Page 26 - Chip Scale Review_May June_2023-digital

Figure 2: Schematic of inspection optics: a) Conventional macro inspection setup; and b) CF inspection setup.
residual ABF can also affect the integrity
of the electrical signal. Etching the Cu
seed layer is the final process step of each
build-up layer; this step defines the routing
of Cu trace lines. Under- or over-etching
the Cu seed layer can lead to shorts and
open circuits. Preventing shorts and open
circuits is a critical issue when addressing
decreases in yield.
Conventional macro inspection is
typically done using one of two techniques:
bright-field (BF) and dark-field (DF)
illumination (Figure 2a). Both BF and DF
techniques use light-emitting diode (LED)
sources that cover the visible wavelength
region. In BF illumination, the camera
objective and illumination source are
positioned on a common axis perpendicular
to the surface of the substrate so that the
camera sees the specular reflection of the
source illumination. Therefore, the BF
image is formed by the reflected light from
the sample and is therefore a strong function
of light attenuation and reflection between
differing materials on the sample. In DF
illumination, the camera is positioned away
from the direction of the specular reflection
of the illumination source. On a flat, mirror-
like surface, the specular reflection from
the substrate is directed away from the
camera, and the field becomes dark. But
any particle or surface irregularity that

24 Chip Scale Review May • June • 2023 [ChipScaleReview.com]
24