A. Transient ESD Measurements We applied the optical electric-field probe to the transient measurement of the ESD signals transmitting on and around a microstrip line (MSL) as shown in Fig. 3. In the DUT, a 50mm long MSL was patterned on the FR4 wafer with a size of 100 mm x 30 mm and a thickness of 1 mm. An ESD signal were generated with an ESD generator (ESS-2002/ NoiseKen) and input to the DUT via an ESD gun (TC-815R/ NoiseKen). The generated ESD signal was 0.7-ns duration pulse with a peak voltage of 2 kV as shown with a black solid line, which is calibrated by NoiseKen prior to shipment. The ESD signal transmitted on the MSL of the DUT and output directly to an oscilloscope through an electrical cable at the measurement for the time domain waveform of the output signal from the MSL. The output signal of the MSL is deteriorated since its high frequency components are filtered out with the degradation of the electrical connector soldered in the output port on the wafer. The first peak of the output signal decreases comparing with that of the input signal. The output port of the MSL was terminated at the transient measurement with the optical electric-field probe. The optical system for the optical electricfield probe was the same with the evaluation system with the capacitor as described in Sec. 2. The tip of the optical electricfield probe was located at a height of 0.5 mm apart from the surface of the wafer. The measurement points were positioned at a distance of 0, 5, 10 mm along the perpendicular line to the MSL. The time domain waveforms of the electric field on the MSL were provided with a single-shot measurement of the sampling oscilloscope. Fig. 4 shows the time domain waveform of electric field along x axis measured with the optical electric-field probe at a distance of 0(blue line), 5(pink line), and 10(yellow line) from the MSL. At the distance of 0, the time domain waveform of the electric field agrees well with that the input signal. This indicates that the optical electric-field probe can measure the electric field with less deterioration. The electric field is almost zero at a distance of 5 and 10 mm. This indicates that a substrate radiation mode of the electric field along x axis is below the measurement limit of the probe at the side of the MSL above the wafer. From these results, we obtained a lot of transient information about electric field transmitting and radiating above and around the MSL and the wafer, and can effectively utilize them in the diagnosis of the electric circuits damaged with ESD and the design process for defeating an ESD noise.