HELP: Contactless resistance measurement

[modamag]

Guys, I got a project that is in need of a contactless measurement technique and tool.
Anyone have any information on this would be extremely helpful.

Thanx,
Jonathan

 

[Mr Happy]

Can you give more specific details?

To measure resistance you have to pass a current through the device to be measured. It would be hard to do that without some kind of contact being made.

 

[modamag]

I need to measure the resistance of a conductive oxide film without touching it.
If I touch it then it will cause deformation in the film and increase the shunt resistance, not good.

 

[Mr Happy]

I'd guess you need to put in place the normal electrical contacts with the oxide film, and then measure the resistance through those contacts?

 

[Darkpower]

I need to measure the resistance of a conductive oxide film without touching it.
If I touch it then it will cause deformation in the film and increase the shunt resistance, not good.

Sounds like a fascinating project. I'm a retired electrical engineer and I worked R&D for many years on display systems such as electroluminescent displays. We use to use thin-film sputtered Indium-Tin Oxide as a transparent front electrode for displays and electroluminescent devices and I think what you want to do can be done but its going to be very elaborate and you will require specialized equipment and jigs. You will very likely have to develop your own methodology or contact people in the industry that make thin film sputter conductive coatings. Can I ask you what the oxide is? Most thin film sputtered (CVD Chemical Vapor Deposition) coatings are literally a few hundred molecules thick, although they were very delicate we were able to contact them physically, generally in the scrap area of the displays, and by depositing heavier thick films, generally polymeric conductive silver as a test contact point.

Anyhow, most sheet resistivity can be measure by its ability to attenuate RF signals. The higher the conductivity (lower sheet resistivity) the more attenuation you get. For example the interior of Fighter Planes have a thin-film coating of Indium-Tin Oxide to prevent electromagnetic radiation from leaking in or out of the cockpit windshield. Indium-Tin Oxide is used because Tin Oxide is transparent and Indium is added to stabilize the Tin Oxide.

You would have to set up a system with an RF generator in a shielded RF room, and make some sort of jig where you have a Radio Frequency wave-guide propagating electromagnetic waves, and a receiver that is able to measure radio wave amplitude. You would then have to calibrate with a sheet of known material at various conductivity. Plus take into any account leakage. If you can develop a scale where you can correlate attenuation with sheet resistivity then I think you can make the measurement without contact. I believe that equipment may already exist for the industry but I haven't got a clue who makes it and the thin-film oxide coating industry may keep that information as a trade secret.

I hope I didn't ramble on uselessly and that the information is useful to you.

 

[Darkpower]

I'd guess you need to put in place the normal electrical contacts with the oxide film, and then measure the resistance through those contacts?

A two point contact may not work with what is known as "sheet resistivity" which is a dimensionless measurement of an area of conductivity. In normal thin-film and thick-film industry sheet resistivity is measured as (ohms per square) and it specifically does not specify inches or millimeters. He would have to first define a square, then deposit two bus-bars at each end of the defined square, then he can measure the sheet resistivity in terms of ohms/Square.

A little more info from Wiki > http://en.wikipedia.org/wiki/Sheet_resistance

 

[Mr Happy]

A two point contact may not work with what is known as "sheet resistivity" which is a dimensionless measurement of an area...

Indeed, thanks. Your first reply was very informative. Not being an electrical engineer, I had not considered the potential applications of thin film conductive coatings that you outlined. The omission of words like resistivity or specific resistance earlier had thrown me off the scent.

 

[modamag]

Thank guys for the input.

To my limited knowledge there are several ways of getting the resistance value.
1) Eddy Current measurement
2) AC Attenuation
3) RF (from Darkpower)

I'm not an EE so terms might not be totally correct

Right now, currently I'm trying to purchase a complete solution but just don't know where to find them. Pricing is not an issue (well within limits) but I suspect such system would cost ~70K-200K.

 

[Mr Happy]

Make sure you have someone with sufficient expertise to operate whatever expensive system you get, though. An expensive system in untrained hands may deliver no better results than a cheap system

Regarding terminology, make a note of what Darkpower said. Resistance is a property of a device that you can measure between two points. The value may vary according to which two points you pick to measure at. Resistivity (or specific resistance) is a general property of a material, and for a uniform material always has a single value.

Equivalent terms are conductance (between two points) and conductivity (property of a material). Because conductivity is a property you can look it up in tables for known materials such as pure copper or pure silver.

 

[Darkpower]

Well if its any help, I found a paper written by Koreans that have developed a contactless method much like I described. Quoted is their abstract on the paper. But this technique has been used in the past. I know because our vendors supplied us with rolls of Indium-Tin Oxide coated heat stabilized polyester (PET) film and they told us that the measured sheet resistivity continuosly on a web coater by measuring its ability to attenuate an RF (microwave) signal passed through it.

I'm sure if you keep googling more terms like 'non-contact' 'sheet resistence' 'thin film' 'oxide coatings', you might be able to narrow down a peice of equipment and a specific methodology that is time tested and proven.

http://www.sciencedirect.com/scienc...serid=10&md5=a5412b7297778d8b178f872906bb8606

Noncontact characterization of sheet resistance of indium–tin-oxide thin films by using a near-field microwave microprobe


Soonil Yuna, Sunguk Naa, Arsen Babajayana, Hyunjung Kima, Barry Friedmanb and Kiejin Leea, ,
aDepartment of Physics and Interdisciplinary Program of Integrated Biotechnology, Sogang University, Seoul 121-742, South Korea
bDepartment of Physics, Sam Houston State University, Huntsville, TX 77341, USA
Received 29 August 2005; revised 20 March 2006; accepted 20 March 2006. Available online 4 May 2006.


Abstract

The sheet resistance of indium–tin-oxide (ITO) thin films with different microstructures and morphologies was measured by using a near-field scanning microwave microprobe (NSMM). The NSMM system was coupled to a dielectric resonator with a distance regulation system at an operating frequency f = 5.2–5.5 GHz. ITO thin films with different microstructures and morphology were characterized by X-ray diffraction and atomic force microscopy. As the sheet resistance increased, the intensity ratio of the (222) to the (400) diffraction peak increased and the surface roughness increased.

 

[modamag]

DP, can you check if this tool is a custom in-house tool that your vendor used. Or is it off the shelf somewhere.

I found ISRA and are currently working with them, but more resource the better.

 

[HarryN]

Hi Modamag

Here is the tool you need. Widely used in the compound semi industry.

http://www.lehighton.com/

Good luck

Harry