For a thick, quick, blanket coating of material, it is sometimes possible just to pour it over the surface! The technique is familiar from photolithography, where photoresist-polymer precursors are dissolved in a solvent. The wafer is spun at high speed (up to several thousand rpm) and a droplet is dripped onto its centre. This is thrown outwards, coating the surface instantly. The wafer is then baked to drive off the solvents and leave a solid film, perhaps 1 mm thick. This spin-on technique is not just used for polymers, however. Recently, solvent solutions of silicon dioxide have been developed that allow spin-on glass (SOG) to be deposited in just the same way.
The technique is not suitable for critical layers: the solvent solution is likely to contain many impurities; uniformity is poor, with a thick ‘edge bead’ at the wafer edge; and since film thickness depends critically on the viscosity, control is difficult. For high-quality films, a vacuum deposition technique is required.