Neutron Transmutation Doping of Silicon
The MIT-NRL Silicon Program irradiates silicon material for the semiconductor industry for use in the manufacture of microcircuits. We treat the silicon material provided by the customer per the customer’s requirements. The Silicon program utilizes the 4TH1-3 and 6TH1-2 horizontal through ports for Neutron Transmutation Doping (NTD) of single crystal silicon at the MIT Research Reactor (MITR-II). These through ports are tangent to the D2O Reflector Tank. The 4TH1-3 port can accommodate the 4-inch crystals and the 6TH1-2 port can accommodate 4, 5 and 6-inch crystals.
The NTD process takes place when undoped (high purity) silicon is irradiated in a thermal neutron flux. The purpose of semiconductor doping is to create free electrons (low resistivity). The thermal neutron is captured by the 30Si atom, which has a 3% abundance in pure Si. Due to the high neutron/proton ratio of 31Si, it will release a beta and, by converting a neutron to a proton, the Si-31 atom transmutes to a P-31 atom.
Overall the result is a lower resistivity with little variance from the target resistivity. The doped Si is used in a variety of electronic devices, such as transistors, diodes, and IC chips. The silicon crystals are loaded into 400 mm long magnesium cans and then placed on the loading conveyor. These cans are then transferred to the airlock (see figure above) and pushed through the rotated through port to the unload side. The cans are then placed on another conveyor for radioactive decay. The speed at which the crystals are pushed through the port is determined by the reactor power, the final and initial ingot resistivity, and the port in which the silicon is processed.
Once the Si ingots have been irradiated they are removed from the conveyor, carefully cleaned, surveyed for radiation, and inspected for quality control before being packaged and shipped to the customer.