Noncontact Sheet Resistance Instrumentation


Sheet Resistance

Many applications require objects that are sheets, wafers, panels, or plates of conductive or conductively coated material. For many of these applications, the resistance of the entire object (which could be a massive quantity) and/or the resistivity of the material are of secondary importance. Of primary importance is:

  1. The resistance of the material as measured in the direction of its depth (the z dimension). For these users, the focus is on the material resistivity at a certain point, and they are not interested in the xy plane or the full dimensions of the sheet, disc, panel, plate, etc.
  2. The resistance of the material as a sheet (any particular shape), regardless of the dimensions of the material as a whole. This is also a function of the thickness and resistivity of the material and not dependent on the size of the plane of the material.

For use in these applications, sheet resistance (Rs) was created as a unit of measurement. The inherent value of sheet resistance is that it is a measure of the electrical properties of a material independent of the xy dimensions.

Sheet resistance is resistivity divided by the thickness of the material.

XYZ Planes
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Measuring Thickness and Resistivity

Eddy current meters can measure sheet resistance, thickness, resistivity, and more.

Measuring Thickness and Resistivity

Delcom meters do not directly measure the resistivity of material—instead, they measure the sheet conductance of a conductive layer. However, if the material is of uniform thickness and if that thickness is known, a Delcom meter can be used to measure, record, display, and even map the material’s resistivity.

Delcom meters do not directly measure the thickness of material—instead, they measure the sheet conductance of a conductive layer. However, if the material is of uniform resistivity and if that resistivity is known, a Delcom meter can be used to measure, record, display, and even map the material’s thickness.

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Eddy Current Technology

The essential element of an eddy current meter is an inductor in a ferrite core. Typically there are two inductor and core assemblies: one located above and one located below the material to be tested. A current—oscillating in the low megahertz (RF) region—flows through the inductor coils, generating an oscillating magnetic field. When a conductive material is introduced between the two inductors and into the magnetic field, a circular flow of electrons (an eddy current) begins to move in the conductive layer due to magnetic induction. The difficulty of maintaining this eddy current is used to deduce the sheet resistance of the material being measured.

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Eddy Current Meters vs. Four Point Probes

Four point probes are the traditional instruments for determining sheet resistance. However, for many applications, Delcom’s eddy current meters are much more useful.

A Delcom eddy current meter:

  • is nondestructive
  • reads through insulating layers
  • measures moving material
  • provides nearly instantaneous readings
  • provides real-time process inspection

Additionally, Delcom meters provide better repeatability and involve less cost and hassle than four point probes.

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Each Delcom sensor is calibrated against a National Institute for Standards and Technology (NIST) standard. This calibration takes place at the order of magnitude in which the sensor has the most significant digits. If all environmental factors are held constant and the calibration standard is inserted back into the sensor at exactly the same location, the meter will read the exact value of the standard. For this reason, Delcom guarantees 99.9% accuracy.

Any eddy current instrument should be nearly 100% accurate when measuring the standard that was used to calibrate it. Therefore any question from a user about the accuracy of a Delcom meter is really a question of whether the user’s standards match the standards used by Delcom. To make this question moot, any Delcom instrument can be quickly calibrated to a standard by the user upon receipt of the instrument. Calibration is a simple process that takes less than one minute.


Resolution is a function of the significant digits available at each order of magnitude in a meter’s range.

Each meter’s range spans different orders of magnitude. Users should be familiar with the number of significant digits to be expected in each order of magnitude for the meter range they are using.

If the user is using a Delcom meter in mhos/square as the unit of measurement, keeping track of significant digits is easy: every digit is significant.

However, if a user is using a meter with ohms/square as the unit of measurement, care must be taken to keep track of—and understand—which digits are significant and which are not.

Each meter range spans different orders of magnitude. For this reason, the user should be familiar with the number of significant digits to expect in each order of magnitude for the meter range in use.

Significant Digits


Linearity is the answer to the question, “If a meter is calibrated with a standard at one point in the meter’s range, how accurate will the meter be when reading other possible values in its range?”

Delcom calibrates each meter at the order of magnitude in which it has the most significant digits. Delcom then characterizes each meter’s linearity against NIST and other standards over the entire range of the meter. Delcom guarantees no more than 3% deviation from the true sheet resistance value of tested material. The graph below shows a Delcom sensor tested against 10 NIST, VLSI, and MSA standards.

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Meter Ranges

Theoretically, a Delcom eddy current meter could measure from zero to infinity ohms/square. However, in reality, such a range is not practical or necessary. Over 30 years of research, Delcom has pushed eddy current technology so that the range of each of our sensors spans five orders of magnitude. This range is more than enough for most applications. Ranges are physically set in the sensors before they leave our factory, so the user must carefully choose the best range for the intended application prior to purchase.

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Elevation in Gap

Eddy current meters usually have a top and a bottom inductor. Material reads higher sheet resistance in the exact middle elevation between the top and the bottom halves of the sensor. If the material is moved towards either the top or the bottom half of the sensor, the material will read a lower sheet resistance. Thus, it is imperative that material being measured is either introduced to the sensor at the magnetic center or that the sensor is calibrated to expect the material at another elevation and that the material is consistently inputted at that exact elevation. Delcom makes stages for benchtop sensors which aid greatly in achieving elevation consistency.

Elevation in Gap
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Lift-Off Effect for S3 Sensors

The S3 meter has a single-sided sensor. As such, the magnetic field is thrown off from the face of the sensor in a fountain-like pattern. As conductive material is elevated away from the face of the sensor, the field becomes less and less dense. This causes the sensor to read the material as having a higher and higher sheet resistance. This effect is called “lift-off.” Because of the lift-off effect, a single-sided sensor is only useful if the material elevation is carefully controlled. Elevation can be controlled in multiple ways.

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Sensor Size, Gap, and Spatial Resolution

The essential elements of a Delcom eddy current meter’s sensor are inductors in ferrite cores. An inductor/ferrite assembly exists in both the top and the bottom parts of the sensor (except for the single-sided S3 sensor). In general, given a certain diameter of a pair of inductor/ferrite assemblies, the smaller the gap between these sensor halves, the better the performance, and the larger the gap the poorer the performance. Thus, it is best to work with the smallest possible gap suitable to the desired application, and it is important to carefully choose the Delcom meter with the proper gap.

Spatial Resolution
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