The Emerging field of nanoscale science, engineering and technology
The Emerging field of nanoscale science, engineering and technology – that is the ability to work at the atomic, molecular and supramolecular levels, to create large structures with fundamentally new properties and functions have lead to an unrivalled understanding and control over basic building blocks of all natural and man-made things [roco]. This rapid advancement has lead to an increased demand for technological development on a nanoscale, which has brought about the birth and improvement of infrastructural changes aimed at representing and observing these features. The world wide focus over this time has been the evolution of methods including SEM (Scanning Electron Microscope), TEM (Transmission Electron Microscope), FIB (Focus Ion Beam) etcetera for the detailing of features at the nanoscale.
1.1 History of the Focus Ion Beam (FIB) Technology
Focus Ion Beam (FIB) systems have been commercially produced, mostly for manufacturers of large semiconductors for about 20 years [www.fibics.com]. In 1982, Anazawa et al. produced a 35Kv Ga- source and about three years later Orloff and Sudruad proposed FIB system for implantation and lithography [sudruad], even though as of 1959, Feyman had suggested the use of ion beams [www.nanofib.com]. In 1985, Kato et al. have pointed out the advantages of the FIB technology in the fabrication of sub-micro structures.
1.2 Operational Overview
The operation of the FIB are same as that of SEM (Scanning Electron Microscope), except that the focus ion beam system employs the use of focussed beam of ions instead of beam of electrons utilised in the SEM systems.
Commercialised nanoscience is limited by availability of tools. Using focussed ion beam system allows specified fabrication and imaging abilities which reduces greatly the characterization cycles and development required in the nano-technological field by scientist. The capabilities within focus ion beam ( FIB) are valued highly for rapid prototyping application. The deposition combination / direct etching of FIB in combination with digitally addressed patterning system allows nano prototyping engine with capabilities that will help researches in nano technology , because the operation of FIB is on both micro and nano scale, it can be used in creating the required structures.
FIB has precised control over deposition and milling parameter and as such, it is the proper tool for creating small structures for nano technology in the top –down approach. It is a highly flexible, mask-less technique which is fast for serial techniques, thus allowing the FIB instrument very efficient for design modifications. Most conventional methods of sample preparation used today in life sciences are compatible with investigations by using FIB.
1.3 Using Focus Ion Beam Systems
The direct applicability obtained in using FIB instrument is highly relevant in industrial applications. FIB instrument and its application have contributed immensely to industrial researches carried out in several analysis laboratories – For instance in the polymer industry, metallurgy industry, nuclear research etcetera. The ability to image, mill and deposit material by using FIB instrument depends largely on the nature of the ion beam- solid interactions. Milling occurs as a result of physical sputtering of the target. In understanding the mechanism of sputtering we need to consider the interaction between an ion beam and the target. Sputtering usually takes place when there is elastic collision in series when momentum is transferred from the incident ions to the target atoms in the region of collision cascade. Ionization of a portion of the ejected atoms can be collected for mass analysis or image formation. Production of plasmons (in metals), phonons and emission of secondary electrons can occur as a result of inelastic scattering. Imaging in the focus ion beam is carried out by detecting the secondary ions/electrons typically, sputtering in focus ion beam processes occurs within energy ranges that are dominated by nuclear energy losses.
Focus Ion beam devices are used to scan the surfaces of samples using simple focussed ion beams. The detection of secondary ions allows the processed surface of samples and microscopic images to be observed. The ion beam is generated by using liquid metal ion source (LMIS) when a beam of ion is irradiated on the surface of a specimen by finding the secondary ions with a detector – a two dimensional distribution which shows the microscopic images of the surface of the specimen can be observed.
1.4 The Focus Ion Beam Instrument
The Operation of the FIB technology uses a similar principle as the SEM (Scanning Electron Microscope) / TEM (Transmission Electron Microscope) but differs in the use of ions and this introduces consequences of enormous magnitude for interaction which occur at the surface of the specimen. Using Focus Ion Beam (FIB) instrument involves two major parameters – penetration of ion into material and the rate of sputtering of ion of the material.
When the emitted liquid metal ion source (LMIS) primary ion beam hits the surface of the specimen, it splutters a small amount of material this will leave the specimen surface as either neutral atoms or secondary ions – Secondary beams are also produced using the primary beam. Signals from the sputtered ion or secondary electron are collected to produce an image as the primary beam raster on the specimen surface.
Liquid metal ion source (LMIS) development is crucial for the development of Focus Ion Beam (FIB) [www.dspace.cam.ac.uk] , application of electric field that are very high into a steering quadrupole, octupole deflector, two electrostatic lenses in the column to focus ions in a beam and scan the beam on the specimen. Liquid metal ions source (LMIS) generates ions; these ions are focussed on electrostatic lenses. When specimen surfaces are bombarded using ions that have been extracted from the liquid metal ion source (LMIS) this generates ions, secondary electron and sputtered material and the various generated items serve different purpose in the focus ion beam.
At high primary currents a large amount of material can be removed by sputtering thus allowing precision milling of the specimen down to the submicron scale, while less material is removed at low primary beam currents. The use of ions in focus ion beam instruments means that they cannot penetrate with ease individual atoms of the specimen because ions are large. So interaction usually occurs within outer shell interaction which causes chemical band breakage of the substrate atom and atomic ionization. Inner shell electrons of the specimen cannot be reached by an incoming ion. The probability of an interaction with atoms that are within the specimen is much higher because of the large ion size and this result in rapid loss of energy of the ion. This means that the depth of penetration is much lower.
