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1 Introduction of Focused Ion Beam (Spring 2013) Ningyuan Wang, 3550808 EEE-5425 Introduction to Nanotechnology Spring 2013 Abstract—The Focused Ion Beam (FIB) is a tool that using the focused Gallium ion beam directed towards the samples, and upon interaction it generates signals that are used to create high magnification images of the sample by mapping that signal to the beam position. The ions contain many times more energy and heavier than electrons on the surface of sample so when they impact a material they sputter away atoms from the surface. Thus, the FIB system can use as an etching tool. Also, a chemistry in gas form can be injected close to the surface and allow material deposition. An example of using all above three applications is doing the micro-sampling for Transmission Electron Microscope. Index Terms—Focused Ion Beam, FIB, Liquid Metal Ion Source, Gallium, Applications of FIB, FIB imaging, etching, deposition, Micro-sampling for TEM I. INTRODUCTION T HE Focused Ion Beam (FIB) system is a significant method of the Nanotechnology. Nowadays, the FIB is the only method that actually operating in Nano-scale. The minimum size of the metal deposition is typically around 15nm, and for etching, the size can be even a little bit small than deposition. The following sections will talk about some details about the FIB system, which including the history, instrument, and applications. TABLE I DETAILS OF FOCUSED ION BEAM FIB Particle Beam Penetration depth Average electrons Type Elementary charge Particle size mass size In polymer at 30 kV In polymer at 2 kV In iron at 30 kV In iron at 2 kV Secondary electron Ga+ Ion +1 0.2nm 1.210-25kg nm range 60 nm 12 nm 20 nm 4 nm 100-200 This paper is submitted on March 16th, 2013. Ningyuan Wang is with the Florida International University, Miami, FL 33199 USA (e-mail: [email protected]). II. HISTORY OF FOCUSED ION BEAM When the Focused Ion Beam (FIB) system first built, the technology using is not the Liquid Metal Ion Source (LMIS) which we use today. In 1975, the first FIB system based on field emission technology developed by Levi-Setti [1] and based on gas field ionization sources by Orloff [2] and Swanson. Also, in the same year, Krohn and Ringo [3] produced first high brightness ion source, which we still us today in the Focused Ion Beam system: Liquid Metal Ion Source. Then, three years later, in 1978, the first FIB based on an LMIS was built by Seliger et al [4]. III. INSTRUMENT The basic FIB instrument consists of a vacuum system and chamber, a liquid metal ion source, an ion column, a sample stage, detectors, gas delivery system, and a computer to run the complete instrument as shown schematically in Figure 1. The typical FIB system may have three vacuum pumping regions, one for the source and ion column, one for the sample and detectors, and a third for sample exchange. The liquid metal ion source in the ion column which can move vertically provides the finely focused ion beam that makes possible high lateral resolution removal of material. The detector is used to catch the sputtering electrons and ions for the imaging. Five axis motorized eucentric stage motion allows rapid sputtering at various angles to the specimen. The ion beam interaction with organo-metallic species facilitates site specific deposition of metallic. 2 There are several metallic elements or alloy sources that can be used in a LMIS. Gallium (Ga) is currently the most commonly used LMIS for commercial FIB instruments for a number of reasons: (i) its low melting point minimizes any reaction or inter diffusion between the liquid and the tungsten needle substrate, (ii) its low volatility at the melting point conserves the supply of metal and yields a long source life, (iii) its low surface free energy promotes viscous behavior on the (usually W) substrate, (iv) its low vapor pressure allows Gallium to be used in its pure form instead of in the form of an alloy source and yields a long lifetime since the liquid will not evaporate, (v) it has excellent mechanical, electrical, and vacuum properties, (vi) its emission characteristics enable high angular intensity with a small energy spread, and (vii) A high brightness is obtained due to the surface potential, the flow properties of the Gallium, the sharpness of the tip, and the construction of the gun which results in both ionization and field emission.[5] TABLE II CHEMISTRY PROPERTIES OF GALLIUM Gallium Melting point Boiling point Energy of first ionization Figure 1. (a) A schematic diagram of a basic FIB system. (b) A single beam FEI 200TEM A. Liquid Ion Source The LMIS has the ability to provide a source of ions of approximately 5 nm in diameter. Figure 2a shows a schematic diagram of a typical LMIS which contains a tungsten needle attached to a reservoir that holds the metal source material. Figure 2. (a) A schematic diagram of a liquid metal ion source (LMIS) and (b) an actual commercial Ga LMIS (courtesy of FEI Company). 