Techniques which visualise the location of failures in electronic components are particularly useful in forensic fault analysis. Electron Beam Induced Current (EBIC) is a tool which can be used to do just that.

EBIC is a technique used in SEM (scanning electron microscope) analysis to visualize the electronic behaviour of a component. It can be used to map critical features and to highlight critical characteristics such as:

	locating the position of a junction in a circuit or subcomponent (e.g. a diode)
	showing how a circuit is functioning
	locating a point of failure in an integrated circuit, when there is no visible surface damage
	determining whether a crack goes through a critical point

How does EBIC work?
When the electron beam in the SEM strikes a semiconductor, electron – hole pairs are formed at the point of impact. In most places these rapidly recombine, but where a p – n junction comes up to the surface of the semiconductor the small internal voltage separates these charges. By suitable connection of a sensitive current amplifier to the device, the charges recombine through the amplifier. The net result is that the output signal can be used to modulate the brightness of the display providing a map of the positions where the p – n junctions reach the surface. The signal is also modified by the other electronic components in the path of the current.

Interpretation of the image is sometimes difficult but comparisons between known good and bad circuits can show the positions of faults. Circuits are often formed of repeating parts or where one side is the mirror image of the other side. Comparison of these parts can show where there are differences and help to locate problems.

Case studies - how ERA has applied EBIC to real failures

Diode contamination: Power rectifier diodes, Tranzsorbs and Schottky diodes tend to run at high temperatures. In extreme circumstances contaminants can be driven into the silicon under high local currents. It is necessary to find whether the contaminants have reached the p-n junction, as that would cause an increase in leakage current. Figure 1 shows a pair of images of a cross-section through a Schottky diode with the EBIC signal superimposed on the right hand one.

Back scatter image		EBIC image superimposed on image at left to reveal the device junction as a dark line (indicated by the arrow)
Figure 1. Cross-section through one end of a solder joint to a Schottky diode. In this case the junction is shallow, showing that contaminants would not have to penetrate far to cause damage. Integrated circuits: The internal operation of pairs of circuits can be compared. For example figures 2 and 3 show the same central parts of two ICs.

The good one in figure 2 has several transistors displayed as black features.
		Figure 2. The central region of the good device
The EBIC signal from the failed device in figure 3 highlights several resistors which do not appear prominent in figure 2. There are clearly functional differences between these two circuits.
		Figure 3. The central region of the failed device
Diode cracks: A large area diode had a high leakage current and a small crack in the corner. It was required to check the relative position of the crack and the p-n junction. The result is shown in figure 4. The back of the chip with a wire soldered on for contact is shown in a). The EBIC image on its own is shown in b). The two images are superimposed in c) to show the position of the junction on the surface. Finally a faulty one is shown in d). The line from the junction due to the EBIC signal crosses the crack.

a) Surface image of good device		b) EBIC image of good device
c) Surface image of good device with EBIC signal superimposed		d) Surface image of failed device with EBIC signal superimposed and highlighting crack

Figure 4. A series of images showing the position of the junction superimposed on the surface of a power diode with the crack crossing the junction in the failed sample

A crack crossing a p-n junction disrupts the charge carriers and local electric field. The result is a higher leakage current. In this case the high leakage current was caused by the crack and not by contamination.

Electrostatic Discharge Damage: ESD damage can be difficult to locate. The current pulses causing the damage are very short – of the order of nanoseconds. The heat generated at the damage point has little time to spread out and so the point may be too small to see. It may also be beneath some of the layers of tracks, making it impossible to see under optical microscopes. The EBIC signal spreads sideways slightly and this may make the damage visible.

Further information

The Reliability and Failure Analysis group has found many situations over the years where this technique can help to demonstrate the cause or location of a fault in a component. We have recently installed a new EBIC system on our new SEM.
To find out more call us on +44 (0)1372 367444 or email.