Under the microscope

Exploring a patent for enhancing atomic force microscopy.

Atomic force microscopy (AFM) is a type of scanning probe microscopy in which an Angstrom-scale probe tip, mounted on a cantilever beam, is physically scanned across a sample surface and interactions between the tip and the surface are used to form an image.  

In contact imaging mode, surface contours are determined by moving the probe tip and measuring either the cantilever beam’s deflection, or an electronic feedback signal that maintains the cantilever at a constant height above the surface.  

In tapping imaging mode, the cantilever oscillates at or near its resonance frequency. Changes in the amplitude of oscillation, due to interactions with the surface, are monitored for imaging. In this mode, the probe tip may be brought into direct contact with the sample surface, or it may only be brought close enough to interact via van der Waals or other long-range forces.

Atomic force microscopy can produce topographic images of insulating or conductive surfaces with atomic-level precision, without the need for lenses or irradiation beams (as compared to optical and electron-based microscopies), and with no limitations in spatial resolution due to diffraction and aberration. The technique can also be used in force spectroscopy to characterise van der Waals, Casimir, dissolution and dispersion forces. For these reasons, AFM has found diverse applications across Physics, Chemistry and Biology.  

Nevertheless, AFM does suffer from some drawbacks. 

One issue is that the potential for artifacts in the measurement data is greatly increased when working at very small length scales. The design and set-up of the apparatus must therefore be precisely controlled, and interactions between the device components and the environment should be accounted for. 

For instance, unintended probe bending or deflection can be caused by phenomena such as drift and creep, or through thermal interactions with the sample, deflection monitoring system or the environment.  

Bruker Nano Inc. was granted European patent EP3111239B1 in April 2025, which claims protection for addressing one of these issues. In particular, a method of compensating for a thermal deflection artifact of a bimorph probe in a scanning probe microscope operating in an oscillating (i.e. tapping) mode.  

Bimorph probes include a cantilever microfabricated from two different materials – typically, a base made of silicon, SiN₃ or SiO₂, and a metal layer that creates a reflective surface for an optical detection system. As these materials exhibit different thermal expansivities, changes in temperature can cause probe bending or deflection towards or away from a sample surface. 

Although this deflection is typically in the range of sub-nanometre to 100nm, this can result in a deflection artifact when resolving surface features on the sub-nanometre scale. Deflection can also be caused by differences in thermal conductivity between the probe and the sample, and between the probe and different parts of a non-homogeneous sample.

The solution to this in the patent involves generating relative oscillating motion between the probe and the sample in an intermittent contact mode. This provides relative scanning motion between the probe and the sample, and detects probe deflection as the probe and the sample interact.  

The probe-sample proximity is then controlled based on the detected motion, generating a ‘SPM scanner measured height’. This height is the distance between a fixed end of the scanner and a probe holder, where the holder is supported by the SPM scanner. Thereafter, a DC component of a cantilever shape change is determined during the scanning step, and the cantilever shape change is converted to a displacement. The displacement with the SPM scanner measured height is then added at each scan location to compensate for the deflection artifact, in turn, yielding the true sample topology.

The patent also describes how the cantilever shape change can be detected. When operated in a tapping mode, existing AFMs typically monitor the probe’s relative motion (e.g. oscillation amplitude) by detecting the corresponding relative motion in a laser beam reflected from the probe and impinging on a photodetector.  

The precise beam location on the photodetector (such as the centre position about which the beam oscillates) is not typically monitored. The inventors, however, have found that the centre position on the detector changes when the probe deflects due to thermal effects. The offset in this position can be taken as a measure of the probe shape change, and can also be converted to the probe’s spatial displacement.  

The patent further explains that measurements of the cantilever deflection in this way can be used to generate a thermal image of the sample, in addition to a topographic image.

Read the patent in full online.