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Materials Engineering

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Focused Ion Beams



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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