Moiré atomic force sensors for robust nanoscale sensing

To improve the resolution of force measurement using an atom force microscope (AFM), a new method based on using nanostructures as a material scale, which inspired from the optical moiré effect and so-called moiré atomic force sensors, would in this presentation proposed. This approach utilizes a fabricated Moiré pattern on the AFM cantilever tip, which scans a grating sample of same pitch. The system incorporates a second cantilever of the same type, arranged at a 90° phase shift relative to the first, for signal up-counting.

The fundamental principle is as follows: the detected deflection of cantilever, caused by interaction between cantilever tip and sample, is at minimum when the ridges of one grating align with the grooves of the other. Conversely, the signal reaches its maximum when the ridges of both gratings are in direct alignment. During scanning process, the signal exhibits a sinusoidal variation with a specific period. To enhance signal contrast, a large effective contact area on the tip is required. We address this by using spherical-tipped cantilevers with diameters up to 85 µm, which also mitigates the tilting issues often associated with plateau tips.

The advantages of this method are: firstly, the measurement resolution for side wall height could be improved significantly with 16-bit AD conversion. Secondly, tip-wear as a major limitation of traditional AFM would not be a problem for long-time scanning in this case.

At Institute for Process Measurement and Sensor Technology at the Technische Universität Ilmenau an atomic force microscope from company NanoAnalytik GmbH is available, which equipped with a precision bottom scanner within range of 50 μm 50 μm 20 μm along the xyz-axes, respectively and self-actuating and self-sensing active microcantilever probs. Since there are no commercially manufactured active cantilever probes with spherical tips, we have custom-fabricated them by mounting microspheres onto the cantilevers. The chosen SiO? spheres, sourced from micro particles GmbH, have diameters of 20 µm, 50 µm, and 85 µm and were attached using UV-curing adhesive NOA61 from Thorlabs.

Force-displacement measurements were conducted using these custom cantilevers to confirm the expected increase in interaction force with larger sphere diameters. The results show that compared to a commercial sharp tip with 80 nm diameter, the pull-off force by force spectrometer increased by about 40 times with 20 µm-sphere-tip, by about 3.5 times with 50 µm-sphere-tip and by around 80 times with 85 µm-sphere-tip. The suboptimal performance of the 50 µm tip was attributed to surface contamination with small particles, detected via scanning electron microscopy (SEM). Nevertheless, the results demonstrate the potential of larger spheres for maximizing interaction forces.

As the next step, a 300 nm pitch grating was fabricated on these spherical tips of cantilevers by focused ion beam (FIB) with operation parameters of 240 pA current and 5 kV voltage for 1690 s.  The key part, scanning with 300 nm-pitch Moiré cantilever of a prepared 300 nm-pitch sample should be operated and detected signal would be analyzed more in detail.

Nan Deng is a Ph.D. candidate at Technische Universität Ilmenau in Germany. Her research focused on developing novel nanoscale sensors for high-precision metrology. She is currently working on the research topic of Moiré atomic force sensors for robust nanoscale sensing, which has applied the optical Moiré effect on AFM scanning with high resolution. Her work is expected to make significant contribution to the field of nanometrology.