News Article

FIB Process Takes Materials Analysis Into New Realms

The novel integration of ion and electron beams into one tool with precision sample handling abilities is providing a one-stop shop for many characterization needs, writes Lloyd Peto.
The examination and analysis of multi-layered films and complex structures is often challenging. Site-specific analysis, as required for single-site semiconductor device failures, is especially demanding. Problems are exacerbated when working with materials that are sensitive to the preparation techniques being used and/or environmental factors during preparation. Laser devices, for example, are almost impossible to remove from can-type packages without damage, so the detailed analysis of laser facets or failure sites on returns is normally only attempted when essential. Recent advances in equipment and analysis techniques are now removing many of the existing limitations that are imposed by complex structures and sensitive materials.

Focused ion beam (FIB) instruments are widely used in the semiconductor and data-storage industries for many applications, including 3D materials analysis, photolithographic mask repair, circuit edit and general nanoengineering. The ability to focus a beam of ions onto an area of a sample with precision and accuracy means that specific sites can be machined by the beam to modify the structure by sputtering away material. Buried structures such as HBTs can also be cross-sectioned and imaged using the ion beam. Imaging the area of interest is accomplished by scanning the ion beam across the sample and detecting the electrons emitted as a result of the interaction between the ion beam and the surface. Scanning ion microscopy has evolved to a point where it rivals high-resolution SEM as a microscopy technique in its own right.
FIB sample preparation
Preparing thin samples by FIB machining for subsequent TEM analysis is a widely accepted technique in the silicon-based IC industry (see figure 1). The benefits of using FIB technology in this way are very clear; the process is very fast and labor saving when compared with sample preparation by mechanical grinding and polishing. It is very site specific (placement accuracies down to ±500 Å), independent of different material hardnesses within a single foil and is the only fully instrumental technique that does not require any mechanical preparation of the bulk sample. These physical benefits are now routinely combined with multi-site automatic operation of the FIB instrument.

Recent developments at FEI Company, a manufacturer of FIB instruments and a provider of FIB analysis services, have resulted in a unique dual-beam system that combines a field emission (FE) electron beam with an ion beam. This gives the ability to prepare a thin section of material and perform electron-beam analysis of that section within a single tool called a DualBeam instrument. The developments described within this article are specific to FEI s Strata DB235 DualBeam tool, and the TEM images were taken by the materials characterization group at QinetiQ in the UK.
The DualBeam advantage
The advantage of the electron and ion beam integration system developed at FEI s laboratories is that all of the benefits offered by FIB and high-resolution FESEM instruments can be used without compromising either technique. The beams can also be used simultaneously in a single-point operation. This mix-and-match capability allows the user the freedom to select the excitation method, imaging technique used and the analysis preferred depending on what is available from the specimen. This affords a level of instantaneous investigation that was not previously possible. However, the effectiveness and precision of a dual-beam instrument is highly dependent on the way the instrument is designed and operated. The design of an instrument with co-located ion and electron beams, each with its own requirement for electrostatic beam steering and focusing optics, is a non-trivial exercise. High-resolution imaging places stringent requirements on vibration isolation, and the ability to move thinned samples inside the instrument requires precision in mechanical placement and alignment, combined with high levels of repeatability.

FIB systems are used for an increasingly wide variety of materials including ceramics, polymers, superconductors, piezo-electrics and many different types of compound semiconductors. Some aspects of the FIB-SEM process offer unique advantages for compound semiconductors. The most commonly used foil size (20 x 10 x 0.1 µm) makes this process ideal for looking at structures fabricated on the surface of any type of semiconductor substrate and for analyzing thin films or surface treatments. Compound semiconductors are generally more sensitive than silicon to crystalline damage at the surface when prepared using FIB techniques. During the final ion-beam polishing stages of making a TEM foil with a single-beam FIB instrument, it has become common practice to avoid imaging the face of the foil with the ion beam to minimize any unnecessary crystalline disturbance. The effect of this practice means that the foil surface cannot be quality checked by imaging after the final polish.

Although high-quality foils are possible with both single- and dual-beam instruments, only the dual-beam systems can give direct feedback through the use of their electron-beam imaging capability during this final sensitive part of the foil production process. This extra degree of control permits thinner, higher quality foils to be made routinely, with a higher degree of confidence in the final results. Optimization of the final polishing methodologies in both FIB and dual-beam instruments means that high-quality foils with a thickness of 50-100 nm can be produced routinely, with no observable crystalline modification in the final specimen. The facility to take electron images in immersion (ultra-high resolution) mode while milling is possible because the electron and ion columns have been designed so that the beams are coincident at the sample stage s eucentric tilt axis, which is only 5 mm from the electron-beam lens.

Figure 2 shows a TEM image of part of a TEM foil sectioned from a GaAs/InGaP HBT structure in a single-beam instrument. The speckled appearance is the result of milled material that has been redistributed during the final polishing step. This might have been avoided or removed if the system operator had been aware of it. Figure 3 is a GaAs/AlGaAs QW laser structure prepared in a dual-beam instrument, which permits the final foil quality to be guaranteed and even examined (here by STEM) before transfer for TEM analysis.
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