How LEDs are made

Grow crystal layers from gas

A sapphire wafer rests on a heated holder called a susceptor while a carrier gas brings metalorganic molecules across it. They react on the surface and build single-crystal films layer by layer. For blue and green LEDs the film is indium gallium nitride.

Pattern the contacts

Liquid photoresist is dripped on the spinning wafer and hardened. Ultraviolet light through a mask copies the contact pattern onto it, and a developer washes the exposed areas away.

Evaporate on metal

In a vacuum chamber, a chunk of metal is heated until it vaporizes and condenses on the open areas of the wafer, the way steam fogs a cold window. Solvent lifts off the resist, and a furnace bonds metal to crystal so it will not flake.

Cut out the dies

The wafer is snapped along a crystal plane or sawn with a diamond blade. This step is hard and error prone, and it costs far more dies than the pattern promises.

up to 6,000 on a 2-inch wafer

Mount and wire

Each die goes onto its package, its metal back touching one lead. A tiny gold wire runs from the other lead to the top contact, pressed on with a fine needle so the soft gold deforms and sticks.

Coat with yellow phosphor

For white light, a blue die is covered with cerium-doped yttrium aluminium garnet crystals suspended in the package. Some blue light passes through and some turns yellow, and the mix looks white.

Seal in plastic

The die and wires sit in a mold while clear plastic seals around them. The plastic's shape and tint decide whether the light scatters or leaves in a narrow beam.

Test brightness and color

Every LED is checked to see that a set current gives the right brightness. Color is tested batch by batch, and some units go off for lifetime, heat and damage tests.