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14

Chip Scale Review September • October • 2019

[ChipScaleReview.com]

to the periphery of the reconstructed chip, one RDL layer is designed

and produced. The minimum L/S is 20μm/20μm, which requires high

accuracy lithography technology.

Process flow.

The manufacturing flow is shown in

Figure 1

.

First, the silicon wafers containing the shielding layer and silicon

cavities were formed. Second, the thinned known good dies (KGDs)

from device wafers were embedded into the cavities. The narrow

trenches between the chips and sidewalls of the cavities were filled

through a vacuum film lamination process. Meanwhile, the pads

of the embedded die were opened using a lithography process. In

order to improve the performance of the device, the thickness of

the dry film is increased compared to the common eSiFO

®

. At this

point, a new wafer is reconstructed with no significant differences

from the ones by a standard WLP process. The following process

flow is similar with the WLP processes, such as the RDL formation

and ball drop. The difficulties for the product were how to open the

pads of the embedded die in the RDL process and how to ensure the

continuity of the seed layer in the opening hole (30μm) of the thick

first-passivation layer, which has a high aspect ratio (≥1.5:1).

In this project, a 12-inch blank wafer was used as the carrier

wafer. The metal layer (Cu) is used for a shielding cap so that the

signal cannot transmit to the silicon and be reduced.The 1μm Cu

layer was deposited by physical vapor deposition (PVD) rather than

the Cu plating process because the thickness is enough according

to the electromagnetic simulation results. Additionally, the thick Cu

may cause stress and wafer warpage. Then the cavities were formed

by lithography and the wet-etch metal process. In order to decrease

the stress of the large-area metal layer, the scribe lines were opened.

The cavity formation has two steps, including oxide etch and silicon

etch processes. To avoid the tilt and cracking of die during the pick and

place process, a smooth bottom surface without any salient point is critical

for this product. After process optimization, we found that having no

oxide and no photoresist (PR) residual are essential to enable a smooth

bottom surface after the oxide etch process. Therefore, two effective

measures should be carried out, including the oxygen plasma process after

lithography and the oxide etch process with extra oxide etch cycles. The

cavities were formed using the Bosch process. The specification of the

cavities is 104μmwith a TTV of ±7μm.

Because die attach has a huge influence on the pad exposure and

final yield, it is a significant process needed for 40GHz mmWave chip

packaging that uses eSiFO® technology. We used a dedicated die attach

tool from ASM, called NUCLEUS, for the attachment; the tool has a

high accuracy of ±5μm.

Figure 2

shows the image after attachment. The

die shift and rotation of die attach is below 5μm, which meets the design

requirement.

The formation of the first passivation layer is another essential step in

the 40GHz mmWave chip packaging process flow. In this production,

we used a vacuum lamination process with the photo-patternable dry

film. The narrow trenches between the dies and sidewalls of the cavities

are completely filled, as shown in

Figure 3c

. The dry film material

has a big influence on the warpage and the electrical performance. After

development process optimization, the pads of the embedded chip are

opened completely, as shown in

Figure 3

. As a result, the pads were well

connected with the RDL.

After the first passivation process is formation of the RDL. The RDL

formation includes three main processes: 1) seed layer formation,2)

patterning lithography, and 3) Cu layer plating. A thick seed layer with

a 0.3μm metal Ti layer and a 0.5μm metal Cu layer were deposited by

PVD in order to ensure that the seed layer in the high-aspect ratio hole

maintained continuity. The development of patterning lithography is

Figure 1:

The process flow of 40GHz mmWave chip packaging.

Figure 2:

The whole wafer image after the die attach process.

Figure 3:

a) The optical microscopy image after the first passivation process; b)

The optical microscopy image with CD dimension; c) the cross-sectional SEM image

of the filled trench filling and the first passivation layer; and d) the SEM image of the

first RDL metal connecting the pads on the embedded chip.