Hybrid bonding of thinned semiconductor dies

Abstract

To provide a method for placing a semiconductor die on a substrate and a bonding tool for placing the semiconductor die on the substrate.SOLUTION: When a semiconductor die is placed on a substrate, the die is captured and carried by a die holding surface of a bonding tool having a protrusion. The protrusion of the bonding tool is configured to be movable between a retracted position within the die holding surface and an extended position protruding from the die holding surface. When the protrusion is placed in the extended position, the die bends when the bonding tool is carrying the die. The bonding tool is then moved to flatten the die against the substrate while the substrate urges the protrusion to retract from the extended position toward the retracted position.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a method for placing a semiconductor die on a substrate and to a bonding tool having protrusions for bonding a semiconductor die onto a substrate. [Background technology]

[0002] The copper hybrid bonding process (sometimes called hybrid bonding) provides a bumpless interconnect between the bond pads of a silicon die and a silicon substrate (or between the bond pads of a pair of silicon substrates) by direct copper-to-copper bonding at room temperature for three-dimensional integrated circuit applications.

[0003] However, performing such hybrid bonding between a silicon die and a silicon substrate can encounter several problems when attempting to provide void-free bonding of the die in environments requiring contaminants to be below a particularly low threshold amount. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2021 / 100185 [Patent Document 2] International Publication No. 2018 / 147147 [Patent Document 3] Japanese Patent Publication No. 2020-191457 [Patent Document 4] International Publication No. 2017 / 155002 Summary of the Invention [Problem to be solved by the invention]

[0005] It is therefore an object of the present invention to seek to provide an arrangement that overcomes at least some of the aforementioned problems of the prior art. [Means for solving the problem]

[0006] According to a first aspect of the present invention, there is provided a method of placing a semiconductor die on a substrate, comprising the steps of capturing and carrying the die by a die-holding surface of a bonding tool having a protrusion configured to be movable between a retracted position within the die-holding surface and an extended position protruding from the die-holding surface, the protrusion being disposed in the extended position to bend the die when the bonding tool is carrying the die; and moving the bonding tool to flatten the die against the substrate while the substrate urges the protrusion to retract from the extended position towards the retracted position.

[0007] The first aspect recognizes that a problem with some existing technologies is that the tools used to bend the die to provide the required void-free contact area between the die and the substrate may be unsuitable for use in applications requiring contaminants to be below a certain threshold amount and / or may not be able to contact the die with the substrate in a manner that provides void-free contact across the entire surface of the die.

[0008] Thus, a method is provided. The method may be for placing a die on a substrate. The method may include capturing and carrying or holding the die. The die may be captured and carried by a die-holding surface of a bonding tool. The die-holding surface of the bonding tool may have a protrusion, projection, or extension. The protrusion may be configured to be movable between a retracted position in which the protrusion is stored or maintained within the die-holding surface and an extended position in which the protrusion protrudes or extends from the die-holding surface. The protrusion may be disposed in the extended position to flex, curve, or deform the die when the bonding tool carries the die. The method may include moving or translating the bonding tool to flatten, smooth, or restore the shape of the die relative to the substrate. The movement may urge the protrusion to retract from the extended position toward the retracted position. In this manner, a bonding tool less susceptible to inappropriate levels of contaminants may be provided, with the protrusion causing an initial deformation of the die that provides a required void-free initial contact zone between the die and the substrate. The contact zone can then be enlarged by moving the bonding tool towards the substrate to flatten the die against the substrate while retracting the protrusions into the bonding tool to enlarge the contact front between the substrate and the die, providing void-free contact across the die.

[0009] The moving step can include moving the bonding tool towards the substrate to contact the die against the substrate.

[0010] By placing the protrusions in an extended position, the die can be bent from a planar to a convex configuration. Thus, the protrusions can serve to bend or deform the die from its natural, stress-free, planar configuration to a convex or curved configuration.

[0011] The protrusions can be arranged to bend the die into a convex configuration by displacing a central region of the die relative to a peripheral or edge region of the die, such that the central region can rise toward the substrate compared to the edge of the die.

[0012] The extended position allows the die to bend away from the die-holding surface by a standoff amount.

[0013] The method can include biasing or directing the protrusions to the extended position with a biasing mechanism disposed within the die-retaining surface, such that the protrusions can be initially biased toward the extended position to bend the die.

