An Introduction to Focus Ion Beam Nanofabrication

Plasmonic device of oligomer array directly fabricated into Au film by focused ion beam with excellent homogeneity and placement accuracy over a large area. The remaining ribbons between adjacent circles scale down to 30 nm in the 80 nm-thick Au film. Sample courtesy of University of Stuttgart, 4^th Physics Institute, and patterning done by Raith GmbH. Image credit: Raith

The focused ion beam (FIB) system, ranging from the industrial processing of semiconductors to fabricating the latest microfluidic devices in academic laboratories, has established itself as an indispensable tool for fabricating and controlling at the nanoscale. The system can operate on almost any material without needing a mask, directly fabricating 3D and 2D features spanning from a few nanometers to hundreds of micrometers in size.

Similar to a scanning electron microscope (SEM), FIB systems use a beam of ions, usually gallium ions rather than electrons. If this ion beam is scanned over a substrate, it will change the surface of the substrate in many different ways, enabling the material to be included, removed, or reformed, based on the properties of the beam and the substrate.

This makes FIB a highly flexible and versatile fabrication method that can be used for sample preparation, nanoscale machining and device processing. The main development of FIB instruments was driven by their exclusive capabilities for circuit alteration and computer chip repair in the semiconductor industry. However, FIB has been adopted by a much wider range of scientific and technological domains because of their ability to fabricate complex nanoscale structures and patterns. FIB instruments are presently being used to develop the latest generation of micro electromechanical systems (MEMS), microfluidic chips, photonic devices and nanopore membranes for DNA sequencing.

This article, based on Essential Knowledge Briefing published by Wiley: Microscopy and Analysis, provides an introduction to the nanofabrication applications of FIB, describing how the method works, detailing how patterning can be regulated, and summarizing the kind of structures it can create. The article also outlines numerous useful issues associated with the method, explains potential problems that may occur and how to resolve them, and also provides examples of how FIB is being employed by researchers in their study. Finally, it shows how the method is set to develop and advance in terms of technology as well as applications in the next few years.

History and Background

Thanks to its flexibility and versatility, FIB is a popular fabrication technique. This stems from the fact that scanning a focused beam of ions across a substrate can modify its surface in several different ways, all of which can be useful for nanofabrication. Eliminating and dislodging atoms is the most obvious way in which the ion beam can alter the surface. This enables the FIB system to etch patterns with a precision ranging from hundreds of micro meters down to a few nanometers without the need for a mask. In addition, the beam can also be used to position the material onto the surface as well as to embed ions into the surface, making changes to the chemical composition and physical structure of the substrate. Due to this versatility and flexibility, FIB finds applications beyond fabrication, albeit these will not be comprehensively discussed in this article.

Reflecting its similarity to SEM, FIB can produce high-resolution images of a surface by collecting the secondary electrons created when the beam of ions bombards the substrate. In addition, FIB is frequently used for site-specific preparation of samples, including almost any sample in material and biological sciences, for transmission electron microscope (TEM) investigation, by removing unwanted material and exposing exciting features.

The use of FIB can be traced to 1975, when G Roy Ringo and VE Krohn at the Argonne National Laboratory in Illinois, USA, initially produced an ion beam with high brightness (intensity) with liquid gallium as the ion source. Later, Laboratories in Malibu, USA, employed gallium as the ion source in the first scanning ion microscope which was effectively utilized for direct patterning. They created a 100 nm resolution beam and used it to mill 100 nm wide lines in a 40 nm thick gold film placed over a silicon substrate.

By the mid 1980s, developments in ion beam optics had allowed researchers to improve the resolution to 10 nm, which ultimately led to FIB’s acceptance by the semiconductor sector for analyzing and repairing circuits, and for machining devices. For example, the ion beam can be used to deposit conductive material in order to rewire it, or cut electrical connections on a circuit. More recently, academic researchers have used FIB to fabricate a wide range of nanoscale and microscale structures, such as microtools, nanorotors, nanopores, and superconducting thin film devices.

The exclusive capabilities of FIB derive from the complex phenomena that take place as the ions, fast-tracked at high energy, bombard the surface of a solid sample. Sputtering is one important physical effect, whereby the ions remove substrate atoms from the surface. When the beam is scanned over the substrate, it can remove the material and thus etch 2D and 3D shapes very precisely. This is also referred to as milling.

However, when a sample is hit by a high-energy ion beam, the entire surface atoms are not removed but a few atoms will remain on the surface and these will be triggered into an excited state. This effect provides mostly emission of secondary electrons that can be used for depositing materials onto a substrate through gas assisted deposition, which involves exposing the sample to molecules in the gas phase whilst targeting the surface with the ion beam. These materials can include metals such as platinum and tungsten, and also insulator materials such as silicon dioxide.

