New requirements for sapphire LED etching have been introduced as new applications for LED lighting are pushing the rightness, lifetime and stability limits. Quantum efficiency improvements are driving new vertical structures requiring additional etching requirements. There are uniquely high emperature equirements for etching these structures and achieving a 10x throughput improvement over conventional methodologies. Although most companies are using a dry etch process to create patterned urfaces, the downside of dry is not minimal, including the high cost of processing equipment, low throughput, and poor scalability to name a few. That downside is driving many to renew their interest in wet etching.
Historically, there has been a 190°C limitation for standard process baths. Sapphire etching rates increase geometrically with temperature, so a system that will achieve a 300-degree process temperature is needed.
The high-temperature wet-etching process is comparatively much cheaper than dry etching – and faster, too.
During high-temperature wet etching, wafers masked with SiO2 are placed in a tank with a mixture of etching and buffering agents: sulfuric and phosphoric acids, typically in a 1:1 or 3:1 ratio. Before submersion, a plasma-enhanced chemical vapor process spins a silicon dioxide mask onto the sapphire substrate, and lithography exposes the required pattern. The mixture is brought to temperatures ranging between 260 and 300 °C – much higher than those used in traditional semiconductor fabricating, which typically run between 150 and 180 °C.
Etching rates do not increase along a linear scale as temperatures rise. Instead, they increase exponentially, so that a 300 °C temperature may make etching twice as fast as at 260 °C. On the other hand, “the etching rate increased linearly when the H2SO volume ratio increased from 0 to 75 percent,” as reported by D.S. Wuu of National Chung Hsing University in Taiwan and colleagues.2
High-temperature wet-etching rates can be measured in microns per minute; a rate of more than 1 µm/min is achievable under the correct conditions, according to Sinmat’s Singh, who added that it is reasonable to expect full etching of a standard 2-in. wafer in five minutes. And with regard to cost, a process tank for a batch of 6-in. wafers is only slightly more expensive than a tank designed for a batch of 2-in. wafers – and it can hold the same number of wafers.
Of course, using extremely hot chemicals can be challenging for manufacturers: safety is paramount with chemicals hot and powerful enough to rapidly etch sapphire surfaces. To maintain safety, any system must feature a suitable chemical tank, designed not to react with any of the chemicals. Preferably it will be constructed of a sturdy substance such as high-purity virgin annealed quartz. No plastics should come into contact with the chemical mixture, and built-in temperature sensors should feed precise readings back to the systems management equipment. As added safety features, some tanks include a cool-down module to house the hot chemistry while it cools and overflow tanks that can hold 120 percent of the main tank’s volume in case of an accident. Fume exhaust must also be considered with high temperature etching because fumes can be more severe; and operator safety must be newly reviewed when working with these elevated levels.
At IMTEC Acculine, we have developed solutions to these challenges using our custom quartz tanks and specially designed automated stations that make sapphire wet etching safe, reliable, and suitable for high volume manufacturing.