Questions on Rs measurement by four point probe device

[Hanyuri]

Hi all,
I am working on electronics material (using metal oxide as an active layer in TFTs). Currently, I'm trying to measure the sheet resistance (Rs) of CuO thin film (~100nm in thickness). My lab uses Agilent B1500 device for measuring the Rs. I met some problems when carrying out the measurement:

For my case, CuO nanoparticle solution is deposited and is annealed by 2 step annealing ( in inert gas first and then is oxidized in very low % O2). At the first step, the samples has very low and specific Rs, which showed as a metallic material (Cu), but after the second step, the Rs is not clearly. The Rs drop at every different current points are varied widely and not specific from a few ohms/sq. (I~1-100mA) to Gohms/sq. (I~1-100nA). If they are insulating, there should be no voltage drops between the probes. I wonder why it happened like that and whether there is any wrong in my measurement.

I alse referred some other scientific articles. They reported that CuO thin film has Rs ~ tens of kohm/sq (the deposition method is RF sputtering).

This measurement is very important for my research. I hope that some of you have done this measurement before.
I will greatly appreciate if somebody can help me for this case. Please feel free to contact me if you nees any further information.

 

[Nidum]

CuO is mechanically very weak . It is very easy to punch right through it or at least reduce local thickness with probes .

Pointed probes are particularly problematic . Small diameter spherical end probes give a variable contact area and indentation depending on applied load .

Copper oxide can react with some probe metals to produce a small voltage .