Controlling electrostatic contamination in cleanroom manufacturing
Eliminating static charge in high technology and life science applications results in improved yield and quality, and lower costs for maintenance and rework
By Arnold J. Steinman
The undesirable effects of electrostatic charge make it more difficult to maintain high levels of product quality and yield in cleanrooms. Particle contamination, electrostatic discharge (ESD) damage, and equipment problems are the result of failing to control static charge.
Electrostatic attraction (ESA) increases contamination of critical product and equipment surfaces, causing defects and increasing maintenance costs. Electrostatic discharge damages semiconductors, medical devices, and thin film products directly. It also interferes with the operation of the production equipment.
This article explores problems caused by static charge and how charge generation in cleanrooms is unavoidable. But there are methods of controlling static charge, including the use of air ionizers, which will be discussed using examples from several industries.
Static charge problems
Modern filtration keeps most external particles from entering the cleanroom. But particles are still produced inside the cleanroom by personnel, production equipment, and parts of the production process. Unfortunately, all of these particle sources are usually close to the product. If surfaces are charged, ESA attracts and holds particles that would otherwise remain airborne in the cleanroom laminar airflow. Sub-micron-sized particles that affect high-technology products are difficult to remove once they are attracted to surfaces.
Particle defects are well known in the semiconductor, disk drive, and FPD industries. They result in failed devices, or damaged magnetic heads and disk media. Flat panel display screens can be destroyed by a single, static-attracted particle. The consequences of static-attracted particles on medical device quality can be even more serious. Medical, pharmaceutical, food processing, and other life science industries must consider microorganisms attached to the particles.
Ionizing blowers in a disk drive assembly facility.
Besides making the elimination of particles from the cleanroom more difficult, static charge causes other production problems. The uncontrolled transfer of static charge—an electrostatic discharge, or ESD event—can damage product directly. Semiconductors, disk drive components, medical devices, and many types of thin films and coatings will be damaged.
ESD events also generate electromagnetic interference (EMI) that can interrupt the operation of production equipment, particularly microprocessor-based robotics. An ESD event in one piece of equipment may affect the operation of other nearby equipment, making the source of the problem difficult to locate.
A comprehensive contamination-control program must include measures to deal with static charge as a type of cleanroom contamination.
Static charge generation
Whenever two surfaces in close contact are separated, one surface loses electrons and becomes positively charged, while the other surface gains electrons and becomes negatively charged. This is known as triboelectric charging. Whether the material remains charged depends on its conductivity and the availability of a path for the charge to flow to ground.
Static charge is also generated by induction. Static charge on an object can create or "induce" opposite polarity charges on the surface of another object. These induced charges can attract particles, and contact with ground can result in damaging ESD events.
But static charge generation is unavoidable. It is not possible to prevent the contact or friction between materials in most production areas. The presence of insulating materials in cleanrooms assures that many charged objects will remain charged for a long period of time. Charge can be transferred to other objects by contact (ESD) or induction.
Solving most static problems will require one or more static control methods. Critical applications will require a well-designed static control program.
Cleanroom static charge control
A variety of methods have been developed to deal with static charge. Modern cleanroom environments make extensive use of grounding with conductive and static dissipative materials.
Grounding prevents the generation of static charge and removes it from isolated conductive or static dissipative materials that have become charged. Static dissipative materials have a higher resistance than conductors, but still have a lower resistance than insulators. They are used to slow down the charge removal process and prevent a damaging ESD event. In the cleanroom, grounding methods control charge on people and equipment, as well as on products.
Ceiling ionization in a 300-mm cleanroom.
Unfortunately, cleanrooms use many materials that are insulators, such as Teflon, various plastics and glass. Often, the insulating materials are an essential part of the product itself. Examples include oxide-coated semiconductors, glass hard disks and display substrates, and many medical products. Most insulators are easily charged, retain their charge for long periods of time, and are close to, or part of, the product.
Cleanroom requirements preclude the use of carbon particle or chemical additives to these insulating materials to make them static-dissipative. Chemical sprays and solutions also create a contamination problem. Humidity control was proposed in the past as a static control method, but has been shown to be expensive and ineffective. Finally, it is not possible to remove the electrostatic charge on insulators by connecting them to ground, since charge will not move though insulators.
Neutralizing static charge on insulators (and isolated conductors) requires the use of some type of air ionization. Using only the highly filtered cleanroom air, ionizers create clouds of both positive and negative air ions to neutralize static charges wherever they exist in the cleanroom environment.
Air ionizers to the rescue
By neutralizing static charge, air ionization assists other defect reduction methods in realizing their full potential for increased yields.
Air ions are gas molecules in air that have either lost or gained an electron. Ionizing radiation from nuclear, x-ray or ultraviolet (UV) sources may be used to create air ionization.
The most common method used to create air ions in cleanrooms is corona ionization—applying high voltage to a sharp point to create a very high electric field; the resulting field is sufficient to remove electrons from the air molecules. The polarity of the resulting air ions depends on the polarity of the high voltage on the sharp point.
When the ionized air comes in contact with a charged insulating surface, the charged surface attracts air ions of the opposite polarity. As a result, the static charge on the insulator is neutralized. Air ions of both polarities are required for neutralization because both polarities of static charge are created in the cleanroom.
