Trench isolation structure, semiconductor assembly comprising such a trench isolation, and method for forming such a trench isolation

Method for forming a trench isolation in a semiconductor slab, comprising the steps of:

• providing a slab of semiconducting material, having a first surface and comprising a buried layer parallel to and below the first surface;
• forming a trench groove in the semiconductor slab, the trench groove having a bottom surface and a sidewall, and extending from the first surface through the buried layer and into the slab of semiconducting material;
• filling the trench groove at least with a first insulating material and with a first filler material, wherein the first insulating material covers at least the bottom surface and the sidewall in a layer having a thickness d, and wherein the first filler material at least partially fills a remaining part of the trench groove,
• characterized in that at least in a first part of the trench groove the first part of the trench groove surrounded by the buried layer, the thickness of the first part of the trench groove is larger than the thickness in a second part of the trench groove the second part of the trench groove located below the first part of the trench groove.