Local oxidation procedure: 3D representation the Local Oxidation Nanolithography process. A voltage pulse applied between the AFM tip and the scanned surface yields to the formation of a liquid meniscus that confines a nanometric oxidation reaction.
Local oxidation nanolithography (LON) is a
atomic force microscope
.
The first materials on which LON was demonstrated were
atomic force microscope, which opened the way to applying the technique to a large variety of materials.[3]
Basic principle
Steps of the local oxidation process in noncontact mode. I: The tip is scanning the sample in noncontact mode oscillating at a constant amplitude. II:When the voltage pulse is applied a liquid meniscus between tip and sample is induced by the electrical field. This liquid meniscus acts like a nanometer-size electrochemical cell where an oxidation reaction is held. III:When the voltage pulse is off, the AFM feedbacks withdraw the tip from the sample stretching the liquid meniscus. IV: After the meniscus is broken the tip recovers its original oscillation amplitude and continues the scanning.
Currently, local oxidation experiments are performed with an
contact or noncontact mode with additional circuits to apply voltage pulses between tip and sample. The local oxidation process is mediated by the formation of a water meniscus.[4]
In order to perform local oxidation nanolithography, the
OH−
,O−) needed to form the oxide and confines the lateral extension of the region to be oxidized.
The chemical reactions that govern the local oxidation in a metallic substrate (M) are the following:[5]
while hydrogen gas is liberated at the AFM tip through the reduction reaction:
When the voltage pulse is off the AFM feedback forces the cantilever to recover its original oscillation amplitude withdrawing the tip from the sample and breaking the liquid meniscus. Finally the AFM continues to scan the sample thus allowing to image MOn nanostructure fabricated during the Local Oxidation process with the very same tip used for its fabrication.
The method to form liquid bridges is so precise that water meniscus diameters of 20 nm or below are easily obtained. This has led to the reproducible fabrication of sub-10 nm structures in silicon and other metallic surfaces.
Experimental setup
Local oxidation experiments can be performed with almost any kind of
relative humidity
generally helps to obtain more reproducible results. The size of the fabricated features depends on a number of parameters, such as the distance between the sample and the tip, the amplitude and the duration of the voltage pulse, and the relative humidity of the atmosphere.
Applications
First paragraph of Cervantes'Don Quixote written on a silicon chip. The local oxidation technique would allow to write all the book (more than 1,000 pages) on a surface as big as the tip of one human hair.
The development of nanometer-scale lithographies is the focus of an intense research activity because progress on nanotechnology depends on the capability to fabricate, position and interconnect nanometer-scale structures.
Patterning
Local Oxidation Nanolithography allows to create a large variety of motives like dots, lines and letters with nanometer accuracy. In 2005, researchers at the
information storage or to design etch-resistant nanomasks in order to fabricate nanodevices
as well as many other applications.
Data storage
π number with twenty decimals: 3,1415926535 8979323846 written in binary code by Local Oxidation on a silicon surface.
It is possible to store information using dot-like nanostructures created by the local oxidation of a surface. This storage uses the
Molecular template growth and preferential deposition
Using specific functionalizations it is possible to deposit molecules and nanoparticles only in very small domains over a substrate surface. LON is a powerful technique to fabricate this kind of domains for the preferential growth.Two SiO2 stripes were fabricated by LON over a substrate functionalized with APTES. After the deposition of a 0.1mM solution of Mn12 the single-molecule magnets are deposited only over the regions defined by the AFM.
Local oxidation of silicon surfaces by noncontact atomic-force microscopy is an emerging and promising method for patterning surfaces at the nanometer scale due to its very precise control of the feature size. The features created with this technique can be used for the template growth and preferential deposition of different molecules like single-molecule magnets, biomolecules and conjugated organic molecules.
This method of nanopositioning is an important tool for the fabrication of new nanodevices based on the novel properties exhibited by some
groups (-NH2). Such termination electrostatically repels the Mn12 molecules. Subsequently, a pattern of silicon dioxide is defined by LON. The SMM molecules are predominantly deposited on the oxide motives because of electrostatic attraction. The electrostatic attraction between the silicon oxide fabricated by LON and the Mn12 molecules achieves the preferential deposition of this molecules with a nanoscale accuracy.
Fabrication of nanodevices
In order to fabricate SiNW using the Top-Down approach of the nanotechnology a nanomask is fabricated by LON over a Silicon On Insulator substrate. After the SOI etching a SiNW is defined under the nanomask. Then the nanomask is removed with a HF etching and finally the SiNW is connected to the whole circuit using Electron Beam lithography.SiNW nanotransistor fabricated with the term 'NANO'. This pattern exhibits a good electrical response based on in the SiNW properties only.
By using local oxidation nanolithography as tool for the fabrication of etch-resistant nanomasks, it is possible to fabricate nanoscale electronic devices, such as
top-down fabrication technique allows the fabrication of a large variety of SiNWs with different shapes, from angular to circular. It also allows the precise positioning of the silicon nanowires in any desired position, making easier its integration; indeed, this technique is compatible with the standard silicon CMOS processing technology. Single crystalline silicon nanowires have already shown a great potential as ultrasensitive sensors by detecting changes in the nanowire conductivity when a specific analyte is present.[9]
Local oxidation nanolithography, therefore, is a promising technique to allow the realisation of array of biosensors.
