Resolving issues must begin with an understanding of the fundamentals of both contamination and ESD control
By Roger Welker, R.W. Welker Associates
Both contamination levels and electrostatic discharge (ESD) are widely recognized as critical factors affecting yield and reliability in an ever-increasing number of industries. Although contamination has long been recognized as affecting semiconductors, disk drives, aerospace, pharmaceutical and medical device industries, today other industries-such as automobile and food production-are discovering the benefits of contamination control. ESD control has also experienced a similar growth in applications. In fact, today, control of electrostatic charge on surfaces is a widely recognized method of helping to reduce the impact of contamination, providing a synergistic benefit.
Despite the large number of degreed professionals working in contamination and ESD control, these fields remain misunderstood and underappreciated. The misunderstanding often arises because of the interdisciplinary nature of the two fields. Because so many different academic disciplines are required in order to provide a comprehensive understanding, the problems and solutions often appear confusingly complex when, in fact, the vast majority of contamination or ESD problems can be solved using very simple analysis. In addition, there is a long-standing perception that what is good for contamination control is bad for ESD control and vice versa. The following statements clearly point out why this is not the case:
1. Control of charges on surfaces in cleanrooms derives a benefit for contamination control. In this regard, selecting materials with a low tendency to tribocharge and that can be grounded because they are static-dissipative or conductive is desired. However, alone, these two solutions are insufficient in the vast majority of cases and the use of air ionization provides further benefit. Thus, the use of air ionization can provide a benefit for contamination control, regardless of the ESD sensitivity of products or processes within the cleanroom.
2. Control of electrostatic discharge in cleanrooms can provide a benefit by minimizing ESD and EM-induced microprocessor upset, regardless of the ESD sensitivity of products or processes within the cleanroom.
3. Airflow in mixed-flow cleanrooms is not optimized for performance of air ionizers not equipped with fans or compressed air sources. Fan-powered or compressed-gas ionizers are preferred in mixed-flow cleanrooms for this reason. However, fan-powered or compressed-gas ionizers can increase redistribution of contamination and can have a detrimental effect. The requirements of ESD (e.g., discharge time and float potential) must be balanced with contamination (airborne particle count, surface contamination rates, etc.).
4. Ionizers placed near ceilings or bench-mounted HEPA filters can perform well in unidirectional flow applications, particularly for controlling surface contamination. Conversely, airflow in unidirectional flow cleanrooms and clean benches can cause isolation effects or flow stratification, which prevents ionizers from achieving discharge time performance. Proper ionizer deployment must take into consideration cleanroom airflow effects.
5. Ceiling-mounted, room-type ionizers may not provide rapid discharge times. As the ESD sensitivity of devices tends toward lower voltages, only local fan- or gas-powered ionizers can achieve acceptable ESD performance. In these cases, balancing the requirements of ESD failures and contamination failures becomes precarious.
6. All ESD control materials and equipment must be qualified for contamination performance when they are used in contamination-sensitive applications. Similarly, all contamination control materials and equipment must be qualified for ESD performance when they are used in ESD applications.
7. Continuous monitoring systems are available for both contamination and ESD control applications. Their use in workstations that simultaneously require both forms of control can serve to minimize upsets in processes.
Figure 1. This illustration shows the possible effect on airflow of centering the layout of tooling within the process aisle of a cleanroom on a grade-level floor.
Any attempt at rectifying these two problems should begin with an understanding of the fundamentals of both contamination and ESD control and with defining the specific requirements that must be met. Following this should be an examination of the available analysis methods useful for solving contamination and ESD problems, especially with regard to the selection of materials.
One unconventional approach to building a contamination- and ESD-controlled workplace avoids the conventional approach of discussing the architectural and utility aspects of room construction, and instead focuses on the airflow character within the room and how that airflow is affected by tool and workstation placement (see Figs. 1 and 2).
Figure 2. This illustration shows the possible effect on airflow of bulkhead mounting tools within the process aisle of a cleanroom on a grade-level floor.
Cleaning processes and the equipment needed to support them must also be considered from the perspective of both the supplier and the user. Design and certification of tooling involves materials selection and evaluation of problems common to all industries affected by contamination and ESD. Continuous monitoring systems for contamination and ESD must also be taken into account.
Consumable materials and supplies should be chosen with a view toward requirements of both contamination and ESD control. In the late 1960s throughout the 1970s, many materials that were good for contamination control were bad for ESD control and vice versa. Today this is no longer true.
Of course, contamination originating from people and how to contain it must be major considerations, along with personnel behavior and discipline. Related to this is one area that has been sadly neglected-that of change-room design and layout.
Finally, overall management of the cleanroom and ESD-protected workplace environments must come into play. Companies dealing with contamination and ESD range in size from those having a single sensitive facility to multinational corporations having cleanrooms and ESD-protected workplaces on virtually every continent.
Roger W. Welker is founder and principle scientist at R.W. Welker Associates, an independent consulting firm specializing in complex contamination, electrostatic discharge, and quality control issues. He can be contacted at [email protected]
Editor’s note: This article contains excerpts from the new book Contamination and ESD Control in High-Technology Manufacturing by Roger Welker, R. Nagarajan, and Carl Newberg (John Wiley & Sons, 2006). In it, “the authors set forth a new and innovative methodology that can manage both contamination and ESD, often considered to be mutually exclusive challenges requiring distinct strategies.”