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In situ optical monitoring raises epi reactor productivity

Reflectance anisotropy spectroscopy has helped to improve the productivity of GaAs epitaxy. Elisabeth Steimetz and colleagues describe how it is now being applied to InP-based structures.
Epitaxial growth is the first and often performance-determining step in the manufacturing of GaAs- and InP-based optoelectronic and RF devices. A steadily increasing number of device-grade single- and multi-wafer epitaxy systems are being equipped with real-time optical sensors for calibrating, monitoring and controlling the epitaxial growth process, thereby replacing, at least in part, conventional ex situ characterization techniques. The ability to measure layer parameters during growth, using real-time optical monitoring, can significantly improve the productivity of MBE and MOCVD systems.

Various types of sensors are available for almost any reactor, including such complex systems as multi-wafer production reactors. In combination with emission-corrected pyrometry measurements, even the surface temperature (or in the case of transparent materials, the susceptor) can be accurately measured in situ.

Optical in situ measurements involving reflectance and reflectance anisotropy spectroscopy (RAS) have been extensively applied to the MOCVD and MBE growth of GaAs-based device structures such as HBTs and 850 nm lasers. Efficient on-line reproducibility checks and faster device process developments have been made possible by a combination of in situ RAS, reflectance and pyrometry measurements.

To date there has been very little work on applying reflectance or RAS to the growth of InP-based materials such as InGaAsP. This is perhaps surprising given the increasing use of InP and its derivatives in high-speed microelectronics and optoelectronics, and the attendant demand for high-quality epiwafers. However, close co-operation between the Heinrich-Hertz-Institute and LayTec in Berlin has enabled this gap to be closed using an EpiRAS in situ sensor.

RAS fingerprints of 1.3 µm laser structures have shown the great potential of this method for monitoring the growth of InP-based devices. Real-time doping concentration measurements for all relevant dopants can be performed. In addition, real-time RAS measurements at characteristic photon energies provide composition monitoring of each quantum well in the active region of the laser.

Reflectance anisotropy spectroscopy, also known as reflectance difference spectroscopy, is a normal incidence reflectance technique that, by utilizing the anisotropy of reconstructed semiconductor and metal surfaces, has evolved into one of the most useful optical surface science techniques. Using a wavelength range covering the near-UV, visible and near-infrared part of the spectrum (250 to 850 nm), RAS is capable of sensing the stoichiometry and symmetry of the upper-most atomic monolayers of cubic semiconductors and metals.

RAS measures the difference (Δr) in normal-incidence reflectances (rx and ry) for light polarized in two directions perpendicular to each other (x and y) in the plane of the sample. RAS calculates the complex reflectance anisotropy (Δr/r), where r = (rx+ry)/2, caused by the reduced symmetry at the surface of cubic crystals. The RAS set-up includes a photoelastic modulator (PEM)-based optical system acting as a normal-incidence phase-modulated ellipsometer. The reflected light passes through the PEM and into a monochromator before detection. The signals detected are usually of the order (Δr/r)

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