Widely used in optoelectronic device fabrication, epitaxial growth takes place in a vacuum in a sealed reactor. The need for closed-loop control in epitaxial processes is thus driving the development of in-situ, real-time, vacuum-compatible techniques. A new pan-European in-situ monitoring (ISM) project will have a major impact on the future development of commercial ISM. Funded by the European Community's Fifth Framework Program (FWP), the project will combine real-time optical diagnostic techniques with next-generation software to improve growth of advanced optoelectronic devices.
"Milestones of this shared-cost, three-year project are combining the MOVPE with the embedded optical and x-ray diffraction sensors and making in-situ measurements," says project leader Kurt Hingerl of the University of Linz (Austria). He notes that the data will provide process-control feedback on stoichiometry, roughness, voids, interface and surface quality, growth rate, homogeneity, and doping. "The partners will correlate ex-situ data with that from the in-situ to develop a closed-loop control system."
Aixtron AG (Aachen, Germany) is working closely with LayTec (Berlin, Germany), a commercial spinoff from the Technical University of Berlin. In the Aixtron R&D lab, a reactor provides a development testbed. Within the course of the project, it will be fitted with a sensor for spectral ellipsometry (SE) from Sentech Instruments GmbH (Berlin, Germany), a sensor for reflectance anisotropy spectroscopy (RAS) from LayTec, and a system for x-ray diffraction from Philips Analytical (Eindhöven, the Netherlands). SE and RAS require probing the growth surface with polarized light by coupling light through strain-free windows and measuring the change of polarization and also the intensity change (see oemagazine, June 2001, page 56). "Polarization is extremely sensitive to overlayers, so we can detect even a few angstroms of growing material," Hingerl says.
The ISM systems, including the optics, are mounted outside the reactor, so the beam must pass through a window. For some growth chemistries, window coating has been a problem, but Aixtron has managed to optimize the gas flow in such a way that the windows are not coated. The reactor liner includes either a single aperture (for RAS) or three (RAS + SE). After some process modification, these have been set up so as not to interfere with the epitaxial growth. This setup has proven successful for materials like gallium nitride and gallium arsenide and is undergoing trials for installation in a production environment.
Meanwhile, in the UK, researchers from the University of Bangor (Bangor, Wales) have licensed technology developed at the North East Wales Institute (NEWI; Wrexham, UK) to form ORS Ltd., which currently has a production ISM system based on a single-wavelength laser interferometer.
"NEWI has already installed a system for Boeing's Anaheim, CA, facility for process control of the deposition of cadmium-telluride onto sapphire," says Stuart Irvine, a professor at the University of Bangor and executive director of ORS. "This technology has also formed the basis for a modified system to grow gallium-nitride on sapphire at the University of Gent (Gent, Belgium)."
Throughout the semiconductor industry, optical probing methods are being developed to provide timely alerts to the grower should process fatalities occur. Early warning of such an occurrence will serve to avoid further wasteful processing steps on what will be a useless wafer. ISM will aid in the development of newer devices previously impossible to make without the control provided by the technology, but it is expensive. If the need is there, however, cost-of-ownership will not be a problem, for ISM is also a yield-enhancement tool and will likely recoup initial costs within the first year of operation.