Atomic Force Microscopy

Park Systems XE 100 Atomic Force Microscope

The Park Systems XE 100 Atomic Force Microscope (AFM) is a type of a Scanning Probe Microscope which is a powerful tool that allows imaging at a near atomic resolution for the analysis of surface topography in 2D as well as in 3D. This instrument could be used to study the surface roughness, feature sizes such as step height and the other dimensions involved. Apart from these measurements, the AFM can be used to obtain information regarding physical properties of a given sample such as adhesion, modulus, dopant distribution, surface potential, etc.

Principle

The principle used in the imaging using an atomic force microscope involves the use of a micro machined cantilever with a sharp tip. This tip is subjected to attractive or repulsive forces/interactions depending on the distance between the atoms and the tip. These forces/interactions are recorded by measuring the degree of deflection of the cantilever. This detection is facilitated by a laser beam that is reflected off the back of the cantilever onto a photo sensitive photo detector. During the scanning of a sample, a tube-shaped scanner located under the sample moves the sample in the horizontal direction (X-Y) and in the vertical direction (Z). This motion causes the sample to be scanned line by line, while the photo sensitive photo detector signal is used to control the vertical movement of the scanner via a feedback loop as the cantilever moves across the sample. The AFM is capable of obtaining measurements from conductors, non-conductors as well as some liquids without delicate sample preparation. The Park XE100 can operate in the following modes; contact mode, non-contact mode, lateral force mode, magnetic force microscopy and scanning tunnelling microscopy.

There are 3 modes of operation possible with the instrument, those being contact mode, non-contact mode and tapping mode. The contact mode causes the tip to be "dragged" across the surface of the sample and the contours of the surface are measured using the feedback signal required to keep the cantilever at a constant position. In non-contact mode, the tip of the cantilever does not contact the sample surface as the name suggests. Instead, it oscillated at either its resonant frequency (frequency modulation) or just above (amplitude modulation) where the amplitude of oscillation is typically a few nanometers (<10 nm) down to a few picometers. The van der Waals forces, which are strongest from 1 nm to 10 nm above the surface, or any other long-range force that extends above the surface acts to decrease the resonance frequency of the cantilever. This decrease in resonant frequency combined with the feedback loop system maintains a constant oscillation amplitude or frequency by adjusting the average tip-to-sample distance. Measuring the tip-to-sample distance at each (x,y) data point allows the scanning software to construct a topographic image of the sample surface. The tapping mode was developed to bypass the problem of keeping the probe tip close enough to the sample for short-range forces to become detectable while preventing the tip from sticking to the surface in ambient conditions. The images in tapping mode are produced by imaging the force of the intermittent contacts of the tip with the sample surface.

Strengths

• Available modes:
     o Contact mode
     o True non-contact mode
• 3-D Mapping
• Surface modulus value measurements
• Scanning of sample at high temperatures by heating
• Identification of differences in material by using phase differences in non-contact mode
• Resolution up to 0.5 nm

Limitations

• Slower measurement speed compared to optical microscopes and electron microscopes as the tip of the instrument has to mechanically follow a sample surface
• Measurement errors may be present due to the non-linearity and hysteresis of piezoelectric materials, as piezoelectric ceramic tubes are used as scanners,
• Independent movement in the x, y and z directions is impossible due to the restraints imposed by the tube type scanner resulting in the cross coupling of the individual scan axes
• The measurement of narrow, deep indentations/ steep slopes may be impossible at times as the tip is of finite size (Even if such a measurement is possible, the shape of the tip and the sample profile may result in measurement errors)
• If the sample is protruding on the surface, it may be complicated to scan (e.g. hairy fabrics)

Applications

• AFM can be used to produce topological images with/without making contact with the sample, to identify phase differences in a sample, to study the frictional characteristics of a surface, to study the magnetic properties on a surface, for the conductive mapping of a surface and to scan the electron density across a sample
• This instrument is used in a wide range of fields including solid-state physics, semiconductor science and technology, molecular engineering, polymer chemistry and physics, surface chemistry, molecular biology, cell biology and medicine

Technical Specifications

• Scan range of XY-scanner: 5 μm, 50 μm, and 100 μm
• XY resolution < 0.02 nm (open loop. 50 μm XY scanner) 0.06 nm (closed-loop, 50 μm XY scanner) Noise level reduction of Z 0.02 nm (12 μm Z scanner)
• Working range of XY: 25 mm x 25 mm x 2 μm resolution
• Working range of Z: 27.5 mm Anti vibration chamber for quality imaging