Page 52 - Chip Scale Review_March April_2022-digital

be used and the thermal budget for the
overall processes.
In this paper, a wafer-level polymer/ Therefore, the thermal budget of the
metal hybrid bonding is demonstrated
by using a developmental low-curing- at 250°C. In addition, the merits of
Table 1: Material properties of photosensitive permanent bonding material (PS PBM). Compared to most polymeric bonding
Figure 2: a) Ramp and b) isothermal TGA scan images for PS PBM under nitrogen (with a ramp of 10°C/min).
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temperature photosensitive permanent the photosensitive permanent bonding
bonding material as the dielectric. material including low dielectric constant
and dissipation factor, superior thermal
integration process can be controlled stability, low processing and curing
temperatures, and excellent bonding
strength, make it an attractive candidate
for the future development of polymer/
metal hybrid bonding to replace the
current oxide/metal hybrid bonding.
Photosensitive permanent bonding
material (PS PBM)
A developmental photosensitive
permanent bonding material (PS PBM) is
proposed by Brewer Science, Inc. for the
polymer/metal hybrid bonding application.
The PS PBM can be coated at various film
thicknesses, ranging from 3μm to 20μm
in a single coat, which is good to cover
most bumps or other surface topographies
without causing much stress on the wafer
stack.
materials, which require 300°C or higher
for curing, the cure temperature for the PS
PBM is only 180°C, allowing the thermal
budget of the integration process to be
greatly reduced. As a dielectric, the PS
PBM possesses a low dielectric constant
of 2.5 and a dissipation factor of 0.0016 at
a frequency of 10GHz. The low Young’s
modulus and high elongation ensure
it has the ability to absorb thermally-
induced stress created during thermal
processes, resulting in minimal bowing
of the bonded substrates. The general
mechanical, electrical, and reliability
properties of the PS PBM are summarized
in Table 1. The points outlined above are
discussed in the sections below.
Thermal stability. The thermal
stability of the PS PBM is evaluated using
ramp and isothermal thermogravimetric
analysis (TGA) (see Figure 2). The result
shows the PS PBM has a good thermal
stability with 1% weight loss at 373°C and
a 5% weight loss up to 441°C in a nitrogen
atmosphere. The isothermal TGA for the
PS PBM heated at 300°C for 2 hours in
nitrogen shows there is only about a 1%
weight loss during thermal processing.
The excellent thermal stability of the
PS PBM ensures it has a good thermal
budget, which is required for metal
annealing and other thermal processes
used for hybrid bonding.
Patterning performance. The PS
PBM is designed as negative tone and
sensitive to i-line (365nm) light sources.
To demonstrate the patterning capability,
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