Introduction

From its inception (Binnig et al, 1986), Atomic Force Microscopes (AFM) have been widely used for interrogating the material properties at nano-scale. Especially the emergence of the special mode, Lateral Force Microscopy (LFM) or Friction Force Microscopy (FFM), makes it possible to study the nano-friction behavior in far more details than any attempts before.

However, for as long as AFM has been used to study the friction, there have been difficulties in measuring the absolute lateral force components involved in the friction process. Although tons of papers are appearing everyday concerning this AFM calibration business, the situation has not been entirely satisfactory yet. Many results of LFM experiments are still published in term of Arbitrary Unit or Volts in stead of Newtons ! 

As we tried to implement the calibration methods proposed by prior researchers, there are three major problems we found:

• Inaccurate  (many error sources exist in the calibration, sometimes more than an order of magnitude off)
• Indirect  (as discussed in Background many intrinsic quantities are involved and complicate interpretations; not in situ)
• Complicate setup and procedures  (usually effort-intensive micro fabrications and data processing; applies only to a probe with limit radius of curvature; etc)

To find a robust way to accurately calibrate the absolute lateral forces exerted on an AFM probe, we developed a novel calibration method using a Diamagnetic Lateral-Force-Calibrator (D-LFC), which has the following distinct features:

• Direct  (as discussed in Background, the force constants are treated as input-output parameters of the AFM system and calibrated directly without knowing the detail information of the cantilever and optical sensitivity factor; ideal for in situ calibration without disturbing the AFM system)
• Accurate  (since it's direct, this method minimizes the error sources; with a compliant standard spring provided by D-LFC, it has a much higher accuracy than any other existing techniques; no further assumptions are made for the AFM cantilever system, it can handle the "cross-talk" effect by measuring both force constants)
• Easy and low cost  (D-LFC setup is relatively easy, costing only a few bucks, see in DIY Steps, and the linear-fitting is the only data processing needed, see Lateral Calibration)

A detailed technical paper could be found on the Journal Review of Scientific Instruments (vol. 77, issue 6, 065105, 2006) or you can download the electric copy here :)

Background

DIY Steps

Lateral Calibration