Making LEDs at Home From an Online Wafer, a Shed Cleanroom, and a Laser Bought on eBay

Dr Semiconductor already had working DRAM cells from a class 100 cleanroom built inside a home backyard shed. Packaging those silicon chips onto a circuit board was the next problem, because silicon is opaque and lining up pads by eye is a guessing game. Gallium nitride grown on sapphire is transparent, already contains the layers that make blue light, and happens to be the same material used in commercial LEDs. A two-inch epiwafer costs about $160 and lasts a long time once you dice it.

An LED epiwafer is essentially a thin layer of crystal on a sapphire disk. Underneath is an n-type gallium nitride layer that emits electrons, a thin indium gallium nitride ‘quantum well’ in the middle is where the magic happens, and a p-type gallium nitride layer on top delivers positive charges. When those two combine in the well, you get blue light, which is a really simple process.

To get to the buried n-layer, however, you must cut through the top two layers without damaging the rest. You can’t simply use a chlorine plasma etcher like the big boys do in actual fabs since they’re way too expensive and unsafe for a home setup. The problem is that gallium nitride absorbs 355 nanometer UV radiation whereas sapphire does not, therefore you need a controlled means to ablate the coating. A refurbished glass-engraving laser from eBay does the work. Getting the right settings can take some trial and error on scrap parts, but once you’ve figured it out, the laser can cut nice little boxes down to the n-layer. To ensure that the wafer still works before any fancy lithography begins, simply apply a smear of soft indium metal to the etched spot and another to the unaltered p-surface, then connect a 9-volt battery between the two, which will glow blue in no time.

This process can leave behind some ugly metallic gallium and rough sidewalls, which a warm potassium hydroxide bath can remove by peeling the damaged material away like an onion and leaving the top surface clean and smooth. Then it’s just a matter of following the more common flow from there: spin on the usual lift-off resist and photoresist, bake and expose through a microscope stepper (which can print a YouTube logo or simple contact pads), and after a few developments, sputter on some nickel and then silver for the p-contact in a vacuum chamber. A 400 to 450 degree air anneal converts the nickel into a good semiconductor contact and allows the silver to do its job. Following a second alignment and lift-off in DMSO, the titanium and silver create the n-contact.

When a voltage is applied to the p-side positive and the n-side negative, the dies light up a stunning blue-purple, and the turn-on voltage is an ideal 2.5 volts for this material. One major problem is sawing sapphire at home, which is a hassle to get through, but the same 355 nm laser can do the trick if you aim it so it is absorbed inside the transparent crystal, allowing you to cut the wafer into individual chips over several passes. The edges may become scatter-damaged throughout the procedure, so leave some additional space around each die.

To make the LEDs more practical, you’ll need some custom boards with plated vias, which can then be electroplated with indium bumps. Mask off the regions that must remain clean using Kapton tape and rosins, then apply a little amount of flux to prevent the indium from oxidizing. A micromanipulator aligns the tiny die over the bumps, heat melts the metal, and you have a bonded chip. If you run current through the board vias, the packaged LED will light up just like a factory part. Blue LEDs on their own can appear frigid and uninviting, but when mixed with cerium-doped YAG phosphor powder and hardware-store silicone, the majority of the blue turns yellow. The eye views the combination as white since the more powder you add, the warmer the color becomes, and a small amount of silicon dries rapidly but appears rough, but it still works.