Optical performances of CMOS imagers can be affected by contamination caused by several elements found, even if in extremely low concentration, in semiconductor production environment.
In particular, contamination by noble elements like, for example, Gold or Silver, is extremely critical since these elements have a very low migration rate in the silicon, so they are not easily segregated into the gettering sites. They remain electrically active, and contribute to the so called “hot pixel” defects.
Unfortunately, the noble elements are also very difficult to be effectively detected by VPD/ICP-MS, the technique traditionally and successfully used to detect most of the other elements (like Fe, Cr, Ni, Na, Ca...).
In fact, the chemical mixtures typically used to dissolve noble metals (like Aqua Regia) are very aggressive to the silicon substrate as well, damaging the wafer surface and making recovery by the automated equipment used for this technique (VPD) very difficult and not reliable.
In order to allow the implementation of routine analysis for the full elemental range of interest, alternative chemistries should be defined (through theoretical modeling and analytical test confirmation), capable to assure acceptable analytical recovery for noble elements and the other common elements at same time.