Ferromagnetic behavior in the material
The placement of a non-conductive ferromagnetic material between the faces of the sensing head can reduce the reading value. If the instrument is zeroed, the effect can be seen as a negative conductance value. This meaningless quantity is the result of increased inductance in the sensing head component of the resonant circuit. An increase in inductance results in a higher impedance and a slight frequency shift. This higher impedance results in less energy required to drive the resonant circuit. Hence a smaller number is displayed. As an example, a standard 5-1/4 inch DSDD computer floppy disk produces a value of approximately -0.0002 Mhos. Similarly, if the pole face separation distance is decreased the reading will decrease.

Dielectric loss tangent
A material may have a small conductance when measured at DC or low frequency, however, it can exhibit a large conductance at an RF frequency of several megahertz. Water is an example of a material with this behavior. Pure water is relatively non-conductive at low frequencies, but at RF and microwave frequencies it is an effective power absorber. Even though this type of conductance monitor operates in the low megahertz region, certain oxides of titanium and other metals do have high dielectric losses and consequently high apparent conductance values.

Spatial non-uniformity of the conductive material
Materials that exhibit cracking, crazing, striated surfaces, non-isotropic properties, and island-like structures can cause apparent conductance variations. A D.C. four contact conductance measurement responds to the unidirectional current flow over a given area of material. If the given area is composed of islands which are isolated from each other by surface cracks, the DC conductance value can be small. However, because the confined circulating currents induced by this eddy current type monitor can reside within an island structure, the dynamic conductance can be larger than DC measured values. Similarly, a striated or nonuniform metallized film has produced reading differences of between one and two orders of magnitude.

Non-Linear Behavior
Non-linear behavior of conductance is observed in semiconductors, dielectric-metal mixtures and loosely bound dielectric materials because they can exhibit tunneling, electron hopping, and current decrease due to large compliance voltages, heating, electromigration and high current densities. Consequently, correlation between the RF/dynamic and the four contact DC measurement technique depends on measurement conditions.

Skin effects
At high frequencies, electrical currents are unable to deeply penetrate conductive materials. Consequently, the effective resistance of conductors can sometimes rise to excessive levels. However, the frequency chosen for this device is such that the skin depth properties should rarely affect accuracy.

Other Effects
Highly conductive materials such as common aluminum foil or thick copper foil etc. will completely eliminate the mutual magnetic coupling between the ferrite halves of the sensor element. This results in a net inductance decrease which causes an increase in operating frequency. To avoid erroneous recording, the frequency shift is detected and an overload condition is indicated by flashing of the display.

Miscellaneous Information
The sample conductivity can be calculated by dividing the displayed conductance value by the thickness in appropriate units. The resistance is simply the reciprocal of the conductance.

Principle of Operation
The DELCOM Conductance Monitor utilizes absorption of RF electrical energy to measure the conductance of samples. The sensing head element is composed of virtually identical upper and lower half ferrite cores. These two cores introduce circulating eddy currents in the conductive material. The eddy current is induced by magnetic induction and is switched at an oscillation frequency in the low megahertz region. Greater conductance results in greater eddy current loss. This larger loss causes the oscillator magnitude to diminish. Some conductance monitors have been devised to measure and relate the consequent voltage loss with the corresponding degree of conductance. However, test data show that better results are obtained if the oscillator voltage magnitude is maintained at a constant value and measurements are performed to arrive at the power level required to compensate for the load characteristics of the conductive substance. This compensating power level is the parameter displayed by the panel readout.

Typical applications require careful implementation of operational principles. Stable, drift free resolution of 0.5 millimho is assured for continuous metallization in vacuum chamber applications where zero correction is not possible during the coating process. The sensing head is connected by a single coaxial cable that is an integral part of a parallel tuned L-C circuit. Consequently, cable types and lengths cannot be indiscriminately chosen. Electronic temperature drift characteristics have been minimized by the appropriate choice of special semiconductor devices. The output of an extremely sensitive AGC circuit feeds a special bipolar multiplier circuit which then produces the constant amplitude output signal. All critical components in the electronics module utilize monolithic chip silicon technology to achieve temperature stability and freedom from drift.

Tests have shown that the sensing head exhibits approximately one order-of-magnitude greater temperature drift than the electronics module. Consequently, in vacuum chamber operation a water cooled sensor head may be required and a temperature compensating system is typically used.

Total system power consumption is measured in milliwatts. Hence, very little self-heating is evident and air motion has little effect if no significant radiant source is present. Therefore, an instrument operated in a typical room environment will seldom require re-zeroing.
 

CALIBRATION EQUIPMENT

Following is a list of the equipment utilized for the calibration of DELCOM conductance monitors:

Keithley Instruments, Inc.
28775 Aurora Road
Cleveland, OH 44139

Primary cause of calibration error is possible spacing errors in four point probe assembly. Instruments are calibrated with silicon wafers and ITO coated glass slides. Because of heating effects during four point probe measurements at low conductivities, ITO coated glass slides are used at low conductance values and yield better results than those obtained with silicon wafers.

All calibration standards have been corrected for the finite nature of the size of the standard relative to the probe spacing.

DELCOM Conductance Monitors are also calibrated with flexible metallized films. A four-bar system is used in combination with the above Keithley equipment.

Please consult Reference page for further sources of information regarding measurement techniques.

REFERENCES

Smits, F. M.
"Measurement of Sheet Resistivity with the Four Point Probe" Bell System Technical Journal , May 1958, p. 711

Valdes, L.
"Resistivity Measurements on Germanium for Transistor", Proceedings IRE, Volume 42, Feb. 1954, p. 420

Uhlir, A., Jr.
"Potentials...", Bell System Technical Journal, Volume 34, Jan. 1955, p. 105

ASTM
Ref. #D991-68  "Standards for Testing Via Four Bar System"

van der Pauw, L. J.
Phillips Research Reports, Vol. 13 #1, Feb. 1958, pp. 1-9