Uneven-trench pixel cell and fabrication method

A pixel cell, comprising:

• a semiconductor substrate that includes a floating diffusion region, a photodiode region, and a front surface;
• and
• a transfer gate disposed on the semiconductor substrate, coupling the photodiode region to the floating diffusion region, and including:
• a first vertical gate electrode extending to a first depth along a depth direction from the front surface into the semiconductor substrate, the first depth exceeding a junction depth of the floating diffusion region;
• and
• a second vertical gate electrode extending a second depth from the front surface along the depth direction into the semiconductor substrate, the second depth exceeding the first depth;
• the first vertical gate electrode and the second vertical gate electrode being located between the floating diffusion region and the photodiode region in a first direction, the first vertical gate electrode being laterally displaced from the second vertical gate electrode in a second direction that is perpendicular to the first direction, each of the first direction and the second direction being perpendicular to the depth direction.

A method for fabricating a pixel cell, comprising:

• simultaneously etching (i) a first region of a semiconductor substrate exposed by a small aperture of an apertured mask located on a surface of the semiconductor substrate to yield a shallow trench having a shallow bottom surface, and (ii) a second region of the semiconductor substrate exposed by a large aperture of the apertured mask to yield a deep trench having a deep bottom surface, in a direction parallel to the surface, a width of the large aperture in a first direction exceeding a width of the small aperture in the first direction;
• the semiconductor substrate including a floating diffusion region and a photodiode region;
• the shallow trench being located between the floating diffusion region and the photodiode region and extending into the semiconductor substrate from a front surface of the semiconductor substrate, a distance between the shallow bottom surface and the front surface defining a shallow depth of the shallow trench that exceeds a junction depth of the floating diffusion region relative to the front surface;
• a deep trench located between the floating diffusion region and the photodiode region and extending into the semiconductor substrate from the front surface, a distance between the deep bottom surface and the front surface defining a deep depth, of the deep trench, that exceeds the shallow depth;
• the distance defining the shallow depth being in a depth direction, the shallow trench and the deep trench being located between the floating diffusion region and the photodiode region in a first direction, the shallow trench being laterally displaced from the deep trench in a second direction that is perpendicular to the first direction, each of the first direction and the second direction being perpendicular to the depth direction.