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Static charge is a cleanroom contaminant that must be eliminated to enhance the production of electronic components, medical and optical devices, pharmaceuticals, biologicals, and a variety of other products.

Electrostatic charge attracts contamination to the critical surfaces of products and process equipment, causing defects and increasing maintenance costs. Electrostatic discharge (ESD) damages semiconductors, medical devices and film products directly. ESD also interferes with the operation of production equipment. Charge is a fundamental property of matter – charge movement is the flow of electrons. When electrons flow through a conductor we have the familiar electric current that runs our electronic components, but charge can also be transferred between objects. When electrons leave an object, the object becomes positively charged. When an object gains electrons it becomes negatively charged. If the object is isolated from the ground or an insulator, the charge will remain on the object. This is what we refer to as "static charge". The most common way of generating static charge is known as "triboelectric or friction" charging. Whenever two materials are brought into contact and then separated, a charge exchange occurs between the two materials. One material gains electrons and becomes negatively charged, the other material loses electrons and become positively charged. It is hard to imagine processing in the cleanroom that does not involve contact and separation of materials. Charge on an object may be transferred to ground or another object – this transfer is ESD.

Particle infiltration Improvements in filtration technology keep most external particles from entering the cleanroom, but particles are still produced in the cleanroom by personnel, production equipment and parts of the production process. Dealing with these particles is made more difficult by the presence of static charge, as charged surfaces attract and hold particles that would otherwise remain airborne. ESD occurs when static charge is transferred between two objects with different electrostatic potentials. As a product moves through the production cycle, contact and separation with other objects give it a charge. Once charging occurs, contact with another object creates an ESD event that can damage the product, or cause malfunctions of the equipment that handles the product. ESD causes electronic device failures throughout their life cycle, from production through to final use. As the critical dimensions of components and circuits become progressively smaller, they become less tolerant to ESD. In circuits designed to operate at lower and lower voltages, charges as low as 50 volts can destroy an electronic device, thereby causing a product failure. The electronics industry has been aware of this issue for many years, and has developed ESD tolerant designs as well as specialised handling procedures. Unfortunately these designs and procedures may not have been applied, for example, to new generation nanotechnology, MEMS devices and biological sensors incorporating electronics. While ESD causes catastrophic failures of devices which results in lower production yields, there is a potentially more serious problem. Many devices have shown the existence of "latent" ESD failures – a device is damaged by ESD, but not enough to cause a catastrophic failure. The device passes factory testing and is sold to a customer. The device then fails prematurely in the customer's product. In products such as computers and entertainment devices, this failure is an inconvenience. In medical products, such as pacemakers or monitoring equipment, the loss of function can have life-threatening consequences. In any case, the manufacturer runs the risk of losing customers due to failures of ESD damaged products. This can be a very expensive loss, far more important than simple production yield losses. The variety of materials and processes in the pharmaceutical, medical device, semiconductor production and other industries provides endless opportunities for static charge generation. The following is by no means a complete list of the sources of static charge in manufacturing environments, and a simple static audit with an electrostatic fieldmeter will reveal many more possibilities. • Routine wipe-down of cleanroom surfaces • Moving or handling plastics • Reel to reel component handling • Component transfers or pick-and-place • Tweezers handling small components • Parts, powders, or liquids moving through tubes • Processing plastic film over rollers • Parts in a vibrating hopper • Unrolling laminate from a roll • Cleaning wafers/panels/plastic parts/ glassware/packaging material • Handling or weighing pharmaceuticals. Many methods exist for preventing the generation or dissipation of static charge. Static-susceptible parts can be processed reliably, provided the generally accepted static-safe handling practices are followed. There are three main principles involved in setting up static-safe handling practices: • Electrically bond all conductive items in the workplace, including personnel, and connect them to earth ground • Provide for the neutralisation of static charge on the necessary non-conductors that are in the workplace • Transport all ESD susceptible items between static safe work areas in ESD protective containers and packages. Producing static-sensitive components in cleanrooms can make the implementation of a static control programme more difficult. Additives and carbon particles used in static-dissipative materials may become sources of contamination in cleanrooms where contamination is an issue. Many cleanroom compatible materials, as well as the production tools and parts of the product, are neither conductive nor static dissipative and cannot be earth grounded to remove static charge. Several insulating materials are found in cleanrooms, such as Teflon, various plastics and glass. It is difficult to imagine a product without insulators or isolated conductors. Insulated epoxy IC packages, printed circuit boards, coated media or cables and connectors, medical implants, and pharmaceutical materials and packaging are just a few examples. In addition, cleanroom requirements for low particle production, low outgassing of contaminants and chemical and thermal process compatibility take precedence over static control requirements. Since insulators are required and charged insulators will not lose their charge by grounding, other methods of charge neutralisation are needed. Ionisation is the most often applied technology for the neutralisation of static charge on insulators and conductors that are isolated from ground. Ionisers use the air surrounding the manufacturing process, often of cleanroom quality, to dissipate static charge on insulators and isolated conductors.

