Electro-Optic Sampling and Charge-Density Probe

Abstract

Electro-optic sampling is among the fastest of the current optical techniques available. This technique is based on the “Pockels effect” where the optical properties of a crystal change according to an applied electric field applied across it. By sending the light through the crystal, measuring the polar polarization changed, unknown test-point voltages can be determined. There are two general techniques for electro-optic sampling, namely, the external and the internal electro-optic (e-o) sampling. The external electro-optic method uses a small electro-optic crystal as the electro-optic medium, while the internal electro-optic technique uses the circuit substrate itself as the electro-optic medium. In another optical technique named “charge-density” probing, the plasma-optical effect is utilized where charge-density modulations within devices and parasitic PN junctions cause local refractive index changes. By interferometrically sensing these refractive index variations from the backside of an IC, these measurements can be related to either a current or a voltage signal.