[0014] Moving the bonding tool to flatten the die against the substrate can include moving the bonding tool toward the substrate to initially contact a central region of the die against the substrate.

[0015] Moving the bonding tool to flatten the die against the substrate overcomes the biasing mechanism and retracts the protrusions to a retracted position, allowing contact between the die and the substrate to propagate from the area corresponding to the location of the protrusions towards the periphery of the die. Thus, pressing the bonding tool towards the substrate brings the die into contact with the substrate, which in turn presses the protrusions into the bonding tool.

[0016] The method can include continuing to move the bonding tool to fully retract the protrusion into the die-holding surface.

[0017] The die-holding surface may be planar.

[0018] Retracting the protrusions completely into the die-holding surface may allow the die to return to a planar configuration.

[0019] The bonding tool continues to move, allowing the die to flatten against the substrate.

[0020] The step of supporting the die may include holding a periphery of the die against a die support surface to facilitate bending the die into the convex configuration with the protrusions.

[0021] The step of carrying the die may include generating a vacuum force to hold a periphery of the die against the die-holding surface.

[0022] The vacuum force holding the periphery of the die can be configured so that it does not overcome the biasing mechanism when the protrusions are in the extended position - in other words, the vacuum created at the edge of the die is insufficient to prevent the protrusions from bending the die.

[0023] The method may include ceasing to hold the periphery of the die against the die-holding surface to allow the die to remain in a planar configuration following contact between the die and the substrate, and allowing contact between the die and the substrate to propagate from the area corresponding to the location of the protrusion towards the periphery of the die.

[0024] According to a second aspect of the present invention, there is provided a bonding tool for placing a semiconductor die on a substrate, the bonding tool including: a die-holding surface for capturing and carrying the die, the die-holding surface having a protrusion configured to be movable between a retracted position within the die-holding surface and an extended position protruding from the die-holding surface, the protrusion being disposed in the extended position to bend the die when the bonding tool is carrying the die; and an actuation mechanism configured to move the bonding tool to flatten the die against the substrate while the substrate urges the protrusion to retract from the extended position towards the retracted position.

[0025] By placing the protrusions in an extended position, the die can be bent from a planar configuration to a convex configuration.

[0026] The protrusions can be positioned to deflect the die into a convex configuration by displacing a central region of the die relative to the periphery of the die.

[0027] The bonding tool may include a biasing mechanism disposed within the die-retaining surface configured to bias the protrusions toward the extended position.

[0028] The actuation mechanism can be configured to move the bonding tool toward the substrate to initially contact a central region of the die against the substrate.

[0029] The actuation mechanism can be configured to flatten the die against the substrate, overcome the biasing mechanism, and retract the protrusion to the retracted position, allowing contact between the die and the substrate to propagate from the area corresponding to the location of the protrusion towards the periphery of the die.

[0030] The actuation mechanism can be configured to continue to move the bonding tool to fully retract the protrusion into the die-holding surface.

[0031] The die-holding surface may be planar.

[0032] The actuation mechanism can be configured to continue to move the bonding tool to fully retract the protrusions into the die-retaining surface and allow the die to return to a planar configuration.

[0033] The actuation mechanism can be configured to continue to move the bonding tool to flatten the die against the substrate.

[0034] The die-holding surface can be configured to support the die by holding a periphery of the die against the die-holding surface to facilitate bending the die into the convex configuration with the protrusions.

[0035] The apparatus can include a vacuum generator configured to generate a vacuum force to hold the periphery of the die against the die-holding surface.

[0036] The vacuum generator may be configured to generate a vacuum force when the protrusion is in the extended position such that the vacuum force does not overcome the biasing mechanism.

[0037] The generated vacuum can be configured to cease generating a vacuum force and cease holding the periphery of the die against the die holding surface, allowing the die to remain in a planar configuration following contact between the die and the substrate, and allowing contact between the die and the substrate to propagate from the area corresponding to the location of the protrusion towards the periphery of the die.

[0038] These and other features, aspects, and advantages will become better understood with regard to the following description, appended claims, and accompanying drawings.

