Last month we discussed some Micro- and Nano- Electrical-Mechanical Systems (MEMS and NEMS) with potential for greatly improved contamination sensors. This month we continue with descriptions of some of the sensing tools that are available or are under development.
Atomic force microscopy (AFM) is a key tool for surface characterization of MEMS and NEMS scale devices. Applications include identification of thin film, particulate, and molecular-level contaminants.
AFM is analogous to the operation of a phonograph, in which the movement of a stylus due to the topography of the record groove is converted to electrical signals and then to sound. In AFM, a very small (down to a few nm) diameter tip is attached to a thin cantilever. As the tip is moved over the surface to be studied, small deflections due to repulsive or attractive forces of nearby atoms cause the cantilever to bend. Detection is typically by laser reflection from the cantilever surface.
AFM can be operated in several modes. Contact mode is the most analogous to the record player. The tip ìstylusî is dragged over the surface and the deflection is by the repulsive force associated with the close atomic distances of contact. If the stylus encounters an atom sitting on the surface, the repulsive forces between the atom and the stylus tip cause the stylus to be lifted. Non-contact mode of AFM utilizes the attractive Van-der-Waals forces between atoms when they are close but not in contact. This force is much weaker than the repulsive forces so this technique does not have as fine resolution as contact techniques but does avoid some problems associated with actual contact. A third mode, called TappingModeô,1 is in between contact and non-contact. The cantilever is allowed to vibrate at its natural frequency (50kHz-500kHz), and touches the surface for only a small fraction of the time. In this mode it provides the stronger forces of contact mode while reducing issues such as lateral or dragging forces.
Of particular interest to those concerned with contamination is an extension of the tapping mode to mapping the phase of the cantilever oscillation during the scan. This allows detection of variations in composition, adhesion, friction, viscoelasticity, and other properties.
A number of variants of AFM are in use or in development. Magnetic force microscopy (MFM) is a magnetic analog of AFM; the tip on the cantilever is magnetic. Just as like charges repel and unlike charges attract, magnetic poles behave analogously. MFM can detect small magnetic materials that exert a force on the magnetic tip on the MFM cantilever.
One of the more interesting newer developments for contaminant identification is Magnetic Resonance Force Microscopy that combines the principles of MFM with those of Magnetic Resonance Imaging (MRI) such as are used in medical diagnoses. Each electron possesses the quantum mechanical property of ìspinî in which it acts as a tiny bar magnet that can be either aligned or anti-aligned with a magnetic field. A small electric coil positioned near the tip generates a high-frequency magnetic field that can flip the direction of the spin and therefore change the force generated on the magnetic tip. An advantage of MRFM is that it can look beneath the surface and detect electron spins in localized deeper layers of the region being examined. IBM researchers have recently reported in the journal Nature, the detection of the spin of a single electron by MRFM.2 This is a sensitivity about 10 million times that of medical MRI. Although currently limited to detecting the spin of the electron, research is underway to extend the applications of MRFM to nuclear spins (600 times or more weaker than the electron spin) of protons, C13 and other elements. This would make the technique sensitive to particular molecules and would be a powerful tool in determining the exact nature of surface and near-surface areas.
Note: The authors acknowledge Taniya Das of Veeco Instruments, Inc. for her helpful comments in the preparation of this column.
1 Trade mark of Veeco Corporation
2 D. Rugar, J. Mamin, R. Budakian, B. Chui. Nature, 430, (2004) pp.329-332. Story available at IBM News, http://www.research.ibm.com/resources/news/20040714_nanoscale.shtml.