This invention relates to a method for forming and releasing interconnects and more particularly, though not exclusively, relates to such a method using a dummy substrate with there being a reduced release force to release the interconnects from the dummy substrate.
With trend towards ever finer interconnection feature size and the resultant increase in the effective stress in the interconnects, it has become increasingly challenging for interconnection to meet temperature cycling and drop-impact requirements. At the same time, the introduction of relatively delicate Cu/low-k chips imposes constraints on the level of allowable packaging stress. One solution is to increase the compliance of the interconnection so as to reduce the interconnect stress such as by, for example, using wafer-level stretched interconnections with an hourglass shape. The reliability of the stretched interconnect may be further enhanced by deforming the stretched interconnect into, for example, a spiral or stretched-spring shape.
A key challenge for wafer-level stretched interconnects is the release of the stretched interconnects from a dummy substrate without deforming the delicate, stretched solder columns, especially the stretched-spring columns; and without damaging the delicate Cu/low-k structure. The release method is also preferably capable of high-volume manufacturing while retaining a strong joint between the stretched interconnect and the substrate to which the interconnects are attached.
According to an exemplary aspect, there is provided a method for forming and releasing interconnects by using a dummy substrate, the method comprising: applying metallization to the dummy substrate for creating a relatively strong bond between the metallization and the dummy substrate and a weak bond between a first end of each of the interconnects and the metallization; weakly bonding the first ends to the metallization; shaping the interconnects; and releasing the weak bond between the metallization and the first ends by using a reduced release force to release the first end of the interconnects from the dummy substrate.
The reduced release force may be applied to a fixture attached to the dummy substrate. Brittle fracture may be created in joints between the first ends and the metallization. The fixture may be attached to the dummy substrate by at least one of: bonding, glueing, and use of a thermoplastic.
The metallization may be a combination of metallization layers on the dummy substrate. The release may be along an intermetallic boundary.
The combination of metallization layers may be applied by applying a second layer to the dummy substrate that adheres to the dummy substrate, and applying a first layer to the second layer. The first layer may provide a weak bond with the first ends and a strong bond to the second layer. The first layer may wet with the first ends of the interconnects.
A thermal oxide layer may be applied to the dummy substrate before the second layer. The thermal oxide layer may provide a relatively high strength bond with the dummy substrate. The second layer may have a relatively high strength bond with the thermal oxide layer.
Each of the first ends of the interconnects may have an effective area where they may be bonded to the metallization that may be reduced in comparison with a second end of the interconnects where the interconnects may be attached to a functional substrate.
Reinforcing may be formed around the functional substrate and the second ends. The first ends may have a reduced contact area on the metallization.
Each of the first ends of the interconnects may have a cavity after shaping the interconnects.
Each of the first ends of the interconnects may be profiled to increase the circumference-to-area ratio of each of the first ends.
The profiling of the first ends may be to form a stress riser to facilitate initiation of cracks in the first ends to assist release of the first ends from the metallization. An outer contour of each of the first ends may be maintained. An interior area of the first ends may be reduced.
The fixture may be rigid and the reduced release force may be an impact force provided on the rigid fixture.
The fixture may be flexible and the reduced release force may be applied to edges of the flexible fixture. The force may be at least one of: reciprocating, and alternating.
A plurality of vias may be formed in the dummy substrate prior to metallization. The metallization may be around the vias.
A chemical etchant may be used to perform chemical etching of the first ends through a cavity in the first ends and through the vias so as to weaken attachment of the first ends to the metallization.
According to another exemplary aspect there is provided a method for forming shaped interconnects by using a dummy substrate, the method comprising: shaping a first end of each interconnect to have a reduced contact area with the dummy substrate, the reduced contact area providing stress concentration for facilitating brittle fracture of the attachment of the first ends with the dummy substrate; weakly bonding the first ends to the dummy substrate; shaping the interconnects; releasing the weak bond between the metallization and the first ends by using a reduced release force to release the first end of the interconnects from the dummy substrate.
Each of the first ends of the interconnects may have an effective area where they may be attached to the dummy substrate that may be reduced in comparison with a second end of the interconnects where the interconnects may be attached to a functional substrate. Reinforcing may be formed around the functional substrate and the second ends.
Each of the first ends of the interconnects may be profiled to increase the circumference-to-area ratio of each of the ends. The profiling of the first ends may be to form a stress riser to facilitate initiation of cracks in the first ends to assist release of the first ends from the dummy substrate. An outer contour of each of the first ends may be maintained. An interior area of the first ends may be reduced.
A fixture may be placed over the dummy substrate and a force provided on the fixture to create brittle fracture in the joints between the first ends and the dummy substrate. The fixture may be a flexible fixture. The force may be at least one of: reciprocating, and alternating. A plurality of vias may be formed in the dummy substrate prior to attachment of the first ends.
A chemical etchant may be used to perform chemical etching of the first ends through a cavity in the first ends and through the vias to weaken attachment of the first ends to the dummy substrate.
