A cleanroom is NOT free of particles or contamination, even if many users of cleanrooms hold this opinion. Contamination should not be reduced solely to particles, even if the standards and guidelines define a cleanroom exclusively in terms of the particle concentration in the air. Which contaminants are to be reduced by a cleanroom is determined by the products to be produced only.
According to applicable standards and guidelines, such as ISO 14644, VDI 2083, and others, the operator of a cleanroom or a clean area is obliged to verify the conformity of the parameters with the specifications in the room documentation and, if necessary, to take measures to restore this conformity. This verification is carried out through regular qualification or requalification measurements. These measurements should be performed at least once or twice a year but can also be conducted more frequently, up to continuous monitoring of the cleanroom environment.
In the literature, there are numerous reports over the past years regarding the main sources of contamination in cleanrooms. Depending on perspectives and different experiences, these findings vary significantly.
Most people consider humans to be the largest source of contamination in cleanrooms. However, this statement, which is not fundamentally incorrect, cannot be generalised. It always depends on the extent to which humans are involved in the processes. Are the processes open with manual activities, or are they closed processes with a high degree of automation, which determines the influence of human contamination? Additionally, the type of contamination matters. It can be particles originating from improper cleanroom clothing or particles that do not form an effective barrier between humans and the environment.
In the life sciences sector, “living” particles, such as germs, play an important role. In the semiconductor industry, alongside particle influences, the importance of molecular contamination (CMC) is increasing. In battery manufacturing, for example, the entry of moisture due to human presence in the room plays a significant role. It becomes clear that there is a wide variety of different influences on cleanrooms and thus on processes and products. Therefore, for each of these different applications, a separate risk assessment should be conducted to develop targeted avoidance strategies.
The influence of the manufacturing processes can be divided into two categories. First, the impact on neighbouring processes in the cleanroom: Do the manufacturing processes themselves generate contaminations that can be transferred to adjacent machines via crossflow effects and negatively affect the processes taking place there? If so, this influence should be mitigated through constructive measures, such as extraction or encapsulation of contamination sources, or by reconsidering the placement of the problematic equipment / machine.
In the second category, the process negatively influences itself. Process-specific materials may be released, which can affect the quality of the products. In such cases, it should be determined whether constructive countermeasures can be implemented or if the situation needs to be accepted as it is.
The cause of process-related contamination is usually found in the history of the respective process machine. Often, these machines were developed at a time when the cleanliness requirements were not as high. Technologically, these machines still operate at a high level, but they no longer meet the necessary purity standards. It is also possible that machines from entirely different manufacturing sectors or industries are used, where the cleanliness requirements are not as strict, but all other technical parameters are met.
Closely related are the contaminations that originate from process media and process materials. The purity of these media is essential for a high quality of the products. Therefore, the requirements for their purity are described in various standards and guidelines (e.g., VDI 2083-7). If this purity is ensured at the point of delivery and contaminations related to these media are detected during the process, the causes are usually found in the process machine.
The materials used and to be processed must also meet high standards regarding the entire process or supply chain. Most suppliers of these materials have adapted their manufacturing processes to meet these high requirements, have implemented cleaning procedures, produce these materials in cleanroom conditions, or use closed systems in which contamination from the environment is not possible.
Another approach is to select from a large quantity of material only those ones that meet the purity requirements, discarding the rest. For example, highly pure PMMA is needed for micro embossing in plastics. The delivered PMMA sheets were heavily contaminated on all sides due to handling, transport, and storage. The top two and bottom two sheets were discarded, and approximately one-third of the material from the middle sheets was cut out and used for the embossing process.
An often times overlooked source of particles are the devices, machines, and equipment, as well as materials and surfaces that do not meet the desired requirements. For many years, various guidelines (VDI 2083-9.1; ISO 14644-14) have addressed the topic of cleanroom suitability and cleanliness compatibility. For example, by applying these guidelines and the resulting measurements, it is possible to assess in advance in which cleanroom environment a machine can be used. It can also be determined how many machines can be placed in a cleanroom to ensure that their particle emission burdens the cleanroom to a minimum only, so that it maintains the required specifications during operation.
These measurements are distinguished into cleanroom suitability and cleanliness suitability tests. Cleanroom suitability assesses the impact of the machine, system, or component on the cleanroom - is there a risk of deterioration of the cleanroom parameters? Cleanliness suitability evaluates the impact of the machine, system, or component on the product - is there a risk of contamination and thus a negative impact on the product? Cleanroom suitability is also considered a subcategory of cleanliness suitability, because the primary concern is protecting the products from contamination.
The situation with the materials is somewhat different. The material itself is a passive unit and generally does not actively release particles into the environment. However, if chemical contamination is a problem for the manufacturing of products, the wrong materials can indeed be not suitable for use. It is also possible that the passivity of a material is not immediately assessed in such a way that the movement of the material is not immediately apparent. Here is an example: A new gripper unit for flat workpieces was stimulated using ultrasound, causing an air pocket to form between the workpiece and the gripper, enabling contactless handling. The gripper was made of anodised aluminium, and the measured particle values were extremely high. The anodised surface represents a ceramic with many micropores, from which particles were ejected or broken out at the micro level. After the gripper was made from stainless steel, the values improved but still did not reach the desired range. Despite electropolishing, the microstructure still allowed particles to escape from the surface. Ultimately, the ideal material for this application was found through further testing: glass.
The last category of contamination hotspots is the cleanroom itself. It can be stated that almost all cleanrooms on the market meet the desired requirements. The solutions that have been continuously developed over the past 60 years are so mature that hardly any significant differences can be observed. Solutions where the recirculation opening (air outlet) is placed very close to the filter (air inlet) have become rare. Not only does approximately 30 % of the ultra-clean air seek the shortest path from the air inlet to the air outlet, wasting a lot of energy, but it can also happen that the ultra-clean air flows over the process area to the cleanroom floor, becomes contaminated there, and then flows through the process area to the air outlet, thereby negatively affecting it.
Another remarkable aspect has become evident during the construction of semiconductor fabs. According to Figure 1, the cleanroom has an influence of approximately 2%, and the machinery and equipment (equipment) around 20%. In the procurement process, one of the goals is to improve purity parameters. To optimise the influence of machinery and equipment (e.g. particle emission), discussions with about 100 to 150 different suppliers would be necessary. In the case of the cleanroom, discussions are limited to a single provider. If the machinery and equipment manufacturers could reduce particle emission by about 25%, this would be a very good result. Based on the total particle entry into the cleanroom (see the graph above, on the left), this would correspond to 5%. However, if the cleanroom provider were able to reduce their influence from 2% to 1%, this would mean a reduction in total particle entry of 1%, but a 50% improvement in the cleanroom itself. This raises the question of whether the consideration of these influencing factors is being approached with the right perspective and conclusions. Certainly, all levers should be adjusted to achieve a better overall result. The question to be answered is: How do the efforts compare to the expected outcome, and where should priorities be set – is the cleanroom or the product the decisive factor?
Summary
Contamination sources in cleanrooms are very diverse. It is important to analyse which of these sources have the greatest impact on the processes taking place in the cleanroom. The results of this analysis should not be limited to a single influencing factor to focus on (for example, placing a hood in the cleanroom reduces contamination but does not fundamentally solve a contamination problem), but rather help to set priorities and gradually eliminate or reduce all controllable contamination sources in a meaningful way.