(Transforming Technologies)
Static Control in the Cleanroom
Introduction
Though often overlooked, the presence of static electricity in the cleanroom can create serious issues resulting in reduced overall profitability. All too often, this effect is simply written off through budget changes citing unforeseen issues. However, with proper understanding the issue of static electricity in the cleanroom is wholly preventable.
What makes a cleanroom unique?
Often maintained at a low relative humidity (RH) to optimize production, the cleanroom is designed to exclude most particulate contamination through the use of high efficiency particulate air (HEPA) filters. Any object brought into the cleanroom is first wiped with special cloths or cleaners, which unfortunately
also produce static charge – particularly when coupled with the low relative humidity level.
Usually, static charge is naturally dissipated through conduction to ground via surface contamination or residue. In the cleanroom however, products are handled with gloves and therefore remain clean. Surface charge is also naturally dissipated through ions in the air generated by radioactivity, running water, electrical motors and similar factors. In a cleanroom these ions are unable to pass through the HEPA filter, and they are automatically removed from the air as it enters the room. Use of glass or plastic insulators like TeflonTM initiate further complications with an aggressive hold on static charges.
As a general rule, cleanrooms efficiently generate static without a means of dissipation and the resulting charge far exceeds that present in a conventional environment.
Static charge in the cleanroom
Static charge effectively attracts objects with an opposite charge, repelling objects with the same charge. Not only does electrostatic discharge cause physical damage to electronic devices, but it also radiates high frequency electrical pulses through the room. These pulses can mimic a software bug, corrupting microprocessor-controlled applications, causing further time delays and more potential damage.
Because static charge poses a serious contamination issue, a great deal of effort is placed into establishing airflows to transport any contamination in the cleanroom beyond the product before it has the opportunity to settle. Unfortunately the electrostatic force of a particle in the cleanroom outweighs the aerodynamics of this engineered airflow.
Physical damage – Certain products are very sensitive to static, and can be damaged through accidental discharge during the handling process. Electrostatic discharge can take place in less than one nanosecond, for example in the case metal-to metal discharge. Approximately half of this energy generates local heating of the objects involved, whereas the other half is radiated as electromagnetic interference (EMI). This EMI takes the form of high frequency radio waves and microwaves (~100 MHz-5
GHz), creating short wavelength bursts that are challenging to shield against as they pass through cracks and permeate wires. These can result in short, large amplitude pulses, or glitches in microprocessor circuits.
Generally this type of discharge has no effect on the circuit as it induces a pulse on a bus within a nearby microprocessor. However, in rare circumstances the pulse will result in either a corrupted instruction or data value to be interpreted by the microprocessor, halting production until the fault is rectified. Similar to a software bug, this type of problem can take weeks or even months to resolve but is eventually identified as the result of ESD-induced EMI. The main attributes of this effect are:
- Appearance of a software bug
- Random and infrequent occurrence
- Inconsistent occurrence in identical process tools
Generating static charge
Matter is constructed of atoms and molecules. Some of the electrons in solid and liquid matter are attached to the matter itself rather than to individua atoms. Certain materials surrender electrons easily while others readily accept additional electrons. These characteristics are all related to the surface condition and binding energy of the material.
The triboelectric series is used to indicate the rank of materials in accordance with their tendency to accept or give up electrons. Generally organic materials are electro-positive while metals are neutral with plastics tending toward electro-negative. The further apart two materials in the triboelectric series, the larger the charging effect.
Because the triboelectric charging phenomenon involves surfaces, the relative humidity of an environment affects the rate of charging. In the instance of high relative humidity, a monolayer of water is present on material surfaces throughout the room. As a result, contact of intrinsic materials is reduced and the triboelectric effect is minimized. Alternatively, a low relative humidity environment encourages an increased triboelectric effect.
For many applications, glass and TeflonTM can be replaced by similar alternatives that provide electrical conductivity. Materials with a low electrical resistance (<104 Ω) are called conductors, whereas materials with high electrical resistance (>104 Ω) are insulators. Dissipative materials fall in between these two ranges. While conductors and dissipative materials can be grounded and rendered electrically neutral, insulators can be impossible to discharge due to sufficiently high resistance.
Although most plastics fall into the insulative category, chemically altered dissipative plastics do exist and are used extensively in static control. Metal tools are also given dissipative tips to dissipate static charge slowly, allowing for handling of sensitive parts without damage due to static discharge. Work surfaces themselves can be hazardous, and a grounded dissipative mat should always be placed on surfaces coming in contact with sensitive components.
In summary, there are several main rules surrounding the control of static charge:
- Grounded electrical conductors should be implemented wherever possible
- Insulators should be avoided wherever possible
- Dissipative plastics should be used and grounded wherever possible
Elements of a static charge control program
Any effective static charge control strategy requires significant consideration, dependent on specific production details relating to the particular facility. If a process is primarily manual, emphasis should be placed on operator grounding. Robots and process chamber walls close to the product are a major concern for automated processes. Mobile objects within a cleanroom, such as personnel and carts, must be grounded – a wrist strap for instance is not a practical solution for mobile objects. Many effective solutions employ the use of specially-designed ESD flooring to ensure continuous grounding.
Personnel grounding
Operators themselves create significant static charge, as fields from charged personnel remove contaminants from the unidirectional airflow, defeating its cleansing effect. Operators in contact with product can additionally cause damage through discharge, presenting an issue for electronic and medical devices. Options for grounding personnel include:
- A full-length ESD “bunny suit” worn in conjunction with ESD flooring. These suits are constructed of conducting threads to create continuity from the garment body to the soles of the boots, so the operator remains grounded and neutral.
