Short Info

Purpose and strength of the technique

  • Three-dimensional Imaging and measuring of surface topography
  • Determination of roughness, step height or pitch / slopes
  • Imaging of sample characteristics like magnetic field, capacitance and friction
  • Can be used under ambient condition, under liquids or in vacuum
  • Visualisation of lateral mechanical material contrasts, e.g. in polymer blends or block co-polymers.
  • Imaging of conducting and insulating samples without pretreatment

Technical Data

Vertical Resolution: ~0,1 nm
Lateral Resolution: ~0,1 nm
Information Depth: 1 Monolage
Max. Area and Height: ~100 x 100 µm² lateral, ~6 µm vertical

Applications

  • Assessing wafers (SiO2, GaAs, SiGe, etc.) before and after processing
  • Determining processing effects (e.g. plasma treatment) on biomedical devices such as contact lenses, catheter and coated stents.
  • Examining the impact of surface roughness on adhesion
  • Assessing trench shape/cleanliness on patterned Wafers
  • Determining whether morphology is the source of surface hazes

Details

The Atomic Force Microscopy (AFM) is a scanning probe microscopy method developed in 1985. The AFM is an important tool in surface physics and chemistry and is primarily used for mechanical scanning and Imaging of surfaces and the measurement of forces on the nanometer scale.

AFM Tip and Cantilever (SEM image)

Basic Principle

During a measurement, a tip on a tiny leaf spring (cantilever) is scanned over the sample surface. By measuring the interaction of the tip with the sample and correlation with the lateral position of the tip an image of certain sample properties with very high spatial resolution (typically up to 0.1 nm) can be measured. One distinguishes between various operating modes, which are described briefly below.


Contact-Mode

CD/DVD compression tool

In contact mode the AFM tip is always in repulsive contact with the sample surface. A distinction is made between "constant height mode" and "constant force mode":

The "constant height mode" is the historically older measurement mode. The tip is largely unregulated scanned over the surface and the variation in cantilever deflection is measured. This mode is therefore suitable only for very smooth sample surfaces, however, relatively high measurement speeds can be achieved. The entire information about the topography of the surface is included in the displacement signal of the cantilever.

The second mode is called "constant force mode". Here, the acting force, so the deflection of the cantilever, is kept constant during the measurement. This is achieved by using the displacement signal of the cantilever as the control signal for the height controlling piezoelectric element. In this mode also rougher sample surfaces can be examined. The signal controlling the z-piezo allows directly to derive the height information needed to measure the sample topography.


Non Contact Mode

DNA prepared on mica
The non-contact mode belongs to the family of dynamic excitation modes. In this mode, the cantilever is excited by an external periodic force to oscillate. This typically occurs at, or at least nearby, the resonance frequency of the cantilever.
 
The easiest way to do so is to use the principle of self-excitation: The detected oscillation signal of the cantilever is directly phase shifted by 90° and is fed back again to the exciting piezoelectric element. This creates a closed resonant circuit. Thus, the cantilever vibrates at its resonance frequency. When the tip is approaching to the sample surface, the interaction forces between tip and surface influence the oscillation behavior (frequency / amplitude of the oscillation) of the cantilever. The frequency shift is a measure of the force interaction and is used as a control signal during scanning of the surface. This mode can normally only be used under vacuum. By using the Q-Control developed by nanoAnalytics a true non contact mode can also be used under ambient conditions.

Related to the non-contact mode is the intermittent contact mode (also called tapping mode). In contrast to the real non-contact mode, the external excitation is performed at a frequency close to the resonance frequency of the cantilever ("working point"). If an interaction force between tip and sample changes the vibrational behaviour of the cantilever this can be detected very sensitive. typically the amplitude and phase (regarding the excitation signal) are changed and can be used as a control signal for the measurement.
This mode is typically used under ambient conditions or under liquid.

Further Options

Phase Contrast Microscopy

SIBS Block-Copolymer
left: surface topographie
right: Distribution of hard and soft segments.

As already described in the section about the "non contact mode", the interaction between the sample surface and tip leads to a phase shift between excitation signal and the cantilever oscillation. This phase shift is related to the local mechanical properties of the sample surface. This allows to distinguish between harder and softer areas on the sample.

This is for example very useful when investigating the distribution of hard and soft phases of block copolymers or polymer blend systems.

Magnetic Force Microscopy (MFM)

Hard Disk Surface
left: Surface Topographie,
right: Magnetic field at the same analysis position.

MFM is used to study the local magnetic field strength at the sample surface. This could be used for example in the development of computer hard drives.

The measurement is performed in non-contact mode, wherein the stylus used is additionally coated with a ferromagnetic material. The measurement itself is done in two runs for each image line: In the first pass, the height profile of the sample surface is recorded. The interaction between the tip and the magnetic field of the surface is recordet in the second run (This is also called "Lift Mode"). Therefore the tip follows the sample profile in an additional distance, so that the repulsive forces between sample and tip are reduced but the longer range magnetic forces can still influence the force interaction of the AFM tip. Therefore the information collected will no longer be from the mechanical interaction between tip and sample, but from the magnetic attraction forces between sample and tip, depending on the local field strength.

Lateral or Friction Force Microscopy (LFM or FFM)

The measurement is made in the constant-force mode. During scanning of the surface in addition the tilting signal of the cantilever is recorded. Depending on the friction between stylus and surface, the cantilever twists varying degrees. This allows to distinguish between areas of different friction. This allows conclusions about the material composition on the sample surface.