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Exploring the failure analysis process

Posted: 28 Apr 2015 ?? ?Print Version ?Bookmark and Share

Keywords:failure analysis? DC-DC converter? transistor? PCB? thermal imaging?

Another form of imaging technology is laser signal injection microscopy, which can be used to locate shorts, junction defects, and problems with vias, among other IC defects. A laser beam is scanned through a microscope lens over the die while watching for laser-induced shifts in the device (I-V) current-voltage response. Different effects can be created based on the laser wavelength. Short-wavelength lasers inject photo-currents and can reveal failure sites in transistors and p-n junctions, while longer-wavelength lasers create localized heating that result in temporary resistance changes. In the latter example, leakage paths can be revealed due to the temperature coefficient of resistance (figure 6). The change in resistance can be superimposed on an image of the die to show the location of the leakage path.

Figure 6: Laser signal injection microscopy can be used to locate shorts, junction defects, resistive vias, and die cracks, among other IC defects.

In addition to looking at electrical response through laser signal injection microscopy, it is also useful to look at the reflected laser light image and its amplitude using LTP (laser timing probe) techniques (figure 7). The brightness of the reflected laser actually changes with the voltage on the transistor, so by running a logic pattern through the sample, it is possible to acquire a waveform measurement of the individual transistor. This capability also makes it possible to see what is switching and what isn't, and even identify locations that are switching at a certain clock or data frequency. On a good circuit, each clock will be switching all the way through at the correct clock frequency, but on a failing circuit, it's possible to see where a clock stops halfway through, and precisely where this event happens. Thus, LTP techniques help to localise the defect.

Figure 7: LTP's two main functions include single-point waveform measurement and frequency mapping for defect localisation.

All of the aforementioned techniques will take the investigation to within a few microns of the failure mechanism. Nanoprobe techniques go the rest of the way, providing the ability to measure characteristics and behaviours of the individual transistors. Nanoprobe has been successful on transistors as small as 14 nm.

Nanoprobe can be used to create what is often referred to as a PicoCurrent map. The sample is polished until contacts are exposed and biased probe tip is scanned across the surface. Subtle leakage differences between sources, drains and gates can be revealed in the PicoCurrent image.

The test is similar in concept to scanning electron microscope (SEM) -based Passive Voltage Contrast (PVC) tests, but 10,000 times more sensitive. Figure 8 shows how the test was used to clearly reveal a leaking gate contact. Another option with this test is to drop up to six probes onto the transistor, make contact at the source, gate, drain, and body, then measure the electrical characteristic of each individual transistor: threshold voltage, off leakage, and on current. Once the problem transistor has been identified, the analyst can move on to a physical analysis.

Figure 8: Nanoprobe is a highly productive defect localisation technique that can identify problems at the transistor level including, as this example shows, a single leaking gate contact.

A common imaging tool used after nanoprobe tests is the TEM (transmission electron microscope). To use this tool, samples are first placed under a focused ion beam so that trenches can be dug on each side of the target transistor until there is just a thin (approximately 100 nm-thick) slice remaininggenerally the size of the transistor itself. This slice is then viewed under the TEM to visualise the defects in a way that no other technology can. Figure 9 shows TEM images of an overextended drain in a location where I/V curves had identified a threshold voltage shift. The TEM clearly shows the malformed drain that was causing the problem.

Figure 9: Very high resolution TEM imaging provides crystallographic information about defects, as well as elemental mapping and identification with nanometre resolution.

Electronic system failures have become increasingly difficult to identify and solve, especially as devices become smaller and more complex. In a world where many failure mechanisms can be tied to the dynamics of a single misaligned atom, failure analysis requires a highly disciplined process using numerous sophisticated test and imaging tools. With the right approach, though, it is possible to isolate the root causes of failures that may be occurring down to the transistor level, understand their mechanisms, and resolve problems so they don't happen again.

About the author
Winfield Scott contributed this article.

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