eddy current

Eddy current measurement is a non-contact electromagnetic technique for measuring conductive film thickness and sheet resistance on semiconductor wafers. **Principle**: AC magnetic field from a probe coil induces eddy currents in the conductive film. The eddy currents generate an opposing magnetic field that changes the probe coil impedance. Impedance change relates to film conductivity and thickness. **Sheet resistance**: For thin films, eddy current directly measures sheet resistance (Rs = rho/t). Combined with known resistivity, thickness is calculated. **Materials**: Measures any conductive film - Cu, Al, W, Ti, TiN, Co, doped silicon. Cannot measure insulators. **Non-contact**: Probe does not touch wafer surface. No damage, no consumable tips. Fast measurement. **Proximity**: Probe hovers 0.5-2mm above wafer surface. Sensitive to probe-to-wafer distance (lift-off). **Frequency**: Operating frequency affects measurement depth (skin depth). Lower frequency penetrates deeper. Multiple frequencies can resolve multi-layer stacks. **Applications**: Post-CMP Cu thickness mapping, metal deposition uniformity, sheet resistance monitoring, endpoint detection during CMP. **Wafer mapping**: Automated scanning produces full-wafer thickness or Rs maps at 49+ points. **Throughput**: Very fast (seconds per wafer). Suitable for high-volume inline monitoring. **Limitations**: Cannot measure insulating films. Affected by underlying conductive layers. Edge effects near wafer edge. **Vendors**: KLA (RS-series), CDE (ResMap), Onto Innovation.