PART 6 • YIELD AND METROLOGY

Yield and Metrology

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Defects

The flaws that turn a good die into a bad one

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Metrology

Measuring what was actually built, not just what was designed

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Yield models

Turning defect data into a number that predicts profit

By the end of this part, you can read a wafer map and explain what actually drives yield.

THE DEFINITION

Good die, divided by total die

yield = good die / total die

Yield is the fraction of die on a wafer that pass every test and actually work. A wafer might hold hundreds of die, and yield simply asks how many of them came out good. It sounds simple, but almost everything in a fab exists to push that one number up.

Every process step this course has covered exists, in part, to protect yield.

WHY IT'S EVERYTHING

Cost scales the wrong way with yield

A wafer costs about the same either way Whether 95% or 40% of its die work

Low yield means fewer good die So each good die absorbs a larger share of that fixed cost

A few points of yield Can be the difference between a profitable product and a loss

Yield doesn't just measure quality: it sets the price of every chip that ships.

GOOD DIE PER WAFER

Bigger die, fewer chances

smaller die, more die per wafer

A wafer's usable area is fixed, and die near the very edge are usually thrown out because the pattern there is incomplete. Smaller die fit more chances per wafer and lose a smaller fraction to that edge, which is one reason die size is such a carefully guarded design choice.

Every square millimeter of die size is a square millimeter yield has to defend.

A WAFER MAP

One square, one die, one verdict

After testing, every die on the wafer gets colored by whether it passed. Most of this map is good die, with a scatter of individual failures spread randomly across it.

A wafer map turns a pass/fail list into a picture you can actually read.

WHAT LOWERS YIELD

Three broad causes

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Defects

Particles, pattern flaws, and contamination that physically break a die

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Parametric failures

A die that works, but too slowly or with too much leakage

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Design marginality

A design too sensitive to the process's normal variation

Not every failure is a defect: some are the design itself running out of margin.

UNIT 31 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered what yield means, and why it decides whether a fab makes money.