[0039] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0040] [Figure 1] 1 is a cross-sectional view of a flat thin silicon die and a flat silicon substrate. [Figure 2] FIG. 1 is a cross-sectional view of a polished copper bond pad. [Figure 3] FIG. 10 is a bottom view of a bonding tool illustrating protruding tips and vacuum hole distribution. [Figure 4] FIG. 2 is an out-of-plane cross-sectional view of a bonding tool. [Figure 5] FIG. 1 is a cross-sectional view of a bonding tool with a vacuum channel. [Figure 6] 1 is a cross-sectional view of a bonding tool as it captures a thin silicon die and holds the die against its surface by vacuum suction. FIG. [Figure 7] FIG. 1 is a three-dimensional depiction of the convex surface curvature of a thinned silicon die when held by a bonding tool at its central protruding tip. [Figure 8]1 is a cross-sectional view of a bonding tool and a thinned silicon die when the convex surface of the die is lowered onto and just touches the flat surface of the silicon substrate. FIG. [Figure 9] 1 is a cross-sectional view of a bonding tool and a thinned silicon die when the vacuum suction of the tool is turned off so that the thinned silicon die flattens and recovers to its stress-free state. FIG. [Figure 10] 1 is a cross-sectional view of a bonding tool and a thinned silicon die when a normal compressive force is applied to the tool such that the protruding tip is forced back and retracted. DETAILED DESCRIPTION OF THE INVENTION

[0041] In the drawings, like parts are designated by like reference numerals.

[0042] The following may be useful background information to aid in understanding the technology of some embodiments. As described above, the copper hybrid bonding process (sometimes referred to as hybrid bonding) provides bumpless interconnects to bond pads between silicon dies and silicon substrates by direct copper-to-copper bonding at room temperature for three-dimensional integrated circuit applications. The hybrid bonding process is typically a two-step process. First, the silicon die is pre-bonded onto the silicon substrate at room temperature. Second, a subsequent thermal annealing process for the pre-bonded die and substrate is performed in an oven with a specific annealing profile. In the pre-bonding process, the silicon die is held by a bond tool and precisely aligned to its bonding position on the substrate. The substrate can be a silicon wafer, a silicon interposer, or an active silicon die, which is fixed on a work chuck during the pre-bonding process. Pre-bonding is performed by placing the silicon die on the substrate at the exact desired location with an optimized compressive force at room temperature.

[0043] To achieve pad interconnects with a pitch of less than 10 microns, the alignment accuracy for placing the die on the substrate for the pre-bonding process should be 0.5 microns or better. To achieve a suitable bonding interface for the hybrid bonding process, the surfaces of the silicon die and substrate must be very clean, smooth, and particle-free, and these surfaces must be plasma activated. These surfaces are typically polished by chemical mechanical polishing (CMP) to a roughness (RA) of less than 0.5 nanometers on any dielectric surface layer. These surfaces are typically thoroughly cleaned with megasonic-activated deionized water and then activated by nitrogen plasma treatment. When these smooth surfaces are in close proximity (typically less than 100 nanometers apart), the van der Waals attraction between -OH radicals on these surfaces bonds the silicon die and silicon substrate together. A perfect interface can be achieved if both surfaces are particle-free.

[0044] The bonding interface between the silicon die and the substrate consists of a dielectric passivation and copper pads. The dielectric passivation can be provided by silicon oxide or silicon carbonitride. A pre-bonding process brings the bonding surfaces together, and van der Waals forces induce an initial bond at the dielectric passivation interface. This pre-bonded configuration is then placed in an oven for thermal annealing at a specific profile. The bond between the die's dielectric interface and the substrate is converted to a covalent bond during a subsequent thermal annealing process at a first elevated temperature, typically for more than an hour. The copper pads on both surfaces bond together to form a complete metallurgical interconnect between the silicon die and the silicon substrate when the configuration is annealed in the oven at a second temperature, typically higher according to the annealing profile.

[0045] To achieve a perfect bonding interface, a key requirement is to provide particle- and contaminant-free bonding surfaces for both the silicon die and the substrate before they are joined together. However, other conditions can affect the ability to provide a perfect, void-free bonding interface. The material used for the dielectric layer (usually SiO2 or SiCN), the wet process used to remove the protective coating on the singulated silicon wafer, and the parameters used for plasma surface activation play important roles for void-free copper hybrid bonding. The assembly process for bonding typically begins from the center. Contact between the two bonding surfaces begins at the center and then expands. The contact area between the bonding surfaces typically increases radially from the central region toward the periphery until the contact area extends across the entire surface. This can help achieve a void-free, perfect bonding interface for wafer-to-wafer bonding. However, due to the relatively small die size for a given thickness, this bonding process may not be usable for die-to-die and die-to-wafer bonding with standard bonding tools.

