Conditions required to achieve desired detection limits may also affect depth resolution. The SIMS depth resolution capability is a complex compilation of factors that must be optimized for each application.
Some examples that illustrate advancement in SIMS depth resolution capability, and value to the semiconductor engineer, are given here for HEMT structures. Figure 1 is a SIMS profile of Si, O, In and Al in an AlGaAs/InGaAs HEMT structure. The primary ion is Cs+ and the impact energy of the primary ion is 2 keV. The combination of a heavy primary ion (Cs) and a low impact energy (2 keV) is necessary for optimum depth resolution.
The same sample was analyzed using a 1 keV impact energy, so that a comparison could be made between depth resolutions derived from 1 keV and 2 keV impact energies. Figure 2 is an overlay plot of the Al, O and In signals for 1 keV (solid lines) and 2 keV (dotted lines). First consider the Al signals. At about 1100Å nm the Al signal drops sharply at the first interface of the InGaAs layer. The sharpness of this drop is greater for the 1 keV impact energy and the apparent “valley” of the Al signal is deeper for the 1 keV impact energy. This is a direct result of improved depth resolution. For the In signal we see a similar effect in that the sharpness of the In decrease at the second interface (~1300Å nm) is greater for a 1 keV impact energy. For the O signal we see the width of the interfacial oxygen concentration at about 1750Å nm is more narrow for a 1 keV impact energy, and the sharpness of the falloff of oxygen signal is greater for a 1 keV impact energy.