Bridging the Gaps to Achieve Ultra-Clean Processing and Surface Preparation of Germanium Wafers

Germanium has re-emerged as an important material for optoelectronic, photonic, and space photovoltaic applications because it combines high carrier mobility, favorable optical properties, and radiation tolerance with an important role in multi-junction solar cells that achieve high power conversion efficiency. However, the integration of germanium wafers into silicon CMOS foundries requires ultra-clean surfaces and remains challenging because standard silicon-oriented approaches are not directly transferable to germanium. In this work, we identify critical process gaps and evaluate strategies to improve germanium wafer cleanliness across the wafering flow. Vapor phase decomposition (VPD) coupled with inductively coupled plasma mass spectrometry (ICP-MS) was used to detect and quantify metal contaminants on wafer surfaces and in liquid consumables, while interferometry was used to assess the effect of cleaning chemistries on surface roughness. The results show that cleanliness can be improved not only by optimized cleaning chemistries and process parameters, but also by early process modifications, stringent consumable control, and the addition of advanced cleaning functionalities. These measures reduced surface roughening by about 2 nm and supported a target metal surface contamination level below 5E10 atoms/cm², a conservative threshold based on Si prime wafer targets and within the 10 9 – 10 11 atoms/cm² sensitivity range of the measurement technique. This study highlights the metrologies, process adaptations, and control points identified within the Umicore wafering flow and discusses their broader relevance for cleaner germanium wafer processing aimed at CMOS-compatible applications.