Question

How to check the ohmic contact to the film?

Answer 1

My first hunch would be, that the proper way to measure this is with at least four contacts, because you also need to eliminate the resistivity of the film from your measurement. The most simple geometry is dots A, B, C, D in a linear arrangement. A and B are close together, so are C and D. A and D are the two dots which you use to inject the current, B and C are used to measure the potential on the film. You can improve on this with concentric electrodes or a "parallel plate" geometry

Answer 2

So, my first conclusion that to be made is that the contact in ohm laws and rearfractions made.