Ultra low energy SIMS (ULESIMS) is a special application of dynamic SIMS where the primary ion bombardment energy is significantly reduced, providing superior depth resolution and near surface quantification. Like dynamic SIMS, the detection sensitivity of the technique is in the ppm - ppb range for all elements in the periodic table.

During SIMS depth profiling, primary ions used to sputter the surface become implanted in the target. As the surface recedes these are exposed and themselves sputtered, whilst more ions are implanted. At some time after bombardment begins the mixture of target and primary atoms in the surface reaches an equilibrium concentration, and has a well-defined thickness. Until equilibrium is reached, the signals are dominated by changes in the ionised fraction, which is highly dependent on the local chemical environment from which an ion is ejected.  The local chemistry can cause changes in the fraction of ejected material that is ionised covering many orders of magnitude.

At conventional dynamic SIMS primary ion energies (typically above 4 keV) equilibrium may not be achieved until a significant amount of the target material has been removed (10 ‑ 20nm).  However, reducing the bombardment energy permits equilibrium to be reached more rapidly, often with less than 1 nm of the original sample being removed. This means that ULE SIMS can provide quantified data from within 1 nm of the surface.

The mixed layer is responsible for blurring interfaces in SIMS depth profiling, degrading the depth resolution. The low primary ion energy employed in ULESIMS significantly reduces the mixing depth; consequently ULESIMS provides extremely high depth resolution.

Optimum depth resolution is achieved with planar surfaces, such as glass and silicon wafer.
Valleys in the surface roughness profile cause buried features to become exposed early; similarly, peaks in the surface retain material longer than their surroundings. These effects lead to features becoming broadened – loss of depth resolution. Ion beam bombardment can cause the development of surface topography (this leads to a loss of depth resolution with depth), however, correct choice of analysis protocol can usually overcome this problem.

The depth scale is generally calibrated by measuring the terminal crater depth with either a stylus or optical profilometer. At very shallow depths, or when it is impractical to measure the crater (such as on a wire or relatively rough surface), the depth may be estimated by relating the erosion rate of the sample to that of a known material, typically silicon dioxide on silicon.