In a method for forming a dielectric isolation structure or container, ion implantation is performed to form a buried implant region in a base semiconductor material. Trenches are formed in the base semiconductor material that access the buried implant region. The buried implant region is removed by etching via the trenches to form a lateral undercut region connected with the trenches. The lateral undercut region and the trenches are filled with dielectric material to form a dielectric bottom region and annular dielectric sidewall of the dielectric isolation structure. By forming of the trenches and the filling of the trenches in two or more iterations, with the removal of the buried implant region being performed after one of these iterations, detachment and self-collapse of the contained portion of base semiconductor material is avoided.