an electrical stimulus such as during electrophoresis and/or to provide a chemical stimulus to urge the polymer balls into registration with the demarcated areas.

Preferably the polymer balls are coated with a conductive material so that they act as conductive particles between the bonded substrates. The conductive polymer balls may be used for providing electrical contacts directly between, for example, a substrate of a component and a substrate of a carrier that it is being adjoined to, or directly between a first component and a second component in order to provide a stack of components. The components may be chips, processors, memory units, heat sinks, sensor elements, transmitters, or indeed any other electronic component that a person may wish to couple to a carrier and/or to another component as a bonded assembly. The method preferably includes the step of demarcating areas of the first substrate, ready to receive the polymer balls. In the case of a traditional semiconductor process flow, this is already performed when the contact metallisation step is carried out, followed by a passivation process (where a layer, typically of silicon nitride, is deposited). The method may therefore include a step of depositing a demarcating layer, which may be a passivation layer but more preferably is an organic layer, preferably an organic layer which has some adhesive properties, on the first substrate (which is preferably a patterned metallised substrate). In other applications where electrical contacts are needed, the demarcated areas may be formed by depositing a conductive layer on a substrate, for example, by using a deposition or plating technique to deposit or plate metallised areas, to provide conductive pads for the substrate. This metallisation layer can be followed by a patterned dielectric or other layer for protection or passivation of the metal (or underlying structures), where openings in the dielectric form the demarcated areas.

More preferably the demarcated areas are formed by patterning a dielectric layer which is deposited over a (typically patterned) conductive layer on the substrate. This patterning process can be performed by using a mask to activate selectively a photo-resist or similar material that is activated by light, to define the regions that will become the demarcated areas. The photo-resist that is laid over these regions is then removed, e.g., by etching, to expose the conductive layer below and create the demarcated areas. By providing an electrical potential to the semiconductor substrate with respect to a counter electrode in a particle suspension, the conductive demarcated areas, e.g., in the form of exposed metallised electrical pads, allows the polymer balls to be urged into registration through an electrical stimulus, for example, by using electrophoresis, when the liquid-based dispersion has been applied. Such an electric field will provide a drift velocity of the particles, depending on the surface charge of the particles in the relevant dispersion. The demarcated areas may also comprise areas of a chemical reagent that reacts with or provides an affinity for a coating material of the polymer balls, in order to urge the polymer balls into registration with the demarcated areas through a chemical stimulus. More preferably the chemical stimulus is created by providing an ionic charge on a ball coating and a corresponding but opposite ionic charge on the demarcated areas, such that the two have an affinity for one another. In this case, the attraction forces between demarcated area and particles are very short, and the particles must be brought into proximity by convection in the dispersion. A chemical stimulus might be used in addition to using an electrical stimulus, such as applying an electrical potential, to assist the registration with the demarcated area.

In addition to demarcating the areas, e.g., to define the shape of the electrical pads, topographical structures can be made on the surface to trap the polymer balls as the substrate is slowly removed from the liquid-based dispersion. For example, a film of the liquid may be retracted from the substrate in such a way that the polymer balls are caused to be trapped as they are being drawn along by the thin liquid film. The thickness of the film is only slightly larger than the ball diameter. Once the particle is trapped, further movement would require that the particle would be partly pushed out of the liquid, which is strongly opposed by the liquid surface tension. Thus the topography of the material marking the boundary of the demarcated areas, can also provide a physical stimulus that urges the polymer balls into registration with the demarcated areas. The demarcating layer may also have adhesive properties. In this way the layer can be used as an underfill material to adhere the substrate and an adjoining carrier substrate together. In other embodiments, a photoresist used to pattern the demarcating layer will be removed as in a standard photo process once the pattern is made, and replaced with a layer of adhesive once the polymer balls have been placed in position or covered over by an adhesive, the adhesive then being used to bond the first and second substrates together, with the polymer balls arranged in their predefined positions between them.