[0046] One method for achieving a void-free bond interface utilizes a deformable, convex pickup tool with a substantially convex contact surface. The flexible film or die is captured and held on the deformable tool, held in the convex curvature of the surface by vacuum suction through holes in the tool. When a downward compressive force is applied to the tool while the flexible film or die rests on a flat substrate, the flexible film or die and the deformable tool flatten. However, such deformable pickup tools are typically made of a rubber material that has a low surface roughness and can trap contaminants. Therefore, this approach is not suitable for environments requiring contaminant levels below a threshold. In another approach, the flexible film or die bends to follow the convex curvature of the non-deformable tool. However, in this approach, compressive forces along the peripheral edges at the outer corners of the die are minimal because the tool is not deformable and therefore no direct compressive force acts on these areas.

[0047] Before discussing the embodiments in more detail, an overview will first be provided. Some embodiments provide techniques for placing a die on a substrate. A bonding tool is used to hold the die on a surface of the bonding tool. The surface of the bonding tool has protrusions that can be moved from an extended position, protruding from the die-holding surface to initially bend or flex the die on the bonding tool. The protrusions are moved toward a retracted position to flatten the die on the substrate. Typically, movement of the bonding tool toward the substrate causes the die to contact the substrate, which in turn causes the protrusions to move toward a retracted position as the die flattens on the substrate. This allows an initial area of ​​the die to first contact an adjacent area of ​​the substrate, and this contact area to expand as the protrusions retract into the die-holding surface of the bonding tool. Fully retracting the protrusions allows the bonding tool to apply pressure to the peripheral edge of the die to ensure proper contact with the substrate across the entire surface of the die. This approach allows for reliable initial contact between the die and the substrate with a controlled expanding contact front between the die and the substrate using a bonding tool that is suitable for bonding in environments requiring less than a specified threshold amount of contaminants and that avoids the creation of undesired voids in the contact area between the die and the substrate.

[0048] Thus, some embodiments provide a vacuum pick-and-place tool having a bonding tool with a flat or planar die-holding or contact surface, typically with multiple vacuum holes, and a retractable protruding tip typically located in a central region of the contact surface. A thin die can be captured by the bonding tool using vacuum suction through the vacuum holes. The die is then firmly held on the contact surface by vacuum suction while its central region is pushed out and bent or deformed by the protruding tip. The protruding tip is typically preloaded by a compliant structure that is sufficiently resistant to withstand the compressive force on the die created by the vacuum suction. The protruding tip protrudes or extends from the contact surface of the bonding tool and presses against the contact rear side of the die, forming a convex curvature on the die's bonding surface. This curvature of the bonding surface helps achieve a void-free dielectric bonding interface for hybrid bonding. At the beginning of the bonding process, when the die first contacts the substrate, with minimal contact force during initial adhesion, the convex surface curvature of the die provides an initial minimum contact area between the die and the substrate in the central region, which helps avoid any voids. As the vacuum suction decreases, the die recovers to its stress-free state and flattens on the substrate. The bond front between the die and the substrate grows and propagates toward the edge of the die, closing the bond interface. Additional normal downward compressive force can then be applied to drive the bonding tool toward the substrate until the protruding tip fully retracts into the contact surface of the bonding tool and the flat contact surface of the bonding tool presses the die against the substrate with the appropriate compressive force.

[0049] This configuration avoids the use of rubber or other similar flexible contact surfaces, providing a bonding tool suitable for use in very clean environments where low contamination is required. Additionally, because the protrusions are fully retractable within the bonding tool, this allows the contact surface to provide more uniform compression across the entire bond interface during hybrid bonding, compared to convex bonding tools that cannot provide adequate compression along the edges at the die corners. Therefore, this configuration is more suitable for hybrid bonding of structures with either SiO2 or SiCN interfacial dielectric layers.

