The Effects of Weak Viscoelasticity on Enhanced Particle Removal for CMP

Highly elastic solutions have been shown to increase particulate removal from silicon wafers, indicating the possibility for reduced reliance upon hazardous chemicals and expensive technologies during post-CMP cleaning processes. We explored the onset of this enhanced removal using micron-sized silica particles in rinsing and siphoning experiments with polymer fluids of high molecular weights that ranged from dilute to entangled concentrations. Increases in particle removal with less than 100 ppm of polymer were observed using both methods, with 10% increased removal observed in the rinsing experiment and 4% increased removal in the siphoning experiment relative to water. Further increases in polymer concentration increased removal significantly, as the solutions entered the semidilute regime. Highly concentrated solutions showed “streaking” patterns, consistent with the viscoelastic nature of the solution. Additionally, we examined particle removal as a spatially dependent parameter in siphoning experiments, showing a plateau of significant removal near the siphon with a sharp decay to a constant value at distances farther away from the siphon. Rheological characterization of the siphoning and rinsing polymer solutions was used to explore relationships between observed removal and solution properties. Increases in particle removal with solutions having nearly indistinguishable shear rheology and immeasurable extensional character were observed, indicating the need for more precise methods for extensional characterization of dilute polymer solutions. The application of polymer solutions to particle-removal processes has the potential to simplify post-CMP cleaning and reduce the environmental impact that is associated with present particle-removal chemistries. Figure 1: Removal profile of micron-sized silica particles on glass slides that were subjected to fluid siphoning. Crosshairs indicate the point on the glass slide directly under siphon. Siphoning solutions consisted of water (a), and poly (ethylene oxide) at concentrations of 50, 100, 500, 1000, and 5000 parts-per-million (b-f, respectively). Particulate area was determined by the total pixel count of silica particles in each image. Removal profiles for dilute solutions (a-c) show similar spatial dependence with mild increases in average removal. Removal profiles for semidilute solutions (d-e) show clear radial dependence while the concentrated solution showed behavior associated with significant elastic character. Figure 1