High transmittance overcoat for microlens arrays in semiconductor color imagers

A semiconductor device comprising:

• a plurality of imaging sensors, each imaging sensor having a photosensitive area;
• a first layer formed over the plurality of imaging sensors, said first layer being substantially optically transparent;
• a first color filter layer formed on the first layer, said first color filter layer being registered with the photosensitive areas of a first subset of the plurality of imaging sensors;
• a second layer formed over the first color filter layer;
• a third layer formed upon the second layer;
• a layer of patterned microlens material forming a first plurality of microlenses over the third layer, said first plurality of microlenses being mutually registered with the photosensitive areas of the plurality of imaging sensors;
• and
• an overcoat layer formed over the first plurality of microlenses, said overcoat layer having high transmittance and a graded refractive index profile such that a first portion of the overcoat layer has a first refractive index and a second portion of the overcoat layer has a second refractive index, the first and second refractive indexes being unequal.

A semiconductor device comprising:

• a plurality of imaging sensors, each imaging sensor having a photosensitive area, the plurality of imaging sensors being organized in a plurality of subsets;
• a first color filter layer disposed over the plurality of imaging sensors, said first color filter layer being registered with the photosensitive areas of a first subset of the plurality of imaging sensors;
• a second color filter layer disposed over the first color filter layer, said second color filter layer being registered with the photosensitive areas of a second subset of the plurality of imaging sensors;
• a third color filter layer disposed over the second color filter layer, said third color filter layer being registered with the photosensitive areas of a third subset of the plurality of imaging sensors;
• a layer of patterned microlens material forming a first plurality of microlenses over the third color filter layer, said first plurality of microlenses being mutually registered with the photosensitive areas of the plurality of imaging sensors;
• and
• an overcoat layer formed over the first plurality of microlenses, said overcoat layer having high transmittance;
• said overcoat layer having a top portion and a bottom portion, said bottom portion being disposed adjacent said plurality of microlenses, wherein said bottom portion has an index of refraction that is substantially equal to an index of refraction of at least one of the plurality of microlenses, and said top portion has an index of refraction that is substantially different from the index of refraction of said bottom portion.