Piezoresistive Ring-Shaped AFM Sensors with Pico-Newton Force Resolution

Piezoresistive Ring-Shaped AFM Sensors with Pico-Newton Force Resolution

Zhuang Xiong, Benjamin Walter, Estelle Mairiaux, Marc Faucher, Lionel Buchaillot, Bernard Legrand
Copyright: © 2013 |Pages: 15
DOI: 10.4018/ijimr.2013010104
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A new concept of Atomic Force Microscope (AFM) oscillating probes using electrostatic excitation and piezoresistive detection is presented. The probe is characterized by electrical methods in a vacuum chamber and by mechanical methods in air. The frequency-mixing measurement technique is developed to reduce the parasitic signal level. These probes resonant in the 1MHz range and the quality factor is measured about 53000 in vacuum and 3000 in air. The ring probe is mounted onto a commercial AFM set-up and the surface topography of PMMA sample (2 µm square) is obtained. The force resolution deduced from the measurements is about 10 pN/Hz0.5.
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Structure Design

Mechanical Design

The ring probe is designed to resonate at about 1 MHz (elliptic mode). A simple formula of the ring’s resonance frequency is given by J.Kirkhope (Kirkhope, 1976).

(1) with


Where R is the central radius, E, the Young’s modulus, I, the moment of inertia, A, the ring cross section, ρ, the mass density, G, the shear modulus, and K, the shear coefficient which is 5/6. The elliptic mode is found for n=2. Two rings are designed to resonate around 1 MHz. The outer radii are set to 250 µm and 200 µm, respectively, with a width of 50 µm and a thickness of 20 µm. The tip is designed in a triangular form (200 µm in length and 60 µm in width) and placed in a face-to-face position to keep the structure symmetrical (Figure 1).

Figure 1.

ANSYS modal simulation of the ring probe vibrating in the elliptic mode. The dash lines indicate the non-deformed structure. The simulated frequency is 1.47 MHz.


Table 1 shows the ring dimensions and the elliptic mode resonance frequency given by analytical modeling and Finite Element Modeling (FEM) simulation.

Table 1.
Ring dimensions and corresponding resonance frequency of elliptic mode
Outer radius
Inner radius
Analytical results (MHz)Simulation results(MHz)
Without tipWith tip

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