UNIT 6 • INSIDE THE FAB

Air ten thousand times cleaner

ISO 5 ≈ 3,520 / m³

Ordinary room air holds tens of millions of particles per cubic meter. In an ISO Class 5 cleanroom, particles of 0.5 µm and larger are limited to about 3,520 per cubic meter, roughly ten thousand times cleaner. Critical areas run cleaner still.

One particle on the wrong spot can kill a transistor, so the air is engineered.

KEEPING IT CLEAN

Air, people, and wafers

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Filtered airflow

Ceiling filters push air down in a steady, laminar flow that carries particles away

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Gowning

Suits cover skin and hair, the biggest particle source in the room

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Sealed pods

300 mm wafers travel in sealed pods called FOUPs, moved by overhead robots, so people rarely touch them

The best cleanroom strategy is to keep people and wafers apart.

YELLOW LIGHT

Why lithography rooms glow yellow

no UV, no blue

Photoresist reacts to shorter wavelengths: ultraviolet and blue light. Lithography areas are lit in yellow, which lacks them, so wafers can be handled without accidentally exposing the resist.

Even the lighting is part of the process.

THE VERTICAL STACK

From transistors up to wiring

A chip is built bottom-up. Front-end-of-line (FEOL) makes the transistors in the silicon. Middle-of-line adds contacts. Back-end-of-line (BEOL) stacks many layers of copper wiring, separated by insulator, to connect billions of transistors.

Don't confuse this with a factory's 'back end': assembly and test come later.

THE LOOP

How long a chip takes

Layers Dozens of mask layers, each patterned with lithography

Steps Hundreds to well over a thousand process steps

Time Roughly two to three months from bare wafer to finished wafer

Repeat Add, pattern, remove, modify, again and again

A chip is a stack of layers built one at a time.

DIES PER WAFER

How many chips fit?

A rough formula: dies per wafer ≈ π d² / (4 A) − π d / √(2 A), where d is the wafer diameter and A the die area. A 100 mm² die on a 300 mm wafer gives about 640 dies. Dies cut by the edge are incomplete and wasted. In this picture, 30 × 30 mm dies leave 57 full dies.

Bigger dies waste more of the wafer's edge, which is one reason small dies are cheaper.

UNIT 6 STUDY COMPLETE

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Ready for the Fab Challenge?

You can describe how a cleanroom and the FEOL-to-BEOL stack fit together.