Image sensor comprising a photodiode in a crystalline semiconductor layer and manufacturing method thereof

An image sensor comprising:

• a first substrate comprising a readout circuitry;
• an interlayer dielectric on the first substrate, the interlayer dielectric comprising a lower conductive line therein;
• a crystalline semiconductor layer on the interlayer dielectric;
• a photodiode in the crystalline semiconductor layer, the photodiode comprising a first impurity region and a second impurity region;
• a first via hole passing through the crystalline semiconductor layer and the interlayer dielectric to expose the lower conductive line;
• a plug inside the first via hole connecting the lower conductive line to the first impurity region, wherein a top surface of the plug is below the second impurity region;
• and
• a device isolation region in the crystalline semiconductor layer, the device isolation region separating the photodiode according to a unit pixel.