ESD Phenomena in Integrated Circuits
When two objects at different electrostatic potentials are brought into close proximity
such that their electric field lines interact, transfer of electrostatic charges between
the two objects can occur. This process is called electrostatic discharge or ESD.
ESD events relevant to ICs have a relatively short duration of about 150 ns, and
can generate very high voltage and current transients (up to a few kilovolts and a
few tens of amperes). ESD is essentially a charge-driven mechanism as the event
occurs because of charge imbalance . ESD represents the transient discharge of
static charge which may arise due to human handling or contact with machines. The
theory behind the generation of these transient discharges have been presented in
detail in previous works .
Electronic components that are susceptible to ESD damage are generally referred
to as ESD sensitive devices, also known as ESDS elements . To help prevent
damage to such devices, dedicated on-chip ESD structures must be incorporated to
protect the core circuit against ESD stresses. The ESD protection performance level,
referred to as ESD robustness, is determined by the ESD failure threshold voltage,
ESDV (expressed in units of volts or kilovolts), of an IC component.
There are three principal sources of ESD in IC manufacturing and handling environ-
ments. The first and the most common source is human handling. A human body
can charge up to 20kV by walking across a carpet. This voltage is discharged to
ground through an IC when the person touches the pins of the IC (assuming there
exists a path to ground). Even though, the amount of energy involved is relatively
small (as an ESD event lasts for very short period of time), it may be large enough to
cause damage given the very small dimensions of the IC. The duration of discharge
is typically about 150ns, with currents ranging from 1A to 10A depending on the
conditions. The second source of ESD occurs in handling and testing systems. Such
pieces of equipment can pick up charge due to improper grounding, which can then
be transmitted to the IC during machine handling. The accumulated charge may be
higher than in the cases of human handling. Hence the discharge is a high magnitude
but short duration current pulse. The third source of ESD is the charging of the
IC itself when it comes in contact with highly charged surface or material during
transport. When the IC comes in contact with a grounded surface, the charge is
discharged resulting in failure due to the large currents flowing in the IC. Each of
the above mentioned ESD mechanisms, along with the methods used to test an IC?s
sensitivity to them,
Even though ESD is fundamentally caused by electrostatic potential, the energy dissipation
and damage is due to the current flowing through the IC during the discharge.
Hence, ESD protection structures should be designed so as to withstand high current
levels. ESD robustness is typically stated in tems of an ESDV ; however this is not a
complete description as the above mentioned discharge models can result in different
ESDV for the same circuit.