1310 K 1448 K 1620 K 1838 K 2125 K 2518 K 302.9146 K, 29.7646 , 85.5763 2477 K, 2204 , 3999 578.6kJ·m-1 Vapor pressure 1 Pa 10 Pa 100 Pa 1 kPa 10 kPa 100 kPa B. Ga+ Ion Emission Ga+ ion emission occurs via the following two steps as shown in Figure 3: --First, the heated Gallium flows and wets a tungsten needle having a tip radius of ~2-5um. Once heated, the Gallium may remain molten at ambient conditions for weeks due to its super-cooling properties. An electric field applied to the end of the wetted tip causes the liquid Ga+ to form a point source on the order of 2-5 nm in diameter in the shape of a “Taylor cone.” The conical shape forms as a result of the electrostatic and surface tension force balance that is set up due to the applied electric field. --Second, once force balance is achieved, the cone tip is small enough such that the extraction voltage can pull Gallium from the tungsten tip and efficiently ionize it by field evaporation of the metal at the end of the Taylor cone. 3 IV. APPLICATIONS OF FOCUSED ION BEAM Figure 3 Ga+ ion emission C. The Ion Column Once the Ga+ ions are extracted from the LMIS, they are accelerated through a potential down the ion column. Figure 4 and Figure 5 are the construction of ion column. Typically, the ion column has two lenses. The condenser lens (lens 1) is the probe forming lens and the objective lens (lens 2) is used to focus the beam of ions at the sample surface. A set of apertures of various diameters also help in defining the probe size and provides a range of ion currents that may be used for different applications. In the middle, is the beam blanking. The beam blanking is used to prevent unwanted erosion of the sample by deflecting the beam away from the center of the column. Figure 4 Construction of typical ion column Figure 5 Real FIB ion column The basic applications of Focused Ion Beam system can be divided into three aspects: etching, deposition and imaging. They all depend on the reaction on the surface of sample when the ion beam impinges it. The ability to etch, image, and deposit material using a focused ion beam (FIB) instrument depends critically on the nature of the ion beam, solid interactions. Figure 6 shows a schematic diagram illustrating some of the possible ion beam or material interactions that can result from ion bombardment of a solid. Figure 6 Schematic diagram of the sputtering process and ion-solid interactions A. Etching Etching takes place as a result of physical sputtering of the target. An understanding of sputtering requires consideration of the interaction between an ion beam and the target. Sputtering occurs as the result of a series of elastic collisions where momentum is transferred from the incident ions to the target atoms within a collision cascade region. A surface atom may be ejected as a sputtered particle if it receives a component of kinetic energy that is sufficient to overcome the surface binding energy of the target material. The secondary electrons and secondary ions are collected and detected to form images. However, there is also limitation about the FIB etching. FIBs are most often used to create features of high aspect ratio. Sputtered material and backsputtered ions may therefore deposit on surfaces that are in close proximity to the active milling site. Thus, surface degradation due to redeposition of sputtered material must also be considered during FIB etching. Figures 7 is SEM images of three trenches observed at a 70° tilt. Figure 7 Rectangular FIB trenches in (100) Si milled at normal incidence by 4 applying: (a) single fluence, (b) double fluence, and (c) triple fluence of Ga+ ions at 25 keV. Therefore, controlling or at least predicting the manner in which redeposition of sputtered material will occur can be significant for the successful and rapid production of high quality specimens by FIB techniques. B. Deposition FIB material deposition is currently commonly employed for deposition of conductors and insulators for IC circuit edit and for deposition of material to mask or protect the sample surface during FIB micromachining for cross section analysis, for Transmission Electron Microscope (TEM) sample preparation and other micromachining applications. Fig. 8 shows the spatial relationship of the gas source, the focused Ga+ ion beam, the sample surface, and the volatilized and sputtered species. All deposited films contain not only the desired metals but also incorporate impurities from the incompletely decomposed precursor and contain Ga from the focused ion beam. The percentage of contamination can vary significantly depending on the deposition conditions, but in all cases, the amount of gallium and carbon is a significant amount of the total. TABLE IV COMMON DEPOSITION GASES AND SOURCE TEMPERATURE Metal Composition (metal:C:Ga:O) Resistivity(µohm-cm) W 75:10:10:5 150-225 Pt 45:24:28:3 70-700 C. 3D-Nanofabrication The three-dimensional nanofabrication technology, also can be seen as a special type of FIB deposition, has been jointly developed by Himeji Institute of Technology, NEC, and SII (Matsui, Kaito and Fujita, 2000). [6] Using the FIB-CVD technique allows the formation of 3D structures that cannot be made with existing technologies. Figure 9 shows the 3D fabrication of a spring coil structure. The spring coil was formed by