TEST & MEASUREMENT
more cost-effective configurations by reducing the
thickness of wafers, reducing scribe’s widths/depth
ratio, and increasing roughness of c-Si or thin-film
modules. As cell manufacture requires handling of
still thinner substrate with care, objective work
performing measurements.
A typical laser scribing pattern geometry for thin-
film patterning is shown in Figure 1. Typically, each
250 µm x 250 µm area spans three scribes of both
Issue I 2009
the width and the period between scribes from 30
µm to 50 µm (Figure 2). In each 250 µm x 250 µm
area, 4 measurements should be performed along
each 3 scribes, resulting in 36 measurements. The
current production requires the upper limit of 90
seconds for on-line application, that is, no more
than 2.5 seconds per measurement to obtain a
-pv-management.com required throughput.
.solar
The thin film roughness should be measured as
www
well, with the same unit to avoid time losses
associated with switching between units and
34
possible damage to the cell. The peak-to-valley
depth is typically from several tens of nanometers
to around a micrometer. Antireflection coating
(typically SiN) renders measurement of c-Si solar
cell difficult as the light is absorbed more by the
rough cells to improve on their efficiency (Figure 3
and Figure 4). The surface roughness parameter
(Sa) can reach values as high as several
micrometers with peak-to-peak value of ~10 µm.
For c-Si applications, both the roughness of the
silicon substrate, the width and depth of the laser
Figure 3. Measurement patterning the solar cell layer with scribes that scribe for the contact formation or edge isolation,
of surface roughness separate cells and optical 3D measuring devices to the width and height of the contact as well as the
of c-Si wafer check the conformance of obtained laser scribes extent of the “dead” or “shadow” zone reducing
to manufacturer’s specifications are viable tools for the efficiency of the solar cell, are required
solar cell industry. parameters to be measured. As a typical example,
front-surface scribing on a c-Si solar cell produces
Measurement specifications ~30 µm wide, ~30 µm deep grooves suitable for
Non-contact measurement of the laser scribe buried contact formation. The height and width of
parameters (width and depth of the scribe) is the contact paste (usually Ag of Al contacts) before
necessary to check the laser penetration depth and after co-firing should be accurately measured
and laser spot width especially as wafers move to to meet the solar cell efficiency specifications. The
As the usage of non-contact processing is preferred by
manufacturers, increasing yield requires that non-contact
quality control techniques be used in performing routine
tasks of measuring wafer roughness and surface
properties of the scribed solar cell compounds
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