PART 5 • THE CMOS PROCESS FLOW
The CMOS Process Flow
Front-end, back-end
Every process step sorts into one of two halves
The metal stack
Up to fifteen layers of wiring on top of the transistors
Logic gates
Transistors, paired up, become the gates a chip computes with
THE SPLIT
Every process step is one or the other
A fab flow splits cleanly in two. Front-end-of-line, FEOL, builds the transistors themselves: wells, gate, implants, and the contacts that reach them. Back-end-of-line, BEOL, wires those transistors together with layers of metal, from the first contact up to the pads that connect the finished chip to the outside world.
WHAT COUNTS AS FEOL
Everything up to the first contact
Wells and implants
The doping that sets up each transistor
The gate stack
Oxide and poly, patterned and etched
The anneal
The heat step that activates every implant
WHAT COUNTS AS BEOL
Everything above the first contact
Contacts
Tungsten plugs reaching down to the transistor
The metal stack
Layer after layer of copper wiring
Pads
The metal squares wire bonds attach to later
ONE STACK, TWO HALVES
Transistors below, wiring above
Picture the whole chip as a single tall stack. Near the silicon, front-end work builds the transistors. Above that, back-end work adds contact, then metal layer after metal layer, each one wider and farther apart than the last.
WHY SPLIT THE FLOW AT ALL
Different tools, different rules
FEOL steps run hot, sometimes over 1000 degrees C, and work directly on silicon. BEOL steps run cooler, since high heat would damage the transistors underneath, and work mostly with metal and dielectric instead of silicon. Engineers, equipment, and even entire fab buildings often specialize in one half or the other.
UNIT 25 STUDY COMPLETE
Ready for the Fab Challenge?
You've covered why every fabrication step sorts into front-end-of-line or back-end-of-line.