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metal and simultaneously forms a bond. The complexities involved
during the material preparation make it extremely difficult to
© 2017 Brewer Science, Inc.
implement in a production environment [2]. Another approach, also
proposed by Intel, relies on breaking thin native oxides on In and
Sn surfaces by applying sufficiently high pressure before reaching
Creating Safe the melting point in an inert environment. For some processes,
the temperature is raised close to, but lower than the melting point
of the solder to promote solid-state diffusion. This method also
Environments requires a noble metal finish on one of the mating surfaces, and
additionally requires an inert environment for a reliable bond [3].
Metal-based interconnects. Metal-metal TCB provides the
unique advantage of eliminating the solder materials. Consequently,
the reliance on flux materials to remove oxides can be completely
Laser Release System eliminated. Other than using the noble metal finishes (e.g., Au, Pt,
etc.), most metals (e.g., Cu, Sn, etc.) would require some method to
In the laser release system, the device wafer remove the existing native oxides prior to and during the bonding
is bonded to a transparent glass carrier using process. In addition, metal-metal bonding has some very stringent
requirements for reliable bonding, which include planarity, atomic-
a bonding material and a release material. level flatness and removal of native oxide. The TCB process further
Once processing is completed, the pair is facilitates the solid-state diffusion. Out of available material
choices, Cu is of keen interest because it offers excellent material
separated by exposing the release material properties and is economically viable. Oxidation prevention on
with an excimer laser or solid-state laser. Low- Cu-surfaces, however, is a major challenge. Bajwa, et al. [4], have
shown that using a noble metal finish (e.g., gold) protects it from
stress separation coupled with high throughput oxidation, but this approach adds severe processing complexities.
make the laser release system suitable for all Noble metal finish is currently not a standard practice in foundries
and it will require noble metal finishing on the chips as well as the
production environments. substrates to achieve its purpose.
More recently, plasma pre-treatments and other in situ oxide
reduction techniques (e.g., forming gas reduction) have been used
to reduce and prevent oxidation, but they are more applicable to
wafer-to-wafer (W2W) processes [6-9]. More often, these methods
Laser are employed in controlled environments such as vacuum, N 2 , etc.
Transparent
Carrier Another major hurdle during metal-metal bonding is the roughness
Thin Device Wafer
of the joining surfaces and planarity of the chip to the target
Release Layer Bonding Material substrate. Though chemical mechanical planarization (CMP) can
effectively resolve this issue, it is only applicable to semiconductor
materials (e.g., Si) and cannot be extended to laminates or PCBs.
Other techniques, such as fly-cutting of the copper pillars, can be
potentially useful, but there is very limited data on its applicability.

Formic acid vapor delivery system
Laser Release System Benefits: K&S has developed an FA vapor delivery system that can
be integrated with both our chip-to-wafer (C2W) and chip-
•Highest-throughput system available with a to-substrate (C2S) TCB machines. The delivery system
release time of less than 30 seconds allows injection of FA vapor directly onto the target surfaces
immediately before the TCB process, thereby cleaning the
•Ultraviolet laser does not heat or penetrate metal as well as eliminating solder oxides, so that the use
the bulk bonded structure of flux is eliminated. Localized delivery of FA vapors and
creating a localized reducing mini-environment is achieved
•Low-stress processing through use of CTE- through a custom-designed shroud that fits over a standard
matched carrier and room temperature TCB bond head.
In the next sections we will discuss the chemistry of the FA-
separation based tin oxide reduction process and the functionalities of the
shroud and the gas delivery system. We will also provide a set of
Compatible with: 308 nm 343 nm 355 nm experimental data to clarify the performance of the fluxless TCB
2
process specifically for large die (i.e., 900mm bonding case).
Additionally, future applicability of the process technology for
Cu-to-Cu interconnect will be discussed as will the experimental
www.brewerscience.com data showing Cu-to-Cu bonds for different devices.

38 Chip Scale Review September • October • 2020 [ChipScaleReview.com]
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