Picosecond time-domain measurements

The increasing performance of semiconductor devices and millimetre-wave integrated circuits imposes corresponding requirements on measurement techniques, if characterization is to be achieved directly over the full frequency range, without reliance on extrapolation from lower-frequency measurements. Device and circuit development should not be dependent on measurements on packaged devices but should utilize on-wafer probing methods, which should not be unduly invasive. These requirements can be satisfied using fast laser systems for pump and probe methods in which a subpicosecond laser pulse is divided by a partially-reflecting beam splitter. The 'pump' pulse is used to turn on a fast photoconductive switch and so to inject a current pulse into a transmission line and thence into the device under test; the 'probe' pulse (delayed by travelling a longer optical path) is used to sample the response, either taking a current sample with a second photoconductive switch or optically using an electro-optically active material sensing the electric field above part of the circuit.