When the Chips Are Down, It's Microscopy to the Rescue

Hank Hogan, Contributing Editor

Integrated circuits are modern miracles, cramming tens of millions of transistors into thumbnail-size chips. But there’s a dark lining to this silver cloud — one that a terahertz microscopy technique under investigation by scientists at Okayama University, Osaka University and Riken research institute in Wako, Japan, could play a part in illuminating.

As with other manufactured goods, integrated circuits can have defects; for example, when a connecting trace breaks or a particle is contaminated. The challenge is to find and identify the offender.

That’s difficult to do, given the multiple, stacked layers of metal tracks that cross a chip. In three dimensions, insulating layers on all sides separate those conductors. When viewed from above, however, all that can be seen is a mass of metal. What’s more, a malfunctioning subcircuit may not look any different from one that is working properly.

International Sematech of Austin, Texas, sees the problem of defect and failure analysis as a challenge for tomorrow’s advanced semiconductor manufacturing processes, and the research consortium says that there is a need for nondestructive and automated analysis methods.

To that end, the researchers in Japan are turning to microscopy based on terahertz emissions, a largely unexploited part of the electromagnetic spectrum between the infrared and the microwave regions, with wavelengths hundreds of times that of visible light. Reporting on their work in the Jan. 10 issue of Optics Express, they describe a laser terahertz emission microscope and its use in viewing an operating microprocessor.

Fiber laser

In the laser terahertz emission microscope, the researchers employ a fiber laser producing 100-fs pulses at a wavelength of 790 nm. They split the beam into a pump and trigger pulse, and focus the pump pulse onto the chip sample. The interaction of the near-IR radiation with the chip yields terahertz emissions from the sample that a detector collects. The trigger pulse, which is delayed optically from the pump by picoseconds, activates the detector.

As a result of its construction, the detector responds primarily to emission parallel to the X direction. A full X-Y scan requires rotating the sample 90°, and getting information in the Z direction requires oblique illumination and detection.

The use of a dichroic beamsplitter made of an indium-tin-oxide thin film on glass enabled them to demonstrate a spatial resolution of less than 3 μm with the setup, a nearly tenfold improvement over their previous system’s best. The beamsplitter was transparent to the incoming 790-nm radiation but highly reflective to the outgoing terahertz signal.

Using this instrument, the scientists imaged a running microprocessor. The amplitude of the terahertz emission tracked the direction of the electric field. A different detector and setup, they noted, would enable the user to determine the electric field at any given point, a key piece of information in decoding the operation of local circuitry. Such knowledge is important in deciding whether a defect exists.

Further refinement, the investigators suggest, should ratchet down the resolution even more. By changing the detector layout, it should be possible to perform an X-Y scan without rotating the sample. The next development step would be to employ a backside-illumination scheme, which would enable the nondestructive inspection of chips with multiple levels of interconnects.