Voltage contrast: A powerful tool for VLSI circuit diagnosis

Abstract

The advent of very large scale integration (VLSI) presents many new problems for integrated circuit (I.C.) diagnosis. Some I.C.s have over 100,000 transistors with less than 100 external leads with which the transistors can be tested. The geometries are now as small as 3 μm, smaller than can be probed mechanically. Along with the size, node capacitance and current drive are also reduced making a probe's capacitive load very detrimental to rise-time measurements. New processes have many layers of interconnect, leaving more and more of the circuit below passivation and therefore inaccessible to a mechanical probe, even after the removal of scratch protection. VLSI challenges I.C. diagnosis with more internal circuitry that becomes smaller and less accessible to conventional probes.Voltage contrast is an electron beam voltage probing technique especially suited to VLSI circuits. A properly modified SEM, used in the voltage contrast mode, provides both high resolution images of circuit voltages and also voltage waveforms of particular nodes.