Introduction
The Atomic Force Microscope (AFM) is a scanning probe imaging and sensing device, useful for
physical and chemical studies. In its basic configuration, it measures the microscopic surface profile
of a near-planar target by mechanically scanning a tiny probe across it in a raster pattern. The
probe rises and falls in accordance with the surface profile. As it does so, its position is sensed and
captured digitally. That captured topogram can then be rendered as a photograph-like image,
reminiscent of an optical micrograph.
Although the AFM is but one of a family of scanning probe “microscopes” (see Table 1) the
AFM is far more versatile and capable than the others, viz:
The three dimensional profile of a surface is measured with the AFM by monitoring the position
of the probe in three dimensions as it scans. A feedback control system maintains the force
between the probe and the surface sensibly constant during the scan. The probe tip thus follows
the surface profile, as one might do with a finger tip at a more macroscopic scales.
Soon after the invention of the AFM it was realized that these instruments were capable of measuring
far more than surface topography. It is possible to measure any physically phenomenon at
the nanometer scale for which a suitable sensor can be incorporated in the probe. Demonstrated
examples include magnetic fields, electric fields, contact potential difference, temperature, and
hardness. It is also possible to use the AFM probe as a tool to modify surfaces.
Such non-topographic uses have come to be called AFM “modes”. It is the purpose of this monograph
to elaborate upon these modes, both in principle and in practice. .
AFM “Modes”
This is a white paper on AFM modes. There’s a problem with the notion of “modes”, as this is
really a misnomer. The “mode” terminology is, however, deeply entrenched amongst workers in
this exciting field as a sort of shorthand name for one of the myriad AFM techniques or applications.
Therefore, with an English teacher’s caveat regarding possible bad usage, we present the list
in Table Table 2: as a practical user’s map.
AFM Basic implementation
Figure 1-1 shows the elements of a basic atomic force microscope. The probe tip is positioned in
three dimensions by three mutually orthogonal actuators. Each actuator is a
piezoelectric ceramic
transducer
(PZT). PZTs are well suited to the small motions required by the AFM. Their travel is
approximately linear in the applied voltage when used in the AFMs.
We adopt the convention that the X and Y coordinate axes are nominally parallel to the
surface
under test
(SUT), and the Z axis completes a right handed Cartesian coordinate system. The Z
axis positioner thus moves the probe toward (negative Z) or away (positive Z) from the SUT,
while the X and Y positioner move the probe in a nominal object image plane. In normal operation,
the X and Y position is programmed in a raster scanning path, while the Z position is tightly
controlled relative to the SUT by a feedback control loop. The Z-axis error signal is derived from
a force transducer that measures the force on the probe tip. The transducer output is differenced
against a fixed voltage, which corresponds to a setpoint (target) force. The error signal is amplified
and drives the Z-axis PZT. This control loop acts to reduce the error signal, and hence the probeto-
SUT distance to near-zero. The feedback loop thus causes the probe to closely follow the
undulations of the SUT while the X-Y PZTs scan the probe over a rectangular image area. Meanwhile,
the control voltage applied to the Z-PZT provides a convenient representation of the elevation
of the probe and surface is used to generate and image or topogram of the surface.
 Light lever force sensing
High sensitivity in the force transducer of an AFM can be achieved by a simple geometric optical
device known as the light lever (see Figure 1-2). A low-power laser beam is reflected from the top
of the cantilever to a distant photo detector. Small displacements of the cantilever result in large
displacements of the laser beam at the location of the detector due simply to the large “lever arm”
of the light path. The detector is bifurcated, and the halves are differenced, giving a force-proportional
error signal.
 Probe-surface interactions
While it might be thought that the force between an AFM probe and a hard surface might have
an abrupt brick wall character, this is an over-simplification at the nanoscopic scale. Not only are
fundamental interatomic forces finite in extent, there is also the problem of surface contamination.
AFMs are usually operated in ambient environmental conditions (room temperature, atmospheric
pressure, ambient air). As a result, there is invariably a surface layer comprised of water
and miscellaneous hydrocarbons. This layer is thick enough relative to the nominal probe operating
height that the probe tip is almost always immersed in it (Figure 1-3).
The forces between probe and surface, to the extent that they are position-dependent only, i.e. are
lossless, can be represented by an effective potential energy (see, for example, Figure 1-4)
As shown in Figure 2-9, the interaction between probe and surface falls into one of three zones:
There are two primary methods for measuring the force between a probe and a sample: contact
mode and vibrating mode.
Contact mode entails a direct quasi-static force versus distance measurement, that is, a rather intimate
contact between probe and surface. While producing accurate results, it tends to rapidly
damage both probe and surface.
Vibrating mode may be either large amplitude, in which the probe contacts the surface on every
cycle, or small amplitude non-contact. The latter is generally less stressful on both the probe and
surface, and tends to give superior results. It measures more subtle properties of the surface.
|
