four point probe help

[physicistX]

Hi,

I am new to four point probe measurements and I have a question.

I've deposited 500 nm of Ni on glass substrate to check out the equipment we bought. When I measured its sheet resistance the display showed 13.56 mohm/sqare for 5 mA current.

To check out the measurement I multiplied the sheet resistance with the coating thickness which gave about 1/10 of Ni's bulk resistivity (96.3 nohm.meter). I couldn't figured it out if it's a faulty measurement or is there a logic I am missing. Any help would be appreciated.

thanks.

[Mapes]

Hi physicistX, welcome to PF. A few questions: is your test equipment set to analyze a thin conductive layer (instead of a bulk sample)? Are the length and width of the Ni film much larger than the spacing between the probes? Are you sure of the thickness?

You could also confirm the current and voltage with a multimeter. In fact, a four-point-probe measurement is quite easy to do yourself; the only advantage of the tool is the precise spacing of the probes.

Are you familiar with the equation used to turn current, voltage, and geometry into sheet resistance? See here (http://www.ecse.rpi.edu/~schubert/Course-Teaching-modules/A37-Four-point-probe-measurement-of-semiconductor-sheet-resistance.pdf) or here (http://www.four-point-probes.com/fpp.html), for example.

[Gokul43201]

Could it be that your film is actually much thicker?

Have you measured at smaller currents and established linearity?

I don't see an error in your logic. The film resistivity for 500nm should be pretty close to the bulk value... possibly a little bigger.

[physicistX]

Hi Mapes,

Thank you for your answer. I am quite sure of the thickness although I did not check it out after the deposition process but it was deposited with magnetron sputter equipped with a thickness monitor. The thickness monitor is healty and calibrated. So I am sure it is 500 nm. I checked the linearity of the resistance by using different currents. The current value I used is the current value where I could get stable voltage and sheet resistance.

[Mapes]

Then I think a useful next step is to try the measurement yourself: hold four electrodes in contact with the sample (perhaps with a friend to help) and apply current, measure voltage, apply equation. Also, use a multimeter to confirm what your prober is doing. Finally, check the prober manual to see if you've set it up right. Good luck!