[SI-LIST] ESD is a low frequency event -really??
- From: "Pommerenke, David" <davidjp@xxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 5 Mar 2004 11:18:06 -0600
Dear Group,
ESD is one of my research areas, so please excuse (or ignore) my long
email.
Some basic physical aspects of ESD
==================================
This brief explanation is limited to the source of ESD, it excludes the
coupling path to the circuit or the response of the circuit.
For understanding ESD it is worth to look at the physics in somewhat
greater depth. Let us assume an ESD event between metallic parts and
conclude the impulse response of the structures is linear with respect
to current, i.e, the only non-linearity is the arc.
For voltages below about 30 kV the arc can be modeled quite well as a
time dependent resistor for times from 0-10ns. Later, a resistor and a
voltage source (typically 25-40) need to be taken as model.
From a disturbance point of view in fast circuits, mainly the initial
fast rise is of interest, maybe some structural resonances of the
discharging object (e.g, the structural ringing of a toolbox).
For simplicity, let us just look at the initial rise of the current.
There are two regions. They differ in the physics:
I will give a boundary at about 1.5 kV, but this value will move up if
the metallic parts are approaching fast (often leading to strongly
overvoltaged spark gap conditions) and will move done at low air
pressure and some other secondary parameters.
Above 1.5 kV
============
Surface initiated, but gas discharge physics.
The electrodes are approaching. At some distance they reach the minimal
distance at which a discharge is possible (Paschen's law in homogeneous
fields). But the discharge may not occur due to a lag of seed electrons.
In this case, the electrodes will approach further without sparking,
increasing the field strength. Once the breakdown is initiated the speed
at which the arc resistance will fall depends on how much the gap is
overvoltaged (and the "wave-impedance" of the driving and sinking
metallic parts). If the electrodes have been allowed to come very close
(a long delay before the spark could occur) then the field strength will
be very large, leading to a fast collapse of the voltage. If the spark
occurs over a distance that is close to the one predicted by Paschen's
law, then the voltage collapse will be slower.
The most important factor that contributes to the generation of seed
electrons is humidity. Under high humidity, there are plenty of seed
electrons, i.e., it is difficult to overvoltage the gap. The breakdown
will occur over a distance close to the Paschen value, i.e., the
risetime will be long.
In summary: The risetime depends strongly on the amount of overvoltaging
the gap. It is easier to overvoltage a gap (e.g, by up to a factor of
10) at voltages below maybe 5 kV in dry conditions, due to aspects of
the generation of seed electrons.
Please check [2,3] for details.
Below 1.5 kV
===========
Surface initiated surface explosion driven.
The surface field strengths will become very large, large enough that
areas of local field enhancement will heat up due to the field emission
current. These areas explode and release metallic particles into the
gap. The voltage fall time is extremely fast, it cannot be measured at
the lower end of voltages. If you go below maybe a few tens of volts,
the breakdown may be totally driven by tunneling effect.
Below 1.5 kV the breakdown is pretty independent of which gas is used to
fill the gap or of the gas pressure (if it is not too low). This shows
that gas discharge physics is not relevant anymore. The discharge is
similar to a vacuum discharge. Please check [1] for details.
Fastest risetime
================
There are some methods that calculate the fastest possible rise time for
voltages above 1.5 kV. Please check [3] for methods used, results and
assumptions.
The results indicate that the voltage collapse can occur in about 50 ps
(dry air, clean surfaces, fast approach speeds). It is not likely to go
to shorter values, as the amount of possible overvoltaging is limited by
field emission induced surface explosions.
[1] S.Bonisch, W. Kalkner, D. Pommerenke, 'Modeling of short-gap ESD
under consideration of different discharge mechanisms', IEEE
Transactions on Plasma Science, Vol.31, No.4, August 2003, pp. 736-744
[2] D. Pommerenke and M. Aidam, 'ESD: waveform calculation, field and
current of human and simulator ESD', Journal of Electrostatics, Vol. 38,
Nov. 1996, pp. 33-51
[3] D. Pommerenke, 'ESD: Transient Fields, Arc Simulation and Rise
Time Limit' , Journal of Electrostatics 1995 36 (1995), pp. 31-54
[4] Kai Wang, D.Pommerenke, R.Chundru, J.Huang, K.Xiao, P.Ilvarasan,
M.Schaffer, 'Impact of ESD Generator Parameters on Failure Level in Fast
CMOS System", IEEE EMC Symposium on EMC, Boston, August 2003, pp. 52-57
We do have two more papers on the relationship between ESD generator
design, currents fields and circuit response that will be published in
the transactions on EMC soon. Please check [4].
David Pommerenke
Associate Professor
Electromagnetic Compatibility Group
University Missouri-Rolla
1870 Miner Circle, Rolla MO 65409
ph: 573 341 4531
573 341 5835
fax: 573 341 4532
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