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.'


The high power generated during the EOS/ESD event is equal to Ie2R,  where Ie is the EOS/ESD current and R is the resistance of the metal or resistor line.  If this power produces enough localized heat to bring the EOS/ESD site's temperature above the melting temperature of the conductor or resistor, then the fusing, meltdown, or burn-out of the conductor/resistor occurs.


Figure 2.  Photo of a fused metal line


Conductor/resistor fusing is often just a secondary mechanism of another EOS/ESD failure, such as a dielectric or junction damage that has created a short circuit where large currents can flow to subsequently cause the conductor/resistor line to melt down or burn out.


Junction Damage or Burn-out


Junction damage or burn-out refers to the destruction of a p-n junction due to joule-heating caused by the EOS/ESD event, resulting either in the junction's being open- or short-circuited. This type of damage also involves joule heating, and is more prevalent in bipolar devices.   


Hot spots arise in the junction when it undergoes joule heating, especially in parts where there are non-homogeneities and geometrical shifts.  Silicon where these hot spots arise become intrinsic in nature, whereby its resistivity goes down as temperature goes up. The reduction in resistivity further sinks more current, increasing the temperature further. 


This cycle continues, resulting in a thermal runaway that eventually melts the silicon with the hot spot when its temperature exceeds the melting point of silicon.  The silicon meltdown often creates a short across the junction, although high-energy transient EOS/ESD events can also result in open junctions.


Figure 3. Photo of a junction short


The power that heats up the junction is equal to IeVBD, where Ie is the EOS or ESD current and VBD is the breakdown voltage of the junction. Reverse-biased junctions are more vulnerable to EOS/ESD damage than forward-biased ones because its higher breakdown voltage results in a higher power dissipation in the depletion layer, requiring a smaller current to cause the damage.


See also:   What is ESD?What is EOS? Latch-upDie Failures




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