Local anodic oxidation (LAO)

Nano-lithography by local anodic oxidation (LAO) has successfully been demonstrated on a variety of materials. The basic elements of LAO includes a conductive probe, wettable substrate surafce, humid enviroment, and tunable applied bias circuit. The LAO stepup is depicted in the schematic plot (Fig.1).

LAO1

Applied external bias difference between probe and surafce supplies sufficient energy to dissociate water molecular in the water bridge between probe and surface of interest. Under negative bias on probe, the chemical reaction reads

LAO2

The most widely accepted models for LAO includes Cabrera-Mott model, empirical power-of-time model, and space-charge limited model.

Cabrera-Mott model

Cabrera–Mott model is valid for thin oxide films. The first assumption of the Cabrera–Mott model is that electrons can pass freely from the metal, through the oxide surface, and to react surface atom. Oxide growth under high electric field is described by ion migration .The below figure shows the schematic picture of Cabrera–Mott model.

LAO3

The kinetic of growth could be written by:

LAO4

This model is valid only when h << h1(few nanometer) . When h is small, the oxide rate is vary large. Where

LAO5

We can get

LAO6

hL where oxidation is no longer assisted by the electric field.

a’ is half the distance between two interstitial sites.

Empirical power-of-time model

This model is used to describe how tip’s oxidation writing speed effects the height of oxide.

LAO7

The silicon oxide growth function:

LAO8

Uth is the threshold voltage for growing oxide. vo=1 um/s ,αo=0.78 nm/V,γ=0.23±0.02 for this case.Reference : Appl. Phys. Lett. 67, 3144 (1995); doi: 10.1063/1.114861

Graphene oxide(GO)

In our experiment condition we use the silicon buffer oxide covered with the CVD graphene as the oxidation template. Conductive tip apllied with negative voltage around 16~-5 V to oxidize the graphene sheet under contact mode. The GO experiment set up graph as below.

LAO9

It can be seen in the AFM topography graph. Under stong electrical field, the accelerated OH- not only react with the surface graphene but also the subsurface SiOx/Si template, and results in protrusion much higher than the carbon-oxygen physical stacking. The writing speed which from 0.1-5 um/s has strong effect on the resultant protrusion height. The figures below is shown as example.

LAO10