ExMOD ESD detector

Jon Titus -- Test & Measurement World, 2/1/2001

A new type of ESD detector indicates the level of an ESD event as well as its polarity. These detectors are small enough to mount in IC packages that will record ESD events as they go through a production line or other process. When you suspect a process damages ICs—or other devices—due to ESD, you mount individual detectors in an empty IC package (or other assembly) between the various contacts and ground and pass the test device through the equipment. At the end of the process, an inspector examines the minute detectors to determine whether an ESD event took place, and if so, what intensity it had. Examination takes place using a polarized-light microscope that reveals optical differences between unaffected and "zapped" elements on a detector.

How does it work?
The underlying technology in the magneto-optical event detector, called ExMOD by its developers, relies on the Faraday effect. The effect manifests itself as slight changes in the rotation of a beam of polarized light reflected off a surface subjected to a magnetic field. The surface comprises a magneto-optical thin film and a series of conductive paths that will produce magnetic fields when electrified by a static discharge (Figure 1). When set up as an ESD detector, the magneto-optical material starts off in an "altered" or set condition. A sufficiently large magnetic field switches the material back to its normal state.

Although the polarization changes are small—usually less than 10%—they're easy to see using polarized light and a standard microscope. To observe whether a detector has been reset by an ESD event, you simply illuminate it with polarized light and look for polarization changes that appear as changes in contrast. Figure 2 shows the image of a detector hit by a +150-V and a –50-V ESD pulse that passed through the detector. The size and arrangement of the conductors and thin films regulates the magnetic field, and thus the voltage, needed to "switch" each of the detectors. After using a detector, you return the magnetic material to its set state by placing a permanent magnet about 5 mm from the detector. You can use the detectors over and over again because an ESD event does not damage the magneto-optical thin film or the conductors.