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Figure 5-11 shows the tilted SEM image of the uniform arrays of nickel

nanopatterns over a relatively large area. This particular electroformed nanostructure was created using a current density of 10 mA/cm2 in a nickel bath with a temperature and pH value of 50º and 4.5, respectively. The information on the ingredients of the nickel electrolyte bath, which had been used for this production, is presented in

Table 5-2. It can be clearly seen in the inset of this figure that the average height of

the obtained nanopillars is 100±5 nm.

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Table 5-2 The information on ingredients of the nickel electrolyte bath used in the electroforming process.

Symbol Name Value

Ni(NH2SO3)2 Nickel Sulfamate 380-425 g/L

NiCl2 Nickel Chloride 8-12 g/L

H3BO3 Boric Acid 25-35 g/L

NaC12H25SO4 Sodium Dodecyl Sulphate ~1 g/L

Figure 5-12 and Figure 5-13 show the FIB characterisation results for both

nanostructures which had been fabricated using the single and double layer colloidal lithography methods, respectively. An FEI dual beam FIB (Strata DB235) was used for these experiments. The milling process was conducted using an FIB acceleration voltage of 30 kV and ion current of 30 pA. It is worth noting that the usage of lower ion current did not provide a good focusing on the samples due to the low energy of ions, and the usage of higher ion current caused the development of a re-deposition effect on the samples. An area of 2 µm by 3 µm was milled from the samples in 70 seconds.

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Figure 5-13 SEM images illustrate milled arrays of dual nanoholes created by BCL method.

A thin film of platinum was used as a protective layer to support the soft surfaces of the resist coated samples and reduce the re-deposition effect in the course of the FIB milling process. The depth of nanopatterns can be estimated using the FIB milling results. Random areas were examined on several samples for this investigation.

Figure 5-14 illustrates the resultant measurements of the nanoholes’ depth. In this

figure, the samples with single arrays of nanoholes are distinguished from the samples with dual arrays of nanoholes using different coloured symbols. The small and big blue squares represent the depth of the small and big nanoholes in the dual patterned structure, while the red triangle indicates the depth of nanoholes for the single patterned structure. As it can be seen in this figure, the average depth of nanoholes produced by the MCL method is 820±10 nm, while the average depth of nanoholes produced by the BCL method is 60±3 nm and 350±5 nm for the small and big nanoholes, respectively.

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Figure 5-14 Comparison of the obtained height between single and dual patterned nanostructure.

As discussed earlier in this chapter, Figure 5-11 displayed a nickel nanopillars structure which was fabricated through an electroforming process over a template with single arrays of nanoholes structure. The comparison between the height of nickel nanopillars and the depth of nanoholes shows a massive difference. The height of the nickel nanopillars is about one-eighth of the nanoholes’ depth. The reason for this issue was found in the wettability of the nanoholes. Although SDS surfactant was used to decrease the surface tension of the nickel electrolyte and provide better conditions for the solution to wet the nanotemplate surface, it is difficult to completely fill the fine nanoholes with nickel solution due to the presence of the trapped air at the bottom of them. On the other hand, the higher the aspect- ratio of the nanopatterns, the lower the chance of electrolyte penetration. Since the depth of nanoholes created by the MCL technique is higher than the depth of nanoholes fabricated by the BCL method, it was necessary to use other elements to

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get rid of the trapped air. The use of a vacuum was found as the proper solution. Hence, the desired nanotemplate was first immersed in the electrolyte bath followed by placing it in the vacuum chamber. The sample was then held inside the vacuum for about 30 minutes to ensure that all the fine nanoholes were filled by the nickel solution. Also, all residual bubbles were removed from the nanoholes using the vacuum. Figure 5-15 and Figure 5-16 illustrate the resultant nickel nanostructures. Highly ordered arrays of single and double patterned nanopillars were successfully produced using the nickel nano-electroforming process. As can be seen in Figure 5-

15, the nanopillars are smoothly electroformed which indicates the quality of the

initial nanotemplate.

Figure 5-15 SEM images show the nickel nanopillars fabricated by electroforming process: a) single patterned metallic nanostructure, b) double patterned metallic nanostructure.

The top view SEM images show that the nickel nanopillars inherited the exact features of the photoresist surface. This issue can be clearly observed from Figure 5-

15b in which bigger nanopatterns comprise several finer features on their surfaces.

This confirms that the electroformed structures resemble the photoresist nanotemplate. Hence, nickel electroforming can be used as an effective tool to create

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metallic nanostructures with high precision. The tilted SEM image of a single patterned nanostructure is demonstrated in Figure 5-16. It can be seen that the height of the nanopillars is greatly increased. The experimental results confirmed that a vacuum process can easily assist to improve the electroforming performance at the nanoscale level. Ordered arrays of metallic nanopillars with an average height of 815 nm were successfully fabricated using this technique. The achieved results of the electroformed structures are in a good agreement with the FIB characterisation results which were presented earlier in this chapter. The fabricated metallic nanostructures can be used as a nano-stamp for imprinting lithography.

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5.8 Summary

In this chapter, a nickel electroforming process was used for the production of ordered arrays of metallic nanostructures. A brief introduction was provided to explain the mechanism of electrodeposition. Next, a general overview of the fabrication process was introduced and the electroforming setup was illustrated. It was found that the operating conditions would directly influence the quality of the electroformed nanostructures. Several structural defects were identified as the challenges for the electroforming process at nanoscale. Different approaches were applied to overcome these defects followed by presenting the proper solution for each challenge. FIB milling was conducted to measure the dimension of the nanopatterns which were placed on the template. The characterisation results were subsequently used to justify the dimension of the obtained metallic nanostructures.

The resultant metallic nanopillars confirm that the process is able to mirror the profile of photoresist patterns even for very fine features. The proposed method provides a versatile way for creating metallic nanostructures with high dimensional accuracy. Uniform arrays of single and double patterned metallic nanostructures have been successfully fabricated using a simple and convenient technique without the need for expensive tools.

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