Making a Back-Illuminated Imager With Back-Side Contact and Alignment Markers

A design modification and a fabrication process that implements the modification have been conceived to solve two problems encountered in the development of back-illuminated, back-side-thinned complementary metal oxide/semiconductor (CMOS) image-detector integrated circuits. With respect to such an integrated circuit to be fabricated on a silicon substrate, the two problems are (1) how to form metal electrical-contact pads on the back side that are electrically connected through the thickness in proper alignment with electrical contact points on the front side and (2) how to provide alignment keys on the back side to ensure proper registration of backside optical components (e.g., microlenses and/or color filters) with the frontside pixel pattern. (In this special context, “front side” signifies that face of the substrate upon which the pixel pattern and the associated semiconductor devices and metal conductor lines are formed.)