Steve Thompson wrote:
>>> Some semiconductors I worked with years ago were rated for 25 years
>>> continuous operation at maximum temperature. It was said that the
>>> tracks inside the part migrated to the next nearest track over time
>>> and stress.
>>>
>> I doubt the tracks themselves migrate.
>>
>
> The metal does migrate, dependent on temperature and current
> density.
True. Two pieces of gold placed together will eventually bond. IIRC The
migration of gold through silicon is very slow except at very high
temperatures which would be high enough to create problems with the
doping migration and minority carriers on their own. Of course in
today's devices with very small trace spacing and extremely small traces
a number of problems crop up including conductor erosion from electron
flow, particularly when looking at devices with 40 nanometer traces and
equal spacing and millions or billions of transistors per die.
> Aluminium is much worse than gold (these are the two
> metals mostly used in rf semiconductors). Amongst others, TRW
> published some work on this which was carried as a App note in the
> Motorola RF data books after the takeover.
Aluminum, again IIRC doesn't tend to cold flow or migrate as much under
the same mechanical conditions, but there are some serious problems with
Aluminum as it is an active material with Silicon. Aluminum is a dopant
although not normally used intentionally as such, and contaminant of
which gold is not normally considered to be. So the Aluminum infuses the
base silicon at higher temperatures as if it were a dopant and adds
receptor atoms creating a layer of P type around the trace. I believe
it's P type but if it's N the results are pretty much the same as far as
a containment although electrically the results are a bit different. The
interaction with Aluminum is far more complex than that of gold.
Where the gold mainly creates a wider and more leakage prone trace the
Aluminum is complex and produced what I recall as a doped layer of P
type Silicon around the Aluminun trace. Depending on base and near by
materials you can end up creating a series of rather large diodes which
can make a device act and test strangely besides rendering it pretty
much useless.
Remember I'm drawing on memories from 20 to 30 years back so they may be
off a bit.
> There's also something known as 'purple plague' - not migration,
> but reaction between gold and aluminium that showed up when gold
> metallised die were bonded with aluminium wires.
>
That I'm not familiar with. I would not expect Gold and Aluminum to
readily bond with out creating some kind of interface.
73
Roger (K8RI)
> Steve
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