Page 47 - ChipScale_Jan-Feb_2021-digital

temperature process. Figure
3 shows how the adhesion
strength changes between
t he a d he s i v e a nd t he
adherend during the wafer
fabrication process. For
conventional adhesive tape,
the adhesion layer becomes
sof t a nd its wet t abilit y
to the adherend becomes
greater during the thermal
process so that the adhesion
strength rises after the high-
temperature process. This
increase in the adhesion
strength causes difficulty
during the carrier debonding
and also causes residue on
the wafer after removal of
the adhesive tape. To prevent
Figure 2: a) (top) Structure of SELFA; and b) (bottom) Gas debonding scheme. the increase in adhesion
The gas generator designed for gas Another key SELFA technology strength, two approaches
debonding is one of the key technologies is designing the adhesive to enable can be considered: 1) hardening the
of SELFA. From the perspective of controlling adhesion strength during adhesive layer to prevent softening
safety and to preclude contamination the process. As a temporary bonding during thermal processing; and 2)
of the wafer, the gas used for carrier material used in wafer fabrication decreasing the affinity for the adherend
debonding should be inactive. In self- processes, the adhesive tape must have to reduce wettability.
releasing adhesive, nitrogen compounds chemical and thermal resistance so For the first approach – hardening
are dispersed and serve as the gas as to inhibit delamination, which can t he ad hesive layer – a chem ical
generator. N 2 gas is generated by the occur in the event of the tape dissolving crosslinking structure is introduced
use of UV radiation at a wavelength of or decomposing during the chemical into the adhesive resin. The adhesive
254nm. In addition to the UV reactivity, or thermal processes, respectively. resin contains unsaturated double
the gas generator used in SELFA must Another difficulty of using a temporary bonds in the main chains and a UV
have a thermal resistance of over 250°C, bonding material is that the adhesion initiator, which reacts to 405nm UV
otherwise the tape can be delaminated strength needed to bond to the device irradiation. By using UV radiation,
because of the decomposition of gas wafer increases during the high- u n s at u r at e d double b ond s r e a ct
g e n e r a t o r s d u r i n g
t h e r m a l p ro c e s se s
such as chemical vapor
deposition (CVD) or
reflow. To improve the
thermal resistance of
the gas generator, the
chemical structure of
the nitrogen compound
wa s opt i m i zed. By
tuning the structure of
the nitrogen compound,
the gas generator has
high thermal stability
at t e m p e r at u r e s
over 250°C. I n ou r
tape, heterogeneous
compounds, including
n i t r o g e n a s a g a s
generator, disperse in an
acrylic adhesive layer
and react by way of
the 254nm wavelength
UV irradiation. Figure 3: Image of adhesion strength change during the wafer fabricating process.

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