The placement of the polymer balls described above is preferably part of a process to fabricate an electronic device. Conventionally, when components such as chips are secured to a carrier substrate or to a further component, a non-conductive adhesive called an "underfill" is used to fill the region beneath the component and bond the substrate of the component to the adjoining substrate. The underfill is necessary to provide sufficient mechanical strength, and to prevent thermally induced shear stresses that arise during chip operation from fracturing the bump connections, e.g., through fatigue. The use of solid metal bump connections within the underfill matrix has been found to exacerbate this problem because of the different coefficients of thermal expansion (CTE) between the polymer of the underfill and the metal of the solder bump, particularly in the direction of the connection, which can create stresses that tends to prise the two substrates apart. To overcome this problem, the CTE of the underfill must be adapted to match the solder, currently through being highly filled with micro and nano-particles typically made of silica. However, this increases the viscosity severely, and impedes the flow of the underfill.

An advantage of using polymer balls (in particular metal-coated polymer balls) is that, because of their polymer core, they have a similar CTE to the underfill. This means that the bonding of the substrates is less prone to damage from thermal expansion than a similar connection where solid metal balls are used. Also the initial elastic deformation of the balls prevents any delamination between balls and contact pads.

As the amount of non-organic fillers (eg silica) is strongly reduced or completely eliminated, a further advantage is that the viscosity of the underfill is significantly reduced. This allows much smaller gaps and much more dense contacts to be made without compromising on the time consumed for the underfill process.

Thus preferably, in accordance with a preferred embodiment, the method of depositing polymer balls on demarcated areas of a substrate is used in the fabrication of an electronic device, for example, where a substrate comprising an electronic component is being secured and connected electrically to a carrier substrate or to a substrate of a further component. In the fabrication, the method includes the step of introducing an underfill between the first substrate and an adjoining second substrate as part of the device fabrication. The underfill may comprise a conventional underfill material such as an epoxy resin (however, without the silica filler), and may be introduced in any known manner, for example, through capillary action. Due to the added compliance of the polymer balls, an adhesive sealing only around the edges might in many cases replace a full underfill. The liquid for the dispersion may be water, a water-based solution, alcohol, an alcohol-based solution, acetone or indeed any similar solution that the polymer balls are able to form a reasonably uniform dispersion within. In this regard the density of the liquid may need to be selected to be close to that of the polymer balls (or the coated, e.g. metallised, polymer balls). An advantage of using the metal-coated polymer balls is that for balls in the range of 0.5 to 50pm, water and alcohol have appropriate densities to produce effective dispersions, whereas for solid metal balls, the higher inherent densities can create problems, particularly for the larger ball sizes. The method may include the steps of adding the polymer balls to the liquid and agitating the liquid to form the dispersion prior to applying it to the substrate.

Brief Description of the Drawings

Certain preferred embodiments will now be described in greater detail by way of example only and with reference to the accompanying drawings, in which:

Figure 1 shows an exemplary cross-section through a bonded assembly that has been made in accordance with a preferred embodiment;

Figures 2a, 2b and 2c show examples of how the polymer balls can be placed in the demarcated areas as an individual ball, a group of three or a group of seven balls, and Figures 2d, 2e and 2f illustrate alternative configurations of demarcated areas for the same arrangements of polymer balls;

Figure 3 is a flow diagram illustrating the possible steps in the formation of a

bonded product;

Figures 4a to 4g illustrate a set of steps in the production of the bonded product of Figure 1 ; and

Figure 5 illustrates a perspective view of an array of polymer balls located within demarcated areas provided on a substrate. Detailed Description

Figure 1 illustrates an example of a bonded assembly 10, in which polymer balls 12 have been pre-deposited on demarcated areas 14 of a first substrate 16 prior to being bonded to a second substrate 18 with an adhesive 20 ("underfill"). In the example, the first substrate 16 might be an electronic component which is electrically connected to a carrier substrate 8 by metal-coated polymer balls 12,

In the bonded assembly 10 of Figure 1 , polymer balls 12 are pre-deposited in contact with demarcated areas 14 and sandwiched between first and second substrates 16, 18. The first substrate 16 might be an electronic component and the second substrate 18 might be a support for that component.

The demarcated areas 14 of the first substrate 16 are provided by metallised pads 22. These pads 22 are masked at their perimeter by a demarcating layer 24, which defines the edge or edges of the demarcated areas 14. Corresponding metallised pads 26 are provided on the second substrate 18.

In Figure 1 , the underfill 20 extends under the entire surface of the first substrate 16. The bonding of the substrates 16, 18, is necessary in order to accommodate the large thermally-induced shear stresses that can build up during use of the final assembly. It also encapsulates the contact metals of the metallised pads 22, 26 and the metal coating of the polymer balls 12 to protect this region from

contaminants or the effects of atmospheric conditions.

