Particles that are invisible to the naked eye present serious obstacles to improving yields and quality in the high technology industries. As product features become smaller, such as nanometric drug delivery particles and nanometric gaps in microchips, it becomes more important and more difficult to exclude unwanted particle contamination.
A broad look at particle contamination is required so that its effect on products and processes can be controlled by an integrated approach. Contamination sources must be examined from outside the cleanroom all the way to micro-zones of cleanliness inside the cleanroom. This article discusses containment of particle sources, particle counting and analysis equipment, and particle filtration and removal in a clean and controlled manufacturing environment.
Basics of particle source containment
When looking at the clean facility as a system, particle removal efficiency can be improved by excluding wholesale quantities of particles at their source—before they get into the facility. This requires closer inspection of incoming parts, equipment and supplies, vendor and employee personnel, HVAC systems, and particulate manufacturing processes.
Assuring the cleanliness of incoming parts, equipment, and supplies, begins at the receiving dock. Packing and shipping containers have gathered dust throughout their journey in cargo vans and warehouses and are usually made of particulate or fibrous materials. They should not be delivered directly to the point of use as received. It is important to have a buffer zone between the receiving area and point of use to permit breakdown and removal of incoming packages so they won’t contaminate clean areas.
Tools and equipment to be used in clean areas should be cleaned and stored in isolation before installing them at the point of use. Parts or ingredients to be used in the product should likewise be inspected and cleaned, or purified, before delivery to the clean processing sites.
The trend toward “virtual corporations” where entire assemblies or products are supplied by vendors may require special attention to prevent contamination of clean processing areas by unwanted particles. Inspection of equipment and materials for particulate cleanliness when they are first received is an important QA function.
Employees and vendors are both major sources of particle contamination and require special attention because of their mobility in the facility. “Clean” areas must have strict procedures with regard to entering the clean environment and may include showering, wearing cleanroom clothing, and the use of a transitional room (like an airlock).
The HVAC system may be circulating particles along with the conditioned air, making contamination control an uphill fight. Some modifications can be made to reduce the problem. If the air intake on the roof is close to an exhaust or source of air contamination from an adjacent business or building, or close to a freight-loading zone, excessive populations of unwanted particles might enter the premises. Chemical or dust hoods may generate polluting gases as well as particles. These kinds of problems should be corrected to the extent possible, by hardware changes and/or special filtration systems. Specific areas of the building can be provided with filtration and ducting to reduce the ambient particle count. Such filters should be accessible so they can be inspected and replaced as needed. Special attention can be given to the effects of air balance to avoid the re-entry of contaminated air to clean areas. When the HVAC system has been properly modified, the clean rooms and clean hoods for specific areas can do their work more effectively. (See Figures 1 and 2).
Particulate manufacturing processes are special situations in some industries. Processes such as grinding, milling, and classifying particulate materials may generate such large numbers of ambient particles that they overwhelm typical particle removal processes in adjacent or nearby areas. Chemical operations such as crystallization, precipitation, polymerization, and evaporation may cause similar problems. The main point about these processes is that they must be isolated and contained so their ambient by-products do not create a difficult problem for the particle contamination control program.
Particle removal and filtration systems
Once the particle source containment issues are addressed, the particle removal and filtration processes are employed. Enumerating them in detail is beyond the scope of this article. Briefly, they include particle filtration of clean rooms, cleanliness of critical work areas, water and chemical purification; smoke alarms for clean room machinery, and sterilization of parts and tools before use to prevent biological contamination.
One important trend worth mentioning is the creation of clean zones in critical areas of the cleanroom rather than maintaining the entire cleanroom volume to a specified level of cleanliness. This entails placing clean “boxes” where clean work is required or using specially designed “SMIF” (Standard Mechanical Interface) units where needed. Investment and operating costs can be reduced with this approach along with improving process cleanliness.
Of primary importance in particle filtration and removal processes is the need for frequent or periodic evaluation of their performance. There must be a regular maintenance and inspection schedule to be certain they are performing as expected. Many types of particle monitoring and analysis systems are available to assist with this function in your facility.
Selecting and validating particle analysis equipment
Particle analysis equipment includes particle counters, particle sizers, and various kinds of electronic and optical microscopes. Particle counters are designed to detect trace quantities of particles in air or liquids and classify them according to broad size categories. Sometimes particle counters are confused with particle sizers, which have different principles of operation and are not normally used in particle contamination analysis.
Microscopes are used to examine individual particles to determine their size, morphology, and composition. They can be used to identify the type and source of contamination particles. Optical microscopes can be fitted with fluorescence, polarizing, or phase contrast accessories to identify special properties of particles as small as 0.5 µm in diameter. Various electronic microscopes are available to examine sub-micrometer particles as small as 0.025 µm in diameter. Automated image analysis programs are frequently used with microscopes to reduce the analytical workload.
