Spectroscopy: a New Route Towards Critical-Dimension Metrology of the Cavity Etch of Nanosheet Transistors
The processing of gate all-around Si transistors requires to isolate vertically stacked nanometer-thick Si sheets or wires.
For this purpose, the SiGe layers of a SiGe/Si superlattice are etched selectively and laterally in a process step commonly
called cavity etch1,2. Controlling the quantity of etched SiGe material, i.e. the cavity depth, is critical for optimal device
performance. Unfortunately, this critical dimension (CD) can only be measured by time-consuming cross-sectional
electron microscopy, which results in limited statistics and hence control of the cavity depth across wafers and batches. As
a first step towards the development of fast inline cavity depth measurements, this work evaluates the sensitivity to cavity
depth of conventional CD metrology and alternative top-down spectroscopic techniques on samples with cavity depths
ranging from 0 to 30 nm. As we show, while optical CD scatterometry remains a technique of choice thanks to its high
throughput and sensitivity, Raman and Energy-Dispersive X-ray spectroscopies also show very promising results owing
to their simple sensitivity to the remaining SiGe volume. Finally, Secondary Ion Mass Spectrometry offers unique cavity
profiling capabilities with a very high sensitivity down to SiGe residues, despite being time-consuming and destructive.
Keywords: cavity etch, lateral recess, critical dimension, inline metrology, Raman spectroscopy, Energy dispersive X-ray
spectroscopy, X-ray photoelectron spectroscopy, Secondary ion mass spectrometry