TEST & MEASUREMENT
height of the contact can reach several tens of
micrometers (Figure 5).
Non-contact measurement system
Ideal 3D measurement units for the on-line quality
control of both c-Si and thin-film solar cells include
fast, light-weight and compact non-contact optical
sensors, which combine several capabilities in one
system:
ring6 measurement of roughness from 10 nm to
several micrometers in low-light situations like in
the case of measuring the roughness of SiN
anti-reflection coated c-Si substrates;
ring6 measurement of widths and depths of laser
scribed grooves with depths of several tens
of microns;
ring6 measurement of contact heights (up to tens
of microns) and widths.
Such optical sensors use new high-resolution
digital wavefront sensors. The digital wavefront
sensing technology uses the minimum use of
hardware components and relies on the intensive
35
use of specialized algorithms. The digital wavefront
sensors rely upon measurements of the energy
www
redistribution in the 3D space: they measure the
.solar
variation of the wave’s intensity in the optical – axis
direction, while as interferometers, digital wavefront
-pv-management.com
sensors measure the redistribution of the wave’s
intensity in the transversal direction. However,
unlike interferometers, no reference plane is used:
the measured object itself serves as a self-
referencing tool. The obtained phase shifts in the
wavefront are proportional to the 3D form of the
observed solar cell’s surface in reflection. As
Issue I 2009
technological innovation, the digital wavefront 173 mm) make the sensor an ideal tool for being Figure 4. Measurement
sensing technology is based on the prevalence of integrated in turnkey solar cell non-contact testing of roughness
software as compared to conventional use of systems having arms and motors with relaxed parameters of a c-Si
hardware elements to achieve highest wavefront weight requirements. wafer: surface
sensing performances, resulting in a vibration - roughness (left) and
insensitive, compact and light – weight optical Conclusion profile roughness
sensor (Figure 6). As the usage of non-contact processing is across an arbitrary
preferred by manufacturers, increasing yield profile (right)
With the optical sensor, areas of 250 µm x 250 µm requires that non-contact quality control
can be measured in 1.4 million points in no more techniques be used in performing routine tasks of
than 2.5 seconds, with <50 nm repeatability and measuring wafer roughness and surface properties
40 nm lateral resolution. The working distance of of the scribed solar cell compounds.
the system is 17 mm from the wafer, thus
eliminating the risk of damaging the wafer and With the improving performances of laser scribing
reducing the measurement time (otherwise, the in terms of depth and width of scribes as laser
optic head need be withdrawn from the penetration depth can be controlled by the laser
measurement area before moving to the next area spot’s quality, systems for measurement of
in order to protect the sample). The working patterns made by lasers should feature better
distance of 17 mm is also useful to avoid using the characteristics in terms of both optical
stepper motor with vertical motion capability. Light performance (resolution, accuracy) and use
weight (0.8 kg) and compact housing (135 x 64 x (portability, weight).
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