Dipl.-Ing. Stefan Adamsmair
AFM is a very powerful microscope to measure and manipulate samples in various environments at the nanoscale. It was invented by Binning, Quate, and Gerber in 1986 (1*) . The AFM consists of (see Figure 1):
- a cantilever
- a detection system
- feedback electronics
At the end of the cantilever a sharp tip is attached, which is brought into close proximity of a sample surface. The forces between the tip and the sample lead to a bending of the cantilever. There are several methods to measure the deflection of the cantilever. Typically a laser beam, which is reflected and refracted at the end of the cantilever and picked up by a two or four quadrant photodiode, is used. To avoid that the tip crushes into the sample surface and damages the sample and/or the delicate tip a fast feedback electronic is used to maintain a constant force between the tip and the sample and therefore a resultant constant bending of the cantilever.
Over the years several different modes of operation have been developed. The mostly used modes are:
- contact mode
- dynamic contact mode
- non-contact mode
- dynamic non-contact mode
The dynamic modes, using sinusoidally excited cantilevers, split into two major groups: amplitude modulation and frequency modulation mode. In amplitude modulation mode the cantilever is excited near its resonant frequency and the change of the amplitude is used as a feedback signal to maintain a constant amplitude and therefore also a constant tip sample distance. In frequency modulation mode the cantilever is excited exactly at its resonant frequency and the change of the resonant frequency due to the tip sample forces is used as a feedback signal to maintain a constant shift in the resonant frequency and therefore also a constant tip sample distance.
The institutes of Biophysics and Measurement Technology are involved in an EU-project named FORCETOOL, which proposes to develop a multipurpose tool for quantitative nanoscale analysis and manipulation of biomolecular, polymeric, and heterogeneous surfaces. Key features of the proposed instrument are 1 nm spatial resolution, 1 pN force sensitivity, operation in technological relevant environments (air or liquids), and with no impact on the sample surface. The multifunctionality and flexibility of FORCETOOL will enable characterization, control or manipulation of structures on a nanometer-scale, so it will open new approaches for manufacturing at molecular and nanoscale levels. This tool is based on two innovative concepts:
- the bi-modal AFM (2*) where the cantilever is excited at both the first and the second mode concurrently, and
- the multimaterial methodology
The participants of FORCETOOL are from Spain, Netherlands, Italy, Germany, and Austria.
Our part of the project is to work on a bi-modal control unit realized completely on a DSP to process the photodiode signal. This signal has to be split up into the two excitation frequencies (see figure 2), which are then processed to yield topography, recognition, and phase images. For these digital computations the Blackfin BF561, a dual core processor from Analog Devices, is planned to be used.
Figure 2: The photodiode signal has to be split up into the two excitation frequencies for further computations
We acknowledge Analog Devices for partial support.
(1*) G. Binnig, C.E Quate, Ch. Gerber, Phys. Rev. Lett. 56 , 930 (1986).
(2*) T. R. Rodríguez, R. García, Appl. Phys. Lett. 84 , 449 (2004).