UV LEDs
Extension of industry proven EpiTT and EpiCurve® TT performance to dedicated UV wavelengths.
Epitaxial growth of UV LEDs faces significant challenges: long runs, superlattices, high Al content vs. high doping level, very high growth temperature, large wafer bow and many more. Furthermore, AlGaN buffer layers with high aluminum content for optimal UV-C LED performance exhibit a band edge below 300 nm, so the established 405 nm in-situ reflectance is insensitive to the surface morphology of such AlGaN layers. To precisely monitor both AlGaN growth rate and surface morphology during UV-C LED epitaxy, LayTec's latest generation of EpiTT and EpiCurve® TT offers additional UV wavelengths for UV growth monitoring to address these problems.
Wafer temperature with precision better than ±1K in a range of 600°C to 1400°C
Capability for True Temperature measurements to up to 1500°C
High accuracy in-situ growth rate (film thickness) measurement with XRD accuracy by 405/633/950 nm low-noise 3-wavelength reflectance
24/7 performance with automated data analysis and integrated interfaces to the MES/SPC systems of our customers
Additional UV wavelength for increased sensitivity of UV emitting layers
Increased sensitivity to surface morphology for UV wavelengths
Metrology solutions
Epitaxial growth of UV LEDs faces significant challenges: long runs, superlattices, high Al content vs. high doping level, very high growth temperature large wafer bow and many more. LayTec's latest generation of EpiTT and EpiCurve® TT offers significant technology advances to solve most of these problems. Here are some application examples.
For in-situ monitoring of high temperature AlN templates for UV LEDs, Fig. 1 shows a layer stack of a typical UV LED grown by MOCVD. Hereby, LayTec‘s EpiCurve® TT monitors the wafer curvature, three in-situ reflectance wavelengths as well as emissivity corrected pyrometry.
In Fig. 2, the 405 nm in-situ reflectance transient for a two step MOCVD growth of UV-B LED (305 nm) is shown as monitored in-situ by EpiTT. (HT=high temperature, LT=low temperature, NL=nucleation layer, SPSL=short-period superlattice)
In the course of LayTec's cooperation with FBH in the consortium "Advanced UV for Life", FBH scientists (Group of Prof. Markus Weyers) also have developed a high temperature (HT) MOCVD process for growing low-defect-density AlN/sapphire templates for UV LEDs. The split of the UV LED growth in two separate steps in two different MOCVD systems fully avoids the possible memory effect related interference of AlGaN processes with the growth of high quality AlN buffers. The AlN buffer growth (Fig. 3) was monitored by 3 wavelength reflectance improved for very thick AlN/AlGaN layer stacks and by the emissivity corrected high temperature sensing of EpiTT Gen3. The blue curve is the result of the quantitative analysis of the 3 wavelength reflectance measured by EpiTT Gen3.
The thickness of the AlN nucleation layer is determined from the fitting curves (red) to be 43.2±0.2 nm, and the AlN high temperature buffer layer thickness to be 524.2±0.5 nm. The consistency of the used high temperature nk database is also obvious from the good agreement of the simulated curve segments in the temperature up-ramping and down-cooling steps.
Mapping solutions
EpiX® mapping stations combine an XY-mapping stage with LayTec’s UV-VIS-NIR spectroscopic reflectance and photoluminescence metrology systems for a comprehensive 2D analysis of optical wafer properties by non-contact measurements. The fully automated version (EpiX C2C) with cassette-to-cassette loading provides a high-throughput solution for industrial production lines. LayTec EpiX® mapping stations exhibit a superior measurement performance that stands out in absolute accuracy, signal-to-noise level and 2D-homogeneity over the wafer while maintaining short measurement times. For (UV) LED wafers, the integrated software provides full automated data analysis, including the numeric analysis of photoluminescence peaks including peak intensity, center and centroid wavelength, and width as well as a total stack thickness measurement.
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