started work, together with its project partners, on SUGAR, a project under
the EU’s 7th framework program for ICT. The project’s goal is to
develop a process for the integrated production of photovoltaic modules based
on ultra-thin crystalline Si.
This includes finding a way to produce 50µm thin crystalline Si, and
a process to fabricate modules from this ultra-thin Si with state-of-the-art
conversion efficiency. Imec is SUGAR’s project coordinator; the other
partners are 4PICO (Netherlands), Applied Materials Baccini (Italia), Ferro
(Netherlands), Bosch Rexroth (Netherlands), Dow Corning Europe (Belgium), Universidade
De Lisboa (Portugal), Semilab (Hungary), Cemef-Mines (France), and Fraunhofer
Ultra-thin Si foil, detached from Si tile with SLiM-cut
The optimal thickness of solar cells made from mono-crystalline Si is around
50µm. Reducing the thickness of solar cells from the current de-facto
standard of 180µm to 50µm would result in modules that are approximately
10% cheaper. The SUGAR project aims to solve the two main technological challenges
that stand in the way of reaching that cost reduction. One is to produce solar
wafers of 50 microns, and the second one is to process them into solar cells.
Fabricating 50µm thick wafers
The first goal of SUGAR is to develop a prototype line that takes Si tiles
(wafers with a thickness above 1cm) as input, and that outputs large-area (50cm²)
ultra-thin wafers (~50µm). The line would have a target throughput of
100 wafers per hour, and the wafering process should be with minimal kerf loss.
The basis of that line will be an innovative and proven method, nicknamed SLiM-cut.
With SLiM-cut, a high-stress field is applied to a Si wafer so that a crack
propagates in the Si substrate parallel to the surface at a given depth. The
top Si layer is separated from the parent substrate and processed into a solar
cell. The parent substrate can be re-used for further wafering.
To apply the stress, a material (e.g. a metal) with a thermal expansion coefficient
different from Si is deposited and bonded at a high temperature on top of the
thick crystalline Si wafer. When the wafer cools down, the mismatch of the thermal
expansion coefficient between the top material and the Si induces the stress
field. This field initiates and propagates a crack in the Si, close to and parallel
with the surface. Next, the top layer of the Si and the attached metal layer
snap off from the parent substrate. The metal layer is removed from the Si foil
in a metal-etching solution, resulting in a clean and stress-free ultra-thin
Si foil. The substrate can be re-used to peel off further layers.