Low frequency noise in SIMOX MOSFETs

An extensive study has been made of low-frequency noise in MOSFETs built in a variety of SIMOX (separation by implanted oxygen) processes. Most of the devices studied showed very high noise levels dominated by generation-recombination noise. Among the devices measured were NMOS and PMOS FETs with superficial Si layers ranging in thickness from 0.3 μm (no epi layer) to 2.5 μm (with epi). SIMOX implant doses ranged from 1.4×10¹⁸ cm^-2 to 1.8×10¹⁸ cm^-2. Anneal times varied from 3 to 16 hours, and anneal temperatures varied from 1150°C to 1275°C. The shape and level of the measured noise spectra varied strongly as a function of back or front surface potential. Classic 1/f noise was not observed and was probably masked by other processes. Over the measured frequency range (10 Hz to 10 kHz) the noise was dominated by fluctuations in the channel carrier concentration due to trapping. From the shape of the noise spectra and their strong dependence on surface potential, it is concluded that the traps are not located in either the top or bottom oxide layers but in the bulk. A few distinct trap levels, each with an associated time constant, interact most strongly with channel carriers when the Fermi level aligns with the trap level, which accounts for the strong dependence on surface potential. It appears likely that these traps are associated with the dislocations typically generated in the SIMOX process