TSMC patent application US20110254115 "Inserted reflective shield to improve quantum efficiency of image sensors" joined a growing group of applications proposing to put a reflector in BSI pixel to increase its red and IR QE:
"Examples of reflective metals that can be used for such application include, but are not limited to, tungsten (W), aluminum (Al), and copper (Cu), zinc (Zn, gold (Au), silver (Ag), titanium (Ti), tantalum (Ta), chromium (Cr), tin (Sn), cobalt (Co), and nickel (Ni). The reflective material can also be a metal-containing film or an alloy that is made of two or more metals. Non-metal (or non-metallic) materials that are reflective may also be used. For example, polytetrafluorethylene (PTFE) is a polymer that has high reflectivity."