CFD Simulation for the Semiconductor Industry

Refining our existing solver for phase changes of water vapour, we added the capability to deal with phase changes of arbitrary liquids. This enables the solver to compute, for example, the evaporation of the solvent in a spin-coater. This makes it now possible to develop and optimize such systems using CFD simulations that can describe the spatiotemporal evaporation behaviour for different machines, chemicals and operating conditions.

The solver can be used for all transient optimization questions in which phase changes (evaporation, condensation, ...) are important factors. This is the case for example in various technical printing methods, chemical vapour deposition (CVD; for the production of diamond-, silica carbide-, titan nitride- and similar coatings), extraction processes, technical drying processes, production of certain pharmaceutical drugs and so on. As an example we demonstrate the solver for a photolithographic coating in a spin-coater:

Comparison of eddy formation in experiment and CFD simulation.

Photolithography is the single most important manufacturing technique in micro electronics and micro mechanics (MEMS, MST) for the production of integrated circuits (ICs). Simply put, a carrier material - usually a pure silica waver - is coated repeatedly with various substances. For the quality of the coating it is crucial to achieve a coating with extremely uniform thickness. The dominant technology to achieve this is spin-coating. The circular waver quickly spins around its central axis while the liquid, so-called photoresist is added in the center. Due to the rotation the photoresist spreads out in a thin coating while the solvent evaporates, solidifying the coating.

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Due to the fast rotation, the complex air flow around the waver and the speed of evaporation of the solvent, a complex system evolves, that influences the uniformity of the coating. The picture on top of this page shows one example of such a complex pattern on the left hand side from an experiment [1] and on the right hand side from our simulation (see also video of eddy formation on a rotating plate). The axially symmetric basic structure of spin-coaters means that different rates of evaporation will occur, this in turn, influences the viscosity of the photoresist which causes disturbances in the thickness of the coating. The evaporation process can be described using our solver, opening up new possibilities to develop and optimize the established techniques.

The video above shows the first couple of seconds of a spin-coating process in slow motion looking down on the waver. On the right hand side of the video you can see the rotation of the solver including the spin-up process; on the left side the evaporation of the solvent (remaining non-evaporated solvent) is visualized. It is clearly visible how the solvent dries faster at the edge of the waver due to the higher flow of ambient air.

[1] Gauthier et al., "Axisymmetric propagating vortices in the flow between a stationary and a rotating disk enclosed by a cylinder" in J. Fluid. Mech. (1999), vol. 386, p. 111