Depth resolution

The depth resolution of a SIMS depth profile depends upon a number of factors, not least, the original surface roughness. Except in special circumstances, the depth resolution cannot be expected to be better than the microscopic surface roughness. Assuming a very flat surface, like a semiconductor wafer or glass plate it is the thickness of the altered layer that has the most direct impact on depth resolution as the continual mixing and remixing in the layer results in a reservoir of species which are released at a fixed rate. This leads to the characteristic exponential tail (linear on a log plot) seen in SIMS depth profiles of sharp layers. If the altered layer thickness is reduced, by changing the bombardment conditions such as impact energy, then the decay will become steeper. Depth resolution is measured in terms of the decay length, l_d, the distance over which the signal falls by a factor e, the natural log base. It is also sometimes described in nm/decade as this easy to estimate from a graph, SIMS depth profiles being presented, almost universally, with logarithmic concentration axis due to their high dynamic range.