Capteurs MEMS pour l’AFM

With their ability to access morphology down to the atomic scale, scanning probe microscopes have been at the origin of, and constantly support the development of nanosciences and nanotechnologies. Moreover, more than 20 years ago it was already stated that recording movies with an Atomic Force Microscope (AFM) on biological systems would be of major interest to understand their behavior and relationship with their biological functions. However, the AFM force resolution is relatively low in liquid media, and hinders imaging of the soft matter samples at high scan rate. Actually, the conventional AFM sensor itself hinders such applications. Even if High-Speed AFM of biological nano-systems has been demonstrated by T. Ando et al. following this approach, performance enhancement remains limited as well as its dissemination among the scientific community.

Figure 1: (a) SEM image of a MEMS AFM sensor based on a silicon ring resonator. Vibration is driven and sensed by integrated capacitive transducers featuring sub-100 nm airgaps. A silicon nanotip (apex radius below 10 nm) is located at a maximum of the elliptic vibration mode. (b) and (c): AFM topographic images of DNA origamis acquired by a 10.9 MHz MEMS AFM sensor. AFM tip vibration amplitude is 0.2 nm.

Our approach at IEMN is to change the AFM sensor concept by introducing ring-shaped silicon MEMS as AFM resonant probes with integrated capacitive transducers (Figure 1a). AFM topographic images of DNA origamis were acquired by a 10.9 MHz MEMS AFM sensor as a demonstration of operation (Figure 1b and c). In these MEMS probes, on-chip electrical actuation and readout of the tip oscillation are obtained by means of built-in capacitive transducers.

In recent work Ultramicroscopy, 175, 2017, pp. 46–57, displacement and force resolutions were determined from noise analysis at 1.5 ± 0.15 fm/√Hz and 0.4 ± 0.04 pN/√Hz, respectively. The probe used here has resonance frequency of 13.59 MHz and a quality factor of 760.

Despite the high effective stiffness of the probes (calculated to be around 200 kN/m at the tip location), the tip-surface interaction force can be kept below 1 nN by using a vibration amplitude significantly below 100 pm and a setpoint close to the free vibration conditions.

Imaging capabilities in amplitude- and frequency-modulation AFM modes were demonstrated on block copolymer surfaces. Z-spectroscopy experiments revealed that the tip is vibrating in permanent contact with the viscoelastic material, with a pinned contact line.

 

 

Presently Vmicro leads the industrial transfer of the MEMS sensor. For further testing the behavior of these probes, a cooperation program between this company and our research group has been set up and we developed a modified Bruker multimode AFM in our research group. In that frame, we performed the characterization of a series of reference samples. In figures 2 and 3 are shown first AFM images for SiC, ADN on Mica and Low-density polyethylene (LDPE).

 

Figure 2: (a) 5x5 µm2 image of SiC in AM-AFM tapping mode. Probe resonance frequency = 12,92MHz, 4 µm/s, 256 pixels. (b) 3x3 µm2 image of ADN on Mica in AM-AFM tapping mode. 19μm/s, acquisition time: 5min, vibration amplitude 900 pm, 1024 pixels. (c) same sample 400x400 nm2, 15 μm/s, acquisition time: 13 s, 256 pixels.

Figure 3: (a) 5×5 µm2 image of SiC in FM-AFM tapping mode. Probe resonance frequency = 12,92MHz, 6 µm/s, 512 pixels, acquisition time: 14 min, vibration amplitude 250 pm (b) and (c) 1.5×1.5 µm2 image (topography and PID drive) of LDPE in FM-AFM tapping mode. Probe resonance frequency = 12,92MHz, 1.8 µm/s, 128 pixels, acquisition time: 3.5 min, vibration amplitude 3 nm.