What Are The Purity Requirements For Microelectronics UPW Piping/Distribution Systems?
By Frank Patrick
The article’s title asks about the purity requirements for high-purity distribution systems in microelectronics water systems. We will begin this Backgrounder article with a primer on the evolution of high-purity water piping materials.
The progression of ultrapure water purity (UPW*) requirements has led to many changes in the materials of construction (MOC) used to generate and transport high-purity water (1). In the early 1980s, most major semiconductor companies used polyvinylchloride (PVC) as high-purity water distribution piping. Ball valves with glued socket joints were also commonplace.
PVC distribution systems were inexpensive and adequate while integrated circuit geometries where were well above 1 micron. As the transistors became dense on the product wafer and critical dimensions (CDs) shrank, defects as a result of the trace organics and particles began to severely impact product yield, resulting in lower profit margins.
At that time, the impact of the residual glue from socket PVC joints, organic plasticizers (which are additives used in PVC formulation which increases the fluidity of the material), and the human handling of piping materials during installation were unknown. Trace organics could not be measured via on-line analytical equipment.
The trace organics conveyed by high-purity water to the process tools deposit a fine film on the wafer surface, interfering with photo resist application and preventing the proper imaging of the integrated circuit patterns, resulting in defects (2, 3) known as “missing” (see Figure 1). The impacted wafers became scrap.
Another high-purity water concern is bacteria (4). Poorly designed deionized (DI) water distribution systems were susceptible to biofouling. With trace organics as a food source, biofouling could be detected by routinely analyzing water samples for bacteria. Unlike static water supply systems used to provide residential drinking water, a DI water distribution system continually recirculates treated water to the points of use (POU). The unused supply water is returned to the treatment system via dedicated “return plumbing” to once again flow through water treatment equipment (see Figure 2).
Keep in mind; high-purity distribution systems are designed at flowrates that include “maintenance flows”, which ensure at maximum consumption (fab use) there remains ample water in the system to allow for recirculation.
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