MicroTech Systems of Fremont, CA, USA, which provides engineering, manufacturing and applications support for wet process and chemical distribution tools, has developed a wet process station for the etching of PSS (patterned sapphire substrate) wafers (which are used to increase light extraction and efficiency in high-brightness LEDs).

The firm claims that its wet station can improve manufacturing throughput (a major stumbling block to making LEDs price competitive with fluorescent lighting).

As LED makers look to decrease costs, the use of PSS is becoming more important, MicroTech says. The average light output power is reported to be up to 37% larger on a patterned sapphire substrate than a standard sapphire wafer. Their use reduces the dislocation density in the gallium nitride layer and enhances the light extraction efficiency (LEE) 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. Also, typically, more dry etch tools are needed to keep throughput up as wafer size increases.

In the wet etch process in the MicroTech system, gallium nitride (GaN)- or indium gallium nitride (InGaN)-coated wafers are submerged in the etch tank with a mixture of etching and buffering agents. Prior to submersion, a silicon dioxide mask is patterned using plasma-enhanced chemical vapor deposition (PECVD). A lithography step exposes the desired pattern to etch. The sapphire etch process takes place at 260–300ºC. This ultra-high temperature etches the wafers exponentially more quickly than the standard 150–180ºC process and therefore speeds throughput.

MicroTech says that 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 undertaken to improve the dome shapes created on the wafers using a chemical mechanical polishing (CMP) process. New non-cone shapes are also under development.

MicroTech recently 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.