Page 30 - ChipScale_Jan-Feb_2020-digital

Using laser-induced deep etching to enable

microfeatures in glass

By Roman Ostholt, Norbert Ambrosius, Jean-Pol Delrue, Rafael Santos, Daniel Dunker, Stephan Schmidt [LPKF Laser & Electronics AG]

This article was edited from papers respectively presented at the IMAPS Conferences in Pisa, Italy in Sept. 2019 and in
Boston, USA in Nov. 2019.
G lass is arguably one of

t h e m o s t i n t e r e s t i n g
materials for heterogeneous
integration. Whether it is for its radio
f requency (R F) proper ties, high
surface quality, hermeticity, tunable
coefficient of thermal expansion (CTE)
or low cost, there are several reasons
to favor glass for advanced packaging
applications. Unfortunately, and up to
now, processibility of glass has not been
among those reasons. This is because
current glass processing technologies Figure 1: Schematic drawing of the GEFOP concept [2].
typically induce micro-cracks and
stresses, which contribute to glass’s the die position error and warpage need
reputation of being prone to brittle to be developed. As such, research is
fracture. In contrast, glass without being carried out to take full advantage
surface defects has excellent mechanical of the properties of glass for advanced
properties as can be demonstrated when packaging [1,3]. One of these solutions
it is processed by laser-induced deep is GEFOP [2]. A schematic drawing of
etching (LIDE). GEFOP is shown in Figure 1.
LIDE is a two-step process using Precision glass processing is used
a fast laser beam, which modifies the to manufacture a mounting wafer with
glass with a single pulse, followed by a multiple open die embedding cavities
chemical etching. As previously shown together with micro-holes for TGVs and
by the authors [1,2], LIDE technology integrated passive devices (Figure 2). Figure 2: A scanning electron microscopy (SEM)
is capable of generating high aspect Dies are placed in open cavities before micrograph of the mounting wafer.
ratio and high-quality microfeatures in filling them with a dielectric material.
various alumino-boro-silicate glasses. After this material – an epoxy molding
Therefore, this technology is regularly compound (EMC) – is cured, the dies
used in applications such as through- are fixed in the mounting wafer and
glass vias (TGV), glass spacer wafers, the substrate can be further processed
and microfluidics. like a normal reconstituted wafer.
In contrast to conventional fan-out
packaging through transfer molding of
Passive die alignment in GEFOP the EMC, reconstituted wafers processed
The following sub-sections discuss using the GEFOP concept presented
various considerations with respect to glass here do not show a critical warpage.
embedded fan-out packaging (GEFOP). Typical glass embedding wafers have
Glass embedded fan-out packaging a diameter of 300mm (Figure 3), but Figure 3: LIDE-produced mounting glass wafer with
(GEFOP). Fan-out packaging has substrates with a size of 500 × 500mm² a 300mm diameter.
become a major driver of advanced can also be produced. Depending on
packaging in recent years. In order to the application, embedding glass wafers The GEFOP concept discussed so
use this packaging technology for future can have a nominal thickness between far represents a solution for the two
generations with higher I/O densities and 100µm and 500µm. most pressing problems in fan-out
multiple dies, new techniques to reduce packaging production: warpage and die

28
28 Chip Scale Review January • February • 2020 [ChipScaleReview.com]