(483e) Rinsing Flow Instabilities on Rotating Silicon Wafers – Effect of Viscosity and Wettability

Cleaning of silicon wafers is a crucial step in manufacturing silicon circuits for microelectronic applications. The thrust to develop an efficient and environment-friendly way to remove small contaminants has led to the procedure of rinsing a rotating circular wafer by an impinging jet of liquid (water). The wafer is usually pre-wet by a liquid through spin-coating before the rinsing fluid is impinged on it. A common concern in this procedure is the occurrence of fingering instabilities. They eventually form rivulets that are acted on by the centrifugal forces and spread faster than the radially expanding wave-front. This results in stagnation zones in between rivulets that are potential locations of particle build-up. Therefore, it is preferred that these instabilities are suppressed to ensure efficient rinsing.

Earlier work by our group involved rinsing experiments on pre-wet wafers and the observation that pre-wetting wafers with the same liquid significantly reduced instabilities. The influence of surface tension of the pre-wet, coating fluid on the instabilities was also explored. The wave-front shape is quantified by a parameter called the excess perimeter which is defined as the ratio of the actual perimeter of the wave-front to the circumference of the circle of the same area.

In the present work, we study the effect of the viscosity of the coating fluid and the influence of the Saffman-Taylor instabilities for this specific system. Our study also analyzes the influence of substrate wettability by comparing the rinsing wave-front development on hydrophilic and hydrophobic substrates. The flow profile for a hydrophobic, pre-wet hydrophilic and dry hydrophilic substrates show very different behaviors. The flow rate of the rinsing water and the rotation rate of the silicon wafer were varied to determine their impact. This investigation of the dependence of flow rate, rotation rate, fluid viscosity and substrate wettability strives to determine the exact conditions for instabilities in this system. This work seeks to gain significant physical insight into the fluid dynamics problem of an impinging jet on a rotating substrate where the surface tension, viscosity, centrifugal and Coriolis forces are all present. They all play a key role in shaping the wave-front propagation and associated instabilities that need to be minimized for an effective industrial process development.