Scanning capacitance microscopy

Scanning capacitance microscopy

Scanning capacitance microscopy (SCM) is a variety of scanning probe microscopy in which a narrow probe electrode is held just above the surface of a sample and scanned across the sample. SCM characterizes the surface of the sample using information obtained from the change in electrostatic capacitance between the surface and the probe.

More precisely SCM uses an ultra-sharp conducting probe (often Pt/Ir or Co/Cr metal covering an etched silicon probe) to form a metal-insulator-semiconductor (MIS/MOS) capacitor with a semiconductor sample if a native oxide is present. When no oxide is present, a shottky capacitor is formed. When the probe and surface are in contact, an AC bias is applied, generating capacitance variations in the sample which can be detected using a GHz resonant capacitance sensor. The tip is then scanned across the semiconductor's surface in 2D while the tip's height is controlled by conventional contact force feedback .

By applying an alternating bias to the metal-coated probe, carriers alternately accumulate and deplete within the semiconductor’s surface layers, changing the tip-sample capacitance. The magnitude of this change in capacitance with the applied voltage gives information about the concentration of carriers (SCM amplitude data), whereas the difference in phase between the capacitance change and the applied, alternating bias carries information about the sign of the charge carriers (SCM phase data). Due to the fact that SCM functions even through an insulating layer, a finite conductivity is not required to measure the electrical properties.

Limit of resolution of SCM

On the conducting surfaces, the limit of resolution at the scale of 2nm is estimatedcite journal
url=http://www.iop.org/EJ/abstract/0022-3727/36/5/326
author=Lanyi S
coauthors=Hruskovic M
title=The resolution limit of scanning capacitance microscopes
journal=JOPD
volume=36
pages=598–602
year=2003
doi=10.1088/0022-3727/36/5/326] .For the high resolution, the quick analysis of capacitance of a capacitor with rough electrode is requiredcite journal
url=http://www.iop.org/EJ/abstract/0022-3727/33/22/305
author=N.C.Bruce
coauthors=A.Garcia-Valenzuela, D.Kouznetsov
title=The lateral resolution limit for imaging periodic conducting surfaces in capacitive microscopy
journal=JOPD
volume=33
pages=2890–2898
year=2000
doi=10.1088/0022-3727/33/22/305] cite journal
journal=JOPD
url=http://www.iop.org/EJ/abstract/0022-3727/32/20/317
author=N.C.Bruce
coauthors=A.Garcia-Valenzuela, D.Kouznetsov
title=Rough-surface capacitor: approximations of the capacitance with elementary functions
volume=32
pages=2692–2702
year=1999
doi=10.1088/0022-3727/32/20/317] .The limit of resolution of SCM seems to be an order of magnitude better than that estimated for the atomic nanoscope;however, as other kinds of the probe microscopy, SCM requires careful preparation of the analyzed surface,which is supposed to be almost flat.

References


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