Method and device for CMOS image sensing with separate source formation

A method for making an image sensor, the method comprising:

• forming a first well and a second well in a substrate, the first well and the second well being different;
• forming a gate oxide layer on the substrate;
• depositing a first gate region and a second gate region on the gate oxide layer, the first gate region being associated with the first well, the second gate region being associated with the second well;
• forming a third well in the substrate;
• implanting a first plurality of ions to form a first lightly doped source region and a first lightly doped drain region in the first well;
• implanting a second plurality of ions to form a second lightly doped drain region and a second lightly doped source region in the second well;
• forming a first spacer and a second spacer associated with the second gate region after implanting the second plurality of ions;
• and
• implanting a third plurality of ions to form a source in the second well after forming the first spacer, wherein the source has a depth that is deeper than a heavily doped drain region in the second well.

A device for image sensing, the device comprising:

• a semiconductor substrate;
• a first well, a second well, and a third well in the semiconductor substrate, the first well and the second well being associated with a CMOS, the third well being associated with a photodiode, the first well and the second well being different;
• a gate oxide layer on the semiconductor substrate including the third well,
• a first gate region and a second gate region on the gate oxide, the first gate region and the second gate region being associated with the first well and the second well respectively;
• a first spacer and a second spacer adjacent to the second gate region;
• a first lightly doped drain region in the second well and associated with the second gate region, the first lightly doped drain region is substantially aligned with the second gate region;
• a first heavily doped drain region in the second well, aligned with the first spacer, and associated with the first lightly doped drain region;
• and
• a source in the second well and aligned with the second gate region.