Selecting the right particle counter

Knowing which features are important when specifying a particle counter can be difficult. The right particle counter will depend on the monitoring environment, communications, monitoring purposes, desired flow rate, and particle size to be monitored. The pharmaceutical sector, for example, has special monitoring requirements that are not completely addressed here, but for other industry sectors, selecting an airborne-particle counter requires a basic understanding of the terminology and procedures. All airborne particle counters sample air at specified volumetric flow rates, which are measured in cubic feet per minute (cfm) or litre per-minute (lpm). Since particle counters are calibrated at a specific flow rate, the sizing accuracy depends upon that flow rate. To meet most contamination specifications, particle counters must sample defined volumes of air; this provides confidence in the particle data and is often stated as of statistical significance. The ISO standard refers to cubic metres; to meet this standard, a 1cfm particle counter should sample for 35 min (one cubic foot multiplied by 35 equals one cubic meter). Particle counters with faster flow rates can meet the ISO specifications in less time. A particle counter with a flow rate of 50 lpm can sample one cubic metre in only 20 mins. The ISO standards prescribe limits for contamination and the limits can be converted to the obsolete Federal Standard 209E (FS-209E). In most cases the ISO limits are available for common particle sizes, such as 0.1µm, 0.3µm, and 0.5µm. When the primary application is cleanroom certification, an instrument that has ISO certification features simplifies the process. The particle testing method must be accurate, reliable, and repeatable. Modern cleanrooms consistently meet ISO Class 5 or 6 (FS-209E Class 100 or 1000), and these classifications require set limits (see table 1). Monitoring a cleanroom in accordance with ISO cleanroom classifications requires the particle counter's specification for maximum concentration to exceed ISO limits. For example, to monitor a Class 5 cleanroom for 0.1µm particles, the maximum concentration of the counter must be greater than 100,000 particles/cubic meter (2,841 particles/ cubic foot). Using a 0.3µm particle counter to monitor the same Class 5 cleanroom requires a particle counter maximum concentration value of greater than 10,200 particles/cubic meter (290/cubic foot). These are easily achieved limits with most modern particle counters. Note that there is no ISO specification for 0.1µm particle counts higher than ISO Class 6, so a 0.1µm particle counter is not required for those applications. Continuous or frequent monitoring in the cleanroom still needs to demonstrate compliance to ISO. Frequent monitoring requires sampling at specified time intervals, not exceeding 60 mins, during operation. Manifold systems are the least expensive suitable solution and are best installed during the cleanroom construction process. Standalone particle counters may be installed at any time. A manifold system includes either 16 or 32 sampling ports with a single line that connects to a particle counter; the manifold sequentially samples from each port, sends the samples to the particle counter, and then repeats the process. However, because a manifold cycles through many sample points, a particle event can go unnoticed if the particle counter is not currently monitoring the appropriate port. Continuous monitoring requires constant sampling: this method constantly gathers data so that events are not missed. Sample intervals can be of any duration, but shorter sample intervals will give better time resolution. Short time intervals will provide vast quantities of data that can overwhelm a system. Typical time intervals range from one to 10 mins, and particle counter choices for these applications are diverse. Factors to consider are particle size, flow rate and communication options. If a cleanroom offers network ports (Ethernet's 10Base-T or 100Base-T), select a particle counter with networking capability; if it relies upon serial communications, select a particle counter with RS-232 or RS-485 communication protocols. Choosing between continuous and frequent cleanroom monitoring is a choice of economics and infrastructure. Dedicated particle counters are the best method to detect particle excursions, but come at a high cost per sample point. If short-duration events are not critical and the need is for trending, then a manifold system can be an effective and economical solution. After choosing the type of monitoring method desired, the next step is to determine how many monitoring locations or particle counters are needed. The total number of locations required by ISO can be calculated by determining the area of the cleanroom (in m²) and finding its square root. Using this formula and a typical cleanroom area of 9290msup>2 (100,000ftsup>2), the square root of 9290 is 96; ISO therefore requires 96 monitoring locations. These locations should be evenly distributed and mounted at work height: 76cm (30in). Strictly speaking this guideline applies only to cleanroom certification. Cleanroom operators should evaluate their processes and the sensitivity of their product to contamination to determine the number of sampling locations required. Our advice is to monitor where it counts; that is, measure where your product is exposed and where contamination will cause damage. In the case of semiconductor manufacturers that use standard mechanical interface (SMIF) pods or front opening unified pods (FOUPS) the wafers are not exposed to the general cleanroom environment, making monitoring in this area less critical. However, the wafers are exposed in the minienvironment (used to isolate the product from the main source of particles), so monitoring efforts should be focused there. Monitoring should be concentrated where the risk is highest; in the case of minienvironments, this is often near the load ports where wafers are loaded. Minienvironments isolate the product from the main source of particles (people) and are often classified as ISO Class 1 or 2 (no FS-209E classification exists for these levels of cleanliness). Within these classifications most instruments can easily remain under the maximum concentration limits. Minienvironment particle data often follows trends in differential air pressure, so an instrument's ability to correlate particle and differential pressure data provides trend analysis, yield improvements, and accurately scheduled preventative maintenance cycles. Published minienvironment particle data1 shows particle concentrations clustered near 0.4µm, and since the cost of a particle counter doubles as the sensitivity increases from 0.3µm to 0.1µm, the most cost-effective