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Detection of Static Charges with a Sensitivity of 0.004 Electron

IP.com Disclosure Number: IPCOM000107370D
Original Publication Date: 1992-Feb-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 3 page(s) / 121K

Publishing Venue

IBM

Related People

Martin, Y: AUTHOR [+2]

Abstract

Disclosed is a version of the Atomic Force Microscope (AFM) (1-3) aimed at the detection of static charges. Although the charges are often screened by a nearby conductor, the sensitivity of the technique is high enough to allow the detection of a single electron, or even smaller charges, such as a dipole. Many applications can be derived for the characterization of semiconductors.

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Detection of Static Charges with a Sensitivity of 0.004 Electron

       Disclosed is a version of the Atomic Force Microscope
(AFM) (1-3) aimed at the detection of static charges. Although the
charges are often screened by a nearby conductor, the sensitivity of
the technique is high enough to allow the detection of a single
electron, or even smaller charges, such as a dipole.  Many
applications can be derived for the characterization of
semiconductors.

      The figure shows the experimental configuration for detecting
static charges with the AFM.  Two resonances of the tip are used. The
essential element is the AC generator which biases the tip at the
resonance frequency l1 .  In the absence of static charge, the
electrostatic force by the sample on the tip is the same and has the
same direction during the positive alternance than during the
negative one. The net result is a force at a frequency 2 l1 and no
force at l1 .  In the presence of a static charge, the force
direction is different during each alternance and depends on the sign
of the static charge.  A force at l1 is generated, which can be
measured in the tip vibration; depending on the sign of the charge,
the vibration will be in phase or out of phase with the AC generator.
The adjustable DC supply in series with the AC generator serves
essentially as an offset to compensate for non-symmetry in the
sinusoidal waveform of the AC generator and any contact potential
difference between tip and sample, which would otherwise produce a
force component at l1 .

      The electric force measured by the tip compares with Fo = qE =
qV/d, which is the force by the field on the charge. Electrical
screening by the sample conductor makes the force on the tip smaller
than qV/d.  However, two factors work in our favor:
      The field from the tip is divergent. Its action will be
stronger on the charge than on the charge images generated by the
sample, provided that the static charge is not too close to the
conductor compared to the lateral size of the tip.
      When the tip is close to the charge, it will provide some
screening action, too, that increases the force on the tip.

      For practical purposes, we shall consider a round tip of 200
angstrom radius, separated by 50 angstrom from the charge which is at
50 angstrom from the conductor. A calculation, which takes account of
the multiple images of the charge by the planar sample and the
spherical tip, yields the value for the force on the tip F = 0.32, Fo
= 0.32 qV/d.  This force produces a displacement z of the tip given
by the relation  F = kz/Q,  where k is the stiffness of the
cantilever-tip and Q is the quality factor of th...