Approaching the limit for quantitative SIMS measurement of ultra-thin nitrided SiO2 films

As the demands of device performance force gate oxide thicknesses to 2 nm and less, more and more pressure is placed on conventional characterization techniques to measure such thin films. Although SIMS has been utilized to accurately measure ultra-thin nitrided oxide layers for years, we are approaching the limit at which quantitative measurements can be made, simply because SIMS is a sputtering process. Measurements at a very low beam energies (300–1000 eV) and incidence angles (60–80°) indicate ion mixing depths significantly less than 1 nm can be achieved, with the angle of incidence being more important than the beam energy in improving depth resolution. Despite these limitations, we have achieved excellent agreement between SIMS and XPS for measurements of N areal density in 1.7–2.4 nm thick films. Because the ion mixing depth is significantly less than the film thickness, SIMS can give both the distribution and amount of N within these thin layers.