Contactless Sheet Conductivity Measurements

We have developed a completely novel and highly sensitive probe of sheet conductivity (US Patent 6448795). This method is applicable to investigations of bulk materials and also to in situ measurements of surface conductivity variations that are induced by processes such as gas adsorption, heteroepitaxial growth, and even roughening through homoepitaxial deposition or etching. Our conductivity probe technique also has considerable potential in looking at near-surface conductivity changes on highly conducting substrate materials. This remote contactless method uses a driving RF coil and sensing coils that are driven at a frequency in the range of 10 to 30MHz. The probe measures a signal that arises from an imbalance of mutual impedances (between the two sensor coils and the central driving coil), which arises from induced eddy currents in the surface/material under investigation.

Contactless conductance sensor.

Three coils (L1, L2, L3) are placed parallel to a sample surface. The central coil (L3, driver) is connected to a RF generator. The driver's RF electro-magnetic field induces RF currents in the receiving coils (L1, L2) and also eddy- currents within a skin depth of the probed sample's surface. The induced eddy-currents are proportional to the sample's conductance. Because the two sensor coils, L1 and L2, are different distances from the sample surface, their induced RF voltages are affected differently by the sample's eddy-currents. Hence, the difference in RF voltage amplitudes across L1 and L2 is proportional to the sample's conductance. A schematic circuit diagram of our contactless conductance sensor is shown below.

Ls, Rs -- sample's effective inductance and sheet resistance respectively.
Diodes D1, and D2, and resistors R', and R" -- produce a DC voltage (Vout) equal to the difference of RF voltage amplitudes across L1 and L2.

Example data is shown below of in-situ monitoring of chemical vapor deposition (CVD) of aluminum on a thin film (1000 Å) of gold on silicon wafer material. The CVD precursor, trimethylamine alane decomposes readily on metallic surfaces above 350K, and hydrogen desorbs. Aluminum is deposited.

(CH₃)₃N:AlH₃(g) $\arrow$ (CH₃)₃N:AlH₃ (a) $\arrow$ (CH₃)₃N(g) + 3/2 H₂ (g) + Al(s)

In the above vacuum based studies we have demonstrated sensitivity to changes in sheet conductivity as low as 10^-4 W^-1. This is formally equivalent to less than 1/100^th of a monolayer of ideal bulk aluminum on a substrate. Real films, as grown, are not necessarily continuous and this in-situ technique is therefore an extremely sensitive probe of metallic film quality. The experimentally observed breaks in the gradient of surface conductivity, as a function of deposited thickness, are the subject of further investigation.

Using our patented design, we have also developed a prototype, highly sensitive, handheld, sheet conductivity meter:

This battery-operated, highly portable device readily provides instant, calibrated, sheet conductivity measurements in the following full scale 3 ½ digit ranges: .2 W^-1, .02 W^-1, and .002 W^-1.

The example 3” Si wafer shown above exhibits a sheet conductivity of 0.001204 W^-1. The limiting reproducibility of this meter is ~ 10^-5W^-1.

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