Tip-sample interaction in force modulation microscoscopies: mechanical properties at nanoscale level

Project manager: Prof. Dr.-rer. nat. Gerold A. Schneider
Projekt worker: Dr. Francisco Javier Espinoza Beltrán
Supported by: Alexander von Humboldt Foundation
Collaboration: Dipl. Torben Sholz (TUHH), Dr. Juan Muñoz Saldaña and M.C. Alvaro Vargas, M. C. David Torres (CINVESTAV, Mexico)
Start: September 2005
End: August 2006

Atomic force microscope is a versatile tool currently used to measure local elastic properties including force modulation microscopy, nanoindentation, atomic force acoustic microscopy, and a spectroscopy technique in which contact resonances are measured. In the latter method, the resonance frequencies of the cantilever flexural modes are used to determine the tip-sample contact stiffness. The amplitude of vibration and the phase shift of the cantilever are measured and then introduced into physical models of tip-sample contact to derive the desired mechanical properties. Cantilever behavior and the involved tip-sample contact mechanics may greatly vary depending on the chosen model. The simplest model used to analyze the results considers that the cantilever is a point mass, considering an elastic contact between sample and tip. In this project a more complex model, which considers a realistic geometry of the tip, is applied for the analysis of the interaction tip-sample is developed (Fig. 1). This model is developed by the finite element method, and used to study experimental spectra of ceramic materials (ferroelectric and hard coatings).

Fig.1: Images of a AFM tip used for finite element modeling

We are implementing Force Modulation Microscopy (FMM) and Atomic Force Acoustic Microscope (AFAM) mode using the AFM Nanoscope IV - Dimension 3100 of Digital Instruments system. The excitation is performed directly on the AFM by means of the piezoelectric one of the holder for FMM (Force Modulation Microscopy) of DI (Fig. 2).

Fig. 2: Sketch of Force Modulation Microscopy - FMM – image of ferroelectric domain structure and resonance spectrum of the tip-sample contact.

Ferroelectric ceramic films with graded composition and hard coatings are fabricated by reactive magnetron sputtering techniques (Fig. 3) and characterized using scanning probe microscopy techniques: FFM and AFAM.

Fig 3. This is the setup of the DC and RF magnetron sputtering system. A) Schematic diagram of the effect of the magnetic field generated by the magnet that causes preferential regions of erosion in the target. B) Distribution of glow of plasma emission, which coincides with the preferential regions of erosion. C) Plasma erosion on the target by effect of the use of the magnet. It is in these regions of greater wearing down where we will place the metals: Sr, Ba and Pb.



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    Materials, Volume 7, Issue 8 , Pages 713 – 718. Published Online: 16 Au g 2005