Photonic device (wavelength and coupler) in a silicon substrate created by direct milling with a focused gallium ion beam. Image produced by Raith for Professor Peng, Peking University, China. Image credit: Raith

Initially, a fine nozzle is used to spray a ‘precursor’ gas onto the surface: to deposit tungsten (W), for instance, tungsten hexacarbonyl (W(CO)₆) will be used as a precursor gas. The gas adsorbs onto the substrate, where it creates various products by reacting with the excited surface atoms, secondary electrons, and the ion beam. If these products are unstable, they will simply break free of the substrate and be emitted; however, some products may chemically etch the surface of the substrate or collect on the surface as thin films.

These thin films can either be a part of the desired structure or can be used as a ‘sacrificial layer’ to protect the underlying substrate from the damaging sputtering of the beam. The tiniest features that can be placed are usually in the order of 100 nm (lateral dimension), albeit in some situations this can be reduced to 20 nm, with a marginal thickness of approximately 10 nm.

The FIB systems can even be used to change a material’s surface by embedding ions. When high-energy ions hit the sample, besides sputtering atoms from the surface, some of them will enter into the top few nanometers, thereby getting stuck. Implantation can be employed to carefully dope a substrate with specific ions, such as using gallium ions to dope compound semiconductor devices

The physical structure of a material can also be altered by the dislodgment of surface atoms by the ion beam, along with implantation, making the material highly amorphous. In several cases, this is just an unnecessary damage and has to be ceased, but it can also have beneficial applications. Creating a mask with a distinct shape is one of the benefits. As this mask exhibits a more amorphous structure when compared to the other substrate areas, it can be selectively removed using chemical etching. These methods employing well regulated low-dose ion beam damage involve direct alteration of the sample properties or directing an ensuing processing step, for instance modifying the etch selectivity, rendering yet another fabrication alternative.

In existing FIB systems, liquid metal ion sources (LMIS) mostly produce the ion beam because they yield bright and highly focused beams (when coupled to the appropriate optics). While there are several different types of LMIS, the most extensively used one is based on gallium.

Gallium has several advantages when compared to other LMIS metals, including a low melting temperature (30 °C) and a low vapor pressure. The low melting temperature makes it easy to design and work the source, and the low vapor pressure ensures that evaporation is insignificant.

Magnetic domains defined by specific low-dose ion beam damaging (intermixing of multi-layers) without creating any surface topography. Reproduced with permission from Gierak J, Mailly D, Hawkes P, et al. Exploration of the ultimate patterning potential achievable with high resolution focused ion beams. Applied Physics A 2005;80:187–94 © Springer. Image credit: Raith

A process similar to electrospray forms the ion beam. In this process, the liquid gallium sprays from the tip of an electrically charged tungsten needle, resulting in ionization. The resultant ions are expedited to an energy of 5–50 keV and focused at the sample through electrostatic lenses. LMIS produces ion beams with high current density. An advanced FIB is capable of delivering tens of nanoamperes of current to a sample, or imaging the sample with a spot size of just a few nanometers.

Gallium is the most commonly used ion source for FIB, but there are also other options which can be more suitable for certain applications. These include metal alloys produced from combinations of the metals generally used for LMIS, such as gold with silicon or germanium. Ion beams produced using metal alloys have practical lifetime and stability, but truly gain from the fact that they enable processing with various ions.

Another option is gas field ion sources (GFIS). These form ion beams from hydrogen as well as noble elements, for example helium in the gas phase by condensing the gases onto the electrically charged tungsten needle for field ionization. GFIS can produce narrower beams than are possible with LMIS, boosting pattern resolution, but compromising some stability and a relatively short lifetime. In addition to electrospraying, such as plasmas and electron bombardment, there are also many ways to convert the sources into ions. For example, xenon plasma sources can produce beams that are better at milling than gallium beams as they have higher sputter yields.

In Practice

Among all the potential applications of FIB, nanofabrication requires the highest level of ion beam stability and control, because the process of producing fine patterns and detailed structures over stretched regions could take hours or even days. This means a consistent beam current and ion emission, and fine control over beam size and positioning

Normally, a focused gallium ion beam has an energy of approximately 30 keV, but this can range from 5 keV to 50 keV. The beam’s diameter (spot size) and its current can be as small as below 5 nm and 1pA, respectively. At the other end of the scale, the current can be about 100 nA and the spot size can go beyond 500 nm. Yet, for fabrication applications, beam currents toward the lower end (1 pA to 1 nA) tend to be utilized, because they provide the required resolution.

