Over-sized or agglomerated particles (>1micron) can wreak havoc on polished semiconductor wafers by creating microscratches.
Although the mean diameter of most CMP slurries is on the order of hundreds of nanometers, relatively small amounts of larger particles can render a wafer useless. Single particle optical sensing (SPOS) allows the user to individually size and count these particles with unprecedented accuracy before they cause damage to the wafer. With SPOS single particles flow through a sensor and pass through a focused beam of light. As the particles pass through the laser beam, light is blocked and a photodiode records a decrease in light intensity. This decrease in intensity is proportional to the cross sectional area of the particles. PSS offers a variety of Accusizer systems that use SPOS technology. The online systems provide ease and accuracy while the bench top systems are perfect for smaller scale research projects. The histogram shows the SPOS result of an agglomerated cerium oxide CMP slurry. This portion of the particle size distribution is often referred to as the “tail” of the distribution. This particular distribution contained roughly 7million particles per milliliter greater than 1 micron and 22 thousand particles per milliliter greater than 2 microns. With an Accusizer particle size analyzer, the user is able to obtain high resolution information about a sample that cannot be resolved with conventional dynamic light scattering or light diffraction techniques.
A recent study spiked 1 µm PSL particles into a silica CMP slurry. The slurry was analyzed without dilution on the AccuSizer FX point of use (POU) system both with and without the 1 µm PSL particles. The estimated spike concentration was ~ 5000 particles/mL. The results are shown below.