Transcript No Slide Title

Incomplete Notes
Solid State Electronics
IC Fabrication Technology - PT2
Ronan Farrell
Recommended Book:
Streetman, Chapter 9
Solid State Electronic Devices
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Incomplete Notes
Making a MOSFET
Silicon
surface
N-Well
26
Incomplete Notes
Making a MOSFET
SiO2 grown on either side of our area of interest. It acts
as a buffer between this device and a nearby device.
Field
Oxide
N-Well
27
Incomplete Notes
Making a MOSFET
SiO2 and then polysilicon P++ deposited over all of the wafer
Polysilicon
Oxide
Field
Ox
N-Well
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Incomplete Notes
Making a MOSFET
Using a mask to ensure that we don’t etch the centre piece,
apply an acid to remove all the unexposed photoresist and the
additional layer of SiO2 (note not all of it, leave the boundary
layers behind. We’ve now got our gate.
Field
Ox
N-Well
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Incomplete Notes
Making a MOSFET
Using a mask again to control location, apply doping so that
a lightly doped region exists on either side of the gate. This
means close to gate we won’t get the dopant diffusing under it.
P-minus
Field
Ox
N-Well
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Incomplete Notes
Making a MOSFET
Grow a small area of additional oxide around the gate,
you’ll see why on the next slide.
Oxide Spacer
P-minus
Field
Ox
N-Well
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Incomplete Notes
Making a MOSFET
A heavily doped P+ region is now formed on either side
of the gate. The spacer guaranteed that the N-type channel
under the gate was protected. So note, a heavily P+ region
connected to a lightly doped P- region, an N type channel and
then the same again on the other side.
Oxide Spacer
P-
P+
Field
Ox
N-Well
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Incomplete Notes
Making a MOSFET
Over all of this place a layer of oxide to close off the entire
MOSFET.
Silicon Oxide
Field
Oxide
N-Well
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Incomplete Notes
Making a MOSFET
Using a mask cut three holes through the new oxide layer
to the polysilicon and the two heavily doped P+ regions.
Silicon Oxide
Field
Oxide
N-Well
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Incomplete Notes
Making a MOSFET
Fill these with metal and top it all off with an additional
layer of oxide. Normally tungsten is used for these connections
as it bounds well to the silicon.
Drain
Gate
Source
Silicon Oxide
Field
Oxide
N-Well
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Incomplete Notes
Making a Resistor
Resistors are easy, use a resistive piece of semiconductor,
ie some lightly doped region and vary size accordingly.
Resistors require a lot of space as they need to be long and narrow
to get the resistance up, ie the number of squares.
Top Down View
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Incomplete Notes
Making a Capacitor
Capacitors are also easy, a common technique is to use the metal wiring layers of
the semiconductor process. On two levels make a large square sheet of metal.
The oxide between the layers acts very well as a dielectric. There are refinements
to this and fringing capacitance is a problem but basically capacitance is proportional
to area. Using more than two levels and alternating the polarity increases the capacitance.
Side View
One side (+)
Other side (-)
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Incomplete Notes
Making an Inductor
Inductors are very hard to
make in an integrated chip.
Inductors need coils but we
can’t get this 3D structure in
what’s basically a 2.1D
technology. The best
solution so far is to make a
coil. This does provide an
inductance but it is weak and
lossy.
One end
Other end
The other source of
inductance is actually the
bond wires and that is an
inductance you don’t want as
it leads to oscillations.
Basically avoid having to use
an inductor in your circuit if
you can avoid it.
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