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Microscopy Lab Report 7
Introduction to Focused Ion Beam Spectroscopy
By: Crystal Patteson
As part of a series of laboratory exercises designed to teach students advanced microscopy techniques, I was asked to learn and relate key physical concepts used in Focused Ion Beam (FIB) spectroscopy. A FIB is a scientific instrument that resembles a scanning electron microscope (SEM) that, unlike an SEM, uses a focused beam of ions (instead of electrons) for site -specific analysis, deposition, and ablation of materials. To familiarize myself with key concepts, I made sure to pay careful attention to features such as high-resolution imaging (focusing and stigmatism correction), sputtering of arbitrary patterns in a Silicon wafer, deposition of Tungsten patterns on a sample, and understanding the limitations of FIB patterning. For the purposes of this lab, a Hitachi FB-2100 and silicon wafer was used.
The sample was already mounted and prepared for observation at the beginning of lab from the previous class. To begin, the instructor proceeded to carefully explain the role of the main components of the FIB (Fig. 1-4). Figure 1 showcases an outer view of the FB-2100 while identifying many of the main components. For example, an ion beam source that compromises the top section of the column can be readily identified. This column serves to support the liquid-metal gallium ion source so that emitted ions can transverse the interior of the column. Along the way, ions encounter converging, focusing, scanning, blanking, and astigmatism correction lenses and apertures that are used to control the dimensions and trajectory of the beam as it travels to meet the interface of a loaded sample. Figure 1 also shows the specimen stage and chamber which make for easy loading and transfer samples. For instance, to avoid losing a nicely prepared Transmission Electron Microscope (TEM) sample during manual transfer from the FIB sample holder to another SEM or scanning TEM (STEM), the Hitachi FB2100 offers a compatible sample holder which can be inserted into the both FIB and the SEM/(S)TEM units. Thus, the FIB can be used to carefully mount the specimen on a compatible holder in-situ and then safely and quickly transfer the sample to another complementary microscope. An ion pump is used to clear the chamber of lingering ions and a beam limiting aperture is available to limit the size of the ion beam.
Figure 2 shows a deposition, or “gas injection”, system which holds three different gasses that share a single injection needle. This system provides the ability to deliver one of three selected deposition and etching gases through a single needle, one at a time. The gas injection needle is positioned near the sample surface where it came be easily employed. Intuitive software is used to control the needle position and allow the user to manipulate settings for custom applications of gas induced deposition and etching. Figure 3 is similar to Figure 1, but includes a look at the console and control panel. Figure 4 shows two roughing pumps connected to a vibration isolator to limit vibrations coming into the machine.
To demonstrate the high resolution capabilities of the FIB, images were taken at the following magnifications: 1000X, 5000X, 20,000X, and 100,000X (Fig. 5). Then to exhibit the etching and milling capabilities of the FIB, I etched a rough picture of a rabbit with the drawing tools and then bombarded the sketched out image with the ion beam (Fig. 6). Lastly, to establish the narrowest possible line possible, I etched a single line and measured its width to be 51 nm (Fig.7). Images were acquired by rastering the ion beam across the sample and then saving the resulting scan.
Specimen chamberSpecimen stageFIB columnIon sourceIon pumpBeam limiting aperture
Figure 1. The FB-2100 System.
Figure 2. The OmniGIS Deposition System as installed on a Hitachi FB2100 FIB.
ConsoleEvacuation control panel
Figure 3. The FB-2100 System.
Figure 4. Two roughing pumps connect to a vibration isolator.
Figure 5. Top left) Sampled is imaged at 1000X. top Right) Sampled is imaged at 5000X. Bottom left) Sampled is imaged at 20,000X. Bottom right) Sampled is imaged at 100,000X.
Figure 6. Sputtering was used to etch a rabbit on the samples
Figure 7. A 51 nm wide line that has been etched into the surface the sample.
What is astigmatism?
Crisp, clear, and accurate images are created when the electron beam is circular as it approaches the specimen. Sometimes, the probe cross section can become distorted and forms an ellipse. This can be due to the machining accuracy, the material of the pole-piece, and imperfections in the casting of the iron magnets and the copper winding. This elliptical distortion is called astigmatism and is due to perpendicular axis’s having different focal lengths. A stigmator is used to correct for astigmation and make the electron beam circular. Electromagnetic coils are placed in quadruple, sextuple or octagonal orientations inside of the microscope.
What is the purpose of the extraction voltage? What is its effect on the beam current?
The gallium tip of the FIB is heated until the melting point of the Gallium is met. The Gallium drips down the tip of a tungsten needle where the opposing forces of surface tension and the electric field forms a Taylor cone. The electric field causes the ionization and field emission of the gallium ions. The extraction voltage is used to form this field. Therefore, a higher extraction voltage leads to a higher current since more ions will be able to break free and leave the source.
Is FIB always appropriate to characterize/pattern surface thin films? What are the limitations?
While a Fib has the ability to cross-section small targets, offer fast resolution imaging and precise milling, and serve as a good SEM sample prep, a major drawback of FIB imaging and machining is due to the damage caused by the ion beam. Depending on the material of the sample and the temperature within the chamber, ion beam damage can take the form of sample surface amorphization, point defect creation, dislocation formation, phase formation, grain modification, and other unusual effects. The imaging process itself may spoil subsequent analyses through beam damage that leads to lower resolution and the ion beam can implant residual Gallium into a sample and thus contaminate it. An understanding of the material properties should first be examined to determine if FIB spectroscopy is appropriate for a thin film.
How would you experimentally characterize the sputtering rate of a given material?
Sputtering refers to the process of ejecting particles from a solid from the bombardment of energetic ions. Thus, sputtering is used to etch samples because the incoming ions weaken the bonds in the sample. The electronic stopping power of the ions cause electronic excitations to the sample which leads to the breaking of bonds between atoms. Depending on the material, the strength and longevity of these excitations may vary. For example, the electronic excitations in an inductor are not quenched nearly as fast as they would be in a conductor.
Why does it take up to 30 minutes to form the Ga tip?
You have to wait for the Gallium to melt and cover the tungsten tip as previously described in a previous question.
Why is it important to have several apertures when doing imaging/sputtering?
The more apertures you have the smaller the beam size one can acquire. The smaller the beam size is the greater the resolution capabilities for imagining as well as creating a more refined beam for precision sputtering.
7 I etched a single line and measured its width to be 51 nm (Fig.7). Images were acquired by rastering the ion beam across the sample and then saving the resulting scan.
Figure 5. Top left) Sampled is imaged at 1000X. top R