The effect of triple well implant dose on performance of NMOS transistors

The application of triple well (TW) structures provides important advantages for different types of silicon-integrated circuits. In a triple-well technology, the addition of a high energy implant allows the creation of a separate tub which is junction isolated from the substrate. We study the dependence of NMOS transistor performance and leakage current in diode test structures on the dose of a 1 MeV phosphorus TW implant. We observe that initial increase in the implant dose leads to increase in threshold voltage of the devices with a peak at a dose of about 1 × 10¹⁴ cm^-2. With a further increase in dose the threshold voltage decreases: the decrease corresponds to the onset of Si amorphization induced by the TW implant. We explain this anomalous behavior using a model that accounts for suppression, due to formation of a buried amorphous layer, of transient enhanced diffusion of boron in the n-channel region. The dose dependence of leakage current in junction diode structures is explained by the formation of threading dislocations