Effects of sample cooling on depth profiling of Na in SiO2 thin films

Depth profiling using secondary ion mass spectrometry of a 100 keV Na implant in a 430 nm thick SiO2 film has been studied as a function of temperature from 295 to 93 K. At 295 K, and using a coincident electron flux to reduce the extent of electric field-induced Na migration, accumulation of Na at the interface could be eliminated. However, profiles at 295 K without Na accumulation were difficult to reproduce. In addition, the trailing edges often did not decrease exponentially and are broadened with decay lengths >44 nm. At 93 K and using the electron flux only to maximize the matrix secondary ion signals, accumulation of Na at the interface was eliminated, the trailing edge decreased exponentially and the decay length was reduced to 27 nm. Sample cooling also improved reproducibility and allowed a wider range of sputtering conditions to be used. (C) 1997 by John Wiley & Sons, Ltd.