Measuring and imaging particle deposition in real time

Most particle counters fail to register particles of 5 µm or larger, as they settle out in the air sampling tube. Isabelle Tovena-Pecault, research engineer, CEA CESTA, looks at the benefits of particle deposition monitoring of 5 µm particles and larger, using an innovative, real-time measurement instrument

CLEAPART–100 shown alongside its monitoring application on a smartphone

Particle fallout in cleanrooms is a primary concern for industries manufacturing critical products with surfaces highly susceptible to particulate contamination.1, 2, 3 Particulate contaminants can be hazardous to lasers, optics and to spacecraft in a number of ways including failure of precision mechanisms, light absorption and scattering, points of high local electric field and associated electrostatic discharge, and noise on electrical contacts.

The objective of the ECSS-Q-70-50-C standard, as an example, was to ensure that the particle monitoring of spacecraft systems and cleanrooms utilised in the production of such systems, was carried out in an appropriate manner.4 The device used for the fall out particles is a photometer which measures the optical effect of particle contamination and is not a precise particle classification.

The ISO 14644-3: 2005 standard (in the annex dealing with particle deposition testing) describes the procedures and methods to be used to measure and count particles that have been or may be deposited from the air onto the work surfaces or products inside cleanrooms.5 Deposited particles are collected on suitable witness plates, similar to the surfaces considered to be at-risk, and the size and number of these particles are analysed using an optical microscope, an electron microscope or other surface analysis equipment.

In 2015, the standard was revised in response to the growing preoccupation of industrial operators regarding this form of contamination risk, and Annex B.11 dealing with particle deposition tests has been considerably expanded. We should mention that an international standard is now being prepared on the particle deposition rate monitoring in cleanroom and controlled environment (ISO 146417).

The traditional technique of using witness plates to collect particles and then analysing them with a microscope or photometer could now be replaced by instruments capable of monitoring particle deposition in real time.5 Instruments already available on the market include the PDM (Particle Deposition Meter) developed by SAC and the APMON unit from TNO.6,7

However, none of these instruments is equipped to provide real-time measurements of particles of 5 µm and above and on surfaces large enough to be considered statistically representative. It was this lack of satisfactory equipment that first inspired CEA-CESTA to join forces with the University of Aix-Marseille and Winlight Systems to develop the CLEAPART-100.8

Real-time monitoring

This innovative, patented system for counting deposited particles and monitoring the cleanliness of controlled environments, allows users to detect particles and sort them by size, starting from 5 µm.9 The detector comes in cube form, suitable for cleanroom use. It has a high flatness glass surface (100 cm2 surface) that collects particles, which are then detected by a mobile optical camera positioned underneath the glass plate, on X and Y axes (see Figure 1).

Figure 1: Cumulative particle counting results (unit is particle/cm2)

The benefits of this instrument can be illustrated by looking at some past counting results. As shown in Figure 1, on 14 July at 4 pm, a contamination event occurred that was linked to the maintenance on the air recirculation in the cleanroom. The recorded images can be examined for post analysis; it is also possible to focus on a specific particle and observe its shape and measure the different axis lengths.

According to ISO 14644-1, cleanrooms and controlled environments with a particle class of ISO 5 or cleaner will contain zero or very low concentrations of airborne particles larger than 5 µm.

However, in operating cleanrooms, particles in the size range of 5 µm and 500 µm are found on surfaces. These particles, particularly the larger ones, are not detected by particle counters because their weight means they are deposited within the sampling tubes upon entry.

Particles larger than 25 µm are deposited by gravitational sedimentation onto horizontal surfaces, such as floors, furniture, equipment and products. Although, these particles can be re-dispersed by movements such as walking, sitting and wiping, these particles can only be removed by cleaning. Particles between 5–25 µm have a very specific aerodynamic behaviour. Even having been deposited on a surface, they may be resuspended but nobody can predict when and where.

CLEAPART-100 is an online particle deposition rate detector with less than 5% uncertainty for counting particles of size greater than 5 µm, and it can have a built-in alert system for a smartphone, for example. Thus, the system lets cleanroom operators:

  • know when cleanroom intervention is required to clean contaminated surfaces
  • know when a particle contamination event takes place in a controlled environment or cleanroom
  • quantify the particle deposition rate and surface contamination
  • optimise the cleaning frequency and thus increase productivity by reducing cleaning team interventions
  • know the shape and lengths of surface particles
  • demonstrate cleaning effectiveness.

These functions are helpful in all industries where large deposited particles can be damaging for both products and processes in cleanrooms.

References

  1. Peter S.M. (1995) Particle Fallout in a Class 100,000 High-Bay Aerospace Cleanroom. The Journal of Environmental Sciences (IES), 38(2) pp15-17.
  2. Pui D.Y.H., Ye Y. and Lui B.Y.H. (1990) Experimental study of particle deposition in semiconductor wafers. Aerosol Science and Technology, 122, pp795-804.
  3. Wu J.J., Miller R.J., Cooper D.W., Flynn J.F., Delson D.J. and Teagle R.F. (1989) Deposition of submicron aerosol particles during integrated circuit manufacturing: experiments. The Journal of Environmental Sciences, January/February, pp27-28 and 43-45.,/li>
  4. ECSS-Q-ST-70-50C, Particles contamination monitoring for spacecraft systems and cleanrooms, October 2011
  5. ISO 14644-3 (2005), Cleanrooms and associated controlled environments – Part 3: Test methods
  6. Agricola K., (2015) Practical experiences in practical deposition monitoring. Clean Air and Containment Review, 21, pp4-8.
  7. Agricola K. (2016) Real-time Particle Deposition Monitoring of Operational Cleanroom Quality, The Journal of Environmental Sciences, 59(1) pp1-13.
  8. Tovena Pecault, I., Pasquinelli M. Aerosol and deposited particle measurements in Laser MegaJoule, European Aerosol Congress, Milan September 2015
  9. Patent FR 1550614 (2015), Process and methods for detecting surface contamination by free airborne particles
  10. Tovena-Pécault, I.,Godefroy, P., Escoubas, L., (2017) Qualification testing of an innovative system for monitoring particle contamination fallout, Sensors and Actuators A 253 pp181-187

This October, Cleapart-100 featured as one of the four shortlisted innovative products in the Cleanroom Awards at the Cleanzone show in Frankfurt, Germany.

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