Non-Contact Probing of Active Semiconductor Devices Using Terahertz Waves

Terahertz waves are sensitive to changes in free carriers within semiconductor materials. To date, research in this area has focused on spectroscopic analysis of exposed wafers and inactive devices. In principle, terahertz waves should be able to probe packaged and active semiconductor devices subjected to bias signals, which induce changes in the carrier density across a PN junction. A challenge lies in a discrepancy between the probing spot size on the order of millimeters at 275 GHz, and the sub-wavelength footprint of a PN junction, which results in small changes of any reflected terahertz waves. Although normally buried underneath the phase noise of the terahertz oscillator, these minuscule changes in carrier density can be observed as a baseband signal in complex reflection measurements using a homodyne quadrature receiver. Due to the high sensitivity of this transceiver, sub-wavelength targets can be effectively monitored with conventional terahertz optics. These experimental results demonstrate the ability of terahertz waves to non-invasively monitor semiconductor device activity. We anticipate that this work will be a precursor for non-destructive inspection of chips during production and the covert monitoring of operational devices.