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Aerospace's Electronic Materials Section is heavily involved in the investigation of the function and failure of microelectronic devices. Microelectronic components, as the term suggests, are typically small! The critical dimension of the gate structure in a microwave transistor, for example, may be less than 0.15 micrometers (6 millionths of an inch). In order to understand how such devices work and how they fail, physical processes and chemical reactions must be investigated on this scale and state-of-the-art instrumentation is essential. With a very good electron microscope, investigators can get a view of the microelectronic world. They can look, but they can't touch! It has not been easy to see inside the complicated three-dimensional structure of a modern microelectronic device. FIB systems change the situation dramatically. The FIB uses a controlled beam of ions to remove material from a specimen with surgical precision. Using such a tiny scalpel, cross sectional views of a very large scale integrated circuit or a transistor can be prepared by precisely removing material in locations of interest, e.g., through a suspicious feature identified using an electron microscope. However, the instrument not only slices, it splices as well. Conducting metal lines can be deposited where desired using the ion beam to decompose a metal-containing gas introduced near the specimen. By cutting some connections and creating others, a microcircuit can actually be rewired. Modifications to circuits have permitted contractors to validate design changes before remanufacture and have saved time, money, and schedule. The FIB is not limited to the investigation of microelectronic components. The technique can also be applied to the analysis of optical and metallurgical coatings.

• Field effect transistors
• IMPATT diodes
• Digital and analog integrated circuits
• Microwave integrated circuits
• Solar cells

The FIB is a general-purpose tool for "micromachining". To demonstrate that point, at left is a picture of The Aerospace Corporation logo carved on a human hair (a hair is about 75 micrometers [0.003"] in diameter). The scales that you see are quite typical of hairs. A still smaller logo about 5 micrometers in diameter has been carved within the larger logo.

The FIB is similar in appearance to a scanning electron microscope. Specimens are placed in the vacuum chamber on the left. A liquid gallium ion source sits at the top of the ion optics column on the vacuum chamber. The ion beam in the FIB, like the electron beam in the SEM, can be rastered across the specimen to produce high magnification images. The beam current is kept low (about 6 pA) for imaging. The imaging capability permits accurate location of regions to be modified by the beam with the current set to higher levels (up to 4 nA). The beam position in both the imaging and specimen modification modes is digitally controlled.

For more information on focused ion beam milling contact Dr. Gary Stupian at stupian@law-west.org.