Source: Sina Shahbazmohamadi and Peiman Shahbeigi-Roodposhti-Roodposhti, School of Engineering, University of Connecticut, Storrs, CT
As electron microscopes become more complex and widely used in research labs, it becomes more of a necessity to introduce their capabilities. Focused ion beam (FIB) is an instrument that can be employed in order to fabricate, trim, analyze and characterize materials on mico- and nano-scales in a wide variety of fields from nano-electronics to medicine. FIB systems can be thought of as a beam of ions that can be used to mill (sputter), deposit, and image materials on micro- and nano-scales. The ion columns of FIBs are commonly integrated with the electron columns of scanning electron microscopes (SEMs).
The goal of this experiment is to introduce the state of the art in focused ion beam technologies and to show how these instruments can be used in order to fabricate structures that are as small as the smallest membranes that are found in the human body.
FIB systems use a beam of ions to mill, deposit and image micro- and nano-scale samples. The beam is formed in a high-vacuum environment where selective electric potentials are used to ionize and extract gallium from a liquid metal ion source (LMIS). This beam can be directed and focused with electromagnetic lenses similar to light in a traditional, optical microscope. The beam then rasters to cover an area on the sample. With a different kind of source, an electron beam can be used for nondestructive imaging and characterization without sputtering the sample surface, much like scanning electron microscopy (SEM). The combination of SEM and FIB paves a path for very innovative ion beam millings and characterization. Additionally, three-dimensional information can be obtained by combining the electron and ion beam operations to perform a tomography (i.e. mill a slice with ion beam, image with electron beam, and repeat). Generally, conductive samples are ideal for FIB and SEM because they do not collect charge and thereby affect the pathway to imaging, milling, and deposition. However, non-conductive samples like most polymers and biological samples can be probed with the use of charge correction, conductive coating, variable pressure settings, and low energy beam settings. Having an understanding of the basics of ion beam-solid interactions may improve the ability to achieve optimal results using an FIB system. The mechanics of ion beam-solid interactions consists in the following events: primary ions of the focused beam bombard the surface, sputter material, eject secondary electrons and implant themselves.
Milling occurs due to the physical sputtering of the target. In order to understand the sputtering process, the interactions between the ion beam and the target must be explored. Sputtering takes place as a consequence of a series of elastic collisions in which momentum is transferred from the incident ions to the target atoms within a region that is called cascade region. This process is similar to what happens when a cue ball hits the object balls when the break shot is taken. An atom on the surface of the target may be sputtered if it receives a kinetic energy that exceeds its surface binding energy (SBE). The surface binding energy is the energy required to remove a surface atom from its bulk lattice. A portion of these ejected atoms might be ionized. Because of ion bombardment, inelastic interactions can also happen. These interactions produce phonons, plasmons in metals, and secondary electrons (SE). A standard FIB employs secondary electrons in order to produce an image. Deposition can also be accomplished by deploying small amounts of precursor gas molecules to the surface of the material and using the impinging ions to facilitate a chemical reaction where the material is deposited onto the surface. Though, for this study, milling and imaging are the only mechanisms covered.
1. Fabrication of a perforated filter from a 300nm thick silicon oxide membrane comparable in scale to the kidneys' endothelial cytoplasm
- Load the as-prepared membrane into the FIB chamber. The membranes are often prepared by professionals (when creating Wheatstone bridges) and can be acquired at semiconductor fabrication sites. To prepare one yourself, photolithography must be used. The details of this process can be seen in the photolithography video of the "Bioengineering Collection" on the JoVE website. NOTE: Be certain to wear nitrile gloves when handling the sample or when coming in contact with any internal components of the FIB/SEM. The environment must be kept very clean and free of any skin oils.
- Turn the focused ion beam and electron gun on and adjust the sample to achieve the coincident-eucentric point. This is the point where the area of interest (the membrane) is in the line of electrons and ions for tilt angles ranging from 0-54 degrees.
- Adjust the ion beam current and accelerating voltage of the FIB to 30kV and 100pA and focus on an area close to the area to be milled. Draw a matrix of circles through the FIB milling program of a diameter around 50nm with a center to center distance of 150nm (see Figure 1).
