Complete Report - University of New South Wales

4.3.2.4 Process Engineering – Contact Silicide Formation
Contact resistance and contact adhesion is important for the formation of high-effi ciency
silicon solar cells. In 2005, we studied the formation of nickel silicide for n-type silicon solar
cells. High effi ciency n-type solar cells use both heavy boron contact diffused (BCD) and heavy
phosphorus contact diffused (PCD) regions to form the metal semiconductor regions. Effective
metallization requires the simultaneous deposition of electroless nickel and formation of nickel
silicide in both BCD and PCD regions.
In this study, we form arrays of small-area contacts, formed by laser drilling a small hole
in an oxidized silicon wafer. The resulting inverted, square-based pyramid “pits” are then
diffused with either a BCD or PCD, which are subsequently deglazed with an HF-based etch.
The diffused pits are then subjected to an electroless nickel deposition process, followed by a
350°C sintering process in nitrogen. The small pits afford a better view of the nucleation and
sintering processes compared to grooves.
Figure 4.3.2.22 shows two diffused and nickel plated pits that were formed with insuffi cient
diffusion glass deglazing. Figure 4.3.2.22a shows poor deposition in a poorly deglazed PCD
contact pit, while Figure 4.3.2.22b shows poor nucleation on a BCD pit.
Figure 4.3.2.22: Nickel plated to a pits in a silicon wafer
with (a) PCD and (b) BCD pits.
(a)
(b)
An improved deglazing process leads to improved plating
in the PCD pit, exhibiting a smooth, conformal layer of
nickel plated over the entire contact surface, as shown
in Figure 4.3.2.23a. The nucleation in the BCD is still poor, as shown in Figure 4.3.2.23b, due
to the poor affi nity of the alkali-based nickel solution to plate p-diffused surfaces.
Figure 4.3.2.33: Properly deglazed pits with a heavy a)
PCD and b) BCD.
(a)
(b)
The samples in Figure 4.3.2.22 were sintered in a
furnace at 350°C for 10 minutes in nitrogen to form
a nickel silicide at the interface between the nickel and
the heavily diffused silicon. The un-reacted nickel was
completely removed by etching and the surface of the pits examined (Figure 4.3.2.24). There
is clearly interaction between the silicon and the nickel as is evidenced by the roughening of
the surface on the PCD pit. The nickel was so badly nucleated on the BCD pit that the Si has
been drawn out from the interface such that it almost fi lls the whole pit, forming a poor-quality
physical metal-semiconductor interface.
Figure 4.3.2.24: Pits on the same sample as that shown
in Figure 4.3.2.20 after sintering and nickel removal.
(a)
(b)
Palladium activation is effective in increasing the
nucleation density of electroless nickel plating on both
p++ and n++ diffused silicon surfaces. Figure 4.3.2.25
shows PCD and BCD pits that have been well deglazed,
activated with a palladium solution and subsequently plated with electroless nickel. The
electroless nickel plate is more uniform in the palladium activated BCD pit compared to
without palladium activation.
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