A method of making a non-volatile MOS semiconductor memory device includes a formation step, in a semiconductor material substrate, of STI isolation regions (shallow trench isolation) filled by field oxide and of memory cells separated each other by said STI isolation regions. The memory cells include a gate electrode electrically isolated from said semiconductor material substrate by a first dielectric layer, and the gate electrode includes a floating gate self-aligned to the STI isolation regions. The method includes a formation phase of said floating gate exhibiting a substantially saddle shape including a concavity; the formation step of said floating gate includes a deposition step of a first conformal conductor material layer.