General description of particle deposition measurement
Particle deposition occurs in a clean room with particle sources like people and equipment in operation. When the clean room is at rest there is in general no particle deposition. Particle deposition is determined by measuring the surface cleanliness of a witness surface before and after exposure using the ISO 14644-9 approach.
At critical locations data on particle deposition can be used for risk assessments. Particle deposition is different at each location and depends on the activity in the clean room. The average particle deposition of various locations demonstrates the average operational quality of a clean room.
The Particle Deposition Rate at a certain location is the change of surface cleanliness in number of particles ≥ 1 µm per dm2 per hour, where the time, the time the clean room is in operation is taken. In stead of particle size of 1 µm various sizes d larger than 1 µm can be taken. In that case the number of particles ≥ d µm per dm2 is multiplied by the size d. So PDR is n1 per dm2 per hour = d*nd per dm2 per hour, where nd is there number of particle larger than d µm per dm2 and d is the length of the particle.
The Particle Deposition Rate can be expressed as a particle size distribution for particles > 8 µm and chosen intervals like 20, 50, 100, 200 and 500 µm or as a PDR number being the larger product d*nd in the particle size range of interest.
In some industries the total particle fall out during a project is important. Therefore it is also possible to determine the particle deposition (particle fall out) in ppm coverage by particles or PAC (percentage Area Coverage).