Eddy Current

Eddy Current

There are different methods for measuring the bulk resistivity of semiconductor wafers or blocks. Some techniques need to contact the surface of the sample, e.g. four point probe, and may be considered destructive techniques. The eddy current method is a non-contact, non-destructive and fast technique for measuring the resistivity of semiconductor samples.

Theory of the measurement

When an AC current flows in a coil in close proximity to a conducting material the magnetic field of the coil will induce circulating (eddy) currents in the sample. The conductivity of a material has a very direct effect on the eddy current flow: the greater the conductivity of a material is the greater the flow of eddy currents on the sample is. The eddy current measurement is actually the measurement of the electrical loss in the material.

The output signal of the system is proportional to the power, which is necessary to keep the amplitude of the oscillation in the coil constant. This signal has its maximum at a given resistivity.

Frequency

The eddy current response is greatly affected by the test frequency chosen. This is an important property that we can control. The basic frequency is 125-135 MHz, which is a proper selection for the 0.5-25 Ωcm resistivity range.

Geometry

If the sample is not flat or of infinite size, geometrical features such as curvature, edges, grooves etc. will affect the eddy current response. This should be taken into account when evaluating the eddy current signal. Where the material thickness is less than the effective depth of penetration (see below) this will also affect the eddy current response. If the eddy current probe touches the surface, the influence of the geometry decreases.

Proximity

The closer a probe coil is to the surface the greater will be the effect on that coil. Therefore there is a reduction in sensitivity as the coil to sample spacing increases.

Depth of penetration

The eddy current density, and thus the strength of the response is greatest on the surface of the sample being tested and declines with depth. It is mathematically convenient to define the "standard depth of penetration" where the eddy current is 1/e of its surface value. The standard depth of penetration:

decreases with an increase in frequency
decreases with an increase in conductivity
decreases with an increase in permeability (penetration into ferrous materials is very small at practical frequencies).

It is also common to talk about the "effective depth of penetration" usually defined as three times the standard depth, where eddy current density has fallen to around 3% of its surface value. This is the depth at which there is considered to be no influence on the eddy current field.

The RT-100 is used to monitor bulk resistivity of large pieces of silicon, such as ingots, feedstock material, bricks, and chunks, using the eddy current technique. The RT-110 uses eddy current technology to make single point measurements of wafers. It measures the thickness of the wafer and corrects the eddy current signal, using the thickness information, to report the bulk resistivity. Both of these products are popular among wafer reclaim and silicon reclaim customers, as well as wafer makers and block makers.

The WT-2000PV makes maps of resistivity of PV wafers using the eddy current technique. The WT-2000P and WT-2000D make line scans (and maps) of blocks of silicon, and resistivity measurements are available as an option. For In-Line measurements of PV wafers on a conveyor belt, the WLT and WMT systems make resistivity and thickness measurements.