Aptina's patent application US20110127628 proposes to use ion implantation to change passivation layer refractive index:
"FIGS. 2A illustrates the transmission of incident light 202 through CFA 104, passivation layer 106 (106′) and insulation layer 108, when passivation layer 106 (106′) includes impurities. In particular, FIG. 2A illustrates passivation layer 106 having a single refractive index neff.
In conventional image sensors, a passivation layer typically has a refractive index of about 2.0. In contrast, CFA 104 typically includes a refractive index (n1) of between about 1.5-1.7, whereas insulation layer 108 typically includes a refractive index (n2) of about 1.46. Accordingly, the refractive indices n1 and n2 of CFA 104 and insulation layer 108 are typically similar. Because the refractive index of the passivation layer in conventional devices is different from the refractive indices n1 and n2, incident light 202 may be reflected at an interface 214 (between CFA 104 and a conventional passivation layer) and/or at an interface 216 (between a conventional passivation layer and insulation layer 108)."