Ionisation Air ions are molecules of the gases in air (ie, nitrogen, oxygen, water vapour and carbon dioxide) that have lost or gained an electron, thereby making them positive or negative ions. When ionised air contacts a charged surface, the charged surface attracts ions of the opposite polarity until it is neutralised (Fig. 1). The most common method of producing air ions artificially is "corona discharge", where high voltage is applied to a sharp point (Fig. 2). Bipolar ionisation (both positive and negative ions) is the accepted standard for controlling static charge in all industries, because either polarity of static charge may be generated on a surface. When materials come into contact and are subsequently separated, one material charges positively and the other charges negatively. Bipolar ionisation charges gas molecules in the air to both polarities, neutralizing either polarity of static charge.

Contamination control – there is a connection between airborne particle levels and surface defects, which is why we carry out production in cleanrooms. Installing ionisation can ultimately lower airborne particle counts. Cleanrooms utilise laminar airflow as a means to keeping particles generated in the cleanroom away from critical product surfaces – if particles remain in the airflow, they exit the cleanroom. Static charge on surfaces defeats this process by attracting particles out of the cleanroom airflow. The particles may cause a defect directly, or may be continually stirred up by manufacturing operations or personnel contacting the charged surfaces. The measurable symptoms are higher particle counts, both on surfaces and airborne.

Medical device contamination – contamination created by static charge is one of the most costly problems in the medical device industry. Medical products are generally exposed to the cleanroom environment during most of their production steps. Plastic and glass are used extensively for their inertness and resistance to chemical attack, but unfortunately these materials are also static charge generators. The result is increased levels of contamination attracted to the charged surfaces.

Medical device ESD damage – for medical electronic products, optics, X-ray films or any other thin-film components that may be damaged by ESD, a carefully monitored ESD control programme will drastically reduce failure rates. In the US, the Food and Drug Administration (FDA) is responsible for approving ESD control programmes and the current regulations recommend following the ESD Association ANSI ESD S20.20 static control programme, which is certified by ISO approved auditors.

Pharmaceutical static issues – for the same reasons as medical devices, pharmaceuticals need to be protected from static-attracted contamination. Pharmaceuticals or biologicals produced by culturing are examples. Conversely, personnel must be isolated from the pharmaceuticals themselves in many cases. Static charge attraction must be avoided, both of contaminants to charged powders and of the powders to personnel. Garments worn in the cleanroom may be a consideration as there are several types of cleanroom-compatible, static-dissipative garments available to assure that ESA of particles is minimised. Such garments will need to be monitored over time to assure that they retain their static control properties when subjected to repeated washing.

Filling and packing problems Many other pharmaceutical static charge problems occur during filling and packaging in insulators using high speed process equipment. High charge levels are found on many of the materials used in these areas and requirements for low humidity may make the charge levels even higher. Charges are also found on the curtains and enclosures surrounding the equipment, and these may attract contamination from the air and then transfer it to the product due to physical contact or motion caused by the equipment operators. Parts of the equipment, such as robot arms and conveyer belts, may be made of insulators – such as nylon, Teflon and other plastics – that generate high charges when they contact the product, or its containers, which are often also made from plastic or glass. Static charges also create problems with precision weighing operations, as pharmaceutical powders attracted to critical measurement surfaces of the equipment cause errors in dosages. Where costly chemicals are involved, significant product losses may also occur due to particle attraction to the walls of the work enclosures and other surfaces of the weighing equipment. Bar type and compressed gas ionisers are installed to neutralise static charges on equipment surfaces as well as on the product itself. The results are fewer errors in product dosages and reduced losses of expensive products. While static charge cannot be eliminated, it can be controlled. A complete static control programme includes earth-grounding and ionisation for neutralising charge on insulators and isolated conductors. Such a programme can achieve remarkable results in reducing manufacturing losses and field failures due to ESD and contamination.