Method of manufacturing a semiconductor device comprising MOS-transistors having gate oxides of different thicknesses

Method of manufacturing a semiconductor device comprising MOS-transistors of a first type (A) having a gate oxide (3) of a first thickness and MOS-transistors of a second type (B) having a gate oxide (10) of a second, greater thickness. In this method, active regions (4) and field oxide regions (5) are formed in a silicon body (1). Then a layer of gate oxide (6) of said first thickness is formed on the active regions, on which a layer (7,8) of an electrode material is deposited. In the layer of electrode material, the gate electrodes (9) for the transistors of the second type are formed. Then an oxidation treatment is carried out, in which the thickness of the gate oxide under said gate-electrodes increases to the desired second thickness (10). During these processes, the electrode layer on the active regions of the MOS-transistors of the first type is not disturbed. The gate electrodes for the transistors of the first type are formed after the oxidation treatment. Thus a semiconductor device comprising MOS-transistors having gate oxides of different thicknesses can be manufactured in a simple manner.