LASER ABLATION DELIVERS HIGHER
THROUGHPUT AND BETTER YIELDS
RALPH DELMDAHL and MATTHEW GINGERELLA
The relentless consumer demand for microelectronics products that are more compact, energy-effi- cient, and deliver greater function- ality translates into a need for circuit packaging techniques employing ever-smaller feature sizes.
Traditional methods (e.g., photolithography) used for
a variety of microstructuring tasks in advanced packaging are becoming less effective and more costly as
circuit dimensions shrink, so excimer laser ablation
offers an attractive alternative in many cases. This article looks at the use of excimer laser ablation for drilling glass interposers, and reviews how excimers have
been successfully employed in a turnkey platform from
SÜSS Micro Tec (Corona, CA) for other advanced packaging applications.
2.5D and 3D packaging
So-called “3D packaging” is an important emerging
technology for achieving higher density of packaged
microelectronics. In 3D packaging, multiple individual integrated circuits (ICs) are stacked like a sandwich. An interstitial substrate serves as an intercon-nector and redistribution circuit between the dies.
This substrate, called an interposer, might be constructed of silicon or glass. Currently, actual microelectronics production utilizes 2.5D packaging. This is an intermediate step where just a single set of circuit dies is stacked
on the interposer.
A critical process is the creation of small through-holes (vias),
which are used to make electrical connections between the
circuit components. Target via diameters are currently in the
25µm size range. These vias can be produced using several
techniques, including photolithography, etching, sandblasting,
ultrasound drilling, or laser ablation.
Laser ablation offers advantages over other methods
because it is non-contact, scalable, and enables the production of small, high-precision features on a wide variety
of materials. Furthermore, laser microstructuring often delivers higher process throughput and better yields with superior process consistency. Plus, it is an inherently “greener”
process, avoiding the use of potentially hazardous lithography chemicals.
Because of these benefits, CO2 lasers have been used exten-
sively for drilling vias in microelectronics packaging applica-
tions for some time. However, the long infrared wavelength of
CO2 lasers limits their minimum focused spot size (due to diffraction). The result is a practical lower limit of about 70µm
for the diameter of CO2 laser-drilled vias. Reaching smaller
diameters therefore requires
a shorter wavelength (e.g.,
To investigate the potential
for using excimer lasers for
this application, the Coherent
applications laboratory test-drilled a large number of
tightly spaced holes into
glass wafers. Specifically,
these were 25µm diameter
holes, with a pitch (
hole-to-hole spacing) of 50µm, pro-
duced in glass substrates
ranging in thickness from
100 to 300µm. They used a
193nm laser wavelength with a 600mJ pulse energy in a mask-based process that produced a fluence of 7J/cm2 at the work
surface. The 193nm wavelength was chosen because glass
exhibits strong absorption at this wavelength.
FIGURE 1. Excimer laser (193nm) vias
drilled into glass: The 25µm diameter
entrance hole (a), the exit hole (b), and
the cross-sectional view (c).