Narrow surface transient and high depth resolution SIMS using 250 eV O 2+

Abstract

Ultrashallow junctions in semiconductors and multi-quantum wells (MQW) in lasers demand high depth resolution for accurate depth profiling. SIMS has been widely used in depth profiling and the use of ultralow-energy SIMS has demonstrated a narrower surface transient and an improvement in depth resolution. In this work, we use an ATOMIKA 4500 SIMS depth profiler with O 2 + primary ions at an ultralow-energy ( E p) of 250 eV and incidence angles ( θ) between 0 and 70° without oxygen flooding. A sample with 10 delta layers of Si 0.7Ge 0.3 nominally grown 11 nm apart is used. We observe that for applications like characterizing ultrashallow junctions, θ ∼ 0° provides the narrowest surface transient ( z tr) of 0.7 nm, which is marginally better than at θ ∼ 40° with z tr of 1.0 nm. The depth resolution denoted by the full width at half maximum (FWHM) of the 70Ge + peaks is comparable for both θ ∼ 0 and 40° at 1.6 and 1.4 nm, respectively. However, in the case of MQW profiling, whereby the quantum wells are normally located deeper, θ ∼ 40° is preferable. At this angle, the average sputter rate of 47 nm min −1 nA −1 cm −2 is significantly higher, more than double that at θ ∼ 0° and a better depth resolution with decay length ( λ d) of 0.64 nm compared to 0.92 nm at θ ∼ 0°. Moreover, the dynamic range possible is also better at θ ∼ 40°. θ ∼ 60 ο is not ideal, even though there is no sign of the onset of roughening. Although the higher sputter rate is an advantage, the depth resolution deteriorates as the profile gets deeper.