Overcoming Spin-Coating Challenges for Square Substrates
Typical spin-coating processes involve dispensing a small puddle onto the center of a round substrate. This material spreads due to the centrifugal force and evenly flows across the entire surface of the substrate. Final coating thickness and coating uniformity are affected by final rotational speed, acceleration, airflow turbulence, and fume exhaust. Process engineers have utilized spin-coating techniques for decades producing predictable results with standard round substrates.
Round substrates feature an advantage in airflow dynamics compared to other shapes because the smooth, contoured edges create minimal turbulence. This shape produces relatively uniform evaporation rates, as the fluid moves toward the edge during the spreading and subsequent drying steps. The resulting edge profile effect is very consistent and can be optimized through many standard process techniques.
However, square and rectangular shapes are commonly processed for a wide variety of applications. These square and rectangular substrates create unique and complicated challenges for spin-coating due to increased air turbulence. The leading edge causes significant turbulence which leads to uneven evaporation of the film resin and poor uniformity. Common imperfections seen on square or rectangular substrates are often referred to as “edge buildup,” “fringing,” or corner “interference bands.”
Using a recessed spin chuck, a design that emulates a round substrate, solves these problems by significantly reducing turbulence and thereby increasing coat quality. Additional advantages of a recessed spin chuck includes the ability to auto-center and to provide vacuum grip.