25. Solid State Electronics
IC Fabrication Technology - PT2
Ronan Farrell
Recommended Book:
Streetman, Chapter 9
Solid State Electronic Devices

26. Making a MOSFET
Silicon
surface
N-Well

27. 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

28. Making a MOSFET
SiO2 and then polysilicon P++ deposited over all of the wafer
Polysilicon
Oxide
Field Ox
N-Well

29. 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

30. 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

31. 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

32. 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

33. Making a MOSFET
Over all of this place a layer of oxide to close off the entire
MOSFET.
Silicon Oxide
Field Oxide
N-Well

34. 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

35. 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

36. 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

37. Making a Capacitor Side View One side (+) Other side (-)
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 (-)

38. Making an Inductor One end Other end
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.
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.
One end
Other end