Buried, fully depletable, high fill factor photodiodes
View Patent ↗A semiconductor detector of electromagnetic radiation which utilizes a dual-purpose electrode which extends significantly beyond the edge of a photodiode. This configuration reduces the sensitivity of device performance on small misalignments between manufacturing steps while reducing dark currents, kTC noise, and “ghost” images. The collection-mode potential of the dual-purpose electrode can be adjusted to achieve charge confinement and enhanced collection efficiency, reducing or eliminating the need for an additional pinning layer. Finally, the present invention enhances the fill factor of the photodiode by shielding the photon-created charge carriers formed in the substrate from the potential wells of the surrounding circuitry.
1. A method of making a CMOS active pixel structure, comprising:
providing a semiconductor substrate with dopants of a first conductivity type at a first concentration density, and with an insulating layer at a surface region of the semiconductor substrate;
forming a collection region by introducing dopants of a second conductivity type which is opposite the first conductivity type at a second concentration density region into the surface region of the semiconductor substrate;
forming a detection region not bordering the collection region which defines a barrier region between the detection region and the collection region by introducing dopants of the second conductivity type at a third concentration density into the surface region of the semiconductor substrate;
forming a barrier region of the first conductivity type in the substrate, with a concentration density of dopants being higher than the concentration density of dopants in the substrate;
forming a dual-purpose electrode on the insulating layer with the dual-purpose electrode extending over the surface of at least part of the collection region, and over at least part of the substrate, the dual-purpose electrode being intended to be driven by a first voltage that causes an electrostatic potential which collects in an area of the collection region beneath the dual-purpose electrode charges generated by electromagnetic radiation and by a second voltage, which is higher than the first voltage, for transferring the charges from the collection region into a detection region; and
forming an amplifier integrated in the active pixel structure that is coupled to the detection region to amplifying the collected charge.
2. The method of claim 1 , wherein said barrier region is extending at least partly under the dual-purpose electrode.
3. The method of claim 1 , further comprising:
forming a detection region with dopants of the second conductivity type at a third concentration density, formed in the surface region of the semiconductor substrate and not bordering the collection region and being connected to read-out electronics.
4. The method of claim 1 , wherein the collection region forms a junction with the semiconductor substrate.
5. The method of claim 4 , wherein the junction formed is a photodiode.
6. The method of claim 1 , wherein the surface regions of the semiconductor substrate beyond the collection region are barrier regions which have dopants of the first conductivity type at a concentration density larger that the concentration density of the semiconductor substrate and read-out electronics are formed within shielding regions.
7. The method of 6 , wherein at least part of the charge carriers that are generated in the semiconductor substrate underneath the shielding regions are collected by the collection region.
8. The method of claim 1 , wherein a pinning region with dopants of the first conductivity type at a fourth concentration density is within the surface region.
9. The method of claim 8 , wherein the pinning region is not covered by the dual-purpose electrode.
10. The method of 9 , wherein the pinning region is aligned with the dual-purpose electrode, and extends along the collection region.
11. The method of claim 1 , wherein the barrier region substantially impedes the diffusion of charges to said detection region.