Electrostatic Discharge vs Electromigration

Electrostatic Discharge and Electromigration might sound similar, but refer to different physical phenomenon. I would try to explain the difference between the two.

Electrostatic Discharge (ESD) is the large amount of 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 you by some means a large potential is applied on the Gate of the MOS device, then a large current tend to flow through the gate and this in turn may disrupt the Silicon dioxide of the transistor. As you are aware this this silicon dioxide controls the important parameters like the threshold voltage (Vt) of the transistor, any physical damage would render the functionality of the entire device capricious.

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 or universities, or corporate, care is taken to obviate any excess potential from getting accumulated on any lab material. There's a separate ground for every device, which may 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.

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 different. 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. There are many ways to do it. We would take those up in detail in some other post.