Metrology & Instrumentation : AFM /SPM

The NanoDrive Controller

The NanoDrive controller platform is built on modern state-of-the-art technology and while entirely new from the ground up, also incorporates 20 years of experience only available from the SPM market leader. 

The NanoDrive controller includes two fully independent lock-in amplifiers enabling advanced imaging modes such as scanning capacitance microscopy and surface potential microscopy with maximum productivity and flexibility while completely eliminating the need for external hardware. Thorough optimization of the controller architecture ensures maximum performance and stability while minimizing computer requirements. Eight fast ADCs working in parallel offer generous data acquisition bandwidth with up to 8 images and 1024 x 1024 pixels each. The NanoDrive controller offers a 3-axis stage control, used in Innova for pitch and tilt capabilities. The built-in high-voltage supply enables unipolar tube driving, thus eliminating the danger of piezo depoling. It provides the wide dynamic range and low noise floor demanded by high performance AFMs.

The software available with the NanoDrive controller platform includes Veeco’s intelligent tip approach and our patented lift mode for efficient basic and advanced imaging. Powerful research flexibility beyond established SPM modes comes standard with advanced signal diagnostics and generic signal sweep functions for custom experiments. Software configurable inputs and outputs built into the controller provide extensive signal access and are further augmented by software controlled, user accessible signal routing. Three digital feedback loops are entirely software configurable by the user and form the foundation for the ultra-low noise position control.

Contact Mode
In contact AFM, the tip is in perpetual contact with the sample. The tip is attached to the end of a cantilever with a low spring constant, lower than the effective spring constant holding the atoms of most solid samples together. As the scanner gently traces the tip across the sample (or the sample under the tip) union the contact force causes the cantilever to bend and the Z-feedback loop works to maintain a constant cantilever deflection.

Force Modulation Microscopy (FMM)

Force Modulation imaging is a secondary imaging mode derived from contact AFM that measures relative elasticity/stiffness of surface features, and is commonly used to map the distribution of materials of composite systems. As with LFM and MFM, Force Modulation imaging allows simultaneous acquisition of both topographic and material-property maps.

In Force Modulation imaging mode, the probe tip tracks the sample topography as in normal contact AFM.  In addition, a periodic signal mechanically drives the cantilever (and tip) in the Z-direction.  The amplitude of cantilever modulation that results from this applied signal varies according to the elastic properties of the sample

 

The resulting force modulation image is a map of the sample’s elastic response. The frequency of the applied signal is typically a few kilohertz, which is faster than the z feedback loop is set up to track. Thus, topographic information can be separated from local variations in the sample’s elastic properties, and the two types of images can be collected simultaneously, as shown below.



Contact-AFM (left) and Force Modulation (right) images of a carbon fiber/polymer composite collected simultaneously.
5µm scan.

Force-Distance Measurements
Force-Distance Measurements are performed to study attractive and repulsive forces on a tip as it approaches and retracts from the sample surface. Commonly applied to investigating fundamental force interactions, nano-scale adhesive and elastic response, binding forces, colloidal studies, and chemical sensing.

Lateral Force Microscopy (LFM)

Lateral Force Microscopy (LFM) is a secondary contact AFM mode that detects and maps relative differences in the frictional forces between the probe tip and the sample surface. In LFM, the scanning is always perpendicular to the long axis of the cantilever. Forces on the cantilever that are parallel to the plane of the sample surface cause twisting of the cantilever around its long axis. This twisting is measured by a quad-cell Position Sensitive PhotoDetector (PSPD) union as with TRmode.

AFM tip lateral movement in LFM.

Twisting of the cantilever usually arises from two sources: changes in surface friction and changes in topography. In the first case, the tip may experience greater friction as it traverses some areas, causing the cantilever to twist more. In the second case, the cantilever may twist when it encounters edges of topographical features. To separate one effect from the other, usually three signals are collected simultaneously: the trace and retrace LFM signals, and the AFM height (topography) signal.

LFM applications include identifying transitions between different components in polymer blends and composites, identifying contaminants on surfaces, delineating coverage by coatings, and chemical force microscopy (CFM) using probe tips functionalized for specific chemical or biological species.

