X-ray Evolution From Microfocus to Nanofocus

Estimated reading time: 7 minutes

X-ray technology continues to evolve because sharper images can better determine the core causes of product failure.

By Adrian S. Wilson

"A picture is worth a thousand words." The author of this phrase never had the opportunity to consider its relevance to industrial X-ray, but no truer statement can be used for this technology. For manufacturers of industrial X-ray systems, the quality of a system is judged by the information derived from their pictures.

The higher the image quality, the greater the chance that the failure analysis engineer will be able to identify the cause of a product failure. It is to this end that tube manufacturers have worked on the evolution of X-ray from microfocus to new nanofocus technology. Within the past few months, reliable and repeatable nanofocus imaging has occurred.

Brief Industrial X-ray HistoryIn1951, V.E. Cosslett and W.C. Nixon, working at Cambridge University, England, invented the "X-ray shadow microscope." This X-ray shadow microscope became the forefather of all industrial X-ray systems that use magnification. The basic principle is shown in Figure 1.

Figure 1. An illustration of the basic principle of X-ray shadow microscope magnification.

During their studies, Cosslett and Nixon discovered that at larger magnifications the image resolution is limited by the focal spot size of the X-ray source. Image blurring or "Penumbra" effect, resulting from a loss in image definition caused by the spot's finite size, as shown in Figure 1, causes this limitation. Additionally, they found that the detector primarily affected the image contrast.

Since this time, X-ray scientists and engineers have worked to develop new and higher resolution sources. In the 1950s, X-ray tubes were developed that could produce image resolutions a little above 1 mm. In later years, minifocus tubes with focal spot sizes from about 200 μm to 1 mm were developed. These tubes still are used today in many industries and applications, including oil pipe weld inspection, engine block inspection and jet engine inspection.

Approximately 20 years ago, a new breed of X-ray was introduced, the microfocus X-ray tube. These tubes offered image resolutions down to a few microns (1/25th of a mil). Combined with a new generation of image detectors, this launched the X-ray inspection industry into the realm of real-time X-ray and secured the technology as a "must have" in the electronics industry for failure analysis and process control.

During 2001, a team of engineers and physicists (phoenix|x-ray Systems + Services Inc.) pushed X-ray technology forward with a new type of X-ray tube that produces images well below the microfocus range and hence has become known as the nanofocus X-ray tube. These tubes, which achieve resolutions as low as 200 nm under certain conditions, have several technological advances that allow mass-production, thus opening a new range of fault detection and application potentials that require finer image resolution.

The Nanofocus TubeThe developing team of researchers had to overcome some major engineering challenges during the process and developed some rather unique tube physics.

Figure 2. Block diagram of a microfocus tube (a) and a new nanofocus tube (b).

Figure 2 shows the basic design of a microfocus and nanofocus X-ray tube. While early versions of a nanofocus tube were similar to a microfocus, the team realized that to maintain a stable submicron focal spot size, the tube generator required significant enhancements. Other modifications included the physical design of the X-ray tube. However, the X-ray controller the brain of the tube did not require many modifications.

Figure 3. A nanotube operating as an X-ray microscope.

The most notable difference between the two designs is that the nanofocus tube manipulates the electron beam in a slightly different way than a microfocus tube with the use of additional coils and apertures. By adding the collimating coil unit and the nano-aperture, the electron manipulation results in an extremely small beam hitting the output tungsten target, resulting in a tube that provides an extremely small focal spot. This, in turn, also reduces the Penumbra to provide an extremely high image quality. Figure 3 illustrates the operation of a nanotube as an X-ray microscope from Figure 1. Note how the blurring of the letter B that was present in Figure 1 (with the microfocus) is not visible in this figure.

Putting Nanofocus Technology to WorkLike all electron beams, the more narrow the beam, the finer the details that can be seen. The better microfocus tubes have a resolution of 1 μm while the most common tubes used in X-ray systems output a beam with a focal spot of 5 μm. The nanofocus develops a beam in the range of 0.5 μm. This allows an X-ray system with a nanofocus tube to resolve objects as small as 0.2 μm, opening the door for better failure analysis.

