ESD Failures

Many advanced technologies, such as Plastic Encapsulated Microcircuits (PEMs) for example, are susceptible at less than 100 volts and many disk drive components have sensitivities below 10 volts. To put more circuitry into small packages, the spacing isolating circuitry has been reduced making them more susceptible to ESD. A discharge of static electricity produces enough heat that can burn through microelectronic architecture that is rated to withstand voltage in the order of volts. Figure 2 shows the silicon thickness verses the mean ESD failure voltage of NMOSFETs.

Figure 2. Mean ESD Failure Voltages of NMOSFET’s versus different silicon film thickness under positive ESD stress.

ESD damages are generally classified as either a catastrophic failure or a latent defect. Catastrophic Failure When an electronic device is exposed to an ESD event, it may no longer function. The ESD event may have caused a metal melt, junction breakdown, or oxide failure. The device's circuitry is permanently damaged causing the device fail. Such failures usually can be detected when the device is tested before shipment. If the ESD event occurs after test, the damage will go undetected until the device fails in operation.

Latent Defect

A latent defect, on the other hand, is more difficult to identify. A device that is exposed to an ESD event may be partially degraded, yet continue to perform its intended function. However, the operating life of the device may be reduced significantly. A product or system incorporating devices with latent defects may experience premature failure after the user places them in service. Such failures are usually costly to repair and in some applications may create personnel hazards. Figures 3 and 4 show ESD damage on the input of a device during ESD simulation testing.

Figures 3 & 4. Visual ESD damage of Ball Bond on circuitboard. Source: ADI Reliability Handbook

Failure Mechanisms of Parts

Three failure mechanisms for hard failures have been experimentally noted for semiconductor devices:

• Thermal Breakdown

• Dielectric breakdown

• Metallization Melt

Thermal Breakdown

Thermal breakdown is caused by the injection of an electrical transient, such as an ESD pulse, of sufficient magnitude and duration to initiate a melt in a portion of the junction. Large temperature change, short transient time of ESD pulse, and the lack of diffusion of heat causes hot spots on the silicon and with enough energy melts the silicon, short-circuiting the junction and failing the device. [2]

Dielectric Breakdown

When the voltage across a dielectric region excesses its dielectric tolerances, the result is a puncture of the dielectric. Once the dielectric has been punctured, small amount energy will be enough to create a short circuit. A device, after dielectric breakdown, will usually exhibit lower breakdown voltage or increased leakage current but not a catastrophic failure.

Metallization Melt

Failures can also occur when ESD transients increase the device temperature sufficiently to melt metal of fuse bond wires. Metallization melt is considered a secondary failure mechanism. It occurs when a second dielectric breakdown results in a short circuit, which then draws enough current to melt the metallization.

Device Susceptible to ESD

Different devices are susceptible to ESD to various degrees due to their design. Table 1 lists the device structures that are incorporated into various devices types, which are ESD-sensitive.

Table 1. Representative ESD Sensitive Electronic Devices List

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