Page 12 - Chip Scale Review_January-February_2024-digital

TECHNOLOGY TRENDS

The great lithography debate: Copper clad

laminate or glass substrates?

By Doug Brown [Onto Innovation]

F or many in the semiconductor industry, the future is limits will require process innovation and additional lithography

steps. For instance, laser-drilled vias in ABF will not support the less
clear. Glass substrates are destined to play an important
role in advanced packaging. Compared to organic
substrates, glass offers better thermal management, than 10µm via dimension requirements needed for state-of-the-art
advanced packages with RDLs of 2/2µm l/s. The alternative process
enhanced electrical properties, new form factor possibilities and will be either photo-imageable dielectrics (PID) or ABF with a
improved conductor routing. All of which make glass substrates lithography patterned hardmask.
an innovative advanced packaging option for artificial intelligence The benefits of using lithography for both RDL and via layers are
(AI) and high-performance computing (HPC). significant. Not only will the overlay between via layers and RDL
One thing is certain—the glass substrate future is poised to arrive be improved, the via dimension could easily be reduced to less than
later than originally expected. Previously, many in the industry 5µm. And by using lithography to pattern both the via and RDL
thought that the transition from copper clad laminate (CCL) structures, it will be possible to shrink the design rules of the via to
substrates to glass substrates would occur when redistribution the RDL landing pad, which currently limits package design rules,
layers (RDLs) shrank below 5/5µm line/space (l/s). But with resulting in low interconnect density and additional RDL layers. At
today’s equipment and processing solutions, the life of CCL in the moment, the via to RDL landing pad dimensions for 9/12µm RDL
substrates may be extended beyond the 5/5µm l/s demarcation are more than 50µm, with a contingency for overlay errors between
point and reach 2/2µm l/s. the via and the pad of more than 10µm. If overlay were improved by
Whether or not manufacturers should use substrates with CCL utilizing lithography for both RDL and vias, these dimensions could
(Figure 1) or glass substrates is likely to be the subject of debate shrink significantly. However, this process adjustment would require
for years to come. In fact, the argument may only be resolved a few additional steps. Regardless, the benefits would reduce overall
when one substrate, either those with CCL or made from glass, costs, improve yield and, most importantly, extend the CCL roadmap
reaches the 2/2µm l/s finish line first—and does so while offering to 2/2µm l/s.
more reliability and lower cost. Until then, let’s explore the pros Another lithography challenge for the 2/2µm l/s goal involves
and cons of CCL and glass substrates. photoresist. Currently, the majority of AICS CCL manufacturers use
dry-film negative-tone photoresist. This photoresist is laminated to
the substrate and works well with larger RDL structures. However, at
the 2/2µm l/s RDL node, a positive tone material would provide better
resolution and process latitude. At this time, most positive photoresist
is liquid and will require slit coating, or spray coating, unless a dry-
film laminated version can provide the same imaging performance.
The AICS glass argument
When AICS CCL manufacturers identified the instability of
CCL substrates and RDL design rule limits with their existing
processes, they singled out glass as an attractive alternative
(Figure 2). As it stands, glass has several selling points over CCL.
At the top: glass provides a flat and distortion-free surface on which
to build RDL and micro vias. The benefit here is that it enables even
smaller features to be defined.
Glass, however, comes with its own set of challenges. Number
Figure 1: Copper clad laminate substrate. one, it is fragile. This is especially true when it comes to the large
The CCL argument panel sizes being employed today (510mm x 515mm and 600mm x
With most advanced IC substrates (AICS) currently using CCL 600mm) in advanced packages. Another drawback: glass substrates
and Ajinomoto build-up film (ABF), there is significant industry are also very thin. In some cases, less than 100µm. Given the
momentum to propel CCL technology to its full potential of fragility and thinness of glass substrates, sophisticated handling
2/2µm l/s. After all, CCL has some significant advantages for AICS. equipment will be required to process glass substrates through the
For one, CCL’s properties and limitations are well understood. Two, various process steps to reduce the risk of breakage.
CCL is robust and nearly indestructible. But pushing AICS with From a lithography perspective, a number of issues encountered
CCL beyond the current RDL line/space requirements and overlay with CCL can be immediately resolved by opting for glass

10 Chip Scale Review January • February • 2024 [ChipScaleReview.com]
10