Lesson 13.2Micro & nanoChapter 13 · Micro- and nanofabrication

Photolithography

Projects a pattern with light onto a light-sensitive coating, then etches it

Photolithography prints patterns onto a silicon wafer with light, much like developing a photo through a stencil. It draws the billions of transistors and wires on a computer chip, one layer at a time. Every chip in your phone went through it dozens of times.

Photo: Le hollandais volant, CC BY 4.0, via Wikimedia Commons

One photolithography cycle in six cross-sectionsEach panel is a cut through a tiny piece of the wafer after that step; the circled numbers match the steps below.

How it works

Spin-coat the resist. The wafer already carries a thin layer to be patterned, such as silicon oxide or metal. A few drops of photoresist (a light-sensitive liquid plastic) land on it, and the wafer spins at thousands of rpm, leaving an even film about a micrometer thick or less. A short bake dries it.
Align and expose. A mask (a glass plate carrying one layer's pattern in chrome, also called a reticle) is lined up with the patterns already on the wafer, to within a few nanometers on advanced chips. UV light shines through its clear parts, and a lens shrinks the image, typically four times, onto the resist. The machine, called a stepper or scanner, repeats this field by field across the wafer.
Develop. A developer liquid washes over the wafer. With the most common positive resist, the areas hit by light dissolve, leaving openings shaped like the mask's clear areas.
Etch. In a plasma etcher (a chamber of electrically charged gas), the plasma eats away the layer only where the resist is open; elsewhere the resist shields it. Instead of etching, the openings can also receive new material or implanted atoms.
Strip. The leftover resist is removed with solvents or an oxygen plasma. The pattern now exists in the layer itself.
Repeat. A new layer is added and the cycle starts again with the next mask, each pattern aligned to the ones below. An advanced chip goes through dozens of these cycles.

Key facts

Materials: Silicon wafers coated with photoresist; also glass, compound semiconductors and circuit boards
Typical parts: Flat patterns stacked layer by layer: transistors, wiring, sensors, display backplanes
Part size: Wafers up to 300 mm across; one exposure covers a field of about 26 × 33 mm
Precision: About a micrometer with simple lab aligners; tens of nanometers with 193 nm laser light; near 10 nm with EUV (13.5 nm) light
Surface finish: Leaves no texture of its own; patterned wafers shimmer in rainbow colors
Batch size it suits: Mass production: one mask set serves thousands of wafers, each holding hundreds to thousands of chips
Tooling and machine cost: Lab mask aligners: tens to hundreds of thousands of dollars; EUV scanners: hundreds of millions; an advanced mask set: millions
Speed: A production scanner exposes roughly 100–300 wafers per hour, one layer each

Strengths and limits

Strengths

Patterns a whole wafer in parallel: billions of features at once
Reaches nanometer-scale detail with modern scanners
Very repeatable: the backbone of all chip production
Very low cost per transistor at high volume

Limits

Extremely expensive machines, masks and cleanrooms
The wavelength of the light limits the finest detail
Only flat layers; 3D structures come from stacking dozens of them
Needs dust-free, vibration-free rooms under yellow light

Where it's used

ElectronicsProcessors, memory chips, camera image sensors
DisplaysThe transistor grid behind LCD and OLED screens
Sensors and MEMSAccelerometers, microphones, pressure sensors
PhotonicsLED chips, laser diodes, light guides on chips
Circuit boardsCopper tracks, patterned with dry-film resist
ResearchMicrofluidic "lab-on-a-chip" devices

How to recognize it

The patterns are invisible to the eye. The finest lines are thousands of times thinner than a hair and need an electron microscope to see. They hide inside every chip in your phone, laptop and car, and behind every pixel of your screen.
Rainbow shimmer. Tilt a bare wafer or chip die under light and it flashes rainbow colors: its regular patterns split light the way a CD does.
Yellow-lit rooms. In factory photos, the lithography area glows yellow, because the resist ignores yellow light.
Circuit boards. The fine copper tracks on a green circuit board were also defined by light through a mask, at a much coarser scale.

Compare with

Chemical machining and photochemical etching6.8the same light-and-resist trick at millimeter scale, used to etch parts out of thin metal sheets.LIGA13.7uses X-rays and very thick resist to make tall micro-parts rather than thin layers.Focused ion beam13.5writes or cuts patterns point by point with no mask: flexible, but far too slow for mass production.

Watch it

TED-Ed · A short animated overview of chipmaking and where lithography fits

Branch Education · Detailed 3D animation of every fab step, including the scanner

Conformable Decoders · Real lab photolithography, step by step

ASML · How the most advanced scanners make their EUV light

See real photos

Wikipedia: Photolithography — diagrams of the steps and photos of lithography equipment
Wikimedia Commons photo search

Remember

Photolithography prints a whole chip layer at once, like developing a photo through a stencil.
Coat, expose, develop, etch, strip, then repeat: dozens of cycles per chip.
Shorter-wavelength light prints finer details, which is why EUV machines exist.