Curvature sensor: indispensable and award-winning
Curvature transients during growth of the active region comparing different wafers (above) and histogram of PL wavelength distribution for both wafer (below)
EpiCurveTT sensor on top a multiwafer epitaxy system at FBH
GaN-based devices have taken the markets with an impressive pace: LEDs for lighting, lasers for data storage devices (blu-ray disk) and power transistors for mobile communication.
The manufacture of such devices is troubled by a considerable bow of the semiconductor wafers. The different lattice constants in GaN and (Al,In,Ga)N compounds as well as the thermal expansion mismatch between GaN and the substrate materials sapphire (for LEDs) and silicon carbide (for transistors) cause wafer curvature or even cracking. Here, the amount of bow depends on the thickness of the epitaxial layers (some micrometers) and the size and thickness of the used substrates. This curvature can for example cause significant inhomogeneity of the emission wavelengths of LEDs as well as laser diodes over a wafer [1]. Therefore, understanding and control of wafer curvature are decisive to obtain a flat wafer with a homogeneous temperature distribution in the epitaxial growth process when the light-emitting active region is deposited.
As for transistors, the bow has to be low in the device process to allow for reproducible patterning, for example for gates of only 0.25 µm length [2]. Besides the geometrical properties, strain can also have impact on the material quality of for example quantum well structures in LEDs [3]. Control of wafer bow from the selection of substrates to the epitaxial growth and finally device processing is thus indispensible and routinely applied at FBH.
For its contribution to the development and process qualification of Laytec’s in-situ sensor "EpiCurveTT", FBH has now been awarded the technology transfer prize "wissen.schafft.arbeit". The institute shares this year’s award with its partners Otto-von-Guericke-Universität Magdeburg, where the concept of the curvature sensor had been developed, and LayTec GmbH, who manufactures and installs these tools all over the world.
Publications:
[1] F. Brunner, V. Hoffmann, A. Knauer, E. Steimetz, T. Schenk, J.-T. Zettler, M. Weyers, "<link /fileadmin/fbh-berlin/english/ver07/pub04.htm _blank internal-url-new-window>Growth optimization during III-nitride multiwafer MOVPE using real-time curvature, reflectance and true temperature measurements", J. Crystal Growth vol. 298, pp. 202-206 (2007)
[2] F. Brunner, O. Reentilä, J. Würfl, and M. Weyers, "<link /fileadmin/fbh-berlin/english/ver09/pub30.htm _blank internal-url-new-window>Strain engineering of AlGaN-GaN HFETs grown on 3 inch 4H-SiC", phys. stat. sol. (c), vol. 6, no. S2, pp. S1065-S1068 (2009)
[3] A. Knauer, T. Kolbe, S. Einfeldt, M. Weyers, M. Kneissl, and T. Zettler, "<link /fileadmin/fbh-berlin/english/ver09/pub07.htm _blank internal-url-new-window>Optimization of InGaN/(In,Al,Ga)N based near UV-LEDs by MQW strain balancing with in-situ wafer bow sensor", phys. stat. sol. (a), vol. 206, no. 2, pp. 211-214 (2009).
FBH research: 29.11.2010