Preserving UHP product integrity
How to keep ultra high purity chemicals clean
By Lou Leone, Process Facilities Inc.
The drive to produce electronic chemicals to the highest levels of purity has paralleled the drive to reduce chip geometry. In the 1980s most chemistry was considered “clean” when metal ion contamination could be reported in parts per million (ppm) levels. Today, metal ion contamination is reported at parts per trillion (ppt) levels, with some reporting in the parts per quadrillion (ppq) range. Particulate is reported in single digits at 0.2 micron.
A critical question that must be addressed in the future is: What level of purity is really required by the semi conductor fabs?
The ability to produce ultra high purity (UHP) products is not the primary issue. The manufacturers of electronic process chemicals have demonstrated this ability. Once manufactured, however, products are then put in containers, shipped, and distributed within the end-use facility. When the product reaches the fab, the purity that was reported at the manufacturing point has little resemblance to that which was measured at the point of use.
Consider the factors that are incorporated during the design of a typical UHP manufacturing facility:
1. Selection of materials of construction, which will not contribute to contamination levels through extraction or shedding;
2. Pre-cleaning, qualification and packaging of all wetted components;
3. Implementation of rigorous “clean build” protocols for the facility and all manufacturing systems;
4. Control of the environment in which the product will be manufactured; and
5. Techniques for charging, sampling, and processing raw materials, and intermediates which will preclude the introduction of contamination.
Once construction and commissioning are completed, weeks and sometimes months are needed to clean and stabilize piping systems, equipment, controls, etc. to minimize the potential for contaminant extraction from the materials themselves.
Realizing the effort which must go into preparing materials that will come in contact with UHP chemicals leads to the following question: If all this effort is required to assure that the environment and materials of construction don`t contribute to increased microcontamination, why would commercial containers, which have received little, if any, treatment, not be a source of contamination? The answer simply is that these containers contaminate. Several factors exist to ensure that this happens:
1. The interior and exterior surfaces are either not treated or are superficially treated (i.e,. wash and rinse), and sufficient contact time is not allowed for contaminants to be removed;
2. The environment within the head-space of the container is largely uncontrolled (i.e., non-inerted, variable temperatures, etc.);
3. Contact time of the product with the container and the environment within the container is generally long (i.e., weeks and months); and
4. Vibration associated with travel over roads tends to accelerate contaminant “shedding.”
During recent years, the issue has been partially addressed through the use of returnable containers. Since these containers have the potential to be used for many cycles, it is economically feasible to “invest” in the effort to clean and stabilize these containers and amortize that investment over the life of the container. Similar investment in a single-use disposable container is not possible. Returnable containers do have a downside, however. The cost of returnables is high; administration of the “returnable container fleet” requires significant resources; and waste treatment and safety are concerns.
While final filtration is often provided to eliminate particulate contamination, little or no effort is focused at the removal of metallic or organic contamination. The removal process is not clearly understood in most instances, and is very specific to the product. With specifications today commonly in the “ppt” range, and moving toward “ppq,” the contribution after manufacture becomes a significant value.
Producers of UHP chemicals are able to achieve very low contaminant levels through careful attention to a number of factors:
1. Insistence on the highest purity from raw materials suppliers;
2. Elimination of all contamination sources within the process itself;
3. Use of materials of construction and clean protocols in the design and construction of the manufacturing facilities; and
4. Adherence to operating protocols that ensure that the human interface does not compromise purity.
Even with careful attention to these factors, most UHP manufacturers maintain a “final polishing step” at the end of the process. Many technologies and unit operations are used in this step, depending on the specific chemistry and specification set.
The most effective way to provide product with the highest purity level at point-of-use would be to provide the final polishing step as close to the point-of-use as is technically possible. In most cases, flow rates are small, and the purification process can be designed and installed within a relatively small footprint. The means, resources and technology exist to make this happen today. Turnkey delivery of UHP chemistry on-site may very well provide the next step in total chemical management (TCM). This will be the approach of the future where extremely high purity levels can be demonstrated as providing real benefit. Higher purity will translate into higher costs. It remains for the user to determine where this cost fits within the total cost of ownership.
Louis C. Leone has over 30 years of engineering experience with several Fortune 500 manufacturing organizations, and has been responsible for facilities design and management in North America, Europe, and the Far East. Mr. Leone is currently director of high purity manufacturing systems for Process Facilities Inc. (Boston), an engineering concern specializing in the design and construction of pharmaceutical and other advanced technology chemical manufacturing facilities.