13 June 2022
Optimizing AlInP/AlGaInP laser epilayer uniformity by combining in-situ and ex-situ metrology
The combination of ex-situ wafer mapping and optical in-situ measurements during metal-organic vapor phase epitaxy (MOVPE) for laser devices constitutes a powerful method for obtaining maximum layer and die uniformity across the wafer for the production of devices such as vertical-cavity surface-emitting laser (VCSEL) or edge-emitting laser (EEL) structures, notes in-situ metrology system maker LayTec AG of Berlin, Germany.
This uniformity directly increases the yield in subsequent device processing, with a strong cost impact. Ex-situ mapping measurements by means of white light reflectance (WLR) and photoluminescence (PL) are well-established methods for ensuring sufficient uniformity prior to further processing in expensive dicing and packaging processes. Unfortunately, they do not directly disclose the root-cause of inhomogeneity that might be observed in these measurements.
In contrast, in-situ measurements by means of emissivity-corrected pyrometry (ECP), reflectance and curvature directly reveal the effect of certain process parameter changes like heater or gas flow settings on the wafer temperature or layer composition. However, they never cover the entire wafer area and do not probe the layers under device operating conditions.
Therefore, only a smart combination of both methods constitutes an efficient way for identifying the parameter most affecting the uniformity and tuning the process towards the most uniform layer properties across the wafer for cost-effective production, says LayTec.
Dr André Maaßdorf et al at the Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik (FBH) of Berlin, Germany established this approach by combining LayTec’s EpiX mapping station and LayTec’s in-situ measurement tool EpiCurve TT AR VCSEL+ for optimizing the gas flow profile and the susceptor configuration at their Aixtron AIX 2800 G4 IC2 (12x4”) epitaxial deposition system for AlInP/AlGaInP edge-emitting lasers. The EpiCurve TT AR VCSEL+ combines the curvature, three-wavelength reflectance and emissivity-corrected pyrometry (ECP) of an EpiCurve TT with additional spectral reflectance capability. The EpiX-Mapper equipped with WLR and PL was deployed for wafer mapping to deduce properties like layer thickness and emission wavelength. At the beginning of the optimization process a significant deviation in layer thickness (relative deviations ≤ 8%) was observed. Moreover, in-situ ECP data obtained during deposition revealed that the wafer temperature profile also exhibited a similar non-uniformity at the edges and further in the wafer center.
Based on the results of the comparison of in-situ and ex-situ results, gradual modification of the gas foil rotation (GFR) and absolute gas flow, its distribution over the gas inlet as well as modification of recess of the wafer satellite led to an almost perfectly flat thickness profile.
LayTec says that the results clearly demonstrate the advantage of combining ex-situ wafer mapping to detect and eliminate wafer non-uniformities. The upcoming application note will provide more details about this optimization process and will be available soon on the firm’s website.