Particle counters and, to some extent, microscopes, require spherical particles of known size and properties for initial calibration and periodic validation of their precision and accuracy. Certified particle size standards traceable to the standard meter through NIST (National Institutes of Standards and Technologies) are available for this purpose.
Particle counters, so named because they detect particles one at a time in their sensing zone, are available with a variety of features and specifications. They range from tiny personal models designed to detect only one range of particles, to large scanners designed to monitor particle contamination on silicon wafers in computer chip factories. Although prices vary with requirements, a portable counter with capabilities for spot-checking ambient particle levels down to 0.3 µm throughout your facility would be modestly priced compared to its value in your operations.
Regardless of which particle counters are used, their calibration and precision should be checked against particle size standards and documented on a regular schedule. This is required not only for conformity with quality standards such as ISO 9000, but to be sure the particle counter is measuring correctly. Small changes in calibration or sensitivity can cause particles to be displayed in the wrong channel and mislead you about the number and origin of the particles in question. The proper use of certified reference particles is also important for comparing results with outside parties such as other facilities, customers, or suppliers.
Using challenge particles to check cleaning and filtration systems
Given the availability of validated particle counting and analysis equipment, particle cleaning and filtration systems can be checked with a variety of challenge or test particles. The preferred particles are solid polystyrene spheres, available in many diameters, dyed or nondyed, in fluorescent and non-fluorescent colors. In special cases, quartz sand or phthalate ester droplets can be used but they are not available in some of the smaller sizes needed. They also have special safety, disposal and residual contamination issues.
From an overview standpoint, particle removal processes can be checked at three different stages of their usage: testing of filter raw materials; testing of installed systems before they go into use; and checking the results of operating processes.
Filter materials testing
This type of testing is usually performed by the materials manufacturer for the purpose of establishing retention or efficiency ratings for the materials. Retention refers to the smallest size particles collected by the filter; efficiency refers to the percentage of particles retained from a specified number of particles entering or “challenging” the filter at a specified flow rate. For example, a filter materials producer might use an aerosol of 0.26 µm particles to verify that a 0.3 µm rated filter would retain particles of 0.3 µm or larger. This test would require a particle counter both upstream and downstream from the filter. In another case, a household water filter producer might use 3.0 µm fluorescent particles and a fluorescent microscope to verify that microorganisms such as cryptosporidium or giardia, 4 to 6 µm in diameter, would be retained by that filter material. Many such tests can and should be devised for filtration materials.
Testing installed systems
Filtration systems are usually large-scale assemblies of equipment that include the fans and electrical controls as well as the filtration material mounts and structures. Testing installed systems usually entails checking for leaks in the system, rather than retesting the filter material, although rating test results for the materials should be part of your documentation.
Leak testing is usually done by generating an airborne cloud of 0.26 µm or other sized particles, then monitoring the number of particles in the filtered air with a particle counter. Filter certification firms are available to perform this kind of testing if warranted by the size and complexity of the system. Issues of oily smoke droplets vs. solid particles for airborne testing should be addressed with the testing vendor.
Checking operating processes for cleaning and filtration
Checking of operating systems involves a series of empirical tests of the process results that allow inferences to be made as to how the systems are functioning. For example, if incoming parts or components are subjected to a washing or rinsing process, the rinsing fluid can be analyzed in a particle counter. The particle count in the fluid is then a proxy for the cleanliness of the parts. The number of rinses required to achieve a desired cleanliness level can be determined and then used to establish a cleanliness specification with the vendor, or for checking parts in later stages of production.
Another important check is periodic or continuous monitoring of ambient particle levels using particle counters in cleanrooms, clean water, or other fluids. Intravenous fluids or medicines are checked for particle contamination with particle counters after they are packaged to be sure the total process is functioning properly. Despite all of the efforts to reduce particle contamination in their handling, silicon wafers used to make computer chips are examined before use to be sure they meet the ever more stringent demands for particle defect reduction.
Because of their ubiquitous nature, contamination particles can be anywhere and everywhere. The only way to be sure they are contained and removed is by continuously examining your processes and products with particle counters and microscopes.
The integrated approach advocated in this article involves systematic examination and containment of all incoming sources of particle contamination along with frequent and widespread monitoring of ambient particle levels.
Periodic evaluation of filtration systems and filtrate particle count levels is required, using particle counting equipment that has been carefully checked and validated with NIST traceable particle standards.
In addition to certified particle size standards for checking instrument performance and precision, challenge and test particles are available to validate and monitor particle cleaning and filtration systems.
Vendor claims and specifications regarding particulate cleanliness must be verified before introducing their products into your laboratory or processing areas. Multi-stage packaging should be used so that dirty shipping containers don’t make it all the way to the clean processing areas of your facility.
Novel and creative testing and inspection methods can be devised that take advantage of the many particle analyzers, particle standards, and reference particles that are commercially available.