Use Of A New Online Elemental Instrument For UPW Analysis Through An X-ray Fluorescence-based Approach
By Krag Peterson et al.
In recent years, metals and silica contamination in ultrapure water (UPW) have become a rising concern. The need for detecting metallic impurities in water has grown largely because of the decreasing line-width geometries in the semiconductor industry. This concern is warranted because metallic impurities can cause dielectrics to breakdown and can lead to crystal defects that greatly affect many industries. Silica, on the other hand, is an indicator of anion exchange resin breakdown. Because of these issues, the Semiconductor Equipment and Materials International (SEMI) currently recommends that many metal concentrations to be less than 1 part per trillion (ppt) and less than 500 ppt for silica (1). These stringent conditions call for accurate and precise methods of measurement.
The current approach for measuring metals uses off-line analyses, which often employ inductively coupled plasma mass spectrometry (ICP-MS) as the analytical technique. While this method has low detection limits, the process is time consuming and results can take days or weeks to be reported, which postpones the detection of problems (2). Continuous monitoring could mitigate some of these issues. With a continuous monitor, contamination is identified in real-time, thus minimizing the negative impacts of contamination. Currently, continuous monitors are available for measuring dissolved silica, but not for colloidal or total (dissolved plus colloidal) silica. Furthermore, no on-line commercial metals monitors are currently available at the detection limits required for UPW.
A service company to the microelectronics industry is developing a multi-metal continuous water analyzer (MM-CWA). This instrument represents the combination of two already existing technologies. The first is a series of near-real-time ambient and stack metals monitors, which deploy energy dispersive X-ray fluorescence (EDXRF) to measure up to 24 metals simultaneously. The instruments operate by pulling an air sample through a filter tape in order to capture and pre-concentrate metals. After sampling for a specified length of time (available sampling times include 15, 30, 60, 120, and 240 minutes), the resulting deposit is advanced into the analysis area where it is analyzed by EDXRF while the next sample is being collected. These monitors are used throughout the world by regulatory agencies and researchers including the U.S. Environmental Protection Agency (EPA), the Missouri Department of Natural Resources, and the University of Toronto. The accuracy of the technology has been validated in several studies (3-5).
The water analyzer also utilizes a second technology developed by the service company called the quantitative aerosol generator (QAG). The QAG generates an aerosol stream of known concentration that is normally used to challenge the accuracy and precision of the ambient and stack metals monitors or other particulate matter continuous emissions monitoring systems (PM CEMS). The aerosol stream is created by nebulizing a solution of a known concentration, at a measured rate and drying the resulting droplets in a measured air flow. This technology has been validated by the EPA (4).
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