rotating the ion beam at a 13s cycle. The diameter of the coil is 600 nm, while the diameter of the wire is 80 nm. Figure 8 Schematic drawing of deposition/controlled material removal process. The enhanced etch process is shown. If adsorbed gas decomposes to non-volatile products, then deposition will take place. For FIB induced material deposition to occur, a precursor must have two properties. The precursor must have a sufficient sticking probability to stick to a surface of interest in sufficient quantity, and it must, when bombarded by an energetic ion beam, decompose more rapidly than it is sputtered away by the ion beam. Of the many precursors that have been successfully used for FIB deposition, those whose FIB induced decomposition results in metal deposition (e.g. W, Pt, Au, Al, and Cu) are the most commonly used. The microstructure, composition and resistivity of the deposited metals varies. FIB deposited Pt and W films are amorphous, but FIB deposition of Cu and Au produces polycrystalline films.[5] W and Pt are by far the most commonly deposited metals and are routinely and mostly interchangeably used for semiconductor circuit edits and as a surface protection for SEM or TEM specimen preparation. TABLE III COMMON DEPOSITION GASES AND SOURCE TEMPERATURE Element W Pt Al SiO2 SiO2 C Precursor gas Tungsten Hexcarbonyl, W(CO)6 Methylcyclopentadienyl platinum trimethyl, (CH3)3(CH3C5H4)Pt Trimethylamine Alane, (TMAA), (CH3)3NAlH3 O2 and tetraethoxysilane (TEOS) O2 and tetraemethoxysilane (TMOS), Si(OCH3)4 Phenanthrene 55 30 (melting point) 25 , 1 torr vapor pressure Figure 9 3D fabrication of a spring coil. The example above that the FIB-CVD technique allows for forming three-dimensional structures at a fixed position, which could not be achieved using existing techniques. The SIM2000 series scanning ion microscope has a function of making FIB processing data automatically based upon the 3-D CAD data. This function allows users to form three-dimensional nanostructures very easily. An example of this function is shown in the Figures 9 and 10. First, the user makes a CAD file of any three-dimensional structure as shown in the Figure 9. This CAD data is divided in the Z-axis direction into several pieces of data to make a series of FIB processing data automatically. Then this series of data is loaded to the SMI2000 series scanning ion microscope and deposition begins. The SMI2000 uses the CAD data to automatically form the three-dimensional nanostructure as shown in the Figure 10. 5 2) Figure 9 3D CAD drawing of a feature. 3) beam for imaging the system operates as high resolution field emission SEM. The second imaging mode uses the ion beam while the electron beam is blanked. The FIB imaging mode is used for grain analysis, voltage contrast imaging, and defining of milling areas. It also reveal chemical differences, and are especially useful in corrosion studies, as secondary ion yields of metals can increase by three orders of magnitude in the presence of oxygen, clearly revealing the presence of corrosion. The last imaging mode is the so called CrossBeam operation mode: Both beams are turned on and while the ion beam is milling a defined area, the SEM is used to image the milling process at high resolution in real time. This enables the operator to control the milling process on a nanometer scale and to perform extremely accurate cross sections and device modifications. Figure 10 3D FIB fabrication performed automatically from the CAD drawing in figure 9. D. FIB Imaging The FIB instrument is similar to a scanning electron microscope (SEM), except that the beam that is rastered over the sample is an ion beam rather than an electron beam. Secondary electrons are generated by the interaction of the ion beam with the sample surface and can be used to obtain high-spatial-resolution images. Most modern FIB instruments supplement the FIB column with an additional SEM column so that the instrument becomes a versatile “dual-beam” platform, as shown in Figure 11 for imaging, material removal, and deposition at length scales of a few nanometers to hundreds of microns. Figure 12 Secondary electron imaging (left) and secondary ion imaging (right). V. MICRO-SAMPLING FOR TEM The FIB micro-sampling technique is developed to remove a portion of the micro-sample with a thickness of about 0.1µm for TEM observation. The process of lifting out the micro-sample, mounting the micro-sample on a special carrier, and thinning of the micro-sample by FIB milling are performed in the FIB system continuously. All of the preparing steps are accomplished under vacuum in the same FIB system. The procedure for the FIB micro-sampling is schematically illustrated in Figure 13. Figure 11 Schematic illustration of a dual-beam FIB–SEM instrument. Expanded view The CrossBeam system can operate at three different imaging modes. 1) By blanking the ion beam and only using the electron Figure 13 Schematic illustration of the FIB micro-sampling technique. 