2 3D Packaging Architectures and Assembly Process Design
33
2.4 Assembly Process Flows
3D stacks can be assembled using 3 different approaches. They are commonly
referred to as Wafer-to-Wafer (W2W) attach [68, 69], Die-to-Wafer (D2W) attach
[70–72] or Die-to-Die (D2D) attach.
1. Wafer-to-Wafer Attach (W2W)
In this process, entire wafers are aligned and then bonded, followed by the singulation
of individual die stacks. Two W2W flows are schematically illustrated in Fig. 2.13.
• Back-to-Face Flow: where the inactive side (back) of the top wafer is bonded to the
active (front) side of the bottom wafer. This approach can enable multiple wafers
with TSV’s to be bonded one after the other, while retaining the back-to-face
connectivity between the die.
• Face-to-Face Flow: where the active side of the top wafer is attached to the active
side of the bottom wafer. This approach is less valued for stacking more than 2
wafers because of asymmetry.
A significant advantage of a W2W process is that both bonding surfaces are
extremely flat, and thus there can be excellent wafer to wafer alignment (<3 µm across
the wafer) and hence very fine interconnect pitches can be achieved. In the case of
solder based interconnects, pitch scaling in W2W attach is limited by solder bridging
between interconnects and the implications of intermetallic compound formation.
Another advantage of the W2W process is that it is a batch process with high manufacturing throughput. Key limitations of the wafer W2W process include: (1) all the
die have to be of the same size; (2) the process precludes the ability to bond known
Fig. 2.13 Schematic describing the key steps in a W2W attach process
33
2.4 Assembly Process Flows
3D stacks can be assembled using 3 different approaches. They are commonly
referred to as Wafer-to-Wafer (W2W) attach [68, 69], Die-to-Wafer (D2W) attach
[70–72] or Die-to-Die (D2D) attach.
1. Wafer-to-Wafer Attach (W2W)
In this process, entire wafers are aligned and then bonded, followed by the singulation
of individual die stacks. Two W2W flows are schematically illustrated in Fig. 2.13.
• Back-to-Face Flow: where the inactive side (back) of the top wafer is bonded to the
active (front) side of the bottom wafer. This approach can enable multiple wafers
with TSV’s to be bonded one after the other, while retaining the back-to-face
connectivity between the die.
• Face-to-Face Flow: where the active side of the top wafer is attached to the active
side of the bottom wafer. This approach is less valued for stacking more than 2
wafers because of asymmetry.
A significant advantage of a W2W process is that both bonding surfaces are
extremely flat, and thus there can be excellent wafer to wafer alignment (<3 µm across
the wafer) and hence very fine interconnect pitches can be achieved. In the case of
solder based interconnects, pitch scaling in W2W attach is limited by solder bridging
between interconnects and the implications of intermetallic compound formation.
Another advantage of the W2W process is that it is a batch process with high manufacturing throughput. Key limitations of the wafer W2W process include: (1) all the
die have to be of the same size; (2) the process precludes the ability to bond known
Fig. 2.13 Schematic describing the key steps in a W2W attach process