How are LEDs made?

The Manufacturing Process

The first step in the manufacturing process is the making of a semiconductor wafer.  The composition of the wafer can be, GaAs, GaP, GaAsP or InGaN, this depends on what color LED is being made.  The crystalline semiconductor is grown in a high temperature, high pressure chamber.  Gallium, arsenic, phosphor, Indium and/or Phosphate are purified and mixed together in the chamber.  The heat and pressure liquefy and press the components together so that they are forced into a solution.  So that the gas doesn't escape a layer of liquid boron oxide is used to cover it forcing everything to stick together.  This process is liquid encapsulation, or the Czochralski crystal growth method.  After the elements are mixed in a uniform solution a rod is dipped into the solution and pulled out slowly.  As the solution cools crystallizations grow on the end of the rod as it is pulled out of the chamber, these are called crystal ingot (or boule) of GaAs, GaP, GaAsP, or InGaN.

The second step consists of the ingot or boule being sliced into very thin wafers of semiconductor, approximately 10 mm thick.  The wafers are then polished until the surface is very smooth.  Each wafer should be a single crystal of material of uniform composition.  It is very important that there be as few imperfections as possible. If there are too many irregularities the wafer will not work as a semiconductor.

The third step is the cleaning of the wafers in a rigorous chemical and ultrasonic process using various solvents.  This process removes dirt, dust or organic matter that could have settled on the polished wafer surface.  The cleaner the processing, the better the resulting LED will be.

During the fourth step additional layers of semiconductor crystal are grown on the surface of the wafer.  This is one way to add impurities to the crystal.  This process is call "doping".  The crystal layers are grown this time by a process called Liquid Phase Epitaxy (LPE).  In this technique, epitaxial layers, semiconductor layers that have the same crystalline orientation as the substrate below, are deposited on a wafer while it is drawn under reservoirs of molten GaAsP.  The reservoirs have appropriate dopants mixed through them. The wafer rests on a graphite slide, which is pushed through a channel under a container holding the molten liquid, (melt).  Different dopants can be added in sequential melts or several in the same melt, creating layers of material with different electronic densities.  The deposited layers will become a continuation of the wafer's crystal structure. LPE creates an exceptionally uniform layer of material, which makes it a preferred growth and doping technique.  The layers formed are several microns thick.

Step five consists of adding more dopants to alter the characteristics of the diode for color or efficiency.  If additional doping is done, the wafer is placed in a high temperature furnace tube again, where it is immersed in a gaseous atmosphere containing dopants, nitrogen or zinc ammonium are the most common. Nitrogen is frequently added to the top layer of the diode to make the light more yellow or green.

Step six is when the metal contacts are defined on the wafer.  The contact pattern is determined in the design stage and depends on whether the diodes are to be used singly or in combination.  Contact patterns are reproduced in photoresist, a light-sensitive compound; the liquid resist is deposited in the drops while the wafer spins, distributing it over the surface.  The resist is hardened by a brief, low temperature baking (about 215 degrees Fahrenheit or 100 degrees Celsius).  Next, the master pattern, or mask, is duplicated on the photoresist by placing it over the wafer and exposing the resist with ultraviolet light.  Exposed areas of the resist are washed away with the developer, and unexposed areas remain, covering the semiconductor layers.

Step seven is when the contact metal is now evaporated onto the pattern, filling in the exposed areas.  Evaporation takes place in another high temperature chamber, this time vacuum sealed.  A piece of metal is heated to temperatures that cause it to vaporize.  It condenses and sticks to the exposed semiconductor wafer.  The photoresist can be washed away with acetone, leaving only the metal contacts behind.  Depending on the final mounting scheme for the LED, an additional layer of metal may be evaporated on the back side of the wafer.  Any deposited metal must undergo an annealing process, in which the wafer is heat to several hundred degrees and allowed to remain in a furnace (with and inert atmosphere of hydrogen or nitrogen flowing through it) for periods up to several hours.  During this time, the metal and the semiconductor bond together chemically so the contacts don't flake off.

Step 8, at this point there is a single 2-inch diameter wafer produced that will have the same pattern repeated up to 6000 times on it; this give an indication of the size of the finished diodes.  The diodes are cut apart by snapping the wafer or by sawing with a diamond saw.  Each segment is called a die. A difficult process that is not error proof it does not leave all 6000 LEDs useable.  This is one of the biggest challenges in limiting production costs of the semiconductors.

Step nine involves the dies being mounted on the appropriate package.  If the diode will be used by itself as in indicator or for jewelry, it is mounted on two metal leads about two inched long.  Usually the back of the wafer is coated with metal and forms an electrical contact with the lead it rests on.  A tiny gold wire is soldered to the other lead and wire-bonded to the patterned contacts on the surface of the die.  In the wire bonding, the end of the wire is pressed down on the contact metal with a very fine needle. The gold is soft enough to deform and stick to a like metal surface.

Step ten, the complete assembly is sealed in plastic.  The wires and die are suspended inside a mold that is shaped according to the optical needs of the package and the mold is filled with liquid plastic or epoxy.The epoxy is cured, and the package is complete.The epoxy is cured, and the package is complete.