[0050] Die and Substrate FIG. 1 is a cross-sectional view showing a flat die 10 and a flat substrate 20 in their stress-free, freestanding, or normal state. In this example, both the die 10 and the substrate 20 are silicon. However, it will be understood that other types of die 10 and substrate 20 can be used in the hybrid bond. Both the die 10 and the substrate 20 include a dielectric passivation layer and a copper bond pad on their bonding surfaces. For clarity, these structures are not shown in this schematic diagram. Typically, the dielectric layer can be Si02 made from a TEOS structure or a SiCN layer, typically having a thickness greater than approximately 200 nanometers. These dielectric bonding surfaces are typically polished by a chemical-mechanical polishing (CMP) process to achieve a very flat surface, typically with a surface roughness (RA) of less than approximately 0.5 nanometers. However, the CMP-polished surface of the copper bond pad 30 is typically recessed below the surface of the dielectric layer with a certain roughness, as shown in FIG. 2. The recession in the polished copper pad 30 is often described as dishing of the copper bond pad 30 due to the CMP process. The dishing depth A typically depends on the diameter of the copper bond pad 30 and the process control of the CMP process. Typical values ​​for the dishing depth are approximately 5-8 nanometers for a copper bond pad having a diameter of 5 micrometers.

[0051] Bonding Tools FIG. 3 schematically illustrates a plan view of a bonding tool 40, according to one embodiment. The bonding tool 40 has a tool base 45 defining a die-holding surface 50. The die-holding surface 50 is flat or planar and sized to receive substantially the entire surface of a die 10 (not shown) to be bonded to a substrate 20 (not shown). That is, the die-holding surface 50 is typically sized to extend substantially at least to the outer peripheral edge of the die 10. A central opening 110 located in a central region of the die-holding surface 50 has a convex tip or protrusion 60 disposed therein. A plurality of vacuum openings 100 are formed around the inner periphery of the die-holding surface 50 to generate a vacuum suction force to hold the die 10.

[0052] FIG. 4 schematically illustrates a cross-sectional view through bonding tool 40. Protrusions 60 are displaceable between an extended position, as shown in FIG. 4, and a retracted position (see FIG. 10) in which protrusions 60 are fully recessed within bonding tool 40. In other words, protrusions 60 do not protrude from die-retaining surface 50 when in the retracted or recessed position. A resilient biasing mechanism 70 is provided within tool base 45 to bias protrusions 60 toward the extended position as shown in FIG. 4. However, the resilient characteristics of biasing mechanism 70 are selected such that an appropriate force on protrusions 60 can overcome the biasing force generated by biasing mechanism 70 against protrusions 60, causing protrusions 60 to move from the extended position to a retracted position within bonding tool 40, as described in more detail below.

[0053] FIG. 5 schematically illustrates another cross-sectional view of bonding tool 40. Tool base 45 defines vacuum conduit 80 connecting vacuum port 90 with a plurality of vacuum openings 100 defined by die-holding surface 50. Thus, tool base 45 can be seen to have a flat bottom surface provided by die-holding surface 50, a convex tip provided by protrusion 60 at its center, and a vacuum hole provided on the flat surface around the protruding tip by vacuum opening 100. As noted above, protrusion 60 is connected to biasing mechanism 70, which acts as a spring. Vacuum opening 100 communicates with a vacuum source via vacuum conduit 80 and vacuum port 90. Any force pushing on protrusion 60 will act to retract protrusion 60 into central opening 110 defined in die-holding surface 50 as biasing mechanism 70 bends inward into interior cavity 120 of tool base 45. Because the deformation of the biasing mechanism 70 is elastic in nature, it will recover to its original configuration when the compressive force is removed, and the protrusions will again be biased to protrude from the die-holding surface 50.

[0054] It will be appreciated, therefore, that when the die 10 is maintained in position on the die-retaining surface 50, the protrusions 60 will be biased by the biasing mechanism 70 toward the extended position, causing the die 10 to bend into a convex configuration with a central portion of the die 10 near the protrusions 60 positioned farthest from the die-retaining surface 50. When the die 10 contacts the substrate 20, the central portion of the die 10 will contact the substrate 20 first, and continued movement of the die 10 relative to the substrate 20 will generate a compressive force that overcomes the force generated by the biasing mechanism 70 and moves the protrusions 60 toward the retracted position. This causes the die 10 to gradually flatten against the substrate 20, and the bond front between the die 10 and the substrate 20 expands from an initial central region toward the outer peripheral edges of the die 10. Eventually, the entire die 10 is fully flattened against the substrate 20 by the die-retaining surface 50, as will be described in more detail below.

