The square array four point probe has some differences to consider when compared to the much more commonly used linear array four point probe:
- When measuring a thin film using a square array probe, the voltage signal is one-half of the voltage compared to a linear array probe. When using a square array probe, you cannot use the ohms/square button on the Jandel RM3000+ or the Jandel HM21 Hand Held Meter to provide a valid sheet resistance value. However, the Jandel ResTest Meter has a setting for use with a square array probe.
- To calculate an ohms-per-square value, the formula is 4.5324 x 2V/I (rather than V/I), however, by using a suitable current, the math involved can be minimized. Square array probes are not usually used on bulk materials (e.g., ingots) since the main advantage is the smaller footprint which allows the four tips to fit on a smaller material. For further information about using a square array probe on bulk materials, please see the article by S. B. Catalano, “Correction factor curves for square-array and rectangular-array four-point probes near conducting or nonconducting boundaries”:
http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?reload=true&arnumber=1473476
- Square and rectangle arrays can only be built with needle spacing from 0.635mm up to 1.591mm, not close needle spacing of 0.5mm. The Jandel probe models that can be built using the square array are the Cylindrical probe, Six-Way probe (both CDE and KLA-Tencor/Prometrix models), Four-Pin probe, and the Cartridge with Lead probe. When probing wafers, the use of a square array probe allows the user to measure closer to the edge of a sample before a correction factor is required. The smaller voltage signal must be taken into account when using four point probing electronics and software that are designed to calculate sheet resistance and to read-out directly in ohms-per-square based upon the assumption that a linear array probe is being used.
Jandel references these papers related to square array probes in their various instruction manuals:
Small slice at centre:
A. Marcus and J. J. Oberly, IEEE Trans. Electron. Devices, ED-3, 161 (1956)
Small slice along a radius:
L. J. Swartzendruber, National Bureau of Standards Technical Note 199 (1964)
Square sample:
M. G. Buehler, Solid State Electronics, 10, 801 (1967)
Thick sample near boundary:
S. B. Catalano, IEEE Trans. Electron. Devices, ED-10, 185 (1963)
Thin infinite sheet:
M. G. Buehler, Solid State Electronics, 10, 801 (1967)
Four-Point-Probes is a division of Bridge Technology. To request further information please call Bridge Technology at (480) 219-9007 or send e-mail to Joshua Bridge at: sales@bridgetec.com
