Scanning Capacitance Microscopy
# Scanning Capacitance Microscopy Dopant Profiling
Scanning capacitance microscopy maps dopant type and concentration across a device cross-section by measuring something neither SIMS nor atom probe tomography measures directly: the local capacitance response of a semiconductor to a tiny applied voltage. A conductive atomic force microscope tip is scanned across a polished, beveled cross-section of the device while a small AC voltage modulates the tip-sample junction. The resulting change in capacitance at each point depends on the local carrier concentration and polarity — n-type and p-type regions produce distinguishably different capacitance-voltage responses — so the technique builds a two-dimensional map of where dopants sit and roughly how concentrated they are, directly over the actual device geometry rather than along a single destructive depth line.
The real differentiator against SIMS depth profiling and atom probe tomography is that SCM is comparatively gentle and maps a full 2D cross-section in one scan, trading absolute quantitative precision for speed and geometric context. SIMS sputters material away layer by layer to build a depth profile — destructive, slow, and limited to a single vertical line through the sample. Atom probe tomography reconstructs atomic positions in three dimensions with extraordinary precision but requires a needle-shaped specimen prepared by focused ion beam milling, a painstaking process applicable to a tiny sample volume. SCM instead rides an AFM tip across a bevel-polished cross-section and reads capacitance directly, revealing the full junction shape — where an implanted well actually terminates, whether a diffusion front is uniform across a transistor array — in one pass, at the cost of being a relative rather than absolute dopant concentration measurement.
Fabs rely on SCM specifically to verify that an implanted or diffused junction's actual shape in a finished device matches the process simulation, catching lateral straggle and junction-depth variation that a single-point depth profile would simply miss. Because SCM images the real device cross-section rather than a blanket test wafer, it catches effects that only show up next to actual device structures — junction pull-back near a shallow trench isolation edge, or lateral diffusion spreading further under a gate than the implant mask alone would predict — exactly the kind of geometry-dependent variation that process engineers need validated on real hardware, not inferred from a planar test structure.
Nanobeam electron diffraction maps lattice strain at the nanometer scale inside a device channel; scanning capacitance microscopy maps dopant polarity and relative concentration across the same kind of cross-section — together with SIMS and atom probe tomography, they form a toolkit where each technique answers a different question about where atoms sit and what they're doing, and no single one replaces the others.