| Design and failure analysis engineers are often faced with the tough challenge of testing high frequency signals at internal points of an integrated circuit (IC). They want to verify why and under what conditions do their circuits fail. Accurate magnitude and timing measurements are becoming increasingly harder to perform as IC technology pushes toward 100-nm line widths and gigahertz operating speeds.;A promising new tool for measurements of ultrafast voltages on the nanometer scale has been developed. The probe is based on an atomic force microscope (AFM). An AFM with a conductive probe tip is used to measure electric forces due to voltages present on a chip's surface. In addition, it has been shown that the non-contact probe can sense voltages through passivating overcoat layers, albeit with reduced sensitivity. For dynamic voltage measurements, the probe relies on high-speed mixing and stroboscopic sampling, made possible by the square-law force interaction between the AFM tip and the circuit under test. The mixing allows for measurement of frequencies far above the mechanical resonance of the AFM cantilever. The intrinsic speed of the measurement technique is limited by the extremely small capacitance of the tip and in theory, should exceed 100 GHz. Practically, the probe's bandwidth is limited by the high-speed signal sources used for sampling.;This thesis explains the technique and discusses the overall design of the ultrafast electrical force probe. The fundamental limits to the probe's voltage, spatial, and time resolution are explored. Experimental, time-resolved measurements of very-large-scale-integration (VLSI) and microwave circuits are reported. To date, measurements with 0.5-micron spatial resolution have been achieved and edges with fall times as short as 5 picoseconds (70 gigahertz bandwidth) have been captured using the novel AFM probe system. |
