Introduction to ESD - Electrostatic Discharge

You may find sometimes when you touch another object you feel an electric shock. For example, when walking down a hotel corridor you may get a small shock when you press the elevator button. These shocks, harmless to ourselves, are caused by a build-up of a static electric charge on our bodies which can reach several thousands of volts. The shock we feel is caused by the static rapidly discharging into another object - often the earth.

ESD is normally harmless to us because it is a very fast phenomenon - lasting much less than a millionth of a second - and therefore has a relatively low amount of total energy compared to the size of our bodies. Modern semiconductor devices, in contrast, have ever decreasing geometries and can be very sensitive to even the smallest of ESD events (including events too small for us to feel). This raises a major reliability concern for semiconductor designers which they invariably address by embedding protection structures into their integrated circuits.

The whole subject of semiconductor reliability is very broad, but it is clear that each design must be subjected to a range of tests to ensure that it can meet the robustness and life expectancy demands of its application in the real world. Generally known as qualification testing, checking the correct operation of the ESD protection structures is part of this activity. There are several models which are commonly used to qualify semiconductor devices for susceptibility to ESD:

Human Body Model (HBM)
the so-called Machine Model (MM)
Charged Device Model (CDM)

The HBM and MM are broadly similar, in that a capacitor is charged to a known potential and then discharged into the device under test (DUT) via a known impedance. The HBM uses 100pF discharged through 1,500 ohms and the MM uses 200pF discharged through nominally zero ohms although in practice the impedance must approximate to 10 ohms in series with 0.75uH.

The HBM stems from the US Mil-Std 883 test method 3015.7 and the MM originated from Japan's EIAJ testing standards. It has been said that the Machine Model is actually mis-named and that it is really the Japanese human body model rather than an attempt to model a discharge coming from a charged machine as opposed to a charged human. In reality the history and origins of the test methods does not matter. The standard models simply allow a way to test and compare the ESD robustness of a device in a well-understood and repeatable way.

There has always been much debate about the relative attributes of HBM and MM testing but there is substantial evidence that both are essentially testing the same failure mechanism within an IC. For the same charge voltage HBM testing results in much lower currents than MM (because of the higher series impedance) so a device's HBM failure threshold is often an order of magnitude greater than it's MM failure threshold. As mentioned above, this does not matter as long as a like-for-like comparison can be made. Many IC users are satisfied with an ESD classification of 2kV HBM or 200V MM, although high performance analog ICs may struggle to achieve these levels without a detrimental effect on their performance.

Commercial HBM and MM test systems have a fixture which can hold the DUT in an IC socket and connect each pin to the HBM or MM discharge network using some form of switching mechanism which can be based on mechanics, relays or a combination of the two.

Whereas HBM and MM address external objects discharging into the IC, CDM is based on the IC itself becoming charged and then discharging to earth. As mentioned earlier, ICs are very small relative to most external objects and this has an impact on the speed of the ESD event. HBM and MM events last for times of the order of 100ns with a rise time normally in the range 2ns to 10ns. CDM events are much faster than this, lasting perhaps only one or two nanoseconds with rise times of the order of 100ps (100 trillionths of a second). As might be expected, the type of damage caused by CDM is characteristically different to HBM and MM and the protection techniques deployed by the IC designer are also different.

It isn't feasible to generate proper CDM events when a device is in a socket because the physical characteristics of the socket would overwhelm those of the IC package. Commercial CDM test systems have a fixture which can hold the bare DUT on a ground plane. The DUT is charged up, either directly via a probe or indirectly by induced field before being discharged by another probe.