Nanofabrication of Titanium Thin Films Using Local Oxidation Nanolithography (LON)

SPM-based LON has gained traction as a powerful nanofabrication method to develop innovative nanodevices and nanostructures. The spatial confinement of an anodic oxidation reaction between the sample surface and the tip of an AFM forms the basis of this versatile technique. The application of a voltage between the sample surface and the AFM tip within a water meniscus formed between them leads to the oxidation of the surface, creating ultra-small oxide structures in a myriad of materials.

LON of Titanium Surfaces

LON of titanium surfaces holds potential to develop various kinds of nanodevices, including high-density data storage structures and single electron transistors. Selective nano-oxidation of titanium is a key intermediate stage for further nanostructuration processes. Thin metal films can be fully converted into an oxide down to a substrate through SPM-induced oxidation. This capability is essential to produce non-volatile RAMs using the resistance change of nanoscale titanium oxide structures.

The fast bipolar non-volatile switching performance shown by TiO2 resistive switches can be related to the drift of charged oxygen vacancies in the metal- oxide interface in an electric field. As a result, Metal/TiO2/Metal nanodevices provide the cornerstone for reconfigurable hardware, thus enabling the development of fine grain field programmable gate array (FPGA) devices.

Instrumentation

The efficient application of the AFM tip-induced oxidation method in nanofabrication requires better understanding of the inherent mechanisms of the anodization process on the metal surface. The created nanostructures are topographically measured in order to study the kinetics of scanned probed oxidation. However, more insights into the anodization mechanisms can be achieved with additional characterization of electric properties at the nanoscale. This can be performed with the RESISCOPE II® module, thanks to its huge dynamical range of measurement of up to 10 decades (Figure 1).

Resiscope principle

Figure 1. Resiscope principle

Experimental Procedure and Results

This study involved LON of 10nm-thick titanium thin films using a 5500LS system equipped with a Resiscope module. A Pulsed Laser Deposition technique was used to deposit the thin films onto alumina (isolating) substrates. The topography and associated current and resistance maps of vertical lines and closed patterns (squares) acquired by changing the tip velocity (20nm/s - 1µm/s) are presented in Figure 2.

(a) topographic, (b) current, and (c) resistance maps of the obtained patterns; (d) 3D topographic map in which the color represents the resistance. The numbers in (a) indicate the tip velocity in µm/s.

Figure 2. (a) topographic, (b) current, and (c) resistance maps of the obtained patterns; (d) 3D topographic map in which the color represents the resistance. The numbers in (a) indicate the tip velocity in µm/s.

There is an increasing trend observed for the average height of the patterns with decreasing tip velocity, achieving a plateau at the lowest velocities (20 and 50nm/s). Conversely, the oxidized width monotonically increases with decreasing tip velocity in the full studied range, indicating the limitation to thickness of the oxide growth but not along the surface. However, the maximum oxide thickness value is less than the titanium film thickness.

(a) average height and width of the oxide lines obtained by LON of titanium film as a function of the tip velocity; (b) oxide line width ratios obtained from the topographic (W), resistive (RW) and high resistive (RH) measurements. Inset: topographic (black) and resistance (red) transversal profiles of the oxide lines.

Figure 3. (a) average height and width of the oxide lines obtained by LON of titanium film as a function of the tip velocity; (b) oxide line width ratios obtained from
the topographic (W), resistive (RW) and high resistive (RH) measurements. Inset: topographic (black) and resistance (red) transversal profiles of the oxide lines.

The oxide spreading along the surface can be more precisely evaluated with the local current and resistance measurements performed using the Resiscope module. The ratios between topographic (W), resistive (RW; resistance higher than the average one of titanium surface), and high resistive (RHW; resistance higher than 10G) width quantified on the oxide lines are presented in Figure 3b.

In all cases, the oxides lines’ resistive width is more than by 10% when compared to those measured in the topography map, indicating the additional coverage by the front-side oxide layer a little bit beyond the area wherein topographic modifications are apparent. Sub-stoichiometric oxides most probably form this front-side layer. Conversely, there is an increase in the RHW/W and RHW/RW ratios with decreasing tip velocity, achieving a plateau about 0.55 - 0.6 at 20 - 50nm/s.

The high resistive areas indicate the thickest oxide layer, suggesting a steady state oxide growth. The experimental conditions required to oxide the full thickness of the films can be easily determined from current-resistance maps because the inner area of closed patterns seem to be highly resistive under these conditions. The full thickness oxidation is achieved at tip velocities less than 50nm/s (Figure 2). These conditions are in line with the conditions under which the high resistance/oxide width ratio and measured oxide height become stable with decreasing tip velocity.

Conclusion

The RESISCOPE II® capabilities have been fully demonstrated in this application that demands a large dynamic range. Moreover, the RESISCOPE II® has high sensitivity to detect any deviation of current and resistance when compared to a standard conductive AFM. As the RESISCOPE measurement requires low level of current, tip damage as well as additional oxidation between the tip and the sample can be avoided.

This information has been sourced, reviewed and adapted from materials provided by CSInstruments.

For more information on this source, please visit CSInstruments.

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