continuous monitoring solution is a 0.3µm particle counter with an inclusive differential air pressure (DAP) probe. For validation and certification, a 0.1µm counter is recommended for ISO class 1 and 2 minienvironments. Depending on the level of accuracy required, testing filters may require specialised particle counters. Aerosol spectrometers employ more than 32 channels for particle size distinction and resolution; spectrometers provide the most detailed information regarding particle sizes and distributions, but are expensive. Standard 0.1µm or 0.3µm particle counters can easily monitor filters and valves and are usually installed upstream and downstream of the filter or valve. This technique provides accurate filter efficiency data and alarming for contamination problems, but may not be desirable for testing valves. Filters use an efficiency rating specified at the most penetrating particle size (MPPS). Standard specifications dictate the filter's efficiency at a specific MPPS and velocity. High Efficiency Particulate Air (HEPA) filters have a minimum filtering efficiency of 99.99% at 0.3µm, and Ultra Low Penetration Air (ULPA) filters have a minimum filtering efficiency of 99.999% at 0.12µm. Detecting penetrating particles requires a particle counter with at least 0.3µm sensitivity for HEPA filter testing and 0.1µm sensitivity for ULPA filter testing. Valve testing procedures are outlined by the semiconductor association SEMATECH. By their nature, valves tend to trap and shed particles, so sampling particles from a valve can provide unreliable data. Therefore, some of the particles detected may be generated by the process and others may come from the valve, so valve cleanliness reports are difficult to generate.
Lab counts Lab testing applications do not typically need to meet ISO cleanroom requirements. These applications seek a specific number of particle counts within a certain size range, and this number defines whether the lab components will pass or fail. Particle counter selection is dependant on the components being tested in the lab, so the lab must define the critical particle size limit (in µm) and the acceptable maximum concentration limits. High flow rates are often desirable as they increase throughput, reduce sampling times and gather more data. Since lab tests tend to focus upon sub-micron contamination, the choices narrow for particle counters. Harsh environments require special instrumentation. These environments may include pharmaceutical labs, cleanroom make-up air handling (MUAH) units, fan decks or aerospace launch facilities. These conditions require particle counters that are isolated from the environment but still provide accurate air sampling. Particle counters developed for harsh environments are often housed in NEMA-rated enclosures, which isolate the sensitive optics and electronics, while providing an external probe for monitoring the particle concentrations. The interests of pharmaceutical laboratories lie only in 0.3µm and 3.0µm or 0.5µm and 5.0µm. Some particle counters offer screen/data configurations that only display/print these specific channels. If a pharmaceutical lab contains heavy concentrations of hydrogen peroxide, a particle counter with resilient coating and high maximum sampling concentrations, such as the Airnet 510 XR, is recommended. MUAH's fan decks and launch systems require robust enclosures that can withstand conditions outside normal room environments. Appropriate particle counters for this use have enclosures made from stainless steel or Kydex, which provide superior resistance to damaging external conditions but have proven reliability in particle counting. When choosing a particle counter for counting particles in gas, it is necessary to know if the gas is reactive and what the pressure range is. Reactive gases include, but are not limited to, hydrogen and oxygen. These gases require a special particle counter stored inside a containment vessel, which should be designed to withstand moderate levels of overpressure resulting from detonation between mixtures of hydrogen and oxygen. Usually, the containment vessel is back-filled with nitrogen, an inert gas that neutralises small volumes of reactive gases. It may be possible to monitor other reactive gases, but the user must carefully evaluate the wetted materials of the particle counter to ensure compatibility with the gas. Furthermore, the user should consider additional precautions, such as leak monitoring, purge flow monitoring and any other measure to ensure safe operation. Sampling gases at pressure is preferred, therefore gas instruments employ mass flow controllers to provide constant, volumetric flow rates when connected to gas line pressures between 40~150psig. Particle sizing can differ with pressure and the composition of gas, so gas particle counters must account for these variables. A gas constant entered into the instrument's data system provides correction factors for different gases and allows the mass flow controller to increase/decrease the flow rate based upon the chemistry of the gas. If the gas is reactive and falls within the specified pressure range, it can be sampled using Particle Measuring Systems' High-Pressure Gas Probe (HPGP). The HPGP-101-C offers 0.1µm sensitivity, 0.1cfm flow rate and a containment vessel tested to confine overpressures of 3200psig. Non-reactive gases, such as argon, helium, neon, nitrogen, and xenon, have different monitoring requirements. A dedicated gas particle counter, such as the Micro Laser Particle Counter (MLPC-101-HP), provides accurate measurements and particle counting in pressurised, non-reactive gases. Another option for non-reactive gas monitoring is to connect a high-pressure diffuser (HPD) to a standalone particle counter. The HPD accommodates pressures from 40~100psig and dilutes the gas sample with ambient air, which provides humidity and thus prevents degradation in a particle counter's optics and plumbing. A particle counter combined with an HPD typically has the lowest initial cost. However, monitoring at reduced pressure is not as effective as monitoring at pressure due to the following factors:

• the potential for particle loss in the diffuser • a smaller percentage of the total gas flow is measured • the diffuser can become contaminated and cause artificially high particle measurements • the design of a diffuser wastes gas that is not measured

Dedicated gas particle counters should be used for critical applications and any measurement of reactive gases. HPDs should be used for less critical applications or for occasional monitoring of non-reactive gases. Following these basic guidelines when purchasing a particle counter will ensure that application requirements are met without incurring costs for additional features that are not needed.