Method to improve the control of bird's beak profile of poly in split gate flash

Abstract

A method is provided to improve the control of bird's beak profile of poly in a split gate flash memory cell. The control of the bird's beak profile is achieved in a first embodiment where the polycrystalline layer of the floating gate is annealed at a high temperature. The annealing promotes small grain size and hence smoother surface in the polysilicon, which in turn promotes sharper poly tip. The smoother poly surface also results in thinner inter-poly between the floating gate and the control gate, which together with the sharp poly tip, enhances the erase speed of the split-gate flash memory cell. In a second embodiment, the performance is further enhanced by providing an amorphous silicon for the floating gate, because the amorphous nature of the silicon yields a very smooth surface. This smooth surface is transferred to the recrystallized state of the silicon layer through annealing. Thus, a good control for the bird's beak is achieved. A sharp and short poly tip then results from a well controlled and well-defined bird's beak. Hence, an enhanced split-gate flash memory cell follows.