The polymer balls 12 are compressed to an extent before curing of the underfill 20. This may be in the range of 10 to 50%, possibly even as much as 60%. In so doing the polymer balls 12 will be flattened slightly, increasing their areas of contact with the metallised pads 22, 26.

The polymer balls 12 are less than 50 μΐη in diameter, more preferably less than 20 μηη. A preferred size range is between 1-10 μηη, more preferably between 2 - 6 μιη in diameter. The polymer balls 12 may be uncoated or coated, and they may be transparent or opaque depending on their intended purpose. For example, the polymer balls 12 may be intended to act as light guides, pixels for a security image, or perhaps provide a spacing function between the first and second substrates 16, 18. In such examples, the transparency of the polymer balls (and any coating if present) would be advantageous. More preferably the polymer balls include a coating. For conductive particles, such a coating is preferably a metal coating. Polymer particles can be coated using conventional coating methods, such as those described in US-A-6787233. Suitable metal coating layers can be formed from transition metals or a metal such as Bi, Si, Sb, Sn, Pb, Ga, Ge, In or Al or mixtures thereof. Metals of particular interest include chromium, bismuth, indium, zinc and antimony. Especially preferably the metal may be selected from Ni, Cu, Pd, Pt, Au and Ag, preferably Ni. Mixtures of these metals, e.g. Ni /Au can also be used.

In particular a lead free solder may be employed in one or more layers of the coating. Lead free solders in commercial use may contain tin, copper, silver, bismuth, indium, zinc, antimony, and traces of other metals. Sn-Ag-Cu solders are especially preferred.

The combinations nickel/gold, nickel/palladium, and copper/nickel are preferred along with metals silver, gold, palladium, platinum and chromium.

Preferably the thickness of any coating layer is in the range 5 nm to 10 pm, more preferably 5-500 nm. The total thickness of any coating layers may be 5 nm to 10 μιη, more preferably 5 nm to 5pm. It is preferred if the total thickness is in the range 5 nm to 2 pm, especially preferably 5 nm to 1 pm.

The polymer particles can be coated according to methods known in the art. Such methods include electroplating, electroless plating, substitution plating, barrel coating, sputtering and vapour deposition. Mixtures of these methods may also be employed.

The coating provided on the polymer balls can be monolayer or multilayer.

Preferably the particle comprises a plurality of metal layers. Where multiple coating layers are present it is within the scope of the invention for different coating application methods to be involved in the formation of each layer. In particular the method of electroless plating is the preferred method. For subsequent layers substitution plating may be applied. For larger particles electroplating may be preferred. Electroless plating is a method of depositing a metal on to a substrate using a process of chemical reduction. The advantage of this type of deposit is that the coating is uniform with hardly any variance in the distribution of thickness across the polymer ball.

It may be beneficial to heat-treat the as-deposited electroless plated coating to improve hardness. Electroplating is a plating process that uses electrical current to reduce cations of a desired metal from a solution and coat an object with a thin layer of the metal. Electroplating can be used to build up coating thickness on a conducting base layer. Substitution plating is where a more noble metal replaces a pre-deposited metal, where the existing metal is oxidized and goes into solution and the more noble metal is reduced; for example nickel metal and a gold salt such as KAu(CN)₂.

In a multilayer coating, it is preferred if the layer adjacent the actual polymer particle comprises nickel, copper or silver, especially silver or nickel. This layer is preferably applied by electroless coating. It is preferred if the top layer comprises Sn, in particular comprises lead free solder, palladium or gold. The top layer is preferably one that resists oxidation. The most preferred top layer is Ag, Pd or Au ideally having a thickness in the range 5-2000 nm. Preferably the top layer has a thickness in the range 5-50 nm.

A highly preferred structure involves a Ni layer adjacent the polymer particle and a top layer containing, Pd, Ag or Au. Such a polymer particle may have other intermediate layers.

Any intermediate layers preferably comprise copper or nickel. It is obviously beneficial to use as much low-cost metal and as little expensive metal as possible in the coating. The use of two layers is preferred. It will be appreciated that the different layers can be formed from the same or different metals. If the same metal is used to form multiple layers it may be impossible to distinguish those layers. If layers are indistinguishable then the particle will present simply as having a thicker monolayer made up by multiple depositions. Where the same metal is deposited by different techniques, such layers are often distinguishable.

