Laser scribing tools edge in front
thickness wafers.
As a reference, per-
wafer costs for the
etching tools show
close alignment
to previous stud-
ies
10
, once specific
edge isolation steps
and materials are
extracted.
Figure 2. In laser Edge Isolation (left image), a trench is scribed around the
perimeter on the front surface, between finger grid and cell edges. This results in a
Environmental
fine scribe line visible from above (right image).
comparison has
been predominantly
Figure 1. Edge Isolation is a key process step at the
requires—in theory—material removal of
chemical vs. plasma, with consumables and
cell manufacturing stage within the c-Si value chain.
SiN and n-type layers (~ 300 nm depth),
waste produce specific to etching processes.
x
Three options are available to perform this step: dominant absorption (micron level) occurs
In fact, both etching methods have their
plasma or chemical etching, and laser scribing.
in the bulk p-type c-Si. Figure 4a shows that
own subset of environmental concerns:
short absorption lengths
11
(with minimized
for chemical etching, the requirement
impact on the bulk c-Si) are provided by
choice.” Analyses focused mainly on the for HF, HNO
3
, KOH, DI-water, and acid
green (532 nm) or UV (355 nm) lasers.
limitations of plasma-etching, rather than disposal; for plasma etching, etch gases
Absorption was discussed by Acciarri
12

direct per-wafer ROI analyses, or indeed, with high global warming potential (GWP)
when comparing residual surface quality:
specifics on optimizing a laser tool. For and exhaust gas mitigation. However, it’s
“The silicon absorption curve indicates
example, Correia
7
spelled out plasma’s not just environmental ‘perception’: their
that the ratio of absorption (355 to 1070
limitations in (batch) processing with inter- relative costs form the basis of the disparity
nm) is higher than 10
7
, [which] magnifies
rupted material flow and heavy demand between etching and laser per-wafer costs
undesired thermal effects in [the] case of
on personnel (labor cost). The other factor shown in Figure 3. Chemical etching costs
IR fiber lasers.”
often cited against plasma is environmen- have sizable contributions from each of
In a comprehensive analysis on c-Si
tal: PFC gases. In a recent review, Roth
8
consumables, utility/services and equip-
laser scribing by Schoonderbeek
13
, sub-
commented that “most solar manufactur- ment capital expenses (CAPEX). Plasma
surface carrier lifetimes were measured
ers prefer laser isolation because the step is etching costs are dominated by consum-
before and after irradiation from (nano-
able to be automated and adapted for mass able-costs, followed by equipment CAPEX.
second pulsewidth) lasers operating in the
production. Furthermore, the elimination Laser scribing tools (diode-pumped
infrared (1060 - 1070 nm), green (532 nm)
of the bypass between the p- and n-areas is solid-state lasers) have almost negligible
and UV (355 nm). Figure 4b illustrates why
safer.” consumable or utility/services costs; their
many c-Si laser processes show preference
Comparing the efficiency gains from per-wafer costs almost exclusively governed
for short-wavelength lasers (green or UV).
fully processed cells using each of the by CAPEX. This explains why amortized
Acciarri
12
echoes this for Edge Isolation:
three methods has not provided any clear per-wafer costs are so much lower using
“[lasers] with short wavelengths (e.g. 355
outcome. Edge isolation is not a process to laser scribing for edge isolation.
nm) are the most popular laser sources
enhance cell efficiency; rather, it is a ‘loss-
used, bearing [in mind] the advantage of
preventative’ step. Any fractional differenc- Laser tool optimization
less Heat Affected Zone compared to IR
es in cell efficiency using each of the three The two main components (laser ‘source’
[1060 nm] lasers. Conventional 1060 nm
edge isolation methods can be attributed and tool design) of a laser-based tool for
lasers can create microcracks that emanate
to other efficiency-enhancement steps; a edge isolation can be optimized separately.
comparison by Schneiderlöchner
9
between The laser source is the most important.
chemical etching and laser scribing suggest- Indeed, research labs have compared
ing efficiency parity using production level different laser types for edge isolation
throughput statistics. and similar c-Si front-surface scribing
The decision on whether etching or techniques. Scribing trenches (or grooves)
laser-scribing is best ultimately comes down in silicon requires lasers operating with
to ROI for cell producers. A comparison high peak-power pulses (‘pulsed’ regime)
between plasma and chemical etching and for effective material removal. But what
laser scribing by Preu
11
concluded that ‘kind’ of pulsed lasers? What color (or
“the laser approach leads to the lowest laser wavelength) works best? What power
investment of the alternative techniques.” is needed? How fast should the pulses be
Figure 3 shows the output from Monte produced? And what impact do these have
Carlo modeling with material costs driven on cell efficiency and yield?
on a per-wafer basis. This uses a range of Laser micromachining quality is Figure 3. Per-wafer costs for the three types of Edge
costs per (good) wafer produced using each strongly dependent on wavelength and
Isolation based on a Monte Carlo modeling process
method by varying input parameters be- pulsewidth; edge isolation is no exception.
(5 year period). Capital cost (CapEx) is straight-
tween extreme values. Notably, this model Two factors impact most on wavelength
line depreciated. Standard equipment values for
excludes any handling or ‘jigging’ costs. choice: absorption properties of c-Si and
tool uptime, breakage rates, and yield are included
within the analysis4. Labor is excluded for each of
It includes published specifications from changes in minority carrier lifetimes due to
the methods.
commercial tools
4
and the use of standard- laser-induced bulk damage. While scribing
www.globalsolartechnology.com Global Solar Technology – March/April 2009 – 13
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