Resolution enhancement in probing of high-speed integrated circuits using dynamic electrostatic force-gradient microscopy

Dynamic mode electrostatic force microscopy is a technique capable of measuring the internal voltage signals of high-speed integrated circuits. Circuit signals are extracted by utilizing the localized nonlinear Coulomb force acting on a microfabricated probe that is closely positioned above the circuit test point. Equivalent time sampling of repetitive high-frequency signals, that can have a bandwidth much greater than the mechanical response of the probe, is achieved by driving the probe with amplitude-modulated high-speed pulses. Currently, dynamic mode electrostatic force microscopes (DEFMs) extract circuit voltage signals through direct sensing of the electrostatic interaction which results in a poor spatial resolution and is susceptible to interference due to significant coupling to the tip sidewall and the cantilever support of the probe. This is especially true for large tip-to-sample distances such as when passivated circuits are measured. This article describes a force-gradient method to improve the spatial resolution of DEFM. The force-gradient method is implemented numerically and is based on sensing the force as the tip-sample distance is modulated. The method is shown to reduce the contribution from the tip sidewall and the cantilever. Measurements of high-speed signals up to 500 Mb/s demonstrate a significant reduction of interference signals. (C) 2004 American Vacuum Society.