Defective computer chips are the semiconductor industry’s scourge. Even a little defect in a chip with billions of electrical connections might cause a vital function in a computer or other sensitive electronic equipment to fail.
Researchers at the National Institute of Standards and Technology (NIST) have discovered a way for concurrently locating specific electrical problems in many microcircuits on the same chip by adapting an existing methodology for finding errors. Because the technology is based on a very low-cost and widely used imaging equipment, an atomic force microscope (AFM), it may give a new way to evaluate the linked wiring of computer chips in the factory.
AFMs have a razor-sharp tip linked to a small cantilever that vibrates like a diving board. Scientists often provide an alternating current (AC) voltage to the tip as it scans through individual wires buried in parallel several micrometres (millionths of a metre) below the surface of a silicon chip. The voltage differential between the tip and each wire produces an electric force that manifests itself as variations in the frequency or amplitude (height) of the vibrating tip. A wire break or fault will manifest as a sudden shift in tip vibration.
However, there is a disadvantage to using an AFM to look for faults, which is known as electrostatic force microscopy (EFM). The vibration of the tip is influenced not only by the static electric field of the wire under investigation but also by the voltages of all the nearby wires. These unwanted signals obstruct the ability to clearly see flaws in the wire being scanned.
NIST researchers Joseph Kopanski, Evgheni Strelcov, and Lin You addressed the issue by applying particular alternating current voltages provided by an external generator to individual nearby wires rather than the tip. An alternating current voltage (AC voltage) alternates between positive and negative values; when traced over time, the voltage resembles a wave with peaks and troughs. The voltage reaches its largest positive value (the peak) and then falls to its lowest negative level in a single cycle (the valley).
Using this cyclic nature, the researchers supplied the identical AC voltage to surrounding wires as they did to the wire being scanned, with one critical difference: the voltages to the neighbours were precisely out of phase. The voltages to the adjoining wires were at their lowest whenever the voltage to the wire of interest reached its maximum value.
The out-of-phase voltages produced electrostatic forces on the AFM tip that were in opposition to the force produced by the scanned wire. These opposing pressures translated into high-contrast areas on an AFM picture, making it simpler to detect the signal from the wire of interest.
The scientists showed their approach by using a test chip with four pairs of wires buried 4 micrometres under the surface to obtain clear and accurate photographs of flaws. The researchers also demonstrated that by customising the AC voltages given to each wire to have various frequencies, they could see flaws in many neighbouring wires at the same time. Because the technology relies on an alternating current voltage given remotely to the wires rather than the AFM, the researchers called it remote bias-induced electrostatic force microscopy.
“Applying a voltage to the wires instead of the AFM tip may seem to be a little change, but it makes a significant impact,” Kopanski said. “The approach does not need the development of a new instrument and may be simply used by the semiconductor sector,” he noted.
Other approaches for detecting faults, such as X-rays or magnetic fields, are likewise very precise, but need more expensive equipment, according to Strelcov. The researchers presented their findings on November 3 in Pasadena, California, at the 48th International Symposium for Testing and Failure Analysis.