The following examples illustrate how nanofocus technology can be used in electronic process applications.

IC Packaging Microvia Inspection. Microvia holes, which are laser drilled, have a small diameter. Microvias are used both in package substrates and more dense circuit boards. Figure 4 shows how microfocus (a) and nanofocus (b) compare. The figure shows the nanofocus image with notably less blur around the microvia's edges than the microfocus image. This added clarity gives better measurement of the via's plating, ensuring that there are no large voids or cracks that would result in early life failures.

Because of this poor image quality, via inspection often is overlooked. Yet it is critical because the substrate comprises layers with potentially differing thermal characteristics. If the thermal coefficients are not consistent or tightly controlled, the assembly and bonding can cause vias to sheer or tear. Additional vias may be incompletely plated, resulting in opens within the substrate.

IC Package Bond Wire Inspection. A common failure for bond wires is cracks as a result of excess current passing along the wire. The nanofocus image in Figure 5 clearly shows the cracked wire, whereas the microfocus blurred image could be missed. This ability to reduce the quantity of missed or overlooked errors is a key justifying factor for nanofocus tube use.

The other justifying factor is its ability to see detail simply not visible to a microfocus tube. Figure 6 shows the same wire bond in Figure 5 but at a different angle. Note how the microfocus image is unable to resolve the majority of the fine structure caused by the wire bond fraying.

Other Applications Presently, investigations are underway with research institutes and key industry leaders to identify how nanofocus technology may benefit other applications in the electronics industry and beyond. The following are some examples of how the nanofocus technology may be used in the future:

• Wafer-level inspection: X-ray is no stranger to the front-end process, although its primary use is for lithography. Combining nanofocus technology with a high-contrast digital detector has potential for die fault inspection, including microcracks and microporosity (Figure 7).
• Nanotechnology: With the push for smaller micro machines and other MEMS technology, it has become more difficult to inspect using conventional methods. Because of the inability of most of these techniques (microscopes and automated optical inspection) to measure features within the final encapsulated assembly, nanofocus technology has a bright future in this arena.

Figure 7. Combining nanofocus technology with a high-contrast digital detector has potential for die fault inspection, including microcracks and microporosity.

ConclusionTrue nanofocus X-ray systems have been the "holy grail" of the X-ray inspection industry for many years. Now that the first nanofocus systems are entering the market, the true benefits of this technology are beginning to be discovered. But will this new technology mean the end for microfocus X-ray systems?

No, and for the same reasons microfocus X-ray did not put an end to minifocus. Minifocus technology did not disappear but found its niche for the inspection of large, dense objects because of its ability to create a large X-ray flux not possible with a microfocus source. This is due to localized heat dissipation limitations, or, in everyday terms, because a minifocus has a larger spot size and thicker target, you can pass more current through the tube without the localized target area melting. If you were to pass as much current in a minifocus tube through a microfocus tube and maintain a microfocus spot size, you would melt the target much as focusing sunlight through a magnifying glass can start a fire.

Likewise, you cannot operate a nanofocus tube with the higher microfocus currents without increasing the spot size. However, many electronic applications generally need lower target currents, which means nanofocus technology has a wide range of applications in the package inspection industry.

Nanofocus tubes are unlikely to replace microfocus tubes for all electronics inspection applications. However, the benefits most certainly will result in some segments of the industry replacing microfocus X-ray technology with nanofocus solutions. Additionally, the technology's introduction and further development for new applications in front-end chip processes make the adoption of nanotechnology inevitable.

Clearly a new era of X-ray is at hand. The new world order, however, will not be all that different just with a sharper picture than we have seen before.

Adrian S. Wilson, president, may be contacted at phoenix|x-ray Systems + Services Inc., 3883 Via Pescador Unit A, Camarillo, CA 93012; (805) 389-0911; Fax: (805) 445-9833; E-mail: aswilson@phoenix-xray.com; Web site: www.phoenix-xray.com.