Local oxidation nanolithography (LON) is a tip-based nanofabrication method. It is based on the spatial confinement on an oxidation reaction under the sharp tip of an atomic force microscope. The first materials on which LON was demonstrated were Si(111) and polycrystalline tantalum. Subsequently, the technique has been extended to III–V semiconductors, silicon carbide, metals such as titanium, tantalum, aluminium, molybdenum, nickel and niobium; thin films of manganite in the perovskite form; dielectrics like silicon nitride, organosilane self-assembled monolayers, dendritic macromolecules and carbonaceous films.
History The local oxidation of a surface by means of a scanning probe technique was first observed by Dagata and co-workers in 1990 who locally modified a hydrogen-terminated silicon surface into silicon dioxide by applying a bias voltage between the tip of a scanning tunneling microscope and the surface itself. In 1993 Day and Allee demonstrated the possibility of performing local oxidation experiments with an atomic force microscope, which opened the way to applying the technique to a large variety of materials.
Basic principle
Currently, local oxidation experiments are performed with an atomic force microscope operated in 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. In order to perform local oxidation nanolithography, the relative humidity in the AFM chamber is kept between 30% and 60%. A voltage pulse is applied between a conductive AFM tip and the sample. The applied voltage induces the formation of a water bridge between tip and sample whenever the amplitude of the voltage pulse is above a certain threshold voltage. When the liquid meniscus is created the applied voltage pulse causes an oxidation reaction by breaking the covalent bonds in the water molecules. The liquid bridge provides the oxyanions (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:
M + n H 2 O ⟶ MO n + 2 n H + + 2 n e − {\displaystyle {\ce {M}}+n{\ce {H2O -> MO}}_{n}+2n{\ce {H+}}+2n{\ce {e-}}}
M n + 2 n H 2 O + 2 n e − ⟶ n H 2 + 2 n OH − + M {\displaystyle {\ce {M}}^{n}+2n{\ce {H2O}}+2n{\ce {e- ->}}\ n{\ce {H2}}+2n{\ce {OH- + M}}}
while hydrogen gas is liberated at the AFM tip through the reduction reaction:
2 H + + 2 e − ⟶ H 2 {\displaystyle {\ce {2H+ + 2e- -> H2}}}
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 atomic force microscope. The key requirement is the possibility to apply voltage pulses between the tip and the sample. It is recommendable to enclose the microscope in a chamber where the atmosphere is controlled. In the simplest case, the oxidant is water vapor, which is naturally present in the air. Controlling the 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
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 Spanish National Research Council in Madrid wrote the first ten lines of Cervantes' Don Quixote on a few square micrometres of silicon. This pattern versatility can be used for information storage or to design etch-resistant nanomasks in order to fabricate nanodevices as well as many other applications.
Data storage
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