Effect of Particle Size Distribution on Filter Lifetime in Three Slurry Pump Systems
Mark R. Litchy – CT Associates, Inc., Bloomington, MN
Reto Schoeb – Levitronix GmbH, Zurich, Switzerland
Slurries, suspensions of fine particles dispersed in a liquid, are often used in semiconductor chip manufacturing to planarize wafer surfaces. The effectiveness of these slurries in achieving a flat surface free of scratches is highly dependent upon the physical properties of the slurry. Perhaps the most important physical properties of the slurry are the size of the fine or “working” particles in the slurry and the presence of large particles, often referred to as the large particle tail, which can cause scratches.
Delivery systems are often used to supply the slurry to the planarization tools. These systems pressurize the slurry to deliver it to the tools and circulate it to help keep the particles in suspension. Pressurization and circulation are accomplished by various means including a variety of pumps and pressure-vacuum technology. Typically, the slurry passes through the slurry distribution systems approximately 100 times before it is used to polish wafers1.
Many shear sensitive slurries are easily damaged by mechanical handling. Damage often takes the form of changes in the size distribution of the slurry particles. In a previous study, experiments were performed to determine the effect of circulating Semi-Sperse? 12 (Cabot Microelectronics Corporation) with three different types of pumps (bellows, diaphragm, and centrifugal) on the size distribution of the particles in the slurry. The size of the working particles (particles typically ~ 0.1 ?m in diameter that are performing the planarization) was measured with a NICOMP 380 ZLS Submicron Particle Sizer (Particle Sizing Systems) that determines particle size by dynamic light scattering. The particle size distribution (PSD) of the large particle tail was measured with an AccuSizer 780 optical particle counter (Particle Sizing Systems). Significant differences in the large particle tail of the slurry PSD were observed after circulation with three different types of pumps.
Large particles that can scratch wafer surfaces are removed by filtration. The large particles tend to occlude the filter causing reduced flow rates and large pressure drops. Chemical-mechanical polishing (CMP) filters must be changed regularly, in some cases every few days. The frequency of filter change depends on many factors including the type of slurry, type of filter, pore size rating of the filter, etc. The increase in pressure drop across a filter determines the life of a filter. This paper describes the results of a study that was performed to determine if the changes observed in the large particle tail PSD as a result of pumping correlated to changes in the lifetime of filters used to remove large particles from the slurry. The increase in pressure drop across a 10” Mykrolis PlanargardTM CMP3 filter as a function of delivered slurry was characterized for three types of pumps. The expected results were observed: the higher the concentrations of large particles, the faster the filters clogged.
CTA publication #57: Materials Research Society Symposium Proceedings, Vol. 867, W2.8.1, 2005