  Helium-ion beams (HIB) focused to sub-nanometer scales have emerged as
powerful tools for high-resolution imaging as well as nano-scale lithography,
ion milling or deposition. Quantifying irradiation effects is essential for
reliable device fabrication but most of the depth profiling information is
provided by computer simulations rather than experiment. Here, we use atomic
force microscopy (AFM) combined with scanning near-field optical microscopy
(SNOM) to provide three-dimensional (3D) dielectric characterization of
high-temperature superconductor devices fabricated by HIB. By imaging the
infrared dielectric response we find that amorphization caused by the nominally
0.5 nm HIB extends throughout the entire 26.5 nm thickness of the cuprate film
and by about 500 nm laterally. This unexpectedly widespread structural and
electronic damage can be attributed to a Helium depth distribution
substantially modified by internal device interfaces. Our study introduces
AFM-SNOM as a quantitative nano-scale tomographic technique for non-invasive 3D
characterization of irradiation damage in a wide variety of devices.
