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Figure 2: Multi-layer RDL formation in the original
PID example with 4µm L/S/ structures.
The PID materials are negative tone,
so the UV exposed area of the material
u nde rgoe s t he phot ore a ct ion by
radical polymerization and becomes
insoluble to developer chemicals. For
developer chemicals, weak alkaline Figure 3: Filler size vs. PID resolution.
aqueous solutions such as Na 2 CO 3 and
tetramethyl ammonium hydroxide
( T M A H ) a r e u s e d . Fi n a l ly, U V
cure with an excess amount of UV
irradiation, and thermal cure at 180°C,
are applied to complete photo- and
thermal-polymerization reactions,
respect ively. T he PI D mater ials
are dry-film format, and have high
resolution and low CTE.
Many of conventional photodielectric
materials are in liquid format, and
they conformally cover the Cu wirings
u nder neath, and for m an u neven
surface after the coating process.
Such an uneven surface of dielectric Figure 4: Challenges of the original PID example: a) Micro-cracks after acetone immersion; b) Cu migration
layer limits the fine-pitch wiring with L/S=10/10µm after 100 hours of BHAST.
formation on top. On the other hand, low CTE that is quite important for However, the original PID had two
PID materials in this study are in a high reliability with reduced internal technical challenges for high-density
dry-film format, and a very flat surface stress in packaging applications. A packaging applications, including
is attained after vacuum lamination, reduction in CTE of dielectric materials FOWLP, FOPLP, and high-density
even if there are Cu wiring structures is generally attained by addition of package substrates (Figure 4): 1)
under the PID layer. Therefore, it is inorganic filler particles. Meanwhile, limited resistance to organic solvents
beneficial for fine-Cu patterning, addition of inorganic filler increases for photoresist st r ipping; and 2)
especially for mu lt i-layer R DL. UV light scattering at the inorganic- insufficient BHAST reliability with
Figure 2 shows a cross section of organic interface, thereby exacerbating very fine-pitch Cu wirings.
the demonstrated multi-layered RDL the photolithography resolution of For fine-pitch Cu patterning by
structure on the PID material. The PID. To overcome this trade-off, nano- semi-additive processing (SAP), the
highly uniform PID surface on top sized filler was applied to minimize latest photoresist materials with ultra-
of the underlying Cu wirings made it Mie scattering at a UV exposure high resolution are applied. After Cu
possible to form 4µm L/S structures on wavelength (365nm). Consequently, the plating processes, these photoresist
top of the PID without any additional PID successfully enables both low CTE materials are stripped off by organic
planarization processes. down to 35-45ppm/°C, and down to solvents. However, the original PID
The PID materials have excellent 6µm via resolution (Figure 3). experienced severe damage by organic
patternability and can form <10µm vias A dry-film format with low CTE solvents, especially when acetone was
by photolithography. In addition to the and high resolution are quite useful used as a stripping agent. As a result,
high resolution, the PID materials have features for IC packaging applications. PID materials had microcracks after

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