For both aspects, after release the first ends may be bonded to a third substrate using a bonding agent. The cavity may be filled with the bonding agent during the bonding to the third substrate.
According to a further exemplary aspect there is provided a method for forming shaped interconnects by using a dummy substrate, the method comprising: profiling a first end of each of the interconnects to increase the circumference-to-area ratio of each of the first ends and to form a stress riser to facilitate initiation of cracks in the first ends to assist release of the first ends from the dummy substrate; weakly bonding the first ends to the metallization; shaping the interconnects; releasing the weak bond between the metallization and the first ends by using a reduced release force to release the first end of the interconnects from the dummy substrate.
Each of the first ends of the interconnects may have an effective area where they may be attached to the dummy substrate. The first ends may be reduced in comparison with a second end of the interconnects where the interconnects may be attached to a functional substrate. The first ends may be also shaped to have a reduced contact area with the dummy substrate. The reduced contact area may provide stress concentration for facilitating brittle fracture of the attachment of the first ends with the dummy substrate. Reinforcement may be formed around the functional substrate and the second ends. An outer contour of each of the first ends may be maintained. An interior area of the first ends may be reduced.
A rigid fixture may be placed over the dummy substrate. An impact force may be provided on the rigid fixture to create brittle fracture in the joints between the first ends and the dummy substrate.
A flexible fixture may be attached to the dummy substrate. A force may be applied to edges of the flexible fixture to create brittle fracture in the joints between the first ends and the dummy substrate. The force may be at least one of: reciprocating, and alternating.
A plurality of vias may be formed in the dummy substrate prior to attachment of the first ends. A chemical etchant may be used to perform chemical etching of the first ends through a cavity in the first ends and through the vias to weaken attachment of the first ends to the dummy substrate.
For all foregoing aspects, after release the first ends may be bonded to a third substrate using a bonding agent. The cavity may be filled with the bonding agent during the bonding to the third substrate.
According to a penultimate exemplary aspect there is provided a dummy substrate comprising at least one metallization layer having a relatively strong bond between the metallization and the dummy substrate and may be for providing a weak bond between a first end of each of a plurality of interconnects and the metallization.
The at least one metallization layer may be a combination of metallization layers on the dummy substrate. The combination of metallization layers may comprise a first layer that provides a weak bond with the first ends and a strong bond to a second layer. The second layer may have a relatively strong bond with the dummy substrate. The first layer may wet with the first ends of the interconnects. A thermal oxide layer may be between the dummy substrate and the second layer. The thermal oxide layer may provide a relatively high strength bond with the dummy substrate. The second layer may have a relatively high strength bond with the thermal oxide layer.
The dummy substrate may further comprise a plurality of vias. The at least one metallization layer may be around the vias.
According to a final exemplary aspect there is provided a package comprising a substrate and a first die connected and spaced apart by shaped interconnects formed by one of the methods given above.
The package may further comprise a second die between the substrate and the first die. The second die may be connected to the first die. The second die may be connected to the first die by connections selected from: solder balls and low temperature bonding. The second die and the first die may be connected and spaced apart by another set of shaped interconnects formed by one of the methods given above. The package may further comprise a third die between the second die and the first die. The third die may be connected to the first die by connections selected from: solder balls and low temperature bonding. The package may further comprise a second die connected to and spaced apart from the first die by another set of shaped interconnects formed by one of the methods given above. There may be a third die between the second die and the first die. The third die may be connected to the second die; and a fourth die between the first die and the substrate. The fourth die may be connected to the first die. The third die may be connected to the second die. The fourth die may be connected to the first die. Connections may be selected from: solder balls and low temperature bonding.
In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings.
In the drawings:
The exemplary embodiment relates to a method of separating a plurality of interconnects extending between two substrates. The interconnects may have been elongated or deformed into a specific shape such as, for example, hourglass and/or spiral and/or spring. Alternatively, the interconnects may not have been deformed. The interconnects may be of any suitable form including, but not limited to: hourglass, spiral, spring, post, Cu post and solder, solder only, ball, and so forth.
The substrates may be a pair of silicon wafers, a silicon wafer and a planar carrier, a plural of IC components and a planar carrier, or an IC component and a printed circuit board; and so forth.
In general, one of the substrates may contain active devices while the other may be a dummy. The dummy may be subsequently released from the interconnects.
The method may also be used in a solder transfer process to separate solder from a decal carrier after the solder has been transferred to a target structure. For the purposes of this application, the term dummy substrate is taken to include a decal carrier, while the term interconnect is taken to include solder that has bonded with the target structure.
In an exemplary case, the substrate with the interconnects may be singulated into smaller components. The smaller components may subsequently be attached to a third substrate using the released end of the interconnects. A bonding agent may be used for the attachment.
