Sensing is believing

By Sharon Ann Holgate SEMICONDUCTOR makers could soon have a way to spot faulty chips on the production line. Researchers at the University of Sussex have developed a device that reveals electricity passing through integrated circuits but, unlike other testing devices, does not interfere with the working of the circuit. The most common testing method involves touching a chip with a probe to measure voltages. This shows up failed components, but is no help working out what has gone wrong. Probing millions of components is impractical, so usually only a few points are tested, which limits quality control. Electron-beam scanning is occasionally used but it cannot distinguish between components made of different materials. Both techniques draw current from the circuit and interfere with what they are measuring. The Sussex technique can detect signals passing through any component using a device called a scanning electric potential microscope. The SEPM uses a tungsten probe connected to a novel voltmeter. When mounted a few micrometres above a chip, the probe and circuit form a capacitor. Any AC signal flowing beneath the probe causes a displacement current to flow through this capacitor. The value of this current changes depending on the amplitude and phase of the AC signal, enabling the signal to be imaged. Feedback circuitry ensures that the probe draws displacement currents of only 10-15 amps—too low to affect the current flowing in the circuit—by giving the probe an extremely high resistance to AC current. Taking measurements of components 1 micrometre across means the probe needs a resistance of over 1017 ohms. So far the researchers have scanned an operational amplifier 1 micrometre across (Measurement Science and Technology, vol 9, p 1229). But Terry Clark and Robert Prance of the Sussex team say the instrument will be able to scan 10 nanometre components once it is given a high enough resistance. At this scale, individual electric charges could be detected. Akhtar Rasul, yield improvement manager at semiconductor maker Motorola, says: “If it’s going to give you fast diagnostic capability at high resolution, nondestructively,
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