🔥 0 ⚡ 0 XP
← Back to Path
1 / 8

PART 3 • PRINTING THE PATTERN

Mask, light, resist

🎭

Mask

A plate with the pattern for one layer, drawn as clear and opaque regions

🔦

Light

Shines through the mask and is focused into a tiny image on the wafer

🧴

Photoresist

A coating that chemically changes wherever the light lands

Light carries the pattern, and the resist remembers it.

UNIT 13 • WHAT IS LITHOGRAPHY?

Coat, expose, develop, repeat

1 · Coat spin on photoresist 2 · Expose light through a reticle 3 · Develop dissolve the soft resist 4 · Etch or implant the pattern reaches the wafer 5 · Strip remove the leftover resist repeat for every patterned layer

A wafer gets coated with photoresist, exposed to a patterned image, and developed so part of the resist washes away. The remaining resist protects some areas while the wafer is etched or implanted. Then the resist is stripped and the wafer heads to the next layer.

Lithography is a loop, run once for every patterned layer.

PHOTORESIST

A coating that changes when light hits it

light through the mask develop Positive resist exposed resist washes away light through the mask develop Negative resist exposed resist stays

Resist is a polymer that reacts to light. After exposure, a developer solution washes away either the exposed or the unexposed part, depending on the resist type. What is left is a relief pattern: raised resist over the areas to protect.

Exposure makes a chemical difference that development turns into a physical one.

POSITIVE VS NEGATIVE

Two flavors of resist

Positive resist Exposed areas become soluble and dissolve. Most leading-edge chip patterning uses positive-tone processes
Negative resist Exposed areas cross-link and stay. Widely used for thick films in MEMS and packaging, such as SU-8
Why it matters The mask must be drawn as the opposite pattern depending on which tone you use
Same light, opposite result: pick the tone that fits the layer.

THE RETICLE

A stencil for one layer

🪟

Fused silica plate

A very flat, transparent quartz plate, about 6 inches (152 mm) square

⬛

Chrome pattern

Opaque lines on the plate block light where the pattern says 'dark'

🔍

Four times larger

Features on the reticle are 4× the size they will print on the wafer

📚

One per layer

Each patterned layer of the chip has its own reticle

A reticle is a master copy of one layer, drawn larger than life.

COUNT THE LAYERS

How many masks does a chip need?

Older chips A few tens of masks, printed with simpler tools
Advanced logic Dozens of masks, often well over 50, including some that need several exposures
Each mask is a loop Coat, expose, develop, etch or implant, strip, and clean, so every added mask adds days to the wafer's trip
The consequence Lithography steps dominate a fab's cost, schedule, and equipment count
The more layers a chip has, the more times the fab runs the litho loop.

WHAT THE PATTERN IS FOR

The resist is a temporary stencil

✂️

Etch mask

Resist protects the film below while plasma etches everywhere else

💉

Implant mask

Resist blocks dopant ions from areas that must stay untouched

🪄

Lift-off stencil

Metal is deposited over the resist, and washing the resist away lifts off what is not wanted

Lithography only draws the plan. Etching, doping, and deposition do the building.

UNIT 13 STUDY COMPLETE

🎭

Ready for the Fab Challenge?

You can explain what a reticle and photoresist do in the litho loop.