Utilized in the Contamination Assessment process are the following particle contamination monitoring methods:
1.) Particle Fallout Monitoring or Witness Plate Monitoring uses silicon wafer witness plates for the collection of particle fallout onto horizontal surfaces and critical product process areas.
PFM is a critical and extremely valuable evaluation method and diagnostic tool. It is a true measure of actual particles being deposited in critical areas from the environment, including influent air, equipment, processes, personnel, supplies, and materials. It is a powerful tool in the quest for continuous environment and process improvement, prioritizing application of resources, and identification of corrective actions leading to operational cost savings.
For Airborne Particle Fallout Witness Plate Testing, state-of-the-art laser scanning technology is used to count and size the particles collected on the witness plates to determine the extent of airborne particle fallout throughout the cleanroom and at critical product and process operations.
Laser scanning of witness plates eliminates the problems associated with counting of particles by optical microscopy methods. These being, speed, reliability, repeatability, objectivity, and accuracy.
As laser scanning cannot determine the type and possible origins of particles, such as metallic, plastic, skin flake, and so forth, Optical Microscopy and other analytical tools, such as Scanning Electron Microscopy and Fourier Transform Infrared Spectroscopy (FTIR) are used for further analysis.
2.) Airborne Particle Monitoring (APM) is used to evaluate the quantity and size of airborne particles present in the environment.
Airborne or aerosol particle counters draw in a sample of air and direct a laser or white light across the air sample. Photo detection electronics interpret the resulting light scatterling pattern and determine the size and count the number of particles. Air particle counters can generally detect particles as small as 0.2 microns or as large as 25 microns. However, as particle size increases their fallout rate also increases. Therefore, particle counters are essentially blind to particles over 5 microns. Likewise, smaller partilces will remain airborne for long lengths of time, especially in diffuse or turbulent airflows and may never be detected by a particle counter. Airborne particle counts should therefore not be ilnterpreted as an absolute measure of cleanroom aire cleanliness. Airborne particle counters can be used to not only evaluate the contamination level of a cleanroom, but also find the source of cotamination. They are also used to filnd leaks or contamination sources and verify the effectiveness of a cleanroom and certivy the classification of a cleanroom or critical environment.
