we know all too well, this is not exactly the
case. Regardless of the level of ESD protection
implemented, ESD problems still persist.
How would a diligent ESD specialist in a production
facility verify that his environment is ESD-safe? Many
companies use their yield for such verication.
However, this approach is expensive, offers no real-
time information, and cannot pinpoint specic
problems in the process. As such, it does not lead to
proper and timely corrective measures.
So how do you effectively manage your ESD
environment? How do you verify that your ESD
environment is truly safe? How do you assure that
your components are not exposed to ESD? How do
you prove to your customers that the components and
assemblies that you provide to them have not been
exposed to damaging ESD?
This article outlines a results-based approach to
managing an ESD environment.
The Growing Importance of ESD Management In
Manufacturing
Increasing ESD sensitivity of components is a trend
that we have observed in the recent years. There are
several objective reasons why ESD is fast becoming an
important factor in manufacturing processes:
The geometry of integrated circuits is shrinking. It
takes less energy to damage an internal trace or
device in a 0.1µm IC than in a 0.25µm IC.
High-frequency ICs cannot utilize adequate ESD
protection on-chip because such protection invariably
consists of low-pass ltering that slows down the rise
and fall times, and limits frequency response.
A higher number of pins increases the statistical
probability of ESD damage to an IC. Large die sizes
make such losses very expensive.
Automation in IC handling means more metal-to-
metal contacts and faster movement, thus increasing
the probability of ESD damage.
Magnetic heads, which are already extremely
sensitive to ESD, are projected to have even higher
sensitivity. In 2003, the damage level to magnetic
heads is already within 1V for CDM (charged device
model) or MM (machine model) discharge
(Reference 1).
Flat panels become larger while increasing their
resolution. That dramatically magnies the effect of
ESD damage to even one thin-lm transistor.
These trends will continue making ESD management
an even more important and integral part of your
manufacturing process.
The ultimate goal of an ESD program is to eliminate
ESD exposure to sensitive components, or at least
reduce it to safe levels. Just as with other business
venues, the most efficient approach to managing your
ESD environment is by the results. Simply having
wrist straps and ionizers in your facility does not yet
guarantee a safe ESD environment. Having factual
information about your ESD environment at all times
www.conformity.com
FEATURE
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Effective Management
Of An ESD Environment
In Production
Vladimir Kraz, Credence Technologies and acting upon any problems in real
time, on the other hand, provides you
with assurance of ESD safety.
In most manufacturing environments,
important parameters such as
temperature, humidity, particle count,
and others are monitored on continuous
watch. The ESD environment is no
different. If it is important to establish
and maintain a safe ESD environment, it
is no less important to have real-time
and historical information about the
actual status of ESD-related conditions.
The only realistic way to control ESD
exposure is to provide continuous
monitoring of every parameter of
importance, and to act upon the results.
ESD monitoring at every static-sensitive
step of your manufacturing process will
enable you to spend your ESD budget
wisely by applying your ESD dollars
where it is necessary and with the most
effectiveness.
ESD Events
What constitutes ESD exposure? For
device damage, it means the presence of
ESD events, or discharges. For particle
contamination purposes, it is static
voltage.
The frequency and magnitude of ESD
events is the ultimate metric of the
ESD-safety of your production
environment. Regardless of the level of
ESD protection implemented, if you still
have ESD events, something is not
working. ESD events last an extremely
short time, typically on the order of
nanoseconds. The only residual
information they leave is a damaged
device. Real-time monitoring of ESD
events allows you to identify those
devices, provides you with a continuous
assurance of ESD-safety, and alerts you
to any problems as they occur.
The following properties of an ESD
event are important for understanding
the level of damage they may inict:
Rise time. The faster the rise time, the
more damage inicted on a device.
This is because the energy supplied by
an ESD event to the device cannot
dissipate inside the device as quickly
as it is being supplied, and thus
damages the device by way of
overheating. As an example, it is not
uncommon for an IC to have a
damage threshold of 5000V HBM
(Human Body Model relatively slow
discharge with 10 or more
nanoseconds rise time), and at the
same time only 200V CDM (Charged
Device Model very fast discharge
with sub-nanosecond rise time).
Peak magnitude of the event. The
higher the peak discharge current, the
more damage is inicted on the
device.
The pulse width, or more accurately,
the area under the curve of the
discharge. All other parameters being
equal, the wider the discharge pulse,
the more energy it injects into the
device and the more damage it inicts.
Figure 1 shows an ESD monitor that
takes into account all of the above
properties of ESD events, in addition to
monitoring static voltage and ionization
parameters. Shown are also typical data
collected by an ESD monitor. Please
note the multiple ESD events on the top
line of the strip chart. Many ESD events
in production are multiple. The rst
event is not always the
strongest one. Ability to
differentiate between
individual ESD events and to
assess their strength is another
key parameter in verication of
your ESD environment.
Static Voltage
Presence of static voltage in
production environment
indicates the possibility of
particle contamination caused
by static attraction and
potential for occurrence of
ESD events. Static voltage can
be measured in two distinct
ways, either the voltage on the
charged object or the induced
voltage.
In those instances where
static voltage can cause
particle attraction, the most
relevant type of monitoring
is voltage on charged object.
Examples would include
wafer and at panel
handling. In these cases,
sensor element of the static
voltage monitor should be
placed at xed distance from the
object in question, such as passing
wafer, in order to obtain consistent
repeatable readings.
In cases where induced voltage can
cause discharges, this would be the
most relevant parameter to monitor.
Examples of such applications would
include handling of reticles and
magnetic heads of disk drives. In these
cases, a miniature static voltage
sensor, preferably capable of
monitoring ESD events as well,
should be placed as close as possible
to the location where the sensitive
components are handled. The readings
then would reect the voltage to
which these components are charged
by passing charged objects.
Knowledge of static voltage alone is in
no way an indication of safe ESD
environment. Damaging ESD events can
and do happen when there is no
indication of static voltage. For
example, static charges may develop
extremely quickly when the two
dissimilar materials are separated (i.e.,
an IC is lifted from its tray) and
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Figure 1: ESD monitor and typical collected data immediate discharge may follow. A
static voltmeter has very little chance of
detecting such rapid occurrence. ESD
event monitoring in these cases would
be more appropriate, since it can be
combined with static voltage monitoring
to provide more comprehensive
information for continuous verication
of the ESD environment.
Data collected while monitoring static
voltage exposure would help in
identifying proper placement of
ionizers, selection of materials used in
the process and grounding needs.
Ionization Properties
Ionization is one of the key components
in managing a safe ESD environment.
Properly implemented, ionization offers
substantial reduction in ESD exposure.
However, even the best ionizer may not
be effective if it is not properly installed
and maintained. Some of the specic
problems are:
Poor installation: an ionizer needs to
be installed in such a way that it
provides adequate airow to the area
where sensitive components are being
handled. Often, the ionizer installation
is guided by mounting convenience
rather than by the needs of the
application. The end result is that the
airow doesnt reach the workplace in
a proper way and the ESD safety is
compromised.
Air blockage: if there is anything that
obstructs the air path between the
ionizer and the workplace, it renders
useless an otherwise perfectly working
ionizer.
Lack of maintenance: if the ionizer is
not maintained on a regular schedule
(such as cleaning and replacement of
tips), it may stop functioning properly
after some period of time.
Balance (offset) uctuations: if ionizer
balance has drifted signicantly,
which is not a rare occurrence in
production environment, it may begin
to charge components within its reach
rather than discharging them.
As we see, relying on the mere presence
of an ionizer gives a fals