Pixel structures of CMOS imaging sensors and fabrication method thereof
A method is provided for fabricating a pixel structure of a CMOS transistor. The method includes providing a semiconductor substrate doped with first type doping ions; and forming a trench in the semiconductor substrate by etching the semiconductor substrate. The method also includes forming isolation layers on side surfaces of the trench to prevent dark current from laterally transferring; and forming an epitaxial layer doped with second type doping ions with a doping type opposite to a doping type of the first type doping ions in the trench. Further, the method includes forming a pinning layer on a top surface of the epitaxial layer; and forming a gate structure on a surface of the semiconductor substrate at one side of the epitaxial layer. Further, the method also includes forming a floating diffusion region in the semiconductor substrate at one side of the gate structure far from the epitaxial layer.
1. A method for fabricating a pixel structure of a CMOS imaging sensor, comprising:
providing a semiconductor substrate doped with first type doping ions;
forming a hard mask layer having an opening on the semiconductor substrate;
forming a trench in the semiconductor substrate by etching the semiconductor substrate using the hard mask layer as an etching mask;
etching the hard mask layer to enlarge the opening to form an enlarged opening to expose a surface portion of a top surface of the semiconductor substrate around the trench;
forming isolation layers on side surfaces of the trench to prevent a dark current from laterally transferring;
forming an epitaxial layer doped with second type doping ions with a doping type opposite to a doping type of the first type doping ions in the trench by filling up the trench and covering the surface portion of the top surface of the semiconductor substrate around the trench,
wherein a portion of the epitaxial layer is higher than the top surface of the semiconductor substrate, the portion of the epitaxial layer higher than the top surface of the semiconductor substrate is filled in the enlarged opening, and the portion of the epitaxial layer higher than the top surface of the semiconductor substrate covers the surface portion of the top surface of the semiconductor substrate exposed by the enlarged opening;
forming a pinning layer on the epitaxial layer;
forming a gate structure on the top surface of the semiconductor substrate at one side of the epitaxial layer; and
forming a floating diffusion region in the semiconductor substrate at one side of the gate structure far from the epitaxial layer.
2. The method according claim 1 , wherein:
a photodiode is formed by the epitaxial layer and the semiconductor substrate.
3. The method according to claim 1 , wherein:
the epitaxial layer is made of silicon.
4. The method according to claim 1 , wherein:
a line distance between a side surface of the enlarged opening and a side surface of the trench is in a range of approximately 0.1 μm˜1 μm.
5. The method according to claim 1 , wherein forming the isolation layer further includes:
forming an isolation material layer on side surfaces of the trench; and
performing a mask-less etching process onto the isolation material layer.
6. The method according to claim 1 , wherein:
a thickness of the isolation layers is in a range of approximately 8 nm˜20 nm.
7. The method according to claim 1 , wherein:
the epitaxial layer is formed by a selective epitaxial process with an in situ doping.
8. The method according to claim 2 , wherein:
the first type doping ions are P-type; and
the second type doping ions are N-type.
9. The method according to claim 2 , wherein:
the first type doping ions are N-type; and
the second type doping ions are P-type.
10. The method according to claim 1 , wherein:
the pinning layer is formed by an ion implantation process; and
a doping type of the pinning layer is same as the doping type of the semiconductor substrate.