Research Index / Ultrafast Technology / Potential Measurements

Measurements of electrical potentials inside integrated circuits has been demonstrated using a micromachined photoconductive sampling probe.  This probe is capable of distinguishing absolute microwave signal amplitudes at internal nodes of integrated circuits.  Thus, the signal’s measured value is equal in size to its actual value, both for the dc and the ac component.

Photoconductive Sampling Probe click picture for a larger view SEM image:  the probe is 130 mm x 230 mm (at its widest points) by 1 mm thick; the finger size of the interdigital MSM switch is 1.5 mm.

Experimental Setup:

The measurements are performed using the experimental setup shown in the figure below.  The basic principle used in this setup is equivalent time sampling of the microwave signal, with the optically-activated photoconductive switch acting as a sampling gate.  The setup uses a phase-referencing technique to synchronize the microwave signal source and the pulsed-laser source in order to cancel out phase fluctuations.  The reference signal is generated by a RF mixer with the LO provided by the output signal of a fast photodiode and a fraction of the microwave signal used as the RF.  The IF signal triggers a digitizing oscilloscope that records the down-converted waveform of the sampled microwave signal at the probed node including the DC offset.

Experimental Setup for Digital Circuit Measurements

Temporal Resolution:

The temporal resolution of the probe is measured using a pump-probe style of experiment.  In this case, the Ti:Sapphire laser system optoelectronically generates electrical pulses with durations of ~1 ps on a coplanar strip line on a low-temperature grown GaAs substrate.  The figure below shows the voltage waveform measured by the probe.  The excellent time response of t_fwhm = 4.2 ps corresponds to a bandwidth of at least 150 GHz for this sampling system.  That is, the probe exhibits a frequency response well into the mm-wave regime.

Setup for Measuring Probe Time Response The negative transient and the subsequent positive pulse arise from reflections present on the coplanar strip photoswitch.

Examples:

1. Time wave forms measured inside a digital InP- HBT-circuit

2. Frequency-domain measurements at internal nodes in an analog three-stage SiGe-amplifier

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