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15

Chip Scale Review September • October • 2019

[ChipScaleReview.com]

a key step in this product because the aspect ratio is close to 2:1. After

lithography process optimization, a suitable photoresist is crucial for

such a high-aspect ratio hole. As shown in

Figure 4a

and

Figure 4b

,

no photoresist residues in the opening area can be found. Finally, the

plating Cu process increased the lead thickness to 10μm to ensure a low

resistance circuit. The lead minimum L/S is 20μm/20μm, which should

be controlled within ±3μm.

Following the first passivation process, the second passivation is done

whereby the ball drop layers, and the grind and saw processes are built-up

using the standard WLP processes. The polymer passivation layers were

made to be 10μm thick by the lithography process. The ball diameter is

250μm, while the ball height is 200μm.The final package size is 5000 X

5000μm, and the final package thickness is 600μm, as shown in

Figure 5

.

After intensive optimization work, the yield of the 40GmmWave product

exceeded 98%.

Measurements and results.

The chip is a type of a micrometer

wavelength transceiver that operates in the Ka band—its sweeping

frequency is 2GHz. The receiving and transmitting performance are

strongly affected by packaging. The return loss and transmit power

of the chip are close to the simulated values, as shown in

Table 1

and

Figure 6

.

Figure 4:

Images of the RDL1 fabrication process: a) the RDL patterns; b) the

enlarged view of lithography patterns; c) the overview of the Cu layer; and d) the

minimum line width of the Cu layer.

Figure 5:

a) The front side of a single die; b) The back side view of a single die.

© 2017 Brewer Science, Inc. Compatible with: 308 nm 343 nm 355 nm Creating Safe Environments Laser Release System In the laser release system, the device wafer is bonded to a transparent glass carrier using a bonding material and a release material. Once processing is completed, the pair is separated by exposing the release material with an excimer laser or solid-state laser. Low- stress separation coupled with high throughput make the laser release system suitable for all production environments. www. b r e w e r s c i e n c e . c om Laser Release System Benefits: •Highest-throughput system available with a release time of less than 30 seconds •Ultraviolet laser does not heat or penetrate the bulk bonded structure •Low-stress processing through use of CTE- matched carrier and room temperature separation Laser Release Layer Thin Device Wafer Bonding Material Transparent Carrier

Table 1:

Comparison between simulated and actual values.