# Frequently Asked Questions

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## Questions about Jandel Engineering

##### Who is Jandel Engineering Ltd?

Jandel Engineering was established in 1967. Prior to founding Jandel Engineering, Managing Director John Clark worked for A & M Fell in London. It was at this company that the first commercially available four point probe head, the ‘Fell’ probe head, was designed by John Clark and a colleague. Since this original design, many improvements have been introduced.

##### Who makes Jandelâ€™s probes?

Experts! All probe heads are made by experienced engineers with intensive training in the manufacture of four point probes. Some of our probe builders can boast over 30 years experience.

## Questions about Jandel four point probes

##### Why use Jandel probes?

Jandel probes offer the accuracy and perfection of more than 30 years experience. Tip radii and probe spacings are checked by interferometer and video inspection for certainty of accuracy. Spring loadings are checked by electronic force gauge.

Unique standard specifications include:

- Upper and lower guides are jewelled
- Solid tungsten carbide needles for superior durability and accuracy
- Teflon insulation giving minimum leakage

##### Why are there so many different characteristics?

Different users wish to probe different items, which means it is necessary to provide a wide range of characteristics. For instance someone wishing to measure a layer beneath an oxide would need sharp probes to penetrate the oxide layer, whereas a very thin metallic layer may require a much larger radius on the needles so that they do not puncture the layer. Similarly, some spacings are designed in order to help with the maths involved in working out resistivity, someone else may just require a very close spacing probe in order to fit all the probes on the sample.

##### What model of probe should I use?

Jandel has many different shaped probe heads. The best one for you depends entirely on your application and what machine (if any) you are using. Contact us with details of the probe you have now and we will let you know the best probe for you. Alternatively, if you wish to make a custom set up, we can help you to identify which probe will work best.

##### Which characteristics should I use?

The characteristics needed vary depending on the material being probed. If you contact us we may be able to advise the best characteristics for your samples from past experience. Alternatively, once an order is placed, you have the option of sending us a typical sample of the material you wish to probe and we will test it so that you can be sure to receive the best characteristics for your application. There is no charge for this service.

## Questions about resistivity measurement

##### What are 'ohms per square'?

The unit of measurement when measuring the resistance of a thin film of a material using the four point probe technique. It is equal to the resistance between two electrodes on opposite sides of a theoretical square. The size of the square is unimportant and so strictly the measurement is in ohms, however ohms/square distinguishes measurement of a thin sheet from measurement of, for instance, a wire.

##### What are 'ohms-centimetre'?

The unit of measurement when measuring the bulk or volume resistivity of thick or homogeneous materials such as bare silicon wafers or silicon ingots, using the four point probe technique.

##### Is sheet resistance an 'inherent' property of a material or is it a function of thickness?

Resistivity is the inherent property of the material which gives it electrical resistance. It is sometimes called Specific Resistance. Sheet resistance is the resistance of a thin sheet of material which when multiplied by the thickness (in cm) gives the value of resistivity.

##### How do I convert from ohms per square to ohms-centimetre?

The term Ohms.cm (Ohms centimetre) refers to the measurement of the “bulk” or “volume” resistivity of a semi-conductive material. Ohms.cm is used for measuring the conductivity of a three dimensional material such as a silicon ingot or a thick layer of a material. The term “Ohms-per-square” is used when measuring sheet resistance, i.e., the resistance value of a thin layer of a semi-conductive material. To calculate Ohms.cm using the Jandel RM3000 Test Unit, one needs to know the thickness of the wafer (if it is a homogeneous material) or the thickness of the top layer that’s being measured

The equations for calculating bulk resistivity are different from those used to calculate sheet resistance, however, if one already knows the sheet resistance, bulk resistivity can be calculated by multiplying the sheet resistance in Ohms-per-square by the thickness of the material in centimetres.

##### At what point do you stop multiplying the sheet resistance by the thickness in centimetres to arrive at Ohms-cm?

When the thickness exceeds 0.1 of the spacing between two needles – after which sheet resistance doesn’t apply. So, 0.1mm for a probe head with 1mm needle spacings. However, due to corrections, up to 0.3mm would be ok. If the thickness is equal or greater than five times the probe spacing, the correction factor to be applied to the formula:

resistivity(rho) = 2 x pi x s x V/I is less than 0.1%.

From the sheet resistivity point of view, the correction factor tables we have start at ratio thickness to probe spacing of 0.3, where the correction factor is unity, to a ratio of 2, where the correction factor is x0.6337. I expect these tables can be extended up to a larger ratio, but clearly from a thickness of 2x spacing up to 5 x spacing is a bit of a no-mans land, but if one assumes that the situation is ‘bulk’ there are correction factors covering the ratio of thickness to spacing from 10 down to 0.4 where the correction factor is x0.288.