Written by Dr Mark Dineen, Principal Applications Engineer , Oxford Instruments Plasma Technology
Lighting is one of those things we take for granted, but it forms an integral part of our daily lives, impacting enormously both on us and our environment.
Global lighting consumption is estimated to account for the output of 1000 electric power plants, and costs £100 billion a year, so the drivers for a more efficient light source are both commercial and environmental. This is where the HBLED comes in, replacing the old incandescent lightbulb technology. Theoretically, HBLEDs could use less than 20% of the power needed to produce incandescent light bulb equivalent levels of output for lifetimes up to 50,000 hours – a great leap forward.
Producing an HBLED with the help of Oxford Instruments
Starting with a flat substrate, commonly sapphire, to improve the quality of the device the surface is sometimes patterned with raised features, and this requires etching. The Oxford Instruments Plasmalab®System133-ICP380 is used by a number of leading HBLED manufacturers to perform this function.
GaN material is then grown on the sapphire using MOCVD, MBE or HVPE. The final structure is a p-n junction with the p-type on the top surface and the n-type layer next to the substrate. Because of the insulating nature of the sapphire it is now necessary to etch down to the n-type layer.
The Plasmalab®System133-ICP380 can etch 21 x 2” of these wafers at one time, giving world leading throughput.
Some further surface preparation can now be performed, for example photonic crystal etching, to enhance the light extraction from the devices, this is followed by the device isolation etch. Both these steps are achievable using the Plasmalab®System133-ICP380.
The final process is to enclose the device in a protective dielectric layer and the Plasmalab800Plus DP800 is the system of choice to do this for a large number of HBLED manufacturers. The advantage of the Plasmalab800Plus DP800 is the leading batch size, as it is capable of a processing mammoth 40 x 2” wafers in a single run, giving superb throughput on a reliable platform.
The dielectric isolates the device and protects it from contamination. It is now ready for packaging and making into a light.
What the future holds
As the industry moves on to the larger 4” substrates, it becomes a much more attractive proposition to process single wafers at a time rather than batches. Oxford Instruments already has the technology to perform high yield, high speed processes, and the company is ready to progress to the next stage.
Single Wafer Etching using an Electrostatic Chuck
The benefit of moving to single wafers is that the wafer may now be clamped more readily. New technology allows clamping of sapphire wafers using a piece of equipment called an Electrostatic Chuck. The benefit of this is that the wafer is held without touching the sensitive topside – an important improvement over mechanical clamping, as this reduces particles and gives improved wafer cooling, allowing increased etch rates with photoresist masks.
ICP-CVD Passivation
Traditionally PECVD has been used for the deposition of the final protective layer, however this requires high temperature to give the quality of film needed for HBLEDs. Some studies have shown that HBLEDs perform better if the temperature is kept below a certain limit during the manufacturing process. ICP-CVD allows very high quality films to be grown at low temperature and high rate.
Looking to the future
HBLEDs will have a very positive impact on our lives but technical advances still need to be made to facilitate it. Through technology and experience, Oxford Instruments is uniquely placed to make this future possible.