Contamination control is required for products and processes that are sensitive to particulate contamination. A cleanroom will be used to create an environment with low particle concentration. In contamination control traditionally the focus is on air cleanliness. This works fine for particles smaller than 5 micrometer. However in many cases particles larger than 5 micrometer are the major threat for products like optical systems. Contamination control of macroparticles (< 5μm) especially visible particles (< 25μm) should focus more on surface cleanliness. Surface contamination can become airborne locally and subsequently deposit on critical surfaces or can be transferred during direct and indirect contact. Many years of collecting particle deposition rate data in various industries has demonstrated the presence of macroparticles and visible particles. Next to limiting the generation of particles and removing airborne particles by ventilation, frequent and effective removal of surface particles by cleaning will reduce the likelihood of contamination significantly. Therefore monitoring surface cleanliness of floors, work surfaces, equipment and tools surfaces in the entire cleanroom will demonstrate the quality of cleanroom use and operational procedure, especially the cleaning program. New instrumentation makes this possible. The role of surface contamination in clean controlled environments and monitoring methods is as important as air cleanliness.
For system contamination control often cleanrooms or clean zones are used. Cleanrooms are classified for particles between ≥ 0.1 μm and ≥ 5 μm according to the ISO 14644-1:2015 classification table. In many applications air cleanliness for larger particles is more important. Especially in optical systems contamination by particles ≥ 20 μm is important. The present ISO 14644 cleanroom standards do not offer methods of controlling and monitoring of these particles. The new ISO 14644-17, of which the Draft International Standard is published in January 2020, covers the missing aspects of air cleanliness for macro-particles from ≥ 5 μm to ≥ 500 μm. The particle deposition rate gives a direct relation between air cleanliness and contamination risk. Particle deposition rate data predicts the contamination by particles during exposure, but the data also provides information on how to reduce the particle deposition rate if required. This paper discusses the specification and application of the particle deposition rate in clean controlled environments. As examples the production of a photo conductor and the assembly of a MEMS inkjet device is described.
Recently a real time particle deposition monitoring system is developed. After discussions with optical system engineers a new feature has been added. This enables the real time monitoring of obscuration of exposed optical components by counting the deposited particles and sizing the obscuration area of each particle. This way the Particle Obscuration Rate (POR) can be determined. The POR can be used to determine the risk of product contamination during exposure. The particle size distribution gives information on the type of potential particle sources. The deposition moments will indicate when these sources were present.
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