1. Method of making wide, deep recessed isolation trenches in a semiconductor substrate, where a) narrow, shallow trenches (16) are formed in the surface of the semiconductor substrate, said trenches having vertical sidewalls (18) and being separated from each other by profiles with a mesa cross section ; b) the bottom and sidewall surfaces (20, 18) of the trenches (16) formed in the semiconductor substrate, as well as the surfaces of the mesa profile are coated with a masking material (22) ; c) the masking material is removed from the bottom surfaces (20) of the trenches (16) and from the covering surfaces of the profiles by means of reactive ion etching ; d) the semiconductor substrate is exposed to a reactive ion etching, the masking material (24) remaining at the sidewalls of the trenches being used as an etching mask in order to produce a series of deep trenches separated from each other by means of narrow mesa sidewalls (28) ; e) the width of the sidewalls (28) is determined by the layer thickness of the masking material (22) ; and f) the material of the sidewalls is completely thermally oxidized.