On the whole, the etching resolution is better when the beam has a smaller diameter. The diameter of the beam is mostly established by the beam current, with larger currents producing wider beams, and by the lenses focusing the beam. The current generation of FIB instruments can etch features below 10 nm, based on particular interactions with the sample and the beam size.

The other beam property that has an impact on resolution is beam profile, meaning how the intensity of the beam varies over its width. This profile contains a central peak of maximum intensity enclosed by tails of decreasing intensity; these tails should be as small as possible for high resolution fabrication

The ion dose, which means the quantity of ions delivered by the beam, is usually divided into several portions and delivered in several rounds. The number of repeated scans (loops or passes) can range from 100 to 1 million for gas-assisted deposition and from 1 to 10,000 for milling. For the former, there is usually a set delay, up to about 1 ms, between repeatedly scanned points; this is referred to as the refresh time.

Dwell time refers to the length of time that an ion beam stops at a particular scanned point and can vary from 50 ns to 1 second, whilst the distance between dwell points (step size or pitch) is usually 50 to 200% of the spot size.

The substrate is placed on a stage that can travel in three varying directions, with some stages also capable of tilting and rotating, whilst the ion optics focuses and shapes the ion beam and scans it across the surface of the substrates. Most of stages employ standard position encoding systems. Stages equipped with laser interferometer position control provide the precision and stability for large area nanofabrication. Meanwhile, dedicated FIB nanofabrication systems are integrated with such ultra precise stages, similar to electron beam lithography systems. The stage is usually oriented horizontally, with the ion beam placed vertically above it.

As the ion beam and the stage are moved, patterns defined with computer-aided design tools are etched into the substrate. The dwell points, dwell times, scan path needed to implement the designs are automatically established by the software supplied along with commercial FIB systems. This software can also take into account the composition of the substrates; substrates that are not as strong will require more rounds at shorter dwell times.

The main advantage of FIB for nanofabrication is its capability to perform direct patterning, preventing the necessity for complicated masks and pattern transfer processes. Therefore, the fabrication process is simpler, requiring fewer steps.

This helps to explain why the semiconductor industry was the first industrial user of FIB nanofabrication. A present, FIB is used by the semiconductor industry to define and modify prototype-integrated circuit devices, and also to test physical failures. The ion beam, similar to a micro-soldering iron, can precisely mill a cut to disconnect a wire, yet can also connect two pieces of wire by depositing conducting material in another area. Currently, regular procedures include rewiring interconnects for circuit editing or selectively deprocessing a certain area on a chip to inspect a defective component. All these can be carried out without causing much damage to the silicon substrate.

FIB is also being used by the hard disk drive manufacturing industry to trim magnetic write heads. While read-write heads are mostly fabricated by optical lithography, an additional FIB milling step shapes the magnetic strip on the wafer employed for writing.

Recently, FIB has also become a popular technique for developing high-precision microstructures on MEMS such as pressure sensors and actuators, as well as on photonic devices and scanning probe microscope tips. Milling, etching and deposition are FIB methods that are satiable for MEMS processing, as they enable direct high-resolution patterning on surfaces positioned at different angles, such as vertical or curved surfaces. In addition to surface modifications, milling can also achieve the entire fabrication of structures such as cantilevers.

At present, FIB is the only nanofabrication method that can produce complex, high- resolution 3D surface patterns on any type of solid material. To elaborate, FIB is capable of fabricating layered structures like nanoscale-stacked ‘junction’ devices, which has potential as photovoltaic devices containing two or more semiconductor materials stacked on top of a solar cell.

Plasmonic nano-antenna on top of a gold-coated AFM tip. Direct milling, i.e. without the need for using a resist, enables high-resolution features even on topographic samples. Image produced by Raith for Professor Moerner, Stanford University, USA. Image credit: Raith

Graphite is another layered structure that is amenable to FIB nanofabrication. It consists of atom-thick layers of pure carbon (graphene) heaped on top of each other. Graphene has unique electrical and electronic properties, and as a result it has recently been a subject of intense research. FIB 3D etching can create sub-micrometer stacked junctions that contain bulk graphite materials, and also nano devices such as 3D single electron transistor (SET) devices, graphene nanoribbons, and graphene-based ultracapacitors.

Sometimes, a fabrication operation is exclusively suited for FIB processing. For example, milling and 3D deposition methods can be employed to fabricate microfluidic devices containing a network of small channels, with diameters spanning from 1 µm to 100 µm, for fluids to easily flow through. These devices are used in a wide range of biological and chemical applications, such as cell culturing and DNA analysis.

Considering the small scales available on these devices, it is very difficult to accomplish the blending of fluids. This problem can be resolved by FIB by patterning refined 3D channel shapes that create mixing by exploiting the geometrical patterns in a single fabrication step.

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

For more information on this source, please visit Raith.