|The resist film thickness attained by spincoating represents the equilibration between centrifugal force and solvent evaporation (both increasing with spin speed). Generally, the last two digits of the resist name (e.g. AZ® 6632) indicate the film thickness d attained by spincoating (without gyrset) at 4.000 rpm in 100 nm units. The thickness approximately decreases with the (increasing) square-root of the spin speed (in rpm).
Especially in case of coating thick resist films, a so-called edge bead forms which may cause sticking to the mask as well as an undesired proximity-gap during exposure (with a reduced lateral resolution as a consequence). If no automatic edge bead removal is possible, initial stages for solution to avoid/lower the edge bead are i) an elevated spin-speed for a shorter time, ii) a ‚spin-off‘ of the edge bead by abruptly increasing the spin speed at a certain stage of spin-coating (The resist film should be solvent-poor enough to prevent further thinning, while the edge bead needs a viscosity still low enough for proper spin-off), iii) a multiple coating with an elevated spin speed for each coating cycle, iv) a delay between coating and softbake prevents the edge bead from growing during softbake due to the thermally reduced viscosity, v) in case of round substrates a manual edge bead removal at e.g. 500 rpm with dispensed AZ® EBR solvent, vi) in case of edged substrates, if applicable, removing (breaking) the outer pieces of the substrate bearing the edge bead; alternatively manual cleaning the substrate from the edge bead with clean room wipes.Particles or Bubbles?
The appearance of air bubbles or particles in the coated resist film is very similar to each other. In this case, it has to be checked if a careful dispensing (bubbles), or better substrate cleaning (particles) will help.
Beside optical methods with transmission or reflection data, a simple way to determine the resist film thickness is to scratch the softbaked resist on a dummy wafer with e.g. a scalpel and measure the depth of the scratch with a surface profilometer.|