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Laser scribing tools edge in front
Laser scribing tools edge
in front
by Finlay Colville, Coherent, Inc., Santa-Clara, CA, USA
What is edge isolation?
thanks to semiconductor industry adoption
Laser-based tools are now widely
Currently, most c-Si cells are manufac-
for silicon processing (scribing, dicing, cut-
adopted by silicon cell producers
tured within a standard production line
ting, marking, TSV drilling). Moreover, a
at the edge isolation stage due to
process: front screen-printed and fired
wide range of laser types have been applied,
the economic and environmen-
Ag-contacts and a full aluminum-deposited
making laser optimization for edge isola-
back surface field (BSF). The process flow
tal benefits when compared to
tion a topic in its own right. Laser scribers
and location within the c-Si value-chain is
etching approaches. This article
are forecast to account for ~ 50 % of all
illustrated in Figure 1. While forming the
tool revenues for edge isolation by 2010
2
.
explains how to optimize a laser-
p-n junction, a phosphorous diffusion step
based tool for edge isolation and
is employed near the front-end to create a
Chemical etching‡: Chemical etch-
what level of per-wafer return on thin (~ 200 nm) n-type region. However,
ing for edge isolation is a relatively new
investment (ROI) is available to this doping step not only creates an n-type
approach
3
, favored by some turn-key line
cell producers.
region at the front surface, it results also
suppliers who use chemical etching for
in the formation of phosphorous silicate
other process steps (SDR, texturing, PSG
glass (PSG) on all exposed faces, and an
removal). Chemical etching is promoted
undesired n-type doped layer at the sides
mainly as ‘convenient’ within lines already
Keywords: Laser Scribing, Edge
and rear. Consequently, front and back
employing chemical etching equipment
Isolation, c-Si
contact regions are at risk from electrical
(although “every process step demands a
connection via the edges. This would cre-
specific chemical-bath design”
4
), rather
ate a short-circuit (or shunt) and requires
than having the highest return on invest-
an additional process step to isolate the
ment (ROI). In chemical etching, wafers
surfaces. This extra stage is referred to as
are passed horizontally across the surface
edge isolation (or ‘junction’ isolation).
of an aggressive hydrofluoric (HF) and
Plasma etching : Plasma etching was
nitric (HNO
3
) acid solution, supported by
the dominant process until ~ 2007 due
rollers and surface tension. Contact with
to relatively low capital cost and satisfac-
the acid bath etches PSG material from
tory results. Here, wafers are manually
the rear (and side) surface of the cells, but
‘coin-stacked’ within an Al-etching jig and
“care has to be taken that the reactants
placed inside a barrel-type chamber. The
are not spilled onto the emitter side of the
wafers are rotated in a perfluorocompound
wafers.”
4
Etching is followed by multi-
(PFC) reactive plasma gas mix (by excit-
cascade DI-water rinsing and drying steps
ing fluorine compound with a microwave
to prevent further etching and remove
or RF field) to remove material from the
traces of acid. Plant installation requires a
exposed edges. Installations require a waste
chemical waste treatment system. Recently
gas abatement system.
it was noted that
4
“wet etching is getting a
Laser scribing: Lasers for edge isolation
foot into edge isolation, primarily done by
were proposed in the 1980s. The benefits
laser.”
of non-contact processing at high-speed
were known, but industrial-grade lasers ca-
comparing different approaches
pable of this task were not available. Laser
Due to their market share, early stud-
edge isolation is performed at the back-end
ies compared plasma-etching and laser-
for increased cell efficiency
1
(characterized
scribing. Hauser
5
concluded that “using
by higher fill factors and open circuit volt-
lasers to insert trenches is the smartest way
ages). Figure 2 illustrates how laser scribing
to remove the shunts,” complemented by
creates a thin (< 50 mm) trench through
Schneiderlöchner
6
who claimed that “the
the SiN coating and the n-type region.
x
inherent advantages of laser beam process-
Industrial-grade lasers are available today
ing make laser edge isolation the tool of
*Junction Isolation is also required on various back-contact (BC) cell types, such as the RISE-EWT
design. An analogous term Contact Isolation is sometimes used.
†Plasma etching is often referred to as ‘dry’ etching.
‡Chemical etching is often referred to as ‘wet’ etching.
12 – Global Solar Technology – March/April 2009 www.globalsolartechnology.com
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