Laser scribing tools edge in front
time for a single wafer. This determines the
number of lasers per machine, the optical
design (beam-splitting, number of scan-
ners), and the number of wafers processed
simultaneously. In laser Edge Isolation
tools, typical scanning speeds of 500-1000
mm/sec are routine. The distance scribed
for industry-standard 6” wafers is ~ 0.6 m,
making single-wafer process times less than
one second. Therefore, total process times
(laser scribing plus automation/handling)
are dominated not by laser scribe time
Figure 5. Laser scribing tools for Edge Isolation are
but by wafer handling. Here, increased
equipped typically with turn-key diode-pumped solid-
throughput is obtained by multiple-wafer
state (DPSS) lasers, such as the AVIATM laser.
handling and parallel-processing, with the
stipulation that sufficient power is avail-
pulsewidth collectively: “Laser treatment
able from suitable industrial-grade lasers.
on silicon surfaces requires short wave-
Now laser beam-splitting is possible while
Figure 4. Short-wavelength laser sources operating in
lengths less or equal to 355 nm and short
staying within the (power) process window
the UV (355 nm) and green (532 nm) have signifi-
pulse durations of less than a few tens of
at the workplace(s).
cantly shorter absorption lengths in c-Si than infra-
nanoseconds.”
Industrial-grade lasers for edge isola-
red lasers (~ 1060 nm) (4a). The “large penetration
depth of the 1064 nm wavelength results in a heat
Laser output power and pulse
tion are available with powers 2-4 times
affected zone about 25 micron-deep13”, promoting repetition-rate (pulses per second) are more
that required for single-wafer processing
the use of UV and green lasers(4b). Figures adapted ‘tool-specific’ than ‘process-specific.’ The
(Figure 6). Therefore, single lasers can
from data presented in references 11 and 13.
laser power requirement is a function of
drive parallel wafer processing. Assuming
process ‘threshold’, optical efficiency from
a total ‘handling’ time of ~ 2 seconds, a
process time of ~ 1 second and dual-wafer
from the scribed groove and compro-
source-to-sample, and number of wafers be-
processing/handling, a tool throughput of
mise structural integrity.” Specifically,
ing (parallel) processed to meet throughput
> 2,400 wafers-per-hour is easily provided.
Acciarri
12,14
compared a UV 355 nm laser
demands. Indeed, process time and ‘takt’
Higher throughput from single laser-based
and an IR fiber laser for edge isolation:
time (maximum time to produce a single
tools will benefit multi-line sites (‘GW-fab’
“at 355 nm, backscattering and secondary
wafer) are key issues within overall tool
class
18
) where single process tools can be co
images prove the absence of microcracks;”
design. While spot-overlap, laser repetition-
located beside multi-lane process steps.
“there is more redeposit (shunt) in the IR
rate and scanner speed also need to be op-
treated sample.” Indeed, LBIC (light beam
timized, the findings of Emanuel
17
provide
summary
induced current) measurements confirmed
a useful reference: fill factors stabilize for
Historically edge isolation was performed
improved isolation quality with the UV
laser average powers above ~ 10 W; pulse-
by plasma etching; today, three different
laser and negligible parasitic leakage.
to-pulse overlap ~ 80 %.
approaches are promoted within the in-
These wavelength studies explain the
In designing an optimized laser tool for
dustry. As cell producers design next-gen-
market transition observed with edge
edge isolation, key parameters include:
eration, fully automated lines for sub 180
isolation tools; often starting with IR lasers
• Throughput target (or takt time):
mm wafers, optimization of each process
(typically Nd-based or fiber) and moving to
how many (good) wafers per hour.
step becomes essential. Here, laser scribing
green or UV lasers; substantiated by Kray
15
:
• Process time for a single wafer and
tools are poised to dominate the equip-
“Solid-state lasers with short wavelengths
number of wafers if parallel wafer
ment landscape for edge isolation due to
(e.g. 355 nm) are used [for Edge Isola-
handling is necessary.
significantly higher ROI. This prioritiza-
tion].” Figure 5 shows an AVIATM laser,
• Power required at the wafer sample:
tion will be driven down the supply-chain
currently the dominant source within these
for multi-wafer processing, how
tools.
many lasers per tool required?
continued on page 29
Optimum pulsewidth has been deter-
Should a single high-power laser
mined practically, rather than empirically
be used with beam-splitting and
from research studies. Successful inte-
multiple scan-heads?
gration of ‘standard’ short-nanosecond
• Handling: how are wafers trans-
pulsewidth lasers (pulse durations ~ 20-50
ported into (and out of) the tool,
ns) is similar to analogous laser applica-
for inline compatibility with adja-
tions on semiconductor-grade silicon
cent processes?
wafers. Longer pulses (termed ‘long-pulse’
• Visual alignment: locating cell
lasers) would decrease material removal
edges and tracking beam(s) to cater
rates and increase thermal debris on the
for off-spec sized wafers.
surface: shorter pulses (from ‘picosecond’
• Is the tool migration-roadmap
lasers) do offer cleaner ablation, but the
Figure 6. Increase in power output year-on-year
capable? Optimized for thinner
from the dominant laser source used today for laser
increase in capital cost is not justified
wafers? Suitable for GW-fab inline
Edge Isolation (AVIATM laser), with available-
for Edge Isolation when analyzing ROI.
integration? power several times in excess of the typical per-wafer
Grischke
16
touched on wavelength and
process threshold.
The most important parameter is process
14 – Global Solar Technology – March/April 2009 www.globalsolartechnology.com
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