In the embodiment shown in Figure 1 , each demarcated area 14 is provided by a separate metallised pad 22. However, it is also possible to provide a common metallised pad 22 that extends across the first substrate under the demarcating layer 24, the metallised pad being shared by a plurality of demarcated areas 14. Holes in the demarcating layer 24 may then reveal a grid or other arrangement of demarcated areas 14. Such an arrangement is illustrated in Figure 5. The first and/or second substrate 6, 18 preferably comprises a wafer, for example, a silicon wafer or other semiconductor wafer. It may also comprise a glass or crystalline including ceramic materials (for example, low temperature co-fired ceramic (LTCC) or high temperature co-fired ceramic (HTCC)) that can be formed with sufficient flatness to allow the fine (less than 50 m) size connections to be formed.

The pads 22 may comprise a number of layers. For example, in one example, the pads 22 may comprise a base layer of a first material and include one or more coating layers of a second or third material. For example, the base layer might be an aluminium bond pad, and the second material might be a metal that is less likely to corrode or suffer metallurgical problems from contact with the metal of the coated polymer balls 12 (e.g., a UBM layer).

Where the polymer balls 12 are providing an electrical contact between the first and second substrate 16, 18, an opposing metallised pad 26 is provided on the second substrate 18 to make an electrical contact with the polymer balls. In embodiments where the polymer balls 12 are providing non-electrical functions, such as light guide, security marking or spacing functions, then the opposing metallised pads 26 may not be required. Although in the electronics industry, it is standard practice to fill the entire region underneath a component with underfill, there may be occasions, for example, in non-electronic applications, where it is desirable to bond the first substrate 16 to the second substrate 18 only at specific regions, for example, the perimeter, central regions or at the corners, and not in other regions. In this way the bonded substrates 16, 18 may trap the polymer balls 12 in place at the demarcated areas 14 without adhesive 20 being arranged in contact with some or all of the polymer balls 12. Preferably an electrical connection between the substrates 16, 18 does not require an additional soldering step to be performed. Instead, the electrical connection relies on the mere placement and retention of the metal-coated polymer balls 12 between respective contacts 22, 26 of the substrates 16, 18. Figures 2a to 2f show some examples of how the polymer balls 12 may be placed on the demarcated areas 14 of the first substrate 16.

In Figure 2a, an individual polymer ball 12 has been placed in a circular demarcated area 14. In Figure 2b three polymer balls 12 have been placed within a triangular demarcated area 14. In Figure 2c, seven polymer balls 12 have been placed in a hexagonal close packing arrangement within a hexagonal-shaped demarcated area 14. The shape of the demarcated area 14 is preferably set to mirror the form of the individual balls or clusters in order to minimise the amount of material required for the demarcated areas 14.

However, other shapes of demarcated areas are also possible. For example, rather than a circular demarcated area 14 in Figure 2a, an oval, polygonal or square shape (e.g. as illustrated in Figure 2d) may be provided for the individual polymer ball 12. Similarly, in place of the triangular demarcated area 14 in Figure 2b, a circular, hexagonal, rectangular, square demarcated area 14, for example as illustrated in Figure 2e may be preferred. The demarcated area for a seven ball cluster as shown in Figure 2c can also take other forms such as square, rectangular or circular, e.g. as shown in Figure 2f. ln addition to these possibilities, there may be certain applications where it is desirable to have a line of polymer balls 12 placed along a demarcated area 14. In such embodiments, the maximum width dimension of the demarcated areas 14 would correspond to three ball diameters or less, more preferably two ball diameters and most preferably a single ball diameter or less. Lines of balls which are 1 , 2 or 3 balls deep across the width of the line, may be straight or curved, for example, following an arc of a circle or a more complex curve such as an oval or spiral. Figure 3 is a flow chart illustrating a preferred set of steps for producing the bonded assembly 10.

The first part of the process 100 is to define the position and the size and shape of the demarcated areas 14. For semi-conductor wafers (or components) this is part of the standard processing of the wafers. The only change is that each traditional contact pad is now replaced by a number of very small contacts (the demarcated areas 14), in total occupying an area that is more than one order of magnitude smaller than the original pad. On other types of substrates, where the ultra fine accuracy obtained by

photolithography 102 is not required, this costly process can be replaced, for instance, by ink-jetting 104 or aerosol-jetting 106.

