Focused ion beam

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Focused ion beam, also known as FIB, is a technique used particularly in the semiconductor and materials science fields for site-specific analysis, deposition, and ablation of materials.

The FIB is a scientific instrument that resembles a scanning electron microscope. However, while the SEM uses a focused beam of electrons to image the sample in the chamber, a FIB instead uses a focused beam of gallium ions. Gallium is chosen because it is easy to build a gallium liquid metal ion source (LMIS). In a Gallium LMIS, gallium metal is placed in contact with a tungsten needle and heated. Gallium wets the tungsten, and a huge electric field (greater than 10⁸ volts per centimeter) causes ionization and field emission of the gallium atoms. The FIB can also be incorporated in a system with both electron and ion beam columns, allowing the same feature to be investigated using either of the beams^[1] ^[2].

These ions are then accelerated to an energy of 5-50 keV (kiloelectronvolts), and then focused onto the sample by electrostatic lenses. A modern FIB can deliver tens of nanoamps of current to a sample, or can image the sample with a spot size on the order of a few nanometers.

Photograph of a FIB workstation

Unlike an electron microscope, the FIB is inherently destructive to the specimen. When the high-energy gallium ions strike the sample, they will sputter atoms from the surface. Gallium atoms will also be implanted into the top few nanometers of the surface, and the surface will be made amorphous.

Because of the sputtering capability, the FIB is used as a micro-machining tool, to modify or machine materials at the micro- and nanoscale. FIB micro machining has become a broad field of its own, but nano machining with FIB is a field that still needs developing. The common smallest beam size is 4-6 nm. FIB tools are designed to etch or machine surfaces, an ideal FIB might machine away one atom layer without any disruption of the atoms in the next layer, or any residual disruptions above the surface. Yet currently because of the sputter the machining typically roughens surfaces at the submicron length scales ^[3].

FIB deposition can produce features 200 nm or less in thickness. FIB milling can produce even finer results. This structure required approximately 30 minutes of FIB time to produce.

McMaster University name and logo "tattooed" in two sizes on a hair.

A FIB can also be used to deposit material via ion beam induced deposition. FIB-assisted chemical vapor deposition occurs when a gas, such as tungsten carbonyl (W(CO)₆) is introduced to the vacuum chamber and allowed to chemisorb onto the sample. By scanning an area with the beam, the precursor gas will be decomposed into volatile and non-volatile components; the non-volatile component, such as tungsten, remains on the surface as a deposition. This is useful, as the deposited metal can be used as a sacrificial layer, to protect the underlying sample from the destructive sputtering of the beam. Other materials such as platinum can also be deposited.

FIB is often used in the semiconductor industry to patch or modify an existing semiconductor device. For example, in an integrated circuit, the gallium beam could be used to cut unwanted electrical connections, or to deposit conductive material in order to make a connection.

SEM image of a thin TEM sample milled by FIB.

The FIB is also commonly used to prepare samples for the transmission electron microscope. The TEM requires very thin samples, typically ~100 nanometers. Other techniques, such as ion milling or electropolishing can be used to prepare such thin samples. However, the nanometer-scale resolution of the FIB allows the exact thin region to be chosen. This is vital, for example, in integrated circuit failure analysis. If a particular transistor out of several million on a chip is bad, the only tool capable of preparing an electron microscope sample of that single transistor is the FIB.

The drawback to FIB sample preparation is the above-mentioned surface damage and implantation, which produces noticeable effects when using techniques such as high-resolution "lattice imaging" TEM or electron energy loss spectroscopy. This damaged layer can be removed by further milling with a low voltage argon ion beam after completion of the FIB process ^[4]^[5].

FIB preparation can also be used with cryogenically frozen samples in a suitably equipped instrument, allowing cross sectional analysis of samples containing liquids or fats, such as biological samples, pharmaceuticals, foams, inks, and food products ^[6]

Future FIBs will be much faster than the current FIBs that have a ~100ns dwell time, making them too slow for direction competition in device fabrication. Nanostructures 10 nm in diameter grown serially across a single 12-in. wafer would require more than two years, not including the time to get the FIB from one nanostructure to the next ^[7]. Also the cut and paste type of work the FIB can perform will make it more and more viable.