Spin-coating is a process used to evenly distribute material on a substrate by spinning it at a high speed. Round substrates are commonly used because their smooth edges create minimal turbulence, resulting in uniform evaporation rates. However, square and rectangular substrates create unique challenges due to increased air turbulence and can result in uneven coating thickness and poor uniformity. A recessed spin chuck, which emulates a round substrate, can be used to solve these problems by reducing turbulence and providing auto-centering and vacuum grip capabilities.
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.”
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