VCSELs grown on GaAs are currently emerging as a leading technology in rapidly expanding markets like Gesture Recognition, 3D imaging, datacomm and others. Utilizing the modular concept of our new Gen3 in-situ platform, LayTec has customized and expanded the related in-situ metrology performance for VCSEL epitaxy. Our new family of in-situ tools for VCSEL applications combines spectral reflectance measurements with the robust and industry proven EpiTT and EpiCurve® TT performance.
An upgrade from standard EpiTT to EpiTT VCSEL is simple and straight-forward, because the additional spectral sensing module (R-VCSEL) shares the optical head and the two optical fibers with EpiTT. Therefore, a single viewport at the MOCVD reactor is sufficient for EpiTT VCSEL or EpiCurve® TT VCSEL. The EpiCurve® TT VCSEL is a comprehensive 4-in-1 in-situ metrology tool measuring all 4 key epi parameters on all wafers in the reactor: wafer temperature, growth rates, wafer bow and the VCSEL’s spectral reflectance. Fig. 1 shows this 4-in-1 metrology tool mounted on an Aixtron G3 Planetary reactor top. This combination allows integrating the full EpiCurve® TT performance with the spectral monitoring of DBR (distributed Bragg reflector) stop-bands and cavity dip position.
The additional spectral reflectance module delivers real-time information about the evolving DBR (distributed Bragg reflector) and cavity structures already during epitaxy in a customer chosen spectral range. Fig. 2 shows that the spectral reflectance during the growth of a VCSEL is shifted at growth temperature (Tg=650°C) to a longer wavelength as compared to the room temperature signatures.
Fig. 2: Simulated in-situ data of 980 nm InGaAs/GaAs VCSEL growth (x=12%/90%DBRs) based on A.Mutig, PhD thesis (Technical University of Berlin).
For seamless integration of spectral sensing into EpiTT, it is of importance to adjust the wafer temperature measurement appropriately. The high reflectance of the growing VCSEL structure in the near infra-red range reduces the emissivity of the wafer. Hence, based on the specific requirements of our VCSEL customers, the EpiTT VCSEL optical set-up can be customized for minimizing trade-offs between wafer temperature measurement and spectral sensing.
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