Question

Basic Semiconductor Process Lab Fabrication

9. How do you adjust the spin speed and spin time if you found that the PR solution is thick(not flowing freely)?

Answer 1

Thinckness t

Speed w

t is inversely proportional to square root of w.

t=c (1/sqrt(w))

c= proportionality constant

Spin speed:

The range of spin speeds available is important as it defines the range of thicknesses that can be achieved from a given solution. In general, spin coating can produce uniform films relatively easily from about 1000 rpm upwards, but with care and attention good film quality can be achieved down to around 500 or 600 rpm in most cases (and even lower in some cases). Most common spin coaters will also reach a maximum speed of 6000 to 8000 rpm (although specialist coaters may go to 12000 rpm or higher). As such, a normal range of working spin coating rpm might span a factor of ten (from say 600 rpm to 6000 rpm) which in turn means a maximum variation in film thickness of around a factor √(10)=3.2.

For example, a solution which gives a film thickness of 10 nm at 6000 rpmwill give a thickness of around 32 nm at 600 rpm and if a thicker film is required then the solution concentration would need to be adjusted. In the reverse scenario, if a solution gives a thickness of 100 nm at 600 rpm then the minimum thickness that can be achieved without diluting the solution will be 32 nm (100 x 3.2).

Spin time:

For most standard spin coating techniques the objective is to keep the substrate spinning until the film is fully dry. As such this will mainly depend upon the boiling point and vapour pressure of the solvent that is used but also on the ambient conditions (temperature and humidity) that the spin coating is performed in. For most solvents a spin coating duration of 30 seconds is usually more than adequate and is therefore recommended as a starting point for most processes.