Image sensor with raised photosensitive elements
View Patent ↗An image sensor having a pixel array comprises periphery elements formed over a substrate, an oxide layer formed over the periphery elements, an epitaxial layer formed in an opening in the oxide layer in a pixel array area, and a plurality of photosensitive elements of the pixel array formed in the epitaxial layer. Formation of an initial metallization layer occurs after the formation of the photosensitive elements in the epitaxial layer. The photosensitive elements can thus be formed in the epitaxial layer at a higher level within an image sensor stack than that of the initial metallization layer. This advantageously allows stack height and pixel size to be reduced, and fill factor to be increased. The image sensor may be implemented in a digital camera or other type of digital imaging device.
1. A method of forming an image sensor having a pixel array, the method comprising the steps of:
forming periphery elements of the image sensor over a substrate;
forming an oxide layer over the periphery elements;
forming an opening in the oxide layer in a pixel array area;
forming an epitaxial layer in the opening;
forming at least one well within the epitaxial layer;
forming photosensitive elements of the pixel array within the at least one well in the epitaxial layer including at least one photodiode, wherein the at least one well extends through a thickness of the epitaxial layer into the substrate; and
forming at least one of a transfer gate, a reset transistor, a select transistor, and a source follower output transistor within the epitaxial layer.
2. The method of claim 1 wherein the periphery elements comprise polysilicon gates of periphery transistors.
3. The method of claim 1 further including the step of forming an initial metallization layer comprising periphery metal conductors overlying one or more of the periphery elements.
4. The method of claim 3 wherein the step of forming the initial metallization layer is performed subsequent to the steps of forming the oxide layer, forming the opening in the oxide layer, forming the epitaxial layer in the opening, and forming the photosensitive elements of the pixel array.
5. The method of claim 4 further including the step of forming a plurality of additional metallization layers subsequent to the step of forming the initial metallization layer.
6. The method of claim 1 wherein the step of forming the epitaxial layer in the opening further comprises performing a selective epitaxial growth process such that the epitaxial layer is confined substantially to the pixel array area.
7. The method of claim 6 wherein the selective epitaxial growth process is controlled to provide a designated depletion region characteristic for the photosensitive elements.
8. The method of claim 1 further comprising the step of forming metal conductors interconnecting pixel array circuitry with periphery circuitry.
9. The method of claim 8 wherein the metal conductors comprise final metallization layer conductors.
10. The method of claim 1 further comprising the steps of:
forming a nitride layer over the photosensitive elements of the pixel array;
forming multiple metallization layers and associated vias in periphery areas of the image sensor; and
removing the nitride layer.
11. The method of claim 1 wherein the steps are implemented as part of a wafer level process for forming a plurality of image sensors utilizing an image sensor wafer and the image sensor wafer is subsequently separated into the plurality of image sensors.