Data storage companies have been driven to compete on single digit margins while producing dramatic increases in memory capacity. In the process, they have forced the development of relatively inexpensive recirculating, continuously filtered cleaning systems that ensure the components that go into a disk drive have only a few particles per square centimeter larger than 0.2 microns. In many cases, the cleanliness is automated and part of the cleaning tool. The automotive industry continues to use stagnant cleaning baths in many cases. Some have reported that cleaning is futile because the parts are more contaminated after leaving the cleaning bath than when they went in. Quality assurance is established by visual inspection, or gravimetric analysis of the residue removed from a part by means of spray extraction and collection on a filter patch.
When considering the benefits of switching to automated particle monitoring, one needs to look beyond the initial cost of equipment and evaluate all the ways the instrument can be used to save money. Several years ago, a large semiconductor fab purchased several compression batch samplers. They were assigned to two different cleanroom areas run by different managers. One manager used the sampler almost exclusively to optimize filter selection for each process. The other manager integrated the sampler into the process equipment and monitored for any process upsets. Both reported that the equipment paid for itself within six months. The information was shared throughout the company, allowing them to save a large amount of money.
Measurement Basics
Figure 2. Compression sampler |
A question that frequently arises is, “How do optical particle counters compare to other methods?” The answer is, “It depends.” Consider, for example, the many different ways to measure a particle visually. Most naturally occurring particles are irregular in shape, so how do you classify the size
Referring to Figure 2, is it the longest dimension? The longest dimension in one axis? Perhaps it should be the equivalent area? When examining most particles, each of these different methods will yield a slightly different answer.
Optical particle counters (commonly referred to as OPCs) work on one of two principals; they either measure the amount of light that is obstructed by the particle as it passes through a directional light source of known energy density (light obscuration), or they measure the amount of light that is scattered away from the directional light source (light scattering). The result is compared to the amount of light (obscured or scattered, respectively) by a calibration particle of known size and shape, typically polystyrene latex spheres (PSLs). In other words, an optical particle counter will report natural particles as the optical equivalent diameter of a PSL suspended in water.
Figure 3. Measuring particle size
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While this may result in a slight difference in the measurement obtained by other methods, the repeatability of this measurement is highly reliable. Couple this with its relative ease of use and reduced labor requirements and the benefits become significant.There are a few things to consider when implementing a monitoring program using OPCs. When monitoring on-line, care should be taken to eliminate any source of particle shedding upstream of the particle counter. Valves that are used to isolate the OPC when not in use at a given location should be opened 100% during monitoring. Flow control should be accomplished downstream from the OPC.
Partially open valves contribute to particle counts in two ways:
When performing batch sampling operations, it is important to allow sufficient time for the sampling apparatus to clean up. Proper operation of the equipment is essential. Following the manufacturer’s recommendations will usually ensure quick and simple testing with this type of equipment.
Sample Volume versus Sensitivity
When selecting a particle counter, the value of total particles counted per unit time cannot be over-emphasized. In general, as particle counters approach the limits of technology, the volume of liquid assessed by the instrument will be less than 100% of the flow rate. Some manufacturers of particle counters do not publish this difference in their specifications, so it is important to ask, “What is the sample volume?”
Figure 4. Particle counter with high sensitivity and high flow rate
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If the application of interest requires high sensitivity (ability to detect very small particles) in a very clean fluid, you must evaluate the time required to count a minimum number of particles. In many ultra pure applications that time may be several hours. Unfortunately, a sample interval of this length does not allow the operator to detect temporary upsets of short duration. If the ability to detect these upsets is important, consideration should be given to sacrificing sensitivity for a particle counter with greater sample volume. It should be easy to understand that process upsets will not generate only one size particle, but rather an increase in particles of all sizes proportionately. Therefore, a less sensitive particle counter will be able to detect the upset and report it as it happens because it can be set with a sample interval of only a few minutes.
Conclusion
Optical particle counters are used in many industries. However, they are not always used effectively. Industry benchmarking could help many companies just entering the ultra clean manufacturing environment to make significant strides forward.
While there are differences between OPCs and visual measurement techniques, they are not significant and the highly repeatable nature of OPCs makes them an operator’s tool of preference.
Finally, consideration must be given to the number of particles counted in the sample interval desired. If this number is not enough for statistical significance, the better choice is often a less sensitive particle counter with greater sample volume. •
For more information, contact Dwight Beal, liquid product line manger, Particle Measuring Systems, at dbeal@pmeasuring.com or by phone at 303-443-7100.