SPI Replacement Sputter Coater Cathodes
The relative advantages and disadvantages of the different metals
Trade-Offs between the Metals
For most SEM users, we recommend that which is the nearly unanimous choice
of SEM users generally: gold. The work function of gold is the lowest of
the precious metals, meaning that all else being equal, more gold will be
sputtered in a unit of time than any other metal. For those working with
heat sensitive samples, this is clearly the metal of choice, since the
fragile sample is exposed to an absolute minimum or radiant heat.
The disadvantage of gold is that the grain size starts to become a factor at
the upper magnification ranges of most modern SEMs, and there is a constant
search for metallization with smaller grain sizes. The best way to obtain
minimum grain size is to make use of the
OPC Osmium Plasma Coater because
with this system, one has zero grain size. But we do realize that
many would not have the budget for the purchase of such a system.
Platinum does give a slightly smaller grain size, and for those looking for
the smallest grain size without resorting to the purchase of an osmium
coater, Pt is often times their first choice. However the grain size
reduction, while significant, is still not that much smaller, so one has
little difficulty picking up the grain size in today's modern SEMs as well.
In some instances, because of the higher work function, and therefore longer
sputtering times, the increased radiant heat exposure negates the benefits
of what would otherwise be a smaller grain size.
The allow of 60% gold/40% palladium was popular when evaporations were being
done via a vacuum evaporator (not by sputtering) and indeed, the alloy was
indeed shown to result in a smaller grain size than for gold alone. But the
physics of sputtering is clearly different from the physics of vacuum
evaporation. And we are not aware of anyone who has demonstrated that the
grain size of the alloy, for sputtering is indeed smaller than the grain
size for pure gold. However, the average work function of the alloy is
higher than for pure gold and as a result, longer sputtering times are
needed to reach the same thickness. Furthermore, since the average atomic
number is less, the secondary electron emission is less, and under certain
circumstances, contrast will be less. Our own opinion on this is that it
would be an extraordinary situation for someone to be able to justify a
gold/palladium target instead of using pure gold.
The main advantage of silver is that it is cheap! However, as is well known
by anyone who has had in their home items of sterling silver, the metal
tarnishes easily, and indeed, although one can sputter coat a layer of
silver on a sample, the rate at which the pristine metal surface oxidizes,
or sulfides to a non-conductive skin layer is so fast, that it is just not a
recommended solution for most users. However it might be just fine for
student use in a lab class, where the samples are looked at immediately
after being prepared.
We hope this dialogue has been useful for one new to the field and needing
this kind of guidance in terms of selecting their cathode metal of choice.
Remember also, to take advantage of our cathode recycling program
and keep these non-renewable resources in the stream of commerce and
keep them from being sent off to a land fill.
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Tuesday December 02, 2008
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