  We design and fabricate shallow-etched periodically poled lithium niobate
waveguide to realize highly-efficient broadband spontaneous parametric
down-conversion~(SPDC) on nanophotonic chip. The shallow-etched waveguide is
capable to tolerate the non-uniformities of waveguide width induced by
fabrication imperfections, enabling generation of photon pairs with high count
rate and bandwidth. We demonstrate photon-pair generation with a high
brightness of 11.7~GHz/mW and bandwidth of 22~THz, in a 5.7-mm-long PPLN
waveguide. The generated photon pairs exhibit strong temporal correlation with
a coincidence-to-accidental ratio up to 16262$\pm$850. Our results confirm the
feasibility of shallow etching in fabrication of efficient SPDC device on
platform of lithium niobate on insulator, and benefit quantum information
processing with broadband photon source.
