New wet process for LEDs on patterned sapphire boosts efficiency
Switching from a dry process to wet lowers capital and manufacturing costs, scalability, and better throughput for gallium nitride and indium gallium nitride LEDs.
MicroTech has developed a wet process station for the etching of PSS (Patterned Sapphire Substrate) wafers used to increase light extraction and efficiency in high brightness LEDs.
The wet station can improve manufacturing throughput, a major stumbling block to making LEDs price competitive with fluorescent lighting.
As LED manufacturers look to decrease costs, the use of PSS becomes more important. The average light output power is reported to be up to 37% larger on a PSS than a standard sapphire wafer. The use of patterned sapphire substrates reduces the dislocation density in the GaN layer and enhances the light extraction efficiency from the LED chip.
Traditional dry etching on PSS produces highly efficient, very bright light but throughput is slow and scalability is impacted as wafer sizes increase. Typically, more dry etch tools are needed to keep throughput up as wafer size increases.
In the wet etch process in the MicroTech system, GaN or InGaN coated wafers are submerged in the etch tank with a mixture of etching and buffering agents. Prior to submersion, an SiO2 mask is patterned using plasma enhanced chemical vapor deposition. A lithography step exposes the desired pattern to etch. The sapphire etch process takes place between 260°C and 300°C. This ultra high temperature etches the wafers exponentially more quickly than the standard 150-180°C process and therefore, speeds throughput.
Independent customer evaluations show a significant improvement in light extraction and efficiency in the substrates and a considerable cost savings, even if polishing work is performed on the wafers after etch to increase efficiencies. Development work is also being done to improve the dome shapes created on the wafers with a chemical mechanical polishing process. New non-cone shapes are also under development.
Recently MicroTech announced the delivery of a similar concept process station to the solar industry that delivers high throughput and lower cost-of-ownership benefits by moving from a dry CVD process to an efficient, production-proven, environmentally friendly wet process.