It should be noted that the main advantage of the Focus Ion beam is its ability to produce image of the sample after which it mills the sample precisely away from the areas that are selected[ ].
1.41 Ions in Operation
Ions are slower when paired to electrons for the same energy, because they are much heavier as a result Lorenz force is lower, so the use of magnetic lenses is less effective, and as such the focussed ion beam instrument is equipped with electro static lenses. Ions are positive, slow, large and heavy; so the resulting ion beam will remove atoms from the substrate and because the size, beam position and dwell time are well controlled, it can be used in the removal of materials locally in a manner that is highly controlled down to the nanoscale. As a result of the actions due to the ions used in the Focus ion beam instrument, fabrication and imaging functions are derived. The fabrication function occurs due to the sputtering while the imaging function arises due to the ions and secondary electrons.
1.42 Gallium (Ga+) Ions
The gallium ions are used in the focus ion beam (FIB) instruments for the following reasons [fei];
Due to its surface potential it exhibits very high brightness, the tip sharpness, the flow properties of the gun and the gun construction which results in field emission and ionization. This is an important result for the focussed ion beam. It should be noted that whatever chosen material should be ionized before the formation of the beam and then accelerated.
The element Gallium is metallic and because of its low melting temperature is a very convenient material for compact gun construction with limited heating.
Gallium is the centre of the periodic table and exhibits an optimal momentum transfer capability for a wide range of materials, lithium which is a higher element will not be sufficient in milling of heavier elements.
Gallium element has low analytical interference
2.0 Focus Ion Beam System
In the figure below, the FEJ 200 series type F113 of the FIB system is represented. In the figure are the various components of the system which includes the column, the specimen chamber and the detector;
This is situated above the specimen chambers. It is made up of two electrostatic lenses, a set of beam blanking plates, liquid metal ion source (LMIS), a beam acceptance aperture, steering quadrupole, beam defining aperture and an octupole deflector.
2.2 Lens System
Coming from the source, the beam goes through a beam acceptance aperture after which it goes into the first lens. Above the beam- defining aperture (BDA), the quadrapole adjust the position of the beam in a manner as to allow the beam move through the center of the beam-defining aperture (BDA). The beam is aligned to the optical axis of the second lens’ quadrapole. Beam astigmatism correction, shift and scanning is provided by the octupole which is positioned below the second lens. Between the second lens assembly and the second lenses steering quadrapole we have the beam blanking assembly. This is made up of aperture and electrical path and blanking plates. Beam blanking provides specimens with protections against constant milling.
2.3 Generation of Image
The primary beam is scanned as a raster across the specimen and it is made up of lines in vertical axis (shifted slightly from one another) and lies in the horizontal (in series). With scanning of the beam over the specimen the secondary ions and the secondary electrons that are generated by the specimen are detected. Details of this information are stored in the computer and images are produced from these information.
2.4 Detector, Stage and Gas Injection
Control of rotation and X and Y axis is performed by software and it can be tilted to the XY plane manually. Gases of two types are evolved above the surface of the specimen at about 100µm of distance. One of these gases is used for platinum deposition and the other for enhanced etch. During bombardment of ion in milling, species that are charged are formed and they are attracted to the detector. A glass of millions of arrays of minute channel electron multiplier is the detector; it is a micro channel plate (MCP).
2.5 Liquid Metal Ion Source (LMIS)
LMIS is made of a needle emitter which has an end radius of 1 – 10µm. It is coated with high surface tension metal which at its melting point has a low vapour pressure. This emitter is subjected to heating till the melting point of the metal is attained. A positive high voltage is placed on it. Using the balance between the surface tension forces and the electrostatic the liquid metal is drawn into a conical shape. The source that is commonly used is Gallium [dspace.mit.edu].
By using the scan control system, polygons, circles and lines can be milled. The table below represents the different beam currents and their corresponding milling spot sizes.
The figure below gives us the pixel size and milling spot size and the beam overlap. The overlap can be expressed as the overlapped area where the beam moves from a position to the other. And the time where the beam remains in a position is known as the dwell time.
2.7 Sample Preparation
The three main strategies used in the focus ion beam sample preparation of specimen that will be inspected using TEM are: Ex situ lift-out (EXLO) preparation (Centre Image), H-Bar sample preparation (Left image) and In Situ lift-out (INLO) preparation (Right image) [info.omniprobe.com/blog/bid]
When ion beam is scanned on the surface of the specimen, it causes ions and electrons to be ejected. After scanning through the surface of the specimen the primary Gallium ion penetrate into the surface of the specimen. The depth of the penetration varies from one material to the other. The secondary electron yield is much higher than secondary ion yield during ion milling and thus is the reason why focus ion beam is usually used in the secondary electron mode. Secondary ions and secondary electrons are obtained within regions that are closer the surface of the specimen.
In their work on the future of focus ion beam, the ORSAYPHYSICS group has shown that field of focus ion beam is open to expansion. Their projections with regards the extent to which focus ion beam can be deployed is shown in the figure below:
Fig. Current and Future FIB Technologies
The use of FIB has been developed extensively over the years in applications like super conductor, field emission device, accelerometer etcetera. Armed with imaging capability of high resolution as its recently upgraded technologies, the focus ion beam (FIB) instrument is indeed technology that is providing solutions to problems that has been previously unresolved. This heralds the focus ion beam (FIB) instrument as an important device for the future in the nano science, technology and engineering environment.