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In the old times (up to the 150
mm wafer diameter era), wafers had flats, and the flats told you two
things: |
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- The doping type of the wafer (n- or p-type
- The orientation of the wafer: {100} or {111}
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While this is trivial information,
consider: All wafers, whatever doping type or crystal orientation, look exactly
the same! As soon as a wafer has been removed from its box that carried this
and other information, you can't see anymore what you got. You also cannot
measure it easily (and without destroying the wafer). |
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And if something goes wrong here (and
things that can go wrong will go wrong some day), it may be a horribly
expensive mistake! If you feed wafers of the wrong doping kind into the line,
it will really, really cost you - probably your job. |
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So here is the convention |
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But beware! Wafer manufacturers will
produce whatever the customer wanted, and after n-type Si went
out of style for most mass-produced chips, the only reason for a flat was to
allow the patterns to be made to be aligned with a crystallographic
direction. |
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You then ordered your p-type
{100} wafers with only one flat in the <110> direction. One
reason was that the wafer would easily cleave along this and the respective
perpendicular direction. |
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So wafers with diameters larger or
equal to, say, 100 mm and just one
flat are more likely {100} p-type than the "proper" {111}
p-type. The picture in the
backbone, e.g., shows p -type {100} 150 mm wafer! |
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And wafers with diameters larger or
equal to, say, 200 mm, probably will have no flat at all, but just a
small "notch" - simply
because you loose too much expensive area by cutting of a flat. |
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So, how can you tell what you have -
if you don't trust the one flat there is, or if there is none! There are
extremely simple ways of checking: |
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Checking
doping type: Take an Voltmeter and measure they
thermovoltage
between a hot tip (tale a soldering iron) and a room-temperature tip pressed on
the wafer somewhere. Its sign will tell you
if the wafer is n- or p-type. Which is which results from hard
thinking or from checking a known piece of Si. |
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Checking
orientation: Break your wafer. If the pieces tend to be rectangular,
it was {100}. |
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In either case your wafer is now
"dead", i.e. no longer usable for making IC's. |
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© H. Föll
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5.2.1 Process Integration