  This manuscript proposes a method for characterizing the complex permittivity
of the human finger skin based on an open-ended waveguide covered with a thin
dielectric sheet at sub-terahertz frequencies. The measurement system is
initially analyzed through full-wave simulations with a detailed finger model.
Next, the model is simplified by replacing the finger with an infinite sheet of
human skin to calculate the forward electromagnetic problem related to the
permittivity characterization. Following this, a radial basis network is
employed to train the inverse problem solver. Finally, the complex
permittivities of finger skins are characterized for 10 volunteers. The
variations in complex relative permittivity across different individuals and
skin regions are analyzed, revealing a deviation of $<\pm 1.5$ for both the
dielectric constants and loss factors across 140 to 220 GHz. Repeated
measurements at the same location on the finger demonstrate good repeatability
with a relative estimation uncertainty $<\pm 1.5\%$.
