Browse Prior Art Database

Fast Image Acquisition with Scanning Tunneling Microscope or Atomic Force Microscope

IP.com Disclosure Number: IPCOM000104072D
Original Publication Date: 1993-Mar-01
Included in the Prior Art Database: 2005-Mar-18
Document File: 2 page(s) / 104K

Publishing Venue

IBM

Related People

Martin, Y: AUTHOR [+2]

Abstract

Scanning tunneling microscope (STM) or atomic force microscope (AFM) imaging often takes several seconds or minutes. For qualitative inspection, or for coarse imaging, higher imaging speeds are very desirable. Several solutions are already being implemented:

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Fast Image Acquisition with Scanning Tunneling Microscope or Atomic Force Microscope

      Scanning tunneling microscope (STM) or atomic force microscope
(AFM) imaging often takes several seconds or minutes.  For
qualitative inspection, or for coarse imaging, higher imaging speeds
are very desirable.  Several solutions are already being implemented:

1.  Variable current, or variable force modes [1]: when imaging
    shallow topography (less than 10 angstrom for STM and less than
    100 angstrom for AFM), one can position the tip far enough from
    the surface and scan faster than the response of the feed-back
    loop, without the tip touching the surface.

2.  Contact mode AFM [2]: in this mode, tip and sample are in contact
    when the tip scans over the surface of the sample.  The stiffness
    of the cantilever is low so that the  force  due to the
    cantilever spring stays small.  In addition, the tip is V-shaped
    with a tip cone angle around 90 degree, so that it can be pushed
    up on upward surface slopes.  Imaging speed is high since it is
    not limited by the feed-back mechanism.

3.  Non-contact AFM  [3]: in this mode, the tip is vibrated and
    senses some effect of the much smaller van der Waal forces.  For
    this important type of AFM, the tip has a high aspect ratio and,
    therefore, is fragile and must not touch the sample surface.
    Scanning speed is mostly limited by the speed of the feed-back
    system which adjust the tip-sample spacing.  One published system
    [4]  gives a solution to increase the limited speed of response
    of the vibrating tip, when the force gradient is sensed through a
    frequency shift.  Instead of measuring a change in vibration
    amplitude, this system directly measures a frequency shift, which
    is instantaneous.

This disclosure provides several solutions to increase the  speed of
an AFM (or  STM) when it is limited by the feed-back system (case 3
above).  They address the two major causes which limit speed of the
feed-back mechanism:

o   the response of the tip for sensing forces via tip vibration
o   the control system which moves the tip in a servo feed-back mode.

      The normal response time of the tip, given by  Q/ f sub o
(quality factor over resonance frequency), is typically around 1 ms.
A method to increase this response time consists in operating in a
discontinuous mode, with the excitation set high, and the set-point
(desired amplitude of vibration) set low.  The feed-back servo then
operates in a discontinuous hysteretic mode, following the curve
tip-vibration amplitude versus tip position, which is discontinuous
and hysteretic.  When the tip approaches the surface within 30
angstrom, capillary forces damp the tip vibrating resonance at a
faster rate than normal response time.  The feed-back then pulls the
tip away until the capillary forces disappear.  At this...