Scanning capacitance microscopy combines atomic-force-microscopy techniques and capacitance measurement to map charge carrier levels in semiconductor structures. Developed by engineers at Digital Instruments (Santa Barbara, CA), the technique offers semiconductor developers and manufacturers high-resolution images of the basic operational aspects of their device structures, unobtainable by other methods.
In a scanning capacitance microscope (SCM), a transmission line connects a high-sensitivity capacitance resonator to a cantilever-mounted probe that scans the sample under test. When the probe tip contacts a semiconductor sample, the probe, sensor, transmission line, and carriers in the material become part of the resonator system. Probe-to-sample capacitance and small variations in capacitance load the end of the transmission line, changing the resonant frequency of the system. These frequency variations are magnified in the resonance sensor output signal; the SCM can detect capacitance changes on the order of 10-18 farads, which gives it a charge carrier detection range from 1015 to 1020 per cm3.
A kilohertz-range alternating-current (ac) bias voltage applied to the semiconductor generates capacitance variations between the probe tip and the sample by causing the probe to alternately attract and deplete the free carriers in the surface region of the semiconductor. This depletion and accumulation of carriers under the probe can be thought of as an equivalent moving capacitor plate.
Capacitance varies as the inverse of the capacitor plate separation. The free-carrier concentration controls the depletion depth on the effective "plate" location. High carrier concentration leads to a shallow (10 to 100 Å) depletion depth; as concentration decreases, this depletion moves deeper into the material.
The SCM can map compound semiconductors such as the diode laser. Unlike instruments such as transmission electron microscopes, the SCM in this case requires no sample preparation beyond structural exposure by cleaving. The semi-insulating, iron-doped indium phosphide layer of the laser has no carriers and thus appears dark in the image, while the n-doped regions appear very bright.