Particle Precursors
Particle precursors are predominantly organic compounds released from polymeric materials used in the production and delivery of ultra pure water (UPW). These dissolved compounds are invisible to optical particle counters and are difficult to measure by offline techniques due to their low mass concentrations. A 2021 White Paper by the International Roadmap for Devices and Systems (IRDS) UPW Task Force identified the potential for particle precursors to form particles on wafers of sufficient size to impact yield.
A series of more recent studies by CTA and our partner labs has put CTA in a unique position to help clients measure particle precursor contributions from their materials and components, allowing management to take proactive risk mitigation.
First, studies sponsored by the IRDS Critical Components Task Force have detected particle precursor extracting from PFA and PVDF piping material in hot UPW. These have also been shown to generate particles ≥8 nm on a wafer surface. Shown below are a set of results from the 2022 Ultrapure Micro Conference presentation, “Critical Organics Risk Assessment of High-Purity Polymer Piping”. A series of wafers were prepared using a technique called Sequential Spin Coating (SSC), where increasing numbers of dispense/dry cycles of these extracts were applied. On-wafer measurements were then conducted, using UNISERS Surface Enhanced Particle Sizing (SEPS), to investigate the accumulation and distribution of nanoparticles that adhere to, or form on, the wafer surface.
On-Wafer Particle Size Distribution of PFA and PVDF Extract Prepared with Sequential Spin Coating (SSC) Measured by UNISERS Surface Enhanced Particle Sizing (SEPS)
Another study, sponsored by the SEMI UPW Task Force, has led to the identification of new classes of potential particle precursors, extracted from ion exchange resins in hot UPW. Furthermore, chemical pathways to additional potential particle formation were identified. Development is also in progress for a particle challenge, based on ion exchange resin extract, to evaluate sub-15 nm filters. Shown below are some results of an aging study of a candidate material. Check out our presentation on this topic from the 2023 Ultrapure Micro Conference.
Aging study of a mixture of three different molecular weight polymers (approximately 3, 5, and 8 nm)
Finally, a study on the application of a new SEMI F121 "Guide for evaluating particle precursor metrology" demonstrated how to distinguish between native particles and particle precursors, and the ability to quantify the particle precursor concentration found in “real-world" extracts. Shown below is a series of calibration curves made from different concentrations of diafiltered KCl, as measured by Liquid Nanoparticle Sizing (LNS), which nebulizes and dries the measurement stream for analysis as aerosol. This calibration is used to quantify the particle precursor concentration by normalizing the aerosol distribution of an extract to mass concentration as KCl. For more information, check out the rest of the presentation from the 2023 Ultrapure Micro Conference.
KCl Calibration Curves for Particle Precursor Quantification via Liquid Nanoparticle Sizing (LNS)
