Back-illuminated sensor and a method of manufacturing a sensor

A method of fabricating an image sensor, the method comprising:

• forming a first epitaxial layer on a substrate;
• forming a circuit element on said first epitaxial layer;
• thinning the substrate to generate a thinned substrate, the thinned substrate exposing at least a surface portion of the first epitaxial layer;
• forming a second epitaxial layer on the exposed portion of the first epitaxial layer;
• and
• forming a pure boron layer on the second epitaxial layer,
• wherein forming the second epitaxial layer includes generating a p-type dopant concentration gradient in the second epitaxial layer by gradually increasing a concentration of a p-type dopant used during formation of the second epitaxial layer such that a first layer portion of the second epitaxial layer has a lower p-type dopant concentration than a subsequently formed second layer portion of the second epitaxial layer, and a highest p-type dopant concentration of the second epitaxial layer is adjacent to the pure boron layer, and
• wherein the method further comprises:
• attaching a handling wafer to the first epitaxial layer over the circuit elements prior to thinning the substrate, and
• forming vias in at least one of the first epitaxial layer and the handling wafer prior to thinning the substrate.