Electrostatic Discharge vs Electromigration

This is a guest post by Naman Gupta, a Static Timing Analysis (STA) engineer at a leading semiconductor company in India.

Electrostatic Discharge and Electromigration might sound similar, but refer to two different physical phenomena. Let’s take them up one by one.

Electrostatic Discharge (ESD) is the large current flow between any two points when a large (usually momentarily) potential difference is applied across those two points. In semiconductor terms, let’s say by some means a large potential is applied between the gate and source (typically at ground potential) of the MOS device, it may disrupt the silicon dioxide of the transistor. The silicon dioxide controls the important parameters like the threshold voltage (Vt) of the transistor and any physical damage would impact the functionality of the device and hence that of the entire SoC.

To give you a general perspective:

• The semiconductor industry incurs losses worth millions of dollars just due to ESD and therefore while shipping the parts, each and every IC is packed with utmost care and insulated from the outside world.

• Also, while working in the labs in research centers, universities or corporates, care is taken to obviate any excess potential from getting accumulated on any lab equipment. Ideally, there’s should be separate ground for all equipments which may accumulate charge, even if it might be as small as a metallic needle. Even back in my college days, our professor used to admonish us for touching the pins of any IC with bare hands because sufficient potential can get accumulated on our body, specially, our extremities like fingers. Additionally, engineers wear specially designed suits which ensure that any charge created is discharged back to the ground!

• Expensive chips like FPGAs usually come with an in-built protection in form of diodes with high-surge capability to protect it from ESD.

Note that ESD is a single time event. It can occur maybe while shipping, maybe while you are beginning to use the device or maybe when you are using that device.

Electromigration (EM): Let’s say a device is operating over a long period of time. And there are certain regions in the device, where the current density is pretty high. These electrons have the propensity to displace the atoms of the device and this might create voids in certain regions and hillocks in other regions.

Hillocks can create “shorts” between two metal lines which were otherwise two distinct lines. Voids can create “opens” between two metal lines which were otherwise shorted. Hence it will affect the functionality of the device.

Note that unlike ESD, EM is a gradual phenomenon. It is the main reason you see a dip in the device performance or functional failures in the device when it is operating over a certain period of time.

EM can be mitigated by:

• Reducing or distributing the current density in the metal lines. Current density can be reduced by increasing the width of the metal interconnects.

• The wire material also affects the electromigration. For example: copper wires are typically 4-5 times more robust to the deleterious effect of electromigration as compared to the aluminum wires.

To read more blogs from Naman, visit http://vlsi-soc.blogspot.in/