Reflectance sensor eases the strain
If the high-brightness LED is ever going to replace the light bulb then its price-per-lumen will have to fall substantially. Large savings could result if GaN LED chip makers, which mainly grow their devices on 2 inch sapphire substrates, switch to 4 inch material. However, growth on a foreign substrate causes strain in the GaN epilayers, and managing this strain is a real challenge with larger wafers. Yields suffer if the strain causes the wafer to bend, as any deformation produces a temperature variation across the substrate that leads to inhomogeneities in epilayer composition.
Although it is possible to produce LED epiwafers on large substrates, many growth runs are required to optimize the growth parameters that can cause the wafers to bend. This approach is both time-consuming and expensive, which is why Germany-based LayTec has recently released a sensor for MOCVD reactors that can monitor the wafer s strain in situ. According to the company president Thomas Zettler, the EpiCurve reflectance sensor can slash the time and expense required to perfect a structure: "One customer used 80 runs to modify its recipe for blue LEDs, but for the next generation of devices [developed with an EpiCurve tool] only 20 runs will be necessary." The sensor can also improve production efficiency, as it reveals whether the process stays within the limits required to produce an in-spec epiwafer.
EpiCurve can provide reflectance measurements during most stages of the growth of a typical LED, including deposition of AlGaN layers that typically have a surface roughness of a few nanometers. These measurements are unaffected by isolated defects on the wafer, as their contribution is filtered out by the software. However, the instrument cannot provide meaningful data during growth of the very rough GaN nucleation layer formed at the sapphire-GaN interface.
The sensor was developed a few years ago by Alois Kroist s research team at the Otto-von-Guericke University, Magdeburg, Germany. It determines the strain by monitoring the changes in the separation of two 633 nm laser beams that are reflected from the epiwafer surface - convex and concave wafers are identified by laser spot separation increasing and decreasing, respectively.
"The critical technological breakthrough was to develop a small optical head that was able to measure this effect through only a few millimeters of viewport," says Zettler, who explained that space is severely limited for monitoring equipment installed in MOCVD reactors. The EpiCurve can be fitted to the majority of Aixtron s single-wafer and planetary MOCVD machines - while Veeco s reactors have their own brand of in situ reflectance sensors that can be fitted to these machines.
Although LayTec s EpiCurve sensor can readily determine the wafer s curvature during nearly all the LED growth stages, it is unable to form part of an active feedback system that could automatically adjust growth parameters in real time and maintain optimized conditions during a run. However, Zettler says that in a few years the thickness of a strain engineering layer could be modified by this kind of feedback control mechanism.
Most reactors used for LED manufacturing already have a port for other forms of in situ monitoring, so established growth recipes require no alteration after the addition of an EpiCurve sensor. As the existing ports already use a flow of gas to prevent material growth over this area, the sensor can be used for many uninterrupted runs.
EpiCurve s benefits will probably appeal to many fab managers, but they will have to weigh its advantages against the downtime required to install the sensor onto the reactor. However, Zettler believes that these managers should not be put off by the short delay to production: "For many, many customers the sensor will pay back the effort within a few weeks."
Although the largest target market is GaN LED chip makers, the EpiCurve sensor is also suitable for GaN-on-silicon epitaxy. The prototype EpiCurve sensor built at the Otto-von-Guericke University was actually constructed for this purpose. Today this university s new spin-off, Azzurro Semiconductors, is using the sensor to manufacture crack-free GaN epiwafers of up to 150 mm in diameter, with GaN layers up to 5 μm thick.
