This paper discusses how to ensure that a liquid optical particle counter (OPC) is producing valid data.

Ensuring that your liquid OPC is operating within its designed specification is critical to obtaining quality data. As data from OPCs becomes increasingly important to maintaining process control, it becomes more critical than ever for operators to have confidence in the data being generated. OPC data is frequently used to qualify process improvements, control chemical manufacturing, and maintain quality control for production of complex parts. It is common for operators to wonder if the OPC is working correctly when the data collected does not match their expectations. Common misunderstandings and performance issues about OPCs are discussed to help the operator reach proper conclusions when questions arise about the validity of the data.

Operators can take several easy steps to ensure that quality data is collected from an OPC. All OPCs have defined and easily-controlled specifications. These specifications are:

• flow rate
• background scatter
• concentration limit

Particle size distribution is also important. Unless all these parameters are correct, the data quality should be considered suspect. These parameters and their effects on each other are discussed below, as well as how to ultimately ensure that quality data is achieved when analyzing ambient particle distributions in process chemicals and DI water.

Cleanliness testing for liquid systems can help ensure components meet the standards aimed for.

Contamination from fluid handling components can eventually reach the processing environment either in the form of solid particulates or, through chemical reaction, as a form of ionic contamination. Due to the cost associated with any yield impacts, components must be ensured clean prior to their use in production equipment, which will interact with the final product. Establishing a simple component test program that will confirm initial and long-term cleanliness of critical fluid handling components down to 20 nm for plastic contaminants and < 9 nm for metallic contaminates using Particle Measuring Systems Ultra DI® 20 or Chem 20™ liquid particle counter is discussed here. This simple step can save both time and money in the production environment by selecting the best components for the designated task.

In liquid handling systems components such as valves, pumps, filters, degas modules, etc. are common sources of particles.

A simple test station can be constructed to help determine which component type and supplier has the lowest particle shedding or generation (see full paper for test schematic). This test setup can also be used to determine component- to-component variability. It is an easy set up that can be established by the component manufacturer or component user.

The setup includes a bypass line around the component under test to allow continuous flushing when the test piece is removed, thereby maintaining system cleanliness at all times. It is recommended that after every new component under test is installed or removed, water is flushed through the system to remove any air in the lines before the particle counter is turned on.

Topics discussed in this paper:

• Fluid systems and their sources of contamination
• Valve, pump, filter and degas module testing
• Recommendations for fluid system setups to reduce contamination

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.

Ultra-pure water particle measurements can take a long time to return to a stable baseline after an event interruption. Learn about factors affecting stabilization time.

Modern ultra-pure water (UPW) systems must be purified to meet the increasingly stringent standards of high-end manufacturing processes. It is common to see high-end UPW systems containing particle concentrations in the single-digit particle/ml range or lower for particles larger than 20 nm. Monitoring particles in a UPW system is crucial to ensuring the particle level remains below acceptable control limits and does not threaten production.

When the steady-state “baseline” particle level is low, the time taken for particle counts to return to the baseline level can be considerable and can be attributable to major interruptions such as shutdowns to perform maintenance and install new components or sample lines to the system’s regular operation. Depending on the size and complexity of the system, the stabilization period can last for several weeks or even months. In addition, the high-sensitivity particle counters required to monitor particle levels in UPW systems need time to reach a stable operating temperature after powering up. During this initial warm-up phase, the particle counts reported by the particle counter may be temporarily elevated before decreasing to the actual level as the instrument reaches thermal stability.

This application note presents the results of an evaluation performed using an Ultra DI® 20 (UDI-20) Liquid Particle Counter to investigate various factors that influence the stabilization time of particle measurements in a UPW system. Factors tested include the warm-up period after powering on, the effect of disconnecting and reconnecting sample tubes and renewing the flared connection, the clean-up time after installing new sample tubes of various lengths, and the clean-up time after installing a new PFA diaphragm valve. These findings should help the reader understand why UPW systems can take a significant amount of time to clean up after an interruption and which factors are likely to have the most significant influence.

In this paper, an Ultra-DI 20 Liquid Particle Counter instrument is used to characterize the clean-up time after various actions.

Which Ultrapure Water Particle Counter is Best for Your Application?

Trends of improved cleanliness of liquids and particle size sensitivity have increased the complexity and cost of liquid particle counters. To choose the best particle counter for your ultrapure water application it is important to understand if the goal is to monitor for alarms of extreme out of specification conditions or monitor for both alarming and trend analysis. These factors are discussed and a variety of ultapure water particle counters compared.

Interested in learning more?

Complete the form on this page to receive the paper. 

Looking for an ultrapure water particle counter? Particle Measuring Systems has industry leading 20 nm solutions with the Ultra DI 20 DI water particle counter.

Other solutions include:

• Ultra DI 50 particle counter
• Liquilaz
• HSLIS