Passive and real time measurements of particle deposition will be the game changers in cleanroom technology.
Although surface cleanliness is the main reason to use cleanrooms most attention went to airborne particle measurements instead of deposited particles measurements. Particles that do fall on the product, especially the larger particles, harm the product quality. Even in the best clean rooms deposition of particles is daily reality as it is linked to people working and machines running. In the past witness plates were examined by microscope to get a grip on deposition of particles. Today the PDM, a machine vision automated microscopic inspection system for particle deposition, replaces the human examining of witness plates. Benefits of the witness plate system not requiring power or even the measurement apparatus during the exposure are maintained. PDM, Particle Deposition Monitor, is an award-winning game changing innovation. It can be used to collect particles in vacuum, at high temperatures and at many places at the same time. The glass witness plates allow sterilisation in an autoclave. Optimising towards lower levels of particle contamination at work spaces and process optimising is well facilitated within the feature set of the PDM. The fast measurement results being available within seconds puts the unit in the event catching equipment group. This allows direct feedback for finding root causes and is good support in training staff.
Despite firm flows of highly purified air large particles from 5 micron and up will fall on products. People and machines are the working reality and even in the cleanest rooms particle deposition will take place. Merely the walking and working in a cleanroom close to the critical surfaces will generate particles that finally settle. To reduce the probability of deposition, air flows are designed to get settlement at a lesser extent. Particles least affected by air flows are the larger particles. Fibres, definitely a special class of large particles, are however highly affected and may even become resuspended by air flows.
PDM equipment is capable of capturing deposited fibres, even very large fibres. Micro plastics are often fibres or part of fibres. Still the ratio between surface and weight is very important for their behaviour. Fibres have mostly a low aerodynamic diameter based on their settlement time, however their footprint in deposition is mostly significant. PDM equipment is highly suited to investigate fibre behaviour in deposition. The large completely horizontal detector surface will catch the fibre and the system will image and size the fibre. The position on the surface is logged so that further investigations can take place if required.
Root causes of deposition need to be suppressed in order to increase yield. Measurement of deposition is the key in this battle. Witness plates showing the particle fall-out should be exposed for a minimum period. These witness plates need to be carefully placed and collected, in order not to contaminate them due to handling. After collection they are hand-fed into the PDM unit. Within 30 seconds a particle size distribution that can be translated to a particle deposition class (PDC) is made. Every particle on the witness plates is sized, counted and logged with its position in a raw data file. The big data thus fully automated collected is valuable for yield improvements.
Cees van Duijn contamination expert of Eindhoven the Netherlands has demonstrated with witness plates that the position of tools with respect to the product are of influence on the particle generation and therewith deposition. The large flat detection surface of the PDM glass witness discs ensures that even low amounts of deposition will result in quality data. The ability of the system to show amongst all other results within 30 seconds images of the top 6 of largest particles or fibres found can be of use to improve on the type of cleanroom clothing and garments used. Because the number of witness plates can be increased easily and the low processing time of each witness plate large scale investigations even on various places at the same time in order to find root cause can be carried out fast thus reducing production loss.
PDM, Particle Deposition Monitor has a rich feature set covering most jobs required. An optical system automatically scans the total surface of about 50 cm2 every 30 seconds. Each scan is analysed and can be compared to any previous disc image thus revealing additional particles. This is translated to PDC-levels automatically and continuously. A system consists of a PDM unit and a PC based Vision Analysis Software. The interconnection between PDM and system is network based. Maintenance of the PDM is simple. The system can be verified with NIST particles thus showing good working on real particles. Verification of the calibration can be carried out at site by means of a verification disc.
A deposition monitoring system with a PDM requires a limited amount of labour. Fully automated measurements can be carried out at one spot however the detachable witness plate system allows measuring at many spots at the same time as well. There is no doubt that cleanroom controllers and process engineers will appreciate the PDM tool allowing them to see the deposition of particles in a simple way. Rail systems producing particles amongst other hard to tackle sources will be caught in action by the PDM changing the game in particle deposition monitoring.
N.A.W.J. Thevissen Ing.
Oude Roderweg 17e
+31 45 5410093