Scanning for flaws

An optical beam failure analysis technique that detects integrated circuit defects from both the front and back of the device has recently been developed at Sandia National Laboratories.

The TIVA (thermally-induced voltage alteration)/SEI (Seebeck Effect Imaging) technique finds failures in an integrated circuit quickly and is said to be as fast or faster than any of its predecessors.

The beam from an infrared laser, operating at wavelengths for which silicon is transparent, is focused on the device, heating only a small part of the integrated circuit at a time. The localised heating produces a voltage change on the integrated circuit, which is biased with a constant current source.

An image of the circuit’s response is generated by rastering the laser spot over the circuit with a laser-scanning microscope while recording changes in the power requirements. The laser-scanning optics also generate a reflected light image, which when registered with the TIVA image, allows for rapid localisation of the failure in the circuit.

Faults and failures within the circuit react differently to the heat stimulation than operational components. In an unflawed device, the effects produced by the heat do not change the circuit’s operation. However, if the power demands of the chip change due to the local heating, it is an indication of flaws. The SEI mode detects if a conductor is open, while TIVA locates a short circuit.

‘TIVA is extremely sensitive and allows us to see flaws we either couldn’t detect before or could locate only with significant time and effort,’ said Sandia developer Ed Cole.

One of the significant aspects of TIVA is that it allows for scanning of the integrated circuit from both the front and back.

‘This is important because state-of-the-art chips employ up to seven layers of metal interconnections, preventing direct observation of deeper structures from the front of the device,’ Cole said. ‘Additionally, flip-chip or upside down packaging denies direct access to the front surface. TIVA gives the ability to evaluate the integrated circuit from both sides.’