Complete Report - University of New South Wales

Conductive Paste Firing Tests
Basic cell test structures (Figure 4.3.2.6) were fabricated with a diffused, 100 Ω/sq front
boron emitter. A bare silicon surface was used to avoid added complexities from having to fi re
through an insulating layer. The front grid metallisation was formed by metal stencil printing of
thick-fi lm metal pastes, using stainless steel foil stencil techniques developed in 2004. These
stencils are capable of printing high defi nition contact patterns with fi ngers as narrow as
60 µm in width and up to 30 µm in height. The stencil-printed front contacts were fi red with
varying peak temperatures and durations in a four-zone belt furnace with peak temperature
varied between 700°C and 900°C.
Screen-printed Al contact
p-type emitter
n-type bulk
n+ back surface field
Screen-printed Ag contact
Figure 4.3.2.6: Test structure used for evaluation of front contact shunt resistance.
Figure 4.3.2.7a shows shunt-resistance results obtained for the case of aluminium paste, fi red
with a peak time of 10 s. Results clearly show acceptable shunt resistance for a wide range
of peak temperatures ranging between 720oC and 870oC. Acceptable shunt resistances
were also obtained using silver paste (Figure 4.3.2.7b), however perhaps surprisingly, shunt
resistances obtained for aluminium pastes were often higher than those in cells fabricated
with silver.
Figure 4.3.2.7: Shunt resistances after fi ring a) Ferro 5540 aluminium and b) Ferro 33-466
silver thick-fi lm conductive paste. Peak fi ring time was 10 s.
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