EOS/ESD Failures

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EOS and ESD Failures and their Attributes

Electrical Overstress, or EOS, is a failure mechanism wherein the device is subjected to excessive voltage, current, or power. Electrostatic Discharge, or ESD, is a special type of EOS mechanism in the form of a single-event, rapid transfer of electrostatic charge between two objects. Many people distinguish ESD from other EOS-related but non-ESD mechanisms, so this discussion will do the same and refer to ESD as a separate mechanism from conventional EOS.

EOS and ESD can destroy a semiconductor device in many ways, resulting in observable signs of damage or failure attributes. There are, however, three (3) frequently-encountered and basic mechanisms by which a device is damaged by EOS or ESD. These mechanisms are: 1) dielectric or oxide punchthrough; 2) fusing of a conductor or resistor; and 3) junction damage or burn-out.

Dielectric or Oxide Punchthrough

Dielectric or oxide punchthrough refers to the EOS/ESD mechanism involving a voltage pulse that is large enough to rupture an oxide or dielectric layer. This problem is prevalent in MOS circuits because the thin oxide isolating the gate and the channel of the MOS transistor can easily be 'punched through' by large voltage spikes. Trends in new fab processes that lean towards thinner oxide layers also aggravate the occurrence of this mechanism.

A typical dielectric punchthrough event may occur in the following stages: 1) a high voltage spike occurs between two pins connected to opposite sides of a dielectric layer, in effect applying a large potential difference across the dielectric layer; 2) the breakdown voltage of the dielectric layer is exceeded by the large potential difference across it; 3) the dielectric breaks down and starts conducting current; 4) adiabatic or localized heating of the dielectric at the point of current conduction occurs; and 5) the conduction site melts down forming a filament that shorts the metal layer above the dielectric (connected to one of the pins) and the metal layer below the dielectric layer (connected to the other pin).

Figure 1. Photo of an oxide punchthrough after the

top metal layer has been removed

Dielectric punchthrough is minimized by using adequate ESD protection circuits and prevention of EOS occurrences, such as the inadvertent or random generation of voltage spikes in the circuit.

Conductor / Resistor Fusing

The phrase 'Conductor/Resistor Fusing' literally pertains to a metal line or resistor that acted as a 'fuse', or one that has become open due to excessive current. Such melting of a metal or resistor line is often due to intense heat produced by excessive power dissipation, or joule heating, caused by an EOS/ESD event that involves a large current flow through the conductor or resistor. Conductor/resistor fusing is also sometimes referred to as 'metal burn-out' or 'resistor burn-out.'

See also: What is ESD?; What is EOS?; Latch-up; Die Failures

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