[0055] Bonding tool operation 6, following capture of the die 10, the die 10 is held against the die-holding surface 50 by the vacuum suction provided by the vacuum openings 100. However, the die 10 does not lie flat against the die-holding surface 50 due to the action of the protrusions 60 in the central region 130 of the die 10 pushing the central region 130 of the die 10 away from the die-holding surface 50. Typically, the protrusions 60 extend from the die-holding surface 50 by approximately 30 to 40 microns.

[0056] When the die 10 is held against the die-holding surface 50 by vacuum suction force, the protrusions recede slightly to extend approximately 20 microns from the die-holding surface 50. It will be appreciated that the amount by which the protrusions 60 extend from the die-holding surface 50 and the stiffness of the biasing mechanism 70 should not be too great, otherwise it may not be possible to build up sufficient vacuum suction on the chip 10 during the pick-up process, even for dies less than 250 microns thick.

[0057] The die 10 deforms when held by the vacuum on the die-holding surface 50, forming a convex surface curvature similar to that shown in Figure 7 following successful capture by the bonding tool 40. The convex surface curvature created in the central region 130 of the die 10 relative to its peripheral edge 140 facilitates providing a void-free hybrid bonding process.

[0058] As shown in FIG. 8 , as the bonding tool 40 approaches the substrate 20, initial contact is made between the central region 130 of the die 10 and the substrate 20, forming a void-free interfacial bond area. The size of the initial interfacial bond area is determined by the shape and configuration of the protrusions 60, the biasing force generated by the biasing mechanism 70, and the contact force acting on the bonding tool 40. This contact force introduces initial compression into the protrusions 60, thereby reducing their height. Therefore, the contact force should be low enough to ensure that when the die 10, with its convex central region 130, touches and makes initial contact with the substrate 20, the protrusion height H is large enough to maintain a small initial central contact area. The standoff height H can be optimized for the bonding process and is controllable by the compressive force acting on the bonding tool.

[0059] As shown in FIG. 9 , once initial contact is made, the vacuum is typically turned off, which releases the die 10 from the vacuum and allows it to recover to its stress-free form, returning to a flat, planar configuration. As the die 10 flattens, the interfacial contact area between the die 10 and the substrate 20 increases, expanding from the central region 130 and moving the bonding front outward until it reaches the outer peripheral edge 140 of the die 10. This helps achieve a void-free bonding interface between the dielectric layer of the die 10 and the substrate 20.

[0060] 10, a higher compressive force is then applied by bonding tool 40. This causes protrusion 60 to further overcome the biasing force generated by biasing mechanism 70, causing protrusion 60 to retract completely into tool base 45 so that it no longer protrudes beyond die-retaining surface 50. Die-retaining surface 50 can then apply a uniform bonding pressure across substantially the entire surface of die 10. Once the pre-bonding process is complete, a subsequent thermal annealing process can then occur.

[0061] This configuration allows the height of the protrusions 60 above the die-retaining surface 50 to be controlled during bonding by varying the contact force when the bonding surface of the die 10 makes initial contact with the substrate 20. This allows for optimization of the bond profile and achieves a void-free bond interface for the hybrid bonding process. Additionally, when there is a sufficient amount of compressive force between the die 10 and the substrate 20, the protrusions 60 can be fully retracted into the bonding tool to provide uniform bonding pressure and aid in completing the pre-bonding process.

[0062] It can thus be seen that some embodiments provide a copper hybrid bonding process used to achieve a bumpless interconnect between the bond pads of a silicon die 10 and a silicon substrate 20 through direct copper-to-copper bonding at room temperature. Pre-bonding is achieved by placing the die 10 onto the substrate 20 with an optimized compressive force at room temperature. Contact between the two bonding surfaces begins at the center of the die 10 and then expands. The contact surface area increases radially from the center to the periphery of the die 10 until the contact surface covers the entire die 10. The bonding tool 40 of some embodiments has a flat contact surface 50 with multiple vacuum holes 100 and a retractable protruding tip 60 at its center. The die 10 can then be firmly held onto the flat contact surface 50 of the tool 40 by vacuum suction while its central region 130 is pushed out by the central protruding tip 60 of the tool. The protruding tip 60 is preloaded by a compliant or biasing structure strong enough to withstand the compressive force of the die 10 created by the vacuum suction. The protruding tips 60 protrude from the flat surface 50 of the tool 40 and press against the backside of the die 10, forming a convex curvature on the bonding side of the die 10. When the vacuum suction of the bonding tool 40 is turned off, the die 10 will restore to its stress-free state and lie flat on the silicon substrate. During bonding, the bond front will grow and propagate toward the peripheral edge 140 of the die 10 to completely close the bonding interface, while the protruding tips 60 can retract into the tool 40 due to the compressive bonding force.

