SEMI F121
Guide for Evaluating Metrology for Particle Precursors in Ultrapure Water
The UPW International Roadmap for Devices and Systems (IRDS) has defined particle precursors as dissolved compounds that can form particles when dried on a wafer surface. A recent IRDS study found that particle precursors can form particles that are large enough to impact device yield.
There are potentially multiple sources of particle precursors, which are believed to be primarily organic in nature, in UPW systems including ion exchange resins, tubing and piping, fluid handling components and filter membranes. Given that particle precursors are dissolved, they are difficult to remove with traditional filtration techniques and cannot be detected by conventional light-scattering particle detection technologies such as optical particle counters. Liquid nebulization in conjunction with aerosol-based condensation particle counting has the proven ability to not only detect and count particles as small as 2.5nm, but are a useful tool for quantifying particle precursors in liquids.
When liquid is dried on a wafer surface, native particles in the liquid may adhere to the wafer surface and dissolved compounds may “form” a particle on the surface of the wafer. This process is mimicked by nebulizing the liquid sample to form droplets and evaporating the liquid to form an aerosol. This aerosol contains native particles and “formed” precursor particles.
After normalizing for dilution ratio, the native particle concentration in the original sample is unchanged. As dilution ratio decreases, the particle precursor signal becomes apparent.
When liquid is dried on a wafer surface, native particles in the liquid may adhere to the wafer surface and dissolved compounds may “form” a particle on the surface of the wafer. This process is mimicked by nebulizing the liquid sample to form droplets and evaporating the liquid to form an aerosol. This aerosol contains native particles and “formed” precursor particles.
After normalizing for dilution ratio, the native particle concentration in the original sample is unchanged. As dilution ratio decreases, the particle precursor signal becomes apparent.
The particle precursor signal can be quantified by converting the particle concentration data to mass concentration. This is performed by nebulizing filtered, dissolved KCl solutions at various concentrations and measuring the resultant particle size distributions. Thus, a calibration curve is generated which relates the measured particle concentration to a mass concentration as KCl.
Cumulative particle size distributions of KCl calibration standards, as measured by LNS
