| The higher clock rate and higher integration densities , and the same time further miniaturization of very high frequency and very high speed integrated electronic circuits increase demands to research sensitive testing techniques, which should be able to provide high spatial and temporal resolution. We also hope such technique can provide frequency, simultaneously. In the past, the tools for electric test conventionally based are oscilloscopes or network analyzers, but they are limited in several aspects. The working rate of oscilloscopes and network analyzers can reach 50 GHz and 110 GHz respectively, but to many internal measuring, they are suffered from fitting special mechanical probes and the size of the probes. Besides the difficulty discussed above, another problem is that they will bring significant parasitic impedances.The first measuring tool for very large scale integrated circuits is electron beam testing technique, it's spatial resolution is submicrometer, and it works in a contactless manner. Electron beam testing technique utilities electron beam pulses as sampling gates, and the temporal duration is hundreds of femtosconds, but because the influence of the transit time effect of the secondary electrons, its testing bandwidth is about 24GHz. Furthermore, Electron beam testing technique is not applicable to some devices, such as monolithic microwave integrated circuits(MMICs) ,very high speed electronic circuits on III-V substrates or on silicon-substrates whose working rare up to 210 GHz or 93 GHz.The merit of indirect (external) electro-optic sampling technique and direct (internal) electro-optic sampling technique is that their voltage sensitivity, spatial resolution and temporal resolution are very high, so they are promising testing techniques. The principle of the both technique is electric field-induced birefringence within an e.-o. material. They are based on Pockels-effect, that is in the non-centrosymmetric crystals properties of optical was changed due to electric field. The polarization of a laser beam will suffer a change when the beam travel through a noncentric crystal, by the use of the change, a intensity modulation of the laser beam is done through pockels-effect. Direct electro-optic sampling technique has its drawback when measure devices and circuits based on III-V semiconductor material substrates and internal electric field of the substrates. Indirect electro-optic sampling technique is available to test electric field above the devices because the measuring system is equipped a probe made from electro-optic material. So it is possible testing circuits based on based on III-V semiconductor material substrates or the silicon substrates.In case of indirect electro-optic sampling technique, a electro-optic probe made from electro-optic crystal is immersed into the electro stray field above the testing point, the laser beam is focused on the probe and reflected from the center electrodes or from the ground electrodes. While the laser beam enters into MMIC through the backside and is reflected from the electrodes in case of backside line geometry testing using direct electro-optic sampling technique. The polarization of the laser beam is changed by the electro field in the crystal above the circuits or inside the substrates, and the change is proportional to the signal of the testing point.The spot size of the focused laser beam determinate the spatial resolution of electro-optic sampling system. In the common, the definition of sampling system spatial resolution is Half Maximum Full Width Diameter. In Abbe's classical diffraction theory, the definition of spatial resolution can be written as: . In the air, , can reach at the most, so the maximum Numerical Aperture of microscopy is about 1, and Half Maximum Full Width Diameter of the probing laser beam will reach at its least, that is half of the wavelength. To increase the spatial resolution of the sampling system, we must reduce constant. (1) The more shorter the wavelength is, the higher t...
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