Isolation structure and formation method thereof

A formation method of an isolation structure, the method comprising:

• forming a trench in a substrate, wherein a sidewall of the trench has a first inclined surface and a second inclined surface, the first inclined surface is located on the second inclined surface, a slope of the first inclined surface is different from a slope of the second inclined surface, and a length of the first inclined surface is greater than 15 nanometers, wherein the substrate has a first region and a second region, a length that the first inclined surface of the trench in the first region is projected onto a horizontal plane is a first horizontal length, a length that the first inclined surface of the trench in the second region is projected onto the horizontal plane is a second horizontal length, the first horizontal length is different from the second horizontal length.