Micro‐Raman spectroscopy is used to study mechanical stress in local isolation structures on silicon substrates (poly‐buffered local oxidation of silicon, called PBLOCOS or LOPOS). The influence of processing parameters such as nitride film thickness and width, pad oxide thickness, and field oxide thickness is studied. Also the change of the local stress during the successive processing steps of the isolation is investigated: deposition of the nitride mask, field oxidation, and removal of the nitride mask. The results are explained using a simple analytical model. It is found that stress varies very much during the different processing stages. After deposition of the nitride film, the stress is compressive under the mask lines and tensile outside the lines, close to the line border. The stress magnitude is highly dependent on the thickness of the nitride film. It can be described by edge forces. During field oxidation, this edge‐force‐induced stress nearly completely relaxes. In LOPOS structures with field oxide, the magnitude and distribution of the mechanical stress is mainly determined by the compressive forces exerted by the field oxide and the bird’s beak on the silicon, and tensile forces which are related to the bending of the nitride film at the edges. The influence of nitride film thickness on the stress at the center of the lines, &ohgr;c, is much smaller after field oxidation. For wide lines &ohgr;cis compressive and much higher than before field oxidation. For small widths &ohgr;cbecomes tensile due to the influence of the vertical forces acting at the bird’s beak edge. The tensile stress due to the bending of the nitride film edge is much larger than the field‐oxide‐induced compressive stress, and dominates the stress picture at the bird’s beak. Only when very thick field oxides and thin nitride films are used, the field‐oxide‐induced compressive stress becomes visible. This becomes more clear when, in a further process stage, the nitride film is removed. The stress picture is then completely determined by the field oxide and the stress in the active area is always compressive.