Magnetic Force Microscopy (MFM)
Magnetic Force Microscopy (MFM) is a secondary imaging mode derived from TappingMode mode that maps magnetic force gradient above the sample surface. This is performed through a patented two-pass technique, LiftMode. LiftMode separately measures topography and another selected property (magnetic force, electric force, etc.) using the topographical information to track the probe tip at a constant height (Lift Height) above the sample surface during the second pass.




Lift Mode AFM.
The MFM probe tip is coated with a ferromagnetic thin film. While scanning, it is the magnetic field’s dependence on tip-sample separation that induces changes in the cantilever’s resonance frequency or phase. MFM can be used to image both naturally occurring and deliberately written domain structures in magnetic materials. An image of a hard disk acquired in MFM mode is shown.

Nanoindenting/Scratching
Nanoindenting measures mechanical properties by localized indentions, using a diamond tip to investigate hardness. AFM can also perform nano-scratching and wear testing to investigate film adhesion and durability.

PhaseImaging™
Phase imaging is a secondary imaging mode derived from TappingMode that goes beyond topographical data to detect variations in composition, adhesion, friction, viscoelasticity, and other properties, including electric and magnetic. Applications include contaminant identification, mapping of components in composite materials, differentiating regions of high and low surface adhesion or hardness and regions of different electrical or magnetic properties.
Phase imaging is the mapping of the phase lag between the periodic signal that drives the cantilever and the oscillations of the cantilever. Changes in the phase lag often indicate changes in the properties of the sample surface.



The phase lag varies in response to the properties of the sample surface.

The system's feedback loop operates in the usual manner, using changes in the cantilever's oscillation amplitude to map sample topography. The phase lag is monitored while the topographic image is being taken so that images of topography and material properties can be collected simultaneously.




This figure shows simultaneously acquired topography (left) and phase (right) AFM images of silicone hydrogel in saline solution. The four outer areas were exposed to a sequence of chemical processing steps. The central cross-like region was masked and so protected from the processing steps and hence retained its hydrophobicity.

In the phase image (right) union a marked phase shift is clearly seen across the boundaries. However, the hydrophilic and hydrophobic regions show no topographic contrast (left). The phase image is clearly providing material property contrast on this well-defined experimental hydrogel surface.

The phase signal is sensitive to both short- and long-range tip-sample interactions. Short-range interactions include adhesive forces and visco-elastic forces; long-range include electric fields and magnetic fields.

Phase imaging is a key element in the use of a number of scanning probe techniques, including Magnetic Force Microscopy (MFM) union Electric Force Microscopy (EFM) and Scanning Capacitance Microscopy (SCM). This method of detection provides a more sensitive measurement than other detection methods, such as amplitude detection.

TappingMode™
TappingMode AFM, the most commonly used of all AFM modes, is a patented technique (Veeco Instruments) that maps topography by lightly tapping the surface with an oscillating probe tip. The cantilever’s oscillation amplitude changes with sample surface topography, and the topography image is obtained by monitoring these changes and closing the z feedback loop to minimize them.

TappingMode has become an important AFM technique, as it overcomes some of the limitations of both contact and non-contact AFM. By eliminating lateral forces that can damage soft samples and reduce image resolution, TappingMode allows routine imaging of samples once considered impossible to image with AFM, especially in contact mode.

Another major advantage of TappingMode is related to limitations that can arise due to the thin layer of liquid that forms on most sample surfaces in an ambient imaging environment, i.e., in air or some other gas. The amplitude of the cantilever oscillation in TappingMode is typically on the order of a few 10’s of nanometers, which ensures that the tip does not get stuck in this liquid layer. The amplitude used in non-contact AFM is much smaller, as different forces are being measured. As a result, the non-contact tip often gets stuck in the liquid layer unless the scan is performed at a very slow speed.

In general, TappingMode is much more effective than non-contact AFM for imaging larger scan sizes that may include large variations in sample topography. TappingMode can be performed in gases, liquids, and some vacuum environments.