eTraining Introduction

Specimen Preparation

Leica Ultracut UCT

Histology

JEOL JSM-6400 SEM

Hitachi S-4700 FE-SEM

Hitachi FB-2000A FIB

Microanalysis

Veeco Dim 3000 AFM

Fluorescence Microscopes

Support

Form and Function: Part 2

FIB Applications

The FIB is a highly effective piece of technology that can accelerate the specimen preparation processes for use in a SEM or a TEM through the application of its micro-machining, sectioning, and deposition capabilities.

Milling

Micro-machining, commonly referred to as "milling," is the result of controlled sputtering of specimen material. Generally, the user will start by rough milling, involving larger beams, and then transition into the use of a smaller diameter beam to perform final milling. If materials are rather durable and/or coarse, the user may increase the beam diameter, the beam current, or the time the beam spends scanning the specimen, known as the dwell time. It is often the case that re-deposition occurs, a process which results when the sputtered material accumulates at the margins of the milled area. Re-deposition can be overcome by using a lower beam current and/or many fast scans, in contrast to fewer slow scans. Milling is especially useful in creating cross sections of materials.

Deposition

Deposition, or the application of a material such as metal to a specimen surface, can be achieved with great precision in the FIB system. The Hitachi FB-2000A FIB is equipped with a tungsten deposition gun. During use, the deposition gun nozzle is positioned within a few micrometers of the specimen surface. After opening the deposition gun valve, tungsten hexacarbonyl [W(CO)6] gas is injected into the ion beam-specimen interaction area. In the presence of the ion beam, the tungsten molecules split into W and CO components, depositing tungsten metal onto the specimen surface while the CO gas is scattered out of the region by the vacuum. It is important that the user consider the effects of dwell time on the deposition process. For optimal results, the dwell time should be high enough to apply a coating, but low enough that milling will not occur on the layer of newly deposited material. For precision deposition, operating with a low beam current is also highly advised.

Applications in Materials Science

The electronics industry was the first recognize the utility of the FIB. In the field of materials science, FIBs are utilized for very fast semi-conductor chip modifications, failure and/or structural analysis.

The preparation of TEM specimens from hard materials has traditionally involved cutting, grinding, and polishing steps. Depending on the material, the entire preparation procedure can require up to 8 hours. Using a FIB, scientists are able to rapidly produce transmission electron microscope (TEM) sections from site-specific areas of interest of their material. Another distinct advantage of the FIB is that it does not introduce mechanical damage to the specimen, unlike the previously mentioned grinding/polishing procedures. Powders, particulates, and minerals are also suitable for FIB specimen preparation.

Focused ion beam systems have recently been used for lithography work, also known as pattern generation, which was previously carried out using electron beam (e-beam) lithography equipment. E-beam lithography requires the application of a beam sensitive mask and a development step. Neither of these is required in FIB lithography; the pattern is directly sputtered through the metal film-coated substrate.

Applications in Polymers and Biological Science

Biological and polymeric soft materials are usually prepared for TEM examination by epoxy embedment and sectioning with an ultramicrotome. (See Overview in Leica Ultracut UCT.) Because the ultramicrotome is not an ideal tool for sectioning hard materials, the study of interfaces between hard and soft materials is problematic. The FIB is a suitable tool in this instance. Many biological and polymeric specimens contain water or are suspended in some aqueous solution. As mentioned previously, hydrated specimens cannot be directly processed in a vacuum system. Therefore, soft specimens must be taken through a dehydration treatment before FIB processing. Generally, organic samples must be coated in order to preserve surface detail and to prevent charging. Through the use of FIB cross sectionings, scientists are able to examine the internal microstructures of specimens that range from dental to bioengineered applications.

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