SIMS depth profiling of organic or inorganic samples may suffer from the formation of ion beamed induced surface roughening.
This effect limits the achievable depth resolution and can also lead to different erosion rates. Sample roughening can effectively be avoided by using fast sample rotation during sputtering.
The measurements shown here are of two depth profiles through a GaN/AlGaN multilayer system using 2 keV oxygen for sputtering.
The coating is composed of four 200 nm thick GaN layers and four 100 nm thick AlGaN layers. The first profile was made without sample rotation and the second one was acquired using fast sample rotation.
It can be clearly seen that without sample rotation the sharpness of the Al and AlGaN layers decreases with increasing depth.
This is due to the formation of surface roughening during the measurement which progressively limits the depth resolution.
The surface roughness also leads to a decreasing sputter rate during the profile.
In this example the depth scale of both profiles was established by adjusting the depth of the end of the first AlGaN layer to 300 nm. If the erosion rates are consistent for each layer, the end of the fourth Al layer should be at a depth of 1200 nm.
The profile without sample rotation incorrectly shows the substrate interface at a depth of 1700 nm due to the decrease in erosion rate brought on by surface roughening.
This is avoided by the fast sample rotation and as a consequence the substrate interface is located at the correct depth.