- In lieu of a full-length suit, ESD shoes, heel grounders, or booties provide electrical conductivity between the floor and the operator’s feet when used in conjunction with ESD flooring.
- Special ESD chairs offer continuity to ground, and can be used in conjunction with ESD smocks. The smock provides contact with the operator while the chair ensures grounding.
- ESD wrist straps are placed on the operator’s wrist and then connected to ground through the work station. This is often implemented in an environment without ESD flooring, but requires the user to remember to plug and unplug the device.
Hardware grounding
Carts and shelves must also be grounded. At least two ESD wheels should be used to provide continuity to ground for a cart, which is only effective when used in conjunction with ESD flooring. Alternatively, carts can be placed on grounded ESD mats. Typically carts incorporate black plastic bushings to hold shelves in place, which are usually insulating so the shelves remain floating. At least one bushing should be replaced with dissipative bushing on each shelf.
Surprisingly, process equipment with metal walls often incorporates large, non-grounded panels. Grounding failure can result from something as simple as non-contact due to paint on these surfaces. Each panel should be tested for ground integrity, with ground straps used to rectify any identified faults.
Windows
Constructed of plastic or glass, windows in close proximity to the product are of particular importance due to the threat of discharge and contamination. Window materials can be replaced with dissipative plastic and grounded to offer electrical conductivity. Alternatively, a dissipative film with adhesive backing can be used, or a dissipative coating applied. While coatings can be quite effective, they can wear down with contact over time.
Required insulators and un-groundable conductors
There is no practical dissipative replacement for certain insulating materials. For instance, while acids will attack most plastics they will not affect TeflonTM, which is why it is frequently present in the cleanroom environment. Unfortunately the material is at the very end of the triboelectric series, so it is easily charged and holds that charge persistently. Due to mechanical issues, some metal structures cannot practically be grounded. These problems can be resolved through the use of air ionization.
Air ionization works by converting air molecules to air ions which find their way to the surface of a charged object. The ions then exchange charge with the surface, thus leaving it neutralized. While ionizers eliminate static charge, they require regular maintenance and are not intended to function
alone as a comprehensive ESD solution.
Ionizer technologies
Air ionization technologies most commonly employ the corona effect, using high voltage (5 to 15 kV) on sharp needle points, or emitters. Various AC and DC technologies are used with several emitters per ionizer. These ionizers function as precipitators, which extract chemical contaminants from the air and thus require regular cleaning.
With each cleaning, corona ionizers must be adjusted and reinstalled to ensure the rate of negative and positive ion generation is equal. This is performed with a specialized instrument called a charge plate monitor and without it, the ionizer will not neutralize the objects intended. Some corona ionizers conveniently mount on the ceiling, offering a modest discharge rate (~30-60 sec) to objects within an area of ~3x3 m. These are called ceiling emitters and are often located strategically above an individual workspace. Ceiling emitters rely on the unidirectional airflow of the cleanroom to deliver ionization to materials.
A second form of the corona ionizer can be used within the enclosed process equipment itself. In this circumstance, the ionizer uses pre-existing airflows ithin each machine to discharge components of hat particular machine.
In the form of blowers, corona ionizers produce neutralizing ions delivered via the generated airflow. While such ionizers are quite clean, they disturb the unidirectional airflow present in a cleanroom. However, mounted to a work surface or within a piece of process equipment, blower ionizers deliver substantially faster discharge than ceiling emitters.
Alpha technology is also used in ionization, which ionizes charged particles as they travel through the air. Alpha particles generate approximately 200,000 inherently balanced air ions and can be used in very close proximity (2.5-10 cm) to the product. Outside of this range an alpha ionizer requires driven airflow, or a blower, to achieve ionization. Available in the form of small discs the size of a coin, or bars with alpha elements (~5 mm x 25 mm), alpha ionizers must be replaced annually and require government licensing to own and operate. Alpha ionizers do not require cleaning, adjustment or other maintenance, but are fatal if consumed.
Reference documents
A variety of documents are available in reference ESD control program development.
• IEST recommended practice RP CC 022 (www.iest.org/i5a/pages/index.cfm?pageid=3835) reviews the implementation of ESD control in the workplace.
• ANSI/ESDA S20.20 offers guidance on levels required for ESD controlled facilities and can be downloaded free of charge at www.esda.org/s2020.html
• One of the most comprehensive sources of ESD documents can be found at http://www.esda.org/documents/presscatalog.pdf
Personnel training
Any successful ESD control effort requires the complete support of personnel. If the program is not fully understood by uneducated workers, they may commonly fail to wear wrist straps, or they may routinely switch off air ionizers. If a factory needs to be rearranged, grounding of work stations, shelves or process equipment could easily be neglected.
However, when personnel understands how static charge can impact productivity and work load, they are supportive and willing to cooperate with policies in place. This can be achieved through an online course or a brief, one or two hour seminar. For many companies with successful ESD programs, this is a pre-employment requirement.
Summary
ESD control is an essential component of profitable manufacturing for many cleanroom environments as it resolves contamination control, physical damage, electrostatic repulsion and automation interruption issues. The program should eliminate insulators when possible and implement comprehensive grounding and grounded dissipative materials. Air ionization should be utilized for issues that cannot be resolved through these steps to ensure an effective ESD solution.
Based on an original publication by Larry Levit, Ph.D.