7 Fundamentals and Failures in Die Preparation for 3D Packaging
167
dissipation during wafer-level test and burn-in. In addition, the conventional Electrostatic Chucking (ESC) technology [36] prevalently deployed in various microfabrication equipment (e.g. plasma diffusion, dry etching, thin film deposition, and etc.)
is based on electrostatic force, and could only handle conductive or semiconductor
(e.g. Silicon) wafers. Glass wafers as an insulator require a different interdigitated or
“bipolar” type of ESC chuck [36], which drives dedicated equipment configuration.
However, the need for glass wafer arise with growth of laser debond technology due
to its transparency to Ultraviolet (UV) laser [37, 38]. Glass wafer also enables visual
or optical inspection through glass for process defects, such as adhesive void and
device wafer crack. More recently, laser debond using Infrared (IR) laser has been
successfully demonstrated, where Si wafer could be used instead of glass [38, 39].
The bonding process [1] starts from the preparation of both wafer surfaces.
Surface contamination or condensed moisture trapped in the nanometer scale or
sub-nanometer scale troughs on wafer surface would negatively impact the wetting
of fluid on bonding surface and lead to poor bonding quality. Large surface particles,
which impede normal fluid flow during the spin coating process, could cause comets
and streaks in the coating [40, 41]. Surface cleaning [42] pre-baking to remove moisture and organic volatiles, or applying adhesion promoter coating on the surface [42]
are typical methods for surface preparation. In some cases, additional release layer
or adhesion promoter is needed on either the device wafer side or carrier wafer side
to control of the adhesion at the interface.
After surface preparation, adhesive precursor in solvent solution is coated onto
one of the wafer surfaces. The adhesion at the adhesive to wafer interface is achieved
primarily through covalent bonds and van der Waals bonds, which requires the atoms
of the two surfaces to be brought to less than 0.3–0.5 nm apart [1]. Because of the
surface roughness and topography of the wafers, an effective way is to apply the
adhesive in a liquid form to wet the surface before solidified, so that the polymeric
adhesive would fit the surface profile of the wafer and achieve bonds at the interface.
To apply a viscous liquid to a wafer surface, a spin coating process is often selected
due to the excellent control over thickness and uniformity across the whole wafer, in
comparison to alternatives such as spray coating [1]. The key stages of a typical spin
coating process, as shown in Fig. 7.9, consists of fluid deposition, spin acceleration,
and fluid thinning at a constant spin rate [40, 43]. The film thickness gradually
reduces with longer spin time, until the final film thickness is reached. The final film
thickness depends on the fine balance between the centrifugal force that makes fluid
flow from center to edge, controlled by spin speed, and the viscous force of the fluid
to resist flow, controlled by the evaporation rate. The adhesive thickness tends to be
thicker at wafer edge due to surface tension effect [40, 44], resulting in increased
bond line thickness variation.
Then, the wafer will go through a soft bake process. The main purpose is to bake all
solvent and volatile content out of the adhesive layer, to prevent void and delamination
[45]. Thermoplastic adhesive may undergo partial or full polymerization at this stage,
and still can be remelted during bond step; while thermoset adhesive should be left
with no cure or partial cure at this stage to enable bonding.
167
dissipation during wafer-level test and burn-in. In addition, the conventional Electrostatic Chucking (ESC) technology [36] prevalently deployed in various microfabrication equipment (e.g. plasma diffusion, dry etching, thin film deposition, and etc.)
is based on electrostatic force, and could only handle conductive or semiconductor
(e.g. Silicon) wafers. Glass wafers as an insulator require a different interdigitated or
“bipolar” type of ESC chuck [36], which drives dedicated equipment configuration.
However, the need for glass wafer arise with growth of laser debond technology due
to its transparency to Ultraviolet (UV) laser [37, 38]. Glass wafer also enables visual
or optical inspection through glass for process defects, such as adhesive void and
device wafer crack. More recently, laser debond using Infrared (IR) laser has been
successfully demonstrated, where Si wafer could be used instead of glass [38, 39].
The bonding process [1] starts from the preparation of both wafer surfaces.
Surface contamination or condensed moisture trapped in the nanometer scale or
sub-nanometer scale troughs on wafer surface would negatively impact the wetting
of fluid on bonding surface and lead to poor bonding quality. Large surface particles,
which impede normal fluid flow during the spin coating process, could cause comets
and streaks in the coating [40, 41]. Surface cleaning [42] pre-baking to remove moisture and organic volatiles, or applying adhesion promoter coating on the surface [42]
are typical methods for surface preparation. In some cases, additional release layer
or adhesion promoter is needed on either the device wafer side or carrier wafer side
to control of the adhesion at the interface.
After surface preparation, adhesive precursor in solvent solution is coated onto
one of the wafer surfaces. The adhesion at the adhesive to wafer interface is achieved
primarily through covalent bonds and van der Waals bonds, which requires the atoms
of the two surfaces to be brought to less than 0.3–0.5 nm apart [1]. Because of the
surface roughness and topography of the wafers, an effective way is to apply the
adhesive in a liquid form to wet the surface before solidified, so that the polymeric
adhesive would fit the surface profile of the wafer and achieve bonds at the interface.
To apply a viscous liquid to a wafer surface, a spin coating process is often selected
due to the excellent control over thickness and uniformity across the whole wafer, in
comparison to alternatives such as spray coating [1]. The key stages of a typical spin
coating process, as shown in Fig. 7.9, consists of fluid deposition, spin acceleration,
and fluid thinning at a constant spin rate [40, 43]. The film thickness gradually
reduces with longer spin time, until the final film thickness is reached. The final film
thickness depends on the fine balance between the centrifugal force that makes fluid
flow from center to edge, controlled by spin speed, and the viscous force of the fluid
to resist flow, controlled by the evaporation rate. The adhesive thickness tends to be
thicker at wafer edge due to surface tension effect [40, 44], resulting in increased
bond line thickness variation.
Then, the wafer will go through a soft bake process. The main purpose is to bake all
solvent and volatile content out of the adhesive layer, to prevent void and delamination
[45]. Thermoplastic adhesive may undergo partial or full polymerization at this stage,
and still can be remelted during bond step; while thermoset adhesive should be left
with no cure or partial cure at this stage to enable bonding.