Radioactive Tracers in Semiconductors

IN the manufacture of semiconductor integrated circuits certain undesirable atoms or ions can remain on or in the silicon wafer or the passivating SiO2 layer. These. originate in the chemical and physical operations such as lapping, polishing, photolithographic etching, diffusions and treatments necessary for the formation of metal contacts and interconnections for the devices. Many of these unwanted impurities are capable of degrading device performance under certain circumstances and causing the ultimate failure of the devices. For example, high concentrations of phosphorus in SiO2 layers will result in the lowering of its dielectric strength. (At the same time the phosphorus can also act as a getter for other undesirable impurities so an impurity can be both undesirahle or desirable depending on the type of device being made.) It is often difficult to attribute failures in semiconductors (other than simple mechanical or electrical ones) to any certain causes. Yet, it appears that this class of failures is caused by migration of certain trace chemical species or, perhaps, by imperfections in the crystal or oxide structures or by combinations of these mechanisms. Since the number of atoms or molecules responsible for a failure need not be very large, radioactive tracers offer a general method for studying these kinds of failures. Thus far, the amount of residue left behind the radioactively tagged C14H3OH, H2S35O4, NaCr51O4 and HCl36 has been investigated.