Semiconductor device

A semiconductor device, comprising:

• a substrate including a first region and a second region;
• an isolation structure disposed in the substrate in the first region, wherein the isolation structure extends from a first surface of the substrate toward a second surface of the substrate;
• an alignment mark in the substrate in the second region, wherein the alignment mark extends from the first surface of the substrate toward the second surface of the substrate and at the same level as the isolation structure;
• another isolation structure corresponds to the isolation structure disposed in the substrate in the first region, wherein the another isolation structure extends from the second surface of the substrate toward the first surface of the substrate and aligned to the isolation structure;
• a dielectric region buried in the substrate in the second region and overlapping the alignment mark, wherein heteroatoms are ion implanted and annealed to form the dielectric region having a refractive index (n value) less than 1.5 and coplanar with a top surface of the another isolation structure;
• and
• wherein a thickness of a portion of the substrate from the top surface of the another isolation structure to a top surface of the isolation structure is between 4 μm to 20 μm or more and a thickness of a portion of the substrate from a top surface of the alignment mark towards the second surface of the substrate is less than 4 um and greater than 0 um.