Christopher Long IBM
Larry Levit LBL
Contamination on devices such as semiconduc- tors and disk drive components poses a serious risk for reduced yield and reliability. In many industries there is a general attitude that some
loss is acceptable, resulting in billions of dollars of
lost revenue every year. This loss can be reduced or
eliminated through careful but basic contamination
and static control policies and procedures.
Contamination issues in hi-tech manufacturing
Micro-contamination can be a significant source
of yield and reliability loss. The total number of
particles landing on a surface in manufacturing
can limit the yield and profitability of the process.
Virtually every semiconductor fab in the world
checks each process step frequently in order to
validate that the process is working correctly.
Tools that pass their specification are said to be
in specification with respect to micro-contamination. Some companies have come to the conclusion that any reduction of micro-contamination
creates an amazingly large increase in profitability.
Therefore, the question is not whether there is a
micro-contamination problem, but whether it is
a good return on investment to pursue any given
micro-contamination improvement program.1 See
Figures 1 and 2.
Electric fields and contaminant attraction
Coulomb’s law states that the electrostatic force
acting between two charged objects is directly
proportional to the product of the charges and
inversely proportional to the square of the distance between the two objects. In the case of
oppositely charged objects, the particle will be
attracted to the charged insulating surface. In
fact it will accelerate until it comes in contact
with the insulating surface, the force increasing dramatically as the distance between the
particle and surface is reduced. The charged
particle has three forces acting on it — gravitational, aerodynamic, and electrostatic. For
electric fields greater than 1,000 volts/cm it has
been demonstrated that the dominant force is
the electrostatic force. 2
In clean environments there are many instances
of process-required insulators. These materials can be easily charged to very high levels.
Oppositely charged particles in proximity to these
items would be attracted to the surfaces, rapidly
contaminating them. Even if the surfaces are of
the same polarity as the charged particle, the
trajectory of the particle will be drawn off of the
carefully engineered unidirectional flow direction
and become a contamination candidate. Figures 3
and 4 show the difference between the movement
of particles on the laminar air flow and particles
moving in an environment with charged products
and charged walls. These figures also demonstrate
the importance of a neutral environment, not just
a neutral product.
Effect of particle size
The distribution of particulate sizes tends to follow a 1/X3 defect size distribution. 3 This means
that with a small number of particles at a given
size, there will be many more particulates with an
order of magnitude reduction in particle diameter.
Electrostatic bonding and cleaning
Studies4, 5 have shown that the implementation of
ionization in process environments in which product transits greatly reduces the number of particulates that are added “per wafer pass.” The concept
is that reducing the charge levels, both on surfaces
and on particulates in the environment, greatly
reduces the force of electrostatic attraction and
reduces the electrostatic bond of particulates on a
product surface, making them easier to remove in
subsequent cleaning process steps.
Ionization for process required insulators
Charge on process required insulators can also
be dealt with through the use of air ionization.
These systems reduce electrostatically induced
contamination problems, along with other static
control problems. If these systems aren’t in place
or don’t provide enough charge control, other ionization systems are available such as steady-state
DC ionizing blowers, Alpha emitters, and X-ray
generation systems. The selection of the ionizer type needs to be balanced with other critical
systems such as cleanliness, airflow, and process
issues. Unless a compliance verification procedure
already exists for the ionization systems, one must
be put in place to ensure the ionization system
Static Charge Control is of