  We have realized a microstrip based THz near field cantilever which enables
quantitative measurements of the impedance of the probe tip at THz frequencies
(0.3 THz). A key feature is the on-chip balanced hybrid coupler which serves as
an interferometer for passive signal cancellation to increase the readout
circuit sensitivity despite extreme impedance mismatch at the tip. We observe
distinct changes in the reflection coefficient of the tip when brought into
contact with different dielectric (Si, SrTiO3) and metallic samples (Au). By
comparing to finite element simulations we determine the sensitivity of our THz
probe to be well below 0.25 fF. The cantilever further allows for topography
imaging in a conventional atomic force microscope mode. Our THz cantilever
removes several critical technology challenges and thus enables a shielded
cantilever based THz near field microscope.
