Dynamic SIMS & Dopant Profiling Lab

Dynamic SIMS focuses a continuous energetic primary ion beam (such as Cs+ or O2+) onto a sample surface in ultra-high vacuum. The primary ions trigger collision cascades in the top atomic layers of the solid, sputtering away neutral matrix atoms and a small fraction of secondary ions. These ejected secondary ions are accelerated into a high-transmission mass spectrometer (such as a double-focusing magnetic sector) to detect elemental and isotopic concentrations as a function of depth with parts-per-billion sensitivity.

During Cs+ bombardment, an electropositive sub-monolayer of cesium builds up on the sample surface, reducing the electronic work function by approximately 2.6 eV. In accordance with the surface electron tunneling model, lowering the work function drastically decreases the potential barrier for valence electrons to transfer from the solid onto sputtered neutral dopants (such as phosphorus, arsenic, and carbon), boosting negative secondary ion yields by three to four orders of magnitude.

High-energy primary ions impart forward momentum to matrix and dopant atoms, physically knocking them deeper into the material before they can be sputtered (atomic mixing and cascade recoil knock-in). This broadens sharp interfaces and distorts the apparent trailing slope. By operating at ultra-low impact energies between 250 and 500 eV, the penetration range is restricted below 1.5 nanometers, delivering trailing decay lengths under 1.0 nanometer per decade for sub-5nm junction profiling in nanosheets.

Mass Resolving Power (MRP = M / Delta M) is the ability of a mass spectrometer to separate two ions with the same nominal mass. For example, 31P- (exact mass 30.9738 amu) and 30Si1H- (exact mass 30.9816 amu) have a tiny mass difference of 0.0078 amu at nominal mass 31. Without a high-resolution magnetic sector spectrometer operating at an MRP exceeding 3,955, the silicon hydride peak would obscure phosphorus dopant signals below 10^18 atoms per cubic centimeter.