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AFM Probes and Types

Atomic force microscope (AFM) is an important instrument for surface topography and electromagnetic performance analysis with atomic resolution. The first atomic force microscope was successfully developed in 1985, and its modes can be divided into contact mode and tap mode. Because the application scope of AFM probes is limited to atomic force microscopes, they are consumables for high-tech instruments, and their application areas are not wide.

The operation mode of AFM

With the development of AFM technology, various new applications continue to emerge. Specifically include the following technologies:

  • Contact mode

Contact mode is the earliest mode, the probe is in direct contact with the sample, the probe is easy to wear, so the probe is required to be soft, that is, the elastic coefficient of the cantilever is small, generally less than 1N/m.

  • Tapping mode, also called Dynamic Force or Intermittant-contact.

The probe resonates under the drive of an external force, and the part of the probe’s vibration position enters the repelling zone of the force curve, so the probe intermittently contacts the sample surface. The probe requires a high cantilever elastic coefficient to avoid the micro-layer water film on the sample surface. Tapping mode has a small force on the sample, which is particularly helpful for improving the resolution for soft samples. At the same time, the life of the probe is slightly longer than that of the contact mode.

The above is the most commonly used AFM mode, there are many other modes:

Lateral Force Microscopy (lateral force microscopy, which detects the lateral friction force of the micro area on the sample surface to the probe to obtain the mechanical properties of the material),

Noncontact mode Force (non-contact mode microscope, basically the same as tapping mode, the difference is that the non-contact mode probe works in the attraction zone of the force curve),

Force Modulation (Force modulation microscope, the probe has a lot of force on the micro area of ​​the test sample surface, and the mechanical properties such as the elastic coefficient of the material micro area can be obtained),

CFM chemical force microscopy

EFM electric force microscopy

KFM Kelvin force microscopy

MFM magnetic force microscopy

SThM Scanning thermal microscopy

SCM Scanning capacitance microscope

SCPM Scanning chemical potential microscope

SEcM Scanning electrochemical microscope

SICM Scanning ion conductance microscope

SKPM Scanning Kelvin probe microscope

SThM Scanning thermal microscope

STOS Scanning tunneling optical spectrometer

Various modes and applications require probes with different performance, and the performance index of the probe is the most critical factor that determines the resolution of the microscope.

Pros and cons of probes

AFM probes are basically prepared by processing Si or Si3N4 by MEMS technology. The radius of the probe tip is generally 10 to tens of nm. The cantilever is usually made of a silicon wafer or silicon nitride wafer that is generally 100~500μm long and about 500nm~5μm thick. A typical silicon microcantilever is about 100 μm long, 10 μm wide, and several microns thick.

The use of various interaction forces between the probe and the sample has developed a variety of microscopes for different application fields, such as AFM (van der mana), electrostatic force microscope (EFM), magnetic force microscope (MFM) side force microscope LFM (probe lateral deflection force), etc., so there are corresponding probes for different types of microscopes.

The probes of atomic force microscope mainly include the following types:

(1) Non-contact/tap mode tip and contact mode probe: the most commonly used products, with high resolution and average service life. The probe is constantly worn out during use, and the resolution is easily reduced. Main application and surface topography observation.

(2) Conductive probes: obtained by plating ordinary probes with 10-50nm thick Pt (and other metals that improve the bonding force of the plating, such as Cr, Ti, Pt, Ir, etc.). Conductive probes are used in EFM, KFM, SCM, etc. The resolution of conductive probes is worse than that of tapping and contact mode probes, the conductive coating is easy to fall off during use, and the conductivity is difficult to maintain for a long time. The new conductive tip products include carbon nanotube tip, diamond-coated tip, full diamond tip, and full metal wire tip. These new technologies overcome the short life and low resolution of ordinary conductive tip.

(3) Magnetic probes: Applied to MFM, it is prepared by plating Co, Fe and other ferromagnetic layers on probes in common tapping and contact mode. The resolution is worse than common probes, and the conductive coating is easy to fall off during use.

(4) Probes with large aspect ratio: the tip of large aspect ratio is specially designed and produced for measuring deep grooves and nearly vertical sides. Features: Less commonly used products, with high resolution and average service life. Technical parameters: tip height> 9μm; length to diameter ratio 5:1; tip radius <10 nm.

(5) Diamond-like carbon AFM probe/full diamond probe: One is to add a layer of diamond-like carbon film on the tip of the silicon probe, and the other is made of full diamond material (high price). These two kinds of diamond carbon probes have great durability, reduce the wear of the needle tip and increase the service life.

There are also biological probes (molecular functionalization), force modulation probes, indenter probes etc.





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