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Focused Ion Beams is very favourable tools in micro- and nanotechnology as well as in analytics. Characteristic properties are the sub micrometre spot size, the broad energy range from some eV (low energy FIB) up to 200 keV, the high current density and a broad spectrum of ion species. Focused Ion Beam systems usually use Ga Liquid Metal Ion Sources (Ga-LMIS), but in some cases also Liquid Metal Alloy Ion Sources (LMAIS) are used. Modern FIB-systems are Computer controlled and so structures of arbitrary shape with dimensions on nm-scale can be written.
Focused ion beam, also known as FIB, is a technique used particularly in the semiconductor and materials science fields for site-specific analysis, deposition, and ablation of materials. Focused ion beam (FIB) systems have been produced commercially for approximately twenty years, primarily for large semiconductor manufacturers. FIB systems operate in a similar fashion to a scanning electron microscope (SEM) except, rather than a beam of electrons, FIB systems use a finely focused beam of ions (usually gallium) that can be operated at low beam currents for imaging or high beam currents for site specific sputtering or milling.
Principle of FIB
When energetic ions hit the surface of a solid sample they lose energy to the electrons of the solid as well as to its atoms. The most important physical effects of incident ions on the substrate are: sputtering of neutral and ionized substrate atoms (this effect enables substrate milling), electron emission (this effect enables imaging, but may cause charging of the sample), displacement of atoms in the solid (induced damage) and emission of phonons (heating). Chemical interactions include the breaking of chemical bonds, thereby dissociating molecules (this effect is exploited during deposition).
Type of Focused Ion Beam nanopatterning
For Naonopatterning, there are three methods to generate the nanostructure, materials removal (ablation), materials deposition and Combination of materials removal and addition.
Nanostructuring by materials removal
Using a high ion current beam removal of sample material is achieved, which results in physical sputtering of the sample material. By scanning the beam over the substrate, an arbitrary shape can be etched. The rate of removal is related to the sputtering yield of the material and normal incidence. However, these numbers cannot be used directly to calculate the etch rate, because, depending on the scanning style, re-deposition occurs, which drastically reduces the effective etch rate. Additionally, the sputtering yield is dependent on the angle of incidence: it roughly increases with 1/ cos(c), with c the angle between the surface normal and the ion beam direction.
The resolution of the milling process is a few tens of nanometres. The typical maximum aspect ratio of the milled holes is 10V20. In order to speed up the milling process, or to increase the selectivity towards different materials, gas-assisted etching (GAE) is used, where an etching gas can be introduced into the work chamber during milling. It will increase the etching rate and the selectivity towards different materials by chemically facilitating the removal of reaction products. This type of methods allowed the generation of trenches and hole on the nanostructure. There are problems and limitations in FIB Ablation, for example amorphisation, heating, ion implantation, alloying and dependence with crystal orientation.
Nanostructuring by materials deposition
Using this method, positive nanostructure can be made, as FIB enables the localized mask-less deposition of both metal and insulator materials. The principle is chemical vapour deposition (CVD) and the occurring reactions are comparable to, for example, laser induced CVD. Where the main difference is FIB have better resolution but with lower deposition rate.
The metals that can be deposited on commercially available machines are platinum (Pt) and tungsten (W). In the case of W, the organometallic precursor gas is W(CO)6. The deposited insulator material is SiO2, with 1, 3, 5, 7- tetramethylcyclotetrasiloxane (TMCTS) and oxygen (O2) (or alternatively water vapour (H2O)) as precursors. The deposition process is illustrated in figure 3(c); the precursor gases are sprayed on the surface by a fine needle (nozzle), where they adsorb. In a second step, the incending ion beam decomposes the adsorbed precursor gases. Then the volatile reaction products desorb from the surface and are removed through the vacuum system, while the desired reaction products (W or SiO2) remain fixed on the surface as a thin film. The deposited material is not fully pure however, because organic contaminants as well as Ga+ ions (from the ion beam) are inevitably included.
For generating 2D and 3D nanostructure, the chemical reaction products must be non-volatile and should have a lower sputter-rate than the pure substrate, similarly for the deposition to grow, the chemical reaction rate have to higher than the sputter yield rate or it would lead to trench formation.
FIB deposition enables the deposition of complex three dimensional shapes with overhanging features. The significant of this technique is the possibility to deposit features that extend beyond the already present or previously deposited structure underneath. In this way an overhang is created. As the deposition proceeds layer by layer, in each pass the new material extends a bit more over the previously deposited layer. Various three-dimensional structures have been fabricated using this technique.
For the overhanging depositions, there is a trade-off between having a high accuracy and cleanliness on the structure, and having a short processing time with lower accuracy. If a very fine low-current ion beam and a very small overlap between the deposited layers are used, a very accurate deposition is obtained with little or no debris underneath the overhanging parts. On the other hand, if high deposition rates are required, a higher-current ion beam is desired, which will result in a rougher definition of the deposition pattern and a larger amount of debris. Also 3D Nano-machining can be made using 3D CAD drawing, where pre-programming if all deposition parameters and predictive modelling of growth angle and width of cross-sections, but accumulation of errors are unavoidable.
Combination of materials removal and addition
Direct-write techniques, the high resolution of FIB in combination with a five-axis motorized stage and the possibility to mill and deposit materials yield a powerful tool for maskless micromachining. It should be noted that both subtractive (milling) and additive (deposition) machining are possible in the same machine. FIB deposition is compared to LCVD and micro stereolithography; FIB milling is compared to laser ablation and micro electro discharge machining (MEDM). Depending on the application, FIB compares very favourably to other direct-write micromachining techniques, especially in terms of resolution and accuracy. Its main limitation however, is the processing time involved to machine large structures: it is clear that the FIB deposition and etching rates are low. Dimensions up to some tens of micrometres are easily feasible, but above 100 ?m, the typical processing times become unacceptably high.
FIB is a powerful tool for the inspection of microsystem technology. It is relatively fast, and works on single dies as well as on entire wafers. The power of this approach lies in its versatility: virtually any material type can be milled and visualized in a flexible way, down to a sub micrometre scale. Furthermore, it is shown that FIB milling, implantation and deposition offer unique and powerful techniques for the fabrication of three-dimensional microstructures The main benefits of FIB micromachining are the high flexibility in the shapes that can be realized, and the attainable resolution (below100nm for deposition, and even lower for milling and implantation). The size of the structures that can be obtained is limited by the available processing time. The slow processing is the main drawback of FIB. Hence only relatively small structures (typically tens of micrometres) can be realized within a reasonable time. The technique is best suited for small-scale post-processing or prototype fabrication.