Besides providing electrical contact where that is required, the main purpose of the demarcated area 14 is to provide a local surface where the surface chemistry, surface charge or applied electrical potential (metal pad) is different from that of the surrounding, and thereby provide a local attraction to the polymer balls 12 to be deposited. Next comes the step of moving the particles in proximity of demarcated areas 110. The polymer balls 12 are dispersed in a liquid, typically of water or alcohol or combinations of these. The ability to control pH in the liquid is important to control the surface charge on the polymer balls 12 and the demarcated areas 14. The application of the liquid-based dispersion 28 may be by any method that creates a substantially uniform distribution of the dispersion across the surface of the substrate 16. In one embodiment it includes spinning the substrate to provide centrifugal forces that cause the dispersion 28 to spread out evenly. In another embodiment, the substrate 6 with the demarcated areas 14 is immersed into a liquid-based dispersion 28 of the polymer balls 12.

Polymer balls 12 are transported to the vicinity of the demarcated areas 14 either by random Brownian motion, or by drifting in an electrical field in the bulk liquid dispersion (bulk liquid 1 12). Alternatively, polymer balls 12 are accumulated in a liquid meniscus retracting on the surface of the wafer / substrate 16, due to the internal liquid flow transporting polymer balls 12 to this area 14 as the liquid 28 evaporates (retracting liquid film 1 14). The demarcated areas 14 may be defined by one or more layers to provide topological structures such as hollows on the surface of the substrate 16 having a depth of at least 0.25 polymer ball diameters, more preferably 0.5 polymer ball diameters or greater, for example, 0.6 to 0.9 ball diameters thick, more preferably about 0.7 ball diameters thick. The one or more layers may comprise a mask of photo-resist material. In this way, the pattern of the demarcated areas can be accurately set.

Once the particle is in the proximity of an unoccupied area with the relevant charge or surface chemistry (demarcated area 14) it is forced into physical contact (deposition 120). This may be through chemical affinity 122 or through electrostatic attraction 124. In case of deposition from a retracting liquid meniscus, a physical trap (topographic barrier - surface topology 126) must be incorporated so that the polymer ball 12 is pinned with respect to further movement. After removing from the suspension, the capillary process during drying will cause a further adhesion of the balls to the substrate (adhesion of particles 130 and drying of liquid 132). If necessary, further adhesion can be obtained by for instance capillary bridging or necking 134. If needed, the substrate 16 populated with the polymer balls 12 can now be singulated into individual components using conventional dicing techniques like laser cutting or mechanical sawing. At this point or before, the photo-resist marking out the demarcated areas 4 may be removed, so that it can be replaced with an adhesive. Alternatively it may have adhesive properties and/or be desirable to keep it in place. Where the photo-resist is an adhesive, preferably the layer is greater than 0.5 polymer ball diameters, more preferably between 0.6 and 0.9 ball diameters. Thus in one embodiment, a step of masking the substrate to provide demarcated areas 14 may comprise depositing a layer of adhesive to mask the substrate 16 to define the demarcated areas 14.

The first substrate 16 is then aligned with respect to a second substrate 18, for example, the first substrate may be "flipped" and mounted on to the second substrate 18. The substrates are then pressed together (system alignment and contact pressure 140) to partially compress the polymer balls 12, for example, by 5 to 60%, and form an assembly having a given geometry.

The region between the first and second substrates 16, 18 is then subjected to adhesive bonding 150. The photo-resist may provide the adhesive properties necessary for this (preapplied adhesive 152), or a separate adhesive (underfill) may be used and applied through capillary flow 154.

