  Several technical issues and challenges are identified and investigated for
the planar tunneling spectroscopy of the topological Kondo insulator SmB$_6$.
Contrasting behaviors of the tunnel junctions prepared in two different ways
are analyzed and explained in detail. The conventional approach based on an
AlO$_\text{x}$ tunnel barrier results in unsatisfactory results due to the
inter-diffusion between SmB$_6$ and deposited Al. On the contrary, plasma
oxidation of SmB$_6$ crystals produces high-quality tunnel barriers on both
(001) and (011) surfaces. Resultant conductance spectra are highly reproducible
with clear signatures for the predicted surface Dirac fermions and the bulk
hybridization gap as well. The surface states are identified to reside on two
or one {\it distinguishable} Dirac cone(s) on the (001) and (011) surface,
respectively, in good agreement with the recent literature. However, their
topological protection is found to be limited within the low energy region due
to their inevitable interaction with the bulk excitations, called spin
excitons, consistent with a recent theoretical prediction. Implications of our
findings on other physical properties in SmB$_6$ and also other correlated
topological materials are remarked.
