Semiconductor device with fin structures

A semiconductor device structure, comprising:

• a first fin structure and a second fin structure over a semiconductor substrate;
• a first epitaxial structure over the first fin structure;
• a second epitaxial structure over the second fin structure;
• and
• a dielectric fin over the semiconductor substrate, wherein the dielectric fin is between the first fin structure and the second fin structure, the dielectric fin has an inner portion and a protective layer, the protective layer extends along sidewalls and a bottom of the inner portion, the protective layer has a dielectric constant higher than that of the inner portion, the protective layer has a first average grain size, the inner portion has a second average grain size, and the first average grain size is larger than the second average grain size, and a topmost surface of the inner portion is vertically higher than a bottommost surface of the first epitaxial structure.