FROM BARE WAFER TO SWITCH

Every part of this course, in one device

wafer, wells, gate, implants, anneal

A single transistor pulls in nearly everything covered so far: a pure wafer, a grown or deposited gate oxide, a patterned and etched gate, implants placed and masked by lithography, and an anneal that activates them without smearing the profile.

One finished transistor is the whole course, compressed into a few square micrometers.

THE BUILD, IN ORDER

Seven steps, roughly

Well implant n-well or p-well, doped and annealed first

Gate oxide Grown thermally, a few nanometers thick

Gate poly Deposited, then patterned and etched by lithography

LDD implant Self-aligned to the gate's edge, no extra mask needed

Spacer A deposited film etched back to a sidewall on the gate

Source/drain implant The heavier, deeper implant, set back by the spacer

Spike anneal Activates every implant made so far, all at once

Notice how few extra masks this needs: the gate itself does most of the aligning.

A REAL NUMBER

How small is small?

gate length: tens of nanometers

A modern logic transistor's gate can be well under 20 nanometers long, a few hundred silicon atoms. At that scale, an LDD implant only a few nanometers deeper than intended, or an anneal a few degrees too hot, can measurably change how the device switches.

At this scale, doping isn't approximate: a few nanometers is the whole margin.

UNIT 24 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered putting doping, implantation, and the gate stack together into one transistor.