Here's a look at recent trends in ESD control for two major industries:
Trends in static charge control
Semiconductor industry: Trends in semiconductor manufacturing are reflected in the International Technology Roadmap for Semiconductors (ITRS). Published annually in November, it communicates requirements for the construction and operation of semiconductor factories now and in the next 15 years. Regarding static charge control, ITRS 2003 states:
"Electrostatic charge adversely impacts every phase of semiconductor manufacturing, causing three basic problems. Electrostatic attracted (ESA) contamination increases as particle size decreases, making defect density targets more difficult to attainU Electrostatic discharge (ESD) causes damage to both devices and photomasks. Shrinking device feature size means less energy is required in an ESD event to cause device or mask damageU. Equipment malfunctions due to ESD-related electromagnetic interference (EMI) reduce OEE (overall equipment efficiency), and have become more frequent as equipment microprocessor operating speeds increase. These three problems occur where bare wafers and photomasks are produced, where devices are produced in wafer fabs, and where individual devices are produced in backend packaging, assembly, and test."
The ITRS contains recommendations to reduce static charge to levels that prevent static problems. These recommendations should be included in new facility construction and in new equipment, as well as in existing factories. Since static levels must go down as newer, smaller, technologies are introduced, it is critical that a static control program be implemented in every semiconductor factory. The cost of a static problem is 10 to 100 times greater than the cost of the static control methods.
ITRS 2003 recommends the use of two Semiconductor Equipment and Materials International (SEMI) standards in establishing and verifying a static-control program. The first, E78-1102, "Guide to Assess and Control Electrostatic Discharge (ESD) and Electrostatic Attraction (ESA) for Semiconductor Equipment," makes recommendations for controlling static charge in production equipment, describing static sensitivity levels of products and measurement methods to protect them. Originally issued in 1998, it is being revised to consider the rapid change in semiconductor technology requirements.
The latest document issued by SEMI is E129-1103: "Guide to Assess and Control Electrostatic Charge in a Semiconductor Manufacturing Facility." This document is synchronized with the static control recommendations of ITRS 2003, and recommends static levels to prevent contamination and ESD damage from today's 100-nm technology to the expected 25-nm technology of 2015. Anyone building a semiconductor factory needs to consider both present and future needs for static control.
Disk drive industry: Static charge must be controlled in the disk drive industry to solve the same problems of contamination and ESD damage; however, the problem of ESD damage is considerably more extreme.
The disk drive contains a magneto-resistive (MR) read head that is extremely sensitive to very low levels of ESD. Static charge must be controlled to levels of 5 volts and lower on every object that might transfer charge to the MR head, including the head assemblies themselves.
Assembly procedures require careful grounding techniques and avoiding metal-to-metal contacts. Everything must be made of conductive or selected static dissipative materials, and reliably grounded. Particular attention needs to be paid to personnel grounding issues for cleanroom garments, booties, gloves, and hand tools.
Air ionizers are used extensively to control static charges on process essential insulators. Special ionizers have been developed for this industry using alpha radiation sources or sensor feedback control to keep static to very low levels of 2 volts or less. It is predicted that future generations of MR heads will have even lower ESD sensitivities.
Dealing with increasing complexities
Standard methods of static control, including room ionization and equipment ionization, are used in disk media factories. Here, the problems are contamination and equipment robot malfunctions.
These problems will only become more critical as increasing data density requires more specialized disk materials, and profitability demands higher production equipment operating speeds. III
ARNOLD STEINMAN leads the ESD Task Force for Semiconductor Equipment and Materials International (SEMI), and is chief technology officer at Ion Systems (Berkeley, Calif.).
The S20.20 program
One of the important static control program documents is the standard, ANSI/ESD S20.20, "Protection of Electrical and Electronic Parts, Assemblies and Equipment." The S20.20 program was developed using industry experiences with the ISO 9000 quality program. Rather than defining a single static control program that all must follow exactly, it specifies all the elements of the program and lets the user define their implementation.
The user of the program must determine:
- The static sensitivities of the products that are being protected;
- The program procedures and the static control methods in use;
- How performance of static control methods is being verified;
- How problems in the program are corrected;
- How personnel are trained as part of the program; and
- How to retain records of performance, corrective actions, and training.
The standard S20.20 is a guidance document, which directs the user to existing industry standards for determining the ESD sensitivity of devices. It lists both required and optional static-control methods to protect 100-volt Human Body Model (HBM) sensitive devices, provides specification limits for each static control method, and allows the program to be modified for both more sensitive (for example, disk drives) and less sensitive devices (for example, many semiconductors).
The document also provides references to test methods for performance verification, and directs the user to establish and document the methods used for training and correction of program deficiencies. These methods will be very familiar to any company that has an ISO 9000 certification.
In real manufacturing situations, it is not possible to achieve S20.20 compliance without neutralizing the static charge on process-essential insulators and isolated conductors. A properly designed S20.20 program will include ionization, targeted to where the insulators are being handled. This may involve the use of one or more types of ionization.
The ESD Association has made the S20.20 standard available as a free download at www.esda.org. In addition, the ESD Association has published a handbook to assist users in implementing the S20.20 program.—AS