A scanning probe microscope (SPM), also known as an atomic force microscope (AFM), produces high resolution thee-dimensional images by scanning a sharp physical probe over a sample surface while keeping the tip in close proximity to the sample.  The tip is typically integrated into a cantilever beam which allows for both the physical support of the force sensing tip and displacement sensing via an optical beam bounce system.  The cantilever/tip assembly is in turn mounted on  a three dimensional scanner.  The scanner is piezo actuator that is  physically configured as a tube, which is both rigid and allows for motion in all three directions.  A laser is reflected off the backside of the scanner and the reflected light is placed on a position sensitive photodetector.   Scanning action is produced by applying a voltage to the appropriate part of the tube to produce motion in X, Y, and Z.  The tip-sample proximity is controlled by a feedback loop.  The feedback mechanism, in part, determines the operating mode of the instrument.

AIF has two AFMs that utilize the same controller (only one is available at a time).  Both the Bruker Dimension 3000 and Multimode are conventional optical beam bounce scanning probe microscopes (SPM).  Both instruments utilize commercially available cantilever/tip assemblies.    Both systems utilize a four segment photodetector for sensing cantilever displacement.  A four segment photodetector allows for sensing of both vertical displacement and torsion.   Both instruments are equipped with an extender electronics box, which allows for phase (time lag) imaging.  The D3000 is a scanning tip instrument.  That is, the tip is attached to the scanner and the sample is fixed to a motorized stage.  The sample stage can accomodate up to 150mm diameter samples that are up to 12mm thick.   The Multimode is a scanning sample instrument, i.e., the tip is fixed and the sample is scanned.  The Multimode requires small samples (10x10x5 mm) that are glued or taped to a sample puck that is attached to the scanner.

The most commonly used operating mode is intermittent contact mode, aka tapping.  When operated in standard tapping or contact mode, the SPM is more commonly known as an atomic force microscope (AFM).  Phase imaging can show changes in elasticity or hardness, but may only show a derivative of the topography.  Since the AFM senses heights directly it is trivial to measure step heights and surface roughness.  It is also possible to measure long range forces like magnetic and electrical forces.  Magnetic and electrical forces require special tips that are sensitive to the forces that are to be measured.

Equipment Specifications

Detector
  • Position sensitive photodetector
Capabilities
  • Intermittent Contact (tapping) AFM
  • Phase imaging in intermittent contact mode
  • Contact AFM
  • Magnetic Force Microscopy
  • Electric Force Microscopy, requires either static charge or means of electrical connection for active devices
Cantilever Beam/Tip
  • Standard, commercially available tips are utilized
  • MFM requires magnetic tips
XY Resolution
  • Max image resolution is 512×512
  • Actual resolution is tip dependent
Scan Size, D3000
  • Z:  Max Z-range is 4.8 micrometers
  • XY:  Up to 100 micrometers square
  • Smallest size (XY) is 0, which is used for diagnostics
  • Smallest practical size is dependent upon the tip sharpness and sample feature sizes
Z Resolution, D3000
  • Z digital resolution is scalable in the sub-Angstrom scale
  • Typical resolution settings:
    • Full range: 0.074nm
    • 1um range: 0.015nm
Specimen Stage, D3000
  • x = 150 mm
  • y = 80 mm
  • z = 13 mm
  • Rotate 360º
Scan Size, Multimode
  • Several scanners available
  • J scanner:  XY = 97um, Z = 3.4um
  • E scanner:  XY = 9.3um, Z = 3.5um
  • Smallest size (XY) is 0, which is used for diagnostics
  • Smallest practical size is dependent upon the tip sharpness and sample feature sizes
Z Resolution, Multimode
  • Z digital resolution is scalable in the sub-Angstrom scale
  • Typical resolution settings:
    • Full range: 0.052nm
    • 1um range: 0.015nm
Specimen Stage, Multimode
  • x = 15 mm
  • y = 15 mm
  • z = 5 mm
Specimen Considerations
  • Accepts most samples: Conductors, Semiconductors, and Insulators can be observed
    • Including, but not limited to metals, semiconductor devices, ceramics, biological, polymer, textile, pharmaceuticals, even food, etc.
  • Specimens can be up to 150mm in diameter and up to 12mm thick
    • Sample stage must be rotated in order to observe all the way across

Instrument Photograph and Examples

D3000

Bruker D3000 Scanning Probe Microscope

 

GaN

Gallium nitride observed with the AFM in intermittent-contact mode (tapping). The steps observed are single molecular layers. The large holes are screw dislocations and the small holes are edge dislocations. Sample courtesy Nadia El-Masry.

GaN-3d

Three-dimensional rendering of the GaN image observed above. Since the AFM provided direct height information, 3-d renderings are often used to display the data. An added bonus of direct height information is that roughness values are directly attainable. Sample courtesy Nadia El-Masry.

GaN2

Large scale scan of GaN sample described above. In this case, long range means that the field of view is 5 micrometers square. Sample courtesy Nadia El-Masry.

 

HDPE1

Height (left) and phase (right) images of HDPE. This sample was cut directly from a gallon milk container. The phase image shows changes in material elasticity. Note that the phase in phase image does not mean material phase but rather shows the time lag in the response of the force sensing tip. Higher elasticity materials have a shorter time lag and therefore appear brighter in the phase image. The phase image will also have a component of topography that is essentially a derivative of the topography image.

hdpe2

Height (left) and phase (right) images of an HDPE sample taken from a gallon milk jug. In this case the field of view is 2 micrometers.

 

pearlite

AFM image of etched pearlite. Sample courtesy Lew Reynolds.

pearlite_3d

Three dimensional representation of the etched pearlite image seen above. Sample courtesy Lew Reynolds.