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Chip Scale Review September • October • 2017

[ChipScaleReview.com]

Excimer laser ablation for microvia and fine RDL

routings for advanced packaging

By Habib Hichri, Markus Arendt

[SUSS MicroTec Photonic Systems, Inc.]

xcimer laser ablation is a

direct etching process using

high-energy pulses at short

wavelengths that allows patterning of

most polymers at minimal thermal impact

through bond breaking. Metal layers of

>1µm in thickness serve as a stop layer,

so underlying circuits are not affected by

the excimer laser ablation. With this type

of patterning technology, the advanced

packaging industry gets access to materials

that do not require photo patterning

and allow a wider range of options for

coefficient of thermal expansion (CTE)

matching in the package. A tool, consisting

of a high-power excimer laser source, a

reticle-sized laser mask, and reduction

projection optics on top of a state-of-the-art

wafer stepping stage enables the accurate

and cost-effective replication and placement

of high-resolution circuit patterns. We

will explain the excimer laser technology

and its application for drilling microvias

and trenches that are used in different

packaging applications.

The h i gh - dens i t y s t r uc t u r i ng

challenge

Ever y t ype of advanced package,

including chip-on-chip, wafer-level

packages (WLP; e.g., 2.5D), chip-on-

chip stacking, and embedded IC, all

have a need to structure thin substrates,

redistribution layers, and other package

components at high resolution with

smaller features sizes and tighter pitches.

A very common example is drilling vias

down to contact pads for applications

like fan-in or fan-out WLP (FOWLP).

Another emerging example is trenching

to form embedded connectors in thin

substrates or interposers. When using

organic dielectrics, these structures are

usually created by photolithography, using

an ultraviolet (UV) light source with a

photomask, or by laser direct imaging

(LDI). These photoimageable materials,

such as polyimides (PIs), polybenzoxazoles

(PBOs) and epoxies, face CTE mismatch,

process relies on heat (pyrolytic), melting

and evaporation to ablate polymers [2-

3]. This “cold ablation” allows material

removal without damaging surrounding

areas, and no cracks or heat-affected zones

are observed.

When a high-energy laser pulse is

focused onto a material so that the intensity

(which is measured as the fluence) is above

a material-dependent threshold value, the

high-energy ultraviolet photons directly

excite electrons and break interatomic

bonds. This threshold is quite important

because it can be used to structure polymers

on top of inorganic materials like metal

without destroying the metal underneath

because the threshold of metals is mostly

very different to the threshold of dielectric

materials. Along with the subsequent shock

wave, this causes material to be ejected at

high velocity in the form of mostly gaseous

organic byproducts, and sometimes as

fine powder. Because a typical excimer

laser pulse duration is around 30ns, the

interaction with the material occurs very

rapidly resulting in little time for thermal

transfer in the material.

Post-ablation cleaning

Excimer ablation of polymers produces

little (to no) heat affected zone (HAZ), and

>90% of the ablated material evaporates.

The remaining portion results in the

generation of carbonaceous debris that

will be landing back on the surface of the

dielectric. For these, a cleaning process is

essential to eliminate any interference of

the debris with the subsequent process steps

such as seed layer deposition and plating.

The most common and recommended

cleaning process post ablation is cleaning

with O

2

plasma or desmear.

Figure 1

shows

post-ablation and post-O

2

plasma cleaning

for a via patterned by excimer laser. Another

alternative to O

2

plasma cleaning post

ablation is the use of a protective layer (or

sacrificial layer) that will be coated in a thin

film (1-2µm) on top of the dielectric prior

to ablation. The sacrificial layer is a UV-

E

and pattern integrity control through

developing and curing. This causes stress

damage between these materials and the

chips, resulting in higher warpage, and

finally, impacting the reliability of the

package. Furthermore, these materials

have limitations in the resolution and

via wall angle that they can support. In

the case of vias for example, this limits

the minimum achievable diameter and

interconnect density, as well as their

practical aspect ratios to values <1.5.

Devices that are al ready seei ng an

impact are those involving larger dies,

higher I/O densities, thinner Si chips and

substrates, and, obviously, any high heat

load applications. Other challenges in this

multi-step process involving developers

and other wet chemicals are the increasing

cost-of-ownership (CoO) associated with

these chemicals, as well as their safe

handling and disposal to support a greener

process. A very attractive alternative to

photolithography would be a technique

that can directly structure PIs, PBOs and

epoxies at even higher resolution than

today’s bleeding edge packages, and allows

a wider material selection to contain the

CTE issue, at reduced cost. Excimer laser

ablation now provides that alternative.

Excimer laser ablation process

T he l a s e r a bl a t i on of t h i n f i l m

polymers is i n pr i nciple not a new

technology for wafer–level packaging.

The laser ablation of polymers was

first reported by Srinivasan, et al. [1] in

1982. Low speed and high cost were the

major barrier for further developments

thirty years ago. But the lithographic

approach of excimer laser systems using

quar tz masks on stepping /scanning

platforms has improved this technology

to overcome the limited throughput.

Excimer laser ablation is a one-step,

dry-etch patterning process that differs

from solid-state laser ablation in that the

excimer process is based on photo-chemical

(photolytic) bond breaking; the solid-state