- Change to electron beam and image the area at an accelerating voltage of 5kV.
Figure 1: FIB milled holes in silicon oxide membrane creating particle filter.
2. Milling a logo on a hair
- Put a hair strand on a microscope stub using carbon tape
- Gold/Carbon coat the hair strand using a sputter coater. This tool coats the sample in a few nanometers of a conductive material so it can be imaged/sputtered with minimal charging artifacts.
- Turn the focused ion beam and electron gun on and adjust for the coincident-eucentric point.
- Adjust the ion beam current and accelerating voltage to 30kV with a 100pA aperture, respectively, and focus on an area of about 15um x 15um close to the area to be milled.
- Load the pattern/logo to be milled as a bitmap and adjust the beam current and accelerating voltage and start the milling.
- Change to electron beam and image the area. This is shown in Figure 2.
Figure 2: "Happy Holidays" milled on a spider web with FIB.
The Focused Ion Beam is an instrument that can be used to fabricate, trim, analyze, and characterize materials on micro and nano scales. Focused Ion Beams are used in a wide variety of fields, ranging from electronics to medicine.
Focused Ion Beam Systems accelerate Liquid metal ions in a vacuum to form a beam. Using a series of Electromagnetic lenses, the beam can be focused onto an area of about 10 nanometers in diameter. When the ions from the Focused Ion Beams strike the target, some of the target material is sputtered.
At Low primary beam currents, very little sputtering occurs and the beam can be used for imaging. At higher currents, Surface atoms are ejected. This allows for Site-Specific sputtering or larger scale milling of samples.
Focused Ion Beam Systems create a beam of Liquid metal ions under vacuum in order to mill material from a sample or take an image of it. Inside the Focused Ion Beam System, Liquid metal ions, usually Gallium, are extracted from a filament. The ions are accelerated through application of voltage, and then a series of Electromagnetic lenses focuses the beam on the target. The metal ions collide with the material in the sample much like a cue ball does when striking billiard balls. At low energies, a metal ion knocks away secondary electrons, which can be collected to form an image of the target surface. At higher energies, the ions may transfer enough kinetic energy to atoms in the material to overcome their surface-binding energies and scatter into the vacuum. This is known as Sputtering.
Focused Ion Beams can use sputtering to bore holes at specific sites, mill patterns onto a target, or even remove the surface layer from a sample. By repeatedly and uniformly removing a layer and the imaging the region, three-dimensional images of a sample can be constructed. A percentage of the metal ions used by the beam are implanted in the sample. After the initial impact, an ion continues to lose energy through a series of collisions until it stops inside the sample. Chemical Vapor Deposition can also be accomplished by deploying small amounts of Precursor gas molecules to the surface of the material and using the impinging ions to facilitate a chemical reaction, wherein the Precursor gas breaks down and a portion of it is deposited onto the surface along with some of the impinging ions. Due to the accumulation of metal ions on or within the material, and scattering of secondary electrons from the surface, it is possible that charge can build up on a non-conducting target.
This accumulation of charge can create additional electrostatic fields that alter the beam path. One way to prevent this is by coating non-conducting samples in a conducting material such as Gold, Gold-palladium, or Carbon, before using the Focused Ion Beam System. A standard Focused Ion Beam takes an image of the sample by collecting the scattered secondary electrons from the ion interactions. It is also common to include a Scanning Electron Microscope Beam in the same chamber as the Focused Ion Beam.
For these combined systems, once the Focused Ion Beam has finished, the Scanning Electron Microscope is used to take an image of the sample. The two beams are arranged at a 54 degree angle relative to one another. The sample must be at the focal point of both the ion beam and the electron beam. This is known as the Coincident-Eucentric Point. In the next section, we will use a Focused Ion Beam to mill a logo onto a hair in order to demonstrate the remarkable precision of the technique.
Be sure to wear nitrile gloves when handling the sample or touching internal components of the Focused Ion Beam Scanning Electron Microscope.