Underfill is used to mechanically lock the component to the substrate and thereby avoid the high shear stress in the solder interconnect otherwise causing fatigue failure. However, the thermal expansion of the underfill must be closely matched to that of the solder to avoid the development of tensile stress in the solder. All polymer materials have a high CTE compared to that of solder, so they need to be filled with a large volume fraction of inorganic materials (typically silica) to reduce the expansion coefficient to that of solder. However, the high fill fraction needed causes the viscosity of the underfill to increase by orders of magnitude making the underfill process slow and time consuming and sometimes impractical. The resin and the particles in the prior art can also easily separate during the process, causing an un-homogeneous underfill. Material Use Value [10 C]

Silicon IC component 3 lll-V semiconductors IC component Typically (GaAs, GaP +++ 4-6

Glass - epoxy (in plane) Printed circuit board (PCB) 15-20

Glass - epoxy (in z- Printed circuit board 50-80 T<Tg direction) 150-350 T>Tg

Ceramic Component carrier 7-10

Copper Metal layers on PCB 17

Solder Interconnect 25

Polymer resins (below Tg) Underfill resin 55-80

Silica Filler particles in Underfill resin 0.6

Underfill Polymer resin + silica 25

Polymer core particles Interconnect 50-75

Table 1 - Typical coefficient of thermal expansion (CTE) values

Tg: Glass transition temperature

The advantage with the polymer core balls 12 is that these balls can easily accommodate significant deformations in the z-direction. Typically these polymer balls 12 can be deformed to 50% for thousands of cycles without wearing out. Also, these balls have a CTE, which is much better matched to the pure underfill (resin only). The underfill 20 is then cured while pressure is applied to the substrates 16, 18 to squeeze them together (final curing 160). The curing might be activated by heat or light, for example, UV light or microwave radiation.

The final process step 170 is the end product of the bonded assembly following the curing of the underfill 20. The set underfill retains the polymer balls in a

compressed form in the final electrical junction. This has the added advantage that it increases the electrical contact area between the surface portions of the balls and the surface contacts. The processing sequence described in Figure 3, from the forming of the

demarcated areas 14 through to the bonding of the final assembly, will now be illustrated by reference to Figures 4a to 4g. Figure 4a shows a cross section through a first substrate 16. On a surface of the substrate 16, metallised pads 22 have been formed. These may have been deposited through a printing or plating technique, or there may be metallised areas remaining after a surface layer has been etched. Figure 4b illustrates the first substrate 16 after the application of a demarcating layer 24. In one example, this is a photo-resist material that has been applied across the entire surface of the substrate 16 and then etched to reveal the demarcated areas 14 of the metallised pads 22. In this way, the edges of the demarcated areas 14 can be accurately defined according to a predetermined arrangement or pattern.

Figure 4c illustrates the deposition of the polymer balls 12 on to the demarcated areas 14. The polymer balls 12 are suspended in a liquid-based dispersion 28. The liquid may be water, an alcohol, a water or alcohol-based solution or any other suitable liquid like acetone etc. The low density of the polymer core particles reduces the effect of sedimentation. The polymer balls 12 are urged towards the demarcated areas 14 during electrophoresis. In one example, the metallised pads 14 are charged by applying a potential to the electrical circuits that either connect to the pads, or to the rear face of the device. In another, the pads 22 may be charged with a static charge. In another example, the pads may include a charge as a result of chemical processing. The polymer balls 12 also include a charge and/or have some other affinity for the material of the metallised pads 22. For example, the coating on the polymer balls 12 may be modified through chemical processing to place a charge at the surface.

Once sufficient time has elapsed for the polymer balls 12 to locate into the demarcated areas 14, the liquid of the liquid-based dispersion 28 is removed. This may be performed through a drying process or some other form of liquid-extraction process. As the meniscus 30 is drawn back across the surface of the demarcating layer 24, this may also help to drag the polymer balls 12 into registration with the metallised pads 22, since the edges of the demarcating layer 24 will provide hollows or pits for the polymer balls 12 to be pulled into.

Figure 4d shows the substrate 6 with the polymer balls 2 placed on the demarcated areas 14 between the regions of the demarcating layer 24. The process of drying the remaining liquid helps to adhere the polymer balls 12 to the metallised pads 22 through a "necking" process. This is where residual

components within the liquid are deposited as the last of the liquid dries, which tends to be in crevices such as around the junction where the polymer balls 12 contact the metallised pads 22. The final drying might be part of a stabilization process that helps to adhere the polymer balls 12 in place. The action of drying the substrate has been found to stabilise the polymer balls 2 sufficiently for the subsequent processing in most cases. In Figure 4e, the demarcating layer 24 is removed to leave the polymer balls 12 standing proud on the metallised pads 22 of the first substrate 16.

A second substrate 18, which might be a supporting substrate for an electronic component provided by the first substrate 16, is provided with metallised pads 26 corresponding in position to the metallised pads 22 of the first substrate. These metallised pads 26 may be formed in the same way as the pads 22 on the first substrate 16, or they may be formed by an alternative process.

