A traditional optical microscope can be used to achieve a maximum magnification of about 800–1000x due to the nature of light. Scanning electron microscopes (SEMs) can be used for further magnification, of which the transmission electron microscopes (TEMs) can reveal single atoms and thus offer the highest possible magnification. Taking this information into account, why is the scanning probe microscope (SPM) used as yet another type of microscope?
One reason is that a to-be-investigated sample in a transmission electron microscope must be sliced thinly and hence could be damaged. The SPM method involves imaging surface structures at atomic (height) resolution without ruining the sample. Another reason is the type of imaging provided by SPM microscopes since the outcomes are depicted as a type of 3D image (also in cases where only 2D information is assessed). Since this is the scenario with an optical microscope, it is highly challenging to analyze the surface structure of a sample using an electron microscope. The sample must be sliced to measure a surface profile with the highest resolution. In addition an SPM functions in the absence of vacuum and, in contrast to optical and electron microscopes, it has the ability to measure other physical effects. This includes electrical characteristics, such as Kelvin Probe Force Microscopy (KPM/KPFM) or magnetic characteristics (Magnetic Force Microscopy, MFM).
Types of Scanning Probe Microscopes
The most fundamental AFM operation mode is the purported DC, or contact, mode. In this mode, a force is applied to the cantilever (sensing) tip when the sample surface and the cantilever are close to each other. This results in the bending of the cantilever, which modifies the detection laser’s reflectance angle. A position-sensitive photo detector is used for measuring the deflection of the laser. Attractive forces are applied to the cantilever upon moving the cantilever toward the sample during the process. A surface scan can be carried out by using these negative forces.
At present, a majority of the AFM microscopes are typically operated in the purported AC mode rather than the DC mode to prevent damage to the cantilever. A very low force is used in this mode during the scanning process, and very little interaction takes place between the cantilever and the surface without resolution loss.
The cantilever is made to permanently vibrate with its resonance frequency. This oscillation results in a periodic bending of the cantilever, which can be measured using a reflected laser beam such as in DC mode. There is a change in the resonance frequency when the cantilever is close to the surface and interacts with the surface atoms. (In fact, there is an increase in the resonance frequency when the cantilever reaches the surface.) This results in the dampening of the amplitude and a change in the phase of the cantilever oscillation. The dampening of the cantilever oscillation and the tip-sample interaction force are roughly proportional when the tip is close to the sample.
When compared to the DC mode, the AC mode operates with less interaction force and has certain benefits:
- It is possible to image fragile samples in AC mode, which otherwise would be damaged in a DC measurement
- The surface itself is considerably less influenced by the measurement due to the low interaction forces
- The relation phase shift/damping is attributive to the surface material, hence more information can be gained from this operation mode
- Due to the lower forces, it is possible to normally use a single cantilever for more images
Magnetic Force Microscopy—MFM
A cantilever must be used to detect magnetic forces, which is applied with a magnetic coating. Standard MFM tips come with a magnetic coating that has a comparatively large thickness of about 40 nm. The high thickness leads to a considerably larger tip radius of about 50 nm, which is much larger compared to that of traditional AFM tips. These standard tips can be used to achieve lateral magnetic resolutions of around 100 nm. Hence, the magnetic resolution is considerably less compared to the commonly expected resolution from AFM topography measurements, which are better by a factor of 10.
When the MFM tip is close to the sample, mechanical force and also magnetic force contribute to the force measured by the cantilever. Although the magnetic force is considerably smaller compared to the mechanical force, it is effective over long distances. The two sources of force can be separated by measuring the MFM force from a specific distance of the sample surface, where the contribution from the mechanical force can be ignored.
Kelvin Probe Force Microscopy—KPFM
This mode is a technique for determining the chemical potential between a sample surface and a cantilever, and hence receive information related to the material and the material state on a sample surface. The figure below illustrates the basic framework for performing a KPFM measurement.
When the sample is electrically connected to the output of a frequency generator, an oscillating electrical field is produced between the sample and the cantilever, which can be measured by the AFM’s position sensor. Then, this detected signal is fed into a lock-in amplifier and its output is fed into an integrator. Subsequently, the outcome is added to the oscillation as a constant offset, which is then applied to the sample. This functions as a feedback loop, thereby reducing the electrically induced cantilever oscillation.
Nearly all SPMs have the ability to resolve atomic height steps:
- Normal AFMs are designed for comparatively large scan areas (50–200 µm), and the scanner stages permit analyses of various different sizes and types of samples.
- AFMs with lateral atomic resolution are only used for distinctive applications which necessitate accomplishing a lateral atomic resolution than the loss of flexibility, enabled by the higher stability of the total framework.
- AFMs have the ability to achieve lateral atomic resolution.
- In general, STMs are incorporated into a highly stable scan platform, and the maximum scan area is about 0.5–10 µm.
- Usually, STMs are used in ultra-high vacuum to ensure that a prepared sample surface stays unaffected for a longer period of time.
- STM offers the upper hand that the tunnel current always flows directly over the next atom on the sample surface. This type of “self-focusing” cannot be achieved using an AFM.
This information has been sourced, reviewed and adapted from materials provided by Semilab Semiconductor Physics Laboratory.
For more information on this source, please visit Semilab Semiconductor Physics Laboratory.