Problems Presented By Static Electricity In Cleanrooms

18th February 2006

We are all familiar with the presence of Static Electricity, we are made aware of it when we touch a door handle or filing cabinet and get a small electric shock. It also causes particles to cling to our clothing, particularly man made fibers. In our every day life these are minor irritations, but in modern day cleanrooms these two problems, electrostatic discharge and particle attraction can have a major impact on the operation of a cleanroom and have to be addressed.

What Is Static Electricity?

In simple terms static electricity is caused by motion and the effects caused by materials or gases rubbing together, commonly called tribo-electric charging. This rubbing action causes electrons to be exchanged and one object becomes more or less charged, either positively or negatively than another. This sets up a potential difference between the objects. Nature being what it is then tries to establish a balance by the migration of charges across a conductor or given a chance will cause a sudden discharge between the objects. Additionally this potential difference will act like a magnet and try to attract objects of the opposite potential to try to obtain equilibrium. We call these actions, electrostatic discharge or electrostatic attraction.

Problems Caused By Static Electricity In Cleanrooms

In a cleanroom, electrostatic discharge can cause the degradation of microelectronic products to such a point that they fail to function. The causes and effect of the application of a large charge to small electronic circuits is well understood. Electrostatic attraction causes particles to stick to surfaces such as walls, equipment and products where we don't want them causing contamination. A 5-micron particle sitting across a sub micron circuit is very undesirable. Surface charges, can through electromagnetic fields, cause equipment to receive spurious signals and miss-function, causing damage to products or equipment. Cleanrooms are expensive to build and maintain and these problems can have a deleterious effect on output and yields, steps must be taken to minimize these treats.

We cannot stop these physical phenomenon's from occurring unless everything remains motionless, this obviously cannot happen in a working environment like a cleanroom. Nature can offer us two opportunities to be of assistance in either preventing the charges from building up or causing their rapid dispersal, moisture and conductivity. Moisture in the form of water particles across a surface provide a conductive path and the use of conductive materials that are loaded with a conductive medium such as carbon permit current to flow. Unfortunately both these options are excluded to us in cleanrooms, we cannot tolerate moisture and take steps to lower the humidity and most cleanroom utilize materials that are very non-conductive, such as plastics. So great care must be taken when we design cleanrooms to ensure that where ever possible we use materials that are unlikely to generate or hold static electricity and employ processes that can actively eliminate or conduct charges safely to ground.

Steps To Be Taken To Minimize The Effects Of Static Electricity

The use of Static Dissipative plastics for walls, doors and other process equipment provide conductive surfaces, which will not generate or hold a charge. These materials are endowed with a conductive film to both sides that will not rub off and remain effective over a long period of time, even under tough conditions of use. The materials have a resistivity of between 106 – 107 Ohm and is considered static dissipative per EN 100015-1. These materials are transparent; the conductive coating does not effect clarity and have excellent fabrication, low weight and impact resistance characteristics, so they are most suitable as a building material.

Inside the cleanroom, static dissipative materials should be used for benches and storage containers such as desiccators and glove boxes. People should be hard grounded by the use of wristbands when they are at work at a station. Where ever possible, floors should be conductive to provide a ground plane for people to walk on provided the use of conductive footwear is employed. The use of antistatic clothing and footwear will ensure that as well as providing particulate protection, clothing does not generate it's own charge.

As it is not always possible to use conductive materials or ground surfaces, ionization can help remove surface charges. On a non-conductor pools of either positive or negative charges can be present, as current cannot flow across a non-conductor, these pools will continue to remain and represent a hazard in the form static discharge and as an attraction to air borne particulate. Ionizers generate a balanced flow of both positive and negative charges, which can be brought to the effected surface either by means of the room's vertical laminar air flow or through the use of the Ionizers integral blower. These charges will then eliminate the charges of the opposite polarity and thereby reducing the static charges. Ionizers can either be of the "total room" variety or can just be "spot Ionization" for local control.

Working in a dry air conditioned environment with plastic materials will always present a threat to products caused by static discharge and particle attraction. Careful consideration and selection of materials chosen and the types of process involved will mitigate these problems to a more manageable proportion; these steps are necessary if a company and it's cleanroom is to become world class competitive.

- Alan Waterman-Smith