As shown in Figure 4f, the first substrate 16, with its pre-deposited polymer balls 12, is flipped over and aligned with respect to the second substrate 18. The two substrates 16, 18, are then pressed together to trap the polymer balls 12 between the opposed metallised pads 22, 26. Contact pressure is then applied which causes elastic deformation in the polymer balls and a widening of the contact areas where the polymer balls 12 touch the metallised pads 22, 26.

Then, as shown in Figure 4g, an adhesive 20 is applied to fill the entire remaining surface between the first and second substrates 16, 18 while they are pressed together. Before the contact pressure is removed, the adhesive 20 is cured, for example, through heat or light, to bond the first substrate 16 to the second substrate 18 with the polymer balls 12 trapped in the pre-deposited positions, in contact with the metallised pads 22 and 26. The bonded assembly 10 can then be subjected to any final processing, such as the addition of further components or dicing, before it moves from the production line for use in a final product. Figure 5 illustrates a further possibility which could be used for spacing two substrates at a precise separation, for example, in the formation of screens or fluid channels. In the embodiment, the substrate 16 is provided with a common metallised pad 22 across its entire surface. It would also be possible to have several large metallised pads that are common to groups of polymer balls 12, for example, where the groups are arranged in separate clusters as shown in Figure 5. On top of the metallised pad 22 is provided a demarcating layer 24 which has been etched to reveal demarcated areas 14, exposing the metallised pad 22 beneath. On each demarcated area 14 is placed an individual polymer ball 12 through the previously described electrophoresis process using a liquid-based dispersion of polymer balls 12. Where a common metallised pad 22 is used, a single electrical contact with the pad 22 can be used to supply a charge to many demarcated areas 14 during the electrophoresis deposition process.

In the illustration, the polymer balls 12 are placed individually in their respective demarcated areas 14. Clusters, for example, groups of three or seven polymer balls 12 and lines of one, two or three polymer balls thick, or any combination of individual, clusters/lines of polymer balls 12, could be used according to the final requirements of the product. As before, after removal of the liquid, the demarcating layer 24 can be removed to leave the polymer balls 12 adhered to the metallised pad 22 in their predetermined positions. A second substrate 18 (not shown) can then be placed over and in contact with the polymer balls 12 and an adhesive 20 (not shown in Figure 5) applied between the two substrates 16, 18 to bond them together. That adhesive might be in regions where the polymer balls 12 are present (i.e. the adhesive 20 would encapsulate the polymer balls 12) or the adhesive may be placed at other regions, e.g. at the edges, to trap the compressed polymer balls 12 between the two substrates 16, 18. The adhesive may have different optical properties to the polymer balls 12, for example, where the placement of the polymer balls 12 is being used to generate a security image, or the adhesive 20 may define channels between the two substrates 16, 18 that will be of a precise separation, for example, for use as capillary paths for liquids.

In addition to or as an alternative to the adhesive, the polymer balls may include a coating that is responsible for bonding (in full or in part) the assembly together and fixing the geometry of the assembly. For example, the polymer balls may include a coating of a solder that can melt and bond the substrates together upon application of heat, or they may include combinations of metal elements that diffuse into one another to form a bond, such as, for example, Au and In.

Thus, at least in preferred embodiments, there is provided an advantageous placement method that can place a huge quantity (millions) of small balls (much less than 50 pm) accurately and reliably, and depending on the relative size of the demarcated areas, the polymer balls can be placed individually, i.e., one ball per demarcated area, or in small groups or clusters, for example, of perhaps, two, three, or seven polymer balls. This is not possible with traditional pick and place technology.

Also finer pitch becomes possible, through reducing the area occupied by I/O pads on a component. In many cases (for instance driver ICs for LCD) the size of the components is dictated by the size of the I/O pads. The above described method enables component size to be reduced and accordingly, reduced cost. For ACF with 3 pm particles, the minimum pad size is 20 x 50 pm, to ensure more than 6 balls per pad. With the present technique, 6 polymer balls of the same size can be placed on an area of less than 7 x 1 pm, reducing the occupied area by more than a factor of 10.

In traditional ACF technology, the components must also be "bumped" to provide reliable contacts. This is done by an additional electrolytic gold plating process. This requires a number of additional processes that adds cost to the semiconductor manufacturing. These extra processes become superfluous with the present technology.

