Spin Coating on Freeform Surfaces: A Numerical Study

Abstract Spin coating on flat substrates is a well-understood fluid phenomenon, creating uniform thin films with microscale and nanoscale thicknesses. It is widely used in the microscale and nanoscale fabrication of functional devices. Beyond flat substrates, there is an increasing manufacturing demand for fabricating micro/nanoscale thin films on non-flat substrates, such as spherical and freefrom surfaces. However, it is still challenging to model spin coating on a non-flat substrate. This paper numerically models the spin coating on a freeform substrate as a laminar flow. The centrifugal force from spinning was modeled as a volume force proportional to the spin coating speed square and radius. Various initial thicknesses were simulated and compared. A two-way sinusoidal surface was used as the freeform surface. The simulation results showed that the coating thickness converged to an asymptotic solution, regardless of the initial coating conditions. Also, similar to the spin coating on a flat surface, we found the thickness of the spin coating on a non-flat surface was halved when the spin coating time was increased to four times. These surprising results produce a predictable thin film coating on the freeform surfaces, which enables microfabrication on the freeform substrate.