Microbial Contamination Monitors

Particle Detection in Liquids with High Viscosity or Molecular Weight

Particle detection in liquids with high viscosity or high molecular weight has unique challenges. High viscosity and high molecular weight liquids can scatter laser light, due to the molecules that comprise the chemical. This can interfere with light scattered off particles when using a laser optical particle counter, creating false-positive counts. A method is described to measure high viscosity or high molecular weight liquids using a syringe sampling system and LiQuilaz particle sensor.

Laser-based particle counters have been used extensively to detect particles in DI water and process chemicals such as hydrofluoric acid. Not as many users have applied these tools to monitoring contamination in liquids with higher viscosity and/or molecular weights such as photoresist and antireflective coatings. Monitoring particles in such fluids is important for both the manufacturers and end users of these products. Manufacturers must ensure the cleanliness of their products before they leave the factory. End users need very clean materials which will not increase product defect densities.

Traditionally, the cleanliness of photoresist and ARCs has been measured by applying them to a silicon wafer and inspecting the wafer on a laser-scanning system. This is wasteful of valuable chemicals, silicon
wafers, coating system time and operator effort. A better method would be to use a particle counter on the liquid itself. However, photoresist and other light-scattering media present numerous challenges to particle monitoring equipment. Some sensors can be overwhelmed by the light scattered from the fluid and produce inaccurate results. Other systems work very well for sampling out-gassing liquids but use a debubbling sampler which can be wasteful of very expensive fluids. Furthermore, since the liquids to be sampled were designed to coat surfaces, debubbling samplers can be difficult and time-consuming to clean between samples.

The solution is to use an LS-50 sampler and a LiQuilaz®-S02 sensor which has been optimized for the parameters of the liquid. The LS-50 sampler has a 25ml syringe and is available in materials compatible with Hydrofluoric acid. It has a smaller foot print than a debubbling sampler and does not require a source of pressurized gas for its operation. The high sampling volume of the LiQuilaz® sensor and the minimal internal volume of the LS-50 result in a cost efficient method of gathering particle data. The selectable thresholds of the LiQuilaz® sensor allow it to be used with fluids that have high molecular scatter coefficient.


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