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Chip Scale Review May • June • 2019

[ChipScaleReview.com]

cr it ical applicat ions have requ i red

the development of new sensors and

techniques, including: high-speed laser

triangulation for whole-wafer bump

height metrology and advanced white

light and infrared interferometry for

measurements of film thickness, optical

profiling, substrate metrology, and via

and car rier trench depth metrology.

Additionally, there are new non-visual

defect s t hat t r ad it ional i nspect ion

technologies have not been able to

detect. These defects may be invisible

cracks or residues that may not be caught

during electrical test but cause reliability

issues downstream. In this case, the

development of laser-based contrast

inspection has provided much needed

defect detection capability.

Through-silicon vias

TSVs are used to make connection

between vertically-stacked die by creating a

hole through the silicon substrate and filling

it with metal. There are several types of

TSVs, and many different process sequences

used to form them, such as:

• TSV reveal height: The TSV reveal step

is an etch process that removes the last

bit of silicon after the bulk is removed by

backside grinding. It “reveals” the TSV,

leaving the metal fill protruding slightly

above the surrounding silicon surface.

The height of the protrusion is critical.

• A v i s u a l t h i ck ne s s a nd s h ap e

sensor (VTSS) provides accurate

measurements of reveal height needed

to control the process.

Figure 1

Illustrates the TSV reveal step. The

full wafer plot shows height variations

across the wafer and the histogram

shows the frequency distribution of

individual measurements.

• Recess: After reveal, the wafer surface

and protruding TSVs are covered

with an oxide layer and the oxide is

planarized with chemical mechanical

polishing (CMP), leaving a flat surface

with exposed TSVs. It is important to

avoid excessive dishing of the TSVs,

which erode faster than the surrounding

oxide during CMP. The VTSS sensor

can accurately measure the depth of

the recess and the thickness of the

oxide layer.

Figure 2

illustrates the

process step and compares a VTSS

measurement to a cross-section image

acquired with an electron microscope

showing close correlation.

• O t h e r T SV i n s p e c t i o n s a n d

measurements include bright-field

scans for surface defects (scratches,

met al re sidue s, and pa r t icle s),

dark-field scans for TSV defects,

and VTSS measurements of pad

height and tetraethyl orthosilicate

(TEOS) thickness.

Redistribution layers

RDLs allow signals from a location on

the die surface to be routed to a different

location in WLP and FOWLP packages.

They are typically formed by plating

copper through a patterned photoresist

mask, then isolated by an organic polymer

(polyimide [PI] or benzocyclobutane

[BCB]). Organic contaminants on metal

pads can cause reliability problems that

are often not apparent until a device fails

in the field due to a partial connection

or repeated cycling. These defects are

difficult to detect with conventional

optical inspection systems because they

“disappear” in the noise generated by the

background material. To uncover these

defects, Rudolph developed Clearfind

(CF) technology. CF technology is a

combination of illumination, optics and

image processing capabilities specifically

designed to enhance contrast differences

between metal and organic materials.

When using CF technology, imaged

metals are dark and organics are bright,

eliminating the background noise.

• Shorts/opens/distortions: As shown

in

Figure 3

, the enhanced contrast

provided by CF images facilitates

Figure 1:

TSV Cu reveal step: a) (left) TSV reveal height, b) (middle) full wafer plot of reveal height, and c) (right)

histogram of individual measurements.

Figure 2:

After TSV process step and data: a) (left) TSV oxide CMP, b) (middle) VTSS measurement of TSV

recess, and c) (right) cross-sectional electron microscope image of the TSV recess.

Figure 3:

a) (left) CF image of organic (bright) and metallic (dark) contaminants; b) (right upper) CF image RDL

line distortion; and c) (right lower) CF image of metal residue.