Simultaneously locating multiple defects on microchips
A minor flaw in a computer chip packed with billions of electrical connections can cause a critical operation in a computer or other sensitive electronic equipment to fail.
By modifying an existing technique for identifying objects, researchers at the National Institute of Standards and Technology (NIST) have developed a method that can simultaneously locate individual electrical flaws in multiple microcircuits on the same chip. Because the technique relies on a relatively inexpensive and common imaging tool, an atomic force microscope (AFM), it may provide a new way to test the interconnected wiring of computer chips in the factory.
An AFM features an ultra-sharp tip attached to a tiny cantilever that vibrates like a diving board. In the standard mode of operation, scientists apply an AC (alternating current) voltage to the tip as it scans across individual wires buried in parallel several micrometres (millionths of a metre) below the surface of a silicon chip. The voltage difference between the tip and each wire generates an electric force revealed as changes in the frequency or amplitude (height) of the vibrating tip. A break or defect in a wire will show up as an abrupt change in the vibration of the tip. However, that method of searching for defects with an AFM, known as electrostatic force microscopy (EFM), affects the vibration of the tip because of the static electric field from the wire under study and also because of the voltages from the neighbouring wires. Those extraneous signals interfere with the ability to clearly image defects in the wire undergoing scanning.
NIST scientists Joseph Kopanski, Evgheni Strelcov and Lin You applied specific AC voltages, supplied by an external generator, to individual neighbouring wires instead of to the tip. An AC voltage alternates between positive and negative values; traced over time the voltage resembles a wave with peaks and valleys. In a single cycle, the voltage reaches its maximum positive voltage (the peak) and then falls to its lowest negative voltage (the valley). Taking advantage of this cyclic nature, the researchers applied the same AC voltage to neighbouring wires as they did to the wire undergoing scanning, with one key difference: the voltages to the neighbours were exactly out of phase. Whenever the voltage to the wire of interest reached its highest value, the voltages to the neighbouring wires were at their lowest.
The out-of-phase voltages exerted electrostatic forces on the AFM tip that opposed the force exerted by the scanned wire. Those oppositely directed forces translated into regions of high contrast on an AFM image, making it easier to distinguish the signal from the wire of interest. Using a test chip with four pairs of wires buried four micrometres beneath the surface, the researchers demonstrated that their technique produced clear and accurate images of defects. In tailoring the AC voltages applied to each wire so they would have different frequencies, the researchers showed that they could image defects in several adjacent wires at the same time.
Because the technique depends on an AC voltage applied remotely, to the wires rather than the AFM, the researchers dubbed the technique remote bias-induced electrostatic force microscopy. “Applying a voltage to the wires instead of the AFM tip may seem like a small innovation, but it makes a big difference. The method does not require a new instrument and could be easily implemented by the semiconductor industry,” Kopanski said.
Other techniques used to spot defects, such as X-rays or magnetic fields, are also accurate but can require costly equipment.
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