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UNIT 17 • IMMERSION, MULTI-PATTERNING, AND EUV

Single exposure ran out of room

The limit ArF immersion with one exposure resolves about 40 nm half-pitch
The demand Chip makers wanted features far smaller than that
The options Split the pattern across multiple exposures, or move to a much shorter wavelength
What fabs did Both, in sequence: multi-patterning first, then EUV
When the light cannot shrink further, the process must get more clever.

MULTI-PATTERNING

Print half the pattern at a time

1 Mandrels print lines 2 Spacers conformal film 3 Etch back sidewalls stay 4 Strip spacers remain orange = printed resist lines (mandrels) teal = spacer film grown on their sides 3 printed lines become 6: pitch is halved

In self-aligned double patterning, a coarse pattern of mandrels is printed. A thin spacer film is deposited over them and etched back, leaving spacers on each sidewall. The mandrels are removed, and the spacers form twice as many lines at half the pitch. Repeating the trick gives quadruple patterning.

Deposition and etch can double a pattern that lithography alone cannot print.

THE PRICE OF MORE STEPS

Multi-patterning is not free

Extra steps Every extra exposure, etch, and clean adds cost and cycle time
Overlay risk Multiple exposures must align to each other very precisely
Yield More steps mean more chances for a defect
The pull toward EUV One EUV exposure can replace several immersion exposures
EUV was worth developing partly because multi-patterning got so complicated.

EXTREME ULTRAVIOLET

Printing with 13.5 nm light

CO₂ laser hits a tin droplet about 50,000 droplets per second Tin plasma emits 13.5 nm light a tiny, very hot flash Collector mirror gathers the light Illuminator mirrors shape and aim the beam Reflective reticle the mask is a mirror, too Projection mirrors shrink the image about 4× Wafer resist is exposed all inside a vacuum

EUV light has a wavelength about 14 times shorter than 193 nm light. Almost everything absorbs it, including glass and air, so EUV scanners use only mirrors and operate in a vacuum. Even the reticle is a mirror.

EUV changes almost everything about the machine: source, optics, and mask.

MAKING EUV LIGHT

Tin plasma, 50,000 times a second

The droplets Tiny tin droplets stream through the source chamber
The lasers A CO₂ laser hits each droplet twice, flattening it and then vaporizing it into a hot plasma
The result The plasma radiates EUV light, and a collector mirror gathers it
The efficiency Hundreds of watts of laser-produced light are made, but only a small fraction reaches the wafer
EUV light is expensive to make and mostly lost before it prints.

EUV OPTICS

Mirrors, mirrors, mirrors

🪞

Multilayer mirrors

Alternating molybdenum and silicon layers, roughly 40 pairs, reflect about 70% each

🎯

Atomic smoothness

Mirrors are polished so smooth that errors are measured in fractions of a nanometer

🏭

One supplier

ASML in the Netherlands is today the only company that builds production EUV scanners

💶

The cost

A scanner costs well over 150 million dollars, and high-NA models cost far more

EUV works because of some of the most precise optics ever made.

UNIT 17 STUDY COMPLETE

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Ready for the Fab Challenge?

You can explain immersion, multi-patterning, and how EUV light is made.