October 5, 2009
Groundbreaking work by scientists from Switzerland, Finland and Germany has lead to a new world record in X-ray microscopy. For the first time ever, features below 10 nm in width were resolved.
Key to this new world record was doubling of the effective line density of FZP template (Fresnel Zone Plate, a key component of an X-ray microscope) by extremely conformal thin films created through the use of Atomic Layer Deposition (ALD). ALD work was carried out with a Picosun Sunale reactor at the premises of the University of Helsinki’s Laboratory of Inorganic Chemistry.
ALD is a thin film method which enables completely controlled growth of extremely conformal films through its signature self-limiting, sequential surface reactions. ALD produces complex layer structures with atomic level accuracy. ALD processes can be reproduced with stunning accuracy.
The group produced a modified FZP with structures based on the conformal deposition of high refractive index material by ALD onto the sidewalls of a pre-patterned template made from a low refractive index material. This new focusing structure achieves an unprecedented spatial resolution in X-ray microscopy. Line widths of down to 9 nm were successfully resolved.
Vila-Comamala J., Jefimovs K., Raabe J., Pilvi T., Fink R.H., Senoner M., Maassdorf A., Ritala M., David C.: Advanced thin film technology for ultrahigh resolution X.ray microscopy. Ultramicroscopy. 2009 Oct;109(11):1360-4. Epub 2009 Jul 15.
September 17, 2009
Atomic force microscopy is well-known even in the public as a versatile tool for the production of images on the nanoscale level. Kelvin probe force microscopy is a special type of this imaging technique named after Lord Kelvin. When brought to the market in 1991, a scientific description of how to interprete the images was delivered. To this, physicist Christine Baumgart, a doctoral student of the nanospintronics group at the research center Forschungszentrum Dresden-Rossendorf (FZD), has now added new features.
Christine Baumgart now discovered what exactly is measured by Kelvin probe force microscopy. It is the electric potential which is needed to move electrons or holes from the inside to the surface of a semiconductor. Her new findings will simplify the microscopic technique itself, and will lead to unambiguous and reproducible results concerning the structure and electronic properties of samples. Also, Kelvin probe force microscopy, which has been used mainly in materials science and semiconductor physics so far, is likely to become more attractive for other areas like biotechnology.
But how exactly does a Kelvin probe force microscope work? The tip is deflected by the electrostatic force between cantilever and sample when moved over the sample. By applying bias to the sample, electrons and holes are moved to the surface of the semiconductor and the electrostatic force decreases. The cantilever moves back to its original position and the applied bias is stored as the signal measured. To be more precise, there is a quantitative relation between the measured Kelvin bias and the difference between the calculated Fermi energy and respective semiconductor band edge independent of the work function of the probing microscope tip. Thus, Christine Baumgart’s novel explanation of how the Kelvin probe force microscope works elucidates why the signal depends on the bias necessary for injecting majority charge carriers towards the interface between insulator and semiconductor.
Baumgart C., Helm M., Schmidt H., (2009): Quantitativ dopant profiling in semiconducters: A Kelvin probe force microscopy model. DOI: 10.1103/PhysRevB.80.085305.
Schematic drawing of a Kelvin probe force microscopy probe above a doped semiconductor with a thin oxide layer (grey blue atomic layer). Occupied surface states at the interface between the oxide layer and the semiconductor are animated in red and the same number of unscreened dopant atoms is animated in dark blue. Left: The resulting asymmetric electric dipole causes the deflection of the probe. Center: By applying a bias mobile majority charge carriers are injected into the semiconductor (animated in orange) and screen the unscreened ionized dopant atoms. Right: As a result the electrostatic force onto the cantilever vanishes. The cantilever moves back to its normal position. The applied bias is measured and depends on the concentration of dopant atoms. Picture: Sander Münster, Kunstkosmos