AN APPROACH TO CHIP-INTERNAL CURRENT MONITORING AND MEASUREMENT USING AN ELECTRON BEAM TESTER

Abstract

An innovative measurement and imaging technique for electron beam testers is introduced, that promises to expand the present applicability of such systems for chip-internal voltage measurements by the capability of chip-internal current measurement. The theoretical principles of the method are discussed and the effect is calculated analytically for a model arrangement. For more realistic measurement situations, the results of numerical calculations, showing the strength of the effect and its dependency of situational parameters, are presented. First experimental results are added. 1 MOTIVATION During the last years, the market share of mixed-signal designs could be observed to be continously increasing. This tendency led to the introduction of dedicated mixedsignal test systems. For prototype debugging of purely digital circuits, internal measurement tools like electron beam !esters (EBTs) have proved to be valuable and at the moment efforts are undertaken to enhance these tools also for application to mixed-signal circuils. The main task herewith is to improve the voltage resolution of the EBT by reducing system-inherent noise. Thus the measurement of analog voltage signals becomes available at an acceptable signal-to-noise ratio [Gar 871. However, information in analog circuitry is often carried by currents and therefore not accessible for measurement with an electron beam tester. On the other hand, the capability of measuring supply currents to chip internal function blocks would allow for some kind of chipinternal IDDQ technique also in digital circuits [Haw 891. But the task of current measurement on chip-internal wires has been ncglected so far. To date, no mechanism has been presented that allows contactless rno:iitoring and measurement of chip-internal currents using an electron beam technique. Neither the measuremenl of the voltage drop caused by the rcsistance of chip-internal wires nor the evaluation of the deflection of the primary beam due to the magnetic field around a current carrying wire (in the range of some pm/A) provides a practical access to chip-internal currents [He1 911. The following paper will describe a promising approach to such a technique, that exploits the interaction between the magnetic field around a current carrying wire and secondary electrons emitted from its surface. 2 PRINCIPLES The basic interaction between a current and a moving electron (e.g. secondary electron) is established by the magnetic field caused by this current. Therefore, the interaction process is described by Ampere's Law and the Lorentz force, aA at p = -e--eeV@+evx(ZZxAJ