Page 27 - Chip Scale Review_May June_2023-digital

scatters light out of the specular beam will Imaging is accomplished using
make the field bright. This characteristic a high-resolution line scan
makes DF illumination particularly good at camera. The image pixel size
identifying small particles and defects on a corresponds to 0.7μm at 10X,
flat specular surface. which provides the highest
We also used another inspection level of magnification for
®
illumination technology, Clearfind inspection.
(CF). The optical path of this technology
is also show n i n Figure 2b, but Via inspection
Figure 3 illustrates in more detail how ABF is a widely-used
this technology works. The light source insulating dielect r ic in
for this new illumination technology is a advanced IC subst rates
monochromatic blue laser with a stable used for FC-BGA. To make
wavelength and output power. The laser ele c t r ic a l c o n n e c t io n s
beam is collimated and expanded into between layers, vias are
a horizontal line at the sample and then for med by h ig h- energ y
scanned over the surface. Stimulated by ultraviolet (UV) lasers.
illumination, the sample either reflects the Occasionally, ABF is not
light from the source, or emits secondary completely removed from
photons of lower energy, depending on the via, which results in a
the types of materials involved. Metals poor electrical connection
have continuous energy bands in the th rough the via and, in
visible wavelength regions and will t ur n, negatively affects
either absorb or reflect incident photons. signal integrity. Therefore,
Organic materials, such as polymers, inspecting the via after
exhibit distinctive optical properties that laser ablation is necessary
are not present in the metals or common to find such defects (Step 12
inorganic materials used in IC substrate Figure 3: Illustration of CF technology.
manufacturing. These properties tend to
be unique to organic molecules displaying
a high degree of conjugation, such as
polycyclic aromatic hydrocarbons, and in
linear or branched chain organic polymers
with multiple regularly interspersed
pi bonds [2]. The emission from the
organic surface tends to be anisotropic
and, therefore, less sensitive to surface
topography that could potentially direct
most ordinary BF- or DF-reflected light
away from the detector. Moreover, photons
of the same energy are blocked by the filter;
as a result, any reflection or scattering
from the metal surface cannot be collected
by the imaging camera. This results in
increased sensitivity to organic residue and
reduced sensitivity to interference from the
surrounding features.
The method described above has the
additional advantage of being relatively
insensitive to signal variations caused by
metal grains. The new technology has been
found to be very effective in detecting
invisible defects on fan-out wafer-level
packaging (FOWLP) and fan-out panel-
level packaging (FOPLP) [3,4]. In our
investigation, we employed a high-speed,
near-infrared laser-triangulation autofocus
system that maintains a constant distance
between the imaging optics and the area
being scanned to keep the image focused.

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