Claims

Claims:

1 A method of forming a bonded assembly comprising:

applying a liquid-based dispersion, containing polymer balls having a diameter of less than 50 μηι, to a first substrate having a plurality of demarcated areas, the demarcated areas having a width dimension of less than five ball diameters;

2. A method as claimed in claim 1 , including the step of depositing a demarcating layer on the first substrate, preferably a patterned substrate, to define the plurality of demarcated areas.

3 A method as claimed in claim 1 or 2, wherein the step of urging the balls into registration with the demarcated areas comprises applying an external electric field causing a drift of particles towards the demarcated areas (electrophoresis process).

4. A method as claimed in claim 1 , 2 or 3, wherein the step of urging the balls into registration with the demarcated areas comprises a process in which a meniscus of the liquid-based dispersion is retracted over the first substrate. 5. A method as claimed in any preceding claim, wherein the polymer balls are of between 0.

5 to 20 pm, preferably of between 1 to 10 μητι in diameter and more preferably of between 2 to 6 pm; wherein the polymer balls are of substantially uniform size and preferably they have a size distribution of less than 3% (where the size distribution is equal to 100 times the standard deviation of diameter divided by the average diameter).

6. A method as claimed in any preceding claim, wherein the polymer balls include a metal coating, preferably comprising Au and/or In, and the step of fixing the geometry comprises a thermo compression bonding step.

7. A method as claimed in any preceding claim, wherein the method includes the step of patterning the first substrate, either with a layer of conductive material, or a layer of non-conductive material that has an affinity for the polymer balls, to define and/or provide material for the plurality of demarcated areas.

8. A method as claimed in any preceding claim, wherein the polymer balls are placed individually or in clusters of up to seven polymer balls.

9. A method as claimed in any preceding claim, wherein the demarcated areas are formed with a width dimension of less than 3.5 ball diameters, preferably less than 2 ball diameters, more preferably a single ball diameter or less.

10. A method as claimed in any preceding claim, wherein the method includes the step of depositing a layer of material that also serves to define the demarcated areas prior to applying the liquid-based dispersion, in particular an organic photoresist material that also functions as an adhesive layer.

1 1 . A method as claimed in any of claims 1 to 9, wherein the method includes the step of introducing an adhesive between the first and second substrate by capillary action once the first substrate has been aligned with respect to the second substrate.

12. A method as claimed in any preceding claim, wherein the substrates are pressed together to compress the polymer balls by between 5 to 60% during the fixing step.

13. A method as claimed in any preceding claim, wherein the step of urging the balls into registration with the demarcated areas comprises a chemical/charge affinity between the surface of the balls and the surface of the demarcated areas.

14. A method as claimed in any preceding claim, wherein the polymer balls include a metal coating, preferably solder, and the step of fixing the geometry includes the application of heat to melt solder either on the polymer ball and/or on the substrates.

15. A method as claimed in any preceding claim, wherein the method includes providing an adhesive around an edge(s) of the first substrate and/or between the first and second substrates, and curing the adhesive while pressing the substrates together to fix the geometry and thereby form the bonded assembly.

16. A method as claimed in any preceding claim, wherein the polymer balls are provided with a metal coating, preferably comprising a metal selected from the group consisting of Ni, Cu, Pd, Pt, Au, Ag and In or an alloy selected from their combinations, and/or preferably comprising layers having different metallic compositions.

17. A method as claimed in claim 16, wherein the method is a method of making an electronic product comprising the bonded assembly, where the first substrate comprises an electronic component that is being secured and connected electrically to a second substrate that is a carrier substrate or a further component.

18. A bonded assembly, preferably an electronic component, made by a method as claimed in any preceding claim.

19. A bonded assembly where two or more substrates are bonded together with an adhesive, wherein the adhesive provides a mechanical fixation of the assembly that comprises pre-deposited polymer balls that have been placed individually or in small clusters at pre-determined locations, and where the balls contribute certain functionalities such as electrical, optical or mechanical or combinations of these.

20. A bonded assembly as claimed in claim 19, wherein one of the substrates is an electronic component and the other is a supporting substrate, and wherein the polymer balls include a metal coating and have been placed individually or in clusters of up to twenty one, more preferably up to seven balls.