In this experiment, we will mill the JoVE logo onto a hair. First, stick a strand of hair onto a microscope stub using Carbon tape. Before the hair can be milled, it must be coated in a conductive material. Using a Sputter Coater, coat the hair in a for example nanometers of Gold-palladium. Once the hair is coated, we can load the sample into the Focused Ion Beam. Place the Microscope stub containing the hair into the Focused Ion Beam Loading chamber.
Once the sample is loaded and the Imaging Chamber is pump down, turn on the Focused Ion Beam and the Electron Gun. At a low magnification, and using Secondary Electron Imaging, orient the sample to achieve the Coincident-Eucentric Point. This is typically performed at a five millimeter Working Distance and a 54 degree Stage Tilt.
To find the Eucentric Point, adjust the Upward Stage Motion in the direction of the Tilt or along the m-axis. There should be no loss of field view when the Stage is tilted from zero to 54 degrees. Adjust the Ion Beam Accelerating Voltage to 32 kilovolts, the Aperture current to five picoamperes in order to focus the Beam, and the Dose level to two.
Focus on an area of about 15 micrometers by 15 micrometers. This is where we will mill our logo.
Now adjust the Aperture current to 700 picoamperes to mill the logo. Load the pattern to be milled into the Focused Ion Beam. In this case, the JoVE logo is created using the text function. Once the logo is loaded, begin the milling process. Depending on the complexity of the logo, this process will take between 15 and 30 minutes. Once the milling is complete, an image of the hair can be taken.
Change from the Focused Ion Beam to the Scanning Electron Microscope. Change the angle so that the image is now perpendicular to the SEM and image the area at an Accelerating voltage of five kilovolts. When this process is complete, you are ready to examine the image.
As you can see, the Focused Ion Beam has milled the JoVE logo onto a single strand of hair.
This image demonstrates the precision milling capabilities of Focused Ion Beams. The width of the logo is approximately 30 micrometers by 10 micrometers, with a pixel size of 30 nanometers.
Now that you're familiar with the capabilities of Focused Ion Beam Systems, let's look at some ways Focused Ion Beams are used. Three-dimensional images of Microstructures within a sample can be created through Tomographic Imaging.
The Focused Ion Beam mills a layer of the sample and then an image is taken of the exposed surface. This image of the structures in a section of rat brain consists of 1,600 images, with a depth resolution of five nanometers.
Focused Ion Beams can provide a means for the Nanofabrication of Ohmic Contacts in layered Semiconductors. Through the use of a Precursor gas, sputtering of Semiconductor surface and ion implantation are prevented. The metal ions are deposited on the surface to provide current pathways.
You've just watched JoVE's Introduction to Focused Ion Beams. You should now understand the Principles behind Focused Ion Beams and their Interactions.
You should also be aware of many of the primary applications of Focused Ion Beam technology, which include Imaging, Milling, Sample characterization, and Ion deposition.
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Applications and Summary
This experiment demonstrated how using electron microscopes and focused ion beams enable researchers to manipulate and fabricate microscale structures. The molecular nature of the focused ion beam-material interaction provides FIB with a unique ability to manipulate materials on the micro- and nano-scales. By carefully considering how the beam interacts with the material, mitigating charging artifacts and setting the system for optimal milling quality, a researcher can produce unique patterns on biological and non-biological materials that can, in the case of silicon oxide membrane, perform just like its anatomical counterpart. FIBs show a lot of potential in this area of research but techniques and the materials used should improve a lot more for finding their way into the living organisms. These instruments and techniques alongside tissue engineering techniques can revolutionize the way we approach treatment of the organs in the near future.
This experiment focused on giving an introduction to focused ion beam (FIB) systems and demonstrating what they can do. Their applications are vast. The exercises here highlighted some applications in biology, which can range from micron size cross sectioning to the examination of bone and tissue to three-dimensional reconstruction of small parts of an organ. It is important to note that FIB is not just a tool for tissue engineering. It has much history with microelectronics, geological studies, additive manufacturing, spray coatings, transmission electron microscopy (TEM) sample preparation and general material characterization. Examples within these topics are widespread and can be found in any FIB-related literature.