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specific issues. The organic polymer used as an insulator in RDL is
essentially transparent, making any residues left after critical cleaning
steps nearly invisible with conventional illumination techniques.
Residues on metal contact surfaces can increase contact resistance
or prevent electrical continuity completely. Contact residues can also
degrade reliability, contributing to costly field failures. The graininess
of metals used as conductors interferes with defect detection on
metal surfaces, and the transparency of polymers, which allow
the graininess of underlying metals to show through, extends that
difficulty to insulators as well. Moreover, the automated detection
routines used in optical inspection usually rely on a comparison
of the sample to a “golden” standard. Any differences register as
defects. Random differences in the grain pattern can therefore create
thousands of false-positive nuisance defects, overwhelming the
detection algorithm.

Illumination technologies
Conventional illumination approaches include bright-field (BF)
and dark-field (DF) techniques. The work presented here uses a new
®
technique, known as Clearfind (CF), as implemented in our Firefly
®
macro defect inspection system. Figure 1 illustrates the essential
characteristics of the three techniques. BF and DF systems typically
use a broadband white light source. To simplify the discussion,
assume that the sample surface is essentially flat and the features
of interest – defects – are small and irregularly shaped. In bright
field illumination the camera objective and illumination source are
positioned on a common axis perpendicular to the substrate surface
such that the camera sees the specular reflection of the illumination.
The entire field of view appears to be uniformly illuminated—both
the background surface (the field) and any features on it are bright.
In dark-field illumination, the camera is positioned away from the
direction of the specular reflection of the illumination source. On a
perfectly flat, mirror-like surface, the specular reflection from the
substrate is directed away from the camera and the field is dark. But
any defect or surface irregularity that scatters light out of the specular
beam will be bright. It is this characteristic that makes dark-field
illumination particularly good at seeing small particles and defects on
a flat specular surface, like that of a bare silicon wafer.
The light source for CF illumination is laser based. The light is
monochromatic with stable wavelength and output power. The laser
beam is collimated and expanded into a horizontal line at the sample
and then scanned over the surface. Stimulated by the illumination, the
sample emits light at a different wavelength and a wavelength filter in
the optic path prevents reflected laser light from reaching the imaging
camera. The intensity of the light emitted by the sample depends
on the type of material illuminated. A high-speed, near-infrared
laser-triangulation autofocus system maintains a constant distance
between the imaging optics and the area being scanned. Imaging is
accomplished using a high-resolution line scan camera. The image
pixel size corresponds to 1.4µm on the sample surface at 4X and 0.7µm
at 10X. CF technology is most powerful as part of a comprehensive
inspection regime that may also include bright-field and dark-field
inspection. The BF inspection results shown here were acquired on
the same automated optical inspection (AOI) platform that acquired
the CF images.

Results
The following sections discuss the results obtained by the study.
RDL sample. The sample shown in Figure 2 is a large molding
compound panel. Figure 2a shows a 10X CF image and Figure 2b

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