Revamped Reactor Targets Higher Yields
Many LED manufacturers are looking to move on from making devices for mobile-phone backlights and keypads to producing chips for much larger backlight units. However, the new application places more stringent demands on the LED manufacturing process, as the acceptable spread in emission wavelength is much narrower than it was previously.
According to Rainer Beccard, director of marketing for Aixtron s compound semiconductor technology branch, the company s existing planetary reactors are unable to consistently produce a high enough proportion of LED chips within the specifications required for large backlight units. This has led the company to investigate various ways to improve the reproducibility and growth uniformity of its reactors, and ultimately to release a 42 × 2 inch reactor that can manufacture larger numbers of LED chips with a narrower distribution of electroluminescence wavelengths.
The improvements in emission uniformity resulted from changes to the reactor hardware that were assessed by monitoring the distribution of peak photoluminescence wavelengths from multiple quantum-well epiwafers, which is strongly correlated to the spread in electroluminescence of chips taken from the wafer. The reactor is also claimed to be far more "robust"
and simple to use, which should boost the long-term manufacturing yield.
Beccard says that one of the problems with the company s existing reactors relates to the positioning of the central gas-injection nozzle, which has to be regularly removed for cleaning. According to him, although Aixtron provides a very detailed description of how to adjust this part, the procedure can be carried out incorrectly. However, with the new design, misalignment is impossible because the central gas injection is mechanically fixed.
This new injector, which is water-cooled, also has two separate inlets for the group V gases, in addition to the single inlet for group III material. This is claimed to produce a more laminar flow than before, which improves growth uniformity, delivers greater control of the gas flows within the reactor, and cuts ammonia consumption by half for nitride growth.
Aixtron s latest planetary reactor is also designed to operate at a lower temperature in the center of the growth chamber (see "Aixtron s latest reactor"). "The center is now water cooled and the susceptor is made of quartz, so inductive heating doesn t couple to the center plate," explains Beccard. This modification makes that central region too cold to drive reactions between the gases and prevents any growth of unwanted material on the injector that would have to be removed subsequently. The reduced deposition in the central region improves the run-to-run reproducibility, says Beccard, because it limits any changes in the reactor s thermal profile.
Aixtron s latest planetary reactor features these refinements (collectively referred to as yield+), and has already been ordered by Taiwanese LED manufacturers Highlink Technology and Epistar. However, the company can also update its existing 24 × 2 inch platform for customers who don t want to have to buy a new machine. Internal trials with these modifications have revealed an improvement in peak photoluminescence wavelength uniformity. The average standard deviation in peak photoluminescence from three wafers from one disk, taken over three consecutive runs and using an exclusion zone of 2 mm, fell from 2.70, 2.91 and 2.70 nm, to 1.20, 0.81 and 0.95 nm.
The down time for upgrading to a 24 × 2 inch reactor with the yield+ system is typically one week. "We replace the coil, put in a new set of graphite if it s not already there, and change part of the top-plate in order to fit the injector. That s it," says Beccard. Once Aixtron s engineers have made these changes they stay at the fab and help tweak the LED growth recipes for the upgraded reactor. "We don t want people to start from zero again," Beccard remarks, "so we teach them how the changes in the recipe will affect their uniformity."
Aixtron is hoping that its two options to improve GaN LED manufacturing yield – either installing a new 42 × 2 inch planetary reactor, or upgrading an existing 24 × 2 inch set-up – will tempt LED chip manufacturers to part with their cash. A surge in order-book activity will certainly be welcomed by the German outfit, which is hoping to see LED equipment sales recover after two difficult years.