Mechanical separation offers a simple and inexpensive method of releasing the dummy substrate from the interconnects. However, the separation force is preferably minimised to prevent damage to the stretched interconnects, the substrate, and any delicate interface such as, for example, a Cu/low-k interface. This may be achieved through the use of one or more of:
Weak metallization is a combination of sputtered metallization layers on the dummy substrate. This creates a weak bond between the interconnects and the sputtered metallization layers. The weak bond contributes to the ease of separation of the interconnects from the dummy substrate without damage to the interconnects. The separation may be along the intermetallic boundary.
The metal layers typically consists of a first layer (or wetting layer) that wets with the interconnect solder, and a second layer (or adhesion layer) that adheres the first layer to the substrate. The detached interconnects should be able to be attached to a third substrate using a bonding agent. As such, the detached surface of the interconnects are preferably able to wet readily with the bonding agent.
A number of metallizations and thicknesses have been evaluated. For example, Cr of 200 A as an adhesion layer and Au of 500 A as wetting layer have been identified to give satisfactory results. Other suitable metals may be used. Both adhesion and wetting layer in the weak metallization may range from 0 to 500 A depending on the interconnect material used.
Described below are some exemplary methods and results for designing/selecting/verifying the weak metallization. Other methods may be used. A test specimen is shown in
The test matrix and the test results for the shear and pull test are tabulated in Table 2 and Table 3 respectively.
The Cr (200 A)-Au (500 A) metallization system gave the best results in that it gave the lowest shear and pull strength; and the desired failure interface when using SnCu as the interconnect material.
As shown in
However, excessive reduction of the area of the ends 101 may lead to poor shape formation of the interconnects 102. To address this problem, exemplary pad designs as shown in
All three examples of
A joint between an end 101 of an interconnect 102 and the dummy substrate 103 may exhibit brittle fracture upon impact at high strain rate. As such, the strength of the joint decreases with an increase in the strain rate. Three methods of utilizing this tendency to brittle fracture for the separation of the dummy substrate 103 from the interconnects 102 are illustrated in
These three exemplary techniques shown in
These three techniques require attachment of the dummy substrate 103 to the fixture 315, 317, 319 thereby avoiding clamping stress. This may be by bonding, glueing, or use of a thermoplastic. They are also able to be used with dummy substrates 103 of any size. The methods may be performed at a wafer level.
Reinforcement 326 may be provided over the functional substrate 105 and second ends 104 as a form of strengthening of the joints between the second ends 104 and the functional substrate 105 during the detachment process. This may be by use of known encapsulation techniques. An example of an encapsulation process is shown in
During the process, the fixture 327 is lowered so that the functional substrate 105 is immersed in the reinforcement material 326, as shown in FIG. 3A(ii). It is important that the reinforcement material 326 encapsulates the entire functional substrate 105. However, the interconnects 102 should only be partially encapsulated at the second ends 104. When the reinforcement material 326 has solidified, the encapsulated functional substrate 105 and second ends 104 are released in the direction shown by the arrow 330 by pushing the releasing pins 328 in the direction shown by arrows 332. Examples of reinforcement material 326 that may be used include moulding compound, underfill, flux, wax, and thermoset plastics.
Encapsulation may also help to protect the die after detachment and singulation of the functional substrate 105 into individual dies. This may improve package reliability in case of drop or temperature cycling, for example.
In an exemplary solder transfer process, to detach a decal carrier from solder that has been transferred to a target structure, the separation examples using the flexible fixtures 317 and 319 are suitable. FIG. 3B(i) shows a decal 362 carrying solder 364 and a target structure 366. Solder 364 from the decal carrier 362 is transferred to the target structure 366 using a standard solder transfer step, as shown in FIG. 3B(ii). Depending on the choice of separation technique, a flexible fixture 317 or 319 is then attached to the decal carrier 362 as shown in FIG. 3B(iii), and an appropriate force 380 is applied to the flexible fixture 317 or 319 as described above. FIG. 3B(iv) shows the decal carrier 362 successfully detached from the transferred solder 368, achieving increased solder 368 on the target structure 366.
If desired or required, chemical etching could be used before mechanical release to weaken the attachment of the interconnects 102 to the dummy substrate 103. Chemical etching is especially effective for those interconnects 102 having ends 101 with a reduced interior pad area design (e.g. as shown in
To further enhance the effectiveness of chemical etching prior to mechanical release, through vias can be fabricated on the dummy substrate 103, as shown in
The through vias 421 leading to the interior of the pad area allows the chemical etchant to reach the interior of the ends 101 through the vias 421 to pre-crack the weak metallization layers 423 so as to facilitate subsequent mechanical release.
The formation of the through vias, as shown in
As shown in
Release for 20 mm by 20 mm chips is achievable for aspect ratios of 2 and 3. The pad size and pitch used were 300 μm and 600 μm respectively.
Various package designs as shown in
In the space 80, another die 90 can be bonded to the first die 105 using various types of interconnects, for example, the solder balls 92 shown in
Stacked dies may be formed as shown in
The high standoff provided by the stretch solders 102 therefore provides great flexibility in package design.
Whilst there has been described in the foregoing description exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations in details of design, construction and/or operation may be made without departing from the present invention.