High-Resolution RF Probing

Abstract

In recent years electronics have not only become smaller but also faster. The resulting integrated circuits (IC) often operate at radiofrequencies (RF). As a result of the small distance between features and the high operating frequencies, parasitic coupling can occur between functional elements impacting the functionality of the circuits. Our research therefore presents a novel contactless probe capable of measuring RF voltages and currents with μm-resolution at a bandwidth of 1 to 23 GHz. The probe is fabricated through a multiscale 3D-printing process where the bulk of the chip is printed using Digital Light Processing (DLP) stereolithography. The rest of the probe is printed through a Two-Photon Polymerisation (2PP) process. At the interface, a two-leged cantilever is 2PP printed on top of the DLP print. At the end of the cantilever, a sensing tip is integrated. It contained a triangular loop with a uniform width, and connects the two legs of the cantilever. On top of the triangular loop, a sharp tip is printed. In the last fabrication step, a thin conductive film is deposited by means of thermal evaporation, resulting in a sharp, conductive tip that sits on top of a conductive triangular loop. The loop is connected to the two legs of the cantilever and, hence, a potential over the loop can be measured.

When the probe is brought in proximity to an electrode carrying a time-varying current it inductively couples to the loop. Similarly, when the probe comes close to an electrode subjected to a time-varying voltage, it capacitively couples to the conductive tip. A model-based approach is applied to increase the spatial resolution of the probe. The sharp tip and cantilever allow the probe to be mounted in a modified AFM setup, which is used to accurately position the tip, and determine the tip-circuit distance. The voltage and current are continuously measured while the tip-circuit distance is increased. Sources directly underneath, hence very close to the probe show a non-linear dependence on the tip-circuit distance, whereas parasitic adjacent sources that are further away show an approximately linear dependence on the tip-circuit distance. By fitting the measured data to a mathematical model, the non-linear components can be isolated from the parasitic contributions to the measurement signal. Applying this technique allows us to conduct contactless RF voltage and current measurements with μm-resolution at a bandwidth ranging from 1 to 23 GHz.