Abstract
Complementary field-effect transistor (CFET) architectures stack n- and p-type devices vertically, creating buried features that challenge conventional metrology. A critical module is the selective SiGe lateral recess, which defines the inner-spacer width and strongly impacts device performance. Transmission electron microscopy (TEM) and optical scatterometry provide valuable insight but face limitations in throughput, scalability, or sensitivity for recessed structures. We demonstrate micro-spot X-ray fluorescence (μXRF) as a non-destructive wafer-scale method to quantify SiGe recess depth in device-relevant CFETs. Normalizing the Ge Kα signal using an on-wafer Si/SiGe reference stack isolates the lateral geometry from thickness and Ge concentration variations in the SiGe layers; the normalized signal correlates strongly with top-view critical dimensions and TEM-derived SiGe lengths (R2≈0.996). Applying TEM-based calibration across the full wafer reveals a clear radial non-uniformity of the lateral recess. Repeatability and error propagation show a calibration-limited 3σ depth-resolution floor of ∼1.4 nm, with intrinsic μXRF sensitivity <1 nm at dwell times of about one minute—indicating strong potential for future inline monitoring.