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SPi vFinal DR 2/9/09 11:14 Page 17
MANUFACTURINGCHOICES
Left:
Far left - 20 W; 25 kHz
WF0
1 m/s scan speed
163 mm f-theta lens
80 µm scribe width
Left - 20 W; 125 kHz
WF2
1 m/s scan speed
80 mm f-theta lens
Thin film overlap leading to scalloped edges that can lead to 40 µm scribe width
In thin film processing the use of fibre lasers for defects such as micro cracking, layer delamination
non-contact patterning is becoming increasingly and incomplete isolation. At higher repetition rates
widespread, rapidly replacing conventional (>100kHz) a smoother line edge can be achieved
chemical etching processes, offering greener and which is considered to be of higher quality.
more compact solutions. Such applications are
underpinning the flat panel display (both plasma The beam quality of the laser can be an important
and LCD) and the thin film solar markets by factor as there are a number of potential
enabling higher throughput together with improved compromises that need to be made with beam
yield. These thin film layers have to be line quality. High beam quality, as defined by a low M2
structured to give galvanic separation without typically with a Gaussian energy distribution, offers
17
causing damage to the substrate or adjacent the greatest focusability and hence smallest line
material/layers. Nanosecond pulsed fibre lasers are width, but suffers from high energy concentration
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commonly used for P1 patterning, selectively in the centre of the spot that can result in substrate
.solar
removing either molybdenum in CIGS (copper damage. They also give users a greater depth of
indium gallium selenide) or ITO in a-Si (amorphous field making manufacturing processes more
-pv-management.com
silicon), generating line widths in the order of tolerant. Lasers with higher M2 values have a
30µm. slightly broader energy distribution with lower
central intensity reducing the sensitivity to some
The directly modulated seed design of SPI’s quality features such as substrate damage. The
pulsed fibre laser offers end users greater flexibility choice is very much down to the process Below left:
in optimising the pulse characteristics required for requirements. Holes processed in Si
a particular scribe. A key differentiator is the ability at 50 µm
Issue IV 2009
to vary the length of the pulse which in turn gives Silicon
greater control over parameters such as peak Within the silicon solar cell manufacturing process
power, pulse energy and pulse repetition rate all of there are a number of processes carried out by Below: Pulsed fiber
which can impact the material removal process. fibre lasers. These include simple scribe and laser
The scribe lines are produced by an array of break, hole drilling, edge isolation and cutting.
overlapping spots where the % overlap can be Ribbon silicon cutting using pulsed fibre lasers is a
critical to providing high quality scribes. Low key application where a multi-pass technique is
repetition rates (<100kHz) generally give low adopted to give full depth cutting. This process is
quite slow and does have some quality issues with
recast material and surface
microcracking. Recent
developments at SPI are
looking into
alternative cutting
techniques for
silicon using
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