  A new \textit{D0$_{23}$} metastable phase of Cu$_3$Au is found to grow at the
interfaces of Au/Cu multilayers deposited by magnetron sputtering. The extent
of formation of this novel alloy phase depends upon an optimal range of
interfacial width primarily governed by the deposition wattage of the
dc-magnetron used. Such interfacially confined growth is utilized to grow a
$\sim$ 300 nm thick Au/Cu multilayer with thickness of each layer nearly equal
to the optimal interfacial width which was obtained from secondary ion mass
spectrometry (SIMS) data. This growth technique is observed to enhance the
formation of the novel alloy phase to a considerable extent. SIMS depth profile
also indicates that the mass fragment corresponding to Cu$_3$Au occupies the
whole film while x-ray diffraction (XRD) shows almost all the strong peaks
belonging to the \textit{D0$_{23}$} structure. High resolution cross-sectional
transmission electron microscopy (HR-XTEM) shows the near perfect growth of the
individual layers and also the lattice image of the alloy phase in the
interfacial region. Vacuum annealing of the alloy film and XRD studies indicate
stabilization of the \textit{D0$_{23}$} phase at $\sim$ 150$^{\circ}$C. The
role of interfacial confinement, the interplay between interfacial strain and
free energy and the hyperthermal species generated during the sputtering
process are discussed.
