Method of manufacturing low and high voltage CMOS transistors with EPROM cells

The body regions for the n-channel and p-channel LV transistors, for the n-channel HV transistors, and for the EPROM cells are formed on a silicon substrate; a thermal oxide layer (12) is formed and a layer of polycrystalline silicon (13) is formed thereon; the latter layer is selectively removed to form the floating gate electrodes (13a) of the cells and the gate electrodes (13b) of the HV transistors; the source and drain regions (14) of the cells, the source and drain regions (22) of the n-channel HV transistors, the body regions (24) and the source and drain regions of the p-channel HV transistors are formed; an ONO composite layer (15) is formed; the silicon of the areas of the LV transistors is exposed; a thermal oxide layer (16) is formed on the exposed areas; a second polycrystalline silicon layer (17) is deposited and is then removed selectively to form the gate electrodes of the LV transistors (17c) and the control gate electrodes (17a) of the cells, and the source and drain regions of the LV transistors are formed. By virtue of the use of a material (ONO) which is impermeable to the oxygen atoms of the subsequent thermal oxidation and because the body regions (24) of the p-channel HV transistors and the source and drain regions of all of the HV transistors are produced by separate implantations, components of very good quality are produced with few more masks than a conventional LV method.