Image sensor with improved charge transfer efficiency and method for fabricating the same

An image sensor, comprising:

• a gate structure formed on a semiconductor layer, wherein the semiconductor layer includes a substrate and an epitaxial layer of a first conductive type formed on the substrate;
• a first impurity region of the first conductive type aligned with one side of the gate structure and extending from a surface portion of the epitaxial layer to a first depth in the epitaxial layer;
• a first spacer formed on each sidewall of the gate structure;
• a second impurity region of the first conductive type, aligned with the first spacer and extending from the surface portion of the epitaxial layer to a second depth in the epitaxial layer that is deeper than the first depth;
• a second spacer formed on each sidewall of the first spacer;
• a third impurity region of the first conductive type aligned with the second spacer and extending from the surface portion of the epitaxial layer to a third depth in the epitaxial layer that is deeper than the second depth;
• and
• a fourth impurity region of a second conductive type in the epitaxial layer and beneath the third impurity region;
• wherein:
• the entirety of the fourth impurity region is deeper in the epitaxial layer than the third impurity region;
• the second conductive type is different than the first conductive type;
• and
• the first impurity region, the second impurity region, the third impurity region, and the fourth impurity region form a photodiode of the image sensor.

An image sensor, comprising:

• a gate structure formed on a semiconductor layer, wherein the semiconductor layer includes a substrate and an epitaxial layer of a first conductive type formed on the substrate;
• a first impurity region of the first conductive type aligned with one side of the gate structure and extending from a surface portion of the epitaxial layer to a first depth in the epitaxial layer;
• a first spacer formed on each sidewall of the gate structure;
• a second impurity region of the first conductive type, aligned with the first spacer and extending from the surface portion of the epitaxial layer to a second depth in the epitaxial layer that is deeper than the first depth;
• a second spacer formed on each sidewall of the first spacer;
• a third impurity region of the first conductive type aligned with the second spacer and extending from the surface portion of the epitaxial layer to a third depth in the epitaxial layer that is deeper than the second depth;
• a fourth impurity region of a second conductive type in the epitaxial layer and beneath the third impurity region;
• and
• a floating diffusion region of the second conductive type aligned with the other side of the gate structure and extending to a predetermined depth from another surface portion of the epitaxial layer;
• wherein:
• the entirety of the fourth impurity region is deeper in the epitaxial layer than the third impurity region;
• the second conductive type is different than the first conductive type;
• the first impurity region, the second impurity region, the third impurity region, and the fourth impurity region form a photodiode of the image sensor;
• and
• the gate is configured to transfer charge collected by the photodiode to the floating diffusion region in response to a control signal.