WHY MEASURE

You can't fix what you can't see

measure, then adjust

Every process step in this course has a target: a certain film thickness, a certain line width, a certain alignment. Metrology is the practice of actually measuring what got built, so a fab can catch a process drifting off target long before it ruins a whole lot of wafers.

A target nobody measures against is just a hope.

WHAT GETS MEASURED

The numbers that matter most

📏

Critical dimension (CD)

The width of the smallest, most important features

🎯

Overlay

How precisely one layer lines up with the layer below it

📐

Film thickness

How thick a deposited or grown layer actually came out

These three numbers are checked constantly, often after every single layer.

CD-SEM

An electron microscope, measuring instead of imaging

resolution down to a few nanometers

A CD-SEM, critical-dimension scanning electron microscope, scans a beam of electrons across a feature and measures its width directly from the resulting image, with resolution down to a few nanometers. It's slower than optical methods, so it usually samples a handful of sites per wafer rather than every die.

CD-SEM trades speed for the resolution a modern feature size actually needs.

OVERLAY

Layer two has to land exactly on layer one

measured in nanometers of misalignment

Overlay measures how far one patterned layer's alignment marks sit from the layer beneath it, in nanometers. As features shrink, the tolerance for that misalignment shrinks right along with them, which is why scanners spend so much effort on alignment before every exposure.

A few nanometers of overlay error can be the difference between a working contact and an open circuit.

UNIT 33 STUDY COMPLETE

📏

Ready for the Fab Challenge?

You've covered the tools that measure critical dimension, overlay, and film thickness.