[0063] Although the present invention has been described in considerable detail with reference to certain embodiments, other embodiments are possible.

[0064] Therefore, the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein. [Explanation of symbols]

[0065] 10 Die 20 PCB 30 bond pads 40 Bonding Tools 45 Tool Base 50 die holding surface 60 protrusion 70 Biasing mechanism 80 Vacuum conduit 90 Vacuum port 100 vacuum opening 110 Center opening 120 Cavity 130 Central area 140 Peripheral edge

Claims

1. 1. A method for placing a semiconductor die on a substrate, comprising: capturing and carrying the die with a die-retaining surface of a bonding tool having a protrusion, the protrusion being resiliently attached to the bonding tool via a resilient mechanism and configured to be resiliently movable between a retracted position within the die-retaining surface and an extended position protruding from the die-retaining surface, the resilient mechanism generating a biasing force on the protrusion when the bonding tool is carrying the die, urging the protrusion toward the extended position to bend the die; moving the bonding tool to flatten the die against the substrate while the substrate biases the protrusions to retract from the extended position toward the retracted position, the protrusions being configured to retract in response to a force exerted on them by the substrate to overcome a biasing force generated by the elastic mechanism as the die is flattened against the substrate; A method comprising:

2. The method of claim 1 , wherein placing the protrusion in the extended position causes the die to bend from a planar configuration to a convex configuration.

3. The method of claim 2 , wherein the protrusions are arranged to deflect the die into the convex configuration by displacing a central region of the die relative to a periphery of the die.

4. The method of claim 1 , wherein the resilient mechanism comprises a biasing mechanism disposed within the die-retaining surface to bias the protrusion toward the extended position.

5. 10. The method of claim 1, wherein moving the bonding tool to flatten the die against the substrate comprises moving the bonding tool toward the substrate to initially contact a central region of the die against the substrate.

6. 5. The method of claim 4, wherein moving the bonding tool to flatten the die against the substrate overcomes the biasing mechanism and causes the protrusion to retract to the retracted position, allowing contact between the die and the substrate to propagate from an area corresponding to the location of the protrusion toward a periphery of the die.

7. The method of claim 1 , further comprising the step of continuing to move the bonding tool to fully retract the protrusion into the die-holding surface.

8. The method of claim 1 , wherein the die-holding surface is a flat surface.

9. The method of claim 1 , wherein the die is allowed to return to a planar configuration by fully retracting the protrusion into the die-retaining surface.

10. The method of claim 7 , wherein continuing to move the bonding tool causes the die to flatten against the substrate.

11. The method of claim 1 , wherein supporting the die includes holding a periphery of the die against the die-holding surface to facilitate bending the die into a convex configuration by the protrusions.

12. The method of claim 1 , wherein supporting the die comprises generating a vacuum force to hold a periphery of the die against the die-holding surface.

13. The method of claim 12 , wherein the vacuum force holding the periphery of the die is configured not to overcome a biasing mechanism when the protrusions are in the extended position.

14. 12. The method of claim 11, comprising ceasing to hold the periphery of the die against the die-holding surface to allow the die to remain in a planar configuration following contact between the die and the substrate, and allowing contact between the die and the substrate to propagate from an area corresponding to the location of the protrusion towards the periphery of the die.

15. 1. A bonding tool for placing a semiconductor die on a substrate, comprising: a die-holding surface for capturing and carrying the die, the die-holding surface being resiliently attached to the bonding tool via a resilient mechanism and having a protrusion configured to be resiliently movable between a retracted position within the die-holding surface and an extended position protruding from the die-holding surface, the resilient mechanism acting to generate a biasing force on the protrusion when the bonding tool is carrying the die, urging the protrusion toward the extended position to bend the die; an actuation mechanism configured to move the bonding tool to flatten the die against the substrate while the substrate biases the protrusions to retract from the extended position toward the retracted position, the protrusions being configured to retract in response to a force exerted on the protrusions by the substrate to overcome a biasing force generated by the elastic mechanism when the die is flattened against the substrate; and A bonding tool comprising:

Images