  In this study, we explore the heterointerface of MgB$_2$ film on SiC
substrate at atomic scale using electron microscopy and spectroscopy. We detect
~1 nm MgO between MgB$_2$ and SiC. Atomic-level electron energy loss spectra
(EELS) show MgB$_2$-E2g mode splitting and softening near the MgB$_2$/MgO
interface. Orbital-resolved core-level EELS link the phonon softening to
in-plane boron-atom electron states' changes. Ab initio calculations confirm
this softening enhances electron-phonon coupling at the interface. Our findings
highlight interface engineering's potential for superconductivity enhancement.