6 First, the region of interest is covered with a protective layer of W or Pt by FIB assisted deposition. Then the sample is deep trenched by FIB milling to remove a portion of sample (micro-sample) from the region of interest (Figure 13 a). Next, the micro-sample is separated from the bulk sample and lifted out using a micromanipulator (Figure 13b). The micro-sample is then mounted onto an edge of a micro-sample carrier (Figure 13c). Finally, the mounted micro-sample is secured by an FIB assisted metal deposition (Figure 13d). The micro-sample carrier is mounted on an FIB/TEM(STEM) compatible specimen holder so that thinning of the micro-sample can be performed just after securing it to the micro-sample carrier. [7] A series of FIB images showing the procedure to prepare a micro-sample is shown in Figure 14. localized and characterized by TEM observation. After the plan view TEM observation, the micro-sample is transferred back to the FIB system for in order to prepare a cross sectional TEM specimen. In the FIB system, the carrier is rotated so the incident ion beam is perpendicular to the micro-sample (Fig. 15f). Then, the micro-sample is separated from the carrier by FIB milling, subsequently transferred to the surface of a flat substrate material (Fig. 15g-h), and secured with FIB assisted metal deposition (Fig. 15i). Finally, the site to be characterized is thinned for cross sectional TEM observation (Fig. 15j). [8] Figure 15 Procedure for the micro-sampling of a specific site for cross sectional and plan view TEM observation Figure 14 Series of FIB images showing the procedure for the FIB micro-sample preparation. First, the area to be examined is localized (Figure 4a). Then, metal (W or Pt) is deposited as the protection layer over the region of interest (Figure 4b). The thickness of the deposition layer is normally 0.5 to 1µm. Next, the front side of the region of interest is deep trenched by FIB milling (Figure 4c). After that, the back side and both right and left sides of the region of interest are deep trenched leaving a micro-bridge at the upper left corner of the sample (Figure 4d). The sample is then tilted 60° and the bottom of the sample is milled to separate the bottom portion of the sample from the bulk sample (Figure 4e). The bulk sample is then tilted back to normal to the ion beam (Figure 4f), and a micromanipulator probe is bonded to the micro-sample with W deposition. The size of the deposited W bonding layer is about 2µm2µm and the thickness of the layer is approximately 0.5µm. After the probe bonding, the micro-bridge is cut off with FIB milling to completely separate the micro-sample from the bulk sample (Figure 4g). Then, the micromanipulator probe is elevated to lift out the micro-sample (Figure 4h). The milling and bonding operations are performed using a Ga+ ion beam at an accelerating voltage of 30kV. The beam currents and the beam diameters of the Ga+ ion beam used in these operations ranged from 10-20 nA and 400- 600 nm, respectively. Figure 15 shows the procedure to prepare the specimen for both cross sectional and plan view TEM observation of the same site. First, a micro-sample is prepared from the specific site for plan view TEM observation (Fig. 15a-e). The site is REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] W. H. Escovitz, T. R. Fox and R. Levi-Setti (1975). "Scanning Transmission Ion Microscope with a Field Ion Source". Proceedings of the National Academy of Sciences of the United States of America 72 (5), pp. 1826-1828. Orloff, J. and Swanson, L., (1975). "Study of a field-ionization source for microprobe applications". J. Vac. Sci. Tech. 12 (6), pp. 1209-1214. Jon Orloff and et al, “High Rsolution Focused Ion Beams: Fib and Its Applications: The physics of Liquid Metal Ion Sources and Ion Optics and Their Application to Focused Ion Beam Technology,” Kluwer Academic/Plenum Publish, 2002 edition, pp. 24. Seliger, R., Ward, J.W., Wang, V. and Kubena, R.L. (1979). "A high-intensity scanning ion probe with submicrometer spot size". Appl. Phys. Lett. 34 (5), pp. 310-312. Lucille A. Giannuzzi, “Introduction to Focused Ion Beams,” Springer, 2005, pp. 5. Matsui S, Kaito T, Fujita J, Komuro M, Kanda K and Haruyama Y, “Three-dimensional nanostructure fabrication by focused-ion-beam chemical vapor deposition”, J. Vac. Sci. Technol. B 18, pp. 3181, 2000. Giannuzzi LA, Drown JL, Brown SR, Irwin RB, Stevie FA, “Specimen preparation for Transmission Electron Microscopy of Materials”, Mater. Res. Soc. Symp. Proc. Vol.480, pp. 19, 1997. Yaguchi T, Kamino T, Sasaki M, Barbezat G and Urao R, “Cross sectional specimen preparation and observation of a plasma sprayed coating using an FIB/TEM system,” Microscopy and Microanalysis 6, pp. 218-223, 2000. Ningyuan Wang was born in Shijiazhuang, Hebei Province, China in 1989. She received B.S. degrees in electrical engineering from Florida International University, Miami FL, USA and Hebei University of Technology, Tianjin, China in 2012. From 2010 to 2012, she is an 7 undergraduate student in Florida International University. Her work several project with partner during the study. Including, a power project which using three different power supplies, wind, solar and battery, to accomplish a car runaway-resistance system. And a guiding robot based on RFID technology and Kinect to detective the path.